Inhibitory effects of parathyroid hormone on growth of osteogenic sarcoma cells

Continuous infusion of PTH1-34 delayed fracture healing in mice

Hyperparathyroidism, which is increased parathyroid hormone (PTH) levels in the blood, could cause delayed or non-union of bone fractures. But, no study has yet demonstrated the effects of excess continuous PTH exposure, such as that seen in hyperparathyroidism, for fracture healing. Continuous human PTH_1–34 (teriparatide) infusion using an osmotic pump was performed for stabilized tibial fractures in eight-week-old male mice to determine the relative bone healing process compared with saline treatment. Radiographs and micro-computed tomography showed delayed but increased calcified callus formation in the continuous PTH_1–34 infusion group compared with the controls. Histology and quantitative histomorphometry confirmed that continuous PTH_1–34 treatment significantly increased the bone callus area at a later time point after fracture, since delayed endochondral ossification occurred. Gene expression analyses showed that PTH_1–34 resulted in sustained Col2a1 and reduced Col10a1 expression, consistent with delayed maturation of the cartilage tissue during fracture healing. In contrast, continuous PTH_1–34 infusion stimulated the expression of both Bglap and Acp5 through the healing process, in accordance with bone callus formation and remodeling. Mechanical testing showed that continuously administered PTH_1–34 increased the maximum load on Day 21 compared with control mice. We concluded that continuous PTH_1–34 infusion resulted in a delayed fracture healing process due to delayed callus cell maturation but ultimately increased biomechanical properties.

Teriparatide (human PTH1-34) compensates for impaired fracture healing in COX-2 deficient mice

Genetic ablation of cyclooxygenase-2 (COX-2) in mice is known to impair fracture healing. To determine if teriparatide (human PTH_1-34) can promote healing of Cox-2-deficient fractures, we performed detailed in vivo analyses using a murine stabilized tibia fracture model. Periosteal progenitor cell proliferation as well as bony callus formation was markedly reduced in Cox-2^-/- mice at day 10 post-fracture. Remarkably, intermittent PTH_1-34 administration increased proliferation of periosteal progenitor cells, restored callus formation on day 7, and enhanced bone formation on days 10, 14 and 21 in Cox-2-deficient mice. PTH_1-34 also increased biomechanical torsional properties at days 10 or 14 in all genotypes, consistent with enhanced bony callus formation by radiologic examinations. To determine the effects of intermittent PTH_1-34 for callus remodeling, TRAP staining was performed. Intermittent PTH_1-34 treatment increased the number of TRAP positive cells per total callus area on day 21 in Cox-2^-/- fractures. Taken together, the present findings indicate that intermittent PTH_1-34 treatment could compensate for COX-2 deficiency and improve impaired fracture healing in Cox-2-deficient mice.

Calpain modulates cyclin-dependent kinase inhibitor 1B (p27(Kip1)) in cells of the osteoblast lineage

The ubiquitously expressed calpains-1 and -2 belong to a family of calcium-dependent intracellular cysteine proteases. Both calpains are heterodimers consisting of a large catalytic subunit and a small regulatory subunit encoded by the gene Capn4. Ablation of the calpain small subunit eliminates calpain activity and leads to embryonic lethality. We previously created osteoblast-specific Capn4 knockout mice to investigate a physiological role for the calpain small subunit in cells of the osteoblast lineage. Deletion of Capn4 reduced trabecular and cortical bone, mainly due to impaired proliferation and differentiation of cells of the osteoblast lineage. To further investigate an underlining mechanism by which osteoblast-specific Capn4 knockout mice develop an osteoporotic bone phenotype, we established osteoblastic cell lines stably expressing either control or Capn4 RNA interference for this study. Capn4 knockdown cells showed reduced cell proliferation, accumulation of total and phosphorylated cyclin-dependent kinase inhibitor 1B (p27(Kip1)) on serine 10, and reduced phosphorylation of retinoblastoma protein on threonine 821. Moreover, ablation of Capn4 increased 27 ( Kip1 ) mRNA levels, likely due to stabilized binding of Akt to protein phosphatase 2A, which presumably results in reduced phosphorylation of Akt on S473 and forkhead Box O (FoxO) 3A on T32. Collectively, calpain regulates cell proliferative function by modulating both transcription and degradation of p27(Kip1) in osteoblasts. In conclusion, calpain is a critical modulator for regulation of p27(Kip1) in cells of the osteoblast lineage.

Parathyroid hormone regulation of the human bone sialoprotein gene transcription is mediated through two cAMP response elements

Parathyroid hormone (PTH) regulates serum calcium and inorganic phosphate levels through its actions on kidney and bone. Bone sialoprotein (BSP) is an early marker of osteoblast differentiation and bone metabolism. We here report that two cAMP response elements (CRE) in the human BSP gene promoter are target of PTH. In human osteoblast-like Saos2 cells, PTH (human 1-34 PTH, 10 nM) increased BSP mRNA and protein levels at 3 h. From transient transfection assays, 2- to 2.5-fold increase in transcription by PTH was observed at 3 and 6 h in -184, -211, -428, -868, and -927 luciferase constructs that included the human BSP gene promoter. Effect of PTH was abrogated by 2 bp mutations in either the CRE1 (-79 to -72) or CRE2 (-674 to -667). Luciferase activities induced by PTH were blocked by protein kinase A inhibitor H89 and tyrosine kinase inhibitor herbimycin A. Gel shift analyses showed that PTH increased binding of nuclear proteins to the CRE1 and CRE2 elements. The CRE1-protein and CRE2-protein complexes were disrupted by CRE binding protein 1 (CREB1) antibodies and supershifted by phospho-CREB1 antibody. ChIP assays detected binding of CREB1 and phospho-CREB1 to a chromatin fragment containing CRE1 and CRE2, and increased binding of phospho-CREB1 to the both sites. These studies demonstrate that PTH stimulates human BSP gene transcription by targeting the two CREs in the promoter of the human BSP gene.

Cyclin D1 as a target for the proliferative effects of PTH and PTHrP in early osteoblastic cells

PTHrP induced a proliferative cyclin D1 activation in low-density osteoblastic cells. The process was PKA and MAPK dependent and involved both AP-1 and CRE sites. In ectopic ossicles generated from implanted bone marrow stromal cells, PTH upregulated cyclin D1 after acute or intermittent anabolic treatment. These data suggest a positive role of PTH and PTHrP in the cell cycle of early osteoblasts.

INTRODUCTION

The mechanisms underlying the actions of PTH and its related protein (PTHrP) in osteoblast proliferation, differentiation, and bone remodeling remain unclear. The action of PTH or PTHrP on the cell cycle during osteoblast proliferation was studied.

MATERIALS AND METHODS

Mouse calvarial MC3T3-E1 clone 4 cells were synchronized by serum starvation and induced with 100 nM PTHrP for 2-24 h under defined low serum conditions. Western blot, real-time PCR, EMSAs, and promoter/luciferase assays were performed to evaluate cyclin D1 expression. Pharmacological inhibitors were used to determine the relevant signaling pathways. Ectopic ossicles generated from implanted bone marrow stromal cells were treated with acute (a single 8- or 12-h injection) or intermittent anabolic PTH treatment for 7 days, and RNA and histologic analysis were performed.

RESULTS

PTHrP upregulated cyclin D1 and CDK1 and decreased p27 expression. Cyclin D1 promoter/luciferase assays showed that the PTHrP regulation involved both activator protein-1 (AP-1) and cyclic AMP response element binding protein (CRE) sites. AP-1 and CRE double mutants completely abolished the PTHrP effect of cyclin D1 transcription. Upregulation of cyclin D1 was found to be protein kinase A (PKA) and mitogen-activated protein kinase (MAPK) dependent in proliferating MC3T3-E1 cells. In vivo expression of cyclin D1 in ectopic ossicles was upregulated after a single 12-h PTH injection or intermittent anabolic PTH treatment for 7 days in early developing ossicles.

CONCLUSIONS

These data indicate that PTH and PTHrP induce cyclin D1 expression in early osteoblastic cells and their action is developmental stage specific.

