Activation of the hypothalamic-pituitary-adrenal axis by exogenous and endogenous GDF15.

An acute increase in the circulating concentration of glucocorticoid hormones is essential for the survival of severe somatic stresses. Circulating concentrations of GDF15, a hormone that acts in the brain to reduce food intake, are frequently elevated in stressful states. We now report that GDF15 potently activates the hypothalamic-pituitary-adrenal (HPA) axis in mice and rats. A blocking antibody to the GDNF-family receptor α-like receptor completely prevented the corticosterone response to GDF15 administration. In wild-type mice exposed to a range of stressful stimuli, circulating levels of both corticosterone and GDF15 rose acutely. In the case of Escherichia coli or lipopolysaccharide injections, the vigorous proinflammatory cytokine response elicited was sufficient to produce a near-maximal HPA response, regardless of the presence or absence of GDF15. In contrast, the activation of the HPA axis seen in wild-type mice in response to the administration of genotoxic or endoplasmic reticulum toxins, which do not provoke a marked rise in cytokines, was absent in Gdf15-/- mice. In conclusion, consistent with its proposed role as a sentinel hormone, endogenous GDF15 is required for the activation of the protective HPA response to toxins that do not induce a substantial cytokine response. In the context of efforts to develop GDF15 as an antiobesity therapeutic, these findings identify a biomarker of target engagement and a previously unrecognized pharmacodynamic effect, which will require monitoring in human studies.

GDF15: A Hormone Conveying Somatic Distress to the Brain.

GDF15 has recently gained scientific and translational prominence with the discovery that its receptor is a GFRAL-RET heterodimer of which GFRAL is expressed solely in the hindbrain. Activation of this receptor results in reduced food intake and loss of body weight and is perceived and recalled by animals as aversive. This information encourages a revised interpretation of the large body of previous research on the protein. GDF15 can be secreted by a wide variety of cell types in response to a broad range of stressors. We propose that central sensing of GDF15 via GFRAL-RET activation results in behaviors that facilitate the reduction of exposure to a noxious stimulus. The human trophoblast appears to have hijacked this signal, producing large amounts of GDF15 from early pregnancy. We speculate that this encourages avoidance of potential teratogens in pregnancy. Circulating GDF15 levels are elevated in a range of human disease states, including various forms of cachexia, and GDF15-GFRAL antagonism is emerging as a therapeutic strategy for anorexia/cachexia syndromes. Metformin elevates circulating GDF15 chronically in humans and the weight loss caused by this drug appears to be dependent on the rise in GDF15. This supports the concept that chronic activation of the GDF15-GFRAL axis has efficacy as an antiobesity agent. In this review, we examine the science of GDF15 since its identification in 1997 with our interpretation of this body of work now being assisted by a clear understanding of its highly selective central site of action.

Circulating adipokines are associated with pre-eclampsia in women with type 1 diabetes.

AIMS/HYPOTHESIS: The incidence of pre-eclampsia, a multisystem disorder of pregnancy, is fourfold higher in type 1 diabetic than non-diabetic women; it is also increased in women with features of the metabolic syndrome and insulin resistance. In a prospective study of pregnant women with type 1 diabetes, we measured plasma levels of adipokines known to be associated with insulin resistance: leptin, fatty acid binding protein 4 (FABP4), adiponectin (total and high molecular weight [HMW]; also known as high molecular mass), retinol binding protein 4 (RBP4) and resistin and evaluated associations with the subsequent development of pre-eclampsia. METHODS: From an established prospective cohort of pregnant type 1 diabetic women, we studied 23 who developed pre-eclampsia and 24 who remained normotensive; for reference values we included 19 healthy non-diabetic normotensive pregnant women. Plasma adipokines were measured (by ELISA) in stored samples from three study visits (Visit 1- Visit 3) at different gestational ages (mean ± SD): Visit 1, 12.4 ± 1.8 weeks; Visit 2, 21.7 ± 1.4 weeks; and Visit 3, 31.4 ± 1.5 weeks. All the women were free of microalbuminuria and hypertension at enrolment. All study visits preceded the clinical onset of pre-eclampsia. RESULTS: In all groups, leptin, the ratio of leptin to total or HMW adiponectin, FABP4 concentration, ratio of FABP4 to total or HMW adiponectin and resistin level increased, while total and HMW adiponectin decreased, with gestational age. At Visit 1: (1) in diabetic women with vs without subsequent pre-eclampsia, leptin, ratio of leptin to total or HMW adiponectin, and ratio of FABP4 to total or HMW adiponectin, were increased (p < 0.05), while total adiponectin was decreased (p < 0.05); and (2) in normotensive diabetic vs non-diabetic women, total adiponectin was elevated (p < 0.05). At Visits 2 and 3: (1) the primary findings in the two diabetic groups persisted, and FABP4 also increased in women with subsequent pre-eclampsia (p < 0.05); and (2) there were no differences between the two normotensive groups. By logistic regression analyses after covariate adjustment (HbA1c, insulin kg-1 day-1 and gestational age), the best predictive models for pre-eclampsia were as follows: Visit 1, doubling of leptin, OR 9.0 (p < 0.01); Visit 2, doubling of the leptin:total adiponectin ratio, OR 3.7 (p < 0.05); and Visit 3, doubling of FABP4 concentration, OR 25.1 (p < 0.01). The associations were independent of BMI. CONCLUSIONS/INTERPRETATION: As early as the first trimester in type 1 diabetic women, adipokine profiles that suggest insulin resistance are associated with subsequent pre-eclampsia, possibly reflecting maternal characteristics that precede pregnancy. These associations persist in the second and third trimesters, and are independent of BMI. Insulin resistance may predispose women with type 1 diabetes to pre-eclampsia.

