Resistance to cisplatin is attributed to three molecular mechanisms: increased DNA repair, altered cellular accumulatio...
Accepted Manuscript Title: Cisplatin Resistance and Opportunities for Precision Medicine Author: Lauren Amable PII: DOI: Reference:
S1043-6618(16)00002-5 http://dx.doi.org/doi:10.1016/j.phrs.2016.01.001 YPHRS 3027
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Pharmacological Research
Received date: Accepted date:
28-12-2015 1-1-2016
Please cite this article as: Amable Lauren.Cisplatin Resistance and Opportunities for Precision Medicine.Pharmacological Research http://dx.doi.org/10.1016/j.phrs.2016.01.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cisplatin Resistance and Opportunities for Precision Medicine Lauren Amable, Ph.D. Corresponding author: Lauren Amable, Ph.D. National Institute on Minority Health and Health Disparities National Institutes of Health 9000 Rockville Pike Bethesda, MD 20892 Email:
[email protected] Phone: (301) 451‐6629 Fax: (301) 480‐4490 1
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Abstract Cisplatin is one of the most commonly used chemotherapy drugs, treating a wide range of cancer types. Unfortunately, many cancers initially respond to platinum treatment but when the tumor returns, drug resistance frequently occurs. Resistance to cisplatin is attributed to three molecular mechanisms: increased DNA repair, altered cellular accumulation, and increased drug inactivation. The use of precision medicine to make informed decisions on a patient’s cisplatin resistance status and predicting the tumor response would allow the clinician to tailor the chemotherapy program based on the biology of the disease. In this review, key biomarkers of each molecular mechanism will be discussed along with the current clinical research. Additionally, known polymorphisms for each biomarker will be discussed in relation to their influence on cisplatin resistance. Abbreviations ABC, ATP‐binding cassette; ASE‐1, anti‐sense ERCC1; ATP7A, ATPase copper‐transporting, alpha polypeptide; ATP7B, ATPase copper‐transporting, beta polypeptide; CAST, T‐cell receptor complex subunit CD3‐associated signal transducer; CTR1, copper transporter 1; CTR2, copper transporter 2; ERCC1, excision repair cross‐complementation group 1 gene; GSH, glutathione; GST, glutathione‐s‐ transferase; MRP, multidrug resistance associated protein; MT, metallothionein; NER, nucleotide excision repair; NSCLC, non‐small cell lung cancer; OCT, organic ionic transporter; PCNA, Proliferating cell nuclear antigen; Pol, polymerase; SLC, solute carrier; TFIIH, transcription factor II H; UTR, untranslated region; XPA, xeroderma pigmentosum group A; XPB, xeroderma pigmentosum group B; XPD, xeroderma pigmentosum group D; XPE, xeroderma pigmentosum group E; XPF, xeroderma pigmentosum group F; XPG, xeroderma pigmentosum group G; Keywords: cisplatin resistance, nucleotide excision repair, ERCC1, copper transporters, ABC transporters, glutathione 3
1.0 Introduction Rosenberg and colleagues first discovered in E. coli that the byproducts from platinum electrode activity resulted in the inhibition of cell division [1, 2]. Within 15 years, cisplatin was approved for the treatment of cancer by the FDA. Cisplatin is one of the most widely used anticancer drugs in North America and Europe [3], treating a variety of cancers including: testicular, ovarian, non‐small cell lung cancer (NSCLC), head and neck cancer, bladder, gastric, and other malignancies [4]. The main issue with obtaining the optimum cisplatin cancer treatment is the significant interpatient variability with outcome, efficacy, and toxicity. There are two problems associated with cisplatin usage in the clinic: toxicity and resistance. Cisplatin has a numerous toxicities including renal damage, deafness, and peripheral neuropathy, thus the overall efficacy of the drug could not be reached due to the side effects. This has led to the development of cisplatin analogs that would be clinically effective but without the toxicity. Carboplatin and oxaliplatin, figure 1, are the most popular analogs and reached FDA approval for usage. Interestingly, there is a variation in the cancers treated by the cisplatin analogs. Carboplatin is not as effective in treating germ cell malignancies compared to cisplatin. Oxaliplatin is very effective for the treatment of colon cancer, a cancer where cisplatin is not effective. Understanding the molecular basis for the difference between these three compounds could provide new insights and unlock novel mechanisms into how cancer cells counteract the effects of DNA‐damaging drugs. While the analogs show hope for a better response with less toxicity the scope of this review will not cover cisplatin analog resistance. For a review on cisplatin analog resistance, the author directs the reader to a recent review from Perego & Robert [5]. The second issue associated with cisplatin treatment is resistance to therapy. Initially the tumor responds to cisplatin but then the tumor comes back and is frequently refractory to further platinum therapy. There are two forms of resistance found in the clinic: innate and acquired. Innate resistance is resistance without
