Calcium fluxes at the bone/plasma interface: Acute effects of parathyroid hormone (PTH) and targeted deletion of PTH/PTH-related peptide (PTHrP) receptor in the osteocytes

Calcium ion concentration ([Ca²⁺]) in the systemic extracellular fluid, ECF-[Ca²⁺], is maintained around a genetically predetermined set-point, which combines the operational level of the kidney and bone/ECF interfaces. The ECF-[Ca²⁺] is maintained within a narrow oscillation range by the regulatory action of Parathyroid Hormone (PTH), Calcitonin, FGF-23, and 1,25(OH)₂D₃. This model implies two correction mechanisms, i.e. tubular Ca²⁺ reabsorption and osteoclast Ca²⁺ resorption. Although their alterations have an effect on the ECF-[Ca²⁺] maintenance, they cannot fully account for rapid correction of the continuing perturbations of plasma [Ca²⁺], which occur daily in life. The existence of Ca²⁺ fluxes at quiescent bone surfaces fulfills the role of a short-term error correction mechanism in Ca²⁺ homeostasis. To explore the hypothesis that PTH regulates the cell system responsible for the fast Ca²⁺ fluxes at the bone/ECF interface, we have performed direct real-time measurements of Ca²⁺ fluxes at the surface of ex-vivo metatarsal bones maintained in physiological conditions mimicking ECF, and exposed to PTH. To further characterize whether the PTH receptor on osteocytes is a critical component of the minute-to-minute ECF-[Ca²⁺] regulation, metatarsal bones from mice lacking the PTH receptor in these cells were tested ex vivo for rapid Ca²⁺ exchange. We performed direct real-time measurements of Ca²⁺ fluxes and concentration gradients by a scanning ion-selective electrode technique (SIET). To validate ex vivo measurements, we also evaluated acute calcemic response to PTH in vivo in mice lacking PTH receptors in osteocytes vs littermate controls. Our data demonstrated that Ca²⁺ fluxes at the bone-ECF interface in excised bones as well as acute calcemic response in the short-term were unaffected by PTH exposure and its signaling through its receptor in osteocytes. Rapid minute-to-minute regulation of the ECF-[Ca²⁺] was found to be independent of PTH actions on osteocytes. Similarly, mice lacking PTH receptor in osteocytes, responded to PTH challenge with similar calcemic increases.

3β-Hydroxysterol-Delta24 reductase plays an important role in long bone growth by protecting chondrocytes from reactive oxygen species

Desmosterolosis is an autosomal recessive disease caused by mutations in the 3β-hydroxysterol-Delta24 reductase (DHCR24) gene, with severe developmental anomalies including short limbs. We utilized DHCR24 knockout (KO) mice to study the underlying bone pathology. Because the KO mice died within a few hours after birth, we cultured metatarsal bones from newborn mice. The growth of bones from KO mice was significantly retarded after 1 week of culture. Absence of proliferating chondrocytes in the growth plate and abnormal hypertrophy of prehypertrophic chondrocytes were observed in the bones from KO mice. Hypertrophic differentiation was evidenced by higher expression of Indian hedgehog, alkaline phosphatase, and matrix metalloproteinase 13. Since elevated levels of reactive oxygen species (ROS) during chondrogenesis are known to inhibit proliferation and to initiate chondrocyte hypertrophy in the growth plate, and since DHCR24 acts as a potent ROS scavenger, we hypothesized that the abnormal chondrocyte proliferation and differentiation in KO mice were due to decreased ROS scavenging activity. Treatment with an antioxidant, N-acetyl cysteine, could correct the abnormalities observed in the bones from KO mice. Treatment of bones from wild-type mice with U18666A, a chemical inhibitor of DHCR24, resulted in short broad bones with a disrupted proliferating zone. Treatment of ATDC cells with hydrogen peroxide (H(2)O(2)) induced hypertrophic changes as evidenced by the expression of the marker genes specific for hypertrophic chondrocyte differentiation. H(2)O(2)-induced hypertrophic change was prevented by adenoviral delivery of DHCR24. Induction of chondrocyte differentiation in ATDC cells by insulin was associated with increased ROS production that was markedly enhanced by treatment of ATDC5 cells with DHCR24 siRNA. This is the first demonstration that DHCR24 plays an important role in long bone growth by protecting chondrocytes from ROS.

Identification of oxytetracycline as a chondrogenic compound using a cell-based screening system

To effectively treat degenerative joint diseases including osteoarthritis (OA), small chemical compounds need to be developed that can potently induce chondrogenic differentiation without promoting terminal differentiation. For this purpose, we screened natural and synthetic compound libraries using a Col2GFP-ATDC5 system and identified oxytetracycline (Oxy) as a chondrogenic compound. Oxy induced cartilaginous matrix synthesis and mRNA expressions of chondrocyte markers in ATDC5 cells. In addition, Oxy suppressed mineralization and mRNA expressions of terminal chondrocyte differentiation markers in ATDC5 cells, primary chondrocytes, and cultured metatarsal bones. Oxy's induction of Col2 mRNA expression was decreased by the addition of Noggin and was increased by the addition of BMP2. Furthermore, Oxy increased mRNA expression of Id1, Bmp2, Bmp4, and Bmp6. These data suggest that Oxy induces chondrogenic differentiation in a BMP-dependent manner and suppresses terminal differentiation. Oxy may be useful for treatment of OA and also for regeneration of cartilage tissue.

OIC-A006 promotes osteogenesis in vitro and in vivo

Bone morphogenesis proteins (BMPs) are one of the potent bone-forming factors. However, the safety, utility, and cost effectiveness of BMPs must be considered. Nowadays, there has been substantial interest in developing a chemical compound that safely promotes bone formation and facilitates fracture repair. Based on previous research with high throughout screening assay, we found one potent osteogenic inductive compound, named as OIC-A006 (Osteogenic inducible compound-active 006), which is classified in the amine family. In this study, we aimed to investigate the inducing effects of OIC-A006 on osteogenesis by bone marrow stem cells (BMSCs) in vitro and in vivo. We demonstrated that OIC-A006, at different concentrations, especially at optimal concentration of 6.25 microM, could stimulate BMSCs to express alkaline phosphatase (ALP), core-binding factor a1 (Cbfa1), osteopontin (OPN) and osteocalcin (OC), and to form calcified nodules in vitro. Under the bone tissue culture conditions, OIC-A006 also stimulated new bone formation of murine calvarial and metatarsal bone, indicating that OIC-A006 may exert positive effects on osteogenesis. Furthermore, to elucidate the in vivo osteogenic potential of OIC-A006, we used a rabbit skull defect model treated with sustained release microcapsules (OIC-A006/PLGA-MC) injected s.c. adjacent to the defect. These results revealed, for the first time, that OIC-A006 has the potential to promote osteogenesis in vitro and in vivo. This new compound may provide a new alterative agent for growth factors to promote bone healing and bone regeneration.

IGF-I signalling in bone growth: inhibitory actions of dexamethasone and IL-1beta

OBJECTIVE

To determine if glucocorticoids and proinflammatory cytokines inhibit bone growth through a common mechanism involving impaired IGF-I signalling.

DESIGN

IGF-I (100 ng/ml), dexamethasone (dex) (10(-6)M) and IL-1beta (10 ng/ml) with inhibitors of the PI3K (LY294002) and Erk 1/2 (PD98059 and UO126) IGF-I pathways (all 10 microM) were studied using the ATDC5 chondrocyte cell line and murine fetal metatarsal cultures.

