Ob Cell Research Articles

Influence of Calcium Supplementation against Fluoride-Mediated Osteoblast Impairment in Vitro: Involvement of the Canonical Wnt/β-Catenin Signaling Pathway.

Fluoride (F) is capable of promoting abnormal proliferation and differentiation in primary cultured mouse osteoblasts (OB cells), although the underlying mechanism responsible remains rare. This study aimed to explore the roles of wingless and INT-1 (Wnt) signaling pathways and screen appropriate doses of calcium (Ca2+) to alleviate the sodium fluoride (NaF)-induced OB cell toxicity. For this, we evaluated the effect of dickkopf-related protein 1 (DKK1) and Ca2+ on mRNA levels of wingless/integrated 3a (Wnt3a), low-density lipoprotein receptor-related protein 5 (LRP5), dishevelled 1 (Dv1), glycogen synthase kinase 3β (GSK3β), β-catenin, lymphoid enhancer binding factor 1 (LEF1), and cellular myelocytomatosis oncogene (cMYC), as well as Ccnd1 (Cyclin D1) in OB cells challenged with 10-6 mol/L NaF for 24 h. The demonstrated data showed that F significantly increased the OB cell proliferation rate. Ectogenic 0.5 mg/L DKK1 significantly inhibited the proliferation of OB cells induced by F. The mRNA expression levels of Wnt3a, LRP5, Dv1, LEF1, β-catenin, cMYC, and Ccnd1 were significantly increased in the F group, while significantly decreased in the 10-6 mol/L NaF + 0.5 mg/L DKK1 (FY) group. The mRNA expression levels of Wnt3a, LRP5, β-catenin, and cMYC were significantly decreased in the 10-6 mol/L NaF + 2 mmol/L CaCl2 (F+CaII) group. The protein expression levels of Wnt3a, Cyclin D1, cMYC, and β-catenin were significantly increased in the F group, whereas they were decreased in the F+CaII group. However, the mRNA and protein expression levels of GSK3β were significantly decreased in the F group while significantly increased in the F+CaII group. In summary, F activated the canonical Wnt/β-catenin pathway and changed the related gene expression and β-catenin protein location in OB cells, promoting cell proliferation. Ca2+ supplementation (2 mmol/L) reversed the expression levels of genes and proteins related to the canonical Wnt/β-catenin pathway.

Tumour Interstitial Fluid Pressure May Regulate Angiogenic Factors in Osteosarcoma

We have previously shown that osteosarcomas have states of increased interstitial fluid pressure (IFP) which correlate with increased proliferation and chemosensitivity. In this study, we hypothesized that constitutively raised IFP in osteosarcomas regulates angiogenesis. Sixteen patients with the clinical diagnosis of osteosarcomas underwent blood fl ow and IFP readings by the wick-in-needle method at the time and location of open biopsy. Vascularity was determined by capillary density in the biopsy specimens. We performed digital image analysis of immunohistochemical staining for CD31, VEGF-A, VEGF-C and TPA on paraffin-embedded tissue blocks of the biopsy samples. Clinical results were validated in a pressurised cell culture system. IFPs in the tumours (mean 33.5 +/- SD 17.2 mmHg) were significantly higher (P = 0.00001) than that in normal tissue (2.9 +/- 5.7 mmHg). Pressure readings were significantly higher in low vascularity tumours compared to high vascularity tumours (P <0.001). In the osteosarcoma cell lines, growth in a pressurised environment was associated with VEGF-A downregulation, VEGF-C upregulation and TPA upregulation. The reverse was seen in the OB cell lines. Growth in the HUVEC cell line was not significantly inhibited in a pressurised environment. Immunohistochemical assessment for VEGF-A (P = 0.01), VEGF-C (P = 0.008) and TPA (P = 0.0001) translation were consistent with the findings on PCR. Our data suggest that some molecules in angiogenesis are regulated by changes in IFP.

Myeloablative Therapies Damage and Alter the Composition of the Cells of the Bone Marrow Microenvironment.

