Cell Cycle Perturbation and Acquired 5-Fluorouracil Chemoresistance

Abstract

Acquired chemoresistance is one of the obstacles for success of 5-fluorouracil (5-FU)-based cancer chemotherapy. Some molecular mechanisms of acquired 5-FU resistance are still unknown. We have recently demonstrated down-regulation of a group of cell cycle related genes in acquired 5-FU resistant human cancer cell lines. In this study, the bivariate distribution of propidium iodide versus BrdU in acquired 5-FU resistant colon (H630_R10) and breast (T47D_FU2.5) cancer cell lines was compared with their parental cell lines using flow cytometric analysis. The resistant cell lines showed significantly lower labelling index (T47D_FU2.5) and cell cycle delay in G1 and G1/S boundary and prolonged DNA synthesis time (H630_R10). Both resistant cell lines demonstrated significantly prolonged potential doubling time (T_pot). The protein expression levels of some G1 and S phase transition-related genes were also analysed by Western blot. CDK2 protein and Thr-160 phosphorylated CDK2 were remarkably reduced in the resistant cell lines. Cyclin D3 and cyclin A were also decreased in the resistant cells. Total pRB expression was unaltered but hypophosphorylation of pRB (Ser780, Ser795 and Ser807/811) was detected in the resistant cancer cells. Our data suggest that there may be a slow down in cell cycle traverse preventing incorporation of 5-FU metabolites into DNA and also providing cancer cells with sufficient time to correct the mis-incorporated nucleotides. The cell cycle perturbation may be involved in acquired 5-FU resistance.