The G Protein-Coupled Taste Receptor T1R1/T1R3 Regulates mTORC1 and Autophagy

Abstract

Cells continually assess their energy and nutrient state to maintain growth and survival and engage necessary homeostatic mechanisms. Cell autonomous responses to the fed state require the surveillance of the availability of amino acids and other nutrients. The mammalian target of rapamycin complex 1 (mTORC1) integrates information on nutrient and amino acid availability to support protein synthesis and cell growth. We identify the G protein-coupled receptor (GPCR) T1R1/T1R3 as a direct sensor of the fed state and amino acid availability. Knocking down this receptor, which is found in most tissues, reduces the ability of amino acids to signal to mTORC1. Interfering with this receptor alters localization of mTORC1, downregulates expression of pathway inhibitors, upregulates key amino acid transporters, blocks translation initiation, and induces autophagy. These findings reveal a mechanism for communicating amino acid availability through a GPCR to mTORC1 in mammals.

Ras transformation uncouples the kinesin-coordinated cellular nutrient response

“…part, activated through interaction with Rag small GTPases and the Ragulator complex on lysosomes (16), we examined how removal of amino acids and serum affected lysosomes and mTOR localization. After 60-90 min of starvation in EBSS, lysosomes in HBEC3KT lost the typical perinuclear concentrated pattern and instead gained a more diffuse and symmetric distribution, similar to what has been observed in HEK293 and HeLa cells following removal of amino acids (15,16,23) (Fig. 1 A and B).…”

Muscarinic Control of MIN6 Pancreatic β Cells Is Enhanced by Impaired Amino Acid Signaling

“…ERK1/2 activation enhances insulin gene transcription following nutrient-induced insulin secretion (15)(16)(17)(18). We have reported previously that the GPCR complex T1R1/T1R3 is an early amino acid sensor in the MIN6 pancreatic ␤ cell line and in other cell types (12). Similar to M3R, T1R1/T1R3 activation leads to a rise in Ca 2ϩ i and ERK1/2 phosphorylation that is partially dependent upon phospholipase C ␤ activation.…”

“…Signaling by the M3R also activates ERK1/2 in ␤ cells, most likely downstream of elevated intracellular calcium (12)(13)(14). ERK1/2 activation enhances insulin gene transcription following nutrient-induced insulin secretion (15)(16)(17)(18).…”

“…Reduced expression of T1R3 in MIN6 cells resulted in a decrease of amino acid-induced ERK1/2 and mammalian target of rapamycin complex 1 activation. Signaling defects in cells in which the receptor had been depleted included a reduction in the ability of amino acids to induce changes in Ca 2ϩ i (12). Despite the impaired ability of amino acids to stimulate ERK1/2 in T1R3-depleted MIN6 cells, carbachol, a muscarinic * This work was supported, in whole or in part, by National Institutes of Health Grant DK55310.…”

“…2 To whom correspondence should be addressed: Dept. receptor agonist, activated ERK1/2 better in T1R3-depleted cells than in control cells (12). We explored the underlying mechanisms for the enhanced carbachol response in MIN6 cells to determine whether similar mechanisms were enlisted to compensate for amino acid deficiency.…”