Human rheumatoid synovial fibroblasts promote osteoclastogenic activity by activating RANKL via TLR-2 and TLR-4 activation

Induction of apoptosis of human primary osteoclasts treated with extracts from the medicinal plant Emblica officinalis

Background

Osteoclasts (OCs) are involved in rheumatoid arthritis and in several pathologies associated with bone loss. Recent results support the concept that some medicinal plants and derived natural products are of great interest for developing therapeutic strategies against bone disorders, including rheumatoid arthritis and osteoporosis. In this study we determined whether extracts of Emblica officinalis fruits display activity of possible interest for the treatment of rheumatoid arthritis and osteoporosis by activating programmed cell death of human primary osteoclasts.

Methods

The effects of extracts from Emblica officinalis on differentiation and survival of human primary OCs cultures obtained from peripheral blood were determined by tartrate-acid resistant acid phosphatase (TRAP)-positivity and colorimetric MTT assay. The effects of Emblica officinalis extracts on induction of OCs apoptosis were studied using TUNEL and immunocytochemical analysis of FAS receptor expression. Finally, in vitro effects of Emblica officinalis extracts on NF-kB transcription factor activity were determined by gel shift experiments.

Results

Extracts of Emblica officinalis were able to induce programmed cell death of mature OCs, without altering, at the concentrations employed in our study, the process of osteoclastogenesis. Emblica officinalis increased the expression levels of Fas, a critical member of the apoptotic pathway. Gel shift experiments demonstrated that Emblica officinalis extracts act by interfering with NF-kB activity, a transcription factor involved in osteoclast biology. The data obtained demonstrate that Emblica officinalis extracts selectively compete with the binding of transcription factor NF-kB to its specific target DNA sequences. This effect might explain the observed effects of Emblica officinalis on the expression levels of interleukin-6, a NF-kB specific target gene.

Conclusion

Induction of apoptosis of osteoclasts could be an important strategy both in interfering with rheumatoid arthritis complications of the bone skeleton leading to joint destruction, and preventing and reducing osteoporosis. Accordingly, we suggest the application of Emblica officinalis extracts as an alternative tool for therapy applied to bone diseases.

Cytokine-controlled RANKL and osteoprotegerin expression by human and mouse synovial fibroblasts: fibroblast-mediated pathologic bone resorption

OBJECTIVE

To determine whether proinflammatory cytokine treatment or the complete absence of select cytokines modulates the expression of RANKL and osteoprotegerin (OPG) in synovial fibroblasts.

METHODS

Fibroblasts were isolated from normal and rheumatoid human synovium and from normal or arthritic joints of wild-type and cytokine gene-deficient (interleukin-4-knockout [IL-4 (-/-)] and interferon-gamma-knockout [IFNgamma (-/-)]) mice. Fibroblasts were stimulated with proinflammatory cytokines (tumor necrosis factor alpha [TNFalpha], IL-1beta, and IL-17) or antiosteoclastogenic cytokines (IL-4 and IFNgamma), alone or in combination, and the expression of RANKL and OPG was measured.

RESULTS

Proinflammatory cytokine-stimulated fibroblasts from rheumatoid and arthritic mouse joints expressed higher levels of RANKL and OPG than those from normal joints. IL-4 suppressed RANKL expression and increased OPG expression, IFNgamma reduced the production of both RANKL and OPG, and IL-17 had only a modest effect on the expression of RANKL or OPG. Additive effects of combination treatment (TNFalpha/IL-17 or IL-1beta/IL-17) were observed only in the human system. Extensive destruction was observed in the arthritic joints of IL-4 (-/-) mice, with a corresponding upward shift of the RANKL:OPG ratios. However, an IL-17 deficiency did not attenuate arthritis or reduce bone resorption.

CONCLUSION

Proinflammatory cytokines induce the expression of RANKL and OPG in both human and murine synovial fibroblasts. The RANKL:OPG ratios are shifted in favor of bone protection by IL-4 treatment, and, to a lesser extent, by IFNgamma treatment. Unexpectedly, an IL-17 deficiency alone does not induce reduced inflammatory bone destruction. Our results suggest that synovial fibroblasts may significantly contribute to bone resorption through modulation of RANKL and OPG production in a cytokine-rich milieu of inflamed joints.

