Loss of ovarian function in mice results in abrogated skeletal muscle PPARdelta and FoxO1-mediated gene expression

Abstract

Menopause, the age-related loss of ovarian hormone production, promotes increased adiposity and associated metabolic pathology, but molecular mechanisms remain unclear. We previously reported that estrogen increases skeletal muscle PPARdelta expression in vivo, and transgenic mice overexpressing muscle-specific PPARdelta are reportedly protected from diet-induced obesity. We thus hypothesized that obesity observed in ovariectomized mice, a model of menopause, may result in part from abrogated expression of muscle PPARdelta and/or downstream mediators such as FoxO1. To test this hypothesis, we ovariectomized (OVX) or sham-ovariectomized (SHM) 10-week old female C57Bl/6J mice, and subsequently harvested quadriceps muscles 12weeks later for gene expression studies. Compared to SHM, muscle from OVX mice displayed significantly decreased expression of PPARdelta (3.4-fold), FoxO1 (4.5-fold), PDK-4 (2.3-fold), and UCP-2 (1.8-fold). Consistent with studies indicating PPARdelta and FoxO1 regulate muscle fiber type, we observed dramatic OVX-specific decreases in slow isoforms of the contractile proteins myosin light chain (11.1-fold) and troponin C (11.8-fold). In addition, muscles from OVX mice expressed 57% less myogenin (drives type I fiber formation), 2-fold more MyoD (drives type II fiber formation), and 1.6-fold less musclin (produced exclusively by type II fibers) than SHM, collectively suggesting a shift towards less type I oxidative fibers. Finally, and consistent with changes in PPARdelta and FoxO1 activity, we observed decreased expression of atrogin-1 (2.3-fold) and MuRF-1 (1.9-fold) in OVX mice. In conclusion, muscles from ovariectomized mice display decreased PPARdelta and FoxO1 expression, abrogated expression of downstream targets involved in lipid and protein metabolism, and gene expression profiles indicating less type I oxidative fibers.

Figure 1. OVX mice have altered PPARδ and FoxO1 mediated oxidative gene expression

Real-time PCR was used to determine expression of genes (relative to the endogenous control cyclophilin B) in quadriceps from sham-ovariectomized (SHAM, white bars) and ovariectomized (OVX, black bars) mice. MLC = myosin light chain. Error bars indicate SE of the mean. *p<0.05, **p<0.01, ***p<0.001.

Figure 2

Summary of OVX-induced gene expression changes in mouse quadriceps: OVX decreases PPARδ expression, leading to decreased UCP-2, PDK4 and FoxO1 expression. Attenuated FoxO1 expression is concomitant with decreased myogenin and increased myoD expression, promoting less type I fiber development. Consistent with a fiber type shift, as only type II fibers produce musclin, OVX mice display increased musclin expression. Expression of PPARδ and FoxO1 downstream targets involved in protein metabolism (e.g. MuRF-1, atrogin-1) are similarly decreased.