Abstract 2798: Cytotoxic and cystostatic properties of rapamycin are due to differential effects on the phosphorylation of mTORC1 substrates S6 kinase and 4E-BP1

Abstract

Abstract The mammalian target of rapamycin (mTOR) has been widely implicated in signals that promote cell cycle progression and survival in cancer cells. Rapamycin, which inhibits mTOR with high specificity, has consequently attracted much attention as an anti-cancer therapy. Used clinically at nano-molar concentrations, rapamycin suppresses phosphorylation of mTOR complex 1 (mTORC1) substrate S6 kinase and has been considered a cytostatic drug. However, we have reported that higher (micro-molar) doses are required for complete cell cycle arrest. Such high doses suppress phosphorylation of another mTORC1 substrate, eukaryotic initiation factor 4E (eIF4E) binding protein-1 (4E-BP1). In this state, eIF4E is sequestered and unable to facilitate mRNA translation. Dual ablation of S6 kinase and eIF4e resulted in a larger increase in G1 content than by ablation of S6 kinase alone. This confirmed the complete cytostatic effect of high-dose rapamycin. We also demonstrated that the differential dose effects of rapamycin correlate with partial and complete dissociation of Raptor from mTORC1 at low and high doses, respectively. Therefore, high-dose rapamycin ensures complete non-functionality of mTORC1. Interestingly, in the absence of serum, high dose rapamycin induces apoptosis and dual inhibition of S6 kinase and eIF4e does not prevent it. We have established that the differential cytostatic and cytotoxic effects of rapamycin are due to intact TGFb signaling. In serum, TGFb is activated upon low dose rapamycin (suppression of of S6 kinase phosphorylation). The lack of TGFb in the absence of serum allows cells to progress through the cell cycle; at high rapamycin doses, the inactivity of eIF4e results in cell death. This study reveals the mechanism for both cytostatic and cytotoxic properties of rapmaycin. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2798. doi:1538-7445.AM2012-2798