Na+/K+-ATPase Revisited: On Its Mechanism of Action, Role in Cancer, and Activity Modulation

Abstract

Maintenance of Na⁺ and K⁺ gradients across the cell plasma membrane is an essential process for mammalian cell survival. An enzyme responsible for this process, sodium-potassium ATPase (NKA), has been currently extensively studied as a potential anticancer target, especially in lung cancer and glioblastoma. To date, many NKA inhibitors, mainly of natural origin from the family of cardiac steroids (CSs), have been reported and extensively studied. Interestingly, upon CS binding to NKA at nontoxic doses, the role of NKA as a receptor is activated and intracellular signaling is triggered, upon which cancer cell death occurs, which lies in the expression of different NKA isoforms than in healthy cells. Two major CSs, digoxin and digitoxin, originally used for the treatment of cardiac arrhythmias, are also being tested for another indication-cancer. Such drug repositioning has a big advantage in smoother approval processes. Besides this, novel CS derivatives with improved performance are being developed and evaluated in combination therapy. This article deals with the NKA structure, mechanism of action, activity modulation, and its most important inhibitors, some of which could serve not only as a powerful tool to combat cancer, but also help to decipher the so-far poorly understood NKA regulation.

Keywords: Na+/K+-ATPase activity modulation; anticancer activity; cardiac glycosides; combination therapy; digitoxigenin; digitoxin; digoxin; natural compounds; ouabain; sodium-potassium pump inhibitors.

Figure 1

Structure of Na⁺/K⁺-ATPase (PDB, 3A3Y; [22]) with bound ouabain (green) in a molecular surface and cartoon view mode. Subunits are color-coded: Magenta (α subunit), cyan (β subunit), and orange (FXYD subunit). The image was taken using PyMOL 2.3.3.

Figure 2

Amino acid sequence alignment of seven human isoforms of FXYD subunit of Na⁺/K⁺-ATPase. A conservative FXYD sequence is highlighted in yellow. Other shared amino acids are in turquoise. An elongated N-terminus present in isoform 5 is depicted in italics. The sequences were taken from refs. [41,42,43,44,45,46,47]. * N-terminal extension of FXYD5 isoform.

Figure 3

The human proteome of individual Na+/K+-ATPase isoforms of α subunit (α-1, α-2, α-3, and α-4) β subunit (β-1, β-2, and β-3) and FXYD subunit (FXYD-1, FXYD-2, FXYD-3, and FXYD-6, data for FXYD-4, FXYD-5, FXYD-7 were not available). Data were taken from ProteomicsDB [48,49,50,51,52,53,54,55,56,57,58]. The color scale represents log10 normalized intensity-based absolute quantification (iBAQ) [59] for respective isoforms in a given tissue. The plots were prepared in R software, version 3.4.4.

Figure 4

Chemical structures of ouabain (1).

Figure 5

Chemical structures of digoxin (2) and digitoxin (3).

Figure 6

Predicted functional association network for sodium-potassium ATPase (NKA) isoforms (ATP1B4, ATP1B2, ATP1B3, ATP1A2, ATP1B1, ATP1A4, ATP1A3, ATP1A1) created by STRING 11.0 database [128]. The nodes represent gene products depicted in an evidence view mode. The type of the lines indicates knowledge or prediction of the protein-protein associations: Turquoise = from curated databases, pink = experimentally determined, green = gene neighborhood, red = gene fusions, blue = gene co-occurrence, yellow = text mining, black = co-expression, violet = protein homology. The NKA isoforms association network was generated for Homo sapiens species with the confidence score set to 0.700 with a maximum of 50 interactions. Small and large nodes represent proteins with unknown and known or predicted 3D structures, respectively. A description of the listed gene products is in Supplementary Information Table S1.

Figure 7

Predicted functional association network for cardiac steroids digoxin, digitoxin, and ouabain created by STITCH 5.0 database [135]. The nodes represent gene products depicted in a molecular action view. The type of the lines indicates the predicted mode of action: Green = activation, blue = binding, turquoise = phenotype, black = reaction, red = inhibition, dark blue = catalysis, pink = posttranslational modification, yellow = transcriptional regulation, a line with an arrowhead = positive, a line with a vertical bar = negative, a line with a filled circle = unspecified interaction. The cardiac steroid association network was generated according to the known and predicted interactions for Homo sapiens with the confidence score set to 0.700 with a maximum of 50 interactions. Small and large nodes represent proteins with unknown and known or predicted 3D structures, respectively. A description of the listed gene products is in Supplementary Information Table S2.

Figure 8

Chemical structure of compound 4.

Figure 9

Chemical structures of selected cardiac steroids and related compounds 5–21.