BAG-1 interacts with the p50–p50 homodimeric NF-κB complex: implications for colorectal carcinogenesis

Abstract

Understanding the mechanisms that promote aberrant tumour cell survival is critical for the determination of novel strategies to combat colorectal cancer. We have recently shown that the anti-apoptotic protein BAG-1, highly expressed in pre-malignant and colorectal cancer tissue, can potentiate cell survival through regulating NF-κB transcriptional activity. In this study, we identify a novel complex between BAG-1 and the p50-p50 NF-κB homodimers, implicating BAG-1 as a co-regulator of an atypical NF-κB pathway. Importantly, the BAG-1-p50 complex was detected at gene regulatory sequences including the epidermal growth factor receptor [EGFR] and COX-2 [PTGS2] genes. Suppression of BAG-1 expression using siRNA was shown to increase EGFR and suppress COX-2 expression in colorectal cancer cells. Furthermore, mouse embryonic fibroblasts derived from the NF-κB1 [p105/p50] knockout mouse were used to demonstrate that p50 expression was required for BAG-1 to suppress EGFR expression. This was shown to be functionally relevant as attenuation of BAG-1 expression increased ligand activated phosphorylation of EGFR in colorectal cancer cells. In summary, this paper identifies a novel role for BAG-1 in modulating gene expression through interaction with the p50-p50 NF-κB complexes. Data presented led us to propose that BAG-1 can act as a selective regulator of p50-p50 NF-κB responsive genes in colorectal tumour cells, potentially important for the promotion of cell survival in the context of the fluctuating tumour microenvironment. As BAG-1 expression is increased in the developing adenoma through to metastatic lesions, understanding the function of the BAG-1-p50 NF-κB complexes may aid in identifying strategies for both the prevention and treatment of colorectal cancer.

Inhibition of Transcription by B Cell Leukemia 3 (Bcl-3) Protein Requires Interaction with Nuclear Factor κB (NF-κB) p50

“…In addition to the NF-B subunits, Bcl-3 interacts with a number of regulators of transcription, including c-Jun and c-fos (8); STAT1 (9); PPAR␥ (10); CREB-binding protein/p300 (8); HDAC-1, -3, and -5 (11); steroid receptor coactivator 1(8); TORC3 (12); and retinoic X receptor (13); Fyn (14) and Lck (15); insulin receptor substrate 3 (IRS3) (16); and Bag-1 (17). The contribution of these factors to immune regulation by Bcl-3 has not been determined previously.…”

“…Bcl-3 has also been reported to interact with the AP-1 transcription factors c-Jun and c-fos (8); STAT1 (9); peroxisome proliferator-activated receptor␥ (PPAR␥) (10); CREB-binding protein/p300 (8); HDAC-1, -3, and -5 (11); steroid receptor coactivator 1 (8); TORC3 (12); and the retinoic X receptor (13). Bcl-3 has also been reported to interact with the Src-related kinases Fyn (14) and Lck (15) as well as insulin receptor substrate 3 (IRS3) (16) and Bag-1 (17). Previous data suggest that Bcl-3-mediated stabilization of p50 is required for limiting NF-B transcriptional activity.…”

H3K4me3 mediates the NF-κB p50 homodimer binding to thepdcd1promoter to activate PD-1 transcription in T cells

“…21,30 The p50 homodimer can act as a transcriptional activator by associating with transcriptional coactivators. 39,41 In addition, the p50 homodimer can also function to repress transcription of κB element-dependent target genes through excluding transcriptionally active NF-κB heterodimers binding to the κB sites or by recruiting HDACs to κB element-containing promoters to decrease chromatin histone acetylation. 37 Therefore, it is possible that different co-factors are associated with the p50 NF-κB and p65 NF-κB complexes at the pdcd1 promoter region in activated T cells and macrophages, respectively.…”

“…37,38 Conversely, PD-1 expression level should be diminished in p50 KO T cells if p50 is a transcriptional activator. [39][40][41] Purified CD3 + T cells from WT and p50 KO mice were stimulated with anti-CD3 and anti-CD28 antibodies and CD25 expression was used as a marker for T cell activation. Activated CD4 + CD25 + and CD8 + CD25 + T cells were then analyzed for PD-1 expression levels.…”

Downregulation of cyclooxygenase‑1 stimulates mitochondrial apoptosis through the NF‑κB signaling pathway in colorectal cancer cells

“…Subsequently, p105-RelA is processed to p50-RelA (NF-κB), which translates into the nucleus and binds promoters of target genes to switch on the expression of diverse target genes involved in cell proliferation (such as c-Myc) and anti-apoptosis (such as Bcl-2) (14). The non-classical NF-κB pathway is activated through IκB kinase α, and p100-RelB is processed into p52-RelB heterodimers, which translates into the nucleus to bind the promoter of NF-κB-responsive elements in NF-κB target genes regulating lymphoid organogenesis and maintaining the malignant phenotype in certain types of cancer (15). The activation of the two pathways relies on the inducible phosphorylation-inhibitory proteins IκB (IκBs and p100), and regulates cell survival, death and carcinogenesis.…”