Citations

Abstract 2-Methoxyestradiol (2ME2), a physiologic metabolite of 17β-estradiol (estrogen), has emerged as a promising cancer therapy because of its potent growth-inhibitory and proapoptotic effects on both endothelial and tumor cells. 2ME2 also suppresses osteoclast differentiation and induces apoptosis of mature osteoclasts, and has been shown to effectively repress bone loss in an animal model of postmenopausal osteoporosis. Given these observations, we have examined whether 2ME2 could effectively target metastasis to bone, osteolytic tumors, and soft tissue tumors. A 4T1 murine metastatic breast cancer cell line was generated that stably expressed Far Red fluorescence protein (4T1/Red) to visualize tumor development and metastasis to bone. In an intervention study, 4T1/Red cells were injected into bone marrow of the left femur and the mammary pad. In the latter study, 2ME2 (10, 25, and 50 mg/kg/d) treatment began on the same day as surgery and was continued for the 16-day duration of study. Tumor cell growth and metastasis to bone were monitored and bone volume was determined by micro-computed tomography. 2ME2 inhibited tumor growth in soft tissue, metastasis to bone, osteolysis, and tumor growth in bone, with maximum effects at 50 mg/kg/d. Furthermore, tumor-induced osteolysis was significantly reduced in mice receiving 2ME2. In vitro, 2ME2 repressed osteoclast number by inducing apoptosis of osteoclast precursors as well as mature osteoclasts. Our data support the conclusion that 2ME2 could be an important new therapy in the arsenal to fight metastatic breast cancer. [Cancer Res 2007;67(21):10106–11]

The decrease in estrogen levels that follows the onset of menopause results in rapid bone loss and osteoporosis. The major effect of estrogen deficiency on bone metabolism is an increase in the rate of bone resorption, but the precise mechanism by which this occurs remains unresolved. A recently developed technique for the isolation of avian osteoclasts has been modified to obtain highly purified multinucleated cells from human giant cell tumors. These osteoclast-like cells have been examined for evidence of estrogen receptors (ERs) and responses to 17 beta-estradiol (17 beta-E2). Analysis of giant-cell RNA demonstrated expression of ER mRNA. Furthermore, immunoblot analysis revealed that the giant cells contained a 66-kDa protein that was recognized by a monoclonal antibody specific for the human ER. When isolated multinucleated cells were cultured on slices of bone, there was a dose-dependent decrease in resorption in response to treatment detectable at 10 pM 17 beta-E2. Treatment with 10 nM 17 alpha-estradiol or vehicle (control) did not inhibit resorption. Moreover, the multinucleated cells isolated from these tumors had decreased mRNA levels for cathepsin B, cathepsin D, and tartrate-resistant acid phosphatase (TRAP) as well as secreted cathepsin B and TRAP enzyme activity in response to treatment with 10 nM 17 beta-E2. In contrast to these data, no change in gene expression was detected in mononuclear cells from these tumors in response to 17 beta-E2 treatment. These data support the proposition that human osteoclasts are target cells for estrogen and that estrogen can inhibit bone resorption by human osteoclasts.

Although in vivo effects of 17 beta-estradiol (estrogen) on bone turnover have been shown to occur mainly through influences on osteoclast-mediated bone resorption, the mechanism by which estrogen reduces bone resorption is unclear. To approach this question, we have examined authentic osteoclasts for evidence of a direct osteoclast response to estrogen in vitro. Highly purified (greater than (90%) viable avian osteoclasts from birds maintained on a low calcium diet were obtained using an osteoclast-specific monoclonal antibody coupled to magnetic beads. Isolated cells were either analyzed directly for estrogen receptor (ER) levels or cultured to assess the biological effects of estrogen. Northern blot analysis revealed a 5.2-kilobase mRNA that hybridized with a cDNA to human ER mRNA in the osteoclasts. An anti-human ER antibody recognized proteins of 66 kDa and 140 kDa in osteoclast extracts by Western blot analysis. The 66-kDa size is in close agreement with the reported size of the human ER. Nuclear binding of estrogen to intact viable osteoclasts was steroid-specific and saturable, with 5662 +/- 1420 molecules bound per nucleus (mean +/- SEM). In vitro estrogen responses in osteoclasts included a dose-dependent decrease in resorption as well as an increase in nuclear protooncogene mRNA levels. These observations indicate that osteoclasts are capable of directly responding to estrogen in vivo.

