Deleting Suppressor of Cytokine Signaling-3 in chondrocytes reduces bone growth by disrupting mitogen-activated protein kinase signaling

OBJECTIVE

To investigate the impact of deleting Suppressor of Cytokine Signaling (SOCS)-3 (SOCS3) in chondrocytes during murine skeletal development.

METHOD

Mice with a conditional Socs3 allele (Socs3^fl/fl) were crossed with a transgenic mouse expressing Cre recombinase under the control of the type II collagen promoter (Col2a1) to generate Socs3^Δ/Δcol2 mice. Skeletal growth was analyzed over the lifespan of Socs3^Δ/Δcol2 mice and controls by detailed histomorphology. Bone size and cortical bone development was evaluated by micro-computed tomography (micro-CT). Growth plate (GP) zone width, chondrocyte proliferation and apoptosis were assessed by immunofluorescence staining for Ki67 and TUNEL. Fibroblast growth factor receptor-3 (FGFR3) signaling in the GP was assessed by immunohistochemistry, while the effect of SOCS3 overexpression on FGFR3-driven pMAPK signaling in HEK293T cells was evaluated by Western blot.

RESULTS

Socs3^Δ/Δcol2 mice of both sexes were consistently smaller compared to littermate controls throughout life. This phenotype was due to reduced long bone size, poor cortical bone development, reduced Ki67⁺ proliferative chondrocytes and decreased proliferative zone (PZ) width in the GP. Expression of pMAPK, but not pSTAT3, was increased in the GPs of Socs3^Δ/Δcol2 mice relative to littermate controls. Overexpression of FGFR3 in HEK293T cells increased Fibroblast Growth Factor 18 (FGF18)-dependent Mitogen-activated protein kinase (MAPK) phosphorylation, while concomitant expression of SOCS3 reduced FGFR3 expression and abrogated MAPK signaling.

CONCLUSION

Our results suggest a potential role for SOCS3 in GP chondrocyte proliferation by regulating FGFR3-dependent MAPK signaling in response to FGF18.

Proteinase-activated receptor-2 is required for normal osteoblast and osteoclast differentiation during skeletal growth and repair

Proteinase-activated receptor-2 (PAR(2)) is a G-protein coupled receptor expressed by osteoblasts and monocytes. PAR(2) is activated by a number of proteinases including coagulation factors and proteinases released by inflammatory cells. The aim of the current study was to investigate the role of PAR(2) in skeletal growth and repair using wild type (WT) and PAR(2) knockout (KO) mice. Micro computed tomography and histomorphometry were used to examine the structure of tibias isolated from uninjured mice at 50 and 90 days of age, and from 98-day-old mice in a bone repair model in which a hole had been drilled through the tibias. Bone marrow was cultured and investigated for the presence of osteoblast precursors (alkaline phosphatase-positive fibroblastic colonies), and osteoclasts were counted in cultures treated with M-CSF and RANKL. Polymerase chain reaction (PCR) was used to determine which proteinases that activate PAR(2) are expressed in bone marrow. Regulation of PAR(2) expression in primary calvarial osteoblasts from WT mice was investigated by quantitative PCR. Cortical and trabecular bone volumes were significantly greater in the tibias of PAR(2) KO mice than in those of WT mice at 50 days of age. In trabecular bone, osteoclast surface, osteoblast surface and osteoid volume were significantly lower in KO than in WT mice. Bone marrow cultures from KO mice showed significantly fewer alkaline phosphatase-positive colony-forming units and osteoclasts compared to cultures from WT mice. Significantly less new bone and significantly fewer osteoclasts were observed in the drill sites of PAR(2) KO mice compared to WT mice 7 days post-surgery. A number of activators of PAR(2), including matriptase and kallikrein 4, were found to be expressed by normal bone marrow. Parathyroid hormone, 1,25 dihydroxyvitamin D(3), or interleukin-6 in combination with its soluble receptor down-regulated PAR(2) mRNA expression, and fibroblast growth factor-2 or thrombin stimulated PAR(2) expression. These results suggest that PAR(2) activation contributes to determination of cells of both osteoblast and osteoclast lineages within bone marrow, and thereby participates in the regulation of skeletal growth and bone repair.

Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes

The therapeutic goal of increasing bone mass by co-treatment of parathyroid hormone (PTH) and an osteoclast inhibitor has been complicated by the undefined contribution of osteoclasts to the anabolic activity of PTH. To determine whether active osteoclasts are required at the time of PTH administration, we administered a low dose of the transient osteoclast inhibitor salmon calcitonin (sCT) to young rats receiving an anabolic PTH regimen. Co-administration of sCT significantly blunted the anabolic effect of PTH as measured by peripheral quantitative computer tomography (pQCT) and histomorphometry in the femur and tibia, respectively. To determine gene targets of sCT, we carried out quantitative real time PCR and microarray analysis of metaphyseal samples 1.5, 4 and 6.5h after administration of a single injection of PTH, sCT or PTH+sCT. Known targets of PTH action, IL-6, ephrinB2 and RANKL, were not modified by co-administration with sCT. Surprisingly, at all time points, we noted a significant upregulation of sclerostin mRNA by sCT treatment, as well as down-regulation of two other osteocyte gene products, MEPE and DMP1. Immunohistochemistry confirmed that sCT administration increased the percentage of osteocytes expressing sclerostin, suggesting a mechanism by which sCT reduced the anabolic effect of PTH. Neither mRNA for CT receptor (Calcr) nor labeled CT binding could be detected in sclerostin-enriched cells differentiated from primary calvarial osteoblasts. In contrast, osteocytes freshly isolated from calvariae expressed a high level of Calcr mRNA. Furthermore immunohistochemistry revealed co-localization of CT receptor (CTR) and sclerostin in some osteocytes in calvarial sections. Taken together these data indicate that co-treatment with sCT can blunt the anabolic effect of PTH and this may involve direct stimulation of sclerostin production by osteocytes. These data directly implicate calcitonin as a negative regulator of bone formation through a previously unsuspected mechanism.

152. DEVELOPMENT OF A VAGINALLY APPLIED, NON-HORMONAL CONTRACEPTIVE: THE CONTRACEPTIVE EFFICACY AND IMPACT ON BONE TURNOVER OF PEGLA, A LONG-ACTING LIF ANTAGONIST

The WHO has called for the urgent development of pharmacological, non-hormonal contraceptives. Leukaemia inhibitory factor (LIF) is obligatory for embryo implantation in mice and associated with infertility in women. Injection of a long-acting LIF antagonist (PEGLA) blocks uterine LIF, preventing implantation in mice, making PEGLA a promising non-hormonal contraceptive. LIF and LIFR null mice show decreased bone volume associated with increased osteoclast number and size, suggesting PEGLA may target bone. Vaginally administered PEGLA could be a ‘dual-role’ contraceptive: delivered in a microbicide which blocks the vaginal transmission of sexually transmitted infections. We aimed to establish the contraceptive efficacy of vaginally administered PEGLA and identify non-uterine targets of PEGLA in mice. PEGLA was administered to mated female mice by intraperitoneal (IP) injection or vaginally (n = 4/group) during the peri-implantation period to determine its effect on implantation and bone turnover. The tissue and blood accumulation of 125I-PEGLA or control was identified at various time-points following IP injection (≤120 h) or vaginal administration (≤24 h) (n = 3/group). PEGLA administered via vaginal gel blocked implantation (0.0+0.0 vs 8.5+0.5) at a lower dose (500 μg) than IP injection (1500 μg). PEGLA administered by IP injection resulted in fewer (4.0+0.3% vs 7.7+1.5%; P < 0.05) but larger (20.9+0.9 μm vs 18.1+0.5 μm; P < 0.05) osteoclasts and increased trabecular bone volume (6.8+0.9% vs 3.1+1.1%; P < 0.05) but vaginally administered PEGLA had no effect on bone (P > 0.05). 125 I-PEGLA accumulated more quickly (10 min vs 30 min) and was retained longer (96 h vs 24 h) in blood and tissue following IP injection compared to vaginal administration. This is the first study to show the contraceptive efficacy of a PEGylated compound following vaginal delivery. Local delivery of PEGLA decreased the required dose and eliminated the effect on bone, suggesting that local administration would minimise the non-target effects of PEGLA. Contraceptive trials are now required in non-human primates to progress PEGLA towards human clinical trials.

