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mTORC1 is Required for Brown Adipose Tissue Recruitment and Metabolic Adaptation to Cold

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ABSTRACT

In response to cold, brown adipose tissue (BAT) increases its metabolic rate and expands its mass to produce heat required for survival, a process known as BAT recruitment. The mechanistic target of rapamycin complex 1 (mTORC1) controls metabolism, cell growth and proliferation, but its role in regulating BAT recruitment in response to chronic cold stimulation is unknown. Here, we show that cold activates mTORC1 in BAT, an effect that depends on the sympathetic nervous system. Adipocyte-specific mTORC1 loss in mice completely blocks cold-induced BAT expansion and severely impairs mitochondrial biogenesis. Accordingly, mTORC1 loss reduces oxygen consumption and causes a severe defect in BAT oxidative metabolism upon cold exposure. Using in vivo metabolic imaging, metabolomics and transcriptomics, we show that mTORC1 deletion impairs glucose and lipid oxidation, an effect linked to a defect in tricarboxylic acid (TCA) cycle activity. These analyses also reveal a severe defect in nucleotide synthesis in the absence of mTORC1. Overall, these findings demonstrate an essential role for mTORC1 in the regulation of BAT recruitment and metabolism in response to cold.

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mTORC1 impairs oxidative metabolism in BAT following chronic cold exposure.Control and Ad-RaptorKO mice were cold exposed for 14 days and fasted during 6 hours before BAT collection. (A) Metabolites of the TCA cycle measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (B) TCA cycle gene expression extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (C) Acyl-Carnitine (AcCa) metabolites measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (D) Expression of genes controlling Acyl-Carnitine production extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (E) Schematic summary of transcriptomics and metabolomics data.
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f6: mTORC1 impairs oxidative metabolism in BAT following chronic cold exposure.Control and Ad-RaptorKO mice were cold exposed for 14 days and fasted during 6 hours before BAT collection. (A) Metabolites of the TCA cycle measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (B) TCA cycle gene expression extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (C) Acyl-Carnitine (AcCa) metabolites measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (D) Expression of genes controlling Acyl-Carnitine production extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (E) Schematic summary of transcriptomics and metabolomics data.

Mentions: As presented in Fig. 6A (and Fig. 4A), high levels of pyruvate accumulated in BAT of Ad-RaptorKO mice. Pyruvate molecules produced from glycolysis can be used to produce lactate or can be transported into the mitochondria where they are converted to acetyl-CoA. In the mitochondria, acetyl-CoA is used in the TCA cycle to promote cellular respiration and support thermogenesis (see metabolic map in Fig. 6E). Despite high levels of pyruvate in BAT of Ad-RaptorKO mice, we observed no change in lactate levels, but did measure a reduction in the levels of acetyl-CoA and other TCA cycle intermediates such as citrate and succinate, indicating a defect in pyruvate catabolism (Fig. 6A). This effect was associated with a profound reduction in the expression of genes coding for proteins catalyzing the conversion of pyruvate to acetyl-CoA, as well as almost all the genes encoding proteins of the TCA cycle (Fig. 6B). Interestingly, despite the clear impairment in TCA cycle, we observed that alpha-ketoglutarate (aKG) levels were elevated in Ad-RaptorKO mice (Figure S5A and Fig. 6E). This effect may have been caused by the elevation in glutaminolysis, as we observed very high levels of glutamine and glutamate in Ad-RaptorKO mice (Figure S5B). In addition to pyruvate, acetyl-CoA can be produced from the β-oxidation of acyl-CoA in the mitochondria. The import and catabolism of acyl-CoA depends on the production of acyl-carnitines (AcCa) by carnitine acetyltransferases. As shown in Fig. 6C, the production of AcCa was completely impaired in BAT of Ad-RaptorKO mice. We also observed a significant reduction in the expression of the genes encoding proteins required for AcCa formation and mitochondrial transport (Fig. 6D). Altogether, these results indicate that glucose and fatty acid oxidative metabolism are severely reduced in BAT of Ad-RaptorKO mice. An integrative representation of the metabolomics and transcriptomics data is presented in Figure S6.


