Elevated levels of PGE2 have been reported in synovial fluid and cartilage from patients with osteoarthritis (OA). However, the functions of PGE2 in cartilage metabolism have not previously been studied in detail. To do so, we cultured cartilage explants, obtained from patients undergoing knee replacement surgery for advanced OA, with PGE2 (0.1–10 μM). PGE2 inhibited proteoglycan synthesis in a dose-dependent manner (maximum 25% inhibition (p < 0.01)). PGE2 also induced collagen degradation, in a manner inhibitable by the matrix metalloproteinase (MMP) inhibitor ilomastat. PGE2 inhibited spontaneous MMP-1, but augmented MMP-13 secretion by OA cartilage explant cultures. PCR analysis of OA chondrocytes treated with PGE2 with or without IL-1 revealed that IL-1-induced MMP-13 expression was augmented by PGE2 and significantly inhibited by the cycolooygenase 2 selective inhibitor celecoxib. Conversely, MMP-1 expression was inhibited by PGE2, while celecoxib enhanced both spontaneous and IL-1-induced expression. IL-1 induction of aggrecanase 5 (ADAMTS-5), but not ADAMTS-4, was also enhanced by PGE2 (10 μM) and reversed by celecoxib (2 μM). Quantitative PCR screening of nondiseased and end-stage human knee OA articular cartilage specimens revealed that the PGE2 receptor EP4 was up-regulated in OA cartilage. Moreover, blocking the EP4 receptor (EP4 antagonist, AH23848) mimicked celecoxib by inhibiting MMP-13, ADAMST-5 expression, and proteoglycan degradation. These results suggest that PGE2 inhibits proteoglycan synthesis and stimulates matrix degradation in OA chondrocytes via the EP4 receptor. Targeting EP4, rather than cyclooxygenase 2, could represent a future strategy for OA disease modification.

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The production of nitric oxide (NO) and prostaglandin E2 (PGE(2)) is increased in human osteoarthritis-affected cartilage. These and other inflammatory mediators are spontaneously released by OA cartilage explants ex vivo. The excessive production of nitric oxide inhibits matrix synthesis, and promotes its degradation. Furthermore, by reacting with oxidants such as superoxide anion, nitric oxide promotes cellular injury, and renders the chondrocyte susceptible to cytokine-induced apoptosis. PGE(2) exerts both anabolic and catabolic effects on chondrocytes, depending on the microenvironment and physiological condition. Thus, NO and PGE(2), produced by activated chondrocytes in diseased cartilage, may modulate disease rogression in osteoarthritis, and should therefore be considered potential targets for therapeutic intervention

Interleukin 1 (IL-1), produced by both synovial cells and chondrocytes, plays a pivotal role in the pathogenesis of cartilage destruction in osteoarthritis (OA). We examined the specific expression and function of IL-1 receptor family-related genes in human joint tissues. Gene array analysis of human normal and OA-affected cartilage showed mRNA expression of IL-1 receptor accessory protein (IL-1RAcp) and IL-1 type I receptor (IL-1RI), but not IL-1 antagonist (IL-1ra) and IL-1 type II decoy receptor (IL-1RII). Similarly, human synovial and epithelial cells showed an absence of IL-1RII mRNA. Functional genomic analyses showed that soluble (s) IL-1RII, at picomolar concentrations, but not soluble TNF receptor:Fc, significantly inhibited IL-1␤-induced nitric oxide (NO) and/or prostaglandin E 2 production in chondrocytes, synovial and epithelial cells. In OA-affected cartilage, the IC 50 for inhibition of NO production by sIL-1RII was 2 log orders lower than that for sIL-1RI. Human chondrocytes that overexpressed IL-1RII were resistant to IL-1-induced IL-1␤ mRNA accumulation and inhibition of proteoglycan synthesis. In osteoarthritis, deficient expression by chondrocytes of innate regulators or antagonists of IL-1 such as IL-1ra and IL-1RII (soluble or membrane form) may allow the catabolic effects of IL-1 to proceed unopposed. The sensitivity of IL-1 action to inhibition by sIL-1RII has therapeutic implications that could be directed toward correcting this unfavorable tissue(s) dependent imbalance. Osteoarthritis (OA)1 is considered a non-inflammatory arthritis, characterized by a limited infiltration of neutrophils into the synovial space and, in general, an absence of the classical signs of inflammation. Chondrocytes embedded within articular cartilage that is avascular and aneural reside in a sequestered environment, perhaps more than other cell types, whereby cellular metabolism is regulated by an autocrine mechanism responsive to biomechanical and pericellular signals. Recent observations by this and other laboratories indicate that, despite the general absence of clinical signs of inflammation, chondrocytes derived from patients with OA, show superinduction of proinflammatory genes typically associated with the products of synovial tissues in rheumatoid arthritis, including nitric-oxide synthase, cyclooxygenase-2, TNF␣, IL-6, and IL-8. The spontaneous production of the corresponding gene products and inflammatory mediators promotes a catabolic state, which leads to progressive cartilage damage in OA (1-4). This intraarticular inflammatory response in OA-affected cartilage, which may be considered as an in situ "molecular inflammation," is partially dependent on autocrine IL-1␤ production, which induces and sustains an imbalance of cartilage homeostasis and extracellular matrix synthesis (5). The autocrine production of IL-1 in OA-affected cartilage is amplified by engagement of integrins such as ␣ 5 ␤ 1 by abnormally expressed extracellular matrix proteins, including proteolytic fragments of fibronectin (5, 6)...

Objective. To determine the effects of the antioxidant resveratrol on the functions of human chondrocytes in osteoarthritis (OA).Methods. Chondrocytes and cartilage explants were isolated from OA patients undergoing knee replacement surgery. Effects of resveratrol in the presence or absence of interleukin-1␤ (IL-1␤) stimulation were assessed by measurement of prostaglandin E 2 (PGE 2 ) and leukotriene B 4 (LTB 4 ) synthesis, cyclooxygenase (COX) activity, matrix metalloproteinase (MMP) expression, and proteoglycan production. To explore the mechanisms of action of resveratrol, its effects on mitochondrial function and apoptosis were examined by assessing mitochondrial membrane potential, ATP levels, cytochrome c release, and annexin V staining.Results. Resveratrol inhibited both spontaneous and IL-1␤-induced PGE 2 production by >20% (P < 0.05) and by 80% (P < 0.001), respectively; similarly, LTB 4 production was reduced by >50% (P < 0.05). The production of PGE 2 was inhibited via a 70-90% suppression of COX-2 expression and enzyme activity (P < 0.05). Resveratrol also promoted anabolic effects in OA explant cultures, by elevating proteoglycan synthesis and decreasing production of MMPs 1, 3, and 13. Pretreatment of OA chondrocytes with resveratrol blocked mitochondrial membrane depolarization, loss of mitochondrial biomass, and IL-1␤-induced ATP depletion. Similarly, IL-1␤-mediated induction of the apoptotic markers cytochrome c and annexin V was also inhibited by resveratrol. Exogenous addition of PGE 2 abolished the protective effects of resveratrol on mitochondrial membrane integrity, ATP levels, expression of apoptotic markers, and DNA fragmentation.Conclusion. Resveratrol protects against IL-1␤-induced catabolic effects and prevents chondrocyte apoptosis via its inhibition of mitochondrial membrane depolarization and ATP depletion. These beneficial effects of resveratrol are due, in part, to its capacity to inhibit COX-2-derived PGE 2 synthesis. Resveratrol may therefore protect against oxidant injury and apoptosis, which are main features of progressive OA.