Abstract 4230: Senescence reversion contributes to acquired drug resistance
Authors: Samaraweera Leleesha , Liu Lingling , Suzuki Masako , Horwitz Susan Band , McDaid Hayley M. 
Published: August 2012

Abstract
Cells undergo a diverse range of responses to cytotoxic drugs such as Taxol, including apoptosis, mitotic catastrophe, necrosis and accelerated senescence. Senescent cells are resistant to growth factor-stimulated proliferation and often have a secretory phenotype (the Senescence-Associated Sectetory Phenotype, or SASP) that mediates extracellular matrix remodeling and immune clearance. Although poorly defined, in some situations senescent cells do not get cleared and persist. A growing body of evidence suggests that cancer cells made senescent from cytotoxic therapy have a pro-inflammatory SASP that promotes malignant transformation and EMT of surrounding cells. Furthermore, we have demonstrated that senescent cells undergo reversion (PNAS 2010; 108, p391-396.), although the frequency of this phenomena in human malignancies is unknown. Specifically, we generated a human lung cancer cell line that overcame senescence caused by the microtubule-stabilizing drug discodermolide. These cells are resistant to discodermolide and other cytotoxic drugs; therefore we hypothesized that senescent reverted cells retain some characteristics of their senescent precursors, and that this contributes to drug resistance. To explore this possibility, we profiled global gene expression and promoter methylation of drug-resistant cell lines derived from the A549 lung cancer cells and their senescent counterparts. These were made resistant to different tubulin-polymerizing drugs that induce accelerated senescence. Bioinformatic and subsequent ingenuity pathway analysis identified common pathways that were statistically significantly altered in senescent cells and their resistant counterparts. Candidate genes were selected for validation of gene expression and methylation changes by qRT-PCR and MassArray, respectively. Drug-induced senescent cells had increased levels of the SASP genes IL6 (16.8-fold), IL8 (4.15-fold) and CXCL1 (4.35-fold); as well as other genes not typically associated with SASP phenotypes. Interestingly, all three resistant cells retained high expression of specific genes in pro-inflammatory signaling pathways, including IL8 and CXCL1. The increased expression of CXCL1 in senescent cells was via transcriptional activation, whereas the even higher expression observed in senescent reverted cells (resistant cells) was via promoter hypomethylation. Ongoing experiments to modulate gene expression using shRNA-mediated knockdown will help elucidate the role of CXCL1 in senescence reversion and how this contributes to drug resistance. Additional pathways including Wnt and G-protein coupled receptor signaling were also strongly implicated in senescence reversion and will be presented. This is the first study to document retention of SASP signaling in senescent reverted cells and to explore whether these contribute to acquired drug resistance.
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 4230. doi:1538-7445.AM2012-4230