The nuclear and oncogenic BCL-3 protein activates or represses gene transcription when bound to NF-κB proteins p50 and p52 yet the molecules that specifically interact with BCL-3 and drive BCL-3-mediated effects on gene expression remain largely uncharacterized. of an oncogenic IκB protein and they establish a functional link between the E3 ligase TBLR1 and NF-κB. Numerous oncogenic proteins are aberrantly expressed because the molecules involved in their degradation are not functioning properly (22 24 44 BCL-3 is an IκB protein whose degradation through the proteasome requires GSK3-mediated phosphorylation yet the E3 ligase involved in this pathway is usually unknown (42). BCL-3 was originally identified through molecular cloning of the breakpoint of the t(14;19) chromosomal translocation found in a subset of human B-cell chronic lymphotic leukemias (27). This translocation triggers BCL-3 overexpression and consequently the deregulation of many genes involved in survival and cell proliferation (30). Aberrant BCL-3 expression has also been reported in multiple myelomas and subtypes of lymphomas even in the absence of any t(14;19) chromosomal translocation (6 7 26 29 Deregulated BCL-3 expression has also been seen in breast and nasopharyngeal cancers hepatocarcinomas and familial cylindromatosis. Familial cylindromatosis is usually a genetic disease characterized by benign tumors of hair-follicle Tenacissoside G keratinocytes due to some loss-of-function Tenacissoside G mutations of CYLD a deubiquitinating enzyme that limits the nuclear import of BCL-3 (4 12 25 31 41 Insight into the role of BCL-3 in the immune system has been provided through the analysis of and show a complete block in secondary lymphoid organogenesis as well as defects in thymic stromal cells and they develop lymphocytic infiltrates in multiple organs (51). This severe Tenacissoside G phenotype combined with the overlapping phenotypes of mice deficient in or (15 16 51 BCL-3 activates gene transcription by removing the inhibitory NF-κB p50 and p52 homodimers from DNA and/or by acting as a coactivator for a subset of NF-κB target genes (13 17 23 BCL-3 can also negatively regulate lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α) synthesis in macrophages when bound to histone deacetylase 1 (HDAC1) and HDAC3 and/or because BCL-3 prevents the degradative polyubiquitination of p50 inhibitory homodimers (8 21 46 To learn more about the regulation of BCL-3 we have purified BCL-3-associated proteins and have identified CtBP LSD1 and TBLR1 as proteins that all bind to the N-terminal domain name of this oncoprotein. CtBP is crucial in the biology of BCL-3: this corepressor is required for the stabilization of BCL-3 and for the ability of BCL-3 to repress gene transcription to transform cells and to inhibit UV-mediated cell apoptosis in transformed keratinocytes. Moreover the ITGA8 half-life of BCL-3 is extended in TBLR1-depleted cells due to altered polyubiquitination thus defining TBLR1 as a key molecule for BCL-3 degradation. Therefore our data identified the N-terminal domain of BCL-3 as an essential region for the degradation transcriptional activity and oncogenic potential of this protein and defined CtBP and TBLR1 as key regulators of different properties of the BCL-3 oncoprotein. MATERIALS AND METHODS Cell Tenacissoside G culture biological reagents and treatments. 293 Phoenix NIH 3T3 HeLa and Karpas cells were cultured as described previously (42). HaCat cells were maintained in Dulbecco’s modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS) 1 glutamine and 1% penicillin-streptomycin. MG132 and LiCl were purchased from A&E Scientific (Marcq Belgium) and Sigma (St. Louis MO) respectively. Polyclonal antibodies against hemagglutinin (HA) HDAC3 Hsp90 Myc BCL-3 IκBα IκBβ and IκB? as well as monoclonal antibodies against ubiquitin CtBP and Myc were purchased from Santa Cruz Biotechnology (Santa Cruz CA). Monoclonal anti-FLAG antibodies and beads were purchased from Sigma. Polyclonal antibodies against LSD1 p52/p100 and p105/p50 were from Abcam (Cambridge United Kingdom) and Millipore (Temecula CA) respectively. The monoclonal anti-TBLR1 antibody was from Abnova (Taipei Taiwan). GFP TBLR1 and CtBP Tenacissoside G small interfering RNAs (siRNAs) were purchased from Dharmacon (Lafayette CO) whereas LSD1 siRNAs were from Eurogentec (Liege Belgium). Tenacissoside G Plasmids retroviral constructs and lentiviral constructs..