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Constitutive nuclear factor (NF)-kappaB activation is thought to be involved in survival, invasion, and metastasis in various types of cancers. However, neither the subtypes of breast cancer cells with constitutive NF-kappaB activation nor the molecular mechanisms leading to its constitutive activation have been clearly defined. Here, we quantitatively analyzed basal NF-kappaB activity in 35 human breast cancer cell lines and found that most of the cell lines with high constitutive NF-kappaB activation were categorized in the estrogen receptor negative, progesterone receptor negative, ERBB2 negative basal-like subtype, which is the most malignant form of breast cancer. Inhibition of constitutive NF-kappaB activation by expression of IkappaBalpha super-repressor reduced proliferation of the basal-like subtype cell lines. Expression levels of mRNA encoding NF-kappaB-inducing kinase (NIK) were elevated in several breast cancer cell lines, and RNA interference-mediated knockdown of NIK reduced NF-kappaB activation in a subset of the basal-like subtype cell lines with upregulated NIK expression. Taken together, these results suggest that constitutive NF-kappaB activation, partially dependent on NIK, is preferentially involved in proliferation of basal-like subtype breast cancer cells and may be a useful therapeutic target for this subtype of cancer.

Nuclear factor-kappaBeta (NF-kappaB) binds specifically to NF-kappaB-binding sites (kappaB sites, 5'-GGGRNNYYCC-3'; R, purine; Y, pyrimidine; N, any nucleotide) present in enhancer regions of various genes. Binding of various cytokines, growth factors and pathogen-associated molecular patterns to specific receptors activates NF-kappaB and expression of genes that play critical roles in inflammation, innate and acquired immunity, bone remodeling and generation of skin appendices. Activation of NF-kappaB is also involved in cancer development and progression. NF-kappaB is activated in cells that become malignant tumors and in cells that are recruited to and constitute the tumor microenvironment. In the latter scenario, the TLR-TRAF6-NF-kB pathways seem to play major roles, and NF-kappaB activation results in production of cytokines, which in turn induce NF-kappaB activation in premalignant cells, leading to expression of genes involved abnormal growth and malignancy. Furthermore, NF-kappaB activation is involved in bone metastasis. Osteoclasts, whose generation requires the RANK-TRAF6-NF-kappaB pathways, release various growth factors stored in bone, which results in creation of microenvironment suitable for proliferation and colonization of cancer cells. Therefore, NF-kappaB and molecules involved its activation, such as TRAF6, are attractive targets for therapeutic strategies against cancer.