Nerve growth factor mediates phosphorylation of specific proteins

Abstract

Nerve growth factor (NGF), epidermal growth factor (EGF), insulin, cholera toxin (CT) and cAMP all stimulate the phosphorylation of proteins in the PC12 nerve-like cell line. NGF, CT and cAMP enhance phosphorylation of the same set of proteins including tyrosine hydroxylase, ribosomal protein S6, histones H1 and H3, and the nonhistone chromosomal and cytoplasmic high mobility group (HMG) 17 protein, and reduce phosphorylation of H2A. EGF but not insulin enhances the phosphorylation of tyrosine hydroxylase. Insulin but not EGF enhances the phosphorylation of histone H3 and decreases the phosphorylation of H2A. EGFD and insulin each enhance phosphorylations of both ribosomal protein S6 and histone H1, but neither hormone induces phosphorylation of HMG 17. The extent of these effects depends upon the ligand concentration and is half-maximal at physiological concentrations of the hormones (beta-NGF, 2 ng/ml; EGF, 1 ng/ml. insulin, 0.5 microunits/ml). Maximal effects of NGF are seen within 15 min and persist even after 3 days of culture in the presence of NGF. When phosphorylation of ribosomal protein S6 is maximally stimulated by NGF, no further stimulation can be achieved by adding saturating quantities of either cAMP or CT. However, simultaneous addition of saturating quantities of NGF and either EGF or insulin results in an enhancement of phosphorylation that is equal to the sum of that achieved when the two ligands are added separately. These results suggest that the enhanced phosphorylation of S6 achieved by NGF or cAMP occurs through a common mechanism which differs from those which mediate EGF or insulin-enhanced phosphorylation. The data also provide strong evidence that the action of NGF included protein phosphorylation mediated by cAMP-dependent protein kinase. The phosphorylation of each of these proteins in response to NGF may play an important role in NGF action.