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Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Objective

S100A4 has been shown to be increased in osteoarthritic (OA) cartilage and to stimulate chondrocytes to produce matrix metalloproteinase 13 (MMP-13) through activation of the receptor for advanced glycation end products (RAGE). The aim of this study was to examine the mechanism of S100A4 secretion by chondrocytes.

Methods

Human articular chondrocytes isolated from ankle cartilage were stimulated with 10 ng/ml of interleukin-1β (IL-1β), IL-6, IL-7, or IL-8. Cells were pretreated with either a JAK-3 inhibitor, brefeldin A, or cycloheximide. Immunoblotting with phospho-specific antibodies was used to determine the activation of signaling proteins. Secretion of S100A4 was measured in conditioned media by immunoblotting, and MMP-13 was measured by enzyme-linked immunosorbent assay.

Results

Chondrocyte secretion of S100A4 was observed after treatment with IL-6 or IL-8 but was much greater in cultures treated with equal amounts of IL-7 and was not observed after treatment with IL-1β. IL-7 activated the JAK/STAT pathway, with increased phosphorylation of JAK-3 and STAT-3, leading to increased production of S100A4 and MMP-13. Overexpression of a dominant-negative RAGE construct inhibited the IL-7–mediated production of MMP-13. Pretreatment of chondrocytes with a JAK-3 inhibitor or with cycloheximide blocked the IL-7–mediated secretion of S100A4, but pretreatment with brefeldin A did not.

Conclusion

IL-7 stimulates chondrocyte secretion of S100A4 via activation of JAK/STAT signaling, and then S100A4 acts in an autocrine manner to stimulate MMP-13 production via RAGE. Since both IL-7 and S100A4 are up-regulated in OA cartilage and can stimulate MMP-13 production by chondrocytes, this signaling pathway could contribute to cartilage destruction during the development of OA.

S100 proteins are acidic low molecular weight calcium binding proteins that are only found in vertebrates and are expressed in many tissues in humans (1). The S100 protein family consists of 21 known members and is considered to be one of the largest subgroups of the EF-hand calcium binding protein family (1). S100 proteins regulate numerous intracellular functions, including protein phosphorylation, enzyme activation, cell motility, cell growth and differentiation, and calcium homeostasis (2). Interestingly, S100 proteins are also known to have extracellular functions. Studies have shown that S100B is released into the extracellular environment by neuronal cells, stimulates neurite extension, and promotes cell survival (3). The extracellular functions of S100 proteins are attributed to their ability to be released from cells and to interact with cell surface receptors, including the receptor for advanced glycation end products (RAGE) (4). Recent studies of chondrocytes have shown that when added extracellularly, S100 proteins stimulate the expression of matrix metalloproteinase 13 (MMP-13) (5) and promote chondrocyte hypertrophy (6) through stimulation of RAGE signaling.

S100A4 is a member of the S100 family that was originally isolated as a gene that was differentially expressed in mouse adenocarcinoma cells (7) and was subsequently found in other tissues (8). Recent studies have identified S100A4 in cartilage and have shown it to be up-regulated in tissues from patients with osteoarthritis (OA) or rheumatoid arthritis (RA) (5, 9). Like other members of the S100 family, S100A4 exerts intracellular and extracellular effects. With respect to its intracellular targets, S100A4 binds the p53 tumor suppressor and regulates its functions (10). S100A4 also interacts with the heavy chain of non-muscle myosin II and plays an active role in cell motility and adhesion in metastatic tumor cells (11). When applied extracellularly, S100A4 acts as a potent cytokine that stimulates neurite outgrowth in astrocytes (12) and angiogenesis in endothelial cells (13).

In addition, S100A4 has also been suggested to play an important role in matrix remodeling (14). We have previously shown that extracellular S100A4 binds to RAGE in articular chondrocytes and activates the RAGE signaling cascade, leading to increased production of MMP-13 (5). Recent studies have shown that extracellular S100A4 can induce the up-regulation of several MMPs, such as MMP-1, MMP-3, MMP-9, and MMP-13 in RA synovial fibroblasts (15). Taken together, these findings suggest that S100A4 may play an important role in the degradation of cartilage and the progression of arthritis.

