CHARACTERIZATION OF A CLONAL HUMAN COLON ADENOCARCINOMA LINE INTRINSICALLY RESISTANT TO DOXORUBICIN

Intrinsic low-level resistance to anti-cancer drugs is a major problem in the treatment of gastrointestinal malignancies. To address the pro blem presented by intrinsically resistant tumours, we have isolated tw o monoclonal lines from LoVo human colon adenocarcinoma cells: LoVo/C7 , which is intrinsically resistant to doxorubicin (DOX); and LoVo/C5, which shows the same resistance index for DOX as the mixed parental ce ll population. For comparison, we have included in the study a LoVo-re sistant line selected by continuous exposure to DOX and expressing a t ypical multidrug resistant (MDR) phenotype. In these cell lines we hav e studied the expression and/or activity of a number of proteins, incl uding P-glycoprotein 170 (P-gp), multidrug resistance-associated prote in (MRP), lung resistance-related protein (LRP), glutathione (GSH)-dep endent enzymes and protein kinase C (PKC) isoforms, which have been im plicated in anti-cancer drug resistance. Intracellular DOX distributio n has been assessed by confocal microscopy. The results of the present study indicate that resistance in LoVo/C7 cells cannot be attributed to alterations in P-gp, LRP or GSH/GSH-dependent enzyme levels. Increa sed expression of MRP, accompanied by alterations in the subcellular d istribution of DOX, has been observed in LoVo/C7 cells; changes in PKC isoform pattern have been detected in both intrinsically and pharmaco logically resistant cells.