Extracts of Actinobacillus actinomycetemcomitans induce apoptotic cell death in human osteoblastic MG63 cells

NLRP3 inflammasome is required for apoptosis of Aggregatibacter actinomycetemcomitans-infected human osteoblastic MG63 cells

Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) is a Gram-negative bacterium which is implicated in the pathogenesis of human periodontal disease and in particular aggressive periodontitis. Virulence factors from A. actinomycetemcomitans have been shown to induce apoptosis of osteoblasts, however, the underlying mechanisms of the induction of apoptosis are poorly understood. In the present study, the infection of A. actinomycetemcomitans in human osteoblastic MG63 cells was established. Accordingly, A. actinomycetemcomitans infection enhanced significant apoptosis of MG63 cells. We found that both expression levels of NLRP3 and ASC were increased dramatically after MG63 cell cultures exposed to A. actinomycetemcomitans. Moreover, the secretion of mature interleukin-1β (IL-1β) and IL-18 were extensively induced in A. actinomycetemcomitans-infected cells as compared with non-invasion group of MG63 cell cultures, indicating the activation of the NLRP3 inflammasome during infection. Finally, we found that the knockdown expression of NLRP3 by specific small interfering RNA (siRNA) attenuated apoptosis of A. actinomycetemcomitans-infected MG63 cells. Our data suggest that A. actinomycetemcomitans promotes apoptosis of human osteoblasts at least partially through the NLRP3 inflammasome.

Apoptosis-associated uncoupling of bone formation and resorption in osteomyelitis

The mechanisms underlying the destruction of bone tissue in osteomyelitis are only now being elucidated. While some of the tissue damage associated with osteomyelitis likely results from the direct actions of bacteria and infiltrating leukocytes, perhaps exacerbated by bacterial manipulation of leukocyte survival pathways, infection-induced bone loss predominantly results from an uncoupling of the activities of osteoblasts and osteoclasts. Bacteria or their products can directly increase osteoclast formation and activity, and the inflammatory milieu at sites of infection can further promote bone resorption. In addition, osteoclast activity is critically regulated by osteoblasts that can respond to bacterial pathogens and foster both inflammation and osteoclastogenesis. Importantly, bone loss during osteomyelitis is also brought about by a decline in new bone deposition due to decreased bone matrix synthesis and by increased rates of osteoblast apoptosis. Extracellular bacterial components may be sufficient to reduce osteoblast viability, but the causative agents of osteomyelitis are also capable of inducing continuous apoptosis of these cells by activating intrinsic and extrinsic cell death pathways to further uncouple bone formation and resorption. Interestingly, bacterial internalization appears to be required for maximal osteoblast apoptosis, and cytosolic inflammasome activation may act in concert with autocrine/paracrine death receptor-ligand signaling to induce cell death. The manipulation of apoptotic pathways in infected bone cells could be an attractive new means to limit inflammatory damage in osteomyelitis. However, the mechanism that is the most important in bacterium-induced bone loss has not yet been identified. Furthermore, it remains to be determined whether the host would be best served by preventing osteoblast cell death or by promoting apoptosis in infected cells.

Modulation of connexin signaling by bacterial pathogens and their toxins

Inherent to their pivotal tasks in the maintenance of cellular homeostasis, gap junctions, connexin hemichannels, and pannexin hemichannels are frequently involved in the dysregulation of this critical balance. The present paper specifically focuses on their roles in bacterial infection and disease. In particular, the reported biological outcome of clinically important bacteria including Escherichia coli, Shigella flexneri, Yersinia enterocolitica, Helicobacter pylori, Bordetella pertussis, Aggregatibacter actinomycetemcomitans, Pseudomonas aeruginosa, Citrobacter rodentium, Clostridium species, Streptococcus pneumoniae, and Staphylococcus aureus and their toxic products on connexin- and pannexin-related signaling in host cells is reviewed. Particular attention is paid to the underlying molecular mechanisms of these effects as well as to the actual biological relevance of these findings.

