Synovial IL-6 Research Articles

OP0254 Cytokine-Induced JNK-Dependent Downregulation of the Forkhead Box O Transcription Factor Foxo1 Promotes Survival and Proliferation of Fibroblast-Like Synoviocytes in Rheumatoid Arthritis

BackgroundAberrant regulation of proliferation and survival of immune and stromal cells contributes to the pathogenesis of rheumatoid arthritis (RA). Forkhead box O (FoxO) transcription factors integrate extracellular signals to modulate...

Decreased collagen‐induced arthritis severity and adaptive immunity in MKK‐6–deficient mice

The MAPK kinases MKK-3 and MKK-6 regulate p38 MAPK activation in inflammatory diseases such as rheumatoid arthritis (RA). Previous studies demonstrated that MKK-3 or MKK-6 deficiency inhibits K/BxN serum-induced arthritis. However, the role of these kinases in adaptive immunity-dependent models of chronic arthritis is not known. The goal of this study was to evaluate MKK-3 and MKK-6 deficiency in the collagen-induced arthritis (CIA) model. Wild-type (WT), MKK-3(-/-) , and MKK-6(-/-) mice were immunized with bovine type II collagen. Disease activity was evaluated by semiquantitative scoring, histologic assessment, and micro-computed tomography. Serum anticollagen antibody levels were quantified by enzyme-linked immunosorbent assay. In vitro T cell cytokine response was measured by flow cytometry and multiplex analysis. Expression of joint cytokines and matrix metalloproteinases (MMPs) was determined by quantitative polymerase chain reaction. MKK-6 deficiency markedly reduced arthritis severity compared with that in WT mice, while the absence of MKK-3 had an intermediate effect. Joint damage was minimal in arthritic MKK-6(-/-) mice and intermediate in MKK-3(-/-) mice compared with WT mice. MKK-6(-/-) mice had modestly lower levels of pathogenic anticollagen antibodies than did WT or MKK-3(-/-) mice. In vitro T cell assays showed reduced proliferation and interleukin-17 (IL-17) production by lymph node cells from MKK-6(-/-) mice in response to type II collagen. Gene expression of synovial IL-6, MMP-3, and MMP-13 was significantly inhibited in MKK-6-deficient mice. Reduced disease severity in MKK-6(-/-) mice correlated with decreased anticollagen antibody responses, indicating that MKK-6 is a crucial regulator of inflammatory joint destruction in CIA. MKK-6 is a potential therapeutic target in complex diseases involving adaptive immune responses, such as RA.

Histone deacetylase inhibitors prevent inflammation-mediated inactivation of the forkhead box class o transcription factor FOXO1 in rheumatoid arthritis

Backgroundand objectivesForkhead box O (FoxO) transcription factors, regulated by phosphatidlyinositol 3-kinase and reversible acetylation, integrate environmental signals to orchestrate inflammatory responses, cell cycle and apoptosis. Here, the authors examined the...

Interleukin 17 (IL-17) Increases the Expression of Toll-like Receptor-2, 4, and 9 by Increasing IL-1β and IL-6 Production in Autoimmune Arthritis

To examine the effect of interleukin 17 (IL-17) on the expression of Toll-like receptor (TLR)-2, 4, and 9 in collagen-induced arthritis (CIA) in mice. On Days 28 and 32 after induction of CIA in mice, phosphate-buffered saline (PBS group) or IL-17 (IL-17 group) was injected into both knee joints. On Day 35, mice were sacrificed. The severity of knee joint arthritis, synovial inflammation, and bone destruction was measured by a scoring system using macrography and histological analysis. Synovial expression of TLR-2, 4, 9, IL-17, IL-1beta, tumor necrosis factor-alpha (TNF-alpha), and IL-6 was determined by real-time PCR and immunohistochemistry. Synoviocytes of CIA mice were cultured with IL-17 and with neutralizing antibodies to cytokine, and the expression of TLR-2, 4, 9, IL-1beta, TNF-alpha, and IL-6 was determined by real-time RT-PCR. In CIA mice, knee arthritis scores, synovial inflammation, bone destruction scores, and expression of synovial TLR-2, 4, and 9, IL-17, IL-1beta, TNF-alpha and IL-6 were higher in the IL-17 and PBS groups than in normal DBA1 mice. These variables were also significantly higher in the IL-17 group than in the PBS group. In CIA synoviocytes, IL-17 increased the expression of TLR-2, 4, and 9, and this effect was significantly alleviated by neutralizing antibodies to IL-17, IL-1beta, and IL-6. IL-17 aggravates joint inflammation and destruction, and increases the synovial expression of TLR-2, 4, and 9 by increasing IL-1beta and IL-6. These results imply that the IL-17-induced increase in expression of TLR-2, 4, and 9, and IL-1beta and IL-6 production are involved in the IL-17-induced aggravation of arthritis.

