Biochemical and cross-resistance studies with HeLa cell mutants resistant to cardiac glycoside SC4453. Regulation of the resistant form of Na+/K+-ATPase in the mutant cells

Antiproliferative activity of mammalian lignan derivatives against the human breast carcinoma cell line, ZR-75-1

The effect of each of twelve mammalian lignan derivatives on the growth of human mammary tumor ZR-75-1 cells was examined. At a concentration less than 10 micrograms/ml, tumor cell growth was inhibited from 18-68%. The effect of 2,3-dibenzylbutane-1,4-diol(hattalin) was found to be strongest, inhibiting growth by 50% at a concentration (EC50) of 2.1 micrograms/ml. Hattalin inhibited membrane Na+, K(+)-ATPase of canine kidney cortex. It also inhibited the ATPase of the plasma membrane fraction from both cultured cells and a section of human breast cancer tissue at a concentration ranging from 0.5 to 2.0 mM. However, only a few percent of membrane ATPase from either ZR-75-1 cells or breast carcinoma tissue was inhibited by 2.0 mM of ouabain, suggesting that the target ATPase of hattalin was other than ouabain-sensitive ATPase. The relative incorporation of [3H]thymidine per 1 x 10(5) cells into the acid-precipitable fraction of ZR-75-1 cells was not affected by 1-50 micrograms/ml of hattalin, while a marked decrease resulted from 1-10 micrograms/ml of 5-fluorouracil (5-FU). These results suggest that the suppressive effect of hattalin on tumor cell growth may not occur through inhibition of DNA synthesis but rather partly by inhibition of the plasma membrane ATPase other than Na+ and K(+)-dependent ones.

Development of a specific assay for cardiac glycoside-like compounds based on cross-resistance of human cell mutants

The cross-resistance patterns of single- and two-step mutants of HeLa cells resistant to SC4453 (a digoxin analog) and digoxin, which involve specific alteration in Na+, K+-ATPase towards numerous other compounds, have been examined. The mutants exhibited increased resistance to all of the steroidal compounds known to elicit a digitalis-like positive inotropic response (viz. various cardiac glycosides and their genins, erythrophleum alkaloid cassaine), but they showed no cross-resistance to any of a large number of other compounds which do not show cardiac glycoside (CG)-like biological activity. Based on the above characteristics of the mutants, a new cross-resistance assay for identifying compounds that show CG-like activity has been developed. In this assay, a sample is considered to possess CG-like activity if, in comparison to the parental HeLa cells, the CGR mutants exhibit increased resistance to it. From the known D10 value (drug concentration which reduces cloning efficiency of cells to 10%) of the drug for HeLa cells and the sample dilution necessary to produce equivalent cytotoxicity, the concentration of CGs in a given sample can be estimated. In blind studies the assay correctly identified all of the samples containing CGs; none of the other samples which lacked such activity tested positive. In the blind studies the assay also provided a good estimate of the concentration of CGs (+/- 50% of the actual concentration) that was not affected by the presence of either serum components or a 20-fold excess of various steroidal compounds known to interfere in other assays. In view of the high specificity of the present assay for CG-like compounds, it should prove very useful in establishing/characterizing the presence of such activity in various biological (namely endogenous digitalis-like substances) and other samples.

Human cell mutants affected in the interaction of the 12 beta-OH group of cardiac glycosides with the digitalis receptor

The cross-resistance patterns of two different types of mutants of HeLa cells selected for resistance to the digoxin analog SC4453 (SCR mutants) in which the Na+/K+-ATPase is affected [A. Chopra and R. S. Gupta, J. biol. Chem. 261, 2034 (1986)], and towards numerous cardiac glycosides (CGs) and genins, were examined. One type of SCR mutant (designated as group C) was highly resistant to all CGs and genins investigated. In contrast, the other type of SCR mutant (group D) showed a high degree of cross-resistance towards selected CG derivatives (viz. digoxin, SC4453, digoxigenin, lanatoside C, alpha- and beta-methyldigoxin, dihydrodigoxin, alpha- and beta-acetyldigoxin, alpha,beta-diacetyldigoxin), all of which contained a free 12 beta-OH group in the steroid structure. Slight cross-resistance of the group D mutants was also observed for other compounds (viz. ouabain, ouabagenin, dihydroouabain) that contain a free 11 alpha-OH group in the molecule. However, these mutants exhibited no cross-resistance to other CG derivatives, which either lacked the above groups (viz. digitoxin, digitoxigenin, dihydrodigitoxin, digitoxigenin mono- and bisdigitoside, nerifolin, gitoxigenin, gitoxin, 16-acetylgitoxin, lanatosides A and B, cymarin, convallatoxin, oleandrin, strophanthidin, actodigin and bufalin) or in which the 12 beta-OH group was acetylated (viz. as in the case of 12-acetyldigoxin). Since the 12 beta-OH group is not required for CG-like activity, to account for these observations it is suggested that the genetic lesion in the group D mutant leads to the creation of a new binding site in the digitalis receptor, which specifically interacts with the 12 beta-OH group (the site presumably also interacts weakly with the 11 alpha-OH group) and either prevents or distorts the binding of the compounds to the drug binding site on the receptor. Further investigations with the different classes of CG-resistant mutants at the molecular level should prove very useful in identifying the drug receptor site and in understanding how these drugs interact with it.

Ouabain-resistant mutants of Chinese hamster ovary cells are not directly affected in Na+, K+-ATPase

The nature of biochemical lesions in a number of independent single-step ouabain-resistant (OuaR) mutants of Chinese hamster ovary cells has been examined. The cellular uptake of 86Rb in the mutant cells was found to be resistant to inhibition by ouabain and correlated with the degree of resistance of the mutants to the drug. However, the plasma membrane Na+, K+-ATPase from all five of the OuaR mutants examined showed similar sensitivity to inhibition by ouabain as seen for the enzyme from the parental sensitive cells. These results provide evidence that, contrary to the general assumption, OuaR mutants of Chinese hamster ovary cells, which are widely used for genetic and quantitative mutagenesis studies, are not directly affected in the Na+, K+-ATPase.