Secretion of surfactant protein A from rat type II pneumocytes

Abstract

Secretion of surfactant phosphatidylcholine has been extensively studied and there is evidence that it is a regulated process that can be influenced by a variety of physiological factors and pharmacological agents. In contrast, secretion of the major surfactant protein, surfactant protein A (SP-A), has been investigated to much lesser extent. It is not known whether SP-A secretion is constitutive or regulated and, if regulated, whether its regulation is similar to that of phosphatidylcholine. To address those questions we measured SP-A secretion in primary cultures of type II pneumocytes under conditions identical to those used to study phosphatidylcholine secretion. Freshly isolated cells from adult rats were cultured overnight, washed, and then incubated in fresh medium in the presence and absence of surfactant phospholipid secretagogues. As previously reported for phosphatidylcholine, SP-A secretion was linear with time for up to 4 h. However, the rate of SP-A secretion, approximately 6% of total SP-A (cells+medium) released into the medium per hour, was more than sixfold greater than that of the lipid. Although freshly isolated cells contained 70% more SP-A than cells that were cultured overnight, the rate of SP-A secretion was not significantly different. Secretion of SP-A by freshly isolated or cultured type II cells was not increased by a combination of ATP, terbutaline, the adenosine A2 receptor agonist 5'(N-ethylcarboxyamido)adenosine, 12-O-tetradecanoylphorbol-13-acetate, and ionomycin at concentrations that optimally stimulated phosphatidylcholine secretion. We conclude that secretion of the major lipid and protein components of surfactant are independently regulated.

Maintenance of surfactant protein A and D secretion by rat alveolar type II cells in vitro

“…SP-A found in lamellar bodies occurs only after endocytosis of previously secreted SP-A (32,39,47). Our data support this concept that SP-A is secreted independent of surfactant phospholipids.…”

“…The relationship of the secretion of SP-A to lamellar body phospholipids is more complex and controversial. The current hypothesis is that newly synthesized SP-A is secreted independent of lamellar bodies, and then the extracellular SP-A is endocytosed, routed to lamellar bodies, and secreted with lamellar bodies (15,32,39). However, studies directly measuring the secretion of SP-A and phospholipids have been difficult to perform, because phospholipid secretion by type II cells is usually done under culture conditions (e.g., on tissueculture plastic) that do not support surfactant protein gene expression.…”

Surfactant protein-A levels increase during Pneumocystis carinii pneumonia in the rat

“…SP-A levels were measured using an indirect ELISA protocol with a rabbit anti-human SP-A polyclonal antiserum as the primary antibody [22]. Rat SP-A purified from surfactant by preparative isoelectric focusing was used as a standard.…”

Pathway to lamellar bodies for surfactant protein A

“…In this scenario, the presence of SP-A in lamellar bodies is accounted for entirely by recycling following endocytosis of extracellular 19,28,29,36). Because of its cellular complexity, it has been difficult to evaluate the relative roles for these two potential pathways (regulated vs. constitutive secretion) in the intact lung, and the majority of recent studies related to intracellular SP-A trafficking have been carried out with isolated type II cells (28,29,31). Unfortunately, these cells in primary culture differentiate relatively rapidly following their isolation and appear to transform into type I cells, which lack the machinery for SP-A synthesis and trafficking.…”

“…These studies were interpreted to indicate transfer of newly synthesized SP-A from the endoplasmic reticulum to storage organelles (lamellar bodies) prior to exocytosis, similar to the pathways for processing of lung surfactant phospholipid. However, more recent studies using the intact lung (11,19) or isolated type II cells (28,31) have indicated possible differences in kinetics for the appearance of SP-A in surfactant and lamellar bodies. These latter publications have suggested an alternative pathway in which newly synthesized SP-A is secreted constitutively, i.e., by a nonregulated secretory pathway.…”