The scavenger receptor CD36 is highly expressed on the apical side of enterocytes of the proximal intestine and on endothelial and immune cells throughout the organ. We previously documented the physiological role of CD36 in intestinal uptake of fatty acids (FA) and cholesterol and in promoting chylomicron synthesis. During this funding period, we made several novel observations. We found that CD36 is ubiquitinated on two C- terminal lysines, that ubiquitination is induced by FA and diglycerides and that it occurs during lipid absorption. The same C-terminal lysines were also found crucial for CD36's ability to transduce signals to the src kinase and downstream to the extracellular regulated kinase (ERK1/2). The CD36-ERK pathway is activated in a lipid- dependent manner during early stages of chylomicron formation. Furthermore we found that the src-ERK pathway is also involved in CD36-mediated regulation of the synthesis of arachidonic acid derived pro- inflammatory eicosanoids. Our hypothesis is that during early absorption the CD36-ERK signaling pathway functions to initiate in parallel chylomicron synthesis and an acute inflammatory response to dietary lipid. As digestion proceeds, luminal lipids down-regulate CD36 by inducing its ubiquitination and this feedback loop acts to limit inflammation. This hypothesis will be tested in aim 1. We will examine lipid regulation of ERK1/2 and triglyceride secretion in Caco-2 cells stably expressing CD36 or a mutant form with impaired signaling. Parallel studies will be conducted using intestinal segments and enterocytes from newly generated mice models with intestine-specific CD36 deletion (intCD36-/-) or rescue (intCD36+/+). Based on our new data showing CD36 dimerization and acetylation of the dimer, we will determine the role of these modifications in CD36 signaling and triglyceride secretion.
In aim 2 (A - C) we will examine the role of intestinal CD36 in influencing systemic metabolism, gut immune cells and lipid-induced inflammation using the intCD36-/- and intCD36+/+ mice. Our research approach will also involve a translational component (aim2D). We recently showed that common CD36 single nucleotide polymorphisms (SNPs) contribute to individual differences in fasting blood lipoproteins. The effects of several significant SNPs were related to alterations in CD36 protein expression (determined on monocyte and platelets). We now propose (aim 2D) to test impact of these expression SNPs (eSNPs) on postprandial lipids and on lipid-induced eicosanoid secretion and inflammation. Finally in aim 3 we will explore the mechanisms underlying our novel preliminary data demonstrating that CD36 deletion increases the bile acid pool, decreases bile hydrophobicity and effectively protects against gallstone formation. The findings generated from the work proposed should enhance our understanding of the absorptive function of a key metabolic protein. They should provide insight into the molecular steps involved in intestinal lipid processing and into how lipid processing interconnects with inflammation. We should learn whether targeting intestinal CD36 may be beneficial in disorders like obesity, insulin resistance and gallstone formation.
We previously documented the physiological role of the membrane protein CD36 in intestinal uptake of fatty acids and cholesterol and in promoting chylomicron synthesis. Our recent data suggest additional roles of the protein in the inflammation induced by dietary lipid and in regulation of the biliary circulation. The work proposed should enhance our understanding of the absorptive functions of CD36, of the molecular steps involved in intestinal lipid processing and of how this interconnects with inflammation. We should learn whether targeting intestinal CD36 may be beneficial in disorders like obesity, insulin resistance and gallstone formation.
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