Trafficking and localization of platinum complexes in cisplatin‐resistant cell lines monitored by fluorescence‐labeled platinum

Cisplatin is a chemotherapeutic agent commonly used in the treatment of a wide variety of malignant tumors. Resistance to cisplatin represents a major obstacle to effective cancer therapy because clinically significant levels of resistance quickly emerge after treatment. Based on previous studies indicating abnormal plasma membrane protein trafficking in cisplatin-resistant (CP-r) cells, Fluorescence (Alexa Fluor)-labeled cisplatin was used to determine whether this defect altered the trafficking and localization of cisplatin by comparing drug sensitive KB-3-1 and KB-CP-r cells. Alexa Fluor-cisplatin was readily internalized and localized throughout the KB-3-1 cells, but overall fluorescence decreased in KB-CP-r cells, as detected by flow cytometry (FACS) and confocal microscopy. Only punctate cytoplasmic staining was observed in KB-CP-r cells with less fluorescence observed in the nucleus. Colocalization experiments with a Golgi-selective stain indicate the involvement of Golgi-like vesicles in initial intracellular processing of Alexa Fluor conjugated cisplatin complexes. As detected using an antibody to Alexa Fluor-cisplatin, cisplatin complex-binding proteins (CCBPs) were reduced in membrane fractions of single-step cisplatin-resistant KB-CP.5 cells, and increased in the cytoplasm of KB-CP.5 cells compared to KB-3-1 cells. CCBPs localized to lower density fractions in KB-CP.5 cells than in KB-3-1 cells as determined by iodixanol gradient centrifugation. In summary, inappropriate trafficking of CCBPs might explain resistance to cisplatin in cultured cancer cells, presumably because membrane binding proteins for cisplatin are not properly located on the cell surface in these cells, but are instead trapped in low density vesicles within the cytoplasm.