Role of cellular Ca++ in phosphorylation of 21 K and 19 K polypeptides in cultured thyroid cells: effects of phorbol ester, trifluoperazine, and 8-diethylamino-octyl-3,4,5-trimethoxybenzoate hydrochloride.

Cultured dog thyroid cells contain 21 and 19 kilodalton (K) phosphoproteins which by several criteria have been identified as light chains of myosin (MLC). TSH causes a reduction in the phosphorylation state of the 21 K-19 K proteins, at least in part through activating adenylate cyclase and increasing cAMP levels. We now report that 12-O-tetradecanoyl-phorbol-13-acetate (TPA) also decreases the 21 K-19 K protein phosphorylation state, but in contrast to that due to TSH, the TPA-induced decrease is not associated with elevated cAMP levels. The effect of TPA was not additive to that of TSH. Because Ca++ is a major factor regulating MLC kinase and TPA-stimulated protein kinase C in other systems, the role of Ca++ in the phosphorylation of the 21 and 19 K polypeptides in dog thyroid was examined. In intact cells, both (8-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8) (1 X 10(-4) M) and trifluoperazine (TFP) (4 X 10(-5) M) increase basal 21 K-19 K protein phosphorylation and inhibit the decrease in phosphorylation caused by TSH and TPA without affecting cAMP levels. Ionophore A23187 (5 X 10(-6) M) counteracts TMB-8- and TFP-stimulated phosphorylation as well as TMB-8 and TFP inhibition of TSH- and TPA-reduced 21 K-19 K phosphorylation. Incubation of 32PO4-labeled dog thyroid cells in the absence of extracellular Ca++ or with verapamil does not significantly affect basally phosphorylated 21 K-19 K proteins or the decreased 21 K-19 K phosphorylation state caused by TSH. These results strongly suggest that the phosphorylation state of the 21 and 19 K proteins is affected more significantly by intracellular Ca++ pools than by extracellular Ca++, and implicate a kinase(s) other than Ca++-calmodulin-dependent MLC kinase in the phosphorylation of MLC in the dog thyroid.