Structural Insights into the Interactions of Digoxin and Na+/K+-ATPase and Other Targets for the Inhibition of Cancer Cell Proliferation

Abstract

Digoxin is a cardiac glycoside long used to treat congestive heart failure and found recently to show antitumor potential. The hydroxy groups connected at the C-12, C-14, and C-3′a positions; the C-17 unsaturated lactone unit; the conformation of the steroid core; and the C-3 saccharide moiety have been demonstrated as being important for digoxin’s cytotoxicity and interactions with Na+/K+-ATPase. The docking profiles for digoxin and several derivatives and Na+/K+-ATPase were investigated; an additional small Asn130 side pocket was revealed, which could be useful in the design of novel digoxin-like antitumor agents. In addition, the docking scores for digoxin and its derivatives were found to correlate with their cytotoxicity, indicating a potential use of these values in the prediction of the cancer cell cytotoxicity of other cardiac glycosides. Moreover, in these docking studies, digoxin was found to bind to FIH-1 and NF-κB but not HDAC, IAP, and PI3K, suggesting that this cardiac glycoside directly targets FIH-1, Na+/K+-ATPase, and NF-κB to mediate its antitumor potential. Differentially, digoxigenin, the aglycon of digoxin, binds to HDAC and PI3K, but not FIH-1, IAP, Na+/K+-ATPase, and NF-κB, indicating that this compound may target tumor autophagy and metabolism to mediate its antitumor propensity.

Interaction of (+)-Strebloside and Its Derivatives with Na+/K+-ATPase and Other Targets

“…In addition, compound 1 showed a lower docking score and more potent cytotoxicity than 2 , and consistent trends were observed when comparing 1 with 5, 3 with 4, and 5 with 6 ( Table 1 ). These data are in agreement with those observed for digoxin and its derivatives, of which the docking scores correlated well with their cancer cell cytotoxicity [ 14 ]. Furthermore, correlation between the docking scores from binding to NKA and cancer cell cytotoxicity of compounds 1–11 presented in Table 1 has been tested by Spearman correlation, using the GraphPad Prism 6.0 program.…”

“…Previously, all of the C-3 saccharide moiety, the C-10 formyl substituent, the C-5, C-14, and C-4′ hydroxy groups, the C-17 lactone unit, and the established conformation of (+)-strebloside ( 1 ) were found to be important in the mediation of its cytotoxicity against HT-29 human colon cancer cells and in its binding to NKA [ 9 , 10 ]. To test the role of the C-3 saccharide moiety and the C-10 formyl group in the interaction with NKA, 1 and several selected derivatives have been docked to the human NKA (hNKA) model that had been built in our previous study [ 14 ] using AutoDock Vina [ 15 , 16 ]. These derivatives included (+)-4′- O -acetylstrebloside ( 2 ), (+)-19-hydroxykamaloside ( 3 ), (+)-5-hydroxyasperoside ( 4 ), (+)-3- O -β-D-fucopyranosylperiplogenin ( 5 ), (+)-4′- O -benzoylstrebloside ( 6 ), and (+)-4′- O -benzoyl-19-nor-kamaloside-10-carboxylic acid ( 7 ) ( Figure 1 ).…”

“…Previous investigations have shown that the C-3 saccharide moiety plays an important role in binding between (+)-strebloside ( 1 ) and NKA [ 10 ], and the binding parameters between digoxin and its aglycone and NKA were found to be different [ 14 ]. Thus, the docking profiles for the aglycone of 1 , strophanthidin ( 10 ), and its analogue, (+)-19-nor-5(10),14-dianhydrostrophanthidin-3-yl formate ( 11 ), and NKA have been investigated herein ( Figure 1 ).…”

“…However, the binding pose of 10 seems similar to that of ouabain ( Figure 1 ), and several conserved hydrogen bonds could be formed between the C-3 hydroxy group, the C-10 formyl group, and the C-14 hydroxy group and the Glu125, Gln119, and Thr805 side chains of NKA, respectively. These hydrogen bonds may improve the binding affinity of 10 when compared with digoxigenin [ 14 ]. However, the absence of hydroxy groups at the C-5 and C-14 positions and the formyl group at the C-10 position in 11 results in the loss of the hydrogen bonds formed between these groups and the residues of NKA, as observed in 10 .…”

“…Docking profiles for digoxin and selected derivatives and NKA and other targets have been reported, and digoxin was postulated to interact directly with FIH-1, NF-κB, and NKA to mediate its antitumor activity [ 14 ]. Following these previous investigations, the binding between NKA and (+)-strebloside ( 1 ) and its derivatives ( 2 – 11 ), in which the structure of 1 has been changed at the C-3, C-10, and C-17 positions, has been investigated herein.…”