Plasma membrane as target of alkylating agents

Mucocutaneous cholinergic system is targeted in mustard-induced vesication

The purpose of this research is to elucidate the pharmacological mechanism mediating vesicating effects of sulfur mustard (HD) and identify an antidote to its action. HD causes blisters because epithelial cells lose their attachments. Epithelial cell adhesion is under control of the local cytotransmitter acetylcholine (ACh) working through the muscarinic and nicotinic receptor, mAChR and nAChR, classes expressed by epithelial cells. In this study, nitrogen mustard (NM)-a structural analog of HD-was used to elucidate the mechanism of vesicating effects of mustards in mucocutaneous tissues. NM caused cell detachment and cholinergic agents antagonized its effect. Radioligand binding inhibition experiments showed that NM binds to the ligand-binding site of ACh receptors (AChRs) of both classes. Ligation of AChRs on the cell membrane of keratinocytes (KC) and bronchial epithelial cells (BEC) with NM increased total esterolytic activity of serine proteinases (TEASP). Antagonists of both classes of AChRs, atropine and mecamylamine, diminished NM-induced changes, suggesting that the pathobiological effects of NM on KC and BEC result from an agonist-like degradation of ligated AChRs, predominantly of the muscarinic class. Thus, biological effects of NM on cell adhesion were antagonist-like, whereas its pharmacological effect on TEASP was agonist-like. These findings support a hypothesis that pharmacologic protection from the vesicating action of HD can be achieved by using cholinergic drugs.

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

Inhibition of Na(+)-K(+)-2Cl(-) cotransport by mercury

Mercury alters the function of proteins by reacting with cysteinyl sulfhydryl (SH(-)) groups. The inorganic form (Hg(2+)) is toxic to epithelial tissues and interacts with various transport proteins including the Na(+) pump and Cl(-) channels. In this study, we determined whether the Na(+)-K(+)-Cl(-) cotransporter type 1 (NKCC1), a major ion pathway in secretory tissues, is also affected by mercurial substrates. To characterize the interaction, we measured the effect of Hg(2+) on ion transport by the secretory shark and human cotransporters expressed in HEK-293 cells. Our studies show that Hg(2+) inhibits Na(+)-K(+)-Cl(-) cotransport, with inhibitor constant (K(i)) values of 25 microM for the shark carrier (sNKCC1) and 43 microM for the human carrier. In further studies, we took advantage of species differences in Hg(2+) affinity to identify residues involved in the interaction. An analysis of human-shark chimeras and of an sNKCC1 mutant (Cys-697-->Leu) reveals that transmembrane domain 11 plays an essential role in Hg(2+) binding. We also show that modification of additional SH(-) groups by thiol-reacting compounds brings about inhibition and that the binding sites are not exposed on the extracellular face of the membrane.

Synthesis and in vitro efficacy of transferrin conjugates of the anticancer drug chlorambucil

One strategy for improving the selectivity and toxicity profile of antitumor agents is to design drug carrier systems employing soluble macromolecules or carrier proteins. Thus, five maleimide derivatives of chlorambucil were bound to thiolated human serum transferrin which differ in the stability of the chemical link between drug and spacer. The maleimide ester derivatives 1 and 2 were prepared by reacting 2-hydroxyethylmaleimide or 3-maleimidophenol with the carboxyl group of chlorambucil, and the carboxylic hydrazone derivatives 5-7 were obtained through reaction of 2-maleimidoacetaldehyde, 3-maleimidoacetophenone, or 3-maleimidobenzaldehyde with the carboxylic acid hydrazide derivative of chlorambucil. The alkylating activity of transferrin-bound chlorambucil was determined with the aid of 4-(4-nitrobenzyl)pyridine (NBP) demonstrating that on average 3 equivalents were protein-bound. Evaluation of the cytotoxicity of free chlorambucil and the respective transferrin conjugates in the MCF7 mammary carcinoma and MOLT4 leukemia cell line employing a propidium iodide fluorescence assay demonstrated that the conjugates in which chlorambucil was bound to transferrin through non-acid-sensitive linkers, i.e., an ester or benzaldehyde carboxylic hydrazone bond, were not, on the whole, as active as chlorambucil. In contrast, the two conjugates in which chlorambucil was bound to transferrin through acid-sensitive carboxylic hydrazone bonds were as active as or more active than chlorambucil in both cell lines. Especially, the conjugate in which chlorambucil was bound to transferrin through an acetaldehyde carboxylic hydrazone bond exhibited IC50 values which were approximately 3-18-fold lower than those of chlorambucil. Preliminary toxicity studies in mice showed that this conjugate can be administered at higher doses in comparison to unbound chlorambucil. The structure-activity relationships of the transferrin conjugates are discussed with respect to their pH-dependent acid sensitivity, their serum stability, and their cytotoxicity.

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.

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.

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.

Alkylating antitumor agents reduce histone acetyl-transferase activity

N-Mustard depresses the acetylation of histones in Ehrlich ascites and Walker carcinoma cells. It is demonstrated that this effect is not caused by an accelerated deacetylation but is due to an inhibition of the acetyl-transferase reaction. Employing 4-sulphonatoethylthio-cyclophosphamide it is demonstrated that the alkylating agent affects predominantly the acetylation of a chromatin fraction which is soluble in 0.1M NaCl after digestion with micrococcal nuclease. After removal of the alkylating agent, the recovery of histone acetylation is relatively slow and--in contrast to the repair of DNA cross-links--characterized by a 4-hr lag period. The reduction of histone acetylation by N-mustard is much less expressed in cells which are resistant to the drug than in the sensitive parental lines. This is in contrast to DNA-interstrand cross-links in Walker cells where both N-mustard sensitive and resistant cells inhibit the same cross-link frequency and identical repair rates. Based on these data it is concluded that the inhibition of histone acetylation may be an important part of the mechanism by which alkylating agents inhibit tumor growth.