Role of endosomal Na⁺-K⁺-ATPase and cardiac steroids in the regulation of endocytosis

Abstract

Plasma membrane Na(+)-K(+)-ATPase, which drives potassium into and sodium out of the cell, has important roles in numerous physiological processes. Cardiac steroids (CS), such as ouabain and bufalin, specifically interact with the pump and affect ionic homeostasis, signal transduction, and endocytosed membrane traffic. CS-like compounds are present in mammalian tissues, synthesized in the adrenal gland, and considered to be new family of steroid hormones. In this study, the mechanism of Na(+)-K(+)-ATPase involvement in the regulation of endocytosis is explored. We show that the effects of various CS on changes in endosomal pH are mediated by the pump and correspond to their effects on endosomal membrane traffic. In addition, it was found that CS-induced changes in endocytosed membrane traffic were dependent on alterations in [Na(+)] and [H(+)] in the endosome. Furthermore, we show that various CS differentially regulate endosomal pH and membrane traffic. The results suggest that these differences are due to specific binding characteristics. Based on our observations, we propose that Na(+)-K(+)-ATPase is a key player in the regulation of endosomal pH and endocytosed membrane traffic. Furthermore, our results raise the possibility that CS-like hormones regulate differentially intracellular membrane traffic.

Ouabain-induced Internalization and Lysosomal Degradation of the Na+/K+-ATPase

“…A number of previous studies have also reported that external application of ouabain as well as other CGs results in internalization of both ␣ and ␤ subunits of the Na ϩ /K ϩ -ATPase. One role suggested for this process is acidification of endosomal pH (11,30). Other studies related this process to the activation of caveolar Src and the subsequent ouabain-induced signaling cascade.…”

“…This effect was suggested to be triggered by ouabain-induced activation of Src and to be involved in the signal transduction involved in translocation of the pump to the nucleus (9,10). Alternatively, it was proposed that ouabain-induced internalization may be a means of regulating endosomal pH (11) or a natriuresis mechanism induced by endogenous CG (12).…”

Increased nuclear expression and transactivation of vitamin D receptor by the cardiotonic steroid bufalin in human myeloid leukemia cells

“…The enhancing effect of bufalin on CYP24A1 expression may be mediated by both nuclear VDR expression and Erk MAP kinase activation. Bufalin has been shown to direct cytoplasmic membrane components adjacent to the nucleus by regulating endosomal pH and endocytosed membrane traffic in neuronal precursor cells most likely via a Na + ,K + -ATPase-mediated mechanism [37]. Since the relation between endosomal function and degeneration of exported nuclear proteins remains unknown, further investigation is required to elucidate the role of the Na + ,K + -ATPase in the regulation of nuclear proteins, including VDR.…”

Physiological roles of endogenous ouabain in normal rats

“…Recent studies have shown that the binding of ouabain to Na ϩ -K ϩ -ATPase elicits numerous additional changes in cell function. These include activation of intracellular signal transduction (46), activation of cytoplasmaic Ca 2ϩ oscillation (2), stimulation of endocytosis, and inhibition of endocytosed membrane traffic (13,37), as well as cell proliferation (1) and adhesion (24). Because all of these molecular mechanisms were induced by ouabain in the nanomolar range, any of them could be involved in the physiological processes described in this study.…”