The overall direction of the Molecular Mechanisms of Tumor Promotion Section is to understand the regulation of the signalling pathways downstream from the lipophilic second messenger diacylglycerol, to elucidate the basis for heterogeneity of response to different ligands which function through this pathway, and to exploit this understanding for developing novel ligands with unique behaviour that function through this pathway. A complementary direction is to understand the regulation and structure activity relations for the vanilloid receptor. The vanilloid receptor is a downstream target of the diacylglycerol signalling pathway, shares partial homology in its ligands to this pathway, and shares with the diacylglycerol signalling pathway an important role in inflammation. Both directions impact both our understanding of biological regulation and the potential development of therapeutic agents. Protein kinase C, the best studied downstream target for diacylglycerol, represents the classic system for tumor promotion and is a therapeutic target for cancer chemotherapy. The vanilloid receptor represents a promising therapeutic target for cancer pain, among other indications, and thus represents an important direction in palliative care for cancer patients. Exploiting strong collaborations with groups in computational chemistry and synthetic chemistry, we continue to improve our understanding of the structural basis for ligand - protein kinase C interactions. In a major advance by our collaborative program with the Marquez laboratory, we have used combinatorial chemistry to show that the pattern of hydrophobic/hydrophilic substitution on diacylglycerol lactones can have major effects on its selectivity for different biological responses. For a series of different responses, different structures proved to be selective. Our zip code strategy is designed to exploit the different cellular membrane environments in which the targets of the diacylglycerol lactones are present. Bryostatin is in clinical trials as a protein kinase C activator with a unique pattern of induced response. Whereas it activates protein kinase C in vitro, it antagonizes many of the biological responses mediated by protein kinase C. In collaboration with the chemistry group of Gary Keck, we have shown that a close analog of bryostatin 1 fails to show this antagonism on U937 leukemia cells although it retains comparable potency to bryostatin 1 on protein kinase C. These studies highlight which functional groups determine the unique biological behavior of bryostatin. In further studies, we have shown that, for different responses, the same bryostatin analog may give variable proportions on antagonism, ranging from virtually none to partial. We can thus conclude that the antagonistic behavior is not an all-or-none phenomenon. These results imply that bryostatin analogs can be designed with antagonize a desired subset of protein kinase C responses. PEP005 is another protein kinase C activator in clinical trials for actinic keratosis and non-melanotic skin cancer. Our studies reveal that, whereas it differs from the typical phorbol ester in its modulation of the NFkB response pathway, similar or even more dramatic differences are seen for some other non-promoting phorbol esters. Since PEP005 is in clinical trials, our findings suggest that some of these other derivatives might have similar or superior effect. RasGRP3 is an activator of the Ras pathway directly stimulated by diacylglycerol and phorbol esters. We find that it is expressed at somewhat elevated levels in a range of solid tumors. Overexpression of RasGRP3 enhances the tumor phenotype and suppression of the endogenous expression with siRNA represses the phenotype. In the development of therapeutics targeted to TRPV1, a major problem is designing sufficient specificity of action. We find that its ligand recognition depends on its state of regulatory control. This finding implies that ligands can be designed that will be specific for the vanilloid receptor in a specific regulatory environment, such as at a site of inflammation. Current studies are assessing the roles of individual regulatory elements in the pattern of ligand recognition.
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