PTHrP modulates chondrocyte differentiation through AP-1 and CREB signaling

During the process of differentiation, chondrocytes integrate a complex array of signals from local or systemic factors like parathyroid hormone-related peptide (PTHrP), Indian hedgehog, bone morphogenetic proteins and transforming growth factor beta. While PTHrP is known to be a critical regulator of chondrocyte proliferation and differentiation, the signaling pathways through which this factor acts remain to be elucidated. Here we show that both cAMP response element-binding protein (CREB) and AP-1 activation are critical to PTHrP signaling in chondrocytes. PTHrP treatment leads to rapid CREB phosphorylation and activation, while CREB DNA binding activity is constitutive. In contrast, PTHrP induces AP-1 DNA binding activity through induction of c-Fos protein expression. PTHrP activates CRE and TRE reporter constructs primarily through PKA-mediated signaling events. Both signaling pathways were found to be important mediators of PTHrP effects on chondrocyte phenotype. Alone, PTHrP suppresses maturation and stimulates proliferation of the chondrocyte cultures. However, in the presence of dominant negative inhibitors of CREB and c-Fos, these PTHrP effects were suppressed, and chondrocyte maturation was accelerated. Moreover, in combination, the effects of dominant negative c-Fos and CREB are synergistic, suggesting interaction between these signaling pathways during chondrocyte differentiation.

Development of an animal model for prostate cancer cell metastasis to adult human bone

BACKGROUND

Prostate cancer metastases to bone are associated with significant morbidity and mortality. Presently, there is little known about the biological interaction between prostate cancer cells and bone. Development of an animal model using adult human bone will enhance our ability to study the biology of prostate cancer metastasis to bone.

METHODS

Bone was harvested from patients undergoing total joint arthroplasty and implanted in the hindlimbs of pre-treated SCID mice. Two months after bone implantation 4 x 104 prostate cancer cells (PC-3 or LAPC-4) were injected near the bone implantation site. The animals were sacrificed approximately 8 to 12 weeks after the injections of the cells. Complete histological analysis including immunostaining was performed.

RESULTS

Both the PC-3 and LAPC-4 prostate cancer cells homed to the human bone implant, specifically the reconstituted bone marrow cavity. Analysis of the bone-tumor interaction after injection of PC-3 cells revealed strong labeling for PTHrP, TNF alpha and IL-6, consistent with osteoclast recruitment and osteoclast activity. These cells also were positively stained for CK18. After cellular injection of LAPC-4 cells, there was strong labeling for TNF alpha, IL-6, and IL-1 (osteoclast recruitment and osteolytic activity). PTHrP staining was also noted. The bone cells were strongly stained for osteocalcin, and the tumor cells for PSA.

CONCLUSIONS

These data suggest that the tumor cells may induce an osteolytic response to enhance their ability to metastasize to bone. This animal model allows us to study the biologic interaction between prostate cancer cells and human bone and may enhance our understanding of the events associated with prostate cancer metastasis to bone.

BMP signaling stimulates chondrocyte maturation and the expression of Indian hedgehog

Mutant BMP receptors were transfected into cultured embryonic upper sternal chrondrocytes using retroviral vectors to determine if BMP signaling is required for chondrocyte maturation and the expression of a key regulatory molecule, Indian hedgehog (Ihh). Chondrocytes infected with replication competent avian retroviruses (RCAS) viruses carrying constitutive active (CA) BMPR-IA and BMPR-IB had enhanced expression of type X collagen and Ihh mRNA. Addition of PTHrP, a known inhibitor of chondrocyte maturation, abolished the expression of type X collagen, BMP-6, and Ihh mRNAs in control cells. In contrast, PTHrP treated cultures infected with of CA BMPR-IA or CA BMPR-IB had low levels of BMP-6 and type X collagen, but high levels of Ihh expression. Although dominant negative (DN) BMPR-IA had no effect, DN BMPR-IB inhibited the expression of type X collagen and BMP-6, and decreased alkaline phosphatase activity, even in the presence of exogenously added BMP-2 and BMP-6. DN BMPR-IB also completely blocked Ihh expression. Overall, the effect of DN BMPR-IB mimicked the effects of PTHrP. To determine if there is an autocrine role for the BMPs in chondrocyte maturation, the cultures were treated with noggin and follistatin, molecules that bind BMP-2/-4 and BMP-6/-7, respectively. While noggin and follistatin inhibited the effects of recombinant BMP-2 and BMP-6, respectively, they had only minimal effects on the spontaneous maturation of chondrocytes in culture, suggesting that more than one subgroup of BMPs regulates chondrocyte maturation. The results demonstrate that: (i) BMP signaling stimulates chondrocyte maturation; (ii) BMP signaling increases Ihh expression independent of maturational effects; and (iii) BMP signaling can partially overcome the inhibitory effects of PTHrP on maturation.

