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(Circulation Research. 1999;84:1043-1049.)
© 1999 American Heart Association, Inc.

Original Contributions

Upregulation of Endothelial Receptor for Oxidized Low-Density Lipoprotein (LOX-1) in Cultured Human Coronary Artery Endothelial Cells by Angiotensin II Type 1 Receptor Activation

D. Y. Li, Y. C. Zhang, M. I. Philips, T. Sawamura, J. L. Mehta

From the Departments of Medicine (D.Y.L, J.L.M.) and Physiology (Y.C.Z., M.I.P., J.L.M.), University of Florida and Veterans Affairs Medical Center, Gainesville, Fla; Department of Bioscience (T.S.), National Cardiovascular Center Research Institute, Osaka, Japan; and Department of Molecular Pathophysiology (T.S.), Graduate School of Pharmaceutical Sciences, Osaka University, Japan.

Correspondence to J.L. Mehta, Department of Medicine, University of Florida College of Medicine, 1600 Archer Rd, PO Box 100277 JHMHC, Gainesville, FL 32610. E-mail mehta{at}medmac.ufl.edu

	Abstract

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Abstract—Cross talk between oxidized LDL (ox-LDL) and angiotensin II (Ang II) may be relevant in atherosclerosis. In this study, we examined the presence of a specific endothelial receptor for ox-LDL (LOX-1) and Ang II receptors in human coronary artery endothelial cells (HCAECs). In addition, we studied the effect of Ang II on LOX-1 gene and protein expression. LOX-1 was consistently identified in HCAECs by reverse transcriptase–polymerase chain reaction (RT-PCR), cDNA sequence, Western blot, and ¹²⁵I-labeled ox-LDL binding assay (B_max, 29.7 ng/mg protein). The HCAECs also exhibited Ang II receptors (AT₁>AT₂), as determined by RT-PCR and ¹²⁵I-labeled Ang II binding assay (B_max, 2.21 and 1.19 fmol/mg protein, respectively). Incubation of HCAECs with Ang II markedly increased LOX-1 mRNA (RT-PCR) and protein (Western blot) expression. The increase in LOX-1 expression was dependent on Ang II concentration (10^–12 to 10^–6 mol/L). Ang II caused a concentration-dependent increase in ¹²⁵I-labeled ox-LDL uptake by HCAECs and enhanced ox-LDL–mediated cell injury, as evident from an increase in LDH release and a decrease in cell viability. These effects of Ang II were completely blocked by pretreatment of HCAECs with losartan, a specific AT₁ blocker, but not by PD123319, a specific AT₂ blocker. These observations indicate the following: (1) HCAECs possess abundant LOX-1 as well as Ang II (AT₁>AT₂) receptors, (2) Ang II upregulates LOX-1 receptor and ox-LDL uptake, (3) the effects of Ang II are mediated by AT₁ activation, and (4) Ang II enhances ox-LDL–mediated injury to HCAECs.

Key Words: angiotensin II • endothelial cell • oxidized LDL • receptor

	Introduction

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Low-density lipoprotein, especially its oxidatively modified form, oxidized LDL (ox-LDL), and angiotensin II (Ang II) are 2 critical factors in atherogenesis. Endothelial dysfunction elicited by ox-LDL has been implicated in the pathogenesis of atherosclerosis¹ and its manifestations.^{2 3} LDL is oxidized in vascular endothelial cells into a highly injurious product that results in cellular dysfunction in large arteries and resistant vessels. The endothelial dysfunction (ie, loss of vasodilation, vasoconstriction, thrombosis, and inflammation) occurs before and throughout the development of atherosclerosis and particularly during plaque rupture. ox-LDL appears to induce this cellular dysfunction in a time- and concentration-dependent manner.⁴

