Conditionally immortalized murine osteoblasts lacking the type 1 PTH/PTHrP receptor

Decarboxylated osteocalcin, a possible drug for type 2 diabetes, triggers glucose uptake in MG63 cells

BACKGROUND

Uncarboxylated osteocalcin (GluOC) has been reported to improve glucose metabolism, prevent type 2 diabetes, and decrease the severity of obesity in mice with type 2 diabetes. GluOC can increase glucose uptake in a variety of cells. Glucose metabolism is the main source of energy for osteoblast proliferation and differentiation. We hypothesized that decarboxylated osteocalcin (dcOC), a kind of GluOC, can increase glucose uptake in MG63 cells (osteoblast-like osteosarcoma cells) and influence their proliferation and differentiation.

AIM

To investigate the effects of dcOC on glucose uptake in human osteoblast-like osteosarcoma cells and the possible signaling pathways involved.

METHODS

MG63 cells (human osteoblast-like osteosarcoma cells) were treated with dcOC (0, 0.3, 3, 10, or 30 ng/mL) for 1 and 72 h, and glucose uptake was measured by flow cytometry. The effect of dcOC on cell proliferation was measured with a CCK-8 assay, and alkaline phosphatase (ALP) enzyme activity was measured. PI3K was inhibited with LY294002, and hypoxia-inducible factor 1 alpha (HIF-1α) was silenced with siRNA. Then, GPRC6A (G protein-coupled receptor family C group 6 subtype A), total Akt, phosphorylated Akt, HIF-1α, and glucose transporter 1 (GLUT1) levels were measured by Western blot to elucidate the possible pathways by which dcOC modulates glucose uptake.

RESULTS

The glucose uptake of MG63 cells was significantly increased compared with that of the paired control cells after short-term (1 h) treatment with dcOC at different concentrations (0.3, 3, and 10 ng/mL groups, P < 0.01; 30 ng/mL group, P < 0.05). Glucose uptake of MG63 cells was significantly increased compared with that of the paired control cells after long-term (72 h) treatment with dcOC at different concentrations (0.3, 3, and 10 ng/mL groups, P < 0.01; 30 ng/mL group, P < 0.05). DcOC triggered Akt phosphorylation in a dose-dependent manner, and the most effective stimulatory concentration of dcOC for short-term (1 h) was 3 ng/mL (P < 0.01). LY294002 abolished the dcOC-mediated (1 h) promotion of Akt phosphorylation and glucose uptake without affecting GLUT1 protein expression. Long-term dcOC stimulation triggered Akt phosphorylation and increased the protein levels of HIF-1α, GLUT1, and Runx2 in a dose-dependent manner. Inhibition of HIF-1α with siRNA abolished the dcOC-mediated glucose uptake and substantially decreased GLUT1 protein expression. DcOC intervention promoted cell proliferation in a time- and dose-dependent manner as determined by the CCK-8 assay. Treatment with both 3 ng/mL and 10 ng/mL dcOC affected the ALP activity in MG63 cells after 72 h (P < 0.01).

CONCLUSION

Short- and long-term dcOC treatment can increase glucose uptake and affect proliferation and ALP activity in MG63 cells. This effect may occur through the PI3K/Akt, HIF-1α, and GLUT1 signaling factors.

PERK-mediated Autophagy in Osteosarcoma Cells Resists ER Stress-induced Cell Apoptosis

Osteosarcoma is a bone cancer that develops commonly in children and adolescents. However, osteosarcoma treatments often fail by the development of chemoresistance to apoptosis, and the molecular mechanisms remain unclear. In this study, we propose that autophagy is responsible for osteosarcomatous resistance to apoptosis. We implicate PERK-mediated autophagy as a significant contributor to apoptosis resistance due to ER stress in osteosarcoma cells. By immunostainings and western blots, we identified that PERK activated osteosarcomatous autophagy via inhibiting mTORC1 pathway, thereby preventing cell apoptosis. While using RNAi, we knocked down PERK and found that autophagy was suppressed, result in osteosarcomatous apoptosis. Our results identify a novel role of PERK-mediated autophagy as a significant mechanism for osteosarcoma cell survival. These results will help to understand the mechanism of chemoresistance in osteosarcoma cells, and indicate a novel target for improving osteosarcoma therapy.

Nuclear translocation of CBP/p300-interacting protein CITED1 induced by parathyroid hormone requires serine phosphorylation at position 79 in its 63-84 domain

The transcriptional cofactor CITED1 inhibits osteoblastic differentiation and blunts the stimulation of osteoblastic differentiation by parathyroid hormone (PTH). In the MC3T3-E1 osteoblastic cell line, we found that CITED1 was located predominantly in the cytoplasm and that hPTH(1-34) increased translocation of CITED1 from the cytoplasm to the nucleus. This response to hPTH(1-34) was not observed when all 9 serine residues within the 63-84 domain of CITED1 were mutated to alanines (CITED1 9S>A) or when a single serine to alanine mutation was made at position 79 (CITED1 S(79)>A). CITED1 containing mutations of these 9 serines to glutamic acid (9S>E) retained the same nuclear translocation response to hPTH(1-34) as the wild type CITED1. ALP activity and formation of mineralized nodules were inhibited in cells transfected with pcDNA3-CFP-CITED1 or with pcDNA3-CFP-CITED1 9S>E with or without hPTH(1-34) treatment (all P<0.05); these changes were not observed using CITED1 9S>A. Cells exposed to intermittent treatment with hPTH(1-34) expressed more ALP2, Runx2 and osteocalcin than vehicle-treated cells. These effects of hPTH(1-34) were inhibited in cells transfected with pcDNA3-CFP-CITED1 or pcDNA3-CFP-CITED1 9S>E, but were slightly enhanced by the alanine mutants. PKC activator (TPA) increased nuclear translocation of CITED1, whereas a PKC inhibitor (Go6983) blunted the effect of hPTH(1-34) on the nuclear translocation of wildtype CITED1 but not of CITED1 S(79)>E. The data indicated that serine phosphorylation at position 79 in the 63-84 domain is associated with PKC activation, and is required for both CITED1 nuclear translocation induced by PTH and the negative effects of CITED1 on osteoblastic differentiation and mineralization.