Communication between ephrinB2 and EphB4 within the osteoblast lineage

Members of the ephrin and Eph family are local mediators of cell function through largely contact-dependent processes in development and in maturity. Production of ephrinB2 mRNA and protein are increased by PTH and PTHrP in osteoblasts. Both a synthetic peptide antagonist of ephrinB2/EphB4 receptor interaction and recombinant soluble extracellular domain of EphB4 (sEphB4), which is an antagonist of both forward and reverse EphB4 signaling, were able to inhibit mineralization and the expression of several osteoblast genes involved late in osteoblast differentiation. The findings are consistent with ephrinB2/EphB4 signaling within the osteoblast lineage having a paracrine role in osteoblast differentiation, in addition to the proposed role of osteoclast-derived ephrinB2 in coupling of bone formation to resorption. This local regulation might contribute to control of osteoblast differentiation and bone formation at remodeling sites, and perhaps also in modeling.

Regulatory pathways revealing new approaches to the development of anabolic drugs for osteoporosis

The understanding of cell interactions and genetic controls of bone cells has provided new approaches to drug development for osteoporosis. Current emphasis in the development of new anabolic therapies is directed at modifying the effects of Wnt signalling on osteoblast differentiation and bone formation. Local signalling that results in bone formation during remodelling takes place in several ways. Growth factors released from resorbed bone matrix can contribute to preosteoblast differentiation and bone formation. Osteoclasts in the bone multicellular units (BMUs) might also generate activity that contributes to bone formation. The preosteoblasts themselves, growing in the resorption space, can communicate through cell contact and paracrine signalling mechanisms to differentiate. Osteocytes can sense the need for bone repair by detecting damage and pressure changes, and signalling to surface cells to respond appropriately. These recent insights into cell communication, together with discoveries from human and mouse genetics, have opened new pathways to drug development for osteoporosis. With the anabolic effect of parathyroid hormone on the skeleton having been established, human genetics revealed the major role of Wnt signalling in bone formation, and this has become the target of activity. Current approaches include activation at any of several points in the Wnt pathway, and neutralization of sclerostin, the protein product of the SOST gene that is produced in osteocytes as a powerful inhibitor of bone formation.

Mechanisms involved in skeletal anabolic therapies

Since parathyroid hormone (PTH) is the only proven anabolic therapy for bone, it becomes the benchmark by which new treatments will be evaluated. The anabolic effect of PTH is dependent upon intermittent administration, but when an elevated PTH level is maintained even for a few hours it initiates processes leading to new osteoclast formation, and the consequent resorption overrides the effects of activating genes that direct bone formation. Identification of PTH-related protein (PTHrP) production by cells early in the osteoblast lineage, and its action through the PTH1R upon more mature osteoblastic cells, together with the observation that PTHrP+/- mice are osteoporotic, all raise the possibility that PTHrP is a crucial paracrine regulator of bone formation. The finding that concurrent treatment with bisphosphonates impairs the anabolic response to PTH, adds to other clues that osteoclast activity is necessary to complement the direct effect that PTH has in promoting differentiation of committed osteoblast precursors. This might involve the generation of a coupling factor from osteoclasts that are transiently activated by receptor activator of nuclear factor-kappaB ligand (RANKL) in response to PTH. New approaches to anabolic therapies may come from the discovery that an activating mutation in the LRP5 gene is responsible for an inherited high bone mass syndrome, and the fact that this can be recapitulated in transgenic mice, whereas inactivating mutations result in severe bone loss. This has focused attention on the Wnt/frizzled/beta-catenin pathway as being important in bone formation, and proof of the concept has been obtained in experimental models.

