Role of Nutraceuticals in Cancer Chemosensitization

By Academic Press

Role of Nutraceuticals in Chemoresistance to Cancer, Volume Two, focuses on nutraceuticals, the compounds derived from natural sources, which are usually multi-targeted as a means to overcome chemoresistance. This book discusses the role of several compounds related to nutraceuticals and chemoresistance, such as curcumin, resveratrol, indole 3-carbinol, tocotrienols, ursolic acid, fisetin, celastrol, gambogic, butein, catechins and silymarin. It is a valuable resource for cancer researchers, oncologists and members of several areas of the biomedical field who are interested in understanding how to use nutraceuticals as a sensitizing agent for chemotherapy.

• Brings updated information on natural compounds used as specific inhibitors of cell signaling pathways as reviewed by experts in the field
• Presents experts analysis and summary of reported and novel findings and potential translational application in cancer patients
• Describes molecular mechanisms with new and helpful approaches for the readers to use in their own investigations

• Language: English
• Publisher: Academic Press
• Release date: Oct 18, 2017
• ISBN: 9780128123744

Book preview

patients.

Preface

Although chemotherapy is routinely used in the treatment of almost all cancers, the development of eventual resistance to chemotherapy is one of the major problems in the treatment. Why patient develops resistance to chemotherapy, what is the mechanism of chemoresistance, and how chemoresistance can be overcome are one of the major areas in cancer research. Extensive research during the last two decades has indicated that numerous cell signaling pathways play a major role in chemoresistance. These include inflammatory pathways consisting of NF-kB, STAT3, COX2, 5-LOX, and IL-6; growth factor receptor kinase pathways such as EGFR, IGFR, and insulin receptor pathways; and other cell signaling pathways. Numerous specific inhibitors of these pathways are being employed as drugs to overcome chemoresistance. In this monograph, we focus on the role of nutraceuticals, the compounds derived from natural sources, which are usually multitargeted as the means to overcome chemoresistance. Various groups from around the world have identified numerous agents/phytochemicals derived from Mother Nature that can downregulate the cell signaling pathways linked with chemoresistance and thus can sensitize the tumors to chemotherapy, the focus of this monograph. The current monograph first describes various cell signaling pathways linked to chemoresistance and then covers the role of nutraceuticals such as curcumin, resveratrol, tocotrienols, ursolic acid, fisetin, celastrol, gambogic acid, butein, catechins, silymarin, berberine, emodin, piperine deguelin, garcinol, plumbagin, zerumbone, and ginger in chemosensitization of various types of cancers.

Professor Alok Chandra Bharti, Ph.D., University of Delhi (North Campus), Delhi, and National Institute of Cancer Prevention and Research - Indian Council of Medical Research (NICPR - ICMR), Noida, UP, India

Professor Bharat Bhushan Aggarwal, Ph.D., Inflammation Research Center, San Diego, CA, United States

Chapter 1

Pathways Linked to Cancer Chemoresistance and Their Targeting by Nutraceuticals

Alok Chandra Bharti*,†; Kanchan Vishnoi‡; Sukh Mahendra Singh§; Bharat Bhushan Aggarwal¶    * University of Delhi (North Campus), Delhi, India

† National Institute of Cancer Prevention and Research - Indian Council of Medical Research (NICPR - ICMR), Noida, UP, India

‡ University of Illinois, Chicago, IL, United States

§ Banaras Hindu University, Varanasi, UP, India

¶ Inflammation Research Center, San Diego, CA, United States

Abstract

Current strategy for cancer treatment involves surgery, radiotherapy, and chemotherapy. Among them, either chemotherapy is the only option or it is administered in conjunction with other modalities for treatment for almost all cancers. Anticancer drugs used for the treatment belong to diverse groups such as cell-cycle inhibitors, alkylating agents, antimetabolites, specific monoclonal antibodies, and small molecule inhibitors that can selectively target either rapidly proliferating cells or specific signature molecules on these cells. Naive tumors respond to these chemotherapeutic agents initially, but later, they develop resistance to most of these agents. This phenomenon makes tumors refractory, and management of the disease becomes difficult irrespective of the tumor type. Resistance may be intrinsic to the tumor clones or acquired during the course of treatment. Principal mechanisms of chemoresistance may include increased activity of the drug efflux transporters, decreased drug activation, altered drug targets, increased drug degradation, enhanced DNA-damage repair, and failure of cells to undergo apoptosis as a result of alterations in p53. Several attempts have been made by researchers to develop strategies to target chemoresistance. Among these, the use of nutraceuticals being most notable as they are safe and economic and have the capability of targeting multiple pathways of chemoresistance, and many of them also show independent anticancer activities. Taking this into consideration, several natural products are in different phases of clinical evaluation and have shown positive results in both preclinical and clinical studies. The results demonstrate that nutraceuticals may be developed as clinically useful anticancer chemosensitizers for adjuvant therapy in combination with existing chemotherapy to increase the treatment efficacy.

