MAPK Inhibitors Enhance HDAC Inhibitor-Induced Redifferentiation in Papillary Thyroid Cancer Cells Harboring BRAF V600E: An In Vitro Study

Clinical efficacy of redifferentiation therapy with histone deacetylase inhibitor (HDACi) for lethal radioiodine-refractory papillary thyroid Cancer (RR-PTC) is urgently needed to be improved. Given that the impairment of histone acetylation is a mechanism in BRaf ^V600E-mitogen-activated protein kinase (MAPK)-induced aberrant silencing of thyroid iodine-metabolizing genes, dual inhibition of HDAC and MAPK may produce a more favorable effect. In this study, we treated BRaf ^V600E-mutant (BCPAP and K1) and BRaf-wild-type (BHP 2-7) cells with HDACi (panobinostat) and MAPK inhibitor (dabrafenib or selumetinib), alone or in combination, and we tested the expression of iodine- and glucose-metabolizing genes, radioiodine uptake and efflux, and toxicity. We found that panobinostat alone increased iodine-metabolizing gene expression, promoted radioiodine uptake and toxicity, and suppressed GLUT1 expression in all the cells. However, MAPKi (dabrafenib or selumetinib) induced these effects only in BRaf ^V600E-mutant cells. Combined treatment with panobinostat and MAPKi (dabrafenib or selumetinib) displayed a more robust BRaf ^V600E-dependent redifferentiation effect than panobinostat alone via further improving the acetylation level of histone at the sodium-iodide symporter (NIS) promoter. In conclusion, MAPK inhibitors enhance HDACi-induced redifferentiation in PTC cells harboring BRaf ^V600E, warranting animal and clinical trials.