Cyclic AMP-mediated growth suppression and MAPK phosphorylation in thyroid papillary carcinoma cells

The main regulating systems of thyroid growth are the mitogen-activated protein kinase (MAPK) signaling pathway and the cAMP signaling pathway. Thyroid papillary carcinoma frequently involves mutations in BRAF or RET/PTC without overlap, which are expected to constitutively activate MAPK signaling. On the other hand, it has been reported that cAMP signaling acts in an inhibitory manner on the proliferation of papillary carcinoma cell lines, although the cAMP pathway physiologically promotes the proliferation of normal follicular cells as well as hormonogenesis. The effect of cAMP on proliferation is attributed to crosstalk with MAPK signaling. However, this phenomenon has not been clearly established in papillary carcinoma with BRAF or RET/PTC mutations. In order to elucidate whether activated cAMP signaling inhibits cell proliferation and affects MAPK signaling in papillary carcinoma, we performed in vitro experiments using two representative cell lines, K1 and TPC-1, which have a BRAF and an RET/PTC mutation, respectively. Elevated cAMP caused by an adenylate cyclase activator suppressed the proliferation of both K1 and TPC-1 cells. Examining the crosstalk between cAMP and MAPK signaling, K1 and TPC-1 cells showed opposite responses to cAMP activation. These responses were blocked by an inhibitor of the cAMP-dependent protein kinase (PKA). In K1 cells, B-Raf might predominate over Raf-1, and the elevated cAMP is thought to promote MAPK phosphorylation through the PKA-mediated activation of Rap1. On the other hand, in TPC-1 cells Raf-1 might predominate and could be inhibited by activated Rap1, resulting in the suppression of MAPK phosphorylation. In conclusion, the proliferation of both papillary carcinoma cell types was significantly suppressed by cAMP signaling, regardless of whether MAPK signaling was activated or inactivated by the PKA-mediated cAMP signaling pathway. There could, however, be other mechanisms by which cAMP signaling inhibits the growth of papillary carcinoma cells.