Effects of transient PTH on early proliferation, apoptosis, and subsequent differentiation of osteoblast in primary osteoblast cultures

In primary calvarial osteoblast cultures derived from transgenic mice expressing green fluorescent protein (GFP) under the control of 3.6-kb Col1a1 promoter, the emergence of GFP signal marks the transition of multipotential osteoprogenitors into preosteoblasts. Early transient treatment (days 1-7) of these cultures with parathyroid hormone (PTH) has an anabolic effect that is not associated with an increase in total DNA content or cell number in day 21 cultures. In the present study, the effect of early PTH treatment on cell proliferation and apoptosis was examined in greater detail in GFP(+) and GFP(-) cells using flow cytometry. In preconfluent cultures, PTH significantly reduced the proportion of cells in S phase but increased those in G(0)/G(1) and G(2)+M phases in both GFP(+) and GFP(-) subpopulations. PTH decreased apoptosis only in GFP(-) but not GFP(+) cells, indicating an increased survival of GFP(-) cells. In contrast, PTH did not change the amounts of cell proliferation and apoptosis seen in either compartment after these cultures reached confluence. To further assess the effect of early PTH treatment on osteogenic differentiation, secondary cultures of sorted GFP(+) or GFP(-) cells were obtained from day 7 primary cultures that had been treated for 1 wk with PTH. This treatment resulted in larger areas of GFP expression accompanied by increased xylenol orange/von Kossa staining in the secondary cultures of GFP fractions. Early transient PTH treatment appears to enhance the commitment of progenitor cells to an osteogenic fate and results in a higher proportion of cells that achieve full osteoblast differentiation.

Parathyroid hormone activates PKC-delta and regulates osteoblastic differentiation via a PLC-independent pathway

PTH exerts major effects upon bone by activating PTH/PTHrP receptors (PTH1Rs) expressed on osteoblasts. The PTH1R is capable of engaging multiple signaling pathways in parallel, including Gs/adenylyl cyclase (AC), Gq/phospholipase C/protein kinase C (PLC/PKC) and a distinct mechanism, involving activation of PKC via a PLC-independent pathway, that depends upon ligand determinants within the PTH(29-34) sequence. The involvement of PLC-dependent vs. PLC-independent PKC activation in PTH action was studied in clonal PTH1R-expressing murine calvarial osteoblasts ("Wt9") using two signal-selective analogs, [G1,R19]hPTH(1-28) and [G1,R19]hPTH(1-34). Both analogs lack PLC signaling but differ in their capacity to activate the PLC-independent PKC pathway. Both hPTH(1-34) and [G1,R19]hPTH(1-34), but not [G1,R19]hPTH(1-28), increased differentiation of Wt9 cells during a 16-day alternate-daily treatment protocol. Wt9 cells expressed PKC-betaI, -delta, -epsilon and -zeta, none of which exhibited net translocation to membranes in response to hPTH(1-34) or either analog. hPTH(1-34) induced activation of membrane-associated PKC-delta, however, and a time- and concentration-dependent increase in cytosolic [phospho-Thr505]PKC-delta which was maximal within 40 s at 100 nM in both Wt9 cells and primary osteoblasts. This response was mimicked by [G1,R19]hPTH(1-34) but not by [G1,R19]hPTH(1-28). Increased expression of bone sialoprotein (BSP) and osteocalcin (OC) mRNAs induced by PTH(1-34) and [G1,R19]hPTH(1-34) in Wt9 cells was blocked by rottlerin, a PKC-delta inhibitor. We conclude that PTH1Rs activate PKC-delta by a PLC-independent, PTH(29-34)-dependent mechanism that promotes osteoblastic differentiation.

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.

Parathyroid hormone uses multiple mechanisms to arrest the cell cycle progression of osteoblastic cells from G1 to S phase

Parathyroid hormone (PTH) plays a major role in bone remodeling and has the ability to increase bone mass if administered daily. In vitro, PTH inhibits the growth of osteoblastic cell lines, arresting them in G(1) phase. Here, we demonstrate that PTH regulates the expression of at least three genes to achieve the following: inducing expression of MAPK phosphatase 1 (MKP-1) and p21(Cip1) and decreasing expression of cyclin D1 at both mRNA and protein levels. The induction of MKP-1 causes the dephosphorylation of extracellular signal-regulated kinase and therefore the decrease in cyclin D1. Overexpression of MKP-1 arrests UMR cells in G(1) phase. The mechanisms involved in PTH regulation of these genes were studied. Most importantly, PTH administration produces similar effects on expression of these genes in rat femoral metaphyseal primary spongiosa. Analyses of p21(Cip1) expression levels in bone indicate that repeated daily PTH injections make the osteoblast more sensitive to successive PTH treatments, and this might be an important feature for the anabolic functions of PTH. In summary, our data suggest that one mechanism for PTH to exert its anabolic effect is to arrest the cell cycle progression of the osteoblast and hence increase its differentiation.

Effect of raloxifene and its interaction with human PTH on bone formation

We studied the of effects raloxifene alone or in combination with human PTH (hPTH) 1-34 in mineralizing cultures of SaOS-2 cells. Raloxifene (10(-8)-10(-6) M) increased bone nodule formation in cultures of SaOS-2 cells when added intermittently from day 8 to day 17. A single 24-h treatment with 10(-8) M hPTH (1-34) at day 8 reduced the nodule area by 75.6% at day 17, and raloxifene added concomitantly with hPTH (1-34) reduced this inhibitory effect in a dose-dependent manner. Raloxifene also reduced the hPTH (1-34)-induced inhibition of alkaline phosphatase (ALP) activity. The 10-fold stimulation of c-fos mRNA expression by hPTH (1-34) was not influenced by raloxifene co-treatment. The protein kinase A (PKA) inhibitor 6-22 amide (1.7 nM) and the protein kinase C (PKC) inhibitor-bisindolylmaleimide 1 (10 nM) did not influence the separate effects of PTH and raloxifene on mineralized bone nodule formation. This is the first report on the interaction of PTH and raloxifene in an osteoblast culture system.

Parathyroid hormone-Smad3 axis exerts anti-apoptotic action and augments anabolic action of transforming growth factor beta in osteoblasts

Although several studies indicated that parathyroid hormone (PTH) exerted anabolic action on bone, its precise mechanisms have been unknown. On the other hand, transforming growth factor beta (TGF-beta), abundantly stored in bone matrix, stimulates bone formation with a local injection in rodents. Although our previous study suggested that Smad3 is an important molecule for the stimulation of bone formation, no reports have been available about the effects of PTH on Smad3. In this present study, we examined the effects of PTH on Smad3 and the physiological significance in mouse osteoblastic cells. PTH promoted the expression of Smad3 mRNA within 10 min and the protein level in a dose-dependent manner in MC3T3-E1 and rat osteoblastic UMR-106 cells. Protein kinase A (PKA) activator as well as protein kinase C (PKC) activators increased Smad3 protein level, and both PKA and PKC inhibitors antagonized PTH-induced Smad3, indicating that PTH promotes the production of Smad3 through both PKA and PKC pathways. Next, we examined anti-apoptotic effects of PTH and Smad3 in these cells, employing trypan blue, transferase-mediated nick end labeling, and Hoechst staining. Pretreatment with PTH or overexpression of Smad3 decreased the number of apoptotic cells induced by dexamethasone and etoposide. Moreover, a dominant negative mutant, Smad3DeltaC, abrogated PTH-induced anti-apoptotic effects. On the other hand, PTH augmented TGF-beta-induced transcriptional activity. Furthermore, PTH enhanced TGF-beta-induced production of type I collagen, whereas it did not affect TGF-beta-reduced proliferation in MC3T3-E1 cells. These observations indicated that PTH amplified the anabolic effects of TGF-beta by accelerating the transcriptional activity of Smad3. In conclusion, we first demonstrated that PTH-Smad3 axis exerts anti-apoptotic effects in osteoblasts and reinforces the anabolic action by TGF-beta in osteoblasts. Hence, PTH-Smad3 axis might be involved in the bone anabolic action of PTH.

