CC-BY 4.0
Abbreviations
| AMPK | adenosine monophosphate-activated protein kinase |
| BC | breast cancer |
| Bcl-2 | B-cell lymphoma 2 |
| Bcl-xL | B-cell lymphoma-extra large |
| BCRP | breast cancer resistant protein |
| CDK | cyclin-dependent kinases |
| CXCR4 | C-X-C chemokine receptor 4 |
| DOX | doxorubicin |
| DRAM1 | DNA damage-regulated autophagy modulator 1 |
| EMT | epithelial–mesenchymal transition |
| GEM | gemcitabine |
| HIF-1α | hypoxia-inducible factor 1-alpha |
| HKII | hexokinase II |
| LC3 | light chain 3 |
| LDHA | lactate dehydrogenase A |
| LRP | lung resistance protein |
| MCL-1 | myeloid cell leukemia-1 |
| MDR | multidrug resistance |
| METTL | methyltransferase-like protein |
| MRP1 | multidrug resistance associated protein 1 |
| MRP2 | multidrug resistance associated protein 2 |
| p53 | tumor protein p53 |
| p65 (NF-κB subunit) | RelA (a subunit of nuclear factor kappa-light-chain-enhancer of activated B cells) |
| p-Akt | phosphorylated Akt (protein kinase B) |
| PARP1 | poly (ADP-ribose) polymerase 1 |
| PDHK1 | pyruvate dehydrogenase kinase 1 |
| p-ERK1/2 | phosphorylated extracellular signal-regulated kinases 1 and 2 |
| P-gP | P-glycoprotein |
| p-GSK-3β | phosphorylated glycogen synthase kinase 3 beta |
| p-GSK-3β at serine-9 | phosphorylated glycogen synthase kinase 3 beta at serine-9 |
| p-IκB | phosphorylated inhibitor of nuclear factor kappa B |
| p-PI3K | phosphorylated phosphoinositide 3-kinase |
| p-Rb | hyperphosphorylated retinoblastoma protein |
| PTX | paclitaxel |
| Rho-123 | Rhodamine 123 (a fluorescent dye used to assess mitochondrial function and membrane potential) |
| SQSTM1 | sequestosome 1 |
| TRAIL | TNF-related apoptosis-inducing ligand |
| YB-1 | Y-box binding protein 1 |
1. Introduction
Cancer remains a dominant heterogeneous disease affecting large numbers of individuals worldwide [1]. Recent figures from the International Agency for Research on Cancer and the World Health Organization (WHO) indicate that 18.1 million new cases were reported globally, leading to about 9.1 million deaths. While the disease burden and lethality are mostly associated with men, lung cancer is their leading cause of mortality, followed by colorectal, gastric, liver, and breast cancer (BC) in both sexes. Breast cancer is the most frequent cancer among females, and its incidence increases with age. Over 80% of diagnoses occur in females over the age of 50 years [2]. It is the most frequent cancer diagnosed in the United States among women and accounts for approximately 15% of all cancer incidences in the United Kingdom (with an estimated 55 000 new diagnoses annually). It is estimated that there will be an increase of 2% in this figure by the year 2035 [3, 4]. By the year 2050, it is estimated that there will be cancer-related fatalities of 17.1 million individuals and 27 million new cases per annum [5]. Conventional cancer treatments such as surgery, chemotherapy, radiotherapy, hormone therapy, and targeted therapies have improved patient survival over the past 10 years. Such treatments, however, are related to serious side effects and affect a variety of organs. Irrespective of the advancements made in the field of cancer prevention, detection, and treatment, the numbers across the globe are concerning. Even in advanced and aggressive stages of the disease, current treatment modalities can be characterized by low efficiency, a lack of specificity, and the possibility of subsequent recurrence.
Multidrug resistance (MDR), also called chemoresistance, is one of the key hindrances to effectively treat BC since it decreases the effectiveness of chemotherapy. Tumors that initially respond to treatment often recur and develop resistance to several anticancer drugs with distinct chemical structures and modes of action [6]. Multidrug-resistant cancer is a condition where drug-sensitive cancer cells evolve to withstand other unrelated drugs that are quite diverse in terms of molecular target, structures, and functions [7]. This phenomenon is not unique to BC: it is found in ovarian, hematological, pulmonary, and gastrointestinal cancers alike. Multidrug resistance can be caused by several factors that include the differences between how a drug is absorbed, metabolized, and distributed, which vary due to the type of cancer. In other instances, the delivery of drugs may be further limited by physiological hindrances like high interstitial hydrostatic pressure or altered vasculature, which hinders drug access to tumor cells [8, 9].
Doxorubicin (DOX), paclitaxel (PTX), and fluorouracil are commonly used chemotherapeutic agents for BC. For example, PTX leads to apoptosis and cell death through the inhibition of microtubule disassembly and the normal process of cell division [10, 11]. Resistance, however, arises in response to the overexpression of the ATP-binding cassette (ABC) transporters, in particular P-glycoprotein (P-gp) and other transporters that actively extrude drugs from the cancer cells [12]. Other resistance mechanisms include the upregulation of anti-apoptotic processes and the low activity of topoisomerase enzymes, epithelial–mesenchymal transition (EMT), and tumor microenvironment (TME) adaptations. Due to the heterogeneity of the tumors, it is difficult to treat MDR successfully in BC.
Owing to the challenges presented by traditional treatments, scientists are increasingly investigating natural products. The attractiveness of these substances is related to the fact that they are abundant, are chemically heterogeneous, and are considered less toxic. Numerous studies document a diversity of biological activities by natural compounds, including antimicrobial, antioxidant, antithrombotic, antidiabetic, anti-inflammatory, and tumor-suppressant activities. Importantly, they are already being examined as potential means of overcoming MDR. Compound families that include phenols, alkaloids, flavonoids, and terpenoids are known to interfere with resistance through the reduced expression of resistance-associated proteins, limiting cell proliferation, inhibiting blood vessel formation required by the tumor, and activating apoptosis. Furthermore, a combination of natural compounds with established anticancer drugs has demonstrated positive effects on treatment outcome. Owing to these findings, this review seeks to critically examine the mechanisms that underlie MDR in breast cancer and how the pharmacological actions of plant-derived bioactive molecules such as polyphenols, terpenoids, alkaloids, flavonoid compounds, and related derivatives can overcome resistance.
2. Mechanisms of MDR development
Several factors are associated with the rise of MDR, including increased efflux of drugs, diminished absorption, altered metabolism of drugs such as enhanced detoxification, activation of DNA repair systems, alterations in the TME, suppression of apoptotic pathways, EMT, and induction of autophagy [13, 14]. Figure 1 summarizes these mechanisms and shows the processes leading to MDR in breast cancer.
The underlying mechanisms for MDR in BC encompass TME modifications, improved efficiency of DNA repair, modifications at the epigenetic level, the transition of epithelial cells into a mesenchymal-like state (EMT), and drug efflux system upregulation.
2.1. Drug expulsion mediated by ABC transporters
Among major factors contributing to chemotherapy resistance is the decline in intracellular drug concentration, which results from the increased activity of drug efflux pumps [15]. The overexpression of transporters governs this process such as P-gp, a key member of the ABC transporter family. In humans, this family consists of about 48 proteins grouped into seven subfamilies: ABCA, ABCB, ABCC, ABCD, ABCE, ABCF, and ABCG [16, 17]. These transporters are expressed in both normal and cancerous cells, where they influence drug toxicity and pharmacokinetic properties, including absorption, distribution, metabolism, and excretion [18].
The ABC superfamily serves an essential physiological role by removing harmful endogenous molecules and foreign substances from the body. However, in cancer, their function often contributes to MDR, ultimately leading to poor therapeutic outcomes [19]. At least 15 members of this family, including P-gp, MRP2, and BCRP, have been implicated in MDR. Among the most critical resistance mechanisms is the elevated activity of these transporters, which actively expel cytotoxic and targeted anticancer drugs from tumor cells as illustrated in Figure 2. Cancer cells resistant to chemotherapy frequently display high levels of P-gp or related ABC transporters [20].
A visual representation of ABC transporters expelling drugs out of multidrug-resistant cancer cells [21]. MDR is conferred via ABC transporters, which lower intracellular drug concentration. The ABC transporter is overexpressed or activated by cancer cells, increasing chemotherapeutic efflux. This reduces the concentration of drugs within cells and produces an MDR phenotype.
2.2. Mechanism of cell death dysregulation
2.2.1. Evasion of apoptosis
Apoptosis is a genetically controlled event to eliminate the cells that harbor broken DNA and preserve the integrity of tissues. Various signaling pathways regulate this process, and these pathways may be initiated via intrinsic mitochondrial or caspase-mediated extrinsic signals [22]. In both scenarios, activation results in a cascade of caspase enzymes [23]. The ratio of pro- and anti-apoptotic proteins determines whether apoptosis will occur. In most cancer types, evasion of apoptosis is facilitated by the overproduction of anti-apoptotic proteins from the Bcl-2 family such as MCL-1, Bcl-2, and Bcl-xL. An imbalanced distribution of apoptotic regulators can foster chemoresistance because drug-resistant behavior is deeply intertwined with aberrant expression levels of apoptotic regulators.
