Clin Transl Oncol (2014) 16:511–516 DOI 10.1007/s12094-014-1162-1
EDUCATIONAL SERIES
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BLUE SERIES
AD VANCES IN TRANSLATIONAL ONCOLOGY
Cancer cell resistance mechanisms: a mini review S. Al-Dimassi • T. Abou-Antoun • M. El-Sibai
Received: 19 January 2014 / Accepted: 4 February 2014 / Published online: 20 March 2014 Ó Federacio´n de Sociedades Espan˜olas de Oncologı´a (FESEO) 2014
Abstract Cancer is a leading cause of death worldwide accounting to 13 % of all deaths. One of the main causes behind the failure of treatment is the development of various therapy resistance mechanisms by the cancer cells leading to the recurrence of the disease. This review sheds a light on some of the mechanisms developed by cancer cells to resist therapy as well as some of the structures involved such as the ABC members’ involvement in chemotherapy resistance and MET and survivin overexpression leading to radiotherapy resistance. Understanding those mechanisms will enable scientists to overcome resistance and possibly improve treatment and disease prognosis. Keywords Cancer Chemotherapy resistance Multiple drug resistance Radiotherapy resistance Immunotherapy resistance and cancer stem cells
Introduction Cancer is responsible for one in eight deaths around the world [1]. There are nearly 7.6 million deaths worldwide due to cancer with about 12.7 million new cases per year [2]. According to NCI, about 1.66 million new cancer cases
S. Al-Dimassi M. El-Sibai (&) Department of Natural Sciences, Lebanese American University, Beirut, Lebanon e-mail:
[email protected] T. Abou-Antoun School of Pharmacy, Lebanese American University, Byblos, Lebanon
are expected to be diagnosed in the USA in 2013 and about 0.6 million are expected to die with cancer in the same year on an average of 1,600 patients per day [3]. Cancer is a complex disease with common features, including cell growth, reduction of apoptosis and loss of cell cycle regulation [4]. Cancer cells are known as the basis of cancer disease. These cells initiate tumors by carrying mutations in oncogenes and tumor suppressor genes. The following are the hallmarks of a cancer cell: 1. Sustained proliferation, where cancer cells proliferate independently from environmental signals; 2. Evasion of growth suppressors, where cancer cells evade the negative regulators of proliferation such as P53 and retinoblastoma tumor suppressor proteins; 3. Activation of metastasis and invasion through the inhibition of cell–cell contact and cell–matrix contact in addition to alterations in pathways regulating cell motility; 4. Induction of angiogenesis, where tumors form new blood vessels to support the tumor needs for nutrients and gases and to eliminate waste; 5. Uncontrolled replicative potential; and finally 6. Immortality whereby cancer cells resist cell death [5]. Cancer treatments are recorded in ancient documents. During the Roman period Celsus and Leonides described the mastectomy procedure. Ibn Sina (Avicenna) in the eleventh century used arsenical treatment for cancer and in fact, the first effective chemotherapy for leukemia was potassium arsenite used by Lissauer in 1865. Radiotherapy and surgery played main roles in cancer treatment since 1960s with cure rates as low as 33 % [6]. Despite the prevalence of these treatment methods for a while, the success rates in treatment are still very disappointing. One of the main reasons for treatment failure and disease
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recurrence is the resistance developed by the body to the different treatment methods.
