Parathyroid hormone-related protein in tracheal aspirates of newborn infants

Up-regulation of fetal rat lung parathyroid hormone-related protein gene regulatory network down-regulates the Sonic Hedgehog/Wnt/betacatenin gene regulatory network

Lung development depends on endodermal Sonic Hedgehog (Shh) signaling to mesodermal Wingless/int/beta catenin (Wnt/betacatenin), followed by parathyroid hormone-related protein (PTHrP) signaling from endoderm to mesoderm. Fluid distension of fetal rat lung explants up-regulates PTHrP signaling and down-regulates Shh/Wnt/betacatenin signaling, marked by decreases in Patched, Gli, Frizzled, and Dishevelled, inducing fibroblast triglyceride uptake, type II cell saturated phosphatidylcholine, and surfactant protein-B expression. Bumetanide, which inhibits fluid distension, blocked down-regulation of the Shh/Wnt/betacatenin pathway and up-regulation of the PTHrP pathway, whereas PTHrP (1-34, 5 x 10(-7) M) treatment overcame bumetanide inhibition, and the PTHrP receptor antagonist PTHrP (7-34) amide (5 x 10(-6) M) mimicked bumetanide, indicating that PTHrP signaling mediates fluid distension-induced alveolar differentiation. Fetal rat lung explant automaturation was characterized by decreased Wnt/betacatenin signaling and increased PTHrP/PTHrP receptor signaling, up-regulating fibroblast-specific adipocyte differentiation related protein (ADRP) and peroxisome proliferator-activated receptor gamma. Wnt/betacatenin agonists (LiCl or SB415268) maintained Shh/Wnt/betacatenin signaling, blocking spontaneous up-regulation of the PTHrP pathway, whereas PTHrP or cAMP down-regulated Shh/Wnt/betacatenin signaling and stimulated PTHrP signaling for fibroblast and type II cell differentiation. This is the first evidence that alveolar fluid distension is an organizing principle for PTHrP signaling down-regulation of the Shh/Wnt/betacatenin pathway.

Lower parathyroid hormone-related protein content of tracheal aspirates in very low birth weight infants who develop bronchopulmonary dysplasia

Since parathyroid hormone-related protein (PTHrP) secreted by pulmonary alveolar type II cells is a key physiologic paracrine factor in maintaining alveolar homeostasis, we hypothesized that its levels in the tracheal aspirates (TA) of ventilated very low birth weight infants (VLBWI) would correlate with the development of bronchopulmonary dysplasia (BPD). Therefore, we examined whether TA PTHrP content during the first week of life correlates with the later development of BPD. Forty VLBWI [birth weight, 943 +/- 302 g (mean +/- SD); gestational age, 27 +/- 2 wk; 21 males and 19 females], who were ventilated for respiratory distress syndrome, were studied. The TA were collected once daily until the infants were extubated and immediately frozen at -70 degrees C for subsequent assays for PTHrP and matrix metalloproteinase-8 (MMP-8), a previously described, nonspecific TA biomarker for BPD. The levels of these proteins were correlated with the later development of BPD. PTHrP in the TA during the first week of life was significantly lower in those infants who developed BPD (12/40) than among those who did not (28/40). The PTHrP levels also correlated with the duration of mechanical ventilation needed in these infants. In contrast, MMP-8 levels did not correlate with BPD. We conclude that lower TA PTHrP content during the first week of life in ventilated VLBWI inversely correlates with prolonged ventilation and the later development of BPD.

