Gα12 ablation exacerbates liver steatosis and obesity by suppressing USP22/SIRT1-regulated mitochondrial respiration
Tae Hyun Kim, … , Cheol Soo Choi, Sang Geon Kim
Tae Hyun Kim, … , Cheol Soo Choi, Sang Geon Kim
Published October 9, 2018
Citation Information: J Clin Invest. 2018;128(12):5587-5602. https://doi.org/10.1172/JCI97831.
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Research Article Hepatology Metabolism

Gα12 ablation exacerbates liver steatosis and obesity by suppressing USP22/SIRT1-regulated mitochondrial respiration

Abstract

Nonalcoholic fatty liver disease (NAFLD) arises from mitochondrial dysfunction under sustained imbalance between energy intake and expenditure, but the underlying mechanisms controlling mitochondrial respiration have not been entirely understood. Heterotrimeric G proteins converge with activated GPCRs to modulate cell-signaling pathways to maintain metabolic homeostasis. Here, we investigated the regulatory role of G protein α12 (Gα12) on hepatic lipid metabolism and whole-body energy expenditure in mice. Fasting increased Gα12 levels in mouse liver. Gα12 ablation markedly augmented fasting-induced hepatic fat accumulation. cDNA microarray analysis from Gna12-KO liver revealed that the Gα12-signaling pathway regulated sirtuin 1 (SIRT1) and PPARα, which are responsible for mitochondrial respiration. Defective induction of SIRT1 upon fasting was observed in the liver of Gna12-KO mice, which was reversed by lentivirus-mediated Gα12 overexpression in hepatocytes. Mechanistically, Gα12 stabilized SIRT1 protein through transcriptional induction of ubiquitin-specific peptidase 22 (USP22) via HIF-1α increase. Gα12 levels were markedly diminished in liver biopsies from NAFLD patients. Consistently, Gna12-KO mice fed a high-fat diet displayed greater susceptibility to diet-induced liver steatosis and obesity due to decrease in energy expenditure. Our results demonstrate that Gα12 regulates SIRT1-dependent mitochondrial respiration through HIF-1α–dependent USP22 induction, identifying Gα12 as an upstream molecule that contributes to the regulation of mitochondrial energy expenditure.

Authors

Tae Hyun Kim, Yoon Mee Yang, Chang Yeob Han, Ja Hyun Koo, Hyunhee Oh, Su Sung Kim, Byoung Hoon You, Young Hee Choi, Tae-Sik Park, Chang Ho Lee, Hitoshi Kurose, Mazen Noureddin, Ekihiro Seki, Yu-Jui Yvonne Wan, Cheol Soo Choi, Sang Geon Kim

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Figure 5

Gα12 regulation of SIRT1 via HIF-1α–mediated induction of USP22.

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Gα12 regulation of SIRT1 via HIF-1α–mediated induction of USP22. (A) Inh...
(A) Inhibition of SIRT1 ubiquitination and degradation by Gα12. SIRT1 immunoprecipitates from HepG2 cells infected with Ad-Gα12QL (or Ad-Con) were immunoblotted for ubiquitin (left) and quantified (middle, n = 3). In another experiment, HepG2 cells were treated with 10 μM cycloheximide for indicated times (right, n = 3). (B) Effect of USP22 gene silencing on inhibition of SIRT1 ubiquitination by Gα12. (C) qRT-PCR assays for Usp22 in the liver (left, n = 5/group) or primary hepatocytes (right, n = 4/group). (D) Luciferase reporter assays for USP22 promoter activity in Gα12-overexpressed AML12 cells. The result shown is combined from 3 independent experiments (n = 6–8 replicates/group for each experiment). Box-and-whisker plot shows median (horizontal lines within boxes), 5%–95% percentile (ends of the boxes), and range of minimum to maximum values (whiskers). Each dot represents an outlying value. Mut1 or Mut2, promoter-reporter constructs with deletion of respective HIF-1α response element sites. (E) Increase in SIRT1 level by Gα12 overexpression through HIF-1α/USP22 axis (right, n = 4/group). (F) Effect of HIF-1α or USP22 gene silencing on SIRT1 induction by Gα12. (G) Effect of RhoA/Rock pathway inhibition on HIF-1α induction by Gα12. (H) Effect of Gα12 overexpression in the liver on HIF-1α/USP22/SIRT1 axis. Immunoblottings were done on the liver homogenates obtained from mice as in Figure 4E. (I) Effect of Gα12 overexpression in hepatocytes on HIF-1α/USP22/SIRT1 axis. Immunoblottings were done on mouse primary hepatocytes infected with Ad-Gα12QL (or Ad-Con) and quantified (n = 3/group). Values represent mean ± SEM. Data were analyzed by 2-tailed Student’s t test (A, C, D, and E) or ANOVA followed by Bonferroni’s post hoc test (I). For A, B, and E–I, blots in each panel were run in parallel using the same samples and β-actin was used as a normalization control for densitometric analysis.