A STRUCTURE-FUNCTION RELATIONSHIP AMONG RESERPINE AND YOHIMBINE ANALOGS IN THEIR ABILITY TO INCREASE EXPRESSION OF MDR1 AND P-GLYCOPROTEIN IN A HUMAN COLON-CARCINOMA CELL-LINE

We previously showed that there is a structure-function relationship a mong reserpine and yohimbine analogues in their ability to inhibit the function of P-glycoprotein (P-gp) and reverse multidrug resistance (M DR). Because some P-gp inhibitors (e.g., verapamil and nifedipine) can increase mdr1 and P-gp expression in human colon carcinoma cell lines , we used our reserpine/yohimbine analogues to determine whether there was a structural requirement for this induction. We found that 10 mu M reserpine increased both mdr1 and P-gp expression by 4-10-fold in 48 hr in a human colon carcinoma cell line that expresses moderate level s of mdr1 (LS180-Ad50) but not in several other cell lines that expres sed no mdr1. The reserpine/yohimbine analogues rescinnamine, trimethox ybenzoylyohimbine, and LY191401 (compound G), all of which contain the three structural elements used to describe the MDR pharmacophore, als o increased both mdr1 and P-gp expression significantly. Despite some exceptions, we found that there was a good association between the abi lity of these analogues to induce mdr1 and P-gp expression and their a bility to reverse vinblastine and doxorubicin resistance, revealing a structure-function relationship for this phenomenon. The increased P-g p expressed by these cells appeared to be functional, as determined by flow cytometric detection of rhodamine 123 retention. The increased e xpression was suppressed by 5,6-dichloro-1-beta-D-ribofuranosylbenzimi dazole, an RNA synthesis inhibitor, whereas the protein synthesis inhi bitor cycloheximide enhanced the expression several-fold, suggesting t hat induction of mdr1 by these analogues is regulated at both the tran scriptional and post-transcriptional levels. Given that the effect is seen only in cell lines that express some P-gp and is mediated only by active, P-gp-interacting compounds, we speculate that induction of md r1 and P-gp by these agents may be mediated through a feedback mechani sm.