The Role of ATP-Binding Cassette Transporters in the Chemoresistance of Anaplastic Thyroid Cancer: A Systematic Review

Abstract

Anaplastic thyroid cancer (ATC) is an aggressive type of thyroid cancer with a high mortality rate. Cytotoxic drugs are among the treatment modalities usually used for ATC treatment. However, systemic chemotherapies for ATC have not been shown to have remarkable efficacy. ATP-binding cassette (ABC) transporters have been suggested as a possible mechanism in ATC resistance to chemotherapy. This systematic review was aimed to define the possible roles of ABC transporters in ATC resistance to chemotherapy. Numerous databases, including Scopus, Web of Science, PubMed, Cochrane Library, Ovid, ProQuest, and EBSCO, were searched for papers published since 1990, with predefined keywords. The literature searches were updated twice, in 2015 and 2017. All identified articles were reviewed, and 14 papers that met the inclusion criteria were selected. In the eligible studies, the roles of 10 out of 49 ABC transporters were evaluated; among them, three pumps (ABCB1, ABCC1, and ABCG2) were the most studied transporters in ATC samples. ABCC1 and ABCG2 had the highest expression rates in ATC, and ABCB1 ranked second among the inspected transporters. In conclusion, ABC transporters are the major determinants of ATC resistance to chemotherapy. By identifying these transporters, we can tailor the best treatment approach for patients with ATC. Additional studies are needed to define the exact role of each ABC transporter and other mechanisms in ATC drug resistance.

Anaplastic thyroid cancer (ATC) has one of the worst prognoses of all types of cancer, mainly because of its aggressive nature and resistance to treatment (1). The median survival of ATC is approximately 3 to 5 months, and only 20% of patients live >1 year after diagnosis (1, 2). Current treatment approaches for ATC including surgery, external beam radiotherapy, and chemotherapy are not efficient and have not shown a considerable improvement in survival (3, 4).

The chemotherapy drugs mostly used for ATC are taxanes (e.g., paclitaxel or docetaxel), anthracyclines (e.g., doxorubicin), and platinum compounds (e.g., carboplatin or cisplatin) (4–6). These chemotherapy drugs have different mechanisms of action. Doxorubicin causes cell death via topoisomerase II–mediated DNA double-strand breaks or slowing down or stopping DNA synthesis (7, 8). Docetaxel and paclitaxel work as antimitotic (antimicrotubule) agents (7). Cisplatin and carboplatin cause DNA crosslinks, interfering with mitosis and cell division, and finally cause the apoptotic cell death of tumor cells (7, 9, 10). However, the use of different classes of chemotherapy agents (either alone or in combination) in ATC treatment has not shown significant efficacy and cannot prevent chemoresistance (4, 6, 11). For example, doxorubicin, with or without platinum agents, provides an overall response rate of 0% to 20% (12).

Recently, the US Food and Drug Administration approved a combination therapy with dabrafenib (BRAF inhibitor) and trametinib (MEK inhibitor) for treatment of patients with unresectable or metastatic ATC with BRAFV600E mutation, leading to an overall response rate of 61% (95% CI, 39% to 80%) (13). This new treatment option for ATC highlights the importance of knowing the underlying mutation and mechanism of drug resistance in ATC.

Many different mechanisms at various levels limit the efficiency of current treatments for cancers. For example, the normal tissue toxicity and pharmacokinetic parameters of drugs restrict the recommended dosage of each drug and the amount of drug reaching cancer cells. At the tumor level, cancer resistance can be primary (patients do not respond or respond very poorly to therapy because of preexisting resistant factors) or secondary, which develops after a certain period of exposure to the drug in tumors that were initially sensitive. These phenomena are caused by new mutations or by a subset of resistant tumor cells surviving after the treatment and repopulating the tumor mass. Several mechanisms are involved in primary and secondary drug resistance (14, 15). These mechanisms acting independently or in combination are the reduction of drug influx, drug efflux pumps, alterations in drug metabolism or drug targets, activation of DNA repair mechanisms, interruptions in apoptotic signaling pathways, and emergence of cancer stem cells (CSCs) (14, 16–18). Among the aforementioned mechanisms, ATP-binding cassette (ABC) transporters acting as efflux pumps have attracted a lot of attention (16, 19).