Vascular endothelial cells in culture⁵ and in vivo⁶ internalize and degrade ox-LDL through a putative receptor-mediated pathway that does not seem to involve the classic macrophage scavenger receptor. Sawamura et al⁷ cloned the endothelial receptor for ox-LDL (LOX-1), which is a membrane protein that belongs structurally to the C-type selectin family and is expressed in vivo in vascular endothelium and in vascular-rich organs. We have recently identified the presence of LOX-1 in cultured human coronary artery endothelial cells (HCAECs) and demonstrated that ox-LDL upregulates the expression of LOX-1 mRNA and protein.⁸ This receptor on bovine endothelial cells is induced by shear stress and by tumor necrosis factor- and phorbol 12-myristate 13-acetate.^{9 10} Because endothelial uptake of ox-LDL is important in atherosclerosis, further understanding of the regulation of LOX-1 may be of immense clinical significance.

Ang II, like ox-LDL, is an important factor in atherogenesis. Ang II activates at least 2 distinct types of cell-surface receptors, type 1 (AT₁) and type 2 (AT₂).^{11 12} Most experimental studies have shown that it is the AT₁ activation that mediates most of the known effects of Ang II in the cardiac tissues,^{11 12 13 14} although in some studies¹⁵ the proapoptotic role of AT₂ has also been demonstrated. Whereas the presence of both AT₁ and AT₂ has been confirmed in coronary artery endothelial cells taken from the rat,¹⁶ the distribution of Ang II receptors (AT₁ and AT₂) in HCAECs has not yet been defined.

Recent studies suggest an interaction between hyperlipidemia, activation of the renin-angiotensin system, and atherosclerotic disease.^{16 17 18 19} For example, Ang II facilitates oxidation of LDL¹⁸ and its uptake by scavenger receptor on monocytes/macrophages.¹⁹ It is, however, not clear whether Ang II stimulates uptake of ox-LDL by endothelial cells. We postulated that Ang II may upregulate LOX-1 as a basis of enhanced endothelial uptake of ox-LDL in the presence of Ang II. This study was designed to document the presence of specific endothelial receptors for ox-LDL and Ang II in cultured HCAECs and the regulation of endothelial receptor for ox-LDL by Ang II.

	Materials and Methods

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Cell Culture
HCAECs (initial batch from Clonetics Corp) were pure, as determined by morphology and staining for factor VIII and acetylated LDL. These cell were 100% negative for -actin smooth muscle expression. Microvascular endothelium growth medium consisted of 500 mL of endothelial cell basal medium, 5 ng of human recombinant epidermal growth factor, 5 mg of hydrocortisone, 25 mg of gentamycin, 25 µg of amphotericin B, 6 mg of bovine brain extract, and 25 mL of FBS. HCAECs were seeded in a 25-cm² flask (4000 cells/cm²) and incubated at 37°C in 95% air/5% CO₂. Fifth-generation HCAECs^{8 20} were used to examine expression of mRNA and binding assays for LOX-1 and Ang II receptors. Several groups of HCAECs were incubated with different concentrations of Ang II (10^–12 to 10^–6 mol/L), ox-LDL (40 µg/mL), or both. In other groups of HCAECs, losartan (10^–6 mol/L), a specific AT₁ blocker, or PD123319 (10^–6 mol/L), a specific AT₂ blocker, was added to the culture medium before cells were incubated with Ang II (10^–6 mol/L), ox-LDL (40 µg/mL), or both. The concentrations of Ang II, losartan, and PD123319 were chosen on the basis of previous studies.^{12 13}

Preparation of Lipoproteins
Native LDL and ox-LDL were prepared as described earlier.²¹ In brief, human native LDL was isolated from human blood plasma by discontinuous centrifugation. It was purified by ultracentrifugation (1.063 to 1.210 g/mL) to homogeneity determined by agarose gel electrophoresis. LDL was oxidized by exposure to CuSO₄ (5 µmol/L free Cu²⁺ concentration) in PBS at 37°C for 24 hours. The thiobarbituric acid reactive substances content of ox-LDL was 18.2±0.28 versus 0.56±0.16 nmol/100 µg protein in the native LDL preparation (P<0.01). ox-LDL was radioiodinated with ¹²⁵I by the iodine monochloride method.²² It was then purified on a Biogel P-10 column and extensively dialyzed against Tris-saline. LDL, ox-LDL, and ¹²⁵I-labeled ox-LDL ([¹²⁵I]ox-LDL) were kept in 50 mmol/L Tris-HCl, 0.15 mol/L NaCl, and 2 mmol/L EDTA at pH 7.4 and were used within 10 days of preparation.

