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krox‐20/egr‐2 is up‐regulated following non‐specific and homophilic adhesion in rat macrophages

Macrophages are known to adhere to a plastic dish via β2 integrin (CR3) and scavenger receptors. Although their functions such as phagocytosis, endocytosis, and nitric oxide production have been investigated on adherent macrophages in vitro, very little is known about intracellular signals triggered by adhesion to a plastic dish. Recently we reported that the mRNA level of krox‐20/egr‐2 was significantly increased in rat alveolar macrophages following exposure to fibrous titanium dioxide particles. In the present study we report that up‐regulation of krox‐20/egr‐2 gene expression following adhesion to a plastic dish and homophilic adhesion in rat alveolar macrophages and rat macrophage cell line, NR8383. The mRNA level of krox‐20/egr‐2 increased with a peak 1 hr after adhesion to a plastic dish in both cell types. Piceatannol inhibited tyrosine‐phosphorylation of Syk and decreased both adhesion and krox‐20/egr‐2 mRNA level. In contrast staurosporine, a serine/threonine kinase inhibitor, increased adherence of macrophages and yet prohibited the adhesion‐dependent increase in krox‐20/egr‐2 gene expression. When NR8383 cells are cultured in suspension, the cells aggregated naturally and produced cell clumps. The mRNA level of krox‐20/egr‐2 also increased in response to the homophilic intercellular adhesion. The increased mRNA level of krox‐20/egr‐2 was not caused by inflammatory stimuli, because lipopolysaccharide did not affect the aggregation‐dependent up‐regulation of krox‐20/egr‐2 gene. The up‐regulation of krox‐20/egr‐2 gene due to the homophilic cell aggregation was also inhibited either by piceatannol or staurosporine. Those results suggest that krox‐20/egr‐2 gene expression is triggered by sensing non‐specific and homophilic cellular adhesion and the following phosphorylation of signal transducing proteins including Syk and staurosporine‐inhibitable kinases.