Alveolar hypoxia may impair sodium-dependent alveolar fluid transport and induce pulmonary edema in rat and human lung, an effect that can be prevented by the inhalation of ␤ 2 -agonists. To investigate the mechanism of ␤ 2 -agonist-mediated stimulation of sodium transport under conditions of moderate hypoxia, we examined the effect of terbutaline on epithelial sodium channel (ENaC) expression and activity in cultured rat alveolar epithelial type II cells exposed to 3% O 2 for 24 h. Hypoxia reduced transepithelial sodium current and amiloridesensitive sodium channel activity without decreasing ENaC subunit mRNA or protein levels. The functional decrease was associated with reduced abundance of ENaC subunits (especially ␤ and ␥) in the apical membrane of hypoxic cells, as quantified by biotinylation. cAMP stimulation with terbutaline reversed the hypoxia-induced decrease in transepithelial sodium transport by stimulating sodium channel activity and markedly increased the abundance of ␤-and ␥-ENaC in the plasma membrane of hypoxic cells. The effect of terbutaline was prevented by brefeldin A, a blocker of anterograde transport. These novel results establish that hypoxiainduced inhibition of amiloride-sensitive sodium channel activity is mediated by decreased apical expression of ENaC subunits and that ␤ 2 -agonists reverse this effect by enhancing the insertion of ENaC subunits into the membrane of hypoxic alveolar epithelial cells.

Decrease in alveolar oxygen tension may induce acute lung injury with pulmonary edema. We investigated whether, in alveolar epithelial cells, expression and activity of epithelial sodium (Na) channels and Na,K-adenosine triphosphatase, the major components of transepithelial Na transport, were regulated by hypoxia. Exposure of cultured rat alveolar cells to 3% and 0% O2 for 18 h reduced Na channel activity estimated by amiloride-sensitive 22Na influx by 32% and 67%, respectively, whereas 5% O2 was without effect. The decrease in Na channel activity induced by 0% O2 was time-dependent, significant at 3 h of exposure and maximal at 12 and 18 h. It was associated with a time-dependent decline in the amount of mRNAs encoding the alpha-, beta-, and gamma-subunits of the rat epithelial Na channel (rENaC) and with a 42% decrease in alpha-rENaC protein synthesis as evaluated by immunoprecipitation after 18 h of exposure. The 0% O2 hypoxia also caused a time-dependent decrease in (1) ouabain-sensitive 86Rubidium influx in intact cells, (2) the maximal velocity of Na,K-ATPase on crude homogenates, and (3) alpha1- and beta1-Na,K-ATPase mRNA levels. Levels of rENaC and alpha1-Na,K-ATPase mRNA returned to control values within 48 h of reoxygenation, and this was associated with complete functional recovery. We conclude that hypoxia induced a downregulation of expression and activity of epithelial Na channels and Na,K-ATPase in alveolar cells. Subsequent decrease in Na reabsorption by alveolar epithelium could participate in the maintenance of hypoxia-induced alveolar edema.

Objectives-During inflammation, cell adhesion molecules are modulated or redistributed for leukocyte transmigration.Among molecules at the interendothelial junction, CD146 is involved in cell-cell cohesion and permeability, but its role in monocyte transmigration is unknown. Methods and Results-TNF enhanced CD146 expression at the junction and apical membrane of human umbilical veins endothelial cells (HUVECs) through CD146 synthesis and intracellular store redistribution. In addition, TNF increased the release of a soluble form (sCD146) through a metalloproteinase-dependent mechanism. The redistribution of CD146 to the junction led us to investigate its role in monocyte transmigration using THP1 and freshly isolated monocytes.Evidence that CD146 contributes to monocyte transmigration was provided by inhibition experiments using anti-CD146 antibodies and CD146 siRNA in HUVECs. In addition, sCD146 specifically bound both monocytes and HUVECs and dose-dependently increased monocyte transmigration. Assessment of sCD146 binding on immobilized CD146 failed to evidence any homophilic interaction. Together, our data suggest endothelial CD146 binds heterophilically with a yet unknown ligand on monocytes. Conclusions-Our results demonstrate that CD146 is regulated by the inflammatory cytokine TNF and that CD146 and sCD146 are both involved in monocyte transendothelial migration during inflammation. Key Words: endothelial cells Ⅲ inflammation Ⅲ cytokines Ⅲ human Ⅲ adhesion molecules T he endothelial junctions play a fundamental role in endothelial integrity, vascular permeability, and cellular traffic. 1 At least 2 types of cell-to-cell junctional structures have been identified in the endothelium: adherens junctions (AJ) and tight junctions (TJ). 2 These junctions are tightly regulated structures composed of several adhesion molecules interacting with cytoskeletal proteins. 3 Among the adhesive molecules, endothelial VE-cadherin 4 is localized in AJ and junctional adhesion molecule (JAM) 5 in TJ, whereas other molecules such as PECAM-1/CD31 (platelet endothelial cell adhesion molecule-1) and CD99 are not restricted to 1 type of junctional structure. 6 The inflammatory response is characterized by leukocyte infiltration from the circulation toward the tissues, after a multistep process in which proinflammatory endothelial activation results in increased vascular permeability and then in leukocyte adhesion and transmigration. 7 Endothelial activation is mediated by several inflammatory cytokines. Among them, TNF␣ increases the expression of cell adhesion molecules like ICAM-1 or VCAM-1 and induces the redistribution of junctional adhesion molecules such as PECAM-1, JAM, VE-cadherin, and CD99 which, in turn, promote the transendothelial migration of leukocytes. 8 We have previously shown that CD146 (S-Endo1 Ag) is a component of the endothelial junction localized outside defined junctional structures. 9 CD146, also referred to as MUC18, is a member of the immunoglobulin superfamily (IgSF) constitutively expressed in all typ...