Reverse Transcriptase–Polymerase Chain Reaction (RT-PCR) for LOX-1 mRNA Expression
Total RNA (1 µg) extracted from cultured HCAECs was reverse transcribed with oligo(dT) and Moloney murine leukemia virus reverse transcriptase (both from Promega) at 37°C for 1 hour. Reverse-transcribed material (2 µL) was amplified with Taq DNA polymerase (Promega) using a primer pair specific to human endothelial receptor (sense primer, 5'-TTACTCTCCATGGTGGTGCC-3'; antisense primer, 5'-AGCTTCTTCTGCTTGTTGCC-3'). The PCR product was 193 bp. PCR consisted of 40 cycles of 94°C for 40 seconds, 55°C for 1 minute, and 72°C for 1 minute.⁷ The RT-PCR–amplified samples were visualized on 1.5% agarose gels using ethidium bromide. In some experiments, human ß-actin was amplified as a reference for quantification of LOX-1 mRNA. A primer pair of human ß-actin was used (sense primer, 5'-TCGAATTCTGGAGAAGAGCTATGAGCTGCCG-3'; antisense primer, 5'-TCGGATCCGTGCCACCAGACAGCACTGTGTTG-3'). The PCR product was 201 bp. PCR consisted of 40 cycles at 95°C for 1 minute, 50°C for 1 minute, and 72°C for 1 minute. Relative intensities of bands of interest were analyzed by the NSF-300G scanner (Microtek) and scan analysis software (Biosoft) and were expressed as ratios to the ß-actin mRNA band.

LOX-1 Sequence
The RT-PCR product for direct sequencing was purified using the QIAquick PCR purification kit. Each sequencing reaction (20 µL) contained 14 µL of cDNA (2 to 3 µg), 2 µL of BigDye reaction mix (Perkin Elmer), 1 µL of LOX-1 forward and reverse primers (3.2 pmol/L), and 3 µL of 5x sequencing buffer (400 mmol/L Tris-HCl and 10 mmol/L MgCl₂, pH 9.0). After 25 cycles of 96°C for 10 seconds, 50°C for 5 seconds, and 60°C for 4 minutes, 1/10 volume of 3 mol/L sodium acetate (pH 5.2) and 3 volumes of 95% ethanol were added to each sequencing reaction. The samples were then centrifuged for 20 minutes at room temperature. The pellet was washed once with 250 µL of 70% ethanol and dried in a vacuum drier, and then it was dissolved in 20 µL of template suppression reagent (Perkin Elmer). The sequence was then read with an automated DNA sequencer (ABI 310; Applied Biosystems, Inc). The published sequence of LOX-1⁷ was used to compare homologies with the sequence of LOX-1 obtained in this study.

LOX-1 Binding Assay
To examine binding properties of ox-LDL, studies were conducted with cells at 4°C. Cells were prechilled for 30 minutes in HEPES buffer, pH 7.4, before addition of lipoprotein. [¹²⁵I]ox-LDL was added to each dish for final concentrations of 0.625, 1.25, 2.5, 5, 10, and 15 µg/mL. To determine specific binding of [¹²⁵I]ox-LDL, a 100-fold excess of unlabeled ox-LDL was added in parallel dishes. Incubation was carried out at 4°C for 2 hours. Cells were washed on ice with 150 nmol/L NaCl, 50 mmol/L Tris, and 2 mmol/L EDTA, pH 7.4, containing 2 mg/mL BSA. The wash schedule consisted of 2 rapid washes, 2 10-minute washes, and a final rapid wash. Cells were then rinsed with cold saline without BSA. Cells were lysed at room temperature in 0.5 mol/L NaOH solution. An aliquot of the cell lysate was counted to determine the amount of bound [¹²⁵I]ox-LDL.¹⁸ Protein was then quantified by BCA protein assay kit. LOX-1 B_max and K_d were determined by Scatchard plot.⁸