The human genome includes 49 ABC genes classified into seven subfamilies (designated from A to G) (7, 20). Through ATP hydrolysis as an energy source, ABC transporters can move substrates across the cells’ plasma membranes or intracellular membranes against the direction of their electrochemical gradients (20, 21). They regulate cellular levels of small molecules and are involved in different physiological processes (21). ABC transporters also efflux anticancer agents and indeed are the most common mechanism for evolution of resistance to different drugs (19).

Several studies have been conducted to define the role of ABC transporters in ATC resistance to chemotherapy. However, to date there has been no systematic approach to assess the results of the previous studies. Thus, this systematic review was designed to evaluate the possible roles of ABC transporters in ATC chemotherapy resistance.

Methods

The main databases searched were Scopus, Web of Science, PubMed, Cochrane Library, Ovid, ProQuest, and EBSCO. Furthermore, to find more evidence, other sources such as Science Direct, Wiley Online Library, CRD Database, UpToDate, ClinicalKey, and Google Scholar were reviewed. The main search keywords were “anaplastic thyroid cancer,” “multidrug resistance gene,”, “multidrug resistance-associated protein,” “ATP-binding cassette transporter,” “SLC5A5,” “sodium iodide symporter,” and their logical combinations, related words and phrases, Medical Subject Headings terms, EBSCO thesaurus terms, and Emtree terms. The details of our systematic review protocol are accessible in the International Prospective Register of Systematic Reviews with registration number CRD42016034152 (https://www.crd.york.ac.uk/prospero/). Two updates of the literature searches were carried out in 2015 and 2017.

The inclusion criteria included all studies (preclinical or clinical) evaluating the role of molecular alterations in ATC chemotherapy resistance and published between 1990 and 2017. The exclusion criteria included articles focused solely on the role of molecules other than ABC transports (e.g., sodium iodide pump) in ATC chemotherapy resistance (considered for a separate systematic review) and non–English language studies. The titles and abstracts of articles were reviewed simultaneously by two independent researchers for the inclusion and exclusion criteria, and in case of inconsistency, a third reviewer’s opinion was considered. In the next step, the full texts of the selected articles were reevaluated for meeting the eligibility criteria. Finally, to increase the comprehensiveness of the study, the reference lists of the selected articles were also reviewed to identify other relevant studies.

The following data were extracted from each selected article: the year of publication, the name of the first author, type of samples, methods, the studied ABC transporters, and a summary of the results.

Because no standard checklist is available for the critical evaluation of basic studies, a modified version of the Biological Variation Data Reporting Checklist (22) was used for quality assessment of the included studies. The final quality score of each included article is presented in Table 1.

Results

A total of 1144 published articles were identified in the primary search. Two updated searches were conducted in 2015 (37 articles) and 2017 (117 articles). After the duplicates were removed, 631 articles remained, which were reviewed based on the title and abstract, and 589 articles were excluded because of irrelevant contents, lack of details, or being written in languages other than English. Finally, among the remaining 42 studies, and after a full text review, 14 articles were found to meet the eligibility criteria, and these were selected for the systematic review. The search flowchart and the reasons for exclusion of the studies according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses method are illustrated in Fig. 1 (36).

The results of our systematic review showed that among 49 ABC transporters, the role of 10 ABC transporters were studied in ATC samples, among which three pumps, ABCB1 [also known as multidrug resistance 1 (MDR1) or P-glycoprotein (P-gp)], ABCC1 [also known as multidrug resistance–associated protein (MRP) 1], and ABCG2 [also known as breast cancer resistance protein (BCRP)], were the most studied transporters. The highest expression rates belonged to ABCC1 and ABCG2, and ABCB1 ranked second. In addition, the roles of ABCA8, ABCB10, ABCC5, ABCC10, ABCG1, MRP2, and MDR3 were also evaluated in limited studies that demonstrated the expressions of ABCA8, ABCB10, ABCC5, ABCC10, ABCG1, and MRP2 in ATC (MDR3 not expressed).

