Cortisol pretreatment enhances the lung growth response to tracheal obstruction in fetal sheep

Thrombospondin-1 expression and localization in the developing ovine lung

Fetal lung growth is critically dependent on the degree to which the lungs are expanded by liquid, although the mechanisms involved are unknown. As thrombospondin-1 (TSP-1) can regulate cell proliferation, attachment, spreading and angiogenesis, we investigated the effects of alterations in fetal lung expansion on TSP-1 expression in sheep. TSP-1 mRNA levels were investigated using Northern blot analysis and in situ hybridization, whereas the protein levels were determined by immunohistochemistry. Early growth response 1 (EGR1) mRNA levels were measured by quantitative real-time PCR. TSP-1 was expressed in type-II alveolar epithelial cells and fibroblasts and its mRNA levels increased from 100.0 +/- 14.0% in control fetuses to 347.5 +/- 73.6% at 36 h of increased lung expansion (P < 0.05), and were reduced to 39.4 +/- 6.1% of control levels (100.0 +/- 20.4%) at 20 days of decreased lung expansion (P < 0.05). The percentage of cells positive for TSP-1 mRNA increased from 1.9 +/- 0.4% to 5.2 +/- 0.8% at 36 h of increased fetal lung expansion (P < 0.01). The proportion of tissue stained positive for TSP-1 protein doubled at 36 h of increased lung expansion (23.3 +/- 2.2%) compared to controls (11.7 +/- 3.2%; P < 0.05). Conversely, at 20 days of decreased lung expansion, the percentage of tissue that stained positive for TSP-1 was halved (25.7 +/- 3.2%) compared to controls (39.8 +/- 3.3%; P < 0.05). The increase in TSP-1 expression may be due to increased mRNA levels of the transcription factor EGR1 at 36 h of increased lung expansion (2.7 +/- 0.7-fold of control levels (1.0 +/- 0.2); P < 0.05). Given the known functions of TSP-1 and its localization within the lung, we speculate that TSP-1 may have a significant role in regulating fetal lung growth.

Regulation of alveolar epithelial cell phenotypes in fetal sheep: roles of cortisol and lung expansion

Our aim was to determine whether cortisol's effect on alveolar epithelial cell (AEC) phenotypes in the fetus is mediated via a sustained alteration in lung expansion. Chronically catheterized fetal sheep were exposed to 1) saline infusion, 2) cortisol infusion (122–131 days' gestation, 1.5–4.0 mg/day), 3) saline infusion plus reduced lung expansion, or 4) cortisol infusion plus reduced lung expansion. The proportions of type I and II AECs were determined by electron microscopy, and surfactant protein (SP)-A, -B, and -C mRNA levels were determined by Northern blot analysis. Cortisol infusions significantly increased type II AEC proportions (to 38.2 ± 2.2%), compared with saline-infused fetuses (23.8 ± 2.4%), and reduced type I AEC proportions (to 59.0 ± 2.2%), compared with saline-infused fetuses (70.4 ± 2.4%). Reduced lung expansion also increased type II AEC proportions (to 52.9 ± 3.5%) and decreased type I AEC proportions (to 34.2 ± 3.7%), compared with control, saline-infused fetuses. The infusion of cortisol into fetuses exposed to reduced lung expansion tended to further increase type II (to 60.3 ± 2.1%, P = 0.066) and reduce type I AEC (to 26.6 ± 2.3%, P = 0.07) proportions. SP-A, -B, and -C mRNA levels changed in parallel with the changes in type II AEC proportions. These results indicate that cortisol alters the proportion of type I and type II AECs via a mechanism unrelated to the degree of fetal lung expansion. However, reductions in fetal lung expansion appear to have a greater impact on the proportion of AECs than cortisol.

Cortisol enhances structural maturation of the hypoplastic fetal lung in sheep

Although exogenous corticosteroids advance structural maturation of the fetal lung, they can adversely affect fetal lung and body growth. Our aim was to determine whether cortisol, at physiological doses, can enhance structural maturation of the hypoplastic fetal lung without affecting fetal lung growth. Fetal sheep were divided into four groups (n= 5 for each) and lung hypoplasia (LH) was induced in two groups. Increasing doses of cortisol (1.5-4.0 mg) were infused into one group of fetuses with LH and one group without LH; the other two groups received saline. LH retarded structural development, reduced tropoelastin mRNA levels, reduced hydroxyproline and elastin contents, and increased active matrix metalloproteinase-2 (MMP-2) levels in the fetal lung. Cortisol infusions had no effect on fetal lung growth or body weights. In fetuses with LH, cortisol increased the percentage airspace, reduced the interalveolar wall thickness, increased alveolar number and reduced the increase in active MMP-2 levels. Thus, relatively low doses of cortisol can enhance structural maturation of the fetal lung without adversely affecting fetal lung growth. However, cortisol did not correct the abnormal deposition of elastin within the alveolar parenchyma associated with LH, indicating that secondary septal crest formation remained abnormal.