Interleukin-7 (IL-7) was initially identified as a factor required for the growth of murine B cell precursors (16). However, subsequent studies have shown that IL-7 plays an important role in T cell, dendritic cell, and bone biology in humans (17). IL-7 has been studied in RA because of its elevated levels in RA patient serum (18) and its increased expression in RA synovium and synovial fibroblasts (19). Recently, we found that IL-7 is expressed in chondrocytes and that its expression is up-regulated in OA chondrocytes and in normal chondrocytes with aging (20). In addition, we also found that IL-7 expression was increased in chondrocytes in response to fibronectin fragment and IL-1 stimulation and that chondrocytes responded to IL-7 treatment with increased production of MMP-13 (20). These data suggest that IL-7 may play an important role in the cartilage degradation seen in OA as well as RA.

As discussed above, studies have shown that S100A4 is secreted into the extracellular environment (21); however, the mechanism of secretion is not known. The current study was therefore designed to determine if cytokines that are known to be active in cartilage can stimulate S100A4 secretion and to study the pathway involved in this process. Experiments were performed using human articular chondrocytes treated with different chemical inhibitors to define the involvement of cell signaling pathways, protein expression, and protein secretion. The data presented here demonstrate that chondrocytes secrete S100A4 in response to IL-7 activation of the JAK/STAT signaling pathway.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Reagents.

Recombinant human IL-1β, IL-6, soluble IL-6 receptor, IL-7, IL-8, and an enzyme-linked immunosorbent assay (ELISA) kit for human proMMP-13 were purchased from R&D Systems (Minneapolis, MN). Human S100A4 antibody was purchased from Dako (Carpinteria, CA). JAK-3 and STAT-3 antibodies were from Santa Cruz Biotechnology (Santa Cruz, CA). Cycloheximide was purchased from Sigma (St. Louis, MO). JAK-3 inhibitor (WHI-P154) and brefeldin A were from Calbiochem (La Jolla, CA). Pronase was purchased from Calbiochem (San Diego, CA) and collagenase P from Boehringer (Mannheim, Germany). An RNeasy Mini kit was obtained from Qiagen (Valencia, CA). S100A4 and GAPDH primers and SYBR Green polymerase chain reaction (PCR) Master Mix were from SuperArray Bioscience (Frederick, MD). Nitrocellulose membranes and enhanced chemiluminescence detection kits were purchased from Amersham Biosciences (Piscataway, NJ). Cell culture media and supplements were purchased from Gibco BRL (Gaithersburg, MD).

Tissue acquisition and chondrocyte cell culture.

Human ankle cartilage was obtained from tissue donors (within 48 hours of death) through the National Disease Research Interchange (Philadelphia, PA) in accordance with institutional protocol. Chondrocytes were isolated from normal cartilage by sequential digestion with Pronase and then overnight with collagenase, as previously described (22). Viability of isolated cells was determined by trypan blue exclusion, and cells were counted using a hemocytometer. Monolayer cultures were established by plating cells in 6-well plates at 2 × 106 cells/ml in Dulbecco's modified Eagle's medium (DMEM)/Ham's F-12 medium supplemented with 10% fetal bovine serum. Plates were maintained for 5–7 days, with feedings every 2 days until they reached 100% confluence prior to use in experiments.

Chondrocyte stimulation and immunoblotting.

Confluent primary chondrocyte monolayers were made serum-free overnight before treating with 10 ng/ml of each of the cytokines: IL-1, IL-6 (in combination with 20 ng/ml of soluble IL-6 receptor), IL-7, or IL-8. Cells were incubated with these cytokines for 16–18 hours to examine secretion of S100A4 or for 0–60 minutes to study the activation of signaling proteins. For inhibition studies, cells were pretreated for 45– 90 minutes with inhibitors before incubating them with the cytokines. Inhibitors tested included 10 mM of a JAK-3 inhibitor, 10 μg/ml of cycloheximide, and 1 μg/ml of brefeldin A. After incubation, conditioned medium was collected and analyzed for S100A4 by immunoblotting and for MMP-13 by ELISA.

For signaling studies, cells were washed with phosphate buffered saline and lysed with lysis buffer that contained 20 mM Tris (pH 7.5), 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% Triton X-100, 2.5 mM tetrapyrophosphate, 1 mM glycerol phosphate, 1 mM Na3VO4, 1 μl/ml of leupeptin, and 1 mM phenylmethylsulfonyl fluoride. Lysates were centrifuged to remove insoluble material, and the soluble protein concentration was determined with BCA Reagent (Pierce, Rockford, IL). Samples containing equal amounts of total protein were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis, transferred to nitrocellulose, and probed for signaling proteins. Immunoreactive bands were detected using the enhanced chemiluminescence system. All immunoblotting experiments were performed at least 3 times, and the results were similar.

