Formation and mineralization of extracellular matrix secreted by an immortal human osteoblastic cell line: modulation by tumor necrosis factor-alpha

The effect of TNF-α on osteoblasts in metal wear-induced periprosthetic bone loss

Aims

This study aimed to examine the effects of tumour necrosis factor-alpha (TNF-α) on osteoblasts in metal wear-induced bone loss.

Methods

TNF-α immunoexpression was examined in periprosthetic tissues of patients with failed metal-on-metal hip arthroplasties and also in myeloid MM6 cells after treatment with cobalt ions. Viability and function of human osteoblast-like SaOs-2 cells treated with recombinant TNF-α were studied by immunofluorescence, terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, western blotting, and enzyme-linked immunosorbent assay (ELISA).

Results

Macrophages, lymphocytes, and endothelial cells displayed strong TNF-α immunoexpression in periprosthetic tissues containing metal wear debris. Colocalization of TNF-α with the macrophage marker CD68 and the pan-T cell marker CD3 confirmed TNF-α expression in these cells. Cobalt-treated MM6 cells secreted more TNF-α than control cells, reflecting the role of metal wear products in activating the TNF-α pathway in the myeloid cells. While TNF-α did not alter the immunoexpression of the TNF-receptor 1 (TNF-R1) in SaOs-2 cells, it increased the release of the soluble TNF-receptor 1 (sTNF-R1). There was also evidence for TNF-α-induced apoptosis. TNF-α further elicited the expression of the endoplasmic reticulum stress markers inositol-requiring enzyme (IRE)-1α, binding-immunoglobulin protein (BiP), and endoplasmic oxidoreductin1 (Ero1)-Lα. In addition, TNF-α decreased pro-collagen I α 1 secretion without diminishing its synthesis. TNF-α also induced an inflammatory response in SaOs-2 cells, as evidenced by the release of reactive oxygen and nitrogen species and the proinflammatory cytokine vascular endothelial growth factor.

Conclusion

The results suggest a novel osteoblastic mechanism, which could be mediated by TNF-α and may be involved in metal wear debris-induced periprosthetic bone loss.

Cite this article: Bone Joint Res 2020;9(11):827–839.

Charcot arthropathy of the foot and ankle associated with rheumatoid arthritis

BACKGROUND

Diabetic peripheral neuropathy is now well recognized as the most common cause of Charcot arthropathy of the foot and ankle, but it may be associated with other peripheral neuropathies. While not well known, it is well documented that rheumatoid arthritis is correlated with peripheral neuropathy. However, despite rheumatoid neuropathy, Charcot arthropathy has never been associated with rheumatoid arthritis. We report a series of Charcot arthropathy patients with concomitant rheumatoid arthritis.

METHODS

The medical records of patients treated between 1986 and 2009 with Charcot arthropathy and rheumatoid arthritis were reviewed. Recorded data included neuropathy risk factors, medications, history of ulcerations, ambulatory status, shoe wear, and treatment course. Radiographs of Charcot joints were categorized according to the Brodsky anatomic classification. Patient care was based on published treatment algorithms, emphasizing accommodative, nonoperative treatment with selective surgical interventions. Surgery was indicated for recalcitrant, nonhealing lesions of the soft tissue and/or unbraceable, nonplantigrade feet. A successful outcome was considered an ambulatory patient without amputation and a closed skin envelope at last follow-up.

RESULTS

Four men and 16 women met the diagnostic criteria, resulting in 33 feet in the series. Average age was 61 years, and average follow-up was 4.3 years. In addition to rheumatoid arthritis, 4 patients (7 feet) had hypothyroidism, 4 patients (6 feet) had diabetes, 1 patient (2 feet) had megaloblastic anemia and diabetes, and 1 patient (1 foot) had hypothyroidism and diabetes; however, 17 feet (52%) had no known sources for neuropathy. Charcot involvement was type 1-midfoot in 21 feet (64%), type 2-hindfoot in 7 (21%), type 3a-ankle in 4 (12%), and type 3b-calcaneus in 1 (3%). Twenty-three feet (70%) were treated with conservative modalities. Ten feet (30%) required 15 surgeries, of which an exostectomy was the most common procedure. Of the 33 feet, 3 had persistent ulcerations and 1 underwent major amputation, representing 4 failures.

CONCLUSIONS

Raising awareness within the orthopaedic community, we report a Charcot arthropathy population with a concomitant rheumatoid arthritis diagnosis, emphasizing a relationship between the 2 diseases. Through a conservative treatment regimen combined with selective surgical interventions, satisfactory outcomes were achieved in 88% of the rheumatoid Charcot feet. While several patients had additional neuropathy sources which could cause Charcot arthropathy (eg, diabetes), the majority of feet had no etiologies accounting for neuropathy or neuroarthropathy except rheumatoid arthritis. Further study is required to expand on this relationship between the 2 diseases.

