SARS-CoV proteins decrease levels and activity of human ENaC via activation of distinct PKC isoforms

Department of Anesthesiology, University of Alabama at Birmingham School of Medicine, Birmingham, Alabama 35233-6810, USA.
AJP Lung Cellular and Molecular Physiology (Impact Factor: 4.08). 01/2009; 296(3):L372-83. DOI: 10.1152/ajplung.90437.2008
Source: PubMed


Among the multiple organ disorders caused by the severe acute respiratory syndrome coronavirus (SARS-CoV), acute lung failure following atypical pneumonia is the most serious and often fatal event. We hypothesized that two of the hydrophilic structural coronoviral proteins (S and E) would regulate alveolar fluid clearance by decreasing the cell surface expression and activity of amiloride-sensitive epithelial sodium (Na(+)) channels (ENaC), the rate-limiting protein in transepithelial Na(+) vectorial transport across distal lung epithelial cells. Coexpression of either S or E protein with human alpha-, beta-, and gamma-ENaC in Xenopus oocytes led to significant decreases of both amiloride-sensitive Na(+) currents and gamma-ENaC protein levels at their plasma membranes. S and E proteins decreased the rate of ENaC exocytosis and either had no effect (S) or decreased (E) rates of endocytosis. No direct interactions among SARS-CoV E protein with either alpha- or gamma-ENaC were indentified. Instead, the downregulation of ENaC activity by SARS proteins was partially or completely restored by administration of inhibitors of PKCalpha/beta1 and PKCzeta. Consistent with the whole cell data, expression of S and E proteins decreased ENaC single-channel activity in oocytes, and these effects were partially abrogated by PKCalpha/beta1 inhibitors. Finally, transfection of human airway epithelial (H441) cells with SARS E protein decreased whole cell amiloride-sensitive currents. These findings indicate that lung edema in SARS infection may be due at least in part to activation of PKC by SARS proteins, leading to decreasing levels and activity of ENaC at the apical surfaces of lung epithelial cells.

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    • "The PCL is maintained by the coordinated action of many ion channels, pumps, and transporters (Knowles et al. 1984; Toczylowska-Maminska and Dolowy 2012). Disruption of ion transport can contribute to airway diseases, such as mucus thickening in cystic fibrosis due to PCL dehydration , lung edema due to an inhibition of epithelial Na + channels (ENaC; Chen et al. 2004; Morty et al. 2007; Ji et al. 2009), and interfere with regulatory mechanisms in the airways, such as the release of epithelium-derived relaxing factor which induces relaxation of airway smooth muscle and submucosal blood vessels (Prazma et al. 1994; Fedan et al. 2004; Wu et al. 2004). There is a large body of evidence supporting the notion that NGF can alter ion transport in nonpulmonary cells. "
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    • "Possibly, these interactions could lead to partial relocalization of these proteins and therefore to the observed reduction of ion transport, which may explain decreased voltage activated currents intensity detected by whole-cell patch clamp assays. Importantly, in agreement with our work, it has been shown that E protein does not display ion channel activity in a different experimental system, the plasma membrane of Xenopus oocytes (Ji et al., 2009) but decreases the levels and activity of human epithelial sodium channels, affecting ion transport at the plasma membrane level. Overall, our results from four complementary approaches strongly suggest that SARS-CoV E protein did not accumulate at the plasma membrane during SARS-CoV infection, or when expressed alone, suggesting that the intrinsic activities of E protein should be displayed in the inner organelles and that plasma membrane permeabilization to monovalent ions (Pervushin et al., 2009) or to small weight compounds in E protein expressing cells (Liao et al., 2004, 2006; Madan et al., 2008), is most likely due to indirect effects. "
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    • "PKC is an important negative regulator of ENaC expression. Recently PKC alpha (PKC-α) and zeta isozymes were found to be crucial in ENaC downregulation caused by proteins of SARS-CoV17 and of Influenza A virus18. Recently, also oxidative stress, which often occurs in the lung under conditions such as infection and inflammation, has been demonstrated to interfere with ENaC expression. "
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