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 prote in kinase C or A plays an important role in silica-induced activation of NF -kappaB in macrophages. The question is raised whether PTK stimulation and NF-kappaB activation in silica-stimulated macrophages are directly connecte d through tyrosine phosphorylation of I kappaB-alpha. Results indicate that stimulation of macrophages with silica led to NF-kappaB activation through tyrosine phosphorylation without serine phosphorylation. Specific inhibito rs of protein tyrosine kinase, such as genistein and tyrophostin AG126, pre vented tyrosine phosphorylation of I kappaB-alpha in response to silica. I kappaB-alpha protein levels remained relatively unchanged for up to 60 min after silica stimulation. Moreover, inhibition of proteasome proteolytic ac tivity did not affect NF-kappaB activation by silica Antioxidants, such as superoxide dismutase (SOD), N-acetylcysteine (NAC), and pyrrolidine dithioc arbamate (PDTC), blocked tyrosine phosphorylation of I kappaB-alpha: induce d by silica, suggesting reactive oxygen species (ROS) may be important regu latory molecules in NF-kappaB activation through tyrosine phosphorylation o f I kappaB-alpha. The results suggest that tyrosine phosphorylation of I ka ppaB-alpha represents a proteasome proteolytic activity-independent mechani sm for NF-kappaB activation that directly couples NF-kappaB to cellular tyr osine kinase in silica-stimulated macrophages. This proposed mechanism of N F-kappaB activation induced by silica could be used as a target for develop ment of antiinflammatory and antifibrosis drugs, (C) 2000 Academic Press.