In vitro and in vivo alterations of enzymatic glycosylation in diabetes

Carbohydrate composition changes of glycoconjugates constituting the glycocalix of microvascular cells could be involved in the alterations of cell-cell interactions observed in diabetic retinopathy. In this field, we have recently reported that advanced glycation end products (AGEs) modify galactose, fucose and sialic acid contents of specific cellular glycoproteins. To better understand the mechanisms involved in glycoprotein modifications in diabetes, we now investigate whether glucose and AGEs could affect the activities of enzymes involved in galactose, fucose and sialic acid metabolism : glycosyltransferases (synthesis) and glycosidases (catabolism). For this, bovine retinal endothelial cells (BREC) and pericytes (BRP) were cultured in the presence of high glucose concentration or AGEs, and cell glycosidase and glycosyltransferase activities were measured. The same enzymatic activities were studied in the whole retina from streptozotocin-treated rats. The results show that high glucose concentration did not affect glycosidases and glycosyltransferases neither in BRP nor in BREC except for galactosyltransferase activities in BREC. Concerning BRP, only galactosyltransferase activities were altered by AGEs. In contrast, in BREC, AGEs increased beta-D galactosidase, alpha-L fucosidase and neuraminidase activities (+37%, +56%, 36% respectively) whereas galactosyltransferase, fucosyltransferase and sialyltransferase activities were decreased (-11%, -24% and -23% respectively). In the retina from diabetic rats, beta-D galactosidase, alpha-L fucosidase and neuraminidase activities increased (+70%, +57%, +78% respectively) whereas fucosyl and sialyltransferase decreased (-7% and -15% respectively). The possible consequence of these enzymatic activity changes could be a defect in the carbohydrate content of some glycoproteins that might participate in the endothelial cell dysfunctions in diabetic microangiopathy.