The increased bone mass in deltaFosB transgenic mice is independent of circulating leptin levels

Transgenic mice overexpressing deltaFosB, a naturally occurring splice variant of FosB, develop an osteosclerotic phenotype. The increased bone formation has been shown to be due, at least in part, to autonomous effects of deltaFosB isoforms on cells of the osteoblast lineage. However, abdominal fat and marrow adipocytes are also markedly decreased in deltaFosB mice, leading to low serum leptin levels. Increased bone mass has been linked to the absence of leptin and leptin receptor signaling in ob/ob and db/db mice. Thus, in addition to affecting directly osteoblastogenesis and bone formation, deltaFosB isoforms might increase bone mass indirectly via a decrease in leptin. To test this hypothesis, we restored normal circulating levels of leptin in deltaFosB mice via sc implanted osmotic pumps. Complete histomorphometric analysis demonstrated that trabecular bone volume as well as dynamic parameters of bone formation was unchanged by this treatment in both deltaFosB transgenic mice and control littermates. This demonstration that restoring circulating levels of leptin in deltaFosB transgenic mice failed to rescue the bone phenotype further indicates that the marked increase in bone formation is autonomous to the osteoblast lineage.

Deletion of estrogen receptors reveals a regulatory role for estrogen receptors-beta in bone remodeling in females but not in males

To determine the contributions of estrogen receptor (ER)alpha and ERbeta in bone growth and remodeling in male and female mice, we generated and analyzed full knockouts for each receptor, and a double ER knockout. Although suppression of the ligand to the ERs (i.e., estradiol) after menopause or gonadectomy in females led to a catastrophic increase in bone turnover and concomitant bone loss, deletion of one or both ERs failed to show such an effect. Complete deletion of ERalpha led to a decrease, not an increase, in bone turnover and an increase, not a decrease, in trabecular bone volume in both male and female animals. Deletion of ERbeta led to different responses in males, where bone was unaffected, and in females, where bone resorption was decreased and trabecular bone volume increased. In contrast, deletion of both ERs led to a profound decrease in trabecular bone volume in females, which was associated with a decrease, not an increase, in bone turnover. Finally, deletion of ERalpha, but not ERbeta, led to major changes in circulating levels of estradiol and/or testosterone, indirectly affecting bone remodeling and bone mass. Thus, only ERalpha was shown to regulate bone remodeling in males, whereas in females both receptor subtypes influenced this process and could, at least under basal knockout conditions, compensate for each other.

Regulating DeltaFosB expression in adult Tet-Off-DeltaFosB transgenic mice alters bone formation and bone mass

The DeltaFosB isoforms are naturally occurring AP-1 family members that increase bone volume via a cell-autonomous effect on osteoblastic bone formation. Mice overexpressing DeltaFosB demonstrate a very high level of bone formation, resulting in a progressive osteosclerosis. Despite the linkage of bone formation and resorption in physiological systems, no alteration in bone resorption was detected in mice overexpressing DeltaFosB. To determine whether altering DeltaFosB expression can regulate bone formation independently of bone resorption in adult mice, we used the Tet-Off-inducible transgene system to induce or block transgenic DeltaFosB overexpression and thereby regulate bone formation in vivo. Overexpression of DeltaFosB after skeletal maturity increased trabecular bone volume by increasing bone formation, again without altering bone resorption, indicating that developmental DeltaFosB overexpression is not required for the osteosclerotic phenotype. Similarly, switching off DeltaFosB overexpression after osteosclerosis had developed led to a marked decrease in bone formation and loss of bone mass such that trabecular bone volume approached normal levels. Despite this dramatic reduction, no alteration in bone resorption was detected. These results clearly demonstrate that DeltaFosB regulates bone formation and bone mass in adult mice with no effect on bone resorption.