Rosiglitazone, a peroxisome proliferator-activated receptor-gamma agonist, prevents hyperoxia-induced neonatal rat lung injury in vivo

Molecular disruption of homeostatic alveolar epithelial-mesenchymal interactions results in transdifferentiation of alveolar interstitial lipofibroblasts to myofibroblasts. Although this process was suggested to be a central molecular event in the pathogenesis of bronchopulmonary dysplasia (BPD), so far it has been only demonstrated in vitro; whether it also occurs in vivo is unknown. Our objectives were to determine if exposure to hyperoxia results in pulmonary alveolar lipo-to-myofibroblast transdifferentiation in vivo, and whether treatment with a potent peroxisome proliferator-activated receptor gamma (PPARgamma) (the key lipogenic fibroblast nuclear transcription factor) agonist, rosiglitazone, prevents this process. Newborn Sprague Dawley rat pups were exposed to control (21% O2), hyperoxia alone (95% O2 for 24 hr), or hyperoxia with rosiglitazone (95% O2 for 24 hr + rosiglitazone, 3 mg/kg, administered intraperitoneally) conditions. Subsequently, pups were sacrificed, and lung tissue was analyzed by morphometry, and by reverse transcription-polymerase chain reaction, Western hybridization, and immunohistochemistry for the expression of key lipogenic and myogenic markers. We observed a significant decrease in the expression of lipogenic markers, and a significant increase in the expression of myogenic markers in the hyperoxia-alone group. These hyperoxia-induced morphologic, molecular, and immunohistochemical changes were almost completely prevented by rosiglitazone. This is the first evidence of in vivo lipo-to-myofibroblast transdifferentiation and its almost complete prevention by rosiglitazone, prompting us to conclude that administration of PPARgamma agonists may be a novel, effective strategy to prevent the hyperoxia-induced lung molecular injury that has been implicated in the pathogenesis of BPD.

Parathyroid hormone-related protein and lung biology

Parathyroid hormone-related protein (PTHrP) is expressed in normal and malignant lung and has roles in development, homeostasis, and pathophysiology of injury and cancer. Its effects in developing lung include regulation of branching morphogenesis and type II cell maturation. In adult lung, PTHrP stimulates disaturated phosphatidylcholine secretion, inhibits type II cell growth, and sensitizes them to apoptosis. In lung cancer, PTHrP may play a role in carcinoma progression, or metastasis. The protein could be a useful marker for assessing lung maturity or type II cell function, predicting risk of injury, and detecting lung cancer. PTHrP-based therapies could also prove useful in lung injury and lung cancer.

Arrested pulmonary alveolar cytodifferentiation and defective surfactant synthesis in mice missing the gene for parathyroid hormone-related protein

Parathyroid hormone-related protein (PTHrP) and PTH/PTHrP receptor expression are developmentally regulated in lung epithelium and adepithelial mesenchyme, respectively. To test the hypothesis that PTHrP is a developmental regulator of terminal airway development, we investigated in vivo and in vitro models of alveolar cytodifferentiation using mice in which the gene encoding PTHrP was ablated by homologous recombination. We have determined that fetal and newborn PTHrP(-/-) lungs showed delayed mesenchymal-epithelial interactions, arrested type II cell differentiation, and reduced surfactant lamellar body formation and pulmonary surfactant production. Embryonic PTHrP(-/-) lung buds cultured in the absence of skeletal constriction or systemic compensating factors also exhibited delayed alveolar epithelial (type II cell) and mesenchymal cytodifferentiation, as well as a > 40% inhibition of surfactant phospholipid production (n = 3-5). Addition of exogenous PTHrP to embryonic PTHrP(-/-) lung cultures normalized interstitial cell morphology and surfactant phospholipid production. The importance of PTHrP as an endogenous regulatory molecule in mammalian lung development is supported by the findings that ablation of PTHrP expression in isolated developing lung is sufficient to disrupt normal development of the alveolar ducts and the centriacinar regions.

Monitoring alveolar epithelial function in acute lung injury

OBJECTIVE

Identification of humoral markers of acute lung injury may lead to insights into pathologic mechanisms. In addition, specific markers may be useful for predicting development of acute respiratory distress syndrome (ARDS) or for assessing prognosis. Ultimately, studies of lung injury markers may help define interventions that prevent or moderate ARDS. The alveolar epithelium is important both for the integrity of the blood-gas barrier and for repair of the barrier after lung injury. This article reviews markers that derive from or relate to the alveolar epithelium and that might be used for monitoring alveolar epithelial function in acute lung injury. Surfactant apoproteins may be important markers of injury or for prognosis. Levels of surfactant apoprotein A (SP-A) fall 50-75% in patients with severe lung injury compared to normal patients. Serum levels of SP-A in patients dying of acute respiratory distress syndrome are double serum levels of survivors.