Most of the studies evaluated only the expression level of ABC transporters in ATC. However, three studies evaluated the effect of ABC transporter inhibition on cancer drug resistance. Lopez et al. (11) demonstrated that gefitinib caused downregulation of ABCG2 in the ARO cell line and increased its sensitivity to doxorubicin. Zheng et al. (31) showed that the sensitivity of CSC-rich cell line HTh74R to doxorubicin is partly or completely restored through inhibition of ABCG2 or MDR1 with fumitremorgin C or verapamil. Carina et al. (34) indicated that SOX2 switch-off through ABCG2 downregulation sensitized the SW1736 cell line to cisplatin and doxorubicin.

All research was conducted on tissues and cell lines, but no in vivo studies were performed. The existence of ABC transporters was evaluated at the level of transcription or protein by the appropriate techniques. The characteristics and a summary of the findings of the included studies are shown in Table 1.

Discussion

According to the definition introduced by the US National Cancer Institute, precision medicine is “a form of medicine that uses information about a person’s genes, proteins, and environment to prevent, diagnose, and treat disease” (37). Knowing about the existence and type of ABC transporters in ATC will help us apply appropriate therapeutic strategies for each patient, for example, by choosing chemotherapeutic agents that do not substrate for ABC transporters or by administering the agents inhibiting ABC transporters or modulating their expression (11, 31, 34, 38).

The results of our systematic review revealed that among the ABC transporters evaluated in various studies, ABCB1 (MDR1, P-gp), ABCC1 (MRP1), and ABCG2 (BCRP) have the greatest role in ATC chemotherapy resistance. However, most of the studies only reported the expression level of ABC transporters, and functional studies are needed to confirm the results.

In general, ABCB1 is the most common form of ABC transporter involved in cancer drug resistance, and the other types of ABC transporters are less prevalent (19). However, ABCC1 and ABCG2 seem to be the most highly expressed transporters in ATC (Table 1). Other transporters such as ABCA8, ABCB10, ABCC5, ABCC10, ABCG1, and MRP2 may also play a role in ATC chemotherapy resistance, but the results are based on one report, and additional studies are needed to define their roles (30, 33) (Table 1). Moreover, other ABC transporters have not been studied yet, and other mechanisms may also be involved in ATC drug resistance. For example, lung resistance–related protein, which is not an ABC transporter, has been found to be associated with ATC drug resistance in some reports (27, 35).

ABC transporters and common chemotherapy drugs used in ATC

As efflux pumps, ABC transporters are one of the main mechanisms for cancer drug resistance. They efflux anticancer drugs and as a result decrease the effective level of these drugs in tumor cells (16). Doxorubicin, a common drug used in ATC treatment, is a substrate for all the main expressed ABC transporters in ATC (Table 2). The other ATC chemotherapy drugs (e.g., docetaxel, paclitaxel, and cisplatin) are also substrates for either ABCB1 or ABCG2 (Table 2). Additionally, as shown in Table 1, the coexpression of different ABC transporters is a common finding in ATC, boosting the resistance to single agent or resulting in multidrug resistance to different compounds. The high failure rate of chemotherapy in ATC could be partly explained by these findings.