Normal cells damaged by DNA may initiate apoptosis, which may prevent malignant transformation. Studies have shown that apoptotic dysfunction is closely linked to unregulated cell division and development of tumors, as well as resistance to treatment. Indeed, loss of apoptotic control is regarded as one of the hallmarks of cancer. Approaches to counteracting MDR, thus, should be based on those that reverse apoptotic insensitivity by interfering with signaling cascades involved in resistance. Several anticancer drugs in current clinical use take advantage of the intact apoptotic pathways to induce tumor cell death. Besides these, plant-derived natural compounds such as aloe-emodin, curcumin, genistein, juglone, quercetin, and epigallocatechin-3-gallate have also been found to have strong anticancer properties mediated by apoptosis of resistant cancer cells [24].
2.2.2. Autophagy
Autophagy is a key catabolic pathway in which intracellular material is engulfed and recycled by autophagosomes, which are membrane-bound vesicles. The process is essential for cellular homeostasis. Though apoptosis and necrosis are different, the two processes have shared pathways. Various internal and external stress signals are able to induce the process of autophagy, and it is a frequently aberrant process in cells, especially in cancer cells. Through the disintegration of damaged organelles and proteins, autophagy avoids the buildup of cytotoxic cellular waste [25]. Nevertheless, it is also pivotal in defining tumor behavior, including MDR, immune response, metabolic reprogramming, tumorigenic cell survival, and cell fate in cancer. In multidrug-resistant cancers, autophagy assists cancer cells in resisting therapy, enabling the tumor to survive and remain resistant to chemotherapy. Inhibition of autophagy may enhance apoptosis sensitivity of resistant cells and augment their responsiveness to chemotherapeutic reagents [26].
Targeting autophagy has emerged as a potential therapy approach. Autophagic cell death induced by some chemotherapeutic drugs, such as arsenic trioxide, resveratrol, cyclosporine A, and tamoxifen, could have direct or indirect anticancer activity. Although when used as a treatment strategy, it can be placed in the management of premalignant lesions, inhibiting autophagy is deemed more viable as a pharmaceutical approach to the treatment of advanced cancers. Current clinical research trials are carrying out a study of agents capable of inhibiting autophagy to enhance the therapeutic outcome. The two approaches are pharmacological abrogation and genetic suppression of Atgs [27]. Some examples of pharmacological inhibitors are LY294002, 3-methyladenine, chloroquine, bafilomycin A1, SB202190, and SB203580, whose anticancer effects are caused by impairing autophagy through the inhibition of endosomal–lysosomal acidification or the nucleation and expansion of the phagophore [28].
2.2.3. Activation of DNA repair pathway
Damage of human DNA on exposure to intrinsic and extrinsic genotoxic agents, including ionizing radiation, ultraviolet light, reactive oxygen species (ROS), and environmental and chemical pollutants, occurs on a persistent basis. Unless these lesions are repaired, they tend to build up, causing difficulties in cell survival and genomic integrity as a whole [29]. The DNA repair thus forms a crucial preventive factor against developing cancer. Cells have a number of repair mechanisms, among which mismatch repair, base excision repair, nucleotide excision repair, and repair of double-stranded breaks can be listed [30]. Failure of these systems may result in mutation or chromosomal damage of some tumor suppressor gene, causing cancerous transformation of the tumor (Figure 3) [31].
Mechanism through which cancer develops due to increased DNA damage [31].
The DNA repair pathways are central to chemotherapy or radiotherapy responses in cancer patients, where tumor cell survival is increased through the repair of treatment-induced DNA damage. Indeed, one of the most critical factors in sensitivity to drugs, including cisplatin and other drugs that cause DNA damage, is cellular initiation of the DNA damage response (DDR). The understanding of the mechanisms of DDR pathway alteration is important to enable one to evaluate drug resistance and provide the mechanisms that can resensitize MDR cells [32, 33].
Targeting DNA has remained a prominent therapeutic approach in conventional cancer treatments despite MDR cells repairing DNA damage caused by the drugs. Alkylating agents (e.g., cyclophosphamide and nitrogen mustards), cisplatin, nucleoside analogs (gemcitabine [GEM], fludarabine, and cytarabine), temozolomide, nitrosoureas, thiotepa, and poly(ADP-ribose) polymerase 1 (PARP1) inhibitors (rucaparib, olaparib) exert their effects by causing DNA damage that interferes with proliferation, culminating in cell cycle arrest and apoptosis. Recent studies indicated that a combination of DNA-damaging agents with DDR inhibitors has been associated with an improvement in therapeutic outcomes [34]. For example, Muvarak et al. [35] showed that when combined with PARP inhibitors, DNA methyltransferase inhibitors increase chromatin occupancy of PARP1 and significantly enhance efficacy in AML and BC. Similar results were reported by Liu et al. [36], where resistance of GEM in pancreatic cancer cells was found to correlate with ATR protein and inhibition of ATR restored drug sensitivity.
3. Exploring natural products in cancer treatment
In the past few decades, plant and animal natural products and their derivatives have drawn interest in medicine, especially in the search for novel therapeutic agents. Islam [37] and co-authors tested a wide variety of natural derivatives and showed their efficacy in contrast to traditional BC therapies. Their results indicated that these compounds could enhance patient survival, reduce deaths related to BC, and potentially be used as the foundation of curative approaches. The compounds have provided pronounced anticancer effects through their action on a variety of cellular processes—angiogenesis, differentiation, proliferation, invasion, migration, and metastasis [38].
The antineoplastic flavopiridol, a semi-synthetic flavonoid derived from rohitukine, is among them, since it blocks cell cycle and proliferation through inhibiting cyclin-dependent kinases (CDKs) [39]. Improvements in structural fine-tuning and optimization, coupled with medicinal chemistry and computer-aided drug design, have also resulted in the emergence of new derivatives that have less toxicity and improved antiresistance effect [40]. Moreover, synergy between flavonoids and traditional non-phytochemical anticancer agents has been demonstrated and can result in increased bioavailability of these classical therapies by reducing dosage and thus limiting side effects [14].
Certain natural products counteract drug resistance (MDR) as well. Examples include glaucine, an isoquinoline alkaloid of Corydalis yanhusuo, which does not affect ATPase activity of transporter protein pumps in drug-susceptible MCF-7 BC cells, but potentiates ATPase in MCF-7/ADR drug-resistant cells, and inhibits P-gp- and MRP1-mediated efflux. In the same manner, curcumin has shown to reverse MDR in gastric cancer through regulating NF-κB-related apoptotic mechanisms [41]. In the gastric carcinoma cell line SGC7901/ADR, administration of curcumin reduces the resistance to VCR and ADM due to inhibition of MRP and P-gp [42]. It is due to these properties that natural compounds and their derivatives are becoming central to drug discovery. In fact, it is estimated that more than 64% of new drugs are based on natural sources directly or indirectly [30].
3.1. Natural products with anticancer properties
Medicinal plants have a long history in traditional healthcare systems and, indeed, are known to be used in most parts of Asia and Africa and in some parts of the developed world to provide health benefits; even in the developed world, plants are also taken to promote health. According to the WHO, there are areas that still depend on the use of plant medicine, and other developing countries utilize naturally occurring compounds in the treatment of ailments [43]. Polyphenols, flavonoids, and terpenoids are some of the highest known bioactive molecules that have been extracted from plants demonstrating anticancer potential.
3.1.1. Polyphenols
As shown in Figure 4, polyphenols are an assorted group of compounds that include gallocatechins, flavonoids, tannins, curcumin, and resveratrol, many of which have been recognized for their anticancer properties [44]. These molecules exert their effects partly by inducing apoptosis, and one proposed mechanism involves controlling the release of chromatin-bound copper ions, which leads to DNA fragmentation. Specifically, it has been shown that resveratrol induces DNA degradation when Cu(II) ions are present [44]. In addition, polyphenols can interfere with protein interactions within cancer cells, thereby inhibiting proliferation. Some polyphenols are also known to influence epigenetic modifications, such as acetylation, methylation, and phosphorylation, through direct interactions with regulatory proteins. For instance, curcumin has been shown in multiple cancer cell lines to suppress the expression of tumor necrosis factor (TNF) when exposed to various stimuli [45].
Structure of some representative polyphenols.
3.1.2. Flavonoids
Flavonoids, the predominant category of secondary metabolites, belong to the class of polyphenolic compounds. In 2013, researchers investigated flavonoids extracted from the fern Dryopteris erythrosora (human lung A456 cell line) and reported significant anticancer activity. These compounds displayed cytotoxic properties and strong free radical scavenging effects against cancer cells [46]. Pure flavonoids have also demonstrated inhibitory effects on several cancers, including BC (MCF-7), cervical carcinoma (HeLa), and hepatoma (Hep-G2) [47].
