The role of FOXO3a in the development of chemoresistance in breast cancer

Abstract

Breast cancer is the most common malignancy in women and represents one of the major causes of death worldwide. The PI3K-Akt-FOXO3a signalling pathway has been shown to play a crucial role in tumorigenesis and the development of drug resistance in many cancer types. However, previous studies on FOXO3a using breast cancer tissues were controversial. So this study aims at better understanding of the role of FOXO3a in the development of drug resistance, especially endocrine resistance and anthracycline resistance in breast cancer.

Examination of FOXO3a and phosphorylated-Akt (P-Akt) expressions in breast cancer tissue microarrays revealed nuclear FOXO3a was significantly associated with poor prognosis (p=0.014) and lymph node positivity (p=0.052) in invasive ductal carcinoma. Using the tamoxifen and anthracycline-sensitive and -resistant breast cancer cell lines as models, we found that the nuclear accumulation of FOXO3a was associated with enhanced anthracycline-resistance but not tamoxifen-resistance. This was consistent with the finding that sustained nuclear FOXO3a was associated with poor prognosis, as cytotoxic chemotherapy resistance is linked to limited therapeutic options and poor prognosis. We demonstrated a possible feedback mechanism in which induction of FOXO3a activity in the anthracycline-sensitive MCF-7 cells induced Akt phosphorylation and promoted cell proliferation arrest. Using MDA-MB-231-FOXO3a(A3):ER cells in which FOXO3a activity could be induced by 4-hydroxytamoxifen, we showed that FOXO3a induction could up-regulate PI3K-Akt activity but had little effect on cell proliferation, which indicates impaired Akt-FOXO3a axis in chemoresistant cell models.

To further uncover the precise mechanism of Akt-FOXO3a deregulation in the development of chemoresistance, we have explored the post-translational regulation of FOXO3a by miRNAs. Through a series of Gain-and-Loss functional experiments and luciferase reporter assays in vitro, three miRNAs, including miR-222, miR-221 and miR-29a, were found to suppress FOXO3a protein expression through binding directly to FOXO3a 3’UTR. Moreover, the aberrant expressions of the miR-222/221 cluster and miR-29a in drug resistant cell lines could confer a proliferation advantage to cancer cells through suppressing FOXO3a expression. We further demonstrated that FOXO3a as a transcription factor could transactivate the oncogenic miR-222 and miR-221 expressions under certain chemotherapy stimulation. This suggests the existence of a feedback regulatory loop composed of the miR-222/221 cluster and FOXO3a which may not only play a self-protective role under drug treatment in chemosensitive cells, but also partially explain the tolerated nuclear FOXO3a in the breast cancer with poor prognosis.

Taken together, our study suggested that lymph node metastasis and poor survival in invasive ductal breast carcinoma are linked to an uncoupling of the Akt-FOXO3a signalling axis, as in these breast cancers the nuclear-located FOXO3a was unable to induce cell death or cell cycle arrest. We also demonstrated post-translational regulation of FOXO3a by miR-222/221 and miR-29a, while aberrant expressions of miR-222/221 and miR-29a may promote cell resistance to therapy through directly suppressing FOXO3a. FOXO3a could further contribute to the deregulation of the miR-222/221 cluster as a transcription factor in breast cancer. Studying this Akt-FOXO3a-miRNAs signalling circuit will provide us better understanding in predicting and monitoring treatment response in breast cancer and other malignancies.