Complex Interactions Among Second Messenger Pathways, Steroid Hormones, and Protooncogenes of the Fos and Jun Families Converge in the Regulation of the Nerve Growth Factor Gene

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Abstract
Regulation of the expression of the nerve growth factor (NGF) gene has been reported previously to be mediated via the protooncogene c‐fos. Activation of the protein kinase C pathway and 1,25‐dihydroxyvitamin D3[1,25(OH)2D3] has also been reported to increase the pool of NGF transcripts in L929 fibroblasts. Here we show that activation of the cyclic AMP second messenger pathway antagonized the effect of phorbol 12‐myristate 13‐acetate (PMA) or serum on NGF synthesis, whereas it enhanced that of 1,25(OH)2D3. A positive effect was also observed when serum, PMA, and 1,25(OH)2D3 were added together, but dexamethasone reduced this enhancement. There was no close correlation between the increase in c‐fos mRNA and that in NGF mRNA, suggesting that expression of the c‐fos protooncogene is not necessarily followed by induction of the NGF gene. Rather, these two genes are simultaneously, and not sequentially, induced after forskolin treatment. It appears that regulation of the NGF gene depends on a repertoire of multiple regulatory AP‐1 complexes arising from activation of the second messenger pathways. This suggests that NGF gene expression is under the control of a complex interplay among second messenger pathways, protooncogenes, and steroid hormones such as 1,25(OH)2D3 and glucocorticoids.

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