Lipolytic Mechanisms of PPAR Activation

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DESCRIPTION (provided by applicant): Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-activated nuclear receptors central to the transcriptional regulation of adipogenesis, glucose control and lipid metabolism. More recent work establishes PPAR expression and effects in the vasculature, including endothelial cells (EC). Despite these advances, major unresolved issues persist. One central unanswered question in the PPAR field is the nature of endogenous PPAR ligands, which might recapitulate the effects of synthetic PPAR agonists, or how such natural mediators are produced under physiologic conditions. Similarly, despite extensive studies implicating PPARs in lipid biology, little data exists regarding the links between pathways of lipid metabolism and subsequent PPAR activation. Such insight could have major implications for metabolic disorders and atherosclerosis. Data is presented establishing lipoprotein lipase (LPL), the central enzyme in triglyceride-rich lipoprotein metabolism, as an endogenous mechanism for PPARalpha activation. In vitro and in vivo evidence, using both gain of function (LPL overexpression) and loss of function (PPARalpha-deficiency) models, support this claim. LPL-mediated PPAR-alpha activation is independent of LPL's known non-enzymatic effects and is selective as to PPAR (PPARalpha>>PPARdelta> PPARgamma), lipoprotein substrate (VLDL>>LDL>HDL), and lipase- absent with other fatty acid-releasing lipases tested. Furthermore, monoacylglycerol (MAG), a LPL-specific product, is identified as a novel PPARalpha activator contributing to these LPL responses. Our central hypothesis is that lipolysis is a major pathway for endogenous PPAR activation, with distal effects determined by the varying nature of lipoprotein substrate, lipase, and targeted PPAR. This proposal outlines experiments to define lipolytic mechanisms for PPAR activation across these same parameters. Unique contributors to LPL-mediated PPARalpha activation, as suggested by preliminary data, will be studied, including MAG as a little studied signaling molecule and mechanisms of ligand delivery. Key lipolytic variables of lipoprotein substrate and other tiglyceride lipases will be examined in terms of their PPARalpha, -delta, and -gamma effects. These studies include measures of lipolytic PPAR responses on available well-characterized VLDL and plasma samples from both mice and humans. The existence of genetic LPL variants in mouse models and in humans, which have a range of LPL activity, will be utilized in vitro and in vivo to determine how graded LPL function alters well-established PPAR responses. These LPL models include the otherwise lethal LPL-deficient mice rescued by transient LPL expression. Through these programmatic efforts, insight will be gained into lipolysis as a mechanism for selective endogenous PPAR activation, and its distal transcriptional effects.