A selective estrogen receptor α agonist ameliorates hepatic steatosis in the male aromatase knockout mouse

Male aromatase knockout mice (ArKO; an estrogen-deficient model) present with male-specific hepatic steatosis that is reversible upon 17b-estradiol replacement. This study aims to elucidate which estrogen receptor (ER) subtype, ERa or ERb, is involved in the regulation of triglyceride (TG) homeostasis in the liver. Nine-month-old male ArKO mice were treated with vehicle, ERa -o r ERb-specific agonists via s.c. injection, daily for 6 weeks. Male ArKO mice treated with ERa agonist had normal liver histology and TG contents compared with vehicle-treated ArKO; omental (gonadal) and infra-renal (visceral) fat pad weights were normalized to those of vehicle-treated wild-type (WT). In contrast, ERb agonist treatment did not result in the similar reversal of these ArKO phenotypes. In vehicle-treated ArKO mice, hepatic transcript expression of fatty acid synthase (Fasn) and stearoyl-coenzyme A desaturase 1 (key enzymes in de novo FA synthesis) were significantly elevated compared with vehicle-treated WT, but only Fasn expression was lowered to WT level after ERa agonist treatment. There were no significant changes in the transcript levels of carnitine palmitoyl transferase 1 (required for transfer of FA residues into the mitochondria for b-oxidation) and sterol regulatory element-binding factor 1c (the upstream regulator of de novo FA synthesis). We also confirmed by RT-PCR that only ERa is expressed in the mouse liver. There were no changes in hepatic androgen receptor transcript level across all treatment groups. Our data suggest that estrogens act via ERa to regulate TG homeostasis in the ArKO liver. Since the liver, adipose tissue and arcuate nucleus express mainly ERa, estrogens could regulate hepatic functions via peripheral and central pathways.