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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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 3

Gα12 regulation of SIRT1-dependent mitochondrial respiration in the liver.

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Gα12 regulation of SIRT1-dependent mitochondrial respiration in the live...
(A) SIRT1 inhibition by Gna12 KO. Immunoblottings for SIRT1, SIRT3, and SIRT5 were performed using liver homogenates from 14-week-old WT or Gna12-KO mice fed ND (upper panel). Lower panel shows quantification (n = 3/group). (B) Effects of Gα12 modulation on SIRT1 levels. Immunoblottings for SIRT1 were performed (upper) and quantified (lower) using primary hepatocytes from WT or Gna12-KO mice (left, n = 3/group), HepG2 cells infected with Ad-Gα12QL or control (Ad-Con) (middle, n = 4/group), or AML12 cells stably expressing sh-Gα12 or control (sh-Luci) (right, n = 3/group). (C) Immunoblottings for CPT1 and PGC1α in liver or primary hepatocytes from WT or Gna12-KO mice (upper) and their respective quantifications (lower, n = 3/group each). (D) qRT-PCR assays for PPARα target genes responsible for FA oxidation in the liver or primary hepatocytes (n = 3–11/group). (E) OCR in mitochondria. OCR was measured using the mitochondrial fraction prepared from liver tissues of WT or Gna12-KO mice (n = 3/group). Analyzed OCR was normalized to the protein concentrations for each set of samples determined by the Bradford method. (F) Palmitate oxidation in primary hepatocytes. [3H]-palmitate oxidation rate was determined using primary hepatocytes from WT or Gna12-KO mice, and 5 × 105 cells per well were cultured in 12-well plates. Data shown are from 1 representative experiment of 2 independent experiments (n = 3 mice/group). Each dot represents an individual pool of primary hepatocytes isolated from each mouse. Values represent mean ± SEM. Data were analyzed by 2-tailed Student’s t test (A–F). For A–C, the blots in each panel were run in parallel using the same samples, and β-actin was used as a normalization control for densitometric analysis. For D, box-and-whisker plots show median (horizontal lines within boxes), 5%–95% (ends of the boxes), and range of minimum to maximum.