Vivek Balasubramaniam

Inhaled nitric oxide enhances distal lung growth after exposure to hyperoxia in neonatal rats

Exposure of newborn rats to hyperoxia impairs alveolarization and vessel growth, causing abnormal lung structure that persists during infancy. Recent studies have shown that impaired angiogenesis due to inhibition of vascular endothelial growth factor (VEGF) signaling decreases alveolar and vessel growth in the developing lung, and that nitric oxide (NO) mediates VEGF-dependent angiogenesis. The purpose of this study was to determine whether hyperoxia causes sustained reduction of lung VEGF, VEGF receptor, or endothelial NO synthase (eNOS) expression during recovery, and whether inhaled NO improves lung structure in infant rats after neonatal exposure to hyperoxia. Newborn rat pups were randomized to hyperoxia [fraction of inspired oxygen (Fio2), 1.00] or room air exposure for 6 d, and then placed in room air with or without inhaled NO (10 ppm) for 2 wk. Rats were then killed for studies, which included measurements of body weight, lung weight, right ventricular hypertrophy (RVH), morphometric analysis of alveolarization (by mean linear intercept (MLI), radial alveolar counts (RAC), and vascular volume (Vv), and immunostaining and Western blot analysis. In comparison with controls, neonatal hyperoxia reduced body weight, increased MLI, and reduced RAC in infant rats. Lung VEGF, VEGFR-2, and eNOS protein expression were reduced after hyperoxia. Inhaled NO treatment after hyperoxia increased body weight and improved distal lung growth, as demonstrated by increased RAC and Vv and decreased MLI. We conclude that neonatal hyperoxia reduced lung VEGF expression, which persisted during recovery in room air, and that inhaled NO restored distal lung growth in infant rats after neonatal hyperoxia.

Bronchioalveolar stem cells increase after mesenchymal stromal cell treatment in a mouse model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) remains a major complication of prematurity resulting in significant morbidity and mortality. The pathology of BPD is multifactorial and leads to alveolar simplification and distal lung injury. Previous studies have shown a beneficial effect of systemic treatment with bone marrow-derived mesenchymal stromal cells (MSCs) and MSC-conditioned media (MSC-CM) leading to amelioration of the lung parenchymal and vascular injury in vivo in the hyperoxia murine model of BPD. It is possible that the beneficial response from the MSCs is at least in part due to activation of endogenous lung epithelial stem cells. Bronchioalveolar stem cells (BASCs) are an adult lung stem cell population capable of self-renewal and differentiation in culture, and BASCs proliferate in response to bronchiolar and alveolar lung injury in vivo. Systemic treatment of neonatal hyperoxia-exposed mice with MSCs or MSC-CM led to a significant increase in BASCs compared with untreated controls. Treatment of BASCs with MSC-CM in culture showed an increase in growth efficiency, indicating a direct effect of MSCs on BASCs. Lineage tracing data in bleomycin-treated adult mice showed that Clara cell secretory protein-expressing cells including BASCs are capable of contributing to alveolar repair after lung injury. MSCs and MSC-derived factors may stimulate BASCs to play a role in the repair of alveolar lung injury found in BPD and in the restoration of distal lung cell epithelia. This work highlights the potential important role of endogenous lung stem cells in the repair of chronic lung diseases.

Bone marrow-derived angiogenic cells restore lung alveolar and vascular structure after neonatal hyperoxia in infant mice

Neonatal hyperoxia impairs vascular and alveolar growth in mice and decreases endothelial progenitor cells. To determine the role of bone marrow-derived cells in restoration of neonatal lung structure after injury, we studied a novel bone marrow myeloid progenitor cell population from Tie2-green fluorescent protein (GFP) transgenic mice (bone marrow-derived angiogenic cells; BMDAC). We hypothesized that treatment with BMDAC would restore normal lung structure in infant mice during recovery from neonatal hyperoxia. Neonatal mice (1-day-old) were exposed to 80% oxygen for 10 days. BMDACs (1 x 10(5)), embryonic endothelial progenitor cells, mouse embryonic fibroblasts (control), or saline were then injected into the pulmonary circulation. At 21 days of age, saline-treated mice had enlarged alveoli, reduced septation, and a reduction in vascular density. In contrast, mice treated with BMDAC had complete restoration of lung structure that was indistinguishable from room air controls. BMDAC comprised 12% of distal lung cells localized to pulmonary vessels or alveolar type II (AT2) cells and persist (8.8%) for 8 wk postinjection. Coculture of AT2 cells or lung endothelial cells (luEC) with BMDAC augmented AT2 and luEC cell growth in vitro. We conclude that treatment with BMDAC after neonatal hyperoxia restores lung structure in this model of bronchopulmonary dysplasia.

