Activator protein 2␣ (AP-2␣) 3 is a sequence-specific DNA binding transcription factor that is required for normal growth and morphogenesis (1-3). AP-2␣ has a conserved C-terminal DNA binding motif with an integral helix-span-helix homodimerization motif and a less-conserved proline and aromatic amino acid-rich helix transactivation domain near the N terminus and binds to a consensus DNA sequence, 5Ј-GCCNNNGGC-3Ј (4 -8). AP-2␣ has been shown to regulate many genes involved in variety of biological functions (9).Several lines of evidences indicate that AP-2␣ may act as a tumor suppressor gene. AP-2␣ gene is located in chromosome position 6p22, a region of frequent loss of heterozygosity in breast and other cancers (10). Diminished AP-2␣ function has been correlated with N-Ras oncogene-mediated transformation (11). The functions of AP-2␣ have been shown to be regulated by SV40 T antigen and adenovirus E1A oncoproteins (1, 12). In addition, reduced or loss of AP-2␣ expression has been reported in human cancers of breast, ovary, colon, skin, brain, and prostate (13)(14)(15)(16)(17)(18)(19)(20). In good correlation, expression of dominant negative mutant AP-2␣ resulted in increased invasiveness and tumorigenicity (21).Overexpression of AP-2␣ by transient transfection into cultured cells has been shown to induce p21 WAF1/CIP1 and inhibit cellular DNA synthesis and colony formation (22). Significant correlation between AP-2␣ expression and p21 WAF1/CIP1 has been observed in breast cancer, colorectal cancer, and malignant melanoma (15,18,23). The growth inhibitory activity of AP-2␣ has also been shown to be mediated through direct interaction with p53 (24). Results from our laboratory suggest that adenovirus-mediated overexpression of AP-2␣ inhibits growth of cancer cells by inhibiting cellular DNA synthesis and inducing apoptosis (25). Furthermore, our recent work establishes that AP-2␣ is induced in cancer cells upon treatment with chemotherapeutic drugs, which contributes to chemosensitivity because the simultaneous inhibition of AP-2␣ by siRNA increases the chemoresistance. In addition, the re-expression of epigenetically silenced AP-2␣ in breast cancer resulted in enhanced chemosensitivity and loss of tumorigenicity upon chemotherapy in an AP-2␣-dependent manner (26). These results point out the importance of apoptosis induction by AP-2␣ for its functions, particularly the role in chemosensitivity.The molecular mechanism by which AP-2␣ induces apoptosis is not known. In the present study we have analyzed the pathways and various molecules involved in AP-2␣-induced apoptosis. We found that AP-2␣-induced apoptosis requires primarily the mitochondrial pathway involving a bax/cytochrome c/Apaf1/caspase 9-dependent mechanism. We also found that AP-2␣ binds to the Bcl-2 promoter leading to its transcriptional down-regulation and is essential for the apoptosis induction by AP-2␣. In addition, we provide evidence that the overexpressed AP-2␣ (perhaps functionally inactive) in certain breast cancer cells can be made functio...