Nitrogen mustard interference with potassium transport systems in Ehrlich ascites tumor cells

Cytotoxic and cytostatic effects of antitumor agents induced at the plasma membrane level

A variety of antitumor agents inhibit cell proliferation by interacting with the plasma membrane. They act as growth factor antagonists, growth factor receptor blockers, interfere with mitogenic signal transduction or exert direct cytotoxic effects. The P-glycoprotein encoded by the MDR1 gene represents a transmembrane protein which catalyzes the efflux of various antitumor agents. This membrane protein is the target of compounds acting as Multi-Drug Resistance (MDR)-modulators. Finally, several established antitumor agents which are considered to represent DNA-targeted drugs, including anthracyclines, platinum complexes and alkylating agents, cause a variety of membrane lesions. Their contribution to the antitumor activity of these drugs is discussed.

Effects of nitrogen mustard-N-oxide on ionic activities of inner ear fluid and ionic permeabilities of the cochlear partition in the guinea pig

The effect of nitrogen mustard-N-oxide (NMO) on the endocochlear potential (EP) was investigated from the aspect of the ion concentrations and permeabilities in the cochlea. Compared with the untreated animals, in NMO-treated animals 20 to 30 hours after administration, the EP was decreased (30.8 +/- 3.5 mV in NMO versus 82.4 +/- 1.6 mV in control), the K+ concentration in perilymph of the scala tympani was increased (8.2 +/- 1.0 mM versus 5.3 +/- 0.7 mM), the K+ concentration in endolymph was decreased (128.5 +/- 10.6 mM versus 157.9 +/- 7.9 mM), and the Na+ concentration in endolymph was increased (9.6 +/- 3.6 mM versus 2.5 +/- 0.4 mM). The permeability coefficient for Na+ of the cochlear partition in the NMO-treated animals significantly decreased, while that for Cl- significantly increased. The negative EP, which presumably exists in the normal state, diminished further (-2.7 mV versus -27.8 mV), and the calculated electrogenic potential of the EP was depressed remarkably (33.5 mV versus 110.2 mV). The results suggest that the effects of NMO involved changes in ion permeabilities of the partition and the inhibition of electrogenic transport processes in the cochlea.

Synergistic enhancement of the antiproliferative activity of cis-diamminedichloroplatinum(II) by the ether lipid analogue BM41440, an inhibitor of protein kinase C

The new phospholipid analogue 3-hexadecylmercapto-2-methoxy-methyl-propyl-1-phosphocholine inhibits the phospholipid-calcium-dependent protein kinase, partially purified from Walker carcinoma cells with a Ki value of 0.56 microM. The compound inhibits the phorbol ester stimulated phosphorylation of the ribosomal protein S6 indicating that the depression of Ca2+-phospholipid-dependent protein kinase by the alkyl phospholipid also occurs in intact cells. The dose effect curve for the inhibition of cell proliferation by 3-hexadecylmercapto-2-methoxy-methyl-propyl-1-phosphocholine in Walker cells exhibits a close correlation to the dose effect curve for the depression of Ca2+-phospholipid-dependent protein kinase activity. Although alternative mechanisms cannot be excluded, the data suggest that the growth inhibitory activity of 3-hexadecylmercapto-2-methoxy-methyl-propyl-1-phosphocholine correlates with the inhibition of Ca2+-phospholipid-dependent protein kinase. The antiproliferative activity of 3-hexadecylmercapto-2-methoxy-methyl-propyl-1-phosphocholine is synergistically enhanced by cis-diamminedichloroplatinum(II).

The phospholipid- and calcium-dependent protein kinase as a target in tumor chemotherapy

Evidence for a constitutive activation of protein kinase C (EC 2.7.1.37) in Ha-ras transformed 3T3 cells is presented. Several compounds which inhibit protein kinase C in vitro have been studied with regard to their antiproliferative activity in cultured tumor cells. The following agents were investigated: 3-hexadecyl-mercapto-2-methoxy-methyl-propyl-1- phosphocholine (BM 41440); 1-octadecyl-2-methyl-sn-glycero-3-phosphocholine (ET-18-OCH3); quercetin, tamoxifen and staurosporine. All compounds decrease protein kinase C activity in vitro as well as in intact cells and inhibit cell multiplication within the same dose range. The results suggest a causal relation between the antiproliferative effects and the inhibition of protein kinase C. All inhibitors of protein kinase C synergistically enhance the antiproliferative activity of cis-diamminedichloroplatinum(II). Available data suggest that the effects of protein kinase C inhibitors should be exploitable for tumor chemotherapy.

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.