Cord blood angiogenic progenitor cells are decreased in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, is associated with impaired vascular and alveolar growth. Antenatal factors contribute to the risk for developing BPD by unclear mechanisms. Endothelial progenitor cells, such as angiogenic circulating progenitor cells (CPCs) and late-outgrowth endothelial colony-forming cells (ECFCs), may contribute to angiogenesis in the developing lung. We hypothesise that cord blood angiogenic CPCs and ECFCs are decreased in preterm infants with moderate and severe BPD. We quantified ECFCs and the CPC/nonangiogenic-CPC ratio (CPC/non-CPC) in cord blood samples from 62 preterm infants and assessed their relationships to maternal and perinatal risk factors as well as BPD severity. The CPC/non-CPC ratio and ECFC number were compared between preterm infants with mild or no BPD and those with moderate or severe BPD. ECFC number (p<0.001) and CPC/non-CPC ratio (p<0.05) were significantly decreased in cord blood samples of preterm infants who subsequently developed moderate or severe BPD. Gestational age and birth weight were not associated with either angiogenic marker. Circulating vascular progenitor cells are decreased in the cord blood of preterm infants who develop moderate and severe BPD. These findings suggest that prenatal factors contribute to late respiratory outcomes in preterm infants.

Endothelial colony-forming cell conditioned media promote angiogenesis in vitro and prevent pulmonary hypertension in experimental bronchopulmonary dysplasia

Late-outgrowth endothelial colony-forming cells (ECFCs), a type of circulating endothelial progenitor cell (EPC), may contribute to pulmonary angiogenesis during development. Cord blood ECFCs from preterm newborns proliferate more rapidly than term ECFCs but are more susceptible to the adverse effects of hyperoxia. Recent studies suggest that bone marrow-derived EPCs protect against experimental lung injury via paracrine mechanisms independent of vascular engraftment. To determine whether human umbilical cord blood ECFCs from preterm and term newborns have therapeutic benefit in experimental neonatal lung injury, we isolated cord blood ECFCs from full-term and preterm newborns and prepared ECFC-conditioned medium (CM) to test its therapeutic benefit on fetal pulmonary artery endothelial cell (PAEC) proliferation and function as well as alveolar type 2 (AT2) cell growth. PAECs and AT2 cells were isolated from late-gestation fetal sheep. Additionally, we administered both ECFCs and ECFC-CM to bleomycin-exposed newborn rats, an experimental model of bronchopulmonary dysplasia (BPD). Both term ECFC-CM and preterm ECFC-CM promoted cell growth and angiogenesis in vitro. However, when ECFC-CM was collected during exposure to mild hyperoxia, the benefit of preterm ECFC-CM was no longer observed. In the bleomycin model of BPD, treatment with ECFC-CM (or CM from mature EC) effectively decreased right ventricular hypertrophy but had no effect on alveolar septation. We conclude that term ECFC-CM is beneficial both in vitro and in experimental BPD. During oxidative stress, preterm ECFC-CM, but not term ECFC-CM, loses its benefit. The inability of term ECFC-CM to promote alveolarization may limit its therapeutic potential.

BAY 41-2272, a Direct Activator of Soluble Guanylate Cyclase, Reduces Right Ventricular Hypertrophy and Prevents Pulmonary Vascular Remodeling during Chronic Hypoxia in Neonatal Rats