Abstract 1447Poster Board I-470Patients with a range of blood cell diseases are usually treated with myeloablative therapies such as chemotherapy or irradiation accompanied by bone marrow (BM) transplants. These therapies have severe effects on the hematopoietic system and cause prolonged pancytopenia, which is life-threatening to the patient. The effects of myeloablative therapies on hematopoietic cells are well-documented but their impacts on the cells of the BM microenvironment, which have important roles in regulating hematopoiesis, are largely unknown. Using the mouse model, we have analyzed the effects of 11 Gy irradiation and transplantation of 5 × 106 BM cells/mouse on hematopoietic and BM microenvironment cells: osteoblasts (Obs), endothelial cells (ECs) (both which have been shown to be important positive regulators of hematopoiesis) and adipocytes (known negative regulators of hematopoiesis). Mature and immature hematopoietic cell contents in BM, peripheral blood and spleen were assessed by cell counts together with fluorescence-activated cell-sorting (FACS) analysis and colony-forming cell potential. BM microenvironment Ob and adipocyte cell content were examined by standard histomorphometry methods on non-decalcified plastic-embedded tibiae. BM microenvironment ECs were measured by immunohistochemical staining of paraffin-embedded sections accompanied by FACS-analysis of lineage/CD45/CD48-negative, CD31-positive cells. Quantitative real-time PCR of BM provided further insight into changes in transcripts specific for immature populations of BM microenvironment cells post-transplant. Cohorts of male mice were analyzed at numerous time-points corresponding to: age-matched non-transplanted male mice (day 0); periods of severe pancytopenia (days 2-7); early recovery of hematopoiesis (days 10-35) and the time-point at which stable hematopoiesis was re-established (day 84). The earliest change in the BM microenvironment was to ECs, as massive hemorrhaging was observed, with damaged vasculature throughout the BM cavity occurring as early as day 2 post-transplant. Furthermore, the numbers of ECs in the BM markedly dropped at this time-point, and this was accompanied by 2-fold reductions in transcripts for the BM vascular-specific markers VEGFR2 and VEGFR3. In contrast, the expression of VE-cadherin, a molecule important for endothelial cell-cell interactions, was significantly up-regulated (6-fold, P<0.05 vs day 0), potentially to aid repair of the vasculature. At the same time-point stromal cell-derived factor-1 (CXCL12), a chemokine that retains hematopoietic cells in the BM but also functions as an angiogenic factor, was highly up-regulated in BM (30-fold, P<0.001) but returned to normal levels by day 7 post-transplant. Interestingly, EC cytopenia paralleled that observed for hematopoietic cells, however, full recovery of BM ECs was detected by day 14 post-transplant. There was a significant 2-fold reduction in trabecular bone that persisted from days 7-35 post-transplant (P<0.05 vs day 0). This correlated with significant (2-fold) decreases in transcripts of Runx2 and Osterix (P<0.005 vs day 0), both markers of immature Ob precursors, Furthermore, while non-treated age-matched mice did not have significant numbers of adipocytes in their BM, there was a 20-fold increase in adipocytes visible from days 7-35 post-transplant (P<0.05). This was preceded by a significant increase in expression of the early adipocyte markers peroxisome proliferator-activated receptor-γ (PPARγ) (12-fold) and adiponectin (3-fold) in the BM by day 2 post-transplant (P<0.0005 vs day 0). These inverse changes observed in bone and adipocyte formation may help to explain why both parathyroid hormone and a PPARγ antagonist (which target mesenchymal progenitor cells to reduce adipogenesis and increase bone formation) have recently been shown to aid hematological recovery post-transplant. Preliminary results suggest that similar BM microenvironment cell changes are also observed when mice are treated with the cytotoxic drug 5-fluorouracil. In conclusion, the cells of the BM microenvironment are highly susceptible to myeloablative therapies. Treatments aiding the recovery of the different BM microenvironment cell types might in turn reduce pancytopenia and promote more rapid hematological recovery in patients receiving myeloablative therapies. DisclosuresNo relevant conflicts of interest to declare.

Tumor interstitial fluid pressure may regulate angiogenic factors in osteosarcoma