Real-time PCR analysis.

Total RNA was isolated using an RNeasy Mini kit (Qiagen, Valencia, CA). Two micrograms of RNA was reverse transcribed for 1 hour at 42°C using an avian myeloblastosis virus reverse transcriptase and oligo(dT) primer, then 2 μl of reverse transcriptase was combined in a reaction mixture with 1 μl of an S100A4-specific primer pair (5′-TCTTTCTTGGTTTGATCCTG-3′ [forward] and 5′-GCATCAAGCACGTGTCTGAA-3′ [reverse]), 12.5 μl of 2× SYBR Green PCR Master Mix, and water to a final reaction volume of 25 μl. Reactions were then run in triplicate, with 40 cycles of amplification, on an ABI Prism 7000 real-time PCR instrument (Applied Biosystems, Foster City, CA). A negative control containing primers, water, and Master Mix, but no complementary DNA, was included. An amplification plot was generated using ABI software. PCR specificity was confirmed by dissociation curve analysis.

STAT-3 activation analysis.

STAT-3 DNA binding activity was measured by using the ELISA-based TransAM transcription factor kits (Active Motif, Carlsbad, CA) according to the manufacturer's protocol. The primary antibody used to detect activated STAT-3 recognizes a specific epitope on STAT-3 that is accessible only when STAT-3 is activated and bound to its target DNA. Briefly, after stimulation with IL-7, nuclear extracts were prepared, and the activated STAT-3 in the extracts was captured on the ELISA plate that had been precoated with oligonucleotides corresponding to STAT-3 consensus sequence binding sites (5′-TTCCCGGAA-3′) and detected with a horseradish peroxidase–conjugated secondary antibody.

Analysis of MMP-13 production.

Conditioned medium obtained after 16–18 hours of treatment with IL-7 from various experiments was collected and analyzed for MMP-13 by ELISA (R&D Systems) according to the manufacturer's protocol and using duplicate wells for each sample. Samples were diluted (if needed) with culture medium to get the value within the linear range of the assays.

Chondrocyte survival assay.

Chondrocyte survival was measured using a Live/Dead cell survival assay (Molecular Probes, Eugene, OR) as described previously (23).

Dominant-negative RAGE transfection.

Chondrocytes were transfected by the nucleofection method, using a human chondrocyte nucleofection kit (Amaxa, Gaithersburg, MD) described previously (24). Briefly, isolated cells were resuspended in transfection reagent and nucleofected with 2 μg of plasmid DNA expressing dominant-negative RAGE (a generous gift from Dr. Shi Du Yan, Columbia University, New York, NY). After the recovery period of 48 hours, cells were made serum-free overnight, stimulated with IL-7, and MMP-13 was measured as described above.

Statistical analysis.

Data sets were analyzed by analysis of variance using StatView 5.0 software (SAS Institute, Cary, NC).

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

IL-7 promotion of the secretion and expression of S100A4 by human articular chondrocytes.

To determine whether articular chondrocytes secrete S100A4, cells were treated with IL-1, IL-6, IL-7, or IL-8 (10 ng/ml) for 16–18 hours, and the presence of S100A4 protein in the conditioned media was examined by immunoblotting. This analysis revealed an immunoreactive band corresponding to a molecular mass of 11 kd in conditioned media obtained from cells treated with IL-6, IL-7, or IL-8. However, the intensity of the immunoreactive band was stronger in the media obtained after IL-7 treatment than in media obtained after treatment with the other cytokines (Figure 1A).

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Figure 1. Interleukin-7 (IL-7) stimulation of S100A4 production by human articular chondrocytes. Chondrocytes were isolated from normal cartilage and cultured as monolayers. Cells in serum-free confluent culture were treated with 10 ng/ml of recombinant human IL-1, IL-6, IL-7, or IL-8 for 16–18 hours. A, Secretion of S100A4 into the medium was measured by immunoblotting with anti-S100A4 antibody. Blots were stripped and reprobed with anti-human MMP-2 as a loading control. Molecular weight markers are shown at the left. B, Densitometric analysis was performed on blots obtained in 3 independent experiments similar to the one shown in A. Values are the mean and SEM. P value is versus control, IL-1, IL-6, and IL-8. C, Intracellular S100A4 protein levels in cell lysates were measured by immunoblotting. D, Total RNA was prepared, and the expression of mRNA for S100A4 and GAPDH (control for normalization) was analyzed by real-time polymerase chain reaction using specific primers for S100A4 and GAPDH. Values are the mean and SEM of 3 independent experiments. P value is versus control.

