Ghrelin, an appetite-stimulatory hormone secreted by the stomach, was discovered as a ligand for the growth hormone secretagogue receptor (GHSR). Through GHSR, ghrelin stimulates growth hormone (GH) secretion, a function that evolved to protect against starvation-induced hypoglycemia. Though the biology mediated by ghrelin has been described in great detail, regulation of ghrelin action is poorly understood. Here, we report the discovery of liver-expressed antimicrobial peptide 2 (LEAP2) as an endogenous antagonist of GHSR. LEAP2 is produced in the liver and small intestine, and its secretion is suppressed by fasting. LEAP2 fully inhibits GHSR activation by ghrelin and blocks the major effects of ghrelin in vivo, including food intake, GH release, and maintenance of viable glucose levels during chronic caloric restriction. In contrast, neutralizing antibodies that block endogenous LEAP2 function enhance ghrelin action in vivo. Our findings reveal a mechanism for fine-tuning ghrelin action in response to changing environmental conditions.

The large docking protein IRS-1 is a major substrate for the insulin receptor and other tyrosine kinases. It plays a key role in eliciting many of insulin's actions, including binding and activation of phosphatidylinositol (PI) 3-kinase and the subsequent increase in glucose transport. Gene disruption of IRS-1 in mice is associated with an impaired insulin-stimulated glucose disposal in vivo and glucose transport in vitro, but the survival of the animals and residual insulin sensitivity is dependent on the presence of the alternative docking protein IRS-2. We examined the expression and function of IRS-1 and IRS-2 in adipocytes from healthy and diabetic individuals. Cells from subjects with non-insulin-dependent diabetes mellitus (NIDDM), but not with insulin-dependent diabetes mellitus, had an impaired insulin effect and a marked reduction (70 ؎ 6%) in the expression of IRS-1 protein, whereas IRS-2 was unchanged. In normal cells, IRS-1 was the main docking protein for the binding and activation of insulin-stimulated PI 3-kinase; IRS-2 was also functional but required a higher insulin concentration for a similar binding and activation of PI 3-kinase. In contrast in NIDDM cells with a low IRS-1 content, IRS-2 became the main docking protein. These findings may provide important reasons for the insulin resistance in NIDDM.

The role of protein-tyrosine phosphatase 1B (PTP1B) in diabetes was investigated using an antisense oligonucleotide in ob͞ob and db͞db mice. PTP1B antisense oligonucleotide treatment normalized plasma glucose levels, postprandial glucose excursion, and HbA 1C. Hyperinsulinemia was also reduced with improved insulin sensitivity. PTP1B protein and mRNA were reduced in liver and fat with no effect in skeletal muscle. Insulin signaling proteins, insulin receptor substrate 2 and phosphatidylinositol 3 (PI3)-kinase regulatory subunit p50␣, were increased and PI3-kinase p85␣ expression was decreased in liver and fat. These changes in protein expression correlated with increased insulin-stimulated protein kinase B phosphorylation. The expression of liver gluconeogenic enzymes, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase was also down-regulated. These findings suggest that PTP1B modulates insulin signaling in liver and fat, and that therapeutic modalities targeting PTP1B inhibition may have clinical benefit in type 2 diabetes.