Biphasic effects of epidermal growth factor on bone nodule formation by isolated rat calvaria cells in vitro

Epidermal growth factor (EGF) promotes bone healing in surgically induced osteonecrosis of the femoral head (ONFH)

Angiogenic effects of epidermal growth factor (EGF), a potent mitogen, have been demonstrated previously. Moreover, different in vitro studies showed that EGF affects processes associated with bone healing, such as osteoblast differentiation and bone resorption. The aim of this study was to investigate the effect of combined core decompression (CD) and recombinant human EGF (rhEGF) treatment on early-stage osteonecrosis of the femoral head (ONFH) surgically induced in rats. ONFH was induced by dissecting the cervical periosteum and placing a ligature tightly around the femoral neck. Thirty rats were assigned to one of the following groups (n = 10 each group): sham-operated control, CD, and CD+rhEGF group. rhEGF was injected intraosseously into infarcted areas 2 weeks after the surgery. Preservation of femoral head architecture was assessed at 8 weeks post treatment by radiographic and histomorphological analyses. Osteopontin (OPN) and cluster of differentiation 31 (CD31) were detected by immunochemistry, as indicators of bone remodeling and vascular density, respectively. Inter- and intra-group (non-operated left and operated right femur) differences in radiographic and histomorphological results were analyzed. The femoral head area and sphericity were more preserved in CD+rhEGF compared to CD and sham-control group. CD31 levels were significantly different between the three groups, and were higher in CD+rhEGF compared to CD group. OPN levels were increased in CD and CD+rhEGF groups compared to sham control, but with no significant difference between CD and CD+rhEGF groups. Overall, our results indicate that EGF promotes bone formation and microvascularization in ONFH and thus positively affects the preservation of femoral head during healing.

The reduced osteogenic potential of Nf1-deficient osteoprogenitors is EGFR-independent

Neurofibromatosis type 1 (NF1) is a common genetic disorder caused by mutations in the NF1 gene. Recalcitrant bone healing following fracture (i.e. pseudarthrosis) is one of the most problematic skeletal complications associated with NF1. The etiology of this condition is still unclear; thus, pharmacological options for clinical management are limited. Multiple studies have shown the reduced osteogenic potential of Nf1-deficient osteoprogenitors. A recent transcriptome profiling investigation revealed that EREG and EGFR, encoding epiregulin and its receptor Epidermal Growth Factor Receptor 1, respectively, were among the top over-expressed genes in cells of the NF1 pseudarthrosis site. Because EGFR stimulation is known to inhibit osteogenic differentiation, we hypothesized that increased EREG and EGFR expression in NF1-deficient skeletal progenitors may contribute to their reduced osteogenic differentiation potential. In this study, we first confirmed via single-cell mRNA sequencing that EREG over-expression was associated with NF1 second hit somatic mutations in human bone cells, whereas Transforming Growth Factor beta 1 (TGFβ1) expression was unchanged. Second, using ex-vivo recombined Nf1-deficient mouse bone marrow stromal cells (mBMSCs), we show that this molecular signature is conserved between mice and humans, and that epiregulin generated by these cells is overexpressed and active, whereas soluble TGFβ1 expression and activity are not affected. However, blocking either epiregulin function or EGFR signaling by EGFR1 or pan EGFR inhibition (using AG-1478 and Poziotinib respectively) did not correct the differentiation defect of Nf1-deficient mBMSCs, as measured by the expression of Alpl, Ibsp and alkaline phosphatase activity. These results suggest that clinically available drugs aimed at inhibiting EGFR signaling are unlikely to have a significant benefit for the management of bone non-union in children with NF1 PA.

Cross-talk between EGF and BMP9 signalling pathways regulates the osteogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent progenitors, which give rise to several lineages, including bone, cartilage and fat. Epidermal growth factor (EGF) stimulates cell growth, proliferation and differentiation. EGF acts by binding with high affinity to epidermal growth factor receptor (EGFR) on the cell surface and stimulating the intrinsic protein tyrosine kinase activity of its receptor, which initiates a signal transduction cascade causing a variety of biochemical changes within the cell and regulating cell proliferation and differentiation. We have identified BMP9 as one of the most osteogenic BMPs in MSCs. In this study, we investigate if EGF signalling cross-talks with BMP9 and regulates BMP9-induced osteogenic differentiation. We find that EGF potentiates BMP9-induced early and late osteogenic markers of MSCs in vitro, which can be effectively blunted by EGFR inhibitors Gefitinib and Erlotinib or receptor tyrosine kinase inhibitors AG-1478 and AG-494 in a dose- and time-dependent manner. Furthermore, EGF significantly augments BMP9-induced bone formation in the cultured mouse foetal limb explants. In vivo stem cell implantation experiment reveals that exogenous expression of EGF in MSCs can effectively potentiate BMP9-induced ectopic bone formation, yielding larger and more mature bone masses. Interestingly, we find that, while EGF can induce BMP9 expression in MSCs, EGFR expression is directly up-regulated by BMP9 through Smad1/5/8 signalling pathway. Thus, the cross-talk between EGF and BMP9 signalling pathways in MSCs may underline their important roles in regulating osteogenic differentiation. Harnessing the synergy between BMP9 and EGF should be beneficial for enhancing osteogenesis in regenerative medicine.

Epidermal growth factor and bone morphogenetic proteins upregulate osteoblast proliferation and osteoblastic markers and inhibit bone nodule formation

OBJECTIVE

The aim of this study was to investigate the in vitro osteogenic activity of EGF in association with bone morphogenetic proteins BMP2 and BMP7.

METHODS

SaOS-2 (osteoblast-like cell line from human osteosarcoma) were cultured in the presence of EGF and BMPs for various culture periods to assess (a) cell proliferation by MTT assay, (b) Runx2, alkaline phosphatase (ALP) and osteocalcin (OC) mRNA expression using quantitative RT-PCR and ELISA, and (c) bone tissue mineralization using Alizarin Red staining.

