Angiotensin-converting enzyme: II. Pulmonary endothelial cells in culture

Modulation of inducible nitric oxide synthase by hypoxia in pulmonary artery endothelial cells

The effects of hypoxia on the regulation of inducible nitric oxide synthase (NOS) 2 expression were examined in cultured rat pulmonary microvascular endothelial cells (EC). EC did not express NOS 2 mRNA or protein when exposed to normoxia or hypoxia unless they were pretreated with interleukin (IL)-1beta and/or tumor necrosis factor (TNF)-alpha for 24 h. Induction of NOS 2 by IL-1beta+TNF-alpha was significantly attenuated by concomitant exposure of EC to hypoxia or treatment of EC with antioxidants such as tiron, diphenyliodonium, and catalase, suggesting that NOS 2 expression is dependent on the production of reactive oxygen species. Degradation of IkappaB and activation of NF-kappaB, which were both induced by treatment of EC with cytokines, were not altered when the cells were exposed to hypoxia, suggesting that the modulation of NOS 2 expression by hypoxia is unrelated to NF-kappaB activation. Following stimulation with IL-1beta+TNF-alpha for 24 h, incubation of EC in normoxia resulted in a progressive decline in NOS 2 expression and a calculated half-life of approximately 6 h for NOS 2 mRNA. Hypoxia significantly prolonged the half-life of NOS 2 mRNA (17 h, P < 0.05 versus normoxic EC). The half-life of NOS 2 mRNA was also prolonged by actinomycin D treatment (19.5 and 29.5 h for normoxic and hypoxic EC, respectively), suggesting that transcription of an RNA destabilizing factor or RNAse contributes to NOS 2 mRNA degradation. In EC transiently transfected with the rat NOS 2 promoter, hypoxia and the combination of IL-1beta+TNF-alpha independently increased promoter activity 2.2- and 3-fold, respectively. As opposed to the attenuating effect that hypoxia had on IL-1beta+TNF-alpha- dependent induction of NOS 2 gene expression, the concomitant treatment with IL-1beta+TNF-alpha and hypoxia synergistically increased NOS 2 promoter activity 17.6-fold. Taken together, these results suggest that hypoxia alone does not induce NOS 2 expression in cultured pulmonary microvascular EC, but may modulate cytokine induction of this enzyme at pretranscriptional, transcriptional, and posttranscriptional levels.

Kinin and enkephalin conversion by an endothelial, plasma membrane carboxypeptidase

Utilizing both thin-layer chromatography and high pressure liquid chromatography, it was determined that a vascular plasma membrane preparation contains a carboxypeptidase capable of converting kinins (B2 agonists) to des(Arg)kinins (B1 agonists) by hydrolysis of C-terminal Arg. The plasma membrane carboxypeptidase also converted Leu5-enkephalin-Arg6 to Leu5-enkephalin. Carboxypeptidase activity was significantly higher in cultured endothelial (1.47 +/- 0.4 units/mg) than in cultured smooth muscle cells (0.16 +/- 0.4 units/mg). Both the vascular and endothelial activities had neutral pH optima and were activated 4- to 5-fold by 0.1 mM CoCl2. The carboxypeptidase N inhibitor MERGETPA (D-L-mercaptoethanol-3-guanidino-ethylthiopropanoic acid) inhibited the plasma membrane bound carboxypeptidase with an I50 of 0.3 microM. Conversion was also inhibited by o-phenanthroline and EDTA, whereas inhibitors of aminopeptidases (bestatin, puromycin), endopeptidases (phosphoramidon), "enkephalinase" (ZINCOV) or enkephalin convertase (PCMS) were without effect. The affinity of the endothelial plasma membrane carboxypeptidase for bradykinin (Km = 56.8 +/- 4.7 microM) was higher than that for Leu5-enkephalin-Arg6 (Km = 92.7 +/- 10.1 microM), whereas the maximal rates of conversion (calculated per mg of endothelial plasma membrane protein) were similar (17.1 and 21.3 nmoles/min/mg respectively). These results demonstrate that a carboxypeptidase is present on the cell surface of vascular endothelium which can convert kinins and enkephalins in the micro-environments of vascular cell surface receptors.

Effects of D-valine on pulmonary artery endothelial cell morphology and function in cell culture

The effects of D-valine on the cell culture of bovine pulmonary artery endothelial cells were studied using D-valine-modified Minimal Essential Medium (MEM). D-Valine-treated cultures (46-920 mg/l) were compared with replicate cells grown in L-valine (46 mg/l)-MEM. All media were supplemented with 15% fetal bovine serum (FBS). Endothelial cells were grown for 14 passages with split ratios varying from 1:3 to 1:6. Unlike cells grown in L-valine MEM, cells grown in D-valine MEM did not become contaminated by the growth fibroblasts in primary cultures. D-Valine-treated cells were found to grow in cobblestone array, exhibit contact inhibition and strongly express factor-VIII antigen (F-VIII). D-Valine-grown cells produced PGI2 in greater proportion to PGE2, both constitutively and when stimulated by bradykinin, on comparison with cells grown in L-valine. In addition, cells grown in L-valine, although able to express factor VIII, were not comparable to D-valine cells with respect to other parameters assayed (morphology and growth as a monolayer).

