Relationship between protein kinase C and adenylyl cyclase activity in the regulation of growth hormone secretion by human pituitary somatotrophinomas.

OBJECTIVE

To determine the potential role of protein kinase C (PKC) and its relationship to adenylyl cyclase activity in controlling growth hormone (GH) secretion by human pituitary somatotrophinomas.

METHODS

Twenty-eight somatotrophinomas were placed into cell culture, and the in vitro effects of the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) and the PKC inhibitor staurosporine on basal and GH-releasing hormone (GHRH)-stimulated GH secretion were examined. In addition, the influence of chronic exposure of cultured somatotrophinoma cells to TPA on the rate of inositol 1,4,5-trisphosphate production was determined. Each tumor was assessed for the presence of gsp oncogenes, and thus constitutive adenylyl cyclase activity, by direct sequence analysis of polymerase chain reaction-generated deoxyribonucleic acid. GH secretory responses of tumors with and without these oncogenes were compared.

RESULTS

TPA consistently stimulated GH secretion by cultured somatotrophinoma cells. There was no difference in response between somatotrophinomas with and without gsp oncogenes, and the effects did not correlate with the variable stimulation exerted by GHRH. Tumors in which GHRH had no significant effect nevertheless responded to TPA. In combination, TPA and GHRH exerted additive stimulation. TPA treatment of cultured somatotrophinoma cells eventually resulted in suppression of inositol 1,4,5-trisphosphate production, probably reflecting down-regulation of membrane phosphatidylinositol hydrolysis, a second messenger system that also generates the endogenous PKC activator diacylglycerol. GHRH had no effect on phosphatidylinositol hydrolysis. In contrast to the effects of TPA, the PKC inhibitor staurosporine tended to reduce GH secretion, although this effect was not observed in all tumors examined. As with TPA, the effects of staurosporine did not correlate with presence or absence of gsp oncogenes. Furthermore, staurosporine did not reduce the stimulatory effects exerted by GHRH on GH secretion.

CONCLUSION

These results demonstrate a role for the phosphatidylinositol-PKC second messenger cascade in controlling GH secretion by human pituitary somatotrophinomas. The results also show that the system operates relatively independent of intracellular adenylyl cyclase and, thus, protein kinase A.