It was found that the flavonoids alpinumisoflavone and 4-methoxy licoflavanone, isolated from the stem bark of Erythrina suberosa, induce apoptosis in leukemia HL-60 cells by both intrinsic and extrinsic pathways. The mechanisms include the activation of apoptotic proteins, the significant depletion of mitochondrial membrane potential (MMP), and eventually the death of malignant cells [48]. Likewise, flavonoid extracts of different fern species have displayed good anticancer activity, even at quite modest doses [49]. A naturally derived flavonoid dihydromyricetin has the potential of overcoming drug resistance. It sensitizes 5-FU by suppressing the MDR1 protein and mRNA expression and thus increases the cytotoxicity of the drug. It also suppresses the expression of MRP2 and its promoter activity in colorectal cancer HCT116/OXA and HCT8/VCR cells, and sensitizes gastric carcinoma SGC7901/5-FU cells [50].
The ubiquitous dietary flavonoid apigenin has been documented to suppress P-gp and BCRP, enhancing intracellular cytotoxic drug concentrations in MDR cells treated with DOX and docetaxel (TXT) [51]. In addition, apigenin diminishes tumor formation and colony formation in cancer stem cells (CSCs) and stem cell invasiveness. In CD44 prostate CSCs, it stimulates caspase-8, apoptotic protease-activating factor-1, and p53 expression and suppresses Bcl-2, sharpin, and survivin, which together increase sensitivity to cisplatin (DDP) [52, 53]. Besides, apigenin is reported to inhibit Akt, STAT3, and mitogen-activated protein kinase (MAPK) signaling in glioblastoma multiforme U373MG and U87MG cells and other CSCs [54]. In other cell models of non-small-cell lung cancer (NSCLC)—A549/oxaliplatin (OXA)/bleomycin (BLM) and H1299/OXA/BLM—apigenin activates pro-apoptotic and p53 proteins, furthering apoptosis [55]. The O-methylated form of apigenin, acacetin, was also observed to inhibit MDR1 gene expression at the mRNA level in the same NSCLC models (A549/OXA/BLM and H1299/OXA/BLM) [56]. Other flavonoids with more or less comparable MDR-modulating activity are shown in Figure 5.
Some representative flavonoids.
3.1.3. Alkaloids
These are a highly heterogeneous group of naturally occurring compounds with a large distribution across families of higher plants, including Loganiaceae and Papaveraceae. A typical example of these drugs is evodiamine, a quinolone alkaloid of Evodia rutaecarpa, which has been reported to have significant anticancer activity in both in vitro and in vivo experiments. Evodiamine causes proliferation arrest and apoptosis; furthermore, it prevents angiogenesis, invasion, and metastases. Moreover, it is able to induce autophagy, which is involved in mechanisms of cellular survival [54].
Evodiamine is nontoxic to normal human cells as compared with many other compounds [55]. It has been demonstrated by randomized controlled studies that evodiamine is a highly efficacious agent against the proliferation of Adriamycin-resistant NCI/ADR-RES BC cells in vitro as well as in vivo in the Balb/c-nude mice. An oral dose of 10 mg/kg twice a day considerably delayed tumor progression, and the treatment in mice initiated at day 6 after tumor inoculation resulted in the reduction of lung metastases with no measurable effects on body weight [56]. In addition to evodiamine, a variety of other alkaloids have been examined for their potent anticancer effects, some of which are highlighted in Figure 6.
The chemical structure of some alkaloids.
4. Anticancer mechanisms of plant-based natural products against multidrug-resistant breast cancer
Natural compounds of plant origin are attracting considerable attention as possible anticancer agents since they can be selectively toxic to cancer cells without damaging healthy cells. A wide range of such compounds have been found that exhibit pronounced anticancer activity. As well as exerting cytotoxicity, these molecules can also bypass drug resistance by impacting numerous molecular and cellular pathways. Key mechanisms include the regulation of MDR-associated proteins, autophagy, NF-κB and PI3K/Akt signaling, as well as other processes involved in cancer progression and MDR.
4.1. MDR-gene-related protein regulation
The presence of MDR is particularly linked to transmembrane transporters encoded by the MDR-related genes such as BCRP, P-gP, MRP, and LRP. These proteins extensively reduce the efficacy of chemotherapy through the reduction of intracellular concentration of anticancer drugs. Natural compounds of plant origin have been promising in overcoming this challenge by downregulating or inhibiting the activity of these transporters. The initial human ABC transporter to be identified was P-gP, which is significantly expressed in drug-resistant tumor cells. The P-gP exports drugs by actively transporting them out of the cells, thereby increasing therapeutic resistance by decreasing the accumulation of the drug within cells and by its interference with apoptosis induced by caspases [57]. Some phytochemicals have proved to reverse this process. As one example, the flavonoid quercetin has been reported to reverse resistance in colon cancer cells, inhibiting the glutamine transporter SLC1A5, increasing DOX uptake, and containing P-gp action [58]. Along the same lines, Zheng et al. indicated that myricetin has potent cytotoxic effects on ovarian cancer cells OVCAR3 and A2780 by triggering apoptosis as well as PTX sensitivity due to P-gP inhibition [59].
Naringin, a dihydroflavonoid, also exhibits powerful MDR-reversal activity. It induces pro-apoptotic Bax and cleaved caspase-3 and inhibits anti-apoptotic Bcl-2 in cisplatin-resistant A549/DDP lung cancer cells. Besides, it inhibits MRP1, p-Akt, P-gP, and CXCR4, dose-dependently [60]. Zhu and others in a study showed that naringin reversed cisplatin resistance in Skov3-sp/DDP ovarian cancer cells through inhibiting NF-κB and P-gP expression [61]. Apigenin has been reported to reverse resistance in A549/DDP cells via both suppression of MDR1 mRNA transcription and inhibition of P-gp-mediated drug efflux, with a resulting reversal index of 2.48 and cell proliferation inhibition [62].
Silibinin, a flavonolignan of Silybum marianum (milk thistle), potentiates the action of other drugs (e.g., DOX and etoposide) against small-cell lung cancer. It also induces apoptosis through the repression of P-gp expression, thus making tumor cells hypersensitive to chemotherapy [63]. Together, these data indicate a promising role of natural compounds in the field of adjuvant therapy of MDR diseases, which is summarized in Table 1.
A detailed overview of the impact and molecular pathways of various plant-derived compounds in combating drug-resistant cancer cells
| No. | Molecule | Concentration | Cell culture and in vivo model | Resistant cancer | Effects | References |
|---|---|---|---|---|---|---|
| Polyphenols | ||||||
| 1. |
 |
100 μmol/L | MCF-7/DOX | BC cells with resistance to DOX | Enhanced uptake of 5-carboxyfluorescein diacetate (5-CFDA) and DOX | [64] |
| 2. |
 |
2.5 μmol/L | K562/DOX | Human leukemia resistant to DOX | Characterized by elevated intracellular DOX levels and reduced levels of S100 calcium-binding protein A8 (S100A8) and P-gP | [65] |
| 3. |
 |
0–30 μmol/L | HepG2/ADR | Human hepatoma resistant to Adriamycin | Decreased MDR expression and P-gP | [66] |
| 4. |
 |
15.81 μg/mL | SiHa/DDP | Cervical cancer resistant to cisplatin | Reduced the levels of P-gP and ABC subfamily G member 2 (ABCG2) | [67] |
| Terpenoids | ||||||
| 1. |
 |
20–320 μmol/L | Skov3-sp/DDP | Cisplatin-resistant ovarian cancer | Reduced expression of ABCB1, LRP, and P-gP | [68] |
| 22.02 μg/mL | K562/DOX | Human leukemia resistant to DOX | Reduced levels of P-gP | [69] | ||
| 31.92 μg/mL | K562/DNR | Human leukemia resistant to daunorubicin | Reduced levels of P-gP | [70] | ||
| Flavonoids | ||||||
| 1. |
 |
25 μmol/L | A2780 OVCAR3 |
Malignant ovarian cells | Increased cleaved caspase-3 and Bax. Reduced levels of Bcl-2 and MDR1 |
[59] |
| 2. |
 |
20 μmol/L | MDA-MB-231 MCF-7 |
Human mammary carcinoma cells | Reduced levels of P-gP | [71] |
| 3. |
 |
33 μmol/L | SW620/Ad300 | Colon carcinoma cells in humans with resistance to DOX | Enhanced drug accumulation and apoptosis | [58] |
| 4. |
 |
5–80 mol/L | A549/DDP | Lung carcinoma cells in humans with cisplatin resistance | Reduced levels of LRP and P-gP | [62] |
| 5. |
 |
60 μmol/L | VPA17 | Small-cell lung carcinoma cells with MDR | Reduced drug efflux | [63] |
| Alkaloids | ||||||
| 1. |
 |
20 μg/mL | BALB/c-nude mice bearing MCF-7/DOXFluc-derived xenografts | Breast carcinoma cells with resistance to DOX | Decreased expression of ABCB1 or P-gP and MRP1 or ABCC1 | [72] |
| BGC-823/DDP: 549.6 μmol/L SGC-7901/DDP: 562.1 μmol/L |
Xenograft tumors derived from SGC-7901/DDP cells in BALB/c-nude mice, alongside BGC-823/DDP and SGC-7901/DDP cell lines | Cisplatin-resistant gastric cancer | Upregulated expression of Bax, cleaved caspase-3, and cleaved caspase-9 accompanied by reduced levels of MDR1, MRP1, and key proteins involved in the PI3K/Akt/mTOR signaling pathway | [73] | ||
| 2. |
 |
100, 200 μg/mL | 253J/DOX | Human bladder cancer cells resistant to DOX | Increased expression of cleaved PARP and Bax. Reduced P-gP levels |
[74] |
| 3. |
 |
32.5 μg/mL | BIU-87/ADM | Bladder tumor cells from humans resistant to Adriamycin | Reduced levels of P-gP and Adriamycin efflux | [75] |
4.2. Apoptosis induction via the PI3K/Akt signaling pathway
The intracellular apoptotic pathway, which mainly controls apoptosis by the mitochondria, can be controlled by pro-apoptotic proteins. The PI3K/Akt/mTOR signaling pathway has a central involvement in inhibiting apoptosis homeostasis, downregulating all pro-apoptotic and upregulating anti-apoptotic substances. The involvement of KT in this phenomenon is attained through KT-induced phosphorylation of certain targets, among which is caspase-9, which makes it impossible to initiate the process of passing through the cascade of apoptotic events. The PI3K pathway regulates vital cellular activities such as proliferation, survival, growth, and angiogenesis; its dysregulation is a common aberration in human malignancies and a critical component in tumor development [76].
