Inhibition of BRAF kinase suppresses cellular proliferation, but not enough for complete growth arrest in BRAF V600E mutated papillary and undifferentiated thyroid carcinomas

The aim of our study was to inhibit BRAF kinase expression and investigate its effect on cellular functions in thyroid carcinomas. 8505C (BRAF ^V600E/V600E) undifferentiated thyroid carcinoma cell line and B-CPAP (BRAF ^V600E/V600E) papillary thyroid carcinoma cell line were used to develop doxycycline-inducible anti-BRAF shRNA stable cell lines. The inhibitions of BRAF expression in these cells were confirmed with qPCR and Western blot. Impacts of BRAF protein inhibition on cellular functions and signalling pathways were observed through Western blot, proliferation and colony formation assays. BRAF kinase expression was inhibited 83 % in undifferentiated thyroid carcinoma and 82 % in papillary thyroid carcinoma (p < 0.05). As a result of BRAF kinase inhibition, reduction in MEK kinase activity was seen (p < 0.05) in both thyroid cancer cell lines (72 and 75 %, respectively). Initially, big drop in proliferation (p < 0.05) was observed (52 and 54 %, respectively), but later an increasing proliferation trend was noticed in BRAF kinase-inhibited cell lines. In addition, reduction in colony formation (p < 0.05) was seen in BRAF kinase-inhibited carcinoma cells (13 and 15 %, respectively). On the other hand, increase in AKT kinase activity (63 and 70 %, respectively; p < 0.05) was discovered in both BRAF kinase-inhibited carcinoma cells. Increased activation of alternative proliferation pathways (as determined by the increase of AKT kinase activity) counteracts the effect of BRAF kinase inhibition in thyroid carcinomas. Thus, alternative proliferation pathways should be inhibited for therapeutic suppression of BRAF-induced proliferation in thyroid carcinomas.