The role of glutamine in the regulation of mTOR signalling in pancreatic β-cells

The mammalian target of rapamycin (mTOR), a serine/threonine kinase, is a master regulator of cellular growth and proliferation. It integrates several signals and inputs, such as nutrient and energy levels, stress, growth factors and amino acids, to regulate cell growth. The mTOR signalling pathway is dysregulated in several disease states, such as cancer and type 2 diabetes. In type 2 diabetes, an increase in demand for insulin is met by an increase in pancreatic β-cell function and mass in a mechanism termed “β-cell compensation”. There is evidence to suggest that this is mediated, at least in part, by the mTOR pathway. However, hyper-activation of mTOR signalling, can lead to its inhibition by a negative feedback loop, and this has been implicated in a reduction in β-cell mass and function which coupled with insulin resistance, could lead to type 2 diabetes. In this work, I investigated the role of L-glutamine in mTOR signalling in β-cells. I show that L-glutamine activates mTORC1. L-glutamine deprivation results in rapid mTORC1 inhibition, the activation of the MAPK pathway and activation of AMPK. In the absence of glutamine, its metabolites glutamate and α-ketoglutarate restore signalling to mTOR and reverses AMPK activation; however, inhibition of glutamine metabolism does not inhibit mTOR activity. Glutamine transporters SNAT 2 and SNAT 3 are the main transporters in INS1e cells, and inhibition of glutamine transport does not inhibit signalling to mTOR. Glutamine withdrawal for 4 hours inhibits signalling to mTORC2 in INS1e, HEK293 and HepG2 cells, but not SH-SY5Y cells; thus, glutamine is required for insulin-stimulated phosphorylation of pPKB Ser 473, a site phosphorylated by mTORC2, but not for PI3K-dependent phosphorylation of pPKB Thr 308. This work provides further insights into how glutamine regulates insulin-dependent signalling to mTORC1 and mTORC2 in pancreatic β-cells.