Abstract 3528: Induction of cytoprotective autophagy and senescence in response to chemotherapy and ionizing-radiation as possible mechanisms of tumor dormancy
Authors: Thekkudan Theresa , Joshi Supriya , Gewirtz David 
Published: July 2016

Proliferative recovery in tumor cells subsequent to chemotherapy and/or radiation-induced prolonged growth arrest may prove to represent a state of autophagy and/or senescence. This may also be useful as an in vitro model of tumor dormancy. To explore this hypothesis, studies were performed in primary mouse mammary carcinoma cells (MMC) treated with Adriamycin (ADR) (0.25uM - 1uM) for 3 days (2h each) and ionizing radiation (IR) (6Gy). Promotion of autophagy was confirmed based on detection of autophagosomes based on acridine orange staining, the appearance of the autophagy marker, LC3.B, associated with autophagosome, degradation of p62/SQSTM1, a hallmark of autophagic flux (i.e. the completion of autophagy and degradation of autophagosome content). Autophagosome formation was observed 24h post ADR treatment and 72h post IR treatment. Furthermore, to understand the function of autophagy (cytoprotective, nonprotective or cytotoxic) autophagy was blocked either pharmacologically using chloroquine (CQ) or genetically by silencing of the key autophagy gene, ATG5. LC3.B appearance and p62/SQSTM1 degradation were more pronounced in wt (autophagy proficient) cells compared to ATG5 silenced cells, confirming that autophagy was inhibited by these pharmacological and genetic strategies. Furthermore, interference with autophagy sensitized the MMC cells to ADR and radiation. These observations indicated that ADR and IR induced autophagy was cytoprotective in the MMC cells. Unexpectedly, apoptosis (as measured by PI/ Annexin V assay) was not increased in response to either ADR alone or the combination of ADR and CQ, suggesting that sensitization was likely occurring through alternative pathways. An evaluation of select markers of senescence indicated enhanced expression of p21 as well as enhanced secretion of the autophagy/senescence associated marker, HMGB1, in response to Adriamycin and IR. Overall, these data suggest that both autophagy and senescence are induced in breast cancer cells by conventional therapies and could contribute to tumor dormancy. Inhibition of autophagy and/or senescence associated signaling pathways could lead to therapeutic strategies for interfering with proliferative recovery of tumor cells after standard modes of therapy. Studies are currently in progress to determine whether the senescence associated secretory phenotype (SASP) accompanies the induction of senescence. SASP has been associated with promotion of tumor growth and alternatively, with activation of an immune response to therapy.

Citation Format: Theresa Thekkudan, Supriya Joshi, David Gewirtz. Induction of cytoprotective autophagy and senescence in response to chemotherapy and ionizing-radiation as possible mechanisms of tumor dormancy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3528.