12-O-tetradecanoyl-phorbol-13-acetate (TPA) counteracts the cAMP up-regulation of the expression of the stimulatory guanine nucleotide binding protein (Gs alpha) and Gs alpha messenger RNA in cultured pig thyroid cells.

In this study, we examined whether the protein kinase C (PKC) pathway could interfere with the regulation of Gs protein in porcine thyroid cells. The two days culture of cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) (0.1 microM) alone neither affected adenylyl cyclase activity, nor the level of Gs alpha protein in membranes when compared with control cells. The co-addition of TPA with thyrotropin (TSH) (1 mU/ml) or forskolin (fk) (10 microM) in the culture medium, abolished the stimulatory effects of either agonists on the activation of adenylyl cyclase by fk or [AlF4]- and on the increase of Gs alpha protein. By contrast, TPA had effects neither on the Gi-dependent inhibition of adenylyl cyclase nor on Gi alpha proteins levels. The level of Gs alpha mRNA measured by Northern blot analysis was increased (200%) in TSH- or fk-treated cells and this increase was counteracted by TPA. The effects of TPA observed after 6-9 h of contact with cells were mimicked by mezerezin, a non-phorbol protein kinase C activator and blocked by bisindolylmaleimide a specific protein kinase C inhibitor (GF 109203X). These results suggest that the activation of the PKC pathway prevents the cAMP-dependent up-regulation of Gs alpha protein and Gs alpha mRNA expression.