Synovial hyperplasia and angiogenesis are prominent hallmarks of inflammatory arthritis. Inflammatory signals promote the recruitment of leukocytes into synovial tissue, increase the proliferation of resident synovial cells, and trigger the degradation of tissue via the activity of matrix-degrading enzymes (1–3). Although the extent of synovitis ranges from an aggressive pannus structure in rheumatoid arthritis (RA) to a mild thickening of the synovial layer in osteoarthritis (OA), these changes exacerbate inflammation within joints and ultimately contribute to the erosion of cartilage and bone. The proinflammatory cytokine tumor necrosis factor α (TNFα) is a major regulator of synovial hyperplasia and angiogenesis (2). Given the clinical benefits of targeting TNFα, it is critical to elucidate the molecular targets downstream of this cytokine pathway.
During the progression of arthritis, inflammation mediators such as TNFα transform synovial fibroblasts (synoviocytes) into aggressive cells that invade surrounding cartilage and bone. Much like tumor cells, transformed synoviocytes demonstrate increased proliferation, loss of contact inhibition, and anchorage-independent cell growth (1–3). A number of genes promoting inflammation, angiogenesis, and tissue degradation are expressed at elevated levels in transformed synoviocytes. TNFα induces factors such as vascular endothelial growth factor and interleukin-8 (IL-8), which promote blood vessel formation and leukocyte recruitment (2). TNFα is also a potent inducer of matrix metalloproteinases (MMPs), which are enzymes that degrade the extracellular matrix and mediate cartilage and bone erosion (4). Synoviocyte transformation is largely regulated by transcription factors activated by inflammatory pathways. For example, NF-κB, c-Myc, CREB, and p53 can modulate synoviocyte proliferation and invasion and MMP gene expression (5–10).
The NR4A subfamily of orphan nuclear receptors is another group of transcription factors with potential regulatory roles in the onset and progression of arthritis. These receptors consist of nerve growth factor–induced protein B (NGFI-B)/Nur-related protein 77 (NUR77, NR4A1), nuclear receptor related 1 protein (NURR1, NR4A2), and neuron-derived orphan receptor 1 (NOR-1, NR4A3). In contrast to other members of the nuclear receptor superfamily, NR4A receptors are ligand-independent transcription factors, and their activity is tightly controlled at the level of expression and posttranslational modifications (11). NR4A receptors induce transcription of target genes by binding to NGFI-B–responsive elements (AAAGGTCA motif) in promoter regions (12). These receptors can also act as transcriptional repressors through mechanisms involving protein–protein interactions with other transcription factors or coregulatory molecules. Although the NR4A receptors have a high degree of sequence homology and use similar transcription mechanisms, distinct roles for these 3 receptors have been described in a variety of cell and tissue types (11).
We previously observed elevated levels of NR4A receptors in synovial tissue and cartilage from patients with RA, patients with psoriatic arthritis, and patients with OA (13–17). Inflammatory signals activating the NF-κB and CREB pathways rapidly and potently induce NR4A expression in cells derived from inflamed joints (13). However, the transcriptional targets of these receptors and their impact on cellular activities associated with arthritis have not been thoroughly explored. In synoviocytes, NR4A2 induces IL-8 transcription (18, 19), potentially linking this transcription factor to cell migration and angiogenesis. In chondrocytes, NR4A2 antagonizes cytokine-induced MMP-1, MMP-3, and MMP-9 expression (14), suggesting a protective role of the receptor within cartilage. In endothelial cells, the NR4A receptors regulate cell survival, proliferation, migration, and angiogenesis (20–24). In some types of cancer, the NR4A receptors contribute to cellular transformation, increased proliferation, and cell survival (25–27). In light of emerging roles information on roles of the NR4A receptors in these processes, we hypothesized that the receptors may also promote synovial hyperplasia and tissue damage during the progression of inflammatory arthritis.
