Effect of dexamethasone upon surfactant phosphatidylcholine and phosphatidylglycerol synthesis in organotypic cultures of type II cells

The effect in premature infants of prenatal corticosteroids on endogenous surfactant synthesis as measured with stable isotopes

Most in vitro studies show that prenatal administration of corticosteroids stimulates the synthesis of surfactant phosphatidylcholine (PC), but studies in animals are controversial. Whether prenatal corticosteroids stimulate surfactant PC synthesis in humans has not been studied. We studied endogenous surfactant PC synthesis in relation to prenatal corticosteroid treatment in 27 preterm infants with respiratory distress syndrome. Infants received a 24-h infusion of the stable isotope [U-(13)C]glucose, starting approximately 5 h after birth. We measured (13)C-incorporation into palmitic acid in surfactant PC from serial tracheal aspirates and in plasma triglycerides and phospholipids by isotope-ratio mass spectrometry. Premature infants had received either zero (n = 11), one (n = 4), or two doses (n = 12) of prenatal betamethasone (12 mg intramuscularly). The fractional synthesis rate (FSR) of surfactant PC from glucose was 1.7 +/- 0.3%/d without corticosteroid treatment, 2.9 +/- 1.4%/d with one dose of prenatal corticosteroid, and 5.8 +/- 1.3%/d after two doses of prenatal corticosteroid. Using multiple regression analysis, we found that the FSR of surfactant PC increased by 40% (confidence interval: 7 to 82%/d, p < 0.02) per dose of corticosteroid and doubled after two doses of corticosteroid. The (13)C-enrichment of plasma triglycerides and phospholipids was not increased by corticosteroid. These data show for the first time that prenatal corticosteroid treatment stimulates surfactant synthesis in the preterm infant.

Metabolism of endogenous surfactant in premature baboons and effect of prenatal corticosteroids

We studied the synthesis of surfactant and the effect of prenatal betamethasone treatment in vivo in very preterm baboons. Ten pregnant baboons were randomized to receive either betamethasone (beta) or saline (control) 48 and 24 h before preterm delivery. The newborn baboons were intubated, treated with surfactant, and ventilated for 6 d. They received a 24-h infusion with the stable isotope [U-(13)C]glucose as precursor for the synthesis of palmitic acid in surfactant phosphatidylcholine (PC). Palmitic acid in surfactant PC became enriched 27 +/- 2 h after the start of the isotope infusion and was maximally enriched at 100 +/- 4 h. The fractional synthesis rate of PC palmitate in the beta group (1.5 +/- 0.2%/d) was increased by 129% above control (0.7 +/- 0.1%/d) (p < 0.02, Mann- Whitney U test). The absolute synthesis rate of PC in the beta group [1.6 +/- 0.3 micromol/kg/d] was increased by 128% above controls [0.7 +/- 0.2 micromol/kg/d] (p < 0.02). These data show that the synthesis of endogenous surfactant from plasma glucose as precursor is a slow process. It is shown, for the first time in vivo, that prenatal glucocorticosteroids stimulate the synthesis of surfactant PC in the very premature baboon.

Improved maintenance of adult rat alveolar type II cell differentiation in vitro: effect of hydrocortisone and cyclic AMP

We have examined the effect of hydrocortisone and cyclic AMP on the maintenance of lipid synthesis in primary cultures of adult rat alveolar type II cells. These hormones were tested in the presence of either 1% or 5% charcoal-stripped rat serum (CS-rat serum). The effect of substratum on responsiveness to these hormones was evaluated by comparing cells cultured for 4 days on tissue culture plastic, on floating type I collagen gels, on rat lung fibroblast feeder layers on floating collagen gels (floating feeder layers), and on Engelbreth-Holm-Swarm (EHS) tumor basement membrane gels. Type II cells cultured on floating feeder layers in medium containing 1% CS-rat serum and 10(-5) M hydrocortisone plus 0.5 mM dibutyryl cyclic AMP exhibited significantly increased incorporation of [14C]acetate into total lipids (238% of control). The hormone combination also increased the relative percentage of acetate incorporated into phosphatidylglycerol (PG; 7.3% versus 1.9%) and saturated phosphatidylcholine (PC; 43.6% versus 37.6%). The percentage of acetate incorporated into neutral lipids was significantly decreased by the addition of hormones (28.6% versus 70.0%). The addition of hydrocortisone and cyclic AMP to medium containing 5% CS-rat serum resulted in an increase in the relative incorporation of acetate into saturated PC (51.2% versus 46.4%), but had no effect on the relative incorporation of acetate into PG or on the incorporation of acetate into total lipids. Type II cells cultured on EHS gels in medium containing 1% CS-rat serum plus hydrocortisone and cyclic AMP showed increased acetate incorporation into total lipids (204% of control) and a relative decrease in the percentage of acetate incorporated into neutral lipids (16.9% versus 47.0%). The hormone combination also increased the relative incorporation of acetate into PG (4.4% versus 2.5%) and saturated PC (49.9% versus 42.1%). Hydrocortisone and cyclic AMP added to medium containing 5% CS-rat serum concentration increased the relative incorporation of acetate into saturated PC by type II cells on EHS gels, but these additions had no effect on acetate incorporation into PG. No responses to these soluble factors were seen when type II cells were cultured on floating type I collagen gels without feeder layers or on tissue culture plastic. These data indicate that there are positive interactions between substratum, soluble factors and serum in the maintenance of differentiated function of adult rat alveolar type II cells in vitro.