RESULTS

EGF alone was able to stimulate osteoblast growth in a time-dependent manner. When mixed with BMP2, BMP7, and their combination, EGF greatly promoted osteoblast growth, compared to the BMP- and EGF-stimulated cells, suggesting a possible synergistic effect between EGF and BMPs on osteoblast growth. Stimulation with EGF, EGF/BMP2, and EGF/BMP2/BMP7 for 7 days upregulated Runx2 mRNA expression by the osteoblasts. EGF downregulated ALP mRNA expression, which was recovered when the BMP2/BMP7 combination was added to the osteoblast culture. Tested on OC mRNA expression, EGF had no effect and inhibited the enhancing effect of BMP2 and BMP7 on osteocalcin expression. The bone mineralization assay showed that EGF reduced both the number and size of the bone nodules. This reducing effect was observable even in the presence of BMP2 and BMP7.

CONCLUSION

This study demonstrated that EGF may act in the early phase to promote osteoblast growth and specific marker expression rather than the late phase involving cell differentiation/mineralization.

A nondestructive method for evaluating in vitro osteoblast differentiation on biomaterials using osteoblast-specific fluorescence

Transgenic mice with a Col1a1-promoter-driven transgene pOBCol2.3GFP were previously developed to visually identify mature osteoblasts through fluorescent expression. Our goal was to determine if this technology could be used to nondestructively evaluate the in vitro differentiation of osteoprogenitor cells on biomaterials such as biomimetic carbonated hydroxyapatite (cHA). Primary osteoprogenitor cells were harvested from calvaria of neonatal Col2.3GFP transgenic mice and cultured on cHA and a tissue culture polystyrene (TCPS) control. The distribution of intensities and area percentage of green fluorescent protein (GFP)-positive cells were quantified using fluorimetry and image analysis of fluorescent microscopy. At 14 days, an increased area and higher mean intensity of GFP-positive cells was observed on cHA as compared to TCPS, indicating more rapid differentiation on cHA. Notably, there were large continuous regions of GFP-positive osteoblasts on cHA, in contrast to the sparse, nodules of osteoblasts on TCPS, implying that cHA provides an osteogenic cue to cells. Xylenol orange staining was capable of distinguishing osteoblast-initiated mineral from the cHA substrate. With this method the unique pattern of osteoblast differentiation on cHA was clearly observed for the first time. Importantly, the generalized method can be used for rapid, high-throughput, nondestructive screening of biomaterials intended to enhance osteogenic differentiation.

Betacellulin inhibits osteogenic differentiation and stimulates proliferation through HIF-1alpha

Cellular signaling via epidermal growth factor (EGF) and EGF-like ligands can determine cell fate and behavior. Osteoblasts, which are responsible for forming and mineralizing osteoid, express EGF receptors and alter rates of proliferation and differentiation in response to EGF receptor activation. Transgenic mice over-expressing the EGF-like ligand betacellulin (BTC) exhibit increased cortical bone deposition; however, because the transgene is ubiquitously expressed in these mice, the identity of cells affected by BTC and responsible for increased cortical bone thickness remains unknown. We have therefore examined the influence of BTC upon mesenchymal stem cell (MSC) and pre-osteoblast differentiation and proliferation. BTC decreases the expression of osteogenic markers in both MSCs and pre-osteoblasts; interestingly, increases in proliferation require hypoxia-inducible factor-alpha (HIF-alpha), as an HIF antagonist prevents BTC-driven proliferation. Both MSCs and pre-osteoblasts express EGF receptors ErbB1, ErbB2, and ErbB3, with no change in expression under osteogenic differentiation. These are the first data that demonstrate an influence of BTC upon MSCs and the first to implicate HIF-alpha in BTC-mediated proliferation.

Betacellulin inhibits osteogenic differentiation and stimulates proliferation through HIF-1alpha

Cellular signaling via epidermal growth factor (EGF) and EGF-like ligands can determine cell fate and behavior. Osteoblasts, which are responsible for forming and mineralizing osteoid, express EGF receptors and alter rates of proliferation and differentiation in response to EGF receptor activation. Transgenic mice over-expressing the EGF-like ligand betacellulin (BTC) exhibit increased cortical bone deposition; however, because the transgene is ubiquitously expressed in these mice, the identity of cells affected by BTC and responsible for increased cortical bone thickness remains unknown. We have therefore examined the influence of BTC upon mesenchymal stem cell (MSC) and pre-osteoblast differentiation and proliferation. BTC decreases the expression of osteogenic markers in both MSCs and pre-osteoblasts; interestingly, increases in proliferation require hypoxia-inducible factor-alpha (HIF-α), as an HIF antagonist prevents BTC-driven proliferation. Both MSCs and pre-osteoblasts express EGF receptors ErbB1, ErbB2, and ErbB3, with no change in expression under osteogenic differentiation. These are the first data that demonstrate an influence of BTC upon MSCs and the first to implicate HIF-α in BTC-mediated proliferation.

Osteoadherin is upregulated by mature osteoblasts and enhances their in vitro differentiation and mineralization

During the process of differentiation, osteoblasts commit through strictly controlled checkpoints under the influence of several growth factors, cytokines, and extracellular matrix (ECM) proteins. The mineralized tissue-specific ECM component osteoadherin (OSAD) belongs to the small leucine-rich repeat protein family of proteoglycans. Proteoglycans modulate cellular behavior either through the attached glycosaminoglycan chains or by direct protein-protein interactions via the core protein sequences. Leucine-rich repeats have been shown to directly interact with cell-surface receptors such as epidermal growth factor receptor, blocking its ability to bind its ligand. In the present study, we investigated the influence of OSAD on the behavior and maturation of MC3T3E1 osteoblasts. OSAD overexpression and repression clones were created by stably transfecting with plasmids coding for either mouse OSAD cDNA or small-hairpin RNA, targeted against mouse OSAD. Overexpression of OSAD resulted in an increase of osteoblast differentiation features, such as increased alkaline phosphatase (ALP) activity and increased in vitro mineralization, as well as reduced proliferation and migration. Bone sialoprotein (BSP) levels were unchanged, while upregulation of osteocalcin (OC) and osteoglycin (OGN) was observed. Conversely, repression of OSAD expression resulted in increased cell proliferation and migration. BSP and OC were unaffected, while OGN was downregulated. ALP activity was reduced, though no change in in vitro mineralization was observed. We conclude that OSAD overexpression enhanced the differentiation and maturation of osteoblasts in vitro.

