Acylated ghrelin induces but deacylated ghrelin prevents hepatic steatosis and insulin resistance in lean rats: Effects on DAG/ PKC/JNK pathway

Abstract

This study investigated the molecular effects of acylated (AG) and unacylated ghrelin (UAG) or their combination on hepatic lipogenesis pathways and DAG/PKC/JNK signaling in the livers of lean rats fed standard diet. Male rats (n = 10) were classified as control + vehicle (saline, 200 μl), AG, UAG, and AG + UAG-treated groups. All treatments were given at final doses of 200 ng/kg of for 14 days (twice/day, S.C). Administration of AG significantly enhanced circulatory levels of AG and UAG turning the normal ratio of AG/UAG from 1:2.5 to 1:1.2. However, while UAG didn't affect circulatory levels of AG, administration of UAG alone or in combination with AG resulted in AG/UAG ratios of 1:7 and 1:3, respectively. Independent of food intake nor the development of peripheral IR, AG increased hepatic DAG, TGs and CHOL contents and induced hepatic IR. Mechanism of action include 1) upregulation of mRNA and protein levels of DGAT-2 and mtGPAT-1, SREBP-1 and SCD-1, and 2) inhibition of fatty acids (FAs) oxidation mediated by inhibition of AMPK/ PPAR-α/CPT-1 axis. Consequently, AG induced membranous translocation of PKCδ and PKCε leading to activation of JNK and significant inhibition of insulin signaling under basal and insulin stimulation as evident by decreases in the phosphorylation levels of IRS (Tyr⁶¹²) and Akt (Thr³¹⁸) and increased phosphorylation of IRS (Ser³⁰⁷). However, while UAG only activated FAs oxidation in control rats, it reversed all alterations in all measured biochemical endpoints seen in the AG-treated group, when administered in combination with AG, leading to significant decreases in hepatic fat accumulation and prevention of hepatic IR. In conclusion, while exogenous administration of AG is at high risk of developing steatohepatitis and hepatic IR, co-administration of a balanced dose of UAG reduces this risk and inhibits hepatic lipid accumulation and enhance hepatic insulin signaling.

Keywords: Diacylglycerol; Ghrelin; Insulin resistance; Liver; NAFLD; PKC.