Relationship between cytotoxic drug response patterns and activity of drug efflux transporters mediating multidrug resistance

Is resistance useless? Multidrug resistance and collateral sensitivity

When cancer cells develop resistance to chemotherapeutics, it is frequently conferred by the ATP-dependent efflux pump P-glycoprotein (MDR1, P-gp, ABCB1). P-gp can efflux a wide range of cancer drugs; its expression confers cross-resistance, termed "multidrug resistance" (MDR), to a wide range of drugs. Strategies to overcome this resistance have been actively sought for more than 30 years, yet clinical solutions do not exist. A less understood aspect of MDR is the hypersensitivity of resistant cancer cells to other drugs, a phenomenon known as "collateral sensitivity" (CS). This review highlights the extent of this effect for the first time, and discusses hypotheses (e.g. generation of reactive oxygen species) to account for the underlying generality of this phenomenon, and proposes exploitation of CS as a strategy to improve response to chemotherapy.

High-throughput screening for analysis of in vitro toxicity

The influence of combinatorial chemistry and high-throughput screening (HTS) technologies in the pharmaceutical industry during the last 10 years has been enormous. However, the attrition rate of drugs in the clinic due to toxicity during this period still remained 40-50%. The need for reduced toxicity failure led to the development of early toxicity screening assays. This chapter describes the state of the art for assays in the area of genotoxicity, cytotoxicity, carcinogenicity, induction of specific enzymes from phase I and II metabolism, competition assays for enzymes of phase I and II metabolism, embryotoxicity as well as endocrine disruption and reprotoxicity. With respect to genotoxicity, the full Ames, Ames II, Vitotox, GreenScreen GC, RadarScreen, and non-genotoxic carcinogenicity assays are discussed. For cytotoxicity, cellular proliferation, calcein uptake, oxygen consumption, mitochondrial activity, radical formation, glutathione depletion as well as apoptosis are described. For high-content screening (HCS), the possibilities for analysis of cytotoxicity, micronuclei, centrosome formation and phospholipidosis are examined. For embryotoxicity, endocrine disruption and reprotoxicity alternative assays are reviewed for fast track analysis by means of nuclear receptors and membrane receptors. Moreover, solutions for analyzing enzyme induction by activation of nuclear receptors, like AhR, CAR, PXR, PPAR, FXR, LXR, TR and RAR are given.

Cytotoxic effects of 100 reference compounds on Hep G2 and HeLa cells and of 60 compounds on ECC-1 and CHO cells. I Mechanistic assays on ROS, glutathione depletion and calcein uptake

In this study fluorometric assays have been used for medium throughput screening on toxicity. Dichlorofluorescein diacetate, monochlorobimane and calcein-AM were fluorophores for the measurement of the formation of reactive oxygen species (ROS), the quantification of glutathione and the membrane stability, respectively. These assays have been carried out in the presence or absence of toxic compounds and with four different cell lines, i.e. human liver (Hep G2), human endometrium (ECC-1), human cervix (HeLa) and Chinese hamster ovary cells (CHO). In these assays the toxic dose of 60 reference compounds was assessed for Hep G2, HeLa, ECC-1 and CHO cells and of 40 pharmaceutical compounds for Hep G2 (ROS, glutathione) or HeLa (calcein) cells, only. These compounds were narcotic analgesics, hypnotics, vasodilators, specific cellular energy blockers, cellular proliferation inhibitors, ion channel blockers, estrogens, antiestrogens, androgens, progestagens and others. The outcome of this study revealed that all four cell lines were responsive to the same set of drugs. Only for some drugs Hep G2 cells appear slightly more sensitive, as compared to the other three cell lines. In general the HeLa cell line was the most sensitive cell line for the calcein uptake, while the Hep G2 cell line shows slightly more sensitivity for dichorofluorescein and monochlorobimane assays than the other three cell lines. Further evaluation at higher toxic dosages with Hep G2 cells for ROS and glutathione depletion and HeLa cells for calcein uptake, demonstrated toxic effects for 56 of the 100 reference compounds in these assays, among which there were estrogens, androgens, progestagens and antiestrogens. In conclusion, almost all tested compounds gave similar dose and toxicity effects on the permanent cell lines used in this study. Only three compounds showed more tissue specific cell responses. This shows that in principle all four cell lines can be used for toxicity screening.

