Adenosine and leukotrienes have a regulatory role in lung surfactant secretion in the newborn rabbit

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Purinergic signalling and development of the autonomic nervous system

Most early studies of the role of nucleotides in development have evidenced their crucial importance as carriers of energy in all organisms. However, an increasing number of studies are now available to suggest that purines and pyrimidines, acting as extracellular ligands specifically on receptors of the plasma membrane, may play a pivotal role throughout pre- and postnatal development in a wide variety of organisms including amphibians, birds, and mammals. Purinergic receptor expression and functions have been studied in the development of many organs, including the autonomic nervous system (ANS). Nucleotide receptors can induce a multiplicity of cellular signalling pathways via crosstalk with bioactive molecules acting on growth factors and neurotransmitter receptors which are fundamental for the development of a mature and functional ANS. Purines and pyrimidines may influence all the stages of neuronal development, including neural cell proliferation, migration, differentiation and phenotype determination of differentiated cells. Indeed, the normal development of the ANS is disturbed by dysfunction of purinergic signalling in animal models. To establish the primitive and fundamental nature of purinergic neurotransmission in the ontogeny of the ANS, in this review the roles of purines and pyrimidines as signalling molecules during embryological and postnatal development are considered.

Effects of cysteinyl-leukotriene receptors' antagonism by montelukast on lung mechanics and olfactory system histology in healthy mice

CONTEXT

At variance with steroid administration, the possible effects of leukotrienes inhibition on basal respiratory mechanics and olfactory system function are still unclear.

OBJECTIVE

To investigate if interference with the leukotrienes activity may influence basal lung mechanics in healthy mammals, as well as the olfactory system.

MATERIALS AND METHODS

We measured lung mechanics by the end-inflation occlusion method in control and in montelukast i.p. treated anaesthetised healthy mice (10 mg/kg/die for a week). A study of olfactory system histology was also conducted.

RESULTS

Elastance and resistive properties of the lung were not affected by montelukast, while a significant increment of lung hysteresis was observed. The analysis of olfactory system histology revealed no significant effects of montelukast compared to controls.

DISCUSSION AND CONCLUSIONS

Leukotrienes' antagonism does not affect respiratory mechanics in basal conditions, except for a hysteresis increment, which might counteract the increase in expiratory flow in asthmatic subjects assuming montelukast.

Pulmonary type II cell hypertrophy and pulmonary lipoproteinosis are features of chronic IL-13 exposure

Interleukin (IL)-13, a key mediator of Th2-mediated immunity, contributes to the pathogenesis of asthma and other pulmonary diseases via its ability to generate fibrosis, mucus metaplasia, eosinophilic inflammation, and airway hyperresponsiveness. In these studies, we compared surfactant accumulation in wild-type mice and mice in which IL-13 was overexpressed in the lung. When compared with littermate controls, transgenic animals showed alveolar type II cell hypertrophy under light and electron microscopy. Over time, their alveoli also filled with surfactant in a pulmonary alveolar proteinosis pattern. At the same time, prominent interstitial fibrosis occurs. Bronchoalveolar lavage fluid from these mice had a three- to sixfold increase in surfactant phospholipids. Surfactant proteins (SP)-A, -B, and -C showed two- to threefold increases, whereas SP-D increased 70-fold. These results indicate that IL-13 is a potent stimulator of surfactant phospholipid and surfactant accumulation in the lung. IL-13 may therefore play a central role in the broad range of chronic pulmonary conditions in which fibrosis, type II cell hypertrophy, and surfactant accumulation occur.

Regulation of surfactant secretion

Lung surfactant is synthesized in the alveolar type II cell. Its lipids and hydrophobic proteins (SP-B and SP-C) are stored in lamellar bodies and secreted by regulated exocytosis. In contrast, the hydrophilic proteins (SP-A and SP-D) appear to be secreted independently of lamellar bodies. Regulation of surfactant secretion is mediated by at least three distinct signaling mechanisms: activation of adenylate cyclase with formation of cAMP and activation of cAMP-dependent protein kinase; activation of protein kinase C; and a Ca(2+)-regulated mechanism that likely results in the activation of Ca(2+)-calmodulin-dependent protein kinase. These signaling mechanisms are activated by a variety of agonists, some of which may have a physiological role. ATP is one such agent and it activates all three signaling mechanisms. There is increasing information on the identity of several of the signaling proteins involved in surfactant secretion although others remain to be established. In particular the identity of the phospholipase C, protein kinase C and phospholipase D isomers expressed in the type II cell and/or involved in surfactant secretion has been established. Distal steps in the secretory pathway beyond protein kinase activation as well as the physiological regulation of surfactant secretion, are major issues that need to be addressed.

