rho(0) tumor cells: A model for studying whether mitochondria are targets for rhodamine 123, doxorubicin, and other drugs

A human osteosarcoma cell line devoid of mitochondrial DNA (rho (o)) and it s wild-type parental cell counterpart (wt) are presented as a model to inve stigate drug targeting. By virtue of the absence of mitochondrial DNA, rho (o) cells cannot perform electron transport or oxidative phosphorylation. S ince most of the drugs studied are transported by the efflux pumping system s controlled by the MDR1 and MRP1 genes, both cell lines were examined for the expression of these genes, and it was found that no MDR1 and only low a mounts of MRP1 were expressed. Growth inhibition experiments indicated that doxorubicin (Dox), vinblastine, and paclitaxel were equitoxic in these cel l lines. On the other hand, the IC50 for rhodamine 123 (Rho 123) in rho (o) cells was 50 times higher than in wt cells. This result correlates with a lower accumulation of Rho 123 in rho (o) cells as measured by fluorescence microscopy and flow cytometry (3 times less than in wt cells). In contrast, when stained with Dox, both cell types accumulated similar amounts. Surpri singly, in these non-P-glycoprotein expressing cells, verapamil increased b oth Dox and Rho 123 retention. Overall, these data suggest that: (i) functi onal mitochondria do not appear to be targets for the growth inhibitory act ivities of Dox, paclitaxel, or vinblastine; (ii) for lipophilic cations lik e Rho 123, however, normal functioning mitochondria and maintenance of a no rmal mitochondrial membrane potential (Delta psi (mt)) appear to play a cri tical role in the intracellular accumulation and subsequent cytotoxicities of these compounds; and (iii) verapamil increases drug accumulation in non- P-glycoprotein expressing cell lines, most likely by direct action on Delta psi (mt) for Rho 123 and safranin O, and on heretofore unidentified plasma membrane transporters, as well as via interaction with low levels of MRP1, for Dox. These results should be considered when Rho 123 and verapamil are used to detect P-glycoprotein. BIOCHEM PHARMACOL 60;12:1897-1905, 2000. (C ) 2000 Elsevier Science Inc.