To elucidate the effects of resistin on human articular chondrocytes and to generate a picture of their regulation at the transcriptional and posttranscriptional levels.
To elucidate the effects of resistin on human articular chondrocytes and to generate a picture of their regulation at the transcriptional and posttranscriptional levels.
Human articular chondrocytes were cultured with resistin. Changes in gene expression were analyzed at various doses and times. Cells were also treated with the transcription inhibitor actinomycin D after resistin treatment or with the NF-κB inhibitor IKK-NBD before resistin treatment. Gene expression was tested by quantitative real-time polymerase chain reaction. Computational analysis for transcription factor binding motifs was performed on the promoter regions of differentially expressed genes. TC-28 chondrocytes were transfected with CCL3 and CCL4 promoter constructs, pNF-κB reporter, and NF-κB and CCAAT/enhancer binding protein β (C/EBPβ) expression vectors with or without resistin.
Resistin-treated human articular chondrocytes increased the expression of cytokines and chemokines. Levels of messenger RNA (mRNA) for matrix metalloproteinase 1 (MMP-1), MMP-13, and ADAMTS-4 also increased, while type II collagen α1 (COL2A1) and aggrecan were down-regulated. The cytokine and chemokine genes could be categorized into 3 groups according to the pattern of mRNA expression over a 24-hour time course. One pattern suggested rapid regulation by mRNA stability. The second and third patterns were consistent with transcriptional regulation. Computational analysis suggested the transcription factors NF-κB and C/EBPβ were involved in the resistin-induced up-regulation. This prediction was confirmed by the cotransfection of NF-κB and C/EBPβ and the IKK-NBD inhibition.
Resistin has diverse effects on gene expression in human chondrocytes, affecting chemokines, cytokines, and matrix genes. Messenger RNA stabilization and transcriptional up-regulation are involved in resistin-induced gene expression in human chondrocytes.
Obesity is associated with alterations in adipose tissue, including the recruitment of macrophages and T cells. Adipose tissue is no longer considered to be an inert tissue, functioning solely for energy storage. Various secreted products of adipose tissue, called adipokines, have recently been characterized, including adiponectin, leptin, resistin, and visfatin (1–3). Adipokines are associated with a chronic inflammatory response syndrome characterized by abnormal cytokine production, increased acute-phase reactant synthesis, and activation of inflammation (1, 3, 4). Recent studies have shown that adipokines represent a potent risk factor for the development and progression of rheumatoid and osteoarthritic joint diseases (5–7).
Resistin is a 12.5-kd cysteine-rich polypeptide that belongs to a family of resistin-like molecules (RELMs) or found in inflammatory zone (FIZZ) molecules (1, 2). Resistin is not only expressed by human adipocytes, but it is also expressed in high levels by macrophages (1). Many aspects of the biologic effects and the regulation of resistin remain subjects of controversy, but studies have provided evidence of a role of resistin in inflammatory processes (1, 3, 8). In rheumatoid and osteoarthritic joint diseases, increased levels of resistin were observed in the synovial fluid and tissue of patients with rheumatoid arthritis (RA) or osteoarthritis (OA) (5, 9, 10), and plasma levels of resistin were significantly correlated with the erythrocyte sedimentation rate and the C-reactive protein level (9). Furthermore, resistin was shown to up-regulate interleukin-1 (IL-1), IL-6, and tumor necrosis factor α (TNFα) in the blood and synovial fluid of patients with RA. Intraarticular injection of resistin was shown to induce arthritis in healthy mouse joints (11).
Cytokines and chemokines are mediators of inflammation and are known to be important in inflammatory diseases, including RA and OA (12–14). Cytokines are a category of signaling molecules that are involved in cellular communication. Chemokines are a specific class of cytokines that mediate chemoattraction (chemotaxis). Chemokines all have a similar protein structure, being 8–10 kd, with the 2 major subclasses having conserved cysteine residues either adjacent to each other (CC) or separated by 1 amino acid (CXC) (15). Using genome-wide expression analysis of human articular chondrocytes, we previously showed that a large site of chemokines was up-regulated by the proinflammatory cytokine IL-1β (12).
