The Regulation of Mammalian Target of Rapamycin

mTOR is a highly conserved Serine/Threonine protein kinase which couples to a variety of stimulatory signals to control both anabolic and catabolic cellular processes. It exists in two biochemically and functionally distinct multi-component complexes termed as mTORC1 and mTORC2. Increasing evidence indicates that both mTOR complexes play crucial roles in the regulation of pancreatic β-cell function and mass. For example, hormones like GLP-1 that augment intracellular cAMP ([cAMP]i) are able to activate mTORC1 in β-cells, and it has been shown that GLP-1 treatment can lead to an enhancement of β-cell proliferation. However, in other cell types such as hepatocytes, neurons and many cancer cell lines, increases in [cAMP]i lead to the inhibition of mTORC1 which coincides with an impairment of cell replication. Therefore, the aims of this thesis were to reveal how cAMP regulates mTOR and to elucidate the molecular mechanism upon which mTOR complexes control pancreatic β-cell mass. My data provides evidence that the PKB-mTORC1 pathway by GLP-1 positively regulates β-cell proliferation, and that prolonged rapamycin treatment results in an induction of β-cell death which is caused by the inhibition of mTORC2 rather than mTORC1. These results further elucidated the mechanism by which mTOR complexes control pancreatic β-cell mass and therefore contribute to the design of future treatment against diabetes mellitus. In addition, I have also shown that cAMP, in contrast to its effects in β-cells, inhibits both mTORC1 and 2 in mouse embryonic fibroblasts (MEFs) and human embryonic kidney (HEK293) cells by promoting complex dissociation and blocking mTOR kinase activity. Increases in [cAMP]i also inhibit protein synthesis and prevent cell cycle progression in MEFs. In conclusion, this work has further unravelled how mTOR complexes control pancreatic β-cell mass, and provided a rationale of using cAMP-inducing agents to treat cancer.