Blockade of the growth hormone (GH) receptor unmasks rapid GH-releasing peptide-6-mediated tissue-specific insulin resistance

The roles of GH and its receptor (GHR) in metabolic control are not yet fully understood. We studied the roles of GH and the GHR using the GHR antagonist pegvisomant for metabolic control of healthy nonobese men in fasting and nonfasting conditions. Ten healthy subjects were enrolled in a double blind, placebo-controlled study on the effects of pegvisomant on GHRH and GH-releasing peptide-6 (GHRP-6)-induced GH secretion before and after 3 days of fasting and under nonfasting conditions (n = 5). Under the condition of GHR blockade by pegvisomant in the nonfasting state, GHRP-6 (1 microg/kg) caused a increase in serum insulin (10.3 +/- 2.1 vs. 81.3 +/- 25.4 mU/L; P < 0.001) and glucose (4.2 +/- 0.3 vs. 6.0 +/- 0.6 mmol/L; P < 0.05) concentrations. In this group, a rapid decrease in serum free fatty acids levels was also observed. These changes were not observed under GHR blockade during fasting or in the absence of pegvisomant. We conclude that although these results were obtained from an acute study, and long-term administration of pegvisomant could render different results, blockade of the GHR in the nonfasting state induces tissue-specific changes in insulin sensitivity, resulting in an increase in glucose and insulin levels (indicating insulin resistance of liver/muscle), but probably also in an increase in lipogenesis (indicating normal insulin sensitivity of adipose tissue). These GHRP-6-mediated changes indicate that low GH bioactivity on the tissue level can induce changes in metabolic control, which are characterized by an increase in fat mass and a decrease in lean body mass. As a mechanism of these GHRP-6-mediated metabolic changes in the nonfasting state, direct nonpituitary-mediated GHRP-6 effects on the gastroentero-hepatic axis seem probable.     

Systemic response of the GH/IGF-I axis in timely versus delayed fracture healing.

The GH-IGF axis has profound effects on the local and systemic regulation of bone metabolism and may be important for quality of fracture healing. To test the hypothesis that deficiency of the GH/IGF axis may play a role in the pathogenesis of fracture non-union we investigated whether alterations of serum concentrations of the GH-IGF axis could be related to failed fracture healing compared to timely fracture healing in trauma patients. Serum probes were prospectively collected from 186 patients with surgical treatment of long bone fractures up to 6 months after surgery. Samples from 14 patients with atrophic type of non-union have been compared to 14 matched patients with normal bone healing. Postoperative time courses of serum concentrations have been analyzed using commercially available chemiluminescence sandwich assays (GH), fully automated assay systems (IGF-I, IGFBP-3) or sandwich immunometric assays (ALS). Comparison between both collectives revealed significantly lower serum concentrations of GH dependent ALS during early (1st week after surgery) and of both IGFBP-3 and ALS during late stages of fracture healing (6 and 8 weeks after surgery) in non-union patients, coinciding clinically with failed fracture healing. Tendentially lower serum levels of IGF-I in the non-union group over the entire investigation period were statistically not significant. We have been able to show time courses of serum concentrations of the GH/IGF-I axis during normal and failed fracture healing in humans. An impairment of the GH/IGF-I axis might be involved in the biochemical mechanisms determining delayed or failed fracture healing.     

Quantification of the soluble leptin receptor in human blood by ligand-mediated immunofunctional assay

Panels of monoclonal antibodies (mAbs) were raised against recombinant human leptin and the recombinant human soluble leptin receptor. Using these mAbs, we established a ligand-mediated immunofunctional assay (LIFA) to quantify concentrations of the soluble leptin receptor, which has been shown to be a major binding protein for leptin in human serum. In performing the assay, a monoclonal antibody (mAb 2H6) against the soluble leptin receptor, which binds an epitope outside the leptin-binding site and equally recognizes both, free and leptin-occupied soluble leptin receptor, is used to capture the soluble leptin receptor on a microtiter plate. Recombinant human leptin is added to saturate all binding sites, and a biotinylated anti-leptin mAb (4D3) detects the amount of leptin (endogenous and exogenous) bound to the soluble leptin receptor. The same procedure, but without adding exogenous leptin, allows for measurement of the circulating endogenous leptin/soluble leptin receptor complexes. The LIFA assay has a linear working range of 0.5-200 microg/liter, intra- and interassay coefficients of variation ranged from 3.2-6.3% and from 5.2-7.9%, respectively. The assay has a linearity of 102.2 +/- 5.2% (mean +/- SD) and a recovery of 100.7 +/- 6.9%. Size-exclusion chromatography revealed that the assay measures a protein with a main peak eluted at 340 kDa. The soluble leptin receptor concentration (63.3 +/- 22.8 microg/liter (mean +/- SD), range 17.9-129.2 microg/liter, n = 43) in normal subjects (body mass index = 22.3 +/- 2.3 kg/m(2)) was not different from the concentration (54.4 +/- 19.8 microg/liter, range 23.7-104.8 microg/liter, n = 34, P > 0.05) found in obese subjects (body mass index = 40.9 +/- 15.7 kg/m(2)). However, the percentage of the total soluble leptin receptor complexed with endogenous leptin was significantly higher in obese subjects, compared with normal subjects (74.9% +/- 23.5% vs. 33.1% +/- 19.5%, P < 0.001). Higher serum leptin levels in obese subjects (38.4 +/- 23.7 microg/liter vs. 7.8 +/- 5.5 microg/liter in normal subjects, P < 0.001) together with comparable soluble leptin receptor levels result in a lower proportion of leptin bound to the soluble leptin receptor in obese subjects (19.3% +/- 19.4%, range 4.9-97.2%) than in normal subjects (39.0% +/- 22.5%, range 15.3-96.5%, P < 0.001). The development of this LIFA for the rapid and accurate quantification of total soluble leptin receptor and circulating leptin/soluble leptin receptor complexes provides a valuable tool for the further understanding of the role of leptin and its soluble receptor in health and disease.