Conditional BRAF Expression Induces DNA Synthesis, Apoptosis, Dedifferentiation, and Chromosomal Instability in Thyroid PCCL3 Cells

The activating mutation BRAF is the most prevalent genetic alteration in papillary thyroid carcinomas (PTC). It is associated with advanced PTCs, suggesting that this oncoprotein confers thyroid cancers with more aggressive properties. BRAF is also observed in thyroid micropapillary carcinomas and may thus be an early event in tumor development. To explore its biological consequences, we established doxycycline-inducible BRAF-expressing clonal lines derived from well-differentiated rat thyroid PCCL3 cells. Expression of BRAF did not induce growth in the absence of thyrotropin despite increasing DNA synthesis, which is likely explained because of a concomitant increase in apoptosis. Thyrotropin-dependent cell growth and DNA synthesis were reduced by BRAF because of decreased thyrotropin responsiveness associated with inhibition of thyrotropin receptor gene expression. These results are similar to those obtained following conditional expression of RET/PTC. However, in contrast to RET/PTC, BRAF activation did not impair key activation steps distal to the thyrotropin receptor, such as forskolin-induced adenylyl cyclase activity or cyclic AMP– induced DNA synthesis. We reported previously that acute RET/PTC expression in PCCL3 cells did not induce genomic instability. By contrast, induction of BRAF expression increased the frequency of micronuclei by both clastogenic and aneugenic events. These data indicate that BRAF expression confers thyroid cells with little growth advantage because of concomitant activation of DNA synthesis and apoptosis. However, in contrast to RET/PTC, BRAF may facilitate the acquisition of secondary genetic events through induction of genomic instability, which may account for its aggressive properties. (Cancer Res 2005; 65(6): 2465-73)