We have previously shown that osteosarcomas (OS) have states of increased interstitial fluid pressure (IFP), which correlate with increased proliferation and chemosensitivity. In this study, we hypothesized that constitutively raised IFP in OS regulates angiogenesis. Sixteen patients with the clinical diagnosis of OS underwent blood flow and IFP readings by the wick-in-needle method at the time and location of open biopsy. Vascularity was determined by capillary density in the biopsy specimens. We performed digital image analysis of immunohistochemical staining for CD31, VEGF-A, VEGF-C, and TPA on paraffin-embedded tissue blocks of the biopsy samples. Clinical results were validated in a pressurized cell culture system. Interstitial fluid pressures in the tumors (mean 33.5 +/- SD 17.2 mmHg) were significantly higher (p = 0.00001) than that in normal tissue (2.9 +/- 5.7 mmHg). Pressure readings were significantly higher in low vascularity tumors compared to high vascularity tumors (p < 0.001). In the OS cell lines, growth in a pressurized environment was associated with VEGF-A downregulation, VEGF-C upregulation, and TPA upregulation. The reverse was seen in the OB cell line. Growth in the HUVEC cell line was not significantly inhibited in a pressurized environment. Immunohistochemical assessment for VEGF-A (p = 0.01), VEGF-C (p = 0.008), and TPA (p = 0.0001) translation were consistent with the findings on PCR. Our data suggests that some molecules in angiogenesis are regulated by changes in IFP.

On the Molecular Mechanism of DKK1 Inhibition of Osteoblast Differentiation in Multiple Myeloma.

Lytic bone destruction is a severe complication of multiple myeloma growth in the bone. We have demonstrated that Dkk1, an antagonist of Wnt signaling which is essential for osteoblast growth and differentiation, is expressed by myeloma plasma cells leading to increased levels of this protein in serum. Myeloma serum can block osteoblast differentiation in-vitro and a neutralizing anti-DKK1 antibody can inhibit this. These data suggest that MM cells disrupt Wnt signaling and that this promotes uncoupled bone turnover. However, the molecular mechanism responsible for this processes remains to be elucidated. Here we investigated Wnt signaling in osteoblast differentiation and identified the molecular mechanism regulating this process. Using qRT-PCR and sequence confirmation we showed that multiple Frizzled (Fz) mRNAs and LRP5/6 co-receptors were expressed in C2C12 cells and three human osteoblast cell lines, hFOB1.19, MG63 and Saos-2. Furthermore, these cells had high levels of total beta-catenin an indicator of active Wnt signaling. Wnt3a conditioned medium (CM) and recombinant Wnt3a protein increased uncomplexed beta-catenin level as assessed by GST-E-cadherin binding assay. Blocking autocrine Wnt signaling with Dkk1 blocked Wnt-3a-induced increase in beta-catenin levels in dose-dependent fashion. Co-culturing of OPM-2 MM cells ectopically expressing Dkk1 inhibited the Wnt-3a induced beta-catenin stabilization in C2C12 cells. Transfection of C2C12 cells with a dominant negative beta-catenin (DNBC) completely blocked endogenous and Wnt-3a-induced TCF/LEF transcriptional activity as evidenced by TOPflash luciferase activity. Furthermore, BMP-2 induced increase in alkaline phosphatase (ALP) activity was completed abrogated in C2C12 cells expressing DNCB. Silencing LRP5/6 mRNA with small inference RNAs blocked BMP-2 induced increase in ALP expression, suggesting that canonical Wnt pathway contributes to BPM-2 induced OB differentiation. BMP-2 induced phosphorylation of Smad 1, 5 and 8 whereas Wnt-3a had no effect on phosphorylation of these Smads. Moreover, Dkk1 did not block BMP-2 induced phosphorylation of Smad-1, −5 and −8 and did not alter transcriptional activity of Cbfa1, an essential transcriptional factor for OB differentiation. Taken together, these data suggest that Wnt induction of OB cell differentiation through canonical Wnt-beta-catenin pathway is independent of the activation of the Smad pathway. Our results suggested that canonical Wnt signaling pathway plays an important role in osteoblastogenesis and that Dkk1 inhibits OB cell differentiation in a way that may contribute to MM bone lesions. New therapies targeting DKK1 in myeloma may represent a novel treatment strategy.

AU-Rich Elements in the Collagenase 3 mRNA Mediate Stabilization of the Transcript by Cortisol in Osteoblasts

