The effect of increasing nucleotide-sugar concentrations on the incorporation of sugars into glycoconjugates in rat hepatocytes.

Treatment of rat hepatocytes with 0.5 mM concentrations of uridine and cytidine results in increased cellular concentrations of UTP, UDP-sugars and CTP, whereas that of CMP-N-acetylneuraminate remained unchanged [Pels Rijcken, Overdijk, Van den Eijnden and Ferwerda (1993) Biochem. J. 293, 207-213]. The incorporation of radioactivity from 3H-labelled sugars into the cell-associated and secreted glycoconjugate fraction was influenced by these altered cellular concentrations of the nucleotides. For [3H]glucosamine, pretreatment with uridine resulted in a reduction of the glycosylation in both fractions. Increases in the secreted fractions were observed for fucose with both uridine and cytidine and for N-acetylglucosamine with uridine only. With [3H]N-acetylglucosamine, similar specific radioactivities for UDP-N-acetylhexosamine and CMP-N-acetylneuraminate were found, regardless of the pretreatment conditions. With [3H]N-acetylmannosamine, the specific radioactivity of CMP-N-acetylneuraminate showed an almost 2-fold increase on pretreatment. The latter increase did not result in an increased incorporation of radioactivity into the glycoconjugates. It was estimated that, in untreated cells, the ratio of radioactivity incorporated from [3H]glucosamine into glycoconjugate-bound N-acetylhexosamine and N-acetylneuraminate amounted to 2:3. In pretreated cells this ratio changed to approx. 2:1. Overall, the data show that pretreatment resulted in an increased incorporation of N-acetylhexosamine into cell-associated and secreted glycoconjugates, accompanied by a reduction in sialylation. It was concluded that an increased availability of UDP-N-acetylhexosamine caused the increased incorporation of N-acetylhexosamine. The elevated cytosolic level of UDP-N-acetylhexosamine (and of compounds like CMP) is suggested to impair the transport of CMP-acetylneuraminate to the Golgi, resulting in reduced sialylation. This study demonstrates that protein glycosylation can be regulated at the level of the availability of the various nucleotide-sugars in the Golgi lumen.