Elevated high-sensitivity C-reactive protein levels are associated with local inflammatory findings in patients with osteoarthritis

C-reactive protein (CRP) has been associated with disease progression in patients with osteoarthritis (OA), but the reasons for this remain unclear. We hypothesized that higher CRP would be related to local inflammatory findings in the joints of patients with OA. Plasma and synovial membrane specimens from 54 OA patients undergoing total hip or knee arthroplasty or arthroscopy were obtained. Synovial fluid was obtained from 25 of these patients. Hematoxylin and eosin stained synovial membrane sections were scored for degree of inflammatory cell infiltration. Plasma high-sensitivity CRP (hsCRP) levels, and serum and synovial fluid interleukin (IL)-6 and IL-1beta levels were measured by enzyme-linked immunosorbent assay. Fifty-seven percent of patients with idiopathic OA had inflammatory infiltrates within the synovial membrane. The mean hsCRP level in patients with inflammatory infiltrates was significantly higher than those without inflammation (4.7 +/- 5.0 mg/L vs 1.7 +/- 3.6 mg/L, P = 0.003). There were significant correlations between hsCRP levels and synovial fluid IL-6 (r = 0.64, P = 0.0006), degree of synovial inflammatory infiltration (r = 0.43, P = 0.002), and body mass index (r = 0.31, P = 0.02). Multivariate analysis indicated that only degree of inflammatory infiltrate was significantly associated with hsCRP level (P = 0.026). These results suggest that systemic hsCRP levels reflect synovial inflammation in OA patients, perhaps by means of synovial IL-6 production. Future studies are needed to clarify how these infiltrates and their products may contribute to disease pathogenesis.

Interleukin‐17 receptor deficiency results in impaired synovial expression of interleukin‐1 and matrix metalloproteinases 3, 9, and 13 and prevents cartilage destruction during chronic reactivated streptococcal cell wall–induced arthritis

To examine the role of interleukin-17 receptor (IL-17R) signaling in cartilage destruction and its interrelationship with synovial IL-1 expression during chronic reactivated streptococcal cell wall (SCW)-induced arthritis. SCW arthritis was repeatedly induced in wild-type (WT) and IL-17R-deficient (IL-17R-/-) mice. At different time points, joint inflammation was assessed by using calipers to measure joint swelling. On day 42, mice were killed, and knee joints were removed for histologic analysis. Quantitative polymerase chain reaction (PCR) analyses for different proinflammatory mediators and matrix metalloproteinases (MMPs) were performed on inflamed synovium from WT and IL-17R-/- mice after 5 repeated injections of SCW fragments. IL-17R signaling did not play a significant role in acute joint swelling induced by a single injection of SCW fragments directly into the joint. However, repeated local injections of SCW fragments into the knee joints of IL-17R-/- mice resulted in fewer infiltrating cells in the joint compared with WT mice. Moreover, histologic analysis on day 42 revealed a significant suppression of the degree of chondrocyte death and an absence of cartilage surface erosion in IL-17R-/- mice. Quantitative PCR analysis revealed impaired synovial expression of IL-1, IL-6, cyclooxygenase 2, stromelysin (MMP-3), gelatinase B (MMP-9), and collagenase 3 (MMP-13) in IL-17R-/- mice. These data show a critical role of IL-17R signaling in driving the synovial expression of proinflammatory and catabolic mediators, such as IL-1 and different MMPs, during progression from an acute, macrophage-driven joint inflammation to a chronic, cartilage-destructive, T cell-mediated synovitis. Prevention of IL-17R signaling warrants consideration as a therapeutic target in chronic destructive arthritis.