Signal-transduction mechanisms of ATP-stimulated phosphatidylcholine secretion in rat type II pneumocytes: interactions between ATP and other surfactant secretagogues

ATP stimulates phosphatidylcholine secretion in type II cells, an effect that is mediated by both adenosine A2 receptors coupled to adenylate cyclase and P2 receptors coupled to phosphoinositide-specific phospholipase C. Activation of these effector enzymes leads to formation of cAMP, diacylglycerols and inositol trisphosphate (IP3). cAMP in turn activates cAMP-dependent protein kinase, diacylglycerols activate protein kinase C and IP3 promotes Ca2+ mobilization. To further investigate the signal-transduction mechanisms mediating the ATP effect, we examined its action in combination with that of other surfactant secretagogues: 5'(N-ethylcarboxyamido)adenosine (NECA), a A2 agonist that activates adenylate cyclase; TPA (12-O-tetradecanoylphorbol-13-acetate), a direct activator of protein kinase C; and ionomycin, an ionophore that increases intracellular Ca2+. The effects of NECA, TPA and ionomycin were additive and thus consistent with independent signaling mechanisms. However, the effects of all combinations of three or four secretagogues that contained ATP were 10-20% less than additive. This suggested that ATP and other secretagogues act via common mechanisms. Calmodulin antagonists decreased the effects of ionomycin and ATP by approx. 60% and 30%, respectively, but did not decrease the effects of NECA, terbutaline or TPA. Complete inhibition of the effect of ATP was achieved with a combination of a calmodulin antagonist, an A2 antagonist and a protein kinase C inhibitor. These and previous data suggest that the stimulatory effect of ATP on phosphatidylcholine secretion in type II cells is mediated by three signal-transduction mechanisms: activation of cAMP-dependent protein kinase; activation of protein kinase C; and a calmodulin-dependent mechanism.

Potentiation of A2 purinoceptor-stimulated surfactant phospholipid secretion in primary cultures of rat type II pneumocytes

Surfactant secretion is mediated by a number of different signal-transduction mechanisms. Positive and negative interactions between different signaling pathways can have an important influence on the overall regulation of secretion. To examine interactions between the adenosine A2 receptor-mediated pathway and those involving activation of protein kinase C and a Ca++/calmodulin-dependent system, we examined the effect of the A2 agonist 5'-(N-ethylcarboxyamido) adenosine (NECA) in combination with 2 activators of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate (TPA) and dioctanoylglycerol, and the Ca++ ionophore ionomycin on phosphatidylcholine secretion in primary cultures of rat type II cells. The individual agonists increased secretion 3-5-fold over the rate in control cells. The stimulatory effects of NECA+TPA, NECA+dioctanoylglycoerol, and NECA+ionomycin were 44%, 20%, and 44% greater, respectively, than expected by addition of the effects of the individual agonists. NECA increased cAMP formation while the other agonists did not. However, the effect of NECA on cAMP formation was significantly enhanced by TPA and dioctanoylglycerol, while the duration of the increase in cAMP level was prolonged by dioctanoylglycerol and ionomycin. Although the possible involvement of other second messenger systems cannot be excluded, we speculate that the synergistic interaction between the agonists in stimulating phosphatidylcholine secretion is mediated by increased cAMP levels.

Cibacron blue stimulation of surfactant secretion in rat type II pneumocytes

1. The effect of cibacron blue, a selective P2y-purinoceptor antagonist in some systems, on the stimulatory effect of adenosine 5'-triphosphate (ATP) on [3H]-phosphatidylcholine secretion was examined in primary cultures of rat type II pneumocytes prelabelled by overnight culture with [3H]-choline. 2. Cibacron blue alone stimulated phosphatidylcholine secretion in a concentration-dependent manner in the range 10(-4)-10(-3) M. At a concentration of 10(-4) or lower, cibacron blue had no effect on ATP-induced phosphatidylcholine secretion but at 10(-4)-10(-3) M it increased the effect of ATP. Enhancement of the ATP effect was apparent whether cibacron blue was added before or together with ATP. Cibacron blue also increased ATP-induced secretion in the presence of the P1-purinoceptor antagonist, xanthine amine congener (10(-5) M). 3. The stimulatory effect of cibacron blue on phosphatidylcholine secretion was additive to those of 5' (N-ethylcarboxyamido) adenosine (NECA) and terbutaline but less than additive to that of ATP. 4. Cibacron blue alone had no effect on formation of cyclic AMP or inositol phosphate and when added simultaneously with ATP it did not affect the ATP-induced increase in these second messengers. Preincubation of the cells with cibacron blue before addition of ATP, however, resulted in antagonism of the ATP-induced increase in cyclic AMP and inositol phosphates. Preincubation with ATP had the same effect. The stimulatory effects of NECA and terbutaline on cyclic AMP formation were enhanced by preincubation with cibacron blue. 5. Thus, ATP-induced phosphatidylcholine secretion in type II cells is not diminished by the P2y-antagonist, cibacron blue.5. Thus, ATP-induced phosphatidylcholine secretion in type II cells is not diminished by the P2yantagonist, cibacron blue. On the contrary, cibacron blue stimulates phosphatidylcholine secretion. Cibacron blue may act as a P2-agonist in type II pneumocytes.

