Stimulation of DNA synthesis by epidermal growth factor in osteoblast-like cells

Individual or combination treatments with lapatinib and paclitaxel cause potential bone loss and bone marrow adiposity in rats

Cancer treatments with cytotoxic drugs have been shown to cause bone loss. However, effects on bone are less clear for ErbB-targeting tyrosine kinase inhibitors or their combination use with cytotoxic drugs. This study examined the effects of individual or combination treatments with breast cancer drugs lapatinib (a dual ErbB1/ErbB2 inhibitor) and paclitaxel (a microtubule-stabilizing cytotoxic agent) on bone and bone marrow of rats. Wistar rats received lapatinib (240 mg/kg) daily, paclitaxel (12 mg/kg) weekly, or their combination for 4 weeks, and effects on bone/bone marrow were examined at the end of week 4. Microcomputed tomographical structural analyses showed a reduction in trabecular bone volume in tibia following the lapatinib, paclitaxel or their combination treatments ( P < 0.05). Histomorphometry analyses revealed marked increases in bone marrow adipocyte contents in all treatment groups. Reverse transcription polymerase chain reaction gene expression studies with bone samples and cell culture studies with isolated bone marrow stromal cells showed that the all treatment groups displayed significantly reduced levels of osterix expression and osteogenic differentiation potential but increased expression levels of adipogenesis transcription factor peroxisome proliferator-activated receptor γ. In addition, these treatments suppressed the expression of Wnt10b and/or increased expression of Wnt antagonists (secreted frizzled-related protein 1, Dickkopf-related protein 1 and/or sclerostin). Furthermore, all treatment groups showed increased numbers of bone-resorbing osteoclasts on trabecular bone surfaces, although only the lapatinib group displayed increased levels of osteoclastogenic signal (receptor activator of nuclear factor κΒ ligand/osteoclastogenesis inhibitor osteoprotegrin expression ratio) in the bones. Thus, inhibiting ErbB1 and ErbB2 by lapatinib or blocking cell division by paclitaxel or their combination causes significant trabecular bone loss and bone marrow adiposity involving a switch in osteogenesis/adipogenesis potential, altered expression of some major molecules of the Wnt/β-catenin signalling pathway, and increased recruitment of bone-resorbing osteoclasts.

(-)-Epigallocatechin gallate but not chlorogenic acid suppresses EGF-stimulated migration of osteoblasts via attenuation of p38 MAPK activity

Phenolic compounds provide health benefits in humans. A previous study by our group has indicated that the epidermal growth factor (EGF)‑induced migration of osteoblast‑like MC3T3‑E1 cells is mediated by the phosphorylation of p44/p42 mitogen‑activated protein (MAPK), p38 MAPK, stress‑activated protein kinase (SAPK)/c‑Jun N‑terminal kinase (JNK) and Akt, and that resveratrol, a major polyphenol in grape skin, suppresses the EGF‑induced migration by attenuating Akt and SAPK/JNK activation. In the present study, the effects of chlorogenic acid, a major phenolic acid in coffee, and (‑)‑epigallocatechin gallate (EGCG), a major flavonoid in green tea, on the EGF‑induced migration of MC3T3‑E1 cells were investigated. EGCG significantly reduced the EGF‑induced migration as evaluated by a Transwell migration assay and by a wound healing assay. However, chlorogenic acid failed to affect the EGF‑induced migration. The phosphorylation of p38 MAPK induced by EGF was significantly suppressed by EGCG; however, the EGF‑induced phosphorylation of p44/p42 MAP kinase, SAPK/JNK or Akt was not affected by EGCG. These results suggest that EGCG, but not chlorogenic acid, suppresses EGF‑induced osteoblast migration through inhibiting p38 MAPK activation.

Osteoblast-specific overexpression of amphiregulin leads to transient increase in femoral cancellous bone mass in mice

The epidermal growth factor receptor ligand amphiregulin (AREG) has been implicated in bone physiology and in bone anabolism mediated by intermittent parathyroid hormone treatment. However, the functions of AREG in bone have been only incipiently evaluated in vivo. Here, we generated transgenic mice overexpressing AREG specifically in osteoblasts (Col1-Areg). pQCT analysis of the femoral metaphysis revealed increased trabecular bone mass at 4, 8, and 10weeks of age in Col1-Areg mice compared to control littermates. However, the high bone mass phenotype was transient and disappeared in older animals. Micro-CT analysis of the secondary spongiosa confirmed increased trabecular bone volume and trabecular number in the distal femur of 4-week-old AREG-tg mice compared to control littermates. Furthermore, μ-CT analysis of the primary spongiosa revealed unaltered production of new bone trabeculae in distal femora of Col1-Areg mice. Histomorphometric analysis revealed a reduced number of osteoclasts in 4-week-old Col1-Areg mice, but not at later time points. Cancellous bone formation rate remained unchanged in Col1-Areg mice at all time points. In addition, bone mass and bone turnover in lumbar vertebral bodies were similar in Col1-Areg and control mice at all ages examined. Proliferation and differentiation of osteoblasts isolated from neonatal calvariae did not differ between Col1-Areg and control mice. Taken together, these data suggest that AREG overexpression in osteoblasts induces a transient high bone mass phenotype in the trabecular compartment of the appendicular skeleton by a growth-related, non-cell autonomous mechanism, leading to a positive bone balance with unchanged bone formation and lowered bone resorption.