Reduction of arthritis severity in protease‐activated receptor–deficient mice

Protease-activated receptor 1 (PAR-1) is the cell surface receptor for thrombin. It is unclear whether thrombin contributes to inflammation other than by effects on coagulation. We investigated the proinflammatory participation of PAR-1 in antigen-induced arthritis (AIA). Arthritis was induced by intraarticular injection of methylated bovine serum albumin (mBSA) in preimmunized PAR-1-deficient (PAR-1(-/-)) and wild-type (WT) mice. The disease was assessed after 7 days by histologic examination of knee joints after decalcification and Safranin O/toluidine blue staining. Serum levels of anti-mBSA IgG, interferon-gamma, and interleukin-4 (IL-4) were determined by enzyme-linked immunosorbent assay. T cell proliferation response was determined by measuring the incorporation of (3)H-thymidine. Cytokine messenger RNA (mRNA) expression was detected in synovial tissues and peritoneal cells by real-time polymerase chain reaction. Arthritis severity was significantly reduced in PAR-1(-/-) mice compared with WT mice (P = 0.017). Analysis of individual aspects of joint histology revealed significant reductions in synovial exudates (P < 0.001), cartilage degradation (P < 0.01), and bone damage (P = 0.05) in PAR-1(-/-) mice. Synovial IL-1, IL-6, and matrix metalloproteinase 13 (MMP-13) mRNA was significantly reduced in PAR-1(-/-) mice. The titers of antigen-specific serum anti-mBSA total IgG, IgG1, and IgG2a were significantly reduced, and serum IL-4 was significantly increased in arthritic PAR-1(-/-) mice. In contrast, no difference was observed in antigen-induced T cell proliferation between PAR-1(-/-) and WT mice. In vitro, thrombin-induced (but not lipopolysaccharide-induced) IL-1, IL-6, and MMP-13 mRNA expression was significantly impaired in PAR-1(-/-) mice compared with WT controls. These data demonstrate the requirement of PAR-1 for the expression of AIA, the development of an antigen-specific Ig response, thrombin-induced macrophage cytokine and MMP expression, and the inhibitory effect of PAR-1 on serum IL-4. We conclude that PAR-1 plays a significant role in this model of arthritis.

Cytokine expression was assessed during antigen-induced arthritis in the synovial membrane, inguinal lymph node, and spleen using competitive RT-PCR and sandwich ELISA

In the synovial membrane, early mRNA elevations of IL-1β and IL-6 (6 hours; 450-fold and 200-fold, respectively) positively correlated with the joint swelling. A sixfold tumour necrosis factor alpha increase was not significant. Not only IL-2 and interferon gamma (IFN-γ) (104-fold and 200-fold, respectively), but also IL-5 and IL-10 increased acutely (6 hours–day 1; threefold and 35-fold, respectively). In general, the protein levels for IL-1β, IL-6, tumour necrosis factor alpha, IFN-γ, IL-4, and IL-10 (increase between fourfold and 15-fold) matched the course of mRNA expression. In the inguinal lymph node there were early mRNA elevations of IL-6 (6 hours; 2.5-fold; positively correlated with the joint swelling) and IL-2 (6 hours; fourfold), as well as later rises of IL-4 and IL-5 (day 3; 2.5-fold and fourfold, respectively). At the protein level no significant elevations were observed in comparison with day 0, except for IL-1β (day 6) and IL-10 (day 1). In the spleen, there were significant mRNA elevations at 6 hours of IL-1β (1.5-fold), IL-6 (fourfold; positively correlated with the joint swelling), IFN-γ (threefold), and IL-2 (sevenfold to 10-fold). IL-5 and IL-10 (twofold and threefold, respectively) peaked from 6 hours to day 3. Increases at the protein level were significant compared with day 0 only in the case of IL-2 (day 6). By day 6 (transition to the chronic phase) the mRNA for cytokines declined to or below pre-arthritis levels in all organs, except for IL-1β in the synovial membrane and IL-6 in the spleen. antigen-induced arthritis is thus characterized by: early synovial activation of macrophages, Th1-like, and Th2-like cells; late, well-segregated Th2-like responses in the inguinal lymph node; late, overlapping Th1-like/Th2-like peaks in the spleen; and chronic elevation of synovial IL-1β mRNA and spleen IL-6 mRNA.

Treatment with a neutralizing anti-murine interleukin-17 antibody after the onset of collagen-induced arthritis reduces joint inflammation, cartilage destruction, and bone erosion.

