New developments in cardiac glycoside structure-activity relationships

Egg toxic compounds in the animal kingdom. A comprehensive review

Covering: 1951 to 2022Packed with nutrients and unable to escape, eggs are the most vulnerable stage of an animal's life cycle. Consequently, many species have evolved chemical defenses and teamed up their eggs with a vast array of toxic molecules for defense against predators, parasites, or pathogens. However, studies on egg toxins are rather scarce and the available information is scattered. The aim of this review is to provide an overview of animal egg toxins and to analyze the trends and patterns with respect to the chemistry and biosynthesis of these toxins. We analyzed their ecology, distribution, sources, occurrence, structure, function, relative toxicity, and mechanistic aspects and include a brief section on the aposematic coloration of toxic eggs. We propose criteria for a multiparametric classification that accounts for the complexity of analyzing the full set of toxins of animal eggs. Around 100 properly identified egg toxins are found in 188 species, distributed in 5 phyla: cnidarians (2) platyhelminths (2), mollusks (9), arthropods (125), and chordates (50). Their scattered pattern among animals suggests that species have evolved this strategy independently on numerous occasions. Alkaloids are the most abundant and widespread, among the 13 types of egg toxins recognized. Egg toxins are derived directly from the environment or are endogenously synthesized, and most of them are transferred by females inside the eggs. Their toxicity ranges from ρmol kg^-1 to mmol kg^-1, and for some species, experiments support their role in predation deterrence. There is still a huge gap in information to complete the whole picture of this field and the number of toxic eggs seems largely underestimated.

Transcriptional profile and differential fitness in a specialist milkweed insect across host plants varying in toxicity

Interactions between plants and herbivorous insects have been models for theories of specialization and co-evolution for over a century. Phytochemicals govern many aspects of these interactions and have fostered the evolution of adaptations by insects to tolerate or even specialize on plant defensive chemistry. While genomic approaches are providing new insights into the genes and mechanisms insect specialists employ to tolerate plant secondary metabolites, open questions remain about the evolution and conservation of insect counterdefences, how insects respond to the diversity defences mounted by their host plants, and the costs and benefits of resistance and tolerance to plant defences in natural ecological communities. Using a milkweed-specialist aphid (Aphis nerii) model, we test the effects of host plant species with increased toxicity, likely driven primarily by increased secondary metabolites, on aphid life history traits and whole-body gene expression. We show that more toxic plant species have a negative effect on aphid development and lifetime fecundity. When feeding on more toxic host plants with higher levels of secondary metabolites, aphids regulate a narrow, targeted set of genes, including those involved in canonical detoxification processes (e.g., cytochrome P450s, hydrolases, UDP-glucuronosyltransferases and ABC transporters). These results indicate that A. nerii marshal a variety of metabolic detoxification mechanisms to circumvent milkweed toxicity and facilitate host plant specialization, yet, despite these detoxification mechanisms, aphids experience reduced fitness when feeding on more toxic host plants. Disentangling how specialist insects respond to challenging host plants is a pivotal step in understanding the evolution of specialized diet breadths.

A structural view on the functional importance of the sugar moiety and steroid hydroxyls of cardiotonic steroids in binding to Na,K-ATPase

The Na,K-ATPase is specifically inhibited by cardiotonic steroids (CTSs) like digoxin and is of significant therapeutic value in the treatment of congestive heart failure and arrhythmia. Recently, new interest has arisen in developing Na,K-ATPase inhibitors as anticancer agents. In the present study, we compare the potency and rate of inhibition as well as the reactivation of enzyme activity following inhibition by various cardiac glycosides and their aglycones at different pH values using shark Na,K-ATPase stabilized in the E2MgPi or in the E2BeFx conformations. The effects of the number and nature of various sugar residues as well as changes in the positions of hydroxyl groups on the β-side of the steroid core of cardiotonic steroids were investigated by comparing various cardiac glycoside compounds like ouabain, digoxin, digitoxin, and gitoxin with their aglycones. The results confirm our previous hypothesis that CTS binds primarily to the E2-P ground state through an extracellular access channel and that binding of extracellular Na(+) ions to K(+) binding sites relieved the CTS inhibition. This reactivation depended on the presence or absence of the sugar moiety on the CTS, and a single sugar is enough to impede reactivation. Finally, increasing the number of hydroxyl groups of the steroid was sterically unfavorable and was found to decrease the inhibitory potency and to confer high pH sensitivity, depending on their position on the steroid β-face. The results are discussed with reference to the recent crystal structures of Na,K-ATPase in the unbound and ouabain-bound states.

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na⁺/K⁺-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.

Na,K-ATPase characterized in artificial membranes. 1. Predominant conformations and ion-fluxes associated with active and inhibited states

The Na,K-ATPase (NKA) system is the receptor for the cardioactive steroids of plant or animal origin. It is not yet known whether passive ion fluxes traverse the inactivated receptor and thereby contribute to the hormonal, pharmacological or toxic actions of these compounds. To look for putative passive ion-fluxes across the ouabain-NKA complex, we incorporated it into the artificial membrane of liposomes. Since this synthetic membrane is virtually impermeable to Na and K ions, the hypothetical ion-fluxes mediated by the NKA can be determined. E2-forms and E2-ouabain-forms of purified NKA were incorporated, in parallel, into separate liposome preparations and the permeability of the resulting E2-liposomes and E2-ouabain-liposomes to K, Na and Ca ions was compared. The E2-liposomes expressed a typical K-permeability which was not observed in the E2-ouabain-liposomes; the latter showed a slightly higher Na-permeability and a similar Ca-permeability as compared to the former. Thus, ouabain does not induce leaks for K or Ca ions in the NKA molecule.

