Regulation of sodium pump endocytosis by cardiotonic steroids: Molecular mechanisms and physiological implications
Introduction
Endocytosis is the vesicle-mediated process used by all cells to internalize extracellular macromolecules and plasma membrane components, and is responsible for the transport of proteins between various compartments of the secretory and endocytic systems. It can be broadly divided into two categories based on the material internalized. Phagocytosis (or cell eating) refers to the internalization of large particles (>200 nm). Pinocytosis (or cell drinking) refers to the internalization of extracellular medium and may occur through four basic mechanisms: clathrin-dependent endocytosis, caveolae-mediated endocytosis, macropinocytosis, and dynamin- and clathrin-independent endocytosis [1]. Although there are several additional types of pinocytosis which have recently been described (e.g., macropinocytosis and non-dynamin mediated pinocytosis [2], [3], [4]), clathrin and caveolin mediated endocytosis are the best described endocytosis pathways.
Section snippets
Mechanisms of endocytosis
Clathrin-coated vesicles are responsible for receptor-mediated endocytosis at the plasma membrane and sorting of proteins at the trans-Golgi network (TGN), and found associated with the cell membrane, the TGN, and on some endosomes [5], [6], [7]. At the plasma membrane, clathrin-mediated internalization is initiated by the self-assembly of clathrin lattice formation to provide an organizing structure. The formation of clathrin coated pits is triggered by the heterotetrameric adaptor complex
Endocytosis and signal transduction
Endocytosis of cell surface receptors is an important regulatory event in signal transduction. The classic concept of receptor downregulation by endocytosis has been established over the past several decades. In general, receptor-mediated endocytosis results in internalization of the receptor and ultimate destruction of the receptor in lysosomes. In this situation, the endocytosis is part of a negative feedback loop meant to attenuate or minimize the signal associated with receptor activation
Na/K-ATPase: an ion pump and a signal transducer
The Na/K-ATPase was first discovered as the molecular machine for the ATP-dependent and -coupled transport of Na+ and K+ across the plasma membranes of all eukaryotic cells [56], [57]. Na/K-ATPase is a heterodimeric membrane protein that belongs to the type II class of P-type ATPases and consists of two noncovalently linked α and β subunits [57], [58], [59]. The α subunit is considered as the “catalytic subunit” containing the binding sites for ATP, ouabain, and other ligands. The β subunit is
Endocytosis of the Na/K-ATPase in the regulation of renal sodium excretion
The regulation of renal tubule epithelial cell sodium transport by endocytosis of the Na/K-ATPase has been extensively studied. Most of this work has been done in the context of G protein receptor mediated signal transduction induced by dopamine. Dopamine stimulates the Na/K-ATPase trafficking and alters renal tubular epithelial sodium handling by decreasing plasmalemmal Na/K-ATPase content [87], [88], [89], [90]. Endocytosis of the Na/K-ATPase in response to dopamine is triggered by the
Ouabain-induced endocytosis of the Na/K-ATPase in LLC-PK1 cells
In LLC-PK1 cells, acute treatment with low concentrations of ouabain or MBG (≤100 nM) do not cause detectable inhibition of the Na/K-ATPase activity, but chronic treatment with ouabain or MBG causes significant decreases in Na/K-ATPase activity and transepithelial Na+ flux without changing in intracellular Na+ concentration [99], [100], [101]. Since simply inhibition of the Na/K-ATPase with low extracellular potassium does not produce these effects, we reasoned that other regulatory mechanism(s)
Ouabain regulates sodium handling in renal proximal tubule
Sodium reabsorption in the proximal tubule involves the coupling of apical sodium entry mainly through the NHE3 (sodium/hydrogen exchanger, isoform 3) and basolateral sodium extrusion primarily through the Na/K-ATPase. Accumulated evidence supports the notion that endogenous cardiotonic steroids may cause a physiologically meaningful regulation of transepithelial sodium transport in the proximal tubule. Since low concentrations of ouabain decrease basolateral sodium extrusion by depletion of
Ouabain-Na/K-ATPase in endocytic pathway: signaling termination or propagation?
Binding of ouabain to the receptor Na/K-ATPase signaling complex activates the Na/K-ATPase-bound c-Src, leading to endocytosis of the signaling complex. In early endosomes, functioning as a ion pump, native or endocytosed Na/K-ATPase may regulate endosomal pH by generation of a interior-positive membrane potential [120], [121]. This regulation kept a mild pH (about 6.0) environment in early endosomes, which is sufficient to dissociate most ligand–receptor complexes and facilitate receptor
Physiological implications
Renal adaptation to both volume expansion and hypertension involves a complicated interplay among different hormonal and cellular regulatory mechanisms. Since the finding of proposed endogenous cardiotonic steroids, accumulated evidences have indicated that these steroids might be related to a number of health conditions such as sodium imbalance, chronic renal failure, hypertension, and congestive heart failure. It is known that CTS are elevated during volume expansion. The cardiotonic steroid
Acknowledgement
Portions of this study were supported by grant from American Heart Association Ohio Valley Affiliate.
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Ouabain-induced internalization and lysosomal degradation of the Na <sup>+</sup>/K<sup>+</sup>-ATPase
2014, Journal of Biological ChemistryCitation Excerpt :Independently, a number of studies have documented internalization of the Na+/K+-ATPase upon incubation of cells with nontoxic doses of ouabain and other CGs. 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).