RESULTS

IGF-I stimulated ATDC5 chondrocyte proliferation (322%; P < 0.001 versus control). Addition of PD or LY individually to IGF-I supplemented ATDC5 cultures partially reduced proliferation by 32% (P < 0.001), and 66% (P < 0.001), respectively. PD and LY in combination blocked all IGF-I stimulated ATDC5 proliferation. LY significantly reversed IGF-I stimulatory effects on metatarsal growth (P < 0.001), whereas PD and UO treatment had no effect. IGF-I induced ATDC5 proliferation was further decreased when Dex (24%; P < 0.01) or IL-1beta (33%; P < 0.001) were added to PD but not LY cultures. Metatarsal growth inhibition by LY was unaltered by Dex or IL-1beta addition.

CONCLUSIONS

Both the PI3K and Erk 1/2 pathways contributed independently to IGF-I mediated ATDC5 proliferation. However in metatarsal cultures, the Erk 1/2 pathway was not required for IGF-I stimulated growth. Dex and IL-1beta may primarily inhibit IGF-I induced bone growth through the PI3K pathway.

Tamoxifen induces permanent growth arrest through selective induction of apoptosis in growth plate chondrocytes in cultured rat metatarsal bones

Estrogen affects skeletal growth and promotes growth plate fusion in humans. High doses of estrogen have been used to limit growth in girls with predicted extreme tall stature; a treatment which has been associated with severe side effects. Selective estrogen receptor modulators (SERMs) could potentially be used as an alternative treatment. We chose to study the effects of Tamoxifen (Tam), a first generation SERM that has been used in the treatment of pubertal gynecomastia or McCune-Albright syndrome. Cultured fetal rat metatarsal bones were used to study the effects of Tam on longitudinal bone growth. In sectioned bones, chondrocyte apoptosis and proliferation were analyzed by TUNEL assay and BrdU incorporation, respectively. We also used a human chondrocytic cell line, HSC-2/8, to study the effects of Tam on apoptosis (FACS analysis and Cell Death detection ELISA) and caspase activation (caspase substrate cleavage and Western immunoblotting). Tam caused a dose-dependent growth retardation of cultured metatarsal bones. No catch-up growth was observed after Tam was removed from the culture medium. Detailed analysis of sectioned growth plate cartilage revealed increased apoptosis of chondrocytes within the resting and hypertrophic zones. HCS-2/8 cells also underwent apoptosis upon Tam treatment. Tam-induced apoptosis was caspase-dependent and completely abrogated by either caspase-8 or -9 inhibitors. A substrate assay revealed that caspase-8 is first activated followed by caspase-9 and -3. Finally, FasL secretion was stimulated by Tam and blocking of either FasL or Fas decreased Tam-induced apoptosis in chondrocytes. We here describe a novel mechanism of tamoxifen-induced apoptosis in chondrocytes, involving the activation of caspases and the FasL/Fas pathway, which diminishes the potential for bone growth.

Anti-rheumatic activities of histone deacetylase (HDAC) inhibitors in vivo in collagen-induced arthritis in rodents

BACKGROUND AND PURPOSE

Rheumatoid arthritis (RA) is a chronic inflammatory disease. Histone deacetylase inhibitors (HDACi), a new class of anti-cancer agents, have recently been reported to exhibit potent anti-inflammatory activities. A proof of concept study was carried out with suberoylanilide hydroxamic acid (SAHA) and MS-275, two HDACi currently undergoing clinical investigations for various oncological indications.

EXPERIMENTAL APPROACH

The anti-rheumatic effects of SAHA and MS-275 were assessed in both mouse and rat collagen induced arthritis (CIA) models.

KEY RESULTS

SAHA exhibited moderate prophylactic efficacy. It attenuated paw swelling due to inflammation, decreased bone erosion in both mice and rats and reduced slightly the RA-induced bone resorption in rats. However, SAHA could not inhibit the onset of arthritis. In contrast, MS-275 displayed dramatic anti-rheumatic activities. In prophylactic intervention, high doses of MS-275 prevented bone erosion and markedly delayed the onset of arthritis; at low doses, MS-275 strongly attenuated paw swelling, bone erosion, and bone resorption associated with RA. Furthermore, the therapeutic efficacy of MS-275 was also documented. After the onset of arthritis, it could stop the disease progression and joint destruction. An anti inflammatory effect of MS-275 was also confirmed through its capacity to decrease serum IL-6 and IL-1beta levels in the CIA induced mouse model. The anti-rheumatic activity of MS-275 was also confirmed through histological observation. No synovial hyperplasia, pannus formation, cartilage or bone destruction were observed in the high dose prophylactic intervention in mice.

CONCLUSION AND IMPLICATION

This study strongly supported HDACi as an innovative therapeutic strategy for RA.

The restricted potential for recovery of growth plate chondrogenesis and longitudinal bone growth following exposure to pro-inflammatory cytokines

Childhood chronic inflammatory disease can be associated with transient and permanent growth retardation. This study examined the potential for spontaneous growth recovery following pro-inflammatory cytokine exposure. Murine ATDC5 chondrogenic cells and postnatal metatarsals were exposed to interleukin (IL)-1beta, IL-6 and tumour necrosis factor-alpha (TNFalpha), and their growth and proliferative capacity were determined following recovery. TNFalpha and IL-1beta reduced chondrocyte proliferation and aggrecan and collagen types II and X expression at minimum concentrations of 10 ng/ml and 0.1 ng/ml respectively. TNFalpha but not IL-1beta exposure led to increased caspase-3 activity and altered cellular morphology, consistent with reduced viability. Cytokine exposure particularly inhibited proteoglycan synthesis. This effect was dose and duration dependent. Compared with the control, IL-1beta and TNFalpha led to a 71% and 45% reduction in metatarsal growth after 8 days of exposure respectively (P < 0.05). An additive effect of IL-1beta combined with TNFalpha was observed (110% decrease; P < 0.05). Metatarsals exposed to IL-1beta or TNFalpha individually for a 2-day period, and allowed to recover spontaneously in the absence of cytokines for a further 6 days, showed normal growth trajectories. In combination, growth was 59% lower (P < 0.01) compared with control metatarsals at the end of the recovery period. Exposure to the combination for 4 days followed by a 4-day recovery period resulted in 87% decrement compared with controls (P < 0.05). IL-6 did not alter any parameter studied. IL-1beta and TNFalpha exert diverse inhibitory effects on ATDC5 chondrocyte dynamics and metatarsal growth. The extent of recovery following cytokine exposure depends on the duration of exposure, and may be incomplete following longer periods of exposure.

The role of insulin in chondrogenesis

The ATDC5 chondrogenic cell line is typically induced to differentiate by exposure to insulin at high concentration (10 microg/ml, approximately 1600 nM). Differentiation can also be induced by physiological concentrations of insulin-like growth factor-I (IGF-I). Unlike previous reports, we observed a stimulation of differentiation, as measured by collagen X expression and Alcian Blue staining for proteoglycan synthesis, upon exposure to insulin at concentrations (10-50 nM) consistent with signaling via the insulin receptor. Analysis of lysates from proliferating and hypertrophic ATDC5 cells demonstrated that exposure to 50 nM insulin induced tyrosine phosphorylation of insulin receptors but not IGF-I receptors or hybrid receptors. In contrast to the potent effects of IGF-I to stimulate both ATDC5 proliferation and differentiation, insulin was not as potent as IGF-I as a proliferating agent but more selectively a differentiating agent. Consistent with this result, insulin was less potent than IGF-I in inducing activation of the Erk1/Erk2 mitogenic signaling pathway. Furthermore, Erk pathway inhibition did not enhance the differentiating effects of insulin as it does in the case of IGF-I exposure. Extending our observations to fetal rat metatarsal explants, we observed significant stimulation of bone growth by 50 nM insulin. This could be accounted for by a disproportionate stimulatory effect on growth of the hypertrophic zone. The proliferative zone was not significantly affected. Based on our results in both ATDC5 cells and metatarsal explants, we conclude that the insulin functioning through insulin receptor has a dominant effect as an inducer of chondrocyte differentiation. These results support assignment of a physiological role for this hormone in linear bone growth.