Most studies with resistin have focused on cells in the inflammatory cascade. It has recently been shown that resistin is elevated following traumatic joint injury and causes the loss of proteoglycan, the production of prostaglandin E2, and the release of inflammatory cytokines from articular cartilage (16). In this study, we investigated the expression levels of cytokines and chemokines in human articular chondrocytes in response to resistin, and we generated an overall picture of their regulation at the levels of transcription and posttranscription.
Dulbecco's modified Eagle's medium and Ham's F-12 medium were obtained from Mediatech. Pronase, collagenase P, and FuGene 6 transfection reagent were from Roche. Recombinant human resistin was from R&D Systems. RNeasy Mini kit, QIAshredder, and DNase I were from Qiagen. Fetal bovine serum (FBS), SuperScript II reverse transcriptase was from Invitrogen. SYBR Green polymerase chain reaction (PCR) Master Mix was from Applied Biosystems. Penicillin/streptomycin solution, ascorbic acid, and actinomycin D were from Sigma. The pGL3-basic vector, reporter lysis buffer, and luciferase assay reagent were from Promega. Cell-permeable NF-κB essential modulator (NEMO) binding domain (NBD) synthetic peptides (IKK-NBD peptide and IKK-NBD control peptide) were from Biomol.
Cartilage was obtained with the approval of the Washington University Human Studies Review Board and with the permission of the patients. Normal chondrocytes were derived from normal articular knee cartilage obtained from a tissue donor (n = 1) with traumatic injury. Cartilage taken from the preserved area of OA cartilage was obtained from patients undergoing total knee replacement surgery (Institutional Review Board protocol no. 05-0279). For the latter, chondrocytes from macroscopically normal–appearing cartilage were used. OA cartilage samples were from male and female patients over the age of 60 years. Cartilage from 2–4 donors was combined prior to cell isolation (n = 19 in patient pool).
Chondrocytes were isolated and plated for 24 hours according to previously published procedures (12). Serum-free medium was added, and cells were allowed to rest for 24 hours before the addition of resistin at the concentrations and times indicated below. Resistin was reconstituted in sterile water. In addition, we also used the T/C-28a2 human chondrocyte cell line (provided by Dr. Mary B. Goldring, Cornell University), which was cultured under the same conditions as the human articular cartilage chondrocytes.
Total RNA was isolated from chondrocytes with the use of an RNeasy Mini kit, with DNase I treatment, according to the protocol recommended by the manufacturer. Total RNA (1 μg) was reverse-transcribed with SuperScript II reverse transcriptase to synthesize complementary DNA (cDNA). The cDNA was then used for the real-time quantitative PCR.
We performed quantitative PCR in a total volume of 20 μl of reaction mixture containing 10 μl of SYBR Green PCR Master Mix, 2.5 μl of cDNA, and 200 nM primers, using a 7300 Real-Time PCR system (Applied Biosystems). Primers used for quantitative PCR were optimized for each gene, and the dissociation curve was determined by the Real-Time PCR system. (Primers for real-time quantitative PCR are available at http://orthoresearch.wustl.edu/Laboratories/Sandell/Overview.aspx.) The parameters of primer design included a primer size of 18–21 bp, a product size of 80–150 bp, a primer annealing temperature of 59–61°C, and a primer GC content of 45–55%. Results were normalized to GAPDH. The threshold cycle (Ct) values for GAPDH and the genes of interest were measured for each sample, and the relative transcript levels were calculated as χ = 2, where ΔΔCt represents Δtreatment − ΔC; Δtreatment represents Ct(treatment) − Ct(GAPDH); and ΔC represents Ct(control) − Ct(GAPDH).
Estimates of changes in mRNA stability were analyzed in 2 ways. First, the pattern of gene expression was measured over a 24-hour period, as described by Hao and Baltimore (17). Second, for genes that remained high at 24 hours, human articular chondrocytes were treated with 100 ng/ml of resistin for 24 hours. The decay of mRNA expression was evaluated in the presence or absence of resistin, using the transcription inhibitor actinomycin D (10 μg/ml). Cells were harvested immediately (time zero) or after 1, 4, 7, or 24 hours of actinomycin D treatment. Levels of mRNA were measured by quantitative PCR as described above, and the results were normalized to GAPDH before the half-lives (i.e., the time when 50% of mRNA remained if the initial value is 100%) were calculated. The half-life (T½) of RNA was calculated from the equation T½ = ln(2)/K, where K represents the degradation rate constant and is equal to −2.303(slope) (18). The slope of the decay curves was obtained by linear regression analysis of the amount of mRNA remaining as a function of time. To facilitate direct comparison, RNA ratios at the respective time points were normalized against the ratio at the beginning of the evaluation (i.e., time 0) in each experiment.
