Synthesis and glycosylation of fibronectin in hepatocytes from vitamin A-deficient rats

Summary

Recent work from our laboratory (Kim and Wolf, J Biol Chem 262: 365–371, 1987) has shown increased uptake of labeled amino acids into fibronectin (FN), increased net synthesis of FN and increased levels of FN-mRNA in primary cultures of hepatocytes from vitamin A-deficient rats compared to controls. We now find, surprisingly, decreased uptake of labeled sugars into the oligosaccharide chains of FN from vitamin A-deficient hepatocytes. This decrease could be reversed by added retinoic acid at physiological concentration. At the same time, FN from deficient hepatocytes (−A.FN) was more susceptible to proteolytic degradation. Decreased uptake of the core sugar mannose into −A.FN was similar to that of glucosamine, yet the percent of label in sialic acid was the same as in +A.FN, suggesting a smaller number of oligosaccharide chains per molecule of −A.FN. Upon enzymatic removal of oligosaccharide and labeling with sodium borotritide, it was found that both −A.FN and +A.FN had biantennary oligosaccharide structures. Selective enzymatic removal of sialic acid showed that +A.FN had both sialic acids in an α2→3 linkage, whereas −A.FN apparently had one α2→3 and one α2→6-linked sialic acid. The borotritide experiments allowed us to calculate that +A.FN appeared to have 5 oligosaccharide chains per FN monomer, whereas the −A. FN showed only 4 chains. These results would account for the decreased glycosylation and increased susceptibility to proteolysis of the −A. FN. We conclude that vitamin A controls both the rate of synthesis of the polypeptide chain of FN via its mRNA, as well as the rate of its glycosylation.