Smad2 and 3 mediate transforming growth factor-beta1-induced inhibition of chondrocyte maturation

Transforming growth factor-beta (TGF-beta) is a multifunctional regulator of a variety of cellular functions, including proliferation, differentiation, matrix synthesis, and apoptosis. In growth plate chondrocytes, TGF-beta slows the rate of maturation. Because the current paradigm of TGF-beta signaling involves Smad proteins as downstream regulators of target genes, we have characterized their role as mediators of TGF-beta effects on chondrocyte maturation. Both Smad2 and 3 translocated to the nucleus upon TGF-beta1 signaling, but not upon BMP-2 signaling. Cotransfection experiments using the TGF-beta responsive and Smad3 sensitive p3TP-Lux luciferase reporter demonstrated that wild-type Smad3 potentiated, whereas dominant negative Smad3 inhibited TGF-beta1 induced luciferase activity. To confirm the role of Smad2 and 3 as essential mediators of TGF-beta1 effects on chondrocyte maturation, we overexpressed both wild-type and dominant negative Smad2 and 3 in virally infected chondrocyte cultures. Overexpression of both wild-type Smad2 and 3 potentiated the inhibitory effect of TGF-beta on chondrocyte maturation, as determined by colx and alkaline phosphatase activity, whereas dominant negative Smad2 and 3 blocked these effects. Wild-type and dominant negative forms of Smad3 had more pronounced effects than Smad2. Our results define Smad2 and 3 as key mediators of the inhibitory effect of TGF-beta1 signaling on chondrocyte maturation.

Tumor metastasis and the reciprocal regulation of prometastatic and antimetastatic factors by nuclear factor kappaB

To investigate the role of the transcription factor nuclear factor kappaB (NFkappaB) in tumor metastasis, we generated a murine lung alveolar carcinoma cell line (Line 1) defective in NFkappaB-signaling by retroviral delivery of a dominant negative inhibitor of NFkappaB. The NFkappaB signal blockade resulted in the down-regulation of prometastatic matrix metalloproteinase 9, a urokinase-like plasminogen activator, and heparanase and reciprocal up-regulation of antimetastatic tissue inhibitors of matrix metalloproteinases 1 and 2 and plasminogen activator inhibitor 2. NFkappaB signal blockade did not affect tumor cell proliferation in vitro or in vivo but prevented intravasation of tumor cells in an in vivo chick chorioallantoic membrane model of metastasis as well as spontaneous metastasis in a murine model. These findings suggest that NFkappaB plays a central and specific role in the regulation of tumor metastasis and may be an important therapeutic target for development of antimetastatic cancer treatments.

Quantitative small-animal surrogate to evaluate drug efficacy in preventing wear debris-induced osteolysis

Individuals who suffer from severe joint destruction caused by the various arthritidies often undergo total joint arthroplasty. A major limitation of this treatment is the development of aseptic loosening of the prosthesis in as many as 20% of patients. The current paradigm to explain aseptic loosening proposes that wear debris generated from the prosthesis initiates a macrophage-mediated inflammatory response by resident macrophages, leading to osteoclast activation and bone resorption at the implant interface. No therapeutic interventions have been proved to prevent or inhibit aseptic loosening. The development of therapeutic strategies is limited due to the absence of a quantitative surrogate in which drugs can be screened rapidly in large numbers of animals. We have previously described a model in which titanium particles implanted on mouse calvaria induce an inflammatory response with osteolysis similar to that observed in clinical aseptic loosening. Here, we present new methods by which the osteolysis in this model can be quantified. We determined that 6-8-week-old mice in normal health have a sagittal suture area of 50 (+/-6) microm2, which contains approximately five osteoclasts. As a result of the titanium-induced inflammation and osteolysis, the sagittal suture area increases to 197 (+/-27) microm2, with approximately 30 osteoclasts, after 10 days of treatment. The sagittal suture area and the number of osteoclasts in the calvaria of sham-treated mice remained unchanged during the 10 days. We also determined the effects of pentoxifylline, a drug that blocks the responses of tumor necrosis factor-alpha to wear debris, and the osteoclast inhibitor alendronate. We found that both drugs effectively block wear debris-induced osteolysis but not osteoclastogenesis. In conclusion, we found the measurements made with this model to be reproducible and to permit quantitative analysis of agents that are to be screened for their potential to prevent aseptic loosening.

Targeted deletion of Minpp1 provides new insight into the activity of multiple inositol polyphosphate phosphatase in vivo

