Mechanism Of Glucocorticoids Regulating Lipid Metabolism In Chicken Skeletal Muscle

The experiments were conducted to study the effects of glucocorticoids (dexamethasone, DEX) on the fatty acids metabolism in skeletal muscle of broiler chickens (Gallus gallus domesticus). The underlying mechanisms regulating lipid metabolism were further investigated in chicken in vitro myoblasts.The effect of glucocorticoids on lipid metabolism in chicken skeletal muscles of initial growth stage Male Arbor Acres chicks (7-d old) were randomly subjected to one of the following three treatments for 3 days: subcutaneous injection of DEX (2 mg/kg BM/d, DEX), sham-treated (2 mg/kg BM/d of saline, control), and sham-treated and mild feed restriction treatment to keep the feed consumption as that of the DEX chickens in the previous day (FR). DEX enhanced circulating very low density lipoprotein (VLDL) level and the lipid accumulation in both adipose and skeletal muscle tissues. Compared with the control, liver-carnitine palmitoyltransferase 1 (L-CPT1) and AMP-activated protein kinase (AMPK)α2 mRNA level of M. biceps femoris (BF) were down-regulated significantly by DEX, while mRNA expression of lipoprotein lipase (LPL), fatty acid transport protein 1 (FATP1), heart-fatty acid binding protein (H-FABP), long chain acyl-CoA dehydrogenase (LCAD), activities of LPL and AMPK in both skeletal muscles were not obviously affected. Feed restriction increased the mRNA expression of LPL, L-CPT1 and LCAD of M. pectoralis major (PM), and FATP1, H-FABP, L-CPT1 and LCAD of BF. In conclusion, DEX retards the growth of body mass but facilitates lipid accumulation in both adipose and skeletal muscle tissues. In contrast to the favorable effect of mild feed restriction, DEX did not alter the uptake of fatty acids in skeletal muscle. The result suggests that DEX may promote intramyocellular lipid accumulation by suppressed fatty acid oxidation while mild feed restriction improved fatty acid oxidation in skeletal muscle, especially in red muscle. Glucocorticoids regulated muscle fatty acid metabolism in a different way from energy deficit caused by mild feed restriction.The effect of glucocorticoids on lipid metabolism in chicken skeletal muscles of maximal growth stage Male Arbor Acres chicks (35-d old) were randomly subjected to one of the following three treatments for 3 days: subcutaneous injection of DEX (2 mg/kg BM/d, DEX), sham-treated (2 mg/kg BM/d of saline, control), and sham-treated and pair-fed treatment to keep the feed consumption as that of the DEX chickens in the previous day (pair-fed). During feeding status, DEX enhanced plasma concentrations of lipid flux, facilitated the lipid accumulation in skeletal muscles, and up-regulated significantly mRNA expressions of glucocorticoid receptor (GR), VLDL receptor (VLDLR), LPL, FATP1, H-FABP, L-CPT1, LCAD and AMPKα2 in BF, and GR, VLDLR, FATP1, H-FABP and LCAD in PM, while down-regulated AMPKα2 mRNA level of PM. In contrast to feeding, DEX did not exert effect on circulating concentrations of triglyceride, transcription levels of VLDLR in both muscles, and LPL, H-FABP, LCAD and AMPKα2 in BF, while decreased LPL, LCAD and AMPKα2 mRNA level in PM, and phosphorylation of AMPKαand CPT1 activity in both muscles. Furthermore, the stimulated effect of DEX on the lipid accumulation was attenuated during fasting status. In conclusion, DEX facilitates lipid accumulation in skeletal muscle, the unmatched lipid uptake and utilisation may contribute, at least in part, to the augmented intramyocellular lipid accumulation.The effect of feed fatty acids on regulation of glucocorticoids Male Arbor Acres chickens (31-d old) were subjected to saturated fatty acid (SFA) or unsaturated fatty acid (UFA) for 7 days, followed by the DEX (2 mg/kg BM/d) or saline administration for 3 days (35-37d old). DEX-induced facilitated lipid accumulation was augmented by PA, while ameliorated by OA. The disbalanced lipid uptake and oxidation, estimated by expression of FATP1 and L-CPT1 respectively, was found in DEX-skeletal muscles. More sensitive than glycolytic muscle, the oxidative muscle in DEX-chickens represented the decreased AMP to ATP ratio, AMPKαphosphorylation and AMPK activity (-22.5%), correspondingly, DEX induced the increased phosphorylation of mammalian target of rapamycin (mTOR) and p70S6K, without AKT activation. In conclusion, DEX facilitated intramyocellular lipid accumulation via the unmatched fatty acid uptake and utilization. The underlying mechanism was the counter-regulation of DEX on AMPK and mTOR pathways, independent of AKT activation.The effect of glucocorticoids and insulin on lipid metabolism in chicken myoblast Myoblasts was prepared from SPF chicken embryos (11-d old), then subjected to following treatments: 1μM DEX, 100nM insulin, 1μM DEX+100nM insulin or control for 2 hours. Although not statistic significantly, DEX induced the increased mRNA level of AMPKα2 and L-CPT1, while decreased mTOR expression. The mRNA level of AMPKα和L-CPT1 were down-regulated by insulin. When cells were treated by cooperation of DEX and insulin, mRNA level of AMPKα2 and L-CPT1 were retarded, while mTOR expression was up-regulated. The result showed that fatty acid oxidation in myoblasts was improved by DEX, while decreased by insulin. The cooperation of DEX and insulin induced the retarded lipid utilization in chicken skeletam muscle.The pathway of glucocorticoids and insulin regulating lipid metabolism in chicken myoblast Myoblasts was prepared from SPF chicken embryos (11-d old), then subjected to following treatments: 1μM DEX, 10μM AMPK inhibitor, 100nM insulin, 20μM AKT inhibitor or control for 12 hours. The expressions of GR, AMPKα2 and L-CPT1 were elevated while mTOR was decreased by DEX, and restored to normal by AMPK inhibitor, except for GR. AKT1 expression was not affected by DEX, but its expression was enhanced by AMPK inhibitor. Insulin induced the increased AKT1 and mTOR transcription and tended to decrease AMPKα2 mRNA level, and they all restored to normal by AKT inhibitor. The results showed that DEX mediated myoblast lipid metabolism through regulation of AMPK and mTOR, independent of AKT, while insulin exert its effect via AKT-mTOR and AKT-AMPK-mTOR pathway.In conclusion, glucocorticoids administration distributes lipid to accumulation, rather than utilization. Glucocorticoids may promote intramyocellular lipid accumulation by increased fatty acid de novo synthesis and availability to tissue, in turn the decreased cellular AMP:ATP ratio. The following down-regulated AMPK induces the active mTOR signal, finally the retarded lipid oxidation. The elevated circulating insulin improves AKT1 pathway, but not associated with mTOR activation. The in vitro results show that glucocorticoids alone increase fat accumulation through regulation of AMPK and mTOR, independent of AKT, while insulin exerts its effect via AKT-mTOR and AKT-AMPK-mTOR pathway.