EGFR-AKT-mTOR activation mediates epiregulin-induced pleiotropic functions in cultured osteoblasts

Epidermal growth factor (EGF) receptor (EGFR) emerges as an essential molecule for the regulating of osteoblast cellular functions. In the current study, we explored the effect of epiregulin, a new EGFR ligand, on osteoblast functions in vitro, and studied the underlying mechanisms. We found that epiregulin-induced EGFR activation in both primary osteoblasts and osteoblast-like MC3T3-E1 cells. Meanwhile, epiregulin activated AKT-mammalian target of rapamycin (mTOR) and Erk-mitogen-activated protein kinase (MAPK) signalings in cultured osteoblasts, which were blocked by EGFR inhibitor AG1478 or monoclonal antibody against EGFR (anti-EGFR). Further, in primary and MC3T3-E1 osteoblasts, epiregulin promoted cell proliferation and increased alkaline phosphatase activity, while inhibiting dexamethasone (Dex)-induced cell death. Such effects by epiregulin were largely inhibited by AG1478 or anti-EGFR. Notably, AKT-mTOR inhibitors, but not Erk inhibitors, alleviated epiregulin-induced above pleiotropic functions in osteoblasts. Meanwhile, siRNA depletion of Sin1, a key component of mTOR complex 2 (mTORC2), also suppressed epiregulin-exerted effects in MC3T3-E1 cells. Together, these results suggest that epiregulin-induced pleiotropic functions in cultured osteoblasts are mediated through EGFR-AKT-mTOR signalings.

Epidermal growth factor receptor (EGFR) signaling promotes proliferation and survival in osteoprogenitors by increasing early growth response 2 (EGR2) expression

Maintaining bone architecture requires continuous generation of osteoblasts from osteoprogenitor pools. Our previous study of mice with epidermal growth factor receptor (EGFR) specifically inactivated in osteoblast lineage cells revealed that EGFR stimulates bone formation by expanding the population of mesenchymal progenitors. EGFR ligands are potent regulators for the osteoprogenitor pool, but the underlying mechanisms are largely unknown. Here we demonstrate that activation of EGFR increases the number of osteoprogenitors by promoting cell proliferation and suppressing either serum depletion-induced or TNFα-induced apoptosis mainly through the MAPK/ERK pathway. Mouse calvarial organ culture revealed that EGF elevated the number of proliferative cells and decreased the number of apoptotic cells, which led to increased osteoblasts. Microarray analysis of MC3T3 cells, an osteoprogenitor cell line, revealed that EGFR signaling stimulates the expression of MCL1, an antiapoptotic protein, and a family of EGR transcription factors (EGR1, -2, and -3). The up-regulation of MCL1 and EGR2 by EGF was further confirmed in osteoprogenitors close to the calvarial bone surface. Overexpression of NAB2, a co-repressor for EGRs, attenuated the EGF-induced increase in osteoprogenitor number. Interestingly, knocking down the expression of EGR2, but not EGR1 or -3, resulted in a similar effect. Using inhibitor, adenovirus overexpression, and siRNA approaches, we demonstrate that EGFR signaling activates the MAPK/ERK pathway to stimulate the expression of EGR2, which in turn leads to cell growth and MCL1-mediated cell survival. Taken together, our data clearly demonstrate that EGFR-induced EGR2 expression is critical for osteoprogenitor maintenance and new bone formation.

EGFR signaling suppresses osteoblast differentiation and inhibits expression of master osteoblastic transcription factors Runx2 and Osterix

The epidermal growth factor receptor (EGFR) and its ligands regulate key processes of cell biology, such as proliferation, survival, differentiation, migration, and tumorigenesis. We previously showed that, EGFR signaling pathway is an important bone regulator and it primarily plays an anabolic role in bone metabolism. In this study, we demonstrated that EGF-like ligands strongly inhibited osteoblast differentiation and mineralization in several lines of osteoblastic cells. Real-time RT-PCR and promoter reporter assays revealed that EGF-like ligands suppressed the expression of both early and late bone marker genes at the transcriptional level in the differentiating osteoblasts via an EGFR-dependent manner. This inhibitory effect of EGFR signaling was not dependent on its mitogenic activity. Furthermore, we demonstrated that EGFR signaling reduced the expression of two major osteoblastic transcription factors Runx2 (type II) and Osterix in osteoblast differentiating cells. EGFR-induced decrease in Runx2 transcriptional activity was confirmed by Runx2 reporter and chromatin immunoprecipitation assays. EGFR signaling increased the protein amounts of transcription co-repressors HDAC4 and 6 and over-expression of HDAC4 decreased Runx2 amount in differentiating osteoblasts, implying that HDACs contribute to the down-regulation of Runx2 by EGFR. Moreover, activation of EGFR in undifferentiated osteoprogenitors attenuated the expression of early bone markers and Osterix and decreased Runx2 protein amounts. Together with our previous data, that EGFR stimulates osteoprogenitor proliferation and that blocking EGFR activity in osteoblast lineage cells results in fewer osteoprogenitors and an osteopenic phenotype, we conclude that EGFR signaling is important for maintaining osteoprogenitor population at an undifferentiated stage.

Epidermal growth factor receptor plays an anabolic role in bone metabolism in vivo

While the epidermal growth factor receptor (EGFR)-mediated signaling pathway has been shown to have vital roles in many developmental and pathologic processes, its functions in the development and homeostasis of the skeletal system has been poorly defined. To address its in vivo role, we constructed transgenic and pharmacologic mouse models and used peripheral quantitative computed tomography (pQCT), micro-computed tomography (µCT) and histomorphometry to analyze their trabecular and cortical bone phenotypes. We initially deleted the EGFR in preosteoblasts/osteoblasts using a Cre/loxP system (Col-Cre Egfr(f/f)), but no bone phenotype was observed because of incomplete deletion of the Egfr genomic locus. To further reduce the remaining osteoblastic EGFR activity, we introduced an EGFR dominant-negative allele, Wa5, and generated Col-Cre Egfr(Wa5/f) mice. At 3 and 7 months of age, both male and female mice exhibited a remarkable decrease in tibial trabecular bone mass with abnormalities in trabecular number and thickness. Histologic analyses revealed decreases in osteoblast number and mineralization activity and an increase in osteoclast number. Significant increases in trabecular pattern factor and structural model index indicate that trabecular microarchitecture was altered. The femurs of these mice were shorter and smaller with reduced cortical area and periosteal perimeter. Moreover, colony-forming unit-fibroblast (CFU-F) assay indicates that these mice had fewer bone marrow mesenchymal stem cells and committed progenitors. Similarly, administration of an EGFR inhibitor into wild-type mice caused a significant reduction in trabecular bone volume. In contrast, Egfr(Dsk5/+) mice with a constitutively active EGFR allele displayed increases in trabecular and cortical bone content. Taken together, these data demonstrate that the EGFR signaling pathway is an important bone regulator and that it primarily plays an anabolic role in bone metabolism.