Role of parathyroid hormone-related protein in the pro-inflammatory and pro-fibrogenic response associated with acute pancreatitis

Pancreatitis is a common and potentially lethal necro-inflammatory disease with both acute and chronic manifestations. Current evidence suggests that the accumulated damage incurred during repeated bouts of acute pancreatitis (AP) can lead to chronic disease, which is associated with an increased risk of pancreatic cancer. While parathyroid hormone-related protein (PTHrP) exerts multiple effects in normal physiology and disease states, its function in pancreatitis has not been previously addressed. Here we show that PTHrP levels are transiently elevated in a mouse model of cerulein-induced AP. Treatment with alcohol, a risk factor for both AP and chronic pancreatitis (CP), also increases PTHrP levels. These effects of cerulein and ethanol are evident in isolated primary acinar and stellate cells, as well as in the immortalized acinar and stellate cell lines AR42J and irPSCc3, respectively. Ethanol sensitizes acinar and stellate cells to the PTHrP-modulating effects of cerulein. Treatment of acinar cells with PTHrP (1-36) increases expression of the inflammatory mediators interleukin-6 (IL-6) and intracellular adhesion protein (ICAM-1), suggesting a potential autocrine loop. PTHrP also increases apoptosis in AR42J cells. Stellate cells mediate the fibrogenic response associated with pancreatitis; PTHrP (1-36) increases procollagen I and fibronectin mRNA levels in both primary and immortalized stellate cells. The effects of cerulein and ethanol on levels of IL-6 and procollagen I are suppressed by the PTH1R antagonist, PTHrP (7-34). Together these studies identify PTHrP as a potential mediator of the inflammatory and fibrogenic responses associated with alcoholic pancreatitis.

Development and characterization of a mouse floxed Bmp2 osteoblast cell line that retains osteoblast genotype and phenotype

Bone morphogenetic protein 2 (Bmp2) is essential for osteoblast differentiation and osteogenesis. Generation of floxed Bmp2 osteoblast cell lines is a valuable tool for studying the effects of Bmp2 on osteoblast differentiation and its signaling pathways during skeletal metabolism. Due to relatively limited sources of primary osteoblasts, we have developed cell lines that serve as good surrogate models for the study of osteoblast cell differentiation and bone mineralization. In this study, we established and characterized immortalized mouse floxed Bmp2 osteoblast cell lines. Primary mouse floxed Bmp2 osteoblasts were transfected with pSV3-neo and clonally selected. These transfected cells were verified by PCR and immunohistochemistry. To determine the genotype and phenotype of the immortalized cells, cell morphology, proliferation, differentiation and mineralization were analyzed. Also, expression of osteoblast-related gene markers including Runx2, Osx, ATF4, Dlx3, bone sialoprotein, dentin matrix protein 1, osteonectin, osteocalcin and osteopontin were examined by quantitative RT-PCR and immunohistochemistry. These results showed that immortalized floxed Bmp2 osteoblasts had a higher proliferation rate but preserved their genotypic and phenotypic characteristics similar to the primary cells. Thus, we, for the first time, describe the development of immortalized mouse floxed Bmp2 osteoblast cell lines and present a useful model to study osteoblast biology mediated by BMP2 and its downstream signaling transduction pathways.

Meningioma 1 is required for appropriate osteoblast proliferation, motility, differentiation, and function

The vitamin D endocrine system is essential for calcium and phosphate homeostasis and skeletal mineralization. The 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) hormone binds to the vitamin D receptor (VDR) to regulate gene expression. These gene products in turn mediate the actions of 1,25(OH)(2)D(3) in mineral-regulating target cells such as the osteoblast. We showed previously that meningioma 1 (MN1) is a novel target of 1,25(OH)(2)D(3) in MG-63 osteoblastic cells and that it is a coactivator for VDR-mediated transcription (Sutton, A. L., Zhang, X., Ellison, T. I., and MacDonald, P. N. (2005) Mol. Endocrinol. 19, 2234-2244). However, the functional significance of MN1 in osteoblastic cell biology is largely unknown. Here, we demonstrate that MN1 expression is increased dramatically during differentiation of primary osteoblastic cells. Using calvarial osteoblasts derived from wild-type and MN1 knock-out mice, we provide data supporting an essential role of MN1 in maintaining appropriate osteoblast proliferation, differentiation, and function. MN1 knock-out osteoblasts displayed altered morphology, decreased growth rate, impaired motility, and attenuated 1,25(OH)(2)D(3)/VDR-mediated transcription as well as reduced alkaline phosphatase activity and mineralized nodule formation. MN1 null osteoblasts were also impaired in supporting osteoclastogenesis in co-culture studies presumably because of marked reduction in the RANKL:OPG ratio in the MN1 null cells. Mechanistic studies supported a transcriptional role for MN1 in controlling RANKL gene expression through activation of the RANKL promoter. Cumulatively, these studies indicate an important role for MN1 in maintaining the appropriate maturation and function of calvarial osteoblasts.

CBP/p300-interacting protein CITED1 modulates parathyroid hormone regulation of osteoblastic differentiation

PTH regulates osteoblastic differentiation and activity and exerts different overall skeletal effects in vivo, depending on the schedule and dose of administration. In clonal Wt9 murine osteoblastic cells, mRNA and protein levels of CITED1 transcriptional coactivator were strongly up-regulated by human (h) PTH(1-34). Stimulation of CITED1 mRNA by PTH was transient, peaking at 4 h, concentration dependent, and blocked by actinomycin D but not cycloheximide. The stimulation was mimicked by forskolin, phorbol ester, and the cAMP-selective PTH analog [G(1),R(19)] hPTH (1-28) and inhibited completely by the protein kinase A inhibitor, H89 and partially by phorbol ester-induced protein kinase C depletion. Increased CITED1 expression was not maintained during persistent (24 h) PTH exposure. Cultured primary calvarial osteoblasts from neonatal homozygous or hemizygous CITED1-knockout (KO) mice achieved 2-fold greater mineralized nodule formation in comparison with wild type (WT) osteoblasts. This effect was blocked by restoration of CITED1 expression via adenoviral gene transfer. Intermittent administration of hPTH(1-34) (10 nm, for 4 h every 48 h) for 3-6 wk increased mineralization up to 2-fold over basal levels in both WT and CITED1 KO mouse calvarial cell cultures. Whereas the cAMP-selective [G(1),R(19)]hPTH(1-28) analog [at 100 nm, equivalent to 10 nm hPTH(1-34)] did not stimulate mineralization in WT cultures, it was twice as effective as hPTH(1-34) in CITED1 KO cultures. Thus, CITED1 negatively regulates osteoblastic differentiation in vitro and inhibits the cAMP-dependent stimulation of differentiation by intermittent PTH. We conclude also that PTH receptor signaling pathways independent of cAMP restrain osteoblastic differentiation, an effect normally obscured in the presence of CITED1 but revealed in its absence.

