Interactions between second messenger pathways influence NGF synthesis in mouse primary astrocytes

Primary mouse brain astrocytes were stimulated with phorbol 12-myristate 13-acetate (PMA), serum, forskolin and ionophore A23187, in order to investigate the effect of distinct signalling pathways on the expression of the nerve growth factor (NGF) gene and of proto-oncogenes encoding 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 barely 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 with corresponding modifications of the levels of c-jun transcripts, a fact which suggests that the c-Jun protein exerts a regulatory role on the expression of the NGF gene. In these cells, however, the regulation of NGF synthesis appears complex, since a pretreatment with forskolin or ionophore A23187 interfered with the promoting effect elicited by PMA or serum in inducing an early decline of the levels of NGF transcripts. This phenomenon was accompanied by a corresponding decrease in the amounts of cell-secreted NGF in cells treated with forskolin and PMA. 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 inhibitory effect was observed with thapsigargin, another compound able to increase the cytosolic concentration of calcium.(ABSTRACT TRUNCATED AT 250 WORDS)