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Suppression of Thyrotropin Receptor-G Protein-Phospholipase C Coupling by Activation of Protein Kinase C in Thyroid Carcinoma Cells¹

Laboratories of Experimental Endocrinology, Department of Medicine, Ruhr University (M.B., M.D.), 44789 Bochum; and Institute for Physiological Chemistry, University Hospital Eppendorf (G.W.M.), 25421 Hamburg, Germany

Address all correspondence and requests for reprints to: Dr. Michael Derwahl, Laboratories of Experimental Endocrinology, University Clinic of Internal Medicine, Buerkle-de-la-Camp-Platz 1, 44789 Bochum, Germany.

In human thyroid follicular cells TSH exerts its action on growth and function at least via two distinct pathways, the adenylate cyclase cascade and the phospholipase Cß (PLCß)-mediated inositol phosphate generation. We investigated the effect of TSH on activation of phosphoinositide hydrolysis and inositol phosphate generation by PLCß 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 cell lines, TSH up to concentrations of 300 mU/ml failed to stimulate myo-inositol 1,4,5-trisphosphate and myo-inositol-tetrakisphosphate generation, but led to a decrease in these compounds within 1 min of stimulation. However, ATP and bradykinin increased concentrations of inositol phosphates in both thyroid carcinoma cell lines. In contrast, in differentiated 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. However, when HTC-TSHr cells were pretreated with calphostin C or staurosporine, inhibitors of protein kinase C, a TSH concentration of 20 mU/ml enhanced generation of inositol phosphates in these cells. From our data we conclude that in HTC-TSHr and HTh74 thyroid carcinoma cells, the coupling within the TSH receptor-G_q protein-PLCß signaling pathway is impaired compared to that in nontransformed cells. It is conceivable that this is at least in part dependent on the level of protein kinase C activation in these cells.

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