Scientists at Sinai Health’s Lunenfeld-Tanenbaum Research Institute (LTRI) and the University of Toronto have identified a new mechanism used by cells during the acquisition of key nutrients, revealing potential vulnerabilities in cancer cells that may lead to the development of targeted therapeutics. The findings were published in the journal Science on October 15th.
To sustain life, human cells require a stable supply of nutrients. Amino acids, the building blocks needed for making proteins, are among the most critical. With proteins making up over half the “dry” mass of cells, they must constantly work to maintain sufficient amino acid levels to enable proper growth and functionality.
While cells normally import amino acids directly from their surrounding milieu, another way they may acquire amino acids in certain situations is through a form of “cellular drinking”, a process termed macropinocytosis. In macropinocytosis, cells take up proteins from their surrounding environment and break them down within the lysosome, a digestive compartment in the cell. How exactly amino acids salvaged from proteins inside lysosomes stimulates cell growth has remained a mystery in the field.
The study was conducted in the laboratory of Dr. Anne-Claude Gingras, a senior scientist at the LTRI and professor in the Department of Molecular Genetics at the University of Toronto. First author Dr. Geoffrey G. Hesketh, a postdoctoral fellow in the Gingras laboratory, focused on the major cellular amino acid sensor known as mechanistic target of rapamycin complex 1 (mTORC1). mTORC1 controls the cellular machineries that make more proteins, and does so only when sufficient nutrients, including amino acids, are present. “We already knew how mTORC1 becomes activated by free amino acids, and this involves several proteins that work in concert,” said Hesketh, “but whether the same proteins also help activate mTORC1 when the macropinocytosis-lysosome pathway is used instead was not known”.
The Gingras lab develops approaches to detect the localization of proteins within different cellular compartments using instruments known as mass spectrometers. Dr. Hesketh applied these approaches to provide a molecular snapshot of all the proteins that reside on the surface of lysosomes inside living cells. This revealed an unsuspected connection between the mTORC1 components and a protein complex already known to be important for lysosome function.
By coupling these experiments with CRISPR gene editing, researchers were able to tease out the alternative mechanism of mTORC1 activation by lysosome-derived amino acids compared to those that are acquired in free form by the cell. “We discovered that free amino acids signal via a different pathway than those coming from inside the lysosome within the cell,” says Dr. Jim Dennis, a study co-author and senior investigator at the LTRI. “Importantly, we have revealed how this new regulatory circuit works as a switch between free and salvaged amino acids in the regulation of mTORC1”.
While these results provided researchers with a better understanding of how nutrients control cell growth, this study also has critical implications for identifying potential cancer vulnerabilities, as certain forms of cancer (such as pancreatic cancer) rely on macropinocytosis to sustain their rapid growth.
“The ability to use whole proteins as a source of amino acids is thought to allow pancreatic cancer cells to grow and survive deep inside tumours that don’t receive a normal supply of nutrients from the blood,” says Hesketh.
“If we can understand the molecular detail of how this alternative nutrient acquisition pathway works inside cancer cells, then we might be able to identify new drugs that could target this pathway – effectively shutting down their food supply and starving the cancer cells to death”.
Future avenues of research will be aimed at developing a more complete understanding of how this alternative mTORC1 activation pathway actually supports cancer cell survival. This may reveal to researchers which parts of this pathway may be good targets for cancer therapeutic development.
This work was made possible by operating funds from the Canadian Cancer Society Research Institute and a Canadian Institutes of Health Research Foundation Grant, as well as funding to the LTRI Network Biology Collaborative Centre through Genome Canada, Ontario Genomics and Canada Foundation for Innovation. Funding of Dr. Hesketh’s salary was partially provided through Parkinson Canada and a gift from the Glowinsky-Sandler family.