Self-differentiation of human fetal lung organ culture: the role of prostaglandins PGE2 and PGF2 alpha

Effects of intrauterine infection or inflammation on fetal lung development

1. Intrauterine infection or inflammation is common in cases of preterm birth. Preterm infants are at risk of acute respiratory distress as a result of lung immaturity; evidence of exposure to infection and/or inflammation before birth is associated with a reduced risk of neonatal respiratory distress syndrome (RDS). Experimentally induced intrauterine inflammation or infection in sheep causes a precocious increase in pulmonary surfactant in the preterm lungs that improves preterm lung function, consistent with the reduced risk of RDS in human infants exposed to infection and/or inflammation before birth. 2. The effects of intrauterine inflammation on fetal lung development appear to result from direct action of proinflammatory stimuli within the lungs rather than by systemic signals, such as the classical glucocorticoid-mediated lung maturation pathway. However, paracrine and/or autocrine production and/or metabolism of glucocorticoids in fetal lung tissue may occur as a result of inflammation-induced changes in the expression of 11β-hydroxysteroid dehydrogenase (types 1 and 2). 3. Likely candidates that mediate inflammation-induced surfactant production by the preterm lung include prostaglandin E₂ and/or other arachidonic acid metabolites. Intrauterine inflammation induces the expression of enzymes responsible for prostaglandin production in fetal lung tissue. Inhibition of prostaglandin production prevents, at least in part, the effects of inflammation on fetal lungs. 4. Our experiments are identifying mechanisms of surfactant production by the preterm lungs that may be exploited as novel therapies for preventing respiratory distress in preterm infants. Elucidation of the effects of inflammation on the fetal lungs and other organs will allow more refined approaches to the care of preterm infants exposed to inflammation in utero.

Prostaglandins mediate the fetal pulmonary response to intrauterine inflammation

Intra-amniotic (IA) lipopolysaccharide (LPS) induces intrauterine and fetal lung inflammation and increases lung surfactant and compliance in preterm sheep; however, the mechanisms are unknown. Prostaglandins (PGs) are inflammatory mediators, and PGE2 has established roles in fetal lung surfactant production. The aim of our first study was to determine PGE2 concentrations in response to IA LPS and pulmonary gene expression for PG synthetic [prostaglandin H synthase-2 (PGHS-2) and PGE synthase (PGES)] and PG-metabolizing [prostaglandin dehydrogenase (PGDH)] enzymes and PGE2 receptors. Our second study aimed to block LPS-induced increases in PGE2 with a PGHS-2 inhibitor (nimesulide) and determine lung inflammation and surfactant protein mRNA expression. Pregnant ewes received an IA saline or LPS injection at 118 days of gestation. In study 1, fetal plasma and amniotic fluid were sampled before and at 2, 4, 6, 12, and 24 h after injection and then daily, and fetuses were delivered 2 or 7 days later. Amniotic fluid PGE2 concentrations increased ( P < 0.05) 12 h and 3–6 days after LPS. Fetal lung PGHS-2 mRNA and PGES mRNA increased 2 ( P = 0.0084) and 7 ( P = 0.014) days after LPS, respectively. In study 2, maternal intravenous nimesulide or vehicle infusion began immediately before LPS or saline injection and continued until delivery 2 days later. Nimesulide inhibited LPS-induced increases in PGE2 and decreased fetal lung IL-1β and IL-8 mRNA ( P ≤ 0.002) without altering lung inflammatory cell infiltration. Nimesulide decreased surfactant protein (SP)-A ( P = 0.05), -B ( P = 0.05), and -D ( P = 0.0015) but increased SP-C mRNA ( P = 0.023). Thus PGHS-2 mediates, at least in part, fetal pulmonary responses to inflammation.