Keywords

Cancer; Chemoresistance; Chemotherapeutics; Apoptosis; DNA damage; Autophagy; Nutraceuticals; Phytochemicals; Drug efflux; Drug inactivation; Alkylating agents; Topoisomerase inhibitors; Cell-cycle inhibitors; Antimetabolites; Targeted therapy

Abbreviations

5-FU  

5-fluorouracil

ABC  

ATP-binding cassette

AP-1  

activator protein-1

BCRP  

breast cancer resistance protein

Caspases  

cysteine-aspartyl-proteases

CSC  

cancer stem cells,

CYP  

cytochrome P450

EGFR  

epidermal growth factor receptor

EMT  

epithelial mesenchymal transition

GST  

glutathione-S-transferase

MDR  

multidrug resistance

MDRPs  

multidrug resistance protein

NF-κB  

nuclear factor-kappa B

PDGFR  

platelet-derived growth factor receptor

STAT-3  

signal transducers and activator of transcription 3

TS  

thymidylate synthase

UGT  

uridine diphospho-glucuronosyl transferase

VEGF  

vascular endothelial growth factor

Acknowledgments

The study was supported by extramural research grants from Department of Science and Technology (DST-PURSE Phase II/RC/2016/944); Department of Biotechnology, Government of India (Grant Support 6242-P34/RGCB/PMD/DBT/ALCB/2015); Indian Council of Medical Research (5/13/38/2014/NCD3); Department of Health Research, Ministry of Health and Family Welfare, Government of India; and intramural funding to ACB from University of Delhi.

Introduction

Cancer is a disease that develops overtime through accumulation of mutations and genetic changes in a cell. The characteristics distinguishing cancerous cells from the normal involve sustained proliferative ability, resistance to cell death, induction of angiogenesis, evasion of growth suppressors, limitless reproductive potential, ability to invade and metastasize, evade immune attack, genomic instability, inflammation, and continuous production of energy that are collectively designated as the hallmarks of cancer [1]. With improvements in longevity and decline of communicable diseases globally, an increase in the number of cancer cases has been observed, and a further increase is estimated in coming years [2]. Conventional approaches for the treatment of cancer include surgery, chemotherapy, and radiotherapy depending upon the cancer type and its clinical stage. Though the combinations and treatment regimens vary, in clinically advanced solid malignancies and hematopoietic cancers, chemotherapy is the upfront method of disease management.

Chemotherapy often employs a limited set of antineoplastic drugs of different categories for the treatment of most of the cancer types. These therapeutic agents principally target bulk of the tumor by killing cells with higher proliferation capacity and may leave behind slow proliferating or quiescent clone of cells that gradually get selected out and are responsible for relapse of drug-refractory tumors over a period of time. These secondary tumors are refractory to drugs initially used to treat cancer and may show a broad-spectrum drug resistance [3–9]. In recent times, several rationally designed anticancer drugs have been introduced that are derived against cancer-specific cellular/molecular targets. However, the use of these selective drugs to treat cancer also results in the development of chemoresistance though the mechanisms may be slightly different. Apart from the onset of chemoresistance, the standard chemotherapeutic drugs are highly toxic and with severe side effects and resulting in poor quality of life to the patient. The ability of cancer cells to develop drug resistance-associated toxicities and high cost of treatment demands an urgent need to develop new treatment strategies that could overcome chemoresistance and sensitize cancer cells to chemotherapy. At the same time, these new therapies should have less drug-associated toxicities and must be economic to cater patients with low resources.

