Skeletal muscle insulin resistance is a key component of the etiology of type 2 diabetes. Caloric restriction (CR) enhances the sensitivity of skeletal muscle to insulin. However, the molecular signals within skeletal muscle linking CR to improved insulin action remain largely unknown. Recently, the mammalian ortholog of Sir2, sirtuin 1 (Sirt1), has been identified as a potential transducer of perturbations in cellular energy flux into subsequent metabolic adaptations, including modulation of skeletal muscle insulin action. Here, we have demonstrated that CR increases Sirt1 deacetylase activity in skeletal muscle in mice, in parallel with enhanced insulin-stimulated phosphoinositide 3-kinase (PI3K) signaling and glucose uptake. These adaptations in skeletal muscle insulin action were completely abrogated in mice lacking Sirt1 deacetylase activity. Mechanistically, Sirt1 was found to be required for the deacetylation and inactivation of the transcription factor Stat3 during CR, which resulted in decreased gene and protein expression of the p55α/p50α subunits of PI3K, thereby promoting more efficient PI3K signaling during insulin stimulation. Thus, these data demonstrate that Sirt1 is an integral signaling node in skeletal muscle linking CR to improved insulin action, primarily via modulation of PI3K signaling.

A connection between diet, obesity and diabetes exists in multiple species and is the basis of an escalating human health problem. The factors responsible provoke both insulin resistance and pancreatic beta cell dysfunction but remain to be fully identified. We report a combination of molecular events in human and mouse pancreatic beta cells, induced by elevated levels of free fatty acids or by administration of a high-fat diet with associated obesity, that comprise a pathogenic pathway to diabetes. Elevated concentrations of free fatty acids caused nuclear exclusion and reduced expression of the transcription factors FOXA2 and HNF1A in beta cells. This resulted in a deficit of GnT-4a glycosyltransferase expression in beta cells that produced signs of metabolic disease, including hyperglycemia, impaired glucose tolerance, hyperinsulinemia, hepatic steatosis and diminished insulin action in muscle and adipose tissues. Protection from disease was conferred by enforced beta cell–specific GnT-4a protein glycosylation and involved the maintenance of glucose transporter expression and the preservation of glucose transport. We observed that this pathogenic process was active in human islet cells obtained from donors with type 2 diabetes; thus, illuminating a pathway to disease implicated in the diet- and obesity-associated component of type 2 diabetes mellitus.

Dissipating excess calories as heat through therapeutic stimulation of brown adipose tissues (BAT) has been proposed as a potential treatment for obesity-linked disorders. Here, we describe the generation of a humanized effector-less bispecific antibody that activates fibroblast growth factor receptor (FGFR) 1/βKlotho complex, a common receptor for FGF21 and FGF19. Using this molecule, we show that antibody-mediated activation of FGFR1/βKlotho complex in mice induces sustained energy expenditure in BAT, browning of white adipose tissue, weight loss, and improvements in obesity-associated metabolic derangements including insulin resistance, hyperglycemia, dyslipidemia and hepatosteatosis. In mice and cynomolgus monkeys, FGFR1/βKlotho activation increased serum high-molecular-weight adiponectin, which appears to contribute over time by enhancing the amplitude of the metabolic benefits. At the same time, insulin sensitization by FGFR1/βKlotho activation occurs even before the onset of weight loss in a manner that is independent of adiponectin. Together, selective activation of FGFR1/βKlotho complex with a long acting therapeutic antibody represents an attractive approach for the treatment of type 2 diabetes and other obesity-linked disorders through enhanced energy expenditure, insulin sensitization and induction of high-molecular-weight adiponectin.

Abstract:Fibroblast growth factor 21 (FGF21) analogs and FGF21 receptor agonists (FGF21RAs) that mimic FGF21 ligand activity constitute the new "FGF21-class" of anti-obesity and anti-diabetic molecules that improve insulin sensitivity, ameliorate hepatosteatosis and promote weight loss. The metabolic actions of FGF21-class proteins in obese mice are attributed to stimulation of brown fat thermogenesis and increased secretion of adiponectin. The therapeutic utility of this class of molecules is being actively investigated in clinical trials for the treatment of type 2 diabetes and non-alcoholic steatohepatitis (NASH). This review is focused on various FGF21-class molecules, their molecular designs and the preclinical and clinical activities. These molecules include modified FGF21 as well as agonistic antibodies against the receptor for FGF21, namely the complex of FGF receptor 1 (FGFR1) and the obligatory coreceptor βKlotho (KLB). In addition, a novel approach to increase endogenous FGF21 activity by inhibiting the FGF21-degrading protease fibroblast activation protein (FAP) is discussed.

