Analysis of the phorbol ester pharmacophore on protein kinase C as a guide to the rational design of new classes of analogs

Abstract

The diterpene diester phorbol 12-myristate 13-acetate and the alkaloid teleocidin B are structurally unrelated natural products that display similar potent irritant and tumor-promoting activities. Computer modeling of these and other structural classes of tumor promoters show a marked similarity in the relative positions of certain heteroatoms and hydrophobic groups. For phorbol this mapping consists of the C-4, C-9, and C-20 hydroxyl groups as well as a hydrophobic region filled by a long-chain acyl functionality attached to either the C-12 or the C-13 positions. Diacylglycerols, thought to be the endogenous activators of the major phorbol ester receptor protein kinase C likewise fit this model in a stereospecific fashion. As an initial test of the utility of the model, members of a new and simplified class of activators were synthesized that possess the predicted essential structural features. These compounds all inhibited specific phorbol ester binding to protein kinase C, albeit with low affinity (10-60 microM); further analysis of one derivative, decylhydroxylindole, confirmed that the inhibition of phorbol ester binding was competitive. This same derivative inhibited epidermal growth factor binding in intact Swiss 3T3 cells and studies with another derivative showed phosphorylation of a 40-kDa protein in platelets. Both of these in vivo responses are characteristic of phorbol esters.