VDUP1: a potential mediator of expansion-induced lung growth and epithelial cell differentiation in the ovine fetus

Lung hypoplasia in newborn rabbits with a diaphragmatic hernia affects pulmonary ventilation but not perfusion

BackgroundA congenital diaphragmatic hernia (DH) can result in severe lung hypoplasia that increases the risk of morbidity and mortality after birth; however, little is known about the cardiorespiratory transition at birth.MethodsUsing phase-contrast X-ray imaging and angiography, we examined the cardiorespiratory transition at birth in rabbit kittens with DHs. Surgery was performed on pregnant New Zealand white rabbits (n=18) at 25 days' gestation to induce a left-sided DH. Kittens were delivered at 30 days' gestation, intubated, and ventilated to achieve a tidal volume (V_t) of 8 ml/kg in control and 4 ml/kg in DH kittens while they were imaged.ResultsFunctional residual capacity (FRC) recruitment and V_t in the hypoplastic left lung were markedly reduced, resulting in a disproportionate distribution of FRC into the right lung. Following lung aeration, relative pulmonary blood flow (PBF) increased equally in both lungs, and the increase in pulmonary venous return was similar in both control and DH kittens.ConclusionThese findings indicate that nonuniform lung hypoplasia caused by DH alters the distribution of ventilation away from hypoplastic and into normally grown lung regions. During transition, the increase in PBF and pulmonary venous return, which is vital for maintaining cardiac output, is not affected by lung hypoplasia.

Differential expression of thioredoxin binding protein-2/Txnip in human placenta: Possible involvement of hypoxia in its suppression during early pregnancy

AIM

Thioredoxin binding protein-2 (TBP-2), which is identical to thioredoxin interacting protein (Txnip), controls cellular proliferation and differentiation. The aim of the present study was to compare TBP-2 protein and mRNA expression in human placenta during the three trimesters of pregnancy and to investigate the role of hypoxia in the change of these expressions in placental tissue. A secondary objective was to determine the gene expression of peroxisome proliferator-activated receptors (PPARs) in TBP-2 deficient placenta using TBP-2 gene disrupted mice (TBP-2^-/- ).

METHODS

Protein and mRNA expression of TBP-2 in human placenta from each trimester were analyzed by immunohistochemistry, Western blots, and by quantitative reverse-transcriptase-polymerase chain reaction. The effect of hypoxia on TBP-2 expression was tested using an explant culture of human placenta. In TBP-2^-/- mouse placenta, we detected PPAR mRNA expression.

RESULTS

TBP-2 was located in syncytiotrophoblasts and cytotrophoblasts, and also in the endothelium in human placenta. Its expression in the placenta was low in the first trimester, and increased in the second and third trimesters. Hypoxia decreased TBP-2 mRNA and protein expression in human placental explant culture. In TBP-2^-/- mice, placental mRNA levels of PPARα and γ were significantly suppressed compared with those in wild-type mice.

CONCLUSION

Hypoxia suppresses TBP-2 gene expression, which may ultimately alter placental development.

First molecular characterisation and expression analysis of a teleost thioredoxin-interacting protein (TXNIP) gene from rock bream (Oplegnathus fasciatus)

Thioredoxin-interacting protein (TXNIP) is an important regulator of glucose metabolism that functions by inhibiting cellular glucose uptake. The full-length rock bream (Oplegnathus fasciatus) TXNIP (RbTXNIP) cDNA (2499 bp) contains an open reading frame of 1188 bp encoding 396 amino acids. Furthermore, multiple alignments showed that the arrestin domain was well conserved among the other TXNIP sequences tested. RbTXNIP was predicted to contain a PxxP and PPxY motif. Phylogenetic analysis indicated that RbTXNIP is most closely related to Fugu rubripes TXNIP. RbTXNIP was expressed significantly in the RBC, intestine, and spleen. RbTXNIP mRNA expression was also examined in several tissues under conditions of bacterial and viral challenge. Generally, all tissues examined from fish infected with Streptococcus iniae, Edwardsiella tarda and red sea bream iridovirus (RSIV) showed significant downregulation in RbTXNIP expression compared to controls. However, RbTXNIP expression showed significant upregulation in the spleen and kidney after injection of recombinant rock bream TRx1 protein. These findings provide a molecular foundation for functional studies and applications in teleosts.

The thioredoxin system in neonatal lung disease

SIGNIFICANCE

Fetal lung development takes place in hypoxia meaning that premature birth is hyperoxia for the prematurely born infant. The most common respiratory morbidity afflicting premature infants is bronchopulmonary dysplasia (BPD). Pathophysiologically, BPD represents the impact of injury, including O2 toxicity, to the immature developing lung that causes arrested lung development.

