Evidence indicates DNA methylation as a part of the regulatory machinery controlling mammalian gene expression. The human melanoma cell line HA-A expresses low levels of transforming growth factor α (TGF-α). TGF-α mRNA accumulated, however, in response to DNA demethylation induced by a nucleoside analog, 5-azacytidine (5-azaC). The importance of DNA methylation in the TGF-α promoter region was examined by a transient transfection assay with luciferase reporter plasmids containing a portion of the TGF-α promoter. 5-azaC treatment of HA-A cells before the transfection caused a significant increase in the luciferase activity. Since input plasmids were confirmed to remain unmethylated, DNA demethylation of the TGF-α promoter itself does not account for the observed increase in TGF-α mRNA. Using an electrophoretic mobility shift assay, enhanced formation of protein-TGF-α promoter complex was detected in response to 5-azaC treatment. This 5-azaC- induced complex was shown to contain the transcription factor Sp1 by the following criteria: the protein-DNA complex formed on the TGF-α promoter contained immunoreactive Sp1; the mobility of the complex in an electrophoretic mobility shift assay was similar to that formed by recombinant Sp1; and DNase I footprinting analysis demonstrated that the 5- azaC-induced complex produced a footprint on the TGF-α promoter identical to that of authentic Sp1. These observations suggest that 5-azaC induces TGF-α expression by augmenting the Sp1 activity. However, neither the Sp1 mRNA nor its protein was induced by 5-azaC. These results suggest that in HA-A cells, TGF-α expression is down-modulated by DNA methylation. In addition, this process may involve the specific regulation of Sp1 activity without altering the amount of the transcription factor.