Lipopolysaccharide alters decorin and biglycan synthesis in rat alveolar bone osteoblasts: consequences for bone repair during periodontal disease

A prime pathogenic agent associated with periodontitis is lipopolysaccharide (LPS) derived from Porphyromonas gingivalis. This study investigated the effects of P. gingivalis LPS on osteoblasts, which are responsible for alveolar bone repair. Bone cells were obtained from explants of rat alveolar bone chips and cultured with 0–200 ng ml⁻¹ of P. gingivalis LPS. Porphyromonas gingivalis LPS significantly increased cell proliferation and inhibited osteoblast differentiation, as judged by reduced alkaline phosphatase activity. Analysis of biglycan mRNA and protein levels indicated that P. gingivalis LPS significantly delayed the normally high expression of biglycan during the early stages of culture, which are associated with cell proliferation and early differentiation of progenitor cells. In the presence of P. gingivalis LPS, decorin expression by the alveolar bone cells was reduced during periods of culture relating to collagen fibrillogenesis and mineral deposition. Analysis of glycosaminoglycan chains conjugated to these proteoglycans suggested that in the presence of P. gingivalis LPS, dermatan sulfate persisted within the matrix. This study suggests that P. gingivalis LPS influences the expression and processing of decorin and biglycan in the matrix, altering alveolar bone cell activity and osteoblast phenotype development. The consequences of this altered expression in relation to hindering bone repair as part of the cycle of events during periodontal disease are discussed.

Cytokine-induced nitric-oxide-dependent apoptosis in mouse osteoblastic cells: Involvement of p38MAP kinase

The apoptotic signalling induced by pro-inflammatory cytokines was examined in mouse osteoblastic MC3T3-E1 cells. Annexin-V/propidium iodine double-staining analysis demonstrated that the combination of tumour necrosis factor-alpha, interleukin-1beta and interferon-gamma caused cell death in osteoblastic cells mediated by apoptosis, not necrosis. Treatment with these cytokines resulted in potent enhancement of inducible nitric-oxide synthase (iNOS) mRNA and nitric-oxide (NO) in the cells. A specific inhibitor of p38 mitogen-activated protein (MAP) kinase, i.e. SB203580, dose dependently inhibited the induction of iNOS mRNA, its enzyme product, NO and DNA fragmentation (as an apoptosis index) in the cytokine-treated cells (P<0.05). In contrast, PD98059, a specific inhibitor of MEK that acts immediately upstream of classic MAP kinase, had no effect on the induction of iNOS, NO or DNA fragmentation in the cells. These results demonstrate that this cytokine-induced apoptosis in mouse osteoblastic cells was mediated by a p38MAP-kinase-dependent iNOS system.

Dual roles for NF-kappaB activation in osteoblastic cells by serum deprivation: osteoblastic apoptosis and cell-cycle arrest

To clarify the mechanisms of osteoblastic cell death, we examined whether serum deprivation would cause activation of the apoptotic signal cascade and arrest of the cell cycle in mouse osteoblastic MC3T3-E1 cells. Serum withdrawal from osteoblastic cell cultures resulted in growth arrest and cell-cycle arrest at G0/G1, which actions were accompanied by transient and potent activation of NF-kappaB, caspase-8, caspase-2, caspase-3, and caspase-9 in this order. Apoptosis, but not necrosis, in serum-deprived cells could be detected by FACS using Annexin-V/propidium iodine double staining. Serum deprivation also resulted in transient activation of the 20S proteasome, which is an important component for regulation of the cell cycle by the ubiquitin-proteasome system. The 20S proteasome inhibitor (PSI) but not NF-kappaB inhibitor SN50 suppressed the activation of proteasomes in serum-deprived cells. Although caspase inhibitors could not prevent the G0/G1 arrest in the serum-deprived cells, SN50 and the 20S proteasome inhibitor could block it. Since SN50, 20S proteasome inhibitor and caspase inhibitor could rescue cells from serum deprivation-induced apoptosis, the pathway for NF-kappaB/caspase activation is independent of the NF-kappaB/cell-cycle pathway, and the events downstream of the NF-kappaB/caspase-9 cascade lead to apoptosis. Taken together, our present results identify a novel role for NF-kappaB in cell-cycle and apoptosis regulation and underscore the significance of each independent signal cascade in serum-deprived osteoblastic cells.