Dysregulated activation of PI3K/Akt signaling may activate several downstream pathways, which further promote tumor progression and drug resistance. Several phytochemicals have been shown to counteract these effects. As another example, curcumin, a rhizome of Curcuma longa (family Zingiberaceae) polyphenolic compound, can reverse MDR in L1210/Adr cells by modulating P-gp expression through PI3K/Akt/NF-κB pathway suppression [77]. Nobiletin, a citrus methoxyflavone found in orange peels (Citrus sinensis), reverses the efflux of ABCB1 substrates in A549/T and A2780/T cells, enhances intracellular accumulation of docetaxel, and inhibits ABCB1 transporter activity. Inhibition of the ATP hydrolase activity and Akt/ERK/Nrf2 pathway by nobiletin makes the cells sensitive to PTX and DOX [78].
Other phytochemicals also show activity against multidrug-resistant cancer via the PI3K/Akt pathway. Emodin has been found to inhibit the PI3K/Akt signaling pathway, ↓Bcl-2 and ↑Bax, and ↑caspase-3 to sensitize 5-fluorouracil (5-Fu) mediated cytotoxicity in SW480 cells. Similar effects on cisplatin resistance are also attributed to shikonin, a naphthoquinone extracted from Arnebiae Radix, which can also reverse cisplatin resistance in HeLa/DDP cells dose-dependently. Du et al. confirmed that levoshikonin, a derivative, had a drug-resistance index of 11.8 in these cells and downregulated Bcl-2, simultaneously overexpressing Bax and cleaved caspase-3 and causing cell cycle arrest at the G0/G1 area [79]. The search results of these plant-derived or other compounds, with their anticancer potential and how they treat drug-resistant cancers, are summarized in Table 2.
A thorough overview of the ways in which various compounds derived from plants impact drug-resistant cancer cells
| No. | Molecule | Concentration | Resistant cancer | Cell line and in vivo system | Mechanism | References |
|---|---|---|---|---|---|---|
| Polyphenols | ||||||
| 1. |
 |
0–45 μmol/L | Mouse leukemia cells with resistance to Adriamycin | L210/ADR | Reduced levels of p-GSK-3β at serine-9, P-gP, and p-Akt at serine-473 | [77] |
| 2. | Grape seed procyanidin | 10–40 μmol/L | Ovarian cancer cells with resistance to PTX | A2780/T | Increased accumulation of Rho-123, p65 (NF-κB subunit), and YB-1. Lower expression of p-ERK1/2, p-IκB, P-gP, and p-Akt |
[80] |
| 3. |
 |
31.62 μmol/L | Ovarian carcinoma cells resistant to PTX | A2780/T | Higher levels of p53. Decreased expression of Akt (protein kinase B), p-Akt, p-ERK1/2, and Nrf2 |
[78] |
| 50 μmol/L | Adriamycin-resistant NSCLC cells | A549/ADR | Reduced levels of MRP1, MYCN, β-catenin, p-GSK-3β, Bcl-2, and survivin. | [81] | ||
| 4. |
 |
20 μg/mL | DOX-resistant breast carcinoma cells | MCF-7/DOX | Lower expression of P-gP, BCRP, and MRP1 | [82] |
| 5. |
 |
5, 10 μmol/L | Leukemia cells resistant to Adriamycin | HL-60/ADM | Increased levels of cleaved PARP and cleaved caspase-3. Decreased levels of MRP1, LRP, Bcl-2, and p-Akt |
[83] |
| 6. |
 |
20–80 μmol/L | Human colon carcinoma cells cultured in low-oxygen conditions | HCT116 | Reduced expression of HIF-1α, HKII, PDHK1, LDHA, p-PI3K, and p-Akt | |
| 7. |
 |
45.9 μmol/L | Human leukemia cells with resistance to Adriamycin | K562/ADR | Increased levels of caspase-3 and caspase-8. Lower expression of P-gP, MDR1, and p-Akt |
|
4.3. Control of the autophagy pathway
Autophagy is a viable therapeutic target in cancer since such regulation can affect both the onset and advancement of cancer. Natural products have been shown to target major intermediates of this process such as Beclin-1, p53, Atg, and PI3K/Akt/mTOR signaling [84]. Clinical and experimental reports have shown that MDR is closely related to autophagy. Autophagic activity in tumors with poor prognoses tends to be high, which implies its role in drug resistance. Beclin-1 and LC3 have gained wide acceptance as markers for autophagy, whereby Beclin-1 is able to evoke both autophagy and apoptosis [85].
Nature has provided several compounds that have been found to control autophagy in cancer. As summarized in Table 3, a flavonoid isoliquiritigenin extracted from licorice also suppresses MES-SA uterine sarcoma cells by arresting the cell cycle at the G1 stage. It sensitizes MES-SA/Dx5-R and MES-SA/Dx5 cells to DOX and causes autophagy through increasing LC3B-II [86]. The second flavonoid baicalein overcomes P-gp-mediated Bel7402/5-Fu resistance in hepatocellular carcinoma by activating autophagy [87]. Likewise, Scutellaria baicalensis extract and cisplatin were described to augment Atg5 and Atg12, contributing to autophagy in the cisplatin-resistant A2780 cell population [88]. In cervical cancer C-33A cells, baicalin, a flavonoid from S. baicalensis, stimulated autophagy through Beclin-1, Atg5, and Atg12, increasing sensitivity to cisplatin [67]. During autophagy, cytoplasmic vacuoles become apparent, allowing the formation of autophagosomes and autolysosomes to be distinguished. During autophagy, cytoplasmic vacuoles become apparent, allowing the formation of autophagosomes and autolysosomes to be distinguished.