Collagenase 3 degrades collagen fibrils and is necessary for bone resorption. Cortisol increases collagenase 3 mRNA in osteoblasts by stabilizing collagenase 3 transcripts. To understand mechanisms involved, we used RNA electrophoretic mobility shift assay and RNA turnover studies. Cortisol increased the binding of Ob cell cytosolic extracts to AU-rich sequences in the collagenase 3 3'-untranslated region (UTR). No cortisol-dependent protein complexes were formed with the coding region or the 5'-UTR. Functional assays, using transient transfections of CMV-driven c-fos collagenase 3'-UTR chimeric constructs, demonstrated that the 3'-UTR of collagenase 3 stabilizes c-fos mRNA in transcriptionally arrested Ob cells, cortisol prolongs the transcript half-life, and mutations of AU-rich sequences destabilize c-fos transcripts precluding the cortisol effect. Purification of osteoblast cytosolic extracts by ultracentrifugation, ion exchange, and RNA affinity chromatography, and polyacrylamide gel electrophoresis followed by mass spectroscopy identified specific proteins. RNA gel mobility supershift assays demonstrated that vinculin and far upstream element (FUSE)-binding protein 2 interacted with collagenase 3 3'-UTR sequences, and RNA interference demonstrated these proteins altered collagenase mRNA stability. In conclusion, AU-rich sequences of the 3'-UTR of collagenase 3 and vinculin and FUSE-binding protein 2 regulate collagenase mRNA stability in osteoblasts.

Metabolic activity of osteoblasts retrieved from osteoarthritic patients after stimulation with mediators involved in periprosthetic loosening

Objective: To compare the effects of polymethylmetacrylate (PMMA), interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) on osteoarthritic (OA) human osteoblasts (Ob). Methods: Ob cell cultures were prepared from metaphyseal trabecular bone of OA patients ( n = 9). Their cellular metabolic activity was characterized by measuring alkaline phosphatase (ALPase), osteocalcin (Oc), osteoprotegerin (OPG), prostaglandin E 2 (PGE 2), IL-6, macrophage colony-stimulating factor (M-CSF) and soluble receptor activator of NF-κb ligand (sRANKL) in supernatants after Ob stimulation with PMMA, IL-1β, IL-6 or TNF-α. Results: IL-6 and PGE 2 production by Ob was significantly increased by all mediators used. PMMA and TNF-α both stimulated M-CSF and sRANKL production whereas PMMA decreased and TNF-α augmented OPG release by Ob. The ratio sRANKL/OPG was significantly increased with PMMA and TNF-α, and treatment with anti-IL-6 antibodies did not alter this ratio. ALPase levels remained constant while only PMMA affected Oc production. Subclassification of patients into high or low endogenous producers of IL-6 and PGE 2 before stimulation of Ob indicated that the two subgroups maintained similar responses to stimulation by all mediators. Conclusion: PMMA and TNF-α have comparable dual action on Ob, lowering their anabolic capacity and favoring production of bone resorption activators, an action not mediated by IL-6. This study confirms the important role of PMMA in implant loosening. However, the pertinence of high and low metabolic activities in OA Ob remains unclear, but could be involved in the pathophysiology of bone disease and aseptic loosening.

IN VITROEFFECTS OF PEMFs ON BONE CELL CULTURES OF NORMAL AND OSTEOPENIC RAT

The aim of this study was to examine the effects of low-frequency, low-energy pulsed electromagnetic fields (PEMFs) on cell proliferation and differentiation in rat osteoblast primary cultures. Cells were obtained from normal and osteopenic rat bone and were named NB and OB, respectively. The osteoblastic phenotype was assessed by stimulation with 1,25(OH2) vitamin D3. NB and OB cells were seeded in multiwell plates and exposed to PEMFs for two different periods. Control cultures of both groups were incubated under the same conditions, with the pulse generator off. Assessment of PEMF effects was performed for the following parameters for each culture: alkaline phosphatase (ALP) activity, osteocalcin level, and MTT test. Results showed that OB and NB cell proliferation was significantly improved (p < 0.03, p = 0.04 respectively) after 48 h of PEMF exposure. Osteocalcin production of OB after 5 days of PEMF exposure was significantly higher than normal (p = 0.007) and osteopenic (p = 0.033) bone-derived controls. These results show that PEMFs act on osteopenic bone-derived osteoblasts, stimulating proliferation of cells and then, after a longer exposure, activating them.

Autocrine down-regulation of collagenase-3 in rat bone cell cultures by insulin-like growth factors

Insulin-like growth factors (IGF)-I and -II are presumed to act as autocrine regulators of bone formation. Recently, we demonstrated that IGF-I and -II inhibit bone collagen degradation and collagenase-3 synthesis in osteoblast cultures. Therefore, we tested the autocrine role of IGFs in the endogenous expression of collagenase-3 in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Steady-state messenger RNA (mRNA) levels were determined by Northern blot analysis and collagenase concentrations in the culture medium were determined by Western immunoblot. Basal level collagenase-3 transcripts decreased in Ob cell cultures, coinciding with an increase in IGF-I and -II protein levels. Removal of the conditioned medium modestly increased collagenase-3 mRNA levels and restored the ability of exogenously added IGF-I to repress collagenase-3 transcripts. IGF neutralizing antibodies and IGF binding proteins-2 and -3 in excess increased and sustained collagenase mRNA, heterogeneous nuclear RNA, and protease levels in Ob cell cultures. In conclusion, IGF-I and -II are autocrine repressors of collagenase-3 synthesis, and this effect may contribute to their actions on the maintenance of a normal bone collagen matrix.