Resistance to chemotherapy Disseminated cancer remains a nearly uniformly fatal disease. While a number of effective chemotherapies are available, tumors inevitably evolve resistance to these drugs ultimately resulting in treatment failure and cancer progression. Chemotherapy treatment uses chemical drugs to kill or halt cancer growth. Most of these drugs act by inducing DNA damage or preventing chromosomal replication, resulting in cancer cell apoptosis. Drugs used in chemotherapy are classified into five groups. These are alkylating agents, anti-metabolites, mitotic inhibitors, topoisomerase inhibitors, and anti-tumor antibiotics. Causes for chemotherapy failure in cancer treatment are due to the following: poor vascularization, hypoxia, intratumoral high interstitial fluid pressure, and phenotypic resistance to drug-induced toxicity through up-regulated xenobiotic metabolism or DNA repair mechanisms and silencing of apoptotic pathways [6] (Fig. 1). A general strategy to prevent the cellular entry of drugs is their active efflux from the cell or the cytoplasmic membrane. Active drug efflux mechanisms can be specific for a given drug, a so-called single-drug, or group-specific efflux system that effluxes a number of drugs. There is also a broad substrate specificity covering a wide range of toxic compounds that are structurally and functionally unrelated [7] (Fig. 2). ATP-binding cassette (ABC) proteins are proteins that play a major role in drug resistance. These proteins bind Fig. 1 The effect of cancer chemical drugs on cell survival. Once the cell is exposed to the drug, this drug will either lead to the enhancement of DNA damage or the inhibition of chromosomal replication, in which both cases will lead to cellular apoptosis
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ATP and mediate translocation of substances in and out of cells. Human ABC gene subfamilies consist of 48 known genes. The most famous genes are ABCA2 (ABC2), ABCB1 (MDR), ABCC1 (MRP1) and ABCC3 (MRP3) [8]. ABC systems couple the energy from ATP hydrolysis to an impressively large variety of essential biological phenomena, comprising not only transmembrane (TM) transport, but also several non-transport-related processes, such as translation elongation and DNA repair [9]. Genetic defects in these proteins lead to many diseases, such as cystic fibrosis, Stargardt disease and Tangier disease, while overexpression of ABCB1 in tumor cells plays a role in drug resistance [10]. ABC Transport subfamilies consist of The MDR system [members of the ABCB subfamily: ABCB1/MDR1/P-glycoprotein (Pgp)], the MRPI protein (members of the ABCC subfamily: ABCC1/MRP1, ABCC2/MRP2, ABCC3–6, and ABCC10–11) and the breast cancer resistance protein (members of the ABCG subfamily: ABCG2/MXR/BCRP). ABCB1 preferentially extrudes large hydrophobic molecules, while ABCC1 and ABCG2 can transport both hydrophobic drugs and large anionic compounds, e.g., drug conjugates. MDR1 can export most neutral and cationic hydrophobic compounds, and cancer cells use this mechanism as a primary shield against chemotherapy [11].
MDR (multidrug resistance) Mammalian cancer cells may develop an MDR phenotype upon exposure to cytotoxic agents used in cancer therapy. The resistance developed protects the cells not only against the drug used in chemotherapy but also against a range of
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Fig. 2 Cellular response to entry of drugs active efflux from the cell or the cytoplasmic membrane via different ABC members. ABCB1 an ATP-dependent drug efflux pump for xenobiotic compounds especially large hydrophobic molecule. ABCG2 It likely serves as a cellular defense mechanism in response to mitoxantrone and
anthracycline exposure, where ABCC1 protein functions as a multispecific organic anion transporter; these three members of the transport system are responsible for decreased drug accumulation in multidrug-resistant cells and often mediate the development of resistance to anticancer drugs
structurally and functionally unrelated toxic agents. MDR transporters can be classified into the following two main groups according to the mode of energy coupling to drug export across the cytoplasm and outer membranes: (1) primary active transporters that belong to the ABC super family and utilize the free energy of ATP hydrolysis to expel the drug from the cell against its concentration gradient and (2) secondary transporters that utilize the PMF (proton motive force) or sodium motive force for drug expulsion [12]. MDR1 gene is located on chromosome 7q21 [13]. It consists of 28 exons, encoding a 1,280–amino acid transporter (170 kDa) Pgp. Transcription of this gene is activated by different factors—antitumor drugs, ultraviolet radiation, inducers of differentiation, phorbol ethers, and carcinogens [14]. This protein has two homologous halves, each containing six TM domains and an ATP-binding site [15]. P-glycoprotein acts as a ‘‘flippase’’ or a ‘‘hydrophobic vacuum cleaner’’ that removes its substrates from the membrane lipid bilayer into the extracellular space [15]. It is able to cause the highest resistance to bulky amphipathic drugs, such as paclitaxel (taxol), anthracyclines, and Vinca alkaloids [16].
