INTERACTIONS BETWEEN 2ND-MESSENGER PATHWAYS INFLUENCE NGF SYNTHESIS IN MOUSE PRIMARY ASTROCYTES

Primary mouse brain astrocytes were stimulated with phorbol 12-myrista te 13-acetate (PMA), serum, forskolin and ionophore A23187, in order t o investigate the effect of distinct signalling pathways on the expres sion of the nerve growth factor (NGF) gene and of proto-oncogenes enco ding transcription factors of the Fos and Jun families. PMA, and to a lesser extent serum, induced a marked accumulation of NGF transcripts, in agreement with published observations [Brain Res., 570 (1992) 316- 322]. The effect of A23187 was less pronounced and that of forskolin b arely detectable. No relationship was observed between the expression of NGF gene and that of c-fos, fos-B, fra-1, jun-B proto-oncogenes. In contrast, changes in the levels of NGF transcripts were associated wi th corresponding modifications of the levels of c-jun transcripts, a f act which suggests that the c-Jun protein exerts a regulatory role on the expression of the NGF gene. In these cells, however, the regulatio n of NGF synthesis appears complex, since a pretreatment with forskoli n or ionophore A23187 interfered with the promoting effect elicited by PMA or serum in inducing an early decline of the levels of NGF transc ripts. This phenomenon was accompanied by a corresponding decrease in the amounts of cell-secreted NGF in cells treated with forskolin and P MA. A23187 had a much more striking effect on the production of mature NGF since this compound maintained the level of cell-secreted NGF to basal values, irrespective of the presence of PMA. A similar inhibitor y effect was observed with thapsigargin, another compound able to incr ease the cytosolic concentration of calcium. The drop in NGF productio n is probably not due to a corresponding block of the synthesis or sec retion of macromolecules. These data suggest that activation of protei n kinase C (PKC) in the presence or absence of calcium mobilization ex erts contrasting effects on the synthesis of NGF.