LEVEL OF EVIDENCE

Level IV, retrospective case series.

Bone damage in rheumatoid arthritis: mechanistic insights and approaches to prevention

In rheumatoid arthritis (RA), cells within the inflamed synovium and pannus elaborate a variety of cytokines, including tumor necrosis factor (TNF) alpha, interleukin (IL)-1, IL-6, and IL-17, that contribute to inflammation, and may directly affect bone. The receptor activator of NF-kappaB (RANK) ligand/RANK/osteoprotegerin pathway plays a critical role in regulating osteoclastogenesis in articular bone erosions in RA. Proinflammatory cytokines can modulate this pathway, and may also affect the ability of the osteoblast to repair bone at sites of articular erosion. In this review, the authors discuss the current understanding of pathogenic mechanisms of bone erosion in RA and examine current therapeutic approaches to prevent this damage.

Bone changes and fracture risk in individuals infected with HIV

The life expectancy of individuals infected with HIV has improved greatly since the institution of combination antiretroviral therapy. However, many metabolic derangements have been discovered with long-term combination antiretroviral treatment, including lipodystrophy; insulin resistance; and, more recently, abnormal bone metabolism. It is well-documented that bone mineral density (BMD) in HIV-positive patients is lower compared with the expected BMD in non-HIV-positive patients. The underlying cause of lower BMD is unknown but is thought to be a multifactorial process. Conflicting evidence exists regarding the effect of antiretroviral exposure and duration of treatment, antiretroviral type, and cumulative HIV viral exposure on bone health. Here we review the bone changes that occur with HIV infection and treatment.

Modulation of osteoclast function in bone by the immune system

Osteoclast differentiation and function is regulated by cellular signals and cytokines that also play significant roles in the immune system. There is much scope, therefore, for immune cell influence on osteoclasts and bone metabolism. Many examples of this have been identified and T cells in particular are a source of factors affecting osteoclast formation and activity, a number which have either pro-osteolytic or anti-osteolytic actions depending on the cellular and microenvironmental context. For example, IL-12 and IL-18 participate in inflammatory processes that can lead to highly destructive osteolysis, yet these cytokines potently block osteoclast formation through mediation of T cells. IL-23 participates in chronic inflammatory processes, but lack of this cytokine results in reduced bone mass in mice, pointing to an influence on physiological regulation of bone mass. Such insights suggest that therapies that target immune responses may significantly influence osteolysis. Investigations into links between the immune system and bone metabolism are thus uncovering important information about the functioning of both systems.

First line zidovudine/lamivudine/lopinavir/ritonavir leads to greater bone loss compared to nevirapine/lopinavir/ritonavir

OBJECTIVE

We studied changes in bone mineral density (BMD) and bone turnover after initiation of combination antiretroviral therapy (cART) and the contribution of zidovudine/lamivudine (ZDV/3TC) in particular.

DESIGN

Randomized clinical trial comparing lopinavir/ritonavir(LPV/r) + ZDV/3TC with LPV/r + nevirapine (NVP) in 50 cART-naive men.

METHODS

Dual energy X-ray absorptiometry (DXA) and quantitative computed tomography scans (QCT) were performed at baseline and 3, 12, and 24 months after cART initiation. Serum 25-hydroxy-vitamin D3, parathyroid hormone (PTH), osteocalcin, and urine deoxypyridinoline (DPD)/creatinine ratio were measured.

RESULTS

BMD decreased rapidly in both femoral neck and lumbar spine after cART initiation. BMD loss during 24 months measured by DXA, but not by QCT, was greater in the ZDV/3TC/LPV/r group compared to the NVP/LPV/r group [femoral neck: -6.3% +/- 1.0% (P < 0.0001) compared to -2.3% +/- 0.9% (P = 0.01), between-group P = 0.0006); lumbar spine: -5.1% +/- 0.8% (P < 0.0001) compared to -2.6% +/- 0.7% (P = 0.0006), between-group P = 0.07]. Osteocalcin [+1.60 +/- 0.32 (P < 0.0001) and +1.81 +/- 0.29 (P < 0.0001) nmol/l] and the urine DPD/creatinine ratio [+1.35 +/- 0.44 (P = 0.0029) and +1.19 +/- 0.38 nmol/mmol (P = 0.0024)] increased in both groups over 24 months, with no significant difference between groups. PTH increased to a greater degree in the NVP/LPV/r group [+2.0 +/- 0.31 pmol/l (P < 0.0001)] compared to [+0.81 +/- 0.33 pmol/l (P = 0.021) in the ZDV/3TC/LPV/r group].