Expression profiling of human fetal cytosolic sulfotransferases involved in steroid and thyroid hormone metabolism and in detoxification

Protection against chemical insult is essential for normal development of the fetus, however many detoxification enzymes are poorly expressed during fetal development. A major exception is the sulfotransferase (SULT) family, which appears to be widely expressed in the developing human. These enzymes also play a key role in biosynthesis and homeostasis of a number of hormones, including estrogens and iodothyronines. We therefore examined the enzyme activity, protein and mRNA expression of SULT 1A, 1B, 1C, 1E and 2A families in a variety of human fetal and adult tissues. Our results show that these SULTs are expressed in the human fetus, with most present at levels equivalent to or higher than the adult. As there are no isoform-selective substrates for SULTs 1B1 and 1C2 we used immunoblot analysis to show for the first time expression of SULT1B1 at high levels in fetal small intestine, and expression of SULT1C2 in fetal liver, kidney and small intestine. SULT1C2 was not expressed in adult liver or colon. Sulfotransferase expression in the developing fetus is therefore more widespread than in the adult, and this has significant implication for our understanding of human developmental physiology.

Iodothyronine levels in the human developing brain: major regulatory roles of iodothyronine deiodinases in different areas

Thyroid hormones are required for human brain development, but data on local regulation are limited. We describe the ontogenic changes in T(4), T(3), and rT(3) and in the activities of the types I, II, and III iodothyronine deiodinases (D1, D2, and D3) in different brain regions in normal fetuses (13-20 wk postmenstrual age) and premature infants (24-42 wk postmenstrual age). D1 activity was undetectable. The developmental changes in the concentrations of the iodothyronines and D2 and D3 activities showed spatial and temporal specificity but with divergence in the cerebral cortex and cerebellum. T(3) increased in the cortex between 13 and 20 wk to levels higher than adults, unexpected given the low circulating T(3). Considerable D2 activity was found in the cortex, which correlated positively with T(4) (r = 0.65). Cortex D3 activity was very low, as was D3 activity in germinal eminence and choroid plexus. In contrast, cerebellar T(3) was very low and increased only after midgestation. Cerebellum D3 activities were the highest (64 fmol/min.mg) of the regions studied, decreasing after midgestation. Other regions with high D3 activities (midbrain, basal ganglia, brain stem, spinal cord, hippocampus) also had low T(3) until D3 started decreasing after midgestation. D3 was correlated with T(3) (r = -0.682) and rT(3)/T(3) (r = 0.812) and rT(3)/T(4) (r = 0.889). Our data support the hypothesis that T(3) is required by the human cerebral cortex before midgestation, when mother is the only source of T(4). D2 and D3 play important roles in the local bioavailability of T(3). T(3) is produced from T(4) by D2, and D3 protects brain regions from excessive T(3) until differentiation is required.

Stretch-stimulated surfactant synthesis is coordinated by the paracrine actions of PTHrP and leptin

Intrauterine lung development, culminating in physiological pulmonary surfactant production by epithelial type II (TII) cells, is driven by fluid distension through unknown mechanisms. Differentiation of alveolar epithelial and mesenchymal cells is mediated by soluble factors like parathyroid hormone-related protein (PTHrP), a stretch-sensitive TII cell product. PTHrP stimulates pulmonary surfactant production by a paracrine feedback loop mediated by leptin, a soluble product of the mature lipofibroblast (LF). When LFs and TIIs are stretched in coculture, there is a fivefold increase in surfactant phospholipid synthesis that can be "neutralized" by inhibitors of PTHrP or leptin, implicating a paracrine feedback loop in this mechanism. Stretching LFs stimulates PTHrP binding (2.5-fold) and downstream stimulation of triglyceride uptake quantitatively (15-25%) due to upregulation of adipose differentiation-related protein expression. Stretching TII cells increases leptin stimulation of their surfactant phospholipid synthesis threefold, suggesting that retrograde signaling by leptin to TII cells is also stretch sensitive. We conclude that the effect of stretch on alveolar LF and TII differentiation is coordinated by PTHrP, leptin, and their receptors.

Prostaglandin E2 integrates the effects of fluid distension and glucocorticoid on lung maturation

