Elevation of bovine endothelial cell angiotensin converting enzyme by cationophores and inhibition by ouabain

Ouabain at Nanomolar Concentration Promotes Synthesis and Release of Angiotensin II from the Endothelium of the Tail Vascular Bed of Spontaneously Hypertensive Rats

The effects of 1 nM ouabain (OUA) on the contractile actions of phenylephrine (PHE, 0.001-100 microg) and functional activity of the sodium pump (NKA) in isolated-perfused tail vascular beds from WKY and SHR were investigated. In preparations from SHR, perfusion with OUA in the presence of endothelium (E+) increased the sensitivity (pED50) of PHE (before: 2.14 +/- 0.06 versus after: 2.47 +/- 0.07; P < 0.05) without altering the maximal response (Emax). After endothelial damage, OUA reduced the Emax of PHE in SHR (before: 350 +/- 29 versus after: 293 +/- 25 mm Hg; P < 0.05). In SHR/E+, pretreatment with losartan (10 microM) or enalaprilat (1 microM) prevented the increased sensitivity to PHE induced by OUA. OUA increased NKA activity in SHR/E+ (before: 45 +/- 6 versus after: 58 +/- 5%, P < 0.05). Losartan (10 mg/Kg, i.v.) also abolished the increment in systolic and diastolic blood pressure induced by OUA (0.18 microg/Kg, i.v.) in anesthetized SHR. OUA did not alter the actions of PHE in either anesthetized WKY rats or vascular preparations. Results suggest that 1 nM OUA increased the vascular reactivity to PHE only in SHR/E+. This effect is mediated by OUA-induced activation of endothelial angiotensin converting enzyme that promotes the local formation of angiotensin II, which sensitizes the vascular smooth muscle to the actions of PHE.

Bleomycin upregulates gene expression of angiotensin-converting enzyme via mitogen-activated protein kinase and early growth response 1 transcription factor

Pulmonary fibrosis is a progressive disorder characterized by the loss of alveolar architecture through epithelial and endothelial cell apoptosis and fibroblast proliferation. Recent studies showed that angiotensin-converting enzyme (ACE) activity is increased in fibrotic tissues, and ACE inhibitors administered in vivo ameliorate fibrosis, suggesting that ACE may play a critical role. However, the regulation of ACE expression is not well understood. In the present study, we demonstrate that bleomycin, a chemotherapeutic agent which induces pulmonary fibrosis in animals and humans, increases gene expression of ACE. Treatment of primary bovine pulmonary artery endothelial cells with 0.1 to 1.0 microg/ml bleomycin increased ACE enzymatic activity and ACE mRNA, as monitored by hippuryl-L-histidyl-L-leucine assay and competitive quantitative reverse transcriptase polymerase chain reaction (RT-PCR), respectively. Luciferase reporter constructs showed that upregulation of ACE transcription by bleomycin is mediated through element(s) in the 97-bp ACE promoter. Bleomycin activated p42/p44 mitogen-activated protein kinase (MAPK) and induced nuclear translocation and activation of the early growth response (Egr)-1 transcription factor, a factor previously shown to positively regulate ACE expression. The MAPK kinase1/2 (MEK1/2) inhibitor U0126 blocked MAPK and Egr-1 activation by bleomycin, suggesting that Egr-1 activation is MAPK dependent. These data provide the first evidence that bleomycin activates ACE gene expression through the MAPK pathway and Egr-1.

Evidence that human endothelial cells express different isoforms of Na,K-ATPase

BACKGROUND

The catalytic alpha and smaller beta subunits of the plasma membrane Na,K-ATPase occur in various molecular forms (alpha1, alpha2, alpha3, beta1 and beta2). The alpha isoforms of the enzyme have varying affinities for ouabain and exist in different tissues with particular distribution patterns.

OBJECTIVE

To document the existence of isoforms of the Na,K-ATPase in cultured human umbilical vein endothelial cells.