Epidermal Growth Factor as a Candidate for Ex Vivo Expansion of Bone Marrow-Derived Mesenchymal Stem Cells

Bone marrow mesenchymal stem cells (BMMSCs) are pluripotent cells capable of differentiating into several cell types and are thus an attractive cell source for connective tissue engineering. A challenge in such a use is expansion and directed seeding in vitro, requiring proliferation and survival, and directed migration, respectively, prior to functional differentiation. The epidermal growth factor (EGF) receptor (EGFR) is the prototypal growth factor receptor and elicits these responses from a wide variety of stromal, epithelial, and endothelial cells. Ligands for this receptor are appealing for use in tissue engineering because they are relatively resistant to biological extremes and amenable to high-volume production. Therefore, we determined whether an EGFR ligand, EGF, could be used for ex vivo expansion of BMMSCs. EGF stimulated motility in rat and immortalized human BMMSCs. EGF-induced proliferation was observed in immortalized human BMMSCs but was not apparent in rat BMMSCs under our experimental conditions. EGF did not, however, rescue either type of BMMSC from apoptosis due to lack of serum. During our examination of key signaling intermediaries, EGF caused robust phosphorylation of extracellular signal-regulated protein kinase (ERK) and protein kinase B/akt (AKT) but only minimal phosphorylation of EGFR and phospholipase C-gamma in rat BMMSCs, whereas in the human BMMSCs these intermediaries were all strongly activated. EGF also induced robust ERK activation in primary porcine mesenchymal stem cells. EGF pretreatment or cotreatment did not interfere with secondarily induced differentiation of either type of BMMSC into adipogenic or osteogenic lineages. Platelet-derived growth factor (PDGF) effects were similar to but not additive with those elicited by EGF, with some quantitative differences; however, PDGF did interfere with the differentiation of these BMMSCs. These findings suggest that EGFR ligands could be used for ex vivo expansion and direction of BMMSCs.

Amphiregulin Is a Novel Growth Factor Involved in Normal Bone Development and in the Cellular Response to Parathyroid Hormone Stimulation

Parathyroid hormone (PTH) is the major mediator of calcium homeostasis and bone remodeling and is now known to be an effective drug for osteoporosis treatment. Yet the mechanisms responsible for its functions in bone are largely unknown. Here we report that the expression of amphiregulin (AR), a member of the epidermal growth factor (EGF) family, is rapidly and highly up-regulated by PTH in several osteoblastic cell lines and bone tissues. Other osteotropic hormones (1alpha,25-dihydroxyvitamin D3 and prostaglandin E2) also strongly stimulate AR expression. We found all EGF-like ligands and their receptors are expressed in osteoblasts, but AR is the only member that is highly regulated by PTH. Functional studies demonstrated that although AR is a potent growth factor for preosteoblasts, it completely inhibits further differentiation. AR also strongly and quickly stimulated Akt and ERK phosphorylation and c-fos and c-jun expression in an EGF receptor-dependent manner. Moreover, AR null mice displayed significantly less tibial trabecular bone than wild-type mice. Taken together, we have identified a novel growth factor that is PTH-regulated and appears to have an important role in bone metabolism.

Modulation of growth factor/cytokine synthesis and signaling by 1alpha,25-dihydroxyvitamin D(3): implications in cell growth and differentiation

Distinct from its classic functions in the regulation of calcium and phosphorus metabolism as a systemic hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is involved in the local control and regulation of cellular growth and differentiation in various tissues, including epidermis (keratinocytes) and bone (osteoblasts and osteoclasts). In this review, the impact of 1alpha,25(OH)(2)D(3) on growth factor/cytokine synthesis and signaling is discussed, particularly as it pertains to bone cells and keratinocytes. 1alpha,25(OH)(2)D(3) not only regulates growth factor/cytokine synthesis but may also alter growth factor signaling. Recently discovered examples for such interactions are the interactions between the vitamin D receptor and the mothers against decapentaplegic-related proteins that function downstream of TGFbeta receptors. Inhibitory effects of 1alpha,25(OH)(2)D(3) on keratinocytes through TGFbeta activation and IL-1alpha, IL-6, and IL-8 suppression may provide a rationale for its beneficial effects in the treatment of hyperproliferative skin disorders, whereas stimulatory effects through the epidermal growth factor-related family members and platelet-derived growth factor may be operative in its beneficial effects in skin atrophy and wound healing. Modulation of cytokines and growth factors by 1alpha,25(OH)(2)D(3) during bone remodeling plays an important role in the coupling of osteoblastic bone formation with osteoclastic resorption to maintain bone mass.

Mechanisms of the regulation of EGF receptor gene expression by calcitriol and parathyroid hormone in UMR 106-01 cells

BACKGROUND

We have previously demonstrated that parathyroid hormone (PTH) and calcitriol increase the expression of epidermal growth factor receptors (EGFR) in UMR 106-01 osteoblast-like cells. The effect of PTH is mediated by cAMP and it involves an increase in the level of EGFR mRNA. The present studies were designed to investigate the mechanisms involved in the regulation of EGFR expression by PTH and calcitriol.

METHODS

To examine the mechanism of the effect of calcitriol on EGFR expression, confluent cultures of UMR 106-01 cells were exposed to calcitriol and levels of EGFR mRNA were determined by reverse transcription-polymerase chain reaction (RT-PCR). In order to study the effect of calcitriol on EGFR gene transcription, a candidate vitamin D-responsive element (VDRE) was identified in the EGFR gene promoter and complimentary 30-mer oligonucleotides spanning this region were tested for binding to recombinant VDR using EMSA. Transcriptional activity in response to calcitriol and PTH was tested in UMR 106-01 cells stably transfected with a luciferase reporter construct containing the full length EGFR gene promoter. The effect of calcitriol on EGFR mRNA stability was examined in transcriptionally arrested cells.

RESULTS

Treatment with calcitriol resulted in a time and dose dependent increase in EGFR mRNA levels in confluent cultures of UMR 106-01 osteoblast-like cells. Using EMSA, we demonstrated that the putative human EGFR VDRE binds to recombinant VDR in a retinoid X receptor (RXR)-dependent manner; however, calcitriol failed to increase transcriptional activity from a luciferase reporter construct containing the full-length EGFR gene promoter in stably transfected UMR 106-01 cells. Therefore, EGFR mRNA degradation was examined in transcriptionally arrested cells and calcitriol was found to prolong the half life of EGFR mRNA. Treatment of the cultures with PTH resulted in a ninefold increase in luciferase activity after four hours of exposure, a finding that was reproduced by treatment with forskolin.

