NF-κB Inhibits Sodium Transport via Down-regulation of SGK1 in Renal Collecting Duct Principal Cells

Abstract

Tubulointerstitial inflammation is a common feature of renal diseases. We have investigated the relationship between inflammation and Na؉ transport in the collecting duct (CD) using the mCCD cl1 and mpkCDD cl4 principal cell models. Lipopolysaccharide (LPS) decreased basal and aldosterone-stimulated amiloride-sensitive transepithelial current in a time-dependent manner. This effect was associated with a decrease in serum and gucocorticoid-regulated kinase 1 (SGK1) mRNA and protein levels followed by a decrease in epithelial sodium channel (ENaC) ␣-subunit mRNA levels. The LPS-induced decrease in SGK1 expression was confirmed in isolated rat CD. This decreased expression of either SGK1 or the ENaC ␣-subunit was not due to enhanced degradation of mRNA. In contrast, LPS inhibited transcriptional activity of the SGK1 promoter measured by luciferase-reporter gene assay. The effect of LPS was not mediated by inhibition of mineralocorticoid or glucocorticoid receptor, because expression of both receptors was unchanged and blockade of either receptor by spironolactone or RU486, respectively, did not prevent the down-regulation of SGK1. The effect of LPS was mediated by the canonical NF-B pathway, as overexpression of a constitutively active mutant, IKK␤ (inhibitor of nuclear factor B kinase-␤) decreased SGK1 mRNA levels, and knockdown of p65 NF-B subunit by small interfering RNA increased SGK1 mRNA levels. Chromatin immunoprecipitation showed that LPS increased p65 binding to two NF-B sites along the SGK1 promoter. In conclusion, we show that activation of the NF-B pathway down-regulates SGK1 expression, which might lead to decreased ENaC ␣-subunit expression, ultimately resulting in decreased Na ؉ transport.

Bestrophin 1 Promotes Epithelial-to-mesenchymal Transition of Renal Collecting Duct Cells

“…27,39,40 Notably, membrane expression of TGF-␤ strongly depends on cell polarization, which may explain why in the present study only cells at the edge of a monolayer expressed Best1 and proliferated, since they are able to expose their basolateral TGF-␤ receptors to serum factors. 41 Similar observations were done in a colonic carcinoma cell line (T 84 ), that changed from a polarized growth pattern to fast growing cells with augmented intracellular Ca 2ϩ signaling.…”

Aldosterone Activates NF-κB in the Collecting Duct

“…40,49 -51 More recently, we demonstrated in CD cells that prolonged NF-B activation inhibits transepithelial sodium transport via downregulation of SGK1 expression and subsequent decreased expression of aldosterone-induced sodium transporters. 52 Interestingly, inhibition of sodium transport by IL-1␤ is more pronounced in mineralocorticoid-than in glucocorticoid-stimulated CD cells. 40 This observation might be explained by an additional stimulation of NF-B by activated MR and by its inhibition by activated GR.…”

Functional Regulation of the Epithelial Na+ Channel by IκB Kinase-β Occurs via Phosphorylation of the Ubiquitin Ligase Nedd4-2

“…Certain studies suggest that stimulation of the IKK/NF-B pathway by either tumor necrosis factor-␣ or LPS inhibits ENaC activity, at least over relatively long time intervals (hours to days) (17,18). This inhibition may occur via down-regulation of SGK1 in renal collecting duct epithelial cells and is earliest seen at 6 h (18).…”

“…Amiloride-sensitive alveolar fluid clearance, a function of ENaC activ-ity, increases in a rat model of pneumonia by a tumor necrosis factor-␣-dependent mechanism (16). In contrast, prolonged (12-24 h) exposure of alveolar epithelial cells to tumor necrosis factor-␣ (17) or of mouse collecting duct cells to LPS (18) results in down-regulation of ENaC activity and expression. We have found that aldosterone, the major hormonal regulator of ENaC, increases IKK␤ levels in renal epithelia and activates NF-B and have suggested that long term stimulation of NF-B might represent a negative feedback loop modulating long term ENaC activation in conditions of oxidative or metabolic stress (10).…”