Activation of caspases is required for osteoblastic differentiation

Previous studies have shown that mouse osteoblastic MC3T3-E1 cells undergo apoptosis when exposed to a mixture of proinflammatory cytokines. Bone morphogenetic protein (BMP)s are important regulators of osteoblast differentiation. Because regulation of osteoblastic differentiation is poorly understood, we sought to determine if BMP-4-induced differentiation of osteoblastic cells depends on the activity of the key apoptotic proteases, i.e. the caspases. BMP-4 induced the growth arrest and differentiation of osteoblastic cell line MC3T3-E1, as evidenced by the appearance of osteoblastic phenotypes such as alkaline phosphatase (ALP) activation and parathyroid hormone (PTH)-dependent production of cAMP. Surprisingly, BMP-4 induced transient and potent activation of caspase-8, caspase-2, and caspase-3, in this order. However, no apoptosis or necrosis in BMP-4-treated cells could be detected by FACS using annexin-V/propodium iodine double staining. Peptide inhibition of caspase activity led to a dramatic reduction in ALP activation and PTH-induced production of cAMP in BMP-4-treated cells. Although BMP-4 treatment resulted in cell-cycle G0/G1 arrest as detected by FACS cell-cycle analysis, caspase inhibitors (caspase-8, caspase-2, and caspase-3 inhibitors) could block the G0/G1 arrest in MC3T3-E1 cells. Taken together, these results confirm a unique and unanticipated role for the caspase-mediated signal cascade in the differentiation of osteoblasts.

Attachment of Periodontal Ligament Cells to Chlorhexidine-Loaded Guided Tissue Regeneration Membranes

BACKGROUND

Early exposure of a guided tissue regeneration (GTR) membrane in the oral cavity results in bacterial contamination, which may lead to failure or incomplete regeneration. Incorporation of antimicrobial agents in GTR membranes may be valuable to control membrane-associated infection during GTR therapy. The purpose of this study was to evaluate whether the incorporation of chlorhexidine into various GTR membranes improves the attachment of periodontal ligament cells in the presence of Actinobacillus actinomycetemcomitans.

METHODS

The possible effects of chlorhexidine on the viability of primary human periodontal ligament (PDL) cells were determined using 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide (MTT), which measures cellular metabolic activity. An expanded polytetrafluoroethylene (ePTFE) membrane, glycolide fiber membrane, and collagen membrane were loaded with chlorhexidine and characterized. Attachment of PDL cells to the chlorhexidine-loaded membranes with or without A. actinomycetemcomitans was examined using scanning electron microscopy (SEM) analysis.

RESULTS

Relative cellular viability of PDL cells was reduced to approximately 50% when 15 microg/ml (0.0015%) of chlorhexidine was used. Chlorhexidine released from the coated GTR membranes inhibited the growth of A. actinomycetemcomitans. At the concentration used in this study, chlorhexidine incorporated into the GTR membranes did not interfere with the attachment of PDL cells. The inhibitory effects of A. actinomycetemcomitans on cellular attachment were reduced using chlorhexidine-loaded membranes, including ePTFE, glycolide fiber, and collagen membranes.

CONCLUSIONS

These results suggest that incorporation of chlorhexidine into GTR membranes is beneficial in reducing bacterial effects on cellular attachment. The future application of chlorhexidine-loaded membranes during GTR therapy may be of value.

Apoptosis: an introduction for the endodontist

Apoptosis plays an important role in many aspects of endodontics, yet there is a paucity of information in this regard in the endodontic literature. Apoptosis is a single deletion of scattered cells by fragmentation into membrane-bound particles that are phagocytosed by other cells. It is a key process in the embryological development of the tooth, periodontal ligament and supporting oral tissue in the progression of oral disease, bone resorption, immunological response and inflammation, and in wound healing and certain pharmacological effects. The understanding of the ability of clinical materials to induce or inhibit apoptosis and the investigation of apoptosis as it relates to the pathogenesis of pulpal and periradicular pathology may eventually lead to new treatment approaches for the endodontist. The purpose of this review is to familiarize the clinical endodontist with current knowledge on apoptosis as it relates to the pulp and periradicular tissues.

Actinobacillus actinomycetemcomitans induces apoptosis of T lymphocytes by the Fas and Fas ligand pathway

