Regulation of macrophage inflammatory protein-1alpha mRNA by oxidative stress

Accumulation of inflammatory cells within the lung has been implicated in oxidative injury. Recruitment of these cells to a tissue site is a complex process that depends in part upon the local expression of appropriate proinflammatory chemokines. Macrophage inflammatory protein-1alpha (MIP-1alpha), a member of the CC subfamily of chemokines, has been shown to contribute to monocyte/macrophage and neutrophil chemotaxis and activation. Our previous work demonstrated that MIP-1alpha mRNA expression in macrophages is induced by bacterial endotoxin. The objective of this study was to test the hypothesis that an oxidative stress alone may trigger expression of MIP-1alpha mRNA in macrophages and to determine the mechanism leading to increased expression. A rat alveolar macrophage cell line (NR8383) was exposed to H2O2 or menadione (2-methyl-1,4-naphthoquinone (MQ)), a quinone compound that undergoes redox cycling and generates reactive oxygen species continuously. Steady-state mRNA levels encoding MIP-1alpha were markedly increased (3-fold) in these cells after 1 h of exposure to 0.5 mM H2O2, remained higher than control levels after 4 h, and decreased after 6 h. Similarly, MQ (25 or 50 microM) caused a significant increase of MIP-1 alpha mRNA with a maximal induction after 4 h of exposure (5-fold). Both H2O2 and MQ-induced up-regulation of MIP-1 alpha mRNA was suppressed by co-treatment with N-acetylcysteine, a synthetic antioxidant. Co-treatment with actinomycin D reduced the MQ induction of MIP-1alpha mRNA to a greater extent than the H2O2-induced increase. Transcription of the MIP-1alpha gene was increased by exposure to both H2O2 and MQ. H2O2 treatment also induced a marked increase of the MIP-1alpha mRNA half-life, indicating post-transcriptional stabilization. These observations indicate that an oxidative stress can regulate MIP-1alpha mRNA expression by two distinct mechanisms: transcriptional activation of the MIP-1alpha gene and post-transcriptional stabilization of MIP-1alpha mRNA.