In this study, we investigated the function of the NR4A receptors in human synovial tissue and synoviocytes. NR4A2 is expressed at elevated levels in vivo and in synoviocytes stimulated with TNFα and prostaglandin E2 (PGE2). Overexpression of NR4A2 enhances synoviocyte proliferation, survival, anchorage-independent growth, migration, and cellular invasion. At the molecular level, NR4A2 induces MMP-13 transcription through a mechanism that requires the DNA binding domain of the receptor. Depletion of endogenous NR4A receptors attenuates synoviocyte proliferation, reduces migration, and suppresses MMP-13 expression. Taken together, these results show that NR4A2 promotes an aggressive phenotype in synoviocytes, suggesting that this receptor may have promise as a therapeutic target in inflammatory arthritis.
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- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
NR4A mRNA levels in human synovial tissue derived from normal, OA, and RA knee joints were measured by quantitative RT-PCR. All 3 NR4A transcripts were detected, with mean expression levels of 0.4 (NR4A1), 350 (NR4A2), and 1.5 (NR4A3) copies per 1,000 copies of GAPDH. In normal synovial tissue, absolute levels of NR4A2 greatly exceeded the levels of NR4A1 and NR4A3 (P < 0.001) (Figure 1A). Results consistent with this finding were observed in OA and RA tissue, with NR4A2 levels elevated to the same extent above NR4A1 and NR4A3 levels in normal, OA, and RA tissue.
Figure 1. Nuclear receptor 4A (NR4A) expression in human synovial tissue and primary synoviocytes. A, Mean NR4A mRNA levels (per 1,000 copies of GAPDH mRNA) in normal, osteoarthritis (OA), and rheumatoid arthritis (RA) synovial tissue specimens from human knee joints, as measured by quantitative reverse transcription–polymerase chain reaction (RT-PCR). ∗∗∗ = P < 0.001 by Mann-Whitney test. B, RA synovial tissue (sublining layer) stained with anti–Ki-67 (fluorescein isothiocyanate [FITC]), anti-NR4A2 (Cy3), and Hoechst. Original magnification × 400. C, Mean NR4A mRNA levels in activated (passages 1–4) and quiescent (passages 5–7) primary synoviocytes from RA synovial tissue, as measured by quantitative RT-PCR. ∗ = P < 0.05 by Mann-Whitney test.
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To further examine expression patterns of the NR4A receptors, pooled RNA from another set of patients was hybridized to high-density microarrays. NR4A2 levels were 2-fold higher in OA and RA tissue compared with normal control tissue, while the levels of NR4A1 and NR4A3 in OA and RA tissue were not different from the level in normal tissue (data not shown). Thus, NR4A2 exhibited a unique expression pattern in synovium, where it was the most highly expressed of the NR4A receptors and, as demonstrated by microarray analysis, was present at elevated levels in advanced OA and RA. Consistent with this expression pattern and with previous results (13, 15, 17), the expression of NR4A2 protein was increased in inflamed synovium, with abundant nuclear expression in synoviocytes from the sublining and lining layers (Figure 1B and results not shown).
To examine potential links between NR4A2 expression and synovial hyperplasia, colocalization of NR4A2 and a cell cycle–associated protein, Ki-67, was examined in RA synovial tissue. Ki-67 expression was low or absent in tissue with little or no inflammation, as measured by CD68 staining (n = 4; results not shown). However, inflamed tissue stained positive for both NR4A2 and Ki-67 expression in the synovial sublining and lining layers (n = 6) (Figure 1B and results not shown). Nuclear colocalization of these factors suggested an in vivo relationship between NR4A2 expression, active proliferation, and inflammation. To investigate this further, NR4A mRNA expression was measured in activated and quiescent synoviocytes derived from patients with RA. NR4A2 levels were elevated in activated synoviocytes undergoing rapid proliferation (Figure 1C). However, they were sharply decreased in quiescent cells (P < 0.05). NR4A1 expression followed a similar trend, while NR4A3 levels remained constant (data not shown). The results for RA tissue and primary synoviocytes highlighted a correlation between NR4A2 levels and proliferation, suggesting that NR4A2 may promote cellular changes associated with synovial hyperplasia.
