Effects of hyperoxia on DNA synthesis in cultured porcine aortic endothelial cells

The effects of hyperoxia on microvascular endothelial cell proliferation and production of vaso-active substances

BACKGROUND

Hyperoxia, an arterial oxygen pressure of more than 100 mmHg or 13% O2, frequently occurs in hospitalized patients due to administration of supplemental oxygen. Increasing evidence suggests that hyperoxia induces vasoconstriction in the systemic (micro)circulation, potentially affecting organ perfusion. This study addresses effects of hyperoxia on viability, proliferative capacity, and on pathways affecting vascular tone in cultured human microvascular endothelial cells (hMVEC).

METHODS

hMVEC of the systemic circulation were exposed to graded oxygen fractions of 20, 30, 50, and 95% O2 for 8, 24, and 72 h. These fractions correspond to 152, 228, 380, and 722 mmHg, respectively. Cell proliferation and viability was measured via a proliferation assay, peroxynitrite formation via anti-nitrotyrosine levels, endothelial nitric oxide synthase (eNOS), and endothelin-1 (ET-1) levels via q-PCR and western blot analysis.

RESULTS

Exposing hMVEC to 50 and 95% O2 for more than 24 h impaired cell viability and proliferation. Hyperoxia did not significantly affect nitrotyrosine levels, nor eNOS mRNA and protein levels, regardless of the exposure time or oxygen concentration used. Phosphorylation of eNOS at the serine 1177 (S1177) residue and ET-1 mRNA levels were also not significantly affected.

CONCLUSIONS

Exposure of isolated human microvascular endothelial cells to marked hyperoxia for more than 24 h decreases cell viability and proliferation. Our results do not support a role of eNOS mRNA and protein or ET-1 mRNA in the potential vasoconstrictive effects of hyperoxia on isolated hMVEC.

Additive toxicity of limonene and 50% oxygen and the role of glutathione in detoxification in human lung cells

Limonene has many commercial applications and has been introduced as an environmentally acceptable solvent replacing halogenated hydrocarbons. Occupational exposure to limonene presumably occurs simultaneously with other chemicals including oxidative agents and may exert a heavy strain on cellular detoxifying capacity resulting in synergistic effects. The present study used oxygen as an example of an ubiquitous oxidative and radical forming agent and investigated the combination effects with limonene on human lung cells. Mechanistic information was gained by comparing the toxicity of limonene with a major oxidation product, limonene 1,2-epoxide, and by the involvement of glutathione in cellular detoxification. At cell culture conditions most similar to the in vivo situation oxygen did not increase the toxicity of limonene beyond an additive effect. The results further indicated that limonene 1,2-epoxide was not the active compound in limonene toxicity. Experimental evidence suggests that detoxification of limonene in human lung cells primarily occurs by mechanisms not involving the glutathione system and point to possible long-term effects of limonene exposure. The present knowledge indicates clearly that the mechanism of action of limonene on biological systems and particularly in combination with oxidative compounds still remains to be elucidated. In light of the frequent exposure of humans to such combinations further investigations into this issue are highly recommended.

Hyperoxia induces S-phase cell-cycle arrest and p21(Cip1/Waf1)-independent Cdk2 inhibition in human carcinoma T47D-H3 cells

Little is known about cell-cycle checkpoint activation by oxidative stress in mammalian cells. The effects of hyperoxia on cell-cycle progression were investigated in asynchronous human T47D-H3 cells, which contain mutated p53 and fail to arrest at G1/S in response to DNA damage. Hyperoxic exposure (95% O(2), 40-64 h) induced an S-phase arrest associated with acute inhibition of Cdk2 activity and DNA synthesis. In contrast, exit from G2/M was not inhibited in these cells. After 40 h of hyperoxia, these effects were partially reversible during recovery under normoxic conditions. The inhibition of Cdk2 activity was not due to degradation of Cdk2, cyclin E or A, nor impairment of Cdk2 complex formation with cyclin A or E and p21(Cip1). The loss of Cdk2 activity occurred in the absence of induction and recruitment of cdk inhibitor p21(Cip1) or p27(Kip1) in cyclin A/Cdk2 or cyclin E/Cdk2 complexes. In contrast, Cdk2 inhibition was associated with increased Cdk2-Tyr15 phosphorylation, increased E2F-1 recruitment, and decreased PCNA contents in Cdk2 complexes. The latter results indicate a p21(Cip1)/p27(Kip1)-independent mechanism of S-phase checkpoint activation in the hyperoxic T47D cell model investigated.