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Immunoblots from 3 independent experiments were quantified, and the results demonstrated a significant increase in S100A4 levels in response to IL-7 stimulation (Figure 1B). Analysis of cell lysates by immunoblotting did not show any significant change in the intracellular S100A4 protein levels upon treatment with any of the cytokines used (Figure 1C), suggesting that the increase in the media in response to IL-7 was due to secretion accompanied by increased expression, such that the intracellular levels were stable. Alternatively, treatment of chondrocytes with IL-7 may have promoted stability or may have inhibited reuptake of S100A4, resulting in increased levels in the media.

To further study the expression of S100A4, we examined whether IL-7 would induce S100A4 messenger RNA (mRNA) expression. Total RNA isolated from chondrocytes that had been stimulated for 16–18 hours with IL-7 showed increased expression of S100A4 mRNA, as analyzed by real-time PCR (Figure 1D). This finding suggests that increased production of S100A4 kept pace with secretion, resulting in stable intracellular levels of S100A4.

Effect of cycloheximide and brefeldin A on IL-7–mediated S100A4 secretion.

Since our data showed that IL-7 induced both S100A4 secretion and mRNA expression in chondrocytes, we wanted to examine whether de novo protein synthesis is required for the increase in extracellular S100A4. Pretreatment of chondrocytes with cycloheximide, a chemical inhibitor of protein synthesis, followed by treatment with IL-7 reduced the amount of S100A4 secreted by chondrocytes (Figure 2A). To ensure that the decrease in S100A4 secretion observed in chondrocytes treated with cycloheximide was not due to cytotoxicity, we performed a cell survival assay. We did not observe any cell death at the cycloheximide concentration or the incubation period used for our experimental conditions (data not shown).

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Figure 2. Effect of cycloheximide and brefeldin A on the secretion of S100A4 by human articular chondrocytes. Cultured chondrocytes were pretreated in the presence or absence of A, 10 μg/ml of cycloheximide for 90 minutes or B, 1 μg/ml of brefeldin A for 60 minutes, and then stimulated with 10 ng/ml of interleukin-7 (IL-7) for 16–18 hours. Secretion of S100A4 into the medium was measured by immunoblotting with anti-S100A4 antibody. Blots were stripped and reprobed with anti-human matrix metalloproteinase 2 (MMP-2) as a loading control.

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To further understand the molecular mechanism of S100A4 secretion by chondrocytes, we used brefeldin A, a chemical compound that blocks protein translocation from the endoplasmic reticulum (ER) to the Golgi complex (25). Pretreatment of chondrocytes with brefeldin A followed by stimulation with IL-7 did not block the secretion of S100A4 (Figure 2B). The data suggest that the secretion of S100A4 in chondrocytes follows an alternative pathway of protein secretion that is independent of the classic ER–Golgi pathway. Interestingly, treatment of chondrocytes with brefeldin A reduced the secretion of MMP-2 protein, which we have been using as our loading control.

Role of the JAK/STAT signaling pathway in S100A4 secretion.

Studies have shown that IL-7 can activate the JAK/STAT signaling pathway (26). Thus, we were interested in examining the role of JAK/STAT signaling in the secretion of S100A4 by chondrocytes. Stimulation of normal chondrocytes with IL-7 induced increased phosphorylation of JAK-3 and STAT-3 in a time-dependent manner, with a significant increase in the phosphorylation of JAK-3 observed within 15 minutes, and a maximal increase over control levels observed at 60 minutes (Figure 3A). The phosphorylation of STAT-3 was more transient, with a decline in phosphorylation observed within 45 minutes. In addition, we observed delayed phosphorylation of JAK-1 upon IL-7 stimulation of chondrocytes (data not shown). The increased phosphorylation of STAT-3 correlated with an increased DNA binding activity of STAT-3 (Figure 3B).