Potential regulatory DNA surrounding the cytokine and chemokine genes was analyzed by the promoter analysis pipeline model (19, 20). Promoters (defined as 10 kb upstream and 5 kb downstream of the transcription start site) were acquired from 6 species (human, chicken, chimp, dog, mouse, and rat), and repetitive elements in the promoters were masked. Promoters were aligned and transcription factor binding sites were identified using the TransFac 11.2 database, a curated database of transcription factor profiles (20).
Probability scores for each promoter and each transcription factor binding site were calculated, and a distribution of probability scores was generated for each transcription factor. R scores were then computed using these distributions (19). This system was used to predict the transcription factors that are most likely to bind to and regulate the set of genes. For each transcription factor binding site motif (identified by the TransFac accession number) and each promoter in the genome, the probability score of the transcription factor binding to the promoter was computed by summing the exponential score of each site predicted in the promoter on either strand. This score was set to a minimum value of 1 for a promoter with no sites that exceeded the cutoff. The rank of this score was converted to the R score, which is related to the fraction of promoters with a higher rank, using the formula R score = lnN − ln(rank). Promoters ranked in the top half have R scores >ln2 (0.693), those in the top 10% have R scores >ln10 (2.302), and those in the top 1% have R scores >ln100 (4.605). The R score for a set of n promoters, or the average R score, was calculated using the following formula:
The CCL3 and CCL4 promoter 5′-deletion constructs were generated by PCR using pGL2-CCL3 (−1972/+75) and pGL3-CCL4 (−1281/+12). The CCL3 and CCL4 promoter constructs, CCAAT/enhancer binding protein β (C/EBPβ) and IκB kinase 2 (IKK-2) expression vectors, and pNF-κB luciferase reporter were provided by the following: human pGL2-CCL3 (−1972/+75) was from Dr. G. David Roodman (University of Pittsburgh); human pGL3-CCL4 (−1281/+12) was obtained from Dr. Sheau-Farn Yeh (National Yang-Ming University, Taipei); human IKK-2 in the pCDNA3 vector and pNF-κB luciferase reporter were provided by one of us (YA-A); human C/EBP (full-length) in the pCDNA3 vector was from Dr. Erika Crouch (Washington University).
DNA transfections of T/C-28a2 cells were performed using FuGene 6 transfection reagent. A total of 2 × 105 T/C-28a2 cells were cultured overnight in a 6-well plate. The transfection mixture containing FuGene 6 (6 μl), various promoter constructs (500 ng), and pCMV-β-gal (200 ng) was then added, and the cells were cultured for 24 hours. For the cotransfection assay using IKK-2 and C/EBPβ expression vectors, the expression vectors or empty vectors were added to the 100-μl transfection mixtures as indicated. FBS was added to transfection medium 4 hours later (final concentration 10%). After 24 hours of incubation, medium was replaced with fresh complete medium and incubation continued for additional time, with or without added resistin, as indicated below. The cells were then harvested with reporter lysis buffer, and the lysate was analyzed for luciferase activity using Promega luciferase assay reagent. The β-galactosidase activities were also measured to normalize variations in transfection efficiency. Each transfection experiment was performed in triplicate and was repeated at least twice.
Resistin induced the expression of genes for multiple cytokines and chemokines in human articular chondrocytes (1 normal sample and 3 patient pools from the preserved area of OA cartilage). The response of proinflammatory cytokines, chemokines, and matrix molecules to resistin (100 ng/ml) was not significantly different between normal cartilage and the preserved area of OA cartilage (Figure 1). (Data on the dose response of matrix molecules to resistin are available at http://orthoresearch.wustl.edu/Laboratories/Sandell/Overview.aspx.) With the exception of CXCL12, resistin stimulated the expression of the other 20 cytokines and chemokines tested. Seventeen genes were up-regulated more than 10-fold (Figure 1B). Bone morphogenetic protein 2 (BMP-2), TNFα, CCL2, and CX3CL1 were up-regulated 2–10-fold (Figure 1A). A selection of other genes related to cartilage growth and degradation were also monitored. The levels of mRNA for matrix metalloproteinase 1 (MMP-1) and MMP-13 increased, whereas those for the matrix genes type II collagen α1 (COL2A1) and aggrecan were down-regulated slightly (Figure 1A).
