Robert C. Noland, Ph.D.
Project 2: Exploration of the role of carnitine octanoyltransferase in preventing lipid-induced insulin resistance
Carnitine octanoyltransferase, lipid catabolism and insulin resistance
Mechanisms underlying insulin resistance remain unclear, but evidence implicates a key role for abnormal lipid metabolism. Circulating medium chain acylcarnitines (MCACs), which are byproducts of lipid catabolism, are elevated in insulin resistance and a cluster of MCACs has been found to be associated with high heritable risk for premature cardiovascular disease and insulin resistance in humans. Carnitine octanoyltransferase (CrOT) is a peroxisomal matrix enzyme that converts the incomplete products of peroxisomal β-oxidation (MC-CoAs) to MCACs, which are in turn exported to the cytosol (Fig 1). The physiological importance of this process and the ultimate metabolic fate of these MCACs are yet unknown, but defining this is a goal of our current project. We predict MCACs are simply a marker of metabolic disease, rather than a causative factor since diets enriched in medium chain fatty acids (MCFAs) actually promote weight loss and insulin sensitivity. Unfortunately MCFA content in the typical diet is very low, thus it is likely that the MCFA intermediates required for these biological processes are derived from incomplete fat oxidation of long chain precursors. Based upon this evidence we feel that incomplete peroxisomal β-oxidation is a likely candidate to produce the MCFAs that are important to various biological processes and that CrOT serves as a key distributor of these chain-shortened moieties. As such, our laboratory is using gene manipulation strategies in cell culture and in vivo to define the importance of CrOT in global lipid metabolism. We hypothesize CrOT overexpression will improve peroxisomal and mitochondrial function and provide MCACs that facilitate insulin sensitization, whereas silencing CrOT will impede lipid catabolism and promote insulin resistance. To test this hypothesis we are pursuing the following specific aims:
- Determine the impact of manipulating CrOT activity on peroxisomal and mitochondrial lipid metabolism and overall function.
- Assess whether altering CrOT expression alters responses to lipid-associated metabolic stresses (fasting & exercise) &/or alters susceptibility to high fat diet-induced insulin resistance.
- Identify if MCACs produced by CrOT act as metabolic messengers affecting signaling cascades involved in the metabolic syndrome. Specifically, experiments will assess whether MCACs serve as 1) substrates for Ghrelin acylation, or 2) ligands for G-protein coupled receptors involved in insulin secretion (GPR40; islet β -cells) &/or lipolysis (GPR109B; adipocytes & immune cells).
Peroxisomal β-oxidation is unique in that it is inherently incomplete, yielding MCACs. Historically these products have been thought to simply be directed toward the mitochondria for use as metabolic fuel. However, this is called into question because MCFAs are becoming increasingly recognized as important regulators of several biological processes, yet they are not being provided in substantial quantities through a normal diet. This suggests the MCFAs involved in these processes must be produced endogenously, which has focused our attention on peroxisomal β-oxidation. CrOT facilitates the export of the incomplete products of peroxisomal β-oxidation; thus study of this enzyme provides a unique model to examine the importance of the MCACs derived from peroxisomal metabolism. Moreover, results from these studies are expected to yield profound insight into the physiological importance of incomplete lipid oxidation and greatly expand our understanding of peroxisomal biology.