Autocrine down-regulation of collagenase-3 in rat bone cell cultures by insulin-like growth factors.

Insulin-like growth factors (IGF)-I and -II are presumed to act as autocrine regulators of bone formation. Recently, we demonstrated that IGF-I and -II inhibit bone collagen degradation and collagenase-3 synthesis in osteoblast cultures. Therefore, we tested the autocrine role of IGFs in the endogenous expression of collagenase-3 in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Steady-state messenger RNA (mRNA) levels were determined by Northern blot analysis and collagenase concentrations in the culture medium were determined by Western immunoblot. Basal level collagenase-3 transcripts decreased in Ob cell cultures, coinciding with an increase in IGF-I and -II protein levels. Removal of the conditioned medium modestly increased collagenase-3 mRNA levels and restored the ability of exogenously added IGF-I to repress collagenase-3 transcripts. IGF neutralizing antibodies and IGF binding proteins-2 and -3 in excess increased and sustained collagenase mRNA, heterogeneous nuclear RNA, and protease levels in Ob cell cultures. In conclusion, IGF-I and -II are autocrine repressors of collagenase-3 synthesis, and this effect may contribute to their actions on the maintenance of a normal bone collagen matrix.

Cortisol represses insulin-like growth factor II receptor transcription in skeletal cell cultures.

Glucocorticoids have a number of effects on bone cell function, some of which might be mediated by changes in the synthesis or activity of insulin-like growth factors (IGFs). Glucocorticoids inhibit IGF-I, but not IGF-II, synthesis in osteoblasts and decrease the expression of selected IGF-binding proteins. The effects of glucocorticoids on IGF-I and -II receptor messenger RNA (mRNA) expression in osteoblasts are not known, and changes in IGF-I or -II receptor levels could result in changes in IGF activity. We examined the effects of glucocorticoids on IGF-I and -II receptor mRNA expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Cortisol at 1 microM for 2-48 h did not alter IGF-I receptor transcripts, as determined by Northern blot analysis and ribonuclease protection assay. In contrast, cortisol caused a time- and dose-dependent inhibition of IGF-II receptor mRNA levels. The effect was maximal at 0.1-1 microM for 24-48 h and was accompanied by a decrease in IGF-II receptor levels, as determined by affinity labeling, cross-linking and polyacrylamide gel electrophoresis, Western immunoblot, and Scatchard analysis. The effect of cortisol on IGF-II receptor transcripts was not dependent on de novo protein synthesis. Cortisol did not modify the IGF-II receptor mRNA half-life in transcriptionally arrested Ob cells and decreased the rate of IGF-II receptor RNA transcription in nuclear run-on assays. In conclusion, cortisol decreases transcription of the IGF-II receptor in Ob cell cultures, an effect that could mediate selected actions of glucocorticoids in bone.

Displacement and translocation of osteoblast-like cells by osteoclasts

Rabbit osteoclasts and rabbit osteoblast-like stroma cells (OB cells) were placed onto plastic surfaces and the migration patterns of individual osteoclasts and osteoclast-OB interactions were analyzed with time-lapse recording. To induce directed migration, the cultures were exposed to an electrical field of 0.01 or 0.1 V/mm. At 0.1 V/mm, osteoclasts moved directly toward the anode in some cases, clearing OB cells from their path of migration. In other cases, osteoclasts migrated toward the anode for part of the time but then changed direction and moved toward groups of OB cells. Observations were made on osteoclasts interacting with single OB cells or small colonies and on osteoclasts interacting with OB monolayers, at both field strengths; the results were independent of field strength. There were several characteristic behaviors. With single OB cells and small OB colonies, retraction of OB cells upon contact with the osteoclast was the predominant mechanism whereby these cells begin to move out of the path of the osteoclast. A pronounced ruffling or blebbing of the OB cell membrane often followed retraction. When osteoclasts displaced OB cells that were part of a monolayer, extension of an osteoclast lamellipodium underneath the edge of the OB cell layer generally preceded partial retraction of the OB cells involved. It sometimes appeared as if the detached or partially detached OB cells were "pushed" by the osteoclast, which in some cases resulted in OB cells being moved hundreds of microns in a period of a few hours, at rates comparable to the normal speed for osteoclast migration (congruent to 100 microns/h), much faster than the normal speed for OB cells (congruent to 10 microns/h).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of insulin-like growth factor-II synthesis in bone cell cultures by skeletal growth factors.