Activated parathyroid hormone/parathyroid hormone-related protein receptor in osteoblastic cells differentially affects cortical and trabecular bone

Parathyroid hormone (PTH), an important regulator of calcium homeostasis, targets most of its complex actions in bone to cells of the osteoblast lineage. Furthermore, PTH is known to stimulate osteoclastogenesis indirectly through activation of osteoblastic cells. To assess the role of the PTH/PTH-related protein receptor (PPR) in mediating the diverse actions of PTH on bone in vivo, we generated mice that express, in cells of the osteoblastic lineage, one of the constitutively active receptors described in Jansen's metaphyseal chondrodysplasia. In these transgenic mice, osteoblastic function was increased in the trabecular and endosteal compartments, whereas it was decreased in the periosteum. In trabecular bone of the transgenic mice, there was an increase in osteoblast precursors, as well as in mature osteoblasts. Osteoblastic expression of the constitutively active PPR induced a dramatic increase in osteoclast number in both trabecular and compact bone in transgenic animals. The net effect of these actions was a substantial increase in trabecular bone volume and a decrease in cortical bone thickness of the long bones. These findings, for the first time to our knowledge, identify the PPR as a crucial mediator of both bone-forming and bone-resorbing actions of PTH, and they underline the complexity and heterogeneity of the osteoblast population and/or their regulatory microenvironment.

Bone homeostasis in growth hormone receptor-null mice is restored by IGF-I but independent of Stat5

Growth hormone (GH) regulates both bone growth and remodeling, but it is unclear whether these actions are mediated directly by the GH receptor (GHR) and/or IGF-I signaling. The actions of GH are transduced by the Jak/Stat signaling pathway via Stat5, which is thought to regulate IGF-I expression. To determine the respective roles of GHR and IGF-I in bone growth and remodeling, we examined bones of wild-type, GHR knockout (GHR(-/-)), Stat5ab(-/-), and GHR(-/-) mice treated with IGF-I. Reduced bone growth in GHR(-/-) mice, due to a premature reduction in chondrocyte proliferation and cortical bone growth, was detected after 2 weeks of age. Additionally, although trabecular bone volume was unchanged, bone turnover was significantly reduced in GHR(-/-) mice, indicating GH involvement in the high bone-turnover level during growth. IGF-I treatment almost completely rescued all effects of the GHR(-/-) on both bone growth and remodeling, supporting a direct effect of IGF-I on both osteoblasts and chondrocytes. Whereas bone length was reduced in Stat5ab(-/-) mice, there was no reduction in trabecular bone remodeling or growth-plate width as observed in GHR(-/-) mice, indicating that the effects of GH in bone may not involve Stat5 activation.

Decreased c-Src expression enhances osteoblast differentiation and bone formation

c-src deletion in mice leads to osteopetrosis as a result of reduced bone resorption due to an alteration of the osteoclast. We report that deletion/reduction of Src expression enhances osteoblast differentiation and bone formation, contributing to the increase in bone mass. Bone histomorphometry showed that bone formation was increased in Src null compared with wild-type mice. In vitro, alkaline phosphatase (ALP) activity and nodule mineralization were increased in primary calvarial cells and in SV40-immortalized osteoblasts from Src(-/-) relative to Src(+/+) mice. Src-antisense oligodeoxynucleotides (AS-src) reduced Src levels by approximately 60% and caused a similar increase in ALP activity and nodule mineralization in primary osteoblasts in vitro. Reduction in cell proliferation was observed in primary and immortalized Src(-/-) osteoblasts and in normal osteoblasts incubated with the AS-src. Semiquantitative reverse transcriptase-PCR revealed upregulation of ALP, Osf2/Cbfa1 transcription factor, PTH/PTHrP receptor, osteocalcin, and pro-alpha 2(I) collagen in Src-deficient osteoblasts. The expression of the bone matrix protein osteopontin remained unchanged. Based on these results, we conclude that the reduction of Src expression not only inhibits bone resorption, but also stimulates osteoblast differentiation and bone formation, suggesting that the osteogenic cells may contribute to the development of the osteopetrotic phenotype in Src-deficient mice.

