Effects of interleukin-1 on calcium signaling and the increase of filamentous actin in isolated and in situ articular chondrocytes

Objective. To determine whether interleukin-1 (IL-1) initiates transient changes in the intracellular concentration of [Ca²⁺]_i and the organization of filamentous actin (F-actin) in articular chondrocytes. Methods. Articular chondrocytes within cartilage explants and enzymatically isolated chondrocytes were loaded with Ca²⁺-sensitive fluorescence indicators, and [Ca²⁺]_i was measured using confocal fluorescence ratio imaging during exposure to 10 ng/ml IL-1α. Inhibitors of Ca²⁺ mobilization (Ca²⁺-free medium, thapsigargin [inhibitor of Ca-ATPases], U73122 [inhibitor of phospholipase C], and pertussis toxin [inhibitor of G proteins]) were used to determine the mechanisms of increased [Ca²⁺]_i. Cellular F-actin was quantified using fluorescently labeled phalloidin. Toxin B was used to determine the role of the Rho family of small GTPases in F-actin reorganization. Results. In isolated cells on glass and in in situ chondrocytes within explants, exposure to IL-1 induced a transient peak in [Ca²⁺]_i that was generally followed by a series of decaying oscillations. Thapsigargin, U73122, and pertussis toxin inhibited the percentage of cells responding to IL-1. IL-1 increased F-actin content in chondrocytes in a manner that was inhibited by toxin B. Conclusion. Both isolated and in situ chondrocytes respond to IL-1 with transient increases in [Ca²⁺]_i via intracellular Ca ²⁺ release mediated by the phospholipase C and inositol trisphosphate pathways. The influx of Ca²⁺ from the extracellular space and the activation of G protein-coupled receptors also appear to contribute to these mechanisms. These findings suggest that Ca²⁺ mobilization may be one of the first signaling events in the response of chondrocytes to IL-1.