Negative regulation of transactivation function but not DNA binding of NF-KB and AP-1 by IκBβ1 in breast cancer cells

The transcription factor NF-kappa B regulates the expression of genes involved in cancer cell invasion, metastasis, angiogenesis, and resistance to chemotherapy, In normal cells NF-kappa B is maintained in the cytoplasm by protein-protein interaction with inhibitor I kappa Bs, In contrast, in cancer cells a substantial amount of NF-kappa B is in the nucleus and constitutively activates target genes, To understand the mechanisms of constitutive NF-kappa B activation, we have analyzed the function of I kappa B alpha and I kappa B beta in breast cancer cells, In most cases, constitutive NF-kappa B DNA binding correlated with reduced levels of either I kappa B alpha Or I kappa B beta isoforms, Overexpression of I kappa B alpha but not I kappa B beta 1 resulted in reduced constitutive DNA binding of NF-kappa B in MDA-MB-231 cells. Unexpectedly, I kappa B beta 1 overexpression moderately increased 12-O-tetradecanoylphorbol-13-acetate and interleukin-1-inducible NF-kappa B DNA binding, 12-O-Tetradecanoylphorbol-13-acetate and interleukin-1-induced transactivation by NF-kappa B, however, was lower in I kappa B beta 1-overexpressing cells. Mutants of I kappa B beta 1 lacking the C-terminal casein kinase II phosphorylation sites, which form a stable complex with DNA bound NF-kappa B without inhibiting its transactivation in other cell types, repressed the transactivation by NF-kappa B in MDA-MB-231 cells, Consistent with the results of transient transfections, the expression of urokinase plasminogen activator, an NF-kappa B target gene, was reduced in I kappa B beta 1-overexpressing cells. These results suggest that depending on the cell type, I kappa B beta 1 represses the expression of NF-kappa B-regulated genes by inhibiting either DNA binding or transactivation function of NF-kappa B.