Abstract 18327: Early Estrogen Replacement After Ovariectomy Limits Liver DAG and Promotes Insulin Sensitivity in a Manner That Requires Hepatic Estrogen Signaling

The cardiovascular effects of postmenopausal estrogen replacement have been controversial. Estrogen replacement at the time of menopause may protect from cardiovascular disease, due to improvements in the complications of obesity. We designed studies to define mechanisms by which estrogen replacement may alter the impact of high-fat feeding, when given at the time of surgical menopause by ovariectomy (OVX) in C57/B6 mice. OVX-mice had a 2-fold increase in adiposity after 5 weeks of high-fat diet feeding, compared with sham mice (sham 16.5±1.2% vs. OVX 32.4±3.1% adiposity). The weight gain and increased adiposity were prevented with subcutaneous replacement of estradiol (E2) at the time of OVX (OVX-E2 5.4±0.4% vs OVX 32.4±3.1% adiposity). We defined the impact of OVX and E2 replacement on insulin sensitivity with hyperinsulinemic-euglycemic clamp studies. For OVX mice, insulin sensitivity was reduced 25% compared with sham mice, while OVX-E2 mice had a 2.2-fold increase in insulin sensitivity (sham 694±51; OVX 519±125; OVX-E2 1584.0±197 mg•ml•[kg•min•mU]-1). Compared to OVX, after hyperinsulinemia, in the liver of OVX-E2 mice, we found: 1) increased phosphorylation of ACC (inactive form), increased protein amounts of DGAT 2 (promotes TG synthesis from DAG). This potential increase in fatty-acid oxidation, coupled with increased TG synthesis was associated with reduced liver DAG content in OVX-E2 mice (OVX 4.0±0.7 vs. OVX-E2 1.74±0.53 µg•mg-1), and increased phosphorylation of ser473-AKT (insulin-signaling activated). In order to define the requirement of liver estrogen signaling in the protective effects of E2 replacement, we performed a parallel study with OVX and E2 replacement in mice with liver estrogen receptor alpha knockout (LKO). While E2 replacement reduced adiposity in LKO mice similar to controls, E2 replacement in LKO mice failed to improve whole-body insulin resistance (746±138 mg•ml•[kg•min•mU]-1), and failed to prevent liver DAG accumulation (3.2±1.1 µg•mg-1) compared to OVX mice. This study demonstrates that estrogen replacement at the time of OVX promotes insulin sensitivity by increasing TG synthesis, thereby limiting liver DAG content, and lipotoxicity. This protective effect requires hepatic estrogen signaling.