Structural basis of protein kinase C activation by tumor promoters.

Protein kinase C (PKC) is an important enzyme that helps govern cell metabolism and growth. The enzyme is physiologically activated when an (S)-diglyceride binds to its own regulatory domain. The saturable binding site of the regulatory domain can also be bound by any of a group of structurally diverse tumor promoters, including debromoaplysiatoxins (DATs), phorbol esters, ingenols, teleocidins, and bryostatins. The question of how the same binding site can be the target of these structurally diverse molecules is of considerable importance and is addressed in this article. The relatively rigid structure of DAT and the fact that it possesses a diglyceride moiety renders it an ideal starting template. Structure-activity studies with PKC reveal that the C29 but not the C30 stereocenter of DAT is critical for activity. Furthermore, 3-deoxy-DAT and DAT are equipotent as PKC activators, hence the C3 hydroxyl group of DAT is not critical for activity. Straightforward structural considerations show that the C30 hydroxyl group of DAT matches the C3 hydroxyl group of diglyceride, the C29 stereocenter of DAT matches the C2 stereocenter of (S)-diglyceride, and the C1 ester moiety of DAT matches the C2 ester moiety of diglyceride. Based on these studies and on published structure-activity observations on other tumor promoters, a structural hypothesis is developed to account for the chemical mechanism of tumor promoter action. Experimentally testable predictions are made concerning the interactions with PKC of several classes of tumor PKC activators.