Increased formation and decreased resorption of bone in mice with elevated vitamin D receptor in mature cells of the osteoblastic lineage

The microarchitecture of bone is regulated by complex interactions between the bone-forming and resorbing cells, and several compounds regulate both actions. For example, vitamin D, which is required for bone mineralization, also stimulates bone resorption. Transgenic mice overexpressing the vitamin D receptor solely in mature cells of the osteoblastic bone-forming lineage were generated to test the potential therapeutic value of shifting the balance of vitamin D activity in favor of bone formation. Cortical bone was 5% wider and 15% stronger in these mice due to a doubling of periosteal mineral apposition rate without altered body weight or calcium homeostatic hormone levels. A 20% increase in trabecular bone volume in transgenic vertebrae was also observed, unexpectedly associated with a 30% reduction in resorption surface rather than greater bone formation. These findings indicate anabolic vitamin D activity in bone and identify a previously unknown pathway from mature osteoblastic cells to inhibit osteoclastic bone resorption, counterbalancing the known stimulatory action through immature osteoblastic cells. A therapeutic approach that both stimulates cortical anabolic and inhibits trabecular resorptive pathways would be ideal for treatment of osteoporosis and other osteopenic disorders.

Overexpression of DeltaFosB transcription factor(s) increases bone formation and inhibits adipogenesis

Members of the AP-1 family of transcription factors participate in the regulation of bone cell proliferation and differentiation. We report here a potent AP-1-related regulator of osteoblast function: DeltaFosB, a naturally occurring truncated form of FosB that arises from alternative splicing of the fosB transcript and is expressed in osteoblasts. Overexpression of DeltaFosB in transgenic mice leads to increased bone formation throughout the skeleton and a continuous post-developmental increase in bone mass, leading to osteosclerosis. In contrast, DeltaFosB inhibits adipogenesis both in vivo and in vitro, and downregulates the expression of early markers of adipocyte differentiation. Because osteoblasts and adipocytes are thought to share a common precursor, it is concluded that DeltaFosB transcriptionally regulates osteoblastogenesis, possibly at the expense of adipogenesis.

The tyrosine phosphatase SHP-1 is a negative regulator of osteoclastogenesis and osteoclast resorbing activity: increased resorption and osteopenia in me(v)/me(v) mutant mice

Naturally occuring inactivating mutations of the Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 (SHP-1) in mice give rise to the motheaten (me) phenotype. me/me mice have multiple hematopoietic abnormalities, suggesting that this phosphatase plays an important role in hematopoiesis. SHP-1 binds to and is activated by several hematopoietic surface receptors, including the colony-stimulating factor type 1 receptor. We have examined the role of SHP-1 in osteoclastogenesis and osteoclast function using mice with the viable motheaten (me(v)/me(v)) mutation, which has markedly decreased SHP-1 activity. Histomorphometric analysis of 6-week-old me(v)/me(v) mice and control littermates showed a marked osteopenia with an increase in bone resorption indices. The number of formed osteoclast-like cells (OCLs) in cocultures of me(v)/me(v) hematopoietic cells with normal osteoblasts was significantly increased. In contrast, the number of OCLs formed in the coculture of normal bone marrow cells with the me(v)/me(v) osteoblasts was not significantly different from controls. The bone-resorbing activity of me(v)me(v) OCLs and authentic osteoclasts was also found to be increased. Finally, Western blotting of proteins from me(v)/me(v) and control OCLs revealed an overall increase in tyrosine phosphorylation in the me(v)/me(v) lysates. These in vivo and in vitro results suggest that SHP-1 is a negative regulator of bone resorption, affecting both the formation and the function of osteoclasts.