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out any prior drug exposure. Acquired resistance is a result of drug exposure. The differences between innate and acquired resistance are not clear but it is generally thought that each operates through different signaling pathways. This review will only focus on resistance as a whole since discerning between the two requires further studies. In the clinic, the definition of whether a patient is sensitive or resistant to cisplatin is generally as follows. If a patient is more than two years from the last platinum dose, the patient is considered sensitive. There is a greater than 70% likelihood that the patient will respond to treatment with platinum‐based therapy [6]. The percentage of patients who will respond to cisplatin decreases with the shortening of the disease free period. Patients who have disease recurrence within the first months after the recent platinum dose will have a low likelihood of treatment response with cisplatin and are considered to have platinum resistant disease. When cisplatin is transported into the cell, cisplatin has several fates, figure 2. Frist, cisplatin can be exported from the cell using a transmembrane transporter system. Second, cisplatin can be chemically neutralized by binding sulfhydryl groups in proteins such as glutathione or metallothioneins. Finally, cisplatin nonspecifically reacts with a variety of subcellular components: proteins, RNA, and DNA. RNA is most sensitive to react with cisplatin, followed by DNA, and then protein. The primary and widely accepted mechanism of action for cisplatin is the binding to cellular DNA, resulting in DNA‐platinum adducts. This prevents the cell from replicating its DNA until the damage is repaired. If the cell cannot repair the DNA or the damage is too severe, then the cell dies. Resistance to cisplatin occurs by the following molecular mechanisms: altered cellular accumulation of drug, altered DNA repair, and cytosolic inactivation of drug. The processes of resistance were studied in L1210 mouse leukemia cells and human ovarian cancer cells [7‐10]. The observations were similar in both
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cell models: all three components contributed to cisplatin resistance. There were differences of each mechanism regarding the relative contributions. At low levels of cisplatin resistance, about 10‐15 fold above baseline, the primary mechanism of resistance was DNA repair. Intermediate levels of resistance, up to 40‐50 fold over baseline, was due to reduced cellular cisplatin accumulation. At very high levels of resistance, cytosolic inactivation of cisplatin was the primary mechanism. However, in many cell lines it has been observed that more than one mechanism can be in play here. The goal of precision medicine is to generate better responses in the clinic. Making an informed decision on predicting the tumor response to cisplatin as well as the type of resistance that is occurring allows for tailoring the chemotherapy program based on the biology of the disease. Here in this review, we will comprehensively discuss the mechanisms of cisplatin resistance‐ altered DNA repair, altered cellular accumulation, and drug inactivation. For each mechanism, the most promising biomarkers identified so far will be discussed and are summarized in table 1. Polymorphisms of each biomarker that correlate with cisplatin resistance from current clinical studies will also be presented, and are summarized in table 2. 2.0 Altered DNA Repair Once inside the cell, cisplatin binds to DNA and forms adducts. The primary form of DNA damage are N7‐ d(GpG) and N7‐d(ApG) intrastrand DNA‐platinum adducts. These bulky adducts result in substantial kinking of the DNA (12), which is recognized and repaired by the nucleotide excision repair (NER) pathway, shown in figure 3. In this pathway, which requires more than 30 proteins, the DNA‐platinum adduct is first recognized by XPE and XPC‐DDB1/2. The transcription factor II H (TFIIH) complex verifies the damage and assembles the pre‐incision complex: RPA, XPA, and XPG. The DNA is then unwound by the XPB and XPD helicases. ERCC1‐XPF and XPG endonucleases create an excision several bases upstream