RECENT ADVANCES

The thioredoxin (Trx) system, which is predominantly expressed in pulmonary epithelia in the newborn lung, acts as an antioxidant system; however, it is increasingly recognized as a key redox regulator of signal transduction and gene expression via thiol-disulfide exchange reactions.

CRITICAL ISSUES

This review focuses on the contribution of Trx family proteins toward normal and aberrant lung development, in particular, the roles of the Trx system in hyperoxic responses of alveolar epithelial cells, aberrant lung development in animal models of BPD, O2-dependent signaling processes, and possible therapeutic efficacy in preventing O2-mediated lung injury.

FUTURE DIRECTIONS

The significant contribution of the Trx system toward redox regulation of key developmental pathways necessary for proper lung development suggests that therapeutic strategies focused on preserving pulmonary Trx function could significantly improve the outcomes of prematurely born human infants.

Thioredoxin/Txnip: redoxisome, as a redox switch for the pathogenesis of diseases

During the past few decades, it has been widely recognized that Reduction-Oxidation (redox) responses occurring at the intra- and extra-cellular levels are one of most important biological phenomena and dysregulated redox responses are involved in the initiation and progression of multiple diseases. Thioredoxin1 (Trx1) and Thioredoxin2 (Trx2), mainly located in the cytoplasm and mitochondria, respectively, are ubiquitously expressed in variety of cells and control cellular reactive oxygen species by reducing the disulfides into thiol groups. Thioredoxin interacting protein (Txnip/thioredoxin binding protein-2/vitamin D3 upregulated protein) directly binds to Trx1 and Trx2 (Trx) and inhibit the reducing activity of Trx through their disulfide exchange. Recent studies have revealed that Trx1 and Txnip are involved in some critical redox-dependent signal pathways including NLRP-3 inflammasome activation in a redox-dependent manner. Therefore, Trx/Txnip, a redox-sensitive signaling complex is a regulator of cellular redox status and has emerged as a key component in the link between redox regulation and the pathogenesis of diseases. Here, we review the novel functional concept of the redox-related protein complex, named “Redoxisome,” consisting of Trx/Txnip, as a critical regulator for intra- and extra-cellular redox signaling, involved in the pathogenesis of various diseases such as cancer, autoimmune disease, and diabetes.

Thioredoxin and thioredoxin target proteins: from molecular mechanisms to functional significance

The thioredoxin (Trx) system is one of the central antioxidant systems in mammalian cells, maintaining a reducing environment by catalyzing electron flux from nicotinamide adenine dinucleotide phosphate through Trx reductase to Trx, which reduces its target proteins using highly conserved thiol groups. While the importance of protecting cells from the detrimental effects of reactive oxygen species is clear, decades of research in this field revealed that there is a network of redox-sensitive proteins forming redox-dependent signaling pathways that are crucial for fundamental cellular processes, including metabolism, proliferation, differentiation, migration, and apoptosis. Trx participates in signaling pathways interacting with different proteins to control their dynamic regulation of structure and function. In this review, we focus on Trx target proteins that are involved in redox-dependent signaling pathways. Specifically, Trx-dependent reductive enzymes that participate in classical redox reactions and redox-sensitive signaling molecules are discussed in greater detail. The latter are extensively discussed, as ongoing research unveils more and more details about the complex signaling networks of Trx-sensitive signaling molecules such as apoptosis signal-regulating kinase 1, Trx interacting protein, and phosphatase and tensin homolog, thus highlighting the potential direct and indirect impact of their redox-dependent interaction with Trx. Overall, the findings that are described here illustrate the importance and complexity of Trx-dependent, redox-sensitive signaling in the cell. Our increasing understanding of the components and mechanisms of these signaling pathways could lead to the identification of new potential targets for the treatment of diseases, including cancer and diabetes.