Human and murine osteocalcin gene expression: conserved tissue restricted expression and divergent responses to 1,25-dihydroxyvitamin D3 in vivo

Human and murine osteocalcin genes demonstrate similar cell-specific expression patterns despite significant differences in gene locus organization and sequence variations in cis-acting regulatory elements. To investigate whether differences in these regulatory regions result in an altered response to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] in vivo, we compared the response of the endogenous mouse osteocalcin gene to a bacterial reporter gene directed by flanking regions of the human osteocalcin gene in transgenic mice. Transgene expression colocalized with endogenous osteocalcin expression in serial sections, being detected in osteoblasts, osteocytes and hypertrophic chondrocytes. In calvarial cell culture lysates from transgenic and nontransgenic mice, the endogenous mouse osteocalcin gene did not respond to 1,25-(OH)2D3 treatment. Despite this, transgene activity was significantly increased in the same cells. Similarly, Northern blots of total cellular RNA and in situ hybridization studies of transgenic animals demonstrated a maximal increase in transgene expression at 6 h after 1,25-(OH)2D3 injection (23.6+/-3.6-fold) with a return to levels equivalent to uninjected animals by 24 h (1.2+/-0.1-fold). This increase in transgene expression was also observed at 6 h after 1,25-(OH)2D3 treatment in animals on a low calcium diet (25.2+/-7.7-fold) as well as in transgenic mice fed a vitamin D-deficient diet containing strontium chloride to block endogenous 1,25-(OH)2D3 production (7.5+/-0.9-fold). In contrast to the increased transgene expression levels, neither endogenous mouse osteocalcin mRNA levels nor serum osteocalcin levels were significantly altered after 1,25-(OH)2D3 injection in transgenic or nontransgenic mice, regardless of dietary manipulations, supporting evidence for different mechanisms regulating the response of human and mouse osteocalcin genes to 1,25-(OH)2D3. Although the cis- and trans-acting mechanisms directing cell-specific gene expression appear to be conserved in the mouse and human osteocalcin genes, responsiveness to 1,25-(OH)2D3 is not. The mouse osteocalcin genes do not respond to 1,25-(OH)2D3 treatment, but the human osteocalcin-directed transgene is markedly upregulated under the same conditions and in the same cells. The divergent responses of these homologous genes to 1,25-(OH)2D3 are therefore likely to be due to differences in mouse and human osteocalcin-regulatory sequences rather than to variation in the complement of trans-acting factors present in mouse osteoblastic cells. Increased understanding of these murine-human differences in osteocalcin regulation may shed light on the function of osteocalcin and its regulation by vitamin D in bone physiology.

Estradiol treatment transiently increases trabecular bone volume in ovariectomized rats

The effect of short-term estradiol treatment, administered from the time of ovariectomy, on increased bone turnover and subsequent bone loss was studied in the rat. Adult female Sprague-Dawley rats were ovariectomized and administered daily subcutaneous (s.c.) injections of 17 beta-estradiol at 8 micrograms/ kg per day (Low) and 20 micrograms/kg per day (High) or vehicle alone (Veh). Femoral trabecular bone volume (BV/TV) and trabecular number (Tb.N) in the distal femur were transiently increased at 6 days postoperation in a dose-dependent manner following estradiol administration [mean +/- SEM: BV/TV (%), day 0, 6.6 +/- 0.2; day 6, Veh 7.8 +/- 0.4, Low 10.2 +/- 2.2, High 12.8 +/- 1.7 (p < 0.05); Tb.N (/mm), day 0, 2.30 +/- 0.24; day 6, Veh 2.89 +/- 0.33, Low 3.4 +/- 0.7, High 4.39 +/- 0.34 (p < 0.05)]. Estradiol prevented the ovariectomy-induced decrease in BV/TV and Tb.N between 9 and 15 days observed in Veh rats. Both serum alkaline phosphatase and urine hydroxyproline excretion were maintained at preoperative levels or lower from day 6 postoperation with high dose estradiol. Serum osteocalcin, however, rose above preoperative levels with estradiol at days 6 and 9, but returned to these values on days 15 and 21 postoperation. These results suggest that estradiol, administered from the time of ovariectomy, immediately suppressed markers associated with osteoblast proliferation/matrix synthesis and bone resorption. Mineralization does not appear to be so rapidly suppressed by estradiol with relatively high levels immediately following administration, resulting in a transient increase in trabecular bone volume and trabecular number.