As the response to exposure to 100 ng/ml of resistin was reproducibly strong, we determined the effect of different resistin concentrations, ranging from 20 ng/ml to 500 ng/ml. It has been reported that the physiologic concentrations of resistin in OA and RA patients range from 22.1 ng/ml to as much as 70 ng/ml in synovial fluid, and from 10 ng/ml to more than 25 ng/ml in serum (9, 10, 11, 16). Human articular chondrocytes were exposed to resistin at 0, 20, 100, and 500 ng/ml. The COL2A1 gene showed a dose-dependent down-regulation beginning at 20 ng/ml of resistin. Aggrecan, MMP-1, MMP-13, and ADAMTS-4 were induced by 100 ng/ml (data available at http://orthoresearch.wustl.edu/Laboratories/Sandell/Overview.aspx.) At a resistin concentration of 100 ng/ml, many of the cytokine and chemokine mRNA were dramatically increased (Figures 1C and D). The genes that continued to increase at 500 ng/ml of resistin were TNFα, IL-1α, IL-1β, CCL2, CCL3, CCL3L1, CCL4, CCL5, CCL8, CXCL1, CXCL2, CXCL3, and CXCL6. Levels of IL-1β, CCL3, and CCL8 were increased and reached 400–600 fold (Figure 1C). In contrast, the induction of BMP-2, IL-6, IL-8, CCL20, CXCL5, and CX3CL1 reached their maximum levels with the 100 ng/ml concentration of resistin (Figure 1D).
In order to begin to ascertain which genes are coordinately regulated by resistin, RNA was isolated at 0, 1, 4, 8, and 24 hours after treatment. The expression of the genes we tested was changed significantly at 4 hours, but 3 patterns of regulation emerged. The expression of genes in group I (Figure 2A) was highest at 4 hours, but then quickly decreased during the remaining time period. Genes in group II (Figure 2B) were also induced quickly after resistin stimulation, but thereafter, their high expression was sustained. Genes in group III (Figure 2C) were induced more slowly, and they gradually and steadily increased, not reaching peak expression even by the end of the 24-hour observation period. Thus, there appear to be a number of pathways that lead to the phenotype changes induced by resistin.
To begin to determine the mechanism of gene regulation by resistin, the ability of resistin to alter mRNA half-life was measured in human articular chondrocytes. For some of the group I genes (TNFα, IL-6, and CXCL2), previous studies by Hao and Baltimore (17) showed that they are primarily regulated by mRNA stability. We have shown that BMP-2 gene expression induced by TNFα is also regulated by mRNA stability (21). Here, we investigated the mRNA stability of cytokines and chemokines in groups II and III by blocking transcription with actinomycin D after 24 hours of treatment with resistin. The results showed that the extension of half-lives in group II was more significant than that in group III, with extension varying from ∼2-fold to 10-fold (Table 1). Thus, the involvement of a posttranscriptional mechanism in the induction of these genes by resistin in human chondrocytes is indicated. Hao and Baltimore (17) showed that multiple Au-rich elements (AREs) were present in chemokine genes that were regulated by mRNA stability. We found that the average number of AREs present in these groups of transcripts correlated with mRNA stability (Table 1).
