Thyroid cancer is the most frequently seen endocrine malignancy, accounting for 3% of all newly diagnosed cancers worldwide (1). The rising incidence has been observed, particularly in papillary thyroid cancer, over the last decades (2). Papillary and follicular thyroid cancers are considered differentiated thyroid cancers (DTC) and account for 95% of all thyroid malignancies. These tumors have quite a good prognosis, and cure is commonly possible even in advanced stages. However, anaplastic thyroid cancer (ATC) is the least common (1–2%) but most aggressive type of thyroid cancer and is called undifferentiated cancer (3). Potentially, ATC arises from the full dedifferentiation of DTCs after gaining new mutations. Because over 50% of ATC patients have co-existent DTC at histopathological analysis (4). The mean survival rates of ATC cases have been given as 4–5 months after the diagnosis, and survival beyond 2 years was given at 12% after multimodal treatment including surgery, radiotherapy, and cytotoxic chemotherapy including adriamycin, docetaxel, or cis-platinum (5, 6). Despite advances in several anti-cancer treatment modalities, ATC remained incurable, and mortality is close to 100 percent (7). Patients mostly die due to airway obstruction due to trakeal compression, vocal cord paralysis, extensive pulmonary invasion, and brain metastasis (3, 8). According to the final edition of the American Joint Committee on Cancer (AJCC/TNM eighth edition), all anaplastic thyroid cancer cases were classified as stage 4 disease, even the tumor was limited to the thyroid gland due to its aggressive behavior (8, 9). Survival advantages, i.e., 8–10 months, can be obtained only with multimodal treatment, including surgery, high-dose accelerated radiotherapy, and combination chemotherapy (8, 10, 11).
Besides extremely high proliferation rates, epithelial-to-mesenchymal transition, and cancer stem characteristics, ATC cells display chemo-resistance to all cytostatics (12, 13). Taxanes and doxorubicine have been used with limited efficacy to improve survival (14, 15). Particularly for this type of cancer, much more effective treatment strategies are urgently needed. Recently, research focus and therapeutic target shifted to some thyrosine kinase receptor inhibitors, increasing chemotherapy efficacy via less toxic compounds, immunotherapies, and drugs that overcome multidrug resistance mechanisms (8, 16).
Accumulating data supported that two dysregulated signaling cascades involve the pathogenesis of differentiated thyroid cancers: mitogen-activated protein kinase (MAPK) activation plays a main role in PTC development through some mutations of the RAF and RAS genes or gene fusions of RET/PTC and TRK (17). On the other hand, phosphatidylinositol-3 kinase (PI3K)/AKT activation through RAS, AKT1, and PTEN mutations plays key roles in the development of follicular thyroid cancer (FTC) (17). Conversion to the undifferentiated stage involves several additional mutations affecting other signaling pathways, such as p53 and Wnt/B catenine (17, 18). Towards ATC progression, the main identified expresional alterations were related to epithelial-to-mesenchymal transition, negative regulation of p53, loss of thyroid differentiation, and activation of ERK and PI3K/mTOR signaling (18). Compared with PDCs, ATC cases are associated with remarkably higher mutation rates of the TERT promoter (10 vs. 73%), TP53 (1 vs. 59%), and the PIK3CA-PTEN-AKT-mTOR pathway (1,4 vs. 39%) (19). The frequency of oncogenic driver mutations involving BRAF, specifically BRAFV600E, the most common mutation seen in papillary thyroid cancer (PTC), can be found in up to 40% of ATC cases (20). Targeting receptor proein kinases (RTKs), which are involved in the activation of MAPK and PI3K/AKT pathways, has yielded very promising results in particularly differentiated thyroid cancer (21). However, one drug application received clinical approval. The combination of ATP-competitive inhibitor dabrafenib and MEK inhibitor trametinib was approved by the FDA in 2018 for BRAFV600E mutation-carrier ATC patients after a published phase II study (16, 22). Out of 16 patients, the overall response rate (ORR) and the one-year OS rate were 69% and 80%, respectively. The MEK inhibitor trametinib has been adding BRAF inhibitors because of paradoxal MAPK activation, and dabrafenib/trametinib treatment has started to use other aggressive BRAF-mutated cancers, including biliary tract cancers and gliomas (23). Therapies targeting other key players of the MAPK pathway, such as RAF and MEK, have shown encouraging results, but inhibitors targeting the PI3K/Akt/mTOR pathway have shown disappointing clinical results (24, 25). Unfortunately, multi-drug resistance is inevitable for many RTK inhibitors. ATC cells become resistant against these targeting agents via paradoxically re-activation of their own signaling pathways due to genomic instability, autocrine secretion of some growth factors, and non-clarified mechanisms (26). Also, some targeting agents may have potential for trans-membrane drug efflux proteins (ABC transporters). For example, a recently published study demonstrated that VCAM-1 is upregulated in thyroid cancer cells treated with BRAF-inhibitor vemurafenib contributes to drug resistance via increasing ABCG2 in BRAF-mutant thyroid cancer cells (27). Moreover, significant toxicities have been observed during BRAF and MAPK/RAS-targeted therapies in thyroid cancer patients (28).
Modulating the expression of multidrug resistance (MDR) proteins by using less toxic compounds and increasing the efficacy of cytostatics or targeting compounds has become a critical strategy for the curative treatment of ATC. One of the main mechanisms of MDR is thought to be the overexpression of ATP-binding cassette (ABC) transporters, which increases drug efflux and drastically lowers the intracellular concentration of antineoplastic drugs. Over the last decades, three groups of ABC transporters (ABCB1(P-gp), ABCG2(BCRP), and ABCC1(MRP1)) and diverse compounds with ABC inhibitory activity have been identified to sensitize drug-resistant cells. For example, the multi-kinase inhibitor Motesanib (AMG706), which has an anti-tumor effect on thyroid cancer, inhibits the ABCB1 transporter and increases paclitaxel concentration in the KB-C2 cell line, which overexpresses ABCB1 (29). In a systematic review evaluating overexpression of ABC transporters, ABCC1, ABCG2, and, to a lesser extent, ABCB1 play essential roles in ATC resistance against docetaxel, adriamycine (doxorobicine), and cis-platinum (30). Several naturally occurring compounds, including resveratrol, quercetin, epigallocatechin-3-gallate, genistein, and curcuminoids, have been shown to induce apoptosis, re-differentiation, and cell growth inhibition in all types of thyroid cancers (31, 32). These compounds are also capable of altering cellular signaling pathways and the expression of ABC transporters in drug-resistant cancer cells (33). Curcumin is a natural compound extracted from Curcuma longa rhisomes, and its multiple anti-cancer properties have been widely demonstrated for many types of cancer. Its derivatives have also been shown to have drug-sensitizing functions in some cancers. As a most studied phytochemical, curcumin and its derivatives have also been shown to have drug-sensitizing and MDR-suppressing functions in various drug-resistant tumors, including breast, prostate, pancreas, lung, and head and neck cancers (34). Recently, some complated clinical phase I/II trials investigating the efficacy of curcumin alone and combined with chemotherapeutic agents revealed encouraging results in the aspects of side effects of chemo/radiotherapy, QoL, and tolerability (35). However, no prospective clinical trial addressing the chemosensitizing impact of curcuminoids on ATC cases has been published yet.
In the present study, the efficacy of a curcumin derivative, tetrahydrocurcumin (THC), on adriamycin and docetaxel resistance in two ATC cell lines was investigated. Additionally, THC's anti-cancer effectiveness was compared with these two chemotherapeutic agents in vitro.