Thioredoxin binding protein (TBP)-2/Txnip and α-arrestin proteins in cancer and diabetes mellitus

Thioredoxin binding protein -2/ thioredoxin interacting protein is an α-arrestin protein that has attracted much attention as a multifunctional regulator. Thioredoxin binding protein -2 expression is downregulated in tumor cells and the level of thioredoxin binding protein is correlated with clinical stage of cancer. Mice with mutations or knockout of the thioredoxin binding protein -2 gene are much more susceptible to carcinogenesis than wild-type mice, indicating a role for thioredoxin binding protein -2 in cancer suppression. Studies have also revealed roles for thioredoxin binding protein -2 in metabolic control. Enhancement of thioredoxin binding protein -2 expression causes impairment of insulin sensitivity and glucose-induced insulin secretion, and β-cell apoptosis. These changes are important characteristics of type 2 diabetes mellitus. Thioredoxin binding protein -2 regulates transcription of metabolic regulating genes. Thioredoxin binding protein -2-like inducible membrane protein/ arrestin domain containing 3 regulates endocytosis of receptors such as the β(2)-adrenergic receptor. The α-arrestin family possesses PPXY motifs and may function as an adaptor/scaffold for NEDD family ubiquitin ligases. Elucidation of the molecular mechanisms of α-arrestin proteins would provide a new pharmacological basis for developing approaches against cancer and type 2 diabetes mellitus.

The oncogene Trop2 regulates fetal lung cell proliferation

The factors regulating growth of the developing lung are poorly understood, although the degree of fetal lung expansion is critical. The oncogene Trop2 (trophoblast antigen 2) is upregulated during accelerated fetal lung growth, and we hypothesized that it may regulate normal fetal lung growth. We investigated Trop2 expression in the fetal and neonatal sheep lung during accelerated and delayed lung growth induced by alterations in fetal lung expansion, as well as in response to glucocorticoids. Trop2 expression was measured using real-time PCR and localized spatially using in situ hybridization and immunofluorescence. During normal lung development, Trop2 expression was higher at 90 days gestational age (GA; 4.0 ± 0.8) than at 128 days GA (1.0 ± 0.1), decreased to 0.5 ± 0.1 at 142 days GA (full term ∼147 days GA), and was positively correlated to lung cell proliferation rates (r = 0.953, P < 0.005). Trop2 expression was regulated by fetal lung expansion, but not by glucocorticoids. It was increased nearly threefold by 36 h of increased fetal lung expansion (P < 0.05) and was reduced to ∼55% of control levels by reduced fetal lung expansion (P < 0.05). Trop2 expression was associated with lung cell proliferation during normal and altered lung growth, and the TROP2 protein colocalized with Ki-67-positive cells in the fetal lung. TROP2 was predominantly localized to fibroblasts and type II alveolar epithelial cells. Trop2 small interfering RNA decreased Trop2 expression by ∼75% in cultured fetal rat lung fibroblasts and decreased their proliferation by ∼50%. Cell viability was not affected. This study demonstrates that TROP2 regulates lung cell proliferation during development.

Thioredoxin-interacting protein inhibits hypoxia-inducible factor transcriptional activity

Vascular endothelial growth factor (VEGF) is required for proper lung development and is transcriptionally regulated in alveolar epithelial cells by hypoxia-inducible factor (HIF). Previous findings in a newborn mouse model of bronchopulmonary dysplasia (BPD) suggest that thioredoxin-interacting protein (Txnip) is a novel regulator of VEGF expression. The present studies were designed to test the hypothesis that Txnip negatively regulates VEGF through effects on HIF-mediated gene expression. To test this hypothesis, we first examined the levels of VEGF and Txnip protein in the lungs of 1-day-old newborn mice and E19 embryos and detected a significant inverse correlation. To elucidate the mechanisms underlying this relationship, we studied the effects of Txnip overexpression on HIF-mediated transcription using murine lung epithelial (MLE-12) cells. Overexpression of Txnip inhibited HIF-mediated reporter activity in both hypoxia and room air. Suppression of HIF activity by Txnip seemed to be independent of the ability of Txnip to bind to thioredoxin. Thus, our studies support a model in which Txnip is a potentially critical regulator of HIF-mediated gene transcription in the murine lung. Alterations in Txnip expression could alter lung VEGF expression in prematurely born human infants and contribute to the development of BPD.

Alterations of the thioredoxin system by hyperoxia: implications for alveolar development