Interleukin-17 (IL-17) is a proinflammatory cytokine that is expressed in the synovium of rheumatoid arthritis (RA) patients. This T cell cytokine is implicated in the initiation phase of arthritis. However, the role of IL-17 during the effector phase of arthritis has still not been identified; this was the objective of the present study. Mice with collagen-induced arthritis (CIA) were treated with polyclonal rabbit anti-murine IL-17 (anti-IL-17) antibody-positive serum or normal rabbit serum after the first signs of arthritis. In addition, during a later stage of CIA mice were selected and treated with anti-IL-17 antibody or control serum. Arthritis was monitored visually, and joint pathology was examined radiologically and histologically. Systemic IL-6 levels were measured by enzyme-linked immunosorbent assay, and local synovial IL-1 and receptor activator of NF-kappaB ligand (RANKL) expression was analyzed using specific immunohistochemistry. Treatment with a neutralizing anti-IL-17 antibody after the onset of CIA significantly reduced the severity of CIA. Radiographic analysis revealed marked suppression of joint damage in the knee and ankle joints. Histologic analysis confirmed the suppression of joint inflammation and showed prevention of cartilage and bone destruction after anti-IL-17 antibody therapy. Systemic IL-6 levels were significantly reduced after anti-IL-17 antibody treatment. Moreover, fewer IL-1beta-positive and RANKL-positive cells were detected in the synovium after treatment with neutralizing IL-17. Interestingly, initiation of anti-IL-17 antibody therapy during a later stage of CIA, using mice with higher clinical arthritis scores, still significantly slowed the progression of the disease. IL-17 plays a role in early stages of arthritis, but also later during disease progression. Systemic IL-6 was reduced and fewer synovial IL-1-positive and RANKL-positive cells were detected after neutralizing endogenous IL-17 treatment, suggesting both IL-1-dependent and IL-1-independent mechanisms of action. Our data strongly indicate that IL-17 neutralization could provide an additional therapeutic strategy for RA, particularly in situations in which elevated IL-17 may attenuate the response to anti-tumor necrosis factor/anti-IL-1 therapy.

Requirement of local synovial IL-17 receptor signaling in the progression of chronic synovitis and bone erosion in arthritis

IL-17 is a proinflammatory cytokine that is suspected to be involved in development of rheumatoid arthritis (RA). However, the role of IL-17–IL-17 receptor signaling in the effector phase of arthritis, including requirement of this signaling pathway in synoviocytes, is still not fully elucidated. Here we demonstrate, using IL-17 receptor deficient (IL-17R-/-) mice, a requirement for IL-17R signaling in immune reactivity and progression of synovitis and bone erosion in a T cell mediated mBSA arthritis model and chronic streptococcal cell wall arthritis. Of great interest, chimeric mice of host wild-type (wt) and donor IL-17R-/- bone marrow (BM) cells developed chronic synovitis similar to that in wt/wt chimeras. In contrast, chimeric mice of host IL-17R-/- and donor wt BM cells were protected from progressive synovitis similar to IL-17R-/-/ IL-17R-/- chimeras, suggesting a crucial role of resident synovial cells. Weakened mobilization of neutrophils into the joint and downregulation of synovial mRNA expression for leukocyte specific chemokines and selectins, and MMP-1 was found in IL-17R-/- mice. These data suggest that, in addition to the role played by IL-17–IL-17R signaling in development of immunity, synovial IL-17–IL-17R signaling plays a critical role in the effector phase of arthritis. Activation of IL-17R on fibroblast-like synoviocytes appears pivotal.

A novel therapeutic approach targeting articular inflammation using the filarial nematode-derived phosphorylcholine-containing glycoprotein ES-62.

Understanding modulation of the host immune system by pathogens offers rich therapeutic potential. Parasitic filarial nematodes are often tolerated in human hosts for decades with little evidence of pathology and this appears to reflect parasite-induced suppression of host proinflammatory immune responses. Consistent with this, we have previously described a filarial nematode-derived, secreted phosphorylcholine-containing glycoprotein, ES-62, with immunomodulatory activities that are broadly anti-inflammatory in nature. We sought to evaluate the therapeutic potential of ES-62 in vitro and in vivo in an autoimmune disease model, namely, collagen-induced arthritis in DBA/1 mice. ES-62 given during collagen priming significantly reduced initiation of inflammatory arthritis. Crucially, ES-62 was also found to suppress collagen-induced arthritis severity and progression when administration was delayed until after clinically evident disease onset. Ex vivo analyses revealed that in both cases, the effects were associated with inhibition of collagen-specific pro-inflammatory/Th1 cytokine (TNF-alpha, IL-6, and IFN-gamma) release. In parallel in vitro human tissue studies, ES-62 was found to significantly suppress macrophage activation via cognate interaction with activated T cells. Finally, ES-62 suppressed LPS-induced rheumatoid arthritis synovial TNF-alpha and IL-6 production. Evolutionary pressure has promoted the generation by pathogens of diverse mechanisms enabling host immune system evasion and induction of "tolerance." ES-62 represents one such mechanism. We now provide proof of concept that parasite-derived immunomodulatory strategies offer a novel therapeutic opportunity in inflammatory arthritis.