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and downstream from the DNA‐platinum adduct. This releases the oligonucleotide containing the adduct. The gap is filled in by the DNA repair synthesis complex containing RPA, RFC, PCNA, and Pol /. In the final step, the DNA is ligated by DNA ligase I, thus completing the DNA repair. The balance of DNA damage and DNA repair dictates death versus survival after cisplatin therapy [11]. Changes in the ability to repair the DNA adducts results in changes in cisplatin sensitivity. 2.1 ERCC1 ERCC1 is one of the most highly studied biomarkers for cisplatin resistance to date. The DNA damage excision step is catalyzed by the ERCC1‐XPF dimer is the rate limiting step in the NER pathway. High ERCC1 levels are associated with increased removal of DNA‐platinum adducts and resulting in increased resistance to cisplatin [12]. There is a linear correlation of ERCC1 expression and cisplatin sensitivity, with resistant cells expressing more ERCC1 compared to sensitive. It was first demonstrated in ovarian cancer that ERCC1 levels are increased in cancer tissue in comparison to normal [13]. Even higher levels of ERCC1 mRNA are found in patients with clinically resistant cancer. Lower levels are found in patients that are clinically sensitive to platinum therapy. In another study comparing the six histologic types of ovarian cancer, there is an upregulation of NER genes that correlates with cisplatin resistance. Clear cell tumors are known to be the most chemoresistant to cisplatin, and they displayed the highest levels of ERCC1 [14]. The evaluation of ERCC1’s potential role as a cisplatin resistance biomarker has been explored in other cancers. High ERCC1 levels that correlate with increased resistance to cisplatin have been observed in: ovarian [15, 16], NSCLC [15, 17‐23], nasopharyngeal [24, 25], esophageal [26], cervical [27, 28], head and neck squamous carcinoma [29, 30], liver [31], osteosarcoma [32], lung adenocarcinoma [33], advanced biliary tract adenocarcinoma [34], mesothelioma [35], pulmonary adenocarcinoma [36], and gastric [11]. Thus, the expression of ERCC1 makes an attractive biomarker for cisplatin resistance since the increased expression has been observed in a variety of cancer types.
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There are two polymorphisms of ERCC1 that appear to have clinical significance with sensitivity to cisplatin treatment. The first polymorphism, rs11615, is located in codon 118, that codes for the same amino acid‐ asparagine, was first described by Reed and colleagues [37, 38]. A point to note for this polymorphism: there is a discrepancy in the literature on whether the change is from C to T or T to C. The reader should take caution in evaluating studies, as these alleles are switched in the analyses. Here this review the rs11615 polymorphism is from C to T. The rs11615, or N118N, polymorphism was originally thought to result in reduced levels of ERCC1 mRNA and protein, as the codon is an infrequently used codon [37‐39]. A clinical molecular correlative study in ovarian cancer was confirmatory [40] but in another recent study it was shown to not change the expression or function of ERCC1 but rather may be linked to other causative variants [41]. There are conflicting data as to whether or not this polymorphism determines sensitivity to cisplatin. In ovarian cancer, the T allele was associated with an increased response to cisplatin therapy [40]. This was also observed in colorectal cancer [42], pancreatic cancer [43], osteosarcoma [44] and NSCLC [45]. Yet in another study, C allele was associated with a higher response rate to cisplatin, progression free survival, and overall survival [46, 47]. Thus two opposite results have been observed. The second ERCC1 polymorphism relating to cisplatin sensitivity is C8092A. This polymorphism was first identified in gliomas and is located in the 3’ UTR of ERCC1 [48]. The A allele is thought to result in decreased mRNA stability of ERCC1. This polymorphism results in an A substitution in two additional genes, nucleolar protein ASE‐1 and t‐cell receptor complex subunit CD3‐associated signal transducer (CAST) [48]. Thus the exact role of this polymorphism is not fully understood as these genes may have an effect that has not been evaluated. Studies in the C8092A polymorphism are few and are additionally associated with conflicting data. The clinical implication of the C8092A ERCC1 polymorphism has been
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studied in lung [49] and esophageal cancer [50] and both studies demonstrated the A polymorphism results in increased cisplatin response. There is also conflicting data as to the A allele and clinical resistance to platinum‐based therapy. In a nasopharyngeal cancer study, the A allele of C8092A was associated with an increased risk of disease progression with patients on cisplatin‐based chemotherapy [51]. In a malignant pleural mesothelioma study, the A polymorphism of C8092A was associated with a shorter progression free survival [35]. Yet, in a meta‐analysis of 39 NSCLC studies, there was no relationship of survival and sensitivity to treatment with platinum‐based chemotherapy [52]. 