Transcriptional activation of human manganese superoxide dismutase (MnSOD) mRNA in A549 human lung carcinoma cells induced by a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was examined to identify the responsive transcriptional regulator within the 5-flanking region of the human MnSOD gene promoter. Deletion of a region between - 1292 and -1202 nucleotides upstream of the transcription start site abolished TPA responsive induction, whereas deletion of the putative binding sequence for NF-kB or AP-1 did not. The region between -1292 and -1202 contains a cAMP-responsive element-like sequence, TGACGTCT, which we identified as the human manganese superoxide dismutase TPA- responsive element, MSTRE. Site-specific mutation of the human MSTRE abolished the TPA responsive induction, validating the critical role of this sequence. We detected specific MSTRE-activity that is related to the activation of CREB-1/ATF-1. The phorbol ester treatment did not change the binding activity, but it rapidly induced phosphorylation of this CREB-1/ATF-1-like factor via the protein kinase C pathway. These results suggest that the human MnSOD gene is induced by TPA, in part, via activation of a CREB-1/ATF-1-like factor and not via either NF-kB or AP-1, which are known as redox-sensitive transcription factors. In addition, we found that this induction was blocked by inhibitors of flavoproteins/NADPH oxidase, indicating the involvement of enhanced generation of superoxide radical anion as an upstream signal. This or a similar mechanism involving superoxide radical as an upstream signal for the induction of MnSOD may be congruent with the role of the enzyme against superoxide radical toxicity in vivo. - EPR, free radicals, oxidative stress, MnSOD, transcription, superoxide radical