PTH2 receptor-mediated inhibitory effect of parathyroid hormone and TIP39 on cell proliferation

The parathyroid hormone (PTH) has dual mitogenic and inhibitory effects on cell proliferation, depending on the cell type and experimental conditions. PTH can signal via two different receptors, both positively coupled to the adenylyl cyclase/cyclic AMP pathway which can mimic some of the proliferative effects of PTH. We evaluated the role of the type-2 PTH (PTH2) receptor on cell proliferation in clonal human embryonic kidney HEK293 cells stably expressing the human PTH2 receptor. Using a cyclic AMP-responsive gene-reporter, we confirmed that the tuberoinfundibular peptide (TIP39) and various human (h) PTH fragments including hPTH-(1-34) were potent agonists (EC(50) in the range of 0.01-0.3 nM) whereas the bovine (b) PTH peptides b(Tyr(34))PTH-(7-34) and its tryptophan derivative b[D-Trp(12),Tyr(34)]PTH-(7-34) behaved as antagonists (IC(50)=117 and 249 nM, respectively). hPTH-(1-34) produced a dose-dependent inhibition of cell proliferation (EC(50)=8.5+/-0.4 nM) after 3 days and this effect was fully reversed by the tryptophan derivative antagonist. The same effect was observed with TIP39 which caused a 30% maximal inhibition. These findings reveal that PTH2 receptor activation can inhibit cell proliferation and might explain the dual functionality of PTH on cell proliferation.

Local secretion of parathyroid hormone-related protein by an osteoblastic osteosarcoma (UMR 106-01) cell line results in growth inhibition

Parathyroid hormone-related protein (PTHrP) has been implicated as being important in the growth of tumor cells responsive to the peptide. We utilized a rat osteoblastic osteosarcoma cell line, UMR 106-01, which has PTHrP receptors and a PTHrP-responsive adenylate cyclase/cAMP messenger system, to produce a modified cell line that overexpresses PTHrP. The human PTHrP cDNA sequence was transfected by electroporation into UMR 106-01 cells and the stable cell lines UMR-36 and UMR-34 were established. The modified cell line, UMR-36, had increased levels of PTHrP mRNA compared with control cell lines and secreted PTHrP into the culture medium at levels of 0.01-0.1 pmol/10(7) cells in 12 h. The secreted peptide was biologically active as indicated by its ability to activate adenylate cyclase. The number of UMR-36 cells following 9 days in culture was reduced by up to 80% compared with control lines, which was associated with decreased (3)H-thymidine incorporation into genomic DNA. Addition of 1000-fold excess of the PTHrP antagonist, PTHrP(7-34), to UMR-36 cells resulted in the escape of growth inhibition and increased rate of growth. In vivo, tumors derived from UMR-36 cells were smaller in size compared with tumors derived from control cells. In conclusion, increased autocrine secretion of, and responsiveness to, PTHrP results in inhibited growth kinetics of an osteoblast-like bone tumor cell line in vitro and in vivo.

Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. PTH and PTH-related protein (PTHrP) indirectly activate osteoclasts resulting in increased bone resorption. During this process, PTH changes the phenotype of the osteoblast from a cell involved in bone formation to one directing bone resorption. In addition to these catabolic effects, PTH has been demonstrated to be an anabolic factor in skeletal tissue and in vitro. As a result, PTH has potential medical application to the treatment of osteoporosis, since intermittent administration of PTH stimulates bone formation. Activation of osteoblasts by PTH results in expression of genes important for the degradation of the extracellular matrix, production of growth factors, and stimulation and recruitment of osteoclasts. The ability of PTH to drive changes in gene expression is dependent upon activation of transcription factors such as the activator protein-1 family, RUNX2, and cAMP response element binding protein (CREB). Much of the regulation of these processes by PTH is protein kinase A (PKA)-dependent. However, while PKA is linked to many of the changes in gene expression directed by PTH, PKA activation has been shown to inhibit mitogen-activated protein kinase (MAPK) and proliferation of osteoblasts. It is now known that stimulation of MAPK and proliferation by PTH at low concentrations is protein kinase C (PKC)-dependent in both osteoblastic and kidney cells. Furthermore, PTH has been demonstrated to regulate components of the cell cycle. However, whether this regulation requires PKC and/or extracellular signal-regulated kinases or whether PTH is able to stimulate other components of the cell cycle is unknown. It is possible that stimulation of this signaling pathway by PTH mediates a unique pattern of gene expression resulting in proliferation in osteoblastic and kidney cells; however, specific examples of this are still unknown. This review will focus on what is known about PTH-mediated cell signaling, and discuss the established or putative PTH-regulated pattern of gene expression in osteoblastic cells following treatment with catabolic (high) or anabolic (low) concentrations of the hormone.

Stimulation of extracellular signal-regulated kinases and proliferation in rat osteoblastic cells by parathyroid hormone is protein kinase C-dependent

Parathyroid hormone (PTH) is known to have both catabolic and anabolic effects on bone. The dual functionality of PTH may stem from its ability to activate two signal transduction mechanisms: adenylate cyclase and phospholipase C. Here, we demonstrate that continuous treatment of UMR 106-01 and primary osteoblasts with PTH peptides, which selectively activate protein kinase C, results in significant increases in DNA synthesis. Given that ERKs are involved in cellular proliferation, we examined the regulation of ERKs in UMR 106-01 and primary rat osteoblasts following PTH treatment. We demonstrate that treatment of osteoblastic cells with very low concentrations of PTH (10(-12) to 10(-11) m) is sufficient for substantial increases in ERK activity. Treatment with PTH-(1-34) (10(-8) m), PTH-(1-31), or 8-bromo-cAMP failed to stimulate ERKs, whereas treatment with phorbol 12-myristate 13-acetate, serum, or PTH peptides lacking the N-terminal amino acids stimulated activity. Furthermore, the activation of ERKs was prevented by pretreatment of osteoblastic cells with inhibitors of protein kinase C (GF 109203X) and MEK (PD 98059). Treatment of UMR cells with epidermal growth factor (EGF), but not PTH, promoted tyrosine phosphorylation of the EGF receptor. Transient transfection of UMR cells with p21(N17Ras) did not block activation of ERKs following treatment with low concentrations of PTH. Thus, activation of ERKs and proliferation by PTH is protein kinase C-dependent, but stimulation occurs independently of the EGF receptor and Ras activation.

Long-term effects of parathyroid hormone, 1,25-dihydroxyvitamin d(3), and dexamethasone on the cell growth and functional activity of human osteogenic alveolar bone cell cultures

The proliferation-differentiation behaviour of human alveolar bone cell cultures grown for 32 days in conditions that allowed the complete expression of the osteoblastic phenotype was significantly affected by the continuous presence of parathyroid hormone, 1, 25-dihydroxyvitamin D(3), or dexamethasone. Parathyroid hormone and, in particular, dexamethasone significantly induced the differentiation of osteoblastic cells. Moreover, cultures exposed to these hormones presented an earlier appearance and higher levels of alkaline phosphatase, and an increased ability to form calcium phosphate deposits in the extracellular matrix.

Parathyroid hormone [PTH(1-34)] and parathyroid hormone-related protein [PTHrP(1-34)] promote reversion of hypertrophic chondrocytes to a prehypertrophic proliferating phenotype and prevent terminal differentiation of osteoblast-like cells

The effects of parathyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) on late events in chondrocyte differentiation were investigated by a dual in vitro model where conditions of suspension versus adhesion culturing are permissive either for apoptosis or for the further differentiation of hypertrophic chondrocytes to osteoblast- like cells. Chick embryo hypertrophic chondrocytes maintained in suspension synthesized type II and type X collagen and organized their extracellular matrix, forming a tissue highly reminiscent of true cartilage, which eventually mineralized. The formation of mineralized cartilage was associated with the expression of alkaline phosphatase (ALP), arrest of cell growth, and apoptosis, as observed in growth plates in vivo. In this system, PTH/PTHrP was found to repress type X collagen synthesis, ALP expression, and cartilage matrix mineralization. Cell proliferation was resumed, whereas apoptosis was blocked. Hypertrophic chondrocytes cultured in adherent conditions in the presence of retinoic acid underwent further differentiation to osteoblast-like cells (i.e., they resumed cell proliferation, switched to type I collagen synthesis, and produced a mineralizing bone-like matrix). In this system, PTH addition to culture completely inhibited the expression of ALP and matrix mineralization, whereas cell proliferation and expression of type I collagen were not affected. These data indicate that PTH/PTHrP inhibit both the mineralization of a cartilage-like matrix and apoptosis (mimicked in the suspension culture) and the production of a mineralizing bone-like matrix, characterizing further differentiation of hypertrophic chondrocytes to osteoblasts like cells (mimicked in adhesion culture). Treatment of chondrocyte cultures with PTH/PTHrP reverts cultured cells in states of differentiation earlier than hypertrophic chondrocytes (suspension), or earlier than mineralizing osteoblast-like cells (adhesion). However, withdrawal of hormonal stimulation redirects cells toward their distinct, microenvironment-dependent, terminal differentiation and fate.