2.2 Other NER genes While the majority of the studies have focused on ERCC1, there are several studies that suggest other NER genes are involved in cisplatin resistance. Dabholkar et al., showed that other NER genes are additionally upregulated in patients who responded to cisplatin therapy [53]. XPA, which is part of the pre‐incision complex, and XPB, a helicase, displayed increased expression in cisplatin resistant ovarian cancer tumors [53, 54]. However, this has not been explored further in other clinical studies. Neither XPA nor XPB polymorphisms have been discovered that correlate with cisplatin resistance. It is logical to think that XPF would be an additional cisplatin resistance marker since it is dimerizes with ERCC1 to catalyze the incision of the damaged DNA strand. However, XPF has been overlooked in many studies as to whether or not it is a valid biomarker for cisplatin resistance. In ovarian and colon cancer cell lines, the increased protein expression of XPF was correlated with increased cisplatin resistance [55]. There have only been two clinical studies that have examined XPF [56, 57]. Both studies examined head and neck cancer and increased XPF expression was correlated with cisplatin resistance. Vaezi and colleagues went on further to examine XPF polymorphisms, however the four polymorphisms they
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identified showed marginal association with treatment failure [57]. Further studies, examining the role of XPF in other cancers are needed. The helicase XPD was additionally identified to have a strong correlation between increased expression and cisplatin resistance in NSCLC and glioma cell lines [58, 59]. The expression of XPD has not been evaluated in clinical samples. The majority of the clinical studies have examined the effects of XPD polymorphisms on cisplatin resistance. Two polymorphisms in XPD have been identified, Asp312Asn and Lys751Gln, both result in decreased DNA repair capacity. Both polymorphisms were found to be potentials markers for clinical outcome in osteosarcomas and lung cancers treated with cisplatin [44, 60]. The Asp312Asn polymorphism was associated with a better survival in osteosarcoma patients treated with cisplatin [61]. The Lys751Gln polymorphism was associated with longer progression free survival in pancreatic and NSCLC [62, 63]. 3.0 Altered accumulation of cisplatin The second mechanism of cisplatin resistance is altered cellular accumulation of cisplatin. It has long been noted that cisplatin resistant cells tend to exhibit decreased levels of cisplatin [64]. Tissue platinum concentrations are correlated with percent reduction of the tumor, meaning reduced tissue platinum concentrations are associated with resistance [65]. Altered accumulation of cisplatin is the result of two independent cellular pathways: decreased uptake or increased export. 3.1 Decreased cellular uptake of cisplatin Cisplatin has a simple chemistry, the core is a single platinum metal bound to two amino groups and two chlorides, figure 1. At physiologic pH, the chlorides of cisplatin are replaced with –OH molecules, resulting in a neutral charge. This it makes it possible for diffusion across the cellular membrane, flowing from the
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high concentration of cisplatin outside to the lower concentration inside the cell. Thus, cisplatin uptake was first thought to be via passive diffusion as uptake was not saturated against time or drug concentrations, up to 3 mM [66, 67]. However, it was discovered that cisplatin mostly enters the cell by membrane transporters. This would explain why it has been observed that low levels of transporters correlate with decreased levels of cellular cisplatin. Cisplatin uptake is performed by the copper transporters CTR1 and CTR2, as well as the organic cationic transporter (OCT) family [68]. 3.1.1 Copper transporters CTR1 and CTR2 Copper transporter protein 1 (CTR1) was shown to be one of the primary cisplatin transporters. It was first discovered in yeast, noting that knocking down CTR1 resulted in reduced uptake in cisplatin [69]. CTR1 primarily transports copper, which is important in a variety of biological functions within a cell. Interestingly, resistance to cisplatin is accompanied by resistance to copper and cisplatin resistant cells display reduced levels of copper [70]. Cisplatin resistant cells show decreased levels of CTR1 mRNA and decreased cisplatin uptake [71, 72]. In the clinic, CTR1 has been evaluated in two cancer types, ovarian and lung, both resulting in the same observation. In ovarian cancer patients treated with cisplatin chemotherapy, high levels of CTR1 mRNA expression was correlated with increased disease free survival [73]. In NSCLC, the same pattern has emerged, high CTR1 protein levels were associated with a favorable cisplatin response [74, 75]. Only one study has examined the relationship of CTR1 polymorphisms with cisplatin resistance. Xu et al. found two polymorphisms in CTR1 that correlate with platinum resistance and survival: rs7851395 and rs12686377 [76]. These two polymorphisms are located in the intron region of CTR1 and are hypothesized to play a role in the epigenetic regulation of CTR1. Currently, it is not known what effect these polymorphisms have on the function of CTR1, as this article was the first to describe them.