CONCLUSIONS

These studies demonstrate that the calciotropic hormones PTH and calcitriol increase EGF receptor expression by different mechanisms. The former increases EGFR gene transcription whereas the latter increases EGFR mRNA stability.

Patterns of integrin expression in a human mandibular explant model of osteoblast differentiation

Cell-matrix interaction is crucial in regulating osteoblast differentiation and function. These interactions are themselves regulated, at least in part, by integrins. Although there are some data from mammalian models, few studies have compared integrin expression at different stages of the osteoblast lineage. Here, primary human mandibular osteoblast cultures were grown in the presence of epidermal growth factor (EGF), giving a proliferative, less differentiated phenotype, or of vitamin D(3) and hydrocortisone (D+Hc), giving a more differentiated phenotype. These cultures were compared with those of cells prepared in the absence of EGF or D+Hc by fluorescence-activated cell sorter using a panel of monoclonal antibodies to specific integrin heterodimers. To provide in vivo correlation, the same panel of antibodies was used to stain fresh-frozen, undemineralised sections of human mandibular bone. Under baseline conditions the alpha(3), alpha(5), alpha(v), alpha(v)beta(3), beta(3) and beta(1) integrin subunits were expressed strongly by the cells, with low-level expression of the alpha(1), alpha(2) and alpha(4) subunits. In the presence of EGF there was increased alpha(2) expression. With D+Hc, alpha(3) and alpha(5) expression was elevated. Immunohistochemical analysis demonstrated alpha(2), alpha(3), alpha(5), alpha(v)beta(3), beta(1) and beta(3) subunits in cells of the osteoblast lineage; alpha(2) staining was restricted to cells close to the bone surface whilst alpha(v)beta(3) and beta(3) were most frequently localised in the osteocytes. The results provide evidence that cells at successive stages of the osteoblast lineage show different patterns of integrin expression. These integrins may be important in cell-matrix interactions leading to osteoblast differentiation.

Pulse application of platelet-derived growth factor enhances formation of a mineralizing matrix while continuous application is inhibitory

Platelet-derived growth factor (PDGF) stimulates chemotaxis and proliferation of osteoblasts, and induces bone formation in vivo. To determine how PDGF might regulate these cells, the effect of PDGF on long-term mineralizing cultures of fetal rat osteoblastic cells was examined. Although PDGF increased cell proliferation in these cultures, continuous treatment with PDGF caused a dose-dependent decrease in mineralized nodule formation. When cells were treated with multiple, brief (1 day) exposures to PDGF at the osteoblast differentiation stage, there was a significant 50% increase in mineralized nodule area. Based on modulation of alkaline phosphatase activity it appears that longer-term exposure to PDGF reduces mineralized nodule formation largely by inhibiting differentiated osteoblast function, while short-term exposure enhances proliferation without inhibiting the differentiated phenotype. Thus, the ultimate affect of PDGF on bone formation is likely to reflect two processes: a positive effect through enhancing cell number or a negative effect by inhibiting differentiated function. The inhibitory effect of PDGF on formation of a mineralized matrix is unlikely to be simply a result of enhanced proliferation of "fibroblastic" cells since cultures treated with PDGF for 3 days and then transferred to new plastic dishes exhibited a 70% increase in mineralized nodule area compared to controls. These results would predict that multiple, brief exposures to PDGF would enhance bone formation in vivo, while prolonged exposure to PDGF, which is likely to occur in chronic inflammation, would inhibit differentiated osteoblast function and limit bone regeneration.

Low-energy laser irradiation stimulates bone nodule formation at early stages of cell culture in rat calvarial cells

Although the acceleration of bone regeneration by laser treatment has been reported, the mechanisms of action of laser on bone are unclear. To determine the target cells responsible for the action of laser irradiation and roles of irradiation on these cells during bone formation, we investigated the effects of low-energy laser irradiation at various cell culture stages on cellular proliferation, bone nodule formation, alkaline phosphatase activity, and osteocalcin gene expression, employing rat calvarial cells. Osteoblast-like cells isolated from fetal rat calvariae were irradiated once with a low-energy Ga-Al-As laser (830 nm, 500 mW) at various cell culture stages (days 1-16). Laser irradiation at early stages of culture significantly stimulated cellular proliferation, ALP activity, and osteocalcin gene expression thereafter. Furthermore, laser irradiation at earlier stages of culture significantly stimulated a greater number (1.7-fold) and larger area (3.4-fold) of bone nodules that had developed in the culture dish on day 21. However, these effects could not be found by irradiation at a later date. These results suggest that laser irradiation may play two principal roles in stimulating bone formation. One is stimulation of cellular proliferation, especially proliferation of nodule-forming cells of osteoblast lineage, and the other is stimulation of cellular differentiation, especially to committed precursors, resulting in an increase in the number of more differentiated osteoblastic cells and an increase in bone formation. Both bone-formation-stimulating roles may be exhibited by laser irradiation to immature cells only.

Retinoids modulate the effect of PTH and calcitriol on EGF receptor expression in UMR 106-01 cells

Although osteoblast proliferation is a prominent feature of osteitis fibrosa, studies in vitro using osteoblast-like cells have shown that parathyroid hormone (PTH) impairs cell growth. Recent studies in our laboratory have shown that PTH increases epidermal growth factor (EGF) receptor expression in UMR 106-01 osteoblast-like cells, and thus, osteoblast proliferation may occur as a result of an enhanced response of the osteoblast to EGF. In the present studies we investigated the effect of calcitriol and the influence of retinoids on the regulation of EGF receptors. Calcitriol increased 125I-EGF binding 2.5-3-fold after 72 hours of incubation and was maximal at a calcitriol dose of 100 nM. Scatchard analysis showed that this effect was due to increased receptor number. In contrast, all-trans retinoic acid or 9-cis retinoic acid alone, even at 10 microM, caused less than a 50% increase in 125I-EGF binding. However, the effect of calcitriol was totally abolished in the presence of all-trans retinoic acid. 9-cis retinoic acid was equivalent with all-trans retinoic acid in this regard. In the presence of either retinoid, the stimulatory effect of PTH was totally eliminated and EGF binding was actually decreased below control values. Additional studies revealed that retinoic acid decreased PTH-stimulated cAMP generation in a dose-dependent manner. These data are consistent with our previous studies which showed that the effect of PTH on the induction of EGF receptors was mediated by a cAMP-dependent mechanism. The inhibition of the calcitriol effect by retinoids is consistent with the requirement of the retinoid-X-receptor (RXR) for binding of the vitamin D receptor (VDR) to its target sequences in DNA. These data indicate that EGF receptors in UMR 106-01 cells are up-regulated by PTH and calcitriol and that this process can be modulated by retinoids. Retinoids, therefore, may play a major role in the regulation of osteoblast function by PTH and calcitriol.