Multiple inositol polyphosphate phosphatase (Minpp1) metabolizes inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) and inositol hexakisphosphate (InsP(6)) with high affinity in vitro. However, Minpp1 is compartmentalized in the endoplasmic reticulum (ER) lumen, where access of enzyme to these predominantly cytosolic substrates in vivo has not previously been demonstrated. To gain insight into the physiological activity of Minpp1, Minpp1-deficient mice were generated by homologous recombination. Tissue extracts from Minpp1-deficient mice lacked detectable Minpp1 mRNA expression and Minpp1 enzyme activity. Unexpectedly, Minpp1-deficient mice were viable, fertile, and without obvious defects. Although Minpp1 expression is upregulated during chondrocyte hypertrophy, normal chondrocyte differentiation and bone development were observed in Minpp1-deficient mice. Biochemical analyses demonstrate that InsP(5) and InsP(6) are in vivo substrates for ER-based Minpp1, as levels of these polyphosphates in Minpp1-deficient embryonic fibroblasts were 30 to 45% higher than in wild-type cells. This increase was reversed by reintroducing exogenous Minpp1 into the ER. Thus, ER-based Minpp1 plays a significant role in the maintenance of steady-state levels of InsP(5) and InsP(6). These polyphosphates could be reduced below their natural levels by aberrant expression in the cytosol of a truncated Minpp1 lacking its ER-targeting N terminus. This was accompanied by slowed cellular proliferation, indicating that maintenance of cellular InsP(5) and InsP(6) is essential to normal cell growth. Yet, depletion of cellular inositol polyphosphates during erythropoiesis emerges as an additional physiological activity of Minpp1; loss of this enzyme activity in erythrocytes from Minpp1-deficient mice was accompanied by upregulation of a novel, substitutive inositol polyphosphate phosphatase.

Cloning of the chick BMP1/Tolloid cDNA and expression in skeletal tissues

The astacin-related metalloproteases Bone Morphogenetic Protein-1 (BMP1) and Tolloid possess multiple functions in the maturation of extracellular matrices containing fibrillar collagens. We are interested in developing an in-vitro model system to study the role of BMP1 and Tolloid in chondrocytes and osteoblasts. Cloning of the cDNAs for chick BMP1 and Tolloid reveals that the two gene products are more than 80% identical to their human and mouse homologs and are similarly derived from the same genetic locus. Anti-BMP1/Tolloid antibodies have been developed, and detect two proteins of 80 and 116kDa. Chick BMP1 and Tolloid message and proteins are found in a variety of embryonic and juvenile tissues, including chondrocytes and osteoblasts. Tolloid message and protein are generally less abundant than BMP1 message; this discrepancy is greatest in growth plate chondrocytes. Tolloid protein is more tightly bound than BMP1 to the extracellular matrix produced by cultured osteoblasts. The Chordin gene is also expressed in chondrocytes and osteoblasts, suggesting that BMP1 and Tolloid influence BMP signaling as well as matrix maturation during skeletogenesis.

PTHrP expression in chondrocytes, regulation by TGF-beta, and interactions between epiphyseal and growth plate chondrocytes

Although PTHrP has been identified as a key regulator of chondrocyte differentiation in the growth plate, the factors directly regulating PTHrP expression have not been identified. Furthermore, while cells from the epiphysis are considered the physiologic source of PTHrP, the relative expression of PTHrP in epiphyseal and growth plate chondrocytes has not been defined. PTHrP expression was examined in chondrocytes isolated from 3- to 5-week-old chick long bones. The expression of PTHrP mRNA was 10-fold higher in epiphyseal chondrocytes compared to cells from the growth plate. Growth plate chondrocytes were isolated into populations with distinct maturational characteristics by countercurrent centrifugal elutriation and analyzed for PTHrP expression. The expression was highest in the least mature cells and progressively declined with the onset of maturation. The regulation of PTHrP expression was further examined in epiphyseal chondrocytes. Both TGF-beta1 and cis-retinoic acid stimulation markedly increased PTHrP mRNA levels, while BMP-2 and PTHrP stimulation decreased the expression of this transcript. The effects of TGF-beta1 (8.9-fold stimulation) and TGF-beta3 (9.2-fold) were slightly greater than the effects of TGF-beta2 (4.9-fold). The effect of TGF-beta was dose-dependent and increases could be detected after 68 h of treatment. To analyze the paracrine effect of epiphyseal and growth plate chondrocytes on each other, these cells were placed in coculture and the mRNA from each of the populations was harvested separately after 24 h. Following coculture the PTHrP mRNA levels increased in the epiphyseal cells while the expression of type X collagen and Indian hedgehog transcripts decreased in growth plate chondrocytes. The results demonstrate potentially important paracrine interactions between these cell populations, possibly mediated by TGF-beta and PTHrP.

Tumor necrosis factor-alpha/nuclear transcription factor-kappaB signaling in periprosthetic osteolysis