Release of heat shock protein 70 and the effects of extracellular heat shock protein 70 on the production of IL-10 in fibroblast-like synoviocytes

It has recently been suggested that heat shock protein (Hsp) 70, an intracellular protein, can be released into the extracellular compartment and exert important immunomodulatory functions. Although elevated Hsp70 has been found in synovial fluid from patients with rheumatoid arthritis (RA), its sources and extracellular functions remain unclear. In this study, we explored whether stress response such as heat stress or exposure to tumor necrosis factor-alpha (TNF-alpha) could induce Hsp70 release from RA fibroblast-like synoviocytes (FLSs) and whether extracellular Hsp70 would stimulate cytokine production in RA FLSs. Cultured FLSs were obtained from patients with RA. The expression of intracellular Hsp70 was studied by Western blot. Hsp70 release and the production of interleukin (IL)-6, IL-8, and IL-10 by RA FLSs were studied by specific enzyme-linked immunosorbent assays. The levels of Toll-like receptor (TLR) 2 and 4 mRNA and protein in FLSs were analyzed using reverse transcription-polymerase chain reaction and Western blotting. Treatment with sublethal heat shock or TNF-alpha results in the up-regulation of intracellular Hsp70 in FLSs and Hsp70 release from RA FLSs. In vitro studies show that extracellular Hsp70 can induce anti-inflammatory cytokine IL-10 production in FLSs. The mRNA and protein expression of TLR2 and TLR4 was demonstrated in FLSs, and TLR4 blocking abrogated the up-regulatory effects of Hsp70 on IL-10 production. Thus, these results lend support to the hypothesis that Hsp70 is actively released from FLSs in response to heat shock or TNF-alpha and Hsp70 may be a major paracrine/autocrine inducer of IL-10 production in FLSs via TLR4.

The roles of the classical and alternative nuclear factor-kappaB pathways: potential implications for autoimmunity and rheumatoid arthritis

Nuclear factor-κB (NF-κB) is an inducible transcription factor controlled by two principal signaling cascades, each activated by a set of signal ligands: the classical/canonical NF-κB activation pathway and the alternative/noncanonical pathway. The former pathway proceeds via phosphorylation and degradation of inhibitor of NF-κB (IκB) and leads most commonly to activation of the heterodimer RelA/NF-κB1(p50). The latter pathway proceeds via phosphorylation and proteolytic processing of NF-κB2 (p100) and leads to activation, most commonly, of the heterodimer RelB/NF-κB2 (p52). Both pathways play critical roles at multiple levels of the immune system in both health and disease, including the autoimmune inflammatory response. These roles include cell cycle progression, cell survival, adhesion, and inhibition of apoptosis. NF-κB is constitutively activated in many autoimmune diseases, including diabetes type 1, systemic lupus erythematosus, and rheumatoid arthritis (RA). In this review we survey recent developments in the involvement of the classical and alternative pathways of NF-κB activation in autoimmunity, focusing particularly on RA. We discuss the involvement of NF-κB in self-reactive T and B lymphocyte development, survival and proliferation, and the maintenance of chronic inflammation due to cytokines such as tumor necrosis factor-α, IL-1, IL-6, and IL-8. We discuss the roles played by IL-17 and T-helper-17 cells in the inflammatory process; in the activation, maturation, and proliferation of RA fibroblast-like synovial cells; and differentiation and activation of osteoclast bone-resorbing activity. The prospects of therapeutic intervention to block activation of the NF-κB signaling pathways in RA are also discussed.

The role of resident synovial cells in destructive arthritis

Infiltration by inflammatory cells, thickening of the lining layer, and destructive invasion into cartilage and bone are pathognomic features of the synovium in rheumatoid arthritis (RA). However, the most common cell types at the sites of invasion are resident cells of the joint, in particular synovial fibroblasts. These cells differ from healthy synovial fibroblasts in their morphology, their expression of proto-oncogenes and antiapoptotic molecules, and in their lack of certain tumor suppressor genes. Through their production of proinflammatory cytokines and chemokines mediated by signaling via Toll-like receptors, they are not only effector cells but also active parts of the innate immune system attracting inflammatory immune cells to the synovium. Most importantly, by producing matrix-degrading molecules they contribute strongly to the destructive mechanisms operative in RA.

