Group 1B phospholipase A2 (PLA2g1B) is synthesized by both exocrine acinar cells and endocrine B-islet cells of the pancreas. It is generally thought to act only as a lipolytic enzyme, facilitating lipid digestion and absorption in the digestive tract. However, our Preliminary Results in characterization of PLA2g1B knockout mice suggested that PLA2g1 B plays a minor role in dietary lipid absorption. However, the PLA2g1 B(-/-) mice are more insulin sensitive and glucose tolerant than PLA2g1B+/+ mice under both low fat and high fat dietary conditions. Weight gained by fat-fed PLA2g1B(-/-) mice was significantly lower than that of fat-fed PLA2g1 B(+/+) mice and similar to that of chow-fed animals. This data indicates that PLA2g1 B is contributory to diet-induced insulin resistance and obesity. The goal of this research project is to identify the mechanism by which PLA2g1B has this physiological role. The overall hypothesis is that PLA2g1B, through its enzymatic digestion of phospholipids in the intestinal lumen, contributes postprandial lysophosphatidylcholine (LPC) to portal and plasma circulation and negatively regulates insulin action in insulin-sensitive tissues.
Specific Aim 1 will test the hypothesis that PLA2g1B(-/-) mice are resistant to diet-induced obesity due to their increased energy expenditure after high fat feeding in comparison to similarly-fed wild type mice. Fat and glucose utilization by various tissues in PLA2g1 B(+/+) and PLA2g1 B(-/-) mice will be compared by measuring fatty acid oxidation in vivo and glucose uptake in hyperinsulinemic- euglycemic clamp experiments.
Specific Aim 2 will test the hypothesis and identify the mechanism by which LPC generated from PLA2g1B hydrolysis of phospholipids negatively regulate insulin signaling in peripheral tissues. In vitro cell culture experiments with hepatocytes, myotubules, and adipocytes will be performed to test the possibility that LPC inhibits UCP expression and activates JNK and/or protein kinase C-alpha, thereby inhibiting insulin and insulin receptor signaling cascade.
Aim 3 will produce pancreatic acinar-specific-and islet (B-cell-specific PLA2g1B transgene in PLA2g1B(-/-) mice to examine the importance of exocrine PLA2g1B versus endocrine PLA2g1B in contributing to diabetes and obesity. Collectively, these studies will add novel insights toward mechanisms that contribute to diet-induced insulin resistance and obesity, thus offering potentially new therapeutic targets for treatment of diabetes, and obesity. ? ? ?
