Abstract A64: Mechanistic insights into the misregulation of p27 in HER2-positive breast cancers

Although it is often stated that low amounts of the cell cycle inhibitory protein p27 are predictive of a poor outcome in breast cancers, this is an oversimplifcation of the data. Typically, tumor cells exhibit decreased expression of p27 in the nucleus, while preserving or even increasing the pool of p27 in the cytoplasm. Whereas nuclear p27 is tumor suppressive by inhibiting cell proliferation, newer evidence suggests that cytoplasmic p27 has an oncogenic action: it interacts with the GTPase RhoA to promote cellular migration and motility. Thus, cytoplasmic p27 may be an indicator of tumor cell invasiveness and metastatic potential. Understanding the mechanism of p27 mislocalization in breast cancers may offer insight into pathways of oncogenic transformation, and provide new markers for predicting clinical responses to specific therapies. Toward this goal, we developed antibodies that reliably detect cytoplasmic p27, and also identified a subset of breast cancers positive for the growth factor receptor HER2 that exhibited increased amounts of cytoplasmic p27. In parallel experiments we discovered an E3 ubiquitin ligase, Trim62 that regulates p27 stability, and found that it is overexpressed in HER2+ breast cancers. siRNA knock down of Trim62 in HER2+ breast cancer cell lines not only induced cell cycle arrest but also caused p27 to relocate entirely to the nucleus. We compared knock down of Trim62 to that of the well-characterized p27 regulator, the F box protein Skp2. Unlike Trim62, Skp2 inhibition had no detectable effect on p27 in these cells. As pharmacological inhibition of HER2 has also been shown to increase nuclear p27 protein levels, we investigated the role of Trim62 in modulating the response of breast cancer cells to anti-HER2 therapeutics. Like Trim62 siRNA, the HER2/EGFR inhibitor, Lapatinib increased p27 amounts, and this increase in p27 was located exclusively in the nucleus. Furthermore, we showed that knockdown of Trim62 increased cellular sensitivity to Lapatinib. Overexpression of Trim62 had the opposite effect. Collectively, our results suggest Trim62 underlies the misregulation of p27 in HER2+ breast cancer cell lines. By modulating p27 expression and cellular localization, Trim62 may mediate the antiproliferative effect of HER2 antagonists in breast cancer cells. Moreover, Trim62 could be a potential biomarker to predict biological response of breast cancer cells to anti-HER2 therapeutics such as Lapatinib. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A64.