Due to irreversible joint destruction caused by the various arthritides, more than 400,000 total joint arthroplasties are performed each year in the United States. As many as 20% of these require revision surgery because of aseptic loosening. The current paradigm to explain aseptic loosening is that wear debris generated from the prosthesis stimulates the release of proinflammatory cytokines (i.e., tumor necrosis factor-alpha and interleukins 1 and 6) following phagocytosis by resident macrophages. These cytokines, in turn, initiate an inflammatory response, with the development of an erosive pannus that stimulates bone resorption by osteoclasts. In support of this model, we have previously shown that human monocytes produce large quantities of tumor necrosis factor-alpha in response to titanium particles in vitro. In the current study, we characterized the role of tumor necrosis factor-alpha/nuclear transcription factor-kappaB signaling in the proinflammatory response to titanium particles in vitro and in vivo. Using the mouse macrophage cell line J774, we showed that these cells produce an amount of tumor necrosis factor-alpha in response to titanium particles similar to that produced by human peripheral blood monocytes. The production of tumor necrosis factor-alpha was preceded by a drop in cellular levels of inhibitory factor-kappaBalpha protein and translocation of p50/p65 nuclear transcription factor-KB to the nucleus 30 minutes after stimulation. Levels of tumor necrosis factor-alpha and inhibitory factor-kappaBalpha mRNA increased 30 minutes after stimulation, consistent with the activation of nuclear transcription factor-kappaB. Interleukin-6 mRNA was first seen 4 hours after the addition of the titanium particles, indicating that the production of this cytokine is secondary to the immediate nuclear transcription factor-kappaB response. To test the relevance of tumor necrosis factor-alpha/nuclear transcription factor-kappaB signaling in response to titanium particles in vivo, we adopted an animal model in which the particles were surgically implanted on the calvaria of mice. The animals displayed a dramatic histological response to the debris, with the formation of fibrous tissue and extensive bone resorption after only 1 week. With use of immunohistochemistry and tartrate-resistant acid phosphatase staining, tumor necrosis factor-alpha and osteoclasts were readily detected at the site of inflammation and bone resorption in the calvaria of the treated mice. By testing mice that genetically over-produce tumor necrosis factor-alpha (hTNFalpha-Tg), those defective in tumor necrosis factor-alpha signaling (TNF-RI-/-), and those that are nuclear transcription factor-kappaB1-deficient (NFkappaB1-/-), we evaluated the importance of tumor necrosis factor-alpha/nuclear transcription factor-kappaB signaling in the biological processes responsible for aseptic loosening. The hTNFalpha-Tg mice had a grossly exaggerated response, the TNF-RI(-/-) mice showed little evidence of inflammation or bone resorption, and the nuclear transcription factor-kappaB1(-/-) mice had an inflammatory response without bone resorption. On the basis of these results, we propose a model for periprosthetic osteolysis in which wear debris particles are phagocytosed by macrophages, resulting in the activation of nuclear transcription factor-kappaB and the production of tumor necrosis factor-alpha. Tumor necrosis factor-alpha directly induces fibroblast proliferation and tissue fibrosis and recruits or activates, or both, osteoclasts to resorb adjacent bone.

BMP-6 is an autocrine stimulator of chondrocyte differentiation

While parathyroid hormone-related protein (PTHrP) has been characterized as an important negative regulator of chondrocyte maturation in the growth plate, the autocrine or paracrine factors that stimulate chondrocyte maturation are not well characterized. Cephalic sternal chondrocytes were isolated from 13-day embryos, and the role of bone morphogenetic protein-6 (BMP-6) as a positive regulator of chondrocyte maturation was examined in monolayer cultures. Progressive maturation, which was accelerated in the presence of ascorbate, occurred in the cultures. During maturation, the cultures expressed high levels of BMP-6 mRNA which preceded the induction of type X collagen mRNA. Treatment of the cultures with PTHrP (10(-7) M) at the time of plating completely abolished BMP-6 and type X collagen mRNA expression. Removal of PTHrP after 6 days was followed by the rapid (within 24 h) expression of BMP-6 and type X collagen mRNA, with BMP-6 again preceding type X collagen expression. The addition of exogenous BMP-6 (100 ng/ml) to the cultures accelerated the maturation process both in the presence and absence of ascorbate and resulted in the highest levels of type X collagen. When exogenous BMP-6 was added to PTHrP containing cultures, maturation occurred with the expression of high levels of type X collagen, despite the presence of PTHrP in the cultures. Furthermore, BMP-6 did not stimulate expression of its own mRNA in the PTHrP treated cultures, but it did stimulate the expression of Indian hedgehog (Ihh) mRNA. These latter findings suggest that while PTHrP directly inhibits BMP-6, it indirectly regulates Ihh expression through BMP-6. Other phenotypic changes associated with chondrocyte differentiation were also stimulated by BMP-6, including increased alkaline phosphatase activity and decreased proliferation. The results suggest that BMP-6 is an autocrine factor that initiates chondrocyte maturation and that PTHrP may prevent maturation by inhibiting the expression of BMP-6.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

Differential activation by cytokines of mitogen-activated protein kinases in bovine temporomandibular-joint disc cells

