Separate Roles for the Golgi Apparatus and Lysosomes in the Sequestration of Drugs in the Multidrug-resistant Human Leukemic Cell Line HL-60

The sequestration of drugs away from cellular target sites into cytoplasmic organelles of multidrug-resistant (MDR) cancer cells has been recently shown to be a cause for ineffective drug therapy. This process is poorly understood despite the fact that it has been observed in a large number of MDR cancer cell lines. Analysis of drug sequestration in these cells has traditionally been done using fluorescent anthracycline antibiotics (i.e. daunorubicin, doxorubicin). This narrow selection of substrates has resulted in a limited understanding of sequestration mechanisms and the intracellular compartments that are involved. To better characterize this phenotype, we chose to examine the sequestration of molecules having different acid/base properties in the MDR HL-60 human leukemic cell line. Here we show that weakly basic drug daunorubicin is sequestered into lysosomes according to a pH partitioning type mechanism, whereas sulforhodamime 101, a zwitterionic molecule, is sequestered into the Golgi apparatus through a drug transporter-mediated process. Quantitative intracellular pH measurements reveal that the lysosome-tocytosol pH gradient is expanded in the MDR line. Moreover, the MDR cells overexpress the multidrug resistance-related protein (MRP1), which is localized to the Golgi apparatus. These results demonstrate, for the first time, that two distinct mechanisms for intracellular compartmentalization are operational in a single MDR cell line

Weak Base Permeability Characteristics Influence the Intracellular Sequestration Site in the Multidrug-resistant Human Leukemic Cell Line HL-60

A number of organelles contained within mammalian cells have been implicated in the selective sequestration of chemical entities including drug molecules. Specifically, weakly basic molecules have been shown to selectively associate with either the mitochondrial compartment or lysosomes; however, the structural basis for this differentiation has not been understood. To investigate this, we have identified a series of seven weakly basic compounds, all with pK(a) near neutrality, which have different sequestration sites within the multidrug-resistant HL-60 human leukemic cell line. Three of the compounds were selectively sequestered into the mitochondria of the cells, whereas the remainder were predominantly localized within lysosomes. Using specific chemical inhibitors to disrupt either mitochondrial or lysosomal accumulation capacity, we demonstrated that accumulation of these compounds into respective organelles are not competitive processes. Comparison of the permeability characteristics of these compounds as a function of pH revealed striking differences that correlate with the intracellular sequestration site. Only those compounds with significantly reduced permeability in the ionized state relative to the un-ionized state had the capacity to accumulate within lysosomes. Alternatively, those compounds with relatively pH-insensitive permeability selectively accumulated into mitochondria. Using novel quantitative assays for assaying drug accumulation into subcellular organelles, we demonstrated a correlation between these permeability characteristics and the lysosomal versus mitochondrial accumulation capacity of these compounds. Together, these results suggest that the selective accumulations of weakly basic compounds in either lysosomes and mitochondria occur via exclusive pathways governed by a unique permeability parameter