Endothelial insulin receptors differentially control insulin signaling kinetics in peripheral tissues and brain of mice.

Insulin receptors (IRs) on endothelial cells may have a role in the regulation of transport of circulating insulin to its target tissues; however, how this impacts on insulin action in vivo is unclear. Using mice with endothelial-specific inactivation of the IR gene (EndoIRKO), we find that in response to systemic insulin stimulation, loss of endothelial IRs caused delayed onset of insulin signaling in skeletal muscle, brown fat, hypothalamus, hippocampus, and prefrontal cortex but not in liver or olfactory bulb. At the level of the brain, the delay of insulin signaling was associated with decreased levels of hypothalamic proopiomelanocortin, leading to increased food intake and obesity accompanied with hyperinsulinemia and hyperleptinemia. The loss of endothelial IRs also resulted in a delay in the acute hypoglycemic effect of systemic insulin administration and impaired glucose tolerance. In high-fat diet-treated mice, knockout of the endothelial IRs accelerated development of systemic insulin resistance but not food intake and obesity. Thus, IRs on endothelial cells have an important role in transendothelial insulin delivery in vivo which differentially regulates the kinetics of insulin signaling and insulin action in peripheral target tissues and different brain regions. Loss of this function predisposes animals to systemic insulin resistance, overeating, and obesity.

SHP-1 activation inhibits vascular smooth muscle cell proliferation and intimal hyperplasia in a rodent model of insulin resistance and diabetes.

AIMS/HYPOTHESIS: Accelerated migration and proliferation of vascular smooth muscle cells (VSMCs) enhances arterial restenosis after angioplasty in insulin resistance and diabetes. Elevation of Src homology 2-containing protein tyrosine phosphatase 1 (SHP-1) induces apoptosis in the microvasculature. However, the role of SHP-1 in intimal hyperplasia and restenosis has not been clarified in insulin resistance and diabetes. METHODS: We used a femoral artery wire injury mouse model, rodent models with insulin resistance and diabetes, and patients with type 2 diabetes. Further, we modulated SHP-1 expression using a transgenic mouse that overexpresses SHP-1 in VSMCs (Shp-1-Tg). SHP-1 agonists were also employed to study the molecular mechanisms underlying the regulation of SHP-1 by oxidised lipids. RESULTS: Mice fed a high-fat diet (HFD) exhibited increased femoral artery intimal hyperplasia and decreased arterial SHP-1 expression compared with mice fed a regular diet. Arterial SHP-1 expression was also decreased in Zucker fatty rats, Zucker diabetic fatty rats and in patients with type 2 diabetes. In primary cultured VSMCs, oxidised LDL suppressed SHP-1 expression by activating Mek-1 (also known as Map2k1) and increased DNA methylation of the Shp-1 promoter. VSMCs from Shp-1-Tg mice exhibited impaired platelet-derived growth factor (PDGF)-stimulated tyrosine phosphorylation with a concomitant decrease in PDGF-stimulated VSMC proliferation and migration. Similarly, HFD-fed Shp-1-Tg mice and mice treated with the SHP-1 inducer, Icariside II, were protected from the development of intimal hyperplasia following wire injury. CONCLUSIONS/INTERPRETATION: Suppression of SHP-1 by oxidised lipids may contribute to the excessive VSMC proliferation, inflammatory cytokine production and intimal hyperplasia observed in arteries from diabetes and insulin resistance. Augmenting SHP-1 levels is a potential therapeutic strategy to maintain stent patency in patients with insulin resistance and diabetes.

Insulin Downregulates the Transcriptional Coregulator CITED2, an Inhibitor of Proangiogenic Function in Endothelial Cells.

In patients with atherosclerotic complications of diabetes, impaired neovascularization of ischemic tissue in the myocardium and lower limb limits the ability of these tissues to compensate for poor perfusion. We identified 10 novel insulin-regulated genes, among them Adm, Cited2, and Ctgf, which were downregulated in endothelial cells by insulin through FoxO1. CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2), which was downregulated by insulin by up to 54%, is an important negative regulator of hypoxia-inducible factor (HIF) and impaired HIF signaling is a key mechanism underlying the impairment of angiogenesis in diabetes. Consistent with impairment of vascular insulin action, CITED2 was increased in cardiac endothelial cells from mice with diet-induced obesity and from db/db mice and was 3.8-fold higher in arterial tissue from patients with type 2 diabetes than control subjects without diabetes. CITED2 knockdown promoted endothelial tube formation and endothelial cell proliferation, whereas CITED2 overexpression impaired HIF activity in vitro. After femoral artery ligation, induction of an endothelial-specific HIF target gene in hind limb muscle was markedly upregulated in mice with endothelial cell deletion of CITED2, suggesting that CITED2 can limit HIF activity in vivo. We conclude that vascular insulin resistance in type 2 diabetes contributes to the upregulation of CITED2, which impairs HIF signaling and endothelial proangiogenic function.