Abstract 2638: Association between Adriamycin-induced autophagy and senescence in breast cancer cells

Abstract

Abstract Previous studies from this laboratory demonstrated that the topoisomerase II poison, Doxorubicin, at a clinically relevant concentration of 1µM, induced a pronounced senescence response in breast cancer cells. Doxorubicin was also observed to generate a time-dependent increase of autophagy within 72 hrs by multiple approaches, including acridine orange staining (AOS), downregulation of p62 (indicative of autophagic flux), LC3 puncta formation and electron microscopy. The current studies were designed to investigate the potential relationship between senescence and autophagy in response to Doxorubicin in breast cancer cells. Silencing of ATM by the pharmacological inhibitors, caffeine or KU55933, or by small interfering RNA abrogated doxorubicin-induced senescence, as demonstrated by decreased β -galactosidase staining as well as down-regulation of p21 expression. At the same time, silencing/attenuation of ATM suppressed autophagy, based on decreased conversion of LC3-I to LC3-II by Western blotting, and reduced acridine orange staining. Free radicals are well-established to induce senescence, and Doxorubicin-induced senescence has been shown to be associated with free radical generation. We demonstrate that the antioxidants, glutathione (GSH) or N-acetyl cysteine (NAC), were able to collaterally suppress Doxorubicin-induced senescence and autophagy, based on reduced β-Gal staining, LC3-I to II conversion and activation of ATG5, respectively. Similarly, both senescence and autophagy induced by the topoisomerase I inhibitor, camptothecin, were suppressed by GSH or NAC. Silencing of p53 or p21 expression inhibited both Doxorubicin-induced senescence and autophagy in both breast and colon cancer cells. In contrast, a blockade of autophagy either by the pharmacological inhibitors, 3-methyl adenine or chloroquine, or genetic silencing of ATG5 or ATG7 expression, only served to delay the onset of Doxorubicin-induced senescence. Similarly, a delayed senescence response was observed when autophagy induced by camptothecin or H2O2 was blocked by chloroquine or bafilomycin in MCF-7 cells. Our studies suggest that: 1. Free radicals, ATM, p53 and p21 are involved in the signaling pathways that promote the autophagic and senescence responses to chemotherapeutic agents; 2. autophagy acts to accelerate senescence; and 3) while chemotherapy induced autophagy and senescence are closely associated, senescence can likely occur in the absence of autophagy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2638. doi:10.1158/1538-7445.AM2011-2638