Tracheal occlusion stimulates cell cycle progression and type I cell differentiation in lungs of fetal rats

Fetal tracheal occlusion (TO) has been reported to stimulate lung growth but decreases number and maturation of type II cells, effects that vary with gestational age and duration of TO. We examined effects of a novel method of TO (unipolar microcautery to seal the trachea) produced at 19.5-20 days (d) of gestation in fetal rats; fetuses were delivered at term, 22 d. Controls were sham operated and unoperated littermates. TO increased wet lung weight but not dry lung weight or lung DNA and protein. To evaluate further the effects of TO, we examined the cell cycle regulators, cyclins D1 and A, in fetal lungs. Cyclin D1 increased with TO (P < 0.005). TO also increased expression of the type I epithelial cell marker RTI40 (mRNA and protein). TO decreased mRNA for surfactant proteins (SP)-A and -C but did not affect protein levels of SP-A and -B and of RTII70, a type II epithelial cell marker. We conclude that TO by microcautery, even of short duration, has diverse pulmonary effects including stimulating increased levels of cyclin D1 with probable cell cycle progression, type I cell differentiation, and possibly inhibiting type II cell function.

Determination of alveolar epithelial cell phenotypes in fetal sheep: evidence for the involvement of basal lung expansion

The factors that control the differentiation of alveolar epithelial cells (AECs) into type-I and type-II cells in vivo are largely unknown. As sustained increases in fetal lung expansion induce type-II AECs to differentiate into type-I cells, our aim was to determine whether reduced fetal lung expansion can induce type-I AECs to trans-differentiate into type-II AECs. Chronically catheterised fetal sheep were divided into two age-matched control groups and three experimental groups (n = 5 for each). The experimental groups were exposed to either: (1) 10 days of increased lung expansion induced by tracheal obstruction (TO), (2) 10 days of TO followed by 5 days of reduced lung expansion induced by lung liquid drainage (LLD), or (3) 10 days of TO followed by 10 days of LLD. Following 10 days of TO, 5 days of LLD reduced the proportion of type-I AECs from 89.4 +/- 0.9 % to 68.4 +/- 2.8 %, which was similar to control values (64.8 +/- 0.5 %), and increased the proportion of type-II AECs from 1.9 +/- 0.3 % to 21.9 +/- 2.8 %, which remained below control values (33.4 +/- 1.7 %). The same treatment increased surfactant protein (SP)-A, SP-B and SP-C mRNA levels (expressed as a percentage of control values) from 26.7 +/- 6.0 %, 40.0 +/- 7.3 % and 10.3 +/- 1.8 % to 78.1 +/- 10.3 %, 105.8 +/- 12.7 % and 121.0 +/- 14.1 %, respectively. Similar results were obtained after 10 days of LLD, which followed 10 days of TO. These results indicate that the phenotypes of type-I and type-II AECs are strongly influenced by the basal degree of lung expansion in fetal sheep. Furthermore, the coincident increase in type-II AEC proportions and SP mRNA levels in response to LLD suggests that type-I AECs can trans-differentiate into functional type-II cells, and hence are not terminally differentiated.

Fetal lung growth after short-term tracheal occlusion is linearly related to intratracheal pressure

Prenatal tracheal occlusion (TO) has been shown to accelerate fetal lung growth, yet the mechanism is poorly understood. The goal of this study was to determine the relationship between fetal intratracheal pressure (Pitr) and fetal lung growth after TO. Fetal lambs underwent placement of an intratracheal catheter and a reference catheter at 115--120 days gestation (term, 145 days). Fetal Pitr was continuously controlled at three levels (high, 8 mmHg; moderate, 4 mmHg; low, 1 mmHg) by a servo-regulated pump. The animals were killed after 4 days, and the parameters of lung growth were compared. Lung volume (136.0 +/- 16.7, 94.9 +/- 9.7, 55.5 +/- 12.4 ml/kg), lung-to-body weight ratio (6.31 +/- 0.70, 4.89 +/- 0.38, 3.39 +/- 0.22%), whole right lung dry weight (3.01 +/- 0.29, 2.53 +/- 0.15, 2.07 +/- 0.24 g/kg), right lung DNA (130.0 +/- 11.3, 116.7 +/- 8.6, 97.5 +/- 10.9 mg/kg), and protein contents (1,865.5 +/- 92.5, 1,657.6 +/- 106.8, 1,312.0 +/- 142.5 mg/kg) in high, moderate, and low groups, respectively, all increased in the moderate compared with the low group and increased further in the high compared with the moderate group. Morphometry confirmed a stepwise increase in the volume of respiratory region and alveolar surface area. We conclude that lung growth in the first 4 days after TO is closely correlated with fetal Pitr, offering additional evidence that an increase in lung expansion is one of the major factors responsible for TO-induced lung growth.