Peroxisome proliferator-activated receptor γ coactivator-1 α (PGC-1α) regulates mitochondrial DNA replication and mitochondrial gene expression by interacting with several transcription factors. White adipose tissue (WAT) mainly comprises adipocytes that store triglycerides as an energy resource and secrete adipokines. The characteristics of WAT vary in response to systemic and chronic metabolic alterations, including obesity or caloric restriction. Despite a small amount of mitochondria in white adipocytes, accumulated evidence suggests that mitochondria are strongly related to adipocyte-specific functions, such as adipogenesis and lipogenesis, as well as oxidative metabolism for energy supply. Therefore, PGC-1α is expected to play an important role in WAT. In this review, we provide an overview of the involvement of mitochondria and PGC-1α with obesity- and caloric restriction-related physiological changes in adipocytes and WAT.

Cholesterol is an essential lipid in vertebrates, but excess blood cholesterol promotes atherosclerosis. In the liver, cholesterol is metabolized to bile acids by cytochrome P450, family 7, subfamily a, polypeptide 1 (CYP7A1), the transcription of which is negatively regulated by the ERK pathway. Fibroblast growth factor 21 (FGF21), a hepatokine, induces ERK phosphorylation and suppresses Cyp7a1 transcription. Taurine, a sulfur-containing amino acid, reportedly promotes cholesterol metabolism and lowers blood and hepatic cholesterol levels. However, the influence of long-term feeding of taurine on cholesterol levels and metabolism remains unclear. Here, to evaluate the more chronic effects of taurine on cholesterol levels, we analyzed mice fed a taurine-rich diet for 14–16 weeks. Long-term feeding of taurine lowered plasma cholesterol and bile acids without significantly changing other metabolic parameters, but hardly affected these levels in the liver. Moreover, taurine upregulated Cyp7a1 levels, while downregulated phosphorylated ERK and Fgf21 levels in the liver. Likewise, taurine-treated Hepa1-6 cells, a mouse hepatocyte line, exhibited downregulated Fgf21 levels and upregulated promoter activity of Cyp7a1. These results indicate that taurine promotes cholesterol metabolism by suppressing the FGF21/ERK pathway followed by upregulating Cyp7a1 expression. Collectively, this study shows that long-term feeding of taurine lowers both plasma cholesterol and bile acids, reinforcing that taurine effectively prevents hypercholesterolemia.

Most mitochondrial proteins encoded in the nuclear genome are synthesized in the cytoplasm. These proteins subsequently undergo maturation through the cleavage of a signal sequence at the N-terminus by one or two mitochondrial signal peptidases, which is essential for their function within mitochondria. The present study demonstrates that adipocyte-specific knockout of one mitochondrial signal peptidase, mitochondrial intermediate peptidase (MIPEP), resulted in disordered mitochondrial proteostasis of MIPEP substrate proteins and their defective maturation. MIPEP deficiency in white and brown adipocytes suppressed the expression of adipocyte differentiation, lipid metabolism, and mitochondrial biogenesis genes. These alterations led to lipoatrophy in white adipose tissue and the whitening of brown adipose tissue. Additionally, it induced an atypical mitochondrial unfolded protein response and local inflammation in white and brown adipose tissue. Furthermore, it induced fatty liver and splenomegaly and caused systemic impairments in glucose metabolism and inflammation. These findings indicate that maturation defects of certain mitochondrial proteins and subsequent proteostasis disorders in white and brown adipocytes cause chronic and systemic inflammatory and metabolic dysfunctions.

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a member of the homologous to E6AP C-terminus-type E3 ubiquitin protein ligase family. Although WWP1 plays a role in several human diseases, including infectious diseases, neurological disorders, and cancers, there is emerging evidence that WWP1 is also associated with metabolic disorders. In this review, we discuss the regulation and molecular function of WWP1 and its contribution to obesity-related metabolic disorders, particularly in white adipose tissue and the liver. We highlight the need for further research to deepen our understanding of how WWP1 may be implicated in metabolic dysfunction and facilitate the development of novel therapeutic strategies that target WWP1.

Obesity is defined as abnormal or excessive accumulation of body fat and contributes to several metabolic disorders. White adipose tissue (WAT) releases energy as free fatty acids and glycerol from triglycerides through a process called lipolysis. People with obesity have impaired catecholamine-stimulated lipolysis, but comprehensive understanding of this lipolysis is still unclear. We previously showed that expression of WW domain-containing E3 ubiquitin ligase 1 (WWP1), a member of the HECT-type E3 family of ubiquitin ligases, was increased in WAT of obese mice. In this study, we generated Wwp1 knockout (KO) mice to evaluate the effect of WWP1 in WAT of obese mice. The mRNA levels of beta-3 adrenergic receptor (Adrb3), which were decreased with a high-fat diet, were increased by Wwp1 KO in WAT. Moreover, Wwp1 KO mice showed increased phosphorylated hormone-sensitive lipase levels in WAT. In contrast, noradrenaline and its metabolism were not altered in WAT of obese Wwp1 KO mice. These findings indicate that WWP1, which is increased in adipocytes because of obesity, is a candidate for suppressing lipolysis independently of noradrenaline metabolism. We anticipate that inhibition of WWP1 is a promising approach for a new treatment of obesity and type-2 diabetes using Adrb3 agonists.

Zinc finger protein 746 (ZNF746), also known as PARKIN-interacting substrate (PARIS), is a KRAB-ZFP family transcriptional repressor implicated in Parkinson’s disease and adipogenesis. Its unclear role in systemic metabolism led us to investigate PARIS-mediated metabolic function by generating mice with a knockout of Znf746 exon 7 (P7KO), containing the DNA-binding domain. Under a high-fat diet, P7KO mice exhibited larger gains in body and adipose tissue weight alongside improved insulin sensitivity and reduced adipocyte size compared with wild-type mice, suggesting hyperplastic expansion of white adipose tissue. P7KO mice also showed elevated anti-inflammatory markers in adipose tissues, in contrast to accumulation of lipids and upregulation of lipogenic genes, without changes in glucose transporter or lipid uptake genes, in the liver. These findings indicate a dual role for PARIS: suppressing adipose hyperplasia and restricting hepatic lipogenesis. Identification of the evolutionary conservation of PARIS among mammals, particularly the shared DNA-binding domain between mice and humans, support its translational relevance. Collectively, our results identify PARIS as a previously unrecognized regulator of metabolic homeostasis that coordinates lipid storage and utilization in a tissue-specific manner. Targeting PARIS may offer a novel therapeutic strategy for obesity-related metabolic disorders, such as type 2 diabetes and non-alcoholic fatty liver disease.