A gene activated in mouse 3T3 cells by serum growth factors encodes a protein with "zinc finger" sequences

Co-localization of elements required for phorbol ester stimulation and glucocorticoid repression of proliferin gene expression

Proliferin (PLF) gene expression provides a model of growth-related transcriptional activation in mouse cells. Transcription from a cloned PLF promoter is inducible by phorbol esters, and this induction involves a region of 31 bp that includes an AP-1 site and a cluster of sites similar to the simian virus 40 (SV40) SphI element. The mutation of either the AP-1 or the SphI sites abolishes phorbol ester induction, and the transfer of this 31-bp sequence to a site upstream of a minimal promoter is sufficient to confer phorbol-ester responsiveness. In contrast to phorbol esters, glucocorticoids repress PLF transcription, which results in a reduced accumulation of PLF mRNA in serum-stimulated cells. Repression is dependent on the glucocorticoid receptor, which binds to the PLF promoter in a region that includes the AP-1 site, and the 31-bp phorbol ester 12-O-tetra decanoylphorbol-13-acetate (TPA)-inducible region is sufficient to mediate glucocorticoid repression. In addition, extracts from glucocorticoid-treated and untreated mouse cells are found to differ in the nature of the protein complexes that interact with the AP-1 site.

Multiple intracellular pathways induce expression of a zinc-finger encoding gene (EGR1): relationship to activation of the Na/H exchanger

Intracellular pathways that rapidly stimulate the expression of a mitogen-inducible, zinc-finger encoding gene, EGR1 (Sukhatme et al., Cell 53:37-43), have been characterized in two human fibroblasts strains (WI-38 and HSWP). Serum and epidermal growth factor (EGF) were each found to strongly stimulate EGR1 expression in both cell types. Comparably high levels of expression could also be induced by treatment with the phorbol ester TPA. In cells rendered deficient in PK-C, serum and EGF were each still capable of inducing high levels of EGR1 mRNA, demonstrating that additional non-protein kinase C pathways are capable of stimulating EGR1 expression. In both fibroblasts strains, stimulation of EGR1 expression by all these agents exhibited rapid, transient kinetics and could be superinduced if protein synthesis was inhibited through the addition of cycloheximide. Finally, various agents, known to stimulate/inhibit the activation of another early mitogenic response, the activation of Na/H exchange, were analyzed for their effect on EGR1 expression. Interestingly bradykinin, vasopressin, and Ca ionophores, which dramatically stimulate Na/H exchange, were only weak stimulants of EGR1 expression. Conversely, EGF, which stimulates Na/H exchange poorly, strongly activated EGR1 expression. Hence while EGR1 expression could be triggered by multiple intracellular pathways, its expression does not appear to require the prior activation of Na/H exchange.

Growth factors and membrane depolarization activate distinct programs of early response gene expression: dissociation of fos and jun induction

A set of early response genes has been identified whose transcription in fibroblasts is rapidly induced in response to growth factors. Prototype members of this group, c-fos and c-jun, encode products that form a heterodimer and have been implicated in the regulation of gene expression and cell growth. It is thought that other early response genes also encode critical mediators of the cell's response to external stimuli. We have used PC12 pheochromocytoma cells as a model system to test the hypothesis that different extracellular signals induce distinct patterns of expression of early response genes. Our results indicate that membrane depolarization, induced either by potassium chloride or by the neurotransmitter analog nicotine, activates a program of gene expression distinct from that activated by nerve growth factor or epidermal growth factor. Notably, c-fos and c-jun activation can be dissociated; whereas c-jun is coinduced with c-fos and jun-B after growth factor stimulation, membrane depolarization activates c-fos and jun-B without stimulating c-jun. Fos may therefore form transcription complexes with alternative cofactors under different stimulation conditions. nur/77 and zif/268, which encode putative transcription factors, also show markedly different responses to growth factors and depolarization. We conclude that multiple nonconvergent signal transduction pathways control early response gene expression. Our findings also indicate that the diversity and specificity of cellular response to environmental change can be accounted for by the differential combinatorial induction of a relatively small number of early response genes.

A gene inducible by serum growth factors encodes a member of the steroid and thyroid hormone receptor superfamily

We previously identified, by cDNA cloning, a set of genes that are expressed during the G0/G1 transition (cell cycle reentry) in mouse fibroblasts. These immediate early genes are transcriptionally activated within minutes of addition of serum or purified growth factors, and their mRNAs are superinduced in the presence of protein-synthesis inhibitors. We report here that one of these genes, represented by nur/77 cDNA (originally called 3CH77), encodes a member of the superfamily of ligand-binding transcription factors that includes the steroid and thyroid hormone receptors. The nur/77 cDNA sequence encodes a protein of 601 amino acids containing two regions of sequence similarity to members of this nuclear receptor superfamily, corresponding to their DNA-binding and ligand-binding domains. These results suggest that the growth factor-inducible immediate early gene nur/77 encodes a ligand-binding protein that regulates the genomic response to growth factors.