CONCLUSION

BMD in both femoral neck and lumbar spine decreased rapidly after initiation of cART, in parallel to an increase in bone turnover. The greater bone loss in the ZDV/3TC/LPV/r group compared to the NVP/LPV/r group suggests that ZDV/3TC contributes to this process. The PTH increase does not explain this greater bone loss.

Galanin treatment offsets the inhibition of bone formation and downregulates the increase in mouse calvarial expression of TNFalpha and GalR2 mRNA induced by chronic daily injections of an injurious vehicle

We have previously shown that after bone fracture, galanin (GAL) and GAL receptor expression is increased in osteoblast-like cells of callus; however, the role of elevated GAL/GAL receptors in this instance of bone injury is not known. We hypothesize that in injury, GAL may facilitate bone formation by suppressing the production of cytokines such as TNFalpha and IL-1alpha, thereby affecting bone collagen formation and collagenolysis by key matrix metalloproteinases (MMPs). In studies to explore this hypothesis, we used a mouse calvarial injection model to (1) investigate whether mild injury caused by a daily subcutaneous injection of a glycerol-containing vehicle onto calvaria affected osteoblast/bone formation-associated histomorphometric parameters and gene expression (mRNA encoding GAL, GAL receptors, TNFalpha, IL-1beta, collagen type I, MMP-2 and -13) compared to non-injected, control mice and (2) determine the effect of GAL+vehicle treatment on these entities. Five groups of 4-week-old mice were used: a non-injected control group; a vehicle (50/50 solution of 10 mM PBS+0.025% BSA/5.4 M glycerol)-treated group; and 3 GAL-treated groups (0.2, 2 and 20 ng doses). Solutions were injected subcutaneously onto calvaria in a 10 mul volume, every day for 2 weeks. Vehicle injection reduced calvarial periosteal osteoblast cell height (P<0.001), osteoblast number (P<0.001) and osteoid thickness (P<0.01), relative to values in non-injected animals at 2 weeks. Vehicle injection also inhibited BFR in this periosteal bone relative to values in non-injected animals at both 1 and 2 weeks (P<0.05 and P<0.001, respectively). Increasing concentrations of GAL reversed the above-listed inhibitory effects caused by vehicle. This reversal was demonstrated by a dose-dependent effect of GAL on osteoblast cell height (Pearson's r=0.330; P<0.05), osteoblast number (Pearson's r=0.715; P=0.000), osteoid thickness (Pearson's r=0.516; P=0.000) and BFR (Pearson's r=0.525; P<0.05) after 2 weeks of GAL+vehicle treatment; with the 20 ng/day GAL+vehicle injection schedule returning these measured parameters toward non-injected control values. All GAL+vehicle treatments had no effect on calvarial expression of GAL, GALR1, GALR3, collagen type 1 and MMP-2 mRNAs compared to levels in vehicle-injected controls. GAL treatment did, however, produce dose-dependent effects on calvarial expression of GALR2 (Pearson's r=0.763; P=0.000), MMP-13 (Pearson's r=0.806; P=0.000), IL-1beta (Pearson's r=0.807; P=0.000) and TNFalpha (Pearson's r=0.542; P=0.000) mRNAs with 20 ng/day of GAL+vehicle producing the strongest reversal of vehicle-associated changes. Thus, the 20 ng/day GAL+vehicle regimen offset the inhibition of osteoblastic activity, and therefore bone formation caused by daily glycerol-containing vehicle injection. This effect on bone formation may be due in part to the peptide suppressing the formation and associated activity of TNFalpha, IL-1beta and MMP-13, as TNFalpha and IL-1beta are known inhibitors of bone formation and MMP-13 is involved in collagenolysis. Furthermore, these effects may be due to the action of GAL via GALR2, as it was the only GAL receptor affected by this GAL treatment regimen. These results indicate that GAL can facilitate bone formation associated with injury and reveal potential efficacy for GAL in treating osseous conditions where bone formation may be inhibited due to excess TNFalpha and IL-1beta production.

Diverse biological functions of extracellular collagen processing enzymes

Collagens are abundant proteins in higher organisms, and are formed by a complex biosynthetic pathway involving intracellular and extracellular post-translational modifications. Starting from simple soluble precursors, this interesting pathway produces insoluble functional fibrillar and non-fibrillar elements of the extracellular matrix. The present review highlights recent progress and new insights into biological regulation of extracellular procollagen processing, and some novel functions of byproducts of these extracellular enzymatic transformations. These findings underscore the notion that released propeptides and other proteolytic products of extracellular matrix proteins have important biological functions, and that structural proteins are multifunctional. An emerging concept is that a dynamic interplay exists between extracellular products and byproducts with cells that helps to maintain normal cellular phenotypes and tissue integrity.

