Her-2/neu overexpression induces NF-kappa B via a PI3-kinase/Akt pathway involving calpain-mediated degradation of I kappa-B-alpha that can be inhibited by the tumor suppressor PTEN

The Nuclear Factor (NF)-kappaB family of transcription factors controls exp ression of genes which promote cell growth, survival, and neoplastic transf ormation, Recently we demonstrated aberrant constitutive activation of NF-k appaB in primary human and rat breast cancer specimens and in cell lines. O verexpression of the epidermal growth factor receptor (EGFR) family member Her-2/neu, seen in approximately 30% of breast cancers, is associated with poor prognosis. Previously, Her-2/neu has been shown to signal via a phosph atidylinositol 3 (PI3)-kinase to Akt/protein kinase B (PKB) pathway. Since this signaling pathway was recently shown to activate NF-kappaB, here we ha ve tested the hypothesis that Her-2/neu can activate TF-kappaB in breast ca ncer, Overexpression of Her-2/neu and EGFR-4 in Ba/F3 cells led to constitu tive PI3- and Akt kinase activities, and induction of classical NF-kappaB ( p50/p65). Similarly, a tumor cell line and tumors derived from MMTV-Her-2/n eu transgenic mice displayed elevated levels of classical NF-kappaB, Engage ment of Her-2/neu receptor downregulated the level of NF-kappaB. NF-kappaB binding and activity in the cultured cells was reduced upon inhibition of t he PI3- to Akt kinase signaling pathway via ectopic expression of kinase in active mutants, incubation with wortmannin, or expression of the tumor supp ressor phosphatase PTEN, Inhibitors of calpain, but not the proteasome, blo cked I kappaB-alpha degradation. Inhibition of Akt did not affect IKK activ ity, These results indicate that Her-2/neu activates NF-kappaB via a P13- t o Akt kinase signaling pathway that can be inhibited via the tumor suppress or PTEN, and is mediated by calpain rather than the I kappaB kinase complex .