Functional characterization of prostaglandin E2 inducible osteogenic colony forming units in cultures of cells isolated from the neonatal rat calvarium

Prostaglandin E2 (PGE2) increases the number of mineralized nodules that form in cultures of rat calvarial (RC) cells. The purpose of our study was to characterize PGE2-inducible osteogenic colony forming units (CFU-Os) by determining their number, the cell populations from which they were released, their specific responsive period to PGE2, and their proliferating and differentiating characteristics under the stimulation of PGE2. Limiting dilution analysis was used to determine the number of PGE2-inducible CFU-Os. Sequential digestion of intact rat parietal bones with collagenase isolated 5 subpopulations of RC cells that were used to estimate the cell populations where PGE2-inducible CFU-Os resided. The responsive period of PGE2-inducible CFU-Os to PGE2 was evaluated by treating cultures of mixed RC cells for all possible combinations of days 1-10, 11-20, and 21-30. PGE2 effects on proliferation and differentiation of CFU-Os were evaluated by comparing the DNA synthesis and AP activity in subpopulations I and IV on days 3, 6, and 9. Results showed: (1) PGE2-inducible CFU-Os represent 0.27% of cells in the mixed RC population, (2) the majority of determined and PGE2-inducible CFU-Os were found in the subpopulations released during the 60-100 min digestion periods, (3) the response of PGE2-inducible CFU-Os is limited to the first 10 days of culture, and (4) PGE2-stimulated nodule formation is associated with an early increase in DNA synthesis and a sustained increase in alkaline phosphatase activity. We conclude that, functionally, PGE2-inducible CFU-Os are slowly proliferating AP negative cells primarily found in the subpopulations III-V. PGE2 stimulates them to proliferate and become AP+, and function as determined CFU-Os to form mineralized nodules in vitro.

Epidermal growth factor induces Egr-1 messenger RNA and protein in mouse osteoblastic cells

The nuclear signaling events activated when epidermal growth factor (EGF) interacts with osteoblasts to produce effects on growth and differentiation are not clearly understood, and may include induction of immediate early genes such as Egr-1, a zinc finger transcription factor. In the present study, Northern analyses were performed to define the effects of EGF on the expression of Egr-1 mRNA in MC3T3-E1 mouse osteoblastic cells. Following treatment of quiescent, subconfluent MC3T3-E1 cells with 0.1-100 ng/ml EGF for various periods, maximal induction of Egr-1 mRNA occurred when cells were treated for 30-60 minutes with 1-10 ng/ml EGF. Inhibition of protein kinase C activity by pretreatment with 1 microM chelerythrine chloride or by prolonged stimulation with 50 ng/ml tetradecanoyl phorbol acetate (TPA) partially diminished the induction of Egr-1 by EGF. Using an immunohistochemical approach, 10 ng/ml EGF was observed to induce Egr-1 protein within 30-60 minutes and this induction was localized to the nucleus. These observations indicate that EGF induces Egr-1 mRNA and protein via protein kinase C and other signaling pathways, and that Egr-1 may be part of the regulatory network mediating the actions of EGF on growth and differentiation of osteoblasts.

Platelet-derived growth factor-modulated guided tissue regenerative therapy

The goal of this study was to develop an effective regenerative therapy capable of achieving periodontal regeneration of Class III furcation defects. We attempted to achieve this goal by combining three therapeutic approaches. First, the lesion was protected by an expanded polytetrafluoroethylene barrier membrane that prevents migration of gingival fibroblasts as well as osteogenic cells from the mucoperiosteal flaps. Second, platelet-derived growth factor-BB (PDGF-BB), which has potent chemotactic and mitogenic effects on periodontal ligament fibroblasts (PDL), was used to promote migration of fibroblasts and their proliferation on the root surface. Third, the root surface, demineralized by citric acid conditioning, was chosen as the primary site for PDGF-BB application. The demineralized root surface appeared to have the capability of providing a sustained release of the applied growth factor. This seemed to facilitate rapid repopulation of PDL fibroblasts on the root surface and new PDL formation in the early stages of repair, which contributed to complete periodontal regeneration without root resorption and ankylosis in later stages. Combining these approaches, we developed a therapy referred to as "PDGF-modulated guided tissue regenerative therapy." Unlike guided tissue regenerative therapy alone (without PDGF-BB), this therapy effectively promoted periodontal regeneration of Class III furcation defects in the beagle dog without significant ankylosis or root resorption.

Effects of genistein, tyrphostin, and pertussis toxin on EGF-induced mitogenesis in primary culture and clonal osteoblastic cells

Epidermal growth factor (EGF) has been found to stimulate proliferation in a variety of cell types. The EGF receptor is known to have tyrosine kinase activity [1], however, the role of this signal mechanism has not been established in bone cells. The aim of this study was to determine whether tyrosine kinase activity and G inhibitory (Gi) proteins are involved in EGF-stimulated proliferation in the osteoblastic cell line G292 and in primary culture osteoblasts isolated from neonatal rat calvaria. Cell proliferation was measured by 3H-thymidine incorporation using liquid scintillation spectrometry. EGF stimulates a dose-dependent increase in proliferation of G292 and primary culture cells above control. Genistein was able to inhibit the effects of EGF in the G292 cells. In the primary culture cells, genistein with EGF appeared to enhance proliferation compared with EGF alone or genistein alone. Tyrphostin 25, on the other hand, inhibited the EGF response in both of these cell types. Inactivation of Gi proteins with pertussis toxin was able to inhibit EGF-induced mitogenesis in the neonatal rat osteoblasts but did not appear to specifically inhibit this response in the G292 cells. These results suggest that although both of these osteoblastic cell types increase proliferation in response to EGF, their signal pathways are different.

