Dietary betaine activates hepatic VTGII expression in laying hens associated with hypomethylation of GR gene promoter and enhanced GR expression.

Background: Vitellogenin (VTG) is a precursor of egg yolk proteins synthesized within the liver of oviparous vertebrates. Betaine is an important methyl donor that is reported to improve egg production of laying hens with an unclear mechanism. In the present study, we fed betaine-supplemented diet (0.5%) to laying hens for 4 wk and investigated its effect on VTGII expression in the liver.

Results: Betaine did not affect chicken weight, but significantly ( P  < 0.05) increased egg laying rate accompanied with a significant ( P  < 0.05) increase in hepatic concentration and plasma level of VTGII. Plasma estrogen level did not change, but the hepatic expression of estrogen receptor α ( ERα ) mRNA was significantly ( P  < 0.05) up-regulated. Betaine did not affect the protein content of ERα, but significantly ( P  < 0.05) increased hepatic expression of glucocorticoid receptor ( GR ) at both mRNA and protein levels. Also, ERα/GR interaction tended to be enhanced in the liver nuclear lysates of betaine-supplemented hens as determined by co-immunoprecipitation. Furthermore, dietary betaine supplementation significantly increased ( P  < 0.05) the hepatic expression of methyl-transfer enzymes, such as BHMT, GNMT, and DNMT1, which was associated with higher SAM/SAH ratio and hypomethylation of GR promoter regions.

Conclusions: Betaine activates hepatic VTGII expression in association with modified DNA methylation of GR gene promoter, GR expression and ERα/GR interaction. Activation of hepatic VTGII expression may contribute, at least partly, to improved egg production in betaine-supplemented hens.