METHODS

Microsomal fractions were prepared by differential ultracentrifugation from primary cultures of human umbilical vein endothelial cells and from such cells obtained after three passages. Na,K-ATPase activity was assayed using the coupled assay method and sensitivity to ouabain was determined in the presence of varying concentrations of ouabain. Specific antibodies for the various Na,K-ATPase isoforms were used to label these different proteins by immunocytochemistry in endothelial cells and by Western blotting in isolated membranes.

RESULTS

In plotting the dose-response curves for Na,K-ATPase activity in response to ouabain we assumed the existence of two independent sites exhibiting different affinities for ouabain (in the micromol/l and the nmol/l ranges). The contribution of low-affinity sites was threefold that of high-affinity sites. After three passages in culture, a specific increase in Na,K-ATPase activity of the high-affinity sites was observed compared with that of cells from primary cultures. Confocal microscopy revealed the existence of beta1, beta2, and alpha1 subunit proteins in human umbilical endothelial cells. Staining for alpha3 isoform was less pronounced and no obvious alpha2 was detected.

CONCLUSION

These findings suggest that human umbilical vein endothelial cells contain beta1, beta2, a large amount of alpha1 isoform with an apparently low affinity for ouabain, and a lesser amount of high-affinity sites, which may correspond to the alpha3 protein.

Na-K-ATPase alpha-isoform expression in heart and vascular endothelia: cellular and developmental regulation

The Na pump (Na-K-ATPase) is important for regulation of membrane potential and transport in smooth muscle and heart. The alpha (catalytic)-subunit of this pump has three isoforms: alpha 1 is ubiquitous, but alpha 2 and alpha 3 are mainly localized to excitable tissue. Physiological differences between isoforms are not completely understood, but alpha 3 pumps appear to have a lower affinity for intracellular Na and a higher ouabain affinity than alpha 1 pumps. The alpha 2-and alpha 3-isoform mRNAs are expressed at high levels in the normal adult rat cardiac conduction system. Although alpha 1 and alpha 3 are both globally expressed in neonatal rat myocardia, there is a switch in the myocardial isoform pattern from alpha 3 to alpha 2 after birth. There are also important species differences in cardiac isoform patterns. Furthermore, changes in Na-K-ATPase isoforms in heart and vascular tissue have been reported in association with hypertension, but little is known about isoform expression in normal endothelia. We therefore studied the cellular distribution of Na pump protein isoforms in neonatal and adult myocardia and endothelia. Immunohistochemical analysis of rat tissues showed that the alpha 1-isoform was expressed throughout atrial and ventricular myocardium, with alpha 1 the only isoform detectable in the adult t tubule system. Although alpha 2 was also present in ventricular myocytes, the signal was markedly stronger in conduction tissue and papillary muscle. In hearts from neonatal rats, the alpha 3-isoform predominated in the cardiac conduction system, whereas alpha 2 was not detectable in any structure except vascular endothelium. In tissues and in cell lines representing a variety of species and vessel sizes, endothelia of large vessels expressed primarily alpha 1, whereas alpha 2 could be detected in endothelia of small vessels in rat heart. No evidence of alpha 3 expression in endothelium was found. Thus the complex spatial and developmental regulation of Na pump isoform expression in cardiovascular tissues may provide additional correlates to distinct physiological roles of these transporters.

Electrogenic Na+/K(+)-transport in human endothelial cells

Na+/K+ pump currents were measured in endothelial cells from human umbilical cord vein using the whole-cell or nystatin-perforated-patch-clamp technique combined with intracellular calcium concentration ([Ca2+]i) measurements with Fura-2/AM. Loading endothelial cells through the patch pipette with 40 mmol/l [Na+] did not induce significant changes of [Ca2+]i. Superfusing the cells with K(+)-free solutions also did not significantly affect [Ca2+]i. Reapplication of K+ after superfusion of the cells with K(+)-free solution induced an outward current at a holding potential of 0 mV. This current was nearly completely blocked by 100 mumol/l dihydroouabain (DHO) and was therefore identified as a Na+/K+ pump current. During block and reactivation of the Na+/K+ pump no changes in [Ca2+]i could be observed. Pump currents were blocked concentration dependently by DHO. The concentration for half-maximal inhibition was 21 mumol/l. This value is larger than that reported for other tissues and the block was practically irreversible. Insulin (10-1000 U/l) did not affect the pump currents. An increase of the intracellular Na+ concentration ([Na+]i) enhanced the amplitude of the pump current. Half-maximal activation of the pump current by [Na+]i occurred at about 60 mmol/l. The concentration for half-maximal activation by extracellular K+ was 2.4 +/- 1.2 mmol/l, and 0.4 +/- 0.1 and 8.7 +/- 0.7 mmol/l for Tl+ and NH4+ respectively. The voltage dependence of the DHO-sensitive current was obtained by applying linear voltage ramps. Its reversal potential was more negative than -150 mV. Pump currents measured with the conventional whole-cell technique were about four times smaller than pump currents recorded with the nystatin-perforated-patch method.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of second messenger systems in stimulation of angiotensin converting enzyme of bovine endothelial cells