An overview of the effects and underlying mechanisms associated with drug-resistant cancer
| No. | Molecule | Concentration | Resistant cancer | Cell culture and in vivo models | Effects | References |
|---|---|---|---|---|---|---|
| Flavonoids | ||||||
| 1. |
 |
0–100 μM/L | Uterine sarcoma cells exhibiting resistance to DOX | MES-SA/Dx5 and its resistant variant MES-SA/Dx5-R |
Decreasing the levels of PARP, Bcl-2, caspase 7, and SQSTM1/p62 while enhancing the levels of p-mTOR, cleaved caspase 7, LC3B-I, LC3B-II, and cleaved PARP | [86] |
| 2. |
 |
0–10 μg/mL | Hepatocellular carcinoma (HCC) cells resistant to 5-fluorouracil | Bel7402 cells resistant to 5-fluorouracil | Reducing the expression of P-gP and Bcl-xL | [87] |
| 3. |
 |
20–40 μM/L | Cervical carcinoma with acquired cisplatin resistance | C-33A cells resistant to cisplatin | Boosted the ratio of MAPLC3II/MAPLC3I, Atg5, Atg12, and Beclin-1 | [67] |
| 4. |
 |
25 μM/L | Human leukemia exhibiting resistance to Adriamycin | K562/ADR | Lowering the NF-κB activity, p-JNK, p-Akt, cyclin B1, and MDR1. Accumulation of Adriamycin and elevating the levels of LC3-II, Beclin-1, and caspase 3 |
[89] |
| 5. |
 |
50–200 μM/L | Ovarian adenocarcinoma exhibiting resistance to cisplatin | A2780/DDP | Decline in Bcl-2, AMPK, LC3, Beclin-1, and mTOR levels. Increase in the levels of caspase 3, p62, Bax, and p53 |
[67] |
| Alkaloids | ||||||
| 1. |
 |
0.2 μg/mL | Human leukemia cells resistant to Adriamycin | K562/ADM | Boosted the expression of LC3-II while suppressing p62 expression | [90] |
| 2. |
 |
6 μM/L | Imatinib-resistant human leukemia | K562/IM | Decreasing Atg5, Beclin-1, P-gP levels, and LC3-II. Increasing the levels of p62 | [67] |
| 3. |
 |
8 μM/L | Icotinib-resistant NSCLC | PC-9/IR | Reducing p-mTOR/mTOR ratios. Increasing the ratio of Beclin-1, p53, and LC3-II/LC3-I |
[91] |
| 4. |
 |
100 μM/L | Breast carcinoma cells resistant to DOX | MCF-7/ADR, xenografts in BALB/c mice | Elevated the levels of Akt, mTOR, and p62 while lowering the levels of MDR1, LC3-II/I, and PTEN | [92] |
| Terpenoids | ||||||
| 1. |
 |
15 μg/mL | Human lung cancer cells resistant to cisplatin | SPC-A-1/DDP | Increased the levels of Beclin-1 while reducing the levels of MRP1 and P-gP | [67] |
| 120 μg/mL | NSCLC resistant to gefitinib | PC9GR and HCC827GR | Elevated the levels of LC3B-II and SQSTM1 while suppressing the levels of METTL3 | [93] | ||
| 2. |
 |
0–100 nmol/L | Ovarian cancer cells with resistance to cisplatin | Skov3-sp/DDP | Reduced the N-cadherin and MMP9 levels while increasing the Beclin-1, E-cadherin, LC3, and LC3-II/LC3-I ratios | [94] |
| Polyphenols | ||||||
| 1. |
 |
15–240 μg/L | Cervical carcinoma with resistance to cisplatin | HeLa/DDP | Boosted the ratio of Beclin-1, p-AMPK/AMPK, Bax, and LC3-II while lowering LC3-I levels | [95] |
| Compounds | ||||||
| 1. | Extract of S. baicalensis | 0–400 μg/mL | Ovarian carcinoma resistant to cisplatin | A2780Cis | Decreased the levels of DRAM1, p53, Atg5, and Beclin-1. Increased the levels of Atg12 |
[88] |
| 2. | Kang-ai injection | 100.07 g/L | Human lung carcinoma resistant to cisplatin | A549/DDP | Increased the LC3-I, Atg7, cleaved caspase 3 levels, LC3-II, Beclin-1, and ratio of Bax/Bcl-2 | [67] |
A similar mechanism has occurred in other bioactive molecules like puerarin, an isoflavone of Pueraria lobata, which triggers cell cycle arrest, apoptosis, and autophagy in K562/ADR with increased autophagosomes and elevated LC3-II and Beclin-1 proteins [89]. Hesperetin induces apoptosis, AMPK/mTOR and p53 signaling inhibition, Bcl-2 repression, and Bax, p53, and caspase-3 expression to sensitize A2780/DDP ovarian cancer cells to cisplatin [67]. Matrine is also reported to target MDR in K562/ADM leukemia cells. It augmented autophagic vacuoles and LC3-II expression and diminished p62, later recovering its vincristine and DOX sensitivity gaining resistance-reversal indices of 4.91 and 10.12, respectively [90]. Alkaloid lycorine II was shown to inhibit the proliferation of both imatinib-sensitive and imatinib-resistant K562/IM cells in a concentration-dependent manner via induction of G0/G1 phase cell cycle arrest and promotion of apoptosis. At the mechanistic level, it caused downregulation of Bcl-2, elevation of the Bax/Bcl-2 ratio, and activation of caspase-3. In resistant cells, lycorine also overcame MDR by reducing the expression of Beclin-1, Atg5, LC3-II, and P-gp while increasing p62 levels [67]. Cepharanthine enhanced the responsiveness of PC-9/IR cells to icotinib through the activation of p53-mediated autophagy, increasing Beclin-1, the LC3-II/LC3-I ratio, and autophagosome formation while suppressing the p-mTOR/mTOR ratio [91].
The dual role of intervening in autophagy by means of 95-elemene has been widely investigated. Zhou et al. demonstrated that 8-elemene enhances the sensitivity of SPC-A-1/DDP lung cancer cells to chemotherapy by promoting the expression of Beclin-1 and inhibiting the expression of MRP1 and P-gp to induce autophagy [67]. Meanwhile, β-elemene inhibited autophagy in gefitinib-resistant NSCLC cells by suppressing lysosomal acidification, thereby impairing autophagic flux and increasing SQSTM1 and LC3B-II protein levels. It also decreased m6A methylation in these resistant cells [93].
Other compounds work in different ways. As an example, triptolide can exert an antimigratory and anti-invasive effect on Skov3-sp/DDP ovarian cancer cells by suppressing JAK2/STAT3 signaling and activating autophagy [94]. When used with cisplatin, naringenin upregulates AMPK-induced autophagy through the increased phosphorylation status of Beclin-1, AMPK, Bax, and LC3-II, and a reduction in LC3-I [95]. Lastly, Kang-ai injection was found to induce autophagy as well as apoptosis in A549/DDP cells [67].
4.4. Regulation of MAPK and NF-κB mediated signaling pathways
As summarized in Table 4, NF-κB is one of the key players in signaling processes that are implicated in cancer development and the development of drug resistance. Natural products have been reported to revamp MDR through NF-κB mediated pathways. For example, hesperidin lowers the post-activation of P-gp as well as NF-κB, thus increasing the responsiveness of cancerous lung cells to cisplatin [96]. Matrine, an alkaloid, downregulates the ABCB1 resistance gene by targeting NF-κB, making leukemia cells become more sensitive to DOX [97]. Furanodiene was also reported to cause apoptosis of DOX-resistant BC cells by downregulating anti-apoptotic proteins Bcl-xl and activating caspases. Similarly, triptolide induces resistance through time- and dose-dependent downregulation of NF-κB activity, and downregulates drug-resistance genes, including FLIP and Bcl-2 [98]. Schisandrin, a compound extracted from Schisandra chinensis, reinforces the activity of DOX in BC cells by affecting NF-κB signaling and downregulating resistance-related proteins [99].
An overview of the effects and mechanisms associated with drug-resistant cancer
| No. | Molecule | Concentration | Resistant cancer | Cell culture and in vivo model | Effect | References |
|---|---|---|---|---|---|---|
| Polyphenols | ||||||
| 1. |
 |
0.6–160 μM/L | Human lung carcinoma exhibiting cisplatin resistance | A549/DDP | Reduction observed in the p-p65/t-p65 ratio, P-gP, and P-IκB/IκB ratio | [96] |
| 2. |
 |
100 μM/L | Osteosarcoma exhibiting resistance to Adriamycin | U2OS/ADR | Lower levels of p-p38, P-gP, and acetylated p65 | [100] |
| 3. |
 |
46.47 μg/mL | NSCLC exhibiting resistance to cisplatin | A549/DDP | Reduction in P-gP, Bcl-2, and caspase 3 Elevation detected in Bax, p-C-Jun, cleaved caspase 3, p53, and p21 |
[101] |
| 4. |
 |
8 μg/mL | Human lung cancer with resistance to cisplatin | A549/DDP | Reduced expression of p-Akt and MARK2 | [102] |
| 5. |
 |
20 μM/L | Esophageal carcinoma exhibiting resistance to vincristine | Eca-109/VCR | Suppressed levels of P-gP, ERCC1, p38MAPK, and p-p38MAPK | [103] |
| 0–60 μg/mL | Liver cancer exhibiting resistance to Adriamycin | HepG2/ADM | Downregulation of p38MAPK | [104] | ||
| 6. |
 |
BGC-823/AP: 53.64 μg/L MGC-803/AP: 56.83 μg/L |
Stomach cancer unresponsive to apatinib | BGC-823/AP MGC-803/AP |
Reduced expression of MAPK1, HKII, LDHB, and GLUT1 | [105] |
| Lignans | ||||||
| 1. |
 |
20 μM/L | Human mammary carcinoma unresponsive to DOX treatment | MCF-7/DOX | Increased levels of PARP and cleaved caspase 9. Reduction in P-gP expression |
[99] |
| 2. |
 |
0–16 μM/L | Lung cancer exhibiting resistance to cisplatin | A549/DDP, xenografts of A549/DDP in BALB/c-nude mice | Downregulation observed in p65, P-gP, MRP1, p50, and p52 | [106] |
| Alkaloids | ||||||
| 1. |
 |
0–300 μM/L | Human leukemic cells unresponsive to Adriamycin treatment | K562/ADR | Reduced expression of phosphorylated NF-κB, ABCB1, survivin, ABCC1, ABCG2, and Bcl-xL. Elevated levels of cleaved caspase 9 |
[97] |
| 2. |
 |
0–8 μM/L | Stomach cancer unresponsive to vincristine therapy | SGC-7901/VCR | Increased expression of p-JNK, cleaved-caspase 9, cleaved-caspase 3, p-p38, and activation of PARP. Downregulation observed in Bcl-2, phosphorylated ERK (p-ERK), and phosphorylated CDC2 (p-CDC2) |
[107] |
| Terpenoids | ||||||
| 1. |
 |
0.200 μM/L | Breast carcinoma unresponsive to DOX treatment | MCF-7/DOX | Elevated levels of IKKα/β, Bad, caspase 3/7/8, p65, and PARP. Reduced expression of Bcl-xl |
[108] |
| 2. |
 |
0.03–3 μM/L | Human adenocarcinoma of the lung unresponsive to Taxol therapy | A549/Taxol | Downregulation observed in COX-2, FLIP, NF-κB, Bcl-xL, P62, XIAP, and Bcl-2 | [98] |
| 3. |
 |
0–4 μg/mL | Human leukemia with resistance to Adriamycin | K562/ADM | Increased phosphorylation of p38 (p-p38) and ERK1/2 (p-ERK1/2). Lowered levels of MDR1 and P-g |
[109] |
| 4. |
 |
HCT116-OxR: 5.13 μM/L HT29-OxR: 9.46 μM/L |
Colorectal carcinoma unresponsive to oxaliplatin therapy | HCT116-OxR HT29-OxR |
Downregulated levels of CDC2, cyclin B1, Bid, Bcl-2, and Bcl-xL. Elevated expression of CHOP, p21, GRP78, p27, DR4, DR5, p-JNK, p-p38, Bim, and Bax |
[110] |
The other signaling that remains imperative in drug resistance is MAPK, which is categorized into p38, ERK, and JNK subpathways. In different scenarios, these cascades may work towards enhancing or suppressing cancer cell survival. Natural compounds are also well researched because they modulate the signaling of MAPK and subsequently reverse resistance. As an example, resveratrol enhances the sensitivity of osteosarcoma to Adriamycin by inhibiting NF-κB as well as MAPK [100]. Rosmarinic acid triggers apoptosis both in lung cancer and cisplatin-resistant lung cancer cells, as well as downregulates HSP70; its mechanism is via activating MAPK signaling and retaining tumor suppressor proteins like p53 [101]. A combination of baicalin and cisplatin demonstrated induction of apoptosis in resistant lung carcinoma cells by suppressing survival routes that are mediated by MAPK [102].
