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Cystic fibrosis (CF) is characterized by impaired Cl- secretion and increased Na+ reabsorption in several tissues including respiratory epithelium. Many CFTR mutations have been identified over the past years. However, only a poor correlation between the genotype and lung phenotype was found suggesting additional factors influencing the phenotype and course of the disease. The serine/threonine kinase SGK1 has recently been shown to stimulate the activity of the epithelial Na+ channel ENaC. A variety of stimuli such as aldosterone, cell shrinkage, insulin or TGF-β1 stimulate transcription and activate the SGK1 kinase. Here we further examined the effects of SGK1 on ENaC and CFTR which have mutual interactions and we analyzed sgk1 mRNA abundance in lung tissue from CF patients. Coexpression of CFTR and h-SGK1 in Xenopusoocytes increased ENaC currents as previously described. In addition CFTR mediated currents were also stimulated. h-SGK1 accelerated the expression of the amiloride sensitive Na+- current in Xenopusoocytes paralleled by increased ENaC-protein abundance in the oocyte membrane, an effect which was reversed by a h-SGK1K127R mutation lacking the ATP-binding site. The cation selectivity or Na+ affinity were not affected. However, coexpression of h-SGK1 with ENaC altered the sensitivity of the Na+-channel to the inhibitors amiloride and triamterene. The inhibitory effect of CFTR expression on ENaC current was not affected by coexpression of h-SGK1 in Xenopus oocytes. Lung tissue from CF patients strongly expressed the serine/threonine kinase h-sgk1 which was not the case for non-CF lung tissue. Loss of CFTR function itself in a CF lung epithelial cell line did not increase SGK1 expression. In summary, enhanced expression of h-SGK1 in epithelial cells of CF-lung tissue may be a novel pathophysiological factor contributing to increased Na+ channel activity and thus to increased Na+ transport in CF. .

1System L is the major Na+‐independent amino acid transporter of mammalian cells. It is constituted of the type II membrane protein 4F2hc (CD98) which is covalently linked to the polytopic membrane protein LAT1 via a disulfide bridge. The transporter is known to be regulated by the mineralcorticoid aldosterone in XenopusA6 cells. To understand the regulation of the transporter, the 4F2hc/LAT1 heterodimer was functionally expressed in Xenopus laevisoocytes and its transport properties were analysed using flux measurements and the two‐electrode voltage‐clamp technique.2Expression of 4F2hc/LAT1 resulted in a rapid increase in a Na+‐independent neutral amino acid antiport activity and simultaneously gave rise to a cation conductance. The cation channel was non‐rectifying and non‐selective, conducting Li+> Cs+= Na+> K+. After replacement of Na+by NMDG, however, the currents were suppressed almost completely. The cation channel was not inhibited by amiloride, Ba2+, TEA, Hoe293B, flufenamic acid or substrates of the system L amino acid transporter. Significant inhibition, however, was observed in the presence of La3+, Gd3+and quinidine. Channel activity was upregulated by coexpression of 4F2hc/LAT1 with the aldosterone‐regulated protein kinase sgk‐1.3The cation conductance was sensitive to changes in the redox potential, being inhibited following incubation of the oocytes with DTE for 30 min. Mutation of either of the disulfide bridge‐constituting cysteines to serine resulted in a loss of ion channel activity whereas amino acid transport was unaffected.4It is concluded that the 4F2hc/LAT1 heterodimer regulates a closely associated cation channel or even constitutes a cation channel itself.