Identification and activation of mitogen-activated protein (MAP) kinase in normal human osteoblastic and bone marrow stromal cells: attenuation of MAP kinase activation by cAMP, parathyroid hormone and forskolin

The mitogen-activated protein (MAP) kinases (p44mapk and p42mapk), also known as extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2), are activated in response to a variety of extracellular signals, including growth factors, hormones and, neurotransmitters. We have investigated MAP kinase signal transduction pathways in normal human osteoblastic cells. Normal human bone marrow stromal (HBMS), osteoblastic (HOB), and human (TE85, MG-63, SaOS-2), rat (ROS 17/2.8, UMR-106) and mouse (MC3T3-E1) osteoblastic cell lines contained immunodetectable p44mapk/ERK1 and p42mapk/ERK2. MAP kinase activity was measured by 'in-gel' assay using myelin basic protein as the substrate. Mainly ERK2 was rapidly activated (within 10 min) by bFGF, IGF-I and PDGF-BB in normal HOB, HBMS and human osteosarcoma cells, whereas both ERK1 and ERK2 were activated by growth factors in rat osteoblast-like cell lines, ROS 17/2.8 and UMR-106. The ERK1 activation was greater than the ERK2 in ROS 17/2.8 cells. Furthermore, ERK2 was also activated by bFGF and PDGF-BB in the mouse osteoblastic cell line, MC3T3-E1. This is the first demonstration of inter-species differences in the activation of MAP kinases in osteoblastic cells. Cyclic AMP derivatives or cAMP generating agents such as PTH and forskolin inhibited ERK2 activation by bFGF and PDGF-BB suggesting a 'cross-talk' between the two different signalling pathways activated by receptor tyrosine kinases and cAMP-dependent protein kinase. The accumulated results also suggest that the MAP kinases may be involved in mediating mitogenic and other biological actions of bFGF, IGF-I and PDGF-BB in normal human osteoblastic and bone marrow stromal cells.

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.

Parathyroid hormone exerts disparate effects on osteoblast differentiation depending on exposure time in rat osteoblastic cells

It has been reported that PTH exerts bone-forming effects in vivo when administered intermittently. In the present study, the anabolic effects of PTH(1-34) on osteoblast differentiation were examined in vitro. Osteoblastic cells isolated from newborn rat calvaria were cyclically treated with PTH(1-34) for the first few hours of each 48-h incubation cycle. When osteoblastic cells were intermittently exposed to PTH only for the first hour of each 48-h incubation cycle and cultured for the remainder of the cycle without the hormone, osteoblast differentiation was inhibited by suppressing alkaline phosphatase activity, bone nodule formation, and mRNA expression of alkaline phosphatase, osteocalcin, and PTH/PTHrP receptor. Experiments using inhibitors and stimulators of cAMP/protein kinase A (PKA) and Ca2+/PKC demonstrated that cAMP/PKA was the major signal transduction system in the inhibitory action of PTH. In contrast, the intermittent exposure to PTH for the first 6 h of each 48-h cycle stimulated osteoblast differentiation. Both cAMP/ PKA and Ca2+/PKC systems appeared to be involved cooperatively in this anabolic effect. Continuous exposure to PTH during the 48-h incubation cycle strongly inhibited osteoblast differentiation. Although both cAMP/PKA and Ca2+/PKC were involved in the effect of continuous exposure to PTH, they appeared to act independently. A neutralizing antibody against IGF-I blocked the stimulatory effect on alkaline phosphatase activity and the expression of osteocalcin mRNA induced by the 6-h intermittent exposure. The inhibitory effect induced by the 1-h intermittent exposure was not affected by anti-IGF-I antibody. These results suggest that PTH has diverse effects on osteoblast differentiation depending on the exposure time in vitro mediated through different signal transduction systems. These in vitro findings explain at least in part the in vivo action of PTH that varies with the mode of administration.

An intact N terminus is required for the anabolic action of parathyroid hormone on adult female rats

Intermittent administration of parathyroid hormone (PTH) peptides increases bone density in animal and human models of osteoporosis. In vitro studies have demonstrated that PTH analogs lacking the first two amino acids can stimulate cell proliferation in certain cell systems, whereas fragments with an intact N terminus can be antimitogenic. We have tested whether the truncated PTH(3-38) fragment may be a better "anabolic analog" than PTH(1-38) by monitoring bone density and biomechanical properties of the femur in 6-month-old ovariectomized (OVX) rats. Either PTH fragment was administered subcutaneously (8 micrograms/100 g of body weight) 5 days/week, for 4 weeks, starting 1 week after surgery. During the entire study, untreated OVX rats lost 12.1 +/- 4.4% of their initial bone density. PTH(1-38) reversed the initial bone loss, leading to complete restoration of presurgery values after 4 weeks of treatment. Conversely, administration of PTH(3-38) resulted in 13.2 +/- 5.8% bone loss, while continuous estrogen infusion (10 micrograms/kg/day) prevented bone loss but did not reverse it. Sham-operated animals also experienced significant bone loss in the vehicle and PTH(3-38)-treated groups (-4.5 +/- 6.7%, and -7.6 +/- 2.8%, respectively), whereas a significant gain in bone density (+4.4 +/- 5.6%) was observed in the rats treated with PTH(1-38). A bone quality factor (index of strain energy loss) and the impact strength (resistance to fracture) were 25% and 44% lower in femurs explanted from OVX animals treated with either vehicle or PTH(3-38), compared with sham-operated animals. On the contrary, no difference was observed between OVX and control animals after treatment with PTH(1-38), indicating a preservation of the capacity to withstand mechanical stress. Thus, PTH(1-38) counteracts estrogen-dependent loss of mineral density and bone biomechanical properties and increases bone density in estrogen-replete animals. An intact N terminus sequence is necessary for this anabolic action of PTH.

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.

Effects of parathyroid hormone and agonists of the adenylyl cyclase and protein kinase C pathways on bone cell proliferation

The anabolic effect of parathyroid hormone (PTH) on bone is partly due to a stimulation of osteoblast proliferation. The PTH signal is transduced by the pathways of adenylyl cyclase (AC)/protein kinase (PK) A and phospholipase C/PKC/Ca++. There is still uncertainty about the relative contribution of the two pathways to the proliferative effects of the hormone. In our study, PTH(1-34), AC/PKA agonists, and phorbol 12-myristate-13-acetate (PMA, a PKC activator) stimulated cell proliferation in cultured mouse calvariae. In isolated osteoblasts, only PMA stimulated proliferation, whereas AC/PKA agonists and PTH(1-34) inhibited it. As already known, PTH in the presence of supramaximal concentrations of transforming growth factor-beta (TGF-beta) stimulated osteoblast growth; under these same conditions, AC/PKA agonists reproduced the stimulatory effect of PTH(1-34), whereas PMA became inhibitory. PTH(1-31), which stimulates AC without affecting PKC, acted similarly to the fully active PTH(1-34) in both calvaria and isolated osteoblasts. On the contrary, midregion fragments that activate only PKC stimulated calvaria cell proliferation faintly in comparison with PTH(1-34); no effect was seen in osteoblasts, either with or without TGF-beta. Our study shows that the effects of PTH on proliferation can be mimicked by agonists of the AC/cAMP pathway. Although PMA is indeed able to stimulate cell growth in tissue explants, its effects on isolated osteoblasts markedly diverge from those of PTH. We conclude that activation of the AC/PKA pathway is the main component of the proliferative effects of PTH.

