beta-Adrenergic receptor regulation of N-linked protein glycosylation in rat parotid acinar cells

We have investigated the relationship between beta-adrenergic receptor stimulation and protein glycosylation and secretion in rat parotid gland cells in vitro. The potent beta-adrenergic agonist (-)-isoproterenol increases [3H]mannose incorporation into newly synthesized glycoproteins. This effect is enhanced if cells are first preincubated with dolichyl phosphate and is not observed after muscarinic-cholinergic or alpha-adrenergic stimulation of cells. The increase in [3H]mannose incorporation is abolished by incubation of cells with tunicamycin, suggesting that the glycosylation events being studied involved asparagine-linked oligosaccharides. The extent of increase in glycosylation is dependent on the concentration of (-)-isoproterenol to which cells are exposed. (+/-)-Propanolol totally abolishes the (-)-isoproterenol-induced increase in [3H]mannose incorporation, in a manner similar to its effects on exocrine secretion. Our findings suggest that beta-adrenergic receptor activation has a profound influence on N-linked protein glycosylation in rat parotid cells in addition to eliciting exocrine protein release.

Dolichyl phosphate supplementation increases N-linked protein glycosylation in rat parotid acinar cells without increasing glycoprotein secretion

N-linked protein glycosylation was increased three- to five-fold, in dispersed rat parotid acinar cells in vitro, by supplementation with exogenous dolichylphosphate. Despite this increase, glycoprotein secretion from both control and dolichylphosphate-supplemented cells was comparable.

Relationship between oligosaccharide-lipid synthesis and protein synthesis in mouse LM cells

Previous studies from several laboratories have reported that inhibition of protein synthesis results in a concomitant reduction in synthesis of oligosaccharide-diphosphoryldolichol. We have investigated this phenomenon in LM cells grown in defined culture medium. The results of this study indicate that incubation of LM cell with cycloheximide at a concentration sufficient to totally arrest polypeptide synthesis, results in a rapid reduction in the synthesis of [3H]mannose-labeled or [3H]glucosamine-labeled oligosaccharide-lipid within 2 min. Cycloheximide treatment had only a slight inhibitory effect on synthesis of GDP-Man. Furthermore, during the first 5 min after the addition of cycloheximide to LM cells, when [3H]mannose incorporation into oligosaccharide-lipid was maximally reduced, the specific activity of the GDP-Man pool was identical to that observed in control cells. Labeling in vivo of the lipid-linked saccharide precursors of oligosaccharide-lipid revealed that cycloheximide had virtually no effect on the synthesis of Man-P-Dol, GlcNAc-PP-Dol, GlcNAc2-PP-Dol, and beta-Man-(GlcNAc)2-PP-Dol (Dol = dolichol). In agreement with this finding, results of experiments in vitro using microsomes prepared from LM cells indicate that cycloheximide did not directly inhibit the enzymes responsible for the synthesis of these lipid-linked saccharide precursors. Supplementation of mouse LM cells by preincubation for 1 h with dolichyl phosphate (5 micrograms/ml) resulted in a 300% stimulation of oligosaccharide-lipid synthesis when compared to non-supplemented cells. However, dolichyl phosphate supplementation of cycloheximide-treated cells failed to restore oligosaccharide-lipid synthesis to the level observed in control cells. In control cells pre-labeled oligosaccharide-lipid turned over rapidly and, as expected, cycloheximide addition to the chase medium significantly retarded this turnover process. However, preincubation of cells with exogenous dolichyl phosphate had little or no effect on the turnover of oligosaccharide-lipid in control or cycloheximide-treated cells. These findings argue against dolichyl phosphate deficiency as the primary cause of reduced oligosaccharide-lipid synthesis when protein synthesis is blocked. The implications of these results, and an alternative hypothesis to explain the effect of inhibition of protein synthesis on oligosaccharide-lipid synthesis based on elevation of intracellular GTP levels, are discussed.