Hydrogen peroxide induces chondrocyte apoptosis via Ca2+ signaling, and ERK is involved in these signal transduction pathways.

Objective. We have previously demonstrated that reactive oxygen species (ROS) are involved in cartilage degradation. Decreased size of hyaluronan (HA), the major macromolecule in synovial fluid, to which it imparts viscosity, is reported in patients with arthritis. The purpose of this study was to determine the alteration in the molecular weight range of HA as a result of mechanical deformation loaded on the chondrocytes, as well as the involvement of ROS in this action.Methods. ROS were generated via the oxidation of hypoxanthine by xanthine oxidase. Cyclic tensile stretch was loaded using a vacuum-operated instrument. Levels of HA were measured using a sandwich enzyme-binding assay. Superoxide dismutase (SOD) activity and ROS were measured using water-soluble tetrazolium and a chemiluminescent probe, respectively.Results. ROS depolymerized HA molecules. Cyclic tensile stretch depolymerized HA and induced ROS. SOD inhibited not only ROS induction but also HA depolymerization caused by the mechanical stress.Conclusion. ROS play an important role in mechanical stress-induced HA depolymerization.

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It is known that the excessive generation of reactive oxygen species (ROS) is a significant factor in tissue injury observed in many disease states. To determine whether extreme levels of mechanical stress applied to osteoblasts enhances ROS synthesis, we loaded cyclic tensile stretch on osteoblast-like HT-3 cells. Cyclic tensile stretch loaded on these cells clearly enhanced ROS synthesis in a time- and magnitude-dependent fashion. Cyclic tensile stretch also enhanced superoxide dismutase (SOD) activity. The disruption of microfilaments with cytochalasin D abolished the stress-induced ROS synthesis. Rotenone, an inhibitor of the mitochondrial electron transport chain, enhanced stress-induced ROS synthesis. These data suggest that actin filament and mitochondria are involved in this action.

Recent reports indicate the alteration of nitric oxide (NO) synthesis with mechanical stress loaded on the osteoblast and NO is considered to have a significant role in mechanotransduction. We found the involvement of guanine-nucleotide-binding regulatory proteins (G proteins), especially Gi, in stress-inhibited NO release of osteoblast-like cells (JOR: 17393-597, 1999). To determine further the mechanism involved in this process, we measured c-Jun N-terminal kinasehtress-activated protein kinase (JNWSAPK) activity under cyclic tensile stretch loaded on osteoblast-like cells. Cyclic stretch significantly enhanced JNWSAPK activity and pertussis toxin clearly reversed stress-enhanced JNWSAPK activity. Cytochalasin D, actin microfilament disrupting reagent, also abolished the stress activation of JNWSAPK. We propose a model for signaling events induced by cyclic tensile stretch, namely a transmembrane mechanosensor which couples Gi-protein, actin cytoskeleton and finally activates JNWSAPK activity of osteoblasts.