Effects of intralipid and hydrocortisone upon human fetal lung cell cultures

Organotypic cell culture systems of human fetal lungs of 15, 18, and 26 weeks' gestational age were treated with Intralipid, a phosphatidylcholine-containing lipid mixture, and with hydrocortisone of varying concentrations. The lamellar bodies found in the pneumocytes type II were ultrastructurally identified. Their amount was quantitated by point-counting, a morphometrical method. Intralipid had a stimulating effect upon the surfactant production depending on the concentration admitted. This effect was quantitatively compared to the known effect of hydrocortisone. Intralipid at a concentration of 10(-2%) produced a significant increase of the relative volume of lamellar bodies (P = 0.05) at a gestational age of 18 weeks. This effect is comparable to hydrocortisone treatment at a concentration of 10(-1%) (P = 0.05) and 10(-3%) (P = 0.01). At a gestational age of 26 weeks, Intralipid at a concentration of 10(-1%) (P = 0.01) stimulated lamellar body production. Hydrocortisone had a similar effect at a concentration of 10(-1%) (P = 0.01). Intralipid does not pass the placenta-barrier and is locally applied by amniocentesis. Therefore, complications to the maternal organism and probably to the fetuses are negligible. The application of Intralipid represents an alternative method to accelerate antenatal surfactant production and to improve the rate of survival of preterm infants.

Effect of fibroblast-pneumonocyte factor on the synthesis of surfactant phospholipids in type II cells from fetal rat lung

The effect of fibroblast-pneumonocyte factor on the formation of surfactant-associated phospholipids was studied in fetal rat lung type II cells in primary culture. Exposure of the cells to fibroblast-pneumonocyte factor enhanced the incorporation of radioactively labelled choline, glycerol and palmitate into total and saturated phosphatidylcholine. Fibroblast-pneumonocyte factor also stimulated the formation of phosphatidylglycerol from labelled glycerol and palmitate but did not significantly affect the synthesis of the other phospholipids. This indicates that fibroblast-pneumonocyte factor specifically stimulates the production of surfactant lipids in fetal lung.

Effects of cyclic AMP analogues and phosphodiesterase inhibitors on phospholipid biosynthesis in fetal type II pneumocytes

Purified type II pneumocytes grown in monolayer cultures after isolation from fetal rabbit lung organotypic cultures were employed to investigate effects of cAMP analogues and phosphodiesterase inhibitors on [methyl-14C]choline and [9-10(n)3H]palmitate incorporation into cell lipids. After 24 h exposure to 0.5 mM N6,O2-dibutyryl-cAMP or 8-bromo-cAMP, a significant increase was found in the rate of incorporation of choline into phospholipids. Addition of 1 mM 1-methyl-3-isobutylxanthine or aminophylline also increased incorporation of choline into phospholipids but did not significantly change the incorporation of choline into sphingomyelin. These effects were not due to increased uptake of choline or changes in the pool size of the precursor. Cyclic AMP analogues also stimulated the rate of incorporation of palmitate into most lipid fractions but did not alter the relative percentages of incorporation of either precursor into any of the phospholipids. Phosphodiesterase inhibitors did not significantly change the rate of incorporation of palmitate into neutral lipids and most phospholipids, except for a decrease into sphingomyelin, phosphatidylinositol and phosphatidylethanolamine. However, they increased the percentage of incorporation of palmitate into phosphatidylcholine and decreased the percentage of incorporation into most other phospholipids. These data clearly indicate that cAMP can stimulate the synthesis of phospholipids within the type II pneumocytes. This effect is probably a general stimulation effect for the cAMP analogues but methylxanthines may selectively increase the synthesis of surfactant lipids such as phosphatidylcholine while decreasing that of other membrane-associated phospholipids.

Incorporation of [3H]palmitate into disaturated phosphatidylcholines in alveolar type II cells isolated by centrifugal elutriation

In order to study synthesis of pulmonary surfactant materials, we measured incorporation of [3H]palmitate into disaturated phosphatidylcholines (PC) in alveolar type II cells isolated by centrifugal elutriation. The time course for this process is not linear and, at high external palmitate levels (1 mM), incorporation is maximal in 4-5 h. Incorporation is dependent on extracellular palmitate with a Vmax (at 1 mM) of 1.66 nmol palmitate incorporated into disaturated PC/4.2 X 10(5) cells per 2 h and a K1/2 of 0.1 mM palmitate. Addition of an optimal amount of extracellular choline (0.05 mM) increases Vmax and decreases K1/2 for palmitate. Incorporation of palmitate is dependent upon cell number, inhibited by extracellular Ca2+ and stimulated by external Mg2+. Cholinergic and beta-adrenergic agonists do not increase incorporation. Pulmonary lavage fluid inhibits incorporation of palmitate into disaturated PC, suggesting there is negative feedback involved. Disaturated PC which has been recently synthesized (i.e., over a 2 h period) is broken down intracellularly by type II cells when they are suspended in palmitate-free medium. These results indicate that (1) several factors, such as substrate levels, cell number, Ca2+, Mg2+ and amount of surfactant present, are involved in the regulation of palmitate incorporation into disaturated PC; (2) disaturated PC which has been recently synthesized may be broken down by type II cells; and (3) surfactant synthesis in freshly isolated cells differs slightly from that reported by other investigators in type II cells maintained in primary cell culture.

