Protein kinase C in pig thyroid cells: activation, translocation and endogenous substrate phosphorylating activity in response to phorbol esters

Abstract

The phorbol ester, TPA, induced the intracellular redistribution of protein kinase C in intact thyroid cells; it caused within 5 min of incubation a 90% decrease of the cytosolic protein kinase C and an increase of the membrane-associated enzyme activity which appeared to be fully activated by TPA. TSH at concentrations which gave the maximal stimulation on various parameters of iodine metabolism induced the translocation of only 10–15% of protein kinase C from the cytosol to the membrane fraction. TPA induced a 2-fold increase in the incorporation of [ 32P]phosphate into cellular proteins and selectively activated the phosphorylation of two molecular species: a 180000 Da protein and to a lesser extent a 170000 Da protein in dispersed pig thyroid cells prelabeled with [ 32P]orthophosphate. The effect of TPA was maximum after 5 min of incubation and was concentration-dependent between 1 nM and 1 μM. The two phosphorylated substrates were only found in the cytosolic fraction. The TPA-induced phosphorylation of the 180000 Da protein was observed in thyroid cells in suspension, in thyroid cell monolayers and follicle-like reassociated cells. In these three experimental situations, the 180000 Da protein was not phosphorylated in response to TSH. Incubation of thyroid cell cytosolic fraction in the presence of [ 32P]ATP with calcium and phospholipid led to the phosphorylation of few proteins among which a 180000 Da component. These proteins were not phosphorylated in the cytosol of TPA-treated cells, a finding in agreement with the translocation of protein kinase C. These results indicate that (1) the activation-translocation of thyroid protein kinase C induced by TPA is associated with the phosphorylation of selective substrates, and (2) TSH, even at high concentration, failed to exert the same action as TPA on protein kinase C in pig thyroid cells.