Effects of sialoadenectomy and exogenous EGF on molar drift and orthodontic tooth movement in rats

Effects on bone remodeling have been attributed to epidermal growth factor (EGF). Sialoadenectomy (SX) removes the major source of EGF in rodents and decreases both salivary and serum EGF levels. EGF effects on rat alveolar bone remodeling manifested by molar drift (MD) and orthodontic tooth movement (OTM) were examined using the following two approaches: 1) EGF depletion by SX and replacement by orally administered EGF (50 micrograms.animal-1.day-1); 2) sham rats supplemented with matching amounts of EGF. MD and OTM were measured using cephalometric radiographs; bone formation was measured histomorphometrically using tetracycline labeling. Normal MD was not detected after SX, and alveolar bone formation was significantly reduced both around the tooth and in nondental sites. Replacement EGF given to SX rats and supplemental EGF administered to sham rats changed the direction and enhanced the rate of MD. A mesially directed orthodontic force applied to the molars of SX animals increased bone formation on the distal aspect of the tooth roots. Supplemental EGF did not significantly affect OTM. EGF affects alveolar bone remodeling, as manifested clinically by alterations in normal maxillary MD.

Evidence for up-regulation of epidermal growth-factor receptors on rat periodontal ligament fibroblastic cells associated with stabilization of phenotype in vitro

This study sought to understand the role of epidermal growth factor receptor (EGF-R) in periodontal ligament (PDL) fibroblasts. Rat PDL fibroblastic cells and ROS 17/2.8 cells (highly differentiated osteoblastic osteosarcoma cells) were cultured and treated with transforming growth factor-alpha (TGF-alpha), EGF, dexamethasone (Dex) or a combination of EGF and Dex. Alkaline phosphatase (ALP) activity, an early differentiation marker for mineralized tissue-forming cells, was measured using p-nitrophenylphosphate as a substrate. For Scatchard analysis of [125I]-EGF binding, cells were incubated in Dulbecco's modified Eagle's medium containing 0.2% bovine serum albumin and 0-64 ng/ml of [125I]-EGF for 4 h at 4 degrees C. Also, the synthesis of EGF-R protein and the expression of mRNA for EGF-R were measured by immunoprecipitation and Northern blot analysis, respectively. Untreated PDL fibroblastic cells showed a gradual increase in spontaneous ALP activity from 32.4 U/10(6) cells at 2 days to 49.6 U/10(6) cells at 7 days of culture. ALP activity was further increased to 70.8 U/10(6) cells at 7 days after treatment with Dex, whereas EGF treatment reduced it to 19.4 U/10(6) cells. Culture of PDL fibroblastic cells in the presence of a combination of Dex and EGF decreased the Dex-induced ALP activity from 70.8 U to 41.8 U/10(6) cells at 7 days. A similar inhibitory effect on ALP activity was found after treatment with TGF-alpha. In contrast, ROS cells maintained a high ALP activity (1748 U/10(6) cells) throughout culture, unaffected by EGF. Scatchard analysis demonstrated that PDL fibroblastic cells have both high- and low-affinity forms of EGF-R, while ROS cells did not have any detectable EGF-R. Treatment of PDL cells with Dex for 2 days decreased the synthesis of EGF-R protein, the expression of EGF-R mRNA and the number of EGF-R. In contrast, EGF treatment increased the expression of EGF-R mRNA. These data suggest that PDL fibroblastic cells express numerous EGF-R, but the number decreases during their differentiation into mineralized tissue-forming cells under the influence of Dex. Thus, EGF-R may function in the stabilization of phenotype in PDL fibroblastic cells.

Formation of mineralized nodules by bone derived cells in vitro: a model of bone formation?

The identification of the factors which regulate the proliferation and differentiation of cells of the osteoblast lineage remains one of the major challenges in the field of bone cell biology. Although considerable progress has been made in the isolation and culture of cells of the osteoblast lineage from both animal and, more recently, human bone, uncertainties have persisted as to the extent to which these cell populations retain the ability to differentiate into functional osteoblasts in vitro. The formation in vitro of mineralized nodules that exhibit the morphological, ultrastructural and biochemical characteristics of embryonic/woven bone formed in vivo, represents the first evidence that the differentiation of functional osteoblasts can occur in cultures of isolated animal bone-derived cell populations. It is clear, however, that the culture conditions employed at present only permit a small number of cells to differentiate to the extent of being capable of organising their extracellular matrix into a structure that resembles that of bone. Moreover, it has generally been found that the reproducible mineralization of this extracellular matrix requires supplementation of the culture medium with mM concentrations of beta-GP, which raises doubts as to the physiological relevance of this process. The formation of nodules has also been observed in cultures of human bone-derived cells. As found in cultures of animal bone-derived cells, reproducible mineralization of these nodules will occur in the presence of beta-GP. We have shown, however, that in the presence of the long acting ascorbate analogue Asc-2-P, the formation and mineralization of nodules can occur in the absence of beta-GP. The nodules formed in human bone-derived cell cultures have yet to be characterized as rigorously as those formed in cultures of animal bone-derived cells and thus it remains to be shown that they resemble bone formed in vivo.

Modulation of plasminogen activators and plasminogen activator inhibitors by TGF-beta, IL-1 alpha and EGF in fetal rat calvaria cells at different times of culture

Fetal rat calvaria cells (RC cells) grown in long term culture in the presence of ascorbic acid and organic phosphate proliferate and differentiate to form mineralized nodules of bone. Since transforming growth factor beta (TGF-beta), interleukin 1-alpha (IL-1 alpha) and epidermal growth factor (EGF) affect both bone resorption and bone formation, we have studied the ability of these growth factors to affect plasminogen activators and plasminogen activator inhibitors release by RC cells at different times throughout this proliferation/differentiation sequence. Cultures in log phase growth (day 4), when first multilayering (day 7) and when bone nodules were forming (day 13) were exposed to either TGF-beta, IL-1 alpha, EGF or vehicle. Conditioned medium was collected after 6 and 24 h and plasminogen activators and plasminogen activator inhibitors were analysed by fibrin autography and reverse fibrin autography. TGF-beta-mediated changes in plasminogen activator were apparent at day 4. By day 7 two molecular weight species of plasminogen activator were noted; a 65 kDa species, prominent at 24 h exposure was blocked by anti-tPA antibody, and a 38 kDa plasminogen activator, prominent after 6 h of stimulation was not blocked by anti-tPA antibody. Plasminogen activator-plasminogen activator inhibitor complexes are also increased. IL-1 alpha caused similar increases in plasminogen activator and plasminogen activator inhibitor with maximal activity measured at day 13, coincident with the time when bone nodules were forming. EGF-mediated changes were less by comparison. TGF-beta significantly decreased bone nodule formation after both a 6 and 24 h serum-free exposure, whereas IL-1 alpha and EGF decreased nodule number only after the 24 h exposure. The data suggest that the three factors influence the expression of plasminogen activator and plasminogen activator inhibitor by RC cells and their effect is different at different times of culture.