The EGFR network in bone biology and pathology

The resorption, formation and maintenance of bone are coordinated by the action of several hormones, growth factors and transcription factors. Recent experiments based on genetically modified mouse models, gene microarrays and pharmacological intervention indicate that the epidermal growth factor receptor (EGFR) system plays important roles in skeletal biology and pathology. This network, including a family of seven growth factors - the EGFR ligands - and the related tyrosine kinase receptors EGFR (ERBB1), ERBB2, ERBB3 and ERBB4, regulates aspects such as proliferation and differentiation of osteoblasts, chondrocytes and osteoclasts, parathyroid hormone-mediated bone formation and cancer metastases in bone. Here, we summarize and discuss the role of the EGFR and its ligands in skeletal biology and pathology.

The epidermal growth factor receptor ligands at a glance

The epidermal growth factor receptor (EGFR) regulates key processes of cell biology, including proliferation, survival, and differentiation during development, tissue homeostasis, and tumorigenesis. Canonical EGFR activation involves the binding of seven peptide growth factors. These ligands are synthesized as transmembrane proteins comprising an N-terminal extension, the EGF module, a short juxtamembrane stalk, a hydrophobic transmembrane domain, and a carboxy-terminal fragment. The central structural and functional feature is the EGF module, a sequence containing six cysteines in a conserved spacement which is responsible for binding to the EGFR. While the membrane-anchored peptide can be biologically active by juxtacrine signaling, in most cases the EGF module is proteolytically cleaved (a process termed ectodomain shedding) to release the soluble growth factor, which may act in an endocrine, paracrine, or autocrine fashion. This review summarizes the structural and functional properties of these fascinating molecules and presents selected examples to illustrate their roles in development, physiology, and pathology.

EGF-like ligands stimulate osteoclastogenesis by regulating expression of osteoclast regulatory factors by osteoblasts: implications for osteolytic bone metastases

Epidermal growth factor (EGF)-like ligands and their receptors constitute one of the most important signaling networks functioning in normal tissue development and cancer biology. Recent in vivo mouse models suggest this signaling network plays an essential role in bone metabolism. Using a coculture system containing bone marrow macrophage and osteoblastic cells, here we report that EGF-like ligands stimulate osteoclastogenesis by acting on osteoblastic cells. This stimulation is not a direct effect because osteoclasts do not express functional EGF receptors (EGFRs). Further studies reveal that EGF-like ligands strongly regulate the expression of two secreted osteoclast regulatory factors in osteoblasts by decreasing osteoprotegerin (OPG) expression and increasing monocyte chemoattractant protein 1 (MCP1) expression in an EGFR-dependent manner and consequently stimulate TRAP-positive osteoclast formation. Addition of exogenous OPG completely inhibited osteoclast formation stimulated by EGF-like ligands, while addition of a neutralizing antibody against MCP-1 exhibited partial inhibition. Coculture with bone metastatic breast cancer MDA-MB-231 cells had similar effects on the expression of OPG and MCP1 in the osteoblastic cells, and those effects could be partially abolished by the EGFR inhibitor PD153035. Because a high percentage of human carcinomas express EGF-like ligands, our findings suggest a novel mechanism for osteolytic lesions caused by cancer cells metastasizing to bone.

Requirement for ErbB2/ErbB signaling in developing cartilage and bone

During endochondral ossification, the skeletal elements of vertebrate limbs form and elongate via coordinated control of chondrocyte and osteoblast differentiation and proliferation. The role of signaling by the ErbB family of receptor tyrosine kinases, which consists of ErbB1 (epidermal growth factor receptor or EGFR), ErbB2, ErbB3 and ErbB4, has been little studied during cartilage and bone development. Signaling by the ErbB network generates a diverse array of cellular responses via formation of ErbB dimers activated by distinct ligands that produce distinct signal outputs. Herstatin is a soluble ErbB2 receptor that acts in a dominant negative fashion to inhibit ErbB signaling by binding to endogenous ErbB receptors, preventing functional dimer formation. Here, we examine the effects of Herstatin on limb skeletal element development in transgenic mice, achieved via Prx1 promoter-driven expression in limb cartilage and bone. The limb skeletal elements of Prx1-Herstatin embryos are shortened, and chondrocyte maturation and osteoblast differentiation are delayed. In addition, proliferation by chondrocytes and periosteal cells of Prx1-Herstatin limb skeletal elements is markedly reduced. Our study identifies requirements for ErbB signaling in the maintenance of chondrocyte and osteoblast proliferation involved in the timely progression of chondrocyte maturation and periosteal osteoblast differentiation.

Roles of epidermal growth factor family in the regulation of postnatal somatic growth

Ligands of the epidermal growth factor receptor (EGF-R), known to be important for supporting tissue development particularly in the gut and brain, have also been implicated in regulating postnatal somatic growth. Although optimal levels of both milk-borne and endogenous EGF-R ligands are important for supporting postnatal somatic growth through regulating gastrointestinal growth and maturation, supraphysiological levels of EGF-R ligands can cause retarded and disproportionate growth and alter body composition because they can increase growth of epithelial tissues but decrease masses of muscle, fat, and bone. Apart from their indirect roles in influencing growth, possibly via regulating levels of IGF-I and IGF binding proteins, EGF-R ligands can regulate bone growth and modeling directly because they can enhance proliferation but suppress maturation of growth plate chondrocytes (for building a calcified cartilage scaffold for bone deposition), stimulate proliferation but inhibit differentiation of osteoblasts (for depositing bone matrix), and promote formation and function of osteoclasts (for resorption of calcified cartilage or bone). In addition, EGF-like ligands, particularly amphiregulin, can be strongly regulated by PTH, an important regulatory factor in bone modeling and remodeling. Finally, EGF-R ligands can regulate bone homeostasis by regulating a pool of progenitor cells in the bone marrow through promoting proliferation but suppressing differentiation of bone marrow mesenchymal stem cells.