To address the cellular functions of NR4A2 in vitro, we used the normal human synoviocyte cell line, K4IM. This cell line has been used as a model to study synoviocyte activation and responses to inflammation (18, 19, 28). Consistent with the findings of our analysis of intact synovial tissue, absolute levels of NR4A2 mRNA in K4IM cells exceeded the levels of NR4A1 and NR4A3 (Figure 2A). Previous studies demonstrated that the NR4A receptors are rapidly and transiently induced by signals that promote synovial inflammation and tissue damage (13, 16, 18). To confirm this, we stimulated K4IM cells with physiologic concentrations of TNFα or PGE2 for 1 hour. TNFα selectively induced NR4A2 expression by 2-fold (P < 0.005) (Figure 2A), while the expression of NR4A1 and NR4A3 was not significantly altered. PGE2 potently induced expression of all 3 receptors and resulted in the highest absolute level of NR4A2 mRNA (P < 0.005) (Figure 2A). NR4A2 expression patterns in the K4IM cells paralleled those in inflamed synovial tissue, where NR4A2 was the most highly expressed member of the NR4A receptor subfamily and was potently regulated by inflammatory mediators.
Figure 2. NR4A2 alters synoviocyte morphology and increases proliferation and survival. A, Human K4IM synoviocytes were treated in triplicate with tumor necrosis factor α (TNFα; 10 ng/ml) or prostaglandin E2 (PGE2; 1 μM) for 1 hour, and the expression of NR4A mRNA per 1,000 copies of GAPDH was measured by RT-PCR. B, Untreated K4IM cells, K4IM cells treated with PGE2 for 1 hour, and K4IM cells stably expressing NR4A2 (clone 1) were immunofluorescence stained for NR4A2 (red), with DAPI counterstaining (blue). Original magnification × 200. C, Stable K4IM clones expressing LacZ (control) or NR4A2 cDNA (clones 1–3) were stained with toluidine blue, and the surface area was measured. In the histologic images, original magnification × 200. D, The proliferation rates of stable NR4A2 clones cultured in complete growth medium for 6 days were determined. E, Flow cytometry was performed, revealing resistance of NR4A2 clones to CdCl2-induced apoptosis. Values in A are the mean ± SD; values in C and D are the mean ± SEM. ∗ = P < 0.05; ∗∗ = P < 0.005 versus untreated, by Student's t-test. ∗∗∗ = P < 0.005 versus control, by analysis of variance. PI = propidium iodide (see Figure 1 for other definitions).
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To study the impact of NR4A2 on cellular processes linked to synovial hyperplasia and tissue degradation, NR4A2 was overexpressed in K4IM cells. Ectopic expression was confirmed by quantitative RT-PCR, and transcriptional activity of the receptor was validated using a consensus NBRE reporter construct (results not shown). Robust nuclear localization of ectopic NR4A2 protein paralleled localization of the endogenous receptor induced by PGE2 (Figure 2B).
TNFα-stimulated release of resorptive agents, such as MMPs, from synoviocytes occurs in association with a change from a fibroblast-like morphology to a stellate morphology (32). We previously observed that synoviocyte transformation following TNFα stimulation occurred with a concomitant increase in the expression of NR4A2 protein (13). In the absence of TNFα, stable NR4A2 clones exhibited a distinctive stellate morphology and a 50% reduction in cell surface area and nuclei circumference (Figure 2C and results not shown), suggesting that the cellular changes induced by TNFα may be mediated in part by NR4A2. Furthermore, the proliferation rates of NR4A2 clones exceeded those of control cells, reaching a 5-fold difference after 6 days (P < 0.005) (Figure 2D). In support of this finding, an increased percentage of NR4A2-overexpressing cells was observed in the S phase of the cell cycle by flow cytometry (data not shown). The cell viability of control cells and NR4A2 clones remained constant (>90%) during the course of the proliferation assays. Furthermore, NR4A2 clones exhibited resistance to cadmium chloride–induced apoptosis, suggesting that this receptor promotes cell-survival pathways (Figure 2E).
