Identification of new Nerve Growth Factor-responsive immediate-early genes.

Stimulation of the PC12 pheochromocytoma cell line with the prototypical neurotrophin Nerve Growth Factor (NGF) induces a cellular response of neuronal differentiation and is therefore a widely used model to gain molecular insight into this process. Classically, the transcriptional response to extracellular stimuli such as NGF is divided in genes that require no protein synthesis prior to their induction (immediate-early genes) and genes that do (delayed-response genes). Because an increasing number of studies have reported important roles for immediate-early genes (IEGs) in neuronal differentiation, the goal of the present study was to identify previously unrecognized NGF-responsive IEGs. Stimulation with NGF for 15, 30, 60 and 120 min resulted in a typical transient induction of many known NGF-responsive IEGs. To identify candidate new genes, we analyzed 27000 measured expression profiles and selected 10 genes for further study. Five genes, including Cbp/p300-interacting transactivator 2 (Cited2), Kruppel-like factor 4 (Klf4), v-Maf musculoaponeurotic fibrosarcoma oncogene family, protein F (Maff), Kruppel-like factor 10 (Klf10 or Tieg) and Activating transcription factor 3 (Atf3) were selected and positively validated by qPCR. NGF-induced activation of all five genes seems to be mediated by MAPK and PI3K-mediated pathways. Additionally, we tested translation-independent induction and showed that NGF induced upregulation of these genes in both the subclonal Neuroscreen-1 PC12 and parental PC12 cell line. These 5 transcription factors have not been previously reported as NGF-responsive IEGs, however have previously been reported as important regulators of cell differentiation and proliferation in different systems. These observations may therefore provide important new information on the molecular mechanisms underlying NGF-induced differentiation.

Steroid receptor coactivator-1 splice variants differentially affect corticosteroid receptor signaling.

The mechanisms of receptor- and cell-specific effects of the adrenal corticosteroid hormones via mineralo- (MRs) and glucocorticoid receptors (GRs) are still poorly understood. Because the expression levels of two splice variants of the steroid receptor coactivator-1 (SRC-1) 1a and 1e, can differ significantly in certain cell populations, we tested the hypothesis that their relative abundance could determine cell- and receptor-specific effects of corticosteroid receptor-mediated transcription. In transient transfections, we demonstrate three novel types of SRC-1a- and SRC-1e-specific effects for corticosteroid receptors. One is promoter dependence: SRC-1e much more potently coactivated transcription from several multiple response element-containing promoters. Mammalian 1-hydrid studies indicated that this likely does not involve promoter-specific coactivator recruitment. Endogenous phenylethanolamine-N-methyltransferase mRNA induction via GRs was also differentially affected by the splice variants. Another type is receptor specificity: responses mediated by the N-terminal part of the MR, but not the GR, were augmented by SRC-1e at synergizing response elements. SRC fragment SRC(988-1240) by the MR but not the GR N-terminal fragment in a 1-hybrid assay. The last type, for GRs, is ligand dependence. Due to effects on partial agonism of RU486-activated GRs, different ratios of SRC-1a and 1e can lead to large differences in the extent of antagonism of RU486 on GR-mediated transcription. Furthermore, we show that SRC-1e but not SRC-1a mRNA expression was regulated in the pituitary by corticosterone. We conclude that the cellular differences in SRC-1a to SRC-1e ratio demonstrated in vivo might be involved in cell-specific responses to corticosteroids in a promoter- and ligand-dependent way.