Interleukin 1-induced calcium signalling in chondrocytes requires focal adhesions

The cytokine interleukin 1 (IL-1) is an important mediator of connective-tissue destruction in arthritic joints but the mechanisms by which IL-1 mediates signal transduction in chondrocytes is poorly understood. Previous results have indicated that IL-1 receptors co-localize with focal adhesions [Qwarnstrom, Page, Gillis and Dower (1988) J. Biol. Chem. 263, 8261–8269], discrete adhesive domains of cells that function in cell attachment and possibly in signal transduction. We have determined whether focal adhesions restrict IL-1-induced Ca²⁺ signalling in primary cultures of bovine chondrocytes. In cells grown for 24 h on fibronectin, the basal intracellular Ca²⁺ ion concentration ([Ca²⁺]_i) was 100±3 nM. Optimal increases of [Ca²⁺]_i above baseline were induced by 10 nM IL-1 (183±30 nM above baseline). There was no significant difference between cells plated on fibronectin or type II collagen (P > 0.2; 233±90 nM above baseline). Ca²⁺ transients were significantly decreased by the inclusion of 0.5 mM EGTA in the bathing buffer (74±11 nM above baseline), and 1 μM thapsigargin completely blocked Ca²⁺ transients. Cells plated on poly-(l-lysine) or suspended cells showed no Ca²⁺ increases, whereas cells grown on fibronectin exhibited IL-1-induced Ca²⁺ responses that corresponded temporally to the time-dependent cell spreading after plating on fibronectin. Cells plated on poly-(l-lysine) and incubated with fibronectin-coated beads exhibited vinculin staining in association with the beads. In identical cell preparations, IL-1 induced a 136±39 nM increase of [Ca²⁺]_i above baseline in response to 10 nM IL-1β. There were no IL-1-induced Ca²⁺ increases when cells on poly-(l-lysine) were incubated with fibronectin-coated beads for only 15 min at 37 °C, in cells maintained for 3 h at 4 °C, in cells incubated with BSA beads for 3 h at 37 °C, or in cells pretreated with cytochalasin D. Labelling of IL-1 receptors with ¹²⁵I-IL-1β showed 3-fold more specific labelling of focal adhesion complexes in cells incubated with fibronectin-coated beads compared with cells incubated with BSA-coated beads, indicating that IL-1 receptor binding or the number of IL-1 receptors was increased in focal adhesions. These results indicate that, in chondrocytes, IL-1-induced Ca²⁺ signalling is dependent on focal adhesion formation and that focal adhesions recruit IL-1 receptors by redistribution in the cell membrane.