Differential sensitivity of histone acetylation in nitrogen-mustard sensitive and resistant cells. Relation to drug uptake, formation and repair of DNA-interstrand cross-links

Cultivation of Ehrlich-ascites tumor cells in the presence of N-mustard leads to a selection of cells with a defective choline carrier. As N-mustard employs the choline carrier for transport, this results in reduced drug uptake and in a decrease in drug sensitivity which is specific for N-mustard. Walker carcinoma cells with a stable pleiotropic resistance to a variety of alkylating agents and adriamycin exhibit no evidence for an impaired drug transport and show the same frequency of DNA-interstrand cross-links as the sensitive parental line. Both sensitive and resistant Walker cells exhibit equal capacities for repair of N-mustard induced DNA-interstrand cross-links. The inhibition of histone acetylation by N-mustard, however, was found to be significantly lower in the resistant Walker or Ehrlich cells compared to sensitive counterparts. Although the difference between N-mustard concentrations leading to half maximal inhibition of histone acetylation in sensitive and resistant cells is considerably smaller than the difference between N-mustard doses required for half maximal inhibition of cell proliferation the data suggest that--besides DNA-DNA cross-linking--the inhibition of histone acetylation has to be considered as an important alternative mechanism responsible for the cytotoxic activity of alkylating agents. Inhibition of histone acetylation is not due an accelerated deacetylation and is predominantly expressed in chromatin fractions soluble in 0.1 M NaCl after digestion with micrococcal nuclease.

Enhancement of the antiproliferative effect of cis-diamminedichloroplatinum(II) and nitrogen mustard by inhibitors of protein kinase C

Quercetin (3,3',4',5,7-pentahydroxyflavone) has been shown to inhibit a variety of enzymes including the calcium- and phospholipid-dependent protein kinase (protein kinase C) in vivo and in vitro. We show that this compound synergistically enhances the antiproliferative activity of cis-diamminedichloroplatinum(II) (cis-DDP) and nitrogen mustard. Quercetin does not affect the repair of DNA interstrand cross-links introduced by cis-DDP. Long-term exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA), which reduces total protein kinase C activity, also amplifies the growth-inhibitory effect of cis-DDP and acts synergistically with quercetin. A synergism is also observed if tamoxifen or staurosporine are combined with cis-DDP. For both drugs the dose-effect curves for the inhibition of protein kinase C closely resemble the dose-effect curves for the antiproliferative activities. Although alternative mechanisms cannot be definitively excluded, the effects of quercetin, TPA, tamoxifen and staurosporine may result from the inhibition of protein kinase C.

Amiloride and 5-N,N-dimethylamiloride inhibit the carrier mediated uptake of choline in Ehrlich ascites tumor cells

Amiloride and 5-N,N-dimethylamiloride (DMA) inhibit the choline uptake of Ehrlich ascites tumor cells. The inhibition by DMA is competitive with a KI value of 20 microM. The apparent KM value for choline was determined as 15 microM. Amiloride is approximately three times less potent. Amiloride uptake is not antagonized by choline or impaired in cells characterized by a deficient choline carrier. This indicates that amiloride is not transported into the cell by the choline carrier. The inhibition of the choline uptake by DMA cannot be attributed to a depression of choline kinase (EC 2.7.1.32) and is therefore considered to be due to a direct interaction between DMA and the choline carrier. DMA does not compete with sodium ions for its effect on the choline carrier. It is suggested that the choline carrier of Ehrlich ascites tumor cells exhibits a binding site for DMA similar to the one on the Na+/H+ antiporter.

Plasma membrane as target of alkylating agents

N mustard resistant Walker cells exhibit the same frequency of DNA interstrand cross-links and the same rate of cross-link removal as the sensitive parental line. Employing cytostatically active concentrations of chlorambucil covalently bound to polyethyleneimine, the extent of DNA cross-linking is reduced to levels observed in the presence of nontoxic concentrations of free chlorambucil. It is concluded, therefore, that DNA cross-links alone are not sufficient to explain the inhibition of cell multiplication by alkylating agents and that additional mechanisms have to be considered. Evidence for an interference of alkylating agents with several enzymes of the plasma membrane is presented. An inhibition by N mustard of the furosemide-sensitive Na+/K+/Cl- -cotransport and the Na+/H+-antiport is described in greater detail. Considering the fact that the enzymes which are affected by alkylating agents are controlled by growth factors it was investigated whether a synergism between inhibitors of early growth-factor-controlled reactions and alkylating agents is to be seen. It is demonstrated that mepacrine, an inhibitor of phospholipase C, and the calmodulin binding drugs, chlorpromazine and flunarizine, amplify the action of N mustard.