ZBP-89 represses vimentin gene transcription by interacting with the transcriptional activator, Sp1

Hypoxia-induced Activation of the Retinoic Acid Receptor-related Orphan Receptor α4 Gene by an Interaction between Hypoxia-inducible Factor-1 and Sp1

Hypoxia plays a key role in the pathophysiology of many disease states, and expression of the retinoic acid receptor-related orphan receptor alpha (RORalpha) gene increases under hypoxia. We investigated the mechanism for this transient hypoxia-induced increase in RORalpha expression. Reverse transcription-coupled PCR analysis revealed that the steady-state level of mRNA for the RORalpha4 isoform, but not the RORalpha1 isoform, increased in HepG2 cells after 3 h of hypoxia. Transient transfection studies showed that the hypoxia-induced increase in RORalpha4 mRNA occurs at the transcriptional level and is dependent on a hypoxia-responsive element (HRE) located downstream of the promoter. A dominant-negative mutant of hypoxia-inducible factor-1alpha (HIF-1alpha) abrogates the transcription activated by hypoxia as well as the transcription activated by exogenously expressed HIF-1alpha, demonstrating the direct involvement of HIF-1alpha in the transcriptional activation. However, HIF-1 alone was not sufficient to activate transcription in hypoxic conditions but, rather, required Sp1/Sp3, which binds to a cluster of GC-rich sequences adjacent to the HRE. Deletion of one or more of these GC boxes reduced or eliminated the HIF-1-dependent transcription. Together, these results suggest that the hypoxia-responsive region of the RORalpha4 promoter is composed of the HRE and GC-rich sequences and that the transcriptional activation under hypoxia is conferred through the cooperation of HIF-1 with Sp1/Sp3.

Stat3 enhances vimentin gene expression by binding to the antisilencer element and interacting with the repressor protein, ZBP-89

Vimentin exhibits a complex pattern of developmental- and tissue-specific expression and is aberrantly expressed in most metastatic tumors. The human vimentin promoter contains multiple DNA elements, some of which enhance gene expression and one that inhibits. A silencer element (at -319) binds the repressor ZBP-89. Further upstream (at -757) is an element, which acts positively in the presence of the silencer element and, thus, is referred to as an antisilencer (ASE). Previously, we showed that Stat1alpha binds to this element upon induction by IFN-gamma. However, substantial binding and reporter gene activity was still present in nontreated cells. Here, we have found that Stat3 binds to the ASE element in vitro. Transfection experiments in COS-1 cells with various vimentin promoter--reporter constructs show that gene activity is dependent upon the cotransfection and activation of Stat3. Moreover, activated Stat3 can overcome ZBP-89 repression. Coimmunoprecipitation studies demonstrate that Stat3 and ZBP-89 can interact and confocal microscopy detects these factors to be colocalized in the nucleus. Moreover, a correlation exists between the presence of activated Stat3 and vimentin expression in MDA-MB-231 cells, which is lacking in MCF7 cells where vimentin is not expressed. In the light of these results, we propose that the interaction of Stat3 and ZBP-89 may be crucial for overcoming the effects of the repressor ZBP-89, which suggests a novel mode for Stat3 gene activation.

Transcription factor ZBP-89 is required for STAT1 constitutive expression

IFNgamma is a pro-inflammatory cytokine that potentiates p53-independent apoptosis in a variety of cell types. STAT1 is the primary mediator of IFNgamma action. ZBP-89 is a transcription factor that binds to the G/C-rich elements and mediates p53-independent apoptosis. In this study, site-directed mutagenesis revealed that a G-rich element from +171 to +179 within the first intron of the STAT1 gene is critical for optimal STAT1 promoter activity. Electrophoretic mobility shift assays and promoter analysis revealed that ZBP-89 binds directly to this STAT1 G-rich element along with Sp1 and Sp3. Reduction of ZBP-89 with siRNA attenuated both basal and IFNgamma-induced STAT1 expression and subsequently diminished the activation of apoptotic markers, e.g. caspase-3 and PARP. Taken together, we conclude that ZBP-89 is required for constitutive STAT1 expression and in this way contributes to the ability of cells to be activated by IFNgamma.