Hyperglycemia regulates post-transcriptional expression and phosphopalmnxylation of the oxidized-ldl/thrombospondin receptor, cd36

Hyperglycemia is a risk factor for (be development of vascular changes leading to atherosclerosis, but the mechanisms involved are not well defined. We have investigated whether high glucose growth conditions can produce changes in the expression or function of the membrane protein CD36 one of several receptors for oxidized LDL, collagen and thrombospondin. Regulation of CD36 ligand-binding function has been previously shown to be mediated by the state of phosphorylation of threonine-92 in the protein's ectodomain (Asch et al. Science 262:1436-1440). Ptaosphoamino acid analysis of recombinantly expressed CD36 revealed the presence of a large phosphorytated species in addition to phosphothreonine. Gas Chromatograpby-mass spectrometry (GCMS) analysis of the phosphorylated species revealed the presence of palmitate as 16:0, and smaller quantities of 18:1, 18:0 and 20:0 species suggesting that the phosphorylated species detected by autoradiography is O_-32p-O-Palmitate resulting from partial hydrolysis of a phosphoester linkage in the labeled CD36 molecule where threonine is the site of phospho-acylation (rareonine-O-P-O-CiePalmitate). Mutagenesis of wild type CD36ibr92 to CD36aia92 results in expression of a non-phospbopalmitoylated CD36 molecule. Growth in high glucose conditions (600mg%) resulted in increased surface expression of CD36 without change in steady state mRNA levels, suggesting that posttranscriptional regulation of CD36 expression was critical for the differences observed. GCMS performed on CD36 immunoisolated from the two phenotypes after 24 hours of labeling with [3,3-2H2]palmitic acid,demonstrated the presence of 220pm [3,3-₂H2]palmitic acid in immunoprecipitates from the cells grown in higa glucose and 50pm [3,3-2H2]palmitic acid in the cells grown under low glucose conditions. To investigate whether hyperglycemia can regulate expression of CD36 in vivo, we rendered SpragueDawley rats diabetic by injection of streptozotocin (STZ, 70 mg/kg) and followed CD36 expression in several tissues. Rats receiving STZ had blood glucose ranging from 200 to 460 mg%. Although CD36 protein levels were significantly increased, there was no effect of diabetes on CD36 mRNA from muscle or adipose tissue for op to 7 days following STZ injection. Our findings suggest a role for glucose-mediated regulation of CD36-paImitoylation and expression and function in a variety of tissues. In particular, upregulation of CD36 in the vascular tissues of hyperglycémie patients may play a role in their accelerated atherosclerosis by altering the net flow of free fatty acids and lipid particles across vascular cell membranes.