Semaphorin 3B is a 1,25-Dihydroxyvitamin D3-induced gene in osteoblasts that promotes osteoclastogenesis and induces osteopenia in mice

The vitamin D endocrine system is important for skeletal homeostasis. 1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] impacts bone indirectly by promoting intestinal absorption of calcium and phosphate and directly by acting on osteoblasts and osteoclasts. Despite the direct actions of 1,25(OH)(2)D(3) in bone, relatively little is known of the mechanisms or target genes that are regulated by 1,25(OH)(2)D(3) in skeletal cells. Here, we identify semaphorin 3B (SEMA3B) as a 1,25(OH)(2)D(3)-stimulated gene in osteoblastic cells. Northern analysis revealed strong induction of SEMA3B mRNA by 1,25(OH)(2)D(3) in MG-63, ST-2, MC3T3, and primary osteoblastic cells. Moreover, differentiation of these osteogenic cells enhanced SEMA3B gene expression. Biological effects of SEMA3B in the skeletal system have not been reported. Here, we show that osteoblast-derived SEMA3B alters global skeletal homeostasis in intact animals and osteoblast function in cell culture. Osteoblast-targeted expression of SEMA3B in mice resulted in reduced bone mineral density and aberrant trabecular structure compared with nontransgenic littermates. Histomorphometry studies indicated that this was likely due to increased osteoclast numbers and activity. Indeed, primary osteoblasts obtained from SEMA3B transgenic mice stimulated osteoclastogenesis to a greater extent than nontransgenic osteoblasts. This study establishes that SEMA3B is a 1,25(OH)(2)D(3)-induced gene in osteoblasts and that osteoblast-derived SEMA3B impacts skeletal biology in vitro and in vivo. Collectively, these studies support a putative role for SEMA3B as an osteoblast protein that regulates bone mass and skeletal homeostasis.

Synthesis and characterization of novel biotinylated carboxyl-terminal parathyroid hormone peptides that specifically crosslink to the CPTH-receptor

Parathyroid hormone (PTH) regulates calcium, phosphorous and skeletal homeostasis via interaction with the G protein-coupled PTH/PTHrP receptor, which is fully activated by the amino-terminal 34 amino-acid portion of the hormone. Recent evidence points to the existence of another class of receptors for PTH that recognize the carboxyl (C)-terminal region of intact PTH (1-84) (CPTHRs) and are highly expressed by osteocytes. Here we report the synthesis and characterization of two novel bifunctional CPTH ligands that include benzoylphenylalanine (Bpa) substitutions near their amino-termini and carboxyl-terminal biotin moieties, as well as a tyrosine(34) substitution to enable radioiodination. These peptides are shown to bind to CPTHRs with affinity similar to that of PTH (1-84) and to be specifically and covalently crosslinked to CPTHRs upon exposure to ultraviolet light. Crosslinking to osteocytes or osteoblastic cells generates complexes of 80 and 220 kDa, of which the larger form represents an aggregate that can be resolved into the 80 kDa. The crosslinked products can be further purified using immunoaffinity and avidin-based affinity procedures. While the molecular structure of the CPTHR(s) remains undefined, these bifunctional ligands represent powerful new tools for use in isolating and characterizing CPTHR protein(s).

Role of calcium channels in carboxyl-terminal parathyroid hormone receptor signaling

Parathyroid hormone (PTH), an 84-amino acid polypeptide, is a major systemic regulator of calcium homeostasis that activates PTH/PTHrP receptors (PTH1Rs) on target cells. Carboxyl fragments of PTH (CPTH), secreted by the parathyroids or generated by PTH proteolysis in the liver, circulate in blood at concentrations much higher than intact PTH-(1–84) but cannot activate PTH1Rs. Receptors specific for CPTH fragments (CPTHRs), distinct from PTH1Rs, are expressed by bone cells, especially osteocytes. Activation of CPTHRs was previously reported to modify intracellular calcium within chondrocytes. To further investigate the mechanism of action of CPTHRs in osteocytes, cytosolic free calcium concentration ([Ca2+]i) was measured in the PTH1R-null osteocytic cell line OC59, which expresses abundant CPTHRs but no PTH1Rs. [Ca2+]iwas assessed by single-cell ratiometric microfluorimetry in fura-2-loaded OC59 cells. A rapid and transient increase in [Ca2+]iwas observed in OC59 cells in response to the CPTH fragment hPTH-(53–84) (250 nM). No [Ca2+]isignal was observed in COS-7 cells, in which CPTHR binding also cannot be detected. Neither hPTH-(1–34) nor a mutant CPTH analog, [Ala55–57]hPTH-(53–84), that does not to bind to CPTHRs, increased [Ca2+]iin OC59 cells. The [Ca2+]iresponse to hPTH-(53–84) required the presence of extracellular calcium and was blocked by inhibitors of voltage-dependent calcium channels (VDCCs), including nifedipine (100 nM), ω-agatoxin IVA (10 nM), and ω-conotoxin GVIA (100 nM). We conclude that activation of CPTHRs in OC59 osteocytic cells leads to a rapid increase in influx of extracellular calcium, most likely through the opening of VDCCs.

Response to mechanical strain in an immortalized pre-osteoblast cell is dependent on ERK1/2

Mechanical strain inhibits osteoclastogenesis by regulating osteoblast functions: We have shown that strain inhibits receptor activator of NF-kappaB ligand (RANKL) expression and increases endothelial nitric oxide synthase (eNOS) and nitric oxide levels through ERK1/2 signaling in primary bone stromal cells. The primary stromal culture system, while contributing greatly to understanding of how the microenvironment regulates bone remodeling is limited in use for biochemical assays and studies of other osteoprogenitor cell responses to mechanical strain: Stromal cells proliferate poorly and lose aspects of the strain response after a relatively short time in culture. In this study, we used the established mouse osteoblast cell line, conditionally immortalized murine calvarial (CIMC-4), harvested from mouse calvariae conditionally immortalized by insertion of the gene coding for a temperature-sensitive mutant of SV40 large T antigen (TAg) and support osteoclastogenesis. Mechanical strain (0.5-2%, 10 cycles per min, equibiaxial) caused magnitude-dependent decreases in RANKL expression to less than 50% those of unstrained cultures. Overnight strains of 2% also increased osterix (OSX) and RUNX2 expression by nearly twofold as measured by RT-PCR. Importantly, the ERK1/2 inhibitor, PD98059, completely abrogated the strain effects bringing RANKL, OSX, and RUNX2 gene expression completely back to control levels. These data indicate that the strain effects on CIMC-4 cells require activation of ERK1/2 pathway. Therefore, the CIMC-4 cell line is a useful alternative in vitro model which effectively recapitulates aspects of the primary stromal cells and adds an extended capacity to study osteoblast control of bone remodeling in a mechanically active environment.