Increased bone resorption precedes increased bone formation in the ovariectomized rat

This study describes an increase in biochemical and histomorphometric markers of bone resorption prior to increased bone formation and trabecular bone loss in the ovariectomized rat. Six-month-old, female Sprague Dawley rats were either sham operated or ovariectomized (Ovx) and killed at 0, 6, 9, 15, 18, 21, and 42 days postoperation when femora were collected and trabecular bone volume (BV/TV) was determined from von Kossa silver-stained sections using the Quantimet 520 image analysis system in the distal region. A number of these sections were also examined unstained for fluorochrome labels, and stained for acid phosphatase to detect osteoclast-like cells (ACP surface). At 18 days postoperation, lumbar vertebrae were examined. Blood and urine specimens were analyzed for bone-related biochemical variables. ACP surface was significantly greater in Ovx rats compared with sham at 6 days postoperation (mean ACP surface (%TS) +/- SEM: sham 36.4 +/- 1.9; Ovx 40.3 +/- 1.2, P < 0.05) as was urinary hydroxyproline excretion. Serum osteocalcin and alkaline phosphatase activity were not elevated in Ovx rats compared with Sham until 9 days postoperation. Mineral apposition rate (MAR) was increased at 12 days after ovariectomy (mean MAR (microm/day) +/- SEM: sham 0.85 +/- 0.06; Ovx 1.23 +/- 0.06, P < 0.05). Trabecular bone volume (BV/TV) at a specific site in the metaphyseal-diaphyseal core area was significantly lower at 15 days postoperation (mean (%) +/- SEM: Sham 7.40 +/- 1.23, Ovx 4.25 0 0.65, P < 0.05). There was no difference in lumbar vertebral BV/TV between the two groups at 18 days postoperation, however, ACP surface was elevated in the Ovx rats (P < 0.05). A systemic increase in bone resorption at 6 days postovariectomy precedes increased formation whereas the length of time required for the dissolution of trabeculae postoperation is determined locally.

Parathyroidectomy does not prevent bone loss in the oophorectomized rat

The evidence for a role of parathyroid hormone in the bone loss after the menopause remains controversial. This study examines the effect of parathyroidectomy on femoral trabecular bone volume, thickness, and spacing and biochemical markers of bone turnover in the oophorectomized rat. Female Sprague-Dawley rats 3 months old were double sham operated (sham), oophorectomized (OPX), parathyroidectomized (PTX), or oophorectomized and parathyroidectomized (O/P) under halothane anesthesia. At 9 weeks postoperation, femoral trabecular bone volume (BV/TV) was lower in OPX and O/P rats compared with sham or PTX animals (BV/TV, %, mean +/- SEM): sham 25.9 +/- 0.5, OPX 15.1 +/- 0.9, PTX 24.1 +/- 0.9, O/P 17.3 +/- 0.5; p < 0.001). Urinary hydroxyproline excretion, serum osteocalcin, and alkaline phosphatase activity were higher in OPX and O/P rats compared with control animals at 3 weeks postoperation (OHPE microM GF, mean +/- SEM: sham 1.37 +/- 0.16, OPX 2.16 +/- 0.26, PTX 0.95 +/- 0.21, O/P 1.92 +/- 0.22, p < 0.005; osteocalcin, microgram/liter, sham 31.8 +/- 1.8, OPX 33.7 +/- 2.7, PTX 24.5 +/- 2.1, O/P 34.3 +/- 2.1, p < 0.025; alkaline phosphatase, U/liter, sham 90 +/- 3, OPX 125 +/- 9, PTX 87 +/- 9, O/P 116 +/- 11, p < 0.005). These data indicate postoophorectomy bone loss is not prevented by parathyroidectomy.