That cells sense extracellular nutrients to affect their own metabolism is a key topic for many human metabolic diseases . GPCR signaling has been reported to regulate glucose and lipid metabolism and energy homeostasis in obesity and type 2 diabetes . Class C GPCRs responsible for amino acid sensing, such as T1Rs, are considered as potential targets for the treatment of related diseases . However, the role of T1Rs in various tissues and organs of the whole body is not completely understood . Here we focused on the role of umami taste receptor T1R1/T1R3 on lipid metabolism, in order to explain the mechanism of cellular amino acid sensing and its effects.
Consistent with previous studies of T1R2-KO and T1R3-KO mice, we found T1R1-KO mice also showed insignificant body weight change, reduced fat mass and smaller adipocyte area . Unlike T1R2-KO mice showing decreased liver TG level following a high-fat diet, our results showed T1R1-KO mice had lower serum TG and TC . Besides, in contrast to T1R3 deleted mice exhibiting glucose intolerance and insulin resistance, it was observed that T1R1-KO mice exhibited better glucose tolerance and insulin sensitivity . That’s because in addition to the function of umami taste receptors, T1R3 also forms sugar-sensitive receptors with T1R2 to perform functions, which mediates the sense of cells to glucose, and has been reported to affect the secretion of insulin by pancreatic islets to regulate sugar metabolism [ 20, 28, 29].
As the center organ of lipid metabolism, liver absorbs fatty acids from the blood and synthesizes them into triglycerides and cholesterol, the excess part of which is then stored in adipose tissue through blood circulation . Therefore, when the body's lipid metabolism changes, the liver and adipose tissue are always checked first. Consistent with the adipose tissue performance of T1R2-KO and T1R3-KO mice, T1R1-KO mice also had deceased iWAT and gWAT fat mass and adipocyte area [20, 30]. Meanwhile, we found reduced lipid droplets in liver and BAT.
Generally, the decrease in lipid accumulation is attributed to the fact that the rate of lipogenesis is lower than that of lipolysis . These processes are mainly regulated by a series of genes, for example the PPARγ, C/EBPα and SREBP1 in adipogenesis, and ATGL, LPL and HSL in lipolysis [32–35]. Changes of these key genes in adipose and liver tissue suggested that the process of adipogenesis was significantly inhibited in T1R1-KO mice, which may partly explain the decrease in lipid deposition. Besides, consistent with the function of mTOR signaling as the downstream of T1R1/T1R3, we also found its reduction in T1R1-KO mice [13, 14].
Proteomics Sequencing analysis could explore extensive biological changes in cells or tissues, so as to easily probe the ways that genes affecting physiological changes . These tools have never been used in previous studies on T1Rs family members’ function in cells or mice. Therefore, the use of proteomics sequencing was made in this study to further reveal the mechanism by which T1R1 affects body lipid metabolism. In accord with our prediction, the up-regulated proteins were enriched in processes of regulation of lipid metabolism, steroid metabolism and cellular ketone metabolism, which showed that the ablation of T1R1 may disturbed the lipid metabolism in liver. Besides, muscle cell development pathway has also been enriched with up-regulated proteins, which is consistent with the functional role of T1R3 as the downstream of muscle regulatory factors . As the amino acid receptor which sense glutamate mostly, it’s predictable that the ablation of T1R1 would affect Glutamatergic synapse pathway . The functional expression of T1R1/T1R3 in mouse neutrophils also showed its potential roles in immune signaling pathway .
Combined with the verification results of proteomics analysis by PRM, we found that CYP7B1 and IGFBP2 act as key regulators of T1R1 regulating liver lipid metabolism (Fig. 5A-C). The process in which the cholesterol is conversed into bile acids and excreted from the body is one of the important ways of cholesterol metabolism in mammals, of which oxysterol 7 α-hydroxylase (CYP7B1) acts as a key regulator . As a secreted protein in the liver, IGFBP2 is believed to increase insulin sensitivity and reduce adipogenesis [40, 41]. Therefore, the increase in CYP7B1 and IGFBP2 explained the improvement of liver lipid metabolism.
Since T1R1/T1R3 could also sense branched chain amino acids (BCAAs), the reduction of BCAAs metabolic enzyme activity (BCKDHA and BCKDHB) in T1R1-KO mice (Fig. 5D and E) was found, which decrease in the production of acetyl-CoA, thus leading to the reduction of raw materials of fat de novo synthesis [11, 42]. These changes showed the ablation of T1R1 in the liver of mice may reduce the de novo lipid synthesis process.
In summary, our results showed that the disruption of T1R1 in mice could reduce body lipid accumulation through reduce the lipid synthesis process and increase the lipid metabolism. In the liver, the loss of T1R1 causes the increase of CYP7B1 and IGFBP2 to enhance lipid metabolism, and causes the reduction of BCAA metabolic enzymes (BCKDHA and BCKDHB) to reduce the lipid synthesis process. Our study gives insights into the effect of amino acid sensing on the body's metabolism, and provides a supplement for T1Rs as a potential drug target for the treatment of type 2 diabetes.