Type II alveolar epithelial eicosanoid metabolism: predominance of cyclooxygenase pathways and transcellular lipoxygenase metabolism in co-culture with neutrophils

Arachidonic acid (AA) metabolism was studied in freshly isolated type II alveolar epithelial cells of rabbits. Substantial basal secretion of prostanoids with predominance of prostaglandin (PG) I2 was noted. Challenge with the calcium ionophore A23187 resulted in a time- and dose-dependent increase in the generation of all AA cyclooxygenase products to severalfold values following the rank order of 12-heptadecatrienoic acid (12-HHT) greater than PGI2 greater than PGE2 greater than or equal to thromboxane A2 greater than PGF2 alpha approximately PGD2. Even larger augmentation of prostanoid generation was evoked by challenge with free exogenous AA. Generation of the different AA cyclooxygenase products was inhibited by acetylsalicylic acid with IC50 in the range between 250 and 500 microM. In addition to the prostanoid release, ionophore-challenged type II pneumocytes liberated substantial amounts of AA lipoxygenase products with leukotriene (LT) B4 greater than 15-hydroxyeicosatetraenoic acid (HETE) greater than 12-HETE greater than 5-HETE. Generation of LTs and HETEs was markedly increased upon simultaneous disposal of free exogenous AA. No omega-oxidation of LTB4 was noted, and no evidence for secretion of intact LTA4 was obtained. The epithelial cells displayed avid uptake of exogenously offered LTA4 with subsequent enzymatic conversion to LTB4. Co-stimulation of pneumocytes with neutrophils (PMN) resulted in an amplification of LTB4 generation, paralleled by a decrease in nonenzymatic decay products of PMN-derived LTA4; both phenomena were dose dependent on the pneumocyte-PMN ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Addition of PGE2, but not PGF2 alpha, to fetal lung organ cultures accelerates the process of self-differentiation with increased dilatation of terminal airsacs and differentiation of the epithelial lining. Indomethacin reduces the endogenous production by organ cultures of PGE2, PGF2 alpha, 13,14-dihydro-15-keto-PGE2, and 13,14-dihydro-15-keto-PGF2 alpha and retards the process of self-differentiation. Prolonged exposure of cultures to indomethacin results in cell necrosis. Indomethacin inhibition of self-differentiation can be reversed and accelerated by the addition of PGE2. Addition of PGF2 alpha in the presence of indomethacin prevents indomethacin-associated cell necrosis but does not accelerate dilatation or differentiation beyond that of cultures in sera-free media without additions. We propose that the endogenous production of PGE2 is a key process in the mechanism of self-differentiation of human fetal lung in organ culture.

Perinatal changes in lung surfactant calcium measured in situ

Calcium ion is thought to play a role in the structure and function of pulmonary surfactant after secretion into the alveolar space. Since fetal lung liquid calcium concentration is inadequate for this hypothesized role, at a time when optimal surfactant function is necessary for survival, we speculated that the necessary calcium is secreted with the surfactant material, i.e., in the lamellar body. Lungs from rat fetuses at 20, 21, and 22 d gestation, and also from newborn rats at 3-5 h, 1 and 3 d, were rapidly frozen, sectioned, freeze-dried, and examined cold (-100 degrees C) in a transmission electron microscope equipped with a fully quantitative energy-dispersive x-ray detector and analyzer. X-ray spectra were collected from the lamellar bodies and cytoplasm of type II cells at each time point. Lamellar body calcium concentration in the fetus was approximately twice that of the adult (70 +/- 4 vs. 37 +/- 2 mmol/kg dry wt +/- SEM, P less than 0.01), and it decreased rapidly after birth to adult levels. Apically located lamellar bodies in the fetus have a significantly higher calcium concentration than those in a perinuclear position (76 +/- 4 vs. 52 +/- 3, P less than 0.01). There is significant correlation of calcium and chloride concentrations in lamellar bodies, suggesting that factors responsible for the distribution of chloride, i.e., pH, may also be responsible for the accumulation of calcium by these organelles. These results show that mature calcium transport in lamellar bodies is achieved prenatally in the rat, and suggest that the calcium required for normal surfactant function at birth is secreted with the lamellar body.