Involvement of calcium in the stimulation of phosphatidylcholine secretion in primary cultures of rat type II pneumocytes by Escherichia coli lipopolysaccharide

Long-term exposure to LPS enhances the rate of stimulated exocytosis and surfactant secretion in alveolar type II cells and upregulates P2Y2 receptor expression

Bacterial LPS is a potent proinflammatory molecule. In the lungs, LPS induces alterations in surfactant pool sizes and phospholipid (PL) contents, although direct actions of LPS on the alveolar type II cells (AT II) are not yet clear. For this reason, we studied short- and long-term effects of LPS on basal and agonist-stimulated secretory responses of rat AT II by using Ca(2+) microfluorimetry, a microtiter plate-based exocytosis assay, by quantitating PL and (3)H-labeled choline released into cell supernatants and by using quantitative PCR and Western blot analysis. Long term, but not short term, exposures to LPS led to prolonged ATP-induced Ca(2+) signals and an increased rate in vesicle fusions with an augmented release of surfactant PL. Most notably, the stimulatory effect of LPS was ATP-dependent and may be mediated by the upregulation of the purinergic receptor subtype P2Y(2). Western blot analysis confirmed higher levels of P2Y(2), and suramin, a P2Y receptor antagonist, was more effective in LPS-treated cells. From these observations, we conclude that LPS, probably via Toll-like receptor-4, induces a time-dependent increase in P2Y(2) receptors, which, by yet unknown mechanisms, leads to prolonged agonist-induced Ca(2+) responses that trigger a higher activity in vesicle fusion and secretion. We further conclude that chronic exposure to endotoxin sensitizes AT II to increase the extracellular surfactant pool, which aids in the pulmonary host defense mechanisms.

Regulation of surfactant secretion in alveolar type II cells

Molecular mechanisms of surfactant delivery to the air/liquid interface in the lung, which is crucial to lower the surface tension, have been studied for more than two decades. Lung surfactant is synthesized in the alveolar type II cells. Its delivery to the cell surface is preceded by surfactant component synthesis, packaging into specialized organelles termed lamellar bodies, delivery to the apical plasma membrane and fusion. Secreted surfactant undergoes reuptake, intracellular processing, and finally resecretion of recycled material. This review focuses on the mechanisms of delivery of surfactant components to and their secretion from lamellar bodies. Lamellar bodies-independent secretion is also considered. Signal transduction pathways involved in regulation of these processes are discussed as well as disorders associated with their malfunction.

Role of platelet-activating factor in phosphatidylcholine secretion in primary cultures of rat type II pneumocytes

This study was designed to investigate the effect of platelet-activating factor (PAF) in the secretory response of type II pneumocytes, that are involved in the synthesis and secretion of the pulmonary surfactant. PAF increased phosphatidylcholine secretion in a concentration-dependent manner in the 10(-5) - 10(-10) M range, with a maximum phosphatidylcholine secretion of up to 3.3 fold the basal values (3.4 +/- 0.3% phosphatidylcholine secreted). This effect was prevented by the synthetic PAF-receptor antagonist WEB 2086. A study of the mechanism through which PAF exerts its stimulatory effect was carried out adding different agents that are well known stimulants of phosphatidylcholine secretion. Thus, PAF increased the TPA- and terbutaline-stimulated phosphatidylcholine secretion, that are PKC and PKA activators respectively, suggesting the involvement of both protein kinases in the process. This involvement was further supported by the use of inhibitors of protein kinases and by the stimulation of cAMP production in type II pneumocytes incubated with PAF.