Both glucocorticoids and alveolar fluid distension affect the rate of fetal lung maturation, possibly representing a common cellular pathway. In an explant culture, there is a spontaneous increase in triglyceride incorporation into saturated phosphatidylcholine over time. This mechanism is stimulated by prostaglandin (PG) E2, blocked by both bumetanide and indomethacin, and overridden by exogenous PGE2. Type II cells synthesized and produced PGE2 between days 16 and 21 postconception, increasing fourfold between days 19 and 21. Fetal rat lung fibroblasts released triglyceride in response to PGE2, increasing 10- to 14-fold between days 19 and 21 postconception; phloretin (1 x 10(-5) M) completely blocked this effect of PGE2 on triglyceride release. Dexamethasone stimulated both type II cell PGE2 synthesis (threefold) and fibroblast triglyceride release in response to PGE2 (60%) by day 20 cells. Stretching type II cells also increased PGE2 synthesis (approximately 100% at 1, 2, and 3 h vs. static cultures). Recombination of [3H]triglyceride-labeled fibroblasts with type II cells in an organotypic culture resulted in progressive incorporation of label into saturated phosphatidylcholine by type II cells. This process was also blocked by the addition of indomethacin and overridden by exogenous PGE2. These data suggest that the combined effects of alveolar fluid dilatation and glucocorticoids may coordinate the timely transfer of triglyceride from fibroblasts to type II cells for augmented surfactant production through their effects on PGE2 production and action as term approaches.

Differential expression and immunohistochemical localisation of the phenol and hydroxysteroid sulphotransferase enzyme families in the developing lung

Reversible sulphation, catalysed by sulphotransferases and sulphatases, of biologically active compounds such as androgens and oestrogens is a sensitive mechanism for regulating their bioavailability, and we have previously hypothesised that this process plays a significant role in the regulation of human fetal lung development. Sulphation is also a major detoxification reaction, contributing significantly to the body's chemical defence mechanism. We have used qualitative and semiquantitative immunological studies to determine the temporal expression and localisation of phenol and hydroxysteroid sulphotransferases during human lung development. Our results show that in the early fetal lung, phenol sulphotransferase expression is at its highest, and is most widely distributed throughout the developing respiratory epithelium. With later development, expression levels decrease and become predominantly restricted to the more proximal airways. In contrast, hydroxysteroid sulphotransferase is present only at very low levels in the early-gestation lung but expression increases rapidly through gestation to reach an apparent peak by 1 year postnatal age. The proximal-to-distal gradients of phenol and hydroxysteroid sulphotransferase expression were similar in mature respiratory epithelium, with immunoreactivity in ciliated cells, non-ciliated secretory cells and basal cells, but with no apparent expression in mucus-secreting cells. These studies provide supporting evidence for the hypothesis that hydroxysteroid sulphotransferase, an androgen-inactivating enzyme, contributes to the role of androgens in retarding the maturation of human lung in utero.

Differential release of prostaglandins by organ cultures of human fetal trachea and lung

Human fetal lung at 16-19 weeks gestation has a partially differentiated epithelium, and in organ culture, distal airsacs dilate and the epithelium autodifferentiates to type I and II pneumatocytes, processes regulated by endogenous prostaglandin PGE2. Human fetal trachea, at the same gestation, has a terminally differentiated mucociliary epithelium but after 4-6 d in organ culture, develops squamous metaplasia. Tracheal cultures restricted to 3 d have normal phase-contrast and light microscopy appearances and immunohistochemical reactivities (epithelium: cytokeratin 7,8,18; glutathione S-transferase pi-isozyme; epithelial membrane antigen and mesenchyme; desmin; vimentin). In human fetal trachea organ cultures, the predominant prostaglandins released are 6-keto-PGF1 alpha, PGF2 alpha, and PGE2, a pattern similar to that previously described for human adult trachea and lung. In fetal lung cultures, 13,14-dihydro-15-keto-PGF2 alpha is the major prostaglandin released with lesser amounts of 13,14-dihydro-15-keto-PFG2 alpha,PGF2 alpha,PGE2, and 6-keto-PGF1 alpha. Human fetal lung in vitro has the competence to self-differentiate, as early as 12 weeks gestation and presence of high levels in fetal lung of the inactive metabolite 13,14-dihydro-15-keto-PGE2 relative to PGE2 suggests that active prostaglandin catabolism may be one of the mechanisms to retard this stage of maturation in vivo by limiting PGE2 availability. Surprisingly, the profile of prostaglandins released from fetal lung organ culture does not change to that of a mature lung with terminal differentiation of the epithelium, and this may indicate differences in the expression of key prostaglandin-metabolizing enzymes in developing human fetal lung in culture and with in utero ontogeny.

