A growth stimulatory effect of iodide is suggested by its effects on c-myc messenger ribonucleic acid levels, [3H]thymidine incorporation, and mitotic activity of porcine follicle cells in suspension culture.

In the present study the effect of iodide on thyroid cell growth was investigated in primary suspension cultures of porcine thyroid cells capable of organifying iodide. The addition of a high dose of iodide (10(-4) M) to such cultures caused a marked increase in c-myc mRNA levels, [3H]thymidine incorporation, and mitotic activity. The incorporation of [3H]thymidine started 30-36 h after the addition of iodide. The stimulatory effect was abolished by a simultaneous incubation with methimazole. The concentration dependence of the iodide-induced stimulation of [3H]thymidine incorporation was similar to that of an inhibitory effect on adenylate cyclase activity. W-7, an inhibitor of calmodulin activity, as well as epinephrine, agents that reduce cAMP levels, also stimulated [3H]thymidine incorporation. Moreover, the stimulatory effect of iodide was reduced in the presence of forskolin. The results suggest that an organic form of iodine stimulates thyroid cell growth by reducing cAMP levels and demonstrate the presence of a growth stimulatory pathway in porcine thyroid cells that is independent of exogenous polypeptide growth factors or hormones.