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Estrogen Treatment Restores Muscle Mitochondrial Function and Redox Homeostasis, reversing the Pro‐Diabetogenic State induced by Ovariectomy

Menopause comes with a significant decline in 17β‐estradiol (E2) levels, increased adiposity and a higher risk for type‐2 diabetes, but hormone therapy can help revert these effects. However, we still lack a comprehensive understanding of the mechanism(s) by which E2 modulates insulin sensitivity. Herein, 10 week‐old C57BL‐NJ mice were studied after 2 weeks of ovariectomy to avoid the development of obesity (OVX‐2w), and following 2 weeks of E2 replacement (OVX+E2, 1μg/day) administered via a subcutaneous miniosmotic pump. Control groups included normally cycling females (NC) in the Proestrus stage (high physiological E2 levels) and an OVX group implanted with a saline control pump (OVX‐ctl). E2 treatment effectively reversed fasting hyperglycemia developed in the OVX‐2w and OVX‐ctl groups (+25%, p<0.05), decreased fat mass by 48% relative to OVX‐ctl (p<0.05), and restored ex‐vivo insulin‐stimulated muscle glucose uptake. Accompanying these effects, we demonstrate that soon after ovarian E2 withdrawal a decrease in mitochondrial function and an oxidative shift in the cellular redox environment occur; both conditions associated with the etiology of insulin resistance. Furthermore, spectrophotometric assessment of specific activities of the electron transport complexes in isolated skeletal mitochondria revealed a ~40% decrease in Complex I (p<0.005) and Complex III (p<0.05) activities in OVX mice. While Complex II activity was not altered, transfer of electrons between Complex I and III, as well as between II and III was reduced by 50% in OVX‐2w (p<0.05), and 35% in OVX‐ctl (p<0.05), but completely restored in OVX+E2. Concomitant with a decrease in mitochondrial membrane fluidity (~‐50%, p<0.05), OVX‐2w mice also showed reduced electron transport supercomplex formation (p<0.05), but all changes were fully rescued in OVX+E2. Overall, we show that E2 protects mitochondrial function and redox homeostasis via non‐genomic pathways; by increasing Complex I and Complex IIII activities, enhancing electron transfer supercomplex assembly, promoting electron transfer efficiency within the Q‐pool, preventing mitochondrial free radical leak, and increasing fluidity of mitochondrial membranes. Our findings offer new insights into the mechanism(s) by which menopause sets, and E2 therapy reverses a pro‐diabetogenic state. A deeper understanding of E2 mechanisms of action in metabolism is of paramount importance to advance in the development of alternative non‐hormonal therapies; that can optimize the metabolic benefits, without the reproductive, oncogenic, and cardiovascular risks associated with HRT, for natural or surgically‐induced menopausal women.