Growth/differentiation factor 15 (GDF15), also known as MIC-1, is a distant member of the transforming growth factor-β (TGF-β) superfamily and has been implicated in various biological functions, including cancer cachexia, renal and heart failure, atherosclerosis and metabolism. A connection between GDF15 and body-weight regulation was initially suggested on the basis of an observation that increasing GDF15 levels in serum correlated with weight loss in individuals with advanced prostate cancer. In animal models, overexpression of GDF15 leads to a lean phenotype, hypophagia and other improvements in metabolic parameters, suggesting that recombinant GDF15 protein could potentially be used in the treatment of obesity and type 2 diabetes. However, the signaling and mechanism of action of GDF15 are poorly understood owing to the absence of a clearly identified cognate receptor. Here we report that GDNF-family receptor α-like (GFRAL), an orphan member of the GFR-α family, is a high-affinity receptor for GDF15. GFRAL binds to GDF15 in vitro and is required for the metabolic actions of GDF15 with respect to body weight and food intake in vivo in mice. Gfral mice were refractory to the effects of recombinant human GDF15 on body-weight, food-intake and glucose parameters. Blocking the interaction between GDF15 and GFRAL with a monoclonal antibody prevented the metabolic effects of GDF15 in rats. Gfral mRNA is highly expressed in the area postrema of mouse, rat and monkey, in accordance with previous reports implicating this region of the brain in the metabolic actions of GDF15 (refs. 4,5,6). Together, our data demonstrate that GFRAL is a receptor for GDF15 that mediates the metabolic effects of GDF15.

Hypothalamic melanocortins are among several neuropeptides strongly implicated in the control of food intake. Agonists for melanocortin 4 (MC-4) receptors such as -melanocyte-stimulating hormone ( -MSH), a product of proopiomelanocortin (POMC), reduce food intake, whereas hypothalamic agouti-related protein (AgRP) is a MC-4 receptor antagonist that increases food intake. To investigate whether reduced melanocortin signaling contributes to hyperphagia induced by uncontrolled diabetes, male Sprague-Dawley rats were studied 7 days after administration of streptozotocin (STZ) or vehicle. In addition, we wished to determine the e ffect of diabetes on muscle uncoupling protein 3 (UCP-3), a potential regulator of muscle energy metabolism. STZ diabetic rats were markedly hyperglycemic (31.3 ± 1.0 mmol/l; P < 0.005) compared with nondiabetic controls (9.3 ± 0.2 mmol/l). Insulin treatment partially corrected the hyperglycemia (18.8 ± 2.5 mol/l; P < 0.005). Plasma leptin was markedly reduced in STZ diabetic rats (0.4 ± 0.1 ng/ml; P < 0.005) compared with controls ( 3 . 0 ± 0.4 ng/ml), an effect that was also partially reversed by insulin treatment (1.8 ± 0.3 n g / m l ) . Untreated diabetic rats were hyperphagic, consuming 40% more food (48 ± 1 g/day; P < 0.005) than controls (34 ± 1 g/day). Hyperphagia was prevented by insulin treatment (32 ± 2 g/day). In untreated diabetic rats, hypothalamic POMC mRNA expression (measured by in situ hybridization) was reduced by 80% (P < 0.005), whereas AgRP mRNA levels were increased by 60% (P < 0.01), suggesting a marked decrease of hypothalamic melanocortin signaling. The change in POMC, but not in A g R P, mRNA levels was partially reversed by insulin treatment. By comparison, the effects of diabetes to increase hypothalamic neuropeptide Y (NPY) expression and to decrease corticotropin-releasing hormone (CRH) expression were normalized by insulin treatment, whereas the expression of mRNA encoding the long form of the leptin receptor in the arcuate nucleus was unaltered by diabetes or insulin treatment. UCP-3 mRNA expression in gastrocnemius muscle from diabetic rats was increased fourfold (P < 0.005), and the increase was prevented by insulin treatment. The effect of uncontrolled diabetes to decrease POMC, while increasing AgRP gene expression, suggests that reduced hypothalamic melanocortin signaling, along with increased NPY and decreased CRH signaling, could contribute to diabetic hyperphagia. These responses, in concert with increased muscle UCP-3 expression, may also contribute to the catabolic effects of uncontrolled diabetes on fuel metabolism in peripheral tissues. D i a b e t e s 49:244-252, 2000 H yperphagia (1,2) and altered fuel metabolism (3,4) are prominent features of uncontrolled diabetes. Although the pathogenesis of diabetic hyperphagia is incompletely understood, several observations suggest a key role for reduced signaling by insulin and leptin, hormones involved in the regulation of energy balance via their effects on the central nervous system (CNS) to regulate ...

