Abstract 996: Retinoic acids activate autophagy in breast cancer cells through the FOXO3A-RARA pathway and blocking autophagy enhances retinoic acid-mediated cell death

Abstract

Abstract A series of pre-clinical studies showed promising anti-cancer effects of retinoic acids (RA) in breast cancer models. However, clinical studies were mostly disappointing possibly also due to the trial design and emerging resistances. One way to confer resistance to a given treatment is activation of protective autophagy. Autophagy is a cellular recycling mechanism that is characterized by the formation of double-membraned autophagosomes. Autophagosomes engulf bulk cytoplasm or selected contents thereof and transfer them to lysosomes for degradation. Activation of autophagy upon anti-cancer treatment may allow prolonged survival of the tumor cells. We therefore asked if autophagy is activated by all-trans retinoic acid (ATRA), a pan-retinoic acid receptor (RAR) agonist, in mammary tumor cells and if autophagy might protect the cells from the pro-apoptotic effects of ATRA. We found that ATRA induces autophagic flux in ATRA-sensitive SKBR3 but not in -resistant MD-MBA- 453 human breast cancer cells as seen by increased LC3B-II protein abundance and turnover of long-lived proteins. Moreover, using RA receptor alpha (RARA), RARB and RARG agonists as well as RARA knockdown breast cancer cells we demonstrated that activation of autophagy by RA depends on RARA expression. RA-treatment also caused induction of the transcription factor FOXO3A, a known transcriptional target of RARA. Since the expression of several autophagy genes namely the ATG8 family members, Beclin-1 and PI3KC3 are controlled by FOXO3A, we tested if FOXO3A might link ATRA-mediated signaling to autophagy activation. Indeed, depleting FOXO3A in SKBR3 cells resulted in significantly decreased autophagic activity and autophagy gene expression upon RA treatment. Importantly, blocking autophagy by knocking down the key autophagy genes ATG5 or ATG7 in breast cancer cells resulted in significantly increased apoptosis under ATRA treatment. Of note, inhibition of autophagy also attenuated cyto-differentiation similarly to the phenotype seen in RARA knockdown breast cancer cells. In summary, our data demonstrate for the first time that retinoic acids induce autophagy in breast cancer cells via the FOXO3A-RARA axis. Furthermore, inhibition of autophagy results in enhanced ATRA-mediated apoptosis representing a potential new treatment strategy for a selected group of breast cancer patients. Citation Format: Anna M. Schlaefli, Enrico Garattini, Mario P. Tschan. Retinoic acids activate autophagy in breast cancer cells through the FOXO3A-RARA pathway and blocking autophagy enhances retinoic acid-mediated cell death. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 996. doi:10.1158/1538-7445.AM2015-996