ORIGINAL ARTICLE Physical and functional cooperation between AP-1 and b-catenin for the regulation of TCF-dependent genes K Toualbi 1,2 , MC Gu¨ ller 1,2 , J-L Mauriz 1,2 , C Labalette 3,4 , M-A Buendia 3,4 , A Mauviel 1,2 and D Bernuau 1,2 1 INSERM, U697, Paris, France; 2 Universite´Paris 7 Denis Diderot, Paris, France; 3 Institut Pasteur, Unite´de Recombinaison et Expression Ge´ne´tique, Paris, France and 4 INSERM, U163, Paris, France Stabilization of cytoplasmic b-catenin is a hallmark of a variety of cancers. The stabilized b-catenin is able to translocate to the nucleus, where it acts as a transcrip- tional activator of T-cell factor (TCF)-regulated genes. b-Catenin may cross-talk with many signalling cascades to activate target genes. Whether b-catenin cooperates with AP-1, another transcriptional complex activated during tumorigenesis is not fully clarified. We show that b-catenin co-immunoprecipitates with c-Jun and c-Fos. GST pull-down experiments indicate a physical associa- tion of the armadillo repeat domain of b-catenin with the DNA-binding domain of c-Jun and of the C-terminal domain of b-catenin with the N-terminal domain of c-Fos. Promoter studies indicate that overexpression of AP-1 activates the transcription of two b-catenin target genes, cyclin D1 and c-myc, by a mechanism independent of the AP-1 site, and fully dependent on the TCF-binding site. We further demonstrate that AP-1/b-catenin synergism is involved during serum-induced cyclin D1 transcriptional activation. We identify a TCF-binding site on the cyclin D1 promoter which binds in vivo a complex induced by serum, containing b-catenin, TCF4, c-Fos, c-Jun, JunB and JunD. This novel mechanism of interaction between two signalling cascades might contribute to the potentia- tion of malignancy. Oncogene (2007) 26, 3492–3502. doi:10.1038/sj.onc.1210133; published online 4 December 2006 Keywords: AP-1; b-catenin; Fos; Jun; cyclin D1; hepatoma cells Introduction b-Catenin is an important nuclear effector of the Wnt/ Wingless signalling pathway involved in the establish- ment of the dorso-ventral axis or the segmentation pattern in embryos (Gumbiner, 1995). In the absence of a Wnt signal, the cytosolic pool of b-catenin is bound to Axin and the adenomatous polyposis coli protein, phosphorylated at the N-terminal Ser/Thr residues by casein kinase Ia and glycogen synthase kinase 3b and then rapidly degraded by the ubiquitination-proteasome system. In response to a Wnt signal, b-catenin phos- phorylation is prevented, leading to the accumulation of the protein in the cytoplasm and in the nucleus, where it binds to and enhances transcriptional activation by proteins of the lymphoid enhancer factor (LEF)/T-cell factor (TCF) family (Giese and Grosschedl, 1993; Gumbiner, 1995; Clevers and van de Wetering, 1997; Billin et al., 2000). Binding of b-catenin to LEF/TCF causes displacement of core-repressor complexes con- taining histone deacetylases (Billin et al., 2000), and heterodimerization that allows interaction with the C-terminal transactivation domain of b-catenin leading ultimately to the activation of transcription of Wnt- responsive genes (van de Wetering et al., 1997; Moon et al., 2002). The coactivator function of b-catenin is induced by recruitment of the basal transcription machinery (Hecht et al., 1999) and is enhanced by synergistic cooperation with a number of other proteins such as coactivators, (Zhurinsky et al., 2000; Martin et al., 2002; Grueneberg et al., 2003; Wei et al., 2003) or proteins involved in chromatin remodeling and histone modification (Hecht et al., 2000; Miyagishi et al., 2000; Sun et al., 2000; Takemaru and Moon, 2000; Barker et al., 2001). Not surprisingly, mounting evidence indicate that b-catenin is able to cross-talk with a variety of signalling cascades, such as those involving Rac GTPase, forkhead box O transcription factors or transforming growth factor (TGF)b (Eger et al., 2004; Esufali and Bapat, 2004; Chakladar et al., 2005; Essers et al., 2005). AP-1 is a dimeric transcription factor composed of one protein of the Jun family (c-Jun, JunB and JunD) and one protein of the Fos family (c-Fos, FosB, Fra-1, Fra-2), also involved in important regulatory cell functions, such as differentiation, proliferation and apoptosis (Eferl and Wagner, 2003). Fos/jun hetero- dimers or Jun/Jun homodimers recognize a cis-acting element, the phorbol 12-myristate 13-acetate (TPA)- responsive element (TRE: TGAG/CTA), which is found in a number of promoters. Recent data suggest that the b-catenin and the AP-1 pathways may interact. Received 16 May 2006; revised 7 September 2006; accepted 5 October 2006; published online 4 December 2006 Correspondence: Dr D Bernuau, Unite´ INSERM U 697, Pavillon Bazin, Hoˆpital Saint-Louis, 1 Avenue Claude Vellefaux, 75010, Paris, France. E-mail: bernuau@stlouis.inserm.fr Oncogene (2007) 26, 3492–3502 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc