Investigation of the effects of different mTOR inhibitors on protein synthesis

Abstract

The mammalian target of rapamycin (mTOR), which controls diverse cellular processes, is regulated by the integration of many signals. Rapamycin strongly inhibits the proliferation of many cancer cell lines and there is a high level of interest in its potential use as an anti-cancer agent. However, some tumours and cancer cells are resistant to rapamycin. This has prompted the development of mTOR kinase inhibitors (mTOR-KIs), such as PP242 and AZD8055, which compete with ATP for binding to the kinase domain in mTOR. In this research, I have studied whether the effects of mTOR-KIs on cell signalling and protein synthesis differed in comparison to those of rapamycin. My data shows that mTOR-KIs have strikingly different effects on proteins (including formation of the eIF4F translation factor complex) that control mRNA translation. Furthermore, while rapamycin only has a very small inhibitory effect on the rate of protein synthesis, mTOR-KIs have a much bigger effect. A new mass spectrometric approach, ?pSILAC‘, was applied to explore the effects of rapamycin and mTOR-KIs on the synthesis of specific proteins. The data from pSILAC reveal (i) mTOR-KIs impair synthesis of many proteins; (ii) rapamycin always inhibits less than mTOR-KIs; (iii) their effects are strongest for proteins encoded by 5‘-TOP mRNAs, but mTOR-KIs again inhibit more strongly; (iv) synthesis of some other proteins which are not encoded by known 5‘-TOP mRNAs shows a similar pattern of inhibition to 5‘-TOP mRNAs. These data show that pSILAC is a valuable tool for studying the control of the synthesis of specific proteins. I have also investigated the effects of disruption of eukaryotic translation initiation factor 4E (eIF4E) phosphorylation on (i) its modification by SUMO-1 (ii) TNF? biosynthesis in macrophages and (iii) the interaction with specific mRNAs encoding protumourigenic factors.

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