BCL6-C-JUN-CAX · cooperation between BCL6 and c-Jun in lymphoma development and progression

cooperation between BCL6 and c-Jun in lymphoma development and progression

AP-1 activity is strongly induced in response to numerous signals, including growth factors, cytokines and extracellular stresses1. The proto-oncoprotein c-Jun belongs to the AP-1 group of transcription factors and it is a crucial regulator of intestinal progenitor proliferation and tumourigenesis2,3,4. An important mechanism of AP-1 stimulation is phosphorylation of c-Jun by the Jun N-terminal kinases (JNKs)1. N-terminal phosphorylation of the c-Jun transactivation domain increases target gene transcription5,6 but a molecular explanation was elusive. We previously described a yeast three hybrid approach designed to identify proteins that interact with c-Jun depending on its phosphorylation status7. Mbd3 was cloned as an interactor that specifically bound to unphosphorylated c-Jun. Mbd3, together with Mbd1, 2 and 4, was originally characterised as a protein containing a region with high homology to the methyl-CpG-binding domain (MBD) of MeCP2. Mbd2 and Mbd3 assemble into mutually exclusive distinct NuRD complexes8. NuRD mediates gene repression through histone deacetylation via HDAC1 and 2, and chromatin remodeling ATPase activities through its CHD3 (Mi2a) and CHD4 (Mi2b) subunits9,10,11. Whereas Mbd2 recruits NuRD to methylated DNA, the MBD of Mbd3 fails to bind methylated DNA12,13. Thus how Mbd3 interacts with chromatin to regulate transcription is not known. We have shown that unphosphorylated, but not N-terminally phosphorylated c-Jun interacts with Mbd3 and thereby recruits the Nucleosome Remodeling and Histone Deacetylation (NuRD) repressor complex. Our study identified c-Jun as the first transcription factor that uses Mbd3 to recruit NuRD to specific target genes. Mbd3 depletion in colon cancer cells increased histone acetylation at AP-1-dependent promoters which resulted in increased target gene expression. The intestinal stem cell marker lgr5 was identified as a novel target gene controlled by c-Jun/Mbd3. Gut-specific conditional deletion of mbd3 (mbd3ΔG/ΔG mice) stimulated c-Jun activity and increased progenitor cell proliferation. In response to inflammation, mdb3 deficiency resulted in colonic hyperproliferation and mbd3ΔG/ΔG mice showed dramatically increased susceptibility to colitis-induced tumourigenesis. Notably, concomitant inactivation of a single allele of c-jun reverted physiological and pathological hyperproliferation as well as increased tumourigenesis in mbd3ΔG/ΔG mice. Thus the transactivation domain of c-Jun recruits Mbd3/NuRD to AP-1 target genes to mediate gene repression, and this repression is relieved by JNK-mediated c-Jun N-terminal phosphorylation. This novel mechanism is important in regulating intestinal progenitor homeostasis and tumourigenesis. The JNK/c-Jun pathway is a crucial regulator of intestinal progenitor proliferation and tumourigenesis. Understanding the JNK/c-Jun signalling pathway and the components that regulate it, such as Mbd3/NuRD, will help us to identify new targets against which to develop new drugs that would prevent or cure intestinal tumourigenesis. References 1 Davis, R. J. Signal transduction by the JNK group of MAP kinases. Cell 103, 239-252. (2000). 2 Eferl, R. & Wagner, E. F. AP-1: a double-edged sword in tumorigenesis. Nat Rev Cancer 3, 859-868 (2003). 3 Sancho, R. et al. JNK signalling modulates intestinal homeostasis and tumourigenesis in mice. EMBOJ (2009). 4 Nateri, A. S., Spencer-Dene, B. & Behrens, A. Interaction of phosphorylated c-Jun with TCF4 regulates intestinal cancer development. Nature 437, 281-285 (2005). 5 Pulverer, B. J., Kyriakis, J. M., Avruch, J., Nikolakaki, E. & Woodgett, J. R. Phosphorylation of c-jun mediated by MAP kinases. Nature 353, 670-674 (1991). 6 Behrens, A., Sibilia, M. & Wagner, E. F. Amino-terminal phosphorylation of c-Jun regulates stress-induced apoptosis and cellular proliferation. Nat Genet 21, 326-329 (1999). 7 Nateri, A. S., Riera-Sans, L., Da Costa, C. & Behrens, A. The ubiquitin ligase SCFFbw7 antagonizes apoptoti

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