Intersubunit disulfide bridge is not required for the protective role of SP-B against lung inflammation

STAT-3 regulates surfactant phospholipid homeostasis in normal lung and during endotoxin-mediated lung injury

Acute lung injury associated with surfactant deficiency remains a major cause of pulmonary morbidity and mortality. Since signal transducer and activator of transcription-3 (STAT-3) plays an important role in protecting respiratory epithelial cells during injury, we hypothesized that STAT-3 may regulate gene expression in type II cells that mediate surfactant phospholipid synthesis. Conditional deletion of Stat-3 in respiratory epithelial cells in the lung of transgenic mice ( Stat-3Δ/Δmice) decreased surfactant phospholipid synthesis and secretion. Deletion of Stat-3 was associated with decreased expression of Akt2, Srebf-1, and other genes expressed in type II cells that may influence surfactant phospholipid synthesis ( Glut-1, Slc34a2, Gpam, Acox2, and Cds2). Stat-3Δ/Δmice were more susceptible to intratracheal lipopolysaccharide (LPS). Saturated phosphatidylcholine and surfactant protein B levels were significantly decreased in bronchoalveolar lavage fluid from LPS-treated Stat-3Δ/Δmice. Alveolar capillary leak, proinflammatory cytokine expression, and perturbations of lung mechanics caused by LPS were exacerbated after deletion of STAT-3. STAT-3 plays a critical role in the regulation of surfactant lipid synthesis in the normal lung and during lung injury caused by LPS.

Reversibility of lung inflammation caused by SP-B deficiency

Whereas decreased concentrations of surfactant protein (SP)-B are associated with lung injury and respiratory distress, potential causal relationships between SP-B deficiency and lung inflammation remain unclear. A transgenic mouse in which human SP-B expression was placed under conditional control of doxycycline via the CCSP promoter was utilized to determine the role of SP-B in the initiation of pulmonary inflammation. Adult mice, made SP-B deficient by removal of doxycycline, developed severe respiratory failure within 4 days. Deficiency of SP-B was associated with increased minimal surface tension of the surfactant and perturbed lung mechanics. Four days of SP-B deficiency did not alter SP-C content or surfactant phospholipid content or composition. SP-B deficiency was associated with lung inflammation and increased soluble L-selectin, STAT-3, and phosphorylated STAT-3 in alveolar macrophages and alveolar epithelial cells. Alveolar IL-6, IL-1beta, and macrophage inflammatory protein-2 concentrations were increased after removal of doxycycline, indicating pulmonary inflammation. Restoration of SP-B expression following administration of doxycycline rapidly reversed SP-B-dependent abnormalities in lung mechanics and inflammation. SP-B deficiency is sufficient to cause lung dysfunction and inflammation in adult mice. SP-B reversed inflammation and maintained lung function in vivo, indicating its potential utility for the prevention and treatment of pulmonary injury and surfactant deficiency.

Alterations in SP-B and SP-C expression in neonatal lung disease

The hydrophobic surfactant proteins, SP-B and SP-C, have important roles in surfactant function. The importance of these proteins in normal lung function is highlighted by the lung diseases associated with abnormalities in their expression. Mutations in the gene encoding SP-B result in severe, fatal neonatal lung disease, and mutations in the gene encoding SP-C are associated with chronic interstitial lung diseases in newborns, older children, and adults. This work reviews the current state of knowledge concerning the lung diseases associated with mutations in the SP-B and SP-C genes, and the potential roles of abnormal SP-B and SP-C expression and genetic variation in these genes in other lung diseases.