Lipophilic character of cardiac glycosides: correlation between RM values and acute toxicity data in different animal species

The RM for a new series of cardiac glycosides were calculated by means of some of the delta RM values previously derived from another series of compounds. The experimental or calculated RM values of both series of derivatives were correlated with the acute toxicity data (log 1/C). The slopes of the linear equations for cats, dogs, guinea-pigs and frogs are very close, showing that the dependence of toxicity on the lipophilic character is the same in these animal species.

14 beta-Hydroxyprogesterone binds to the digitalis receptor, inhibits the sodium pump and enhances cardiac contractility

1. Certain derivatives of progesterone are potent inhibitors of high affinity, specific binding of 3H-cardiac glycosides. The steroids interact at the cardiac glycoside site on Na,K-ATPase and inhibit the enzyme (the sodium pump) in cardiac and other tissues. However, the active congeners identified previously have been, unlike the cardiac glycosides, predominantly cardiodepressant. 2. Because a 14 beta-hydroxy substituent is an important determinant of activity of the cardiotonic cardiac glycosides, we synthesized 14 beta-hydroxyprogesterone. This derivative has about one-tenth the potency of the aglycone, ouabagenin, in a [3H]-ouabain binding assay. 3. Like ouabagenin, but in contrast to the cardiodepressant congeners of progesterone, 14 beta-hydroxyprogesterone consistently elicited positive inotropy in isolated cardiac muscle and enhanced both the magnitude and frequency of fluctuations in scattered light (an index of oscillatory intracellular release of calcium). 4. Thus, at least one hydroxylated derivative (and putative endogenous metabolite) of progesterone, mimics the cardiac effects of cardiac glycosides including enhanced contractility.

Structure-activity relationships of progesterone derivatives that bind to the digitalis receptor: modifications in A and B rings

A number of progesterone derivatives, having a 17 alpha-acetoxy group and various functions at C-3 and C-6, interact at the cardiac glycoside (CG) binding site, using [3H]ouabain in a radioligand binding assay (RBA) with membranes from dog myocardium. We now report on results of structure-activity studies concerned with modification of the A and B rings as they influence potency in the RBA. Some progesterone derivatives with 5 alpha- or 5 beta-stereochemistry show weak receptor competing activity. Among the congeners highest potency is associated with the presence of C-4 or C-4,6 unsaturation and a C-6 substituent (CH3, Cl, Br) whose importance appears to reside in its steric rather than electronic character. The C-3 function may be carbonyl, 3 beta-hydroxy or 3 beta-acetoxy when associated with C-4 or C-4,6 unsaturation. In compounds with other substituents that promote activity, C-6 alpha substitution with -CH3, -Cl, or -Br strongly enhances activity; -F, -OCH3, carbonyl, or the unsubstituted compound promotes weak binding; and -OC2H5, -OAc, -OCOOCH3, or -OH eliminates binding activity. Receptor interaction with the double bond at C-4, but not C-5, appears to be particularly important for binding. The most potent analog identified thus far is chlormadinone acetate (17 alpha-acetoxy-6-chloropregna-4,6-diene-3,20-dione), which has 1/20 the potency of ouabain in the RBA. Studies to determine optimal structural requirements for CG-receptor binding by these hormonal steroid congeners, in conjunction with appropriate biological assays, may provide insight into the nature of a putative endogenous counterpart, lead to a better understanding of the mode of action of the CG and yield CG-like compounds with superior therapeutic properties.

Origin of differences of inhibitory potency of cardiac glycosides in Na+/K+-transporting ATPase from human cardiac muscle, human brain cortex and guinea-pig cardiac muscle

The inhibitory potency of altogether 95 steroidal compounds (including cardenolides, bufadienolides and their glycosides) on the Na/K-ATPases (Na+/K+-transporting ATPases, EC 3.6.1.37) from human cardiac muscle, human brain cortex and guinea-pig cardiac muscle was compared to probe the complementary chemotopology of the inhibitor binding site areas on the three enzyme variants. The changes of potency, resulting from systematic variations of the geometry of steroid skeleton and the character as well as the structure of side chains at C3 or/and C17 of steroid backbone, allowed the following major conclusions. With the human cardiac and cerebral enzyme forms, the paired K0.5 (K'D) values for 77 steroid derivatives, covering seven orders of ten, were highly correlated. On an average, the total of compounds showed a 1.5-fold higher affinity to the cardiac enzyme. This tiny differentiation did not appear to be connected with an important difference in the chemotopology of the complementary subsites for steroid nucleus binding on the two enzyme forms. With the human and guinea-pig cardiac enzyme variants, the K0.5 values for 69 steroid derivatives, covering six orders of ten, were determined. For 41 5 beta, 14 beta-androstane derivatives only, the paired K0.5 values showed a close correlation. Here, the human enzyme variant exhibited 27-fold higher affinity. However, the paired K0.5 values determined on both enzymes for 28 steroid derivatives of differing structural features were but poorly correlated. Essentially, the geometries of the steroid nucleus determined the differential contributions of the side chains at C3 and C17 to the integral inhibitory potency on the two enzyme variants. Thus, the species differences in the potency of cardiac glycosides were traced to species differences in the complementarity of the steroid binding subsites. Hence, estimates of the potency of new steroidal compounds obtained on the guinea-pig cardiac enzyme can be neither quantitatively nor qualitatively easily extrapolated to the human cardiac enzyme. The extrathermodynamic analysis of the data opened major new insights in the structure-activity relationships concerning the role of C14 beta-OH, the character of the lead structure in cardioactive steroid lactones, and the significance of the configuration of A/B ring junction.