’We realized we had almost the entire pathway in our hands’’(7).
Introduction: mTOR mammalian target of rapamycin is a central controller of cell growth, metabolism and protein synthesis.The discovery story starts in 1972 when a group of scientists found Rapamycin magic pill, (also known as Sirolimus) by isolating bacterium Streptomyces hygroscopic species found on the soil sample of a remote island of Rapa Nui (2). They found out that it suppressed the immune system, a finding that led to intense, multifaceted research in the therapies for cancer field(4). Rapamycin discovery provided the stimulus for research on the complex and pivotal mTOR cell signaling pathway which controls a range of vital biological processes. The dissection of the labyrinthine molecular networks has led to improved understanding of the transcription, protein synthesis, and metabolic reprogramming processes that serve as a foundation for oncogenic transformation and therefore can be exploited for targeted therapies. Yeast saccharomyces cerevisiae was used by Joe Heitman and N. Rao Movva to study rapamycin action (4). Rapamycin killed most of the cells on this place, mutants that were resistant to rapamycin were selected. This mutants were defective in any of the three genes: TOR1, TOR2 and FKBP (4).The orthologue molecules to humans were therefore found in yeast first. (4)
Figure 1.Yeast and rapamycin experiment .Mutant strains were grown in YPD liquid medium overnight. This image is adapted from (4).
Figure 2.Experiment with mutant drosophila. This image is adapted from (4).
The role of TOR is to control cell growth therefore growth cell size and gaining mass in individual tissues. TOR mutant fly is smaller showing the importance of TOR in growth. mTOR is like the central processing unit of a computer which sits in the centre of the cell and makes the complicated decisions in response to different inputs(4). According to Lin, the beneficial effects seen from the ketogenic diet are potentially due to the inhibition of the nutrient sensor mTOR, which has shown to effect lifespan extension and health promotion. In addition to the ketogenic diet, Lin said, “ mTOR can also be inhibited by simple caloric restriction or the pharmaceutical rapamycin. This drug should be taken selectively in order to prolong longevity (3).” Animal data from Dog Aging project showed dog with low ejection fraction (percentage of the diastolic left ventricular blood volume that is ejected from the left during systole) had 10 percent improvement after rapamycin was given by Matt Kaeberlein (5)(6).
Image 3. Rapamycin chemical structure adapted from (12).
MECHANISMS OF mTOR Activation: It is known that not only types of cancers differ between people but also that cancer cells differ from each other within an individual (tumor heterogeneity) making the cancer therapies effectiveness difficult. mTOR signaling regulates amino acid, glucose, nucleotide, fatty acid and lipid metabolism. How signaling rewires cancer cell metabolism and how changes in metabolism, in turn, sustain mTOR signaling and tumorigenicity is the question that is most fascinating scientists. They are researching ways of defining signatures for cancer subtypes to classify patients for personalized treatment options by first understanding this complex pathways(1)(8).
Figure 4. mTOR signaling pathway shows: Amino acid nutrients promote the lysosomal location of mTORC1 by the use of RAS related GTP binding proteins(RAGs) by promoting the meeting of mTORC1 and RHEB molecules.RAGS form 4 possible obligate hetrodimers which in active state bind to mTOR complex 1 RAPTOR and bring mTORC1 in the surface of the lysosome where P13K-AKT signalling takes place.Leucine,argenine and glutamine are the most important and effective activators of mTORC1. Image adapted from (1).
Figure 5&6 Yoshinori Ohsumi representing autophagosome located inside the lysosome organelle .Sack-like membranes called autophagosomes capture the cellular junk and get transported to lysosomes and fuse .Lysosomes contain enzymes for digestion of the cellular content .Images adapted from(11).
Serine/threonine kinase mTOR is made up of two distinct complexes: mTOR complex 1 mTORC1 and mTORC2. mTOR complex 1 contains the regulatory protein of mTOR which is rapamycin sensitive (RAPTOR) while mTORC2 contains rapamycin insensitive companion of mTOR (RICTOR). mTOR is found at the interface of the lysosome membrane (1). Autophagy mechanisms discovered by Yoshinori Ohsumi (who won a Nobel Price in 2016) is a catabolic cellular process that degrades and recycles cellular junk in response to starvation.When cells are starved they can use up their own proteins for fuel,this process takes place also in cancer once it has spread.Life is therefore an equilibrium state between synthesis and degradation of proteins. Yoshinori Ohsumi found out the main genes and proteins which contribute in the formation of autophagosome membrane(7).
Cathepsins degrade the junk proteins into amino acids and introlysosomal nutrients will regulate mTOR.mTOR complex 1 regulates the balance between cell growth and death by activating protein S6 ribosome subunit kinase and inhibiting eIf4E necessary for polypeptide translation into ribosome binding protein in order to increase translation .mTOR2 does this mainly by AKT activation.In cancer cells defects in mTOR are followed by defective autophagy and allow prolonged survival (1).
Figure 7. Role of Autophagy on tumour growth .Oncogene activation promote tutor proliferation which is supported by defective apoptosis and sustained.Chronic necrosis and inflammation are stimulated in tumour regions which experience metabolic stress ,which in turn stimulates tumour growth(10).Cancer is activated by mutations in upstream regulators which include gain of function mutation of PI3K and loss of function of tumour suppressing PTEN.Anticancer strategy example : inhibition of mTORC1 has shown limited success(1).Image adapted from (10)
Conclusion: This studies show that scientist are only in the beginning of understanding the role of tumour cell metabolism pathways and our immune system.A research scientist Strong says “I think there are some gaps in our knowledge,” he says. “That’s why these things are, to us, paradoxical. When we finally figure it out, they won’t be so paradoxical anymore.”(9)
COPYRIGHT: This article is the property of We Speak Science, a non-profit institution co-founded by Dr. Detina Zalli and Dr. Argita Zalli (Imperial College London). The article is written by Kasandra Malasi, University of Leicester, UK.