Cell-specific bifunctional role of Jun oncogene family members on glucocorticoid receptor-dependent transcription.
Interaction between protein kinase C (PKC)- and glucocorticoid receptor (GR)-mediated signaling is suggested by the ability of the PKC activating phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to inhibit GR-dependent transcription of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). Here we report that this interference is cell specific, as TPA augmented dexamethasone-induced transcriptional activation of the MMTV LTR in several T cell lines but was inhibitory in NIH-3T3 fibroblasts. TPA-GR synergism was determined to have occurred at the GR-responsive element (GRE) level by functional analysis of deletion mutants or synthetic GRE oligonucleotides driving chloramphenicol acetyl-transferase expression. Synergism required an intact GR DNA-binding domain, whereas amino- or carboxyl-terminal domains were dispensable. The effect was abrogated by the PKC inhibitor staurosporine, suggesting a role for PKC. Increased c-jun, jun-B, and jun-D expression above basal levels and increased transcriptional activity of AP-1/TPA responsive elements fused to chloramphenicol acetyl-transferase vectors were observed in T cells treated with TPA alone or in combination with dexamethasone. The ability of Jun proteins to cooperate with GR in T cells has been investigated after transfection of c-jun, jun-B, or jun-D expression vectors, which augmented GR-dependent transcription from either MMTV LTR or GRE. Conversely, c-jun and jun-B transfection blunted GR-dependent transcription in HeLa cells. The presence of c-fos had a negative influence on GR function and correlated with the cell-specific synergistic or antagonistic activity of Jun with respect to GR; high basal expression of c-fos as well as AP-1 DNA binding and transcriptional activity were observed in HeLa cells, but not in T cells. Furthermore overexpression of exogenous c-fos has an inhibitory effect on GR-dependent transcription from GRE in T cells. We propose that Jun plays a bifunctional role on GR-dependent transcriptional activation of GRE, selecting either synergistic or antagonistic activity depending on the cell-specific microenvironment. In this regard, intracellular levels of c-fos appear to be influential.
Department of Experimental Medicine, University La Sapienza, Rome, Italy.
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Maroder
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Farina
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Vacca
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Felli
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Meco
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Screpanti
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Frati
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Gulino
D016475:3T3 Cells; D000818:Animals; D004247:DNA / Q000378:metabolism; D004279:DNA, Viral / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D016758:Genes, jun / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D008324:Mammary Tumor Virus, Mouse / Q000235:genetics; D051379:Mice; D011493:Protein Kinase C / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000502:physiology*; D012091:Repetitive Sequences, Nucleic Acid; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured
D004279:DNA, Viral; D011965:Receptors, Glucocorticoid; D003907:Dexamethasone; D004247:DNA; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate