Drug Resistance

The resistance of cancer cells to cytostatic agents has been a significant impediment to the effective chemotherapy of cancer. Studies with model systems, where cancer cells grown in vitro were selected for resistance to specific anticancer drugs, identified two major classes of drug-resistant cells: (i) cells resistant to a single class of drugs with the same mechanism of action, and (ii) cells resistant to chemically diverse drugs with multiple mechanisms of action. The latter phenomenon was called multiple drug resistance (MDR). Some metastatic cancers are intrinsically resistant to chemotherapy, whereas others respond to treatment initially but subsequently acquire resistance, not only to the chemotherapeutic agents used against them, but to other chemically unrelated drugs as well (1). Numerous factors have been associated with MDR. These include elevated levels of protein kinase C, enhanced sodium pump activity, mutations of topoisomerase II and tubulin, and altered levels of calcium and calmodulin. It has not been possible, however, to correlate any of these clearly with the MDR phenotype. A large body of evidence, on the other hand, derived from studies of cells in culture, transfection experiments, histochemical studies of a wide variety of tumors graded for resistance to chemotherapy, and studies in reconstituted systems, strongly implicates energy-dependent pump systems that either exclude or extrude chemotherapeutic agents from MDR cells.

Besides human cancer cells and their rodent models, MDR pumps have been found in a wide array of organisms: bacteria, protozoa, and fungi, and mammalian cells; even plant cells have MDR pumps. These share the unique feature of excluding a fairly broad range of chemically unrelated compounds from the cell. Only some of these belong to the ATP Binding Cassette (ABC( superfamily of transporters, and they do not ubiquitously use ATP as their energy source. The majority of the bacterial pumps, for example, use the energy of the proton motive force. The ATP-dependent system that has been most extensively characterized is the MDR1 gene product, the multidrug transporter or P-glycoprotein (P-gp).

 Many cells selected for drug resistance do not show increased levels of P-gp but nonetheless are resistant to a broad range of natural product drugs. Another member of the ABC superfamily, the MDR-associated protein (MRP1), is expressed in some of these cell lines at elevated levels. MRP1 is similar to P-gp in that it is capable of decreasing intracellular levels of drugs and is ATP-dependent. While P-gp has two membrane-spanning domains, MRP has three. The presence of a third such domain is observed in several other ABC proteins, all of which are more closely related to MRP than to any other member of this superfamily of transporters. There are now at least a dozen ABC proteins that together constitute the MRP branch of the superfamily. The most important members are (i) the human MRP, which is an anionic conjugate transporter, (ii) the multispecific organic anion transporter, MOAT or MRP-2, and (iii) SUR1, the sulfonylurea receptor. There are other significant differences between MRP and P-gp: (i) MRP1 catalyzes the efflux of anionic or neutral drugs conjugated to glutathione, glucoronate, or sulfate; (ii) some substrates of P-gp, eg, taxol, are not substrates for MRP; (iii) MDR effected by P-gp is reversed by verapamil and cyclosporin A, but not that caused by MRP. It is thus quite likely that both P-gp and MRP are drug transporters but that their detailed mechanisms of action may differ (2).

 Additionally, a 110-kDa protein, the lung resistance protein, has been identified in non P-gp-resistant lung carcinoma cells. This protein is a major constituent of an intracellular ribonucleoprotein (vault( complex that consists of three other proteins of 210 kDa, 190 kDa, and 54 kDa, plus an RNA  molecule. There is increasing evidence that the vault complex is involved in MDR (3).

References

1. M. M. Gottesman and I. Pastan (1993) Ann. Rev. Biochem. 62, 385–427. 

2.D. Lautier, Y. Cantrot, R. G. Deeley, and S. P. C. Cole (1996) Biochem. Pharmacol. 52, 967–977.

3. G. L. Scheffer, P. J. Wijngaard, M. J. Flens, M. A. Izquierdo, M. L. Slovak, H. M. Pinedo, C. J. L. M. Meijer, H. C. Clevers, and R. J. Scheper (1995) Nature Medicine 1, 578–582.