INDUCTION OF NERVE GROWTH-FACTOR SYNTHESIS BY SPHINGOMYELINASE AND CERAMIDE IN PRIMARY ASTROCYTE CULTURES

Astrocytes synthesize nerve growth factor (NGF) in response to pro-inf lammatory cytokines. To further study the signaling mechanism involved in this induction of NGF production, the sphingomyelin (SM) pathway w as studied. Addition of exogenous neutral SMase (Staphylococcus aureus ) or C-2-ceramide to primary cultures of newborn rat cortical astrocyt es elicited a dose-response increase of NGF synthesis, with maximal ef fect at 1 U/ml and 25 mu M, respectively. Induction of NGF synthesis b y SMase and ceramide was shown to be independent of classical PKC acti vity. Intracellular cAMP-raising agents, such as forskolin and 3-isobu tyl-1-methylxanthine, partially prevented the SMase- and C-2-ceramide- induced secretion of NGF to the cell supernatant. PD098059 and apigeni n, inhibitors of the mitogen-activated protein (MAP) kinase pathway, p roduced a dose-response inhibition of the SMase- and C-2-cer-induced r elease of NGF. This observation points to the possibility that regulat ion of NGF synthesis and secretion by the SMase pathway may be mediate d downstream by the MAP kinase cascade. As a matter of fact, pre-treat ment of astrocytes with SMase or C-8-ceramide led to an increased phos phorylation of raf-1. Moreover, MAP kinase activity was enhanced in as trocytes treated with SMase or both ceramides. In conclusion, results suggest that the SMase pathway may control NGF synthesis in the centra l nervous system, and raise the possibility of an involvement of the M AP kinase cascade in this process. (C) 1997 Elsevier Science B.V.