Wdr5, a WD-40 protein, regulates osteoblast differentiation during embryonic bone development

Wdr5 accelerates osteoblast and chondrocyte differentiation in vitro, and is developmentally expressed in osteoblasts as well as in proliferating and hypertrophic chondrocytes. To investigate the role of Wdr5 during endochondral bone development, transgenic mice overexpressing Wdr5 under the control of the 2.3-kb fragment of the mouse alpha(1) I collagen promoter were generated. The transgene was specifically expressed in the osteoblasts of transgene positive mice and was absent in the growth plate. Histological analyses at embryonic day 14.5 demonstrated that the humeri of transgene positive embryos were longer than those isolated from wild-type littermates largely due to an expansion of the hypertrophic chondrocyte layer. Acceleration of osteoblast differentiation was observed with greater and more extensive expression of type I collagen and more extensive mineral deposition in the bone collar of transgene positive embryos. Acceleration of vascular invasion was also observed in transgene positive mice. Postnatal analyses of transgenic mice confirmed persistent acceleration of osteoblast differentiation. Targeted expression of Wdr5 to osteoblasts resulted in earlier activation of the canonical Wnt signaling pathway in the bone collar as well as in primary calvarial osteoblast cultures. In addition, overexpression of Wdr5 increased the expression of OPG, a target of the canonical Wnt signaling pathway. Overall, our findings suggest that Wdr5 accelerates osteoblast differentiation in association with activation of the canonical Wnt pathway.

PTH(1–84)/PTH(7–84): a balance of power

This review considers many new basic and clinical aspects of parathyroid hormone (PTH). We focus especially on the identification of PTH fragments and how they may relate to renal failure, diagnosis, and treatment of secondary hyperparathyroidism and renal osteodystrophy. The biosynthesis and metabolism of PTH, measurement of circulating forms of PTH, the effects of PTH on receptor activation and turnover, the relationship between PTH levels and bone turnover in renal failure in humans, and the involvement of PTH in experimental models of renal failure are discussed. Despite these developments in understanding the etiology of renal failure and the availability of new assays for bioactive PTH, no adequate surrogate for bone biopsy and quantitative bone histomorphometry has been developed.

Ubiquitin ligase Cbl-b downregulates bone formation through suppression of IGF-I signaling in osteoblasts during denervation

Unloading can prevent bone formation by osteoblasts. To study this mechanism, we focused on a ubiquitin ligase, Cbl-b, which was highly expressed in osteoblastic cells during denervation. Our results suggest that Cbl-b may mediate denervation-induced osteopenia by inhibiting IGF-I signaling in osteoblasts.

INTRODUCTION

Unloading, such as denervation (sciatic neurectomy) and spaceflight, suppresses bone formation by osteoblasts, leading to osteopenia. The resistance of osteoblasts to growth factors contributes to such unloading-mediated osteopenia. However, a detailed mechanism of this resistance is unknown. We first found that a RING-type ubiquitin ligase, Cbl-b, was highly expressed in osteoblastic cells after sciatic neurectomy in mice. In this study, we reasoned that Cbl-b played an important role in the resistance of osteoblasts to IGF-I.

MATERIALS AND METHODS

Cbl-b-deficient (Cbl-b(-/-)) or wildtype (Cbl-b(+/+)) mice were subjected to sciatic neurectomy. Bone formation in these mice was assessed by calcein labeling and histomorphometric analyses. We examined IGF-I signaling molecules in femora of these mice by Western blot and immunohistochemical analyses. We also examined the mitogenic response of Cbl-b-overexpressing or -deficient osteoblastic cells to various growth factors.

RESULTS

In Cbl-b(+/+) mice, denervation decreased femur mass and bone formation, whereas it increased the expression of Cbl-b protein in osteoprogenitor cells and in osteocalcin-positive cells (osteoblastic cells) in hindlimb bone. In contrast, in Cbl-b(-/-) mice, bone mass and bone formation were sustained during denervation. Denervation inhibited the mitogenic response of osteoprogenitor cells most significantly to IGF-I. Therefore, we focused on Cbl-b-mediated modification of IGF-I signaling. Denervation decreased the amounts of insulin receptor substrate-1 (IRS-1), phosphatidly inositol 3-phosphate kinase (PI3K), and Akt-1 proteins in femora of Cbl-b(+/+) mice, whereas the amounts of these IGF-I signaling molecules in femora of Cbl-b(-/-) mice were constant after denervation. On a cellular level, primary osteoblastic cells from Cbl-b(-/-) mice were more stimulated to proliferate by IGF-I treatment compared with those from Cbl-b(+/+) mice. Furthermore, overexpression of Cbl-b increased ubiquitination and degradation of IRS-1 in primary Cbl-b(-/-) osteoblastic cells, leading to their impaired mitogenic response to IGF-I.

CONCLUSIONS

These results suggest that Cbl-b induces resistance of osteoblasts to IGF-I during denervation by increasing IRS-1 degradation and that Cbl-b-mediated modification of IGF-I signaling may contribute to decreased bone formation during denervation.

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.

The receptor for parathyroid hormone and parathyroid hormone-related peptide is hydrolyzed and its signaling properties are altered by directly binding the calpain small subunit

We show calcium-dependent, direct binding between the N-terminal portion of the PTH/PTHrP receptor (PTH1R) C-terminal intracellular tail and the calpain small subunit. Binding requires, but may not be limited to, amino acids W474, S475, and W477. The wild-type, full-length rat (r) PTH1R, but not rPTH1R with W474A/W477A substitutions, copurifies with the endogenous calpain small subunit in HEK293 cells. Calpain hydrolyzes delta Nt-rPTH1R, a receptor with a 156-amino acid N-terminal deletion, in a calcium-dependent manner in vitro and in intact cells. Most importantly, PTH stimulation increases the cleavage of delta Nt-rPTH1R and rPTH1R-yellow fluorescent protein in HEK293 cells, and of talin in HEK293 cells expressing rPTH1R-yellow fluorescent protein and in ROS17/2.8 osteoblast-like cells that express rPTH1R endogenously. The absence of calpain in Capn4-null embryonic fibroblasts and the lowered calpain activity in MC3T3-E1 osteoblastic cells due to stable expression of the calpain inhibitor, calpastatin, reduce PTH-stimulated cAMP accumulation. The calpain small subunit is the second protein, in addition to the sodium-hydrogen exchanger regulatory factor, and the first enzyme that binds the PTH1R; PTH1R bound to both of these proteins results in altered PTH signaling.