We have demonstrated previously that a variety of agents including corticosteroids, thyroid hormone, cationophores, methylxanthines, and analogues of cAMP--all of which have diversified functions in various tissues--elevate cellular angiotensin converting enzyme (ACE) activity of bovine endothelial cells in culture. In addition to these agents, we have now found that direct and receptor-mediated stimulators of adenylate cyclase, i.e., forskolin and cholera toxin, increase cellular ACE activity after 48 h incubation in culture. In an attempt to search out a more unifying concept of these stimulatory effects, we have further investigated the roles of second messengers in the stimulatory actions. Ca2+ ionophore A23187 produced significant increases in both intracellular Ca2+ and ACE of endothelial cells. In contrast to Ca2+ ionophore, agents that transiently mobilize Ca2+ from intracellular reserves such as bradykinin, acetylcholine, and ATP have no effect on the level of cellular ACE. Representative agents that elevate cellular cAMP (e.g., isobutyl methylxanthine [IBMX] and dibutyryl cAMP) elevated cellular ACE, but the slightly increased [Ca2+]i produced by these agents did not reach statistical significance. While IBMX, cholera toxin, and forskolin elevated cellular cAMP, other ACE stimulatory agents (hormones and cationophores) had no effect on cAMP. Ca2+ ionophore and the agents that elevated intracellular cAMP potentiated the effect of dexamethasone, thyroid hormone, and aldosterone in elevating cellular ACE activity. Increases in ACE activity produced by all stimulants were inhibited by the presence of 10-50 nM ouabain in the culture medium. Inhibition of ACE elevation by oubain was reversed by increasing the extracellular [K+], thereby implicating Na+, K(+)-ATPase in the ACE regulatory mechanism. These results support the presence of multiple independent mechanisms for the regulation of cellular ACE. In addition to possible involvement of intracellular Ca(2+)- and cAMP-dependent pathways, ACE is also increased by corticosteroids and thyroid hormone through mechanisms unrelated to Ca2+ and cAMP.

Elevation of angiotensin converting enzyme in bovine endothelial cells quantitated by an ELISA

Levels of angiotensin converting enzyme (ACE) in cultured bovine pulmonary artery endothelial cells treated with dexamethasone, aldosterone, 3,3',5'-triiodo-L-thyronine, Ca2+ ionophore, 3-isobutyl-1-methylxanthine, dibutyryl cAMP and forskolin were quantitated by an enzyme linked immunosorbent assay (ELISA). The configuration for the ELISA consisted of purified bovine lung ACE adsorbed to a solid phase competing with endothelial cellular ACE for a limited amount of anti-ACE immunoglobulin. ACE-IgG complex on the solid phase was detected by goat anti-rabbit IgG-alkaline phosphatase conjugate with enzymatic activity measured by p-nitrophenylphosphate as substrate. This ELISA detected ACE with a sensitivity of 32 ng/ml cellular ACE. Elevation in cellular ACE catalytic activity as measured by fluorescent assay of detergent extracts from bovine endothelial cells corresponded well with an increase in ACE protein as determined by the ELISA. These results provide direct evidence that increases in catalytic activity of ACE produced in endothelial cells by a variety of agents result from enhancement of the synthesis of ACE protein.