variety of proteins some of which have been designated as multidrug resistance-associated proteins (MRPs). Human ABCC sub-family consists of 13 members, nine of which are transporters: MRP1, MRP2, MRP3, MRP4, MRP5, MRP6, MRP7, MRP8, and MRP9 [17]. The multidrug resistance protein, MRP1, was discovered in 1992. It belongs to the ABC super family of transporter proteins, to which Pgp also belongs. MRP1 and Pgp are 15 % identical. Human MRP1 gene is located on chromosome 16p13.1, and its sequence consists of 1531 amino acids [15]. The MRP1 is of 190-kDa size encoded by ABCC1. Increased expression of this protein was reported in a variety of hematological and solid tumors. It was also found that MRP1 and Pgp are expressed in nonmalignant tissues and are believed to play a role in protecting tissues from xenobiotic accumulation and resulting toxicity [18]. Mammalian MRP1 is identified in a wide variety of eukaryotic organisms ranging from plant and yeast to mammals. MRP1 protein possesses two features that distinguish it from other ABC super family members. The first is the structure of NH2 proximal NBD (NBD1), and the second feature in MRP1-related proteins is that they are typically larger than other full-length ABC proteins [15]. MRP1 has three membrane spanning domains (MSDs) and two nucleotide-binding domains (NBDs). While the NBDs of MRP1/ABCC1 function as ATPase to hydrolyze ATP and provide energy for the transport activity, the MSD provides support for drug binding, putative drug transport
MRP1 (multidrug resistance-associated protein) The ABCC sub-family has been associated with drug resistance in organisms. This sub-family is composed of a
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channel, dimerization and trafficking [19]. MRP1 is capable of transporting many lipophilic anions, including conjugates of glutathione, glucuronic acid and sulfate [20]. In fact, it is known as a multi-specific anion transporter (MOAT) [21]. Mammalian MRP1 also transports known immunomodulating agents such as leukotriene C4 (LTC4) with high affinity and it has also been implicated in T cell activation [22]. MRP1 couples ATP binding and hydrolysis to solute transport across the biological membrane. Cells overexpressing MRP1 are resistant to a wide variety of anticancer drugs, such as doxorubicin, daunomycin, epirubicin, gramicidin D and some heavy metal anions. MRP1 is not able to transport unmodified anticancer drugs unless a physiological amount of glutathione (GSH) was added to the reaction mixture [23]. Several studies have shown that MRP1 overexpression is associated with lower levels of intracellular GSH and higher levels of extracellular GSH [24]. It is suggested that increased MRP1 expression without an increase in GSH biosynthesis would not cause any drug resistance in tumor cells, but would result in cell death [21].
Breast cancer resistance protein The 72-kDa breast cancer resistance protein (BCRP) is the second member of the subfamily G of the human ABC transporter superfamily and thus also designated as ABCG2 [25]. It is a 655 amino acid protein that contains an ATP-binding domain and six TM domains [26]. ABCG2 is expressed in many normal human tissues such as the epithelium of the small intestines and the colon, liver canalicular membranes, and ducts and lobules of mammary tissue. Increased expression of ABCG2 is frequently seen in both drug resistant cancer cell lines and clinical tumor tissues. Overexpression of BCRP is also associated with high levels of resistance to a variety of anticancer agents, including anthracyclines, mitoxantrone, and the camptothecins, through enhancing drug efflux [25]. ABCG2 is also considered a critical element of tumorigenic stem cells, and has been used as a selective biomarker for cancer stem cells [27]. The expression of ABCG2 in normal and cancer cells appears to be regulated at different levels, including gene amplification, epigenetic modifications, transcriptional and posttranscriptional regulation [28].