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Cisplatin is also transported by another copper transporter, CTR2. While CTR2 is in the same family as CTR1, they only share a 33% homology on the protein level [77]. CTR2 is found on the cellular membrane like CTR1, but it is also found on intracellular organelle membranes and may have alternative cellular functions [78]. The opposite effect has been observed with CTR2 in terms of its correlation with cisplatin resistance. Unlike CTR1, knockdown of CTR2 in cells results in increased uptake, increased cytotoxicity, and increased sensitivity to cisplatin [79]. In two clinical studies, lower levels of CTR2 are associated with a better outcome to cisplatin therapy [80, 81]. Interestingly, it has been suggested that the CTR1/CTR2 ratio may be a useful biomarker for identifying tumors which may be more sensitive to cisplatin than on one of the transporters alone [81]. No CTR2 polymorphisms have been identified to influence cisplatin sensitivity. 3.1.2 Organic cation transporters The solute carrier (SLC) transporter family, specifically the SLC22 family members, also transport cisplatin into cells. Members of this family, OCT1, OCT2, and OCT3, have been shown to uptake platinum‐ compounds into cells, but vary in the expression and substrate for each transporter [68]. OCT1 has been indicated to transport cisplatin, however the evidence is weak [68]. OCT1 does transport the cisplatin analogs carboplatin and oxaliplatin. OCT3 is known to primarily transport oxaliplatin. Cisplatin is primarily transported by OCT2, or SLC22A2, and this transporter is found in the kidney. OCT2 transfection into cells results in increased cellular levels of cisplatin [82]. There is not a lot of clinical studies examining OCT2’s role in cisplatin resistance, primarily due to the fact it is expressed in the kidney. In the NCI‐60 panel of cancer cell lines, OCT2 was the most frequently expressed gene but its expression in clinical ovarian cancer specimens was low and did not correlate with the treatment outcome of a
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platinum‐based regimen [83]. In one gastric cancer study, higher levels of OCT2 were observed in responders to cisplatin‐based neoadjuvant therapy in comparison to non‐responders [84]. The majority of OCT2 polymorphism studies have primarily focused on the effect on nephrotoxicity. There is one study in lung cancer that evaluated the OCT2 polymorphisms. The polymorphisms rs195854 and rs186941 were associated with increased response to cisplatin [85]. 3.2 Increased cellular export of cisplatin The export of cisplatin has been suggested to occur via passive efflux, however the issue with studies to examine this phenomena are performed with high concentration of cisplatin. Thus it is thought that export of cisplatin from cells occurs via membrane transporters. There are two major pathways in which cisplatin is removed from the cell: removal by P‐type ATPase transporters or removal by ATP‐binding cassette transporters. 3.2.1 P‐type ATPase transporters Cisplatin is exported by ATP7A and ATP7B, which belong to the transporter family of P‐type ATPases which use ATP to export. These transporters are associated with removing excessive copper from cells. Under normal conditions within the cell, ATP7A and ATP7B are found in the trans‐Golgi network and are trafficked to the cell membrane to remove copper. As mentioned earlier, copper levels are lower in cisplatin resistant cells which is additionally regulated by ATP7A and ATP7B. Defects in these transporters are associated with diseases with excessive copper accumulation: ATP7A is defective in Menkes disease while ATP7B is defective in Wilson’s disease [86]. ATP7A is found is most tissues, aside from the liver. Increased ATP7A expression is found in cancer cells but not in normal tissue [87]. Increased expression of ATP7A in cells renders the cells resistant to cisplatin
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but interestingly this was not due to altered cisplatin export [88, 89]. There are only a few clinical studies evaluating the role of ATP7A and cisplatin resistance. In NSCLC and ovarian cancer, increased ATP7A expression is associated with a poorer response to cisplatin [87, 89]. ATP7A levels are additionally higher in NSCLC tumors that are resistant to cisplatin [90]. No ATP7A polymorphisms have been identified with sensitivity to cisplatin, though studies are very limited in examining ATP7A polymorphisms in general [91]. ATP7B is found mostly in the liver, kidney and brain. Similar observations found with ATP7A in terms of cisplatin resistance have also been observed with ATP7B. ATP7B was first proposed to be a biomarker of cisplatin resistance, as transfection of ATP7B into cells resulted in an increase in cisplatin resistance accompanied by reduced cisplatin accumulation [92]. Increased expression of ATP7B is associated with poorly differentiated tumors and are poor responders to cisplatin therapy in a variety of cancers including: gastric, hepatocellular, esophageal, oral, breast, endometrial, lung, and ovarian [93‐101]. Currently, there are no identified ATP7B polymorphisms that are associated with cisplatin resistance. 3.2.2 ATP‐binding cassette (ABC) transporters Multidrug resistance‐associated proteins (MRPs), belong to the ABCC subfamily of ABC (ATP binding cassette) transporters and been implicated in cisplatin resistance [102]. MRPs are membrane transporters responsible for the efflux of glutathione‐platinum conjugates, in an ATP‐dependent fashion. MRP1 was first explored as a cisplatin transporter as it was found that cisplatin resistant cells displaying increased levels of glutathione concurrently had increased levels of MRP1 [103]. Reports from other groups suggested that MRP1 alone was not enough to confer cisplatin resistance and there is no relationship between MRP1 and cisplatin accumulation and cytotoxicity [104‐106]. There are no polymorphisms of MDR1 associated with cisplatin response as well. Thus, MRP1 is generally not thought to play a role in cisplatin resistance.