Curcumin, which is the product of turmeric, restores MDR in liver and esophageal cancer by targeting MAPK signaling [103, 104]. Paeonol targets gastric cancer cell metastasis, growth, and resistance through certain RNA and MAPK pathways [105]. In leukemia cells, inhibition of resistance-associated proteins and MAPK components restores drug sensitivity [109]. Isolinderalactone has been shown to stimulate apoptosis, which occurs as a distal effect of oxidative stress induction in pro-death proteins as signified by the MAPK signaling pathway [110]. What is more, a combination of tetrandrine and vincristine has been also mentioned to interfere with MAPK signaling and overcome gastric cancer resistance, which ultimately leads to inducing the apoptotic processes [107].
4.5. Cell cycle
Uncontrolled cell proliferation is a hallmark of cancer, making cell cycle disruption a crucial strategy in the treatment of drug-resistant malignancies [111]. Regulation of cells by passing through various processes in the cycle is highly guided by cyclins and allied kinases (like CDKs) [112]. Both cyclin D1 and cyclin E are involved in the G1 phase, wherein they activate CDK2/4/6, and low activity of cyclin B1 and CSK1/2 is linked to cell cycle arrest during the G2/M phase [113, 114]. Natural products have proven to be promising in terms of intervening with such mechanisms. Shikonin, which is an extract of Lithospermum erythrorhizon Sieb. et Zucc., in synergy with cisplatin targets the G1/S phase and acts as an apoptotic agent on cisplatin-resistant ovarian Skov3-sp/DDP cancer cells. It counteracts resistance by augmenting Bcl-2, cyclin D1, p-Rb, and CDK2 and reducing p18, Bax, and cleaved caspase-3. Plumbagin is another compound isolated from Plumbago zeylanica (chitrak) roots that displays potent anticervical cancer activity by inhibiting p-Rb, activating apoptosis, ROS production, and a block during S and G2/M phases [115]. Tetramethylpyrazine has also been noted to reverse MDR through inhibition of proteins like Bcl-2, MRP1 as well as GST and stimulating the TPOP-II expression in a concentration-dependent manner. It also triggers G1 arrest in the cells of T24/DDP and Pumc-91/ADM. Another ester-based derivative of cephalotaxine is the growth-inhibitory melanoma drug homoharringtonine, which induces G0/G1 arrest and reduces levels of ERK, IRS4, PI3K, Akt, cyclin E1, and CDK2 [67]. The rest of the compounds have a different mechanism of action. A triptolide analog prepared by Yang et al. showed cell-specific toxicity in cisplatin-resistant cells, blocking the G2/M checkpoint, via RS homodimerization and likely p53–MDM2 interaction interference [116]. Isodon excisus, an ent-kaurane diterpenoid, caused G2 arrest in resistant human liver cancer cells and inhibited cell migration by downregulation of the MDR1 and PTEN–Akt pathways [67]. Ginsenoside Rh2, the main active ingredient of Panax ginseng, inhibited G0/G1 arrest in colorectal cancer by downregulating resistance proteins including MRP1, MDR1, and GST to improve chemotherapy activity [117]. Curcumin has also demonstrated a dual impact by producing ROS and causing a halt in ICC; it can thus help overcome 5-Fu resistance in colorectal carcinoma [118]. In line with this finding, cisplatin in combination with isoxanthohumol was used to treat cisplatin-resistant lung cancer cells and the cells were found to undergo arrest in the G0/G1 cell cycle phase inhibiting the PI3K/Akt pathway [67]. Metformin was revealed to bypass gastric cancer oxaliplatin resistance via apoptosis and reduction in cyclin D1 expression [67]. The various ways and the mechanisms through which the cell cycle is regulated by natural compounds to overcome drug resistance have been further briefly discussed in Table 5.
An overview of the effects and mechanisms associated with multidrug-resistant cancer
| No. | Molecule | Concentration | Resistant cancer | Cell culture and in vivo model | Mechanism | References |
|---|---|---|---|---|---|---|
| Quinones | ||||||
| 1. |
 |
4 μM/L | Ovarian cancer exhibiting resistance to cisplatin | Skov3-sp/DDP | Upregulation of Bax, cleaved caspase 3, and P18 along with decline in cyclin D1, CDK2, p-Rb/Rb ratio, and Bcl-2 expression | [67] |
| 2. |
 |
90.8 μM/L | Cervical squamous carcinoma in humans with cisplatin resistance | A431/Pt | Increased levels of ROS, phosphorylated Rb (p-Rb), and sub-G0 phase populations | [115] |
| Alkaloids | ||||||
| 1. |
 |
73 μM/L | Human bladder carcinoma exhibiting MDR | Pumc-91 (ADM) and T24 (DDP) | Elevated levels of S phase populations and Topo-II. Reduced expression of Bcl-2, MRP1, and GST |
[119] |
| 2. |
 |
18.57 ng/mL | Melanoma unresponsive to vemurafenib therapy | A375R | Increased G0/G1 phase cell populations Decreased levels of p-ERK, cyclin E1, IRS4, PI3K, p-Akt, and CDK2 |
[67] |
| Terpenoids | ||||||
| 1. | Triptolide succinate monoester | 33.32 μM/L | Human pulmonary adenocarcinoma unresponsive to cisplatin therapy | A549/DDP | Elevated levels of G2/M phase populations and apoptosis | [116] |
| 2. |
 |
62.97 μM/L | Liver cancer exhibiting resistance to Adriamycin | HepG2/ADM | Increased expression of Bax, phosphorylated PTEN (p-PTEN), and G2 phase cells. Reduced expression of p-Akt, Bcl-2, and MDR1 |
[67] |
| Saponin | ||||||
| 1. |
 |
20 μM/L | Human colorectal carcinoma exhibiting resistance to 5-fluorouracil | HCT-8/5-Fu and LoVo/5-Fu | Elevated levels of cleaved-caspase 3, phosphorylated IκB (p-IκB), G1 phase populations, and E-cadherin. Reduced expression of cyclin D1, CDK2, phosphorylated Rb (p-Rb), GST, N-cadherin, MMP9, MRP1, LRP, Bcl-2, vimentin, and MDR1 | [117] |
| Polyphenol | ||||||
| 1. |
 |
13.69 μM/L | Colorectal carcinoma exhibiting NNMT-associated resistance | SW480/NNMT | Elevated levels of phosphorylated Rb (p-Rb), G2/M phase populations, and ROS. Reduced expression of phosphorylated STAT3 (p-STAT3), CDK1, and CDK2 |
[118] |
| Flavonoid | ||||||
| 1. |
 |
42.1 μM/L | Lung adenocarcinoma in humans exhibiting resistance to cisplatin therapy | A549/DDP | Elevated levels of G0/G1 phase populations. Reduced expression of P-gP, LRP, MRP, PI3K, and phosphorylated Akt (p-Akt) |
[67] |
| Compounds | ||||||
| 1. |
 |
28.02 μg/mL | Gastric cancer unresponsive to oxaliplatin therapy | SGC-7901 (L-OHP) | Reduced expression of cyclin D1 | [67] |
| 2. | Buzhong yiqi decoction | 0.25 g/mL | Human stomach cancer resistant to cisplatin | SGC-7901/CDDP | Elevated levels of G1/S phase cell populations. Decreased levels of NF-κB, survivin, and Bcl-2 |
[120] |
| 3. | Digitoxin | 132.65 nmol/L | Human liver carcinoma with resistance to Adriamycin | HepG2 (ADM) | Upregulation of cyclin B1, phosphorylated CHK2 (p-CHK2), phosphorylated ATR (p-ATR at Ser428), and γH2AX foci. Bax accompanied by an increased proportion of cells in the G2/M phase. Lowered expression of Bcl-2, phosphorylated CDK1 (p-CDK1 at Thr14), and CDC25C |
[121] |
4.6. Targeting the proliferation of CSCs
The CSCs represent a particular subpopulation of malignant cells characterized by their capacity to differentiate into multiple cell types and renew themselves. They have been considered the key element in the initiation, progression, recurrence, and resistance to therapy in cancer [122]. As summarized in Table 6, a naturally occurring flavonoid (3,4,5,7-tetrahydroxyflavone), luteolin, has been demonstrated to prolong the survival rate and decrease the stem cell marker (Sox2) and CSC features by inhibiting the PI3K/Akt pathway. Furthermore, it inhibits epithelial–mesenchymal switch, the process related to tumor development, lowering the amount of vimentin, slug, and N-cadherin, whereas increasing E-cadherin. The combination of these effects augments apoptosis in EC109/PTX and TE-1/PTX paclitaxel-resistant esophageal cancer cells [67]. Zhao et al. reported inhibition of Sox2 by luteolin and attenuation of CSC-like markers by inhibiting Akt and UBR5 phosphorylation in similar resistant cells [123].