Maturation of undifferentiated lung epithelial cells into type II cells in vitro: a temporal process that parallels cell differentiation in vivo

BACKGROUND

Formation of alveolar-like structures (ALS) by mature fetal rabbit type II pneumocytes (day 29 gestation) and long-term differentiation on Engelbreth-Holms-Swarm mouse tumor extract or EHS gel (Matrigel) were reported by our group (Blau et al., 1988. J. Cell Physiol., 136:203-214). We now describe structural organization and differentiation of immature lung epithelial cells, isolated at day 22 gestation, into mature type II cells in vitro.

METHODS

Peripheral pulmonary tissue was pooled and undifferentiated epithelial cells isolated for primary culture on Matrigel. Cells were examined 12-16 h after plating and on days 1, 3, 5, and 7 of culture and assessed by phase contrast and by transmission electron microscopy after fixation in situ.

RESULTS

Cells formed ALS 12-16 h after plating. Spherule diameter increased about four to eight times from day 1-7 in culture. There was rapid transformation of tall columnar cells to cuboidal, normal polarization of cells with respect to cell-free lumen of ALS, progressive reduction of glycogen zones, apparent gradual increase of cell organelles such as Golgi apparatus, rough endoplasmic reticulum and mitochondria, and apparent extrusion of lipidic figures into the lumen. These morphologic transformations in vitro temporally paralleled cell differentiation in vivo. The relative increase of 14C-acetate precursor into phosphatidylcholine in contrast to cardiolipin was consistent with these transformations.

CONCLUSIONS

Under the conditions of our culture system, maturation of undifferentiated pulmonary epithelial cells is reproduced in vitro along the same time course and according to the same developmental sequence of fetal lungs in vivo.

Endogenous prostaglandin E2 modulates calcium-induced differentiation in human skin keratinocytes

The concentration of extracellular calcium appears critical to the initiation of keratinocyte differentiation. Prostaglandins (PGs) have also been implicated in cell differentiation. Consequently, the participation of endogenous eicosanoids in calcium-induced differentiation of human keratinocytes was evaluated in vitro. Our results demonstrate that: (1) exogenously introduced PGE2, the major keratinocyte-derived eicosanoid, but not prostaglandin I2 (PGI2) or its stable metabolite 6-keto-PGF1 alpha, enhances calcium-induced cornified envelope formation, an established marker of keratinocyte differentiation; (2) increasing extracellular calcium increased endogenous PGE2 synthesis by cultured keratinocytes; (3) blocking endogenous PGE2 synthesis with indomethacin significantly suppresses calcium-induced formation of the cornified envelope; and (4) adding back PGE2 to indomethacin-treated keratinocytes is able to re-establish the control level of cornified envelope formation following stimulation by calcium. These data document the participation of endogenously generated PGE2 in the modulation of calcium-induced differentiation by human keratinocytes.

Exposure of rabbit fetal lung to glucocorticoids in vitro does not enhance transcription of the gene encoding pulmonary surfactant-associated protein-B (SP-B)

We have investigated the ontogeny and hormonal regulation of both synthesis rates and cellular accumulation of the mRNA for surfactant-associated protein B (SP-B) in rabbit fetal lung. The developmental pattern for SP-B mRNA synthesis increased as a function of gestational age and paralleled that for SP-B mRNA levels except on days 22-26 of gestation where relatively higher levels of gene transcription were observed. Time-course studies with explants from 26- and 30-day fetal lung maintained in culture revealed a gradual increase in mRNA levels and a much smaller increase in gene transcription relative to adult values. Within 48 h of exposure of 26-day explants to dexamethasone at 10(-8) M there was a rapid increase in SP-B mRNA levels to 7-fold adult levels. A similar overall although somewhat slower and attenuated pattern was observed with 30-day explants. Dexamethasone at 10(-8) M had no effect on SP-B gene transcription with explants of either gestational age. We conclude that the major effect of dexamethasone treatment in vitro on SP-B mRNA levels appears to be post-transcriptional and there are small but distinct differences in the effects of glucocorticoids on SP-B mRNA levels with explant cultures from early and late stages of fetal lung maturation.

