Approach to the chemotopography of the digitalis recognition matrix in Na+/K(+)-transporting ATPase as a step in the rational design of new inotropic steroids

Inhibition of the Na+/K+-ATPase by cardiac glycosides suppresses expression of the IDO1 immune checkpoint in cancer cells by reducing STAT1 activation

Despite extensive basic and clinical research on immune checkpoint regulatory pathways, little is known about the effects of the ionic tumor microenvironment on immune checkpoint expression and function. Here we describe a mechanistic link between Na+/K+-ATPase (NKA) inhibition and activity of the immune checkpoint protein indoleamine-pyrrole 2',3'-dioxygenase 1 (IDO1). We found that IDO1 was necessary and sufficient for production of kynurenine, a downstream tryptophan metabolite, in cancer cells. We developed a spectrophotometric assay to screen a library of 31 model ion transport-targeting compounds for potential effects on IDO1 function in A549 lung and MDA-MB-231 breast cancer cells. This revealed that the cardiac glycosides ouabain and digoxin inhibited kynurenine production at concentrations that did not affect cell survival. NKA inhibition by ouabain and digoxin resulted in increased intracellular Na+ levels and downregulation of IDO1 mRNA and protein levels, which was consistent with the reduction in kynurenine levels. Knockdown of ATP1A1, the ɑ1 subunit of the NKA and target of cardiac glycosides, increased Na+ levels to a lesser extent than cardiac glycoside treatment and did not affect IDO1 expression. However, ATP1A1 knockdown significantly enhanced the effect of cardiac glycosides on IDO1 expression and kynurenine production. Mechanistically, we show that cardiac glycoside treatment resulted in curtailing the length of phosphorylation-mediated stabilization of STAT1, a transcriptional regulator of IDO1 expression, an effect enhanced by ATP1A1 knockdown. Our findings reveal cross talk between ionic modulation via cardiac glycosides and immune checkpoint protein expression in cancer cells with broad mechanistic and clinical implications.

Identification and characterization of novel sodium/potassium-ATPase inhibitors by virtual screening of a compound database

The medicinal value of cardiac glycoside inhibitors for the treatment of congestive heart failure symptoms stems from their ability to specifically inhibit the ion transport activity of the transmembrane enzyme sodium/potassium-ATPase (Na/K-ATPase) in myocardial cells. In this study, we used the inhibitory potencies of 39 cardiac glycoside analogues for the development of a quantitative structure-activity relationship (QSAR) model for Na/K-ATPase inhibition. In conjunction with a substructure and similarity search, the QSAR model was used to select ten potential inhibitors from a commercial compound database. The inhibitory potencies of these compounds were measured and four were found to be more active than the commonly used inhibitor ouabain. The results of the bioassays were incorporated into a second QSAR model, whose physical interpretation suggested that the nature of substituents in positions 10, 12, and 17 at the cyclopentanoperhydrophenanthrene core of the inhibitors was critical for enzyme inhibition. All descriptors of the QSAR models were conformation-independent, making the search protocol a suitable tool for the rapid virtual screening of large compound databases for novel inhibitors.

Cardenamides from Cardenolides: Cardiac and Anticancer Activities

Two ways are shown to transform cardioactive cardenolides into cardenamides [17β-(5-oxo-2,5-dihydropyrrol-3-yl)-5β,14β-androstane derivatives], and their derivatives by replacement of their ring oxygen by N-R. Cardioactivity is strongly decreased by this transformation. The comparatively easily accessible 21-oxocardenamides [17β-(2-maleimidyl)steroids] are strongly thiol reactive and show remarkable anticancer activity.

A hybrid between Na+,K+-ATPase and H+,K+-ATPase is sensitive to palytoxin, ouabain, and SCH 28080

Na(+),K(+)-ATPase is inhibited by cardiac glycosides such as ouabain, and palytoxin, which do not inhibit gastric H(+),K(+)-ATPase. Gastric H(+),K(+)-ATPase is inhibited by SCH28080, which has no effect on Na(+),K(+)-ATPase. The goal of the current study was to identify amino acid sequences of the gastric proton-potassium pump that are involved in recognition of the pump-specific inhibitor SCH 28080. A chimeric polypeptide consisting of the rat sodium pump alpha3 subunit with the peptide Gln(905)-Val(930) of the gastric proton pump alpha subunit substituted in place of the original Asn(886)-Ala(911) sequence was expressed together with the gastric beta subunit in the yeast Saccharomyces cerevisiae. Yeast cells that express this subunit combination are sensitive to palytoxin, which interacts specifically with the sodium pump, and lose intracellular K(+) ions. The palytoxin-induced K(+) efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proton pump-specific inhibitor SCH 28080. The IC(50) for SCH 28080 inhibition of palytoxin-induced K(+) efflux is 14.3 +/- 2.4 microm, which is similar to the K(i) for SCH 28080 inhibition of ATP hydrolysis by the gastric H(+),K(+)-ATPase. In contrast, palytoxin-induced K(+) efflux from cells expressing either the native alpha3 and beta1 subunits of the sodium pump or the alpha3 subunit of the sodium pump together with the beta subunit of the gastric proton pump is inhibited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensitivity by the chimera indicates that the Gln(905)-Val(930) peptide of the gastric proton pump is likely to be involved in the interactions of the gastric proton-potassium pump with SCH 28080.

Synthesis and inotropic activity of hydroindene derivatives

A synthetic approach to hydroindenic inotropic agents has been developed, starting from enantiopure Hajos-Parrish (1). Hajos-Wiechert (2), and related diketones. Their transformation into C-1 formyl derivatives and other subsequent synthetic targets is described. The results of the thermodynamic equilibration between both epimers of each formyl derivative are analysed. The inotropic activities of selected compounds on right and left atrial preparations are also evaluated and discussed.

Partial synthetic derivatization of canrenone and characterization of its impact on the inhibitory effect on Na+/K(+)-ATPase activity in human heart muscle

To improve the weak inhibitory effect of 3-oxo-17 alpha-pregna-4,6-diene-21,17-carbolactone (canrenone, II) on Na+/K(+)-ATPase activity in human heart muscle, we have investigated the impact of hydrogenation, reduction, glycosidation, and the introduction of a 3-sulfonamido residue on the inhibitory potency of canrenone. The greatest increase in potency (> 20 times) was found for 3 beta-(alpha-L-rhamnopyranosyloxy)-5 beta, 17 alpha-pregnane-21, 17-carbolactone (IX). The 3-O-glycosides IX-XI are the first representatives of C/D-trans steroids with effector-receptor complex decay half-times longer than those of therapeutically used cardenolides.

Reinstatement of the ATP high energy paradigm

The paradigm that the hydrolysis of ATP releases high Gibbs energy able to perform work has increasingly been questioned over the last two decades. Results from theoretical and experimental studies have been interpreted to indicate that the synthesis of ATP from ADP and P(i) does not require energy supply and that binding of ATP per se can transmit utilizable energy to an enzyme. As has recently been concluded, all this has led to a change of the ATP high energy paradigm in bioenergetics. Starting from this challenge, the present review singles out the striking sources of the apparent dichotomy in bioenergetics, and endeavours to eliminate the apparent contradictions by the application of the prior knowledge on both the participation of the enzyme protein in energy exchange processes and the particular reactivities of phosphorus that make it an outstanding element for functionally variable work assignments in enzymatic systems.

Affinity labeling of a sulfhydryl group in the cardiacglycoside receptor site of Na+/K(+)-ATPase by N-hydroxysuccinimidyl derivatives of digoxigenin

Na+/K(+)-ATPase from pig kidney is inactivated by protein-reactive N-hydroxysuccinimidyl derivatives of digoxigenin. Like digoxigenin, its protein-reactive derivatives N-hydroxysuccinimidyl digoxigenin-3-methylcarbonyl-epsilon-aminocaproate (HDMA), 3-amino-3-deoxydigoxigenin hemisuccinimide succinimidyl ester (ADHS), 3-iodoacetylamino-3-deoxydigoxigenin (IAD) and digoxigenin-3-O-succinyl-[2-(N-maleimido)]ethylamide (DSME) inhibited the sodium pump in the presence of Na+, Mg2+ and ATP. At 37 degrees C, half-maximal inhibition of Na+/K(+)-ATPase was seen by HDMA at 0.47 microM, by ADHS at 5.8 microM, by IAD at 8 microM and by DSME at 94 microM. Thus, all compounds bind to the cardiac steroid receptor site of Na+/K(+)-ATPase. Affinity labeling of the alpha subunit by 'front door' or 'back door' phosphorylation was only seen with HDMA or ADHS in the range 0.1 microM. Excess of ouabain protected against affinity labeling. All the other protein-reactive derivatives of digoxigenin labeled the enzyme independent of the formation of a phosphointermediate at much higher concentrations. This labeling was not suppressed by an excess of ouabain. Tryptic hydrolysis of the HDMA-modified Na+/K(+)-ATPase gave peptides of the apparent molecular masses 20, 12.5 and 11.2 kDa. The 11.2-kDa and 12.5-kDa peptides started amino-terminally with Asp68, and the 20-kDa peptide with Asp24. Thus, the HDMA-labeled peptides originate from the cardioactive steroid-binding site formed by the first and second transmembrane helix. N-Hydroxysuccinimidyl esters such as HDMA are normally thought to modify lysine and arginine residues covalently. Since such residues do not exist in the putative cardiac glycoside-binding site, the possibility of a thioester formation of the digoxigenin derivatives HDMA and ADHS with Cys104 in the H1 transmembrane domain was tested. In fact, hydroxylaminolysis led to the release of the covalently bound HDMA, and the formation of a free sulfhydryl group. This could be labeled by [2-14C]ICH2COOH. We therefore propose, consistent with a recent conclusion from a site-directed mutagenesis experiment [Canessa, C. M., Horisberger, J.-D., Louvard, D. & Rossier, B. C. (1992) EMBO J. 11, 1681-1687], that a cysteine residue (probably Cys104) participates in the structure and function of the cardiac glycoside binding.