Mutants of HeLa cells resistant to ouabain and cassaine: genetic evidence for the common site of action of cardiac glycosides and erythrophleum alkaloids

Five new cassane diterpenes from the seeds and bark of Erythrophleum suaveolens

Five new cassane-type diterpenoid heterosides, i. e. two cassane-type amides (1-2), two erythrophlamine-type amine esters (3-4) and a non‑nitrogenous erythrophlamine analogue (5) were isolated from the root barks (1-2) and the seeds (3-5) of Erythrophleum suaveolens. Their structures were unambiguously established by interpretation of their HRESIMS, 1D and 2D NMR data, and chemical degradation for sugar determination. Compounds 3-5 were evaluated for their cytotoxicity against a panel of three cell lines, revealing modest to strong activities.

Progress on Cassaine-Type Diterpenoid Ester Amines and Amides (Erythrophleum Alkaloids)

The structures, spectral characteristics, and bioactivities of 39 natural cassaine-type diterpenoid ester amines and amides (Erythrophleum alkaloids) and 31 synthetic analogues are reviewed. Cassaine-type diterpenoid ester amines and amides, the so called Erythrophleum alkaloids, have the skeleton of cassane-type diterpenoids with a N-containing side chain, and are classified into two groups, ester amines and amides. Cassaine-type diterpenoid ester amines and amides show remarkable inotropic action on the heart, inhibition of Na+/K+-ATPase, cytotoxities, and other major bioactivities. Structural modification of cassaine-type diterpenoid ester amines and amides has been carried out to furnish many derivatives to study the structure-activity relationships.

Chapter 1 Allelochemical Properties or the Raison D'être of Alkaloids

This chapter provides evidence that alkaloids are not waste products or functionless molecules as formerly assumed, but rather defense compounds employed by plants for survival against herbivores and against microorganisms and competing plants. These molecules were developed during evolution through natural selection in that they fit many important molecular targets, often receptors, of cells, which are seen in molecules that mimic endogenous neurotransmitters. The chapter discusses that microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. Many herbivores have potent mechanisms to detoxify xenobiotics, which allow the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. It needs more experimental data to understand fully the intricate interconnections between plants, their alkaloids, and herbivores, microorganisms, and other plants.

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.

Cellular basis for the species differences in sensitivity to cardiac glycosides (digitalis)

The relative toxicity of numerous cardiotonic steroids (viz. ouabain, digitoxin, digoxin, convallatoxin, SC4453, bufalin, gitaloxin, digoxigenin, actodigin, oleandrin, digitoxigenin, gitoxin, strophanthidin, gitoxigenin, lanatosides A, B and C, alpha- and beta-acetyl digoxin, alpha- and beta-methyl digoxin) and related compounds towards a number of independent cell lines established from human, monkey, mouse, Syrian hamster, and Chinese hamster have been determined. All cardiac glycosides and their genins, as well as the cardiotonic alkaloid cassaine, exhibited greater than 100-fold higher toxicity towards cultured human and monkey cells in comparison to the cell lines of mouse, Syrian hamster, and Chinese hamster origins. These differences are species-related as all cell lines (both normal as well as transformed) from any one species, as well as cells from the closely related species (e.g., man and monkey or mouse, Chinese hamster, and Syrian hamster), showed similar sensitivity towards these drugs. The failure to see any significant differences in cellular toxicity for a larger number of other compounds which either bear limited structural resemblance to cardiac glycosides (viz. estradiol 17-beta-acetate, testosterone propionate, 21-acetoxy pregnenolone, beta-estradiol, digitonin, tigogenin, and tomatine) or interact with the Na+/K+ ATPase in a different manner (viz. veratridine, sanguinarine nitrate, penicillic acid, vanadium pentoxide, harmaline-HCI,5,5'-diphenyl hydantoin, quindonium bromide, and methyl quinolizinum bromide) provides strong evidence that the observed species-related differences are highly specific for cardiotonic steroids. Studies on the binding of [3H]ouabain show that, in comparison to human and monkey cell lines, no significant binding of the drug is observed in cells derived from the resistant species (i.e., mouse and Chinese hamster). The Na+/K+ ATPase from cells of the resistant species is inhibited at much higher concentrations of ouabain and digitoxin in comparison to the enzyme from human cells, and a good correlation is observed between these concentrations and those reported for inhibition of the enzyme from isolated heart muscles of the same species. These results provide strong evidence that the species-related differences in sensitivity to digitalis have a cellular basis and that the cultured cells from various mammalian species provide a useful model system for investigating the mechanism of action of cardiac glycosides.