Alveolar proteinosis: a disease of mice and men

Expression of the amino acid transporter ATB 0+ in lung: possible role in luminal protein removal

Normal lung function requires transepithelial clearance of luminal proteins; however, little is known about the molecular mechanisms of protein transport. Protein degradation followed by transport of peptides and amino acids may play an important role in this process. We previously cloned and functionally characterized the neutral and cationic amino acid transporter ATB(0+) and showed expression in the lung by mRNA analysis. In this study, the tissue distribution, subcellular localization, and function of the transporter in native tissue were investigated. Western blots showed expression of the ATB(0+) protein in mouse lung, stomach, colon, testis, blastocysts, and human lung. Immunohistochemistry revealed that ATB(0+) is predominantly expressed on the apical membrane of ciliated epithelial cells throughout mouse airways from trachea to bronchioles and in alveolar type I cells. Electrical measurements from mouse trachea preparations showed Na(+)- and Cl(-)-dependent, amino acid-induced short-circuit current consistent with the properties of ATB(0+). We hypothesize that, by removing amino acids from the airway lumen, the transporter contributes to protein clearance and, by maintaining a low nutrient environment, plays a role in lung defense.

Carbon monoxide modulates endotoxin-induced production of granulocyte macrophage colony-stimulating factor in macrophages

The stress-inducible gene heme oxygenase-1 (HO-1) provides protection against oxidative stress. Although the mechanisms by which HO-1 exerts its cytoprotection are not clearly understood, it has been speculated that carbon monoxide (CO), a catalytic byproduct following heme catabolism by HO-1, may mediate cellular cytoprotection via its anti-inflammatory properties. Granulocyte macrophage colony-stimulating factor (GM-CSF) is a potent cytokine generated in response to bacterial endotoxin (lipopolysaccharide [LPS]) to stimulate proliferation, maturation, and effector functions of leukocytes, contributing to the proinflammatory responses to LPS. We hypothesized that HO-1 and/or CO could regulate the expression and production of GM-CSF. HO-1 overexpression, as well as exposure to a low concentration of CO, inhibited LPS-induced GM-CSF production in macrophages. Furthermore, CO inhibited LPS-induced GM-CSF induction via inhibition in the activation of the transcription factor NF-kappaB. CO inhibited LPS-induced activation of NF-kappaB, which has been shown to regulate GM-CSF transcription, by preventing the phosphorylation and degradation of the regulatory subunit IkappaB-alpha. These data raise the intriguing possibility that CO at low concentrations may play an important role in inflammatory disease states and thus has potential therapeutic implications.