Study of FOXO3a and the development of tamoxifen resistance in breast cancer

Abstract

Some breast cancer patients with prolonged tamoxifen treatment may develop acquired resistance to this drug; however the mechanism involved remains unclear (Johnston SR, 1997). The PI3K/Akt-kinase signaling pathway regulates cellular response to tamoxifen (Frogne T et al, 2005). FOXO3a is a downstream target of this signaling pathway, vital in a variety of cellular processes(Myatt SS and Lam EW, 2007). Recently however, PIK3CA was unexpectedly found to be a direct FOXO3a target (Hui RC et al, 2008) with FOXO3a engaged in a feedback mechanism which results in hyperactivation of the PI3K/Akt pathway. We hypothesized that FOXO3a may be responsible for acquired tamoxifen resistance. As our in-vitro model, we used a tamoxifen-sensitive cell-line MCF-7, and LCC2 which was derived from MCF-7 by step-wise in-vitro selection and confirmed to be tamoxifen-resistant by proliferation and cell-cycle assay. Compared with MCF-7, enhanced basal phosphorylated-Akt (p-Akt) was found in tamoxifen-resistant LCC2, with unexpected increased FOXO3a expression with nuclear accumulation. On treatment with tamoxifen, given the already high basal levels of FOXO3a and p-Akt in LCC2, no further increased expression was observed. The effect of treatment on MCF-7 still needs to be confirmed. To delineate the role of FOXO3a in modulating sensitivity to tamoxifen, the effect of overexpressing FOXO3a in tamoxifen-sensitive MCF-7 and knockdown FOXO3a in tamoxifen-resistant LCC2 will be investigated. Immuno-histochemical analysis was performed on tissue microarray of 143 breast cancer and 42 non-tumor cases. The expression pattern or sub-cellular localization was compared between tumor and non-tumor as well as correlated with pathological parameters of tumor type, grade and stage, ER, PR, c-erbB2 status, lymph node metastasis, and survival. Our results demonstrated that nuclear FOXO3a expression in breast cancer tissue is statistical significantly associated with lymph node metastasis (p=0.029). Patients with nuclear FOXO3a expression were also found to be statistical significantly associated with shorter survival therefore poorer prognosis (p=0.035). Contrary to a previous report (Hu MCT et al, 2004), our results are in keeping with our hypothesis and preliminary findings that nuclear FOXO3a may be associated with tamoxifen resistance. Further correlation with tamoxifen responsiveness will be done to confirm this.