Temporomandibular disorders affect a significant proportion of the population. While their aetiology is not well defined, recent histological studies suggest that the majority are similar to the osteoarthritis seen in other joints. Inflammatory cytokines such as interleukin-1 and tumour necrosis factor-alpha appear to be important in the cascade of events leading to joint destruction in osteoarthritis. Here, cells from the disc of bovine temporomandibular joint were used to examine the response to various cytokines in vitro. Disc cells were stimulated with interleukin-1alpha, tumour necrosis factor-alpha, transforming growth factor-beta, platelet-derived growth factor, and basic fibroblast growth factor. Their effects were monitored by assessing the phosphorylation of selected signal-transduction intermediates using western blot. Mitogen-activated protein kinases (Erk 1, Erk 2) were rapidly phosphorylated by exposure to basic fibroblast growth factor, platelet-derived growth factor, and tumour necrosis factor-alpha, while interleukin-1alpha showed a weak response. Transforming growth factor-beta failed to activate these kinases. Examination of the effect of these cytokines on p38 (an intermediate in the stress-activated protein-kinase pathway) showed an increase in phosphorylated p38 when stimulated with tumour necrosis factor-alpha and interleukin-1alpha. The amounts of phosphorylated signal transducer and activator of transcription-3 did not significantly increase when the cells were exposed to any of the cytokines.

Interleukin-10 inhibits cytokine synthesis in monocytes stimulated by titanium particles: evidence of an anti-inflammatory regulatory pathway

The anti-inflammatory mediator interleukin-10 was investigated as a potential inhibitor of proinflammatory cytokine release in human peripheral blood monocytes activated with titanium particles. It inhibited the secretion of both tumor necrosis factor-alpha and interleukin-6 in a dose-dependent manner, with complete inhibition observed at 2 ng/ml. Co-culture experiments were performed to determine whether this cytokine may have functional importance as an inhibitor of the inflammatory response. When unstimulated lymphocytes and monocytes were co-cultured with titanium-stimulated monocytes, they significantly suppressed the secretion of both interleukin-6 and tumor necrosis factor-alpha. The inhibitory effect of these co-cultured cells could be partially blocked with the addition of an interleukin-10 neutralizing antibody. Interleukin-10 levels were measured in monocyte cultures treated with titanium particles as well as in fresh monocyte cultures treated with conditioned medium from titanium-stimulated monocytes. The latter experiments demonstrated marked stimulation of interleukin-10 secretion in conditioned medium-treated cultures, an effect that was related to the presence of tumor necrosis factor-alpha in the conditioned medium. The addition of titanium to conditioned medium-treated cultures markedly reduced the secretion of interleukin-10, suggesting that the most responsive cells are unstimulated monocytes exposed to agents released from activated monocytes. Altogether, the expression and responsiveness to interleukin-10 suggest a potential role for anti-inflammatory cytokines in regulation of the inflammatory response to wear debris.

Stimulation of bone formation in vivo by insulin-like growth factor-II in rats

Insulin-like growth factor-II (IGF-II) plays an important role in skeletal remodeling, however, little is known about its effect on bone formation in vivo. In our study of the stimulation of bone formation in vivo by IGF II we injected recombinant human IGF-II into the parietal bones of neonatal rats once a day for 12 days. The bone mineral density measured by dual energy X-ray absorptiometry and the thickness of IGF-II-injected parietal bones increased in a dose-dependent manner. The layers of osteoblasts were observed along the IGF-II-injected side.

HiPER1, a phosphatase of the endoplasmic reticulum with a role in chondrocyte maturation

We have previously identified and partially cloned Band 17, a gene expressed in growth plate chondrocytes transiting from proliferation to hypertrophy. We now rename this gene HiPER1, Histidine Phosphatase of the Endoplasmic Reticulum-1, based on the results reported here. HiPER1 encodes two proteins of 318 (HiPER1(318)) and 449 (HiPER1(449)) amino acids, which are 20–21% identical to a group of yeast acid phosphatases that are in the histidine phosphatase family. HiPER1(449) is significantly more abundant than HiPER1(318), correlating with the abundance of the alternatively spliced messages encoding HiPER449 and HiPER318. Anti-HiPER1 antibodies detect two proteins of 53 and 55 kDa in growth plate chondrocytes that are absent in articular chondrocytes. We confirm that the 53 and 55 kDa proteins are HiPER1(449) by heterologous expression of the HiPER1(449) coding sequence in chick embryo fibroblasts. The 53 and 55 kDa proteins are glycosylated forms of HiPER1(449), as N-glycosidase F digestion reduces these proteins to 48 kDa, the predicted size of HiPER1(449) without the N-terminal signal sequence. Immunocytochemistry demonstrates that HiPER1(449) is found in chondrocytes maturing from proliferation to hypertrophy, but is not detectable in resting zone, deep hypertrophic zone or articular chondrocytes, a distribution that is consistent with the message distribution. HiPER1(449) was predicted to localize to the lumen of endoplasmic reticulum by an N-terminal signal sequence and by the C-terminal sequence Ala-Asp-Glu-Leu, which closely matches the consensus signal for ER retention, Lys-Asp-Glu-Leu. We confirm this prediction by demonstrating colocalization of HiPER1(449) with the ER protein HSP47 using dual-label immunofluorescence. PTHrP, a peptide that prevents hypertrophy in chondrocytes, suppressed HiPER1 and HiPER1(449) expression in vitro, an observation that further supports a role for HiPER1 in chondrocyte maturation. The yeast phosphatase homology, localization to the endoplasmic reticulum and pattern of expression suggest that HiPER1 represents a previously unrecognized intracellular pathway, involved in differentiation of chondrocytes.

