Inhibition of tumor growth by alkylation of the plasma membrane

Serotoninergic modulation of cell volume response to estramustine: an image-analysis study on perifused individual glioma cells

A technique of microscopy with computerised detection of early morphological changes during continuous perifusion was used to monitor the geometry changes of cultured glioma cells (MG-251) when exposed to 40 mg/L estramustine phosphate (EMP) alone or in combination with granisetron (0.1 mumol/L), ondansetron (0.1 mumol/L), or serotonin (1 mumol/L). When the cells were exposed to EMP, cell volume measured as projected cell area (PCA) rapidly increased. Serotonin and ondansetron, but not granisetron, prevented the acute EMP response (PCA). Serotonin, but none of the 5-HT3 receptor antagonists, protected against the cytotoxicity of EMP to the glioma cells as measured by a fluorometric microculture assay. Our results demonstrate hitherto unknown differences between selective 5-HT3 receptor antagonist on the cellular response to EMP and shows the necessity to study the receptor antagonists from viewpoint of interference with the antitumour drug effects on malignant cells. The perifusion technique could be used to study the effects of serotoninergic agonists and antagonists on cell volume regulation of cells exposed to anticancer drugs.

Identification of potassium flux pathways and their role in the cytotoxicity of estramustine in human malignant glioma, prostatic carcinoma and pulmonary carcinoma cell lines

Clinically-used drugs such as furosemide, bumetanide and cardiac glycosides, are modulators of transmembrane fluxes of cations. Recently, it has been suggested that the regulation of intracellular cation concentrations could be a primary target for anti-neoplastic drugs, and that the cytotoxic activity may be altered by inhibitors of cation fluxes at the level of the plasma membrane. Therefore, we investigated the mechanisms by which cations are translocated across the plasma membrane of malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines. The interactions between cation flux inhibitors and the cytotoxicity of estramustine were also evaluated. Ouabain, the classical inhibitor of Na+, K+ATPase, markedly reduced 86Rb (K+) influx in all three lines, indicating that this ion transport system is present in the cells. Furosemide and especially bumetanide inhibited the 86Rb influx, indicating the presence of the Na+, K+, Cl- co-transport system. The potassium channel blocker, tetraethylammonium, but not apamin reduced the influx of 86Rb showing that high conductance K+ channels are present, but that channels of low conductance probably do not exist in these cell lines. The Na+, K+, Cl- co-transport inhibitors furosemide and bumetanide significantly reduced cytotoxicity of estramustine in P31 cells, whereas no interaction between other K+ flux inhibitors and the anti-neoplastic drugs were detected in any of the cell lines investigated. Thus, the data show that Na+, K+, ATPase and NA+, K+, Cl- co-transport systems and K+ channels of high conductance are present in malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines, and that inhibition of the Na+, K+, Cl- co-transport system in P31 is associated with reduced cytotoxicity of estramustine. The results justify further studies evaluating the role of these cation flux pathways in terms of targets for anti-neoplastic therapy.

Early cytotoxic effects induced by bis-chloroethyl sulphide (sulphur mustard): [Ca2+]i rise and time-dependent inhibition of B77 fibroblast serum response

Early cytotoxic events were studied on B77 fibroblasts. Cells were treated with sulphur mustard (SM) in short-term experiments in which cell viability was unchanged, as evaluated by the neutral red cytotoxicity test. This treatment was correlated to two early signs of cytotoxicity. The intracellular Ca2+ concentration [Ca2+]i level in SM-treated Fura-2-loaded fibroblasts showed a significant dose-dependent increase. This observed rise was sustained, in contrast to the Ca2+ signal induced by serum, and was already visible 5-10 min after the addition of SM to cell suspensions in vitro. Modification of the extracellular Ca2+ concentration in the medium had no effect on the cytosolic calcium rise caused by SM, suggesting release from intracellular Ca2+ pools. Furthermore, a time-dependent inhibition of the [Ca2+]i transient increase induced by growth-factors (as evaluated by the fetal calf serum (FCS) response) was observed within the first hour of exposure. These latter results suggest that early alterations of calcium distribution induced by SM could be one of the earliest markers of SM intoxication.

Amiloride but not bumetanide protects against the cytotoxic effects of estramustine and bleomycin in cultured fibroblasts

The effects of the clinically used diuretics amiloride (an inhibitor of Na+/H+ exchange) and bumetanide (an inhibitor of Na+, K+, Cl- co-transport) were tested on the cytotoxicity of estramustine and bleomycin in cultured fibroblasts. Both estramustine (50 micrograms/ml) and bleomycin (10 micrograms/ml) reduced the number of surviving clones. Amiloride (100 microM) but not bumetanide (100 microM) partly protected against the cytotoxic effect of both estramustine and bleomycin. The protective effect of the combination of amiloride and bumetanide was not stronger than the effect of amiloride alone. Amiloride or bumetanide alone did not affect the clonal survival. It is suggested that the protective effect of amiloride is not mediated by an effect of the molecular mechanism of cytotoxicity but may rather be due to changes in cellular metabolism and/or drug handling.

