Antagonism of glucocorticoid receptor transactivity and cell growth inhibition by transforming growth factor-beta through AP-1-mediated transcriptional repression.
We have examined the interaction of the glucocorticoid receptor (GR) and transforming growth factor-beta (TGF-beta) signal pathways because of their mutual involvement in the regulation of cell growth, development and differentiation. Most studies of this cross-talk event have focused on the effects of glucocorticoids (GCs) on TGF-beta responses. In this work, we show that TGF-beta can antagonize dexamethasone (Dex)-mediated growth suppression in mouse fibrosarcoma L929 cells. TGF-beta also repressed GR-mediated reporter (pMMTV-CAT) gene expression in a concentration-dependent manner, with an IC(50) of 5 ng/ml of TGF-beta. Maximal inhibition (76%) was observed at 10 ng/ml of TGF-beta. Conversely, Dex inhibited TGF-beta-mediated promoter (p3TP-Lux) activity in these same cells. As TGF-beta inhibition of GR-mediated gene expression occurred after Dex-mediated nuclear translocation of GR, we conclude that TGF-beta inhibition of GR signaling occurs at the level of GR-mediated transcription activity. However, TGF-beta did not repress GR-mediated gene expression using the pGRE(2)E1B-CAT minimal promoter construct, suggesting that TGF-beta did not inhibit intrinsic GR activity but, rather, required DNA-binding factor(s) distinct from GR. As the MMTV promoter contains several putative AP-1 binding sites, we hypothesized that AP-1, a transcription factor composed of c-jun and c-fos proteins, might be involved in the TGF-beta inhibition of GR functions. Curcumin, a potent inhibitor of AP-1 expression, completely abolished the inhibitory effect of TGF-beta on GR-mediated gene expression without affecting GR activity in the absence of TGF-beta, and this drug blocked TGF-beta-induced binding of AP-1 to a response element derived from the MMTV sequence. Furthermore, curcumin abolished TGF-beta inhibition of Dex-induced growth suppression. Taken as a whole, our data suggest that TGF-beta can antagonize the growth inhibitory properties of GR by blocking GR transactivity at complex promoters through a mechanism involving transcriptional repression by DNA-bound AP-1.
Periyasamy
Sumudra
S
Department of Pharmacology, Medical College of Ohio, 3035 Arlington Avenue, Toledo, OH 43614, USA. speriyasamy@mco.edu
Sánchez
Edwin R
ER
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics; D003907:Dexamethasone / Q000494:pharmacology; D015536:Down-Regulation / Q000187:drug effects*; D017930:Genes, Reporter / Q000235:genetics; D008324:Mammary Tumor Virus, Mouse; D051379:Mice; D011401:Promoter Regions, Genetic / Q000235:genetics; D021381:Protein Transport / Q000187:drug effects; D011965:Receptors, Glucocorticoid / Q000037:antagonists & inhibitors* / Q000378:metabolism; D018808:Transcription Factor AP-1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D011965:Receptors, Glucocorticoid; D018808:Transcription Factor AP-1; D016212:Transforming Growth Factor beta; D003907:Dexamethasone; D015500:Chloramphenicol O-Acetyltransferase