Mineralocorticoid hormone signaling regulates the 'epithelial sodium channel' in fibroblasts from human cornea

We investigated the regulation of sodium absorption by steroid hormones in embryologically diverse cells from the human eye. A cell extract from human corneal fibroblasts was positive for both the epithelial sodium channel (E NaC) and the mineralocorticoid receptor (MCR) as 82- to 85-kD and 102-kD ba nds, respectively, by the Western blot technique. In fluorescent, confocal and electron microscopy, the MCR was revealed as a nucleocytoplasmic protei n, whereas the ENaC was almost exclusively membrane bound; both appeared al igned along actin filaments of corneal keratocytes, and both were widely co localized in various cell types of human cornea in situ. Following reverse transcription and amplification of total RNA isolated from corneal fibrobla sts, the ENaC and MCR genes in the PCR product were evident as predicted ba nds of 520 and 843 bp, respectively, whose sequence exhibited 100% identity with those from known human sources. The multiplication of corneal fibrobl asts was influenced by both the MCR-specific antagonist RU 26752 and the na tural hormone aldosterone, and these steroids also stimulated protein phosp horylation. In quantitative PCR, both the basal and aldosterone-induced lev els of ENaC were diminished by the MCR-specific antagonist ZK 91587. Conseq uently, the ocular sodium channel appears to be regulated by steroid signal ling in cells of diverse embryological origins, contrary to the existing no tions where (a) this process would be limited exclusively to the epithelial cells and (b) ocular sodium transport would be regulated via the Na+-K+-AT Pase in the basolateral membrane. Copyright (C) 2001 S. Karger AG, Basel.