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.

Epidermal growth factor receptor-deficient mice have delayed primary endochondral ossification because of defective osteoclast recruitment

The epidermal growth factor receptor (EGFR) and its ligands function in diverse cellular functions including cell proliferation, differentiation, motility, and survival. EGFR signaling is important for the development of many tissues, including skin, lungs, intestines, and the craniofacial skeleton. We have now determined the role of EGFR signaling in endochondral ossification. We analyzed long bone development in EGFR-deficient mice. EGFR deficiency caused delayed primary ossification of the cartilage anlage and delayed osteoclast and osteoblast recruitment. Ossification of the growth plates was also abnormal resulting in an expanded area of growth plate hypertrophic cartilage and few bony trabeculae. The delayed osteoclast recruitment was not because of inadequate expression of matrix metalloproteinases, including matrix metalloproteinase-9, which have previously been shown to be important for osteoclast recruitment. EGFR was expressed by osteoclasts, suggesting that EGFR ligands may act directly to affect the formation and/or function of these cells. EGFR signaling regulated osteoclast formation. Inhibition of EGFR tyrosine kinase activity decreased the generation of osteoclasts from cultured bone marrow cells.

Gene expression profiles and transcription factors involved in parathyroid hormone signaling in osteoblasts revealed by microarray and bioinformatics

Parathyroid hormone (PTH) binds to its receptor PTH1R (parathyroid hormone 1 receptor) in osteoblastic cells to regulate bone remodeling and calcium homeostasis. While prolonged exposure to PTH causes increased bone resorption, intermittent injections of PTH have an anabolic effect on bone. The molecular mechanisms regulating these processes are still largely unknown. Here, we present our results on gene expression profile changes in the PTH-treated osteoblastic cell line, UMR 106-01, using DNA microarray analysis. A total of 125 known genes and 30 unknown expressed sequence tags (ESTs) were found to have at least 2-fold expression changes after PTH treatment at 4, 12, and 24 h. 14 genes were previously known to be PTH-regulated but many were unknown to be regulated by PTH prior to our experiments. Real-time reverse transcriptase-PCR confirmed that 90 and 50% of the genes are regulated more than 2-fold by PTH in UMR 106-01 and rat primary osteoblastic cells, respectively. Most genes belong to the following protein families: hormones, growth factors, and receptors; signal transduction pathway proteins; transcription factors; proteases; metabolic enzymes; structural and matrix proteins; transporters; etc. These results provide a comprehensive and deeper knowledge about PTH regulation of osteoblastic gene expression. Next, we designed a computational method to extract information about transcription factors likely involved in regulating these genes. These factors include those previously known to be involved in PTH signaling (AP-1 and the cAMP response element-binding protein), those that were identified by microarray data (C/EBP), and some novel transcription factors (AP-2, AP-4, SP1, FoxD3, etc.). Our results suggest that a reliable bioinformatics approach can be easily applied for other systems.

Immunohistochemical localization of epidermal growth factor in cat paradental tissues during tooth movement

Epidermal growth factor enhances proliferation and differentiation of cells during growth, maturation, and tissue healing. The objectives were to localize the epidermal growth factor in paradental cells and to determine the effect of orthodontic treatment on its concentrations in periodontal ligament fibroblasts, alveolar bone surface lining cells, and epithelial rests of Malassez. Sixty male cats, 1 year old, were divided into 2 groups: active and sham, and further divided into 10 time groups. In the active group, 1 maxillary canine was retracted by 80 g force; in the sham group, the animals received an inactive appliance. Sagittal sections of each half maxilla were stained for epidermal growth factor; staining intensity was measured microphotometrically in 10 periodontal ligament fibroblasts, alveolar bone surface lining cells, and epithelial rests of Malassez cells in sites of periodontal ligament tension and compression, and in corresponding sites near control and sham canines. The overall mean staining intensity of the cells of the active group animals was 30.47%, whereas that of the sham group was 21.78% (P <.0001). In all 3 types, cells near the actively treated canines stained significantly darker (P <.0001) than cells near the sham or control canines, particularly between 12 hours and 7 days. These results demonstrate that orthodontic forces increase epidermal growth factor concentrations in paradental cells, suggesting that epidermal growth factor participates in the tissue remodeling that facilitates tooth movement.

Use of autologous growth factors in lumbar spinal fusion

The results of spinal fusion, especially posteriorly above the lumbosacral junction, have been mixed. Autologous growth factor concentrate (AGF) prepared by ultraconcentration of platelets contains multiple growth factors having a chemotactic and mitogenic effect on mesenchymal stem cells and osteoblasts and may play a role in initiating bone healing. The purpose of this retrospective study is to review our results with AGF in lumbar spinal fusions. To date, AGF has been used in 39 patients having lumbar spinal fusion. The study group consisted of the first 19 consecutive cases to allow at least 6 months follow-up. The average follow-up was 13 months (range 6 to 18 months). Follow-up compliance was 91%. There were 7 men and 12 women. Average age was 52 years (range 30-72 years). Nine patients had prior back surgery. There were 8 smokers. AGF was used in posterior (n = 15) or anterior intradiscal (n = 4) fusions. AGF was used with autograft and coraline hydroxyapatite in all posterior fusions, and autograft, coral, and intradiscal spacer (carbon fiber spinal fusion cages or Synthes femoral ring) in intradiscal fusions. Posterior stabilization was used in all cases. Eight cases were single-level fusions, 6 were two-level, and 1 was a three-level fusion. Autologous iliac crest bone graft was taken in 14 cases and local autograft used in 5 cases. Posteriorly, a total of 23 levels were fused; of these, nine were at L5-S1, eight at L4-L5, five at L3-L4, and one at L2-L3. No impending pseudoarthroses were noted on plain radiographic examination at last follow-up visit. Solid fusion was confirmed in 3 patients having routine hardware removal, and in 2 patients who had surgery at an adjacent level. There was one posterior wound infection, which was managed without sequelae. When used as an adjunct to autograft, AGF offers theoretical advantages that need to be examined in controlled studies. Further study is necessary to determine whether coralline hydroxyapatite used as a bone graft extender in lumbar spinal fusion may help to obviate the need for secondary site graft harvesting.