Hyperbaric oxygen therapy for deep second degree burns: an experimental study in the guinea pig

Most previous animal studies reporting improved epithelialisation and healing of burn wounds under hyperbaric oxygen (HBO) did not include the conventional treatment with topical antibiotics as part of the protocol, and did not compare the effectiveness of HBO therapy with that of normobaric 100% oxygen (NO). The purpose of our study was to compare the results of combined treatment with HBO + silver sulfadiazine (SS) and those of treatment with NO + SS or SS alone. Deep second degree burns were produced on the depilated backs of 54 guinea pigs using a validated burn protocol. The animals were assigned to three treatment groups: HBO + SS, NO + SS, and SS. Dressings were changed daily. HBO was administered at 2 atmospheres absolute (ATA) for 90 min BID, and NO for 90 min BID. The parameters compared among the groups were laser Doppler flowmetry, and burn wound contracture and re-epithelialisation data derived from computerised planimetry of photographs of the wound. No differences in laser Doppler flowmetry results or the magnitude of contracture were found between the groups. Significantly increased re-epithelialisation was observed under NO + SS starting 10 days after the burn (P = 0.02, ANOVA). This significance stems from the difference between the HBO + SS and NO + SS groups (Tukey test). These data indicate that excessively high levels of tissue PO2 might compromise burn healing, and explain our results. A further study comparing combined treatment using a milder HBO protocol + SS and NO + SS is indicated in the search for the optimal HBO regimen.

Influence of hyperoxia on in vitro growth of rabbit middle ear epithelium and auditory meatal epithelium

The oxygen partial pressure of middle ear gas increases more than 3-fold upon insertion of ventilation tubes, while the carbon dioxide partial pressure decreases. Whereas the middle ear gas is normally equilibrated to venous gases and has an oxygen partial pressure of 43 mmHg, 138 mmHg is measured in ventilated ears. The present study was undertaken to compare the effects of these oxygen tensions on in vitro growth and glycoprotein secretion of rabbit middle ear epithelium and for comparison auditory meatal epithelium. Cultures were incubated in atmospheres of 7, 21 or 75% O2 in 5% CO2 and the remnant N2. The cell layer protein mass, [3H]thymidine-incorporation, DNA content and [3H]glucosamine-incorporation was measured in identical subcultures every third day during a 15-day period. In middle ear epithelium the DNA content, DNA synthesis and cell layer protein mass were significantly higher at 7% oxygen compared to 21% and 75%. In conclusion hyperoxia leads to decreased growth of middle ear epithelium in vitro. If applicable to in vivo conditions, this might contribute to the mechanism of action of ventilation tubes. Moreover the proliferation rate of auditory meatal epithelium exceeds that of middle ear epithelium both at 7 and 21% oxygen, an interesting point with regards to cholesteatoma pathogenesis.

Hyperoxia, unlike phorbol ester, induces glutathione peroxidase through a protein kinase C-independent mechanism