Radiolabelled interleukin-1 receptor antagonist for detection of synovitis in patients with rheumatoid arthritis.

To investigate the distribution of radiolabelled interleukin-1 receptor antagonist (IL-1ra) in patients with rheumatoid arthritis (RA) and to assess whether this cytokine is suitable for scintigraphic visualization of synovitis. In patients with active RA, scintigraphy was performed after a single i. v. dose of [(123)I]IL-1ra. Clearance and organ distribution of radiolabelled IL-1ra were studied. To assess whether radiolabelled IL-1ra targets the synovial IL-1 receptors, the scintigraphic images obtained with IL-1ra were compared with those obtained by the use of a non-specific control agent. In addition, autoradiography was performed in mice with antigen-induced arthritis that were injected with either radiolabelled IL-1ra or a size-matched, non-receptor-binding protein. Radiolabelled IL-1ra allowed clear visualization of inflamed joints. Specificity in the detection of synovitis was high, whereas a number of painful and swollen joints were not visualized by scintigraphy. The procedure was well tolerated and [(123)I]IL-1ra was rapidly cleared from the circulation (t(1/2)alpha 11 min, t(1/2)beta 612 min) and excreted mainly in the urine. The definition of synovial contours by IL-1ra scintigraphy was not better than that observed with a non-specific agent. Although radiolabelled IL-1ra retained its affinity for IL-1 receptors, no binding to synovium was observed by autoradiography. Radiolabelled IL-1ra allows the visualization of synovitis in patients with RA. However, neither the imaging nor the autoradiographic studies indicate that joint accumulation of radiolabelled IL-1ra is due to specific IL-1 receptor targeting. IL-1ra has proved its therapeutic value in RA, but with the dose schedule in this study it does not behave as a specific radiopharmaceutical that is suitable for scintigraphic detection of inflammation.

Effect of tumor necrosis factor antibody on synovial fluid cytokine activities in equine antebrachiocarpal joints injected with endotoxin

SUMMARY Six horses received intra-articular injections of a mixture of 1 μg of endotoxin/5 mg of equine tumor necrosis factor (eqTNF) monoclonal antibody in 1 antebrachiocarpal joint and an equal volume (2 ml) of 1 μg of endotoxin/5 mg of control antibody in the opposite joint. Synovial fluid sample collection (1 ml) was accomplished by use of an indwelling, intra-articular catheter at postinjection hours (pih) 0, 1, 1.5, 2, 5, and 8, and by arthrocentesis at pih 24. Joint fluid samples were analyzed for nucleated cell count, protein concentration, and tnf, interleukin 6 (il-6), il-1, and il-1-inhibitory activities. To monitor local inflammation, each carpus was graded semiquantitatively for swelling prior to each sample collection. Tumor necrosis factor, il-1, or il-1-inhibitory activity was not detected in any synovial fluid sample collected before endotoxin/antibody was administered. However, low il-6 activity (&lt; 100 U/ml) was found in 2 of 12 preinjection samples. In joints injected with endotoxin/control antibody mixture, maximal mean ± sem activities for tnf (1,019 ± 310 U/ml), il-1 (173 ± 102 U/ml), and il-6 (10.8 ± 3.1 × 104 U/ml) were observed at pih 2, 5, and 8, respectively. Tumor necrosis factor and il-1 activities returned to baseline values by pih 8 and 24, respectively; however, il-6 activity remained high. Interleukin 1-inhibitory activity (27.4 ± 2.25 IU/ml) was detected in all pih-24 samples from control joints, but was not detected at any other time in control joints (limit of detection, 20 IU/ml). Tumor necrosis factor activity was not detected in any synovial fluid sample from joints treated with endotoxin/eqTNF antibody. In contrast, endotoxin-induced mean synovial il-1 and il-6 activities were not reduced significantly by eqTNF antibody. Mean il-1-inhibitory activity (pih 24) was higher in eqTNF antibody-treated joints (41.0 ± 7.7 IU/ml) than in control joints, but the difference was not significant. Mean wbc count and protein concentration in control and treated joints were maximal at pih 8. The curves for mean values of wbc count and total protein concentration were not significantly different in treated versus control joints. Swelling in each treated joint was either less than or the same as that in the opposite control joint at every time in the initial 8 pih. There was significant (P = 0.043) difference between treated and control joints at pih 5 and 8. These results describe a profile of synovial fluid tnf, il-1, il-6 bioactivities, and il-1-inhibitory activity during the initial 24 hours of synovitis induced by intra-articular administration of endotoxin in horses. Our eqTNF monoclonal antibody was effective in neutralizing tnf activity in synovial fluid when administered intra-articularly with endotoxin in horses. The induction of il-1, il-1-inhibitory activity, il-6, wbc, and total protein concentration responses are largely independent of tnf activity in synovial fluid of horses receiving endotoxin intra-articularly.

