Forskolin mimics TSH action on the expression of protein kinase C isozymes in pig thyroid cell cultures

In porcine thyroid cell cultures, phospholipid-dependent protein kinases (PKCs) have the same characteristics as intact glands. The overall PKC activity, presence of PKC isozymes, chromatographic pattern and endogenous substrates specificity were not modified during the two-day culture period. Three PKC isozymes (cPKCε{lunate}, nPKCε{lunate} and aPKCξ) were identified by immublot analysis in the two cellular fractions: cytosol and particulate extract, both in intact glands and two-day-old culture. In cells cultured for two days in the presence of TSH (0.1 mU/ml), the overall PKC activity was stimulated (ca. 200%) in the two compartments. This stimulation was parallel to the increase in protein expression of the three PKC isoforms (as demonstrated by immunoblot analysis) and was accompanied by a redistribution of cPKCα and nPKCε{lunate} toward the particulate fraction. In TSH-treated cells, hydroxyapaptide chromatography of cytosolic PKC revealed an additional peak of PKC activity eluted at 195 mM potassium phosphate. Its elution molarity did not correspond to the molarity of any known PKC isozyme, and it did not cross-react with antibodies directed against cPKC isozymes-: α, β, or γ. When TSH was replaced by forskolin (10^-5 M), identical quantitative and qualitative modifications were obtained, suggesting that, in thyroid cells, the cyclic AMP-dependent regulatory cascade could be involved in the control of PKC isoforms expression by TSH.