Exposure to hypoxia during the first weeks of life in newborn rats decreases vascular growth and alveolarization and causes pulmonary hypertension (PH). BAY 41-2272 is a novel direct activator of soluble guanylate cyclase independent of nitric oxide, effective as an acute pulmonary vasodilator in an animal model of persistent pulmonary hypertension of the newborn, but whether prolonged BAY 41-2272 therapy is effective in the setting of chronic PH is unknown. We hypothesize that BAY 41-2272 would prevent PH induced by chronic exposure to neonatal hypoxia. At 2 days of age, newborn rats were randomly exposed to hypoxia (FiO2, 0.12) or room air, and received daily intramuscular treatment with BAY 41-2272 (1 mg/kg) or saline. After 2 weeks, rats were killed for assessment of right ventricular hypertrophy (RVH), wall thickness of small pulmonary arteries, vessels density, radial alveolar counts and mean linear intercepts. In comparison with control, hypoxia increased RVH and artery wall thickness, reduced vessels density, decreased radial alveolar counts and increased mean linear intercepts. In comparison with hypoxic controls, prolonged BAY 41-2272 treatment during chronic hypoxia reduced RVH (0.67 ± 0.03 vs. 0.52 ± 0.05; p < 0.05), and attenuated artery wall thickness (48.2 ± 2.8% vs. 35.7 ± 4.1 µm; p < 0.01). However, BAY 41-2272 did not change vessels density, radial alveolar counts or mean linear intercepts. We conclude that BAY 41-2272 prevents the vascular structural effects of PH and reduces RVH but does not protect from hypoxia-induced inhibition of alveolarization and vessel growth. We speculate that BAY 41-2272 may provide a new therapy for chronic PH.

Hyperoxia disrupts vascular endothelial growth factor-nitric oxide signaling and decreases growth of endothelial colony-forming cells from preterm infants

Exposure of preterm infants to hyperoxia impairs vascular growth, contributing to the development of bronchopulmonary dysplasia and retinopathy of prematurity. Disruption of vascular endothelial growth factor (VEGF)-nitric oxide (NO) signaling impairs vascular growth. Endothelial progenitor cells (EPCs) may play an important role in vascular growth. Endothelial colony-forming cells (ECFCs), a type of EPC, from human preterm cord blood are more susceptible to hyperoxia-induced growth impairment than term ECFCs. Therefore, we hypothesized that hyperoxia disrupts VEGF-NO signaling and impairs growth in preterm ECFCs and that exogenous VEGF or NO preserves growth in hyperoxia. Growth kinetics of preterm cord blood-derived ECFCs (gestational ages, 27-34 wk) were assessed in room air (RA) and hyperoxia (40-50% oxygen) with or without VEGF, NO, or N(omega)-nitro-l-arginine. VEGF, VEGF receptor-2 (VEGFR-2), and endothelial NO synthase (eNOS) protein expression and NO production were compared. Compared with RA controls, hyperoxia significantly decreased growth, VEGFR-2 and eNOS expression, and NO production. VEGF treatment restored growth in hyperoxia to values measured in RA controls and significantly increased eNOS expression in hyperoxia. NO treatment also increased growth in hyperoxia. N(omega)-nitro-l-arginine treatment inhibited VEGF-augmented growth in RA and hyperoxia. We conclude that hyperoxia decreases growth and disrupts VEGF-NO signaling in human preterm ECFCs. VEGF treatment restores growth in hyperoxia by increasing NO production. NO treatment also increases growth during hyperoxia. Exogenous VEGF or NO may protect preterm ECFCs from the adverse effects of hyperoxia and preservation of ECFC function may improve outcomes of preterm infants.

Moderate postnatal hyperoxia accelerates lung growth and attenuates pulmonary hypertension in infant rats after exposure to intra-amniotic endotoxin

To determine the separate and interactive effects of fetal inflammation and neonatal hyperoxia on the developing lung, we hypothesized that: 1) antenatal endotoxin (ETX) causes sustained abnormalities of infant lung structure; and 2) postnatal hyperoxia augments the adverse effects of antenatal ETX on infant lung growth. Escherichia coli ETX or saline (SA) was injected into amniotic sacs in pregnant Sprague-Dawley rats at 20 days of gestation. Pups were delivered 2 days later and raised in room air (RA) or moderate hyperoxia (O₂, 80% O₂ at Denvers altitude, ∼65% O₂ at sea level) from birth through 14 days of age. Heart and lung tissues were harvested for measurements. Intra-amniotic ETX caused right ventricular hypertrophy (RVH) and decreased lung vascular endothelial growth factor (VEGF) and VEGF receptor-2 (VEGFR-2) protein contents at birth. In ETX-exposed rats (ETX-RA), alveolarization and vessel density were decreased, pulmonary vascular wall thickness percentage was increased, and RVH was persistent throughout the study period compared with controls (SA-RA). After antenatal ETX, moderate hyperoxia increased lung VEGF and VEGFR-2 protein contents in ETX-O₂ rats and improved their alveolar and vascular structure and RVH compared with ETX-RA rats. In contrast, severe hyperoxia (≥95% O₂ at Denvers altitude) further reduced lung vessel density after intra-amniotic ETX exposure. We conclude that intra-amniotic ETX induces fetal pulmonary hypertension and causes persistent abnormalities of lung structure with sustained pulmonary hypertension in infant rats. Moreover, moderate postnatal hyperoxia after antenatal ETX restores lung growth and prevents pulmonary hypertension during infancy.

