The cyclooxygenase-2 inhibitor celecoxib induces apoptosis by blocking Aktactivation in human prostate cancer cells independently of Bcl-2

This study investigates the apoptotic activity of the cyclooxygenase-2 (COX -2) inhibitor celecoxib in prostate carcinoma cells. COX-2 is constitutivel y expressed in androgen-responsive LNCaP and androgen-nonresponsive PC-3 ce lls. Exposure of these cells to celecoxib induces characteristic features o f apoptosis, including morphological changes, DNA laddering, and caspase-3 activation, whereas piroxicam, a COX-1-specific inhibitor, displays no appr eciable effect on either cancer cell line even after prolonged exposure. Mo reover, the potency of celecoxib in apoptosis induction is significantly hi gher than that of other COX-2 inhibitors examined despite the observation t hat these inhibitors exhibit similar IC50 in COX-2 inhibition. It is notewo rthy that normal human prostate epithelial cells, expressing a marginally d etectable level of COX-2, are insensitive to the induction of apoptosis by celecoxib, These data suggest a correlation between COX-2 expression and se nsitivity to the apoptotic effect of the COX-2 inhibitor. In an effort to d elineate the underlying mechanism, we examined the effect of celecoxib on t he expression of Bcl-2 as well as the activation of the key anti-apoptotic kinase Akt. In contrast to an earlier report that attributed the apoptotic activity of NS398 in LNCaP cells to Bcl-2 down-regulation, we provide evide nce that the induction of apoptosis by celecoxib in LNCaP and PC-3 cells is independent of Bcl-2. First, treatment with celecoxib does not alter the c ellular Bcl-2 level in both cell lines. Second, enforced Bcl-2 expression i n PC-3 cells does not confer protection against the induction of apoptosis by celecoxib. Our data show that celecoxib treatment blocks the phosphoryla tion of Akt, This correlation is supported by studies showing that overexpr ession of constitutively active Akt protects PC-3 cells from celecoxib-indu ced apoptosis. Nevertheless, how celecoxib down-regulates Akt is not clear because the drug does not adversely affect phosphoinositide 3-kinase activi ty in vivo and okadaic acid, a protein phosphatase 2A inhibitor, cannot res cue the inhibition, In summary, our data demonstrate that inhibition of Akt activation may play a crucial role in the induction of apoptosis by celeco xib.