One of the strategies to overcome chemoresistance could be to target specific molecular mediators using chemosensitizers that can be used in combination with existing chemotherapeutics for the better treatment outcome. Incidentally, the cancer cells can simultaneously employ multiple pathways and molecular mediators for manifestation of resistance. So, single target approach for chemosensitization may not be effective and rewarding. In recent years, search for the drugs that can target cancer cells has highlighted a series of natural products. Several nutraceuticals such as curcumin, resveratrol, berberine, and ursolic acid have received the attention of investigators as stand-alone anticancer agents, but their travel to clinic has not been equally successful as they could not get a place in established chemotherapy regimen as single agents. Alternatively, these nutraceuticals are being tested for their chemosensitizing effects in combination with established chemotherapeutic drugs [10–13]. Interestingly, studies demonstrate that the use of nutraceuticals can enhance the anticancer activity of the chemotherapeutics and lower their effective doses. The evidence for potential chemosensitizing activities of nutraceuticals is presented in individual chapters of the present book.

In this chapter, we provide an overview of the common chemotherapeutic drugs and their mode of action along with the possible mechanisms of chemoresistance, intrinsic or acquired during treatment of cancer cells, and conclude with opportunities and challenges of using the nutraceuticals for chemosensitization.

Chemotherapy

Chemotherapy employs a wide group of drugs that are given alone or in combination with other drugs that preferentially should have cytotoxic effects on the rapidly dividing cancer cells. These drugs are broadly classified on the basis of their origin or mechanisms of action such as alkylating agents, cell-cycle inhibitors, antimicrotubule agents, antimetabolites, antibiotics, topoisomerase inhibitors, targeted antibodies, and small molecule inhibitors (Table 1). These chemotherapeutics may act at different levels in order to target highly proliferating cancer cells (Fig. 1). Groups of chemotherapeutics and the basis of their mode of action are summarized in this section.

Fig. 1 Mode of action of different classes of chemotherapeutic drugs available for treatment of cancer. Though conventional drugs primarily target DNA and microtubules, recently developed rationally designed drugs are more target-specific and supposedly less toxic.

Table 1

Representative List of Different Classes of Chemotherapeutic Drugs Used for the Treatment of Different Malignant Conditions

Alkylating Agents

Alkylating agents are most widely used anticancer drugs and are major components of combination chemotherapy [14]. These are small molecules that can covalently bind an alkyl group to electron-rich nucleophilic moieties to form adducts and, hence, have acytotoxic effect on cells by binding to DNA molecules [15]. Cisplatin, oxaliplatin, carboplatin, chlorambucil, cyclophosphamide, mechlorethamine, and melphalan are some of the most important alkylating agents used for chemotherapy. The exposure of cells to alkylating agents causes a specific reduction in DNA synthesis, chromosomal aberrations, and genetic mutations [15].

Cell Cycle Inhibitors/Antimicrotubule Agents

Antimicrotubule agents are plant-derived antimitotic chemicals that block cell proliferation by acting on the polymerization dynamics of spindles, which are essential for the proper spindle function of microtubules [16]. Microtubules are an important part of the intracellular cytoskeleton structure and have unique polymerization dynamics that are critical for many cellular functions including cell division [17]. Vinca alkaloids and taxanes are two different classes of antimicrotubule agents that cause microtubule dysfunction [18]. Vinca alkaloids such as vincristine and vinblastine bind to tubulin dimers and prevent them from polymerization. On the other hand, taxanes such as paclitaxel and docetaxel have opposite mechanisms of action. These stabilizing agents bind to microtubules and prevent them from depolymerization. The suppression of spindle microtubule dynamics results in cell-cycle arrest through slowing or blocking of mitosis at the metaphase-anaphase transition that leads to the induction of apoptotic cell death [18,19].

Antimetabolites

Antimetabolites are group of anticancer agents that exert their cytotoxic effects by interfering with the DNA synthesis. Some of the important drugs from this class are 5-fluorouracil (5-FU), capecitabine, floxuridine, cytarabine, gemcitabine, decitabine, and vidaza. These molecules are pyrimidine or purine analogues with altered chemical groups [20] and induce cell death during the S phase of cell growth when incorporated into RNA and DNA or inhibit enzymes needed for nucleic acid production [21]. The primary intracellular targets of the antimetabolites are DNA polymerases, thymidylate synthetase, and ribonucleotide reductase. The inhibition of enzymes involved in DNA synthesis or misincorporation of antimetabolites in DNA prevents mitosis and induces apoptosis in dividing cells.

