Unacylated ghrelin stimulates fatty acid oxidation to protect skeletal muscle against palmitate-induced impairment of insulin action in lean but not high-fat fed rats

Abstract

Background Ghrelin is a gut hormone that spikes in circulation before mealtime. Recent findings suggest that both ghrelin isoforms stimulate skeletal muscle fatty acid oxidation, lending to the possibility that it may regulate skeletal muscle’s handling of meal-derived substrates. It was hypothesized in the current study that ghrelin may preserve muscle insulin response during conditions of elevated saturated fatty acid (palmitate) availability by promoting its oxidation. Methods and results Soleus muscle strips were isolated from male rats to determine the direct effects of ghrelin isoforms on fatty acid oxidation, glucose uptake and insulin signaling. We demonstrate that unacylated ghrelin (UnAG) is the more potent stimulator of skeletal muscle fatty acid oxidation. Both isoforms of ghrelin generally protected muscle from impaired insulin-mediated phosphorylation of AKT Ser 473 and Thr 308 , as well as downstream phosphorylation of AS160 Ser 588 during high palmitate exposure. However, only UnAG was able to preserve insulin-stimulated glucose uptake during exposure to high palmitate concentrations. The use of etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase (CPT-1) abolished this protection, strongly suggesting that UnAG’s stimulation of fatty acid oxidation may be essential to this protection. To our knowledge, we are also the first to investigate the impact of a chronic high-fat diet on ghrelin’s actions in muscle. Following 6 wks of a high-fat diet, UnAG was unable to preserve insulin-stimulated signaling or glucose transport during an acute high palmitate exposure. UnAG was also unable to further stimulate 5′ AMP-activated protein kinase (AMPK) or fatty acid oxidation during high palmitate exposure. Corticotropin-releasing hormone receptor-2 (CRF-2R) content was significantly decreased in muscle from high-fat fed animals, which may partially account for the loss of UnAG’s effects. Conclusions UnAG is able to protect muscle from acute lipid exposure, likely due to its ability to stimulation fatty acid oxidation. This effect is lost in high-fat fed animals, implying a resistance to ghrelin at the level of the muscle. The underlying mechanisms accounting for ghrelin resistance in high fat-fed animals remain to be discovered.

Ghrelin, via corticotropin‐releasing factor receptors, reduces glucose uptake and increases lipid content in mouse myoblasts cells

“…Our results support the idea that the CRF family regulates or mediates the ghrelin isoforms' effects on muscle metabolism. A recent study supports our findings in showing that a decline in CRF-R2 levels in soleus muscle isolated from high-fatfed rats resulted in inhibition of the palmitate oxidation stimulated by ghrelin (Cervone et al, 2020).…”

“…Recent studies that support our findings have shown that in isolated muscles from rats, des-acyl ghrelin increases fatty acid oxidation in vitro (Cervone et al, 2020;Kraft et al, 2019). Furthermore, under high fatty acid conditions, des-acyl ghrelin protects isolated mature rat skeletal muscle from insulin resistance by activating the AMP-activated protein kinase/ACC axis, resulting in increased oxidation of fatty acids (Cervone et al, 2020). It is important to note that while those studies found that des-acyl ghrelin increases fatty acid oxidation (Cervone et al, 2020;Kraft et al, 2019), the present study found increased fat accumulation in C2C12 cells.…”

“…Our data suggest that high levels of des‐acyl ghrelin for an extended period of time inhibit the glucose metabolic pathway and increase fat utilization. Recent studies that support our findings have shown that in isolated muscles from rats, des‐acyl ghrelin increases fatty acid oxidation in vitro (Cervone et al., 2020; Kraft et al., 2019). Furthermore, under high fatty acid conditions, des‐acyl ghrelin protects isolated mature rat skeletal muscle from insulin resistance by activating the AMP‐activated protein kinase/ACC axis, resulting in increased oxidation of fatty acids (Cervone et al., 2020).…”

“…Another paper supported these findings by showing that des‐acyl ghrelin inhibits antral motility, and that this effect may be mediated by CRF‐R2 in the brain (Fujimiya et al., 2012). A recent paper further supported these data by demonstrating that reduction in CRF‐R2 in the soleus muscle of rats fed a high‐fat diet for 6 weeks leads to a reduction in the palmitate oxidation induced by ghrelin stimulation (Cervone et al., 2020).…”

“…In myotubes, des‐acyl ghrelin lowered mitochondrial ROS production and enhanced insulin signaling (Gortan Cappellari et al., 2016). Both isoforms of ghrelin were further shown to increase fatty acid oxidation in rat soleus muscle in vitro (Cervone et al., 2020; Kraft et al., 2019). Des‐acyl ghrelin was also shown to conserve glucose uptake in isolated mature rat skeletal muscle exposed to fatty acids (Cervone et al., 2020).…”