Activation of phospholipase D signaling pathway by epidermal growth factor in osteoblastic cells

The receptor-mediated activation of phospholipase D (PLD) is a major signaling pathway in several cell systems. This study determined the effects of epidermal growth factor (EGF) on PLD activity in normal rat osteoblastic cells. Primary cultures were obtained from fetal rat calvaria by sequential collagenase digestion and seeded in BGJb media supplemented with 10% fetal calf serum. PLD activity was assayed by the transphosphatidylation reaction in [H3]myristic acid (5 microCi/ml)-labeled cells treated with EGF in the presence of 5% ethanol and measuring the production of phosphatidylethanol (PEtOH). Lipids were extracted and separated by thin-layer chromatography, detected by iodine staining, and the areas of interest were scraped off and transferred to vials for scintillation counting. EGF significantly increased PEtOH production in a dose-dependent manner and at short (10-60 s) and long (up to 30 minutes) incubation periods (p < 0.05). Phosphatidic acid levels were also significantly increased (p < 0.05) compared with unstimulated controls, but the levels were approximately 60% less than those of PEtOH. 4b-phorbol 12-myristate, 13-acetate (PMA) also produced a significant increase in PEtOH levels when compared with unstimulated control cultures, but when PMA was added together with EGF, the production of PEtOH was reduced about 30%. Pretreatment of cells with the protein kinase C (PKC) inhibitor H-7 caused a significant increase in PEtOH levels, compared with cells stimulated with EGF alone. Preincubation of cells with pertussis toxin produced a partial decrease in PEtOH levels. This study demonstrates that EGF activates the PLD signaling cascade in normal rat osteoblastic cells and that the pathway appears to involve, at least in part, a PKC- and Gi protein-dependent mechanism.

The mitogenic effect of parathyroid hormone is associated with E2F-dependent activation of cyclin-dependent kinase 1 (cdc2) in osteoblast precursors

Injections of parathyroid hormone (PTH) have been reported to stimulate skeletal accretion through increased bone formation in several species, and osteoblast proliferation is a critical component of bone formation. However, the biological mechanisms of PTH-stimulated bone cell proliferation are largely unknown. In this study, we demonstrated that PTH stimulates proliferation of the osteoblast precursor cell line, TE-85, in association with increasing cdc2 protein levels and its kinase activity. cdc2 antisense oligonucleotides blocked PTH-induced DNA synthesis and cell cycle progression. Analysis of the time course of PTH-stimulated cdc2 message levels demonstrated that cdc2 mRNA levels were increased 1.5- to 4-fold between 3-18 h following release from cell synchronization. Transfections of TE-85 cells with a series of cdc2 promoter-luciferase deletion constructs revealed PTH stimulation of the cdc2 promoter. Promoter constructs containing a mutation in the E2F binding site were not stimulated by PTH. Gel mobility shift assays demonstrated increased free E2F levels in TE-85 nuclear extracts in response to PTH. Furthermore, the ratios of hyperphosphorylated to hypophosphorylated forms of Rb protein were increased by PTH treatment. These results demonstrate that PTH-stimulated cdc2 expression was associated with TE-85 cell proliferation and that the mechanism of stimulating cdc2 gene expression involved increasing the levels of free E2F.

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.

EGF-mediated phosphorylation of extracellular signal-regulated kinases in osteoblastic cells

Epidermal growth factor (EGF) induces a rapid increase in the phosphorylation of extracellular signal-regulated kinases (ERKs) in the human osteosarcoma osteoblastic cell line G292 and in primary cultures of rat osteoblastic cells. This phosphorylation is transient and time-dependent. Maximal stimulation is attained within 1 min in G292 and within 5 min in rat osteoblastic cells. Enzymatic activity in G292 cells is also induced rapidly after EGF stimulation. Western blot analysis revealed that enhancement of the phosphorylation of ERKs in the EGF-stimulated cells is not due to an increase in ERK protein, since EGF-treatment does not lead to an increase in the absolute amount of ERKs present even after 2 days of stimulation. The pattern of expression of the ERKs observed in the two cell types differs in the apparent molecular weights observed. The most slowly migrating immunoreactive protein (approximately 45 kDa) in normal rat osteoblastic cells is ERK1, identified by an ERK1-selective antiserum. The same antiserum reacts only weakly with one of the ERK proteins (44 kDa) blotted from the human osteosarcoma cell line G292. Phorbol 12-myristate 13-acetate (PMA) is also capable of inducing ERK phosphorylation, albeit to a lasser degree. The combination of PMA and EGF does not produce a greater response than EGF alone. The role of protein kinase C (PKC) in the EGF-stimulated ERK signaling pathway was further examined by inhibition of PKC with the staurosporine analog, CGP41251, and by down-regulation of PKC via chronic treatment with PMA. Chronic PMA treatment results in a partial inhibition of the EGF-mediated phosphorylation. CGP41251 completely abolishes the increased ERK activity produced by PMA, but the effect of EGF in this regard is potentiated. We conclude that PKC and EGF act through parallel pathways to stimulate ERK phosphorylation and activity. The inhibitor studies, in addition, indicate that activation of PKC may moderate the actions of the EGF pathway via a tonic inhibitory feedback.