Development and characterization of two human tumor sublines expressing high-grade resistance to the cyanoguanidine CHS 828

The cyanoguanidine CHS 828 has shown promising antitumor properties and is currently in early clinical trials, although the mechanism of action still is largely unknown. In this study, resistant sublines of the histiocytic lymphoma cell line U-937 GTB and the myeloma line RPMI 8226 were developed by culturing under gradually increasing concentrations of CHS 828 until reaching 25 times the parental line EC50s. The new phenotypes demonstrate more than 400-fold resistance to CHS 828 and cross-resistance to six cyanoguanidine analogs, but no resistance to nine standard drugs of different mechanistic classes or to the cytotoxic guanidines m-iodobenzylguanidine and methylglyoxal-bis(guanylhydrazone). The resistant phenotypes were stable for several months even if cultivated in drug-free medium and no difference in proliferation, ultrastructural or morphologic appearance in the sublines could be detected. Neither was decreased accumulation of tritium-labeled CHS 828 observed. Furthermore, the new U-937 phenotype was not accompanied by changes in differentiation or an altered cell-cycle distribution. In the myeloma cell line, esterase activity was shown to be moderately enhanced. Two-dimensional protein electrophoresis was undertaken to unmask possible resistance-mediating proteins and/or the target molecule(s) for CHS 828. In the myeloma cell line, lambda light chain immunoglobulin (down-regulated) and a fatty acid-binding protein (up-regulated) were identified. The findings presented here indicate that development of specific cellular alterations is responsible for the gained CHS 828 resistance.

Multidrug resistance protein-6 (MRP6) in human dermal fibroblasts. Comparison between cells from normal subjects and from Pseudoxanthoma elasticum patients

Multidrug resistance protein-6 (MRP6) is a membrane transporter whose deficiency leads to the connective tissue disorder Pseudoxanthoma elasticum (PXE). In vitro dermal fibroblasts from normal and PXE subjects, homozygous for the R1141X mutation, were compared for their ability to accumulate and to release fluorescent calcein, in the absence and in the presence of inhibitors and competitors of the MDR-multidrug resistance protein (MRP) systems, such as 3-(3-(2-(7-choro-2 quinolinyl) ethenyl)phenyl ((3-dimethyl amino-3-oxo-propyl)thio) methyl) propanoic acid (MK571), verapamil (VPL), vinblastine (VBL), chlorambucil (CHB), benzbromarone (BNZ) and indomethacin (IDM). In the absence of chemicals, calcein accumulation was significantly higher and the release significantly slower in PXE cells compared to controls. VBL and CHB reduced calcein release in both cell strains, without affecting the differences between PXE and control fibroblasts. VPL, BNZ and IDM consistently delayed calcein release from both control and PXE cells; moreover, they abolished the differences between normal and MRP6-deficient fibroblasts observed in the absence of chemicals. These findings suggest that VPL, BNZ and IDM interfere with MRP6-dependent calcein extrusion in in vitro human normal fibroblasts. Interestingly, MK571 almost completely abolished calcein release from PXE cells, whereas it induced a strong but less complete inhibition in control fibroblasts, suggesting that MRP6 is not inhibited by MK571. Data show that MRP6 is active in human fibroblasts, and that its sensitivity to inhibitors and competitors of MDR-MRPs' membrane transporters is different from that of other translocators, namely, MRP1. It could be suggested that MRP1 and MRP6 transport different physiological substances and that MRP6 deficiency cannot be overcome by other membrane transporters, at least in fibroblasts. These data further support the hypothesis that MRP6 deficiency may be relevant for fibroblast metabolism and responsible for the metabolic alterations of these cells at the basis of connective tissue clinical manifestations of PXE.

