Regulation of breast tumour cell survival by AP-2 transcription factors

Abstract

AP-2 transcription factors are crucial regulators of embryonic development and also play important roles in human neoplasia. Over-expression of AP-2α and AP-2γ in primary breast cancer (BC) correlates with expression of two major breast markers, ERBB2 and oestrogen receptor. High AP-2γ expression is associated with reduced survival in BC patients, including those treated with hormone therapy. Our aim is to define the pathways regulated by AP-2 factors in breast epithelial cells. Data from AP-2γ depleted MCF-7 cells suggested a role in cell cycle control. Here, regulation by AP-2α and AP-2γ in additional breast cancer cell lines with differing genetic background is investigated. Cell cycle analysis of synchronized T47D cells, which express both AP-2 isoforms but mutant p53, showed a reduction in G1 but increased S and G2/M-phase populations when AP-2α and AP-2γ were silenced either independently or together. Despite the lack of growth arrest, p21cip protein levels increased following AP-2 silencing. ChIP analysis showed AP-2α and AP-2γ binding at the p21cip/CDKN1A promoter. In addition, cyclin D3 protein levels increased following AP-2 silencing and ChIP analysis showed AP-2α and AP-2γ binding to its promoter. Luciferase reporter constructs carrying CCND3 promoter sequences were repressed when co-transfected with AP-2α or AP-2γ expression constructs. These findings demonstrate the importance of AP-2 factors in the control of cell cycle regulation but illustrate cell-type differences in their mode of action. Further work focused on MCF10A immortalised breast epithelial cells, which express both AP-2 isoforms and wild-type p53. AP-2 silenced MCF10A cells adopted a more rounded phenotype suggestive of changes in cell adhesion which was also supported by microarray analysis of their gene expression profile. KIAA1324, a protein of unknown function with features of a membrane-bound growth factor receptor, was found to be significantly down-regulated following AP- 2γ silencing in MCF-7 cells. Functional assays using i) inducible knock down of KIAA1324 in MCF-7 cells, and ii) stable KIAA1324 overexpression in MCF10A investigated if altering KIAA1324 expression level could affect cell growth. KIAA1324 did not affect breast epithelial cell proliferation on plastic but may contribute to the ability of MCF-7 cells to display anchorage-independent growth.