Locally produced estrogen promotes fetal rat metatarsal bone growth; an effect mediated through increased chondrocyte proliferation and decreased apoptosis

The importance of estrogens for the regulation of longitudinal bone growth is unequivocal. However, any local effect of estrogens in growth plate cartilage has been debated. Recently, several enzymes essential for estrogen synthesis were shown to be expressed in rat growth plate chondrocytes. Local production of 17beta-estradiol (E2) has also been demonstrated in rat costal chondrocytes. We aimed to determine the functional role of locally produced estrogen in growth plate cartilage. The human chondrocyte-like cell line HCS-2/8 was used to study estrogen effects on cell proliferation (3H-labeled thymidine uptake) and apoptosis (cell death detection ELISA kit). Chondrocyte production of E2 was measured by RIA and organ cultures of fetal rat metatarsal bones were used to study the effects of estrogen on longitudinal growth rate. We found that significant amounts of E2 were produced by HCS-2/8 chondrocytes (64.1 +/- 5.3 fmol/3 days/10(6) cells). The aromatase inhibitor letrozole (1 microM) and the pure estrogen receptor antagonist ICI 182,780 (10 microM) inhibited proliferation of HCS-2/8 chondrocytes by 20% (P < 0.01) and almost 50% (P < 0.001), respectively. Treatment with ICI 182,780 (10 microM) increased apoptosis by 228% (P < 0.05). Co-treatment with either caspase-3 or pan-caspase inhibitors completely blocked ICI 182,780-induced apoptosis (P < 0.001 vs ICI 182,780 only). Moreover, both ICI 182,780 (10 microM) and letrozole (1 microM) decreased longitudinal growth of fetal rat metatarsal bones after 7 days of culture (P < 0.01). In conclusion, our data clearly show that chondrocytes endogenously produce E2 and that locally produced estrogen stimulates chondrocyte proliferation and protects from spontaneous apoptosis. In addition, longitudinal growth is promoted by estrogens locally produced within the epiphyseal growth plate.

Smad4 is required for the normal organization of the cartilage growth plate

Smad4 is the central intracellular mediator of transforming growth factor-beta (TGF-beta) signals. To study the role of Smad4 in skeletal development, we introduced a conditional mutation of the gene in chondrocytes using Cre--loxP system. We showed that Smad4 was expressed strongly in prehypertrophic and hypertrophic chondrocytes. The abrogation of Smad4 in chondrocytes resulted in dwarfism with a severely disorganized growth plate characterized by expanded resting zone of chondrocytes, reduced chondrocyte proliferation, accelerated hypertrophic differentiation, increased apoptosis and ectopic bone collars in perichondrium. Meanwhile, Smad4 mutant mice exhibited decreased expression of molecules in Indian hedgehog/parathyroid hormone-related protein (Ihh/PTHrP) signaling. The cultured mutant metatarsal bones failed to response to TGF-beta1, while the hypertrophic differentiation was largely inhibited by Sonic hedgehog (Shh). This indicated that Ihh/PTHrP inhibited the hypertrophic differentiation of chondrocytes independent of the Smad4-mediated TGF-beta signals. All these data provided the first genetic evidence demonstrating that Smad4-mediated TGF-beta signals inhibit the chondrocyte hypertrophic differentiation, and are required for maintaining the normal organization of chondrocytes in the growth plate.

Pyridinium-1-yl Bisphosphonates Are Potent Inhibitors of Farnesyl Diphosphate Synthase and Bone Resorption

We report the design, synthesis and testing of a series of novel bisphosphonates, pyridinium-1-yl-hydroxy-bisphosphonates, based on the results of comparative molecular similarity indices analysis and pharmacophore modeling studies of farnesyl diphosphate synthase (FPPS) inhibition, human Vgamma2Vdelta2 T cell activation and bone resorption inhibition. The most potent molecules have high activity against an expressed FPPS from Leishmania major, in Dictyostelium discoideum growth inhibition, in gammadelta T cell activation and in an in vitro bone resorption assay. As such, they represent useful new leads for the discovery of new bone resorption, antiinfective and anticancer drugs.

Co-ordination of TGF-beta and FGF signaling pathways in bone organ cultures

Transforming growth factor-beta (TGF-beta) is known to regulate chondrocyte proliferation and hypertrophic differentiation in embryonic bone cultures by a perichondrium dependent mechanism. To begin to determine which factors in the perichondrium mediate the effects of TGF-beta, we studied the effect of Insulin-like Growth Factor-1 (IGF-I) and Fibroblast Growth Factors-2 and -18 (FGF2, FGF18) on metatarsal organ cultures. An increase in chondrocyte proliferation and hypertrophic differentiation was observed after treatment with IGF-I. A similar effect was seen after the perichondrium was stripped from the metatarsals suggesting IGF-I acts directly on the chondrocytes. Treatment with FGF-2 or FGF-18 resulted in a decrease in bone elongation as well as hypertrophic differentiation. Treatment also resulted in a decrease in BrdU incorporation into chondrocytes and an increase in BrdU incorporation in perichondrial cells, similar to what is seen after treatment with TGF-beta1. A similar effect was seen with FGF2 after the perichondrium was stripped suggesting that, unlike TGF-beta, FGF2 acts directly on chondrocytes to regulate proliferation and hypertrophic differentiation. To test the hypothesis that TGF-beta regulates IGF or FGF signaling, activation of the receptors was characterized after treatment with TGF-beta. Activation was measured as the level of tyrosine phosphorylation on the receptor. Treatment with TGF-beta for 24h did not alter the level of IGFR-I tyrosine phosphorylation. In contrast, treatment with TGF-beta resulted in and increase in tyrosine phosphorylation on FGFR3 without alterations in total FGFR3 levels. TGF-beta also stimulated expression of FGF18 mRNA in the cultures and the effects of TGF-beta on metatarsal development were blocked or partially blocked by pretreatment with FGF signaling inhibitors. The results suggest a model in which FGF through FGFR3 mediates some of the effects of TGF-beta on embryonic bone formation.

Effects of valproic acid on longitudinal bone growth

Valproic acid is widely used in the treatment of children with epilepsy. Evidence indicates that valproic acid has teratogenic effects on the skeletal system. In addition, the use of valproic acid in children has been associated with short stature. Thus, we hypothesized that valproic acid could also affect bone growth after skeletal morphogenesis is completed. To test this hypothesis, we cultured fetal rat metatarsal bones in the presence of valproic acid. Valproic acid markedly suppressed metatarsal longitudinal growth. To determine the underlying mechanisms, we studied cell proliferation and hypertrophy in the growth plate, the site where bone growth takes place. At the end of the culture period, valproic acid-treated bones exhibited narrow growth plate proliferative and hypertrophic zones and an expanded ossification center. In addition, valproic acid suppressed chondrocyte proliferation in the metatarsal rudiments. Our results suggest that valproic acid suppresses longitudinal bone growth by inhibiting cartilage formation and accelerating ossification of the growth plate. These findings warrant a prospective study on the effects of valproic acid on growth in children treated with valproic acid.

