Uracil ribonucleotide metabolism in rat and human glomerular epithelial and mesangial cells

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Abstract
Uridine diphosphosugars (UDP-sugars: UDP-N-acetylglucosamine, UDP-glucose, and UDP-glucuronic acid) are essential coenzymes for the synthesis of glomerular basement membrane and mesangial matrix (GBM-MM). This study has characterized UDP-sugar metabolism in rat and human glomerular cells in tissue culture. Culture of rat mesangial cells in medium containing dialyzed fetal calf serum resulted in UTP loss (28 +/- 4 nmol.mg DNA-1.h-1); the addition of 2 microM orotate to this medium resulted in net UTP accretion (5.42 +/- 0.06 nmol.mg DNA-1.h-1). Rat mesangial cells demonstrated 16- and 29- to 46-fold greater UTP and UDP-sugar pools, respectively, than whole glomeruli. In human mesangial cells, 6-azauridine (500 microM) decreased UDP-sugar pools by 48% (P less than 0.05), whereas uridine (50 microM) produced a 2.5-fold increase. Human and rat mesangial cells had greater (1.8- to 6.1-fold) UDP-sugar pools than epithelial cells and 1.7-3.4 times greater labeled precursor incorporation into UDP-sugars. In conclusion, glomerular cells utilize both exogenous orotate and uridine for ribonucleotide synthesis, and the extracellular concentration of these precursors markedly influence the formation and cellular content of UDP-sugars. Prominent differences exist between separate glomerular cell populations in their metabolism of UDP-sugars. This may represent diverse activity of glycosylating reactions.