Human selenium-dependent glutathione peroxidase (GP) is implicated as a mechanism of resistance against oxygen free radicals. The 5' flanking sequence upstream from the coding region of GP contained an oxygen-responsive element termed ORE1 that is responsive to hypoxia, as well as several copies of the activator protein-1 (AP-1)- and AP-1-like-binding sites. In this study, we sought to define the molecular events that lead to GP gene transcription in response to hyperoxia in human umbilical-vein endothelial cells, and asked whether such induction is mimicked and sustained by activation of protein kinase C (PKC) by phorbol esters. Treatment of cells with 100 nM phorbol 12,13-dibutyrate (PdBu) induced a delayed (24-48 h) but significant (2-fold) increase in steady-state GP mRNA levels. Steady-state GP mRNA levels also rose after exposure to 95% O2, again after considerable delay (48-72 h). For both PdBu and oxygen, induction was transcriptionally regulated, as demonstrated by nuclear run-on experiments. The simulations by PdBu and oxygen were additive. In contrast with PdBu, hyperoxia did not stimulate translocation of PKC from the cytosol to the particulate fraction, although the specific activity of both cytosolic and particulate-associated PKC was increased 2-fold in cells exposed to 95% O2 for 5 days. In addition, gel mobility-shift assays using double-stranded tumour-promoting-agent-responsive element (TRE) and nuclear extracts derived from phorbol- and oxygen-treated cells revealed that PdBu, but not hyperoxia, increased AP-1 DNA-binding activity. On the other hand, the up-regulation of GP expression by oxygen could not be accounted for by the ORE1 core sequence, since no specific protein-DNA binding activity could be detected using nuclear extracts from hyperoxic cells and ORE1. Taken together, these results suggest that there may be different molecular mechanisms controlling GP expression. After exposure to PdBu, GP undergoes transcriptional activation via a process that can be readily explained by a classic AP-1 interaction with the TRE sites in the GP promoter. During hyperoxia, GP also undergoes transcriptional activity, but via a process that appears to involve neither TRE nor ORE1.

Species difference in the resistibility of embryonic fibroblasts against oxygen-induced growth inhibition

The growth of fibroblasts, which were isolated from human, rabbit, rat, mouse, and chick embryos, was inhibited partially under 50% oxygen and nearly completely under 95% oxygen. There was species difference in the resistivity of these cells against oxygen-induced growth inhibition. The extent of the resistivity was in the following order: chick cells > rat cells > human cells > rabbit cells approximately mouse cells. The order of their ability to recover from oxygen-induced growth inhibition was similar to the above order of species. There was also species difference in their antioxidant enzyme activities, including superoxide dismutase, catalase, and glutathione peroxidase activities, and their reduced glutathione concentration. Chick cells, having the highest resistivity against oxygen-induced growth inhibition, were at the lowest activity levels of antioxidant enzymes and at the highest concentration level of reduced glutathione. The species difference in resistivity against oxygen-induced growth inhibition seems to depend on the reduced glutathione concentration, but not on the antioxidant enzyme activities.

Overexpression of manganous superoxide dismutase (MnSOD) in pulmonary endothelial cells confers resistance to hyperoxia

Treatment of cells or organisms with agents that increase the expression of MnSOD confers resistance to certain types of oxidative damage. However, since these treatments also affect other cellular systems with antioxidant capabilities, the role of MnSOD remains uncertain. To better determine whether increased MnSOD expression confers increased resistance to oxidant stress, a eukaryotic expression vector harboring a mouse MnSOD cDNA was constructed. Bovine lung microvessel endothelial cells were co-transfected with the MnSOD expression vector and pSV2-neo, which contains the neor gene and provides a dominant selectable marker. Control clones were generated by transfecting the cells with psV2-neo alone. Stably transfected cell lines were selected and cell lines overexpressing MnSOD were confirmed by Northern blotting, immunoblot analysis, and activity gels. The activities of copper/zinc superoxide dismutase, catalase, and glutathione peroxidase were examined, and no increase in activity of any of these enzymes was detected. Cells were exposed to hyperoxic challenge by treatment with 95% O2 and 5% CO2 for 24 h. Viability was assessed by a clonogenic assay. The cell lines that overexpressed MnSOD showed a twofold increase in survival compared to control cells. These results demonstrate a significant resistance to hyperoxia induced oxidative stress in endothelial cells overexpressing MnSOD.

