Cholinephosphate Cytidylyltransferase in Fetal Rat Lung Cells: Activity and Subcellular Distribution in Response to Dexamethasone, Triiodothyronine, and Fibroblast-Conditioned Medium

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Abstract
The initiation of pulmonary surfactant synthesis during fetal development has been shown to be under hormonal control. Using cultured lung cells isolated from 19-day-gestation fetal rats, we evaluated the effects of various hormones on the activity and subcellular distribution of cholinephosphate cytidylyltransferase, a rate-controlling enzyme in phosphatidylcholine synthesis. The cells were incubated in medium containing 10% carbon-stripped fetal bovine serum to which dexamethasone, triiodothyronine, and/or conditioned medium from dexamethasone-treated fetal rat lung fibroblasts were added for 48 h. Dexamethasone and fibroblast-conditioned medium increased microsomal enzyme activity 169% ± 6% (mean ± SE, p < 0.01) and 150% ± 2% (p < 0.05) over control levels, respectively. Further, dexamethasone increased cytosolic specific activity 160% ± 17% (p < 0.05). Addition ofT3 to the fibroblast-conditioned medium caused a further increase in microsomal activity, but T3 alone had no effect. Increased microsomal cytidylyltransferase activity correlated with an increased rate of [3H]choline incorporation into disaturated phosphatidylcholine. Hormonal induced increases in enzyme activity were not adequately explained by simple translocation of enzyme from cytosol to microsomes. Cycloheximide (5 μg/ml) inhibited enzyme stimulation by dexamethasone and fibroblast-conditioned medium, suggesting that protein synthesis of new enzyme or regulatory proteins is involved. We conclude that hormones modulate cytidylyltransfease activity of isolated fetal lung cells. Dexarnethasone and fibroblast-conditioned medium exert their major efects by stimulating microsomal activity.

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