Bone marrow cells are targets for the anabolic actions of prostaglandin E2 on bone: induction of a transition from nonadherent to adherent osteoblast precursors

Although prostaglandin E2 (PGE2) is known to stimulate bone formation in vivo, its mechanism of action is not well understood. Circumstantial evidence suggests that bone marrow cells (BMC) may well be involved in this, and in order to investigate this further we have studied the effect of PGE2 on proliferation and matrix synthesis in high-density BMC cultures and on colony-forming unit (CFU-f) formation efficiency by BMC in vitro. High-density cultures of BMC formed a collagenous, calcified matrix, synthesized osteocalcin and expressed alkaline phosphatase activity. The addition of PGE2 caused a concentration-dependent increase in total (but not specific) APase activity, cell number, and collagen accumulation. It was found that PGE2 need only be present during the first 48 hours of the culture period and that longer exposure had no additional effect. PGE2 also caused a concentration-dependent increase in CFU-f formation, and it was found that this was due to the recruitment of new mesenchymal precursor cells from the nonadherent fraction of the BMC. Once again, the presence of PGE2 for only the first 48 hours of the culture period was enough to precipitate a maximal response. We conclude that one mechanism for the anabolic actions of PGE2 may be the recruitment of OB precursors from a population of nonadherent mesenchymal precursor cells present in the bone marrow.

Second messenger signaling of c-fos gene induction by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: its role in osteoblast proliferation and osteoclast-like cell formation

The present study was performed to clarify second messenger signaling in parathyroid hormone (PTH)-induced c-fos gene expression, to characterize the participation of the c-fos gene in the regulation of osteoblast proliferation and function as well as osteoclast-like cell formation by PTH and to compare these effects of PTH with those of PTH-related peptide (PTHrP). Both human (h) PTH-(1-34) and hPTHrP-(1-34) at 10(-8) M induced a transient c-fos gene expression to a similar degree in osteoblastic osteosarcoma cells, UMR-106. N6,O2'-dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) as well as Sp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Sp-cAMPS), an activator of cAMP-dependent protein kinase (PKA), induced a weak c-fos gene expression. Although Rp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), an inhibitor of PKA, almost completely antagonized dbCAMP- and Sp-cAMPS-induced expression of c-fos gene, it did not cause an obvious inhibition of PTH- or PTHrP-induced expression. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), induced an intense expression of the c-fos gene, while 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD), incapable of activating PKC, and calcium ionophores (A23187 and ionomycin) did not. Protein kinase C inhibitor (H-7, 50 microM) completely blocked the expression of the c-fos gene by PTH as well as by PTHrP). Antisense oligodeoxynucleotides (as-ODN) complementary to c-fos mRNA, which have been shown to inhibit its mRNA translation, at 1 microM significantly antagonized PTH- and PTHrP-induced inhibition of [3H] thymidine incorporation and stimulation of osteoclast-like cell formation in the presence of osteoblasts, but not an increase in alkaline phosphatase activity, compared to control oligodeoxynucleotides with same nucleotides as as-ODN but with a random sequence. The present study indicates the involvement of PKC system in c-fos gene expression by PTH as well as PTHrP and also indicates the involvement of the c-fos gene in the regulation of bone cell physiology by PTH and PTHrP.

Time-dependent effects of parathyroid hormone and prostaglandin E2 on DNA synthesis by periosteal cells from embryonic chick calvaria

Both PGE2 and PTH (1-34) caused a time- and concentration-dependent stimulation of proliferation by embryonic chick periosteal cells. Cells were exposed to the agents for different periods of time, the medium was replaced with fresh medium, and 3H-TdR incorporation was measured after 16 hours. Challenge with 10(-6) M prostaglandin E2 (PGE2) or 10(-7) M parathyroid hormone (1-34) (PTH) for 5 minutes produced 4- and 5.5-fold increases in 3H-TdR incorporation, respectively. Longer exposures, however, produced diminishing responses and after 45 minutes, only minimal effects or slight inhibitions were seen. These time-dependent effects were also seen with forskolin and dibutyryl-cAMP; TPA on the other hand stimulated DNA synthesis after both short- and long-term exposure. Both PGE2 and PTH (1-34) stimulated cAMP synthesis in periosteal cells but neither could be shown to stimulate protein kinase-C (PKC) at concentrations required for stimulation of proliferation, and dibutyryl-cyclic AMP (cAMP) effectively inhibited endogenous PKC activity. It is possible that the stimulation of proliferation by short-term exposure to PGE2 and PTH (1-34) is mediated by cAMP and that the time dependency possibly stems from the inhibition of endogenous PKC activity by increased intracellular cAMP levels.

Hormonal regulation of Na(+)-Ca2+ exchange in osteoblast-like cells

We proposed a role for Na-Ca exchange in hormonally mediated bone resorption and recently characterized Na-dependent Ca transport in an osteoblast-like rat osteosarcoma cell line (UMR-106). To test whether calcemic agents alter Na(+)-Ca2+ exchange in osteoblasts, UMR cells were treated acutely or cultured in the absence or presence of calcemic agent for 24 h. Cells were then loaded with the Ca-sensitive dye fura-2 in the presence of 140 mM NaCl, no Ca, and the absence or presence of 0.3 mM ouabain. Cells were resuspended at 22 degrees C, and the fluorescence ratio at excitation wavelength of 340 and 380 nm was measured. An outward Na gradient was generated by removing extracellular Na and maintaining isotonicity with choline chloride. Na(+)-Ca2+ exchange was demonstrated by enhanced Ca uptake in ouabain-treated (Na-loaded) cells after the addition of 1.5 mM Ca. Acute addition of 10(-7) M PTH or 10(-6) M PGE2 had no effect on Na-dependent Ca uptake. However, 24 h treatment of cells with PTH, PGE2, or 1,25(OH)2D3 caused a dose-dependent inhibition of Na(+)-Ca2+ exchange. Using the Na-sensitive dye, SBFI, we also demonstrated that the effect was bidirectional; PTH inhibited Ca-dependent Na uptake comparably to its inhibition of Na-dependent Ca uptake. The effects of the calcemic agents were mimicked by 24 h treatment of the cells with 1 microM forskolin or 2 microM PMA.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction pathways mediating parathyroid hormone regulation of osteoblastic gene expression

Parathyroid hormone (PTH) plays a central role in regulation of calcium metabolism. For example, excessive or inappropriate production of PTH or the related hormone, parathyroid hormone related protein (PTHrP), accounts for the majority of the causes of hypercalcemia. Both hormones act through the same receptor on the osteoblast to elicit enhanced bone resorption by the osteoclast. Thus, the osteoblast mediates the effect of PTH in the resorption process. In this process, PTH causes a change in the function and phenotype of the osteoblast from a cell involved in bone formation to one directing the process of bone resorption. In response to PTH, the osteoblast decreases collagen, alkaline phosphatase, and osteopontin expression and increases production of osteocalcin, cytokines, and neutral proteases. Many of these changes have been shown to be due to effects on mRNA abundance through either transcriptional or post-transcriptional mechanisms. However, the signal transduction pathway for the hormone to cause these changes is not completely elucidated in any case. Binding of PTH and PTHrP to their common receptor has been shown to result in activation of protein kinases A and C and increases in intracellular calcium. The latter has not been implicated in any changes in mRNA of osteoblastic genes. On the other hand activation of PKA can mimic all the effects of PTH; protein kinase C may be involved in some responses. We will discuss possible mechanisms linking PKA and PKC activation to changes in gene expression, particularly at the nuclear level.