Carboxy-terminal PTH fragments stimulate [3H]thymidine incorporation in vascular endothelial cells

We have previously reported that the intact PTH molecule (1-84) stimulates proliferation of human umbilical vein endothelial cells (HUVECs). To define the bioactive portion of the PTH molecule we utilized amino, mid and carboxy-terminal PTH fragments. Carboxy- but not amino-terminal fragments were equivalent to the intact PTH molecule in stimulating [3H]thymidine incorporation in HUVEC. Carboxy- but not amino-terminal PTH fragments increased intracellular calcium. Blocking the rise in intracellular calcium with calcium chelators abolished PTHs proliferative effect on HUVEC. In contrast to PTH 1-84, the carboxy-terminal fragment effect on [3H]thymidine incorporation was blocked by KN-93 an inhibitor of CaM kinase II. Taken together, these data suggest that the carboxy-terminal PTH is (or contains) the bioactive fragment responsible for the changes in intracellular calcium and thymidine incorporation in HUVEC stimulated with the intact PTH molecule.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Functional cooperation between CCAAT/enhancer-binding proteins and the vitamin D receptor in regulation of 25-hydroxyvitamin D3 24-hydroxylase

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] induces the synthesis of 25-hydroxyvitamin D(3) 24-hydroxylase [24(OH)ase], an enzyme involved in its catabolism, thereby regulating its own metabolism. Here we demonstrate that CCAAT enhancer binding protein beta (C/EBPbeta) is induced by 1,25(OH)(2)D(3) in kidney and in osteoblastic cells and is a potent enhancer of vitamin D receptor (VDR)-mediated 24(OH)ase transcription. Transfection studies indicate that 1,25(OH)(2)D(3) induction of 24(OH)ase transcription is enhanced a maximum of 10-fold by C/EBPbeta. Suppression of 1,25(OH)(2)D(3)-induced 24(OH)ase transcription was observed with dominant negative C/EBP or osteoblastic cells from C/EBPbeta(-/-) mice. A C/EBP site was identified at positions -395 to -388 (-395/-388) in the rat 24(OH)ase promoter. Mutation of this site inhibited C/EBPbeta binding and markedly attenuated the transcriptional response to C/EBPbeta. We also report the cooperation of CBP/p300 with C/EBPbeta in regulating VDR-mediated 24(OH)ase transcription. We found that not only 1,25(OH)(2)D(3) but also parathyroid hormone (PTH) can induce C/EBPbeta expression in osteoblastic cells. PTH potentiated the induction of C/EBPbeta and 24(OH)ase expression in response to 1,25(OH)(2)D(3) in osteoblastic cells. Data with the human VDR promoter (which contains two putative C/EBP sites) indicate a role for C/EBPbeta in the protein kinase A-mediated induction of VDR transcription. From this study a fundamental role has been established for the first time for cooperative effects and cross talk between the C/EBP family of transcription factors and VDR in 1,25(OH)(2)D(3)-induced transcription. These findings also indicate a novel role for C/EBPbeta in the cross talk between PTH and 1,25(OH)(2)D(3) that involves the regulation of VDR transcription.

Butyrate response factor 1 is regulated by parathyroid hormone and bone morphogenetic protein-2 in osteoblastic cells

Parathyroid hormone (PTH) exerts potent and diverse effects in bone and cartilage through activation of type 1 PTH receptors (PTH1R) capable of coupling to protein kinase A (PKA) and PKC. We have used macroarrays to identify zinc finger protein butyrate response factor-1 (BRF1) as a novel PTH regulated gene in clonal and normal osteoblasts of human and rodent origin. We further demonstrate that in human osteoblast-like OHS cells, biologically active hPTH(1-84) and hPTH(1-34) stimulate BRF1 mRNA expression in a dose- and time-dependent manner, while the amino-terminally truncated hPTH(3-84) which does not activate PTH1R has no effect. Moreover, using specific stimulators or inhibitors of PKA and PKC activity, the PTH-elicited BRF1 mRNA expression is mediated through the PKA signaling pathway. In mouse calvarial osteoblasts, BRF1 mRNA levels are upregulated by PTH(1-84) and reduced in response to bone morphogenetic protein 2 (BMP-2). Hence, our data showing that BRF1 is expressed in osteoblastic cells and regulated by PTH and BMP-2, suggest an important role for BRF1 in osteoblasts within the molecular network of PTH-dependent bone remodeling.

Osteoblasts lacking the vitamin D receptor display enhanced osteogenic potential in vitro

1,25-dihydroxyvitamin D plays an important role in the regulation of osteoblast gene expression, regulating the expression of bone matrix proteins as well as that of Runx2, a key regulator of osteoblast differentiation. Studies in mice lacking the vitamin D receptor (VDR) have revealed that the actions of the VDR on the skeleton are not required in the setting of normal mineral ion homeostasis. Since paracrine and endocrine factors can compensate for gene defects in vivo, studies were performed to determine whether ablation of the VDR alters the program of osteoblast differentiation in vitro. Studies in primary calvarial cultures revealed that ablation of the VDR enhanced osteoblast differentiation. The cells from the VDR null mice exhibited an earlier onset and increased magnitude of alkaline phosphatase activity, as well as an earlier and sustained increase in mineralized matrix formation, demonstrating that this enhancement persists throughout the program of osteoblast differentiation. The expression of bone sialoprotein, which enhances mineralization, was also increased in the VDR null cultures. To determine whether the increase in osteoblast differentiation was associated with an increase in the number of osteogenic progenitors, the number of osteoblastic colony forming units (CFU-OB) was evaluated. There was a twofold increase in the number of CFU-OB in the cultures isolated from the VDR null mice. Furthermore, the VDR null CFU-OB demonstrated an earlier onset and higher magnitude of expression of alkaline phosphatase activity when compared to the CFU-OB from their wild-type control littermates. These studies demonstrate that the VDR attenuates osteoblast differentiation in vitro and suggest that other endocrine and paracrine factors modulate the effect of the VDR on osteoblast differentiation in vivo.

Hedgehog promotes primary osteoblast differentiation and increases PTHrP mRNA expression and iPTHrP secretion

We used both clonal osteoblast-like cells and primary calvarial osteoblastic cells to examine the role of Hedgehog in osteoblast biology. Primary osteoblasts and several clonal osteoblast-like cell lines express Indian hedgehog (Ihh), and genes encoding both components of its receptor, patched (Ptc) and smoothened (Smo). Moreover, Ihh is relatively increased in phenotypically mature clonal cells and it increases by fivefold in primary osteoblasts as they mature in culture. Recombinant N-terminal Sonic Hedgehog (rSHH-N) upregulates Ptc and Gli-1 in osteoblasts, classical transcriptional targets. Furthermore; in response to rSHH-N, immunoreactive parathyroid hormone-related peptide (iPTHrP) secretion is transiently increased in medium conditioned by primary osteoblasts. Changes in PTHrP expression mirror those of iPTHrP, except in late cultures, when mRNA levels remain relatively elevated in response to rSHH-N. Gli-1, but not Ptc, becomes resistant to treatment with rSHH-N over a time course paralleling that of PTHrP, suggesting that mechanisms regulated by Gli-1 affect PTHrP. Last, rSHH-N increases formation of mineralized bone nodules and it accelerates expression of alkaline phosphatase, alkaline phosphatase activity, and mineralization. Taken together, these data suggest a functional role for Hedgehog protein in osteoblast recruitment and differentiation, which includes stimulation of PTHrP expression and secretion.