Alterations in vascular endothelial growth factor (VEGF) contribute to alveolar simplification seen in animal models of bronchopulmonary dysplasia, and VEGF expression is redox regulated by thioredoxin (Trx)-1 in other diseases. The present studies tested the hypothesis that exposure to 85% O2 negatively impacts the Trx1 system and VEGF expression in the lungs of newborn mice. There was no effect of fraction of inspired oxygen on lung Trx1 or Trx reductase-1 protein levels; however, lung Trx1 protein was predominantly oxidized in the lungs of newborn mice exposed to 85% O2 by 24 hours of exposure. In room air (RA), lung Trx interacting protein (Txnip) levels decreased developmentally through Day 7 (1.0 +/- 0.06 [Day 1] vs. 0.49 +/- 0.10 [Day 3] vs. 0.29 +/- 0.03 [Day 7]; P < 0.01), whereas VEGF expression increased (1.25 +/- 0.16 [Day 1] vs. 4.35 +/- 1.51 [Day 3] vs. 13.23 +/- 0.37 [Day 7]; P < 0.01). Newborn mice exposed to 85% O2 had no developmental decrease in Txnip protein levels and a delayed increase in VEGF protein levels. Lung Txnip and VEGF protein levels were different than in corresponding RA controls at Day 3, before the detection of lung morphologic abnormalities in our model. Txnip and VEGF protein levels were inversely correlated in both the RA and hyperoxia-exposed groups (n = 18; R = -0.66; P = 0.003). In conclusion, oxidation of Trx1 and sustained Txnip expression in the lungs of newborn mice exposed to 85% oxygen is likely to severely attenuate normal Trx1 function. The inverse correlation of Txnip with VEGF expression suggests that decreased Trx1 function contributes to the observed lung developmental abnormalities.

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.

Investigation of TXNIP (thioredoxin-interacting protein) and TRX (thioredoxin) genes for growth-related traits in pigs

It is well known that TRX and its endogenous inhibitor TXNIP help sustain the cellular reduction/oxidation balance in response to various stresses and both play a crucial role in cell proliferation and growth. Five SNPs were found in TXNIP and these allowed us to map it by linkage to SSC4. Three of the SNPs were used for association analyses in three commercial pig populations (Duroc, Hampshire, and synthetic line) with more than 1200 animals. Both the single-marker and haplotype analyses revealed significant effects of TXNIP on hot carcass weight, test daily gain, and lifetime daily gain. TRX was mapped on SSC1 but no significant associations with growth-related traits were found in the synthetic pig line in which the SNP was informative. The expression levels of TXNIP and TRX were then detected in two groups (fast growth and slow growth, respectively) with different genetic backgrounds for growth. Compared with the slow-growth group, TXNIP expression was significantly lower in the fast-growth group, whereas a marked increase in TRX expression was observed in fast-growth group. Our findings suggest that TXNIP has effects on growth-related traits in pigs and further investigations will be necessary to elucidate the underlying mechanisms involved.

Tracheal occlusion: A review of obstructing fetal lungs to make them grow and mature

Fetal lung growth and functional differentiation are affected strongly by the extent that pulmonary tissue is distended (expanded) by liquid that naturally fills developing future airspaces. Methods that prevent normal egress of this lung fluid through the trachea magnify mechanical stretching of lung parenchymal cells, thereby promoting lung development. Indeed, experimental observations demonstrate that in utero tracheal occlusion (TO) performed on fetuses during the late canalicular-early saccular stage potently stimulates pulmonary growth and maturation. In this review, we present the four principle non-human animal models of TO/obstruction and discuss them in relation to their utility in elucidating lung development, in remedying congenital diaphragmatic hernia (CDH) as well as in investigating the stretching effects on growth and remodeling of the fine vasculature.

Gene expression profiling during increased fetal lung expansion identifies genes likely to regulate development of the distal airways

Growth and development of the fetal lungs is critically dependent on the degree to which the lungs are expanded by liquid; increases in fetal lung expansion accelerate lung growth, whereas reductions in lung expansion cause lung growth to cease. The mechanisms mediating expansion-induced lung growth are unknown but likely include alterations in the expression of genes that regulate lung cell proliferation. Our aim was to isolate and identify genes that are up- or downregulated by increased fetal lung expansion. In chronically catheterized fetal sheep at 126 days gestational age (GA), the left lung was expanded for 36 h, while the right lung remained at a control level of expansion. Subtraction hybridization was used to isolate genes differentially expressed between the left and right lungs. Screening of ∼6,000 clones identified 1,138 and 118 cDNA fragments that were up- and downregulated by increased lung expansion, respectively. Northern blot analyses in separate groups of control fetuses and fetuses exposed to increased lung expansion were used to verify differential expression. Increased fetal lung expansion upregulated heat shock protein 47, thrombospondin-1, TROP2, tropoelastin, and tubulin-α3 in fetal lung tissue by ∼200–300%; connective tissue growth factor and cysteine-rich angiogenic inducer 61 were increased by 20–30%. Genes downregulated by increased fetal lung expansion included CCSP-related protein-1, elongation factor-1α and vitamin D3upregulated protein 1. We conclude that an increase in fetal lung expansion differentially regulates the expression of numerous genes in lung tissue, many of which have important putative roles in lung development, while the functions of others are currently unknown.