A role for lysyl oxidase regulation in the control of normal collagen deposition in differentiating osteoblast cultures

Differentiation of phenotypically normal osteoblast cultures leads to formation of a bone-like extracellular matrix in vitro. Maximum collagen synthesis occurs early in the life of these cultures, whereas insoluble collagen deposition occurs later and is accompanied by a diminished rate of collagen synthesis. The mechanisms that control collagen deposition seem likely to include regulation of extracellular collagen biosynthetic enzymes, but expression patterns of these enzymes in differentiating osteoblasts has received little attention. The present study determined the regulation of lysyl oxidase as a function of differentiation of phenotypically normal murine MC3T3-E1 cells at the level of RNA and protein expression and enzyme activity. In addition, the regulation of BMP-1/mTLD mRNA levels that encodes procollagen C-proteinases was assayed. The role of lysyl oxidase in controlling insoluble collagen accumulation was further investigated in inhibition studies utilizing beta-aminopropionitrile, a specific inhibitor of lysyl oxidase enzyme activity. Results indicate that lysyl oxidase is regulated as a function of differentiation of MC3T3-E1 cells, and that the maximum increase in lysyl oxidase activity precedes the most efficient phase of insoluble collagen accumulation. By contrast BMP-1/mTLD is more constitutively expressed. Inhibition of lysyl oxidase in these cultures increases the accumulation of abnormal collagen fibrils, as determined by solubility studies and by electron microscopy. Taken together, these data support that regulation of lysyl oxidase activity plays a key role in the control of collagen deposition by osteoblast cultures.

Regulation of collagen deposition and lysyl oxidase by tumor necrosis factor-alpha in osteoblasts

Tumor necrosis factor-alpha (TNF-alpha) inhibits osteoblast function in vitro by inhibiting collagen deposition. Studies generally support that TNF-alpha does not inhibit collagen biosynthesis by osteoblasts but that collagen deposition is in some way diminished. The study investigated TNF-alpha regulation of biosynthetic enzymes and proteins crucial for posttranslational extracellular collagen maturation in osteoblasts including procollagen C-proteinases, procollagen C-proteinase enhancer, and lysyl oxidase. The working hypothesis is that such regulation could inhibit collagen deposition by osteoblasts. We report that in phenotypically normal MC3T3-E1 osteoblasts, TNF-alpha decreases collagen deposition without decreasing collagen mRNA levels or procollagen protein synthesis. Analyses of the cell layers revealed that TNF-alpha diminished the levels of mature collagen cross-links, pyridinoline and deoxypyridinoline. Further analyses revealed that the mRNA expression for lysyl oxidase, the determining enzyme required for collagen cross-linking, is down-regulated by TNF-alpha in a concentration- and time-dependent manner by up to 50%. The decrease was accompanied by a significant reduction of lysyl oxidase protein levels and enzyme activity. By contrast, Northern and Western blotting studies revealed that procollagen C-proteinases bone morphogenic protein-1 and mammalians Tolloid and procollagen C-proteinase enhancer were expressed in MC3T3-E1 cells and not down-regulated. The data together demonstrate that TNF-alpha does not inhibit collagen synthesis but does inhibit the expression and activity of lysyl oxidase in osteoblasts, thereby contributing to perturbed collagen cross-linking and accumulation. These studies identify a novel mechanism in which proinflammatory cytokine modulation of an extracellular biosynthetic enzyme plays a determining role in the control of collagen accumulation by osteoblasts.

An Increased Risk of Osteoporosis during Acquired Immunodeficiency Syndrome

Osteoporosis is characterized by decreased bone mineral density and mechanistic imbalances of bone tissue that may result in reduced skeletal strength and an enhanced susceptibility to fractures. Osteoporosis in its most common form affects the elderly (both sexes) and all racial groups of human beings. Multiple environmental risk factors like acquired immune deficiency syndrome (AIDS) are believed to be one of the causes of osteoporosis. Recently a high incidence of osteoporosis has been observed in human immunodeficiency virus (HIV) infected individuals. The etiology of this occurrence in HIV infections is controversial. This problem seems to be more frequent in patients receiving potent antiretroviral therapy. In AIDS, the main suggested risk factors for the development of osteoporosis are use of protease inhibitors, longer duration of HIV infection, lower body weight before antiretroviral therapy, high viral load. Variations in serum parameters like osteocalcin, c-telopeptide, levels of elements like Calcium, Magnesium, Phosphorus, concentration of vitamin-D metabolites, lactate levels, bicarbonate concentrations, amount of alkaline phosphatase are demonstrated in the course of development of osteoporosis. OPG/RANKL/RANK system is final mediator of bone remodeling. Bone mineral density (BMD) test is of added value to assess the risk of osteoporosis in patients infected with AIDS. The biochemical markers also aid in this assessment. Clinical management mostly follows the lines of treatment of osteoporosis and osteopenia.

