Expression of the multidrug resistance protein gene MRP, which confers non-P-glycoprotein-mediated multidrug resistance, has been found in many drug-resistant variants and tumor samples. Recent studies have demonstrated that MRP functions as an ATP-dependent transporter functionally related to the previously described glutathione-conjugate (GS-X) pump. We have shown recently that the MRP and γ-glutamylcysteine synthetase (γ-GCS) heavy subunit mRNA levels are coordinately overexpressed in cisplatin (CP) resistant human leukemia cells (Ishikawa et al., J Biol Chem 271: 14981-14988, 1996) and frequently co-elevated in human colorectal tumors (Kuo et al., Cancer Res 56: 3642-3644, 1996). In the present study, we showed the coexpression patterns of thirteen additional human drug-resistant cell lines representing different tumor cell origins selected with different agents, except for one doxorubicin-selected line which demonstrated minor elevation in MRP mRNA with no detectable increase in γ-GCS mRNA, suggesting that the increase of MRP mRNA preceded the increase in γ-GCS mRNA. Furthermore, in seventeen randomly selected untreated tumor cell lines, the overall correlation coefficient between MRP and γ-GCS mRNA levels was 0.861. In normal mice, the correlation coefficient of mrp and γ-gcs mRNA was 0.662 in fourteen tissues (kidney and liver were not included) analyzed. Kidney and liver expressed low levels of mrp relative to γ-gcs; however, these two tissues expressed high levels of a functionally related mrp homologue, mrp2 (cMoat or cMrp), which may have compensated for the underexpressed mrp in maintaining the total GS-X pump activities. Altogether, these results demonstrated the frequent coexpression of these two genes in various cell settings.
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