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Figure 3. Activation of the JAK/STAT pathway by interleukin-7 (IL-7). A, Human articular chondrocytes in serum-free confluent monolayer cultures were stimulated in the presence or absence of 10 ng/ml of IL-7 for 0–120 minutes. Cell lysates were immunoblotted for phosphorylated and total JAK-3 and STAT-3 signaling proteins. B, DNA binding activity of STAT-3 in nuclear extracts was measured at the indicated time points using a TransAM enzyme-linked immunosorbent assay. Values are the mean and SEM of 3 independent experiments. P values are versus 0 time point. OD = optical density. C, Chondrocytes were pretreated for 45 minutes in the presence or absence of a JAK-3 inhibitor (10 μM) and then stimulated with IL-7. Cell lysates were immunoblotted for phosphorylated and total JAK-3 and STAT-3 signaling proteins. D, DNA binding activity of STAT-3 was measured in nuclear extracts following a 30-minute incubation with IL-7. Values are the mean and SEM of 3 independent experiments. P value is versus control.

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Pretreatment of normal chondrocytes with a JAK-3 inhibitor blocked the phosphorylation of both JAK-3 and STAT-3 (Figure 3C) as well as the DNA binding activity of STAT-3 (Figure 3D). In addition, pretreatment of normal chondrocytes with a JAK-3 inhibitor blocked the IL-7–mediated secretion of S100A4 (Figure 4). Taken together, these data suggest that activation of the JAK/STAT pathway is essential for the IL-7–mediated secretion of S100A4 by chondrocytes.

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Figure 4. Effect of JAK-3 inhibitor on S100A4 secretion. Chondrocytes in serum-free medium were pretreated for 45 minutes in the presence or absence of 10 μM JAK-3 inhibitor, followed by stimulation with 10 ng/ml of interleukin-7 (IL-7) for 16–18 hours. Secretion of S100A4 into the medium was measured by immunoblotting with anti-S100A4 antibody. Blots were stripped and reprobed with anti-human matrix metalloproteinase 2 (MMP-2) as a loading control.

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IL-7 stimulation of the increased production of MMP-13 by chondrocytes via S100A4 binding to RAGE.

We have previously shown that treatment of chondrocytes with extracellular S100A4 resulted in increased production of MMP-13 through the activation of RAGE (5). Since treatment of normal chondrocytes with IL-7 resulted in the secretion of S100A4, we were interested in examining the relationship between the IL-7-mediated secretion of S100A4 and the increased production of MMP-13. Treatment of chondrocytes with IL-7 resulted in increased production of MMP-13 in conditioned media (Figure 5A). Pretreatment of cells with JAK-3 inhibitor blocked the increased production of MMP-13 (Figure 5A), suggesting that the increased production of MMP-13 observed in response to IL-7 required activation of the JAK/STAT pathway, as was found for IL-7–mediated S100A4 secretion. Pretreatment of cells with the JAK-3 inhibitor but without IL-7 did not have any effect on the production of MMP-13 (data not shown).

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Figure 5. Role of JAK-3 and receptor for advanced glycation end products (RAGE) in the interleukin-7 (IL-7)–mediated production of matrix metalloproteinase 13 (MMP-13). A, Chondrocytes were pretreated in the presence or absence of 10 μM JAK-3 inhibitor for 45 minutes, followed by stimulation with 10 ng/ml of IL-7 for 16–18 hours. Secretion of MMP-13 into the medium was measured by enzyme-linked immunosorbent assay (ELISA). Values are the mean and SEM of 3 independent experiments. P value is versus control. B, Chondrocytes were left untransfected or were transfected with a dominant-negative RAGE (DN-RAGE) construct and then treated with IL-7 for 16–18 hours. Secretion of MMP-13 into the medium was measured by ELISA. Values are the mean and SEM of 3 independent experiments. P values are versus control (P = 0.016) and versus IL-7 (P = 0.013), respectively.

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Since RAGE has previously been shown to mediate MMP-13 production by S100A4 (5), we examined its role in the IL-7–mediated production of MMP-13. Overexpression of dominant-negative RAGE in chondrocytes not only blocked the ability of IL-7 to stimulate chondrocyte MMP-13 production (Figure 5B), it also reduced the basal level of MMP-13 production (Figure 5B). This suggests that both IL-7–mediated MMP-13 production and some of the basal production of MMP-13 by chondrocytes required RAGE.

DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Members of the S100 family of calcium binding proteins are unique in having both intracellular and extracellular functions. S100 proteins lack the classic signaling sequence for secretion, yet are released into the extracellular environment. The molecular mechanism of secretion of these proteins is poorly understood. In the present study, we found that S100A4 is secreted by human articular chondrocytes in response to IL-7 stimulation and that activation of the JAK/STAT pathway is required. IL-7–mediated S100A4 secretion can then act in an autocrine manner to stimulate MMP-13 production via RAGE.

