Skeletal muscles represent the largest tissue mass within the body and are primarily involved in the generation of force for voluntary movement. Skeletal muscles have a remarkable capacity to repair, due primarily to the actions of muscle stem cells (MuSCs). MuSCs are normally quiescent in adult skeletal muscle; however, in response to myotrauma (trauma to muscle tissue) from muscle injury or exercise, MuSCs become activated, either undergo self-renewal to replenish the quiescent population or commit to the myogenic lineage as myoblasts, proliferate, and differentiate into myotubes in vitro or fuse to existing myofibers in vivo. This process of generating new myofibers from quiescent MuSCs is termed myogenesis and a full understanding of how myogenesis is regulated remains to be understood. Mounting evidence suggests that amino acids, particularly the essential amino acid leucine, play a role in MuSC regulation. Leucine is specifically translocated and sensed by the L-type amino acid transporter 1 (LAT1); which facilitates leucine uptake in mature myofibers. Inside the cell, leucine activates mammalian or mechanistic target of rapamycin complex 1 (mTORC1) to stimulate cell growth, proliferation, and protein synthesis. Whether leucine has direct effects on myoblast function via LAT1 is unknown. Thus, our overall objective was to begin to characterize the role of LAT1 in myogenesis. Our results indicate that myoblasts differentially expressed LAT1 throughout myogenesis with peak protein content occurring during differentiation (p<0.05 vs. early proliferation). Further, our results indicate thatpharmacological LAT1 inhibition reduced myoblast expansion and differentiation in vitro (both p<0.05 vs. control). Interestingly, myoblast LAT1 protein content did not change in response to leucine supplementation in vitro; however, was lower under in vitro atrophic conditions (p<0.05 vs. control). Based on these findings, we conclude that LAT1 plays an important role in regulating myogenesis. As such, we uncover a novel role for LAT1 in regulating muscle mass via contributing to the control of MuSC function.

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Dans le but de maintenir leur métabolisme et leur prolifération exacerbée, les tumeurs sont dépendantes d’un apport accru en acides aminés. Afin d'optimiser cet apport, les tumeurs surexpriment certains transporteurs clés tels que l'hétérodimère multifonctionnel CD98/LAT1. CD98 (SLC3A2) agit comme co-récepteur des intégrines β et amplifie leur signalisation régulant ainsi la migration et l'adhésion cellulaire. La protéine LAT1 (SLC7A5) est quant à elle responsable du transport des acides aminés (AA) essentiels. Des études antérieures ont suggéré que la fonction CD98/intégrines du complexe est essentielle à la croissance tumorale, alors que LAT1 aurait un rôle mineur dans ce contexte. Cependant, les besoins nutritifs accrus des cellules tumorales nous ont conduit à émettre l’hypothèse contraire selon laquelle l’avantage prolifératif donné par ce complexe serait en réalité supporté par l’activité du transporteur LAT1 et non pas par l’interaction CD98/intégrine. Dans ce contexte, j’ai montré que l’invalidation génétique ou pharmacologique de LAT1 dans différentes lignées tumorales entraine une suppression totale du transport de la leucine, sodium-indépendant. Ceci entrainant une perte d’homéostasie des AA avec l’activation de la voie de stress GCN2, l’inhibition de mTORC1 et la suppression de la croissance tumorale. De plus, l’invalidation génétique de CD98 ne s’est traduite par aucun phénotype visible. Cependant, la suppression de l'activité résiduelle de LAT1 de ces cellules est suffisante pour abolir leur potentiel tumoral. Ainsi, mes résultats démontrent le rôle clé de LAT1 dans la croissance tumorale en faisant ainsi une cible thérapeutique prometteuse. / Tumours rely on external amino acids (AA) uptake to maintain their exacerbated metabolism and proliferation. To optimize AA uptake, tumors overexpress key carriers such as the multifunctional CD98/LAT 1 heterodimer. CD98 (SLC3A2) acts as a co-receptor of β integrins and enhances signaling that promotes cellular migration and invasion. LAT1 (SLC7A5) is responsible for the transport of essential AA. Previous studies have suggested that the CD98/integrin axis of the complex is essential for tumour growth, while LAT1 activity is dispensible. However, the increased nutritional requirements of tumor cells led us to hypothesize that the proliferative advantage given by this complex is in fact supported by the AA transporter activity of LAT1 and not by the CD98/integrin activity. In this context, I have shown that genetic or pharmacological invalidation of LAT1 in various tumor cell lines leads to a complete removal of the sodium-independent leucine transport. This leads to a loss of AA homeostasis with activation of the GCN2 stress pathway, inhibition of mTORC1 and supression of tumour growth. In addition, genetic invalidation of CD98 did not result in any detectable phenotype. However, inhibition of the residual activity of LAT1 in CD98 knockout cells is sufficient to abolish their tumorigenicity. Thus, my results clearly demonstrate the fundamental role of LAT1 in tumour growth and advocate the pharmacology development of LAT1 transporter inhibitors as very promising anticancer agents.