Glucocorticoid-stimulated lung epithelial Na⁺transport is associated with regulated ENaC andsgk1expression

Abstract

H441 cells, a bronchiolar epithelial cell line, develop a glucocorticoid-regulated amiloride-sensitive Na(+) transport pathway on permeable supports (R. Sayegh, S. D. Auerbach, X. Li, R. Loftus, R. Husted, J. B. Stokes, and C. P. Thomas. J Biol Chem 274: 12431-12437, 1999). To understand its molecular basis, we examined the effect of glucocorticoids (GC) on epithelial Na(+) channel (ENaC)-alpha, -beta, and -gamma and sgk1 expression and determined the biophysical properties of Na(+) channels in these cells. GC stimulated the expression of ENac-alpha, -beta, and -gamma and sgk1 mRNA, with the first effect seen by 1 h. These effects were abolished by actinomycin D, but not by cycloheximide, indicating a direct stimulatory effect on ENaC and sgk1 mRNA synthesis. The GC effect on transcription of ENaC-alpha mRNA was accompanied by a significant increase in ENaC-alpha protein levels. GC also stimulated ENaC-alpha, -beta, and -gamma and sgk1 mRNA expression in A549 cells, an alveolar type II cell line. To determine the biophysical properties of the Na(+) channel, single-channel currents were recorded from cell-attached H441 membranes. An Na(+)-selective channel with slow kinetics and a slope conductance of 10.8 pS was noted, properties similar to ENaC-alpha, -beta, and -gamma expressed in Xenopus laevis oocytes. These experiments indicate that amiloride-sensitive Na(+) transport is mediated through classic ENaC channels in human lung epithelia and that GC-regulated Na(+) transport is accompanied by increased transcription of each of the component subunits and sgk1.

Clinical implication of lung fluid balance in the perinatal period

“…Indeed, glucocorticosteroids have been shown to induce a-ENaC transcription, 57,58 and in human fetal lung explant cultures all three ENaC subunits were upregulated by glucocorticosteroids. 59,60 Glucocorticosteroids also increase the number of available sodium channels by increasing ENaC trafficking and decreasing the rate of degradation of existing channels 61,62 and improving lung responsiveness to b2 stimulants and thyroid hormones. 63 Stutchfield et al 64 have shown that two doses of betamethasone given 48 h before cesarean section significantly decreased admissions due to respiratory distress.…”

Anti-inflammatory functions of glucocorticoid-induced genes

“…On the other hand, GILZ stimulated rather than suppressed the expression and activity of the epithelial sodium channel α (ENaCα) (Bhalla et al, 2006;Soundararajan et al, 2005), possibly via inhibition of the ERK pathway (Ayroldi et al, 2002;Soundararajan et al, 2005). ENaCα expression is thought to contribute to GC-induced hypertension (Fuller et al, 2000;Itani et al, 2002;Sayegh et al, 1999;Stokes and Sigmund, 1998). This raises the interesting possibility that a GCinduced gene could contribute to both anti-inflammatory and harmful effects in a single cell type.…”

Differentiation of Epithelial Na+ Channel Function

“…The Clara-like H441 lung papillary adenocarcinoma cell line has been used as a glucocorticoid-and cAMP-regulated cellular model of pulmonary epithelial sodium absorption (5)(6)(7)(8)(9)(10). These cells produce the Clara cell 10-kDa protein (CC-10) and the surfactant proteins A, B, and D and have ultrastructural characteristics of nonciliated bronchiolar cells (11).…”