Cross talk of dual-signal transduction systems in the regulation of DNA synthesis by parathyroid hormone in osteoblastic osteosarcoma cells

There has been recent evidence that calcium/protein kinase C (Ca/PKC) messenger system as well as adenylate cyclase are involved in the signal transduction stimulated by PTH. We therefore examined the role of these dual-signal transduction systems and the interaction of these systems in the regulation of DNA synthesis by PTH in the osteoblastic osteosarcoma cells, UMR-106. As recently reported, 10(-4) M Sp-cAMPS, a direct activator of cAMP-dependent protein kinase (PKA), and 10(-4) M dibutyryl-cAMP, as well as hPTH-(1-34), caused the significant inhibition of [3H]thymidine incorporation (TdR). Both A23187 and ionomycin (10(-8)-10(-6) M) inhibited TdR in a dose-dependent manner, with a minimal effective dose at 10(-7) M. Although 10(-6) M phorbol 12-myristate 13-acetate (PMA) caused slight but significant stimulation of TdR by itself, it augmented not only dibutyryl-cAMP- but also Sp-cAMPS-induced inhibition of TdR. On the other hand, 4 alpha-phorbol 12,13-didecanoate, incapable of activating PKC, failed to augment these cAMP analogs-induced effects. Pretreatment with 50 microM H-7, an inhibitor of PKC, not only abolished the PMA-induced augmentation of effect by cAMP analogs but also significantly blocked the PTH-induced inhibitory effect on TdR. Pretreatment with 10(-6) M PMA, which downregulates PKC, significantly inhibited the PTH-induced suppression of TdR. Combined treatment with cAMP analog (dibutyryl-cAMP or Sp-cAMPS) and calcium ionophore (A23187 or ionomycin) caused additive effects on TdR, and PMA used in combination with both cAMP analog and calcium ionophore induced the further inhibition of TdR.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of ornithine decarboxylase by parathyroid hormone in osteoblastic cell systems

Parathyroid hormone (PTH) has been shown to induce osteoblastic activity via a complex signal transduction process which is mediated either by cAMP or cytosolic calcium ([Ca2+]i), or a combination thereof. One of the PTH functions in osteoblasts is the induction of ornithine decarboxylase (ODC) activity. We have analyzed the second messengers involved in this process. 8-Bromo cAMP, a cAMP derivative, enhanced ODC activity in UMR106-01 osteoblastic cell system. The calcium ionophore A23187 and the protein kinase stimulator phorbol-12-myristate 13-acetate did not alter ODC activity. ODC activity was increased by bPTH-(1-34), PGE1, and PGE2 which stimulated both cAMP and [Ca2+]i. In contrast, PTH-(2-34), propionyl bPTH-(2-34), bPTH-(3-34), bPTH-(7-34), and PGF2 alpha, which only enhanced [Ca2+]i but not cAMP, had no effect on ODC activity. Thus, the stimulation of ODC in UMR106 cells by PTH appeared to be mediated primarily via the cAMP signal transduction pathway, and the mere increase in intracellular calcium could not account for the stimulation of ODC activity. ODC mRNA level was found to be increased by PTH treatment. Therefore, translation of ODC may be stimulated by PTH. Moreover, PTH also stimulated ODC antizyme activity, suggesting that the ODC degradation rate was increased.

Interrelationship between parathyroid hormone and insulin: effects on DNA synthesis in UMR-106-01 cells

UMR-106-01 osteoblast-like cells respond to high concentrations of parathyroid hormone (PTH) in vitro by decreasing thymidine incorporation, a marker of DNA synthesis and cell proliferation. This response is different from in vivo conditions, such as primary and secondary hyperparathyroidism, in which high PTH levels are associated with an increased number of osteoblasts. When the response of UMR-106-01 cells to PTH is evaluated in vitro, however, these cells are exposed to only a single hormone. The present study was designed to evaluate the combined effects of two hormones, PTH and insulin, on the DNA synthesis of UMR-106-01 cells. PTH is known to decrease and insulin to increase thymidine incorporation by UMR-106-01 cells. To examine the interaction of these hormones, acute studies, defined as a 24 h exposure to hormone, and chronic studies, defined as a 7 day exposure to hormone, were performed. Both acute and chronic exposure to 10(-9) M PTH decreased thymidine incorporation by UMR-106-01 cells, with suppression ranging from 27 to 81% (P < 0.05). Both acute and chronic exposure to 10(-8) M insulin (INS) increased thymidine incorporation by UMR-106-01 cells; this ranged from 26 to 58% (P < 0.05). However, chronic exposure to 10(-9) M PTH followed by an acute exposure to 10(-8) M INS resulted in a 710% increase in thymidine incorporation (P < 0.01). Reversing the sequence by chronically exposing UMR-106-01 cells to 10(-8) M INS followed by acute exposure to 10(-9) M PTH resulted in a 53% decrease in thymidine incorporation (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of parathyroid hormone on cultured human dental pulp cells

The responsiveness of human dental pulp (HDP) cells to parathyroid hormone (PTH) was investigated by measuring their cyclic AMP (cAMP) content, DNA synthesis, alkaline phosphatase activity, collagen synthesis, and glycosaminoglycan (GAG) synthesis. PTH dose-dependently increased the intracellular cAMP 1 min after the addition of PTH. Confluent HDP cells on day 14 expressed a high level of cAMP production after addition of 3 units/ml PTH. The hormone did not affect DNA synthesis by HDP cells. Alkaline phosphatase activity was suppressed by PTH to 81% of control (p < 0.01), and addition of dibutyryl cAMP to the medium mimicked the effect of PTH (79% of control, p < 0.01). The hormone inhibited collagen synthesis (15% decrease of [3H] proline incorporation, p < 0.01), and stimulated non-collagen protein synthesis (10% increase, p < 0.05). The increase of non-collagen protein by PTH was in accordance with the enhancement of GAG synthesis (17% increase of [35S]sulfate incorporation, p < 0.01). Dibutyryl cAMP caused further increase of GAG synthesis, to 155% of control (p < 0.01). Observations of the radiolabeled proteins on 10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis after metabolic labeling with [14C]proline and [35S]sulfate revealed a similar tendency to the quantitative determinations in which PTH inhibited collagen synthesis and stimulated GAG synthesis. These findings suggest that HDP cells have some osteoblastic characteristics in terms of PTH responsiveness, and that this culture system is a useful model for studies of human dental pulp.

Regulation of gene transcription and proliferation by parathyroid hormone is blocked in mutant osteoblastic cells resistant to cyclic AMP

We employed a cyclic AMP-resistant subclone of UMR 106-01 osteoblastic osteosarcoma cells (UMR 4-7) with a regulated, dominant-negative mutation of cyclic AMP-dependent protein kinase (PK-A), to examine the mechanism(s) whereby parathyroid hormone (PTH) regulates growth of these cells. Expression of a transiently transfected CAT reporter gene controlled by the cAMP response element of the rat somatostatin gene ('SST-CAT') was used to monitor PK-A activation in intact cells. Agonist-stimulated SST-CAT expression was specific for agents known to activate adenylate cyclase, required an intact cAMP response element and was specifically blocked following induction of the mutant cAMP-resistant phenotype in UMR 4-7 cells. Inhibition of the proliferation of UMR 106-01 cells by PTH, which is mimicked by forskolin and 8-bromo-cAMP, was blocked completely in mutant cyclic AMP-resistant UMR 4-7 cells. We conclude that control of proliferation in UMR 106-01 cells by PTH involves the cAMP messenger system and requires activation of PK-A.

Effect of protein and steroidal osteotropic agents on differentiation and epidermal growth factor-mediated growth of the CFK1 osseous cell line

The effects of factors known to influence bone metabolism were examined using the osseous cell line CFK1. Parathyroid hormone (PTH) and dexamethasone (DEX) appeared to enhance the formation of cell foci of CFK1 cells in culture whereas retinoic acid (RA) caused a marked alteration in individual cell morphology. Bone morphogenetic protein (BMP-2) and PTH increased alkaline phosphatase activity, however, this index of differentiation was suppressed by epidermal growth factor (EGF), DEX, and RA. BMP-2 and EGF each stimulated DNA synthesis in a dose-dependent manner and enhanced cell numbers, but, no synergistic response of EGF and BMP-2 was observed. PTH and DEX failed to significantly alter cell number or EGF-stimulated DNA synthesis or cell proliferation. Although RA treatment of CFK1 cells resulted in a reduction in cell number compared to control, pretreatment with RA enhanced EGF-stimulated DNA synthesis and proliferative effects. At least part of this effect was by increasing the EGF receptor binding capacity of the cells. Furthermore, using cell cycle analysis, addition of EGF stimulated the progression of RA-treated cells into the DNA synthesis (S) phase with a reduced lag time. EGF and BMP-2, therefore, appear to exert a role in the expansion dynamics of the CFK1 population although BMP-2 may also enhance differentiation. PTH and DEX may act primarily to modulate the differentiated function of the CFK1 cells. RA inhibited cell proliferation and may mediate differentiation towards a less established cell population with upregulation of EGF receptors. The CFK1 cell model may, therefore, provide insight into microenvironmental control of growth and differentiation of precursor osseous cells.