Response of human endothelial cell antioxidant enzymes to hyperoxia

To explore the level of regulation of the expression of the major antioxidant enzymes in response to hyperoxia, we exposed human umbilical vein endothelial cells to 95% O2 for 3 and 5 days and measured (1) the steady-state mRNA levels, (2) the activities, and (3) the immunoreactive content of CuZn and Mn superoxide dismutases (SOD), catalase (CAT), and glutathione peroxidase (GP). We found that a 3-day exposure to 95% O2 caused (1) an increase in CuZnSOD mRNA (by 41%), CAT mRNA (by 26%), and GP mRNA (by 173%); (2) an increase in CuZnSOD activity (by 30%), a decrease in CAT activity (by 37%), and an increase in GP activity (by 60%); and (3) an increase in CuZnSOD immunodetectable protein (by 26%) and a loss in CAT immunoreactive protein (by 27%). After a 5-day exposure to 95% O2, there was (1) a 93% increase in CuZnSOD mRNA, a 71% increase in CAT mRNA, and a 127% increase in GP mRNA; (2) a 56% increase in CuZnSOD activity, a 70% decrease in CAT activity, and an 89% increase in GP activity; and (3) a 35% increase in CuZnSOD immunoreactive protein and a 55% loss in CAT immunoreactive protein. There was no change in the steady-state MnSOD mRNA level after 3 days in 95% O2, but a 100% increase was observed on day 5 of oxygen exposure. MnSOD activity was unchanged in cells exposed to hyperoxia for 3 and 5 days. These data suggest that, in human umbilical vein endothelial cells, the regulation of antioxidant enzymes expression in response to O2 is complex and exerted at different levels.

Fatty acid supplementation protects pulmonary artery endothelial cells from oxidant injury

Although supplemental fatty acids have been shown to alter the susceptibility of experimental animals to oxidant gases, the relationship between the degree of tissue fatty acyl unsaturation and resistance to oxidant exposure remains undefined. Because vascular endothelial cells have been demonstrated to be sensitive cellular targets in oxidant-induced lung injury, we evaluated the effects of a supplemental fatty acid on the lipid composition and oxidant susceptibility of pulmonary artery endothelial cells (PAEC) in monolayer culture. PAEC were incubated in culture medium supplemented with an ethanolic solution of 0.1 mM cis-vaccenic acid (CVA), an 18-carbon monounsaturated fatty acid, or with the ethanol vehicle alone for 3 h. Cells were then exposed to either control or oxidant (hyperoxia: 95% O2; or hydrogen peroxide: 100 microM) conditions. Oxidant-induced cell injury was assessed by phase-contrast microscopy and by measuring the release of intracellular lactate dehydrogenase. Incubation with CVA increased the CVA content of PAEC lipids and protected cells from oxidant-induced injury for up to 72 h after supplementation. CVA had no effect on nonoxidant-induced cell injury. Although the mechanism by which CVA protects cells against oxidant injury remains undefined, evidence is presented that indicates the mechanism does not involve induction of antioxidant enzyme activity, alterations in the physical state of PAEC membranes, or enhancement of PAEC nucleic acid repair mechanisms. These results define a useful model for exploring the relationship between lipid composition and oxidant susceptibility and suggest that fatty acid modifications may constitute an important strategy for protecting cells against oxidant injury.

Responses of type II pneumocyte antioxidant enzymes to normoxic and hyperoxic culture

Cultured type II pneumocyte responses to in vitro normoxia (95% air:5% CO2) or hyperoxia (95% O2:5% CO2) were quantified. Normoxic culture (0 to 96 h) of rabbit type II cells resulted in enhanced cell-monolayer protein and DNA content. During this same time, cellular activities of superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH Px) decreased. Compared to cultures maintained in normoxia, hyperoxic exposure of cultures resulted in decreased cell-associated protein and DNA content. Exposure to hyperoxia also resulted in cytotoxicity as demonstrated by elevated cellular release of DNA, lactate dehydrogenase (LDH), and preincorporated 8-[14 C]adenine. Cellular catalase and GSH Px activities in hyperoxic cells decreased similarly to normoxic controls. In contrast, cellular SOD activity in hyperoxic cells decreased less than in normoxic cultures. Cellular SOD activity in hyperoxic cultures, when normalized for cellular protein, but not DNA, was greater than normoxic values after 24 to 96 h of exposure. Unlike the decrease in cellular antioxidant enzymes during normoxic and hyperoxic culture, cellular LDH activity increased during both these exposures. Cellular LDH activity in 24 to 96 h hyperoxia-exposed cells increased to a lesser extent than normoxic controls. The extent of depression in LDH activity was dependent on whether the activity was normalized for cellular protein or DNA. Type II pneumocytes, which normally undergo hyperplasia and hypertrophy during hyperoxia in vivo, exhibited oxygen sensitivity in vitro. Exposure of type II cells to hyperoxia in vitro resulted in alterations in cellular SOD and LDH activities, but recognition of such changes were dependent on whether enzymatic activities were normalized for cellular DNA or protein.

