Reutilization of phosphatidylglycerol and phosphatidylethanolamine by the pulmonary surfactant system in 3-day-old rabbits

A Common Pathway for the Uptake of Surfactant Lipids by Alveolar Cells

The uptake of different surfactant lipids-dipalmitoylphosphatidylcholine (DPPC), phosphatidylglycerol (PG), or phosphatidylinositol (PI)-and liposomes with a surfactant-like composition by alveolar type II cells (alveolar type II cells) and macrophages (alveolar macrophages) was studied in vitro. Fluorescent-labeled liposomes containing either 86% of the studied lipid, i.e., DPPC, PG, PI, and 6% labeled phosphatidylethanolamine (PE) and 8% cholesterol or a lipid mixture similar to surfactant (DPPC, PG, PI, phosphatidylcholine, PE, and cholesterol in a weight ratio of 55:8:2:21:8:6) were incubated with alveolar macrophages and alveolar type II cells. The cell-associated fluorescence assessed by flow cytometry demonstrated a higher uptake of PG and PI by both alveolar macrophages and alveolar type II cells, and a lower uptake of DPPC by alveolar macrophages. In addition, fewer alveolar type II cells take up DPPC, whereas there are no differences for the alveolar macrophages in the number of cells involved in the uptake. Competition experiments with Texas Red-labeled liposomes and either DPPC liposomes or PI liposomes labeled with Bodipy indicated that all these liposomes are internalized via the same pathway by alveolar cells. Thus, lipid composition directly influences the (re)uptake of surfactant.

Circadian rhythm of surfactant protein A, B and C mRNA in rats

BACKGROUND

All organisms have developed an internal timing system capable of reacting to and anticipating environmental stimuli with a program of appropriately timed metabolic, physiologic and behavioral events. The alveolar epithelial type II cell of the mammalian lung synthesizes, stores, and secretes a lipoprotein pulmonary surfactant, which functions to stabilize alveoli at low lung volumes.

METHODS

The authors investigated the diurnal variation of surfactant protein A, B and C mRNA accumulation. The diunal variation on gene expression of surfactant protein A, B and C was analysed using filter hybridization at 9 a.m., 4 p.m. and 11 p.m. Lung SP-A protein content was determined by double sandwich ELISA assay using a polyclonal antiserum raised in rabbits against purified rat SP-A.

RESULTS

1. The accumulation of SP-A mRNA at 4 p.m. was significantly decreased by 23.5% compared to the value at 9 a.m. (p < 0.05). 2. The accumulation of SP-B mRNA at 4 p.m. and 11 p.m. was decreased by 15.1% and 5.7%, respectively, compared to the value at 9 a.m. (p = 0.07, p = 0.69). 3. The accumulation of SP-C mRNA at 4 p.m. and 11 p.m. was decreased by 6.8% and 7.7%, respectively, compared to the value at 9 a.m. (p = 0.38, p = 0.57). 4. Total lung SP-A content at 4 p.m. and 11 p.m. was increased by 5.3% and 15.9%, respectively, compared to the value at 9 a.m. (p = 0.64, p = 0.47).

CONCLUSION

These findings represent the diurnal variation of surfactant proteins mRNA expression in vivo. These results indicated that the diurnal variation of significant gene expression is observed in hydrophilic surfactant protein rather than in hydrophobic surfactant proteins.

Metabolism of phosphatidylglycerol by alveolar macrophages in vitro

In whole animal studies, it has been shown that turnover of surfactant dipalmitoylphosphatidylglycerol (DPPG) is faster than that of dipalmitoylphosphatidylcholine (DPPC). The goal of this investigation was to characterize the metabolism of DPPG by alveolar macrophages and to determine whether they contribute to the faster alveolar clearance of DPPG. Isolated rat alveolar macrophages were incubated with liposomes colabeled with [(3)H]DPPG and [(14)C]DPPC. Macrophages internalized both lipids in a time- and temperature-dependent manner. The uptake of both lipids was increased by surfactant protein (SP) A and by adherence of the macrophages to plastic slides. The isotope ratio of DPPC to DPPG internalized by macrophages in suspension in the absence of SP-A was significantly lower than the isotope ratio in liposomes, suggesting that macrophages preferentially internalize DPPG when SP-A is absent. Phospholipase activity in macrophage homogenate was higher toward sn-2-labeled DPPG than toward sn-2-labeled DPPC. These studies show that alveolar macrophages play an important role in catabolizing surfactant lipids and may be partially responsible for the relatively faster clearance of DPPG from the lung.

Experimental neonatal respiratory failure induced by lysophosphatidylcholine: effect of surfactant treatment

The purpose of this study was to characterize the toxic effects of lysophosphatidylcholine (lyso-PC) on neonatal lung function. Various doses of lyso-PC (from 0 to 40 mg/kg) were administered to near-term newborn rabbits. Lung-thorax compliance during mechanical ventilation was significantly decreased by doses >/=10 mg/kg, and static lung volumes during deflation were decreased by doses >/=20 mg/kg. Using the same experimental model, we investigated the effects of modified porcine surfactant (Curosurf, 200 mg/kg). Animals exposed to lyso-PC at birth and treated simultaneously with surfactant showed a satisfactory therapeutic response, whereas those treated after 30 min failed to respond. These animals also had a much larger leak of albumin into the air spaces and an elevated minimum surface tension of the lavage fluid in a pulsating bubble surfactometer, suggesting inactivation of the exogenous surfactant. Timing of surfactant administration may thus be essential for the therapeutic effect in this experimental model of acute lung injury.

