Variation in insulin receptor messenger ribonucleic acid expression in human and rodent tissues

The expression of insulin receptor mRNA was studied in human and rodent tissues by Northern analysis. Human EBV-transformed lymphocytes contained four receptor mRNA species of sufficient length to encode the entire proreceptor: 9.5, 7.9, 7.1, and 5.7 kb. In human fibroblasts, the same four species were observed; however, the 7.9 and 5.7 kb mRNAs were markedly decreased. In mouse liver, rat hepatoma cells, and normal rat brain, kidney, liver, and muscle only two mRNA species (7.4 and 9.6 kb) were detected. Each of these human and rodent mRNAs hybridized equally well with cDNA sequences encoding the binding and kinase domains of the insulin receptor. Several smaller polyadenylated mRNAs (approximately 1.8 to 3.3 kb) were also identified in human cell lines that appeared to separately encode either alpha- or beta-subunit sequences of the receptor. In rats, liver had the highest content of insulin receptor mRNA, followed by kidney, brain, and muscle. The relative amount of the two mRNA species also varied among the rat tissues. The ratio of the 9.6-7.4 kb species was 2.7 in brain but only 1.0 to 1.6 in the other tissues (P less than 0.025). Dexamethasone treatment increased the content of the two insulin receptor mRNAs in rat liver by 2-fold. The half-life of both mRNA species was 70 min in rat hepatoma cells. These findings indicate that insulin receptor gene expression is complex and regulated with differential expression of insulin receptor mRNA and/or alterations in mRNA processing among various tissues.     

Growth-associated glycosylation of transferrin secreted by HepG2 cells.

Growth-associated alterations in the oligosaccharide structures of transferrin secreted by HepG2 cells were examined by concanavalin A-crossed affinoimmuno-electrophoresis. Slowly dividing, confluent cultures produced transferrin with a approximately 4.5-fold greater proportion of biantennary complex-type oligosaccharides than rapidly dividing, subconfluent cultures. The activity of N-acetylglucosaminyltransferase V (GlcNAc-T V) and galactosyltransferase were measured in cell extracts from subconfluent and confluent cultures. While the activity of galactosyltransferase remained relatively constant, the activity of GlcNAc-T V was approximately 3.2-fold lower in confluent cultures than in subconfluent cultures. These results suggest that the growth-associated alteration in the oligosaccharides of transferrin is due, at least in part, to the regulation of GlcNAc-T V activity.