Downregulation of heat shock protein 70 protects rheumatoid arthritis fibroblast-like synoviocytes from nitric oxide-induced apoptosis
© Kang et al.; licensee BioMed Central Ltd. 2009
Received: 22 April 2009
Accepted: 27 August 2009
Published: 27 August 2009
Heat shock protein 70 (Hsp70) is a well-known anti-apoptotic protein that blocks multiple steps in the stress-induced apoptotic pathway. Enhanced Hsp70 expression has previously been demonstrated in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLSs). The authors investigated the role of Hsp70 in the survival of RA FLSs in a sodium nitroprusside (SNP)-treated environment.
Targeted knock-down of Hsp70 was performed by RNA interference in RA FLSs at passage 3-7. After SNP treatment, the morphological features of apoptosis were observed by phase-contrast microscopy. Cell survival was measured by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays and by flow cytometric analysis after propidium iodide (PI) staining. Bcl-2 expression and signaling pathways (Akt, extracellular signal-regulated kinase, p38, c-Jun N-terminal kinase) were examined with or without Hsp70 downregulation.
Hsp70 downregulation in RA FLSs, induced by small interfering RNA (siRNA), was confirmed by reverse transcriptase-polymerase chain reaction and Western blotting. When treated with SNP, Hsp70 downregulated cells showed markedly less cell blebbing, cytoplasmic condensation, and nuclear shrinkage than non-downregulated control cells. Furthermore, Hsp70 downregulated cells were found to survive better than control cells in MTT assays (mean of absorbance ratio, 4.39 in target cells versus 1.00 in control siRNA-treated cells versus 1.09 in lipofectamine-treated cells, P = 0.001) and according to PI staining results (mean M1 ratio, 0.21 in target cells versus 1.00 in control siRNA-treated cells versus 1.03 in lipofectamine-treated cells, P = 0.001). Bcl-2 expression and Akt phosphorylation were higher in Hsp70 downregulated RA FLSs than in control cells. When cells were treated with LY294002, a potent phosphoinositide 3-kinase inhibitor, Akt phosphorylation and Bcl-2 levels were reduced and Hsp70 downregulation no longer had a cytoprotective effect.
Knock-down of Hsp70 protects RA FLSs from nitric oxide-induced apoptosis by activating the Akt signaling pathway. These results suggest that Hsp70 has a pro-apoptotic role in RA FLSs.
Rheumatoid arthritis (RA) is a chronic inflammatory disorder that involves mainly joint synovium. One of the major characteristics of RA synovium is the tumor-like growth of fibroblast-like synoviocytes (FLSs) that invade adjacent articular cartilage and bone . Although the mechanism of FLS hyperplasia in RA is not fully understood, it is explained in part by excessive survival and/or anti-apoptotic signals to FLSs transmitted by inflammatory cells and cytokines. For example, it has been well established that tumor necrosis factor-alpha (TNF-α), a key cytokine in RA, activates genes that mediate proliferative and inflammatory responses . Other anti-apoptotic apparatuses expressed in RA FLSs include FLIP (Fas-associated death domain-like interleukin [IL]-1β-converting enzyme-inhibitory protein) , sentrin , mutated p53 [5, 6], and the activation of the nuclear factor-kappa-B or the Akt signaling pathways or both [7, 8].
Heat shock protein 70 (Hsp70) is a molecular chaperone that is rapidly induced by physical and chemical stresses. The anti-apoptotic function of Hsp70 depends on its ability to interact with protein substrates that are not always associated with the chaperoning activity. The mechanisms by which Hsp70 exerts its anti-apoptotic function encompass the inhibition of the c-Jun N-terminal kinase (JNK) signaling pathway, caspase activation, mitochondrial cytochrome c release, and apoptosome formation . Although the anti-apoptotic role of Hsp70 has been demonstrated in a number of studies in various cell types and under different conditions, several other studies have shown that the overexpression of Hsp70 promotes cell death, which suggests that Hsp70 has dual functionality depending on cell and stimulus type [10–12]. It has been reported that the expression of Hsp70 is higher in both tissue and cultured RA FLSs than in the FLSs of osteoarthritis and that inflammatory cytokines, such as TNF-α and IL-1β, that exist abundantly in RA joint fluid further increase Hsp70 expression in cultured RA FLSs . However, no study has investigated the actual role of Hsp70 in the survival of RA FLSs. In this study, we investigate the effect of Hsp70 downregulation on RA FLS apoptosis induced by sodium nitroprusside (SNP), a nitric oxide (NO) donor.