The effects of insulin and hyperglycemia on surfactant phospholipid synthesis in organotypic cultures of type II pneumocytes

Organotypic cultures of fetal type II epithelial cells were incubated in media containing insulin at concentrations ranging from 10 to 400 microunits/ml. Exposure to insulin resulted in increased glucose uptake from the media and in the rate of glucose conversion to CO2. Furthermore, both glucose uptake and CO2 production were dependent on the glucose concentration in the media. Surfactant and residual phosphatidylcholine fractions were isolated from the organotypic cultures by sucrose density centrifugation. The presence of low doses of insulin (10-25 microunits/ml) caused a significant increase in the incorporation of glucose into both surfactant and residual phosphatidylcholine. Insulin at levels of 100 microunits/ml or higher resulted in a significant decrease in glucose incorporation into both phosphatidylcholine fractions. Increasing the media glucose concentration from 5.6 to 20 mM caused a 2- to 2.5-fold increase in glucose utilization for surfactant and residual phospholipid synthesis, but did not produce any significant changes in choline incorporation into either surfactant or residual phosphatidylcholine. The addition of 400 microunits/ml of insulin to media containing 20 mM glucose, however, resulted in a 20% decrease in choline incorporation into surfactant phosphatidylcholine but had no effect on choline incorporation into residual phosphatidylcholine. These results suggest that insulin is an important hormone regulating fetal lung maturation and that hyperinsulinemia may be responsible for the delayed lung development in infants of diabetic mothers.

Delayed pulmonary phosphatidylglycerol synthesis and reversal by prenatal dexamethasone in fetal rats of streptozotocin-diabetic mothers

Lung slices from fetal rats of streptozotocin-diabetic mothers incorporated [3H]glycerol and [3H]choline into phosphatidylglycerol and disaturated phosphatidylcholine, respectively. When compared to age-matched fetuses from nondiabetic mothers, lung phosphatidylglycerol synthesis of 21-day fetuses of diabetic mothers was significantly diminished, although [3H]glycerol incorporation into other phospholipids was not impaired. Synthesis of disaturated phosphatidylcholine was not diminished in lungs of 20-, 21-, or 22-day fetuses of diabetic mothers. Prenatal dexamethasone partially reversed the diminished phosphatidylglycerol synthesis at 21 days of gestation; the degree of stimulation was the same as that seen in 21-day fetuses of normal mothers but the maximal rate of [3H]glycerol incorporation was about 60% of that in 21-day fetuses of normal mothers. Fetal lung disaturated phosphatidylcholine synthesis was not stimulated by dexamethasone in diabetic pregnancies, in contrast to that seen in nondiabetic pregnancies. These data suggest that maternal diabetes interferes with the ability of fetal lungs to synthesize phosphatidylglycerol, a finding consistent with the delayed appearance of phosphatidylglycerol in the amniotic fluid of human diabetic pregnancies. In addition, maternal diabetes impairs the responsiveness of disaturated phosphatidylcholine synthesis to dexamethasone. Since phosphatidylglycerol synthesis is enhanced by prenatal dexamethasone, this therapy may still be effective for reducing the adverse impact of maternal diabetes on fetal lung development.

On the suitability of organotypic cultures of fetal rat lung type II cells for biochemical studies concerning development

When organotypic cultures of fetal rat lung epithelial cells are initiated with undifferentiated cells, the cells differentiate into type II cells (Douglas, W.H.J., McAteer, J. A., Smith, J.R. and Braunschweiger, W.R. (1979) Int. Rev. Cytol., Suppl. 10, 45-65). This conclusion was based only on morphologic studies. The present study was undertaken to investigate whether such maturation in culture could also be demonstrated biochemically. In organotypic cultures initiated with epithelial cells from fetal rat lungs at 17-days gestation, the amount of phospholipids increased for at least 10 days. However, no change took place in the percentage of phosphatidylglycerol nor in the ratio of disaturated to total phosphatidylcholine. In cultures initiated with cells obtained at day 17 of gestation the specific activity of cholinephosphate cytidylyltransferase reached a maximum after approximately 3 days, followed by a decrease. A similar profile was obtained, however, if the culture was started at day 20 of gestation. This indicates that the activity profiles obtained in the organotypic cultures reflect changes caused by the culture conditions rather than changes caused by maturation. From these investigations it is concluded that biochemical studies on type II cell development using organotypic cultures as model should be interpreted with caution.