Human retinal pigment epithelial cells cultured in hyperglycemic media accumulate increased amounts of glycosaminoglycan precursors.

Confluent human retinal pigmented epithelial cells were cultured on microcarrier beads in the presence of 5.6 or 26 mmol/l glucose with or without the aldose-reductase inhibitor Sorbinil (200 microM) for 2 wk. At the end of the incubation period, perchloric acid extracts were prepared and analyzed by 31P nuclear magnetic resonance spectroscopy. As assessed by this method, the phosphorylated metabolites of cells incubated with 5.6 or 26 mmol/l glucose differed significantly in the concentrations of a number of uridine diphosphate (UDP)-conjugated monosaccharides, which were elevated two- to threefold in cells incubated in 26 mmol/l glucose over control samples. The affected metabolites were identified (through a series of spiking experiments) to be UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, and UDP-glucuronic acid. Coincubation of the cells with Sorbinil 200 microM in the presence of 26 mmol/l glucose had no effect on this accumulation. Under normal circumstances, these molecules selectively and sequentially are incorporated into the polysaccharide chains of glycosaminoglycans (GAGs), whose presence and distribution in the basement membranes is affected adversely by diabetes mellitus. These data suggest that the availability of the monosaccharide precursor is not the rate-limiting step for GAG synthesis in the presence of pathologic glucose concentrations. Thus, the lost GAG content in the basement membranes of diabetic patients may be caused by changes elsewhere in the biosynthesis and/or catabolism of the polysaccharide-linked protein molecules.