Silica induces nuclear factor-kappa B activation through tyrosine phosphorylation of I kappa B-alpha in RAW264.7 macrophages.

It was previously reported that protein tyrosine kinase (PTK) but not protein kinase C or A plays an important role in silica-induced activation of NF-kappa B in macrophages. The question is raised whether PTK stimulation and NF-kappa B activation in silica-stimulated macrophages are directly connected through tyrosine phosphorylation of I kappa B-alpha. Results indicate that stimulation of macrophages with silica led to NF-kappaB activation through tyrosine phosphorylation without serine phosphorylation. Specific inhibitors of protein tyrosine kinase, such as genistein and tyrophostin AG126, prevented tyrosine phosphorylation of I kappa B-alpha in response to silica. I kappa B-alpha protein levels remained relatively unchanged for up to 60 min after silica stimulation. Moreover, inhibition of proteasome proteolytic activity did not affect NF-kappa B activation by silica. Antioxidants, such as superoxide dismutase (SOD), N-acetylcysteine (NAC), and pyrrolidine dithiocarbamate (PDTC), blocked tyrosine phosphorylation of I kappa B-alpha induced by silica, suggesting reactive oxygen species (ROS) may be important regulatory molecules in NF-kappa B activation through tyrosine phosphorylation of I kappa B-alpha. The results suggest that tyrosine phosphorylation of I kappa B-alpha represents a proteasome proteolytic activity-independent mechanism for NF-kappa B activation that directly couples NF-kappa B to cellular tyrosine kinase in silica-stimulated macrophages. This proposed mechanism of NF-kappa B activation induced by silica could be used as a target for development of antiinflammatory and antifibrosis drugs.