RelB/p52 NF-κB Complexes Rescue an Early Delay in Mammary Gland Development in Transgenic Mice with Targeted Superrepressor IκB-α Expression and Promote Carcinogenesis of the Mammary Gland

Abstract

NF-B/Rel is a structurally and evolutionary conserved family of transcription factors distinguished by the presence of an N-terminal 300-amino-acid region, termed the Rel homology domain. The Rel homology domain is responsible for DNA binding, dimerization, nuclear translocation, and binding of Rel factors to the IB inhibitory proteins (reviewed in reference 25). Mammals express five NF-B members, of which RelB, c-Rel, and p65 (RelA) are synthesized as mature products and contain a C-terminal transactivation domain. In contrast, p50 and p52 are synthesized as longer precursors that have C-terminal ankyrin repeats and can serve as inhibitory molecules. The p105 and p100 precursor proteins require proteolytic processing to produce the mature p50 and p52 subunits, respectively, which lack a transactivation domain (46). In most untransformed cells other than B lymphocytes, NF-B complexes are sequestered in the cytoplasm bound to specific inhibitory proteins, termed IBs (2), which have been found to display specificity of interaction (reviewed in references 54 and 59). For example, IB-␣ most strongly interacts with p65 and c-Rel and only weakly with RelB, p50, and p52. Activation of NF-B can occur via multiple pathways. Stimuli such as tumor necrosis factor (TNF) and interleukin-1 lead to induction of p65/p50 complexes with IB-␣ via the canonical pathway involving activation of the IKK complex, consisting of the kinases IKK␣ and IKK␤ (33). In particular, activation of the IKK␤ kinase leads to phosphorylation of IB-␣ on S32/36 and its rapid proteasome-mediated degradation (7, 12), allowing for translocation of the free NF-B to the nucleus (reviewed in reference 59). Thus, an IB-␣ protein with the S32/36A mutation, which is resistant to phosphorylation and subsequent degradation, is termed superrepressor (SR)-IB-␣. A recently reported alternative pathway involves induction of RelB/p52 complexes upon stimulation by such TNF family receptors as lymphotoxin ␤ and CD40 (15,34,62). This signaling leads to activation of the NF-B-inducing kinase (NIK) and subsequent IKK␣-mediated phosphorylation of the p100 component of RelB/p100 cytoplasmic complexes (44, 61), resulting in its processing to p52. Studies by Bravo and coworkers (18) indicate that the resulting RelB/p52 complexes do not interact well with IB-␣ and, thus, are largely free to migrate to the nucleus.NF-B factors have been implicated in the pathogenesis of breast cancer and in development of the normal mammary gland. We and others have demonstrated aberrant constitutive activation of NF-B factors in human and rodent breast cancers (14,29,35,41,51). High levels of nuclear NF-B were found in the majority of primary human and rodent breast tumor tissue samples, breast cancer cell lines, and carcinogentransformed mammary epithelial cells (14, 29). Inhibition of

A single NFκB system for both canonical and non-canonical signaling

“…IKK1 AA mice that do not have functional IKK1 kinase activity are defective in lactation during pregnancy [82]. RelB:p52 activation was proposed to rescue the delay in the early mammary gland development observed in transgenic mice overexpressing the IκBα super-repressor [83]. Defective osteoclastogenesis observed in nik −/− mice can be restored by overexpressing RelB, but not RelA, indicating a specific function of RelB in osteoclast differentiation [84].…”

IKK1 and IKK2 cooperate to maintain bile duct integrity in the liver

“…Although at this stage it is not clear why the combined ablation of IKK1 and IKK2 or IKK1 and NEMO does not result in HCC development, it is likely that the additional ablation of IKK1 interferes with liver cancer development in this model. Although the mechanisms by which IKK1 might control liver tumorigenesis in NEMO LPC-KO mice remain elusive, recent studies implicated IKK1 and the alternative NF-B pathway in oncogenesis (27)(28)(29)(30)(31). In the mammary gland, IKK1, p100/p52, and relB were shown to regulate cyclin D1 expression and epithelial cell proliferation, and mice lacking inducible IKK1 kinase activity showed delayed development of ErbB2-induced tumors, implicating alternative NF-B signaling in mammary epithelial hyperplasia and oncogenesis (27,29,31).…”

“…Although the mechanisms by which IKK1 might control liver tumorigenesis in NEMO LPC-KO mice remain elusive, recent studies implicated IKK1 and the alternative NF-B pathway in oncogenesis (27)(28)(29)(30)(31). In the mammary gland, IKK1, p100/p52, and relB were shown to regulate cyclin D1 expression and epithelial cell proliferation, and mice lacking inducible IKK1 kinase activity showed delayed development of ErbB2-induced tumors, implicating alternative NF-B signaling in mammary epithelial hyperplasia and oncogenesis (27,29,31). In addition, IKK1 was recently shown to antagonize p53-mediated gene transcription by mediating phosphorylation of CBP and switching its preference toward NF-B-induced gene expression promoting cell growth (28).…”

Role of the progesterone receptor (PR) in the regulation of inflammatory response pathways and aromatase in the breast

“…Additionally, increased NF-κB p50 DNAbinding activity was found to be associated with poor clinical outcome in ER(+) breast tumors [47]. Furthermore, increased levels and DNA-binding activity of NF-κB proteins RelB and p52 were observed in mouse mammary tumors induced by treatment with 7,12-dimethylbenz (a) anthracene (DMBA) [48].…”