Lack of correlation between mdr-1 expression and volume-activation of cloride-currents in rat colon cancer cells

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Abstract
Correlation between expression of the mdr-1 genes (a and b) at the mRNA and protein level and volume-activation of chloride-channels was studied in rat colon cancer CC531 cells by means of RT-PCR, Western blotting and patch clamp, respectively. Three different kinds of cell lines were used: CC531-PAR, CC531-COL and CC531-REV. At the mRNA level, the parental cell line CC531-PAR showed significantly less mdr-1a expression in comparison with CC531-COL, a drug-resistant cell line induced from the parental CC531 cells by growth in the presence of colchicin. The third cell line, CC531-REV, was a spontaneous revertant of the drug-resistant cell line to a drug-sensitive one, but with a maintained level of mdr-1a mRNA. In none of the three cell lines, mdr-1b mRNA could be detected At the protein level, a clear difference in mdr1 expression between CC531-PAR/REV and CC531-COL was observed. Although the amount of mdr-1a mRNA detected in CC531-REV was comparable to that found in CC531-COL, the amount of mdr-1 encoded protein in CC531-REV was remarkably reduced. In all three cell types, cell swelling activated chloride-currents which could be blocked by NPPB. Chloride-currents measured at the K+ reversal potential of-90 mV were not significantly different (−86.1±19.1 pA/pF, n=5 in CC531-PAR, −59.5±13.1 pA/pF, n=6 in CC531-COL and −68.1±15.3 pA/pF, n=7 in CC531-REV). It is proposed that the mdr-1 genes do not code for the volume-activated chloride-current in these rat colon cancer cells.