Antibiotics

Antibiotics used for chemotherapy inhibit cell division. They have different modes of action. Some of them are intercalating agents, and some of them directly introduce damage in DNA. Anthracyclines, bleomycins, actinomycin D, and mitomycins are a few anticancer antibiotics in use. Doxorubicin and daunorubicin were the first anthracyclines discovered in 1963 [22]. Both doxorubicin and daunorubicin form complexes with DNA by intercalation and interfere with the topoisomerase II activity to produce persistent DNA cleavable complexes [23]. Another common antibiotic used for cancer therapy is actinomycin D. Actinomycin is a complex molecule that binds DNA at the transcription initiation complex and prevents RNA synthesis [24]. It also stabilizes cleavable complexes of topoisomerases I and II with DNA [25]. Bleomycin, another class of antibiotic, is a glycopeptide, and its mode of action is exerted using sequence selective metal-dependent oxidative cleavage of DNA in the presence of oxygen [26].

Topoisomerase Inhibitors

These anticancer drugs specifically interfere with the function of topoisomerases (topoisomerase I and topoisomerase II), which are considered essential to maintain the topology of the DNA. Topoisomerase inhibitors are classified into two groups depending on the type of topoisomerase targeted by them. Type I topoisomerase inhibitors stabilize DNA single-strand breaks initially introduced by topoisomerase I [27] and include irinotecan, topotecan, and camptothecin. On the other hand, type II topoisomerase inhibitors stabilize DNA double breaks introduced by topoisomerase II and include etoposide (VP-16), teniposide, amsacrine, and ellipticines. These inhibitors stabilize topoisomerase-DNA complexes and resulting in DNA cleavage stimulation [28]. Irreversible double-stranded DNA breaks generated by the action of these topoisomerase inhibitors eventually lead to apoptosis and cell death [29,30].

Targeted Therapy

Targeted cancer therapies are a new generation of specific drugs developed to interfere with specific molecules that are critical in growth, progression, and spread of cancer [31]. Targeted therapy involves the use of therapeutic antibodies and rationally designed inhibitors to specifically target rapidly dividing cancer cells with minimum effect on normal cells.

Target-Specific Antibodies

Monoclonal antibodies have emerged as important therapeutic agents for several different malignancies [32]. These antibodies may trigger activation of the immune system. For example, trastuzumab, rituximab, and alemtuzumab are the monoclonal antibodies that target specific tumor cells through complement-dependent cytotoxicity or antibody-dependent cellular cytotoxicity [33,34]. Some of the monoclonal antibodies may target cancer cells by blocking autocrine/paracrine growth stimulatory signals. This can be done through their interaction with growth factor receptors present on the cell surface to inhibit phosphorylation of receptor-protein tyrosine kinases that further lead to inhibition of downstream signaling events [35,36]. Trastuzumab targeting erbB2, cetuximab targeting epidermal growth factor receptor (EGFR), and bevacizumab blocking angiogenic signaling by vascular endothelial growth factor (VEGF) are some of the monoclonal antibodies functioning through this mechanism.

Small Molecule Inhibitors

Several small molecule inhibitors targeting extracellular cell surface ligand-binding receptors and intracellular proteins in cancer cells have been developed [37]. Most of these drugs inhibit critical cancer targets such as serine/threonine/tyrosine kinases and other proteins playing a role in signal transduction pathways. Imatinib targets BCR-ABL tyrosine kinases; gefitinib and erlotinib target EGFR; sorafenib targets B-Raf VEGF receptor (VEGFR), EGFR, and platelet-derived growth factor receptor (PDGFR); navitoclax targets Bcl-xL, Bcl-2, and Bcl-w; and marimastat targets MMPs. These are some of the example of small molecule inhibitors that are clinically being used for the cancer treatment.

Chemoresistance: Problem in Curing Cancer

The ultimate goal of chemotherapy is the elimination of tumor cells. Therefore, almost all of the anticancer mechanisms listed in previous sections converge to the pathway leading to cell death that can be classified as intrinsic or extrinsic depending upon the apoptotic signal whether it is intracellular or mediated through death receptors, respectively (Fig. 2). Occasionally, these drugs also introduce autophagy. It is believed that over 90% of the cancer patients that are treated with chemotherapies experience treatment failure with metastatic cancer at some point of time [38]. Initially, cancers are susceptible to the chemotherapy but later on develop chemoresistance through adaptation of tumor cells to survive in the adverse environmental conditions of chemotherapy. There could be different possible molecular mechanisms by which a cancer cell develops resistance to the drug (Fig. 3). Either the resistance in cancer cells could be intrinsic or it could be acquired during the course of treatment [39]. The intrinsic chemoresistance depends upon the cells intrinsic ability such as inactivation of the drug and alteration of drug targets and DNA-damage repair [40]. On the other hand, cancer cells that initially show sensitivity to the drug may acquire resistance for that drug over the time. These cancer cells not only become resistant to the drug used to treat them but also show cross-resistance to other drugs. Below, we summarized common mechanisms of chemoresistance that are intrinsic or acquired by the cells to escape the cytotoxic effect of the drug.