|Genes||No. of AREs in the 3′-UTR||Half-life, hours|
|Without resistin||With resistin|
|IL-8||6||3.04 ± 0.02||16.09 ± 3.84|
|CCL2||1||3.48 ± 1.17||7.40 ± 1.14|
|CCL20||3||4.43 ± 1.61||44.11 ± 4.10|
|CXCL1||3||1.70 ± 0.49||3.25 ± 0.66|
|CXCL5||10||3.60 ± 0.02||8.92 ± 0.09|
|CXCL6||5||1.57 ± 0.42||5.91 ± 2.00|
|CX3CL1||2||1.50 ± 0.73||2.78 ± 0.71|
|IL-1α||5||2.49 ± 0.08||15.66 ± 0.86|
|IL-1β||4||2.32 ± 0.71||7.01 ± 2.22|
|CCL3||3||1.71 ± 0.37||2.39 ± 0.004|
|CCL3L1||3||5.93 ± 3.17||6.16 ± 3.04|
|CCL5||0||7.89 ± 0.40||8.22 ± 0.85|
|CCL8||5||5.47 ± 0.69||16.35 ± 1.62|
Genes that are transcriptionally coexpressed often contain common regulatory motifs in their DNA flanking domains. To begin to analyze the regulatory mechanism of the cytokines and chemokines by human chondrocytes in response to resistin, the up-regulated cytokines and chemokines were subdivided into 2 groups: group A mRNA were increased more than 10-fold when exposed to 100 ng/ml of resistin, and group B mRNA were increased 2–10-fold. The promoters of group A genes were analyzed (Table 2). The R score indicates the probability that the transcription factor corresponding to this motif will bind to the promoter of these genes: the higher the R score, the more likely it is to bind. Although the binding must be verified experimentally, R scores over 2 have been demonstrated to have a high likelihood of functional significance (19, 20). Overall, several transcription factor binding motifs known to be involved in the expression of proinflammatory cytokine–induced genes were identified: NF-κB, p65, c-Rel, myocyte enhancer binding factor 3 (MEF-3), Ikaros 1 (Ik-1), and C/EBPβ.
|TransFac motif accession no.||Transcription factor||R score (average)|
In order to verify experimentally the transcription factor regulation predicted by computational analysis in human chondrocytes, we examined NF-κB function directly by using a pNF-κB luciferase reporter in TC-28 chondrocytes. The TC-28 cells showed a similar response to resistin as did the human primary chondrocytes. (Data on the response of CCL3 and CCL4 to resistin in the TC-28 cell line are available at http://orthoresearch.wustl.edu/Laboratories/Sandell/Overview.aspx.) The activity of the pNF-κB luciferase reporter in the presence of resistin was up-regulated at 1 hour, remained up-regulated at 8 hours, but was reduced by 24 hours (Figure 3A).
Because other transcription factors are potentially important in cytokine and chemokine gene expression, we also investigated the role of C/EBPβ. To examine the function of NF-κB and C/EBPβ in detail, C/EBPβ and IKK-2 (IKKβ) expression vectors were cotransfected with −1395-bp CCL3 (a group III gene) and −1281-bp CCL4 (a group I gene) promoter constructs. These constructs contain several high-probability candidate C/EBPβ and NF-κB binding sites (Figure 3B). The promoter activity of −1395-bp CCL3 and −1281-bp CCL4 constructs was up-regulated in a dose-dependent manner, suggesting that C/EBPβ and IKK-2 are both acting as activators (Figures 3C and D).
To confirm the potential role of NF-κB in resistin-induced cytokine and chemokine activation, IKK-NBD, a specific NF-κB inhibitor, was added to the human articular chondrocyte cultures before treatment with resistin. Following 4 hours of resistin treatment, the mRNA from these cells showed a modest, but dose-dependent, suppression of cytokine and chemokine activity (Figures 4A–C). As a control, we found that following 4 hours of IL-1β stimulation, the inhibitory effects of IKK-NBD on well-known NF-κB–responsive genes, such as IL-1β, IL-6, IL-8, CCL2, CCL5, and CCL20, were similar (Figure 4D). Therefore, this modest IKK-NBD suppression was not resistin-specific. The modest suppression can be attributed to the use of primary chondrocytes in the present study, as opposed to previous experiments, where only cell lines were used. To test this possibility, similar experiments were performed in the T/C-28a2 cell line where the inhibition was greater. (Data on IKK-NBD peptide inhibition of the activity of pNF-κB Luc reporter in the TC-28 cell line are available at http://orthoresearch.wustl.edu/Laboratories/Sandell/Overview.aspx.)