Plant-derived natural products targeting proliferation in resistant cancer
| No. | Molecule | Concentration | Resistant cancer | Cell culture and in vivo model | Effect | References |
|---|---|---|---|---|---|---|
| Flavonoids | ||||||
| 1. |
 |
20 μmol/L | Esophageal carcinoma exhibiting resistance to PTX | TE-1 (PTX) and EC109 (PTX) | Elevated expression of E-cadherin. Reduced levels of N-cadherin, slug, and vimentin |
[67] |
| 0–40 μmol/L | Esophageal carcinoma exhibiting resistance to PTX | TE-1 (PTX) and EC109 (PTX) | Increased expression of p-PI3K, p-Akt, and E-cadherin. Downregulated levels of NANOG, Sox2, and OCT4 |
[123] | ||
| 2. |
 |
0.7 μmol/L | Human breast carcinoma with resistance to DOX | MCF-7/DOX | Higher levels of PTEN expression. Decreased population of CD44+/CD24−/low, and p-Akt |
[124] |
| 3. |
 |
10 μmol/L | Hepatic cancer resistant to multiple drugs | SMMC-7721 | Lowered levels of CD133, CD44, Gli1, and ABCG2 | [125] |
| Alkaloids | ||||||
| 4. |
 |
10 μg/mL | Colon CSCs unresponsive to 5-fluorouracil | HCT116/5-Fu | Increased level of apoptosis. Reduced expression of P-gP, ABCB1, p-PI3K, p-Akt, and p-mTOR |
[67] |
| 5. |
 |
Neferine: 6.5 μmol/L Isoliensinine: 12.5 μmol/L |
ColonCSCs with resistance to cisplatin | HCT-15/DDP-CSCs | Elevated levels of Bax, cytochrome c, ROS, caspase 3, PARP, and sub-G0 phase populations. Decreased expression of PI3K, phosphorylated Akt (p-Akt), mTOR, and BCl-2 |
[126] |
| Terpenoids | ||||||
| 1. |
 |
9–32 μg/g | Drug-resistant ovarian CSCs studied in nude mouse models | Skov3-sp | Reduced expression of ABCG2 | [127] |
| 2. |
 |
A2780Cis: 1.46 μg/mL Skov3-spPac: 1.8 μg/mL | Ovarian carcinoma exhibiting resistance to therapeutic drugs | A2780CisSkov3-spPac | Elevated levels of Ptch1. Lowered levels of Sox2, Nanog, Bmi1, and Shh |
[128] |
| Polyphenol | ||||||
| 1. |
 |
10, 20 μmol/L | Drug-refractory colorectal cancer | HCT116 (OX) and HCT8 (5-Fu) | Reduced levels of CD133, ALDH1, and CD24 | [67] |
| 10 μmol/L | Gastric carcinoma exhibiting resistance to 5-fluorouracil | BGC-823 (5-Fu) | Elevated expression of E-cadherin and GSK-3β. Decreased expression of vimentin, Wnt2, and β-catenin |
[67] | ||
| Compound | ||||||
| 1. | Yiqi Huoxue Jiedu decoction | 2.5 g/mL | Gastric carcinoma resistant to 5-fluorouracil treatment | BGC823/5-Fu | Reduced expression of P-gP, MRP1, Nrf2, p-PI3K, and p-Akt | [129] |
Natural products and alkaloids of natural origin also possess activity against CSCs. Synergistically, quercetin with DOX restores the PTEN/Akt pathway-mediated resistance of MCF-7/DOX BC cells. It increases PTEN expression and prevents the production of Akt phosphorylation, which effectively eliminates the breast CSCs and restores sensitivity to DOX [124]. Similarly, matrine—an alkaloid in Sophora flavescens—reduces ABCB1-related resistance-associated gene expression and the protein levels of p-mTOR, p-Akt, P-gP, and p-PI3K. By these mechanisms, it potently restores the effect of chemoresistance in HCT116/5-Fu cells and activates apoptosis through the PI3K/Akt/mTOR pathway [67].
Manogaran et al. have established that the pulsed exposure of cells to cisplatin generates colorectal CSCs that exhibit heightened sensitivity to isoliensinine, neferine, and cisplatin. Isoliensinine specifically downregulated PI3K/p-Akt/mTOR, interfered with the cell cycle, and elevated ROS, causing mitochondrial-dependent apoptotic cell death [126]. Ursolic acid increased the apoptosis-inducing effects of chemotherapy, inhibited ABCG2 expression, and abrogated tumor growth in ovarian CSC xenografted mouse models [127]. Besides, Li et al. noted that resistant colon cancer cell lines expressed more stem cells, including Nanog1, CD133, ALDH1, Sox2, OCT-4, and CD24. Similarly, curcumin offset this through the downregulation of CD133, ALDH1, and CD24, thus stopping the proliferation of CSCs [67]. The Yi-qi-hua-yu-jie-du decoction suppressed both MDR1 and MRP1 inhibiting the PI3K/Akt/Nrf2 axis, reversing the resistance and triggering apoptosis in gastric CSC (BGC823/5-Fu) [129].
Possible CSC signaling pathways include others as well. There is increased activity of the Hedgehog signaling pathway that controls cell division and differentiation, aberrantly expressed in CSCs, which contributes to recurrence. Inhibition of this pathway and CSC markers CD133 and CD44 by genistein was found to sensitize fluorouracil-resistant liver cancer cells (SMMC-7721) to fluorouracil [125]. Likewise, the norcantharidin agent circumvented resistance to treat ovarian cancer by inhibiting CSC markers Nanog, Sox2, and Bmi1 in addition to targeting the Hedgehog pathway [128]. Another critical regulator of CSC traits, the Wnt/β-catenin pathway, also acts in a bipolar manner, with inhibition leading to cell death and overactivation abetting tumor spread and resistance. Curcumin has also been implicated in restraining EMT as well as preventing Wnt signaling and cisplatin-sensitizing 5-fluorouracil-resistant gastric cancer cells [67].
5. Examples of promising plant-based natural products
5.1. Curcumin
C. longa (turmeric) is a well-known bioactive compound associated with various pharmacological effects known as curcumin. Curcumin shows antibacterial, anti-inflammatory, antioxidant, and antiviral properties and has been widely studied as a chemopreventive candidate and for its anticancer aspects of various cancer types [130, 131]. Its anticancerous activity lies in many different factors such as proliferation inhibition, apoptosis promotion, and inhibition of metastasis. Curcumin also has an effect on cell signaling. For example, it exerts its anticancer effect on HCT-116/HT-29 colon cancer cells by upregulating death receptor 5 (DR5), the death receptor recruiter of the TNF-related apoptosis-inducing ligand (TRAIL) pathway, inducing apoptosis [132]. The latter has been validated in both in vitro and animal models in colorectal and breast cancers [133]. Additionally, curcumin has been demonstrated to upregulate Bax and reduce Bcl-2 in colon adenocarcinoma HT-29 cells due to p53 induction and phosphorylation at Ser15 [134], and HCT-116 [135] and COLO-205 cells [136]. It triggers the Fas-mediated apoptotic pathway in HT-29 cells by initiating caspase-8 and suppresses Wnt/β-catenin signaling in HCT-116 colon cancer cells and APC-mutant mice, while also inactivating NF-κB in HT-29 cells.
