The activator protein-2 (AP-2) family of DNA-binding transcription factors are developmentally regulated and also play a role in human neoplasia. In particular, the AP-2γ protein has been shown to be overexpressed in a high percentage of breast tumours. In the present study, we report the complete sequence determination of the human TFAP2C gene encoding the AP-2γ transcription factor plus the mapping of the transcription start site used in breast tumour-derived cells. The 5′-end of the gene lies within a CpG island and transcription is initiated at a single site within a classical initiator motif. We have gone on to investigate why some breast tumour-derived cell lines readily express AP-2γ, whereas others do not, and show that the proximal promoter (+191 to −312) is differentially active in the two cell phenotypes. DNase footprinting led to the identification of three Sp1/Sp3-binding sites within this region, two of which are absolutely required both for promoter function and cell-type-specific activity. By Western blotting a panel of expressing and non-expressing breast tumour lines we show that the latter have higher levels of Sp3. Furthermore, increasing Sp3 levels in AP-2γ-expressing cells led to the repression of AP-2γ promoter activity, particularly when Sp3 inhibitory function was maximized through sumoylation. We propose that differences in the level and activity of Sp3 between breast tumour lines can determine the expression level of their AP-2γ gene.

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