Bone morphogenetic protein-7 in growth-plate chondrocytes: regulation by retinoic acid is dependent on the stage of chondrocyte maturation

Although the bone morphogenetic proteins stimulate chondrogenesis, little is known regarding their expression and regulation in growth-plate chondrocytes. The expression of bone morphogenetic protein-7 was examined in chick growth-plate chondrocyte cultures. Low basal levels of bone morphogenetic protein-7 mRNA and protein expression were stimulated by increasing doses of all-trans retinoic acid, a metabolite of vitamin A. The addition of 10 microM retinoic acid resulted in approximately a 6-fold increase in bone morphogenetic protein-7 mRNA levels. In contrast, other growth regulators, including basic fibroblast growth factor, transforming growth factor-beta, vitamin D, bone morphogenetic protein-6, bone morphogenetic protein-7, and parathyroid hormone-related peptide, did not alter bone morphogenetic protein-7 transcript levels. The increase in bone morphogenetic protein-7 transcripts, although present at 6 hours, was maximal following a 12-hour exposure to retinoic acid. Retinoic acid induction of bone morphogenetic protein-7 transcript levels was dependent on protein synthesis because the induction could be blocked by cyclohexamide. In maturationally distinct subpopulations of chondrocytes separated by countercurrent centrifugal elutriation, retinoic acid markedly induced bone morphogenetic protein-7 mRNA levels in the least differentiated chondrocytes but had no effect in the most terminally differentiated hypertrophic chondrocytes. Immunohistochemical localization of bone morphogenetic protein-7 demonstrates its expression throughout the developing and adolescent growth plate consistent with the constitutive pattern of expression seen in isolated chondrocytes. The addition of exogenous bone morphogenetic protein-7 to chondrocyte cultures stimulated maturation in undifferentiated chondrocyte populations. The data support a role for bone morphogenetic protein-7 as an autocrine regulator of chondrocyte maturation in the growth plate. Regulation of bone morphogenetic protein-7 by retinoic acid may be important in normal growth and development as well as in pathologic conditions of an excess or deficiency of vitamin A.

Modulation of the production of cytokines in titanium-stimulated human peripheral blood monocytes by pharmacological agents. The role of cAMP-mediated signaling mechanisms

Cytokines secreted by activated macrophages play a role in the development of osteolysis adjacent to prosthetic joints. To determine whether the synthesis of cytokines can be inhibited by pharmacological agents, we studied the role of the cAMP-protein kinase A signal transduction pathway in the synthesis of interleukin-6 and tumor necrosis factor-alpha and examined the effect of potential pharmacological regulators of this pathway in human peripheral blood monocytes stimulated with titanium particles. Dibutyryl cAMP enhanced the synthesis of interleukin-6 by titanium-stimulated monocytes and resulted in a marked increase (maximum, seventyfold) in the synthesis of interleukin-6 even in the absence of titanium particles. However, the active analogs (agonists) of cAMP, dibutyryl cAMP and Sp cAMP, inhibited the production of tumor necrosis factor-alpha by titanium-stimulated monocytes (the maximum effects resulted in complete inhibition), while the cAMP antagonist, Rp cAMP, enhanced the production of tumor necrosis factor-alpha. Additional agents that alter the intracellular levels of cAMP were examined for their effects on the synthesis of cytokines. Prostaglandins E1 and E2 were potent inhibitors of the synthesis of tumor necrosis factor-alpha but stimulated the synthesis of interleukin-6. In contrast, indomethacin enhanced the stimulatory effects of titanium particles on tumor necrosis factor-alpha, resulting in a more than threefold increase in the maximum levels of tumor necrosis factor-alpha. Phosphodiesterase inhibitors, such as isobutyryl methylxanthine and pentoxifylline, which increase intracellular levels of cAMP, caused a decrease in the production of tumor necrosis factor-alpha and an increase in the production of interleukin-6. In contrast, the fluoroquinolone antibiotic ciprofloxacin, which is also a phosphodiesterase inhibitor, caused a dose-dependent inhibition of the synthesis of both tumor necrosis factor-alpha and interleukin-6 by titanium-stimulated monocytes, suggesting that ciprofloxacin suppresses the synthesis of interleukin-6 through a mechanism that is independent of cAMP.

P-glycoprotein expression in cartilaginous tumors

BACKGROUND AND OBJECTIVES

Malignant cartilage tumors demonstrate chemotherapeutic resistance through undetermined mechanisms. P-glycoprotein is the protein product of the multiple drug resistance gene 1 (MDR-1) and confers multidrug chemotherapeutic resistance in a variety of malignancies.

METHODS

MDR-1 expression was examined in 55 benign and malignant cartilage tumor specimens by immunohistochemistry using C219, C494, and JSB-1 antibodies, and by in situ hybridization with an MDR-1 specific oligonucleotide cDNA probe.