We have recently identified IL-7 in chondrocytes and showed that IL-7 is produced by chondrocytes in response to fibronectin fragment and IL-1β stimulation (20). In the current study, we show that treatment of human articular chondrocytes with IL-7 resulted in an increase in both the mRNA expression and the secretion of S100A4. Pretreatment of chondrocytes with cycloheximide abolished the IL-7–mediated secretion of S100A4, suggesting that S100A4 mRNA expression and de novo protein synthesis play an important role in the IL-7–mediated secretion of S100A4 by chondrocytes. However, pretreatment of chondrocytes with brefeldin A did not have any effect on the IL-7–mediated secretion of S100A4, suggesting that the secretion of S100A4 in chondrocytes does not follow the ER–Golgi secretory pathway. Previous studies of other members of the S100 family of proteins have likewise demonstrated an alternative secretory pathway that is independent of the classic ER–Golgi route (27, 28) and the requirement for de novo protein synthesis (29).

Previous studies have shown that members of the S100 family of proteins can be secreted in response to external stimuli that activate signaling proteins, such as protein kinase C (27, 28) or ERK-1/2 (30). In the present study, IL-7 signaling required phosphorylation of JAK-3 and the downstream transcription factor STAT-3, which resulted in an increased DNA binding activity of the STAT. In addition, our studies also showed a delayed phosphorylation of JAK-1. Studies have shown that JAK-3 is constitutively associated with the carboxy-terminal region of the γ-chain component of the IL-7 receptor and that activation of JAK-3 is considered to be the first step in the signal transduction cascade induced by IL-7 binding (27). Activation of JAK-3 is followed by phosphorylation of JAK-1, which is associated with the α-chain of the receptor, and then leads to the recruitment of STAT protein (31, 32). Activated STATs translocate into the nucleus and activate their target genes (33, 34).

The JAK/STAT pathway has been shown to be operative in chondrocytes. Stimulation of chondrocytes with oncostatin M resulted in increased phosphorylation of JAK-3 and increased DNA binding activity of STAT-1. Inhibition of this signaling pathway by a JAK-3 inhibitor blocked the oncostatin M–induced expression of MMP-1, MMP-3, and MMP-13 genes in chondrocytes (35). Additionally, in support of our observation, a recent study has shown that IL-7 can stimulate increased phosphorylation of Pyk-2, a non–receptor tyrosine kinase and member of the focal adhesion kinase family, in chondrocytes (20). Studies have previously shown that Pyk-2 is activated by IL-7 via a JAK signaling pathway (36), and Pyk-2 was found in association with JAK-3, thus implicating Pyk-2 as an important component of the JAK/STAT signaling pathway (37). Taken together, these studies suggest an important role for IL-7–mediated JAK/STAT signaling in the secretion of S100A4 by chondrocytes.

Treatment of chondrocytes with IL-7 also resulted in the induction of MMP-13 production in conditioned media, which is consistent with our previous report (20). Pretreatment of chondrocytes with a JAK-3 inhibitor blocked both the IL-7–mediated secretion of S100A4 and the production of MMP-13, suggesting a relationship between the secretion of S100A4 and the production of MMP-13 in the presence of IL-7. Previously, we showed that stimulation of MMP-13 production by extracellular S100A4 required activation of RAGE signaling (5). In the present study, transient overexpression of dominant-negative RAGE in chondrocytes blocked the IL-7–mediated production of MMP-13, which is consistent with the hypothesis that S100A4 released by chondrocytes in response to IL-7 stimulation functions as an autocrine or paracrine factor to induce MMP-13 production via RAGE activation.

In summary, our data showed that IL-7 stimulates chondrocyte secretion of S100A4 via activation of the JAK/STAT signaling pathway. Extracellular S100A4 then functions as an autocrine factor and stimulates MMP-13 production via RAGE. Previous studies have shown that S100A4, RAGE, and IL-7 are increased in OA cartilage. Thus, IL-7 and S100A4 may contribute to cartilage destruction and the development of OA.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Dr. Yammani had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study design. Yammani, Loeser.

Acquisition of data. Yammani, Long.

Analysis and interpretation of data. Yammani, Long, Loeser.

Manuscript preparation. Yammani, Loeser.

Statistical analysis. Yammani.

Acknowledgements

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

We are grateful to the National Disease Research Interchange (Philadelphia, PA) for providing the normal tissue samples and to Mary Zhao for technical assistance.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES
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