Progesterone antagonist RU 486 has bone-sparing effects in ovariectomized rats

Sex-Dependent, Osteoblast Stage-Specific Effects of Progesterone Receptor on Bone Acquisition

The role of the progesterone receptor (PR) in the regulation of sexual dimorphism in bone has yet to be determined. Here we utilized genetic fate mapping and Western blotting to demonstrate age-dependent PR expression in the mouse femoral metaphysis and diaphysis. To define sex-dependent and osteoblast stage-specific effects of PR on bone acquisition, we selectively deleted PR at different stages of osteoblast differentiation. We found that when Prx1-Cre mice were crossed with PR floxed mice to generate a mesenchymal stem cell (MSC) conditional KO model (Prx1; PRcKO), the mutant mice developed greater trabecular bone volume with higher mineral apposition rate and bone formation. This may be explained by increased number of MSCs and greater osteogenic potential, particularly in males. Age-related trabecular bone loss was similar between the Prx1; PRcKO mice and their WT littermates in both sexes. Hormone deficiency during the period of rapid bone growth induced rapid trabecular bone loss in both the WT and the Prx1; PRcKO mice in both sexes. No differences in trabecular bone mass was observed when PR was deleted in mature osteoblasts using osteocalcin-Cre (Bglap-Cre). Also, there were no differences in cortical bone mass in all three PRcKO mice. In conclusion, PR inactivation in early osteoprogenitor cells but not in mature osteoblasts influenced trabecular bone accrual in a sex-dependent manner. PR deletion in osteoblast lineage cells did not affect cortical bone mass. © 2017 American Society for Bone and Mineral Research.

Transforming Growth Factor Beta is regulated by a Glucocorticoid-Dependent Mechanism in Denervation Mouse Bone

Bone growth and remodeling is inhibited by denervation in adults and children, resulting in alterations of linear growth and bone mass and increased risk for osteoporosis and pathologic fractures. Transforming growth factor beta (TGF-β) isoforms are a key group of growth factors that enhance bone formation. To explore the relation between denervation-induced reduction of bone formation and TGF-β gene expression, we measured mRNA levels of TGF-β in denervation mouse bone and found decreased mRNA levels of TGF-β1, TGF-β2 and TGF-β3. These changes were accompanied by diminishing weight loss, bone mineral density (BMD), trabecular thickness, trabecular separation and trabecular number of femur and lumbar, serum osteocalcin, total calcium, intact parathyroid hormone, and increased serum C telopeptide. Recombinant human TGF-β1 (rhTGF-β1) prevented denervation-induced reduction of BMD further supporting our hypothesis that denervation-induced reduction of bone formation is a result of inhibition of TGF-β gene expression. In addition, antiprogestins RU 38486 blunted the denervation-induced decrease in mRNA levels of TGF-β group, while dexamethasone (DEX) decreased TGF-β group mRNA levels in normal mice. Furthermore, the denervated-mice exhibited a threefold increase in plasma corticosterone. These results suggest that denervation-induced reduction of bone formation may be regulated by glucocorticoids via inhibition of TGF-β gene expression at least in part.

Inactivation of the Progesterone Receptor in Mx1+ Cells Potentiates Osteogenesis in Calvaria but Not in Long Bone

The effect of progesterone on bone remains elusive. We previously reported that global progesterone receptor (PR) knockout mice displayed high bone mass phenotype, suggesting that PR influences bone growth and modeling. Recently, Mx1+ cells were characterized to be mesenchymal stem cell-like pluripotent Cells. The aim of this study was to evaluate whether the PR in Mx1+ cells regulates osteogenesis. Using the Mx1-Cre;mT/mG reporter mouse model, we found that the calvarial cells exhibited minimal background Mx1-Cre activity prior to Cre activation by IFNα treatment as compared to the bone marrow stromal cells. IFNα treatment significantly activated Mx1-Cre in the calvarial cells. When the PR gene was deleted in the Mx1-Cre;PR-flox calvarial cells in vitro, significantly higher levels of expression of osteoblast maturation marker genes (RUNX2, Osteocalcin, and Dmp1) and osteogenic potential were detected. The PR-deficient calvariae exhibited greater bone volume, especially in the males. Although Mx1-Cre activity could be induced on the bone surface in vivo, the Mx1+ cells did not differentiate into osteocytes in long bones. Bone volumes at the distal femurs and the bone turnover marker serum Osteocalcin were similar between the Mx1-Cre;PR-flox mutant mice and the corresponding wild types in both sexes. In conclusion, our data demonstrates that blocking progesterone signaling via PRs in calvarial Mx1+ cells promoted osteoblast differentiation in the calvaria. Mx1+ was expressed by heterogeneous cells in bone marrow and did not differentiate into osteocyte during long bone development in vivo. Selectively inactivating the PR gene in Mx1+ cells affected the membrane bone formation but did not affect peripheral skeletal homeostasis.

