FoxO transcription factors are well known as key tumor suppressors in mammalian cells. The suppression of FoxOs in cancer cells is thought to be mainly due to activation of multiple onco-kinases, such as Akt and SGK, by a phosphorylation-ubiquitylation-mediated cascade. On the other hand, phosphorlyation of FoxO by JNK relocalizes it from the cytoplasm to the nucleus allowing transcription of FoxO target genes. In this study, we applied human breast cancer cells to determine whether the activity of FoxO3a is regulated during the quercetin mediated apoptosis. After treatment with the indicated amount of quercetin (5-80uM) for 5days, the growth of human breast cancer cell lines including MDA-MB-231, MDA-MB-486 and MCF-7 cells was reduced in dose- and time-dependent manner. FACS analysis with AnnexinV/PI double staining showed that the proportion of early apoptotic cancer cells were significantly increased in quercetin treated cells. It represents the potent apoptotic effect of quercetin in breast cancer cells. Furthermore, a marked elevation of FoxO3a transcriptional activity was detected at 20μM of quercetin using FHRE-reporter assay, as well as increased protein level of FoxO3a was detected by western blotting. And also it was determined that the activation of FoxO3a lead to the upregulation of FasL expression during the apoptotic process. To identify the signaling mechanism of FoxO3a regulation, we examined the activation of several well known upstream kinases such as Akt, Erk, IKKβ, AMPK and JNK. Notably, a significant increase of JNK phosphorylation was observed in quercetin treated cells. In addition, the inhibition study with pharmacological inhibitor (SP600125) showed that the involvement of JNK for the regulation of FoxO3a activity is critical in quercetin treated breast cancer cells. These results demonstrate that quercetin induces FoxO3a-related apoptosis by activation of JNK in MDA-MB-231 cells, and suggest the possibility that JNK-FoxO3a signaling pathway contributes mainly to the apoptotic process of breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 248. doi:1538-7445.AM2012-248