u-0126 and Osteoarthritis--Knee

Pain during inflammatory joint diseases is enhanced by the generation of hypersensitivity in nociceptive neurons in the peripheral nervous system. To explore the signaling mechanisms of mechanical hypersensitivity during joint inflammation, experimental arthritis was induced by injection of complete Freund's adjuvant (CFA) into the synovial cavity of rat knee joints. As a pain index, the struggle threshold of the knee extension angle was measured. In rats with arthritis, the phosphorylation of extracellular signal-regulated kinase (ERK), induced by passive joint movement, increased significantly in dorsal root ganglion (DRG) neurons innervating the knee joint compared to the naïve rats that received the same movement. The intrathecal injection of a MEK inhibitor, U0126, reduced the phosphorylation of ERK in DRG neurons and alleviated the struggle behavior elicited by the passive movement of the joint. In addition, the injection of U0126 into the joint also reduced the struggle behavior. These findings indicate that the ERK signaling is activated in both cell bodies in DRG neurons and peripheral nerve fibers and may be involved in the mechanical sensitivity of the inflamed joint. Furthermore, the phosphorylated ERK-positive neurons co-expressed the P2X3 receptor, and the injection of TNP-ATP, which antagonizes P2X receptors, into the inflamed joint reduced the phosphorylated ERK and the struggle behavior. Thus, it is suggested that the activation of the P2X3 receptor is involved in the phosphorylation of ERK in DRG neurons and the mechanical hypersensitivity of the inflamed knee joint.

Topics: Adenosine Triphosphate; Animals; Arthritis, Experimental; Axonal Transport; Butadienes; Disease Models, Animal; Extracellular Signal-Regulated MAP Kinases; Freund's Adjuvant; Ganglia, Spinal; Hyperalgesia; Injections, Intra-Articular; Injections, Spinal; Male; Neurons, Afferent; Nitriles; Osteoarthritis, Knee; Pain; Phosphorylation; Protein Processing, Post-Translational; Purinergic P2 Receptor Antagonists; Range of Motion, Articular; Rats; Rats, Sprague-Dawley; Receptors, Purinergic P2; Receptors, Purinergic P2X3; Signal Transduction; Stifle; Stress, Mechanical

To understand the molecular mechanisms that lead to increased MMP-13 expression and cartilage degeneration during the progression of osteoarthritis (OA), we have investigated the expression of the transcription factor RUNX2 in OA cartilage and the regulation of MMP-13 expression by RUNX2 and FGF2 in articular chondrocytes.. RUNX2 and MMP-13 expression in human OA and control cartilage was analyzed by immunohistochemistry. The effects of RUNX2 over-expression, with or without FGF2 treatment, on MMP-13 promoter activity and enzyme accumulation were measured in articular chondrocytes. Inhibitors of MEK/ERK were assayed for their ability to block FGF2 and RUNX2 up-regulation of the MMP-13 promoter. We analyzed RUNX2 phosphorylation in response to FGF2.. Fibrillated OA cartilage exhibited increased RUNX2 immunoreactivity when compared to control cartilage. RUNX2 co-localized with MMP-13 in clusters of chondrocytes in fibrillated OA cartilage. RUNX2 over-expression in cultured chondrocytes increased their responsiveness to FGF2 treatment, which led to increased MMP-13 expression. Inhibitors of MEK/ERK signaling blocked up-regulation of the MMP-13 promoter by RUNX2 and FGF2, and also blocked the activation of RUNX2 by FGF2. FGF2 treatment of articular chondrocytes increased RUNX2 phosphorylation approximately 2-fold.. Increased expression of RUNX2 in OA cartilage may contribute to increased expression of MMP-13. FGF2, which is present in OA synovial fluid, activated RUNX2 via the MEK/ERK pathway and increased MMP-13 expression. However, it is unlikely that RUNX2 is a substrate of ERK1/2. RUNX2 expression and activation may be a significant step in the progression of OA by promoting changes in gene expression and chondrocyte differentiation.

Topics: Animals; Blotting, Western; Butadienes; Cartilage, Articular; Case-Control Studies; Cattle; Cells, Cultured; Chondrocytes; Collagenases; Core Binding Factor Alpha 1 Subunit; Fibroblast Growth Factor 2; Flavonoids; Humans; Immunoprecipitation; Matrix Metalloproteinase 13; Mitogen-Activated Protein Kinase Kinases; Naphthalenes; Neoplasm Proteins; Nitriles; Osteoarthritis, Knee; Phosphorylation; Protein Kinase C; Signal Transduction; Transcription Factors