Inhibition of the progesterone nuclear receptor during the bone linear growth phase increases peak bone mass in female mice

Augmentation of the peak bone mass (PBM) may be one of the most effective interventions to reduce the risk of developing osteoporosis later in life; however treatments to augment PBM are currently limited. Our study evaluated whether a greater PBM could be achieved either in the progesterone nuclear receptor knockout mice (PRKO) or by using a nuclear progesterone receptor (nPR) antagonist, RU486 in mice. Compared to their wild type (WT) littermates the female PRKO mice developed significantly higher cancellous and cortical mass in the distal femurs, and this was associated with increased bone formation. The high bone mass phenotype was partially reproduced by administering RU486 in female WT mice from 1–3 months of age. Our results suggest that the inhibition of the nPR during the rapid bone growth period (1–3 months) increases osteogenesis, which results in acquisition of higher bone mass. Our findings suggest a crucial role for progesterone signaling in bone acquisition and inhibition of the nPR as a novel approach to augment bone mass, which may have the potential to reduce the burden of osteoporosis.

Bone growth and turnover in progesterone receptor knockout mice

The role of progesterone receptor (PR) signaling in skeletal metabolism is controversial. To address whether signaling through the PR is necessary for normal bone growth and turnover, we performed histomorphometric and microcomputed tomography analyses of bone from homozygous female PR knockout (PRKO) mice at 6, 12, and 26 wk of age. These mice possess a null mutation of the PR locus, which blocks the gene expression of A and B isoforms of PR. Body weight gain, uterine weight gain, and tibia longitudinal bone growth were normal in PRKO mice. In contrast, total, cancellous, and cortical bone mass were increased in the humerus of 12-wk-old PRKO mice, whereas cortical and cancellous bone mass in the tibia was normal. At 26 wk of age, cancellous bone area in the proximal tibia metaphysis of PRKO mice was 153% greater than age matched wild-type mice. The improved cancellous bone balance in 6-month-old PRKO mice was associated with elevated bone formation and a tendency toward reduced osteoclast perimeter. Taken together, these findings suggest that PR signaling in mice is not essential for bone growth and turnover. However, at some skeletal sites, PR signaling attenuates the accumulation of cortical and cancellous bone mass during adolescence.

Tamoxifen attenuates the effects of exogenous glucocorticoid on bone formation and growth in piglets

Tamoxifen (Tam) has been shown to inhibit dexamethasone (Dex)-mediated effects on bone formation in vitro. Our objective was to determine whether Tam would block Dex-induced osteopenia and growth inhibition in growing piglets. Four-day-old male Yorkshire piglets were adapted to a liquid formula diet (400 ml/kg x day) and randomized to one of four groups (n = 5/group): Dex (0.5 mg/kg x day), Tam (1 mg/kg x day), Dex plus Tam, or placebo control (vehicle only). Both drugs were administered by orogastric gavage twice daily for 12 days. At baseline and at the end of treatment, whole body bone mineral density (BMD) was determined by dual energy x-ray absorptiometry (Hologic QDR1000W). Plasma osteocalcin and PTH were measured on days 0 and 12, and urinary N-telopeptide was measured on day 12. Changes in axial length and daily weight were also measured. Delta whole body BMD was 29% lower (P < 0.05) in Dex alone treated piglets than in controls (0.033 vs. 0.047 g/cm2, respectively), whereas the maximum change in BMD in Dex plus Tam group (0.046 g/cm2) was similar to that in controls. Concurrent Tam administration reduced the Dex-induced deficit in weight gain by 56% (P < 0.05) and the deficit in axial length gain by 72% (P < 0.01). In Dex alone treated piglets, PTH was significantly elevated (7-fold), whereas osteocalcin and N-telopeptide were significantly reduced compared with control values. These effects were prevented by Tam. These data suggest that the suppression of growth and other changes in parameters of bone metabolism induced by glucocorticoids in vivo can be attenuated by Tam.