Functional analysis of an arthritogenic synovial fibroblast

Increasing attention has been directed towards identifying non-T-cell mechanisms as potential therapeutic targets in rheumatoid arthritis. Synovial fibroblast (SF) activation, a hallmark of rheumatoid arthritis, results in inappropriate production of chemokines and matrix components, which in turn lead to bone and cartilage destruction. We have demonstrated that SFs have an autonomous pathogenic role in the development of the disease, by showing that they have the capacity to migrate throughout the body and cause pathology specifically to the joints. In order to decipher the pathogenic mechanisms that govern SF activation and pathogenic potential, we used the two most prominent methods of differential gene expression analysis, differential display and DNA microarrays, in a search for deregulated cellular pathways in the arthritogenic SF. Functional clustering of differentially expressed genes, validated by dedicated in vitro functional assays, implicated a number of cellular pathways in SF activation. Among them, diminished adhesion to the extracellular matrix was shown to correlate with increased proliferation and migration to this matrix. Our findings support an aggressive role for the SF in the development of the disease and reinforce the perspective of a transformed-like character of the SF.

Parathyroid hormone and parathyroid hormone-related protein exert both pro- and anti-apoptotic effects in mesenchymal cells

During bone formation, multipotential mesenchymal cells proliferate and differentiate into osteoblasts, and subsequently many die because of apoptosis. Evidence suggests that the receptor for parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), the PTH-1 receptor (PTH-1R), plays an important role in this process. Multipotential mesenchymal cells (C3H10T1/2) transfected with normal or mutant PTH-1Rs and MC3T3-E1 osteoblastic cells were used to explore the roles of PTH, PTHrP, and the PTH-1R in cell viability relative to osteoblastic differentiation. Overexpression of wild-type PTH-1R increased cell numbers and promoted osteocalcin gene expression versus inactivated mutant receptors. Furthermore, the effects of PTH and PTHrP on apoptosis were dramatically dependent on cell status. In preconfluent C3H10T1/2 and MC3T3-E1 cells, PTH and PTHrP protected against dexamethasone-induced reduction in cell viability, which was dependent on cAMP activation. Conversely, PTH and PTHrP resulted in reduced cell viability in postconfluent cells, which was also dependent on cAMP activation. Further, the proapoptotic-like effects were associated with an inhibition of Akt phosphorylation. These data suggest that parathyroid hormones accelerate turnover of osteoblasts by promoting cell viability early and promoting cell departure from the differentiation program later in their developmental scheme. Both of these actions occur at least in part via the protein kinase A pathway.

Tissue culture models for studies of hormone and vitamin action in bone cells

Osteoporosis is a major health care concern and levies a serious financial burden on the world health care system. For this reason, many physicians and scientists are engaged in research to better understand and treat this disease. To this end, numerous in vitro bone cell models have been developed to explore the cellular and molecular mechanisms of skeletal biology and for the identification and characterization of new drug targets and therapies. In this chapter, we review many of these cellular models as tools to study the hormonal regulation of bone metabolism. In particular, we pay special attention to new human bone cell models, since these have the greatest relevance to osteoporosis research and drug discovery. These new models include (1) the use of peripheral blood mononuclear cells as progenitors of osteoclasts and primary cultures of mesenchymal stem cells as precursors of osteoblasts; (2) the development of conditionally immortalized preosteoclastic and osteoblastic cell lines using temperature-sensitive large T-antigens; and (3) the establishment of the first osteocytic cell lines. Thus, we now have at our disposal many good in vitro models to investigate the regulation of bone resorption and formation by hormones, vitamins and drugs. These models should accelerate our understanding of bone physiology and pathophysiology as well as our ability to develop important new therapies to prevent and treat skeletal diseases.

Cloning and characterization of a novel WD-40 repeat protein that dramatically accelerates osteoblastic differentiation

The bone morphogenetic proteins (BMPs) play a pivotal role in endochondral bone formation. Using differential display polymerase chain reaction, we have identified a novel gene, named BIG-3 (BMP-2-induced gene 3 kb), that is induced as a murine prechondroblastic cell line, MLB13MYC clone 17, acquires osteoblastic features in response to BMP-2 treatment. The 3-kilobase mRNA encodes a 34-kDa protein containing seven WD-40 repeats. Northern and Western analyses demonstrated that BIG-3 mRNA and protein were induced after 24 h of BMP-2 treatment. BIG-3 mRNA was expressed in conditionally immortalized murine bone marrow stromal cells, osteoblasts, osteocytes, and growth plate chondrocytes, as well as in primary calvarial osteoblasts. Immunohistochemistry demonstrated that BIG-3 was expressed in the osteoblasts of calvariae isolated from mouse embryos. To identify a role for BIG-3 in osteoblast differentiation, MC3T3-E1 cells were stably transfected with the full-length coding region of BIG-3 (MC3T3E1-BIG-3) cloned downstream of a cytomegalovirus promoter in pcDNA3.1. Pooled MC3T3E1-BIG-3 clones expressed alkaline phosphatase activity earlier and achieved a peak level of activity 10-fold higher than cells transfected with the empty vector (MC3T3E1-EV) at 14 days. Cyclic AMP production in response to parathyroid hormone was increased 10- and 14-fold at 7 and 14 days, respectively, in MC3T3E1-BIG-3 clones, relative to MC3T3E1-EV clones. This increase in cAMP production was associated with an increase in PTH binding. Expression of BIG-3 increased mRNA levels encoding Cbfa1, type I collagen, and osteocalcin and accelerated formation of mineralized nodules. In conclusion, we have identified a novel WD-40 protein, induced by BMP-2 treatment, that dramatically accelerates the program of osteoblastic differentiation in stably transfected MC3T3E1 cells.

Parathyroid hormone-related protein is required for normal intramembranous bone development

It is well established that parathyroid hormone-related protein (PTHrP) regulates chondrocytic differentiation and endochondral bone formation. Besides its effect on cartilage, PTHrP and its major receptor (type I PTH/PTHrP receptor) have been found in osteoblasts, suggesting an important role of PTHrP during the process of intramembranous bone formation. To clarify this issue, we examined intramembranous ossification in homozygous PTHrP-knockout mice histologically. We also analyzed phenotypic markers of osteoblasts and osteoclasts in vitro and in vivo. A well-organized branching and anastomosing pattern was seen in the wild-type mice. In contrast, marked disorganization of the branching pattern of bone trabeculae and irregularly aligned osteoblasts were recognized in the mandible and in the bone collar of the femur of neonatal homozygous mutant mice. In situ hybridization showed that most of the osteoblasts along the bone surfaces of the wild-type mice and some of the irregularly aligned osteoblastic cells in the homozygous mice expressed osteocalcin. Alkaline phosphatase (ALP) activity and expression of osteopontin messenger RNA (mRNA) in primary osteoblastic cells did not show significant differences between cultures derived from the mixture of heterozygous mutant and wild-type mice (+/? mice) and those from homozygous mutant mice. However, both mRNA and protein levels of osteocalcin in the osteoblastic cells of homozygous mutant mice were lower than those of +/? mice, and exogenous PTHrP treatment corrected this suppression. Immunohistochemical localization of characteristic markers of osteoclasts and ruffled border formation did not differ between genotypes. Cocultures of calvarial osteoblastic cells and spleen cells of homozygous mutant mice generated an equivalent number of tartrate-resistant acid phosphatase-positive (TRAP+) mononuclear and multinucleated cells and of pit formation to that of +/? mice, suggesting that osteoclast differentiation is not impaired in the homozygous mutant mice. These results suggest that PTHrP is required not only for the regulation of cartilage formation but also for the normal intramembranous bone development.