Next, we investigated the impact of NR4A2 on the processes of anchorage-independent growth, migration, and invasion. NR4A2 overexpression promoted formation of synoviocyte colonies in soft agar, and NR4A2 colonies were 3-fold larger than controls (P < 0.005) (Figures 3A and B), indicating persistent survival and growth in a detached state. In addition, the NR4A2 clones showed increased migration through Transwell filters relative to control cells (P < 0.005) (Figure 3C). NR4A2 potently increased synoviocyte invasion through Transwell filters coated with type II collagen or Matrigel (P < 0.005) (Figure 3D and data not shown). Taken together, these results showed that NR4A2 induced a transformed phenotype in normal synoviocytes, potentially by activating transcriptional pathways involved in hyperplasia, cell survival, and extracellular matrix degradation.
Figure 3. NR4A2 induces anchorage-independent growth, migration, and invasion. A, Control and stable NR4A2 clones were cultured in 0.35% agarose and complete growth medium for 7 days. Viable cells were stained with alamarBlue, and the optical density (OD) of each well was measured at 570 nm. B, Following culturing in 0.35% agarose, cells were stained with 0.005% crystal violet, and the diameter of 20 colonies/well was measured using Image-Pro Plus software. In the histologic images, original magnification × 40. C, Control and NR4A2 clones were cultured on Transwell filters with growth medium in the top and bottom chambers. After 48 hours, cells that migrated to the basolateral side of the filters were stained with crystal violet and counted. D, Control and stable NR4A2 clones were cultured on type II collagen–coated Transwell filters with complete growth medium in the top and bottom chambers. After 72 hours, cells that invaded to the basolateral side of the filters were stained with crystal violet and counted. Bars show the mean ± SEM. Results are representative of 3 independent experiments. ∗∗∗ = P < 0.005 versus control, by analysis of variance.
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We hypothesized that NR4A2 may mediate the invasive potential of synoviocytes by regulating the expression of MMPs and TIMPs. K4IM cells express MMP-1, MMP-2, MMP-9, and MMP-13, and we investigated the effects of TNFα and NR4A2 on these genes, using quantitative RT-PCR. Stimulation with TNFα and NR4A2 induced MMP-13 expression and resulted in the synergistic induction of mRNA levels by 40-fold (P < 0.05) (Figure 4A). MMP-1 and MMP-9 were significantly induced by TNFα, but NR4A2 did not regulate these genes alone and, when administered with TNFα, did not increase their induction beyond that observed with the cytokine alone. MMP-2 expression was constitutively low and was not regulated by TNFα or NR4A2. TIMP-2 levels were suppressed by 70% by the combination of NR4A2 and TNFα (P < 0.01), while TIMP-1 was unaffected) (Figure 4B). In support of the results of our mRNA analysis, NR4A2 and TNFα synergistically induced MMP-13 protein secretion by 3-fold (P < 0.01) (Figure 4C). Taken together, these results showed that NR4A2 induced MMP-13 expression and antagonized TIMP-2 expression in K4IM cells. The net impact of these changes could ultimately promote extracellular matrix degradation and cell invasion.
Figure 4. NR4A2 induces matrix metalloproteinase 13 (MMP-13) expression and antagonizes tissue inhibitor of metalloproteinases 2 (TIMP-2) expression. A and B, K4IM cells were transiently transfected with control LacZ cDNA or NR4A2 cDNA. Transfected cells were serum-starved for 24 hours, followed by treatment with tumor necrosis factor α (TNFα; 10 ng/ml). RNA was harvested after 48 hours, and the relative expression of MMP-1, MMP-2, MMP-9, and MMP-13 mRNA (A) and TIMP-1 and TIMP-2 mRNA (B) was determined. C, K4IM cells were transduced with lentiviral particles containing control or NR4A2 cDNA for 24 hours, followed by treatment with TNFα for 48 hours. Active and total MMP-13 protein levels were measured in conditioned medium, using a fluorescent substrate assay. Bars show the mean ± SD. ∗ = P < 0.05; ∗∗ = P < 0.01 versus control, by Student's t-test.
