Induction of nerve growth factor synthesis by sphingomyelinase and ceramide in primary astrocyte cultures.

Astrocytes synthesize nerve growth factor (NGF) in response to pro-inflammatory cytokines. To further study the signaling mechanism involved in this induction of NGF production, the sphingomyelin (SM) pathway was studied. Addition of exogenous neutral SMase (Staphylococcus aureus) or C2-ceramide to primary cultures of newborn rat cortical astrocytes elicited a dose-response increase of NGF synthesis, with maximal effect at 1 U/ml and 25 microM, respectively. Induction of NGF synthesis by SMase and ceramide was shown to be independent of classical PKC activity. Intracellular cAMP-raising agents, such as forskolin and 3-isobutyl-1-methylxanthine, partially prevented the SMase- and C2-ceramide-induced secretion of NGF to the cell supernatant. PD098059 and apigenin, inhibitors of the mitogen-activated protein (MAP) kinase pathway, produced a dose-response inhibition of the SMase- and C2-cer-induced release of NGF. This observation points to the possibility that regulation of NGF synthesis and secretion by the SMase pathway may be mediated downstream by the MAP kinase cascade. As a matter of fact, pre-treatment of astrocytes with SMase or C8-ceramide led to an increased phosphorylation of raf-1. Moreover, MAP kinase activity was enhanced in astrocytes treated with SMase or both ceramides. In conclusion, results suggest that the SMase pathway may control NGF synthesis in the central nervous system, and raise the possibility of an involvement of the MAP kinase cascade in this process.