Characterization of fibroblasts responsible for cartilage destruction in arthritis

In the pathogenesis of rheumatoid arthritis (RA), synovial fibroblasts (SF) play a key role as they secrete distinct patterns of cytokines and express variable levels of costimulatory and adhesion molecules. The murine fibroblast cell line LS48 has been shown to be invasive in the cartilage destruction models in vivo and in vitro. The purpose of this study was to examine in detail the LS48 phenotype, to obtain a better understanding of the SF-mediated cartilage destruction in RA. The destructive fibroblasts line LS48 and the nondestructive 3T3 cells were cultured and characterized with slide-based and flow cytometry, using antibodies against several adhesion molecules, immunological acting molecules, and marker proteins. The invasive LS48 fibroblasts are characterized by significantly higher expression of adhesion molecules such as CD47 (IAP), CD51 (integrin alpha V), CD61 (GPIIIa), and CD147 (EMMPRIN), and immunological acting molecules such as CD40 (Bp50), CD55 (DAF), and TLR-2. The results from the slide-based and flow cytometry analyses were exactly the same, except for the selected CD147 and TLR-2. This study demonstrated that the destructive fibroblast cell line LS48 has the characteristics of RA SFs. The high expression of specific costimulatory and adhesion molecules underlines the aberrant phenotype of these cells when compared with noninvasive fibroblasts. Furthermore, slide-based and flow cytometry complement each other in fibroblast phenotyping. Overall, this study shows that LS48 is an excellent tool to gain a deeper understanding of SF in RA.

The immune response is involved in atherosclerotic plaque calcification: could the RANKL/RANK/OPG system be a marker of plaque instability?

Atherogenesis is characterized by an intense inflammatory process, involving immune and vascular cells. These cells play a crucial role in all phases of atherosclerotic plaque formation and complication through cytokine, protease, and prothrombotic factor secretion. The accumulation of inflammatory cells and thus high amounts of soluble mediators are responsible for the evolution of some plaques to instable phenotype which may lead to rupture. One condition strongly associated with plaque rupture is calcification, a physiopathological process orchestrated by several soluble factors, including the receptor activator of nuclear factor (NF)κB ligand (RANKL)/receptor activator of nuclear factor (NF)κB (RANK)/osteoprotegerin (OPG) system. Although some studies showed some interesting correlations with acute ischemic events, at present, more evidences are needed to evaluate the predictive and diagnostic value of serum sRANKL and OPG levels for clinical use. The major limitation is probably the poor specificity of these factors for cardiovascular disease. The identification of tissue-specific isoforms could increase the importance of sRANKL and OPG in predicting calcified plaque rupture and the dramatic ischemic consequences in the brain and the heart.

Th17 cell differentiation: the long and winding road

The characterization of the new lineage of IL-17-producing CD4+ T helper (Th17) cells has revolutionized our current understanding of T cell-mediated immunity. Over the past five years, there have been many twists and turns as the pathways that lead to Th17 cell differentiation have been elucidated. Not least of these was the discovery that TGF-beta is a crucial cytokine for Th17 cell development, suggesting that Th17 and regulatory T cell subsets share reciprocal developmental pathways during the pathogenesis or control of inflammation. This review aims to bring together the observations that have formed current opinion on factors that promote and contain Th17 cell development, in both mouse and man. Unresolved controversies in this field are also discussed: For example, IL-23 is absolutely required for disease pathogenesis in many models of Th17-cell-mediated autoimmunity, yet its role in Th17 cell development is relatively unclear.

Epigenetic clues to rheumatoid arthritis

The innate immune response needs to be tightly regulated to balance elimination of microorganisms with the magnitude of inflammation. The rupture of this balance is crucial for the outcome of diseases such as rheumatoid arthritis (RA) in which an overflowed proinflammatory response is associated with self-damage. Epigenetics alludes to systems controlling gene expression and silencing independent of the germline, but stable enough to be inherited by daughter cells upon mitosis. We will show in this review how pathological processes in RA can be shaped by epigenetics, which may in turn explain differences in phenotypes between subgroups of patients and also between subsets of fibroblasts within the joint. On the whole, the concourse of epigenetic mechanisms can precipitate the aggressive behaviour of cells and the rupture of peripheral tolerance. Targeting these emerging regulatory pathways is a promising approach for RA therapeutics.