Insulin-like growth factor-II (IGF-II) is a growth factor secreted by bone cells and presumed to act as an autocrine regulator of bone formation. Although hormones and growth factors regulate the synthesis of skeletal IGF-I, hormones do not seem to modify the synthesis of skeletal IGF-II. We postulated that skeletal IGF-II is regulated by growth factors, and we tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor-beta 1 (TGF beta 1), and platelet-derived growth factor-BB (PDGF-BB) on IGF-II messenger RNA (mRNA) expression and polypeptide concentrations in cultures of osteoblast-enriched (Ob) cells from 22-day-old fetal rat calvariae. Steady state IGF-II mRNA levels were determined by Northern blot analysis, and IGF-II concentrations were determined in acidified and fractionated culture medium by a specific RIA. Treatment of Ob cells with bFGF, TGF beta 1, and PDGF-BB decreased IGF-II mRNA levels after 24-48 h. A continuous 48-h treatment with bFGF at 0.6-6 nM, TGF beta 1 at 0.04-1.2 nM, and PDGF-BB at 0.3-3.3 nM caused a dose-dependent decrease in steady state IGF-II mRNA. The effects of bFGF, TGF beta 1, and PDGF-BB on IGF-II mRNA were dependent on protein synthesis and decreased in the presence of cycloheximide at 3.6 microM, but were independent of cell division, because they were observed in the presence and absence of 1 mM hydroxyurea. Treatment with bFGF, TGF beta 1, and PDGF-BB for 24 h did not cause a change in IGF-II polypeptide levels. PDGF-BB at 3.3 nM and TGF beta 1 at 0.04-0.4 nM for 48 h decreased IGF-II polypeptide levels by about 50%, although bFGF had no effect. In conclusion, bFGF, TGF beta 1, and PDGF decrease skeletal IGF-II transcript levels, and this effect may contribute to their actions on selected aspects of Ob cell function.

Growth factors regulate the synthesis of insulin-like growth factor-I in bone cell cultures.

Insulin-like growth factor-I (IGF-I), a prevalent growth factor secreted by bone cells, has important effects on bone remodeling. Hormones are known to regulate the synthesis of skeletal IGF-I, but there is limited information about the actions of growth factors on IGF-I synthesis. We tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor-beta 1 (TGF beta 1), and platelet-derived growth factors (PDGF) AA and BB on IGF-I mRNA expression and polypeptide concentrations in cultures of osteoblast-enriched (Ob) cells from 22-day-old fetal rat calvariae. Steady state IGF-I mRNA levels were determined by Northern blot analysis, and IGF-I concentrations were determined in acidified and fractionated culture medium by a specific RIA. Treatment of Ob cells with bFGF at 0.06-6 nM, TGF beta 1 at 0.04-4 nM, and PDGF BB at 0.3-3.3 nM caused a dose-dependent decrease in steady state IGF-I mRNA. A smaller effect was observed with PDGF AA. The effect was initially observed after 6-8 h of treatment and was maximal after 16 h. Treatment with bFGF at 0.6-6 nM, TGF beta 1 at 0.4-4 nM, and PDGF BB at 0.3-3.3 nM for 24 h decreased IGF-I polypeptide concentrations by 40-80%. The effects of bFGF, TGF beta 1, and PDGF BB and AA on IGF-I mRNA were independent of protein synthesis and cell division, as they were observed in the presence and absence of cycloheximide at 3.6 microM or hydroxyurea at 1 mM. Similarly, their inhibitory actions on immunoreactive IGF-I were not prevented by hydroxyurea. In conclusion, bFGF, TGF beta 1, PDGF BB, and, to a lesser extent, PDGF AA decrease skeletal IGF-I synthesis by reducing IGF-I transcript levels, and this effect may contribute to their actions on selected aspects of Ob cell function.

