AP-1 blockade prevents breast cancer development by suppressing cyclin D and E2F factors.

A120

The AP-1 transcription factor transduces growth signals through signal transduction pathways to the nucleus, leading to the expression of genes involved in proliferation and malignant transformation in many cell types. We have previously shown that overexpression of a dominant-negative form of the cJun proto-oncogene, Tam67, blocks AP-1 transcriptional activity, induces a G1 cell cycle block, and inhibits breast cancer cell growth. We also found that in vivo inducible AP-1 blockade downregulates cyclin D1 expression in mammary cells and suppresses mammary gland development. Further, AP-1 blockade suppresses and partially prevents oncogene-induced mammary tumors. To study the mechanism how AP-1 regulates mammary cell proliferation, we expressed inducible Tam67 in breast cancer cells. We found that AP-1 blockade by Tam67 in MCF-7 breast cancer cells downregulates cyclin D1 transcriptional activity by two mechanisms: by suppressing transcription at the known AP-1 binding site (-934/-928) and by suppressing growth factor-induced expression through suppressing E2F activation at the E2F-responsive site (-726/-719). AP-1 blockade also led to reduced expression of E2F1 and E2F2, but not E2F4 at the mRNA and protein levels. ChIP and supershift assays demonstrated decreased binding of E2F1 protein to the E2F site in the cyclin D1 promoter. We also found that the E2F1 dimerizing partner, DP1, was suppressed by Tam67. In addition, Tam67 suppressed the expression of E2F-upregulated cell cycle genes (cyclins E, A, B and D3), and enhanced the expression of E2F-downregulated cell cycle genes (cyclins G1 and I). Reduced expression of other E2F-regulated genes was also seen with AP-1 blockade and E2F suppression. Thus, the AP-1 factor regulates the expression of cyclin D and E2F (the latter in turn regulates E2F-downstream genes), leading to cell cycle progression and breast cancer cell proliferation. These results suggest that AP-1 is a critical connecting node for growth signals and cell proliferation, and therefore is a promising target for the prevention and treatment of breast cancer.