Seipin, encoded by BSCL2 gene, is a protein whose physiological functions remain unclear. Mutations of BSCL2 cause the most-severe form of congenital generalized lipodystrophy (CGL). BSCL2 mRNA is highly expressed in the brain and testis in addition to the adipose tissue in human, suggesting physiological roles of seipin in non-adipose tissues. Since we found BSCL2 mRNA expression pattern among organs in rat is similar to human while it is not highly expressed in mouse brain, we generated a Bscl2/seipin knockout (SKO) rat using the method with ENU (N-ethyl-N-nitrosourea) mutagenesis. SKO rats showed total lack of white adipose tissues including mechanical fat such as bone marrow and retro-orbital fats, while physiologically functional brown adipose tissue was preserved. Besides the lipodystrophic phenotypes, SKO rats showed impairment of spatial working memory with brain weight reduction and infertility with azoospermia. We confirmed reduction of brain volume and number of sperm in human patients with BSCL2 mutation. This is the first report demonstrating that seipin is necessary for normal brain development and spermatogenesis in addition to white adipose tissue development.

Agonist-induced activation of peroxisome proliferator-activated receptor-γ (PPARγ) stimulates adipocyte differentiation and insulin sensitivity. Patients with heterozygous PPARγ dominant-negative mutation develop partial lipodystrophy and insulin resistance. Inconsistent with this evidence in humans, it was reported that heterozygous PPARγ knockout mice have increased insulin sensitivity and that mice with heterozygous PPARγ dominant-negative mutation have normal insulin sensitivity and improved glucose tolerance. In the context of the interspecies intranslatability of PPARγ-related findings, we generated a PPARγ mutant rat with a loss-of-function mutation (Pparg(mkyo)) without dominant-negative activity by using the ENU (N-ethyl-N-nitrosourea) mutagenesis method. Heterozygous Pparg(mkyo/+) rats showed reduced fat mass with adipocyte hypertrophy and insulin resistance, which were highly predictable from known actions of PPARγ agonists and phenotypes of patients with the PPARγ mutation. This report is the first in our knowledge to clearly demonstrate that both alleles of PPARγ are required for normal adipocyte development and insulin sensitivity in vivo. Furthermore, the study indicates that PPARγ regulates mainly adipocyte number rather than adipocyte size in vivo. The choice of appropriate species as experimental models is critical, especially for the study of PPARγ.

restores the insulinotropic effect of exenatide in a mouse model of type 2 diabetes with increased adiposity induced by streptozotocin and high-fat diet. Am J Physiol Endocrinol Metab 307: E712-E719, 2014. First published August 26, 2014; doi:10.1152/ajpendo.00272.2014.-Leptin may reduce pancreatic lipid deposition, which increases with progression of obesity and can impair ␤-cell function. The insulinotropic effect of glucagon-like peptide-1 (GLP-1) and the efficacy of GLP-1 receptor agonist are reduced associated with impaired ␤-cell function. In this study, we examined whether leptin could restore the efficacy of exenatide, a GLP-1 receptor agonist, in type 2 diabetes with increased adiposity. We chronically administered leptin (500 g·kg Ϫ1 ·day Ϫ1 ) and/or exenatide (20 g·kg Ϫ1 ·day Ϫ1 ) for 2 wk in a mouse model of type 2 diabetes with increased adiposity induced by streptozotocin and highfat diet (STZ/HFD mice). The STZ/HFD mice exhibited hyperglycemia, overweight, increased pancreatic triglyceride level, and reduced glucose-stimulated insulin secretion (GSIS); moreover, the insulinotropic effect of exenatide was reduced. However, leptin significantly reduced pancreatic triglyceride level, and adding leptin to exenatide (LEP/EX) remarkably enhanced GSIS. These results suggested that the leptin treatment restored the insulinotropic effect of exenatide in the mice. In addition, LEP/EX reduced food intake, body weight, and triglyceride levels in the skeletal muscle and liver, and corrected hyperglycemia to a greater extent than either monotherapy. The pair-feeding experiment indicated that the marked reduction of pancreatic triglyceride level and enhancement of GSIS by LEP/EX occurred via mechanisms other than calorie restriction. These results suggest that leptin treatment may restore the insulinotropic effect of exenatide associated with the reduction of pancreatic lipid deposition in type 2 diabetes with increased adiposity. Combination therapy with leptin and exenatide could be an effective treatment for patients with type 2 diabetes with increased adiposity. drug therapy; combination; insulin secretion LEPTIN, AN ADIPOCYTE-DERIVED hormone, has therapeutic potential for treating diabetes and obesity (7, 13 19, 27, 32, 34). In our previous clinical trial in patients with lipodystrophy (6), we confirmed the therapeutic usefulness of leptin as a glucose-lowering agent, and it was first approved for the treatment of lipodystrophy in Japan in March 2013. Given these glucoregulatory effects of leptin, we and others have reported the therapeutic usefulness of leptin for various forms of diabetes, including type 2 diabetes, in rodent models (20,23,26,28,47). The glucoregulatory effects of leptin are associated with the reduction of ectopic lipid deposition, which increases with progression of obesity (36,39,46). The reduction of ectopic lipid deposition in the liver and skeletal muscle could improve insulin sensitivity (42). In the pancreas, the reduction of ectopic lipid deposition could improve ␤-cell function ...

