Hypoxia regulates gene expression of alveolar epithelial transport proteins

Alveolar hypoxia occurs during ascent to high altitude but is also commonly observed in many acute and chronic pulmonary disorders. The alveolar epithelium is directly exposed to decreases in O-2 tension, but a few studies have evaluated the effects of hypoxia on alveolar cell function. The alveolar epithelium consists of two cell types: large, flat, squamous alveolar type I and cuboidal type II (ATII). ATII cells are more numerous and have a number of critical functions, including transporting ions and substrates required for many physiological processes. ATII cells express I) membrane proteins used for supplying substrates required for cell metabolism and 2) ion transport proteins such as Na+ channels and Na+-K+-ATPase, which are involved in the vectorial transport of Na+ from the alveolar to interstitial spaces and therefore drive the resorption of alveolar fluid. This brief review focuses on gene expression regulation of glucose transporters and Na+ transport proteins by hypoxia in alveolar epithelial cells. Cells exposed to severe hypoxia (0% or 3% O-2) for 24 h upregulate the activity and expression of the glucose transporter GLUT-1, resulting in preservation of ATP content. Hypoxia-induced increases in GLUT-1 mRNA levels are due to O-2 deprivation and inhibition of oxidative phosphorylation. This regulation occurs at the transcriptional level through activation of a hypoxia-inducible factor. In contrast, hypoxia downregulates expression and activity of Na+ channels and Na+-K+-ATPase in cultured alveolar epithelial cells. Hypoxia induces time- and concentration-dependent decreases of alpha-, beta-, and gamma-subunits of epithelial Na+ channel mRNA and beta(1)- and alpha(1)-subunits of Na+-K+-ATPase, effects that are completely reversed after reoxygenation. The mechanisms by which O-2 deprivation regulates gene expression of Na+ transport proteins are not fully elucidated but likely involve the redox status of the cell. Thus hypoxia regulates gene expression of transport proteins in cultured alveolar epithelial type II cells differently, preserving ATP content.