Abstract LB-340: Inhibition of autophagy enhances the anticancer activity of the retinoic acid derivative VN/12-1 in endocrine resistant human breast cancer cell lines

Abstract

Abstract Breast cancer is the most common cancer and the second leading cause of cancer related deaths among women in the United States. Although endocrine therapies such as anti-estrogens and aromatase inhibitors have improved the overall survival, resistance to this therapy remains a major concern. Autophagy is a unique, regulated mechanism of cell survival by catabolism of proteins under nutrient deprivation or chemotherapeutic stress. It is an important cause of resistance to chemotherapeutic agents. VN/12-1, a novel 4-imidazolyl methyl ester of retinoic acid synthesized in our laboratory, showed excellent anti-proliferative activity (as shown by MTT assay) in endocrine resistant human breast cancer cell lines SK-BR-3 and MDA-MB-231. Its IC50 in SK-BR-3 cells was 5.2 μM compared to 15.2 μM (all-trans retinoic acid-ATRA) and 45.6 μM (letrozole). For MDA-MB-231 cells, IC50 of VN/12-1 was 7.3 μM compared to 53.7 μM (ATRA) and 74.3 μM (letrozole). At low micromolar concentrations of VN/12-1 treatment on these cells, western blot analysis and imaging studies confirmed the presence of autophagosomes. This was associated with up-regulation of proteins involved in endoplasmic reticulum (ER) stress, markers for DNA damage and downregulation of cyclin D1 and phosphorylated Akt. Co-treatment with chloroquine (an autophagy inhibitor) resulted in increased sensitivity of cells to VN/12-1 as indicated by decrease in cell viability from 71.22% for VN/12-1 (3 μM) alone, 90.18% chloroquine (3 μM) alone to 34.31% for the combination of VN/12-1 (3 μM) and chloroquine (3 μM) in SK-BR-3 cells. It also resulted in induction of apoptosis which was caspase mediated. In summary, ER stress and DNA damage induced by VN/12-1 resulted in cell cycle arrest and induction of autophagy. Inhibition of autophagy by chloroquine overwhelmed this cellular stress and resulted in apoptotic cell death in endocrine resistant breast cancer cell lines. Significance: VN/12-1 and its combination with autophagy inhibitor chloroquine is a novel therapeutic alternative to treat endocrine resistant breast cancer and hence warrants further pre-clinical and clinical development. This is the first in vitro study which showed an enhancement of anti-cancer activity of a retinoic acid derivative by an autophagy inhibitor in endocrine resistant human breast cancer cell lines. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-340.