Regulation of membrane-associated tyrosine phosphatases in UMR 106.06 osteoblast-like cells

Protein tyrosine phosphatases play an important role in cell metabolism. Three distinct protein tyrosine phosphatase activities have been identified in an osteoblast-like cell line, UMR 106.06. These activities comprised two membrane-associated phosphatases and one cytosolic phosphatase of apparent molecular mass > 153 kDa, 80 kDa and 40 kDa respectively, estimated by gel filtration. On the basis of differences in apparent molecular mass, proteolytic-digestion profiles, substrate specificities and responses to a range of extracellular influences and inhibitor molecules, the two membrane-associated tyrosine phosphatases are distinct proteins. Tyrosine phosphatase activity in UMR 106.06 cells was sensitive to cell density. Cells at confluence contained membrane protein tyrosine phosphatase with specific activity 9-fold higher than cells at medium or low cell density. This elevation in membrane tyrosine phosphatase activity was due specifically to an increase in the high-molecular-mass enzyme. This phosphatase was also responsive to extracellular matrix components. This activity was elevated in cells grown on a collagen type-I matrix independently of cell density. Membrane and cytosolic protein tyrosine phosphatases were differentially regulated by a variety of agents including phorbol 12-myristate 13-acetate, parathyroid hormone, epidermal growth factor, okadaic acid and transforming growth factor beta. These observations suggest that regulatory influences control tyrosine phosphorylation in UMR 106.06 cells including cell-cell contact, cell-matrix contact and signal transduction involving tyrosine and serine/threonine phosphorylation events.

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.

Characterization of bone cells isolated on discontinuous Percoll gradients: distribution in sequentially derived populations

Bone cell populations obtained by sequential digestion of newborn mouse calvariae remain morphologically heterogeneous despite well-documented biochemical differences. Fractionation of these populations on Percoll gradient reveal three major cell groups of low, intermediate, and high buoyant density (1.056, 1.070, and 1.095 g/ml) that are present in different ratios in early and late released populations. Cells of low and intermediate density dominate in early released populations. In contrast, late released populations contain mostly high-density cells. Basal levels of alkaline phosphatase are highest in cells of intermediate buoyant density. All cells respond to PTH with cAMP production and morphologic transformation, but biochemical responses to PTH, such as secretion of insulin-like growth factor I (IGF-I) and stimulation of alkaline phosphatase activity, occur mostly in cells of intermediate density. These data suggest that (1) subclasses of osteoblasts can be further separated by density and (2) PTH effects on alkaline phosphatase activity and IGF-I secretion are probably expressed by osteoblasts of a certain subclass and/or stage of development.

Modulation by epidermal growth factor of the basal 1,25(OH)2D3 receptor level and the heterologous up-regulation of the 1,25(OH)2D3 receptor in clonal osteoblast-like cells

The effects of epidermal growth factor (EGF) on basal 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor level and on parathyroid hormone (PTH)-induced 1,25-(OH)2D3 receptor up-regulation were studied in the phenotypically osteoblastic cell line UMR 106. EGF in concentrations exceeding 0.1 ng/ml reduced the number of 1,25(OH)2D3 binding sites without changing the binding affinity. Maximal reduction was 30% at about 1 ng/ml. This reduction was independent of a change in cAMP content. EGF dose-dependently attenuated both PTH-induced 1,25(OH)2D3 receptor up-regulation and PTH-stimulated cAMP production, without an effect on the ED50 of the PTH effects. For both PTH responses the IC50 and the maximal effective dose were similar, 0.1 ng/ml and 1 ng/ml EGF, respectively. Reduction was first seen at 0.01 ng/ml EGF. At this concentration, EGF reduced PTH-stimulated 1,25-(OH)2D3 receptor binding without an inhibition of the cAMP response. Time-course studies with 1 ng/ml EGF revealed that at 2 h preincubation EGF reduced the heterologous up-regulation by PTH, and maximal inhibition was seen after 4 h. In contrast, PTH-stimulated cAMP production was just significantly inhibited only after 6 h, with 60% inhibition after 24 h preincubation. The effects of prostaglandin E2 and forskolin on both 1,25(OH)2D3 binding and cAMP production were inhibited in a similar fashion. On the other hand, dibutyryl cAMP- and 3-isobutyl-1-methylxanthine-stimulated 1,25(OH)2D3 binding were not affected by EGF. Taken together, our results demonstrate that EGF reduces both the basal number of 1,25(OH)2D3 binding sites and the heterologous up-regulation of the 1,25(OH)2D3 receptor. The current data suggest that EGF reduces heterologous up-regulation of the 1,25(OH)2D3 receptor independent of as well as dependent on the cAMP messenger system. The EGF effect is nor primarily located at the PTH receptor, at cAMP phosphodiesterase, or at protein kinase A level.

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 transforming growth factor-beta and epidermal growth factor on clonal rat pulp cells

These factors influence proliferation and differentiation in various cell types. Their effects on a clonal cell line (RPC-C2A) having high ALPase activity were examined by assay of [3H]-thymidine incorporation and ALPase activity. Neither factor (at a dose of 0.5 ng/ml) altered the shape of the pulp cells. DNA synthesis was not affected by transforming growth factor-beta either in growing cells or in those nearly confluent, but epidermal growth factor, in doses ranging from 0.5 to 10 ng/ml, stimulated the incorporation of [3H]-thymidine in nearly confluent cells. Both factors inhibited ALPase activity in a dose-dependent manner. Indomethacin did not affect this inhibition, suggesting that this effect of growth factors may not be mediated by prostaglandin synthesis. Inhibitory effects of ALPase antagonists (L-phenylalanine, L-homoarginine, levamisole) were not affected by transforming growth factor-beta. Thus epidermal growth factor stimulates DNA synthesis and both transforming growth factor-beta and epidermal growth factor inhibit ALPase activity of clonal rat pulp cells, suggesting that both factors may act as regulators of biological function, including cell differentiation, in pulp cells.

