The PI3K/Akt and mTOR signaling pathways are important for cell survival and growth, and they are highly activated in
cancer cells compared with normal cells. Therefore, these signaling pathways are targets for inducing
cancer cell death. The dual PI3K/Akt and mTOR inhibitor
NVP-BEZ235 completely inhibited both signaling pathways. However,
NVP-BEZ235 had no effect on cell death in human
renal carcinoma Caki cells. We tested whether combined treatment with natural compounds and
NVP-BEZ235 could induce cell death. Among several chemopreventive agents,
curcumin, a natural biologically active compound that is extracted from the rhizomes of Curcuma species, markedly induced apoptosis in NVP-BEZ235-treated cells. Co-treatment with
curcumin and
NVP-BEZ235 led to the down-regulation of Mcl-1
protein expression but not
mRNA expression. Ectopic expression of Mcl-1 completely inhibited
curcumin plus NVP-NEZ235-induced apoptosis. Furthermore, the down-regulation of Bcl-2 was involved in
curcumin plus NVP-BEZ235-induced apoptosis.
Curcumin or
NVP-BEZ235 alone did not change Bcl-2
mRNA or
protein expression, but co-treatment reduced Bcl-2
mRNA and
protein expression. Combined treatment with
NVP-BEZ235 and
curcumin reduced Bcl-2 expression in wild-type p53 HCT116 human colon
carcinoma cells but not p53-null HCT116 cells. Moreover, Bcl-2 expression was completely reversed by treatment with
pifithrin-α, a p53-specific inhibitor. Ectopic expression of Bcl-2 also inhibited apoptosis in NVP-BE235 plus
curcumin-treated cells. In contrast,
NVP-BEZ235 combined with
curcumin did not have a synergistic effect on normal human skin fibroblasts and normal human mesangial cells. Taken together, combined treatment with
NVP-BEZ235 and
curcumin induces apoptosis through p53-dependent Bcl-2
mRNA down-regulation at the transcriptional level and Mcl-1
protein down-regulation at the post-transcriptional level.