Simultaneous gene expression analysis of steady-state and actively translated mRNA populations from osteosarcoma MG-63 cells in response to IL-1alpha via an open expression analysis platform

Pro-inflammatory cytokines play a key role in various forms of metabolic bone diseases, including osteopenia and osteoporosis. Human MG-63 cells treated with IL-1alpha were used as a model system to identify potential marker genes that are differentially expressed. This study is designed to quantitate gene expression of actively translated mRNAs as compared to the steady-state mRNA population. Both steady-state mRNAs and actively translated mRNAs from control MG-63 cells and MG-63 cells treated with IL-1alpha were isolated and converted to cDNA. The gene expression analysis from these samples was then quantitated with an open expression analysis platform with no requirement for a priori knowledge of sequence information. As a result, many differentially regulated genes were discovered via IL-1alpha treatment. Some of the genes have been described previously as playing important roles in the regulation of inflammation and cell adhesion. These comparisons provided a panoramic overview of gene expression at both the total transcript and post-transcriptional levels. In addition, the quantitation of actively translated mRNAs associated with polysomes also provided a better estimation of protein expression levels. This methodology allows for the identification of genes acutely regulated during translation. Furthermore, the process may aid in the identification of new drug targets or biomarkers.

Increased levels of biochemical markers of bone turnover in relation to persistent immune activation in common variable immunodeficiency

BACKGROUND

Based on the involvement of cytokines and growth factors in bone homeostasis, we hypothesised that patients with common variable immunodeficiency (CVI), characterised by persistent immune activation in vivo, may have disturbed bone metabolism as evaluated by biochemical markers of bone turnover.

MATERIALS AND METHODS

Serum levels of tumour necrosis factor alpha (TNFalpha), interleukin-6 (IL-6), bone-specific alkaline phosphatase (B-ALP), osteocalcin, carboxyterminal crosslinking telopeptide of type I collagen (CTX-I), insulin-like growth factor (IGF)-I and IGF binding protein-3 (IGFBP-3) were measured in 25 patients with CVI and compared to 25 age- and sex-matched healthy controls.

RESULTS

Patients with CVI had significantly higher serum levels of CTX-I and B-ALP, and significantly lower serum levels of IGF-I and IGFBP-3 compared to controls as shown in cross-sectional, and as for B-ALP and CTX-I, also during longitudinal testing. No differences were observed for osteocalcin between the two groups. The elevated B-ALP and decreased IGF-I and IGFBP-3 levels were most pronounced in a subgroup of CVI patients characterised by persistent activation of proinflammatory cytokines in vivo. Raised B-ALP and decreased IGF-I and IGFBP-3 were also significantly correlated with enhanced IL-6 and TNF-alpha levels in these patients.

CONCLUSIONS

The present study suggests that persistent immune activation in vivo, with raised levels of proinflammatory cytokines, may be related to disturbed bone homeostasis in CVI patients, further supporting an interaction between immune related mediators and bone metabolism in humans.

Proliferative response of different human osteoblast-like cell models to proinflammatory cytokines

Children with inflammatory bowel disease are known to be at risk of osteopenia. The cause of this osteopenia is likely to be multifactorial, but the inflammatory process with its characteristic overproduction of cytokines has been implicated. To investigate this possible contribution of the disease activity to the development of osteopenia, we performed in vitro assays of the proliferation of osteoblast-like cells of differing origins in response to the inflammatory cytokines tumor necrosis factor-alpha and IL-1/beta. Osteoblast-like cells derived from pediatric bone explants, adherent stromal cells derived from bone marrow (osteoprogenitors), MG-63 osteosarcoma cells, and SV-40 virally transformed osteoprogenitor cells (HCC1) were studied. Tumor necrosis factor-alpha stimulated the proliferation of cells in primary cultures (i.e. from explants and marrow samples) in a linear, dose-dependent manner. In contrast, inhibition of proliferation was observed with the established cell lines (MG-63 and HCC1). IL-1beta stimulated proliferation of all cells apart from the immortalized human bone marrow cell line, HCC1, in which case potent inhibition was observed. We conclude that proinflammatory cytokines are potent regulators of osteoblast-like cell proliferation, and that the responses are specific to cell type. The opposite results obtained with established cell lines compared with the primary cultures suggest that careful consideration should be given to choosing the most suitable cell line for in vitro studies relating to in vivo mechanisms predisposing to osteopenia.

