Non-promoting 12-deoxyphorbol 13-esters as potent inhibitors of phorbol 12-myristate 13-acetate-induced acute and chronic biological responses in CD-1 mouse skin.

In previous experiments, pretreatment of CD-1 mouse skin with prostratin (12-deoxyphorbol 13-acetate) inhibited hyperplasia, induction of ornithine decarboxylase and edema in response to acute treatment with phorbol 12-myristate 13-acetate (PMA). We report here that prostratin inhibits biological responses induced by multiple (chronic) PMA treatment. A typical chronic treatment schedule consisted of five applications of 3.2 nmol (2 micrograms) PMA at 48 h intervals. Most effective inhibition could be achieved when the first PMA treatment was preceded 48 h before by a lower dose of prostratin (256 nmol = 100 micrograms) and each PMA treatment was preceded 15 min before by a higher dose (2.56 mumol = 1 mg) of prostratin. Under this schedule hyperplasia was completely blocked, as was keratin K6 expression (a marker of hyperproliferative epidermis), whereas myeloperoxidase activity (a marker of neutrophil granulocyte infiltration) was reduced to 36%. 12-Deoxyphorbol 13-phenylacetate (dPP), a non-promoting 12-deoxyphorbol derivative that binds to protein kinase C with two orders of magnitude higher potency than does prostratin, showed the same pattern of inhibition as did prostratin for a single PMA treatment but with a corresponding two orders of magnitude higher potency. In the case of chronic PMA treatment, however, dPP failed to inhibit hyperplasia fully, though it reduced keratin K6 expression and inflammation. Dissociation of K6 expression from hyperplasia was unexpected, since expression of these two responses was thought to be closely coupled. We conclude that 12-deoxyphorbol 13-monoesters are functional antagonists for a class of protein kinase C-mediated responses closely correlated to tumor promotion.