Estrogens repress PGC1-α expression in the uterus

The estrogen-related receptors (ERRs): potential targets against bone loss

Bone loss and the resulting skeletal fragility is induced by several pathological or natural conditions, the most prominent of which being aging as well as the decreased levels of circulating estrogens in post-menopause females. To date, most treatments against bone loss aim at preventing excess bone resorption. We here summarize data indicating that the estrogen-related receptors (ERRs) α and γ prevent bone formation. Inhibiting these receptors may thus constitute an anabolic approach by increasing bone formation.

Peroxisome proliferator-activated receptor gamma, coactivator 1α enhances local estrogen biosynthesis by stimulating aromatase activity in endometriosis

CONTEXT

Endometriosis is an estrogen-dependent disease, and estrogen is overproduced by abnormally elevated aromatase in endometriotic tissues. Peroxisome proliferator-activated receptor gamma, coactivator 1α (PGC-1α) is a transcriptional coactivator-modulating steroid hormone.

OBJECTIVE

To investigate the effect of PGC-1α on aromatase activity in endometriosis.

DESIGN

Specimens from ovarian endometrioma (OE), endometrium with endometriosis (EE), and normal endometrium (NE) were analyzed for PGC-1α and aromatase expression. PGC-1α-dependent changes in aromatase expression in primary cultured stromal cells (SCs) were identified using luciferase and enzymatic assays, exon I-specific RT-PCR, and real-time PCR. Environmental stimulus-induced changes in PGC-1α were also examined.

RESULTS

PGC-1α was more highly expressed in OE than in EE and NE (P < .01). In OE, PGC-1α was coexpressed with aromatase, and their mRNA expressions were also correlated (r = 0.56, P = .02). PGC-1α was recruited to the nuclear receptor half-site between PI.3 and PII in the aromatase promoter. PGC-1α overexpression enhanced aromatase promoter activity (P < .01), mRNA expression (P < .05), and enzymatic activity (P < .01) in SCs from OE, but not in SCs from EE or NE. The levels of PI.3, PII, and exon II mRNA increased and transcriptional enhancement was abolished by mutation of the PGC-1α-interacting site. PGC-1α expression was enhanced in SCs from OE by tumor necrosis factor (TNF)-α (P < .05) but not by hypoxia or 17β-estradiol.

CONCLUSIONS

PGC-1α stimulated by TNF-α regulates aromatase expression and activity to promote local estrogen biosynthesis in OE, suggesting that PGC-1α is a promising candidate for novel targeted therapies in endometriosis treatment.

Estradiol stimulates mitochondrial biogenesis and adiponectin expression in skeletal muscle

Sexual dimorphism has been found in mitochondrial features of skeletal muscle, with female rats showing greater mitochondrial mass and function compared with males. Adiponectin is an insulin-sensitizing adipokine whose expression has been related to mitochondrial function and that is also expressed in skeletal muscle, where it exerts local metabolic effects. The aim of this research was to elucidate the role of sex hormones in modulation of mitochondrial function, as well as its relationship with adiponectin production in rat skeletal muscle. An in vivo study with ovariectomized Wistar rats receiving or not receiving 17β-estradiol (E2) (10 μg/kg per 48 h for 4 weeks) was carried out, in parallel with an assay of cultured myotubes (L6E9) treated with E2 (10 nM), progesterone (Pg; 1 μM), or testosterone (1 μM). E2 upregulated the markers of mitochondrial biogenesis and dynamics, and also of mitochondrial function in skeletal muscle and L6E9. Although in vivo E2 supplementation only partially restored the decreased adiponectin expression levels induced by ovariectomy, these were enhanced by E2 and Pg treatment in cultured myotubes, whereas testosterone showed no effects. Adiponectin receptor 1 expression was increased by E2 treatment, both in vivo and in vitro, but testosterone decreased it. In conclusion, our results are in agreement with the sexual dimorphism previously reported in skeletal muscle mitochondrial function and indicate E2 to be its main effector, as it enhances mitochondrial function and diminishes oxidative stress. Moreover, our data support the idea of the existence of a link between mitochondrial function and adiponectin expression in skeletal muscle, which could be modulated by sex hormones.