Involvement of nitric oxide and biopterin in proinflammatory cytokine-induced apoptotic cell death in mouse osteoblastic cell line MC3T3-E1

We previously demonstrated that the addition of proinflammatory cytokines (tumor necrosis factor-alpha, interleukin-1beta, and interferon-gamma) caused induction of mRNAs for inducible nitric oxide (NO) synthase and GTP cyclohydrolase I, a rate-limiting enzyme for 5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis, and produced their end-products, NO and BH4, in osteoblastic cells. In the present study, we examined whether NO and BH4, biologically active substances produced in response to proinflammatory cytokines, are involved in the effect of these cytokines on cell viability and apoptotic cell death involving DNA fragmentation. Cytokines as well as S-nitroso-N-acetyl-d,l-penicillamine, an NO generator, decreased cell viability, whereas sepiapterin, which was converted intracellularly to BH4, increased it. The examination of cytotoxicity measured in terms of lactate dehydrogenase release and apoptotic cell death assessed by flow cytometric analysis showed that cytokine-induced reduction of cell viability may be based upon cell death by apoptosis, but not lytic death as in necrosis. In the presence of sepiapterin, cytokine treatment resulted in a statistically pronounced reduction in the amount of DNA fragmentation. Furthermore, this fragmentation could be blocked by 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide, an NO scavenger. These results suggest that cytokine-induced apoptotic cell death is attributed to NO and is protected by BH4, and that osteoblastic cells in response to proinflammatory cytokines operate both a stimulatory process resulting in NO production and an inhibitory one resulting in BH4 production for apoptotic cell death. Cytokine-induced apoptotic cell death may be a consequence of the predominance of the stimulatory process over the inhibitory process.

Decreased bone formative and enhanced resorptive markers in human immunodeficiency virus infection: indication of normalization of the bone-remodeling process during highly active antiretroviral therapy

As cytokines and 1,25-dihydroxyvitamin D [1,25-(OH)2D] appear to have an important role in bone homeostasis, we examined the possibility that human immunodeficiency virus (HIV)-infected patients, characterized by enhanced levels of proinflammatory cytokines and 1,25-(OH)2D deficiency, have disturbed bone metabolism by analyzing serum markers of bone formation (osteocalcin) and bone resorption (C-telopeptide) in 73 HIV-infected patients. HIV-infected patients with advanced clinical and immunological disease and high viral load were characterized by increased C-telopeptide and particularly by markedly depressed osteocalcin levels. HIV-infected patients had enhanced activation of the TNF system. Serum concentrations of p55 and p75-TNF receptors were negatively correlated with osteocalcin, and p75-TNF receptor was positively correlated with C-telopeptide. HIV-infected patients with advanced disease also had decreased serum concentrations of 1,25-(OH)2D, but this parameter was not correlated with osteocalcin or C-telopeptide. During 24 months with highly active antiretroviral therapy there was a marked rise in serum osteolcalcin levels together with a profound fall in viral load and TNF components and a marked rise in CD4+ T cell counts. Also, there was a shift from no correlation to a significant correlation between osteocalcin and C-telopeptide levels during such therapy. The present study suggests disturbed bone formation and resorption during HIV infection. Our findings indicating synchronization of bone remodeling during highly active antiretroviral therapy may represent a previously unrecognized beneficial effect of such therapy and expand our knowledge of the interactions between cytokines and bone in the bone-remodeling process.

Tumor necrosis factor activates a nuclear inhibitor of vitamin D and retinoid-X receptors

Tumor necrosis factor-alpha (TNF) is an important contributor to the pathophysiology of bone loss in osteoporosis. Previous work has revealed that TNF inhibits 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) action. We have shown that TNF decreases binding of the vitamin D receptor (VDR) and its heterodimeric partner, the retinoid-x receptor (RXR), to the vitamin D response element (VDRE) of the osteocalcin gene. Here we test the hypothesis that TNF induces a nuclear inhibitor of RXR/VDR binding to DNA and that this inhibitor can have independent effects on RXR. The effect of TNF on RXR and VDR binding to their cognate response elements and stimulation of transcription was studied in VDR deficient CV-1 and COS-7 cells. In CV-1 cells transfected with VDR and RXR expression vectors, TNF-alpha inhibited 1,25(OH)2D3 stimulated transcription of a VDRE-CAT reporter and 9-cis-retinoic acid (9cRA) stimulated transcription of an RXRE-CAT reporter. Inhibition of transcription was associated with decreased binding of VDR and RXR to their cognate response elements. To determine if TNF-alpha induced a nuclear inhibitor of VDR and RXR binding to DNA, nuclear extract was isolated from TNF treated receptor deficient COS cells and mixed with nuclear extract from ligand treated receptor replete COS cells. Receptor binding to DNA was inhibited by the extract from TNF treated COS-7 cells. The inhibitory activity rapidly appeared in nuclear extracts following TNF stimulation. We conclude that TNF activates a nuclear inhibitor of VDR and RXR.

