Recombinant fibroblast growth factor (FGF)21 has antihyperglycemic, antihyperlipidemic, and antiobesity effects in diabetic rodent and monkey models. Previous studies were confined to measuring steady-state effects of FGF21 following subchronic or chronic administration. The present study focuses on the kinetics of biological actions of FGF21 following a single injection and on the associated physiological and cellular mechanisms underlying FGF21 actions. We show that FGF21 resulted in rapid decline of blood glucose levels and immediate improvement of glucose tolerance and insulin sensitivity in two animal models of insulin resistance (ob/ob and DIO mice). In ob/ob mice, FGF21 led to a 40-60% decrease in blood glucose, insulin, and amylin levels within 1 h after injection, and the maximal effects were sustained for more than 6 h despite the 1- to 2-h half-life of FGF21. In DIO mice, FGF21 reduced fasting blood glucose and insulin levels and improved glucose tolerance and insulin sensitivity within 3 h of treatment. The acute improvement of glucose metabolism was associated with a 30% reduction of hepatic glucose production and an increase in peripheral glucose turnover. FGF21 appeared to have no direct effect on ex vivo pancreatic islet insulin or glucagon secretion. However, it rapidly induced typical FGF signaling in liver and adipose tissues and in several hepatoma-derived cell lines and differentiated adipocytes. FGF21 was able to inhibit glucose release from H4IIE hepatoma cells and stimulate glucose uptake in 3T3-L1 adipocytes. We conclude that the acute glucose-lowering and insulin-sensitizing effects of FGF21 are potentially associated with its metabolic actions in liver and adipose tissues.
Leptin has been shown to improve insulin sensitivity and glucose metabolism in normoinsulinemic healthy or obese rodents. It has not been determined whether leptin may act independently of insulin in regulating energy metabolism in vivo. The present study was designed to examine the effects of leptin treatment alone on glucose metabolism in insulin-deficient streptozotocin (STZ)-induced diabetic rats. Four groups of STZ-induced diabetic rats were studied: 1) rats treated with recombinant methionine murine leptin subcutaneous infusion with osmotic pumps for 12-14 days (LEP; 4 mg x kg(-1) x day(-1), n = 10); 2) control rats infused with vehicle (phosphate-buffered saline) for 12-14 days (VEH; n = 10); 3) pair-fed control rats given a daily food ration matching that of LEP rats for 12-14 days (PF; n = 8); and 4) rats treated with subcutaneous phloridzin for 4 days (PLZ; 0.4 g/kg twice daily, n = 10). Phloridzin treatment normalizes blood glucose without insulin and was used as a control for the effect of leptin in correcting hyperglycemia. All animals were then studied with a hyperinsulinemic-euglycemic clamp (6 mU x kg(-1) x min(-1). Our study demonstrates that leptin treatment in the insulin-deficient diabetic rats restored euglycemia, minimized body weight loss due to food restriction, substantially improved glucose metabolic rates during the postabsorptive state, and restored insulin sensitivities at the levels of the liver and the peripheral tissues during the glucose clamp. The effects on glucose turnover are largely independent of food restriction and changes in blood glucose concentration, as evidenced by the minimal improvement of insulin action and glucose turnover parameters in the PF and PLZ groups. Our results suggest that the antidiabetic effects of leptin are achieved through both an insulin-independent and an insulin-sensitizing mechanism.
Fibroblast growth factor 21 (FGF21) is a promising drug candidate for the treatment of type 2 diabetes. However, the use of wild type native FGF21 is challenging due to several limitations. Among these are its short half-life, its susceptibility to in vivo proteolytic degradation and its propensity to in vitro aggregation. We here describe a rationale-based protein engineering approach to generate a potent long-acting FGF21 analog with improved resistance to proteolysis and aggregation. A recombinant Fc-FGF21 fusion protein was constructed by fusing the Fc domain of human IgG1 to the N-terminus of human mature FGF21 via a linker peptide. The Fc positioned at the N-terminus was determined to be superior to the C-terminus as the N-terminal Fc fusion retained the βKlotho binding affinity and the in vitro and in vivo potency similar to native FGF21. Two specific point mutations were introduced into FGF21. The leucine to arginine substitution at position 98 (L98R) suppressed FGF21 aggregation at high concentrations and elevated temperatures. The proline to glycine replacement at position 171 (P171G) eliminated a site-specific proteolytic cleavage of FGF21 identified in mice and cynomolgus monkeys. The derived Fc-FGF21(RG) molecule demonstrated a significantly improved circulating half-life while maintaining the in vitro activity similar to that of wild type protein. The half-life of Fc-FGF21(RG) was 11 h in mice and 30 h in monkeys as compared to 1-2 h for native FGF21 or Fc-FGF21 wild type. A single administration of Fc-FGF21(RG) in diabetic mice resulted in a sustained reduction in blood glucose levels and body weight gains up to 5-7 days, whereas the efficacy of FGF21 or Fc-FGF21 lasted only for 1 day. In summary, we engineered a potent and efficacious long-acting FGF21 analog with a favorable pharmaceutical property for potential clinical development.
Circulating levels of fibroblast growth factor 21 (FGF21), a metabolic regulator of glucose, lipid, and energy homeostasis, are elevated in obese diabetic subjects, raising questions about potential FGF21 resistance. Here we report tissue expression changes in FGF21 and its receptor components, and we describe the target-organ and whole-body responses to FGF21 in ob/ob and diet-induced obese (DIO) mice. Plasma FGF21 concentrations were elevated 8- and 16-fold in DIO and ob/ob mice, respectively, paralleling a dramatic increase in hepatic FGF21 mRNA expression. Concurrently, expression levels of βKlotho, FGF receptor (FGFR)-1c, and FGFR2c were markedly down-regulated in the white adipose tissues (WAT) of ob/ob and DIO mice. However, dose-response curves of recombinant human FGF21 (rhFGF21) stimulation of ERK phosphorylation in the liver and WAT were not right shifted in disease models, although the magnitude of induction in ERK phosphorylation was partially attenuated in DIO mice. Whole-body metabolic responses were preserved in ob/ob and DIO mice, with disease models being more sensitive and responsive than lean mice to the glucose-lowering and weight-loss effects of rhFGF21. Endogenous FGF21 levels, although elevated in diseased mice, were below the half-maximal effective concentrations of rhFGF21, suggesting a state of relative deficiency. Hepatic and WAT FGF21 mRNA expression levels declined after rhFGF21 treatment in the absence of the increased expression levels of βKlotho and FGFR. We conclude that overt FGF21 resistance was not evident in the disease models, and increased hepatic FGF21 expression as a result of local metabolic changes is likely a major cause of elevated circulating FGF21 levels.
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