The role of AIB-1 and PEA-3 in a signalling pathway mediating breast cancer invasion

Tumour invasion and metastasis is mediated in part through the action of Cyclooxygenase-2 (COX-2) and Matrix Metalloproteinases (MMP). COX-2 mediated production of prostaglandins drives tumour growth through cell proliferation and angiogenesis, while MMPs have a role in degrading the extracellular membrane, which in turn promotes cell motility and invasiveness. The over-expression of these genes has been shown to be a marker of cancer cells with a higher metastatic potential. Unfortunately, the exact mechanism involved in their regulation is yet to be fully elucidated. The transcriptional regulatory region of COX-2 and MMP-9 genes contains binding sites for Ets transcription factors, such as PEA-3. It has previousIy been reported that expression of PEA-3 is associated with the co-activator AIB-1, HER2 positivity, metastasis, and reduced disease free survival. Furthermore, an AIB-1 knockout mouse model demonstrates PEA-3 dependant MMP production, resulting in reduced metastatic potential. Translational and molecular approaches were combined to study the role of PEA-3 and AIB-1 in the transcriptional regulation of MMP-9 and COX2 in breast cancer.

The expression levels of HER-2, p-ERK, PEA-3, AIB-1, MMP-9 and COX-2 were assessed in a tissue microarray of breast cancer patients (n=560). Strong associations were present between AIB-1, PEA-3, and the target proteins COX-2 and MMP-9 (p<0.001, p=0.017 and p=0.004 respectively). In addition, PEA-3 was found to significantly associate with MMP-9 expression (p=0.008). In HER2 positive patients PEA-3, but not AIB-1 or MMP-9, was associated with reduced disease free survival (p<0.0044, p= 0.4491, and p= 0.0666 respectively). A stable breast cancer cell line over-expressing PEA-3 was created. Morphological changes suggesting an invasive and motile phenotype were observed with the development of this cell line. Furthermore, the production of the target proteins COX-2 and MMP-9 was increased compared to parental MCF-7 cells. In addition, up-regulation of the growth factor pathway in these cells further augmented this increase, strengthening our hypothesis that the proteins of interest are downstream targets of this pathway. Concomitant silencing of AIB-1 was found to result in reduced levels of both COX-2 and MMP-9. The expression of HER-2 and phospho-ERK also increased in the PEA-3 transfectant cell line, supporting the role of PEA-3 in the signalling pathway.

Understanding signalling networks important in the development and progression of breast cancer enables the identification of new markers of response to treatment. This data supports a role for the transcription factor PEA-3 and its co-activator AIB-1 in the production of MMP-9 and COX2 in breast cancer. Hence, targeting the PEA-3/AIB-1 pathway is a point of potential intervention in the management of breast cancer and may in turn contribute to halting its progression.