Differing effects of rapamycin and mTOR kinase inhibitors on protein synthesis

Abstract

mTOR (mammalian target of rapamycin) forms two distinct types of complex, mTORC (mTOR complex) 1 and 2. Rapamycin inhibits some of the functions of mTORC1, whereas newly developed mTOR kinase inhibitors interfere with the actions of both types of complex. We have explored the effects of rapamycin and mTOR kinase inhibitors on general protein synthesis and, using a new stable isotope-labelling method, the synthesis of specific proteins. In HeLa cells, rapamycin only had a modest effect on total protein synthesis, whereas mTOR kinase inhibitors decreased protein synthesis by approx. 30%. This does not seem to be due to the ability of mTOR kinase inhibitors to block the binding of eIFs (eukaryotic initiation factors) eIF4G and eIF4E. Analysis of the effects of the inhibitors on the synthesis of specific proteins showed a spectrum of behaviours. As expected, synthesis of proteins encoded by mRNAs that contain a 5'-TOP (5'-terminal oligopyrimidine tract) was impaired by rapamycin, but more strongly by mTOR kinase inhibition. Several proteins not known to be encoded by 5'-TOP mRNAs also showed similar behaviour. Synthesis of proteins encoded by 'non-TOP' mRNAs was less inhibited by mTOR kinase inhibitors and especially by rapamycin. The implications of our findings are discussed.

Rheb Inhibits Protein Synthesis by Activating the PERK-eIF2α Signaling Cascade

“…Whether Rheb mediated reduction of Erk signaling is a feedback loop to regulate protein synthesis remains unclear. Nevertheless, rapamycin which increases Erk signaling (Carracedo et al, 2008) actually inhibits rather than promotes, protein synthesis in certain cells (Huo et al, 2011). Thus the significance of crosstalk between Rheb and Erk signaling and what role they play in protein synthesis remains unclear.…”

Translation Regulation as a Therapeutic Target in Cancer

“…In an attempt to overcome resistance pathways, investigators have developed mTORC1/2 inhibitors (e.g., PP242, WYE-132, Torin1, AZD8055, OSI-027, and INK128) and inhibitors targeting mTORC1/2 and PI3K (e.g., PI-103, NVP-BEZ235, WJD008, and GSK2126458), and these agents have shown antitumor activity superior to that of rapalogs in various in vivo and in vitro cancer models, including lymphoma, leukemia, glioma, breast, lung, and renal carcinoma (110)(111)(112)(113)(114)(115)(116)(117)(118)(119). mTORC1/2 inhibitors (e.g., PP242) were more efficient in the inhibition of global protein synthesis and in eIF4F complex formation than rapamycin, illustrating the more-effective approach of targeting cap-dependent translation in cancer (120). It is worth mentioning that inhibition of PI3K/AKT/mTOR by PI-103 and rapamycin was more effective against malignant melanoma compared with a single-agent treatment, indicating that "vertical inhibition" is a promising strategy to improve therapeutic approaches (121).…”

mTOR inhibitors in cancer therapy

“…They are phosphorylated by mTORC1 and this induces their release from eIF4E, thereby alleviating such inhibition 7 . There are other links from mTORC1 to the activation of protein synthesis and to the production of ribosomes 9, 10 .…”