Modulation by Verapamil of Doxorubicin Induced Expression of Multidrug Resistance Gene (MDR-1/P-Glycoprotein) in Murine Tumor Cells
12(3) : 221-227
Article Type: Original Article
Abstract: Overexpression of the transmembrane drug efflux pump p-glycoprotein (p-gp) is one of the major mecha-nisms by which cancer cells develop multidrug resistance (MDR) against natural product anticancer drugs including Doxorubicin (DOX). In this study, the effects of DOX and/or DOX plus the calcium channel blocker, Verapamil, on mdr-l/p-gp expression in Ehrlich ascites carcinoma cells (EAC-cells) were studied using Rhodamine 123 (RD 123), Western blotting and Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR). Tumor-bearing mice treated with DOX (2 mg/kg I.P. every other day for a total of 3 doses) showed 34.1 days median survival time with 20% long-term survivors. However, pretreatment with Verapamil (20 mg/kg I.P. daily for 5 days), before DOX increased the long-term survivors to 60% with medi-an survival time of 44.4 days. Verapamil pretreatment in-creased the cellular levels of DOX at all time points tested with maximum level appeared at 6 hours after treatment (5.5 pg/108 cells compared to 3.4 pg/108 for DOX alone). The expression of mdr-l/p-gp was significantly increased 6 hours after treatment with DOX. Verapamil pretreatment (20 mg/kg) for 5 days before DOX significantly decreased mdr-l/p-gp expression and decreased p-gp function from 37% to 16%. On the basis of our findings we may con-clude: (1) DOX induced mdr-l/p-gp expression in sensi-tive EAC-cells at both transcriptional and translational levels at only 6 hours post treatment. (2) Verapamil block DOX-induced mdr-l/p-gp expression with the consequent increase in DOX cellular uptake and cytotoxicity. (3) Ve-rapamil can potentiate the cytotoxic activity of DOX against the growth of EAC cells by mdr-l/p-gp indepen-dent mechanism.