Actinobacillus actinomycetemcomitans expresses a number of toxins capable of inducing apoptotic cell death of T lymphocytes. However, the exact mechanism(s) has not been elucidated. The present study investigated the involvement of the Fas (CD95)-mediated apoptotic pathway in A. actinomycetemcomitans-induced T-cell apoptosis. To that end, peripheral blood mononuclear cells (PBMC) were cultured with or without A. actinomycetemcomitans cell-free culture supernatant (CFCS) for 0-96 h. The cells were then labeled with specific monoclonal antibodies and flow cytometry was performed. Results demonstrated up-regulation of Fas and activation of caspase-3 in T cells in response to A. actinomycetemcomitans CFCS. Monocytes were the only cells analyzed to express Fas ligand (FasL) constitutively, and this was further up-regulated in response to A. actinomycetemcomitans CFCS, while T cells expressed FasL only after this stimulation. Depletion of monocytes prior to stimulation with A. actinomycetemcomitans CFCS led to a marked decline in apoptosis. Blocking of Fas-FasL interactions with anti-Fas monoclonal antibody or Fas:Fc fusion protein lead to a significant decline, but not abolition, of T-cell apoptosis. Nearly all T cells expressed Bcl-2 at the outset of culture, and Bcl-2 expression declined in T cells stimulated with A. actinomycetemcomitans CFCS. Collectively, these data provide evidence for the induction of T-cell apoptosis by A. actinomycetemcomitans via the Fas-mediated pathway, involving caspase-3 and Bcl-2. Moreover, this apoptotic response was dependent on the presence of monocytes.

Permeability of Streptococcus mutans and Actinobacillus actinomycetemcomitans Through guided tissue regeneration membranes and their effects on attachment of periodontal ligament cells

BACKGROUND

Microbial colonization on barrier materials used in guided tissue regeneration (GTR) may adversely affect treatment outcomes. The purposes of this study were: 1) to compare the invasion of Streptococcus mutans and Actinobacillus actinomycetemcomitans through 3 GTR membranes, composed of expanded polytetrafluoroethylene (ePTFE; non-resorbable), a glycolide fiber composite, and type I collagen (both bioabsorbable), and 2) to explore the effects of bacteria on the attachment of periodontal ligament (PDL) fibroblasts onto these membranes.

METHODS

Bacterial permeability was analyzed using a tube capped with a GTR membrane as a septum and filled with media. The tube was then placed in a bigger tube inoculated with S. mutans or A. actinomycetemcomitans. The passage of bacteria through the membranes into the inner tube was monitored. For cellular attachment experiments, primary human PDL cells were placed onto the GTR membranes with or without bacteria. Attached cells were analyzed by scanning electron microscopy (SEM) analysis.

RESULTS

The ePTFE membrane had the best barrier effects followed by the collagen membrane and then the glycolide fiber composite membrane. Moreover, S. mutans passed through these membranes faster than A. actinomycetemcomitans. The attachment of PDL cells on the 3 membranes was also varied. The ePTFE membrane was the worst substrate for PDL fibroblast attachment. Moreover, both bacteria influenced the cellular attachment on the GTR membranes.

CONCLUSIONS

Differences in the behavior of 3 GTR membranes penetrated by S. mutans and A. actinomycetemcomitans were demonstrated. The results suggest that attachment of PDL cells was affected on bacterial-contaminated GTR membranes, which may alter healing following membrane exposure.

Differential expression of the Fas-Fas ligand system on cytokine-induced apoptotic cell death in mouse osteoblastic cells

Apoptotic signalling, particularly in the Fas-Fas ligand (FasL) system, was studied in a mouse osteoblastic cell line, MC3T3-E1. A combination of the cytokines tumour necrosis factor-alpha, interleukin-1beta and interferon-gamma activated the Fas-FasL-dependent cell-death system. The cytokines caused significant enhancement of Fas mRNA and Fas protein, and led to apoptotic cell death. Western blot demonstrated that FasL protein was continuously present in MC3T3-E1 cells, although the cytokines had no effect on the induction of FasL. Exogenous FasL caused a decrease in cell viability and a large increase in apoptotic cell death in cells pre-treated with cytokines, indicating that the Fas-FasL system has the potential to cause apoptosis in osteoblastic cells. Treatment with anti-Fas IgG (antagonistic antibody) inhibited the DNA fragmentation induced by cytokines in a dose-dependent manner, suggesting that cytokine-induced Fas may cause apoptotic cell death in MC3T3-E1 cells. Taken together, these findings show that cytokine-induced apoptotic cell death was mediated by the autocrine or paracrine Fas-FasL system in mouse osteoblastic cells, and suggest that cytokine-induced apoptosis could have an important role in localised bone destruction associated with inflammatory bone diseases such as periodontal disease.

