Abstract 689: Short-term exposure of tumor cells to chemotherapeutic agents is associated with changes in microRNA expression
Authors: Ho Tsui-Ting , He Xiaolong , Arslan Ahmet D. , Beck William T. 
Published: June 2012

Abstract
The acquisition of resistance to anticancer drugs is a key obstacle to successful cancer therapy. An increasing number of studies have investigated the roles of microRNAs (miRs) in drug resistance. We reported that the upregulation of miR-135b/-196b correlated positively with acquired drug resistance as well as expression of the ATP-binding cassette (ABC) transporter ABCB1 in the human T-cell leukemic CCRF-CEM cell line (T-T Ho et al, Proceedings of the 101st Annual Meeting of the AACR 2010). We now report that when we exposed cells to etoposide for 48 h, we found similar increases in miR-135b/-196b in leukemias (RPMI 8226 [myeloma], HL-60 [acute promyelocytic leukemia], Jurkat and MOLT-4 [both acute T cell leukemia]), but not in solid tumors (MCF7 breast cancer and A2780 ovarian cancer), suggesting that induction of expression of these miRs may be histiotype-specific. To develop a mechanistic understanding of differentially expressed miRs, we have studied miR-196b, which maps between the HOXA9 and HOXA10 genes on chromosome 7p15.2. We observed upregulation of HOXA9 mRNA in relation to short-term chemotherapeutic drug exposure in CCRF-CEM cells, indicating that the expression of miR-196b is linked to HOXA gene transcription. To investigate the kinetics of expression of miR-135b/-196b, ABCB1 and ABCC1, we have performed rechallenge experiments in which CCRF-CEM cells were repeatedly exposed to etoposide for 48 h with a 3-day drug-free incubation in between. We found that changes in miR-135b/-196b expression were paralleled by expression of the ABCB1 gene, but not the ABCC1 transporter gene. In addition, subsequent etoposide (IC50, 300 nM) rechallenges (2nd and 3rd challenge, days 5 and 10) led to progressively attenuated expression of the miR-135b/-196b and ABCB1 compared to that seen after the first challenge. These results further support the observation that miR-135b/-196b expression is associated with ABCB1 upregulation, and suggest that miR-135b/-196b are involved not only in the initial response of cancer cells to chemotherapeutic challenge, but also in the development of chemoresistance via mechanisms associated with expression of efflux pump genes. We consequently measured expressions of miR-135b/-196b, ABCB1 and ABCC1 in CCRF-CEM cells exposed to repeated drug challenge after either 15 or 20 passages in drug-free medium. Treatment of these previously-exposed cells again with 300 nM etoposide resulted in unexpected attenuation of miR-135b/-196b and ABCB1 expression. We further determined that the etoposide IC50 in these cells had increased to 1 µM. Our results offer insights into the establishment of the drug-resistant phenotype and suggest that induction of miR-135b/-196b and the drug-resistant phenotype is transiently acquired, but its loss after drug exposure seems to re-set the IC50 to higher levels. (Supported in part by 5RO1CA40570 from NCI to WTB, and in part by UIC.)
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 689. doi:10.1158/1538-7445.AM2011-689