High glucose and diabetes modulate cellular proteasome function: Implications in the pathogenesis of diabetes complications

► Retinal endothelial cells exhibit higher proteasome peptidase activity compared to pericytes. ► High glucose treatment increased the levels of ubiquitinated proteins in retinal vascular cells. ► High glucose treatment enhanced the PA28 protein levels in pericytes but not endothelial cells. ► Diabetic retina and kidney have higher levels of PA28 proteasome regulatory proteins. ► The UPS components are increased within the glomerular capillaries of diabetic kidney. The precise link between hyperglycemia and its deleterious effects on retinal and kidney microvasculature, and more specifically loss of retinal perivascular supporting cells including smooth muscle cell/pericytes (SMC/PC), in diabetes are not completely understood. We hypothesized that differential cellular proteasome activity contributes to sensitivity of PC to high glucose-mediated oxidative stress and vascular rarefaction. Here we show that retinal endothelial cells (EC) have significantly higher proteasome peptidase activity compared to PC. High glucose treatment (HGT) increased the level of total ubiquitin-conjugated proteins in cultured retinal PC and EC, but not photoreceptor cells. In addition, in vitro proteasome activity assays showed significant impairment of proteasome chymotrypsin-like peptidase activity in PC, but not EC. The PA28-α/-β and PA28-β/-γ protein levels were also higher in the retina and kidney glomeruli of diabetic mice, respectively. Our results demonstrate, for the first time, that high glucose has direct biological effects on cellular proteasome function, and this modulation might be protective against cellular stress or damage induced by high glucose.