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MRP2, also known as cMOAT (canalicular multispecific organic anion transporter), is the most favored MRP transporter contributing to cisplatin resistance. Overexpression of MRP2 is found in a variety of cisplatin resistant cells lines [107‐109]. The expression of MRP2 is induced by cisplatin as well [110]. In the clinic, there are different observations of correlating MRP2 expression with cisplatin sensitivity, which may reflect the tissue specific nature of the transporter. Increased MRP2 expression is associated with cisplatin resistance in colorectal, esophageal, and hepatocellular cancers [111‐113]. However, in ovarian and lung cancer, MRP2 did not predict cisplatin response [100, 114, 115]. One polymorphism in MRP2, C‐ 24T, has been correlated with increased response to platinum‐based chemotherapy in lung cancer [116, 117]. The C‐24T polymorphism is found in the promoter of MRP2 and its function is not currently known. 4.0 Cytosolic inactivation of cisplatin Finally, the last resistance mechanism is cytosolic inactivation of cisplatin. This inactivation results in the inability of cisplatin to react with DNA. Less damage is produced and the cancer cell survives the drug treatment. The primary form of inactivation is conjugation of cisplatin with glutathione (GSH), resulting in cellular export by the MRP transporters, discussed in the prior section. The secondary form of inactivation is by binding to metallothioneins. 4.1 Inactivation by glutathione conjugation Glutathione‐S‐transferases (GSTs) catalyze the conjugation of glutathione (GSH) to cisplatin. The formation of platinum‐glutathione conjugates inactivates the drug by increasing its solubility, leading to excretion. Inside the cell, glutathione acts as antioxidant. It maintains the redox environment by keeping reduced sulfhydryl groups [118]. Depletion of cellular GSH in cisplatin resistant cells enhances the cytotoxicity of cisplatin [119]. However, the cisplatin sensitivity is not restored to levels of the parental
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cell lines. In ovarian cancer cells, increased levels of GSH were observed in platinum resistant cell lines [120]. There are two families of GST enzymes involved in cisplatin detoxification‐ GSTP1 and GSTM. 4.1.1 Glutathione‐S‐transferase Pi 1 GSTP1, also called GST Pi 1, is expressed in different epithelial tissues. The cellular and clinical studies of GSTP1 are inconclusive as to whether or not it is an indicator of cisplatin resistance. In colon, lung, and glioblastoma cancer cell lines, the levels of GSTP1 are correlated between high GSTP1 expression and cisplatin resistance [121]. In ovarian and head and neck carcinoma patient samples, there is a correlation between high expression of GSTP1 and cisplatin resistance [122‐124]. Several studies in NSCLC have demonstrated that low levels of GSTP1 are associated with increased sensitivity to cisplatin [125‐129]. However, in other clinical studies of ovarian and cervical cancer there was no association of GSTP1 levels and response to cisplatin chemotherapy [130‐132]. GSTP1 has two polymorphisms: rs1695 and rs1138272. Similar to the expression data, GSTP1 polymorphism data is conflicting and inconclusive. The rs1695 polymorphism affects the ability of GSTP1 to conjugate GSH to cisplatin [133]. Studies in NSLC have yielded multiple responses for the GSTP1 rs1695 polymorphism: associated with a favorable response to cisplatin therapy [116, 134], associated with reduced survival to cisplatin therapy [135], and no association with survival [136]. The rs1138272 polymorphism additionally has different responses to cisplatin therapy: one study found it was associated with greater median survival [136] and another study correlated to the polymorphisms to a shorter event free survival and shorter overall survival in osteosarcoma patients [61].
