The overall direction of the Molecular Mechanisms of Tumor Promotion Section is to elucidate the pathways for signal transduction of the lipophilic second messenger sn-1,2-diacylglycerol and its ultrapotent mimetics, the phorbol ester tumor promoters. Protein kinase C (PKC) is the major receptor for these compounds, and our emphasis is correspondingly directed at this family of isozymes. Analysis is directed at four interlocking issues. First, understanding the nature of the ligand - receptor interactions. Major advances include the analysis of the complex by X-ray crystallography and molecular modeling, with predictions tested by site-directed mutagenesis and evaluation of synthetic analogs. Second, characterization of novel receptors with structural homology to the regulatory domain of PKC; Unc-13, like the chimaerins, proved to be virtually indistinguishable by numerous approaches from PKC. Third, mechanisms of control of PKC. Phosphorylation of PKC on tyrosine was characterized and sites of phosphorylation were identified; consequences of this phosphorylation for the activity and specificity of the isozymes began to be defined. Contrary to accepted dogma, ligand binding to PKC was shown not to require lipid, although its affinity was enhanced by it; likewise, phorbol esters were found not to induce irreversible translocation of PKC into the cell membrane. Fourth, understanding the very different mechanisms for the phorbol-related derivative resiniferatoxin (RTX), which functions as an ultrapotent capsaicin analog. Specific receptors were demonstrated for the first time in the central nervous system. Intact cells displayed similar binding characteristics to membrane preparations, validating the conclusions of the earlier studies on the membranes. Different structure-activity relations were established for the high affinity receptors and for a specific response, uptake of calcium, in intact cells, cogently suggesting the existence of receptor subclasses. Our findings provide exciting leads at several levels for novel strategies of drug development.
