Inhibitory effect of 17β-estradiol on triglyceride synthesis in skeletal muscle cells is dependent on ESR1 and not ESR2

The present study aimed to investigate the inhibitory effects and the mechanisms underlying 17 beta-estradiol (E-2) effects on triglyceride synthesis and insulin resistance in skeletal muscle tissues and cells. Ovariectomy (OVX) was performed on 6-month-old female rats treated with or without E-2. Subsequently, various serum biochemical markers were measured. Additionally, pathological alterations of the uterus, liver and skeletal muscle were analyzed, and the content of triglycerides (TG) in muscle was detected. Differentiated myotubes formed by C2C12 cells were treated with palmitic acid (PA) or pretreated with E-2, estrogen receptor (ESR) 1 agonist propylpyrazoletriol (PPT) and ESR2 agonist diarylpropionitrile (DPN). Subsequently, the mRNA or protein expression levels of ESR1/2, peroxisome proliferator activated receptor alpha (PPAR alpha), CD36 molecule (CD36), fatty acid synthase (FASN), perilipin 2 (PLIN2), phosphorylated acetyl-CoA carboxylase alpha (p-ACACA), p-AKT serine/threonine kinase (p-AKT) and p-mitogen-activated protein kinase 8 (p-MAPK8) were analyzed in skeletal muscle or in C2C12 cells by reverse transcription-semi-quantitative polymerase chain reaction and western blotting. The present results suggested that treatment with E-2 inhibited OVX-induced body weight gain, TG accumulation and insulin resistance. The protein or mRNA expression levels of ESR1, CD36, PPAR alpha, p-ACACA and p-AKT were decreased, whereas the protein or mRNA expression levels of ESR2, PLIN2, FASN and p-MAPK8 were increased in the OVX group. Of note, treatment with E-2 restored the expression levels of the aforementioned factors. In C2C12 cells, treatment with E-2 or PPT reversed the alterations induced by treatment with PA. In contrast, pretreatment with DPN did not influence the effect of PA. Collectively, E-2 was able to interact with ESR1, thus activating the CD36-PPAR alpha pathway, decreasing the level of TG in the muscles and improving insulin resistance in skeletal muscles and C2C12 cells.