The mTOR complex (mTORC1) regulates cell growth and metabolism in response to multiple environmental signals. Nutrients signal via Rag proteins (GTPases) to promote the localization of mTORC1 to the lysosomal surface, its activation site. The mTOR1 complex is composed of more than a dozen a subunits and its complexity appears to increase over time. It has sensors for most aminoacids, especially the essential ones that trigger protein synthesis (such as branched-chain aminoacids and arginine or glutamine). But not all sensors for each aminoacid are unknown. Sestrin1 and Sestrin2 detect leucine, while CASTOR1 detects cytosolic arginine. In 2017, scientists identified SAMTOR, a previously uncharacterized protein, that inhibits mTORC1 signaling by interacting with GATOR1, the GTPase activating protein (GAP) for RagA/B.
The methyl donor S-adenosylmethionine (SAM) was found to disrupt the SAMTOR-GATOR1 complex by directly binding to SAMTOR. In cells, methionine deficiency reduces SAM levels below the dissociation constant and promotes the association of SAMTOR with GATOR1, thereby inhibiting mTORC1 signaling. Methionine-induced mTORC1 activation requires the SAM-binding ability of SAMTOR. Several properties of SAMTOR suggest that it works as a SAM sensor that signals cellular methionine sufficiency: (1) SAMTOR binds SAM with an affinity compatible with the drop in intracellular SAM concentrations caused by methionine starvation, (2) it is required for methionine starvation to inhibit mTORC1 signaling, and (3) SAMTOR mutants that do not bind SAM cannot signal methionine sufficiency to mTORC1.
Since SAM levels can be influenced by the availability of folate, betaine, and vitamin B12, SAMTOR may also link mTORC1 signaling to the availability of these metabolites. The Rag GTPase pathway senses and integrates the presence of multiple aminoacids upstream of mTORC1. Unlike Sestrins and CASTOR1, which bind to GATOR2, SAMTOR interacts with GATOR1-KICSTOR. Unlike leucine and arginine, which directly bind upstream sensors of mTORC1, methionine is sensed indirectly via SAM. Methionine is a central metabolite required for most methylation reactions, including those for DNA, histones and phospholipids, and recent data highlight its additional role as a signaling molecule. But how does methionine do all this? It does so indirectly: the cellular enzyme protein-arginine methyltransferase 1 (PRMT1) uses SAM as a cofactor for enzymatic regulation of mTORC1 signaling.
In conditions of sufficient methionine, in fact, elevated cytosolic SAM releases SAMTOR from GATOR1, which confers the association of PRMT1 with GATOR1. Subsequently, SAM-loaded PRMT1 methylates NPRL2, the catalytic subunit of GATOR1, thereby suppressing its GAP activity and leading to mTORC1 activation. In particular, genetic or pharmacological inhibition of PRMT1 prevents hepatic methionine sensing by mTORC1 and improves insulin sensitivity in aged mice, establishing the role of PRMT1-mediated methionine sensing at physiological levels. Thus, PRMT1 coordinates with SAMTOR to form the methionine sensing apparatus of mTORC1 signaling. Despite the complexity of this information for the general public, it has practical importance in the field of oncology. Virtually all tumors are dependent on methionine for survival.
Deprivation of tumor cells with this aminoacid can lead to programmed cell death (apoptosis). Both in vitro and in vivo studies have been conducted on the effects of bacterial methioninase (from Pseudomonas putida) on tumor cells, such as colon carcinoma cells (cultured HCT-11 cells). Methioninase causes a greater decrease in SAM in cancer cells than in normal cells, due to the dependence of cancer cells on methionine resulting in reduced SAM levels under methionine restriction, which can lead to inhibition of mTOR via SAMTOR and apoptotic cell death. Recombinant methioninase (rMETase), in combination with rapamycin, has previously been shown to synergistically eradicate a breast osteosarcoma transplanted into a patient-derived mouse model without toxicity.
This result suggests the possibility that mTOR via SAMTOR and SAM may have a very different effect in tumor cells due to acute SAM deficiency under methionine restriction, which can significantly inhibit mTOR protein kinase activity. This, in contrast to normal cells where methionine restriction does not cause acute SAM deficiency. In fact, in the study by Ardjmand et al. (2024), methionine deficiency via rMETase kills tumor cells, but does nothing to normal fibroblasts. Likewise, the synergistic effect of rMETase + rapamycin appears only in the former but not in the latter. A second ongoing study by the same research group also specified that the administration of rapamycin + methioninase must be simultaneous and not sequential: in the second case the inhibition on tumor cells in vitro is 40%, while in the case of simultaneous administration the percentage rises to 72%.
This opens the possibility of a better application of an oncological biotherapy that can be free from all the known side effects of conventional chemotherapy.
- edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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