Inhibition by phorbol esters and other tumor promoters of the response of the Sertoli cell to FSH: evidence for dual site of action.

Several tumor promoters exert their effects by activating a Ca2+-phospholipid-dependent protein kinase (protein kinase C). To study the role of this protein kinase in the regulation of Sertoli cell function, we have evaluated the effect of phorbol esters, mezerein, and teleocidin on the response of the Sertoli cell to FSH. Cells were treated for different time intervals with the tumor promoters, and cell response was measured by stimulating the cell with FSH. 12-O-Tetradecanoylphorbol 13-acetate (TPA) had no significant effect on basal cAMP production but markedly inhibited the cAMP response to FSH. Significant inhibition of cAMP accumulation was observed after 15 min treatment with 100 nM TPA, and maximal inhibition developed within 1 h. The decrease in cAMP accumulation was dependent on the dose of phorbol ester used, with an estimated ED50 of 10-20 nM TPA. In a manner similar to TPA, mezerein and teleocidin also inhibited the cAMP response of the Sertoli cell, while the phorbol ester 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), inactive as a tumor promoter and unable to stimulate protein kinase C activity, was devoid of effect. The promoters that inhibited cAMP response also inhibited the FSH-stimulated androgen aromatization. The dose of TPA producing half-maximal inhibition of estrogen accumulation was again 10-20 nM TPA, mezerein, and teleocidin inhibited estrogen accumulation whether FSH, forskolin or cholera toxin was used to stimulate the Sertoli cell. In contrast, only FSH-dependent cAMP accumulation was inhibited by the tumor promoters, while forskolin and cholera toxin stimulations were not affected. These data suggest that tumor promoters which activate protein kinase C act at two sites of the Sertoli cell response. They alter receptor-mediated signal transduction across the membrane and affect steroidogenesis at a site distal to cAMP accumulation.