Inducible synthesis of collagenase and other neutral metalloproteinases by cells of aortic origin

In view of the possible link between collagenase and the formation of aortic aneurysms we have determined whether cells within the aorta are able to synthesize this enzyme. Explanted cells obtained from fragments of lapine abdominal aorta secreted little or no collagenase. Two related metalloproteinases, gelatinase and stromelysin, were also produced at very low levels. Treatment with purified human monocyte interleukin-1β, partially purified lapine, synovial IL-1 or phorbol myristate acetate strongly induced the synthesis of all these enzymes. These activators also increased synthesis of prostaglandin E 2. The identity of collagenase was confirmed by detection of the characteristic TC A and TC B breakdown fragments of collagen and by demonstration of collagenase mRNA within activated aortic cells. Unactivated aortic cells contained no detectable collagenase mRNA, suggesting a pretranslational level of regulation. Aortic cells thus possess the ability to express several neutral metalloproteinases and, if a sufficient inflammatory stimulus was present, they might do so in arteries undergoing aneurysmal degeneration.

Synovial IL-1 in early, localized arthritis—A role for surgical synovectomy?: Thomas D. Rosenberg, Daniel C. Wnorowski, and Vernon Cooley. Salt Lake city, Utah, Syracuse, New York, and Boston, Massachusetts, U.S.A.

Synovial IL-1 in early, localized arthritis—A role for surgical synovectomy?: Thomas D. Rosenberg, Daniel C. Wnorowski, and Vernon Cooley. Salt Lake city, Utah, Syracuse, New York, and Boston, Massachusetts, U.S.A.

Evidence that responses of articular chondrocytes to interleukin-1 and basic fibroblast growth factor are not mediated by protein kinase C.

After exposure to human recombinant interleukin-1 (hrIL-1), chondrocytes increase their synthesis of prostaglandin E2 (PGE2) and neutral metalloproteinases (NMPs). This response, known as chondrocyte activation, is also elicited by partially purified preparations of rabbit synovial IL-1, known as 'chondrocyte activating factors' (CAF). CAF activates chondrocytes more strongly than does hrIL-1, probably because it contains additional growth factors. Basic fibroblast growth factor (bFGF) is one such factor, although CAF also contains others which modulate the activation of chondrocytes. Chondrocyte activation by hrIL-1 is strongly potentiated by bFGF and phorbol myristate acetate (PMA). A series of experiments was conducted to examine the possible role of protein kinase C (PKC) in mediating these effects. Inhibitors of PKC partially blocked the induction of NMPs by CAF and completely suppressed the potentiating effect of PMA. However, induction by 10 units of hrIL-1/ml and potentiation by bFGF were not affected by these inhibitors. Furthermore, cells whose PKC had been down-regulated by prolonged exposure to PMA remained responsive to IL-1. Surprisingly, inhibitors of PKC greatly increased the amounts of NMPs produced in response to a low dose (1 unit/ml) of hrIL-1. These inhibitors also enhanced the synthesis of PGE2 by cells responding to 1 and 10 units of hrIL-1/ml. Phosphorylation of the 80 kDa substrate for PKC was augmented by PMA and CAF, but not by hrIL-1 or bFGF. Moreover, Western-blotting techniques, which confirmed translocation of PKC in response to PMA and CAF, did not detect translocation in cells treated with hrIL-1 or bFGF. Western blotting also demonstrated the presence of PKC isoenzyme type III (alpha), but not types I (gamma) or II (beta). These data argue that PKC does not mediate the effects of hrIL-1 or bFGF in chondrocytes. However, CAF contains additional substances which activate this enzyme and whose effects may in part be mediated by PKC.