RESULTS

Constitutive expression of P-glycoprotein was observed in all benign and malignant cartilage tumor specimens with a similar pattern of immunohistochemical staining present with all three antibodies. In benign tumors and low grade chondrosarcomas, the staining pattern was weak to intermediate and localized to clusters of cells. However, higher grade-tumors (Grade II and III) expressed P-glycoprotein in a higher percentage of cells and with more intense staining. P-glycoprotein expression was absent in normal human articular cartilage, but was focally present in costal and growth plate cartilage. The immunohistochemistry results were confirmed by in situ hybridization in 10 cases.

CONCLUSIONS

P-glycoprotein is expressed constitutively in cartilaginous tumors, with greatest expression in high grade malignancies. The findings may account for the resistance of cartilage tumors to chemotherapeutic agents.

Characterization of voltage-sensitive calcium channels in growth plate chondrocytes

Growth plate chondrocytes (GPCs), cells integrally involved with the process of endochondral bone formation, facilitate Ca2+ infux to provide a source of ion for processes such as Ca2+ signaling and matrix vesicle loading. We hypothesize that this Ca2+ entry into GPCs is achieved through the action of voltage-sensitive Ca2+ channels. This hypothesis was tested by measuring intracellular [Ca2+] changes in fura 2-loaded GPCs that were depolarized by challenge with a K(+)-containing medium. KCl doses between 55 and 95 mM evoked significant Ca2+ responses that were blocked by addition of extracellular EGTA. The Ca2+ response evoked by 95 mM K+ was insensitive to 100 microM doses of nifedipine or nitrendipine, ruling out L-type channel involvement. This finding was corroborated by the observation that 10 microM BAY K 8644 did not activate a Ca2+ response of its own. However, 10 microM Cd2+ significantly inhibited the 95 mM Ks(+)-evoked effects, suggesting N-type channel activity. Use of 1 microM Ni+ in an attempt to block possible T-type channel activity caused nonspecific cellular effects, precluding pharmacological assessment of a possible T-type channel activity. These data (i) provide the first direct evidence for voltage-sensitive Ca2+ channel activity in GPCs and (ii) suggest at least partial facilitation of that activity through N type channels.

Osteoclasts constitutively express regulators of bone resorption: an immunohistochemical and in situ hybridization study

Bone resorption is controlled by the local production of soluble regulatory molecules within the marrow microenvironment that mediate osteoclast recruitment, differentiation, and activation. Under normal conditions osteoclasts are rarely seen; in many pathologic states, however, the number of osteoclasts is dramatically increased, resulting in a net-loss of bone mass. The role of the osteoclasts as autocrine regulators of bone resorption in either normal or pathologic conditions has not been extensively investigated. The expression of IL-1 beta, IL-6, and TNF-alpha was examined in osteoclasts by immunohistochemistry under conditions of normal, reactive, and pathologic bone resorption, including growth plate (3 cases), fracture callus (5 cases), osteomyelitis (3 cases), Paget's disease (6 cases), giant-cell tumor of bone (14 cases), and brown tumor of hyperparathyroidism (2 cases). In each case, osteoclasts demonstrated immunoreactivity for IL-1 beta, IL-6, and TNF-alpha. In areas of active bone resorption, the intensity and uniformity of staining among the various conditions were similar, suggesting constitutive expression of these cytokines by activated osteoclasts. Giant-cell tumors of bone showed cytokine reactivity in over half of the giant cells, whereas stromal cells showed scattered staining. In acute osteomyelitis, inflammatory cells (mainly macrophages) and osteoclasts were intensely positive for all three cytokines. The immunohistochemical findings were confirmed by in situ hybridization using probes specific for IL-6 and TNF-alpha, the pattern of mRNA expression paralleled that of immunoreactivity for these cytokines. These findings support the notion of autocrine/paracrine regulation of bone remodeling by osteoclasts. Because overproduction of these cytokines may enhance bone resorption through the stimulation of osteoclast progenitor cells as well as mature osteoclasts, pathologic bone lesions with a large increase in the number of osteoclasts may be self-perpetuating. Alteration in the synthesis, secretion, or activity of these important regulatory molecules may in turn alter bone remodeling and loss.