Differentiating the mechanisms of antiresorptive action of nitrogen containing bisphosphonates

Bisphosphonates (BPS) inhibit bone resorption and are divided into two classes according to their chemical structure and mechanism of action: nonnitrogen containing BPS such as etidronate and clodronate that are of low potency and inhibit osteoclast function via metabolism into toxic ATP-metabolites and nitrogen-containing BPS (NBPS), such as alendronate and risedronate that inhibit the enzyme of the mevalonate biosynthetic pathway farnesyl pyrophosphate synthase (FPPS), resulting in inhibition of the prenylation of small GTP-binding proteins in osteoclasts and disruption of their cytoskeleton. Previously, studies in various cell types suggested, however, that pamidronate functions by mechanism(s) additional or independent of the mevalonate pathway. To examine if such mechanism(s) are also involved in the action of NBPS on osteoclastic bone resorption, we examined the action of alkyl and heterocyclic NBPS with close structural homology on FPPS/isopentenyl pyrophosphate isomerase (IPPI) activity, on osteoclastic resorption, and on reversibility of this effect with GGOH. As expected, both pamidronate and alendronate suppressed bone resorption and FPPS/IPPI activity, the latter with greater potency than the first. Surprisingly, however, unlike alendronate, the antiresorptive effect of pamidronate was only partially reversible with GGOH, indicating the involvement of mechanism(s) of action additional to that of suppression of FPPS. Comparable results were obtained with the heterocyclic NBP NE-21650, a structural analog of risedronate. Thus, despite an effect on FPPS, the actions on bone resorption of some NBPS may involve mechanisms additional to suppression of FPPS. These findings may lead to identification of additional pathways that are important for bone resorption and may help to differentiate among members of the NBP class which are currently distinguished only according to their potency to inhibit bone resorption.

Interleukin 17 synergises with tumour necrosis factor alpha to induce cartilage destruction in vitro

BACKGROUND

Interleukin 17 (IL17) is produced by activated T cells and has been implicated in the development of bone lesions and cartilage degradation in rheumatoid arthritis (RA).

OBJECTIVE

To determine whether IL17, alone or together with tumour necrosis factor alpha (TNFalpha), induces cartilage destruction in vitro.

METHODS

Fetal mouse metatarsals stripped of endogenous osteoclast precursors were used to study the effect of IL17 on cartilage degradation independently of osteoclastic resorption. Cartilage destruction was analysed histologically by Alcian blue staining.

RESULTS

IL17 alone, up to 100 ng/ml, had no effect on the cartilage of fetal mouse metatarsals. IL17 (>/=0.1 ng/ml), however, induced severe cartilage degradation when given together with TNFalpha (>/=1 ng/ml). The cytokine combination decreased Alcian blue staining, a marker of proteoglycans, throughout the metatarsals and induced loss of the proliferating and early hypertrophic chondrocyte zones. TNFalpha alone also decreased Alcian blue staining, but not as dramatically as the cytokine combination. In addition, it did not induce loss of chondrocyte zones. Treatment with inhibitors of matrix metalloproteinase (MMP) activity and nitric oxide synthesis showed that MMP activity played a part in cartilage degradation, whereas nitric oxide production did not.

CONCLUSIONS

IL17, together with TNFalpha, induced cartilage degradation in fetal mouse metatarsals in vitro. IL17 may, therefore, participate in the development of cartilage destruction associated with RA by enhancing the effects of TNFalpha and may provide a potential therapeutic target.

TGFβ2 mediates the effects of Hedgehog on hypertrophic differentiation and PTHrP expression

The development of endochondral bones requires the coordination of signals from several cell types within the cartilage rudiment. A signaling cascade involving Indian hedgehog (Ihh) and parathyroid hormone related peptide (PTHrP) has been described in which hypertrophic differentiation is limited by a signal secreted from chondrocytes as they become committed to hypertrophy. In this negative-feedback loop, Ihh inhibits hypertrophic differentiation by regulating the expression of Pthrp, which in turn acts directly on chondrocytes in the growth plate that express the PTH/PTHrP receptor. Previously, we have shown that PTHrP also acts downstream of transforming growth factor β (TGFβ) in a common signaling cascade to regulate hypertrophic differentiation in embryonic mouse metatarsal organ cultures. As members of the TGFβ superfamily have been shown to mediate the effects of Hedgehog in several developmental systems, we proposed a model where TGFβ acts downstream of Ihh and upstream of PTHrP in a cascade of signals that regulate hypertrophic differentiation in the growth plate. This report tests the hypothesis that TGFβ signaling is required for the effects of Hedgehog on hypertrophic differentiation and expression of Pthrp. We show that Sonic hedgehog (Shh), a functional substitute for Ihh, stimulates expression of Tgfb2 and Tgfb3 mRNA in the perichondrium of embryonic mouse metatarsal bones grown in organ cultures and that TGFβ signaling in the perichondrium is required for inhibition of differentiation and regulation of Pthrp expression by Shh. The effects of Shh are specifically dependent on TGFβ2, as cultures from Tgfb3-null embryos respond to Shh but cultures from Tgfb2-null embryos do not. Taken together, these data suggest that TGFβ2 acts as a signal relay between Ihh and PTHrP in the regulation of cartilage hypertrophic differentiation.

Design of a variant of vascular endothelial growth factor-A (VEGF-A) antagonizing KDR/Flk-1 and Flt-1

Because of its central role in pathological angiogenesis, vascular endothelial growth factor (VEGF) has become a major target for anti-angiogenic therapies. We report here the construction of a heterodimeric antagonistic VEGF variant (HD-VEGF). In this antagonist, binding domains for the VEGF-receptors KDR/Flk-1 and Flt-1 are present at one pole of the dimer, whereas the other pole carries domain swap mutations, which prevent binding to either receptor. As HD-VEGF can only bind to monomeric receptors, it does not lead to signal transduction. Moreover, it antagonizes VEGF and possibly other members of the VEGF family, which are KDR/Flk-1 and Flt-1 ligands. We show here that HD-VEGF is a potent inhibitor of VEGF-mediated proliferation and tissue factor induction in endothelial cell cultures, requiring only a 20-fold and a 4-fold excess, respectively, to block the activity of wtVEGF completely. A 4-fold excess of HD-VEGF over wtVEGF was also sufficient to abrogate vascular permeability as determined in the Miles assay in vivo. Furthermore, HD-VEGF inhibited fetal bone angiogenesis in an ex vivo assay. Thus, HD-VEGF blocks KDR- and Flt-1-mediated VEGF activities that are crucial in the angiogenic process and is therefore a promising, multipotent compound in the treatment of angiogenesis-related diseases.

The perichondrium plays an important role in mediating the effects of TGF-beta1 on endochondral bone formation

Endochondral bone formation is complex and requires the coordination of signals from several factors and multiple cell types. Thus, chondrocyte differentiation is regulated by factors synthesized by both chondrocytes and cells in the perichondrium. Previously, we showed that expression of a dominant-negative form of the transforming growth factor beta (TGF-beta) type II receptor in perichondrium/periosteum resulted in increased hypertrophic differentiation in growth plate chondrocytes, suggesting a role for TGF-beta signaling to the perichondrium in limiting terminal differentiation in vivo. Using an organ culture model, we later demonstrated that TGF-beta1 inhibits chondrocyte proliferation and hypertrophic differentiation by two separate mechanisms. Inhibition of hypertrophic differentiation was shown to be dependent on Parathyroid hormone-related peptide (PTHrP) and expression of PTHrP mRNA was stimulated in the perichondrium after treatment with TGF-beta1. In this report, the hypothesis that the perichondrium is required for the effects of TGF-beta1 on growth and/or hypertrophic differentiation in mouse metatarsal organ cultures is tested. Treatment with TGF-beta1 inhibited expression of type X collagen mRNA in metatarsal cultures with the perichondrium intact. In contrast, hypertrophic differentiation as measured by expression of Type X collagen was not inhibited by TGF-beta1 in perichondrium-free cultures. TGF-beta1 added to intact cultures inhibited BrdU incorporation in chondrocytes and increased incorporation in the perichondrium; however, TGF-beta1 treatment stimulated chondrocyte proliferation in metatarsals from which the perichondrium had been enzymatically removed. These results suggest that the TGF-beta1-mediated regulation of both chondrocyte proliferation and hypertrophic differentiation is dependent upon the perichondrium. Thus, one or several factors from the perichondrium might mediate the way chondrocytes respond to TGF-beta1.

