Nerve growth factor gene expression: Involvement of a downstream AP-1 element in basal and modulated transcription

Regulation of nerve growth factor (NGF) production in neuronal targets is critical for the differentiation and survival of NGF-responsive neurons. A principal cell type that produces NGF in neuronal targets is the fibroblast. Using primary kidney and established L929 cell fibroblast cultures we have studied the regulation of NGF production at the transcriptional level. A reporter gene containing the NGF promoter region including a downstream AP-1 element was efficiently expressed in stably transfected L929 cells. DNase-1 footprinting and gel shift analyses showed that the AP-1 element was bound by L929 cell nuclear factors. Mutation of the AP-1 element prevented nuclear factor binding and severely reduced expression in stably transfected L929 cells. Similarly, a reporter gene lacking the AP-1 element and carried by transgenic mice is not expressed in cultured kidney fibroblasts although the endogenous NGF gene is expressed. In addition to basal transcription, the AP-1 element may also be involved in modulation of NGF production. In support of this role, the phorbol ester TPA was shown to induce NGF mRNA in L929 cells by Northern analysis. The induction was preceded by transient increases in c-fos and jun-B mRNAs. TPA treatment also increased the binding of nuclear proteins immunoreactive with anti-fos and anti-jun antisera to the AP-1 element. A reporter gene containing the AP-1 element was transactivated by c-fos and c-jun in cotransfection experiments. Thus, the NGF promoter region including the AP-1 element is important in basal and modulated NGF gene expression in cells within neuronal targets.