Protein kinase C activators modulate differentiated thyroid function in vitro

Exposure of porcine thyroid cells to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) leads to inhibition of differentiated thyroid function. We investigated whether this effect is mediated via protein kinase C activation. TPA, phorbol 12,13-didecanoate and phorbol 12,13-dibutyrate inhibited TSH-stimulated iodine organification in porcine thyroid cells by 98, 96 and 45%, respectively. Non-tumour promoting phorbol esters had no effect. The diacylglycerol analogue, sn-1,2-dioctanoylglycerol had similar but quantitatively less activity than TPA. Dibutyryl cAMP could not reverse any inhibition noted. Under conditions that caused significant inhibition of differentiated function, TPA caused translocation of thyroidal protein kinase C from the cytosol to its membrane-bound form. These data provide evidence that the mechanism of phorbol action on thyroid function in vitro includes activation of protein kinase C.