Western Analysis for LOX-1 in HCAECs
HCAEC lysates from each experiment (20 µg per lane) were separated by 12% SDS-PAGE and transferred to nitrocellulose membranes. After incubation in blocking solution (4% nonfat milk, Sigma), membranes were incubated with a 1:750 dilution monoclonal antibody to LOX-1 for 2 hours at room temperature. Membranes were washed and then incubated with a 1:3000 dilution of second antibody (Amersham Life Science) for 1 hour, and the membranes were detected with the enhanced chemiluminescence system (Amersham Life Science). To correct for differences in protein loading, the membranes were washed and reprobed with 1:1000 dilution monoclonal antibody to human ß-actin (Sigma). Relative intensities of protein bands were analyzed by scanner (model MSF-300G, Microtek Laboratory).²⁰

RT-PCR for Ang II Receptors
Total RNA (1 µg) extracted from HCAECs was reverse transcribed with oligo(dT) and Moloney murine leukemia virus reverse transcriptase (both from Promega) at 37°C for 1 hour. The reverse-transcribed material (2 µL) was amplified with Taq DNA polymerase (Promega) using a primer pair specific to human AT₁ (forward primer, 5'-TCATTTACTTTTATATTGTAA-3'; reverse primer, 5'-TGAATTTCATAAGCCTTCTT-3'). The PCR product was 532 bp. PCR consisted of 40 cycles at 94°C for 1 minute, 50°C for 1 minute, and 72°C for 2 minutes.²³ A primer pair specific to human AT₂ was as follows: forward primer, 5'-AATATGAAGGGCAACTCCAC-3'; reverse primer, 5'-TTAAGACACAAAGGTCTCCAT-3'. The PCR product was 1100 bp. PCR consisted of 35 cycles at 94°C for 1 minute, 58°C for 1 minute, and 72°C for 2 minutes.²⁴ The RT-PCR–amplified samples were visualized on 1.5% agarose gels using ethidium bromide. ß-Actin was amplified as a reference for quantification of AT₁ and AT₂ mRNA. Previous studies^{25 26} have demonstrated that Ang II does not affect ß-actin expression.

Ang II Receptor Binding
Cells (1x10⁵) were washed twice in ice-cold PBS, pH 7.2, before addition of reaction mixture (consisting of radiolabeled Ang II antagonist ¹²⁵I-labeled (Ser1, Ile8) Ang II and 2% BSA in PBS, pH 7.2). Radiolabeled Ang II antagonist ¹²⁵I-labeled (Ser1, Ile8) Ang II was added to each dish in a final concentration of 12.5, 25, 50, 100, 200, and 400 pmol/L. Cells were incubated with 1 mL of reaction mixture in the absence and presence of 1 µmol/L unlabeled Ang II, the specific AT₁ antagonist losartan, or the specific AT₂ antagonist PD123319 to determine total, nonspecific AT₁ and AT₂ binding, respectively. Incubation was carried out at room temperature for 90 minutes. Cells were then washed in ice-cold PBS, pH 7.2, containing 1% BSA. Cells were then rinsed with cold saline without BSA. Cells were lysed at room temperature in 0.5 mol/L NaOH solution. An aliquot of the cell lysate was counted to determine the amount of bound radiolabeled Ang II antagonist ¹²⁵I-labeled (Ser1, Ile8) Ang II. AT₁ and AT₂ B_max and K_d were determined by Scatchard analysis.²⁷