Clonal distribution of osteoprogenitor cells in cultured chick periostea: functional relationship to bone formation

Folded explants of periosteum from embryonic chick calvaria form bone-like tissue when grown in the presence of ascorbic acid, organic phosphate, and dexamethasone. All osteoblast-like cells in these cultures arise de novo by differentiation of osteoprogenitor cells present in the periosteum. To study the spatial and functional relationships between bone formation and osteoprogenitor cells, cultures were continuously labeled with [3H]thymidine for periods of 1-5 days. Radioautographs of serial 2-microns plastic sections stained for alkaline phosphatase (AP) showed maximal labeling of 30% of fibroblastic (AP-negative) cells by 3 days while osteogenic cells (AP-positive) exhibited over 95% labeling by 5 days. No differential shifts in labeling indices, grain count histograms of fibroblastic and osteogenic cells or numbers of AP-positive cells were observed, indicating no significant recruitment of cells from the fibroblastic to the osteogenic compartment. Despite the continuous presence of [3H]thymidine, less than 35% of both osteoblasts and osteocytes were labeled at 5 days, indicating that only one-third of the osteoprogenitor cells had cycled prior to differentiation. Spatial clustering of [3H]thymidine-labeled cells was measured by computer-assisted morphometry and application of the Poisson distribution to assess contagion. Cluster size and number of labeled cells per cluster did not vary between 1-3 days, but the number of clusters increased 20-fold between Day 1 and Day 3. Clusters were predominantly AP-positive and located close to bone. Three-dimensional reconstruction from serial sections showed that clusters formed long, tubular arrays of osteogenic cells up to eight cells in length and located within 2-3 cell layers from the bone surface. Selective killing of S-phase cells with two pulse labels of high specific activity [3H]thymidine at 1 and 2 days of culture completely blocked bone formation. These data indicate that a very small population of cycling osteoprogenitor cells is essential for bone formation in vitro and give rise to relatively small numbers of clonally distributed progenitors with limited proliferative capacity. The progeny of these clusters undergo restricted migration and differentiate into osteoblasts.

Factors that promote progressive development of the osteoblast phenotype in cultured fetal rat calvaria cells

Rat calvaria osteoblasts derived from 21-day-old fetal rat pups undergo a temporal expression of markers of the osteoblast phenotype during a 5 week culture period. Alkaline phosphatase and osteocalcin are sequentially expressed in relation to collagen accumulation and mineralization. This pattern of expression of these osteoblast parameters in cultured rat osteoblasts (ROB) is analogous to that seen in vivo in developing fetal rat calvaria tissue (Yoon et. al: Biochem. Biophis. Res. Commun. 148:1129, 1987) and is similar to that observed in cultures of subcultivated 16-day-old embryonic chick calvaria-derived osteoblasts (COB) (Gerstenfeld, et.al: Dev. Biol. 122:46, 1987). While the cellular organization of subcultivated COB and primary ROB cultures are somewhat different, the temporal expression of the parameters remains. Both the rat and chick culture systems support formation of matrix mineralization even in the absence of beta-glycerol-phosphate. A systematic examination of factors which constitute conditions supporting complete expression of the osteoblast phenotype in ROB cultures indicate requirements for specific serum lots, ascorbic acid and the ordered deposition of mineral in the extracellular matrix. The present studies suggest that formation of a collagenous matrix, dependent on ascorbic acid, is requisite for expression of the osteoblast phenotype. In ROB cultures, expression of osteocalcin synthesis occurs subsequent to initiation of alkaline phosphatase activity and accompanies the formation of mineralized nodules. Thus, extracellular matrix mineralization (deposition of hydroxyapatite) is required for complete development of the osteoblast phenotype, as reflected by a 200-fold increase in osteocalcin synthesis. These data show the temporal expression of the various osteoblast parameters during the formation and mineralization of an extracellular matrix can provide markers reflective of various stages of osteoblast differentiation/maturation in vitro.

The effects of fluoride on osteoblast progenitors in vitro

The number of discrete, three-dimensional bone nodules formed in vitro from a class of osteoprogenitor cells present in fetal rat calvaria cell populations (RC cells) is linearly related to the number of cells plated, implying that this system functions as a colony assay for the expression of osteoprogenitor cells. To determine the effect of fluoride on the expression of these cells, primary RC cells were grown for periods of up to 21 days in alpha-MEM (minimal essential medium) containing 5-15% heat-inactivated fetal bovine serum (FBS), 50 micrograms/ml ascorbic acid, 10 mM Na beta-glycerophosphate, and NaF at concentrations from 10 microM to 5 mM. The continuous presence of NaF resulted in an increase in the number of bone nodules with maximal response occurring at 500 microM (p less than 0.001). A similar response at 500 microM NaF was observed also with regard to alkaline phosphatase activity. NaF levels up to 500 microM did not affect the growth of the mixed RC cell population, however, higher concentrations (1 mM) significantly reduced cell numbers (p less than 0.001) suggestive of cytotoxicity. Plating efficiency tests for colony formation in the presence of 0.5 to 2 mM NaF showed that the decreases in nodule formation observed at concentrations above 500 microM correlated with cytotoxicity. NaC1 at 1 mM had no effect on nodule formation, alkaline phosphatase activity, or cell growth. The results show that NaF stimulates osteoprogenitor cell number in vitro and that the maximal effect occurs at concentrations close to toxic levels.

Effects of epidermal growth factor on bone formation and resorption in vivo

The effects of mouse epidermal growth factor (EGF) on bone formation and resorption were examined in male mice. EGF administration (2-200 ng.g-1.day-1 ip for 7 days) induced a dose-dependent rise in plasma EGF levels that remained within physiological range. Histomorphometric analysis of caudal vertebrae showed that EGF (20 and 200 ng.g-1.day-1) reduced the endosteal matrix and mineral appositional rates after 5 days of treatment as measured by double [3H]proline labeling and double tetracycline labeling, respectively. This effect was transitory and was not observed after 7 days of EGF administration. EGF administered for 7 days induced a dose-dependent increase in the periosteal osteoblastic and tetracycline double-labeled surfaces. At high dosage (200 ng.g-1.day-1) EGF administration increased the osteoclastic surface and the number of acid phosphatase-stained osteoclasts, although plasma calcium remained normal. The results show that EGF administration at physiological doses induces distinct effects on endosteal and periosteal bone formation and that the effects are dependent on EGF dosage and duration of treatment. This study indicates that EGF at physiological dosage stimulates periosteal bone formation and increases endosteal bone resorption in the growing mouse.