Bone anabolic effects of sonic/indian hedgehog are mediated by bmp-2/4-dependent pathways in the neonatal rat metatarsal model

A neonatal rat metatarsal organ culture model has been employed to study the anabolic effects of Sonic/Indian hedgehog and BMP-4. In this culture system, a significant increase in endochondral ossification is measured by an increase in length of mineralized bone, after daily treatment for 7 days with Sonic hedgehog protein (Shh-N). Previous evidence indicated that PTH related protein (PTHrP) is a critical target of hedgehog function in endochondral ossification. Using a PTHrP blocking antibody, it is shown that hedgehog mediates this activity via pathways other than through PTHrP. A dose-related increase in endochondral ossification is observed when metatarsals are treated with 25 ng/ml recombinant human bone morphogenetic protein 4 (BMP-4). However, this activity is not evident at higher doses of BMP-4 (200 ng/ml). High doses of BMP-4 resulted in increased expression of noggin messenger RNA and cotreatment of noggin and Shh-N resulted in reversal of the anabolic action of Shh-N. This observation suggests that BMP-4 signaling can influence the Shh-N mediated increase in endochondral ossification. Finally, we show that the Shh-N and BMP-4 mediated increase in endochondral ossification is reversed by treatment with antisense oligonucleotides targeted against Cbfa1. Thus, this report identifies Shh-N as an inducer of endochondral ossification that mediates its effect via BMP-4 and Cbfa1-dependent pathways.

Regulation of growth plate chondrogenesis by bone morphogenetic protein-2

Bone morphogenetic proteins (BMPs) regulate embryonic skeletal development. We hypothesized that BMP-2, which is expressed in the growth plate, also regulates growth plate chondrogenesis and longitudinal bone growth. To test this hypothesis, fetal rat metatarsal bones were cultured for 3 days in the presence of recombinant human BMP-2. The addition of BMP-2 caused a concentration-dependent acceleration of metatarsal longitudinal growth. As the rate of longitudinal bone growth depends primarily on the rate of growth plate chondrogenesis, we studied each of its three major components. BMP-2 stimulated chondrocyte proliferation in the epiphyseal zone of the growth plate, as assessed by [(3)H]thymidine incorporation. BMP-2 also caused an increase in chondrocyte hypertrophy, as assessed by quantitative histology and enzyme histochemistry. A stimulatory effect on cartilage matrix synthesis, assessed by (35)SO(4) incorporation into glycosaminoglycans, was produced only by the highest concentration of BMP-2. These BMP-2-mediated stimulatory effects were reversed by recombinant human Noggin, a glycoprotein that blocks BMP-2 action. In the absence of exogenous BMP-2, Noggin inhibited metatarsal longitudinal growth, chondrocyte proliferation, and chondrocyte hypertrophy, which suggests that endogenous BMPs stimulate longitudinal bone growth and chondrogenesis. We conclude that BMP-2 accelerates longitudinal bone growth by stimulating growth plate chondrocyte proliferation and chondrocyte hypertrophy.

FGF signaling inhibits chondrocyte proliferation and regulates bone development through the STAT-1 pathway

Several genetic forms of human dwarfism have been linked to activating mutations in FGF receptor 3, indicating that FGF signaling has a critical role in chondrocyte maturation and skeletal development. However, the mechanisms through which FGFs affect chondrocyte proliferation and differentiation remain poorly understood. We show here that activation of FGF signaling inhibits chondrocyte proliferation both in a rat chondrosarcoma (RCS) cell line and in primary murine chondrocytes. FGF treatment of RCS cells induces phosphorylation of STAT-1, its translocation to the nucleus, and an increase in the expression of the cell-cycle inhibitor p21WAF1/CIP1. We have used primary chondrocytes from STAT-1 knock-out mice to provide genetic evidence that STAT-1 function is required for the FGF mediated growth inhibition. Furthermore, FGF treatment of metatarsal rudiments from wild-type and STAT-1(-/-) murine embryos produces a drastic impairment of chondrocyte proliferation and bone development in wild-type, but not in STAT-1(-/-) rudiments. We propose that STAT-1 mediated down regulation of chondrocyte proliferation by FGF signaling is an homeostatic mechanism which ensures harmonious bone development and morphogenesis.

Effects of fibroblast growth factor-2 on longitudinal bone growth

In vivo, fibroblast growth factor-2 (FGF-2) inhibits longitudinal bone growth. Similarly, activating FGF receptor 3 mutations impair growth in achondroplasia and thanatophoric dysplasia. To investigate the underlying mechanisms, we chose a fetal rat metatarsal organ culture system that would maintain growth plate histological architecture. Addition of FGF-2 to the serum-free medium inhibited longitudinal growth. We next assessed each major component of longitudinal growth: proliferation, cellular hypertrophy, and cartilage matrix synthesis. Surprisingly, FGF-2 stimulated proliferation, as assessed by [3H]thymidine incorporation. However, autoradiographic studies demonstrated that this increased proliferation occurred only in the perichondrium, whereas decreased labeling was seen in the proliferative and epiphyseal chondrocytes. FGF-2 also caused a marked decrease in the number of hypertrophic chondrocytes. To assess cartilage matrix synthesis, we measured 35SO4 incorporation into newly synthesized glycosaminoglycans. Low concentrations (10 ng/ml) of FGF-2 stimulated cartilage matrix production, but high concentrations (1000 ng/ml) inhibited matrix production. We conclude that FGF-2 inhibits longitudinal bone growth by three mechanisms: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and, at high concentrations, decreased cartilage matrix production. These effects may explain the impaired growth seen in patients with achondroplasia and related skeletal dysplasias.

Influence of sex steroids on development of cultured fetal rat metatarsal bones

The effects of 17 beta-estradiol (E), dihydrotestosterone (D, a non aromatisable androgen), and progesterone (P) on osteogenesis were studied on fetal rat cartilaginous anlagues cultivated in vitro. The three medial metatarsal rudiments were harvested at day 19 of gestation and grown in 1% BSA MEM medium (MO 643, Sigma) without serum nor antibiotics. After a 18h preincubation period, hormones were added for 8 days. Paired controls were incubated in the same volume of medium. The length, the metacarpal thickness and the size of the mineralized zone were measured every day, using a calibrated eyepiece (magnification X 40). DNA and protein synthesis, cartilage metabolism and mineralization were evaluated by monitoring the incorporation of 3H-Thymidine, 3H-Proline, 35S and 45Ca into anlagues for the last three hours of incubation, respectively. The dose/response effect of each steroid was studied at the concentrations of 10(-4) M, 10(-6) M, 10(-7) M and 10(-9) M. No difference was observed between male and female fetuses. A significant positive effect on total length (% of length measured at harvesting day) was observed with the 10(-7) M dose of E (163% +/- 2 vs 148% +/- 4 in controls) or D (158% +/- 3). Endochondral growth was not modified by P treatment. The effect of the three steroids (given at a dose of 10(-7) M) alone or as combinations (E, D, P, EP, ED, PD, EPD) confirmed the positive effect of E on endochondral growth and, to a lesser extend, of D and the association ED. Nevertheless, D had a better effect than E on endomembranous growth. On the contrary, P did not affect growth neither administrated alone nor in combination with E or D, while a positive effect of P on mineralization was demonstrated. The treatment associating the three steroids slowed down all the parameters concerning growth but strongly stimulated calcification.