Fig. 2 Primary pathway(s) engaged by standard chemotherapeutic agents for elimination of proliferating cancer cells. Potential of chemotherapeutics to damage DNA, to inhibit cell proliferation, and to trigger apoptosis and autophagy determines the degree of success achieved by the treating agent.

Fig. 3 Schematic representation of intracellular mechanisms and their cooperative interaction that facilitate cancer cells to become drug resistant. Activation of survival factor such as BCl-2 in drug-resistant cell leads to dysfunctional apoptosis. Cell may rescue DNA-damaging agent through enhanced DNA-damage repair response and inactivation of p53. Increased activity of drug efflux transporter and detoxification system cause reduced accumulation of the drug in cells. Constitutive expression of prosurvival transcription factors, NF-κB, STAT-3, and AP-1, provides additional advantage. Autophagy induced may promote quiescence and slow cycling that in conjunction with activation of self-renewal pathways (hedgehog, Notch, and Wnt) lead to epithelial-mesenchymal transition to escape the cytotoxic effect of drug and in turn induce/promote stem-cell-like character in these cells.

Mechanism of Chemoresistance

Drug Transporters

The adaptation of tumor cells for increased drug efflux is one of the main factors for chemoresistance against a variety of currently used chemotherapeutic drugs. Drug efflux is efficiently mediated by the increased expression of multidrug transporters belonging to the ATP-binding cassette (ABC) family of transporters expressed on the cell surface [41–43]. The human genome encodes 48 ABC transporter genes that are categorized into seven subclasses, ranging from ABCA to ABCG [44]. Several members of the multidrug resistance protein family (MRPs), such as MDR1, P-glycoprotein, and breast cancer resistance protein (BCRP), have been demonstrated to confer chemoresistance or multidrug resistance in cancer cells [41,45,46]. Over the last decade, cancer stem cells (CSCs) have received attention to play a critical role in carcinogenesis, which are functionally characterized by their enhanced expression and functional activity of ABC transporters [47,48]. CSCs are resistant to conventional chemotherapies and may play a key role in tumor progression, recurrence, and metastasis [49–51].

Enhanced DNA Repair System

Many chemotherapeutic drugs such as cisplatin and topoisomerase inhibitors act by inducing DNA damage. A cell responds to any DNA damage either by cell-cycle arrest to repair the damage or by inducing cell death through apoptosis. An intrinsic ability of a cancer cell to repair any DNA damage caused by chemotherapeutics determines its potential for resistance against that drug. One of the important factors that is associated with intrinsic drug resistance is the mutation of p53 in most of the cancers [52]. As p53 plays a crucial role in regulating cell-cycle checkpoint and in induction of apoptosis in normal cells [3], any loss in its activity through either mutation or otherwise proves detrimental to cell-cycle control. Another factor that contributes to the resistance is the enhanced DNA-damage response in cancer cells, which can easily reverse the DNA damage caused by the drug. The primary DNA repair mechanisms that include nucleotide excision repair and homologous recombination reverse the DNA damage caused by platinum drugs and confer resistance against these drugs [53,54]. The dysregulation of DNA-damage repair genes is common in many cancers [55] that demonstrate the targeting of DNA-damage repair genes along with chemotherapy is essential to maintain the sensitivity of drugs. MGMT (O6-alkylguanine-DNA alkyltransferase), an enzyme in DNA-damage repair system that functions to repair alkylated nucleotides, is expressed at high levels in many tumors [56] and is responsible for resistance against alkylating agents inducing guanine O6 alkylation.

Drug Inactivation

Most of the chemotherapeutics that are given to the patient as prodrugs have lower activities against a specified pharmacological target but get metabolically transformed into a compound with the desired activity. These anticancer drugs must get metabolically activated in cells and involved their interaction with different proteins. For example, capecitabine is an antimetabolite, which is available as a prodrug and is converted to 5-FU by thymidine phosphorylase [57]. However, gene-encoding thymidine phosphorylase can get inactivated in a cancer cell and may confer resistance against 5-FU [58]. Alkylating agents and platinum drugs can be inactivated by metallothionein [59]. Platinum drugs can also be inactivated by thiol glutathione [60]. Cytochrome P450 (CYP), glutathione-S-transferase (GST), and uridine diphospho-glucuronosyl transferase (UGT) are other important constituents that can inactivate drugs.