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We focused on the mechanism of NR4A2-induced regulation of MMP-13, because this enzyme plays a critical role in the degradation of type II collagen molecules in articular cartilage (4). NR4A2 transactivated human MMP-13 promoter constructs in transiently transfected K4IM cells (Figure 5A), indicating that regulation of this gene occurs at least in part at the level of transcription. The full-length MMP-13 promoter (−3.4 kb) was induced 2-fold by NR4A2, consistent with the effects of NR4A2 on endogenous MMP-13 mRNA and protein shown in Figure 4. The −405-bp and −181-bp promoter constructs were induced 4-fold by NR4A2, suggesting that NR4A2 targets the proximal region of the MMP-13 promoter. A putative NBRE site exists within the proximal promoter at position −28 bp; however, mutation of the core nucleotides in this sequence did not abrogate induction by NR4A2 (Figure 5B). Next, we tested a point mutation in the DNA binding domain of NR4A2, C283G, for its ability to regulate the MMP-13 promoter. This mutation disrupts a zinc finger motif in the receptor that is critical for DNA interactions (14, 18, 31). The C283G mutant failed to transactivate the −181-bp promoter (Figure 5C), suggesting that regulation of MMP-13 by NR4A2 requires a functional DNA binding domain.
Figure 5. NR4A2 induces transcription of MMP-13 through the proximal promoter. K4IM cells were transiently cotransfected in triplicate with human MMP-13 promoter–luciferase constructs and control or NR4A2 cDNA. Relative luciferase units (RLUs) were measured in whole cell extracts 48 hours following transfection. A, NR4A2 induced the full-length MMP-13 promoter (−3.4 kb) and the shorter proximal promoter fragments (−405 bp and −181 bp). B, Mutation of the core nucleotides in the −181-bp MMP-13 promoter construct containing a wild-type or mutant nerve growth factor–induced protein B–responsive element (NBRE) site at −28 bp did not abrogate induction by NR4A2. C, The C283G mutant failed to transactivate the −181-bp promoter. Bars show the mean ± SD. ∗∗ = P < 0.01; ∗∗∗ = P < 0.001 versus control, by Student's t-test. See Figure 4 for other definitions.
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To investigate the roles of the endogenous NR4A receptors in K4IM synoviocytes, levels of NR4A1, NR4A2, and NR4A3 were reduced by transduction with shRNA. We used this strategy because all 3 receptors were expressed in these cells (Figure 2), and the high degree of homology among these receptors could allow for compensatory functions (11). Endogenous NR4A1, NR4A2, and NR4A3 mRNA levels were reduced by 50–65% in K4IM cells stably transduced with specific shRNA molecules (Figure 6A). Depletion of the receptors attenuated proliferation by 35% at time points as early as 36 hours (P < 0.001) (Figure 6B), indicating that the endogenous receptors intersect with cell proliferation pathways. Furthermore, NR4A-depleted synoviocytes demonstrated a defect in wound-healing assays, in which they failed to migrate into monolayer scratches after 12 hours (Figure 6C). We also investigated regulation of MMP-13 by the endogenous receptors and observed a 50% reduction in TNFα-induced levels of MMP-13 in NR4A-depleted cells (Figure 6D). Taken together, these results suggest that the endogenous NR4A receptors are critical regulators of synoviocyte proliferation, migration, and MMP-13 gene expression.
Figure 6. Depletion of endogenous NR4A reduces proliferation, migration, and MMP-13 expression. Stable K4IM cells were generated with a control scrambled (Scram) short hairpin RNA (shRNA) construct and shRNA constructs specific for NR4A1, NR4A2, and NR4A3. A, Endogenous NR4A1, NR4A2, and NR4A3 mRNA levels were reduced by 50–65% in K4IM cells stably transduced with specific shRNA molecules, as determined by quantitative reverse transcription–polymerase chain reaction (RT-PCR). B, Depletion of the receptors attenuated proliferation by 35% at time points as early as 36 hours. Results are representative of 3 independent experiments. C, Wound-healing assays were conducted on stable cells. Results are representative of 3 independent experiments. Original magnification × 40. D, Stable clones were left untreated or were treated with TNFα (10 ng/ml) in triplicate for 24 hours. MMP-13 expression was measured by quantitative RT-PCR. Results are representative of 4 independent experiments. Values in A, B, and D are the mean ± SEM. ∗ = P ≤ 0.05; ∗∗∗ = P < 0.001 versus scrambled, by Student's t-test. See Figure 4 for other definitions.
