Historically, fatty acid-binding protein 4 (Fabp4), commonly called aP2, has been viewed as a strictly intracellular protein. Recently, however, secreted, soluble forms have been identified that can influence systemic metabolism. In a 2015 preclinical study published in the journal Science Translational Medicine investigators as Harvard’s T.H. Chan School of Public Health (Boston, MA) and UCB (Union Chimique Belge) demonstrate that monoclonal antibodies can interrupt Fabp4/aP2 adipokine signaling from adipose tissue and improve glucose metabolism in two mouse models of type 2 diabetes (T2D) (Burak et al., 2015). Blocking Fabp4/aP2 signaling increased systemic insulin sensitivity and glucose uptake by tissues, decreased hepatic glucose production, and reduced liver steatosis. This study highlights a promising new therapy for treating multiple metabolic defects associated with T2D as well as dyslipidemia associated with atherosclerosis.
Fabp4/aP2 fights obesity but promotes diabetes
C57BL/6J (B6) mice are widely used to study diet-induced obesity (DIO). When fed a high fat diet, the mice gain weight and develop progressively more severe insulin resistance and glucose intolerance, similar to pre-diabetic humans. When homozygous C57BL/6-Fabp4-knockout mice (B6-Fabp4 -/-) are fed a high fat diet, they gain even more weight than wild-type B6 DIO mice. The B6-Fabp4 -/- DIO mice, however, do not develop insulin resistance or other pre-diabetic phenotypes observed in wild-type B6 DIO mice. Further, B6 Fabp4 and ApoE double mutant mice show improved lipid metabolism and decreased susceptibility to atherosclerosis compared to B6-ApoE single mutant mice.
Both obese humans and mice have significantly elevated soluble Fabp4/aP2 levels, and injection of recombinant Fabp4/aP2 into lean wild type mice stimulates hepatic glucose production and gluconeogenesis. Also, humans with genetic haplo-insufficiency in FABP4 are largely protected from developing diabetes and atherosclerosis. Taken together, these observations demonstrate a strong role for Fabp4/aP2 in regulating glucose metabolism and provided the impetus for the Harvard and UCB team to develop a monoclonal antibody (mAb) to inhibit the adipokine function of soluble Fabp4/ap2.
Blocking Fabp4/aP2 signaling in vivo ameliorates diabetes
The investigators developed several mAb’s against human and mouse Fabp4/aP2 peptides and tested their in vivo activity in B6 DIO (380050) mice. B6 mice were placed on a high fat diet at 5 weeks of age and were treated with either vehicle or antibody for 4 weeks beginning when the mice were 20 weeks of age (15 weeks on diet). B6 DIO mice have elevated insulin secretion, increased fed blood glucose, and significant glucose intolerance, but mice treated with one particular anti-Fabp4/aP2 mAb, CA33, showed decreased insulin secretion and lower blood glucose. The CA33 antibody also improved glucose tolerance and insulin sensitivity. CA33-treated B6 DIO mice consumed the same amount of food compared to controls and only showed a slight loss in body weight. Vehicle-treated controls, on the other hand, gained weight. The specificity of the CA33 antibody to inhibit Fabp4/aP2 signaling (and not other secondary or off-target proteins) was confirmed using B6-Fabp4 -/- DIO mice. Treating these mice with CA33 failed to lower blood glucose levels or to improve glucose intolerance compared to vehicle-treated mice, verifying that the efficacious results observed in the CA33-treated wild-type DIO mice were due to specific targeting of Fabp4/aP2.
The effects of the CA33 mAb on metabolism also were tested in the severely obese and diabetic leptin-deficient ob/ob mice (B6.Cg-Lepob/J (000632)) mice. Homozygous ob/ob mice treated with CA33 remained obese, but showed normalized hyperglycemia, decreased hyperinsulinemia, and improved glucose tolerance. Therefore, the CA33 mAb demonstrated specificity for Fabp4/aP2 and improved glucose metabolism in two different preclinical mouse models of human T2D.
Mouse Models of Type 2 Diabetes. [A] (Left) C57BL/6J are sensitive to diet-induced obesity (DIO). B6 DIO mice (380050) (right) are obese, have slightly elevated fed blood glucose, and are glucose intolerant compared to normal diet controls (left). [B] (Right) B6.Cg-Lepob/J (000632), commonly called ob/ob, compared to a littermate control. The ob/ob mice are overtly obese, have severe hyperinsulinemia, transient hyperglycemia and are glucose intolerant.
Blocking Fabp4/aP2 signaling improves liver metabolism
B6 DIO mice develop significant fatty liver disease (steatosis). Livers from CA33-treated B6 DIO mice demonstrated markedly decreased steatosis. Biochemical analysis of the fatty livers from these mice revealed significantly reduced triglyceride, glycerol, and cholesterol content. The CA33-treated livers also showed lower mRNA levels for three genes associated with lipogenesis: stearoyl-CoA desaturase (Scd1), fatty acid synthase (Fasn), and acetyl-CoA carboxylase (Acc1). These results demonstrate that blocking Fabp4/aP2 signaling in vivo decreases liver lipogenesis.
A prior publication demonstrated that Fabp4/aP2 influences liver gluconeogenesis and hepatic glucose production. Livers collected from CA33-treated B6 DIO mice revealed decreases in mRNA and enzyme activity for the gluconeogenic genes phosphoenolpyruvate carboxykinase 1 (Pck1) and glucose 6-phosphatase (G6Pase). Next, the Harvard and UCB team measured glucose production and utilization in CA33-treated mice using a hyperinsulinemic-euglycemic clamp. In this assay, mice are infused with insulin to establish hyperinsulinemia and then are treated with glucose at a rate to establish a state of euglycemia. When steady-state is achieved, the glucose infusion rate is equivalent to the rate at which glucose is taken up by tissues. Under these conditions, B6 DIO mice treated with CA33 required a higher glucose infusion rate, had a greater rate of systemic glucose disappearance, increased glucose uptake in muscle, and decreased hepatic glucose production compared to controls. These results demonstrate that blocking Fabp4/aP2 signaling improved insulin sensitivity, allowing better glucose utilization by tissues and decreased glucose production by the liver.
The findings by the Harvard and UCB researchers provide compelling preclinical evidence that mAb CA33 treatment improves multiple disease phenotypes in two independent mouse models of obesity and T2D. Mechanistically, the antibody was shown to block Fabp4/aP2 adipokine signaling in the liver, leading to decreased lipogenesis and decreased glucose production. Systemically, the antibody treatment increased insulin sensitivity, as demonstrated by lowered blood glucose, and improved glucose utilization by tissues. These results uncover a unique signaling pathway and a novel therapeutic approach that may lead to greatly needed drugs for treating metabolic abnormalities associated with obesity.