Skeletal muscle fatty acids shift from oxidation to storage upon dexamethasone treatment in chickens

Abstract

The effect of an exogenous glucocorticoid on the lipid metabolism and fatty acid pattern of skeletal muscle in broiler chickens (Gallus gallus domesticus) was investigated in vivo and in vitro. Male Arbor Acres chickens were subjected to dexamethasone (DEX) treatment for 3 days. We found that DEX retarded body growth, facilitated lipid accumulation in adipose and skeletal muscle tissues, and elevated the thigh monounsaturated fatty acids (MUFA) to saturated fatty acids (SFA) ratio at fasted state. DEX-treated chickens exhibited increased stearoyl-CoA desaturase-1 (SCD1) activity and decreased carnitine palmitoyltransferase-1 (CPT1) activity in the thigh muscle under fasting conditions and in primary cultured myoblasts. Phosphorylation of AMP-activated protein kinase alpha at Thr172 did not occur in vivo but was increased in vitro by DEX. In cells exposed to DEX, fatty acid transport protein-1 mRNA expression and fatty acid storage were enhanced while fatty acid oxidation was repressed. In conclusion, in oxidative muscle of fasted chickens, DEX stimulated uptake of myocellular fatty acids which was stored with the modified MUFA to SFA ratio in a process that maybe involved SCD1 activation. The altered fatty acid composition together with the inactivation of CPT1 showed an increased tendency towards fatty acid accumulation as opposed to oxidation. These findings provide important insight concerning the influence of glucocorticoids on lipid metabolism.

Introduction

Intracellular fatty acids partitioning from oxidation towards storage contribute to diminished insulin sensitivity in skeletal muscles [2], [13], [33], [34], which use fatty acids and glucose as primary fuels. Furthermore, an improper fatty acid composition, such as an altered ratio of saturated fatty acids (SFA) to monounsaturated fatty acids (MUFA), has been shown to be involved in several physiological disease states including diabetes and obesity [25], [38], [42]. Compared with mammals, birds have higher levels of glucose and lower concentrations of insulin [6], [17]. Broiler chicken of modern strain is characterized by a fast muscle growth rate, heavy body weight and high fat deposition [27]. These characteristics suggest that chickens have a more refractory insulin cascade in skeletal muscle tissues, and the study of this chicken model would be beneficial to our understanding of intramyocellular lipid accumulation.

Excessive amounts of glucocorticoids (GCs) are correlated with predisposition to many metabolic syndromes, including dyslipidemia and insulin resistance [1]. This correlation suggests that GCs play an important role in lipid metabolism. GCs have a crucial effect on the development of obesity [21]. GCs and insulin act together to promote intramyocellular lipid accumulation in mammals [62] and chickens [65]. In our previous study using a chicken model, dexamethasone (DEX, an exogenous synthetic GC) increased hepatic de novo lipogenesis and blood lipid flux [9], [10] and induced skeletal muscle lipid deposition [65]; however, the underlying molecular mechanism involved in intramyocellular lipid accumulation remains unclear.

GCs have been shown to modulate the activity of a number of fatty acid desaturases [23], [43], [44], [45], which are the enzymes responsible for alterations in the fatty acid composition. Synthetic and endogenous GCs repress hepatic Δ5- and Δ6-desaturase activities in microsomes, leading to decreased production of 20:4(n − 6) [7], [23], [43], [44]. Other studies have shown that GCs also stimulate the activity of microsomal Δ9-desaturase, an enzyme involved in the conversion of SFA to MUFA [45]. Stearoyl-CoA desaturase-1 (SCD1) is the rate-limiting enzyme in the catalysis of Δ9-desaturase, as it prefers palmitoyl-CoA (C16:0) and stearoyl-CoA (C18:0) as substrates to produce palmitoleoyl-CoA (C16:1) and oleoyl-CoA (C18:1), respectively [18], [51]. SCD1 deficiency significantly increases insulin sensitivity in skeletal muscle [56].

In mammals, AMP-activated protein kinase (AMPK), a positive regulator of intracellular fatty acid metabolism, is activated by an increase in the AMP to ATP ratio [60]. AMPK catalyzes the phosphorylation of acetyl-CoA carboxylase (ACC) and diminishes the levels of malonyl-CoA, thereby relieving the inhibition of carnitine palmitoyltransferase-1 (CPT1)-mediated β-oxidation of fatty acids [59]. The excess lipid accumulation in skeletal muscle of obesity could be due to deregulation of the AMPK/malonyl-CoA fuel-sensing system [58]. Decreased cardiac AMPK phosphorylation was found to be associated with an altered fatty acids composition and a repressed utilization in a DEX-induced insulin resistance [55]. In addition, AMPK phosphorylation was activated by SCD1 deficiency in skeletal muscle [15]. The functional LKB1/AMPK pathway in chickens is similar to the same pathway in mammals [53].

In the present study, experiments were conducted with a chicken model in vivo and in vitro to explore whether DEX-induced intramyocellular lipid accumulation is associated with the changes (i) in the fatty acid composition or/and (ii) in the lipid oxidation. DEX, a synthetic GC that is specific for glucocorticoids receptor (GR) and has delayed plasma clearance [20], was employed to induce hyperlipidemia [65]. In the in vivo experiment, two types of skeletal muscle tissue, oxidative and glycolytic muscle, were investigated to determine the tissue specificity of lipid metabolism regulation. The investigation was conducted in both the fed and fasted states to evaluate the effect of DEX in the presence or absence of dietary fatty acids, and the fasted state was achieved by the withdrawal of food overnight. To avoid the possible influence of changes in food intake caused by GCs treatment [39], a pair-fed group was used. In the in vitro experiment, the DEX effect was further determined using an inhibitor specific for GR. Our results firstly indicate that an alteration in lipid composition may be involved in a lipid partitioning from mitochondrial oxidation to accumulation in oxidative muscle of DEX-treated chickens at fasted state. These findings reveal a better view of the metabolic perturbations associated with long-term GCs use in a different biological system.