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- Top of page
- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
In this study, we documented expression of the NR4A orphan receptors in synovial tissue and specifically explored the cellular and molecular functions of NR4A2 in synoviocytes. Our results highlight a potential pathogenic role of NR4A2, because overexpression of this receptor induces a phenotypic shift in normal synoviocytes that parallels the cellular transformation and hyperplasia observed during the progression of inflammatory arthritis. This study is the first to demonstrate that NR4A2 increases synoviocyte proliferation, survival, and anchorage-independent growth while enhancing cellular migration and invasion. Furthermore, we identified MMP-13 and TIMP-2 as transcriptional targets of NR4A2 in synoviocytes. These results complement recent studies proposing proinflammatory roles for the NR4A receptors in synoviocytes (18, 19).
During the progression of arthritis, synovial hyperplasia and angiogenesis promote inflammation and degradation of articular cartilage and bone (1–3). Transcription factors such as NF-κB and CREB regulate the expression of genes involved in these pathologic changes (6, 9). We previously demonstrated that NF-κB and CREB induce expression of the NR4A receptors in inflamed synovial and cartilage tissue (13–16). However, the roles of these receptors in synoviocyte transformation and hyperplasia have not yet been established. In addition to MMP-13 and TIMP-2 (identified in this study), other transcriptional targets of NR4A2 in synoviocytes include IL-8, amphiregulin, and Kit ligand (18, 19). Collectively, these gene products promote cell recruitment, proliferation, and survival and tissue damage in inflamed joints, suggesting that NR4A2 controls proinflammatory pathways in vivo.
The enhanced migration and invasion of synoviocytes in response to NR4A2 are likely mediated in part by the up-regulation of MMP-13. This collagenase has been linked to the invasion and metastasis of numerous types of cancer due to its ability to degrade extracellular matrix components and facilitate cell motility (4). Of particular importance in arthritic joints, MMP-13 has the highest catalytic activity against type II collagen, the major protein component of articular cartilage (4). Intraarticular expression of MMP-13 in mice results in synovial hyperplasia and inflammation (33), indicating that this enzyme may also contribute to the onset of inflammatory arthritis. NR4A2 also down-regulates TIMP-2, an endogenous inhibitor of MMPs, which could in turn promote cell migration and invasion. Furthermore, TIMP-2 functions as an inhibitor of angiogenesis by reducing the mitogenic response of microvascular endothelial cells to growth factors independently of MMPs (34). Within inflamed synovium, down-regulation of TIMP-2 could enhance angiogenesis and exacerbate inflammation and tissue damage.
NR4A2 induces MMP-13 transcription through a mechanism that targets the proximal promoter and requires a functional DNA binding domain. The C283G mutation disrupts a zinc finger motif in NR4A2 that is critical for DNA interactions (31), suggesting that NR4A2 induces MMP-13 transcription via direct interactions with the promoter. Several NR4A target genes are induced by receptor binding to promoter NBRE sequences (AAAGGTCA) (11). We identified a putative NBRE site at position −28 bp in the human MMP-13 promoter; however, mutation of the core residues in this sequence did not prevent induction by NR4A2. This site is immediately adjacent to the TATA box at position −37 bp and contains a single nucleotide difference from the consensus element (AAAGGTAA).