Differential regulation of type-II and type-X collagen synthesis by parathyroid hormone-related protein in chick growth-plate chondrocytes

Parathyroid hormone-related protein is a critical autocrine regulator of endochondral ossification in the growth plate, as demonstrated by the severe disruption of growth-plate structure and function in parathyroid hormone-related protein-deficient transgenic mice. In the present study, the effects of parathyroid hormone-related protein on the synthesis of collagen mRNA and protein were studied in short-term cultures of isolated chick growth-plate chondrocytes. Parathyroid hormone-related protein selectively inhibits type-X collagen protein synthesis with no significant effect on type-II collagen protein synthesis. These effects were present in all maturationally distinct populations of chondrocytes separated by countercurrent centrifugal elutriation. In cultures of resting chondrocytes, the onset of type-X collagen expression was inhibited, while the synthesis of type-X collagen was decreased in cultures of hypertrophic chondrocytes. Synthesis of type-II and type-X collagen mRNA was examined by nonradioactive in situ hybridization with synthetic oligonucleotide cDNA probes, and the level of expression was quantified using digital image analysis. Dose-dependent suppression of type-X collagen gene expression by parathyroid hormone-related protein was observed, with no significant effect on type-II collagen mRNA detected. The results were confirmed by analysis of Northern blots of total chondrocyte mRNA. These experiments demonstrated differential transcriptional regulation of type-II and type-X collagen, with selective suppression of type-X collagen expression, by parathyroid hormone-related protein in growth-plate chondrocytes. In addition, excellent agreement was found between traditional protein and mRNA analyses and microscopic digital image analysis techniques, supporting the use of this convenient and sensitive assay method. Parathyroid hormone-related protein inhibits chondrocyte maturation and is known to stimulate proliferation, suggesting that this autocrine factor may function to regulate premature hypertrophy in the growth plate.

The reversal line may be a key modulator of osteoblast function: observations from an alveolar bone wound-healing model

The reversal line demarcates the cessation of osteoclast activity from the commencement of osteoblast activity at a remodeling site in bone. It is a seam between segments of bone that are formed at different times. We believe that the reversal line contains regulatory signals that, in part, control osteoblast activity. We have conducted a pilot study to examine the fate of reversal lines during abnormal bone remodeling in alveolar bone. A surgical periodontal defect was created in a Cynomolgus monkey (Macaca fascicularis), allowed to heal in the presence of plaque, and evaluated histologically. In this model, there is an acute inflammatory reaction followed by compromised bone formation. Woven bone rather than lamellar bone was deposited in the defect. A striking finding in this wound-healing model was the disruption of the carbohydrate material along the reversal line. This supports our theory that disruption of the signaling molecules in the reversal line may be responsible for uneven woven bone formation.

Vertebral body osteopenia associated with posterolateral spine fusion in humans

STUDY DESIGN

Lateral dual-energy x-ray absorptiometry was used to examine isolated changes in vertebral body mineral density in humans after instrumented posterolateral lumbar fusion.

OBJECTIVES

To determine if device-related osteopenia will occur in humans who undergo spinal fusion. Device-related osteopenia is known to occur as a result of local stress shielding after instrumentation in the appendicular skeleton. This effect has not been observed, however, in humans after spine fusion. To evaluate such changes, the vertebral body mineral density was measured in eight patients who had instrumented lumbar fusion and in eight matched control patients who had lumbar surgery with no fusion.

SUMMARY OF BACKGROUND DATA

In previous studies of dogs, vertebral body osteopenia occurred as a result of instrumented spine fusions. Previous studies in humans, however, have been limited by the relative insensitivity of conventional photon absorptiometry to isolated changes in the vertebral body because of overlying posterior elements.

METHODS

Absorptiometry was performed an average of 31.9 months after posterolateral fusion that bridged at least one level in the region of L2-L4. To reduce the effects of individual variations in mineral metabolism, the vertebral values were standardized by using the ratio of vertebral body to femoral neck density for each patient.

RESULTS

The mean density ratio for the group of patients who underwent spine fusion was 0.733. This value was significantly lower than the control ratio of 0.879 (P = 0.048).

CONCLUSIONS

Patients who have undergone instrumented posterolateral lumbar fusions have decreased vertebral body bone mineral density at the level of fusion compared with that of matched controls.

Effects of lead on growth plate chondrocyte phenotype

Lead toxicity is a major public health problem in the United States. The skeleton serves as the major reservoir for ingested lead, where it is incorporated into bone matrix during calcification. While lead in bone has been considered inactive, mounting clinical and epidemiological data has shown a strong correlation between lead exposure and adverse effects on stature in children. These epidemiologic data suggest a direct effect of lead on skeletal development, but whether it reflects a systemic effect, a specific effect on osteoblasts, or an effect on the epiphyseal growth plate is as yet unclear. This study examined the effects of lead on parameters of cartilage biology in isolated chondrocytes. Changes in growth plate chondrocyte phenotype were assessed utilizing an established avian growth plate chondrocyte model. Low, sublethal doses of lead caused specific and significant effects on a number of important markers of growth plate chondrocyte phenotype, including suppression of alkaline phosphatase and both type II and type X collagen expression at the protein and mRNA levels, and a decrease in thymidine incorporation. In contrast, proteoglycan synthesis was stimulated relative to controls in lead-treated cultures, suggesting that the alterations in collagen and DNA synthesis and alkaline phosphatase activity are not due to cytotoxity. The data demonstrate important regulatory effects of lead on growth plate chondrocytes in cell culture and suggest an inhibitory effect on the process of endochondral bone formation. The growth plate may be one of the key target tissues accounting for the adverse effects of chronic lead exposure on skeletal development.