Resistin, the adipocyte-derived cytokine, is a potent link between adipokines and inflammatory diseases (1, 11), including rheumatoid and osteoarthritic joint diseases (9, 11). To provide a view of the effect of resistin on the expression of human articular chondrocyte genes, we analyzed 25 genes related to the inflammatory cascade, including 6 cytokines, 14 chemokines, and 5 matrix genes. We found that the levels of the tested chemokines and cytokines were dramatically increased in human adult articular chondrocytes by exposure to the adipokine resistin. One exception was the lack of effect on CXCL12, which is also known as stromal cell–derived factor 1 (SDF-1). A similar pattern of expression was previously observed for chemokines induced by IL-1β in human articular chondrocytes (12). The expression of mRNA for MMP-1, MMP-13, and ADAMTS-4 was also increased, while that of mRNA for COL2A1 and aggrecan was down-regulated in response to the resistin. The expression of ADAMTS-5 was also monitored, and its expression was reduced by resistin (data not shown).
In inflamed joints, cytokines and chemokines are produced by the synovium, macrophages, and fibroblast-like synoviocytes, and they are thought to be key regulators of the inflammatory process (12, 13, 15, 22, 23). Cytokines both enhance the migration of cells into the joint and stimulate matrix metalloproteinase production in synovial fibroblasts and chondrocytes (22). Chemokines function in the recruitment of neutrophils, monocytes, immature dendritic cells, B cells, and activated T cells (24). Furthermore, it has recently been reported that the CXC family of chemokines is important in the regulation of angiogenesis in RA, and CCL2, CCL3, and CCR2 stimulate osteoclastogenesis (25–27). The production of chemokines and cytokines under the influence of resistin could therefore significantly alter the metabolism of chondrocytes.
Cytokines and chemokines that are highly up-regulated by resistin in inflammation have not previously been shown to be regulated by resistin in human chondrocytes. IL-1α, IL-1β, IL-6, IL-8, CCL2, CCL3, CCL4, and TNFα have been identified in patient serum, synovial fluid, and blood cells following resistin stimulation (9, 11, 16). Lee and colleagues (16) also reported that resistin stimulated the secretion of CCL2 and IL-6 in mouse cartilage. Adipokines are expressed in the joint tissue and serum of patients with rheumatoid and osteoarthritic joint diseases (9, 10, 16, 28–31). Adiponectin is unable to modulate TNFα or IL-1β release in chondrocytes (30), but resistin can up-regulate them, especially IL-1β, which was increased more than 100-fold following treatment with 100 ng/ml of resistin. As an important cytokine in inflammatory joint disease, IL-1β can induce enzymes that degrade the extracellular matrix and reduce the synthesis of the primary cartilage components COL2A1 and aggrecan (12).
The level of gene expression is regulated at both the transcriptional and posttranscriptional levels in eukaryotic cells, fibroblasts, and chondrocytes (17, 21, 32). Modulation of the mRNA decay rate is a strategy widely used by cells to adjust the intensity of expression (33). Recently, Hao and Baltimore (17) reported that mRNA stability influences the levels of genes encoding inflammatory molecules in mouse fibroblasts, providing a temporally controlled process of protein expression. The same trend was observed in our human chondrocyte samples over a 24-hour time period for the cytokine and chemokine genes, including TNFα, IL-1β, IL-6, CXCL1, CXCL2, CCL2, CCL20, CCL5, CX3CL1, and CXCL5. As Hao and Baltimore had reported, we found the expression of genes from group I that were highly related to mRNA stability contained a large number of AREs (Table 1), which are known to destabilize mRNA. The effect of mRNA stability was also important in genes from group II, but mRNA from group III genes was more stable, and mRNA stability did not significantly affect their expression.
In the present study, although IL-1β and CXCL1 were not among the group I genes expressed in human articular chondrocytes, the extension of mRNA stability in these genes indicated that the mRNA stability is also involved in the steady-state level of mRNA. For BMP-2, Fukui and colleagues (21) showed the up-regulation of BMP-2 in chondrocytes via both transcription and mRNA stability. Furthermore, the results of mRNA stability analyses revealed that mRNA stability is also involved in the up-regulation of group II and group III genes. Together, the findings of these studies support the view that mRNA stability is an important determinant in resistin-induced gene expression.
