Phorbol esters stimulate growth and inhibit differentiation in cultured thyroid cells

The potent tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) has biological effects on cell growth and differentiation similar to the effects of epidermal growth factor (EGF) on a variety of cells. Since EGF has been shown recently to stimulate thyroid cell proliferation and inhibit iodine metabolism, we examined the effects of phorbol esters on primary ovine thyroid cultures. TPA stimulated cell growth in a manner similar to EGF. The growth effects of EGF and TPA in combination were not additive. In contrast, TPA (1.6 X 10(-7) M) was a more potent inhibitor of iodine uptake and incorporation than EGF (10(-9) M) at their maximally effective concentrations. The inhibitory effects of TPA were also more rapid and less reversible than those of EGF. TPA and EGF in combination inhibited iodine metabolism more than either agent alone at its maximally effective concentration. Both TPA and EGF reduced the accumulation of cAMP in TSH-stimulated cells, but (Bu)2cAMP and stimulators of adenylate cyclase failed to overcome TPA's inhibition of iodine metabolism. TPA interacted with EGF by reducing the affinity of membrane receptors for [125I]iodo-EGF. Although the alteration in EGF-receptor interaction induced by TPA may play a role in mediating TPA's biological effects, the additive effects of TPA and EGF on iodine metabolism suggest that TPA does not act solely through the EGF receptor-effector system. Agents other than TSH, including phorbol esters and EGF, are potent modulators of thyroid growth and differentiated function. Despite several similarities in biological activity, TPA and EGF do not modulate differentiated function in an identical manner. Both factors act at least partially through a non-cAMP-dependent pathway, providing indirect evidence of another second messenger(s) in the control of thyroid function.