The phosphaturic hormone Fibroblast Growth Factor 23 (FGF23) controls phosphate homeostasis by regulating renal expression of sodium-dependent phosphate co-transporters and cytochrome P450 enzymes involved in vitamin D catabolism. Multiple FGF Receptors (FGFRs) can act as receptors for FGF23 when bound by the co-receptor Klotho expressed in the renal tubular epithelium. FGFRs also regulate skeletal FGF23 secretion; ectopic FGFR activation is implicated in genetic conditions associated with FGF23 overproduction and hypophosphatemia. The identity of FGFRs that mediate the activity of FGF23 or that regulate skeletal FGF23 secretion remains ill defined. Here we report that pharmacological activation of FGFR1 with monoclonal anti-FGFR1 antibodies (R1MAb) in adult mice is sufficient to cause an elevation in serum FGF23 and mild hypophosphatemia. In cultured rat calvariae osteoblasts, R1MAb induces FGF23 mRNA expression and FGF23 protein secretion into the culture medium. In a cultured kidney epithelial cell line, R1MAb acts as a functional FGF23 mimetic and activates the FGF23 program. siRNA-mediated Fgfr1 knockdown induced the opposite effects. Taken together, our work reveals the central role of FGFR1 in the regulation of FGF23 production and signal transduction, and has implications in the pathogenesis of FGF23-related hypophosphatemic disorders.

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ObjectiveFibroblast Growth Factor 21 (FGF21) is a potent stimulator of brown fat thermogenesis that improves insulin sensitivity, ameliorates hepatosteatosis, and induces weight loss by engaging the receptor complex comprised of Fibroblast Growth Factor Receptor 1 (FGFR1) and the requisite coreceptor βKlotho. Previously, recombinant antibody proteins that activate the FGFR1/βKlotho complex were proposed to act as an FGF21-mimetic; however, in vivo action of these engineered proteins has not been well studied.MethodsWe investigated the mechanism by which anti-FGFR1/βKlotho bispecific antibody (bFKB1) stimulates thermogenesis in UCP1-expressing brown adipocytes using genetically engineered mice. Anti-FGFR1 agonist antibody was also used to achieve brown adipose tissue restricted activation in transgenic mice.ResultsStudies with global Ucp1-deficient mice and adipose-specific Fgfr1 deficient mice demonstrated that bFKB1 acts on targets distal to adipocytes and indirectly stimulates brown adipose thermogenesis in a UCP1-independent manner. Using a newly developed transgenic system, we also show that brown adipose tissue restricted activation of a transgenic FGFR1 expressed under the control of Ucp1 promoter does not stimulate energy expenditure. Finally, consistent with its action as a FGF21 mimetic, bFBK1 suppresses intake of saccharin-containing food and alcohol containing water in mice.ConclusionsCollectively, we propose that FGFR1/βKlotho targeted therapy indeed mimics the action of FGF21 in vivo and stimulates UCP1-independent brown fat thermogenesis through receptors outside of adipocytes and likely in the nervous system.

How satellite cells and their progenitors balance differentiation and self-renewal to achieve sustainable tissue regeneration is not well understood. A major roadblock to understanding satellite cell fate decisions has been the difficulty of studying this process in vivo. By visualizing expression dynamics of myogenic transcription factors during early regeneration in vivo, we identify the time point at which cells undergo decisions to differentiate or self-renew. Single-cell RNA sequencing reveals heterogeneity of satellite cells, including a subpopulation enriched in Notch2 receptor expression, during both muscle homeostasis and regeneration. Furthermore, we reveal that differentiating cells express the Dll1 ligand. Using antagonistic antibodies, we demonstrate that the DLL1 and NOTCH2 signaling pair is required for satellite cell self-renewal. Thus, differentiating cells provide the self-renewing signal during regeneration, enabling proportional regeneration in response to injury while maintaining the satellite cell pool. These findings have implications for therapeutic control of muscle regeneration.