β1-integrin mediates asbestos-induced phosphorylation of AKT and ERK1/2 in a rat pleural mesothelial cell line

Integrin-mediated signalling has been implicated in asbestos-induced carcinogenesis. In studies here, we examined signal transduction events associated with integrin-directed cell reactions triggered by crocidolite asbestos in the pleural mesothelial cell line 4/4 RM-4. Crocidolite fibres induced a significant time- and dose-dependent activation of the extracellular-signal-regulated kinases ERK1 and ERK2. ERK activation was specifically inhibited by integrin-blocking agents, that are integrin-binding peptides containing the sequence arginine-glycine-aspartic acid (RGD), and monoclonal antibodies against the integrin B1-chain. Integrin-dependent activation of ERK1/2 in response to asbestos appeared to be independent of focal adhesion kinase pp125(FAK) (FAK) since FAK autophosphorylation remained unaffected in crocidolite-exposed mesothelial cells. Instead, we observed striking similarities in the kinetics of asbestos-induced ERK1/2 responses and phosphorylation of protein kinase B (AKT) at serine 473, a possible target residue for integrin-linked kinase. As with ERK activation, asbestos-induced AKT stimulation was significantly blocked by both the RGD-peptide and the B1-integrin antibodies. These studies are the first to establish that in mesothelial cells ERK1/2 and AKT are simultaneously phosphorylated upon asbestos exposure in a B1-integrin-dependent manner.