Overexpression of Akt or even the constitutively activated myr-Akt enhanced the basal level of phosphorylated Akt, mTOR and downstream molecules. However, curcumin nonetheless proficiently inhibited mTOR and downstream signaling, although to a significantly less extent which is probably attributable to the greater basal phosphorylation level . These benefits, specifically curcumin inhibited Akt downstream signaling while the phosphorylation of myr-Akt was not inhibited in any way, strongly propose the existence of inhibitory mechanism and that is independent of inhibition of Akt. Coincidentally, AMPK was activated by curcumin in the time program comparable for the inhibition of Akt phosphorylation . Overexpression of AMPK in PC-3 cells somewhat potentiated the inhibition of mTOR signaling by curcumin, but neither pharmacological inhibitor nor dominant unfavorable overexpression showed sizeable restoration of curcuminmediated inhibition .
Though curcumin-activated AMPK is not really the most important motive for curcumin-mediated inhibition of Akt/mTOR signaling, how curcumin activates AMPK and its physiological rtk inhibitors significance deserve even more investigation in the future. TSC1/TSC2 complicated inhibits mTOR action by activating the GTPase activity of Rheb, and the two Akt and AMPK converged at TSC1/TSC2 to manage mTOR activity . Steady together with the incompetence of constitutive activation of Akt or inhibition of AMPK to rescue mTOR signaling, disruption of the function of TSC1/TSC2 complicated only marginally rescued curcumin-mediated inhibition . Knockout of TSC1 in MEFs led to hyperphosphorylation of mTOR, 4E-BP1, p70 S6K, and S6; nevertheless, curcumin properly inhibited the phosphorylation using a very similar concentration-dependency to that in wild sort MEFs .
It can be notable that curcumin correctly inhibited mTOR signaling while in the noncancerous MEFs, however to a much less extent than in PC-3 cells, suggesting curcumin-mediated inhibition of Akt/mTOR signaling is independent on PTEN standing. Likewise, knockdown of TSC2 in PC-3 cells by siRNA mildly increased the basal phosphorylation degree of Bleomycin mTOR and 4E-BP1, however the phosphorylation could even now be inhibited by curcumin . Multiple feed back loops exist in the regulation of Akt/mTOR signaling. Importantly, p70 S6K phosphorylates and inhibits IRS-1, resulting in a detrimental feed back to Akt/mTOR signaling . By this mechanism, inhibition of mTOR signaling typically leads to activation of Akt and tumor cells could obtain resistance to mTOR inhibitors .
Having said that, in PC-3 cells curcumin inhibited the two Akt and mTOR similarly . On top of that, the inhibition of Akt phosphorylation at Thr308 occurred considerably earlier than the inhibition of phosphorylation of Akt at Ser473, mTOR and also other downstream parts . Based upon these observations, it really is unlikely that curcumin inhibited Akt/mTOR axis by immediately inhibiting mTOR.