The uptake of 5-hydroxytryptamine in endothelial cells cultured from the pulmonary artery in rats. A cytochemical study

Endothelial cells from the rat pulmonary artery were cultured and identified ultrastructurally and immunocytochemically by the presence of factor VIII antigens. Monolayers of cultured cells were treated according to the Hillarp-Falck technique and studied cytofluorimetrically to estimate intracellular serotonin (5-HT) levels. An uptake of 5-HT from the incubation medium was demonstrated at a concentration of 10(-3) M. However, after inhibition of monoamine oxidase (nalamide 5 X 10(-4) M) to prevent intracellular degradation of the amine an uptake could be demonstrated at 10(-5) M. The amine uptake into endothelial cells utilizes an active pumpmechanism, since it was effectively antagonized by either imipramine (10(-4) M) or ouabain (10(-5) M). These in vitro findings contrast earlier cytofluorimetric in vivo studies, where 5-HT uptake predominantly occurred in interstitial mast cells.

In vitro systems for exposure of lung cells to NO2 and O3

In vitro studies of the effects of NO2 and O3 require development of methods for separation and culture of those lung cells that experience in vivo exposure, and also the design and construction of systems for controlled exposure of the cells to known concentrations of the gases. Separation of lung cell types has been accomplished by enzymatic dispersal of lung tissue and centrifugation of the mixed cells on media of various densities in order to separate the cells on the basis of buoyant density or sedimentation rate. The application of centrifugal elutriation has enabled separation of type II alveolar cells and Clara cells with a high degree of purity. Alveolar macrophages and endothelial cells have also been obtained in good yield. Exposure of cultured cells to test atmospheres requires precise control of pollutant levels, close contact of cells and gas without an intervening layer of medium, capability for prolonged exposure, and maintenance of sterile conditions, so that recovered cells can be cultured further or studied for other biological activity. Systems which meet these criteria include roller bottle cultures, petri dish cultures on rocker platforms, cell cultures on cellulose filters fed by perfusion of medium from the side opposite the cells, and cells grown in dishes with gas-permeable film bottoms. Systems that rely on solution of the gases in the overlaying medium do not resemble exposure conditions in vivo, and may not be suitable for studying effects of the poorly soluble oxidant gases. The cell exposure systems have not been used extensively for studies of the effects of pollutants on freshly isolated specific lung cell types. Such studies should be encouraged.

Induction by glucocorticoids of angiotensin converting enzyme production from bovine endothelial cells in culture and rat lung in vivo

The effect of corticosteroids on angiotensin converting enzyme was investigated in endothelial cell cultures and intact rat lung. Cultured endothelial cells from bovine aorta showed net production of angiotensin converting enzyme (ACE) over 2 d culture in serum-free medium. Dexamethasone (DM) increased cell ACE activity six- to sevenfold at 100 nM with a threshold effect at 0.3 nM. The effect of DM on ACE production was completely inhibited by actinomycin D or cycloheximide. Deoxycorticosterone (DOC) and aldosterone were markedly less active, with a threshold near 100 nM and significant (two to threefold) stimulation of ACE activity at 1 muM. In cells incubated in the presence of 10 nM DM, DOC (10 muM) significantly inhibited ACE production compared with 10 nM DM alone, suggesting that DOC is a partial agonist/partial antagonist in this enzyme system. Protein content of cells or medium was unchanged by steroids at all doses used. In vivo, adrenalectomized rats showed lower pulmonary ACE compared with intact controls, and when injected with DM (40 mug/d for 4 d) showed a significant (twofold, P < 0.002) increase in lung ACE over oil-injected, adrenalectomized controls; serum ACE did not change. Injection with DOC (40 mug/d) or aldosterone (10 mug/d) had no effect on lung or serum ACE. Over a range (0.6 to 2,000 mug) of concentrations of DM administered daily for 7 d, the dose-response curve of DM for induction of pulmonary ACE mirrored that for thymolysis; for both, half-maximal effects were seen at approximately 6 mug DM/d, and plateau levels at 60 mug/d. We conclude that glucocorticoids are potent inducers of ACE activity in endothelial cells in culture and in rat lung in vivo, and that the action of aldosterone and DOC reflects occupancy of glucocorticoid receptors. This effect may be of (patho)physiological relevance in regulating levels of ACE in local vascular beds, and thereby modulating local levels of the vasoactive peptides angiotensin II and bradykinin.