SUPPRESSION OF THYROTROPIN RECEPTOR-G PROTEIN-PHOSPHOLIPASE-C COUPLING BY ACTIVATION OF PROTEIN-KINASE-C IN THYROID-CARCINOMA CELLS

In human thyroid follicular cells TSH exerts its action on growth and function at least via two distinct pathways, the adenylate cyclase cas cade and the phospholipase C beta (PLC beta)-mediated inositol phospha te generation. We investigated the effect of TSH on activation of phos phoinositide hydrolysis and inositol phosphate generation by PLC beta in HTh74 thyroid carcinoma cells that express functional TSH receptors and in HTC-TSHr thyroid carcinoma cells that are devoid of endogenous TSH receptors but express recombinant human TSH receptors. In both ce ll lines, TSH up to concentrations of 300 mU/ml failed to stimulate my o-inositol 1,4,5-trisphosphate and myo-inositol-tetrakisphosphate gene ration, but led to a decrease in these compounds within 1 min of stimu lation. However, ATP and bradykinin increased concentrations of inosit ol phosphates in both thyroid carcinoma cell lines. In contrast, in di fferentiated FRTL5 thyroid cell line and CHO-TSHr cell line expressing recombinant human TSH receptors, TSH elicited a significant increase in myo-inositol 1,4,5-trisphosphate and its metabolic derivatives. How ever, when HTC-TSHr cells were pretreated with calphostin C or stauros porine, inhibitors of protein kinase C, a TSH concentration of 20 mU/m l enhanced generation of inositol phosphates in these cells. From our data we conclude that in HTC-TSHr and HTh74 thyroid carcinoma cells, t he coupling within the TSH receptor-G(q) protein-PLC beta signaling pa thway is impaired compared to that in nontransformed cells. It is conc eivable that this is at least in part dependent on the level of protei n kinase C activation in these cells.