Uptake of [¹²⁵I]ox-LDL by Ang II
HCAECs were incubated with Ang II (10^–12 to 10^–6 mol/L) in the presence or absence of losartan or PD123319 for 24 hours. Cells were prechilled for 30 minutes in HEPES buffer, pH 7.4. [¹²⁵I]ox-LDL was added to each dish in a final concentration of 10 µg/mL. Incubation was carried out at 4°C for 2 hours. Cells were washed 3 times on ice with 150 nmol/L NaCl, 50 mmol/L Tris, and 2 mmol/L EDTA, pH 7.4, containing 2 mg/mL BSA. Cells were then rinsed with cold saline without BSA. Cells were lysed at room temperature in 0.5 mol/L NaOH solution. An aliquot of the cell lysate was counted to determine the amount of bound [¹²⁵I]ox-LDL.

Evaluation of Cell Viability
A small aliquot of cells was incubated in 0.1% trypan blue for a few minutes, and the cells were viewed under a light microscope. Dead cells are permeable to trypan blue and thus become colored, whereas viable cells do not take up the dye. By counting 100 cells, the percentage of viable cells was calculated.²⁸

Measurement of LDH
One milliliter of sample was collected for determination of LDH. A spectrophotometric enzyme activity method based on the oxidation of lactate was used (Sigma). LDH activity was expressed as units per milligram protein.²⁸

Data Analysis
All data are presented as mean±SD of duplicate samples from at least 3 independently performed experiments. Statistical significance was determined in multiple comparisons among independent groups of data in which ANOVA and the F test indicated the presence of significant differences. P0.05 was considered significant.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
LOX-1 Expression in HCAECs
mRNA for LOX-1 was consistently detected in all HCAECs (n=6) (Figure 1). The sequence of RT-PCR product for LOX-1 in HCAECs was the same as the previously published sequence of LOX-1 location at 151 to 343 bp (Table).

View larger version (56K): [in this window] [in a new window]   Figure 1. Identification of LOX-1 in HCAECs by RT-PCR. ß-Actin was amplified as a reference for quantification of LOX-1 mRNA.

View this table: [in this window] [in a new window]   Table 1. Sequence of the RT-PCR Product for LOX-1

As shown in Figure 2, all aliquots of HCAECs were observed to possess high-affinity [¹²⁵I]ox-LDL binding sites, as determined from a reciprocal plot of the data from triplicate experiments. Scatchard analysis indicated high-affinity LOX-1 binding sites (B_max, 29.7 ng/mg cell protein; K_d, 1.71x10^–8 mol/L).

View larger version (19K): [in this window] [in a new window]   Figure 2. Identification of LOX-1 in HCAECs by [125I]ox-LDL binding. LOX-1 Bmax and Kd were determined by Scatchard plot.

Ang II Receptors in HCAECs
mRNA for both AT₁ and AT₂ was detected in all aliquots of cultured HCAECs (n=6). Expression of AT₁ mRNA was consistently higher than that of AT₂ mRNA (Figure 3). This observation was supported by Ang II receptor binding assays that showed that HCAECs possess high affinity Ang II binding sites, as determined from a triplicate reciprocal plot of the data. Scatchard analysis indicated that the K_d values of AT₁ and AT₂ for HCAECs were 168 and 172 pmol, respectively. The B_max values of AT₁ and AT₂ in HCAECs were 2.21 and 1.19 fmol/mg protein, respectively (Figure 4).

View larger version (36K): [in this window] [in a new window]   Figure 3. Identification of AT1 and AT2 mRNA in HCAECs by RT-PCR. ß-Actin was amplified as a reference for quantification of AT1 and AT2 mRNA. Upper panel is from a representative experiment. Data in the graph are mean±SD from 6 different experiments. R indicates receptor.

View larger version (24K): [in this window] [in a new window]   Figure 4. Identification of AT1 and AT2 in HCAECs by 125I-[Sar1, Ile8]-Ang II binding. See Materials and Methods for details. AT1 and AT2 Bmax and Kd were determined by Scatchard plot.