Tamoxifen attenuates glucocorticoid actions on bone formation in vitro

Tamoxifen is a synthetic estrogen analog which may regulate osteogenesis in vivo by virtue of its antiglucocorticoid properties. We have examined tamoxifen regulation of glucocorticoid-induced osteogenesis in two different in vitro bone systems: the chicken periosteal osteogenesis model (CPO) and rat bone marrow stromal cells (RBMC). Hormone uptake studies were conducted with the osteosarcoma cell line, ROS 17/2.8. In the CPO model, alkaline phosphatase (AP) activity and collagen synthesis were stimulated by the glucocorticoid dexamethasone (Dex; 0.1 microM). These Dex-mediated effects were inhibited by increasing concentrations of tamoxifen (10-100 microM). Similarly, in the RBMC model, Dex-dependent (0.01 microM Dex) mineralized tissue formation and AP activity were blocked by tamoxifen (0.1 microM). Although tamoxifen inhibited Dex-mediated increases of AP activity in ROS 17/2.8 cells, it did not inhibit uptake of 3H-Dex or of 3H-estrogen. Northern analyses showed that tamoxifen did not affect messenger RNAs (mRNAs) for AP. Tamoxifen did seem to reduce mRNA for collagen type I, but not bone sialoprotein, osteopontin, and osteocalcin. Dex-induced increases for all proteins mRNAs in the RBMC model were not reduced by tamoxifen. Similarly, tamoxifen had no effects on cellular proliferation. We conclude that tamoxifen has no direct effect on gene expression of bone-related proteins of osteoblastic cells. Further, in the ROS 17/2.8 cell line, the antiglucocorticoid properties of tamoxifen do not appear to be mediated through either Dex or estrogen receptors.

Elevated progesterone during pseudopregnancy may prevent bone loss associated with low estrogen

There is increasing evidence for a role of endogenous progesterone in mineral and skeletal metabolism. The purpose of this study was to compare skeletal changes that occur during a condition of high endogenous progesterone but low estrogen (pseudopregnancy) with a condition of low endogenous progesterone and low estrogen (ovariectomy). Pseudopregnancy was selected over pregnancy to eliminate placental factors that may influence mineral metabolism. Rats were ovariectomized (OVX) or pseudopregnancy initiated, and bones were collected 13 days later. In some animals, blood was collected by indwelling catheters for determination of progesterone and estrogen levels. At mid-pseudopregnancy, there were substantial elevations in progesterone but estrogen was below the level of detection. Progesterone and estrogen were below the level of detection in the OVX rats. Longitudinal growth rates were increased compared with the normal cycling rats in both the pseudopregnant and OVX groups, indicative of decreased estrogen levels, but they were greatest in the OVX rats. Cancellous bone mass was maintained in the pseudopregnant rats compared with normal cycling rats but significantly reduced in the OVX animals. As expected, increased bone formation and turnover rates were observed in the OVX animals although some indices of bone formation were also increased in the pseudopregnant animals. Osteoclasts were significantly increased in the OVX but not the pseudopregnant animals compared with normal cycling rats. Increased periosteal bone formation indices are known to occur following OVX, but the greatest periosteal formation rates were observed in the pseudopregnant animals. While possible roles for some other endocrine agents cannot be excluded at this time, the data from the present study suggest that endogenous progesterone may have a role in the maintenance of bone mass perhaps by decreasing bone resorption while maintaining or increasing bone formation during physiological periods of low estrogen such as occurs during early pregnancy.