Abstract
Preincubation of quiescent BALB/C3T3 cells with uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) in conditioned medium resulted in a time- and concentration-dependent stimulation of uptake of 2-deoxyglucose, uridine, and alpha-aminoisobutyric acid over that seen in conditioned medium alone. The apparent Km values for 2-deoxyglucose and uridine uptake were the same for cells incubated with or without UDP-GlcNAc, whereas the Vmax values were higher for cells pretreated with the nucleotide sugar. The stimulation of uptake was specific for UDP-GlcNAc, and the other nucleotide sugars tested were ineffective; incubation of UDP-GlcNAc-pretreated cells with N-acetylglucosaminidase abolished the stimulatory effect. In all cases, the extent of stimulation of nutrient uptake was comparable to that seen with serum-stimulated cells. Incubation of quiescent cells in situ with UDP-[3H]GlcNAc led to incorporation of radioactive N-acetylglucosamine into the acid-precipitable fraction; a large fraction of the labeled amino sugar was found on the cell surface acceptors. We interpret these data to mean that the cellular acceptors of quiescent cells are "under-glycosylated," at least in terms of N-acetylglucosamine, and that stimulation of uptake of nutrients may be a consequence of restoration of the amino sugar residues on the oligosaccharide chains of acceptors on the cell surface.