Materials and methods
Primary culture of fibroblast-like synoviocyte derived from rheumatoid arthritis patient synovium
Clinical data of five patients with rheumatoid arthritis at the time of joint surgery a
Disease durationb, months
Site of surgery
MTX 15 mg/wk, SSZ 1 g/d, Pd 5 mg/d
Pd 10 mg/d
Pd 10 mg/d
HCQ 200 mg/d
Cells were incubated with TNF-α (R&D Systems, Minneapolis, MN, USA) at 10 ng/mL for 8 hours where indicated. To induce apoptosis, cells were treated with 1 mM SNP (Sigma-Aldrich) for 24 hours in a light-shielded state. LY294002 (Cell Signaling Technology, Inc., Danvers, MA, USA) at 2 to 10 μM was added to culture media to block phosphoinositide 3-kinase (PI3K) activity.
Reverse transcriptase-polymerase chain reaction
Total mRNA was extracted from cultured RA FLSs, and single-stranded cDNA was synthesized from mRNA using reverse transcription kits (Intron Biotechnology, Seoul, Korea). The polymerase chain reaction (PCR) amplification of hsp70 was performed with an initial Taq (Invitrogen Corporation) activation at 95°C for 5 minutes followed by 25 amplification cycles of 15 seconds at 95°C, 30 seconds at 60°C, and 30 seconds at 60°C. The primers used were forward 5'-GGA-GGC-GGA-GAA-GTA-CAA-3' and reverse 5'-GCT-GAT-GAT-GGG-GTT-ACA-3' . The semi-quantitative measurements were performed using gapdh (glyceraldehyde-3-phosphate dehydrogenase) as an internal reference.
Total cell lysates were separated on 10% denaturating polyacrylamide gels and transferred to polyvinylidene difluoride membranes. The primary antibodies used were mouse monoclonal anti-Hsp70 antibody (donated by the Department of Biochemistry, Seoul National University College of Medicine, Korea), rat monoclonal anti-constitutional Hsp70 (Hsc70) antibody (StressGen Biotechnologies Corporation, Victoria, BC, Canada), rabbit monoclonal anti-Bcl-2 antibody (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA), rabbit monoclonal anti-Akt/phospho-Akt, anti-extracellular signal-regulated kinase (ERK)/phospho-ERK, anti-c-Jun N-terminal kinase (JNK)/phospho-JNK, and anti-p38/phospho-p38 antibodies (Cell Signaling Technology, Inc.), and rabbit monoclonal anti-actin antibody (Sigma-Aldrich). The secondary antibodies used were horseradish peroxidase-conjugated anti-mouse IgG (The Jackson Laboratory, Bar Harbor, ME, USA), anti-rabbit IgG (The Jackson Laboratory), or anti-rat IgG (StressGen Biotechnologies Corporation) antibodies. Signals were developed using an ECL (enhanced chemiluminescence) system (Amersham Biosciences, now part of GE Healthcare, Little Chalfont, Buckinghamshire, UK).
Transfection using small interfering RNA
FLSs were seeded at a density of 1.0 to approximately1.5 × 106/mL and incubated overnight in antibiotics-free DMEM supplemented with 10% FBS. The following day, cells were washed with phosphate-buffered-saline, and then OPTI-MEM reduced serum medium (Invitrogen Corporation) was added to cells. Hsp70-specific small interfering RNA (siRNA) (forward 5'-CGA-CGG-AGA-CAA-GCC-CAA-GTT-3' and reverse 5'-CUU-GGG-CUU-GUC-UCC-GUC-GTT-3'; Invitrogen Corporation)  and a nucleic acid transferring agent, lipofectamine 2000 (Invitrogen Corporation), were incubated together in OPTI-MEM reduced serum medium for 15 minutes at room temperature to form an siRNA-lipofectamine complex. The siRNA-lipofectamine complex-containing medium was added to cells to a final siRNA concentration of 25 nM. Six hours later, the complex-containing medium was exchanged with antibiotics-free DMEM supplemented with 10% FBS. The second round of transfection was performed 48 hours after the first transfection, using the same method. Cells transfected twice with 25 nM control siRNA of medium GC content (Invitrogen Corporation) and cells transfected with lipofectamine only were used as controls.
Assessment of cell viability and apoptosis
Morphological changes were observed after SNP treatment under a phase-contrast microscope (Olympus, Tokyo, Japan). In addition, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) (Sigma-Aldrich) assays were performed to assess cell viability. Apoptotic cell fractions (sub-G0/G1 portions or M1 fractions) were measured by propidium iodide (PI) (Sigma-Aldrich) staining and flow cytometry (FACSCalibur; BD Biosciences, San Jose, CA, USA). The absorbance (measured at 570 nm) and apoptotic fraction were normalized to those of control siRNA-treated cells in each experiment.
