Necessary role of phosphatidylinositol 3‐kinase in transforming growth factor β–mediated activation of Akt in normal and rheumatoid arthritis synovial fibroblasts

Objective

Rheumatoid arthritis is a disease that, pathologically, is characterized by the progressive growth and invasion of the synovial pannus into the surrounding cartilage and bone. Many cytokines, including transforming growth factor β1 (TGFβ1), have been implicated in this process, but their mode of action is incompletely understood. The goal of the present study was to better understand the downstream signaling pathways of TGFβ in fibroblasts.

Methods

The role of phosphatidylinositol 3‐kinase (PI 3‐kinase) was determined by chemical inhibition with LY294002 or wortmannin. Activation of protein kinase B (Akt), c‐Jun N‐terminal kinases (JNKs), and extracellular signal–regulated kinases (ERKs) was evaluated by Western blot analysis using phospho‐specific antibodies.

Results

Exposure of fibroblasts to TGFβ rapidly induced activation of a kinase, Akt, that is known to inhibit apoptosis by a variety of pathways. Activation of Akt was blocked by the specific PI 3‐kinase inhibitor, LY294002, indicating that TGFβ‐mediated phosphorylation of Akt was dependent on PI 3‐kinase activation. This activation pathway was relatively selective for Akt, since inhibition of PI 3‐kinase failed to substantially modify activation of ERKs or JNKs in synovial fibroblasts. Inhibition of the PI 3‐kinase/Akt pathway resulted in impaired proliferation of synovial fibroblasts and partial attenuation of the protective effect of TGFβ on Fas‐mediated apoptosis.

Conclusion

TGFβ exerts its growth and antiapoptotic effects on fibroblasts, at least in part, by activation of the PI 3‐kinase/Akt pathway.

     

Crucial role of synovial lining macrophages in the promotion of transforming growth factor beta-mediated osteophyte formation

OBJECTIVE: To investigate in vivo and in vitro whether macrophages have an intermediate role in transforming growth factor beta (TGFbeta)-induced osteophyte formation. METHODS: In vivo, synovial lining macrophages were selectively depleted by injection of clodronate-laden liposomes 7 days prior to injection of 20 ng or 200 ng of TGFbeta into murine knee joints 3 times, on alternate days. Total knee joint sections were obtained on day 7 after the last injection and stained with Safranin O. Production of bone morphogenetic protein 2 (BMP-2) and BMP-4 was determined by immunolocalization. The interaction between murine macrophages and mesenchymal cells (precursors with chondrogenic potential) was studied in vitro using a Transwell system in which RAW macrophages were cocultured with C3H10T1/2 mesenchymal cells. Spheroid neocartilage formation was quantified microscopically after staining with May-Grünwald-Giemsa. RESULTS: Triple injections of 20 ng or 200 ng of TGFbeta into normal murine knee joints induced significant osteophyte formation at the lateral and medial sites of the patella and femur on day 7 after the last injection. Strikingly, removal of synovial lining macrophages prior to TGFbeta injection resulted in a drastic reduction of osteophyte formation (by 70% and 64% after injection of 20 ng and 200 ng of TGFbeta, respectively). Synovial lining cells produced BMP-2 and BMP-4 after TGFbeta stimulation, whereas BMP-2 and BMP-4 were absent in the synovial tissue after macrophage depletion. In vitro, clustering and spheroid formation of C3H10T1/2 was induced by TGFbeta concentrations of >1 ng/ml. However, in the Transwell system, in the presence of murine macrophages, 0.5 ng/ml of TGFbeta was very effective in generating large spheroids, suggestive of macrophage-derived (co)factors. In coculture supernatants, TGFbeta concentrations were not elevated in the presence of macrophages, indicating generation of other growth factors involved in spheroid formation. CONCLUSION: These findings indicate that macrophages are crucial intermediate factors in osteophyte formation induced by TGFbeta, probably by inducing other chondrogenic signals.     

Regulation of tumor necrosis factor alpha-mediated apoptosis of rheumatoid arthritis synovial fibroblasts by the protein kinase Akt

