Requirements of Focal Adhesions and Calcium Fluxes for Interleukin-1-induced ERK Kinase Activation and c-fos Expression in Fibroblasts

Abstract

Interleukin-1 (IL-1) is an important inflammatory mediator and plays a central role in the destruction of connective tissue matrices in diseases such as arthritis and periodontitis. It is well established that IL-1 activation of the mitogen-activated protein (MAP) kinase pathway and induction of c-fos expression is a required step in the induction of matrix metalloproteinase expression involved in tissue degradation. Previous studies in our laboratory showed that IL-1-induced calcium flux is dependent on focal adhesion formation, suggesting a matrix-dependent restriction system for IL-1 signaling. Therefore, in the present study, we examined the consequences of this restriction on IL-1-mediated activation of the MAP kinase family and on c-fos expression. Treatment of human gingival fibroblasts with IL-1 activated extracellular signal-regulated kinases (ERK), c-Jun N-terminal kinase (JNK), and p38 kinase activity and induced c-fos expression in a dose- and time-dependent fashion. Plating cells on poly-L-lysine prevented focal adhesion formation, eliminated IL-1-induced calcium influx, abolished ERK stimulation, and blocked c-fos expression. Cells in suspension and hence with no suitable substratum for focal adhesion formation also showed no ERK activation or enhanced c-fos expression in response to IL-1. In contrast, eliminating focal adhesion formation or calcium depletion in cells plated on fibronectin had no effect on IL-1 stimulation of JNK and p38 kinases, demonstrating that their activation was mediated through pathways independent of focal adhesions and calcium. Calcium depletion abolished IL-1-induced calcium uptake, ERK activation, and c-fos expression. The focal adhesion dependence of IL-1-induced ERK activation and c-fos expression could be circumvented in cells plated on poly-L-lysine by simultaneous incubation with IL-1 and the calcium ionophore ionomycin. In transfection studies, IL-1 stimulation of serum responsive element (SRE) transcriptional activity was dependent on the presence of extracellular calcium. This is consistent with a requirement for calcium in the activation of ERKs and their involvement in the induction of c-fos expression through the SRE site on the 5' promoter of the c-fos gene. Our results demonstrate that in cells attached to substrates by focal adhesions, IL-1-mediated calcium flux is required for ERK activation and c-fos expression but not for JNK or p38 activation. We conclude that cellular interactions with the extracellular matrix play an important role in restricting ERK and c-fos-dependent processes.