Elevated Akt activity protects the prostate cancer cell line LNCaP from TRAIL-induced apoptosis

We find that the prostate cancer cell lines ALVA-31, PC-3, and DU 145 are h ighly sensitive to apoptosis induced by TRAIL ((t) under bar umor-necrosis factor-(r) under bar elated (a) under bar poptosis-(i) under bar nducing (l ) under bar igand), while the cell lines TSU-Prl and JCA-1 are moderately s ensitive, and the LNCaP cell line is resistant. LNCaP cells lack active lip id phosphatase PTEN, a negative regulator of the phosphatidylinositol (PI) 3-kinase/Akt pathway, and demonstrate a high constitutive Akt activity. Inh ibition of PI 3-kinase using wortmannin and LY-294002 suppressed constituti ve Akt activity and sensitized LNCaP cells to TRAIL, Treatment of LNCaP cel ls with TRAIL alone induced cleavage of the caspase 8 and XIAP proteins. Ho wever, processing of BID, mitochondrial release of cytochrome c, activation of caspases 7 and 9, and apoptosis did not occur unless TRAIL was combined with either wortmannin, LY-294002, or cycloheximide, Blocking cytochrome c release by Bcl-2 overexpression rendered LNCaP cells resistant to TRAIL pl us wortmannin treatment but did not affect caspase 8 or BID processing. Thi s indicates that in these cells mitochondria are required for the propagati on rather than the initiation of the apoptotic cascade, Infection of LNCaP cells with an adenovirus expressing a constitutively active Akt reversed th e ability of wortmannin to potentiate TRAIL-induced BID cleavage. Thus, the PI 5-kinase-dependent blockage of TRAIL-induced apoptosis in LNCaP cells a ppears to be mediated by Akt through the inhibition of BID cleavage.