Selective inhibition by bis(2-chloroethyl)methylamine (nitrogen mustard) of the Na+/K+/Cl- cotransporter of murine L1210 leukemia cells

Incubation of L1210 murine leukemia cells in vitro with 10 microM of the bifunctional alkylating agent bis(2-chloroethyl)methylamine (nitrogen mustard, HN2) for 10 min brought about a fall of more than 99.9% in their ability to form colonies when the cells were suspended in 0.5% nutrient agar. Incubation with HN2 also inhibited the influx of the potassium congener 86Rb+ to exponentially proliferating L1210 cells in a concentration-dependent manner. This inhibition was specific and was accounted for by a reduction of a diuretic-sensitive component of 86Rb+ influx, identified in the preceding paper (Wilcock, C. and Hickman, J.A. (1988) Biochim. Biophys. Acta 946, 359-367) as being mediated by a Na+/K+/Cl- cotransporter. Inhibition by 10 microM HN2 was complete after a 3-h incubation. There was no inhibition at this time of the ouabain-sensitive component of 86Rb+ influx, mediated by Na+/K+-ATPase. After 3 h of incubation with 10 microM HN2 there was also no change in the membrane potential of the treated cells as measured by the distribution of the [3H]TPMP+, no decrease in cellular ATP concentration and no change in intracellular pH, and the ability of the cells to exclude the vital dye Trypan blue was not significantly different from control values. These effects of HN2, therefore, appeared to follow lethal damage, but precede cell death. In the stationary phase of L1210 cell growth, the component of HN2 and diuretic-sensitive K+ influx to L1210 cells was reduced, whilst the component constituting the HN2-insensitive ouabain-sensitive sodium pump was increased. The monofunctional alkylating agent MeHN1 (2-chloroethyldimethylamine) which cannot cross-link cellular targets and has no antitumor activity, did not inhibit 86Rb+ influx to L1210 cells when incubated at equimolar or equitoxic concentrations to HN2. Intracellular potassium concentration was maintained close to control values of 138 +/- 10 mM in HN2-treated cells because of an approx. 35% fall in cell volume. The results suggest that the Na+/K+/Cl- cotransporter is a selectively inhibitable target for HN2, and the lesion is discussed with reference to the cytotoxic effects of this agent.

Membrane ordering effects of the anticancer agent VM-26

The effect of the anticancer agent VM-26 on acyl chain order of cellular and model membranes was examined by electron spin resonance techniques. The order parameter for the paramagnetic probe 5-doxyl stearate was increased when VM-26 was incorporated into the bilayer of fluid-phase dimyristoylphosphatidylcholine (DMPC) or gel-phase dipalmitoylphosphatidylcholine (DPPC) liposomes at concentrations up to 4.8 mol%. The ordering effect of VM-26 in DMPC was greater than that of cholesterol on an equimolar basis. The less cytotoxic congener of VM-26, VP-16, was only one-third as active as VM-26 in its ordering effects on DMPC. Higher order parameters for 5-doxyl stearate were also noted in asolectin liposomes, Ehrlich ascites tumor cells, and CCRF-CEM cells treated with VM-26. We conclude that VM-26 has significant membrane associated activity in addition to its previously recognized nuclear effects.

Effects of nitrogen mustard on potassium transport systems and membrane structure of Ehrlich ascites tumor cells

By Ehrlich ascites tumor cells 86Rb+ has been shown to be a suitable tracer for K+-transport. Sixty percent of the total 86Rb-uptake into these cells is ouabain-inhibitable, 30% is sensitive to furosemide and 10% enters the cells by ouabain and furosemide-insensitive systems. N-Mustard inhibits both the ouabain-sensitive and the furosemide-inhibitable systems. The uptake which is resistant to both inhibitors is not affected by the alkylating drug. At N-mustard concentrations below 10 microM, the reduction of the Rb-uptake is predominantly due to the inhibition of the furosemide-sensitive transport. Higher concentrations are required before a significant inhibition of the ouabain-sensitive transport can be observed. The dose response curve of the furosemide-sensitive transport--not, however, of the ouabain inhibitable pump--corresponds to the dose response curve for the antiproliferative activity of N-mustard. The recovery of the furosemide-sensitive transport after a single exposure to N-mustard is relatively slow and--in contrast to the repair of DNA cross-links--is characterized by an initial 4-hr lag period. Furosemide alone does not interfere with cell multiplication. The inhibition of the transport system alone does, therefore, not explain the antitumor activity of N-mustard. The effect is discussed as a marker for membrane lesions after exposure to alkylating agents. In order to investigate the influence of N-mustard on membrane structure, membranes were labelled with diiodofluoresceiniodoacetamide. Anisotropy curves obtained from time-dependent depolarization of delayed fluorescence indicated a mustard induced immobilization of membrane constituents. Lateral diffusion of lipophilic probes was determined by following the quenching of fluorescence of pyrene by cetylpyridinium. The latter studies yielded no evidence for a change in membrane lipid fluidity. The data are interpreted as the results of cross-links of membrane proteins by the bifunctional alkylating agent.