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4.1.2 Glutathione‐S‐transferase Mu GSTM, or GST Mu, is the other GST involved in the inactivation of cisplatin. GSTM is more known for the detoxification of xenobiotics thus there is little research in evaluating GSTM in cisplatin resistance. There are five GSTM genes: GSTM1, GSTM2, GSTM3, GSTM4, and GST5. Earlier studies showed that there was no difference and no contribution by GSTMs to cisplatin resistance [137‐139]. However, recent data using a paired cisplatin sensitive/resistant breast cancer cell line demonstrated decreased GSTM3 and GSTM4 levels were found in cisplatin resistant cells compared to sensitive cells [140, 141]. This has not been investigated further as one would assume that decreased levels of GSTs would show decreased resistance to cisplatin, like observed with GSTP1. However, pharmacologic inhibition of GSTM1 resulted in the increased sensitivity of cells to cisplatin [142]. There are no clinical studies examining the relationship of GSTM levels with cisplatin sensitivity. The majority of GSTM polymorphism studies have focused on the susceptibility to cancer and not so much on the relationship with cisplatin resistance. GSTM2 and GSTM5 do not have any reported polymorphisms. Polymorphisms for GSTM3 and GSTM4 have not been evaluated for their relationship with cisplatin resistance. There are few studies with polymorphisms of GSTM1. Wheeler et al. showed that the rs10431718 GSTM1 polymorphism was associated with the cisplatin IC50 [143]. In a NSCLC meta‐ analysis, the GSTM1 null genotype was associated with improved response to platinum therapy [144]. 4.2 Inactivation by metallothionein binding Finally, cisplatin is also inactivated by binding to metallothionein (MT) proteins. MT proteins are cysteine‐ rich, low molecular weight proteins that bind to metals such as copper, zinc, cadmium, and mercury. While there are multiple MTs, mostly MTI and MTII have studied since they are ubiquitously expressed. However, it is not always evident which MT is being examined. MTs function as regulators of cellular metal
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homeostasis as well as detoxification of heavy metal exposure in cells. In terms of cisplatin resistance, MTs serve as a heavy‐metal detoxifier of cisplatin in tumors. Overexpression of metallothionein has been observed in several cell lines that are resistant to cisplatin [145‐147]. Additionally, the overexpression of MTII confers resistance to cisplatin in cancer cells [148]. Cisplatin treatment also induces the expression of MT [149]. In germ cell tumors, MT expression was higher in cell lines and tumors, but there was no difference between patients who responded to cisplatin based therapy compared to non‐responders [150]. In esophageal cancer, expression of MT was associated with a shorter survival rate after cisplatin therapy [151]. This was additionally observed in ovarian cancer patients receiving cisplatin based therapy [123]. There are no MT polymorphisms that are associated with cisplatin resistance. 5.0 Summary Cisplatin is a clinical mainstay for the treatment of a variety of cancers. Unfortunately, many tumors develop resistance and are refractory to treatment. Resistance stems from three overall mechanisms‐ increased DNA repair, altered drug cellular accumulation, and increased drug cytosolic inactivation. In this review, potential biomarkers and their known polymorphisms for each resistance mechanism were examined and are summarized in tables 1 and 2. While ERCC1 represents the most promising biomarker for cisplatin resistance, as it has been extensively studied in a variety of cancers, there are several opportunities and areas ripe for further study. Other components of NER, CTR1 and CTR2, OCT2, ATP7A and ATP7B, GSTs, and metallothioneins have the potential to be valid cisplatin biomarkers as well, and would benefit from additional clinical studies. While this list is comprehensive, there are several things to consider. Not all biomarkers were examined in a multitude of cancer types, so some may be tumor specific. Many biomarkers displayed conflicting evidence with their role in cisplatin resistance. Some of the conflicting data reflects the fact that many
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studies maybe underpowered for the analyses, and would benefit from having larger sample sizes. Because resistance to cisplatin is multilayered and multifactorial, different mechanisms are likely to be activated depending on the cancer type and stage. It is highly likely that multiple resistance mechanisms will be activated within a patient. While one biomarker may not be completely informative for all cancers, a combination of biomarker expression and polymorphism screening may yield a comprehensive approach to elucidate the resistance status of a patient. The method by which patients are screened is critical. The type of biospecimen used in the evaluation (blood, tissue, etc.), the expression type (DNA, mRNA, protein), and the method used to examine expression (PCR‐based, IHC, etc.) will all need to be standardized for analysis of resistance. The definition of what is considered high versus low expression, as well as the cutoff points between the categories, will also require standardization. A recent paper described the challenge of biomarker based screening. In a round robin analysis of three independent commercial labs, 18 tumor blocks were sent for testing of ERCC1 status, and the results were inconsistent and unreliable [152]. Only 4 of 18 blocks tested were fully concordant with ERCC1 status between all three labs. Thus further evaluation and standardization are needed before these assays become clinical standard‐of‐care. Precision medicine serves two purposes for cisplatin resistance: to determine if resistance is occurring and to determine the nature of the activated resistance mechanism(s). Screening the patient prior to initiation of treatment, and during the course of treatment, allows for the improvement of cancer diagnosis by predicting tumor response. Personalizing this therapy will increase the efficacy and decrease the toxicity of platinum‐based chemotherapy. While there is a long road ahead, several of the biomarkers listed here may serve as a foundation for larger, prospective studies to determine which biomarker, or combination of biomarkers, would result in the best prediction of cisplatin sensitivity and resistance in patients.
