Ghrelin rescues palmitate‐induced decrements in insulin signaling in oxidative skeletal muscle from rats
Authors: Cervone Daniel T , Dyck David J 
Published: June 2021

Introduction

Ghrelin is an orexigenic gastric hormone that also has metabolic effects on insulin‐responsive tissues (eg. skeletal muscle). Existing work from our lab using rat skeletal muscle (unpublished), and others using myoblasts (Han L et al. 2015) demonstrate that ghrelin can stimulate fatty acid oxidation. Our current experimental model is aimed at extending these findings, and determining whether ghrelin can protect against acute, palmitate‐induced impairments in insulin signaling and glucose uptake in skeletal muscle, a major site for insulin‐stimulated glucose disposal.

Methods

To date, oxidative soleus muscle strips have been isolated from healthy, male Sprague‐Dawley rats and incubated in vials containing pre‐gassed (95% O2, 5% CO2) and warmed (30ºC) media. All muscles were equilibrated for 30min, then underwent either low palmitate (0.2mM, LP) or high palmitate (2mM, HP) treatment, with or without acylated (AG) or unacylated (UnAG) ghrelin, for 4h. Muscles were then subjected to either saline or maximal insulin (10mU/ml, 10min) and then snap frozen in liquid nitrogen. Western blots were used to quantify the activation of insulin signaling protein AKT (Ser473, Thr308).

Results

We first confirmed that HP impairs muscle insulin signaling within 4h. Insulin significantly stimulated AKT activation (Ser473) in both palmitate conditions at 1mU/ml (LP: 2.4±0.6; HP: 1.9±0.3) and 10mU/ml (LP: 12.5±1.6; HP: 5.4±0.6) doses. The HP‐induced blunting of AKT phosphorylation was more readily apparent at the maximal insulin concentration; therefore, this was utilized for subsequent incubations to examine the potential effects of ghrelin. For signaling experiments with ghrelin, when compared to control (1.2±0.6), 10mU/ml insulin increased the activation of AKT at both its Ser473 (LP: 15.3±3.23; HP: 6.86±1.01) and Thr308 (LP: 12.1±3.17; HP: 7.7±2.05) residues at both palmitate concentrations (p<0.01). Therefore, consistent with our pilot work, 4h of HP exposure significantly reduced insulin's activation of AKT at Ser473 compared to LP (p<0.01); this trend was present with Thr308, but was not significant (p=0.1). Interestingly, AG (14.8±7.6, p<0.05), but not UnAG,(11.2±7.3, p>0.05) was able to preserve Ser473 AKT activation in the HP condition. Neither AG (8.19±2.32) nor UnAG (5.86±1.20) altered insulin's ability to activate AKT (Thr308) in the context of HP (p>0.05).

Conclusions

AG preserved insulin's ability to fully activate AKT at its Ser473 site under HP conditions. Further incubations will be performed to increase statistical power as UnAG trended towards a similar effect, and has been reported to increase insulin sensitivity in the literature. Future measurements will determine whether this alteration to AKT signaling can manifest as a functional change in glucose transport (i.e. whether AG can protect against palmitate‐induced reductions in glucose transport). Also, we will assess ghrelin's effects on (2mM) palmitate oxidation and whether AMPK is involved. These experiments will contribute to the interpretation of both AG and UnAG's effects in skeletal muscle glucose/fatty acid metabolism and insulin signaling.

Support or Funding Information

DJD is supported by a grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada. DTC holds an NSERC PGS‐D Scholarship.

This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.