David J. Lloyd

OBJECTIVE: Fibroblast growth factor 21 (FGF21) has emerged as an important metabolic regulator of glucose and lipid metabolism. The aims of the current study are to evaluate the role of FGF21 in energy metabolism and to provide mechanistic insights into its glucose and lipid-lowering effects in a high-fat diet-induced obesity (DIO) model. RESEARCH DESIGN AND METHODS: DIO or normal lean mice were treated with vehicle or recombinant murine FGF21. Metabolic parameters including body weight, glucose, and lipid levels were monitored, and hepatic gene expression was analyzed. Energy metabolism and insulin sensitivity were assessed using indirect calorimetry and hyperinsulinemic-euglycemic clamp techniques. RESULTS: FGF21 dose dependently reduced body weight and whole-body fat mass in DIO mice due to marked increases in total energy expenditure and physical activity levels. FGF21 also reduced blood glucose, insulin, and lipid levels and reversed hepatic steatosis. The profound reduction of hepatic triglyceride levels was associated with FGF21 inhibition of nuclear sterol regulatory element binding protein-1 and the expression of a wide array of genes involved in fatty acid and triglyceride synthesis. FGF21 also dramatically improved hepatic and peripheral insulin sensitivity in both lean and DIO mice independently of reduction in body weight and adiposity. CONCLUSIONS: FGF21 corrects multiple metabolic disorders in DIO mice and has the potential to become a powerful therapeutic to treat hepatic steatosis, obesity, and type 2 diabetes.

Nuclear migration and positioning within cells are critical for many developmental processes and are governed by the cytoskeletal network. Although mechanisms of nuclear-cytoskeletal attachment are unclear, growing evidence links a novel family of nuclear envelope (NE) proteins that share a conserved C-terminal SUN (Sad1/UNC-84 homology) domain. Analysis of Caenorhabditis elegans mutants has implicated UNC-84 in actin-mediated nuclear positioning by regulating NE anchoring of a giant actin-binding protein, ANC-1. Here, we report the identification of SUN1 as a lamin A-binding protein in a yeast two-hybrid screen. We demonstrate that SUN1 is an integral membrane protein located at the inner nuclear membrane. While the N-terminal domain of SUN1 is responsible for detergent-resistant association with the nuclear lamina and lamin A binding, lamin A/C expression is not required for SUN1 NE localization. Furthermore, SUN1 does not interact with type B lamins, suggesting that NE localization is ensured by binding to an additional nuclear component(s), most likely chromatin. Importantly, we find that the luminal C-terminal domain of SUN1 interacts with the mammalian ANC-1 homologs nesprins 1 and 2 via their conserved KASH domain. Our data provide evidence of a physical nuclear-cytoskeletal connection that is likely to be a key mechanism in nuclear-cytoplasmic communication and regulation of nuclear position.

The gene encoding nuclear lamins A and C is mutated in at least three inherited disorders. Two of these, Emery-Dreifuss muscular dystrophy (EDMD-AD) and a form of dilated cardiomyopathy (CMD1A), involve muscle defects, and the other, familial partial lipodystrophy (FPLD), involves loss of subcutaneous adipose tissue. Mutations causing FPLD, in contrast to those causing muscle disorders, are tightly clustered within the C-terminal domain of lamin A/C. We investigated the expression and subcellular localization of FPLD lamin A mutants and found no abnormalities. We therefore set out to identify proteins interacting with the C-terminal domain of lamin A by screening a mouse 3T3-L1 adipocyte library in a yeast two-hybrid interaction screen. Using this approach, the adipocyte differentiation factor, sterol response element binding protein 1 (SREBP1) was identified as a novel lamin A interactor. In vitro glutathione S-transferase pull-down and in vivo co-immunoprecipitation studies confirmed an interaction between lamin A and both SREBP1a and 1c. A binding site for lamin A was identified in the N-terminal transcription factor domain of SREBP1, between residues 227 and 487. The binding of lamin A to SREBP1 was noticeably reduced by FPLD mutations. Interestingly, one EDMD-AD mutation also interfered with the interaction between lamin A and SREBP1. Whilst the physiological relevance of this interaction has yet to be elucidated, these data raise the intriguing possibility that fat loss seen in laminopathies may be caused, at least in part, by reduced binding of the adipocyte differentiation factor SREBP1 to lamin A.

Glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) has been identified in multiple genome-wide association studies (GWAS) as a contributor to obesity, and GIPR knockout mice are protected against diet-induced obesity (DIO). On the basis of this genetic evidence, we developed anti-GIPR antagonistic antibodies as a potential therapeutic strategy for the treatment of obesity and observed that a mouse anti-murine GIPR antibody (muGIPR-Ab) protected against body weight gain, improved multiple metabolic parameters, and was associated with reduced food intake and resting respiratory exchange ratio (RER) in DIO mice. We replicated these results in obese nonhuman primates (NHPs) using an anti-human GIPR antibody (hGIPR-Ab) and found that weight loss was more pronounced than in mice. In addition, we observed enhanced weight loss in DIO mice and NHPs when anti-GIPR antibodies were codosed with glucagon-like peptide-1 receptor (GLP-1R) agonists. Mechanistic and crystallographic studies demonstrated that hGIPR-Ab displaced GIP and bound to GIPR using the same conserved hydrophobic residues as GIP. Further, using a conditional knockout mouse model, we excluded the role of GIPR in pancreatic β-cells in the regulation of body weight and response to GIPR antagonism. In conclusion, these data provide preclinical validation of a therapeutic approach to treat obesity with anti-GIPR antibodies.