Ghrelin Enhances in Vivo Skeletal Muscle But Not Liver AKT Signaling in Rats

Abstract

BARAZZONI, ROCCO, MICHELA ZANETTI, MARIA ROSA CATTIN, LUCA VISINTIN, PIERANDREA VINCI, LUIGI CATTIN, MARCO STEBEL, AND GIANFRANCO GUARNIERI. Ghrelin enhances in vivo skeletal muscle but not liver AKT signaling in rats. Obesity. 2007;15:2614-2623. Objective: Ghrelin administration can induce fat weight gain and hyperglycemia (potentially through ghrelin-induced hepatic glucose production), but plasma ghrelin is positively associated with whole-body insulin sensitivity (mainly reflecting muscle insulin action) being increased in lean individuals or after diet-induced weight loss and reduced in obesity or after diet-induced weight gain. To investigate potential mechanisms, we measured in vivo effects of sustained ghrelin administration at a non-orexigenic dose on skeletal muscle and liver insulin signaling at the AKT level and adipokine expression changes. Research Methods and Procedures: Young-adult male rats received 4-day, twice daily subcutaneous ghrelin (200 g/ injection) or saline. We measured skeletal muscle (mixed, gastrocnemius; oxidative, soleus) and liver protein levels of activated [phosphorylated (P)] and total (T) AKT and glycogen synthase kinase (GSK; reflecting AKT-dependent GSK inactivation) and epididymal adipose tissue adipokine mRNA. Results: Ghrelin increased body weight (ϩ1.4%) and blood glucose (both p Ͻ 0.05 vs. saline) but not food intake, plasma insulin, or free fatty acids. Ghrelin, however, enhanced P/T/AKT and P/T/GSK ratios and glucose transporter-4 mRNA in soleus (p Ͻ 0.05), but not in gastrocnemius, muscle. In contrast, ghrelin reduced hepatic P/T-AKT and P/T-GSK. No alterations occurred in adiponectin, leptin, or resistin transcripts or plasma adiponectin. Discussion: Despite moderate weight gain and in the absence of insulin-free fatty acid changes, sustained ghrelin administration enhanced oxidative muscle AKT activation. Reduced liver AKT signaling could potentially contribute to concomitant blood glucose increments. These findings support ghrelin as a novel tissue-specific modulator of lean tissue AKT signaling with insulin-sensitizing effects in skeletal muscle but not in liver in vivo.

Fatty acids acutely enhance insulin-induced oxidative stress and cause insulin resistance by increasing mitochondrial reactive oxygen species (ROS) generation and nuclear factor-κB inhibitor (IκB)–nuclear factor-κB (NFκB) activation in rat muscle, in the absence of mitochondrial dysfunction

“…Protein analysis Total tissue protein was extracted from homogenised tissue as described [20]. Protein concentrations in all samples were measured by spectrophotometer (BCA Protein Assay Reagent, Pierce, Rockford, IL, USA).…”

“…Protein concentrations in all samples were measured by spectrophotometer (BCA Protein Assay Reagent, Pierce, Rockford, IL, USA). Total v-akt murine thymoma viral oncogene homologue 1 (AKT), phospho(Ser473)-AKT, total IκBα and phospho (Ser32)-IκBα were measured by western blotting using commercially available antibodies (Cell Signaling Technology, Beverly, MA, USA) [20]. Individual results were expressed as percentages of the average values in the control group.…”

Des-Acyl Ghrelin Fragments and Analogues Promote Survival of Pancreatic β-Cells and Human Pancreatic Islets and Prevent Diabetes in Streptozotocin-Treated Rats

“…Table 1. List of Des-Acyl Ghrelin Fragments compd peptide sequence 1 des-acyl ghrelin GSSFLSPEHQRVQQRKESKKPPAKLQPR-COOH 2 des-acyl ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) GSSFLSPEHQRVQQRKES-NH 2 3 des-acyl ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) GSSFLSPEHQRVQQ-NH 2 4…”

Neuroendocrine Control of Hepatic Gluconeogenesis

“…Ghrelin administration induces a lipogenic and glucogenic patern of gene expression in the liver and increases hepatic triglyceride content [48]. Also, ghrelin atenuates liver AKT signalling, which may contribute to the blood glucose increments [49].…”