Mitogenic responses to and binding of insulin-like growth factor 1 and/or epidermal growth factor by bone cells

Populations of cells with different biochemical characteristics can be obtained following enzymatic digestion of newborn mouse calvaria. Previous studies had suggested an enrichment for periosteal fibroblasts in early released cells, based on their proliferative response to PGE2 and for osteoblasts in late released cells due to their high cAMP response to PTH. Intermediate cells were presumed to be a mixture of those two phenotypes. We have continued our characterization of these populations by studying their mitogenic responses to IGF-1 and EGF since these polypeptide growth factors have been reported to stimulate proliferation of immature bone cells in organ cultures of bone. [3H]Thymidine incorporation into acid insoluble radioactivity was stimulated by IGF-1 in all populations but the greatest increase was in intermediate populations. These latter cells were also the most highly responsive to EGF, which had a small growth stimulating effect on early released cells and no significant effect on late released cells. In bone cells both low and high affinity EGF binding sites were identified (Kd approximately 4 nM and 0.5 nM) and the total EGF binding sites/cell were approximately two-fold higher in intermediate than in early and late released populations. These data suggest that among sequentially released populations of bone cells the intermediate populations may be enriched for immature bone cells from the proliferative zone of bone.

Growth of rat osteoblast-like cells in a lipid-enriched culture medium and regulation of function by parathyroid hormone and 1,25-dihydroxyvitamin D

To examine the role of lipid metabolism in the growth and function of osteoblast-like cells, we studied ROS 17/2.8 osteosarcoma cells and primary cultures of rat calvarial osteoblasts during growth in a serum-free medium supplemented by purified human lipoproteins or by liposomes. Increase in ROS cell number was measured in sparse (1-5 X 10(3)/cm2) cultures over 6-8 days. Liposomes (0-300 micrograms/ml) and high (HDL), low (LDL), and very low density (VLDL) lipoprotein fractions (0-300 micrograms apoprotein) markedly stimulated cell growth. Cells plated at 5 X 10(3)/cm2 achieved growth rates in the presence of LDL or HDL comparable to 10% fetal bovine serum. Serum-free culture with exogenous lipid maintained the response of cell cyclic AMP accumulation to parathyroid hormone. Cyclic AMP response to parathyroid hormone was enhanced by glucocorticosteroid, and was attenuated by 1,25-dihydroxyvitamin D (1,25(OH)2D) with an EC50 (10(-10) M) comparable to that previously observed in serum-cultured cells (J. Biol. Chem. 258:736, 1985). 1,25(OH)2D also increased the alkaline phosphatase activity in ROS cells cultured in lipid-supplemented serum-free culture. Lipoproteins or liposomes also markedly enhanced the proliferative response of sparse cultures of normal rat osteoblasts to polypeptide mitogens.

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

Osteoblast-like cells isolated from fetal rat calvaria (RC) form three-dimensional nodules in vitro having the morphological and immunohistochemical characteristics of bone. When administered continually over 21 days of culture, EGF caused a dose-related inhibition of nodule formation, as well as a reduction in the proportion of mineralized nodules, at concentrations between 10(-12) to 10(-7) M. The same conditions caused an increase in both cell proliferation and saturation density in the cultures, suggesting that decreased nodule formation was not the result of general cell toxicity. Inhibitory concentrations of EGF caused a reversible alteration in cell shape, although changes in protein synthesis were not detectable. Indomethacin (5 X 10(-7) M) did not affect either nodule formation or its inhibition by EGF. When cultures were pretreated for various time periods with 10(-8) M EGF followed by its removal and continued culture in supplemented medium for 21 days, the number of bone nodules formed with brief exposures to EGF (4 hr up to 48 hr) was increased compared to the numbers formed in supplemented medium alone, whereas exposures to EGF of 4 days or longer decreased the number of nodules formed. These data indicate that EGF can either increase or decrease the osteogenic potential of RC cells in vitro depending on the duration of exposure to the factor.

Hormonal influences on bone cells

The methods for establishing osteoblast-rich rat calvarial cell cultures have been described, together with methods for the use of clonal osteogenic sarcoma cells of osteoblast phenotype. The latter clonal lines are useful for several purposes, but all the precautions and quality control measures necessary in the study of clonal lines must be observed. Some of the techniques for studying biochemical responses to hormones in these cells have also been detailed, but clearly others are applicable, including studies of the synthesis of matrix constituents. Osteoclast-like cells have not been considered in this chapter, because osteoclast culture methods have not yet been developed to the degree of purity and reproducibility necessary for this type of biochemical approach.

Transforming growth factor-beta reduces the phenotypic expression of osteoblastic MC3T3-E1 cells in monolayer culture

Transforming growth factor beta (TGF-beta) regulates cell growth and differentiation. Since it is abundant in bone, we have studied the effect of the polypeptide upon the growth and phenotypic expression of murine osteoblastic cells in monolayer culture. Its actions were compared to those of epidermal growth factor (EGF), another hormonally active polypeptide known to alter bone cell function. Picogram amounts of TGF-beta were found to inhibit the growth and phenotype (alkaline phosphatase and cAMP response to parathyroid hormone) of the clonal nontransformed MC3T3-E1 osteoblastic cell line. EGF also inhibited phenotypic expression, although at higher (nanogram) concentrations, but stimulated cell growth. The low concentration of TGF-beta required to inhibit growth and phenotype of osteoblastic cells together with its abundance in bone suggest that TGF-beta may be an important regulator of bone cell function.

