Drosophila transcription factor cubitus interruptus (Ci) and its co-activator CRE (cAMP response element)-binding protein (CBP) activate a group of target genes on the anterior-posterior border in response to hedgehog protein (Hh) signaling. In the anterior region, in contrast, the carboxyl-truncated form of Ci generated by protein processing represses Hh expression. In vertebrates, three Ci-related transcription factors (glioblastoma gene products (GLIs) 1, 2, and 3) were identified, but their functional difference in Hh signal transduction is unknown. Here, we report distinct roles for GLI1 and GLI3 in Sonic hedgehog (Shh) signaling. GLI3 containing both repression and activation domains acts both as an activator and a repressor, as does Ci, whereas GLI1 contains only the activation domain. Consistent with this, GLI3, but not GLI1, is processed to generate the repressor form. Transcriptional co-activator CBP binds to GLI3, but not to GLI1. The trans-activating capacity of GLI3 is positively and negatively regulated by Shh and cAMP-dependent protein kinase, respectively, through a specific region of GLI3, which contains the CBP-binding domain and the phosphorylation sites of cAMP-dependent protein kinase. GLI3 directly binds to the Gli1 promoter and induces Gli1 transcription in response to Shh. Thus, GLI3 may act as a mediator of Shh signaling in the activation of the target gene Gli1.

Upon transforming growth factor-␤ (TGF-␤) binding to its cognate receptor, Smad3 and Smad4 form heterodimers and transduce the TGF-␤ signal to the nucleus. In addition to the Smad pathway, another pathway involving a member of the mitogen-activated protein kinase kinase kinase family of kinases, TGF-␤-activated kinase-1 (TAK1), is required for TGF-␤ signaling. However, it is unknown how these pathways function together to synergistically amplify TGF-␤ signaling. Here we report that the transcription factor ATF-2 (also called CRE-BP1) is bound by a hetero-oligomer of Smad3 and Smad4 upon TGF-␤ stimulation. ATF-2 is one member of the ATF/CREB family that binds to the cAMP response element, and its activity is enhanced after phosphorylation by stress-activated protein kinases such as c-Jun N-terminal kinase and p38. The binding between ATF-2 and Smad3/4 is mediated via the MH1 region of the Smad proteins and the basic leucine zipper region of ATF-2. TGF-␤ signaling also induces the phosphorylation of ATF-2 via TAK1 and p38. Both of these actions are shown to be responsible for the synergistic stimulation of ATF-2 trans-activating capacity. These results indicate that ATF-2 plays a central role in TGF-␤ signaling by acting as a common nuclear target of both Smad and TAK1 pathways.

By screening a lambda gt11 library with the multimerized sequence of the cAMP response element (CRE), we isolated human clones encoding the CRE binding protein, CRE‐BP1, from a human brain cDNA library. CRE‐BP1 expressed in Escherichia coli bound not only to the CRE element of the somatostatin and fibronectin genes, but also to the CRE element of the adenovirus E4 gene, suggesting that the protein was not distinguishable from the adenovirus transcription factor, ATF. The human CRE‐BP1 clone encoded a 54.5 kd protein similar at its carboxy terminus to the leucine zipper motifs found in other enhancer binding proteins such as C/EBP and c‐jun/AP‐1. CRE‐BP1 mRNA was expressed in all of the cells examined and was abundant in brain. The structure of CRE‐BP1 and its recognition elements suggest that cellular response to extracellular stimuli is controlled by a family of transcription factors that bind to related cis‐active elements and that contain several highly conserved domains.