Actions of a tumor-promoting phorbol ester in a cloned rat pituitary cell : Potential involvement of changes in...

Tumor promoters, such as phorbol myristate acetate (PMA), facilitate carcinogenesis by mechanisms that may involve changes in intracellular Ca(2+) metabolism and distribution of Ca(2+), as well as activation of a Ca(2+)-and phospholipid-dependent protein kinase, referred to as protein kinase C. We compared the actions of PMA on GH3 cloned pituitary cells with those of thyrotropin releasing hormone (TRH), an established Ca(2+)-mobilizing agent. The TRH treatment produced a(45)Ca efflux, inhibited(45)Ca uptake, diminished chlortetracycline fluorescence, and stimulated cAMP accumulation and protein synthesis in a Ca(2+)-dependent manner. Like TRH, PMA produced an efflux of(45)Ca and inhibited(45)Ca uptake; however, the phorbol ester stimulated cAMP accumulation and protein synthesis in the absence of external calcium and failed to alter chlortetracycline fluorescence. The TMB-8, a putative inhibitor of the mobilization of membrane-associated Ca(2+), did not alter PMA-induced stimulation of protein synthesis.The results suggest that PMA-induced changes in Ca(2+) metabolism are not caused by the mobilization of membrane-associated calcium. Alternative proposals are that PMA (1) inhibits Ca(2+) influx and/or (2) mobilizes calcium from nonmembranous storage sites. Further study is needed to characterize the mechanism through which tumor-promoting phorbol esters influence Ca(2+) metabolism and to ascertain the significance of changes in Ca(2+) metabolism to cellular processes affected by these substances.