Calpain contributes to silica-induced I kappa B-alpha degradation and nuclear factor-kappa B activation

Both silica and lipopolysaccharide (LPS) induce a rapid degradation of I kappa B alpha, an intracellular inhibitor of the nuclear factor (NF)-kappa B transcription factor. In this report, we demonstrate that MG132, a relatively specific proteasome inhibitor, is capable of suppressing LPS-induced I kappa B alpha degradation and NF-kappa B activation in mouse macrophage line RAW 264.7 cells, but is unable to influence the same induction produced by silica. In contrast, the lysosome inhibitor chloroquine has little effect on I kappa B alpha degradation induced by either silica or LPS. In fact, chloroquine enhances the signal-induced nuclear expression of NF-kappa B p50/p65 heterodimer by inhibiting the resynthesis of I kappa B alpha. With the use of transient transfection of a plasmid that expresses calpastatin, a natural inhibitor for calpain, the silica-induced degradation of I kappa B alpha and NF-kappa B activation was attenuated. In contrast, no inhibition of LPS-induced I kappa B alpha degradation and NF-kappa B activation was observed by the overexpression of calpastatin. This suggests that calpain contributes to silica-induced I kappa B alpha degradation and NF-kappa B activation but not to LPS-induced I kappa B alpha degradation and NF-kappa B activation.