Interleukin-1 inhibits osmotically induced calcium signaling and volume regulation in articular chondrocytes

Objective: Articular chondrocytes respond to osmotic stress with transient changes in cell volume and the intracellular concentration of calcium ion ([Ca²⁺]_i). The goal of this study was to examine the hypothesis that interleukin-1 (IL-1), a pro-inflammatory cytokine associated with osteoarthritis, influences osmotically induced Ca²⁺ signaling. Methods: Fluorescence ratio imaging was used to measure [Ca²⁺]_i and cell volume in response to hypo- or hyper-osmotic stress in isolated porcine chondrocytes, with or without pre-exposure to 10-ng/ml IL-1α. Inhibitors of IL-1 (IL-1 receptor antagonist, IL-1Ra), Ca²⁺ mobilization (thapsigargin, an inhibitor of Ca-ATPases), and cytoskeletal remodeling (toxin B, an inhibitor of the Rho family of small GTPases) were used to determine the mechanisms involved in increased [Ca²⁺]_i, F-actin remodeling, volume adaptation and active volume recovery. Results: In response to osmotic stress, chondrocytes exhibited transient increases in [Ca²⁺]_i, generally followed by decaying oscillations. Pre-exposure to IL-1 significantly inhibited regulatory volume decrease (RVD) following hypo-osmotic swelling and reduced the change in cell volume and the time to peak [Ca²⁺]_i in response to hyper-osmotic stress, but did not affect the peak magnitudes of [Ca²⁺]_i in those cells that did respond. Co-treatment with IL-1Ra, thapsigargin, or toxin B restored these responses to control levels. The effects were associated with alterations in F-actin organization. Conclusions: IL-1 alters the normal volumetric and Ca²⁺ signaling response of chondrocytes to osmotic stress through mechanisms involving F-actin remodeling via small Rho GTPases. These findings provide further insights into the mechanisms by which IL-1 may interfere with normal physiologic processes in the chondrocyte, such as the adaptation or regulatory responses to mechanical or osmotic loading.