OBJECTIVE: To determine if tumor necrosis factor alpha (TNFalpha)-driven proliferation of rheumatoid arthritis synovial fibroblasts (RASF) is associated with up-regulation of the activity of serine/threonine kinase B/Akt and with survival of RASF. METHODS: Staining of phosphorylated Akt was done using anti-phosphorylated Thr308 Akt antibody. Levels of phosphorylated Akt were analyzed by Western blot and Akt activity was analyzed using a kinase assay. TUNEL staining was used to analyze the cytotoxicity of TNFalpha treatment or TNFalpha combined with either the Akt activity inhibitor wortmannin, an adenovirus expressing dominant-negative mutant (AdAkt-DN), or an adenovirus expressing phosphatase and tensin homolog deleted on chromosome 10 (AdPTEN). RESULTS: The levels of phosphorylated Akt were higher in RASF than in osteoarthritis synovial fibroblasts (OASF), as demonstrated by immunohistochemical staining, immunoblot analysis, and an Akt kinase assay. The levels of phosphorylated Akt and Akt kinase activity were increased by stimulation of primary RASF with TNFalpha (10 ng/ml). Treatment of RASF with the phosphatidylinositol 3-kinase inhibitor wortmannin (50 nM) plus TNFalpha resulted in apoptosis of 60 +/- 8% (mean +/- SEM) of RASF within 24 hours. This proapoptosis effect was specific for Akt, since equivalent levels of apoptosis were observed upon TNFalpha treatment of RASF transfected with AdAkt-DN and with AdPTEN, which opposes the action of Akt. CONCLUSION: These results indicate that phosphorylated Akt acts as a survival signal in RASF and contributes to the stimulatory effect of TNFalpha on these cells by inhibiting the apoptosis response. This effect was not observed in OASF and may reflect the pathophysiologic changes associated with the proliferating synovium in rheumatoid arthritis.     

表皮生长因子对类风湿关节炎滑膜细胞的作用

目的 观察表皮生长因子（ＥＧＦ）对类风湿关节炎（ＲＡ）滑膜细胞的作用和细胞中有丝分裂素激活的蛋白激酶（ＭＡＰＫ）的激活情况。方法 ３～５代体外培养的ＲＡ滑膜细胞,＾３Ｈ－ＴｄＲ掺入检测ＥＧＦ对细胞ＤＮＡ合成的影响;ＥＧＦ刺激后裂解细胞,收获蛋白,Ｗｅｓｔｅｒｎ ｂｌｏｔ检测ＭＡＰＫ的活化。结果 ＥＧＦ刺激组和对照组每分钟计数值差异有显著性（Ｐ〈０．００１）。ＥＧＦ能引起细胞内明显ＭＡＰＫ活化。结论     

DNA hypomethylation in rheumatoid arthritis synovial fibroblasts

Objective

Rheumatoid arthritis synovial fibroblasts (RASFs) are phenotypically activated and aggressive. We undertook this study to investigate whether the intrinsic activation of RASFs is due to global genomic hypomethylation, an epigenetic modification.

Methods

Global genomic hypomethylation was assessed by immunohistochemistry, flow cytometry, and L1 promoter bisulfite sequencing. The levels of Dnmt1 were determined in synovial tissue and cultured SFs by Western blotting before and after treatment with cytokines and growth factors. Normal SFs were treated for 3 months with a nontoxic dose of the DNA hypomethylation drug 5‐azacytidine (5‐azaC), and changes in gene expression were revealed using complementary DNA arrays. The phenotypic changes were confirmed by flow cytometry.

Results

In situ and in vitro, RASF DNA had fewer 5‐methylcytosine and methylated CG sites upstream of an L1 open‐reading frame than did DNA of osteoarthritis SFs, and proliferating RASFs were deficient in Dnmt1. Using 5‐azaC, we reproduced the activated phenotype of RASFs in normal SFs. One hundred eighty‐six genes were up‐regulated >2‐fold by hypomethylation, with enhanced protein expression. These included growth factors and receptors, extracellular matrix proteins, adhesion molecules, and matrix‐degrading enzymes. The hypomethylating milieu induced irreversible phenotypic changes in normal SFs, which resembled those of the activated phenotype of RASFs.

Conclusion

DNA hypomethylation contributes to the chronicity of RA and could be responsible for the limitation of current therapies.

     

Microsatellite analysis in rheumatoid arthritis synovial fibroblasts

OBJECTIVES: Rheumatoid arthritis (RA) is a chronic disease characterised by irreversible destruction of the affected joints. As aggressive transformed-appearing synovial fibroblasts are commonly found at the site of invasion of the rheumatoid synovium into the adjacent cartilage and bone, the presence of microsatellite instability (MSI) and expression of mismatch repair enzymes as a possible mechanism in the alteration of these cells was examined. METHODS: DNA was extracted from the synovial fibroblasts and blood of 20 patients with long term RA undergoing joint replacement, and the presence of MSI was studied at 10 microsatellite loci. In addition, immunohistochemistry was performed to evaluate the expression of the two major mismatch repair enzymes (hMLH1 and hMSH2) in rheumatoid synovium. RESULTS: MSI could not be detected in any of the fibroblast cell populations derived from the 20 different rheumatoid synovial samples. In addition, strong expression of mismatch repair enzymes could be seen in numerous cells, including fibroblasts, throughout the synovium. CONCLUSIONS: Applying the currently used and established markers for MSI, the data show for the first time that MSI does not appear to have an important role in alteration of rheumatoid synovial fibroblasts into an aggressive phenotype. On the other hand, strong mismatch repair enzyme synthesis in rheumatoid synovium supports the hypothesis of continuing DNA repair, presumably due to long term, inflammation induced DNA damage.     