OP-1 (BMP-7) affects mRNA expression of type I, II, X collagen, and matrix Gla protein in ossifying long bones in vitro

In long bone development, a regulating role of OP-1 is suggested by the local correlated expression of both OP-1 ligand and OP-1 binding receptors in developing mouse hind limbs. OP-1 is expressed in the interdigital mesenchyme, whereas OP-1 binding receptors are found in the bordering perichondrium, and both OP-1 ligand and receptors are present in the zone of (pre)hypertrophic chondrocytes. We investigated the role of OP-1 in long bone development experimentally by treating organ cultures of embryonic mouse metatarsals with rhOP-1. The mRNA expression patterns of type I, II, X collagen, and matrix Gla protein (MGP) were studied using in situ hybridization and cell proliferation using [3H]thymidine and BrdU labeling. In the epiphyseal perichondrium, treatment with 40 ng/ml OP-1 enhanced cell proliferation after day 2, while 6-day treatment caused a shift in expression from type I collagen to type II collagen mRNA. This supports previous histochemical findings that OP-1 induced the transition of perichondrium into cartilage. In the center of the rudiment, OP-1 inhibited the expression of type X collagen mRNA, indicating inhibition of chondrocyte hypertrophy. An arrest of differentiation at the (pre)hypertrophic chondrocyte stage was also indicated by the large area of cells expressing MGP mRNA in the OP-1-treated rudiments. We conclude that OP-1 affected the expression of marker genes of chondrocyte differentiation by acting on two steps in endochondral ossification. First, cell proliferation was enhanced, particularly so in the perichondrium where cells started to express the chondrocyte phenotype. Second, the terminal differentiation of mature chondrocytes into hypertrophic chondrocytes was inhibited. These results, combined with the known pattern of OP-1 ligand and BMP receptor expression in the embryo, suggest that OP-1 plays a local role in the cascade of events during endochondral ossification.

Effect of alendronate treatment on the osteoclastogenic potential of bone marrow cells in mice

Bisphosphonates suppress bone resorption by inhibiting the activity of mature osteoclasts as well as the formation of osteoclasts from bone marrow-derived precursor cells. It is not yet known at which level of osteoclast development this latter action is exerted and whether this is due to a systemic effect of circulating bisphosphonate or to an action at the bone surface, given the property of these compounds to concentrate specifically at active bone sites. We addressed these questions in an ex vivo study in mice. The animals were treated with the bisphosphonate alendronate for various periods, and the central compartment of the bone marrow was isolated and cultured together with osteoclast-free fetal mouse bone explants. In this way the capacity of bone marrow cells, exposed previously to alendronate in vivo, to generate osteoclasts and induce resorption in vitro was examined. Alendronate (0.25 mg/kg, subcutaneously) given to mice for periods up to 4 weeks suppressed bone resorption, as expected, but did not affect the capacity of bone marrow cells to develop into mature osteoclasts and resorb the calcified matrix of bone explants. In addition, using monoclonal antibodies specific for different macrophage-granulocyte precursor subsets, we found that alendronate treatment did not affect the pattern of antigen expression of bone marrow cells, which is in line with the lack of an effect of the bisphosphonate on the ability of bone marrow cells to induce resorption in vitro. In contrast, in control experiments, lipopolysaccharide (0.1 mg) treatment induced marked changes in the pattern of antigen expression of bone marrow cells. In conclusion, our studies demonstrate that the inhibition of bone resorption by alendronate does not involve alteration of the osteoclastogenic potential of osteoclast progenitors (precursors) from the central bone marrow compartment. Moreover, this treatment did not alter the expression of markers specific for monocyte-macrophage precursors as well as mature macrophages. These results suggest that the effects of alendronate are exerted at the bone surface at late osteoclast precursors (and mature osteoclasts).

Bone resorption and response to calcium-regulating hormones in the absence of tissue or urokinase plasminogen activator or of their type 1 inhibitor

Plasminogen activators (PA) are implicated in cell migration and tissue remodeling, two components of the bone resorption processes. Using mice with inactivated tissue PA (tPA), urokinase PA (uPA), or type 1 PA inhibitor (PAI-1) genes, we evaluated whether these processes, or their stimulation by parathyroid hormone (PTH) or 1,25-dihydroxyvitamin (1,25[OH]2D3) are dependent on these genes. Two culture models were used, one involving 19-day fetal calvariae, to evaluate the direct resorptive activity of osteoclasis, and the other involving 45Ca-labeled 17-day fetal metatarsals, in which this activity depends on preliminary (pre)osteoclast migration. PTH similarly increased (about 10-fold) PA activity in calvariae from wild-type tPA+/+ and uPA+/+ or deficient uPA-/- and PAI-/- mice; it affected only tPA, not uPA. In tPA-/- bones, the low PA levels, due to uPA, were not influenced by PTH. Calcitonin did not affect PA responses to PTH. No differences were observed between tPA+/+, tPA-/-, uPA+/+, and uPA-/- calvariae for any parameter related to bone resorption (development of lacunae, release of calcium and lysosomal enzymes, accumulation of collagenase, loss of hydroxyproline), indicating similar responses to PTH or calcitonin. The progressive 45Ca release was largely similar in cultures of tPA+/+, tPA-/-, uPA+/+, uPA-/-, PAI+/+, or PAI-/- metatarsals and it was similarly enhanced by PTH or 1,25(OH)2D3. However, uPA-/- metatarsals released 45Ca at a slower rate at the beginning of the cultures, suggesting an impaired recruitment of the (pre)osteoclasts, which migrate at that time from the periosteum into the calcified cartilage. Thus, it appears that the direct resorptive activity of the osteoclasts does not necessitate the presence of either tPA or uPA, but uPA is likely to facilitate the migration of the (pre)osteoclasts toward the mineralized surfaces. Although considerably enhanced by PTH, tPA does not mediate the actions of PTH (nor of 1,25[OH]2D3) evaluated in these models.

Effect of essential trace metal on bone metabolism in the femoral-metaphyseal tissues of rats with skeletal unloading: comparison with zinc-chelating dipeptide

The effect of essential trace metals on bone metabolism was investigated in the femoral-metaphyseal tissues obtained from skeletal-unloaded rats. Skeletal unloading was designed by using the model of hindlimb suspension in rats; the animals were fed for 4 days with the unloading. Femoral-metaphyseal tissues were cultured for 24 hours in a medium containing either vehicle (control), nickel, manganese, cobalt, copper, zinc, or zinc-chelating dipeptide (beta-alanyl-L-histidinato zinc; AHZ) in the concentration range of 10(-6) to 10(-4) M. Bone biochemical components (alkaline phosphatase activity, glucose consumption, and DNA content) were significantly decreased by skeletal unloading. The presence of zinc sulfate or AHZ (10(-5) and 10(-4) M) caused a significant increase of alkaline phosphatase activity in the bone tissues from unloaded rats. This effect was not seen by nickel, manganese, cobalt and copper (10(-6) to 10(-4) M). The culture medium glucose was clearly consumed by the bone tissues. This consumption was inhibited by nickel, manganese, or copper (10(-5) and 10(-4) M), while cobalt, zinc, and AHZ had no effect. DNA content in the bone tissues from unloaded rats was significantly increased by all metal compounds (10(-5) M). The effect of AHZ on bone components was greater than zinc sulfate. The AHZ (10(-5) M)-increased alkaline phosphatase activity in the bone tissues from unloaded rats was clearly blocked by the presence of cycloheximide (10(-6) M), staurosporine (10(-7) M), dibucaine (10(-4) M), or okadaic acid (10(-7) M). The present study demonstrates that, of various essential trace metals, zinc compounds have an unique anabolic effect on bone metabolism in the femoral-metaphyseal tissues of rats with skeletal unloading. Zinc-chelating dipeptide may stimulate bone protein synthesis through the mechanism that is involved in protein kinases.

