Trastuzumab treatment has improved the overall survival of HER2-overexpressing
breast cancer patients. However, many of these patients will eventually become resistant to treatment. The mechanisms that contribute to resistance to
trastuzumab are unknown. In this study, we tested the hypothesis that targeting of the FKHR
transcription factor FOXO1A in HER2-overexpressing
breast tumor cells can overcome the
trastuzumab resistance in vitro. We have shown that overexpression of HER2 leads to activation of
phosphatidylinositol 3-kinase (PI3K)/Akt pathway and subsequent inactivation of FOXO1A in HER2-overexpressing
breast cancer cells SKBR3, BT474, and MCF7-HER2. In wild-type SKBR3 and BT474 cells,
trastuzumab downregulates active Akt and increases FOXO1A expression that leads to increase in p27(kip1) and decrease in
cyclin D1 and finally inhibits cell proliferation. In contrast, the effect of
trastuzumab was eliminated by the reduction of FOXO1A in HER2-overexpressing cells with constitutively active Akt1 (SKBR3/AA28 and BT474/AA9). The downregulation of FOXO1A resulted in nuclear export of p27(kip1). Blocking the constitutively active Akt by a specific
Akt/protein kinase B signaling inhibitor-2 (API-2) significantly increased FOXO1A expression and rendered the cells more responsive to
trastuzumab-induced growth inhibition. Reactivation of FOXO1A by stable or transient transfection also restored the growth-inhibitory effects of
trastuzumab in SKBR3/AA28, BT474/AA9, and MCF7-HER2 cells. Knocking down FOXO1A by
small interfering RNA resulted in reducing
trastuzumab-induced growth inhibition. In summary,
trastuzumab can inhibit proliferation of HER2-overexpressing
breast cancer cells by reactivating FOXO1A through inhibition of the PI3K/Akt pathway. FOXO1A may therefore serve as a target for HER2-overexpressing
breast tumors.