Altered expression of microRNA‐203 in rheumatoid arthritis synovial fibroblasts and its role in fibroblast activation

Objective

MicroRNA (miRNA) are recognized as important regulators of a variety of fundamental biologic processes. Previously, we described increased expression of miR‐155 and miR‐146a in rheumatoid arthritis (RA) and showed a repressive effect of miR‐155 on matrix metalloproteinase (MMP) expression in RA synovial fibroblasts (RASFs). The present study was undertaken to examine alterations in expression of miR‐203 in RASFs and analyze its role in fibroblast activation.

Methods

Differentially expressed miRNA in RASFs versus osteoarthritis synovial fibroblasts (OASFs) were identified by real‐time polymerase chain reaction (PCR)–based screening of 260 individual miRNA. Transfection of miR‐203 precursor was used to analyze the function of miR‐203 in RASFs. Levels of interleukin‐6 (IL‐6) and MMPs were measured by real‐time PCR and enzyme‐linked immunosorbent assay. RASFs were stimulated with IL‐1β, tumor necrosis factor α (TNFα), lipopolysaccharide (LPS), and 5‐azacytidine (5‐azaC). Activity of IκB kinase 2 was inhibited with SC‐514.

Results

Expression of miR‐203 was higher in RASFs than in OASFs or fibroblasts from healthy donors. Levels of miR‐203 did not change upon stimulation with IL‐1β, TNFα, or LPS; however, DNA demethylation with 5‐azaC increased the expression of miR‐203. Enforced expression of miR‐203 led to significantly increased levels of MMP‐1 and IL‐6. Induction of IL‐6 by miR‐203 overexpression was inhibited by blocking of the NF‐κB pathway. Basal expression levels of IL‐6 correlated with basal expression levels of miR‐203.

Conclusion

The current results demonstrate methylation‐dependent regulation of miR‐203 expression in RASFs. Importantly, they also show that elevated levels of miR‐203 lead to increased secretion of MMP‐1 and IL‐6 via the NF‐κB pathway and thereby contribute to the activated phenotype of synovial fibroblasts in RA.

     

Morphologic and mitogenic responses of rabbit synovial fibroblasts to transforming growth factor beta require transforming growth factor alpha or epidermal growth factor

We tested the hypothesis that normal synovial fibroblasts might proliferate in response to transforming growth factors (TGFs), peptides that are extracted with acid-ethanol from bovine kidney or salivary gland and that cause anchorage-independent growth of normal cells. A 72-hour exposure of confluent monolayers of rabbit synovial fibroblasts in 10% fetal calf serum to partially purified TGF-beta in the presence of TGF-alpha gave a 2- to 5-fold increase in incorporation of 3H-thymidine, protein content, and cell number. Similar results were obtained with high pressure liquid chromatography-purified TGF-beta in the presence of epidermal growth factor (EGF) (a type of TGF-alpha). By itself, purified TGF-beta was not mitogenic; it depended absolutely on EGF. However, only TGF-beta along with EGF, and not EGF alone, induced a marked morphologic change: a piling up of cells into foci resembling those commonly seen in primary cultures of rheumatoid synovial cells. Mitogenic responses induced by the TGF-beta-EGF combination were prevented by all-trans-retinoic acid but not by indomethacin or dexamethasone. The data indicate that TGF-beta, a peptide extracted from normal cells, can act in concert with EGF to cause proliferation and piling up of synovial cells and raise the possibility that these factors may play a role in rheumatoid arthritis and other proliferative but nonmalignant diseases as well.     

Morphologic and mitogenic responses of rabbit synovial fibroblasts to transforming growth factor β require transforming growth factor α or epidermal growth factor

We tested the hypothesis that normal synovial fibroblasts might proliferate in response to transforming growth factors (TGFs), peptides that are extracted with acid-ethanol from bovine kidney or salivary gland and that cause anchorage-independent growth of normal cells. A 72-hour exposure of confluent monolayers of rabbit synovial fibroblasts in 10% fetal calf serum to partially purified TGF-β in the presence of TGF-β gave a 2- to 5-fold increase in incorporation of ³H-thymidine, protein content, and cell number. Similar results were obtained with high pressure liquid chromatography-purified TGF-β in the presence of epidermal growth factor (EGF) (a type of TGF-β). By itself, purified TGF-β was not mitogenic; it depended absolutely on EGF. However, only TGF-β along with EGF, and not EGF alone, induced a marked morphologic change: a piling up of cells into foci resembling those commonly seen in primary cultures of rheumatoid synovial cells. Mitogenic responses induced by the TGF-β-EGF combination were prevented by all-trans-retinoic acid but not by indomethacin or dexamethasone. The data indicate that TGF-β, a peptide extracted from normal cells, can act in concert with EGF to cause proliferation and piling up of synovial cells and raise the possibility that these factors may play a role in rheumatoid arthritis and other proliferative but nonmalignant diseases as well.