Effect of cyclophosphamide on gene expression of cytochromes p450 and beta-actin in the HL-60 cell line

Many studies have demonstrated that cyclophosphamide (CPA) can affect hepatic cytochrome p450 (CYP) isoenzyme activity in animals. We have investigated the effect of CPA on gene expression of various CYP enzymes as well as beta-actin in the human acute promyelocytic leukemia cell line (HL-60S) and its multidrug-resistant (MDR) phenotype HL-60R. Cells were incubated at different concentrations of CPA ranging between 50 micro g/ml and 5 mg/ml. In determination of cytotoxicity and resistance factor (RF: IC(50) HL-60R/IC(50) HL-60S), concentrations of 100 and 500 micro g/ml CPA were selected to treat HL-60S and HL-60R up to 72 h. CYP gene expression in the cells prior to and after treatment with CPA was determined using semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Unexposed cell lines did not contain measurable levels of mRNA for CYP2B6, CYP3A4, CYP2C9 and CYP2C19 and no induction was observed after exposure. However, CYP1B1-specific mRNA, which is predominantly expressed in HL-60 cell line, was suppressed after exposure to CPA in a concentration-dependent manner. Beta-actin gene expression was also decreased. The HL-60 RF to CPA was calculated to 0.71, indicating that the multidrug-resistant (MDR) phenotype is not involved in the mechanism of resistance to CPA. No CYPs were induced by CPA in vitro, which probably indicates that the CYP inducibility in blood cells is poor. Our study suggests that suppression of beta-actin gene expression contributes or is involved in the CPA cytotoxicity.

cAMP-Dependent potentiation of the Ca(2+)-activated release of the anionic fluorescent dye, calcein, from rat parotid acinar cells

A recent study indicates that elevation of [Ca(2+)](i) enhances the release of calcein, an anionic fluorescent dye, from isolated exocrine acinar cells, so cytoplasmic calcein is useful for monitoring the secretion of organic anions. In this study, we investigated the effect of cAMP on the calcein release evoked by elevation of [Ca(2+)](i). Isoproterenol, forskolin and dibutyryl cyclic AMP (dbcAMP) did not induce the release of calcein from isolated parotid acinar cells, but they potentiated the carbachol-induced release of calcein. Although cytoplasmic calcein is released through an increase in [Ca(2+)](i), isoproterenol potentiated the carbachol-induced release of calcein without affecting the increase in [Ca(2+)](i) evoked by a high concentration of carbachol (10(-6) M). Charybdotoxin, a K(+) channel blocker, inhibited both the carbachol-induced release and the potentiation by isoproterenol. However, the calcein permeation pathways mediating the carbachol-induced release and the isoproterenol-potentiated release exhibited distinct sensitivities to anion channel blockers. Our results indicate that the calcein release induced by carbachol is potentiated through an increase in intracellular levels of cAMP. Although both the Ca(2+)-activated release and the cAMP-potentiated release may be coupled to Ca(2+)-activated K(+) efflux, increases in both [Ca(2+)](i) and [cAMP](i) may activate the calcein conduction pathway which is not activated by an increase in [Ca(2+)](i) alone.

On the mechanism underlying calcein-induced cytotoxicity

The cellular pharmacology of calcein acetoxymethyl ester (calcein/AM)-induced cytotoxicity was investigated in human tumor cell lines in order to identify tentative mechanisms of action. The activity profile in 10 cell lines with known mechanisms of resistance was compared with the activity profiles of standard drugs and experimental substances. The activity of calcein correlated with that of different topo II inhibitor/intercalating compounds and mitochondrial accumulating compounds, such as Rhodamine 123, Mito Fluor Green and Acridine Orange-10. Using U-937 GTB as a model cell line, calcein was found to distribute throughout the whole cell, nuclei and mitochondria included. In addition, studies of mitochondrial dehydrogenase activity and extracellular acidification rate showed an almost complete lack of dehydrogenase activity and extracellular acidification at 12 and 24 h, respectively. The results indicate that calcein/AM may induce cytotoxicity through interference with both mitochondrial and nuclear DNA.