Mechanisms of uncoupling cellular growth from mTOR pathway activity

Cellular growth and timing of division is coordinated and regulated through specific signalling pathways, growth factors, availability of nutrients and energy status. This is paramount for embryonic development, stem cell maintenance, tissue morphogenesis and homeostasis. All eukaryotic organisms (their cells have a nucleus) contain a kinase known as the mechanistic Target of Rapamycin (mTOR) that senses multiple factors such as nutrients and growth factors availability. In the presence of the above, mTOR promotes basic cellular processes such as protein translation and basic metabolism of lipids and carbohydrates and instructs cells to grow in mass and numbers. Inhibition of mTOR through genetic or pharmacological means has a profound negative effect on cell growth. Mutated and overactivated mTOR kinase forms are implicated in many cancers. Numerous clinical trials are currently ongoing targeting the kinase. However, cells (including cancer cells) can rewire their metabolism and resume growth in states where mTOR is inhibited. This shows that there are mechanisms of bypassing the requirement of mTOR for growth and essentially uncoupling nutrient and growth factor availability from cell division. Nevertheless, these mechanisms have remained elusive. We have identified 102 genes, conserved between fission yeast and humans, that when mutated, cells can bypass mTOR inhibition and continue growing and dividing. These genes point to specific processes within eukaryotic cells. Our aim is to comprehensively analyse the roles and connections of all these genes using cutting edge genetic, molecular biology and computational approaches. Our aim is to form a complete genetic connectivity roadmap that will reveal the molecular mechanisms that are involved in inhibition of mTOR resistance. Our results will directly point towards possible vulnerabilities of resistant cells that can be further exploited in cancer biology and beyond.

The mechanistic target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that acts as a rheostat of energy in all eukaryotes. mTOR senses a variety of inputs such as nutrients, growth factors, oxygen, and stress and, accordingly, regulates core decisions and cellular processes including protein synthesis, growth, metabolism, ageing, and autophagy. Activation of mTOR promotes tumour growth and metastasis. While some of the mTOR inhibitors have been developed and approved to treat human cancer, more mTOR inhibitors are being evaluated in clinical trials. Due to the fundamental roles of mTOR in growth, cell cycle progression and anabolism within all eukaryotes, resistance to mTOR inhibition itself is a major problem, partly attributed to the large heterogeneity of cancer cells. Over-activating mTOR kinase mutations as well as Myc- and PI3K-dependent mTOR resistance have been reported. However, a comprehensive genetic roadmap and involved mechanisms that result in bypassing the requirement for mTOR activity in cellular growth have remained poorly defined. Our preliminary data strongly show that cells are able to uncouple cellular growth to nutrient availability and mTOR activity through diverse pathways. Through the proposed work, we will provide a global view of conserved genetic factors that modulate resistance to mTOR inhibition. We will reveal connections and define underlying mechanisms involved in resistance. We will utilise the relevant and established for cell cycle, growth and ageing studies, fission yeast (Schizosaccharomyces pombe), making use of its vast genetic and genomic resources. In addition, our findings will be extended and validated to established, in our lab, human tissue systems. Our results will lead in a better understanding of nutrient- and growth factor-dependent cellular growth and will highlight the key candidates for targeting resistant cancer cells.