1. ¹Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bethesda, MD 20892, USA
2. ²Alpha-Gamma Technologies Inc., Raleigh, NC 27609, USA
3. ³Medical Oncology Branch, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bethesda, MD 20892, USA
4. ⁴Laboratory of Human Carcinogenesis, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bethesda, MD 20892, USA
5. ⁵Gene Silencing Section, Genetics Branch, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bethesda, MD 20892, USA

Correspondence: Dr SV Ambudkar, E-mail: ambudkar@helix.nih.gov

Revised 21 February 2008; Accepted 4 March 2008; Published online 1 April 2008.

Abstract

Understanding the mechanisms of multidrug resistance (MDR) could improve clinical drug efficacy. Multidrug resistance is associated with ATP binding cassette (ABC) transporters, but the factors that regulate their expression at clinically relevant drug concentrations are poorly understood. We report that a single-step selection with low doses of anti-cancer agents, similar to concentrations reported in vivo, induces MDR that is mediated exclusively by ABCG2. We selected breast, ovarian and colon cancer cells (MCF-7, IGROV-1 and S-1) after exposure to 14 or 21 nM doxorubicin for only 10 days. We found that these cells overexpress ABCG2 at the mRNA and protein levels. RNA interference analysis confirmed that ABCG2 confers drug resistance. Furthermore, ABCG2 upregulation was facilitated by histone hyperacetylation due to weaker histone deacetylase 1-promoter association, indicating that these epigenetic changes elicit changes in ABCG2 gene expression. These studies indicate that the MDR phenotype arises following low-dose, single-step exposure to doxorubicin, and further suggest that ABCG2 may mediate early stages of MDR development. This is the first report to our knowledge of single-step, low-dose selection leading to overexpression of ABCG2 by epigenetic changes in multiple cancer cell lines.