Resistance to radiotherapy Fifty percent of all cancer patients will receive radiotherapy knowing that survival rates after radiotherapy depend
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on cancer type and extent of disease progression at diagnosis. Recent improvements regarding radiation dose delivery lead to targeted delivery to the tumor precisely, while sparing the surrounding tissues. Unfortunately, tumor recurrence after radiotherapy is common and many causes may lead to the failure of radiotherapy and to the recurrence of cancer including metastasis, hypoxia, survivin cell repopulation during treatment and cell resistance to radiation [29]. Overwhelming evidence suggests that many solid cancers are organized hierarchically and contain a subpopulation of cancer stem cells (CSCs). It is believed that only once that sub-population is eradicated can cancer truly be cured. Unfortunately, there is growing evidence that CSCs are inherently resistant to radiation, and possibly even other cancer therapies. Generally speaking, the clinical outcome of standard radiotherapy treatment is determined by the 4 R’s of radiobiology: repair of DNA damage, redistribution of cells in the cell cycle, repopulation, and re-oxygenation of hypoxic tumor areas. DNA repair enzymes and anti-apoptotic proteins are expressed in high levels in radio-resistant cancer cells [30]. Some tumors persist after treatment and become more aggressive [31]. Ionizing radiation stimulates an adaptive phenotype known as stress and recovery response to DNA damage leading to metastatic behavior [32]. In single cells, IR-induced DNA damage leads to the activation of the (ATM)-p53 axis resulting in replication blockage and the activation of DNA repair [33]. Failure of repair results in the activation of tyrosine receptors belonging to HGF receptor (MET) oncogene family [34]. Increased transcription of the MET oncogene and activation of its signaling result in the implementation of a prosurvival and regenerative program that counteracts radiation-induced damage [35]. Inhibition of apoptosis may also play an important role in cell resistance to radiotherapy and chemotherapy [36]. Survivin is a member of the IAP family with a single baculovirus IAP repeat and lacks a carboxyl-terminal RING finger [37]. Survivin binds caspase-3 and caspase-7 reducing caspase activity and apoptosis in cells exposed to apoptotic stimuli. In response to radiation, survivin was found to be a constitutive resistant factor. This suggests that the inhibition of survivin mRNA expression may enhance the effectiveness of radiotherapy [38].
Resistance to immunotherapy Cancer immunotherapy utilizes the anticancer immune response or components of the immune system. Seventeen immunotherapy products have received FDA approval in the past quarter century as a treatment of cancer including cytokines, monoclonal antibodies, radiolabelled antibodies, and immunotoxins [39].
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Host immunity affects cancer development and progression by cancer immunoediting [40], which consists of three steps: (1) elimination and destruction of cancer cells, (2) equilibrium where some tumors remain but their growth is inhibited by immunity; and (3) the last step is escape where cancer cells can escape immunity [41]. To avoid the immune system, tumor cells downregulate the expression of tumor antigens. This leads to the inhibition of activated lymphocytes and resistance to immune effector cell-mediated apoptosis. Additionally, altered transporter expression in cancer cells results in the reduction of surface presentation which leads to impaired antigen presentation and subsequent immune escape [42]. Furthermore, some tumors may escape cell death by becoming insensitive to apoptotic signals derived from immune cells [43].
Concluding remarks This review provides a summary of some of the most important therapy resistant mechanisms developed by cancer cells. ABC transporters play an important role in chemotherapy resistance where MDR utilizes ATP to eject anticancer drugs out of cells. Another ABC member MRP1 transports various types of anionic drugs outside of the cell. As a response for radiotherapy, cancer cells increase the expression of MET that opposes the effect of radiotherapy. In addition to these mechanisms developed by cancer cells to evade chemical and radiological therapy, cells build up a mechanism by which they can escape the immune response and immune-based therapy. Understanding these mechanisms through extensive studies will be important to design future therapies that will aim to overcome such evasion and resistance. Conflict of interest
The authors declare no conflict of interest.
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