Mesenchymal-epithelial interactions in lung development and repair: are modeling and remodeling the same process?

We propose that lung morphogenesis and repair are characterized by complex cell-cell interactions of endodermal and mesodermal origin, leading to (or returning back to) an alveolar structure that can effectively exchange gases between the circulation and the alveolar space. We provide the developmental basis for cell/molecular control of lung development and disease, what is known about growth and transcription factors in normal and abnormal lung development, and how endodermal and mesodermal cell origins interact during lung development and disease. The global mechanisms that mediate mesenchymal-epithelial interactions and the plasticity of mesenchymal cells in normal lung development and remodeling provide a functional genomic model that may bring these concepts closer together. We present a synopsis followed by a vertical integration of the developmental and injury/repair mechanisms.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

Parathyroid hormone-related protein and lung injury after pulmonary thromboendarterectomy

Parathyroid hormone-related protein (PTHrP) is an autocrine growth and differentiation factor for alveolar type II epithelial cells. Type II cells secrete pulmonary surfactant and are pluripotent cells with a role in alveolar epithelial repair after lung injury. The goals of this study were to investigate whether the levels of PTHrP in bronchoalveolar lavage liquid (BAL) varied between patients who did and did not develop lung injury after pulmonary thromboendarterectomy (PTE). BAL was performed in 48 patients undergoing PTE for unresolved pulmonary emboli. Samples were obtained following induction of anesthesia, following separation from cardiopulmonary bypass, and 48 h postoperatively. PTHrP was measured by radioimmunoassay. Lung injury was diagnosed in 23 patients on the basis of hypoxemia (PaO(2)/FIO(2) < 300) and the presence of lung infiltrates in the absence of infection or atelectasis. Patients with lung injury had significantly lower preoperative BAL levels of PTHrP, 21 (21-30) pg/ml (median, interquartile gap), compared to patients without lung injury, 34 (21-41) pg/ml (P < 0.05). Preoperative BAL PTHrP levels < 32 pg/ml predicted lung injury with a positive predictive value of 60% and negative predictive value of 82%. The odds of developing lung injury for patients with preoperative PTHrP levels below this cutpoint were 6 times the odds for patients with higher levels.

Developmental aspects of parathyroid hormone-related protein biology

Parathyroid hormone-related protein (PTHrP) has been discovered as a parathyroid hormone (PTH)-like factor responsible for the humoral hypercalcaemia of malignancies. Further studies revealed that PTHrP is ubiquitously expressed, in mature as well as in developing normal tissues from various species. Although not completely understood, the biological roles of PTHrP concern a variety of domains, including calcium phosphorus metabolism and bone mineralization, smooth muscle relaxation, cell growth and differentiation, and embryonic development. As a poly-hormone, PTHrP is now acknowledged to act via the paracrine, autocrine, and even the intracrine pathways. This review focuses on the main developmental features of the biology of PTHrP. During embryonic development, PTHrP is considered to be involved as a growth factor that promotes cell proliferation and delays cell terminal maturation. PTHrP has been shown to intervene in the development of various tissues and organs such as the skeleton, skin, hair follicles, tooth, pancreas, and the kidney. In addition, through its midregion sequence, which is able to promote an active transplacental calcium transport, PTHrP may intervene indirectly in the mineralization of the foetal skeleton. PTHrP has also been shown to be necessary for the normal development of the mammary gland, while huge amounts of PTHrP are found in the human milk. Finally, observations of physiologic, vasodilating effects of PTHrP in the kidney suggest its involvment in the control of renal hemodynamics, especially in the perinatal period.