Review articleResistance mechanisms associated with altered intracellular distribution of anticancer agents
Introduction
The resistance of tumor cells to anticancer agents remains a major cause of treatment failure in cancer patients. The term multidrug resistance (MDR) is classically used to define a resistance phenotype where cells become resistant simultaneously to different drugs with no obvious structural resemblance and with different cellular targets. While it initially was believed that MDR was due to a single molecular mechanism such as overexpression of P-glycoprotein (P-gp), it is now clear that in drug-selected cell lines, MDR is always multifactorial, with at least two resistance mechanisms present in the same tumor cell (see Larsen & Skladanowski, 1998). This may include resistance associated with decreased drug accumulation (decreased drug uptake and/or increased drug efflux), altered intracellular drug distribution, increased detoxification, diminished drug-target interaction, increased DNA repair, altered cell-cycle regulation, and uncoupling of the pathways linking cellular damage with programmed cell death. Many of these resistance mechanisms are not limited to tumor cells, but may also be expressed in bacteria, yeast, fungi, or parasites, providing a major challenge for future chemotherapy, which might be solved by similar strategies.
The present review covers the various resistance mechanisms associated with altered intracellular drug distribution. Since most anticancer agents target DNA or nuclear enzymes, sequestration of drug in cytoplasmic organelles, such as the trans-Golgi network, the recycling endosomes, and the lysosomes, will lead to decreased drug-target interaction and thereby, decreased cytotoxicity, even if the total intracellular drug concentration remains unchanged. However, as reviewed in the following sections, altered intracellular drug distribution is often closely linked to alterations in drug accumulation, making it difficult to separate the two processes, which seem to be among the most common resistance mechanisms occurring in tumor cells. Another common modification in MDR cells is alkalization of the intracellular pH. The relationship between these different resistance mechanisms is assessed and a model proposed that suggests why these different resistance mechanisms are co-expressed in multiple cell lines.
Section snippets
Membrane trafficking and organization
Many anticancer agents, such as the anthracyclines adriamycin and daunomycin (also called doxorubicin and daunorubicin, respectively), vincristine, vinblastine, and mitoxantrone, are weak lipophilic bases with pKs between 7 and 9. Consequently, a substantial fraction of the molecules are uncharged at normal intracellular pH, allowing them to freely penetrate the membranes of cytoplasmic organelles and vesicles. When the drug encounters an acidic environment, such as the interior of acidic
The ATP-binding cassette superfamily of transport proteins
Resistance to numerous anticancer agents is associated with overexpression of the 170-kDa P-gp or the 190-kDa multidrug resistance protein (MRP). Both transporters belong to the ATP-binding cassette (ABC) superfamily of transporters. Presently, this family includes more than 200 prokaryotic and eukaryotic proteins, and given the progression of ongoing genome projects, this list is likely to expand within the coming years (Cole & Deeley, 1998). ABC transporters are found in bacteria, yeast,
Lung resistance protein
P-gp-negative multidrug-resistant cancer cells frequently overexpress a 104- to 110-kDa LRP protein (originally named Lung Resistance-related Protein). Overexpression of this protein is associated with a poor response to chemotherapy in acute myeloid leukemia and ovarian carcinoma (Izquierdo et al., 1996). Subsequent studies showed that the LRP corresponds to the major vault protein (Scheffer et al., 1995), and later studies have shown that the levels of vault particles are also up-regulated
pH alterations in sensitive and resistant tumor cells
An interesting aspect of drug resistance is the observation that many tumor cell lines, including some of the most frequently used cellular models, such as MCF-7 breast carcinoma cells and HL-60 leukemia cells, have altered intracellular pHs compared with their normal counterparts. It currently is not known if alterations of internal pH gradients are as frequent in human tumors as in experimentally employed tumor cell lines.
As discussed in Section 2, changes in internal pH gradients are
Conclusions and perspectives
Multidrug-resistant cancer cells typically show several phenotypic alterations, including increased expression of plasma membrane drug efflux pumps, such as the P-gp and the MRP. In addition, the intracellular pH may be modified, usually in the alkaline direction, and the secretory network considerably expanded, resulting in increased sequestration of anticancer agents in cytoplasmic vesicles, thereby reducing exposure of nuclear targets. A global model that unifies all these mechanisms can be
Acknowledgements
This work was supported in part by the Fondation de France and by the French-Polish Scientific and Technological Cooperation project (98192) of the Ministère des Affaires Etrangères, France and the Committee for Scientific Research (KBN) Poland.
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