Inhaled nitric oxide improves lung structure and pulmonary hypertension in a model of bleomycin-induced bronchopulmonary dysplasia in neonatal rats

Whether inhaled nitric oxide (iNO) prevents the development of bronchopulmonary dysplasia (BPD) in premature infants is controversial. In adult rats, bleomycin (Bleo) induces lung fibrosis and pulmonary hypertension, but the effects of Bleo on the developing lung and iNO treatment on Bleo-induced neonatal lung injury are uncertain. Therefore, we sought to determine whether early and prolonged iNO therapy attenuates changes of pulmonary vascular and alveolar structure in a model of BPD induced by Bleo treatment of neonatal rats. Sprague-Dawley rat pups were treated with Bleo (1 mg/kg ip daily) or vehicle (controls) from day 2 to 10, followed by recovery from day 11 to 19. Treatment groups received early (days 2-10), late (days 11-19), or prolonged iNO therapy (10 ppm; days 2-19). We found that compared with controls, Bleo increased right ventricular hypertrophy (RVH), and pulmonary arterial wall thickness, and reduced vessel density alveolarization. In each iNO treatment group, iNO decreased RVH (P < 0.01) and wall thickness (P < 0.01) and restored vessel density after Bleo (P < 0.05). iNO therapy improved alveolarization for each treatment group after Bleo; however, the values remained abnormal compared with controls. Prolonged iNO treatment had greater effects on lung structure after bleomycin than late treatment alone. We conclude that Bleo induces lung structural changes that mimic BPD in neonatal rats, and that early and prolonged iNO therapy prevents right ventricle hypertrophy and pulmonary vascular remodeling and partially improves lung structure.

Cystathionine protects against endoplasmic reticulum stress-induced lipid accumulation, tissue injury, and apoptotic cell death.

Background: No known function for the amino acid cystathionine other than as an intermediate in cysteine synthesis. Results: Cystathionine prevents ER stress induced steatotic liver injury, acute tubular necrosis and apoptosis without changing induction of the unfolded protein response. Conclusion: Abolition of cystathionine synthesis may contribute to pathogenesis in homocystinuria. Significance: Cystathionine has therapeutic potential for disease states where ER stress is implicated. Cystathionine (R-S-(2-amino-2-carboxyethyl)-l-homocysteine) is a non-proteinogenic thioether containing amino acid. In mammals, cystathionine is formed as an intermediate of the transsulfuration pathway by the condensation of serine and homocysteine (Hcy) in a reaction catalyzed by cystathionine β-synthase (CBS). Cystathionine is subsequently converted to cysteine plus ammonia and α-ketobutyrate by the action of cystathionine γ-lyase (CGL). Pathogenic mutations in CBS result in CBS-deficient homocystinuria (HCU) which, if untreated, results in mental retardation, thromboembolic complications and connective tissue disorders. Currently there is no known function for cystathionine other than serving as an intermediate in transsulfuration and to date, the possible contribution of the abolition of cystathionine synthesis to pathogenesis in HCU has not been investigated. Using both mouse and cell-culture models, we have found that cystathionine is capable of blocking the induction of hepatic steatosis and kidney injury, acute tubular necrosis, and apoptotic cell death by the endoplasmic reticulum stress inducing agent tunicamycin. Northern and Western blotting analysis indicate that the protective effects of cystathionine occur without any obvious alteration of the induction of the unfolded protein response. Our data constitute the first experimental evidence that the abolition of cystathionine synthesis may contribute to the pathology of HCU and that this compound has therapeutic potential for disease states where ER stress is implicated as a primary initiating pathogenic factor.