Phorbol esters parallel effects on tumor promotion, insulin release and calcium ionophoresis

The tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol 12,13-didecanoate (PDD) both stimulated insulin release from rat pancreatic islets and facilitated ionophore-mediated calcium transport in liposomes, the latter effect not resulting from a change in viscosity of the liposomial matrix. Phorbol and 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), which fail to cause tumor promotion, also failed to stimulate insulin release and ionophore-mediated calcium transport. An alteration of calcium transport may represent a fundamental event in both the early (insulin release) and late (tumor promotion) biological responses to phorbol esters.