Influence of the protein kinase C inhibitor 3-[1-[3-(amidinothio) propyl]-1H-indoyl-3-yl]-3-(1-methyl-1H-indoyl-3-yl) maleimide methane sulfonate (Ro-318220) on surfactant secretion in type II pneumocytes

Exocytosis of lung surfactant: from the secretory vesicle to the air-liquid interface

Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

Calcium ionophore and phorbol ester increase membrane binding of annexin a7 in alveolar type II cells

The fusion of lamellar body with plasma membrane, a distal obligatory step in exocytosis of lung surfactant, may be mediated by annexin a7 (anx a7; synexin). To understand the mechanism of anx a7 action, we tested the hypothesis that anx a7 binding to membranes would increase in order to facilitate membrane fusion during stimulation of lung surfactant secretion. Isolated rat alveolar type II cells were treated with established secretagogues of lung surfactant and the membrane and cytosol fractions were analyzed for in vitro binding of anx a7. In cells treated with calcium ionophore (A23187) or phorbol 12-myristate 13-acetate (PMA), anx a7 binding to the membrane fraction was increased by 120%, while that to the cytosol fraction was decreased by 40%, when compared with binding to corresponding fractions from control cells. Protein kinase inhibitors prevented the PMA effects on anx a7 binding. The lamellar body and plasma membrane fractions of A23187-treated cells also showed increased binding of anx a7. The lamellar bodies of A23187-treated cells showed lower K(m) for Ca(2+) and higher maximal binding of anx a7, when compared with those from control cells. Collectively, our findings suggest that these two agents modify membrane proteins to regulate anx a7 binding, which may facilitate increased membrane fusion activity during stimulation of surfactant secretion.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Regulation of SP-B and SP-C secretion in rat type II cells in primary culture

Secretion of lung surfactant phospholipids is a highly regulated process. A variety of physiological and pharmacological agents stimulate surfactant phospholipid secretion in isolated type II cells. Although the lipid and hydrophobic protein components of surfactant are believed to be secreted together by exocytosis of lamellar body contents, regulation of surfactant protein (SP) B and SP-C secretion has not previously been examined. To address the question of whether secretion of SP-B and SP-C is stimulated by the same agonists that stimulate phospholipid secretion, we measured secretion of all four SPs under the same conditions used to measure phosphatidylcholine secretion. Freshly isolated rat type II cells were cultured overnight and exposed to known surfactant phospholipid secretagogues for 2.5 h, after which the amounts of SP-A, SP-B, SP-C, and SP-D in the medium were measured with immunoblotting. Secretion of SP-B and SP-C was stimulated three- to fivefold by terbutaline, 5'-(N-ethylcarboxyamido)adenosine, ATP, 12-O-tetradecanoylphorbol 13-acetate, and ionomycin. Similar to their effects on phospholipid secretion, the stimulatory effects of the agonists were abolished by Ro 31-8220. Secretion of SP-A and SP-D was not stimulated by the secretagogues tested. We conclude that secretion of the phospholipid and hydrophobic protein components of surfactant is similarly regulated, whereas secretion of the hydrophilic proteins is regulated differently.

Regulation of surfactant secretion

Lung surfactant is synthesized in the alveolar type II cell. Its lipids and hydrophobic proteins (SP-B and SP-C) are stored in lamellar bodies and secreted by regulated exocytosis. In contrast, the hydrophilic proteins (SP-A and SP-D) appear to be secreted independently of lamellar bodies. Regulation of surfactant secretion is mediated by at least three distinct signaling mechanisms: activation of adenylate cyclase with formation of cAMP and activation of cAMP-dependent protein kinase; activation of protein kinase C; and a Ca(2+)-regulated mechanism that likely results in the activation of Ca(2+)-calmodulin-dependent protein kinase. These signaling mechanisms are activated by a variety of agonists, some of which may have a physiological role. ATP is one such agent and it activates all three signaling mechanisms. There is increasing information on the identity of several of the signaling proteins involved in surfactant secretion although others remain to be established. In particular the identity of the phospholipase C, protein kinase C and phospholipase D isomers expressed in the type II cell and/or involved in surfactant secretion has been established. Distal steps in the secretory pathway beyond protein kinase activation as well as the physiological regulation of surfactant secretion, are major issues that need to be addressed.

Role of actin depolymerization in the surfactant secretory response of alveolar epithelial type II cells

Alveolar epithelial type II cells (AET2) respond with exocytosis of surfactant containing lamellar bodies to stimulation with mechanical stretch and secretagogues, a process that is fundamental for maintaining alveolar stability and lung gas exchange. In the present study in cultured rat AET2, we employed botulinum C2 toxin, a binary toxin which ADP ribosylates nonmuscle G-actin, as a specific tool to probe the role of the actin microfilament system in the surfactant secretory process. Incubation of AET2 with C2 toxin caused a dose-dependent decay of the cellular F-actin content to a minimum of 20% of baseline, concomitant with an increase in monomeric actin. In parallel, a significant augmentation of baseline surfactant secretion up to twofold elevated levels above control was noted, as assessed by the release of prelabeled phosphatidylcholine. Pretreatment with phalloidin, which stabilized F-actin and reduced the level of G-actin, prevented the C2 toxin-elicited enhancement of baseline surfactant secretion. Even low C2 toxin concentrations, resulting in a reduction of total cellular F-actin content of approximately 10%, sufficed to augment secretagogue (ATP) and, more impressively, mechanical stress elicited an increase in surfactant secretion; the response to the biophysical challenge more than doubled. When investigated in the absence of toxin, different secretagogues (ATP, phorbol ester, betamimetics) caused a rapid-onset, transient reduction of F-actin in the range between 15 and 25% as a consistent part of their secretory response pattern. These data suggest that the state of actin polymerization is intimately linked to the exocytosis process underlying surfactant secretion in AET2. Microfilament system-related compartmentalization effects and/or or the impact of the state of actin assembly on signaling events may be considered as underlying events.