Mechanisms of cell signaling
Many hormones, neurotransmitters and growth factors produce their physiological actions by stimulating the breakdown of certain phospholipids in their target cells. This leads to an increase in the concentration of Ca2+ ions and changes in lipid messenger molecules that are responsible for the physiological effects observed. We are studying the phospholipase isozymes that are specifically involved in agonist-stimulated phospholipid breakdown and also the G proteins, protein kinases and other factors involved in their regulation. The structural elements required for catalysis and the domains and mechanisms of interaction of the phopholipases and their regulatory proteins are being defined. We are also exploring the molecular mechanisms by which hormones and growth factors stimulate phospholipid breakdown in intact cells, in particular, those mechanisms involving large and small G proteins and protein tyrosine and serine-threonine kinases. We are also examining the roles played by some of the products of phospholipid breakdown (phosphatidic acid and diacylglycerol) in generating physiological responses. The approaches involve the generation of mutant forms of the enzymes and regulatory proteins that are constitutively active or function as dominant negative forms, and the use of cells and animals in which the proteins are selectively depleted. This information is aimed at helping us understand how some of the important physiological reponses to hormones, neurotransmitters and growth factors are generated.