Curcumin revealed anticancer effects in osteosarcoma by inhibiting the JAK/STAT signaling pathway and, in doing so, decreased MG-63 cell proliferation and metastases. Curcumin has emerged as a regulator of multiple signaling pathways in vitro and in vivo, promoting MAPK, EGFR, and NF-κB and modulating p53, a recognized tumor suppressor protein in different BC models as noted by Talib et al. [131]. In head and neck squamous cell carcinoma, curcumin suppresses proliferation by decreasing NF-κB and STAT 3 expression, either of which is commonly hyperactive in these tumors. Animal models of mice grafted with U-87 human glioma cells showed that pretreatment with curcumin inhibits angiogenesis via repression of MMP9 and expression of vascular markers. Curcumin treatment in glioblastoma cells (U-251) shows cell phase arrest at the G2/M level through upregulation of DAPK1, which freezes the cell cycle as well as suppresses STAT3 and NF-κB but activates caspase-3 to augment apoptosis [137].
5.2. Quercetin
It is a naturally occurring flavonoid that belongs to the family of polyphenols and is found in fruits and vegetables, including red and green tea, berries, and apples. It is valued owing to its wide spectrum of therapeutic benefits, such as anti-obesity, anticancer, neuroprotective, antimicrobial, antioxidant, hepatoprotective, cardioprotective, and reproductive health advantages [138].
Quercetin possess antibacterial/anticancer effects due to several cellular mechanisms of action. It downregulates the EGFR, a tyrosine kinase considered to be crucial in the progression of several solid tumors as well as in the inhibition of cell proliferation and the induction of apoptosis [139]. Quercetin enhances TRAIL to induce programmed cell death by increasing DR5. Besides, Lee et al. provided evidence that quercetin directly acts upon Raf/MEK/ERK and inhibits both Raf and MEK, key regulators in cancer progression [140]. Other reports have indicated that quercetin induces apoptosis through caspase activation (caspase-3 and caspase-9), stimulating pro-apoptotic proteins (Bcl-2) and inhibiting the anti-apoptotic Bcl-xL. Srivastava et al. also noted that quercetin directly binds to DNA to promote intrinsic apoptosis in cancer cells and tissues [141]. In addition to these actions, quercetin also has high antioxidant and free radical scavenging effects that help to prove its anticancer effect [142].
A large body of research substantiates quercetin as a cancer treatment option. In BC, it can induce apoptosis and G1 cell cycle arrest in MDA-MB-231/MCF-7 cells, and increase DOX-related sensitivity, decreasing their proliferation and invasion, leading to an increase in apoptotic events. A synergistic anticancer effect has also been reported where quercetin is combined with curcumin in triple-negative BC cell lines where together they modulate tumor suppressor genes [143]. In colorectal cancer, quercetin causes apoptosis in CT-26, MC 38, and CACO-2 cells and inhibits lung metastasis in model animals [144]. It has also been demonstrated to prevent 1,2-dimethylhydrazine induced colon cancer in mice. Likewise, in pancreatic cancer, quercetin inhibits growth and brings about apoptosis and G1 phase arrest [145].
5.3. Resveratrol
Resveratrol is well known for many health-promoting properties, which include antioxidant, antitumor, neuroprotective, and cardioprotective effects in addition to anti-obesity benefits. Its anticancer efficacy has been proven in an array of in vitro and in vivo models, where it has been shown to have an effect on several phases of carcinogenesis: initiation, promotion, and progression [146]. Guo et al. showed that co-loaded docetaxel and resveratrol in the ratio 1:1 (w/w) by nanocarrier polymeric MPEG-poly-d-lactide enhanced cytotoxicity in MCF-7 BC cells significantly. This is indicative of a potential treatment modality when it comes to drug-resistant tumors [147]. Similarly, Sameiyan et al. found that RES can suppress cell viability of drug-resistant leukemia K562/ADM cells through autophagy. In ciprofloxacin-resistant human oral cancer (CAR) cells, RES induced autophagy through inhibition and stimulation of the signaling pathways associated with Akt and AMPK, respectively [148].
Experimental and preclinical data indicate that RES has the ability to overcome MDR in a variety of ways. This encompasses the inhibition of drug-metabolizing enzymes such as CYP1A1 and CYP1B1, assessment of DNA damage, regulation of the cell cycle, manipulation of apoptotic pathways, and activity/expression of drug transporters [149]. Moreover, in oxaliplatin-resistant colon cancer cell lines, RES suppressed NF-κB and inhibited MDR1 expression [150], which lent the drug another dimension as a potential MDR-reversal agent.
5.4. Berberine
The starting material Berberis vulgaris, or barberry, is a plant used as a source of numerous alkaloids, including berberine. Its pharmacological profile is well known and comprises a broad spectrum of effects (antibacterial, antihypertensive, and antiarrhythmic actions) [151]. In addition to these properties, there is substantial research on the ability of berberine to reverse MDR in various types of cancer, both at laboratory and animal model levels [152, 153, 154]. Liu et al. [147] reported inhibition of proliferation in the human ovarian cancer OVCAR3 cells by combined treatment with both berberine and ciprofloxacin, dose- and time-dependently, mainly via necroptosis and apoptosis. Pan et al. noted that low doses of berberine magnified the hypoxic MCF-7 BC cell sensitivity to DOX, but higher doses induced apoptosis through an AMPK-HIF-1 alpha-P-gp pathway for in vitro and in vivo models [155].
5.5. Paclitaxel
Among plant-based natural products, PTX or Taxol is a diterpene alkaloid of very complex chemistry. Its main anticancer effect is its action as a microtubule-stabilizing agent by binding to the β-tubulin subunit, which prevents microtubule depolymerization. This stabilization causes arrest of the mitotic division, stops the cell cycle, and leads to apoptosis [156]. At intratumoral concentrations, PTX may also kill cells by causing chromosome misalignment on multipolar spindles. The result of this process is aneuploid daughter cells, some of which die because they lose essential chromosomes. Further, PTX directly acts on mitochondria and represses resistant apoptotic protein Bcl-2, further increasing its anti-apoptotic action [157, 158, 159, 160, 161, 162].
6. Conclusion and future perspectives
In conclusion, the arsenal of plant-derived natural products presents a powerful and versatile strategy to counteract the pervasive challenge of MDR in breast cancer. Their strength lies not in a singular, targeted action but in their ability to simultaneously engage a multitude of resistance pathways. As extensively discussed, these bioactive compounds can effectively inhibit drug efflux pumps, reinstate apoptotic cell death, modulate critical survival signaling cascades (PI3K/Akt, NF-κB, MAPK), regulate autophagy, and target the resilient CSC population. This multimechanistic approach makes them ideal candidates for use as chemosensitizers, capable of enhancing the efficacy of conventional treatments while potentially mitigating their adverse effects. Looking forward, the translation of these promising preclinical findings into clinical practice requires a concerted effort. Future research must focus on rigorous structure–activity relationship studies and computational modeling to design more potent and bioavailable derivatives. A critical area of development is the implementation of advanced nanocarrier systems (e.g., liposomes, polymeric nanoparticles) to improve the solubility, stability, and tumor-specific delivery of these compounds, thereby enhancing their therapeutic index. Moreover, well-designed clinical trials are imperative to validate the safety and synergistic potential of natural product–drug combinations in human patients. By embracing these innovative approaches, the scientific community can harness the full potential of plant-based natural products, transforming them from promising leads into tangible solutions that redefine the battle against multidrug-resistant breast cancer and ultimately improve patient outcomes.
CRediT authorship contribution statement
Israa J. Hakeem: Methodology, Writing—original draft, Writing—review & editing, Data curation, Conceptualization, Investigation, and Software.
Abdullah Yahya Abdullah Alzahrani: Writing—original draft, Writing—review & editing, Data curation, and Conceptualization.
Munirah M. Al-Rooqi: Writing—review & editing, Data curation, and Conceptualization.
Rabab S. Jassas: Writing—review & editing, Data curation, and Conceptualization.
Manzoor Ahmad Rather: Methodology, Software, Investigation, Writing—review & editing, and Validation.
Jan Mohammad Mir: Methodology, Software, Supervision, and Investigation.
Saleh A. Ahmed: Methodology, Software, Supervision, Investigation, and Funding acquisition.
Declaration of interests
The authors do not work for, advise, own shares in, or receive funds from any organization that could benefit from this article, and have declared no affiliations other than their research organizations.