To explore potential transcriptional regulation of the chemokines and cytokines, they were subclassified according to the extent of their up-regulation at 24 hours and were subjected to a computational analysis for transcription factor binding sites that were highly represented in each set. It has been demonstrated that the computed scores are highly correlated with binding probability, such that promoters with higher combined scores were more likely to be bound by the transcription factor than were promoters with lower scores (19). In the genes that were highly up-regulated, binding sites for factors related to NF-κB had very high scores (>90%). The importance of the NF-κB signaling pathway for resistin-induced inflammation has been reported for blood cells (11). We also showed that the activity of the pNF-κB luciferase reporter in human chondrocytes was increased significantly after resistin treatment. Cotransfection of the IKK-2 expression vector established that IKK-2 could enhance the promoter activity of CCL3 and CCL4 with resistin stimulation. Together, these observations showed that NF-κB signaling in human chondrocytes is involved in cytokine and chemokine expression with resistin treatment.
It has been reported that the NF-κB inhibitor hypoestoxide reduced fibronectin fragment induction of IL-6, IL-8, CCL2, CXCL1, CXCL2, and CXCL3 in human articular chondrocytes (34). Amos and colleagues (35) also demonstrated that inhibition of NF-κB activity inhibited most, but not all, mediators of inflammation. Thus, to address the role of NF-κB in resistin-mediated cytokine and chemokine expression, we used the cell-permeable IKK-NBD peptide; this peptide prevents the association of NEMO/IKKγ with IKKα and IKKβ, which is required for NF-κB activation (36). We showed that IKK-NBD modestly inhibited the resistin-induced cytokine and chemokine mRNA expression, but did not inhibit all of the mRNA expression. However, since IKK-NBD is a potent inhibitor of only canonical IKK signaling, the resistin-induced cytokine and chemokine mRNA up-regulation could also be activating NF-κB subunits by an IKK-independent mechanism, which could be important in further studies.
To begin to account for the additional expression, we investigated the role of another transcription factor with a high binding score, C/EBPβ. Cotransfection of the C/EBPβ expression vector indicated that C/EBPβ could also enhance the promoter activity of CCL3 (group III gene) and CCL4 (group I gene) (37, 38). Since IKK-NBD inhibited ∼40% of the CCL3 and CCL4 mRNA expression, C/EBPβ could also be an important regulator.
C/EBPβ has previously been associated with IL-1β–induced and TNFα-induced changes in chondrocyte gene expression. C/EBPβ is increased in chondrocytes by IL-1β and TNFα, and down-regulates COL2A1 and cartilage-derived retinoic acid–sensitive protein (CD-RAP) (37–39). In addition, C/EBPβ plays an important role in repressing cartilage gene expression in noncartilaginous tissues (40). Hirata and colleagues (41) reported that C/EBPβ promoted the transition from proliferation to hypertrophy in growth plate chondrocytes. A cooperative interaction of C/EBPβ and NF-κB has been demonstrated in other genes. The involvement of both C/EBPβ and NF-κB was recently shown in the expression of IL-1β and IL-8 (42, 43). C/EBPβ regulates the basal transcription activity of IL-8, and C/EBPβ and NF-κB together mediate the IL-8 response to infection by Pseudomonas aeruginosa (43).
In summary, we have shown that many cytokines and chemokines are up-regulated by the adipokine resistin in human articular chondrocytes. These findings begin to provide a molecular mechanism by which the increased levels of resistin that occur following traumatic joint injury (16) could lead to matrix degradation. The mRNA stability of some cytokines and chemokines was increased by resistin, which indicated the potential involvement of a posttranscriptional mechanism in the induction of these genes in human chondrocytes. By computational analysis and experimental studies, NF-κB is the most highly represented transcription factor binding site, but we demonstrate that C/EBPβ is also involved. Considering this finding in combination with our IL-1β results in human chondrocytes (12), it can be expected that this high-level increase in such a wide range of cytokines and chemokines will have a significant impact on cartilage cells and should be considered in the pathophysiology of rheumatoid and osteoarthritic joint diseases. These studies provide the basis for further investigation into the function and regulation of chemokines in synovial joint disease.
All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Sandell 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 conception and design. Zhang, Chang, Liao, Abu-Amer, Sandell.
Acquisition of data. Zhang, Xing, Hensley, Chang.
Analysis and interpretation of data. Zhang, Abu-Amer, Sandell.
The authors would like to thank Drs. John C. Clohisy, Robert L. Barrack, Douglas McDonald, Ryan Nunley, and Rick W. Wright and Head Nurse Keith Foreman for the normal and OA cartilage. The authors would also like to thank Drs. Deb Patra, Chikashi Kobayshi, and Corey Gill at the Washington University School of Medicine for valuable assistance.