Constitutive activation of NF-κB in Ki-ras-transformed prostate epithelial cells

Abstract

The signaling pathway responsible for the activation of nuclear factor-κB (NF-κB) by oncogenic forms of Ras remains unclear. Both, the transactivation and DNA binding activities of NF-κB, were increased in 267B1 human prostate epithelial cells transformed by viral Kirsten-ras (267B1/Ki-ras cells) compared with those in the parental cells. This increased NF-κB activity was attributed to a heterodimeric complex of p50 and p65 subunits. Although the abundance of the inhibitor protein IκBβ was higher in 267B1/Ki-ras cells than in 267B1 cells, an electrophoretic mobility-shift assay suggested that IκBα is responsible for the activation of NF-κB in the former cells. Consistent with this notion, the phosphorylation of IκBα appeared increased in 267B1/Ki-ras cells, and the proteasome inhibitor I abolished the constitutive activation of NF-κB in these cells. The expression of dominant negative mutants of either NIK (NF-κB-inducing kinase) or IKKβ (IκB kinase β) inhibited the activity of NF-κB in 267B1/Ki-ras cells. Furthermore, chemical inhibitors specific for Ras activation, sulindac sulfide and farnesytranferase inhibitor I, markedly reduced IκBα phosphorylation and NF-κB activation in the Ki-ras-transformed cells while transfection of these cells with NIK or IKKβ counteracted the inhibitory effect on NF-κB activation. These results suggest that oncogenic Ki-Ras induces transactivation of NF-κB through the NIK-IKKβ-IκBα pathway.