Nonalcoholic fatty liver disease (NAFLD) is recognized as the hepatic component of the metabolic syndrome. Although NAFLD is a major cause of cirrhosis and cancer of the liver of unknown cause, no established pharmacological treatment for NAFLD has been established yet. It has been reported that leptin treatment improved fatty liver dramatically as well as insulin resistance and hyperphagia in patients with lipodystrophy. However, it is unclear whether leptin improves fatty liver independently of these metabolic improvements. We investigated the liver effect of leptin independently of insulin sensitization and appetite suppression using hepatocyte-specific Pten-deficient (AlbCrePtenff) mouse, a model of severe fatty liver with insulin hypersensitivity. Male AlbCrePtenff mice were infused subcutaneously with leptin (20 ng/g/h) for 2 weeks using osmotic minipumps. Leptin infusion effectively reduced liver weight, liver triglyceride content, and glutamate pyruvate transaminase (GPT) concentrations as well as food intake and body weight without the change of plasma insulin concentration in AlbCrePtenff mice. Pair-feeding also reduced body weight but not liver triglyceride content. Pair feeding reduced α1 and α2 AMP-activated protein kinase (AMPK) activities and PGC1α gene expression in the liver, while leptin infusion unchanged them. The present study clearly demonstrated that leptin improve fatty liver independently of insulin sensitization and suppression of food intake. It was suggested that leptin improves fatty liver by stimulation of β-oxidation in the liver. The present study might provide a further understanding on the mechanism of metabolic effect of leptin.

Therapeutic approach for NAFLD is limited and there are no approved drugs. Pioglitazone (PGZ), a thiazolidinedione (TZD) that acts via peroxisome proliferator activated receptor gamma (PPARγ) is the only agent that has shown consistent benefit and efficacy in clinical trials. However, the mechanism of its therapeutic effect on NAFLD remains unclear. The poor understanding may be due to problems with mouse, a species most used for animal experiments. TZDs exacerbate fatty liver in mouse models while they improve it in rat models like in human patients. Therefore, we compared the effects of TZDs including PGZ and rosiglitazone (RGZ) in ob/ob mice and Lepmkyo/Lepmkyo rats, models of leptin-deficient obesity, and A-ZIP/F-1 mice and seipin knockout (SKO) rats, models of generalized lipodystrophy. Pparg mRNA expression was markedly upregulated in fatty livers of mouse models while it was unchanged in rat models. TZDs exacerbated fatty liver in ob/ob and A-ZIP/F-1 mice, improved it in Lepmkyo/Lepmkyo rats and showed no effect in SKO rats. Gene expression analyses of Pparg and its target gene, Fsp27 revealed that PPARγ in the adipose tissue is the exclusive therapeutic target of TZDs in rats but PPARγ in the liver in addition to the adipose tissue is also a major site of actions for TZDs in mice. Although the response to TZDs in mice is the complete opposite of that in human patients, no report has pointed out the problem with TZD studies using mouse models so far. The present study might provide useful suggestions in research on TZDs.