Death receptor 4 (DR4; also called TRAIL-R1), a member of the tumor necrosis factor receptor superfamily, is a cell surface receptor that triggers the apoptotic machinery upon binding to its ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Although several chemotherapeutic agents were reported to induce DR4 expression, the mechanism of this effect remains largely unknown. To begin to understand its regulation, we cloned a 1.8 Kb 5'-flanking region of the human DR4 gene and identified several putative binding sites for transcription factors including activator protein 1 (AP-1). Among the three putative AP-1 binding sites, the site located at -350/-344 is functionally active as evidenced by a combination of electrophoretic mobility shift and luciferase reporter assays. The AP-1 activator phorbol 12-myristate 13-acetate (TPA) enhanced the binding of this DR4 AP-1 binding site to protein(s) in a nuclear extract from TPA-treated cells, increased luciferase activity of a reporter construct containing this site and induced DR4 expression at the transcription level. These results indicate that AP-1 regulates DR4 expression via the AP-1 binding site located at -350/-344. AP-1 has been implicated in many critical cellular processes including apoptosis, and is a major target of the c-Jun NH(3)-terminal kinase signaling pathway that is activated by many anticancer drugs. Therefore, our findings may increase the understanding of the mechanisms underlying AP-1-mediated apoptosis as well as drug-induced apoptosis.