20.12: mTOR Signaling and Cancer Progression
The mammalian target of rapamycin or mTOR protein was discovered in 1994 due to its direct interaction with rapamycin. The protein gets its name from a yeast homolog called TOR. The mTOR protein complex in mammalian cells plays a major role in balancing anabolic processes such as the synthesis of proteins, lipids, and nucleotides and catabolic processes, such as autophagy in response to environmental cues, such as availability of nutrients and growth factors.
The mTOR pathway or the PI3K/AKT/mTOR signaling pathway begins with the growth-factor-induced phosphorylation of a specific cell-surface receptor. The phosphorylated receptor transmits signals that result in the activation of downstream protein kinases— PI3K, Akt, and mTOR complex 1 or mTORC1.
Upon activation, mTORC1 upregulates protein synthesis mainly through the phosphorylation of the eukaryotic initiation factor 4E-binding protein 1 or 4EBP1 and the p70 ribosomal S6 kinase 1 or S6K1. Via activation of S6K1, mTORC1 also regulates the activity of transcription factor - sterol responsive element binding protein or SREBP, which further regulates the synthesis of lipids in a growing cell. In addition, S6K1 is also involved in the activation of carbamoyl-phosphate synthetase (CAD), which plays a critical role in the de novo pyrimidine synthesis pathway.
Besides synthesis of the macromolecules, mTORC1 has also been shown to regulate mitochondrial metabolism and biosynthesis. It facilitates tumor cell growth by shifting glucose metabolism to glycolysis instead of oxidative phosphorylation, a process called the Warburg effect, to help the tumor cells generate the energy needed for their rapid growth and proliferation.
In the absence of essential nutrients, cells activate autophagy to supply basic components such as amino acids to the cellular machinery. Inhibition of mTORC1 has been shown to increase autophagy, whereas stimulation of mTORC1 reduces autophagy.
Due to its varied roles in cellular functions, deregulation in mTOR signaling has not only been implicated in the progression of cancer but also several other diseases, including, aging and diabetes.