O2 effect on composition of chick embryonic heart and brain

Heart ventricles from chick embryos incubated in 60% O2 (hyperoxia) on the 16th through the 18th days of incubation were 21% heavier than those from control embryos maintained in 21% O2 (normoxia). Heart ventricles from embyros incubated in 15% O2 (hypoxia) were 8% lighter than controls. Changes in ventricular weight were accompanied by proportional changes in protein content (21% more in hyperoxic ventricles; 8% less in hypoxic ventricles). Ventricular tissue DNA content showed a significant increase in hyperoxia. Tissue protein/DNA ratios were significantly higher in hyperoxia and lower in hypoxia. These data suggest that increased O2 availability led to hypertrophy of chick embryo ventricular cells and an increase in the level of DNA synthesis. Cytochrome oxidase activity per mg DNA was 15-25% higher in hyperoxic ventricles than in hypoxic ventricles. This result is consistent with our previous findings that alterations in O2 availability affect the O2 consumption rate of the chick emryo in ovo, and it provides direct evidence that a phenomenon repeatedly observed in vitro is of importance in vivo. In contrast to the heart, O2 availability did not affect the wet weight, protein or DNA contents, or cytochrome oxidase activity of the chick embryo brain.

Differential effects of hyperoxia and hydrogen peroxide on DNA damage, polyadenosine diphosphate-ribose polymerase activity, and nicotinamide adenine dinucleotide and adenosine triphosphate contents in cultured endothelial cells and fibroblasts

The effects of oxidative stress on DNA damage and associated reactions, increased polyadenosine diphosphate-ribose polymerase (PARP) activity and decreased nicotinamide adenine dinucleotide (NAD) and adenosine triphosphate (ATP) contents, have been tested in primary cultures of porcine aortic endothelial cells. The cells were treated with 50-500 microM H2O2 for 20 min or 100 microM paraquat for 3 days or were exposed to 95% O2 for 2 and 5 days. The administration of 250-500 microM H2O2 resulted in a marked increase in PARP activity and a profound depletion of ATP and NAD. Although hyperoxia had no effect on PARP activity and reduced only slightly the ATP and NAD stores, it markedly reduced the ability of endothelial cells to increase PARP activity upon exposure to DNase. Paraquat had a similar effect. Human dermal fibroblasts were also exposed to 50-500 microM H2O2 for 20 min or 95% O2 for 5 days. Their response to H2O2 differed from that of endothelial cells by their ability to maintain the ATP content at a normal level. Fibroblasts were also insensitive to the effect of hyperoxia. These results suggest that the oxidant-related DNA damage is a function of the type of oxidative stress used and may be cell-specific.

Oxygen free radicals and lungs

Some of the metabolites resulting from the monovalent reduction of O2, superoxide anion and hydroxyl radical, are O2, radicals, whereas H2O2, which is not a radical since having no unpaired electron, is also an active O2 intermediate. These O2 metabolites are formed intracellularly as a result of normal metabolism. Their production can increase following exposure to high O2 concentration, radiations or certain drugs. An increased amount of extracellular O2 metabolites occurs after activation of certain inflammatory cells or during the course of the hypoxanthine-xanthine oxidase reaction. To counteract this oxidative stress, antioxidant defenses exist, whether enzymatic (superoxide dismutase, glutathione peroxidase, catalase, etc.) or nonenzymatic (GSH, vitamin E and C, etc.). Oxidative injury can result from an imbalance between oxidative stress and the defense mechanisms. The main targets are protein, DNA and lipids. The cellular response of the lung is stereotyped and involves cell injury (especially endothelial cells and type I pneumocytes), inflammatory reaction and repair processes.

