Tumor necrosis factor-α-induced apoptosis in prostate cancer cells through inhibition of nuclear factor-κB by an IκBα 'super-repressor'

Prostate cancer patients experiencing a relapse in disease often express high serum tumor necrosis factor-α (TNF-α) levels. Many androgen- insensitive prostate cancer cells are TNF-α insensitive because of the expression of antiapoptotic genes as part of the nuclear factor-κB (NF-κB) family of transcription factors. NF-κB stimulates gene transcription when expressed in the nucleus; however, in resting cells, this nuclear import is prevented by association with the cytoplasmic inhibitor IκBα. This cytoplasmic retention of NF-κB is uncoupled by many extracellular signals including low levels of TNF-α. During normal cell activation, nuclear translocation of NF-κB is preceded by phosphorylation and degradation of IκBα. When phosphorylation is blocked, IκBα remains intact, thereby blocking NF-κB translocation to the nucleus and subsequent activation of antiapoptotic genes that cause TNF-α insensitivity. We tested whether a 'super-repressor' of NF-κB activity could be transfected into prostate cancer cells and make them TNF-α sensitive. PC-3 and LNCaP cells were stimulated with TNF-α (10 ng/ml) for 24 h in the presence or absence of the IκBα 'super-repressor' (p6R-IκB(S32A + S36A). NF-κB activity was measured by electrophoretic mobility shift assay and the steady state levels of the cytoplasmic IκBα protein were measured by Western blot. Secretory IL-6 and IL-6 mRNA were measured by ELISA. p6R-IκB(S32A + S36A) blocked the stimulation of NF-κB activity by TNF-α in prostate cancer cells. It also subsequently decreased IL-6 production by TNF-α. We conclude that these data demonstrate that inhibition of NF-κB selectively sensitizes previously insensitive prostate cancer cells to TNF-α.