Ouabain-Induced Cell Death and Survival. Role of α1-Na,K-ATPase-Mediated Signaling and [Na+]i/[K+]i-Dependent Gene Expression

Abstract

Ouabain is of cardiotonic steroids (CTS) family that is plant-derived compounds and is known for many years as therapeutic and cytotoxic agents. They are specific inhibitors of Na,K-ATPase, the enzyme, which pumps Na⁺ and K⁺ across plasma membrane of animal cells. Treatment of cells by CTS affects various cellular functions connected with the maintenance of the transmembrane gradient of Na⁺ and K⁺. Numerous studies demonstrated that binding of CTS to Na,K-ATPase not only suppresses its activity but also induces some signal pathways. This review is focused on different mechanisms of two ouabain effects: their ability (1) to protect rodent cells from apoptosis through the expression of [Na⁺]_i-sensitive genes and (2) to trigger death of non-rodents cells (so-called «oncosis»), possessing combined markers of «classic» necrosis and «classic» apoptosis. Detailed study of oncosis demonstrated that the elevation of the [Na⁺]_i/[K⁺]_i ratio is not a sufficient for its triggering. Non-rodent cell death is determined by the characteristic property of "sensitive" to ouabain α1-subunit of Na,K-ATPase. In this case, ouabain binding leads to enzyme conformational changes triggering the activation of p38 mitogen-activated protein kinases (MAPK) signaling. The survival of rodent cells with ouabain-«resistant» α1-subunit is connected with another conformational transition induced by ouabain binding that results in the activation of ERK 1/2 signaling pathway.

Keywords: Na,K-ATPase; cardiotonic steroids; cell death; gene expression; intracellular Na+ and K+; intracellular signaling; ouabain.

Figure 1

[Na⁺]_i-mediated and [Na⁺]_i, [K⁺]_i-independent signaling pathways involved in the regulation of apoptotic and oncotic modes of cell death by cardiotonic steroids (CTS). α, β – subunits of Na,K-ATPase (different shapes indicate the conformational transition of α-subunit triggered by its interaction with CTS); AdPr – adapter protein interacting with Na,K-ATPase in a CTS-dependent manner; S1 and S2 – hypothetical Na⁺ _i and H⁺ _i sensors, respectively; NaRE – Na⁺-response element;? – unknown steps; —–> and —–| – activatory and inhibitory stimuli, respectively. Different letter cases are used to show the different intracellular concentrations of Na⁺ and K⁺. For more details, see the text. Modified from Orlov and Hamet (2006).

Figure 2

Possible mechanisms representing the absence of cell death signaling due to ouabain binding to α1R-NKA (A); and the appearance of this signaling with distinct potency as result of marinobufagenin (B) and ouabain binding to α1S-NKA (C). Conformational transition induced by ouabain binding to α1R-NKA does not affect the binding of unidentified adaptor protein (AdPr) to NKA (A). Marinobufagenin (MBG) with similar affinity freezes α1S-NKA in conformations E1 and E2P (transition between them is prohibited), AdP is released from complex with E1-conformation providing cell survival and does not release from complex with E2P providing cell death (B,C). For more details, see the text.