Role of keratinocyte growth factor in the control of surfactant synthesis by fetal lung mesenchyme.

Fetal lung maturation is regulated by mesenchymal-epithelial cell communication, which plays a major role in the control of surfactant synthesis by alveolar type II cells. We have recently shown that keratinocyte growth factor (KGF), also called fibroblast growth factor-7, enhances the maturation of fetal alveolar epithelial type II cells. Here, we investigated, among the factors produced by lung mesenchyme, the part attributable to KGF in the control of surfactant synthesis. Using a KGF-neutralizing antibody, we assessed surfactant phospholipid synthesis by measuring choline incorporation into disaturated phosphatidylcholine of isolated fetal type II cells. We found that KGF accounts for about half of the stimulating activity present in fetal lung fibroblast-conditioned medium (FCM). By contrast, the use of an epidermal growth factor-neutralizing antibody did not alter the FCM-stimulating activity. To further delineate KGF properties as a mesenchymal mediator, we wondered about its possibility to relay glucocorticoid-stimulating activity on the synthesis of the phospholipid moiety of surfactant in fetal lung fibroblasts. A 24-h exposure to dexamethasone led us to detect a 50% increase in the level of KGF messenger RNA (mRNA) in isolated fetal lung fibroblasts. Moreover, anti-KGF antibody totally abolished the further increase of FCM-stimulating activity induced by dexamethasone. Thus, KGF seems to be a major player in mediating glucocorticoid stimulation of fetal lung maturation.

Mild vitamin A deficiency delays fetal lung maturation in the rat.

During late pregnancy, the fetal lung stores surfactant in preparation for extrauterine life. Surfactant deficiency, most often due to prematurity, precipitates respiratory distress syndrome (RDS) of the neonate. Although vitamin A (retinol) and retinoic acid have been shown to enhance the synthesis of phospholipid surfactant components, their effect on surfactant-specific proteins is unclear. No attempt has been made to evaluate the consequences of vitamin A restriction on surfactant phospholipid storage or on the expression of the life-essential surfactant protein-B (SP-B). We induced in rats a partial vitamin A deficiency leading to a 30-60% reduction in blood retinol, a status compatible with maintenance of gestation and absence of gross abnormalities in offspring. At term, lung surfactant phospholipids were reduced by 21%, and the major surfactant phospholipid, disaturated phosphatidylcholine (DSPC), was reduced by 27% in vitamin A-deficient (VAD) fetuses. The decrease in surfactant phospholipids and DSPC correlated linearly with plasma retinol, and reached about 50% in fetuses with the lowest retinol concentrations; it was accompanied by reduced expression of the gene for fatty acid synthase, a key enzyme in the synthetic pathway for surfactant-phospholipid lipid precursors. The amounts of SP-A, SP-B, and SP-C messenger RNAs were decreased by 46%, 32%, and 28%, respectively, in VAD fetuses. Consistently, amounts of SP-A and SP-B proteins were diminished as assessed by Western blotting. The proportion of type II cells determined after SP-B labeling was unchanged in VAD as compared with control lungs. Vitamin A deficiency is therefore a cause of lung maturational delay. In view of its rather large incidence in human populations, it may represent an increased risk for RDS and an aggravating factor for prematurity.

Keratinocyte growth factor enhances maturation of fetal rat lung type II cells.

Keratinocyte growth factor (KGF) or fibroblast growth factor (FGF)-7, a peptide produced by stromal cells and in particular by lung mesenchyme, has recently been shown to influence early lung morphogenesis and to be a mitogen for fetal and adult alveolar type II cells. Although contradictory findings have been reported regarding its effects on surfactant protein expression, its effects on surfactant phospholipids have not been studied. We investigated the effects of KGF on the synthesis of surfactant components by cultured fetal rat type II cells isolated during the late gestational period, when surfactant accumulates in preparation for extrauterine life. We show that KGF is a potent stimulus of surfactant phospholipid synthesis, particularly for the major component of surfactant, disaturated phosphatidylcholine (DSPC). KGF increased choline incorporation into DSPC in a dose-dependent manner up to 25 ng/ml (1.3 x 10(-9) M), and this effect was greater for surfactant than for nonsurfactant DSPC. KGF was several times more potent in this respect than acidic FGF at the same molar concentration. KGF, similar to epidermal growth factor, also stimulated acetate incorporation and increased the surfactant phospholipid and DSPC content of cultured cells twofold. These effects correlated with increased choline phosphate cytidylyltransferase activity and increased fatty acid synthase activity and gene expression. KGF also induced a dose-dependent stimulation of surfactant protein-A, -B, and -C gene expression, leading to a 2- to 3-fold increase in their messenger RNAs. KGF therefore stimulates the synthesis of all surfactant components in developing type II cells at the time of surfactant accumulation. Its secretion by lung fibroblasts may thus be an important factor in promoting the maturation of fetal lung epithelium and the synthesis of sufficient surfactant. The results suggest that KGF could provide a new therapeutic agent for the management of the immature or injured lung.