Influence of Ang II on LOX-1 Expression
Incubation of Ang II with HCAECs increased LOX-1 mRNA (RT-PCR) and protein (Western blot) expression. The increase in LOX-1 mRNA and protein expression was dependent on Ang II concentration (10^–12 to 10^–6 mol/L).

Importantly, the increase in LOX-1 mRNA expression in response to Ang II was completely blocked by losartan, a specific AT₁ blocker, but not by PD123319, a specific AT₂ blocker (n=6, Figure 5). The increased LOX-1 protein expression in response to Ang II was also blocked by pretreatment of HCAECs with losartan, but not with PD123319 (n=6, Figure 6).

View larger version (69K): [in this window] [in a new window]   Figure 5. Upregulation of LOX-1 mRNA by Ang II. Cells were incubated with Ang II for 24 hours in the absence or presence of losartan (10-6 mol/L) or PD123319 (10-6 mol/L). Thereafter, the cells were subjected to RT-PCR analysis for LOX-1 mRNA expression. Note the Ang II–mediated concentration-dependent increase in LOX-1 mRNA. Ang II–induced increase in LOX-1 RNA is blocked by losartan but not by PD123319. Upper panel is from a representative experiment. Data in the graph are mean±SD from 6 different experiments.

View larger version (73K): [in this window] [in a new window]   Figure 6. Upregulation of LOX-1 protein by Ang II. HCAECs were incubated with Ang II in the presence or absence of losartan or PD123319 (10-6 mol/L), and cell lysate was subjected to Western analysis. Note that Ang II increases LOX-1 protein expression in a concentration-dependent manner. Effect of Ang II is inhibited by losartan but not by PD123319. Upper panel is from a representative experiment. Data in the graph are mean±SD from 6 separate experiments.

Influence of Ang II on ox-LDL Uptake by HCAECs
Incubation of HCAECs with Ang II increased ox-LDL uptake in a concentration-dependent manner (10^–12 to 10^–6 mol/L). This effect of Ang II was completely blocked by losartan. In contrast, PD123319 did not change Ang II–mediated ox-LDL uptake by HCAECs. Losartan alone had no effect on the uptake of ox-LDL (Figure 7).

View larger version (45K): [in this window] [in a new window]   Figure 7. Uptake of [125I]ox-LDL by HCAECs. Note that incubation of cells with Ang II (10-12 to 10-6 mol/L) increases ox-LDL uptake in a concentration-dependent manner (inset). Ang II–mediated ox-LDL uptake is blocked by losartan (10-6 mol/L) but not by PD123319 (10-6 mol/L). Data are mean±SD from 6 separate experiments.

Cell Viability
Incubation of HCAECs with Ang II (10^–6 mol/L) or ox-LDL (40 µg/mL) decreased HCAEC viability compared with control (P<0.05). On coincubation, Ang II (10^–6 mol/L) potentiated the effect of ox-LDL (40 µg/mL) and further decreased cell viability (P<0.05). Losartan completely blocked the effect of Ang II, whereas PD123319 had no effect (Figure 8).

View larger version (39K): [in this window] [in a new window]   Figure 8. Viability of HCAECs in response to Ang II (10-6 mol/L) or ox-LDL (40 µg/mL), as determined by trypan blue dye exclusion. There is a decrease in cell viability in response to Ang II or ox-LDL. On coincubation, Ang II potentiates the effect of ox-LDL. The effect of Ang II is blocked by losartan (10-6 mol/L) but not by PD123319 (10-6 mol/L). Data are mean±SD from 6 separate experiments.

LDH Release in Medium
Ang II (10^–6 mol/L) and ox-LDL (40 µg/mL) each increased LDH release in the medium compared with control (P<0.05). The presence of both Ang II and ox-LDL caused a marked increase in LDH release compared with Ang II or ox-LDL alone (P<0.05). Losartan, but not PD123319, completely prevented the effect of Ang II (Figure 9). These results are consistent with the change in cell viability with Ang II and ox-LDL (Figure 8).