Effects of DNA and prostaglandin synthesis inhibitors on the stimulation of bone resorption by epidermal growth factor in fetal rat long-bone cultures

We examined two inhibitors of DNA synthesis, hydroxyurea (HU) and aphidicholin (APC), and two inhibitors of prostaglandin cyclooxygenase, indomethacin and flufenamic acid, for their effects on the resorptive responses of fetal rat long-bone cultures to epidermal growth factor (EGF) and parathyroid hormone (PTH). As we have previously found, HU decreased unstimulated 45Ca release but had little effect on the resorptive response to PTH. HU also did not block resorption stimulated by EGF. Addition of the cyclooxygenase inhibitor, indomethacin, did not alter the resorptive responses of unstimulated or PTH-treated cultures in either the presence or absence of HU or the resorptive response of bones cultured with EGF alone. However, indomethacin completely blocked the resorptive response to EGF of bones that were cultured with HU. The effects of indomethacin on EGF-mediated resorption in HU-treated cultures appeared to be related to an inhibition of prostaglandin synthesis since flufenamic acid had similar effects. However, the effects of HU on the resorptive response to EGF may not have resulted solely from its inhibitory action on DNA synthesis since APC, in the absence of cyclooxygenase inhibitors, completely blocked EGF-mediated resorption without significantly affecting the response to PTH. These results demonstrate that the mechanisms regulating PTH- and EGF-mediated resorption in fetal rat long-bone cultures differ, and imply that a component of EGF-mediated resorption in these cultures is dependent on sustained DNA synthesis.

Regulation of prostaglandin production by osteoblast-rich calvarial cells

The effect of various factors upon prostaglandin (PG) production by the osteoblast was examined using osteoblast-rich populations of cells prepared from newborn rat calvaria. Bradykinin and serum, and to a lesser extent, thrombin, were all shown to stimulate PGE2 and 6-keto-PGF1 alpha (the hydration product of PGI2) secretion by the osteoblastic cells. Several inhibitors of prostanoid synthesis, dexamethasone, indomethacin, dazoxiben and nafazatrom, were tested for their effects on the calvarial cells. All inhibited PGE2 and PGI2 (the major arachidonic acid metabolites of these cells) production with half-maximal inhibition by all four substances occurring at approximately 10(-7) M. For dazoxiben and nafazatrom, this was in contrast to published results from experiments in vivo which have indicated that the compounds stimulated PGI2 production. Finally, since the osteoblast is responsive to bone-resorbing hormones, these were tested. Only epidermal growth factor (EGF) was shown to modify PG production. At early times EGF stimulated PGE2 release, however, the predominant effect of the growth factor was an inhibition of both PGE2 and PGI2 production by the osteoblastic cells. The present results suggest that the bone-resorbing hormones do not act to cause an increase in PG by the osteoblast and that any increase in PG production by these cells may be in response to vascular agents.

Epidermal growth factor inhibits morphogenesis and cell differentiation in cultured mouse embryonic teeth

Although local epithelial-mesenchymal tissue interactions which are presumably mediated by extracellular matrix molecules are important regulators of tooth morphogenesis and differentiation, our studies have indicated that these developmental processes also depend on circulating molecules. The iron-carrying serum protein transferrin is necessary for the early morphogenesis of mouse tooth in organ culture (A-M. Partanen, I. Thesleff, and P. Ekblom, 1984, Differentiation 27, 59-66). In the present study we have examined the effects of other growth factors on mouse tooth germs grown in a chemically defined medium containing transferrin. Fibroblast growth factor and platelet derived growth factor had no detectable effects but epidermal growth factor (EGF) inhibited dramatically the morphogenesis of teeth, and prevented odontoblast and ameloblast cell differentiation. EGF stimulated cell proliferation in the explants measured as [3H]thymidine incorporation in DNA. However, when the distribution of dividing cells was visualized in autoradiographs, it was observed that cell proliferation was stimulated in the dental epithelium but was inhibited in the dental mesenchyme. The inhibition of cell proliferation in the dental mesenchyme apparently caused the inhibition of morphogenesis. We do not know whether the dental epithelium or mesenchyme was the primary target for the action of EGF in the inhibition of morphogenesis. It is, however, apparent that the response of the dental mesenchymal cells to EGF (inhibition of proliferation) is regulated by their local environment, since EGF enhanced proliferation when these cells were disaggregated and cultured as monolayers. This indicates that the organ culture system where the various embryonic cell lineages are maintained in their original environment corresponds better to the in vivo situation when the roles of exogenous growth factors during development are examined.

Characterization of an osteoblast-like clonal cell line which responds to both parathyroid hormone and calcitonin

The clonal cell line UMR 106, which was originally derived from a rat transplantable osteogenic sarcoma with an osteoblastic phenotype, was subcloned after the emergence of a calcitonin-responsive adenylate cyclase was noted in late passages. Detailed studies on the stimulation of adenylate cyclase and activation profile of the cyclic AMP-dependent protein kinase isoenzymes in response to parathyroid hormone (PTH) and salmon calcitonin (SCT) were conducted on two subclones (UMR 106-01 and UMR 106-06). Both subclones responded in an identical manner to PTH, which stimulated adenylate cyclase and activated both isoenzyme I and isoenzyme II of cyclic AMP-dependent protein kinase. In contrast, only UMR 106-06 cells responded to calcitonin. At 3 X 10(-8)M SCT, there was a sevenfold stimulation of adenylate cyclase, 84% activation of isoenzyme I, and 44% activation of isoenzyme II. The activation profiles of the isoenzymes to PTH and SCT in UMR 106-06 were similar. Furthermore, their response to SCT correlates with the presence of specific, saturable binding of 125I-labeled SCT. Binding parameters indicate apparent Kd = 0.8 nM and 6,000 receptors/cell. These data point to a significant phenotypic change having taken place in this clonal cell line with prolonged maintenance in culture, with the emergence of a calcitonin receptor linked to adenylate cyclase and protein kinase activation.

Insulin-like growth factor I supports differentiation of cultured osteoblast-like cells

Rat calvaria cells grown in culture for one week had properties of osteoblasts: a high content in alkaline phosphatase and a marked cyclic AMP response to parathyroid hormone (PTH). In short-term experiments, insulin-like growth factor I (IGF I) stimulated the incorporation of [14C] glucose into glycogen. When IGF I was present in the medium during 6 days the cell number increased slightly and there was a substantial, disproportionate rise in alkaline phosphatase activity of the cultures. Thus, IGF I stimulates growth, and in addition, and in contrast to other growth factors, mainly enhances differentiation of osteoblasts.