Abstract. We investigated the relationship of ABCB1/MDR1, ABCC1/MRP1 and ABCG2/BCRP expression and promoter methylation with pancreatic cancer tumorigenesis and drug resistance. Gemcitabine-resistant pancreatic cancer cells, SW1990/ GZ (33.3-fold increased resistance), were obtained by treating SW1990 cells with gemcitabine. The expression of ABCB1/ MDR1, ABCC1/MRP1 and ABCG2/BCRP was determined by quantitative real-time RT-PCR in the cell lines, 3 normal pancreatic tissues, 15 human pancreatic cancer samples and 15 adjacent tissues. Promoter methylation was determined in cell lines by bisulfite genomic sequencing. ABCB1/MDR1, ABCC1/ MRP1 and ABCG2/BCRP were upregulated in SW1990 and SW1990/GZ compared with normal pancreatic tissue, and expression in SW1990/GZ was significantly higher than in SW1990 cells. ABCB1/MDR1, ABCC1/MRP1 and ABCG2/ BCRP were upregulated in pancreatic cancer tissues, compared to adjacent tissues. The ABCB1/MDR1, ABCC1/MRP1 and ABCG2/BCRP promoter were hypomethylated in all the cell lines. ABCB1/MDR1, ABCC1/MRP1 and ABCG2/BCRP expression correlated with pancreatic cancer tumorigenesis and drug resistance in a mechanism that is independent of promoter methylation.
IntroductionPancreatic cancer is among the 10 leading causes of cancerrelated deaths, with a 5-year survival <5% (1). Most patients have advanced stage disease at diagnosis. One fifth of pancreatic cancer patients present in the early stage and are eligible for surgical resection (2), which is a complex operation that is associated with high rates of operative morbidity. Pancreatic cancer also shows resistance to chemotherapy. Gemcitabine, the standard treatment in advanced disease, prolongs survival by 5-6 months. Chemotherapy is largely ineffective for patients with metastases as disease frequently redevelops even after surgery; therefore, there is an urgent need to understand the molecular mechanism of chemoresistance in pancreatic cancer.One well-known mechanism of chemoresistance is the overexpression of energy-dependent pumps, such as adenosine triphosphate-binding cassette (ABC) transporters, which lead to the extrusion of drugs and their metabolites (3). P-gp, a 170,000-Da phosphoglycoprotein that consists of two ATP binding cassettes and two transmembrane regions, which is associated with chemoresistance in cancers of the gastrointestinal tract, liver and kidneys (4). The breast cancer resistance protein (ABCG2/BCRP) is a 655-amino-acid polypeptide transporter with a wide range of substrates associated with breast cancer resistance (5). Additionally, ABCC1/MRP1 is correlated with differentiation grade and tumor size in primary hepatocellular carcinoma and breast cancer (6,7).DNA methylation of CpG dinucleotides is the predominant epigenetic gene expression modification. More than 40% of protein coding genes have 5' CpG rich segments, termed CpG islands (8). The aberrant methylation of gene promoters is frequently observed in human cancers and correlates with carcinogenesis, tumor progression and chemosensitivity (9,10). M...