Our data suggest that NR4A2 utilizes an NBRE-independent mechanism to regulate the MMP-13 promoter. NR4A2 induces IL-8 transcription through an NBRE-independent mechanism that involves receptor interactions with p65 on the proximal promoter (18). Similar to IL-8, MMP-13 is synergistically induced by NR4A2 and TNFα, suggesting that cross-talk between NR4A2 and p65 may also contribute to the induction of MMP-13. Protein–protein interactions have also been implicated in the repression of NR4A target genes in other cell types. For example, NR4A1 interacts with p65 to antagonize the expression of steroidogenic enzymes, and NR4A2 uses similar mechanisms to attenuate inflammatory gene transcription (35, 36). In chondrocytes, NR4A2 represses MMP-1 expression via negative interactions with Ets transcription factors (14). Thus, NR4A2 may repress TIMP-2 expression through similar protein–protein interactions that converge on the promoter.
Although our study is the first to define functions for the NR4A receptors in synoviocyte proliferation, survival, anchorage-independent growth, and migration, these activities have been investigated in other systems. As with several members of the nuclear receptor family, cell- and tissue-specific functions for the NR4A receptors have been demonstrated (11). Differential roles for the receptors have been proposed in angiogenesis, a process that is regulated in part by endothelial cell proliferation and migration. NR4A3 promotes endothelial cell proliferation (22), while NR4A1 mediates cell cycle arrest by regulating p27Kip1 and cyclin A protein levels (37). In vivo, NR4A1 and NR4A2 positively impact angiogenesis by increasing endothelial proliferation, survival, and migration (20, 24). During vascular remodeling, NR4A1 blocks smooth muscle cell proliferation and macrophage recruitment (21, 38), while NR4A3 induces mitogenic activity (39, 40). The NR4A receptors have been implicated in several types of cancer, in which they enhance cell proliferation, survival, and migration (25–27, 41, 42). In colorectal cancer, decreasing NR4A2 levels may promote antitumor activities downstream of cyclooxygenase 2 inhibition (43). In melanoma cells, NR4A1 and NR4A2 have recently been linked to tumorigenicity and regulation of the Wnt/β-catenin pathway (44). Consistent with our findings in synoviocytes, NR4A2 expression is associated with anchorage-independent growth of cervical, prostate, and colon carcinoma cells in soft agar (25).
The NR4A2-dependent cellular activities we documented in synoviocytes may occur in part via changes in previously described target genes (18, 19). Furthermore, cyclin D2, a critical regulator of cell cycle progression and a transcriptional target of NR4A receptors in monocytes and smooth muscle cells (40, 45), may also mediate changes in synoviocyte proliferation. We are currently investigating additional NR4A2 target genes that may impact these activities in synoviocytes.
Because the NR4A receptors exhibit a high degree of sequence homology in their DNA binding domains (11), these receptors may have overlapping and redundant molecular functions. For example, mice deficient in NR4A1 or NR4A3 exhibit mild abnormalities (46, 47), while deletion of both genes leads to lethal acute myeloid leukemia (48). Our investigation focused on the role of NR4A2 in synoviocytes; however, we cannot exclude the possibility that NR4A1 and NR4A3 have similar functions in these cells. We detected expression of all 3 receptors in synovial tissue and synoviocytes, and it is possible that all of the NR4A receptors converge on similar transcriptional targets. To account for this possibility and to avoid potential redundancy issues, we used an shRNA knockdown strategy to reduce endogenous levels of all 3 receptors. Similar approaches have been used to establish roles of the NR4A receptors in DNA damage responses in melanocytes and regulation of the Wnt/β-catenin pathway in melanoma cells (44, 49). Depleting the NR4A receptors in synoviocytes attenuated proliferation and migration and also reduced expression of MMP-13. Importantly, these findings highlight a role of the endogenous receptors in regulating basal activities in synoviocytes.
In response to chronic inflammatory signals, elevated levels of the NR4A receptors may contribute to synovial hyperplasia and tissue damage in vivo. Our results suggest that these orphan nuclear receptors may show promise as therapeutic targets in inflammatory arthritis.
- Top of page
- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
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. Mix 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. Mix, Smyth, Fearon, Veale, Murphy.
Acquisition of data. Mix, McMahon, McMorrow, Walkenhorst, Smyth, Petrella, Gogarty, Fearon, Veale, Attur.
Analysis and interpretation of data. Mix, McMahon, Smyth, Gogarty, Abramson, Murphy.