Ontogenesis of IGF regulation of longitudinal bone growth in rat metatarsal rudiments cultured in serum-free medium

The aim of this study was to compare the effect of two cytokines, IGF-I and IGF-II on skeletal development in the rat. The three medial metatarsal rudiments were dissected out from fetuses at days 19, 20 or 21 of gestation and from newborns at days 1, 3, 6 and 9 after birth, then grown in serum-free MEM medium at 37 degrees C and 5% CO2 in air. From day 19 of gestation to the end of experiment, longitudinal bone growth (mm) was significantly increased by IGF-I (2.975 +/- 0.050) and IGF-II (2.530 +/- 0.062), compared to controls (2.188 +/- 0.060). In the same way, the width (mm) at the last experimental day was 0.360 +/- 0.010 in IGF-I- and 0.327 +/- 0.008 in IGF-II-treated bones, respectively (vs 0.313 +/- 0.012 in controls). Mineralization was also stimulated under both growth factors (length of the calcified diaphysis (mm): 0.691 +/- 0.019 in IGF-I- and 0.446 +/- 0.017 in IGF-II-treated bones; vs 0.383 +/- 0.024 in controls). IGF-I and IGF-II (but to a lesser extent) stimulation was due to an increased DNA synthesis (3H-thymidine uptake) as well as protein anabolism (incorporated proline). In addition, cartilage activity (35S captation) and mineralization (45Ca fixed) were involved in the action of these cytokines. An age dependency of bone response to IGFs was pointed out, the effect being higher during the fetal period than after birth. In conclusion, our results raise the possibility that IGF-II, as well as IGF-I, is involved in the control of osteogenesis.

Effects of experimental conditions on the release of 45calcium from prelabeled fetal mouse long bones

Embryonic/neonatal bones in culture are commonly used for the study of osteoclastic resorption in vitro. For this purpose, the release of 45calcium (45Ca) from prelabeled bones is measured as an index of resorption. We studied 45Ca release from two types of long bone explants after different preparation methods: 17-day-old fetal mouse radii/ulnae with and without cartilage ends (intact radii/ulnae and shafts, respectively), and intact 18-day old metacarpals/metatarsals. In addition, we examined the effect of different culture conditions, such as cultures performed under the surface of the medium or at the interphase of medium and air, on 45Ca release and histology. When intact radii/ulnae were cultured under the surface of the medium, there was always a significant amount (10%) of net basal 45Ca release (corrected for physicochemical exchange) that was not due to osteoclastic resorption, as it could not be suppressed by inhibitors of resorption even at high concentrations. Moreover, histologically TRAcP-positive cells were almost absent after culture and the bone marrow/stromal cells in the center of the bone appeared necrotic, possibly due to a lack of oxygen. Under these culture conditions, osteoclasts could survive in shafts as well as in PTH-stimulated intact radii/ulnae, but a constant amount of 10% 45Ca, not due to resorption, was still released in the medium. When these explants were cultured at the interphase of medium and air, basal and stimulated 45Ca release originated from osteoclastic resorption. In contrast, in 18-day-old fetal mouse metacarpals/metatarsals, the experimental conditions applied did not affect 45Ca release, which was always due to resorption of the explants by osteoclasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteoclast formation from human cord blood mononuclear cells co-cultured with mice embryonic metatarsals in the presence of M-CSF

Investigating the potentiality of cord monocytes to differentiate toward osteoclast-like cells (OCL) in vitro, we previously reported that in the presence of 1,25(OH)2 vitamin D3 (1,25-(OH)2D3), multinucleated-cells generated by cord monocyte cultures though displaying morphological features of OCL failed to resorb devitalized bones. We thus hypothesized that full differentiation of cord monocytes toward bone-resorbing cells may require the presence of factors released from and/or direct interactions with living osteogenic cells. In the present study, we tested these hypotheses using two culture systems supporting the development of bone-resorbing cells in the presence of bone matrix. First, cord mononuclear cells were co-cultured with murine fetal metatarsals depleted of osteoclast progenitor cells (stripped metatarsals) in the presence of 1,25-(OH)2D3. We found that cord mononuclear cells failed to differentiate toward OCL as indicated by the absence of the release of 45Ca previously incorporated in fetal bones and by the absence of formation of TRAP-positive (TRAP[+]) multinucleated cells which have invaded mineralized cartilage during the co-culture period. In the same model, we then investigated the effect of some soluble factors known as stimulators of osteoclast differentiation. Whereas exogenous rhIL6 and rhIL3 were ineffective in this assay, rhM-CSF consistently increased both the number of TRAP(+) multinucleated cells inside the mineralized cartilage and the release of 45Ca into the culture media. The effects of rhM-CSF were time-dependent reaching the maximum after 3 weeks of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Opposite effects of osteogenic protein and transforming growth factor beta on chondrogenesis in cultured long bone rudiments

Osteogenic protein-1 (OP-1, also called BMP-7) is a bone morphogenetic member of the TGF-beta superfamily. In the present study, we examined the effect of recombinant human OP-1 on cartilage and bone formation in organ cultures of metatarsal long bones of mouse embryos and compared the OP-1 effects with those of human TGF-beta 1 and porcine TGF-beta 1 and beta 2. Cartilage formation was determined by measurement of longitudinal growth of whole bone rudiments during culture and by the incorporation of 35SO4 into glycosaminoglycans. Mineralization was monitored by 45Ca incorporation in the acid-soluble fraction and by measuring the length of the calcifying center of the rudiment. Toluidine blue-stained histologic sections were used for quantitative histomorphometric analysis. We found that OP-1 stimulated cartilage growth as determined by sulfate incorporation and that it increased remarkably the width of the long bones ends compared with controls. This effect was partly caused by differentiation of perichondrial cells into chondrocytes, resulting in increased appositional growth. In contrast to OP-1, TGF-beta 1 and beta 2 inhibited cartilage growth and reduced the length of whole bone rudiments compared with controls. In the ossifying center of the bone rudiments, both OP-1 and TGF-beta inhibited cartilage hypertrophy, growth of the bone collar, and matrix mineralization. These data demonstrate that OP-1 and TGF-beta exhibit opposite effects on cartilage growth but similar effects on osteogenesis in embryonic mouse long bone cultures. Since both OP-1 and TGF-beta have been demonstrated in embryonic cartilage and bone, these results suggest that they act as autocrine or paracrine regulators of embryonic bone development.

