Objective: Systemic loss of estradiol (E2) during menopause is associated with increased adiposity which can be prevented with E2 replacement. Rodent studies suggest that E2, or lack of, is a key mediator in menopause-related metabolic changes. We have previously demonstrated that E2 treatment produces a rapid, dose-dependent activation of AMP-activated protein kinase (AMPK) in murine skeletal muscle. Activation of AMPK is implicated in the therapeutic benefits of many insulin sensitizing agents including metformin and thiazolidinediones. Here, we expand our observations and provide novel data which demonstrate that in addition to E2, its metabolite 2-hydroxyestradiol (2-HE2), activate AMPK in C2C12 myotubes.
Methods and procedures: C2C12 myotubes were used to examine the effects on E2 and the by-products of its metabolism on AMPK activation.
Results: Low concentrations of E2 (10 and 100 nmol/l) were found to increase AMPK phosphorylation by approximately 1.6-fold, while a higher concentration (10 micromol/l) resulted in a approximately 3.0-fold increase. In comparison to E2 treatment alone, incubation of myotubes with E2 and 1-aminobenzotriazole (ABT) (a CYP450 inhibitor that blocks metabolism of E2) caused AMPK activation to be enhanced at low E2 concentrations, but attenuated at higher concentrations. The effects of ABT suggested that one or more E2 metabolites contribute to the maximal activation of AMPK at high E2 concentrations. Indeed, the estrogen metabolite 2-HE2, but not 2-methoxyestradiol (2-ME2), directly activated AMPK in C2C12 myotubes.
Discussion: We propose a model where E2, acting through its metabolite 2-HE2 and the estrogen receptors (ERs), activates AMPK in myotubes. Finally, activation is abolished when all E2 is metabolized to 2-ME2.