The goal of the proposal is to define mechanisms responsible for insulin suppression of hepatic secretion of very low density lipoproteins (VLDL) containing apolipoprotein B100 (B100). Postprandial insulin increases B100 degradation and reduces B100 synthesis limiting VLDL secretion. Resistance to insulin results in continuous secretion of large VLDL1 causing hypertriglyceridemia. We were first to observe insulin effects on B100 and recently demonstrated that degradation is mediated by sortilin and directed to autophagy. We plan to further define mechanisms for this novel insulin action. Insulin dependent apo B degradation decreases B100 availability by enhancing post endoplasmic reticulum (ER) presecretory B100 proteolysis. Little is known about how insulin reduces apo B synthesis. Our previous studies established that insulin suppression requires activation of Class IA phosphatidylinositide 3-kinase (PI3K) and product PI (3,4,5) triphosphate (PIP3) in the ER.
Aim 1 is to determine the role of sortilin in insulin-mediated B100 degradation based on evidence showing insulin increases sortilin interaction with B100. We propose that this process involves binding of PIP3 to B100, movement to the Golgi, interaction with sortilin, and stimulation of autophagy leading to degradation of both sortilin and B100 through insulin initiated p110? induced autophagy.
In Aim 2, we propose studies on how PIP3 generation causes downstream effects on B100. These studies are novel as they propose translocation of B100 associated with PIP3 from the cytoplasmic face of the ER membrane where PIP3 is generated during translation of B100. Transient cytoplasmic orientation of B100 occurs as a result of pause-transfer sequences allowing exposure of B100 to PIP3 and incorporation into primordial lipoproteins. A model is presented where B100-PIP3 complexes on immature VLDL are transported and released into Golgi by specific vesicles followed by increased B100 interaction with sortilin.
Aim 3 is to further define insulin effects on B100 synthesis. Insulin dependent B100 silencing involves movement of B100 mRNA into processing bodies (P-bodies) as shown by our collaborator, Dr. Khosrow Adeli. Our studies will define initiating events that are reversible preceding formation of P-bodies. We hypothesize that B100 mRNA binding proteins that interact with coding regions resist movement of B100 transcripts into inactive mRNPs. Preliminary data support a role for insulin-dependent phosphorylation of apobec-1 complementation factor (A1CF) induced nuclear retention which reduces cytoplasmic availability of A1CF for stabilization of B100 mRNA translation thereby favoring formation of inactive B100 mRNPs. Resistance to insulin-regulated hepatic B100 secretion is one of the earliest events in development of metabolic syndrome and results in postprandial hypertriglyceridemia which enhances formation of small easily oxidizable LDL and destabilization of HDL. Defining mechanisms involved in insulin regulation of hepatic VLDL secretion will allow focused therapeutic strategies to be developed to restore lipoprotein balance and reduce atherogenic lipoprotein profile during the postprandial transition.
During the fasting-to-fed transition, insulin reduces hepatic secretion of very low density lipoproteins (VLDL) which are precursors of LDL. High levels of LDL are known to cause atherosclerosis in humans. The mechanism responsible for reducing VLDL secretion is through limiting the availability of apolipoprotein B100 which is required for the assembly of VLDL. Short-term suppression of B100 and VLDL secretion by insulin favors efficient metabolism of dietary fat, and minimizes exposure of the arterial wall to highly atherogenic remnant lipoproteins. This pathway also reduces exposure of peripheral tissues especially muscle to elevated fatty acids which negatively impact tissue insulin sensitivity.