Relationship of the plasminogen activator/plasmin cascade to osteoclast invasion and mineral resorption in explanted fetal metatarsal bones

An attempt was made to establish whether the activation of plasminogen into plasmin is necessary either for the preparatory phases to bone resorption, involving the recruitment of osteoclast precursors, their migration toward mineralized surfaces, and their final differentiation, or for the subsequent osteoclastic resorption phase. 45Ca-labeled fetal (17 day) mouse metatarsals were cultured under conditions in which they pursue their modeling for a few days. In this model, the resorption phase, monitored by the release of 45Ca into the medium, is entirely dependent on the preparatory phases affecting osteoclast precursors. It was, as expected, stimulated by parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 and inhibited by calcitonin. PTH also enhanced the activity of tissue-type plasminogen activator (PA) in extracts of metatarsals but not that of urokinase (which is, however, the main PA present in the mouse fetal metatarsal culture model). The resorption processes were not dependent on the presence of plasminogen in the media, even when the rudiments were precultured with tranexamic acid to remove their endogenous plasminogen. Moreover, they were not influenced by inhibitors of plasmin, either the plasma inhibitors alpha 2-antiplasmin, alpha 2-macroglobulin, and alpha 1-antitrypsin, or aprotinin, which was tested under a variety of conditions. Aprotinin also did not influence the resorption (loss of calcium and hydroxyproline) of 19 day fetal mouse calvariae cultured with PTH in a medium devoid of plasminogen. It is concluded that the various steps implicated in the bone resorption processes that occur in the metatarsals and in the calvariae culture models are not dependent on the activity of plasmin. The function of PAs in bone, however, could be exerted through direct proteolysis of extracellular proteins other than plasminogen or be mediated by a molecular structural domain distinct from their catalytic domain.

The effects of 1,25-dihydroxyvitamin D3 on osteoclast formation in fetal mouse metatarsal organ cultures

We have previously described a model system, using 15-day fetal mouse metatarsals cultured in serumless medium, in which osteoclasts and their precursors develop from in situ progenitors in a manner which is similar, both temporally and spatially, to that which occurs in vivo. In this report we evaluate the role of the osteotropic hormone 1,25-dihydroxyvitamin D3 (1,25-D3) on osteoclast formation in this model system by characterizing its effects on proliferation, differentiation, and fusion of cells of the osteoclast lineage. Morphologic evaluation was used to enumerate osteoclast precursors, mono- and multinucleate osteoclasts, and osteoclast nuclei in serial paraffin sections. Dose response data reveal a significant stimulation of osteoclast formation by 1,25-D3 in the range of 0.6 nM to 40 nM, and kinetic analyses suggest that these effects are on proliferation of osteoclast progenitors as well as on differentiation of precursors to form osteoclasts. A single 48 h exposure between day 4 and 6 of culture is necessary and sufficient to induce maximal osteoclast formation, while continuous exposure beyond this "critical period" inhibits multinucleate osteoclast formation. Simultaneous treatment with indomethacin inhibits the effects of 1,25-D3, while treatment with PGE2 stimulates osteoclast formation without significantly increasing precursor numbers, or inhibiting multinucleate osteoclast formation. These results suggest that the effect of 1,25-D3 to induce differentiation of precursors to form mono- and multinucleate osteoclasts is mediated by endogenous prostaglandin synthesis. On the other hand, the inhibition of polykaryon formation observed with continuous 1,25-D3 treatment, does not appear to be a prostaglandin mediated phenomena.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-related effects of 1,25-dihydroxyvitamin D3 on growth, modeling, and morphology of fetal mouse metatarsals cultured in serum-free medium

A serum-free, fetal bone organ culture model that permits the simultaneous determination of modeling and growth parameters was used to examine the effects of a near physiologic and a pharmacologic dose of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]. The fetuses of pregnant mice were removed on day 17 of gestation, and three medial metatarsal rudiments were cleaned and after preculturing were cultured as pair-matched groups for 4 days in MEM supplemented with 0.2% BSA. 1,25-(OH)2D3 was added to the cultures at concentrations of 10(-12) or 10(-6) M. Cultures treated with the carrier and devitalized bones served as controls. For resorption studies, pregnant mice were given 45Ca on day 17 of pregnancy and fetal metatarsals harvested 24 h later. Resorption was determined by the amount of 45Ca released into the media. DNA synthesis was estimated by determining the incorporation of [3H]thymidine, collagen synthesis by measuring the incorporation of [3H]proline, mineralization by the incorporation of 45Ca, and proteoglycan synthesis by the incorporation of 35S. The amount of radiolabel was detected in media, as well as in noncultured, dead, and cultured rudiments. The total length of the rudiments and length of the calcified diaphyses were measured daily. In addition, rudiments from all experimental groups were prepared for light and electron microscopy. The high dose (10(-6) M) of 1,25-(OH)2D3 suppressed total rudiment growth but not the growth of the calcified diaphysis, 1,25-(OH)2D3 also decreased DNA, collagen, and proteoglycan synthesis, reduced calcification, and increased bone resorption in a dose-related manner. There were morphologic and ultrastructural changes in the osseous tissues and cells, particularly with the high dose of vitamin D, that supported the biochemical findings of suppressed activity of the osteogenic and chondrogenic cells. However, the suppression of collagen production and bone cell proliferation observed with the pharmacologic dose of vitamin D may be partially attributable to the decrease in bone mass (from increased resorption), thus resulting in less osseous tissue surface for these events to occur as endochondral osteogenesis progressed. The lower dose of vitamin D, however, had effects on 35S and 45Ca incorporation that could not be attributed to a decreased osseous tissue mass. This study emphasizes the importance of measuring specialized activities of the various cell populations in bone rudiment culture models to more fully understand the changes in tissue metabolism that result in changes in rudiment growth and modeling.

Bone ossification and carcass characteristics of wethers given silastic implants containing estradiol

Administration of growth promotants with estrogenic activity via hastening closure of the growth plate could have an economic impact on lambs because closure results in ovine carcasses being classified as yearlings. Twelve wether lambs approximately 12 mo old were given silastic implants filled with estradiol-17 beta and allotted randomly to be slaughtered 30, 60, 90, or 220 d after implantation to determine time of growth plate closure in relation to date of implanting. Seven comparable wethers not implanted served as controls. Four wethers implanted at 2 mo of age and slaughtered 220 d later also were included. Implanted wethers had serum concentrations of estradiol averaging 13.9 pg/ml over the 220-d implant period; controls averaged 2.7 pg/ml. Estradiol implants increased carcass maturity scores but fat deposition was not altered. Growth plate widths decreased (P less than .05) as wethers grew older and implants were in place for longer periods of time. Metacarpal growth plates in 12-mo-old lambs were completely ossified 220 d after implanting, but control wethers and wethers implanted at 2 mo maintained growth plate widths. Neither metacarpal nor metatarsal bone lengths differed (P less than .05) between control and implanted wethers. Growth plate ossification was not complete until 570 d of age in implanted lambs, even though bone length had stopped increasing by 408 d. Therefore, even though estradiol and other growth stimulants with estrogenic activity increase rate of ossification of the metacarpal growth plate, bone length or mature size is not limited.(ABSTRACT TRUNCATED AT 250 WORDS)

The metatarsal cytochemical bioassay of parathyroid hormone: validation, specificity, and application to the study of pseudohypoparathyroidism type I

The most sensitive method for assaying the bioactivity of PTH in unextracted plasma is the renal cytochemical bioassay. However, PTH acts on bone as well as kidney and clinical studies have suggested that the actions of circulating PTH level may be different at the two sites. We developed cytochemical bioassay for PTH based on the stimulation of glucose 6-phosphate dehydrogenase activity in the hypertrophic chondrocytes of the growth plate and the osteoblasts lining the metaphyseal trabeculae of rat metatarsal bones. The index of precision was 0.14 +/- 0.02 (SE) and the interassay variation was 31%. With this assay, plasma bioactive PTH levels in normal subjects and patients with primary hyperparathyroidism ranged from 0.5-18 ng/L and from 27-850 ng/L, respectively. Studies of patients with pseudohypoparathyroidism type I indicated that plasma PTH bioactivity in such patients is greater in the metatarsal bioassay than in the renal bioassay; no such differences were found in normal subjects or patients with primary hyperparathyroidism.