CYP represents a family of isozymes responsible for biotransformation of drugs via oxidation [61]. A majority of these isozymes are located in the liver, kidneys, skin, gastrointestinal tract, and lungs. Mutation in the gene-encoding CYP may change the metabolic capacity of these isozymes such as increased breakdown of the drug [62], thereby creating resistance for the drugs. Similarly, GST is another important cellular metabolizing system involved in drug resistance. The GST family comprises detoxifying enzymes that can detoxify a broad class of electrophilic compounds. Increased activity of GST results in detoxification of many alkylating agents and platinum drugs and results in resistance to these drugs [63]. Another important constituent that has a role in chemoresistance is UGT, a family of enzymes catalyzing glucuronidation. The expression of UGTA1 is negatively regulated by DNA methylation of the promoter that favors the activity of the topoisomerase I inhibitor, irinotecan [64]. However, epigenetic changes that increase the expression of UGTA1 may provide resistance to irinotecan.

Altered Drug Targets

The effectiveness of anticancer drugs depends upon their interaction with corresponding targets. However, modifications such as mutation in the target gene and high expression level of its target proteins such as kinases can lead to the resistance for that drug. For example, cancer cells can become resistant to topoisomerase II inhibitors, such as teniposide, amsacrine, and ellipticines by mutation of the topoisomerase II gene [6,65]. The activity of the antimetabolite 5-FU that targets thymidylate synthase enzyme transcription [66] is compromised with high expression levels of the thymidylate synthase enzyme in cancer cells [7]. Imatinib, a tyrosine kinase inhibitor that specifically targets the BCR-ABL proteins and used to treat patient of chronic myeloid leukemia, may fail to show its effect in some of the patients after prolonged therapy. The acquisition of imatinib resistance can be caused by a point mutation in the ABL gene and amplification of the BCR-ABL fusion gene [67]. All of these examples demonstrate that understanding the methods of drug target alterations is essential to develop effective therapies to treat drug-resistant cancers.

Epithelial-Mesenchymal Transition

The onset of chemoresistance and tumor relapse is linked to the acquisition of epithelial-mesenchymal transition (EMT) [68–70]. Several in vitro studies investigated the mechanisms associated with drug resistance and found that the epithelial-mesenchymal characteristics are associated with resistance against a variety of chemotherapeutic drugs such as oxaliplatin, cisplatin, paclitaxel, and 5-FU [5,71–73]. These studies demonstrate that EMT provides a selective growth advantage to cancer cells in the presence of a drug. EMT is a phenomenon in which epithelial cells switch from the epithelial to the mesenchymal phenotype [74], and it plays a crucial role during embryonic development and in the differentiation of multiple tissues and organs. However, it can adversely cause organ fibrosis and can promote cancer [75,76]. The hallmark of EMT is the loss of the epithelial marker, E-cadherin [77], and is accompanied with the appearance of the mesenchymal marker, vimentin [78]. Transcription factors snail, slug, and twist, which interact with E-box elements located within the proximal region of the E-cadherin promoter, lead to transcriptional repression of E-cadherin [79–81]. Accumulating evidence suggests the involvement of EMT-associated transcription factors in chemoresistance in different cancers [82–90]. In fact, the onset of chemoresistance is reported to be the consequence of the acquisition of EMT-like characteristics [91]. The connecting link between the onset of chemoresistance and EMT could be the existence of cancer stemlike cells in the tumor, which are supposed to be responsible for chemoresistance [92].

Inhibition of Apoptosis

The effectiveness of many common chemotherapeutic drugs such as etoposide, doxorubicin, paclitaxel, cisplatin, and oxaliplatin to target cancer cells eventually depends upon their potential to stimulate apoptosis. However, there exists a barrier, which must be crossed to trigger apoptotic cell death in cancer cells. The mechanism of apoptosis is extensively studied and explored, and it is well accepted that a balance between apoptosis regulators and survival factors determines the fate of cells either to undergo death or to survive [93,94]. A tumor microenvironment favoring high level of apoptotic regulators and low level of survival factors in the presence of drug is required for drug-induced cell death. On the contrary, a disturbance in the balance between apoptosis regulators and survival factors leads to apoptosis resistance.