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Conflict of interest. The author has no conflict of interest. Acknowledgements The research was supported by the Intramural Research Program of the NIH, National Institute on Minority Health and Health Disparities. This manuscript is dedicated to my former mentor, Dr. Eddie Reed.
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Figure legends Figure 1. Cisplatin and analogs. Figure 2. Cellular fate of cisplatin (Pt). Cisplatin crosses the cell membrane by passive diffusion or by transmembrane transporters. CTR1, CT2, and OCT2 have been identified as transporters that import cisplatin into the cell. Once inside the cell, cisplatin binds to DNA to cause DNA‐platinum adducts. The damage is repaired by ERCC1 and members of the NER pathway. Cisplatin is also inactivated by glutathione‐s‐transferase, which add a glutathione (GSH) to cisplatin. The conjugated cisplatin‐GSH is then exported via the MRP2 transporters. Cisplatin is also exported by ATP7A and ATP7B. Inactivation of cisplatin can also result from binding metallothionein proteins (MT). Figure 3. Schematic of nucleotide excision repair (NER). DNA‐platinum adducts are removed by the NER pathway. First the DNA‐platinum adduct is detected. Then the damage is verified and the pre‐incision complex is set up containing RPA, XPA, and XPG. DNA is unwound by XPB and XPD. XPF‐ERCC1 and XPG create incisions 5’ and 3’ from the damaged base. The oligonucleotide containing the damaged base is removed. The gap is filled in by DNA repair synthesis complex: RPA, RFC, PCNA, and Pol /. Finally, the DNA is ligated.
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38
39
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Table 1. Proteins associated with cisplatin resistance. Protein Relationship to Cancer resistance NER ERCC1 Increased expression Ovarian, NSCLC, Nasopharyngeal, esophageal, cervical, head and neck squamous carcinoma, liver, osteosarcoma, lung adenocarcinoma, biliary tract adenocarcinoma, mesothelioma, pulmonary adenocarcinoma, gastric XPA Increased expression Ovarian cancer XPB Increased expression Ovarian cancer XPF Increased expression Ovarian and colon cancer cell lines; head and neck carcinoma XPD Increased expression NSCLC and glioma cell lines Cellular Uptake CTR1 Decreased expression Ovarian cancer, NSCLC CTR2 Increased expression Ovarian cancer OCT2 No change Ovarian cancer Decreased expression Gastric cancer Cellular Export ATP7A Increased expression NSCLC, ovarian cancer ATP7B Increased expression Gastric, hepatocellular, esophageal, oral, breast, endometrial, lung, ovarian cancer MRP2 Increased expression Colorectal, esophageal, hepatocellular cancer Drug Inactivation GSTP1 Increased expression Ovarian cancer, head and neck carcinoma, NSCLC No change Ovarian, cervical cancer MT Increased expression Esophageal, ovarian cancer
References 13‐16, 15, 17‐23, 24‐25, 26, 27‐28, 29‐30, 31, 32, 33, 34, 35, 36, 11
53, 54 53, 54 55‐57
58, 59 73‐75 80, 81 83 84 87, 89, 90 93‐101
111‐113 122‐124 130‐132 151, 123
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Table 2. Gene polymorphisms associated with cisplatin resistance. Gene Polymorphism Response to Cisplatin Cancer NER ERCC1 rs11615, Increased response Ovarian cancer, N118N colorectal, pancreatic, osteosarcoma and NSCLC Decreased response NSCLC C8092A Increased response NSCLC, esophageal Decreased response Nasopharyngeal, mesothelioma No relationship NSCLC XPD Asp312Asn Increased response NSCLC, osteosarcoma, pancreatic cancer Cellular uptake CTR1 rs7851395, Increased response NSCLC rs12686377 OCT2 rs195854, Increased response NSCLC rs186941 Cellular Export MRP2 Increased response NSCLC Drug Inactivation GSTM1 rs10431718 Increased response lymphoblastoid cell lines Null allele Increased response NSCLC
Reference 38,40, 42, 43, 44, 45
46, 47 49, 50, 51, 35 52 54,60, 62, 63
76 85 116, 117 143 144
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