In vitro and in vivo alterations of enzymatic glycosylation in diabetes

Carbohydrate composition changes of glycoconjugates constituting the glycoc alix of microvascular cells could be involved in the alterations of cell-ce ll interactions observed in diabetic retinopathy. In this field, we have re cently reported that advanced glycation end products (AGEs) modify galactos e, fucose and sialic acid contents of specific cellular glycoproteins. To b etter understand the mechanisms involved in glycoprotein modifications in d iabetes, we now investigate whether glucose and AGEs could affect the activ ities 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 i n the presence of high glucose concentration or AGEs, and cell glycosidase and glycosyltransferase activities were measured. The same enzymatic activi ties were studied in the whole retina from streptozotocin-treated rats. The results show that high glucose concentration did not affect glycosidases a nd glycosyltransferases neither in BRP nor in BREC except for galactosyltra nsferase activities in BREC. Concerning BRP, only galactosyltransferase act ivities were altered by AGEs. In contrast, in BREC, AGEs increased beta-D g alactosidase, alpha-L fucosidase and neuraminidase activities (+37%, +56%, 36% respectively) whereas galactosyltransferase, fucosyltransferase and sia lyltransferase activities were decreased (-11%, -24% and -23% respectively) . In the retina from diabetic rats, beta-D galactosidase, alpha-L fucosidas e and neuraminidase activities increased (+70%, +57%, +78% respectively) wh ereas fucosyl and sialyltransferase decreased (-7% and -15% respectively). The possible consequence of these enzymatic activity changes could be a def ect in the carbohydrate content of some glycoproteins that might participat e in the endothelial cell. dysfunctions in diabetic microangiopathy.