Plasma-membrane-intercalated heparan sulphate proteoglycans in an osteogenic cell line (UMR 106-01 BSP)

The heparan sulphate (HS) proteoglycans associated with the cell layer of a rat osteosarcoma cell line [UMR 106-01 (BSP)] were compared with similar cell-associated proteoglycans from other cells, and their interaction with the plasma membrane was studied. HS proteoglycans were metabolically labelled by incubation of cell cultures with [3H]glucosamine or [3H]leucine and [35S]sulphate. HS proteoglycan core protein preparation generated by heparitinase digestion of the major species from UMR 106-01 (BSP) cells co-migrated on PAGE with identical preparations from ovarian granulosa cells and parathyroid cells (at approximately 70 kDa). The hydrophobic nature of the major HS proteoglycans from these diverse cell lines, based on elution position from octyl-Sepharose, were also comparable. Linkages of the HS proteoglycan to the cell membrane were investigated by labelling plasma-membrane preparations with a lipid soluble photoactivatable reagent, 3-(trifluoromethyl)-3- (m-[125I]iodophenyl)diazirine (TID), which selectively labels plasma-membrane-spanning peptide domains. Purified HS proteoglycan from UMR 106-01 (BSP) cells was shown to be accessible to the [125I]TID, and the core protein portion of the molecule was labelled, confirming its close association with the plasma membrane. Approx. 36% of 35S-labelled HS proteoglycans were released from the cell surface by phospholipase C (Bacillus thuringiensis), which specifically cleaves phosphatidylinositol-linked proteins. In the presence of insulin, the metabolism of the phospholipase C-sensitive population was unaltered; however, release of the phospholipase C-insensitive population into the medium was increased. These data indicate that a subpopulation of HS proteoglycans are covalently bound to the plasma membrane by a glycosylphosphatidylinositol structure, with the remainder representing those species directly inserted into the plasma membrane via a hydrophobic peptide domain. These observations are similar to those reported for ovarian granulosa cells [Yanagishita & McQuillan (1989) J. Biol. Chem. 264 17551-17558], and thus may represent a general phenomenon for many cell types.

Insulin-like growth factor I stimulates sodium-phosphate cotransport in the rat osteoblastic cell line UMR-106-01

The effect of recombinant insulin-like growth factor I (IGF-I) on Pi uptake by a rat osteoblast-like cell line (UMR-106-01) in culture was investigated. IGF-I (10(-6)-10(-8) M) caused a dose-related stimulation of Na(+)-Pi cotransport. A 30-70% increase in Na(+)-dependent Pi uptake over control values was observed after 1- to 5-h exposure of these cells to 10(-7) M IGF-I. The increase was detected within 45 min, in contrast to the slower action of insulin. This effect of IGF-I was specific for Na(+)-Pi uptake, because Na(+)-independent Pi uptake and Na(+)-alanine cotransport were unaffected by IGF-I. A reversal of IGF-I induced stimulation of Na(+)-Pi cotransport was observed within 1 h of removal of the hormone. Kinetic analysis of the IGF-I effect indicates a significant change only in the apparent maximum velocity (Vmax) of Na(+)-Pi cotransport. The Vmax was 5.22 +/- 0.47 vs. 3.33 +/- 0.45 nmol Pi.mg protein-1.10 min-1 in confluent monolayers exposed to 10(-7) M IGF-I and vehicle alone, respectively, for 3 h (P less than 0.05, group t test). Blocking de novo protein synthesis with cycloheximide had no effect on this stimulatory effect of IGF-I. These observations indicate that IGF-I specifically stimulates Pi uptake in osteoblastic cells. The effect is characterized by an increase in Vmax and is not dependent on de novo protein synthesis. The mechanism remains to be determined.

PTH stimulates the proliferation of TE-85 human osteosarcoma cells by a mechanism not involving either increased cAMP or increased secretion of IGF-I, IGF-II or TGF beta

Injections of parathyroid hormone (PTH) result in increased bone formation in several species. Work in our laboratory and others has shown a stimulation of bone cell proliferation and growth factor production by PTH. Our purpose was to study the effects of PTH on a human bone cell line using TE-85 human osteosarcoma cells as a model. After 24 h treatment, PTH caused an increase in cell proliferation as measured by cell counts and [3H]-thymidine incorporation. Proliferation was not inhibited by an anti-transforming growth factor beta (TGF beta) antibody which could abolish stimulation by exogenous TGF beta. PTH did not stimulate cAMP production, alkaline phosphatase activity or production of insulin-like growth factors I or II (IGF-I or IGF-II) in TE-85 cells. Although basal TE-85 proliferation was slowed by incubation with the calcium channel blocking agent verapamil, PTH still caused an increase in growth rate. We conclude that PTH directly stimulates TE-85 proliferation via a mechanism not involving increased adenylate cyclase activity or increased secretion of IGF-I, IGF-II or TGF beta and may stimulate bone formation in vivo by activating some other mitogenic signal to increase bone cell proliferation.

Regulation of osteoblast proliferation by leukemia inhibitory factor

We recently showed that leukemia inhibitory factor (LIF) stimulates 45Ca release from neonatal mouse calvariae in vitro and that it increases DNA and protein synthesis in this model. To elucidate further the actions of LIF on bone we now report the effects of this cytokine on DNA synthesis and cell proliferation in isolated fetal rat osteoblasts and in the osteogenic sarcoma cell line, UMR-106. In both actively growing and growth-arrested rat osteoblasts, LIF stimulated [3H]thymidine incorporation in a dose-dependent manner. The increase in DNA synthesis was time dependent, was associated with an increase in the number of osteoblasts, and was not blocked by indomethacin. LIF-treated cells showed reduced [3H]thymidine incorporation in comparison with control, as they approached confluence, possibly because of the increased cell density in the LIF-treated cultures. In UMR-106 cells, treatment with LIF inhibited [3H]thymidine incorporation in both actively growing and growth-arrested cultures. The effect was dose dependent and sustained with time. There was a corresponding decrease in cell numbers. It is concluded that although LIF causes an early stimulation of proliferation in isolated osteoblasts, it has opposing effects on UMR-106 cells. It is not possible to determine which of these effects is more relevant to the actions of LIF in vivo. The demonstration of a LIF effect on both these cell types, however, provides further evidence that this cytokine acts directly on osteoblasts.

The activation of cAMP-dependent protein kinase is directly linked to the inhibition of osteoblast proliferation (UMR-106) by parathyroid hormone-related protein

The present study was performed to compare the effect of parathyroid hormone-related protein (PTHrP) on the proliferation of osteoblastic osteosarcoma cells (UMR-106) with that of PTH and characterize the direct involvement of cAMP in the change of osteoblast proliferation by PTHrP. Human(h)PTHrP-(1-34) (10(-11)-10(-7)M) dose-dependently inhibited [3H]thymidine incorporation (TdR) in the same manner as hPTH-(1-34). The simultaneous addition of PTHrP and PTH at a maximal effective dose of 10(-7) M did not cause additive suppressive effect on cell proliferation. Rp-cAMPs, which has been recently shown to act directly as antagonist in the activation of cAMP-dependent protein kinase (PKA), dose-dependently (10(-6)-10(-4)M) antagonized PTHrP-induced suppression of TdR in the same manner as PTH. Present study indicated that PTHrP has the same effect on osteoblast proliferation as PTH and that the activation of PKA is directly linked to the change of osteoblast proliferation by PTHrP.