Estradiol and tamoxifen regulate NRF-1 and mitochondrial function in mouse mammary gland and uterus

Nuclear respiratory factor-1 (NRF-1) stimulates the transcription of nuclear-encoded genes that regulate mitochondrial (mt) genome transcription and biogenesis. We reported that estradiol (E2) and 4-hydroxytamoxifen (4-OHT) stimulate NRF-1 transcription in an estrogen receptor α (ERα)- and ERβ-dependent manner in human breast cancer cells. The aim of this study was to determine whether E2 and 4-OHT increase NRF-1 in vivo. Here, we report that E2 and 4-OHT increase NRF-1 expression in mammary gland (MG) and uterus of ovariectomized C57BL/6 mice in a time-dependent manner. E2 increased NRF-1 protein in the uterus and MG; however, in MG, 4-OHT increased Nrf1 mRNA but not protein. Chromatin immunoprecipitation assays revealed increased in vivo recruitment of ERα to the Nrf1 promoter and intron 3 in MG and uterus 6 h after E2 and 4-OHT treatment, commensurate with increased NRF-1 expression. E2- and 4-OHT-induced increases in NRF-1 and its target genes Tfam, Tfb1m, and Tfb2m were coordinated in MG but not in uterus due to uterine-selective inhibition of the expression of the NRF-1 coactivators Ppargc1a and Ppargc1b by E2 and 4-OHT. E2 transiently increased NRF-1 and PGC-1α nuclear staining while reducing PGC-1α in uterus. E2, not 4-OHT, activates mt biogenesis in MG and uterus in a time-dependent manner. E2 increased mt outer membrane Tomm40 protein levels in MG and uterus whereas 4-OHT increased Tomm40 only in uterus. These data support the hypothesis of tissue-selective regulation of NRF-1 and its downstream targets by E2 and 4-OHT in vivo.

Repression of osteoblast maturation by ERRα accounts for bone loss induced by estrogen deficiency

ERRα is an orphan member of the nuclear receptor family, the complete inactivation of which confers resistance to bone loss induced by ageing and estrogen withdrawal to female mice in correlation with increased bone formation in vivo. Furthermore ERRα negatively regulates the commitment of mesenchymal cells to the osteoblast lineage ex vivo as well as later steps of osteoblast maturation. We searched to determine whether the activities of ERRα on osteoblast maturation are responsible for one or both types of in vivo induced bone loss. To this end we have generated conditional knock out mice in which the receptor is normally present during early osteoblast differentiation but inactivated upon osteoblast maturation. Bone ageing in these animals was similar to that observed for control animals. In contrast conditional ERRαKO mice were completely resistant to bone loss induced by ovariectomy. We conclude that the late (maturation), but not early (commitment), negative effects of ERRα on the osteoblast lineage contribute to the reduced bone mineral density observed upon estrogen deficiency.

Endogenous ovarian hormones affect mitochondrial efficiency in cerebral endothelium via distinct regulation of PGC-1 isoforms

Mitochondria support the energy-intensive functions of brain endothelium but also produce damaging-free radicals that lead to disease. Previously, we found that estrogen treatment protects cerebrovascular mitochondria, increasing capacity for ATP production while decreasing reactive oxygen species (ROS). To determine whether these effects occur specifically in endothelium in vivo and also explore underlying transcriptional mechanisms, we studied freshly isolated brain endothelial preparations from intact and ovariectomized female mice. This preparation reflects physiologic influences of circulating hormones, hemodynamic forces, and cell-cell interactions of the neurovascular unit. Loss of ovarian hormones affected endothelial expression of the key mitochondrial regulator family, peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1), but in a unique way. Ovariectomy increased endothelial PGC-1α mRNA but decreased PGC-1β mRNA. The change in PGC-1β correlated with decreased mRNA for crucial downstream mitochondrial regulators, nuclear respiratory factor 1 and mitochondrial transcription factor A, as well as for ATP synthase and ROS protection enzymes, glutamate-cysteine ligase and manganese superoxide dismutase. Ovariectomy also decreased mitochondrial biogenesis (mitochondrial/nuclear DNA ratio). These results indicate ovarian hormones normally act through a distinctive regulatory pathway involving PGC-1β to support cerebral endothelial mitochondrial content and guide mitochondrial function to favor ATP coupling and ROS protection.