Our previous studies have shown that dietary pigment curcumin [ 1,7- bis( 4- hydroxy- 3- methoxyphenyl)- 1 - 6- heptadine3,5- dione; C21H20O6] sensitizes human prostate cancer cells to tumor necrosis factor- related apoptosis- inducing ligand ( TRAIL/Apo2L)- induced apoptosis by inhibiting nuclear factor ( NF)-kappa B. In the present study, we demonstrate that activated ( phosphorylated) Akt kinase plays a pivotal role in regulation of NF-kappa B and sensitization of LNCaP and PC3 prostate cancer cells to TRAIL by curcumin. Curcumin inhibited the expression of phospho-Akt ( p-Akt), which was not due to activation of phosphatase and tensin homolog deleted on chromosome 10 phosphatase activity by curcumin. Because NF-kappa B is a downstream target of Akt, we investigated whether inhibition of NF-kappa B by curcumin is mediated through suppression of p-Akt. Data demonstrate that treatment of PC3 cells with SH- 6 ( J Am Chem Soc 125: 1144 - 1145, 2003), a specific inhibitor of Akt, or transfection with small inhibitory RNA ( siRNA)- Akt not only inhibited p-Akt but also abrogated the expression and transcriptional activity of NF-kappa B. Furthermore, overexpression of constitutively active Akt1 in cancer cells prevented the inhibition of NF-kappa B by curcumin. In addition, treatment with SH- 6 or transfection with siRNA- Akt sensitized PC3 cells to TRAIL-induced cytotoxicity. On the other hand, SH- 6 does not inhibit NF-kappa B or sensitize DU145 cancer cells to TRAIL because these cells do not express p-Akt. Because expression of antiapoptotic Bcl- 2, Bcl-xL, and X- chromosome- linked inhibitor of apoptosis protein ( XIAP) is regulated by NF-kappa B, both curcumin and SH- 6 decreased the levels of these proteins in PC3 cells through inhibition of NF-kappa B. Furthermore, gene silencing of Bcl- 2 with siRNA- Bcl- 2 sensitized PC3 cells to TRAIL. Collectively, these data define a pathway whereby curcumin sensitizes prostate cancer cells to TRAIL by inhibiting Akt- regulated NF-kappa B and NF-kappa B-dependent antiapoptotic Bcl- 2, Bcl- xL, and XIAP.