Effects of acidic fibroblast growth factor and epidermal growth factor on subconfluent fetal rat calvaria cell cultures: DNA synthesis and alkaline phosphatase activity

The effects of acidic fibroblast growth factor (aFGF) and epidermal growth factor (EGF) were examined in subconfluent fetal rat calvaria cell cultures, in the presence of 2% serum. Maximal effect of aFGF and EGF on DNA synthesis measured by [3H]thymidine incorporation was observed after 18 h. aFGF stimulated DNA synthesis by 3.5-fold with an ED50 of 0.75 ng/ml while a 2.3-fold EGF stimulation was recorded with an ED50 of 0.067 ng/ml. 5-Bromo-2-deoxyuridine staining showed a higher stimulation of proliferation in the scattered cells than in the cell clusters. An 18 h aFGF or EGF treatment decreased alkaline phosphatase (ALP) activity by 40 and 23%, respectively, as compared with control cultures. This inhibition was more pronounced after 48 h in the presence of the effectors but no modification of the ALP electrophoretic mobility was observed. These data suggest that aFGF is a less potent mitogen than EGF and a higher inhibitor of ALP activity in fetal rat calvaria cell culture.

Basic fibroblast growth factor enhances the capacity of bone marrow cells to form bone-like nodules in vitro

The role of basic fibroblast growth factor (bFGF) in the proliferation and differentiation of rat bone marrow cells in culture was studied. bFGF stimulated [3H]thymidine incorporation into these cells by 4-fold at a concentration of 0.3 ng/ml and half-maximal effect was observed at a concentration of 15 pg/ml. In addition to its mitogenic effect, bFGF stimulated alkaline phosphatase activity by 3.6-fold. Continuous treatment with bFGF (for 21 days) resulted in a 6.3-fold increase in the culture dish surface area covered by bone-like mineralized tissue. Maximal bone-like tissue formation was observed in the presence of 3 ng/ml bFGF with half-maximal effect at a concentration of 0.3 ng/ml. These results indicate the possible role of bFGF in the proliferation of osteogenic rat bone marrow cells and their differentiation into cells of osteoblast-like phenotype.

Differentiation of muscle, fat, cartilage, and bone from progenitor cells present in a bone-derived clonal cell population: effect of dexamethasone

RCJ 3.1, a clonally derived cell population isolated from 21-d fetal rat calvaria, expresses the osteoblast-associated characteristics of polygonal morphology, a cAMP response to parathyroid hormone, synthesis of predominantly type I collagen, and the presence of 1,25-dihydroxyvitamin D3-regulated alkaline phosphatase activity. When cultured in the presence of ascorbic acid, sodium beta-glycerophosphate, and the synthetic glucocorticoid dexamethasone, this clone differentiated in a time-dependent manner into four morphologically distinct phenotypes of known mesenchymal origin. Multinucleated muscle cells were observed as early as 9-10 d in culture, lipid-containing adipocytes formed after 12 d, chondrocyte nodules were observed after 16 d, and mineralized bone nodules formed after 21 d in culture. The differentiated cell types were characterized morphologically, histochemically, and immunohistochemically. The formation of adipocytes and chondrocytes was dependent upon the addition of dexamethasone; the muscle and bone phenotypes were also expressed at low frequency in the absence of dexamethasone. The sex steroid hormones progesterone and 17 beta-estradiol had no effect on differentiation in this system, suggesting that the effects of dexamethasone represent effects specific for glucocorticosteroids. Increasing concentrations of dexamethasone (10(-9)-10(-6) M) increased the numbers of myotubes, adipocytes, and chondrocytes; however, when present continuously for 35 d, the lower concentrations appeared to better maintain the muscle and adipocyte phenotypes. Bone nodules were not quantitated because the frequency of bone nodule formation was too low. Single cells obtained by plating RCJ 3.1 cells at limiting dilutions in the presence of dexamethasone, were shown to give rise to subclones that could differentiate into either single or multiple phenotypes. Thus, the data suggest that this clonal cell line contains subpopulations of mesenchymal progenitor cells which can, under the influence of glucocorticoid hormones, differentiate in vitro into four distinct cell types. It is, therefore, a unique cell line which will be of great use in the study of the regulation of mesenchymal stem cell differentiation.

Ultrastructural analysis of bone nodules formed in vitro by isolated fetal rat calvaria cells

When cells enzymatically digested from 21 d fetal rat calvaria are grown in ascorbic acid and Na beta-glycerophosphate, they form discrete three-dimensional nodular structures with the histological and immunohistochemical appearance of woven bone. The present investigation was undertaken to verify that bone-like features were identifiable at the ultrastructural level. The nodules formed on top of a fibroblast-like multilayer of cells. The upper surface of the nodules was lined by a continuous layer of cuboidal osteoblastic cells often seen to be joined by adherens junctions. Numerous microvilli, membrane protrusions, and coated pits could be seen on the upper surface of these cells, their cytoplasm contained prominent RER and Golgi membranes, and processes extended from their lower surfaces into a dense, highly organized collagenous matrix. Some osteocyte-like cells were completely embedded within this matrix; they also displayed RER and prominent processes which extended through the matrix and often made both adherens and gap junctional contacts with the processes of other cells. The fibroblastic cells not participating in nodule formation were surrounded by a less dense collagenous matrix and, in contrast to the matrix of the nodules, it did not mineralize. An unmineralized osteoid-like layer was seen directly below the cuboidal top layer of cells. A mineralization front was detectable below this in which small, discrete structures resembling matrix vesicles and feathery mineral crystals were evident and frequently associated with the collagen fibrils. More heavily mineralized areas were seen further into the nodule. Electron microprobe and electron and X-ray diffraction analysis confirmed the mineral to be hydroxyapatite.(ABSTRACT TRUNCATED AT 250 WORDS)