Growth differentiation factor 15 (GDF15) is an endocrine hormone belonging to the TGFβ superfamily member. GDF15 administration or GDF15 overexpression has been reported to have anti-obesity and anti-diabetic effects. Although non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) is frequently associated with obesity and insulin resistance, the functional role of endogenous GDF15 and therapeutic effect of GDF15 overexpression in NASH and related metabolic deterioration have not been evaluated. Here, we found that GDF15 expression was increased in the livers of NASH animal models and human subjects with NASH. Elevated expression of GDF15 was due to diet-induced hepatic endoplasmic reticulum (ER) stress. Gdf15-knockout mice exhibited aggravated NASH phenotypes such as increased steatosis, hepatic inflammation, fibrosis, liver injury, and metabolic deterioration. Furthermore, GDF15 directly suppressed expression of fibrosis-related genes and osteopontin (OPN), contributing factors for NASH-related fibrosis, in hepatic stellate cells in vitro and in the liver of mice in vivo. Finally, we found that GDF15-transgenic mice showed attenuation of NASH phenotypes and metabolic deterioration. Therefore, our results suggest that induction of endogenous GDF15 is a compensatory mechanism to protect against the progression of NASH and that GDF15 could be an attractive therapeutic candidate for treatment of NASH and NASH-related metabolic deterioration.

Impaired mitochondrial oxidative phosphorylation (OXPHOS) has been proposed as an etiological mechanism underlying insulin resistance. However, the initiating organ of OXPHOS dysfunction during the development of systemic insulin resistance has yet to be identified. To determine whether adipose OXPHOS deficiency plays an etiological role in systemic insulin resistance, the metabolic phenotype of mice with OXPHOS–deficient adipose tissue was examined. Crif1 is a protein required for the intramitochondrial production of mtDNA–encoded OXPHOS subunits; therefore, Crif1 haploinsufficient deficiency in mice results in a mild, but specific, failure of OXPHOS capacity in vivo. Although adipose-specific Crif1-haploinsufficient mice showed normal growth and development, they became insulin-resistant. Crif1-silenced adipocytes showed higher expression of chemokines, the expression of which is dependent upon stress kinases and antioxidant. Accordingly, examination of adipose tissue from Crif1-haploinsufficient mice revealed increased secretion of MCP1 and TNFα, as well as marked infiltration by macrophages. These findings indicate that the OXPHOS status of adipose tissue determines its metabolic and inflammatory responses, and may cause systemic inflammation and insulin resistance. Type 2 diabetes is one of the most challenging health problems in the 21^st century. Although insulin resistance is regarded as a fundamental defect that precedes the development of type 2 diabetes, the nature and cause of insulin resistance remain unknown. Adipose tissue is an important organ that determines whole-body energy metabolism, and its dysfunction is a critical element in the development of systemic insulin resistance. Adipose mitochondrial function is suppressed in the insulin-resistant state, and increased adipose mitochondrial biogenesis is associated with the reversal of insulin resistance by a PPARγ agonist. However, despite these important observations, little is known about how mitochondrial respiratory dysfunction in white adipose tissue (WAT) causes insulin resistance. To determine whether adipose deficiency of mitochondrial respiratory capacity plays an etiological role in systemic insulin resistance, the metabolic phenotype of mice with mitochondrial OXPHOS (oxidative phosphorylation)–deficient adipose tissue was examined. Crif1 is a protein required for the translation of mtDNA–encoded OXPHOS subunits. Interestingly, mice haploinsufficient for Crif1 in adipose tissue showed reduced OXPHOS capacity and developed marked insulin resistance.

Nuclear receptor corepressor 1 (NCoR1) is a transcriptional coregulator that has wide‐ranging effects on gene expression patterns. In the liver, NCoR1 represses lipid synthesis in the fasting state, whereas it inhibits activation of peroxisome proliferator‐activated receptor alpha (PPARα) upon feeding, thereby blunting ketogenesis. Here, we show that insulin by activation of protein kinase B induces phosphorylation of NCoR1 on serine 1460, which selectively favors its interaction with PPARα and estrogen‐related receptor alpha (ERRα) over liver X receptor alpha (LXRα). Phosphorylation of NCoR1 on S1460 selectively derepresses LXRα target genes, resulting in increased lipogenesis, whereas, at the same time, it inhibits PPARα and ERRα targets, thereby attenuating oxidative metabolism in the liver. Phosphorylation‐gated differential recruitment of NCoR1 to different nuclear receptors explains the apparent paradox that liver‐specific deletion of NCoR1 concurrently induces both lipogenesis and oxidative metabolism owing to a global derepression of LXRα, PPARα, and ERRα activity. Conclusion: Phosphorylation‐mediated recruitment switch of NCoR1 between nuclear receptor subsets provides a mechanism by which corepressors can selectively modulate liver energy metabolism during the fasting‐feeding transition. (Hepatology 2015;62:1606–1618)