Betalactam antibiotics interfere with eukaryotic DNA-replication by inhibiting DNA polymerase alpha

Betalactam antibiotics (BLA) are the most widely used antibacterial drugs in practical medicine. Recent experiments suggested that BLA, especially after "aging" in aqueous solutions, have an inhibitory effect on the growth of a variety of cultured human cells by interfering with DNA synthesis (Neftel et al. Cell Biol. Toxicol. 2, 513-521, 1986). Our initial observation that the replicative DNA polymerase alpha might be the target of the action of betalactam compounds (Hübscher et al. Cell Biol Toxicol. 2, 541-548, 1986) is now substantiated due to the following experimental data: (i) extractable DNA polymerase alpha is greatly reduced in cells that had been treated with BLA; (ii) the relative cellular distribution of thymidine and of its phosphorylated derivatives is not affected by BLA; (iii) BLA inhibit crude and highly purified mammalian DNA polymerase alpha; (iv) the inhibitory effect appears to be of the mixed type with a slight deviation from purely non-competitive behaviour towards the four deoxyribonucleoside triphosphates and; (v) the inhibition is evident in aphidicolin sensitive DNA polymerases from mammalian tissues and in DNA polymerases from DNA viruses such as Herpes simplex and Vaccinia. In sum, the results suggest that one of the most commonly used class of drugs has a target within eukaryotic cells being most likely the replicative DNA polymerase alpha.

Comparative study on the selenium- and N-acetylcysteine-related effects on the toxic action of hyperoxia, paraquat and the enzyme reaction hypoxanthine-xanthine oxidase in cultured endothelial cells

The potential protective effect of N-acetylcysteine against various types of oxidative stress (exposure to hyperoxia, treatment with paraquat, incubation in the presence of the hypoxanthine-xanthine oxidase system) was tested in primary cultures of porcine aortic endothelial cells. It was compared to that of selenomethionine (Se-Met), known to increase glutathione peroxidase activity, when given either alone or in combination with N-acetylcysteine. LDH release, 3H-thymidine (TdR) incorporation into DNA and DNA content were measured to assess the cytotoxic effect of the conditions tested. Total and oxidized glutathione content was also determined. Whereas Se-Met had a partial protective effect on all the conditions but paraquat treatment, N-acetylcysteine administration had no effect on the hyperoxia induced changes and significantly worsened the cytotoxic action of paraquat. On the other hand, LDH release following an incubation in the presence of the hypoxanthine-xanthine oxidase was significantly reduced after N-acetylcysteine treatment. No major change in total nor in oxidized glutathione followed N-acetylcysteine treatment in control and experimental conditions. A dose-dependent protective effect of N-acetylcysteine was obtained when this agent was given concomitantly with the xanthine oxidase system. These data suggest that in cultured endothelial cells a N-acetylcysteine-related protective effect, if present, is most likely to result from the direct scavenging action of N-acetylcysteine.

Developmental profiles of protective mechanisms of heart against peroxidative injury

The developmental profiles of the protective mechanisms of heart against peroxidative injury during neonatal growth was examined in the pigs of three different age groups. Lipid peroxidation expressed in terms of malonaldehyde formation was considerably higher in the pig hearts of the 8-10 day age group compared to that either by newborn or adult age groups. The four principal antioxidative enzymes, superoxide dismutase, glutathione peroxidase, glutathione reductase, and glucose-6-phosphate dehydrogenase (G6PD), were enhanced during early neonatal growth and, with the exception of G6PD, all other enzymes were further enhanced during further growth to adulthood. G6PD activity dropped significantly in adult heart. The phospholipid contents of myocardial membrane between newborn and week-old pigs did not vary significantly. Total phospholipids and phosphatidylcholine contents were significantly higher in adult heart compared to those in neonatal heart. The enzymes of phospholipid synthesis and degradation, fatty acyl CoA synthetase (FACS), phospholipase A2 (PLA2), lysophospholipase (LPL), and lysophophatidylcholine acyltransferase (LPCAT) increased during early neonatal growth. During further growth to adulthood, FACS decreased, PLA2 did not change, whereas both LPL and LPCAT increased significantly. Analysis of free fatty acids showed that palmitic and stearic acids decreased during the first week of growth, but increased during further growth to adulthood. Oleic acid did not change with aging, but arachidonic acid dropped in adult heart compared to that in neonatal heart. Linoleic, palmitoleic and free fatty acids increased dramatically during the first week of neonatal growth, but dropped thereafter. These results suggest that the unusual peroxidative status of the week-old pig heart is related to the presence of high concentrations of polyunsaturated fatty acids in the membrane phospholipids and not with the antioxidative defense system.