The effect of N-acetylcysteine (NAC) on levels of hepatocyte growth factor/scatter factor (HGF/SF) gene transcripts was investigated in the human lung embryonic fibroblast cell line, MRC-5. NAC increased expression of HGF/SF mRNA, in a dose- and time-dependent fashion, by a mechanism independent of glutathione synthesis but sensitive to oxidant stress induced by H2O2. Using actinomycin D to block RNA synthesis, it was observed that NAC had no effect on the stability of the HGF/SF mRNA transcripts. NAC increased HGF/SF promoter activity in cells transiently transfected with chloramphenicol acetyltransferase (CAT) reporter genes driven by HGF/SF gene 5'-flanking sequences. Primer extension analysis demonstrated that NAC enhanced the expression of HGF/SF mRNA transcribed from the main transcription initiation site. Although the 5' flanking region of the HGF/SF gene contains a sequence at -1019 to -1011 with homology to the NF-kappaB response element, electrophoretic mobility shift assay demonstrated that this site did not bind nuclear factors in MRC-5 cells in the presence or absence of NAC. In contrast to the effect on HGF/SF mRNA, NAC did not increase HGF/SF protein production by MRC-5 cells,