Transformation and preliminary characterization of primary human pulp cells

The odontoblast is the cell responsible for dentin formation and mineralization during tooth development. A number of primary pulp cell culture systems have been used to study the mechanism of dentinogenesis in vitro. One of the difficulties in using primary cells is the limited number of cell divisions they will undergo. In this study, this problem was addressed by transfecting primary cultures of human pulp cells with an SV40-adenovirus construct. This resulted in the establishment of transformed human pulp cells, which were named HPC-T. A series of preliminary experiments were performed to characterize these cells, including their morphology, cell proliferation, alkaline phosphatase production, and cytogenetic make-up. The results demonstrate that SV40-transformed human pulp cells retain many of the characteristics of the parent primary cells and may be useful in the study of pulp cell function in vitro.

Cytokines stimulate matrix metalloproteinase production by human pulp cells during long-term culture

Interleukin-1 and tumor necrosis factor-alpha are inflammatory cytokines that are known to be potent stimulators of mineralized tissue resorption. One of the mechanisms by which these cytokines induce this loss is through the stimulation of matrix metalloproteinase (MMP) production and secretion by the host cells present at the inflammatory site. We have previously shown that these cytokines have little effect on MMP production by human pulp cells in short-term culture (24 to 48 h). In this study, we examined the production of MMPs by human pulp cells in the presence and absence of interleukin-1 and tumor necrosis factor-alpha in long-term cultures (2 to 16 days) using substrate gel zymography. The major band present in all samples examined migrated at 68 kDa, corresponding to the migration pattern of MMP-2, whereas a minor band migrated at 90 kDa, corresponding to the migration pattern of MMP-9. In the presence of cytokines, elevated levels of MMP-2 and MMP-9 were apparent at days 9 through 16. In addition, a band migrating at 110 kDa was present. This study demonstrates that cytokines stimulate the production of elevated levels of MMPs by human pulp cells in long-term cultures and that these MMPs may play a role in pulpal inflammation.

Acute metabolic acidosis inhibits the induction of osteoblastic egr-1 and type 1 collagen

Metabolic acidosis induces net calcium efflux from bone through a decrease in osteoblastic formation and an increase in osteoclastic resorption. We tested the hypothesis that changes in external pH would alter the expression of genes critical to the function of mouse calvarial bone cells, predominantly osteoblasts. Cells were cultured in physiologically neutral pH medium until confluent and then stimulated with fresh medium at either neutral or acidic pH. Among a group of immediate early response genes, including egr-1, junB, c-jun, junD, and c-fos, only egr-1 stimulation was modulated by changes in medium pH. At pH 7.4, RNA for egr-1 was stimulated approximately 10- to 30-fold, 40 min after medium change. A progressive decrease in pH to 6.8 led to a parallel reduction in egr-1 stimulation, and an increase in pH to 7.6 led to an increase in egr-1 stimulation. The protein synthesis inhibitor cycloheximide led to a superinduction of egr-1 with preservation of the pH dependency of expression. Osteoblasts synthesize collagen, which is subsequently mineralized. RNA for type 1 collagen was stimulated approximately three- to fivefold, 40 min after medium change. Again the stimulation was inhibited by acidosis and increased by alkalosis. Cycloheximide abolished the pH dependency of expression. These results suggest that small changes in external pH have a significant effect on the expression of certain genes important for osteoblastic function.

Nitric oxide acts in conjunction with proinflammatory cytokines to promote cell death in osteoblasts

Proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), and interleukin-1 beta are known modulators of bone remodeling in vitro and in vivo. The same cytokines induce the production of nitric oxide (NO) in various cell types, including osteoblasts and osteoclasts, and NO has recently been implicated in the regulation of bone resorption. We investigated the relationship between NO levels and cell viability in MC3T3-E1, a well-characterized osteoblastic cell line. NO donors at high concentrations (> or = 0.5 mM) produce a significant cytotoxic effect over a 48 h period. Various combinations of the three cytokines strongly promote endogenous NO production, and high NO levels are correlated with the loss of cell viability. Although TNF-alpha produces NO-independent cytotoxicity, NO greatly enhances this cytotoxic effect. Human and mouse TNF-alpha differ in their cytotoxic effects, and human TNF-alpha induces lower levels of NO production. In cocultures of RAW 264.7 mouse macrophages stimulated with lipopolysaccharide and IFN-gamma, and untreated MC3T3-E1 osteoblasts, addition of anti-TNF-alpha antibody and inhibition of NO synthesis have additive, protective effects on osteoblast viability. NO cytotoxicity involves an apoptotic mechanism. Our results underline the importance of NO and TNF-alpha as cytotoxic mediators in the osseous microenvironment and might explain the observed deficiency of bone formation in inflammatory sites.

