Endocrine Abstracts

Thyroid cancer (TC) has one of the fastest increasing incidences worldwide and primarily involves papillary thyroid cancer (PTC). Cancer cells have numerous mechanisms that govern drug resistance, among which activation of multidrug resistance (MDR) genes encoding ABC efflux pumps plays an important role. Acquisition of drug resistance by TC cells is becoming a significant obstacle to effective therapy, leading to insensitivity to classical chemotherapeutics, but also to specific kinase inhibitors. Nevertheless, this process is poorly understood. We aimed to elucidate the role of P-gp (ABCB1) and BCRP (ABCG2), key representatives of ABC transporters, in the biology of PTC. We focused on the crosstalk between the expression of ABCB1 and ABCG2 genes, and activity of MAPK and PI3K/AKT signaling pathways. We used PTC-derived TPC1 (RET/PTC1) and BCPAP (BRAFV600E) cell lines, siRNA-mediated MDR-encoding gene silencing, clinical inhibitors of MAPK (trametinib and dabrafenib) and PI3K/AKT pathways (AKT inhibitor VIII), as well as RT-qPCR, Western blot, protein fractionation, confocal microscopy and RNA sequencing techniques, among others. We observed a strong relation between the expression of MDR genes and the mutational status of PTC cells. Silencing of ABCB1 and ABCG2 resulted in a change in the activity of MAPK and PI3K/AKT pathways. Similarly, inhibitor-driven suppression of signaling cascades changed levels and intracellular localization of P-gp and BCRP proteins. Interestingly, the decreased expression of P-gp, resulting from kinase inhibitor or siRNA treatments, is compensated by increased expression of BCRP, and vice versa. Large-scale transcriptome analysis confirmed the relationship between ABCB1 and ABCG2 expression, and the activity of the MAPK and PI3K/AKT pathways in PTC. Silencing of the MDR genes contributed to alterations in various common cellular processes, such as regulation of gene expression and signaling (including through the MAPK and PI3K/AKT pathways), cell cycle and proliferation. Trametinib and dabrafenib-induced inhibition of the MAPK pathway led to changes in numerous related biological processes, including regulation of proliferation, differentiation, cell cycle, migration and apoptosis. Similarly, PI3K/AKT pathway suppression led to disruption of key biological processes, including apoptosis, angiogenesis and migration. Changes in the various biological processes found in the transcriptome analysis (caused by silencing of the ABCB1 and ABCG2 genes) revealed that the role of drug efflux pumps in PTC is complex and goes beyond the typical function of xenobiotic transporters. Also, regulation of the expression of these genes depends on many factors, which is consistent with the results of studies on other types of cancer. These studies will assist in greater understanding of the drug resistance phenomenon in TC. This research was supported by the National Science Centre (No. 2018/29/B/NZ3/02642).