Cloning and characterization of a novel protein kinase that impairs osteoblast differentiation in vitro

The bone morphogenic proteins (BMPs) play a key role in skeletal development and patterning. Using the technique of differential display polymerase chain reaction (ddPCR), we have identified a novel gene whose expression is increased during BMP-2-induced differentiation of the prechondroblastic cell line, MLB13MYC clone 17, to an osteoblastic phenotype. The 6.5-kilobase mRNA recognized by this ddPCR product is increased 10-fold by BMP-2 treatment of the MLB13MYC clone 17 cells. The mRNA recognized by this ddPCR product is also increased as MC3T3-E1 cells recapitulate the program of osteoblast differentiation during prolonged culture. The full-length transcript corresponding to this ddPCR product was cloned from a MLB13MYC clone 17 cell cDNA library. Analysis of the deduced amino acid sequence demonstrated that this gene encodes a novel 126-kDa putative serine/threonine protein kinase containing a nuclear localization signal. The kinase domain, expressed in Escherichia coli, is capable of autophosphorylation as well as phosphorylation of myelin basic protein. The gene was, therefore, named BIKe (BMP-2-Inducible Kinase). The BIKe nuclear localization signal is able to direct green fluorescent protein to the nucleus in transfected COS-7 cells. When stably expressed in MC3T3-E1 cells, BIKe significantly decreases alkaline phosphatase activity and osteocalcin mRNA levels and retards mineral deposition relative to vector control. This novel kinase, therefore, is likely to play an important regulatory role in attenuating the program of osteoblast differentiation.

Parathyroid hormone-related protein promotes quiescence and survival of serum-deprived chondrocytes by inhibiting rRNA synthesis

Parathyroid hormone-related protein (PTHrP) was initially recognized for its ability to promote parathyroid hormone-like bioactivity in kidney, bone, and squamous epithelial cells. PTHrP is a multifunctional protein in which bioactivity is mediated by two distinct pathways. Its classic parathyroid hormone-like activity results from binding of its amino terminus to cell surface PTH1R and activation of signal transduction pathways. Another less well recognized pathway involves translocation of PTHrP to the nucleus via a mid-region bipartite nuclear targeting sequence (NTS), similar in structure and function to those found in retroviral regulatory proteins. PTHrP was identified in the nucleus of several different cell types in vivo and in vitro, where it has been implicated in cell cycle progression, cellular differentiation, and apoptosis. In previous work we showed that nuclear translocation of PTHrP enhanced the survival of serum-deprived chondrogenic cells, associated with RNA, and localized to a region of the nucleus rich in complexes of newly transcribed ribosomal RNA and protein. In this work we have used two chondrogenic cell lines, CFK2 (PTH1R+) and 27m21 (PTH1R-) to further explore mechanisms whereby PTHrP rescues immature chondrocytes from apoptosis. Endogenous PTHrP and exogenous PTHrP NTS peptide protected serum-deprived cells from apoptosis, in the presence and absence of PTH1R. The survival of cells expressing PTHrP and those treated with PTHrP NTS peptide was associated with a rapid shift into G(o)/G1 accompanied by a significant down-regulation of rRNA synthesis and a decrease in the number of actively translating polyribosome complexes. Together with our previous observations, this work predicts a role for PTHrP in modulating ribosome biogenesis and preventing chondrogenic cells from progressing through the cell cycle in an unfavorable environment.

Fibromodulin is expressed by both chondrocytes and osteoblasts during fetal bone development

Fibromodulin, a keratan-sulfate proteoglycan, was first isolated in articular cartilage and tendons. We have identified fibromodulin as a gene regulated during BMP-2-induced differentiation of a mouse prechondroblastic cell line. Because expression of fibromodulin during endochondral bone formation has not been studied, we examined whether selected cells of the chondrocytic and osteoblastic lineage expressed fibromodulin. Fibromodulin mRNA was detected in conditionally immortalized murine bone marrow stromal cells, osteoblasts, and growth plate chondrocytes, as well as in primary murine calvarial osteoblasts. We, therefore, investigated the temporo-spatial expression of fibromodulin in vivo during endochondral bone formation by in situ hybridization. Fibromodulin was first detected at 15.5 days post coitus (dpc) in the perichondrium and proliferating chondrocytes. Fibromodulin mRNA was also detected at 15.5 dpc in the bone collar and periosteum. At later time points fibromodulin was expressed in the primary spongiosa and the endosteum. To determine whether fibromodulin was expressed during intramembranous bone formation as well, in situ hybridization was performed on calvariae. Fibromodulin mRNA was present in calvarial osteoblasts from 15.5 dpc. These results demonstrate that fibromodulin is developmentally expressed in cartilage and bone cells during endochondral and intramembranous ossification. These findings suggest that this extracellular matrix protein plays a role in both endochondral and intramembranous bone formation.

Signal-selectivity of parathyroid hormone (PTH)/PTH-related peptide receptor-mediated regulation of differentiation in conditionally immortalized growth-plate chondrocytes

Type-1 PTH/PTH-related peptide receptors (PTH1Rs), which activate both adenylyl cyclase and phospholipase C (PLC), control endochondral bone development by regulating chondrocyte differentiation. To directly analyze PTH1R function in such cells, we isolated conditionally transformed clonal chondrocytic cell lines from tibial growth plates of neonatal mice heterozygous for PTH1R gene ablation. Among 104 cell lines isolated, messenger RNAs for PTH1R, collagen II, and collagen X were detected in 28%, 90%, and 29%, respectively. These cell lines were morphologically diverse. Some appeared large, rounded, and enveloped by abundant extracellular matrix; whereas others were smaller, flattened, and elongated. Two PTH1R-expressing clones showed similar PTH1R binding and cAMP responsiveness to PTH and PTH-related peptide but disparate morphologic features, characteristic of hypertrophic (hC1--5) or nonhypertrophic (nhC2--27) chondrocytes, respectively. hC1--5 cells expressed messenger RNAs for collagen II and X, alkaline phosphatase (ALP), and matrix GLA protein, whereas nhC2--27 cells expressed collagen II and Indian hedgehog but not collagen X or ALP. In hC1--5 cells, PTH and cAMP analog, but not phorbol ester, inhibited both ALP and mineralization. PTH1R-null hC1--5 subclones were isolated by in vitro selection and then reconstituted by stable transfection with wild-type PTH1Rs or mutant (DSEL) PTH1Rs defective in PLC activation. ALP and mineralization were inhibited similarly via both forms of the receptor. These results indicate that PLC activation is not required for PTH1R regulation of mineralization or ALP in hypertrophic chondrocytes and are consistent with a major role for cAMP in regulating differentiation of hypertrophic chondrocytes.

