Cytotoxic effect of carbohydrate derivatives of digitoxigenin involves modulation of plasma membrane Ca2+ -ATPase.

Cardiac glycosides, such as digoxin and digitoxin, are compounds that interact with Na+ /K+ -ATPase to induce anti-neoplastic effects; however, these cardiac glycosides have narrow therapeutic index. Thus, semi-synthetic analogs of digitoxin with modifications in the sugar moiety has been shown to be an interesting approach to obtain more selective and more effective analogs than the parent natural product. Therefore, the aim of this study was to assess the cytotoxic potential of novel digitoxigenin derivatives, digitoxigenin-α-L-rhamno-pyranoside (1) and digitoxigenin-α-L-amiceto-pyranoside (2), in cervical carcinoma cells (HeLa) and human diploid lung fibroblasts (Wi-26-VA4). In addition, we studied the anticancer mechanisms of action of these compounds by comparing its cytotoxic effects with the potential to modulate the activity of three P-type ATPases; Na+ /K+ -ATPase, sarco/endoplasmic reticulum Ca2+ -ATPase (SERCA), and plasma membrane Ca2+ -ATPase (PMCA). Briefly, the results showed that compounds 1 and 2 were more cytotoxic and selectivity for HeLa tumor cells than the nontumor cells Wi-26-VA4. While the anticancer cytotoxicity in HeLa cells involves the modulation of Na+ /K+ -ATPase, PMCA and SERCA, the modulation of these P-type ATPases was completely absent in Wi-26-VA4 cells, which suggest the importance of their role in the cytotoxic effect of compounds 1 and 2 in HeLa cells. Furthermore, the compound 2 inhibited directly erythrocyte ghosts PMCA and both compounds were more cytotoxic than digitoxin in HeLa cells. These results provide a better understanding of the mode of action of the synthetic cardiac glycosides and highlights 1 and 2 as potential anticancer agents.

Conformational states of the pig kidney Na+/K+-ATPase differently affect bufadienolides and cardenolides: A directed structure-activity and structure-kinetics study.

There is a renewed interest in the Na+/K+-ATPase (NKA, EC 3.6.3.9) either as a target for new therapeutic uses or for understanding the putative pathophysiological role of its mammalian endogenous ligands. Recent data indicate that bufalin binds to the pig kidney NKA in a way different from ouabain and digoxin, raising the question of a putative class difference between bufadienolides and cardenolides. The purpose of this work was to perform a study of the relationship between structure and both activity and kinetics, focusing mainly on the influence of the lactone ring in C17 (5 vs. 6 membered), the effect of C14-15 cyclization and the carbohydrate moiety in C3. We compared the potency of fourteen related cardiotonic steroids (CTS) for inhibition of the cycling pig kidney NKA in two different concentrations of K+, as well as the affinity for binding to the E2P conformation of the enzyme (Mg-Pi medium) and the potency for inhibiting the E2[2K] conformation of the NKA (K+-pNPPase activity). Cardenolides were clearly sensitive to the antagonistic effect of high K+ concentrations whereas bufadienolides were not or less sensitive. The C14-15 cyclization observed in some bufadienolides, such as resibufogenin and marinobufagin, caused a drastic fall in the affinity for binding to the NKA in the E2P conformation and increased the velocity of K+-pNPPase inhibition. The absence of a carbohydrate moiety in C3 increased the velocity of inhibition. Cardenolides were much more dependent on the E2P conformation for binding than bufadienolides since their ratios of E2[2K] IC50 to E2P Ki were higher than for bufadienolides. Therefore, the present data established the remarkable influence of C14-15 cyclization and of the carbohydrate moiety in C3 on both affinity and kinetics of CTS and indicate that, as a class, bufadienolides would harbor qualitative differences from cardenolides with respect to the NKA conformations to which they can bind.