Chemotherapeutics are the most effective treatment for metastatic tumours. However, the ability of cancer cells to become simultaneously resistant to different drugs--a trait known as multidrug resistance--remains a significant impediment to successful chemotherapy. Three decades of multidrug-resistance research have identified a myriad of ways in which cancer cells can elude chemotherapy, and it has become apparent that resistance exists against every effective drug, even our newest agents. Therefore, the ability to predict and circumvent drug resistance is likely to improve chemotherapy.

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Multidrug resistance in cancer: role of ATP–dependent transporters

Fish bioaccumulation and biomarkers in environmental risk assessment: a review

A surface glycoprotein modulating drug permeability in Chinese hamster ovary cell mutants.

Effective treatment of metastatic cancers usually requires the use of toxic chemotherapy. In most cases, multiple drugs are used, as resistance to single agents occurs almost universally. For this reason, elucidation of mechanisms that confer simultaneous resistance to different drugs with different targets and chemical structures - multidrug resistance - has been a major goal of cancer biologists during the past 35 years. Here, we review the most common of these mechanisms, one that relies on drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters. We describe various approaches to combating multidrug-resistant cancer, including the development of drugs that engage, evade or exploit efflux by ABC transporters.

Targeting multidrug resistance in cancer

The design of cancer chemotherapy has become increasingly sophisticated, yet there is no cancer treatment that is 100% effective against disseminated cancer. Resistance to treatment with anticancer drugs results from a variety of factors including individual variations in patients and somatic cell genetic differences in tumors, even those from the same tissue of origin. Frequently resistance is intrinsic to the cancer, but as therapy becomes more and more effective, acquired resistance has also become common. The most common reason for acquisition of resistance to a broad range of anticancer drugs is expression of one or more energy-dependent transporters that detect and eject anticancer drugs from cells, but other mechanisms of resistance including insensitivity to drug-induced apoptosis and induction of drug-detoxifying mechanisms probably play an important role in acquired anticancer drug resistance. Studies on mechanisms of cancer drug resistance have yielded important information about how to circumvent this resistance to improve cancer chemotherapy and have implications for pharmacokinetics of many commonly used drugs.

Mechanisms of cancer drug resistance

We have investigated factors controlling the rate of adhesion of suspension culture CHO cells to serum coated glass surfaces. Thus, we have examined (1) the effect of metabolic depletion via KCN treatment; (2) effects of colchicine and cytochalasin-B, two drugs which are presumptive inhibitors of microtubule and microfilament activities, respectively; (3) effects of temperature on adhesion; (4) the correlation between changes in cell adhesion rate and alterations of lacteroperoxidase labelled cell surface proteins consequent to treatment with proteases.



We have found that colchicine is completely without effect on the initial adhesion process in CHO cells, indicating that microtubules are not a determinant of adhesion kinetics. Cytochalasin-B and depletion of ATP stores by KCN both diminish CHO cell adhesion, but seem to act in different ways. Treatment with cytochalasin-B results in a dose dependent reduction in the initial slope of adhesion versus time curves, while KCN treatment produces a pronounced lag period during which little cell attachment takes place, followed by a relatively rapid rise to control levels of cell attachment.



Analysis of CHO cell adhesion kinetics as a function of temperature reveals that the inhibition produced by low temperature seems similar to that produced by KCN. Thus at elevated (25°C) temperature, CHO cells adhere rapidly, while at 4°C adhesion is completely inhibited. However, at intermediate temperatures (10°C) a pronounced lag period followed by a rapid rise is noted, a result similar to that seen in KCN treated cells.



Treatment of CHO cells with low doses (10 μg/ml) of crystalline trypsin results in the loss of several major cell surface proteins, without any appreciable effect on adhesion kinetics. High doses (100–1,000 μg/ml) of trypsin result in the cleavage of other proteins whose loss may be related to a parallel loss in adhesive ability. Some of the CHO cell surface proteins seem resistant to doses of trypsin (1,000 μg/ml) which completely abolish the ability of cells to adhere.



Our observations on the effects of inhibitors, temperature, and surface proteolysis on CHO cell adhesion kinetics suggest the involvement of (a) microfilaments, (b) plasma membrane fluidity, and (c) a subclass of the surface proteins as important determinants of the adhesion process.

The adhesion of Chinese hamster cells. I. Effects of temperature, metabolic inhibitors and proteolytic dissection of cell surface macromolecules

An evaluation of techniques for labelling the surface proteins of cultured mammalian cells.

Altered Cell surface glycoproteins in phytohemagglutinin-resistant mutants of Chinese hamster ovary cells

The organization of the plasma membrane of logarithmically growing Chinese hamster ovary (CHO) suspension cells has been probed using surface label techniques in conjunction with subcellular fractionation and sodium dodecyl sulfate gel electrophoresis. Five components of apparent molecular weights 137,000, 121,000, 97,000, 67,000, and 57,000 have been shown to be exposed at the outer surface of the cell. These components fully meet the criteria of being (a) reactive with two or more surface label reagents, (b) enriched in a purified plasma membrane fraction, and (c) sensitive to proteolytic digestion of intact cells. Three other components of molecular weights 200,000, 44,000 and 30,000 are also reactive with certain surface label reagents, but fail to meet other criteria for cell surface components. Two polypeptides of molecular weights 180,000 and 37,000 are substantially enriched in the plasma membrane fraction, but are unreactive with surface label reagents. The organization of the CHO cell membrane and the applicability of surface label techniques to cultured cell systems are discussed.

R. L. Juliano

Papers

A surface glycoprotein modulating drug permeability in Chinese hamster ovary cell mutants.

The adhesion of Chinese hamster cells. I. Effects of temperature, metabolic inhibitors and proteolytic dissection of cell surface macromolecules

An evaluation of techniques for labelling the surface proteins of cultured mammalian cells.

Altered Cell surface glycoproteins in phytohemagglutinin-resistant mutants of Chinese hamster ovary cells

Surface polypeptides of the cultured Chinese hamster ovary cell.