Synergism between transcription factors TFE3 and Smad3 in transforming growth factor-beta-induced transcription of the Smad7 gene

Stoichiometry of active smad-transcription factor complexes on DNA

Transforming growth factor (TGF)-beta signals through a heteromeric complex of serine/threonine kinase receptors. The type I receptor phosphorylates and activates the receptor-regulated Smads (R-Smads), Smad2 and Smad3, which form hetero-oligomeric complexes with the co-Smad, Smad4, and translocate to the nucleus. Smad3 and Smad4 can bind directly to consensus DNA-binding elements in the promoters of target genes, whereas Smad2/Smad4 complexes are targeted to DNA by interacting with sequence-specific DNA-binding transcription factors that contain a well-defined Smad interaction motif (SIM). The exact stoichiometry of Smad homo- and hetero-oligomers both before and after ligand stimulation is controversial. Here we determine the stoichiometry of TGF-beta-induced Smad-transcription factor complexes on DNA. We show that complexes of Smad2/Smad4 with the transcription factors Fast-1 or Fast-3 contain one Fast, two Smad2s, and one Smad4. In contrast, Smad3/Smad4 complexes that bind the Smad-binding element from the c-jun promoter, are heterodimers. Furthermore, these Smad3/Smad4 complexes contain at least two additional components essential for complex formation, one of which contains a SIM. Our data suggest that the R-Smads can form heterodimers or heterotrimers with Smad4, and we propose that the exact stoichiometries of active Smad complexes on DNA may be determined by the transcription factors with which they associate.

Identification of a tightly regulated hypoxia-response element in the promoter of human plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) plays a key role in control of coagulation and tissue remodeling and has been shown to be regulated by a number of cell stimuli, among those hypoxia. In this study we characterize the hypoxia-mediated induction of PAI-1 in human hepatoma cell line HepG2. We found that PAI-1 is tightly regulated in a narrow oxygen gradient. After incubation at oxygen concentrations of 1% to 2%, a 60-fold increase in PAI-1 messenger RNA levels was observed, whereas mild hypoxic conditions of more than 3.5% did not appear to induce transcription. Moreover, increased levels of PAI-1 protein were observed after incubation at low oxygen tensions. Through sequence analysis, several putative hypoxia-response elements (HREs 1-5) were identified in the human PAI-I promoter. Reporter gene assays showed that the HRE-2 (-194 to -187) was necessary and sufficient for the hypoxia-mediated response. By electrophoretic mobility assay we observed hypoxia-dependent binding of a protein complex to the HRE-2 motif. Further analysis demonstrated that HRE-2 was specifically recognized by the hypoxia-inducible transcription factor 1alpha-arylhydrocarbon nuclear translocator complex. Taken together, our data demonstrate that hypoxia-induced transcription is mediated through HIF-1 interaction with the HRE-2 site of the human PAI-1 promoter.

Evidence for a role of the JNK cascade in Smad7-mediated apoptosis

Smad proteins are central mediators of the transcriptional effects of transforming growth factor beta (TGF-beta) superfamily that regulate a wide variety of biological processes. Smad7, an inhibitory Smad protein that prevents TGF-beta signaling by interacting with the activated type I TGF-beta receptor, was recently shown to induce sensitization of cells to different forms of cell death. Here we examined the effect of Smad7 on the c-Jun N-terminal kinase (JNK) cascade and investigated the role of this cascade in both the inhibitory and apoptotic functions of Smad7. The transient and stable expression of Smad7 caused a strong and sustained activation of JNK. Expression of a dominant-interfering mutant of mitogen-activated protein kinase kinase 4, which completely abolished Smad7-induced activation of JNK, had no effect on Smad7-mediated inhibition of TGF-beta signaling, indicating that the inhibitory function of Smad7 is independent of the JNK cascade. In contrast, expression of the dominant-interfering mutant of mitogen-activated protein kinase kinase 4 impaired the ability of Smad7 to promote cell death. These experiments reveal a novel link between Smad7 and the JNK cascade, which is essential for potentiation of cell death by this inhibitory Smad.

Characterization of the DNA-binding property of Smad5

Activation of TGF-beta superfamily receptors leads to phosphorylation of Smad proteins which function as transcription factors to regulate gene expression. Previous studies have indicated that Smad5, together with Smad1 and Smad8, participates in signaling downstream of BMP receptors. To characterize the DNA-binding characteristics of Smad5, we used the GST-Smad5 N-terminal fusion protein to select for random oligonucleotide sequences that were able to binds the protein. As a result, we found that Smad5 is able to bind a consensus sequence TGTGC. We further used the Smad7 promoter sequence that contains a Smad-binding element (SBE), GTCTAGAC to determine how mutations in each nucleotide in the SBE affects the binding with Smad5, compared with the binding with Smad1, Smad2, Smad3, Smad4, and Smad8. Interestingly, Smad5, but not Smad1 and Smad8, was able to bind the SBE, at a level similar to the binding by Smad3 and Smad4. However, mutations at the SBE had different effect on the binding with Smad5, compared to that with Smad3 and Smad4. These studies suggest that even though Smad5 falls into the same subfamily with Smad1 and Smad8 in mediating the signaling by BMP receptors, it has an unique DNA-binding property that is similar to Smad3, which specifically transduces signaling for TGF-beta and activin receptors.