The ability of portal vein insulin to control hepatic glucose production (HGP) is debated. The aim of the present study was to determine, therefore, if the liver can respond to a selective decrease in portal vein insulin. Isotopic ([3H]glucose) and arteriovenous difference methods were used to measure HGP in conscious overnight fasted dogs. A pancreatic clamp (somatostatin plus basal portal insulin and glucagon) was used to control the endocrine pancreas. A 40-min control period was followed by a 180-min test period. During the latter, the portal vein insulin level was selectively decreased while the arterial insulin level was not changed. This was accomplished by stopping the portal insulin infusion and giving insulin peripherally at half the basal portal rate (PID, n=5). In a control group (n=5), the portal insulin infusion was not changed and glucose was infused to match the hyperglycemia that occurred in the PID group. A selective decrease of 120 pmol/l in portal vein insulin was achieved (basal, 150+/-36 to last 30 min, 30+/-12 pmol/l) in the absence of a change in the arterial insulin level (basal, 30+/-3 to last 30 min, 36+/-4 pmol/l). Neither arterial nor portal insulin levels changed in the control group (30+/-6 and 126+/-30 pmol/l, respectively). In response to the selective decrease in portal vein insulin, net hepatic glucose output (NHGO) increased significantly, from 8+/-1 (basal) to 30+/-6 and 14+/-2 micromol x kg(-1) x min(-1) by 15 min and the last 30 min (P < 0.05) of the experimental period, respectively. Arterial plasma glucose increased from 5.9+/-0.2 (basal) to 10.5+/-0.4 micromol/l (last 30 min). Three-carbon gluconeogenic precursor uptake fell from 11.2+/-2.9 (basal) to 5.9+/-0.7 micromol x kg(-1) x min(-1) (last 30 min), and thus a change in gluconeogenesis could not account for any of the increase in NHGO. With matched hyperglycemia (basal, 5.5+/-0.3 to last 30 min, 10.5+/-0.8 micromol/l) but no change in insulin, NHGO decreased from 12+/-1 (basal) to 0 (-1+/-6 micromol x kg(-1) x min(-1), last 30 min, P < 0.05) and hepatic gluconeogenic precursor uptake did not change (basal, 8.0+/-1.7 to last 30 min, 8.9+/-2.2 micromol x kg[-1] x min[-1]). Thus, the liver responds rapidly to a selective decrease in portal vein insulin by markedly increasing HGP as a result of increased glycogenolysis. These studies indicate that after an overnight fast, basal HGP (glycogenolysis) is highly sensitive to the hepatic sinusoidal insulin level.

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We investigated the mechanism by which a selective increase in arterial insulin can suppress hepatic glucose production in vivo. Isotopic (3-3H-glucose) and arteriovenous difference methods were used in overnight-fasted, conscious dogs. A pancreatic clamp (somatostatin, basal portal insulin, and glucagon infusions) was used to control the endocrine pancreas. Equilibration (100 min) and basal (40 min) periods were followed by a 180-min test period. In control dogs (n = 5), basal insulin delivery was continued throughout the study. In the other two groups, peripheral insulin was selectively increased at the beginning of the test period by stopping the portal insulin infusion and infusing insulin peripherally at twice the basal portal rate. One group (INS + FAT; n = 6) received an infusion of 20% intralipid + heparin (0.5 U x kg(-1) x min(-1)) to clamp the nonesterified fatty acid (NEFA) levels during hyperinsulinemia; the other group (INS; n = 7) received only saline during the experimental period. In the INS group, a selective increase in peripheral insulin of 84 pmol/l was achieved (36 +/- 6 to 120 +/- 24 pmol/l, last 30 min) while portal insulin was unaltered (84 +/- 18 pmol/l). In the INS + FAT group, a similar increase in peripheral insulin was achieved (36 +/- 6 to 114 +/- 6 pmol/l, last 30 min); again, portal insulin was unaltered (96 +/- 12 pmol/l). In the control group, basal insulin did not change. Glucagon and glucose remained near basal values in all protocols. In the INS group, NEFA levels dropped from 700 +/- 90 (basal) to 230 +/- 65 micromol/l (last 30 min; P > 0.05), but in the INS + FAT group changed minimally (723 +/- 115 [basal] to 782 +/- 125 micromol/l [last 30 min]). In the INS group, net hepatic glucose output dropped by 6.7 micromol x kg(-1) x min(-1) (P < 0.05), whereas in the INS + FAT group it dropped by 3.9 micromol x kg(-1) x min(-1) (P < 0.05). When insulin levels were not increased (i.e., in the control group), net hepatic glucose output dropped 1.7 micromol x kg(-1) x min(-1) (P < 0.05). In all groups, the net hepatic glucose output data were confirmed by the tracer-determined glucose production data. In the INS group, net hepatic gluconeogenic substrate uptake (alanine, glutamine, glutamate, glycerol, glycine, lactate, threonine, and serine) fell slightly (10.4 +/- 1.3 [basal] to 7.2 +/- 1.3 micromol x kg(-1) x min(-1) [last 30 min]), whereas in the INS + FAT group it did not change (7.3 +/- 1.5 [basal] to 7.4 +/- 0.6 micromol x kg(-1) x min(-1) [last 30 min]), and in the control group it increased slightly (9.6 +/- 1.3 [basal] to 10.3 +/- 1.4 micromol x kg(-1) x min(-1) [last 30 min). These results indicate that peripheral insulin's ability to regulate hepatic glucose production is partially linked to its inhibition of lipolysis. When plasma NEFA levels were prevented from falling during a selective arterial hyperinsulinemia, approximately 55% of insulin's inhibition of net hepatic glucose output (NHGO) was eliminated. The fall in NEFA levels brings about a redirection of glycogeno...