Abstract To examine the functional role of membrane-bound SLF, we evaluated the growing skeletons of WT and SLF mutant (Sl/Sld) mice that do not produce this protein using DXA, bone histomorphometry, cell culture, and flow cytometry. Deletion of membrane-bound SLF delays bone growth, decreases bone mass and BMD, impairs osteoblast function, and increases osteoclast surface in young mice. Introduction: Mutations at the murine steel locus lead to a defect in the development of hematopoietic stem cells, mast cells, and germ cells. Two isoforms of steel factor (SLF), soluble and membrane-associated, have been reported. Soluble SLF increases osteoclast formation and activity in cell culture. The effects of deletion of membrane-bound SLF on bone metabolism in mice have yet to be determined and are the subject of this study. Materials and Methods: Five-, 7-, and 12-week-old male and 5-week-old female WCB6F1/J-KitlSl/KitlSl-d (Sl/Sld) mice and wildtype (WT) littermates were used. BMD and bone mass, growth, architecture, and turnover were evaluated by DXA (males and females) and histomorphometry (males only). Primary osteoblasts isolated from humeri of 5-week-old male WT and Sl/Sld mice were used to determine osteoblast function, and bone marrow cells from tibias and femurs of these mice were analyzed to determine cell surface expression of osteoclast precursors. Results and Conclusions: Young Sl/Sld mice grew more slowly, had a reduced bone mass, and had shorter bones than WT littermates. Male mutants had significantly decreased whole body BMD in all age groups, largely because of a reduction in BMC. Tibial cross-sectional, cortical, and marrow area of cortical bone and cancellous bone volume was reduced in the mutants at all ages. The osteopenia in Sl/Sld was caused by increased osteoclast surface at all ages and decreased osteoblast surface at 5 weeks of age. [3H]thymidine incorporation studies showed that proliferation of osteoblasts derived from mutant mice was significantly suppressed (56%). Moreover, a decrease in mineralization was observed in Sl/Sld osteoblast culture. Fluorescence-activated cell sorting analysis of bone marrow cells from Sl/Sld mice revealed a 65% increase in the percentage of c-Fms+CD11b+RANK+ cells compared with WT controls. These findings suggest that membrane-bound SLF/c-Kit signaling plays a role in the regulation of peak bone mass.

Abstract Isolated osteoclasts from 5-week-old chickens respond to estradiol treatment in vitro with decreased resorption activity, increased nuclear proto-oncogene expression, and decreased lysosomal enzyme secretion. This study examines osteoclasts from embryonic chickens and egg-laying hens for evidence of estrogen responsiveness. Although osteoclasts from both of these sources express estrogen receptor mRNA and protein, estradiol treatment had no effect on resorption activity. In contrast to the lack of effect on resorption, estradiol treatment for 30 minutes resulted in steady-state mRNA levels of c-fos and c-jun increasing in osteoclasts from embryonic chickens and decreasing in osteoclasts from egg-laying hens. These data suggest that a nuclear proto-oncogene response may not be involved in estradiol-mediated decreased osteoclast resorption activity. To examine the influence of circulating estrogen on osteoclast estrogen responsiveness, 5-week-old chickens were injected with estrogen for 4 days prior to sacrifice. Estradiol treatment of osteoclasts from these chickens did not decrease resorption activity in vitro. Transfection of an estrogen receptor expression vector into osteoclasts from the estradiol-injected chickens and egg-laying hens restored estrogen responsiveness. Osteoclasts from 5-week-old chickens and estradiol treated 5-week-old chickens transfected with the estrogen receptor expression vector contained significantly higher levels of estrogen receptor protein and responded to estradiol treatment by decreasing secretion of cathepsins B and L and tartrate-resistant acid phosphatase. In contrast, osteoclasts from embryonic chickens, egg-laying hens, and estradiol-treated 5-week-old chickens either untransfected or transfected with an empty expression vector did not respond similarly. These data suggest that modulation of osteoclast estrogen responsiveness may be controlled by changes in the osteoclast estrogen receptor levels.

For many years it has been recognized that sex steroids have profound effects on bone metabolism. The current perception is that estrogen decreases bone resorption and androgen increases bone deposition. To investigate the potential for androgens to directly modulate bone resorption, we have examined avian osteoclast and human and mouse osteoclast-like cells for androgen responsiveness. There was a dose-dependent decrease in resorption activity in response to α-dihydrotestosterone (α-DHT), β-DHT, testosterone, or the synthetic androgen RU1881. This decrease was blocked by cotreatment with the specific androgen antagonist hydroxyflutamide. Further examination of avian osteoclasts revealed that the cells exhibited specific and saturable nuclear binding of tritiated RU1881 and that α-DHT stimulated the activity of the androgen response element as measured by using a chloramphenicol acetyltransferase reporter plasmid. In addition, avian osteoclasts responded to androgen treatment with elevated production and secretion of transforming growth factor β, a well documented response to androgen exposure in other cell systems. Treatment with either α-DHT or β-DHT for 24 hours resulted in a significant dose-dependent decrease in secretion of cathepsin B and tartrate-resistant acid phosphatase. This response to β-DHT, a stereoisomer of α-DHT that is inactive in other androgen receptor-dependent systems, supports the hypothesis that the osteoclast androgen receptor has unusual ligand-binding properties. Taken together, these results confirm the presence of functional androgen receptors in these cells and support the conclusion that osteoclasts are able to respond directly to androgens in vitro and thus are potential androgen target cells in vivo .