Receptors for the carboxyl-terminal region of pth(1-84) are highly expressed in osteocytic cells

PTH is a potent systemic regulator of cellular differentiation and function in bone. It acts upon cells of the osteoblastic lineage via the G protein-coupled type-1 PTH/PTH-related peptide receptor (PTH1R). Carboxyl fragments of intact PTH(1-84) (C-PTH fragments) are cosecreted with it by the parathyroid glands in a calcium-dependent manner and also are generated via proteolysis of the hormone in peripheral tissues. Receptors that recognize C-PTH fragments (CPTHRs) have been described previously in osteoblastic and chondrocytic cells. To directly study CPTHRs in bone cells, we isolated clonal, conditionally transformed cell lines from fetal calvarial bone of mice that are homozygous for targeted ablation of the PTH1R gene and transgenically express a temperature-sensitive mutant SV40 T antigen. Cells with the highest specific binding of the CPTHR radioligand (125)I-[Tyr(34)]hPTH(19-84) exhibited a stellate, dendritic appearance suggestive of an osteocytic phenotype and expressed 6- to 10-fold more CPTHR sites/cell than did osteoblastic cells previously isolated from the same bones. In these osteocytic (OC) cells, expression of mRNAs for CD44, connexin 43, and osteocalcin was high, whereas that for alkaline phosphatase and cbfa-1/osf-2 was negligible. The CPTHR radioligand was displaced completely by hPTH(1-84), hPTH(19-84) and hPTH(24-84) (IC(50)s = 20-50 nM) and by hPTH(39-84) (IC(50) = 500 nM) but only minimally (24%) by 10,000 nM hPTH(1-34). CPTHR binding was down-regulated dose dependently by hPTH(1-84), an effect mimicked by ionomycin and active phorbol ester. Human PTH(1-84) and hPTH(39-84) altered connexin 43 expression and increased apoptosis in OC cells. Apoptosis induced by PTH(1-84) was blocked by the caspase inhibitor DEVD. We conclude that osteocytes, the most abundant cells in bone, may be principal target cells for unique actions of intact PTH(1-84) and circulating PTH C-fragments that are mediated by CPTHRs.

Sox-4 messenger RNA is expressed in the embryonic growth plate and regulated via the parathyroid hormone/parathyroid hormone-related protein receptor in osteoblast-like cells

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) exert potent and diverse effects in cells of the osteoblastic and chondrocytic lineages. However, downstream mediators of these effects are characterized inadequately. We identified a complementary DNA (cDNA) clone encoding the 5' end of the transcription factor Sox-4, using a subtracted cDNA library enriched in PTH-stimulated genes from the human osteoblast-like cell line OHS. The SOX-4 gene is a member of a gene family (SOX and SRY) comprising transcription factors that bind to DNA through their high mobility group (HMG)-type binding domain, and previous reports have implicated Sox proteins in various developmental processes. In situ hybridization of fetal and neonatal mouse hindlimbs showed that Sox-4 messenger RNA (mRNA) was expressed most intensely in the zone of mineralizing cartilage where chondrocytes undergo hypertrophy, and by embryonic day 17 (ED17), after the primary ossification center was formed, its expression was detected only in the region of hypertrophic chondrocytes. Sox-4 mRNA was detected in osteoblast-like cells of both human and rodent origin. In OHS cells, physiological concentrations (10(-10)-10(-9) M) of human PTH 1-84 [hPTH(1-84)] and hPT(1-34), but not hPTH(3-84), stimulated Sox-4 mRNA expression in a time-dependent manner, indicating involvement of the PTH/PTHrP receptor. Sox-4 transcripts also were detected in various nonosteoblastic human cell lines and tissues, in a pattern similar to that previously reported in mice. The presence of Sox-4 mRNA in hypertrophic chondrocytes within the mouse epiphyseal growth plate at sites that overlap or are adjacent to target cells for PTH and PTHrP, and its strong up-regulation via activated PTH/PTHrP receptors in OHS cells, makes it a promising candidate for mediating downstream effects of PTH and PTHrP in bone.

Progressive osseous heteroplasia

Progressive osseous heteroplasia (POH) is a recently described genetic disorder of mesenchymal differentiation characterized by dermal ossification during infancy and progressive heterotopic ossification of cutaneous, subcutaneous, and deep connective tissues during childhood. The disorder can be distinguished from fibrodysplasia ossificans progressiva (FOP) by the presence of cutaneous ossification, the absence of congenital malformations of the skeleton, the absence of inflammatory tumorlike swellings, the asymmetric mosaic distribution of lesions, the absence of predictable regional patterns of heterotopic ossification, and the predominance of intramembranous rather than endochondral ossification. POH can be distinguished from Albright hereditary osteodystrophy (AHO) by the progression of heterotopic ossification from skin and subcutaneous tissue into skeletal muscle, the presence of normal endocrine function, and the absence of a distinctive habitus associated with AHO. Although the genetic basis of POH is unknown, inactivating mutations of the GNAS1 gene are associated with AHO. The report in this issue of the JBMR of 2 patients with combined features of POH and AHO--one with classic AHO, severe POH-like features, and reduced levels of Gsalpha protein and one with mild AHO, severe POH-like features, reduced levels of Gsalpha protein, and a mutation in GNAS1--suggests that classic POH also could be caused by GNAS1 mutations. This possibility is further supported by the identification of a patient with atypical but severe platelike osteoma cutis (POC) and a mutation in GNAS1, indicating that inactivating mutations in GNAS1 may lead to severe progressive heterotopic ossification of skeletal muscle and deep connective tissue independently of AHO characteristics. These observations suggest that POH may lie at one end of a clinical spectrum of ossification disorders mediated by abnormalities in GNAS1 expression and impaired activation of adenylyl cyclase. Analysis of patients with classic POH (with no AHO features) is necessary to determine whether the molecular basis of POH is caused by inactivating mutations in the GNAS1 gene.