Proteoglycans synthesized by an osteoblast-like cell line (UMR 106-01)

The proteoglycans synthesized by an osteoblast-like cell line of rat origin (UMR 106-01) were defined after biosynthetic labelling with [35S]sulphate and [3H]glucosamine. Newly synthesized labelled proteoglycans were characterized by differential enzymic digestion in combination with analytical gel filtration and SDS/PAGE. UMR 106-01 cells were found to synthesize three major species of proteoglycan: a large chondroitin sulphate proteoglycan of Mr approximately 1 x 10(6), with a core protein of Mr approximately 350,000-400,000; a small chondroitin sulphate-containing species of Mr approximately 120,000 with a core protein of Mr 43,000; and a heparan sulphate proteoglycan of Mr approximately 150,000, with a core protein of Mr approximately 80,000. Over 70% of the newly synthesized intact proteoglycan species are associated with the cell layer of near-confluent cells; however, accessibility to trypsin digestion suggests an extracellular location. Chemical characteristics of the proteoglycans and preliminary mRNA hybridization indicate that the small chondroitin sulphate proteoglycan is probably PG II (decorin). The large chondroitin sulphate proteoglycan is most likely related to a hyaluronate-aggregating species from fibroblasts (versican), and the heparan sulphate proteoglycan bears striking similarities to cell-membrane-intercalated species described for a number of cell types.

The activation of cAMP-dependent protein kinase is directly linked to the regulation of osteoblast proliferation (UMR-106) by parathyroid hormone

In order to characterize the direct involvement of cAMP in the change of osteoblast proliferation by parathyroid hormone (PTH), we employed the diastereoisomers of adenosine 3',5'-cyclic phosphorothioate, Sp-cAMPS and Rp-cAMPS, which have been recently shown to act directly as agonist and antagonist, respectively in the activation of cAMP-dependent protein kinase (PKA). Dibutyryl cAMP (dbcAMP) and cholera toxin as well as human(h) PTH-(1-34) significantly inhibited [3H]thymidine incorporation (TdR) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-6)-10(-4) M) inhibited TdR in a dose-dependent manner. Although Rp-cAMPS (10(-6)-10(-4) M) itself did not affect TdR, it significantly blocked dbcAMP-, cholera toxin- and Sp-cAMPS-induced suppression of TdR. Moreover, Rp-cAMPS (10(-6)-10(-4) M) dose-dependently antagonized hPTH-induced suppression of TdR. Present studies first indicated that the activation of PKA is directly linked to the change of osteoblast proliferation by PTH.

Effects of parathyroid hormone on ornithine decarboxylase activity in human osteosarcoma cells

Ornithine decarboxylase (ODC) is a rate-limiting enzyme of the biosynthesis of polyamines, which are important in cell growth and differentiation. Here, we studied whether parathyroid hormone (PTH) affects the induction of ODC and the proliferation of the human osteoblast-like cell line SaOS2, which is sensitive to PTH. In confluent cells, ODC activity was not detected, but activity was significantly induced by fresh medium, with maximum activity 6 h after the change. PTH potentiated this enzyme induction in a dose-dependent manner at 10(-9) and 10(-8) M at which range the intracellular cAMP level also rose. Dibutyryl cAMP, cholera toxin, and 3-isobutyl-1-methylxanthine each caused an increase in ODC activity similar to that with PTH. The half-life of enzyme activity was about 30 min and was not changed by the addition of PTH. mRNA coding for ODC was detected in the confluent cells and its concentration was increased two- to threefold by the fresh medium. No further increase in mRNA occurred when PTH was added. At 48 h after the change of medium, PTH inhibited the DNA synthesis induced by fresh medium. These results suggest that the increase in ODC activity caused by PTH was caused by enhancement of cAMP synthesis, and that this augmentation involves post-transcriptional regulation.

Bone cell responsiveness to transforming growth factor beta, parathyroid hormone, and prostaglandin E2 in normal and postmenopausal osteoporotic women

We have shown previously that the decreased trabecular bone formation in osteoporotic postmenopausal women results from a reduced ability of osteoblastic cells to proliferate. In this study we have tested the possibility that bone cells from osteoporotic women with low bone formation have an abnormal responsiveness to hormonal or local mitogenic factors. Primary cultures of bone cells with osteoblastic characteristics were obtained by migration from the trabecular bone surface in osteoporotic postmenopausal women with high (n = 7) or low (n = 7) bone formation as evaluated histomorphometrically by the extent of double tetracycline-labeled surface (DLS). Control bone cells were obtained under identical conditions from eight normal age-matched postmenopausal women. Parameters of osteoblastic differentiation (alkaline phosphatase activity and osteocalcin production) were found to be normal and similar in bone cells from osteoporotic women with low or high DLS. In contrast, cell replication as evaluated by [3H]thymidine into DNA was 3.4-fold lower in the low DLS group compared to the high DLS group, confirming our previous findings. Treatment of quiescent bone cells with TGF-beta (0.5-1 ng/ml) for 24 h significantly stimulated DNA synthesis in osteoblastic cells from normal women and in bone cells from osteoporotic patients with low or high DLS, indicating a normal responsiveness to TGF-beta in these patients. We have compared the effect of parathyroidhormone (PTH) on bone cells from normal and osteoporotic women. Basal cAMP levels and the cAMP accumulation in response to (1-34)-hPTH were similar in bone cells from patients with low or high DLS and were not different from normal values.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of insulin and parathyroid hormone on DNA synthesis and ornithine decarboxylase activity in cultured bovine dental pulp

The effects of insulin and parathyroid hormone (PTH) on the proliferation of developing bovine dental pulp in an explant culture system were studied. Dental pulp explants were cultured on siliconized lens paper floating on the serum-free medium for up to 72 h. Ornithine decarboxylase (ODC) activity increased and reached a peak after 24 h. DNA synthesis increased continuously after a lag period of 24 h. Insulin (10 milliunits per ml) stimulated ODC activity 1.3-fold and DNA synthesis 1.5-fold. PTH alone (1 unit per ml) stimulated ODC activity in 1.7-fold, but did not affect DNA synthesis. PTH plus insulin caused greater increases in ODC activity and DNA synthesis in dental pulp explants than insulin alone (ODC, 2.6-fold; DNA, 3.7-fold). These results suggest that insulin and PTH are involved in the regulation of growth of dentinogenically active bovine dental pulp.

Regulation of Na-dependent Pi transport by parathyroid hormone in osteoblast-like cells

In the present work we investigated the influence of parathyroid hormone (PTH) on the transport of inorganic phosphate (Pi) in the osteoblast-like cell line UMR-106. Pi was transferred from the extra- to the intracellular compartment by means of a Na-dependent transport system with an apparent binding affinity for both Pi and Na similar to that recently observed in the osteoblast-like cell line ROS 17/2.8 (Calcif. Tissue Int. 43: 83-87, 1988). Exposure of confluent UMR-106 cells to PTH (10(-9)-10(-7) M) induced a concentration-related stimulation of the Na-dependent Pi transport (NaPiT). An increase in NaPiT was observed after a 1-h exposure to 10(-7) M PTH, with the maximal response occurring at 4-6 h. (PTH, 35.6 +/- 0.3; vehicle, 27.4 +/- 0.2 pmol.microgram DNA-1.4 min-1, P less than 0.001). The stimulatory effect of PTH on NaPiT was not associated with a change in the Na-dependent alanine transport. A positive correlation was observed between the increase of NaPiT and that of cAMP in response to various concentrations of PTH. Stimulation of cAMP by forskolin (10(-4) M) mimicked the effect of PTH on NaPiT. Kinetic analysis of the PTH-induced stimulation of NaPiT indicated an increase in Vmax (PTH, 226.9 +/- 6.9; vehicle, 182.9 +/- 1.9 pmol Pi/microgram DNA, P less than 0.001), with no change in Km. The increase in NaPiT by either PTH or forskolin was followed by a transient inhibition from 6 to 24 h that was associated with a decrease in the Na-dependent alanine transport.(ABSTRACT TRUNCATED AT 250 WORDS)