Glycated and carboxy-methylated proteins do not directly activate human vascular smooth muscle cells

Abstract

Advanced glycation end products (AGEs) accumulate in patients with diabetes, particularly at sites of vascular damage and within atherosclerotic lesions, but whether they have direct actions on vascular smooth muscle cells (VSMCs) is controversial. AGEs were constructed and characterized by protein content, level of modification, fluorescence, and molecular size. Human VSMCs were derived from different vascular beds. Glucose consumption, de novo protein synthesis, and proteoglycan biosynthesis were measured using a colorimetric assay and metabolic radiolabeling. Receptor for AGEs (RAGE) expression was assessed by real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Treatment with AGEs under low or high glucose conditions showed no change in cellular glucose consumption or in cellular protein synthesis under low glucose conditions. Treatment of VSMCs with Nepsilon-(carboxymethyl)lysine in the presence of low glucose increased [35S]-sulfate incorporation into secreted proteoglycans by 72% (P < 0.001) and 67% (P < 0.001); however, the control proteins also increased [35S]-sulfate incorporation into proteoglycans by 56% (P < 0.01), with similar effects observed under high glucose conditions. Human VSMCs showed no difference in response to glycated and non-glycated protein. Protein and gene expression of RAGE in VSMC was approximately 50-fold lower compared to HMEC-1 and U937 cells, consistent with the immunohistochemical staining of RAGE in vivo. VSMCs show very low levels of RAGE expression; thus, activation of VSMCs by AGEs does not occur. In diabetes, RAGE expression in VSM may increase to the extent that it becomes activated by AGEs in a manner that would contribute to the process of atherosclerosis.