Association of surfactant protein C with isolated alveolar type II cells

Surfactant protein C (SP-C) is a small hydrophobic protein that is synthesized and secreted by alveolar type II cells. The mechanism of clearance of SP-C from the alveolar airspace is not well understood, although previous studies demonstrated that recombinant SP-C instilled into the lungs of spontaneously breathing anaesthetized rats was taken up by type II cells and incorporated into lamellar bodies. The current investigation was undertaken to characterize the interaction of a complex of SP-C and surfactant-like lipids with freshly isolated rat alveolar type II cells under conditions in which the extracellular milieu can be regulated. SP-C was isolated from alveolar proteinosis lavage fluid and radiolabeled with 125I-Bolton-Hunter reagent. The radiolabeled protein retained its ability to facilitate adsorption of phospholipids to an air/liquid interface. Labeled human SP-C associated with isolated type II cells in a concentration-dependent manner that was also dependent upon temperature and time. The association of labeled SP-C with isolated type II cells did not saturate up to 150 micrograms/ml. SP-A significantly enhanced the association of SP-C with isolated type II cells. Under the experimental conditions tested, SP-C was not degraded to TCA-soluble products. These results are consistent with the hypothesis that association or uptake of SP-C by type II cells may be enhanced by SP-A and that like SP-A, SP-C is recycled by type II cells.

Transfer of phospholipids by a protein fraction obtained from canine pulmonary lavage

Surfactant phospholipid exists in multicompartment pools within the subphase of the lung. Movement among these pools and back into type II alveolar cells may be catalyzed by a phospholipid transfer protein resident in the subphase. We demonstrate here that a protein fraction obtained from canine lung lavage catalyzes the intermembrane transfer of all the major surfactant phospholipids. The protein is probably not derived from serum and is unrelated to surfactant proteins that have already been described.

Reutilization of surfactant phosphatidylglycerol and lysophosphatidylcholine by adult rabbits

Adult rabbits reutilize the phosphatidylcholine (PC) of surfactant much less efficiently than developing rabbits (22% vs. 95%). Comparisons of reutilization efficiency of other components of surfactant in adult rabbits have not been determined. We injected adult rabbits intratracheally with [3H]dipalmitoylphosphatidylcholine (DPPG) mixed with [14C]lysophosphatidylcholine (lysoPC) and natural surfactant or [14C]DPPC mixed with [3H]dipalmitoylphosphatidylglycerol (DPPG) and natural surfactant. Recovery in the alveolar wash and lamellar bodies of labelled DPPC, lysoPC and DPPG was determined at different times after injection. By plotting the ratio of [3H]DPPG to [14C]DPPC in the alveolar wash versus time after injection we found that phosphatidylglycerol was reutilized with an efficiency of only 0-7% which was much less than the reutilization of PC in these animals. At early times after injection, adult rabbits injected with [14C]lysoPC had a ratio of [14C]PC in their alveolar wash to lamellar bodies that was larger than 1.0. By comparison, 3-day old rabbits injected intratracheally with [14C]lysoPC had a ratio of [14C]PC in alveolar wash to lamellar bodies less than 1.0 at the earliest times measurable. Thus adult rabbits demonstrate a pathway for accumulation of PC in their alveolar space prior to its appearance in lamellar bodies. This was not detected in developing rabbits. As in developing rabbits, adult rabbits reutilize the phosphatidylglycerol of surfactant less efficiently than the PC of surfactant.

The role of sera, growth factors, and hormones in the in vitro production of disaturated phosphatidylcholine and propagation of undifferentiated type II alveolar cells from the fetal rabbit lung

Undifferentiated type II alveolar cells isolated from the fetal rabbit lung were cultured in chemically-defined medium supplemented with 10% carbon-stripped fetal bovine serum for 2-3 days and then placed in serum-free medium, or medium supplemented with either 10% carbon-stripped fetal bovine serum (sFBS) or 10% unstripped fetal bovine serum. Cells exposed to the latter treatment showed the highest growth rate but the lowest level of [3H]choline incorporation into disaturated phosphatidylcholine (DSPC). Cells in serum-free medium showed the lowest rate of growth but removal of serum components stimulated a rapid increase in radioactive precursor incorporation. Cells exposed to the stripped serum were intermediate in both measurements. Incubation of undifferentiated type II cells with EGF stimulated growth but only in the presence of sFBS. EGF also stimulated [3H]choline into DSPC after 48 hours of exposure to the peptide. This latter response could only be elicited in serum-free medium. In the presence of sFBS no effect of the peptide was detected on radioactive precursor incorporation. Similarly dexamethasone (0.55 nM) or 20% medium which had been conditioned by fetal lung fibroblast monolayers did not stimulate [3H]choline in DSPC in the presence of sFBS. In contrast these agents did evoke a significant increase in radioactive precursor incorporation when the undifferentiated type II alveolar cells were incubated in serum-free medium.