ABC transporters and CSCs of ATC

CSCs or tumor-initiating cells are a small population of tumor cells with the ability of self-renewal associated with cancer relapse, metastasis, and drug resistance of many tumors, including those of ATC (31, 43, 44). A limited number of studies have demonstrated that ABCG2, MDR1, and MRP1 are highly expressed by CSCs of ATC (31, 34, 35) (Table 1). Zheng et al. (31) demonstrated that the sensitivity of CSC-rich cell line HTh74R to doxorubicin is restored through inhibition of ABCG2 or MDR1. They proposed that one of the main mechanisms of doxorubicin resistance in ATC is CSCs that highly express ABC transporters (31). In addition, Carina et al. (34) indicated that by silencing SOX2 (a CSC marker), ABCG2 is downregulated, and the resistance of SW1736 cells to doxorubicin and cisplatin decreases. These findings highlight the role of ABC transporters in CSCs resistance to chemotherapy, as a result of which poor ATC outcomes have been obtained so far.

ABC transporters and signaling pathways

Several studies conducted on various types of cancer tissues and cell lines have indicated that signal transduction pathways, including phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) (45), MAPK (46–48), Hedgehog (49), Wnt/β-catenin (50), Notch (51–53), and Hippo (54), play a role in the development of drug resistance through the regulation of gene expression in ABC transporters such as ABCC1, ABCG2, and ABCB1 (Fig. 2). Interestingly, all of these signaling pathways are also involved in the ATC pathogenesis (33, 55, 56); thus, these signaling pathways are also expected to play an important role in ATC drug resistance via the same mechanism. Lopez et al. (11) evaluated the role of epidermal growth factor receptor (EGFR) in the modulation of ABCG2 transporter expression in ATC cell line ARO. They showed that the inactivation of EGFR kinase activity with gefitinib (a tyrosine kinase inhibitor) causes the downregulation of ABCG2 from the plasma membrane and increases the sensitivity to doxorubicin (11). EGFR effects might be partly implicated through interaction with the PI3K/Akt and/or MAPK signaling pathway (45, 57). However, the roles of other aforementioned signaling pathways in the expression of ABC transporter genes have not been evaluated in ATC, and additional studies are needed to determine their exact role.

In conclusion, ATC unresponsiveness to chemotherapy is an important factor associated with the short survival of patients. The results of our study demonstrated that overexpression of ABC transporters, especially ABCC1 (MRP1), ABCG2 (BCRP), and to a lesser extent ABCB1 (MDR1, P-gp), plays an essential role in ATC drug resistance. However, functional studies are needed to confirm these findings. In addition, considering the wide range of ABC transporters, other members of this family may also have a role in ATC unresponsiveness to chemotherapy and should be evaluated in patients with ATC. Knowing about the types of ABC transporters expressed in ATC cells will help us choose the best chemotherapy drugs. Moreover, by direct inhibition of ABC transporters or modification of signaling pathways contributing to the expression of each ABC transporter, drug resistance may be decreased. Finally, drug resistance is not dictated by a single mechanism, and other factors must be considered as underlying mechanisms for the severe chemotherapy resistance phenotype of ATC. Using high-throughput methods for genomic and proteomic profiling and system biology approaches enables us to further clarify the underlying mechanisms of drug resistance in ATC. A better understanding of the molecular basis of drug resistance in ATC will lead to the development of targeted combination therapies capable of producing long-term responses in ATC.

Acknowledgments

The authors thank Mr. Amir Ramezani and Miss Rasha Atlasi for their technical advice and support on the literature search. The authors are grateful to Dr. Mahmood Naderi for his comments on the initial draft of the manuscript. This study was supported by the Endocrinology and Metabolism Research Institute, Tehran University of Medical Sciences.

Financial Support: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Additional Information

Disclosure Summary: The authors have nothing to disclose.

Data Availability: All data generated or analyzed during this study are included in this published article or in the data repositories listed in References.

Abbreviations:

Abbreviations:
 
• ABC

  ATP-binding cassette

 
• ATC

  anaplastic thyroid cancer

 
• BCRP

  breast cancer resistance protein

 
• CSC

  cancer stem cell

 
• EGFR

  epidermal growth factor receptor

 
• MDR

  multidrug resistance

 
• MRP

  multidrug resistance-associated protein

 
• P-gp

  P-glycoprotein

 
• PI3K/Akt

  phosphatidylinositol 3-kinase/protein kinase B

References and Notes