Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar epithelial cells |
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Authors: | Danto, Spencer I. Borok, Zea Zhang, Xiao-Ling Lopez, Melissa Z. Patel, Paryus Crandall, Edward D. Lubman, Richard L. |
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Abstract: | We investigated theeffects of epidermal growth factor (EGF) on activeNa+ absorption by alveolarepithelium. Rat alveolar epithelial cells (AEC) were isolated andcultivated in serum-free medium on tissue culture-treated polycarbonatefilters. mRNA for rat epithelial Na+ channel (rENaC) -, -,and -subunits and Na+ pump 1- and 1-subunits were detected inday 4 monolayers by Northern analysisand were unchanged in abundance in day5 monolayers in the absence of EGF. Monolayerscultivated in the presence of EGF (20 ng/ml) for 24 h fromday 4 to day5 showed an increase in both 1 and 1Na+ pump subunit mRNA but noincrease in rENaC subunit mRNA. EGF-treated monolayers showed parallelincreases in Na+ pump 1- and 1-subunit protein by immunoblotrelative to untreated monolayers. Fixed AEC monolayers demonstratedpredominantly membrane-associated immunofluorescent labeling withanti-Na+ pump 1- and 1-subunit antibodies, withincreased intensity of cell labeling for both subunits seen at 24 hfollowing exposure to EGF. These changes inNa+ pump mRNA and protein precededa delayed (>12 h) increase in short-current circuit (measure ofactive transepithelial Na+transport) across monolayers treated with EGF compared with untreated monolayers. We conclude that EGF increases activeNa+ resorption across AECmonolayers primarily via direct effects onNa+ pump subunit mRNA expressionand protein synthesis, leading to increased numbers of functionalNa+ pumps in the basolateralmembranes. |
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