P0 phosphorylation in nerves from normal and diabetic rats: role of protein kinase C and turnover of phosphate groups.

The effects of phorbol ester and forskolin on the net phosphorylation and turnover of P0 phosphate groups was studied in normal and experimentally diabetic rats. In sciatic nerve segments isolated from normal rats and incubated with [32P]-inorganic phosphate, phosphorylation of the major peripheral myelin protein, P0, was increased 2-5 fold in a time and dose-dependent manner by phorbol 12,13 dibutyrate (PDB). This increase was blocked by the protein kinase inhibitors, H-7 and staurosporine. Both the basal and PDB-stimulated phosphorylation of P0 were significantly greater in segments of sciatic nerve from streptozotocin-induced diabetic rats. Prolonged exposure of nerve segments to PDB abolished the stimulated phosphorylation of P0 and immunoblots of nerve proteins revealed a decrease in the content of the protein kinase C alpha-isoform. The adenylate cyclase activator, forskolin, had no effect on the PDB-stimulated phosphorylation of P0 in normal nerve but decreased phosphorylation in diabetic nerve. To measure turnover of P0 phosphate groups, nerves were incubated with 32P and incorporated label was then chased in radioactivity-free medium for up to 4 hours. P0 from normal nerve prelabeled under basal conditions lost 25% of its radioactivity during this time. In contrast, nearly all of the additional phosphate groups prelabeled in the presence of PDB disappeared after 2 hours of chase. P0 phosphate groups from diabetic nerve displayed similar turnover kinetics. When forskolin was added to the chase medium, the turnover of P0 phosphate moieties was accelerated in normal, but not in diabetic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)