Prostaglandin production and metabolism in self-differentiating human fetal lung organ culture

PGE2 and PGF2 alpha are released into the media of human fetal lung organ cultures in decreasing amounts with time. This decline in PGs is not due to culture failure or loss of synthetic capacity, which can be stimulated by fetal bovine serum, nor is it due to increased catabolism of PGE2 to 13,14-dihydro-15-keto-PGE2 (PGEM) or of PGF2 alpha to 13,14-dihydro-15-keto-PGF2 alpha (PGFM). Immunohistochemically reactive PGs are not retained within lung cells. Antisera against methyl-moximated derivatives of PGEM or PGFM and preceded by derivatization on tissue sections of PGs by methyl-moximation not only demonstrate the localization of PGEM and PGFM in epithelial cells and blood vessels, but also show an overall decline in immunoreactivity with time. In addition electron microscopy of uncultured fetal lung removed directly after termination reveals various degrees of mitochondrial damage and in some cases plasma membrane blebs which resolve during the period in culture and as fetal lung self-differentiates. It is proposed that oxidative and mechanical stresses, occurring during termination of pregnancy or tissue preparation, result in cell damage and increased lung prostaglandin production, which, although decreasing during culture as cells recover, is sufficient to trigger terminal self-differentiation.

Development of human fetal lung in organ culture compared with in utero ontogeny

In utero, at around 23 wk gestation, the progenitor epithelium of distal airway differentiates into type I and type II pneumatocytes. Human fetal lung organ cultures, as early as 12 wk gestation, have the competence to self-differentiate. Distal airway epithelial immunoreactivity to cytokeratins CK 7, 8, and 18 decreases with differentiation both in utero and in organ culture, whereas reactivity to epithelial membrane antigen remains constant in both. As distal airways dilate, the mean percentage airspace of fetal lungs in organ culture increases to 58%, equivalent to lung of gestation 26.0 +/- 7.3 wk. In organ culture, capillary blood vessels, visualized by vimentin immunoreactivity, remodel and more closely approximate the epithelium but without direct invasion. In utero, at 23 wk gestation, elastin appears as condensation around airways and forms a basis for secondary crests which, by 29 wk gestation, evolve into alveolar septae. In organ culture, no elastin is deposited, no secondary or alveolar crests form, and the lung retains a simple saccular structure. Differentiation of the terminal airway epithelium and mesodermal maturational events to facilitate gas exchange, such as capillary invasion or secondary-alveolar crest formation, are almost synchronous in human lung in utero but clearly dissociate in organ culture.

Prostaglandins PGE2 and PGF2 alpha in human fetal lung: immunohistochemistry and release from organ culture

Immunohistochemical studies in human fetal lung have shown that epithelial and endothelial cells are both strongly and equally reactive for PGE2. In contrast, epithelial PGF2 alpha reactivity varied between fetuses, in some as intense as endothelial staining and in others very much less. As lung organ cultures differentiated, the intensity of PGE2 staining declined in airways and blood vessels, although it was still weakly positive at 10 days. In contrast, epithelial cells rapidly became negative for PGF2 alpha, whereas PGF2 alpha positivity was retained in blood vessels, albeit less obviously. PGF2 alpha and PGE2 were released into the media of organ cultures in decreasing amounts as cultures progressed. Amounts of released PGF2 alpha were greater by 2- to 10-fold than PGE2. Our findings suggest that the endogenous production of prostaglandins by human fetal lung in organ culture has a key role in the self-differentiation process that occurs in the absence of sera or added growth factors or hormones.

Estrogen and phenol sulfotransferase activities in human fetal lung

The sulfation of steroid hormones and xenobiotics by human fetal lung cytosol was examined. 1-Naphthol and estrone were extensively sulfated, whereas paracetamol and dehydroepiandrosterone were not good substrates for the pulmonary enzyme. Investigation of the thermostability and inhibition by 2,6-dichloro-4-nitrophenol (DCNP) of the 1-naphthol and estrone sulfotransferase (ST) activities revealed that the estrone ST activity was more thermolabile and more readily inhibited by DCNP than was the 1-naphthol ST activity. Anion exchange chromatography by FPLC resulted in the resolution of two 1-naphthol ST activities, with the estrone ST activity co-eluting with the more basic 1-naphthol ST activity. When human fetal lung cytosol was subjected to gel filtration FPLC, both the 1-naphthol and estrone ST activities had the same native molecular weight of 63,000 Da. this is the first demonstration of estrogen ST activity in human fetal lung. These results suggest that there are at least two forms of sulfotransferase in human fetal lung and that this tissue is capable of sulfating both xenobiotics and endogenous compounds.