Upon receiving stimuli such as the lack of oxygen, stress signal, and the presence of drug or growth factor deprivation, a cell may undergo apoptosis in two ways, through either the intrinsic or the extrinsic pathway. The intrinsic pathway is primarily linked to mitochondrial functions [8]. On the other hand, the extrinsic pathway is activated through binding of a ligand to membrane-linked death receptors [95]. A series of cysteine-aspartyl-proteases (caspases) are the primary executors of apoptosis [96]. Execution of cell death mechanisms are, however, controlled by certain survival factors that counterbalance the apoptotic pathway at different levels. These survival factors are antiapoptotic Bcl-2 family members, inhibitor of apoptosis proteins such as X-linked IAP, cIAP2, neuronal apoptosis inhibitor protein, livin, apollon, and survivin [97]. Fas-associated death domain-like interleukin-1β-converting enzyme (FLICE)-like inhibitory protein (FLIP), a FADD-binding suppressor of apoptosis [98], and PI3K/AKT pathway are other classes of prosurvival factors working as antagonists to apoptotic proteins in a cell. Mutation, overexpression, amplification, and chromosomal translocations of the genes encoding these antiapoptotic proteins have been reported to be associated with a variety of cancers [99–104]. On the other hand, their overexpression/activity is linked with chemoresistance.

Autophagy

Adaptation of a cancer cell to stress induced by the presence of a drug in the tumor microenvironment is essential for its survival. For this, a cancer cell may use a process of autophagy to conserve its resources. Autophagy is a self-degradative process to maintain homeostasis by degrading misfolded or aggregated proteins and by clearing damaged organelles. For this, a portion of the cytosol of damaged organelles is sequestered in double-membrane vesicles and delivered to the lysosome for their destruction [105,106]. The induction of autophagy may mediate cell death in cancer cells that are apoptosis defective [107]. On the contrary, constitutive autophagy in response to the chemotherapeutic drug given to the cancer cells can help in the survival of cancer cells through elimination of damaged organelles and by recycling the macromolecules. There are reports suggesting that autophagy can serve as prosurvival pathway in response to the metabolic stress such as hypoxia and nutrient deprivation and in the presence of chemotherapeutic drugs that can ultimately result in chemoresistance [108–110].

Several drugs used for the treatment of cancer may induce autophagy to protect the cancer cells from apoptosis and, thereby, to facilitate drug chemoresistance. Epirubicin is demonstrated to induce autophagy in breast cancer cells that protected these cells from epirubicin-induced apoptosis and leads to epirubicin resistance [9]. 5-FU and irinotecan resistance in colorectal cancer cells are also demonstrated to be associated with the induction of autophagy through the overexpression of mitogen-activated protein kinase 14 (MAPK14)/p38a [111,112]. Oxaliplatin treatment in hepatocellular carcinoma can induce autophagy as a self-protective mechanism and can contribute to cell survival [113]. Treatment with bevacizumab shows increased autophagic flux and hypoxia-associated growth in human glioblastoma xenografts, thus indicating that hypoxia-mediated autophagy promotes tumor survival and resistance to antiangiogenic therapy [114].

Aberrant Expression and Constitutive Activation of Transcription Factors

The aberrant expression and the constitutive expression of various transcription factors are key events in carcinogenesis and in promoting chemoresistance. Some of these transcription factors are upregulated in cancer cells and are tightly connected with chemoresistance.

Nuclear Factor-Kappa B (NF-κB)

NF-κB, considered as a central regulator of carcinogenesis, plays a key role in mediating chemoresistance. Several chemotherapeutic agents such as doxorubicin, paclitaxel, vinblastine, daunomycin, 5-FU, cisplatin, and bortezomib have been reported to induce NF-κB activation in different cancer cells [115–120]. NF-κB, when active, is a nuclear protein that binds to specific DNA sequences in target genes and regulates the transcription of genes involved in inflammation, cell proliferation, immunoregulation, and cell survival [121]. A number of genes that mediate antiapoptotic and prosurvival activities in cancer cells, such as bcl-2 [122], bcl-xL [123], COX-2 [124], cyclin D1 [125], and survivin [126], have NF-κB-binding sites in their promoters. The control of survival genes by NF-κB provided a rationale to target NF-κB in the cancer cells for their chemosensitization [127].

Signal Transducers and Activators of Transcription 3 (STAT-3)

STAT3 is another important transcription factor that is crucial for carcinogenesis and for chemoresistance. STAT3 is found in the cytoplasm and is activated in response to stimuli from the cytokines. Activated STAT3 regulates the transcription of genes controlling cell survival and proliferation and regulates the expression of antiapoptotic and immune response genes [128–130]. Constitutive activation of STAT3 is necessary for the proliferation and survival of different cancers [128,131–134]. Activation of STAT-3 provides an advantage for survival of the cancer cells. Like NF-κB, the inhibition of STAT-3 in different cancer types has been demonstrated to induce apoptosis and chemosensitization of cells [135–137].