Regulation of pulp cell matrix metalloproteinase production by cytokines and lipopolysaccharides

Little information is currently known regarding the effects of cytokines and lipopolysaccharides (LPS's) on matrix metalloproteinase (MMP) production by pulp cells in vitro. In this study, human pulp cells (HPC's) and clonal rat pulp cells RPC-C2A were treated with interleukin (IL)-1 alpha, IL-1 beta, tumor necrosis factor (TNF)-alpha, and LPS for 24 h. Conditioned medium and cell lysates were collected and analyzed by gelatin zymography. RPC-C2A cells treated with IL-1 beta and TNF-alpha displayed elevated levels of MMP's in conditioned medium fractions. LPS's at increasing concentrations had a similar effect. HPC's treated with either cytokines or LPS's had no change in the pattern of MMP's produced or secreted in either cellular or conditioned medium fractions. These studies indicate that the effects of cytokines and LPS's on pulp cells are not identical for cells from different species and requires further investigation to clarify these variations.

Ultrastructural analysis of mineralized matrix from human osteoblastic cells: effect of tumor necrosis factor-alpha

Recent work by a number of investigators has demonstrated that the process of bone matrix formation and mineralization is under the influence of growth factors and cytokines present in the local environment. Utilizing primary and established osteoblast cell culture systems, these studies have examined the regulation of bone matrix protein synthesis and deposition into the extracellular matrix (ECM) and subsequent mineralization. In previous studies, we have utilized the human osteoblastic cell line, HOS TE85, to study the effects of Tumor Necrosis Factor-alpha (TNF-alpha) on the regulation of matrix proteins and proteolytic function in monolayer cultures as well as during the development and calcification of ECM formed by HOS TE85 cells during extended culture. Our studies demonstrate that TNF-alpha inhibited formation and mineralization of nodules. In the study reported here, we evaluated the ultrastructural morphology of the cell-matrix complex formed by HOS TE85 cells in the presence and absence of TNF-alpha at selected time points during the matrix development process utilizing both transmission electron microscopy and light microscopy. In the presence of TNF-alpha, the cell-matrix complex does not develop normally, with a lack of organization and mineralization, when compared to untreated cells. The lack of mineralization appears to result from the lack of normal collagen fibril deposition and formation of an appropriate ECM essential for the mineralization process. These results support our previous observations that TNF-alpha inhibits HOS TE85 cells from forming a mineralizing ECM by inhibiting incorporation of collagen into the ECM and inducing the synthesis of proteolytic enzymes capable of degrading collagen in the ECM.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Differentiation of human osteoblastic cells in culture: modulation of proteases by extracellular matrix and tumor necrosis factor-alpha

We have followed the synthesis and secretion of urokinase-type plasminogen activator (u-PA) and its inhibitor, PAI-1, and matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMP-1) during differentiation of a human osteoblastic cell line, HOS TE85, and the effect of TNF-alpha on this process. Our results show that the ratio of u-PA/PAI-1 associated with the cell-matrix components increases during differentiation of these cells over a 14-day period. Although TNF-alpha suppresses the induced increase in steady-state mRNA levels of u-PA and PAI-1 during maturation of extracellular matrix (ECM), the u-PA/PAI-1 ratio is altered in such a way that PA activity associated with the ECM is higher than control cells. The expression of MMP-1 is low and remains essentially invariant over a culture period of 14 days. TNF-alpha enhances MMP-1 transcription nearly 12-fold initially, after which mRNA levels drop off but remain significantly higher than the controls. Activities and steady-state mRNA levels of MMP-2 and MMP-9 increase nearly 15-fold during maturation of the ECM, but the level of TIMP-1 mRNA is not appreciably altered. The presence of TNF-alpha suppresses maturation-induced transcription of MMP-2, enhances TIMP-1 transcription, but has little effect on MMP-9 mRNA levels. The data show that chronic exposure to TNF-alpha alters the balance between u-PA/PAI-1 and MMPs/TIMP-1, which favors higher activity of proteinases. Accordingly, the presence of TNF-alpha in chronic inflammatory episodes would be expected to alter bone remodeling by inhibiting maturation of ECM and formation of bone.