Intracellular P-glycoprotein expression is associated with the intrinsic multidrug resistance phenotype in human colon adenocarcinoma cells

The 2 clones, LoVo 5 and LoVo 7, derived from untreated LoVo WT human colon adenocarcinoma cells and exhibiting different sensitivity to doxorubicin ( DOX), were compared in order to identify possible determinants of intrinsic drug resistance. A multidrug resistant variant cell line, selected from Lo Vo WT cells by continuous exposure to DOX (LoVo DX), was also included in t he study. Analysis of the expression and organization of cytoskeletal eleme nts by flow cytometry and fluorescence microscopy evidenced a positive corr elation between vimentin expression and DOX resistance in LoVo 7 and LoVo D X cells, whereas differences in actin, tubulin or cytokeratin did not seem to relate to drug response. The expression and localization of different dr ug transporters commonly implicated in drug resistance, i.e., the MDRI gene product P-glycoprotein (P-gp), the multidrug resistance-related protein MR P and the lung resistance-related protein LRP were also investigated by mea ns of flow cytometry and fluorescence microscopy, following labeling with s pecific: monoclonal antibodies. Surface expression of P-gp was only detecte d in LoVo DX cells, which also exhibited increased MRP and LRP protein leve ls. However, significant amounts of P-gp were found at intracellular sites in the intrinsically resistant LoVo 7 clone. Modulation of P-gp function by cyclosporin A was found to alter DOX accumulation and efflux in LoVo 7 cel ls, indicating that intracellular P-gp plays a functional role in drug traf ficking and suggesting possible implications in determining the intrinsic r esistance displayed by this clone. (C) 2000 Wiley-Liss, Inc.