Increased proliferation of osteoblast precursor cells in estrogen-deficient rats

We have evaluated the in vivo and in vitro changes in osteoblast characteristics induced by estrogen deficiency and 17 beta-estradiol (E2) treatment in ovariectomized (OVX) rats. Estrogen deficiency induced osteopenia and increased bone turnover, as evidenced by bone histomorphometry at 1, 3, and 6 mo postovariectomy. Bone surface osteoblastic cells (OB) isolated from tibias of OVX rats, OVX rats treated with E2 (10 micrograms/kg body wt), and sham rats showed no difference in alkaline phosphatase activity and osteocalcin production in vitro. In contrast the proliferation rate of OB cells was higher in OVX rats compared with sham rats at all time points post-surgery, as shown by [3H]thymidine incorporation and cell number. The proliferation rate of alkaline phosphatase-positive marrow cells was also higher in OVX rats compared with sham rats. E2 treatment of OVX rats corrected histologic indexes of bone resorption and formation and normalized OB cell proliferation. induced by estrogen deficiency in OVX rats is related to an increased proliferation of osteoblast precursor cells present in the marrow stroma and along the endosteal bone surface.

Effects of deflazacort on aspects of bone formation in cultures of intact calvariae and osteoblast-enriched cells

Deflazacort, a synthetic glucocorticoid reported to have bone-sparing properties in vivo, and cortisol were compared for their effects on bone formation in vitro. Deflazacort and cortisol were studied for their effects on DNA and collagen synthesis in cultures of intact fetal rat calvariae and of osteoblast-enriched (Ob) cells from 21- to 22-day-old fetal rat parietal bone. Both steroids were also examined for their effects on skeletal insulin-like growth factor (IGF) I production, which is decreased by cortisol and appears relevant to its mode of action. After 24 h of culture, deflazacort and cortisol had limited effects on the parameters studied, although cortisol at 100 nM decreased [3H]proline incorporation into collagen in intact calvariae. In contrast, after 72 h deflazacort and cortisol at 1-100 nM inhibited the incorporation of [3H]thymidine into DNA and at 100 nM decreased the incorporation of [3H]proline into collagen and noncollagen protein in intact calvariae. Deflazacort and cortisol at 10-1000 nM decreased calvarial collagen degradation to a similar extent. Both steroids had a similar activity, and at 100 nM for 72 h they decreased IGF-I production by calvariae; however, cortisol at 10 nM was somewhat more effective than deflazacort in decreasing IGF-I levels. Deflazacort and cortisol had analogous effects in Ob cell cultures. After 24 h of treatment, deflazacort at 100-1000 nM and cortisol at 10-1000 nM decreased the labeling of DNA, and both steroids at 100-1000 nM caused a similar decrease in [3H]proline incorporation into collagen in Ob cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination and expression of platelet-derived growth factor-AA in bone cell cultures.

We describe a sensitive technique for the extraction and quantitation of platelet-derived growth factor (PDGF)-AA in serum-free culture medium conditioned by fetal rat osteoblast-enriched (Ob) cells and demonstrate the expression of PDGF-A mRNA in Ob cells. Using C18 Sep-Pak chromatography with a methanol step gradient, we extracted immunoreactive PDGF-AA from the culture medium. A RIA protocol employing a recombinant human PDGF-AA standard enabled us to measure picomolar equivalents of PDGF-AA in Ob cell culture medium. The recovery of recombinant human PDGF-AA was 50 +/- 4%, and the coefficient of variation, including chromatographic extraction, was 15% for intraassay and 8.3% for interassay variability. The polyclonal antibody to recombinant human PDGF-AA displayed approximately 20-30% cross-reactivity with PDGF-AB, but did not bind PDGF-BB or other growth factors and cytokines known to be secreted by bone cells. Medium from Ob cells cultured for 24 h contained 0.9-1.3 pM human PDGF-AA equivalents, and exposure to cycloheximide (3.6 microM) decreased those levels by 65%. Treatment of cells with recombinant human transforming growth factor-beta 1 at 0.04-4 nM for 24 h increased PDGF-AA levels by up to 3.5-fold. Northern blot analysis of RNA from Ob cells revealed the expression of PDGF-A, but not PDGF-B, transcripts, and transforming growth factor-beta 1 at 0.04 and 0.2 nM increased steady state PDGF-A mRNA by 3- to 6-fold. Our studies describe a sensitive and reproducible technique for monitoring PDGF-AA levels in cell-conditioned medium and demonstrate that Ob cells synthesize PDGF-AA.

