During the existing research we firstly demonstrated that curcumin also inhibited the phosphorylation of Akt substrates GSK3, FKHR1, TSC2, mTOR too as mTOR downstream targets 4E-BP1, eIF4G, p70 S6K and S6 in a comparable concentration-dependent method as with Akt . In support on the function of Akt/mTOR signaling from the handle of protein synthesis, curcumin inhibited protein synthesis and then DNA synthesis in PC-3 cells , and these inhibitions may be partially but significantly rescued by overexpression of Akt or by restoration of Akt/mTOR signaling by calyculin A . Cyclin D1, and that is important for cell proliferation, has been reported to become regulated by Akt/mTOR posttranscriptionally . In PC-3 cells the expression of cyclin D1 was also inhibited by curcumin and might be restored by overexpression of Akt or by calyculin A . These results are steady together with the critical roles of Akt/mTOR signaling in cell survival and proliferation.
Curcumin continues to be reported to inhibit Akt/mTOR signaling in other cancer small molecule library screening cells , however the underlying mechanism stays unknown. A single serious objective of this study is to delineate the molecular mechanism by which curcumin inhibits Akt/mTOR signaling. First of all we examined the result of curcumin over the p85 subunit of PI3K. The phosphorylation of p85 in PC-3 cells is barely detectable and was not impacted by curcumin treatment method . LY294002, a specific PI3K inhibitor, inhibited the phosphorylation of Akt and mTOR, and this inhibition could possibly be restored by addition of exogenous PIP3. In contrast, exogenous PIP3 failed to restore curcumin-mediated inhibition . In addition, it has been effectively documented that in many cancer cells which include PC-3 cells, the activation of Akt/mTOR signaling axis is significantly less dependent on upstream signals thanks to reduction of PTEN function .
In fact, as reported by other people and confirmed in our lab, curcumin also inhibited Akt/mTOR signaling and proliferation in DU145 prostate cancer travoprost cells which carry wt PTEN. Taken collectively, these evidences recommend that curcumin inhibits Akt/mTOR signaling at downstream of PI3K. As proven in inhibitors 1D, the phosphorylation of Akt at Thr308 was the first for being inhibited. This led on the hypothesis that curcumin could right inhibit PDK1-mediated phosphorylation of Akt and led to your inhibition of downstream signaling. Phosphorylation of PDK1 at Ser241 is critical for its exercise, although could not be the most important regulatory factor . Nonetheless, curcumin didn’t inhibit the phosphorylation of PDK1 S241 .
Moreover, curcumin failed to inhibit the kinase exercise of PDK1 to Akt both in vitro and in vivo , suggesting that PDK1 will not be the direct target of curcumin. Similar observations are already reported that Akt/mTOR signaling will be inhibited independent of PI3K/PDK1 . Following we examined the role of Akt in curcumin-mediated inhibition.