Staphylococcus aureus and Salmonella enterica serovar Dublin induce tumor necrosis factor-related apoptosis-inducing ligand expression by normal mouse and human osteoblasts

Staphylococcus aureus and Salmonella enterica serovar Dublin invade osteoblasts and are causative agents of human bone disease. In the present study, we examined the ability of S. aureus and Salmonella serovar Dublin to induce the production of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) by normal osteoblasts. Normal mouse and human osteoblasts were cocultured with S. aureus or Salmonella serovar Dublin at different multiplicities of infection. Following initial incubation and examination of TRAIL expression, extracellular bacteria were killed by the addition of media containing the antibiotic gentamicin. Lysates and conditioned media from osteoblast cultures were then collected at various times following invasion and analyzed. The results demonstrated that S. aureus and Salmonella serovar Dublin are potent inducers of TRAIL expression by osteoblasts. Mouse and human TRAIL mRNA expression was induced by bacterial infection and demonstrated a dose-dependent response. Analysis of kinetics suggested that TRAIL mRNA was induced within 30 min after exposure to bacteria and that its level of expression remained relatively constant over the time period examined. mRNA molecules encoding TRAIL receptors were constitutively expressed by osteoblasts. Furthermore, TRAIL protein was detected as early as 45 min and up to 24 h following infection. The quantity of TRAIL protein produced also increased in a dose-dependent manner. Collectively, these findings suggest a mechanism whereby bacterial pathogens mediate bone destruction via osteoblast apoptosis.

Evidence for apoptosis of the majority of T cells activated in vitro with Actinobacillus actinomycetemcomitans

Our previous studies had demonstrated that nearly half of all T cells stimulated with Actinobacillus actinomycetemcomitans are activated within a few hours. However, it was not known whether all of these T cells survive. The aim of the present study was to determine whether the T cells activated in response to A. actinomycetemcomitans undergo apoptosis. To that end, peripheral blood mononuclear cells were cultured at different time points in the presence of A. actinomycetemcomitans. Flow cytometric analysis demonstrated that, following exposure to a preparation of A. actinomycetemcomitans, T cells progressively externalized their plasma membrane phosphatidylserine, as measured by annexin V binding. Approximately half of all T cells bound annexin V by 96 h. During this period, Annexin V-positive T cells also incorporated propidium iodide suggesting loss of membrane integrity. The externalization of phosphatidylserine occurred at a higher rate among activated (CD69+) T cells, where roughly two-thirds became Annexin V-positive. Flow cytometric analysis also demonstrated shrinkage of the Annexin V-positive and propidium iodide-positive T cells. The data presented here provides evidence for the induction of apoptosis among the majority of the T cells responding to A. actinomycetemcomitans.

Inhibition of mitosis and induction of apoptosis in MG63 human osteosarcoma-derived cells in vitro by surface proteins from Actinobacillus actinomycetemcomintans

Gentle saline extraction releases a heterogeneous mixture of proteins associated with the cell surface of Actinobacillus actinomycetemcomintans, termed the surface protein fraction (SF). Some SF components are biologically active and may modulate cell behaviour in a manner of putative importance in the aetiology of periodontitis. To further characterize this activity, the ability of the SF to induce mitosis and apoptosis in MG63 cells was investigated. Cells were plated at 10(3)-10(4) cells/cm(2) and allowed to attach before culture in the serum-free medium in the presence of 25 microg/ml SF for 2-24 h. The apoptotic and mitotic figures present were counted and the results expressed as an apoptotic or mitotic index. The apoptotic and mitotic compartments were very small, but there was an inverse correlation between mitosis and apoptosis. In control experiments the mitotic was higher than the apoptotic index, whilst in the presence of SF this was reversed. These results were confirmed using in situ end-labelling. SF, therefore, may stimulate apoptotic, but inhibit mitotic, activity in MG63 cells. This raises the possibility that components of SF might induce subtle changes in the balance between apoptosis and mitosis, which, in turn, could contribute to the progression of periodontitis.

Intracellular Staphylococcus aureus induces apoptosis in mouse osteoblasts

Staphylococcus aureus invades osteoblasts and is the primary cause of osteomyelitis. This study examined the ability of S. aureus to induce apoptosis in a mouse osteoblast cell line. The presence of intracellular S. aureus was demonstrated by transmission electron microscopy. Light microscopy was utilized to examine morphological changes in the osteoblasts following killing of extracellular bacteria. Cell rounding was observed, and dark centers due to condensation of chromatin were noted in cells in infected osteoblast cultures. DNA was isolated from infected osteoblast cultures, and electrophoresis revealed the laddering effect characteristic of cells undergoing apoptosis. Additionally, an in situ cell death detection assay was utilized to label apoptosis-induced DNA strand breaks. Apoptotic nuclei were present, providing further evidence that S. aureus induces apoptosis in osteoblasts.