Activating Protein-1 (AP-1)

The AP-1 transcription factor is a dimeric complex consisting of members of the Jun, Fos, ATF, and MAF family proteins. It can be activated by a large variety of stimuli that include growth factors, oxidative stress, pro-inflammatory cytokines, and tumor promoters [138–141]. AP-1 is involved in cell proliferation, differentiation, oncogene-induced transformation, and cancer cell invasion [141–143]. In addition, AP-1 is reported to be involved in different mechanisms that impart chemoresistance in cancers. AP-1 is shown to mediate multidrug resistance by regulating the expression of MDR1 [144]. It is demonstrated to protect the cancer cells against apoptosis induced by DNA-damaging agents [145]. Moreover, AP-1-dependent pathways are considered to be involved in the regulation of autophagy [146].

Activation of Self-Renewal Pathways

Self-renewal pathways such as hedgehog, Notch, and Wnt are critical for the maintenance of stem cells and play a crucial role during embryogenesis in cell proliferation, differentiation, and apoptosis. However, the activation of these self-renewal pathways has also been implicated in promoting carcinogenesis and tumor progression. The activation of components of the hedgehog signaling pathway, such as GLI and Smo, is found to be associated with the failure of chemotherapy in different cancers [147–149]. GLI1, a transcription factor of the hedgehog signaling pathway, can regulate the expression of MRPs and survival proteins such as survivin and bcl-2 that promote chemoresistance [147]. Moreover, hedgehog signaling also promotes EMT that is considered essential for the metastasis and recurrence of the tumor [150–152]. Failure of chemotherapy is also found to be correlated with the upregulation of Notch and Wnt pathways to promote chemoresistance [153–158]. Likewise, the hedgehog, Notch, and Wnt pathways are also involved in promoting EMT in cancer cells.

Chemosensitization: An Approach to Target Cancer

Chemotherapy, in earlier stages of cancer when invasion is still local, has been more successful in the treatment of naive tumor cells that demonstrate only a low level of intrinsic chemoresistance. However, as the treatment or tumor stage progresses, the frequency of acquiring mutations that provide the cells with selective advantage against different anticancer drugs increases. It is observed that tumor cells utilize more than one mechanism to resist to any class of anticancer agents discussed in earlier section (Table 2). At the same time, the mediators that participate in drug resistance are not drug-specific and participate in response to a broad spectrum of anticancer drugs (Table 3). These findings, therefore, suggest the requirement of a broad-spectrum multitargeting agents to counter chemoresistance. However, the requirement to address the multiplicity of cellular targets promoting chemoresistance is difficult to meet by applying synthetic chemistry principles. Incidentally, the search for novel and more effective anticancer agents has revealed a series of plant-derived small molecules that have the capacity to target multiple pathways that contribute to carcinogenesis [220]. These phytochemicals and their botanical sources are either consumed routinely as food items or used in traditional and folklore remedies for the treatment of inflammatory diseases. A detailed analysis of these pharmacologically active ingredients has shown a multitarget anticancer activity in vitro on different cancer cell lines, in vivo in different tumor models, [221–223], and in some instances in small clinical trials [224–229]. However, due to low bioavailability, quick elimination from the system, and, more importantly, poor pharmacokinetic and pharmacodynamic indexes, these phytochemical-based drugs with a few exceptions like taxanes failed to make a part of the mainstream anticancer treatment regimens.

Table 2

Representative Anticancer Drugs and Multiple and Diverse Mechanisms Employed by the Cancer Cells to Acquire Resistance Against Them

Abbreviations: ABC, ATP-binding cassette transporter; DHFR, dihydrofolate reductase; EMT, epithelial-mesenchymal transition; EGFR, epidermal growth factor receptor; FGF, fibroblast growth factor; IKK, IκB kinase; MRP, multidrug resistance protein; NF-κB, nuclear factor-κB; PIK3CA, the catalytic subunit of phosphatidylinositol 3-kinase; PTEN, phosphatase and tensin homologue; VEGF, vesicular endothelial growth factor.

Table 3

Representative Molecular Mediators Associated With Different Factors Responsible for Manifestation of Chemoresistance and Their Status Against a Broad Spectrum of Chemotherapeutic Agents in Different Cancers