Cortisol Inhibits the Synthesis of Insulin-Like Growth Factor-I in Skeletal Cells*

Supraphysiological levels of cortisol inhibit bone cell replication and matrix synthesis, but its mechanism of action is unknown and could be secondary to an inhibition of local growth factor synthesis. These inhibitory effects of cortisol are the converse of the observed anabolic influences of the endogenously produced insulin-like growth factor-I (IGF-I); therefore, cortisol was examined for its effect on the production of IGF-I in osteoblast (Ob)- and fibroblast/preosteoblast-enriched cell cultures prepared from fetal rat parietal bone. Synthesis of IGF-I was monitored by Northern blot analysis to determine steady state IGF-I mRNA levels and by an IGF-I-specific RIA to quantitate polypeptide levels in acidified and fractionated culture medium. Cortisol at 100 nM decreased IGF-I transcript levels by 60% or more in Ob cultures within 6 h of treatment, and the concentration of immunoreactive IGF-I by 50% after 24 h; these effects were observed in the absence of a change in cellular DNA content. In Ob cultures, PTH at 10 nM increased IGF-I transcripts at 6 h and polypeptide levels at 24 h by 2.5- and 4.1-fold, respectively, and cortisol opposed this effect. The inhibitory effect of cortisol was not specific for the Ob cell population, since at 100 nM it also decreased IGF-I transcript and immunoreactive IGF-I levels in fibroblast/preosteoblast cultures and opposed the stimulation of IGF-I synthesis after treatment with 100 ng/ml GH. In conclusion, high levels of cortisol decrease skeletal IGF-I synthesis by reducing IGF-I transcript levels, and this effect probably contributes to the inhibitory influence of cortisol on bone formation.

Actions of parathyroid hormone and 1,25-dihydroxycholecalciferol on citrate decarboxylation in osteoblast-like bone cells differ in calcium requirement and in sensitivity to trifluoperazine.

The actions of PTH in OB bone cells appear to involve both calcium and cAMP. At present little information exists regarding the relationship, if any, between these two putative second messengers of hormone action in bone cells. In this report the molecular role of calcium in the actions of PTH and 1,25(OH)2D3 has been compared, since like PTH, the steroid 1,25(OH)2D3 is a potent bone resorbing hormone that exerts inhibition of citrate decarboxylation in OB cells, but unlike PTH does not activate adenylate cyclase. It was found that 1,25(OH)2D3 could initiate near maximum inhibition of citrate decarboxylation at extracellular calcium levels as low as 0.05 mM, whereas PTH effects began to be apparent only at 0.1 mM calcium, and maximum inhibition of citrate decarboxylation by PTH required 0.5 mM Ca. In addition, PTH-induced decrease in citrate decarboxylation was inhibited by low doses of TFP, an inhibitor of calmodulin and calcium-dependent, phospholipid-sensitive protein kinases, in contrast to 1,25(OH)2D3, whose effects were not reduced by this agent. These results suggest that: (a) the actions of 1,25(OH)2D3 may not be directly dependent on calcium influx; (b) in OB cell response to PTH a relationship probably exists between cAMP and calcium; and (c) this relationship may involve calmodulin, or calcium-dependent protein kinases that can be inhibited by TFP.

Characterization of subpopulations of OC and OB bone cells obtained by sedimentation at unit gravity.

Immediately after isolation from calvaria, OC and OB bone cells populations 1-6 were individually characterized by sedimentation at unit gravity. This procedure was used to generate from each population 4 fractions that contained cells of different sizes. Sedimentation results suggested that freshly isolated OC cell populations consisted of cells that were generally smaller and demonstrated less size heterogeneity than OB cells. After sedimentation the cells in each fraction were cultured for 6 days and then characterized with regard to cell separation based on basal biochemical characteristics and hormonal responses to PTH and CT. The largest cells in the later released OC cells appeared to be a mixture of OC and OB cells (approx. 15% of populations 2 + 3). All OB cell fractions appeared to be free of OC cells. The highest basal OC activities and hormonal responses occurred in the larger cells of population 2, whereas in OB cells (populations 5 and 6), this occurred in the cells of small to intermediate size. Finally, although the absolute size of the cells in each fraction increased during culture, the size differential within the fractions and between OC and OB cells was maintained even after 6 days.