Continuous values were presented as mean ± standard deviation. Analysis of variance (ANOVA) was used to compare mean values of more than two groups. Two-tailed P values of less than 0.05 were considered significant. All statistical calculations were performed using SPSS version 12 (SPSS Inc., Chicago, IL, USA).
Expression of Hsp70 in cultured rheumatoid arthritis fibroblast-like synoviocytes with or without tumor necrosis factor-alpha
Specific knock-down of Hsp70 expression in rheumatoid arthritis fibroblast-like synoviocyte
Targeted knock-down of Hsp70 by specific siRNA was observed in RA FLSs (Figure 1c). Because knock-down was incomplete after a single transfection, a second round of transfection was performed 48 hours after the first transfection. Almost complete knock-down of Hsp70 expression was then achieved and sustained for up to 72 hours. The expression of Hsc70 was unaffected (observed at 48 hours after the second transfection) by Hsp70-specific siRNA despite its 86% amino acid homology with Hsp70 (Figure 1c).
Effect of Hsp70 knock-down on nitric oxide-induced apoptosis in rheumatoid arthritis fibroblast-like synoviocyte
Intracellular Bcl-2 levels and signaling pathways
Effect of Akt phosphorylation on the survival of rheumatoid arthritis fibroblast-like synoviocyte
This study shows that Hsp70 is expressed in RA FLSs and that knock-down of Hsp70 protects RA FLSs from NO-induced apoptosis via the Akt signaling pathway. This result is consistent with that of Schett and colleagues , who found that RA FLSs express Hsp70 and that this expression is increased by inflammatory cytokines, such as TNF-α. On the other hand, Schick and colleagues  found that Hsc70, and not Hsp70, was overexpressed in RA synovial tissues and cultured FLSs. They suspected that anti-Hsp70 antibody in the study of Schett and colleagues might have cross-reacted with Hsc70. However, the specificity of the anti-Hsp70 antibody used in the present study has been characterized previously . Furthermore, our observation that anti-Hsp70 antibody demonstrated Hsp70 downregulation by siRNA, whereas anti-Hsc70 antibody showed consistent Hsc70 expression, further supports specific antibody reactivity with Hsp70 in the present study.
Unexpectedly, we observed that Hsp70 downregulation protected RA FLSs from NO-induced apoptosis. This finding suggests that Hsp70 may be a pro-apoptotic protein in RA FLSs. Our results indicate that hsp70 interferes with Akt phosphorylation by binding an upstream protein of the Akt signaling pathway in RA FLSs. However, in vivo, abundant intracellular anti-apoptotic apparatuses generated by external survival/growth signals appear to overcome the negative effect of Hsp70 on Akt phosphorylation and on cell survival.
The anti-apoptotic effect mediated by Akt signaling pathway activation after Hsp70 knock-down in RA FLSs is consistent with the findings of the previous report, namely, that Akt phosphorylation is an important mechanism to protect RA FLSs from NO-induced apoptosis . Akt phosphorylation has been shown to regulate Hsp70 expression [21, 22]. However, to the best of our knowledge, this is the first report to suggest that Hsp70 is involved in the regulation of the Akt signaling pathway. However, the direct molecular mechanism by which Hsp70 knock-down promotes Akt phosphorylation remains to be determined.
Recently, anti-cytokine therapies for RA have entered the spotlight [23–25]. However, response to anti-cytokine agents is often only partial and some patients are refractory to these agents. Therefore, treatments with different mechanisms need to be combined. Although blocking anti-apoptotic proteins is a candidate strategy, the results of the present study do not support the notion that the downregulation of Hsp70 promotes the regression of hyperplastic synovium. Alternatively, the in vivo effect of Hsp70 knock-down might differ from that observed in vitro. Recently, the biological role of extracellular hsp70, particularly in terms of immune response, including the cross-presentation of antigenic peptide and maturation of antigen-presenting cells (APCs), was substantively updated [26–28]. Because the synovial fluid of patients with RA contains high levels of soluble Hsp70, which interacts with APCs via a plethora of surface receptors , the effect of Hsp70 knock-down is likely to be complex in vivo.
In summary, this study shows that Hsp70 is expressed in RA FLSs and that knock-down of Hsp70 protects RA FLSs from NO-induced apoptosis by activating the Akt signaling pathway. These results suggest that Hsp70 has a pro-apoptotic role in RA FLS.
Dulbecco's modified Eagle's medium
extracellular signal-regulated kinase
fetal bovine serum
constitutional heat shock protein 70
heat shock protein 70
c-Jun N-terminal kinase
polymerase chain reaction
small interfering RNA
tumor necrosis factor-alpha.
This study was supported by a grant from Seoul National University.
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