Dipyridamole enhancement of doxorubicin‐induced translocation of nucleophosmin and inhibition of cell growth in HL‐60 cells

Dipyridamole (DPM) enhanced sensitivity to doxorubicin (DOX) in a human leukemia cell line that was already relatively sensitive to this agent. Using an immunofluorescence technique, we determined the localization of nucleophosmin (protein B23) in HL‐60 cells after incubation with DOX in the absence and presence of DPM. Bright nucleolar fluorescence was observed in control HL‐60 cells. The addition of DOX (0.1‐0.25 μg/ml) the culture system resulted in time‐ and dose‐dependent induction of nucleophosmin translocation from the nucleolus nucleoplasm and inhibition of cell growth. DPM (5 μM) alone had no effect on nucleophosmin translocation and inhibition cell growth. However, the addition of DPM to the cells enhanced DOX‐stimulated translocation of nucleophosmin. There was a good correlation between the DPM enhancement DOX‐induced nucleophosmin translocation and the increased inhibition of cell growth. The cell number decreased to greater extent within a shorter time period under treatment with DOX in the presence of DPM. Short exposure (0.5 hr) HL‐60 cells to DOX induced dose‐response nucleophosmin translocation and cell growth inhibition. Such effects of a short exposure to DOX were also enhanced by DPM (5 μM) included in the fresh medium after removal of DOX. This was agreement with the observation that DPM could increase the cellular DOX by inhibiting the drug efflux from the cells. These results demonstrate that DPM, being able to increase and retain the intracellular levels of DOX, can markedly enhance the cytotoxicity of DOX, and suggest possible clinical application. „Nucleophosmin translocation”, as observed by immunofluorescence, could be useful in determining the efficacy combinations of DOX and DPM in cancer chemotherapy.