mTORC1 is Required for Brown Adipose Tissue Recruitment and Metabolic Adaptation to Cold
mTORC1 impairs oxidative metabolism in BAT following chronic cold exposure.Control and Ad-RaptorKO mice were cold exposed for 14 days and fasted during 6 hours before BAT collection. (A) Metabolites of the TCA cycle measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (B) TCA cycle gene expression extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (C) Acyl-Carnitine (AcCa) metabolites measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (D) Expression of genes controlling Acyl-Carnitine production extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (E) Schematic summary of transcriptomics and metabolomics data.
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f6: mTORC1 impairs oxidative metabolism in BAT following chronic cold exposure.Control and Ad-RaptorKO mice were cold exposed for 14 days and fasted during 6 hours before BAT collection. (A) Metabolites of the TCA cycle measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (B) TCA cycle gene expression extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (C) Acyl-Carnitine (AcCa) metabolites measured by metabolomics. Metabolites in bold are statistically different between control and Ad-RaptorKO mice (n = 3; mean +/− SEM; t-test; *P < 0.05). (D) Expression of genes controlling Acyl-Carnitine production extracted from the microarray dataset. Genes in bold are statistically different between control and Ad-RaptorKO mice (n = 4; mean +/− SEM; SAM t-test; *FDR < 0.05). (E) Schematic summary of transcriptomics and metabolomics data.
Mentions: As presented in Fig. 6A (and Fig. 4A), high levels of pyruvate accumulated in BAT of Ad-RaptorKO mice. Pyruvate molecules produced from glycolysis can be used to produce lactate or can be transported into the mitochondria where they are converted to acetyl-CoA. In the mitochondria, acetyl-CoA is used in the TCA cycle to promote cellular respiration and support thermogenesis (see metabolic map in Fig. 6E). Despite high levels of pyruvate in BAT of Ad-RaptorKO mice, we observed no change in lactate levels, but did measure a reduction in the levels of acetyl-CoA and other TCA cycle intermediates such as citrate and succinate, indicating a defect in pyruvate catabolism (Fig. 6A). This effect was associated with a profound reduction in the expression of genes coding for proteins catalyzing the conversion of pyruvate to acetyl-CoA, as well as almost all the genes encoding proteins of the TCA cycle (Fig. 6B). Interestingly, despite the clear impairment in TCA cycle, we observed that alpha-ketoglutarate (aKG) levels were elevated in Ad-RaptorKO mice (Figure S5A and Fig. 6E). This effect may have been caused by the elevation in glutaminolysis, as we observed very high levels of glutamine and glutamate in Ad-RaptorKO mice (Figure S5B). In addition to pyruvate, acetyl-CoA can be produced from the β-oxidation of acyl-CoA in the mitochondria. The import and catabolism of acyl-CoA depends on the production of acyl-carnitines (AcCa) by carnitine acetyltransferases. As shown in Fig. 6C, the production of AcCa was completely impaired in BAT of Ad-RaptorKO mice. We also observed a significant reduction in the expression of the genes encoding proteins required for AcCa formation and mitochondrial transport (Fig. 6D). Altogether, these results indicate that glucose and fatty acid oxidative metabolism are severely reduced in BAT of Ad-RaptorKO mice. An integrative representation of the metabolomics and transcriptomics data is presented in Figure S6.

View Article: PubMed Central - PubMed

ABSTRACT

In response to cold, brown adipose tissue (BAT) increases its metabolic rate and expands its mass to produce heat required for survival, a process known as BAT recruitment. The mechanistic target of rapamycin complex 1 (mTORC1) controls metabolism, cell growth and proliferation, but its role in regulating BAT recruitment in response to chronic cold stimulation is unknown. Here, we show that cold activates mTORC1 in BAT, an effect that depends on the sympathetic nervous system. Adipocyte-specific mTORC1 loss in mice completely blocks cold-induced BAT expansion and severely impairs mitochondrial biogenesis. Accordingly, mTORC1 loss reduces oxygen consumption and causes a severe defect in BAT oxidative metabolism upon cold exposure. Using in vivo metabolic imaging, metabolomics and transcriptomics, we show that mTORC1 deletion impairs glucose and lipid oxidation, an effect linked to a defect in tricarboxylic acid (TCA) cycle activity. These analyses also reveal a severe defect in nucleotide synthesis in the absence of mTORC1. Overall, these findings demonstrate an essential role for mTORC1 in the regulation of BAT recruitment and metabolism in response to cold.

No MeSH data available.


Related in: MedlinePlus