View larger version (32K): [in this window] [in a new window]   Figure 9. LDH release in response to Ang II (10-6 mol/L) or ox-LDL (40 µg/mL). Incubation of HCAECs with Ang II or ox-LDL causes an increase in LDH release compared with control. Ang II enhances LDH release in response to ox-LDL. The effect of Ang II is blocked by losartan (10-6 mol/L) but not by PD123319 (10-6 mol/L). Data are mean±SD from 6 separate experiments.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
ox-LDL Receptors on HCAECs
Work done in several laboratories implicates LDL, especially its oxidatively modified form, in the pathogenesis of atherosclerosis.¹ In the vascular tissues, ox-LDL attenuates nitric oxide–mediated dilation and promotes leukocyte deposition.²⁹ ox-LDL also causes changes in the expression of certain genes associated with apoptosis in endothelial cells, such as bcl-2, Fas, and nitric oxide synthase.²⁰ The presence of abundant high-affinity LOX-1 on human endothelial cells provides a structural basis for incorporation of ox-LDL into these cells and resultant cellular dysfunction. Interestingly, ox-LDL also induces expression of P-selectin on the endothelial cells,³⁰ and P-selectin and LOX-1 share some structural homology. These observations may have a bearing on ox-LDL–mediated facilitation of leukocyte deposition on blood vessels as well.³⁰ Other studies from our laboratory indicate that ox-LDL, but not native LDL, upregulates LOX-1 protein and mRNA, suggesting an autoregulatory role of these receptors.⁸ These observations imply a critical role of LOX-1 expression in the uptake of ox-LDL and cell injury.

Ang II Receptors on HCAECs
Both AT₁ and AT₂ have been identified in normal as well as in failing cardiac tissues.^{31 32} Most experimental studies have shown that it is the AT₁ activation that mediates most of the known effects of Ang II in cardiac tissues.^{11 12 13 14} These include superoxide anion generation, adhesion of monocytes/macrophages, oxidation of LDL, and induction of apoptosis. Accordingly, AT₁ activation has been implicated in the pathogenesis of atherosclerosis and coronary heart disease. Actually, a marked increase in AT₁ density has been shown in hypercholesterolemic atherosclerosis in rabbits.³³ A recent study³⁴ showed a total absence of arrhythmias after a brief period of ischemia-reperfusion in AT_1a-knockout mice, further confirming the critical role of AT₁ activation in ischemia-reperfusion injury. However, the distribution of AT₁ and AT₂ in HCAECs has not until now been defined. The present report provides the first definitive evidence for the expression of high-affinity AT₁ and AT₂ in HCAECs, with a predominance of AT₁. The predominance of AT₁ was confirmed in the present study by 2 independent methods, RT-PCR and binding assay, and the results of these methods were complementary.

Preliminary studies from our laboratory¹⁴ have shown that Ang II induces apoptosis of HCAECs and enhances the cell-injurious effects of anoxia-reoxygenation and tumor necrosis factor-. These effects of Ang II are mediated primarily via AT₁ activation, implying that the expression of AT₁ in coronary endothelial cells is linked to endothelial injury. As noted earlier, other studies¹⁵ have shown that certain cell types, such as PC12W (rat pheochromocytoma) and R3T3 (mouse fibroblasts), express primarily AT₂, and in these cell lines apoptosis is mediated by AT₂ activation. It may be speculated that it is the AT₁ activation that mediates apoptosis in cell types that express mainly AT₁, and it is the AT₂ activation that mediates apoptosis in cell types that express mainly AT₂.

Interaction Between ox-LDL and Ang II
Several experimental studies in hyperlipidemic animal models have suggested an interaction of the renin-angiotensin system and hypercholesterolemia.^{17 18 19} Recent work³⁵ shows that LDL upregulates AT₁ gene expression in cultured vascular smooth muscle cells. The AT₁ protein expression is increased 2-fold in hypercholesterolemic rabbits compared with normocholesterolemic animals. Importance of the upregulation of AT₁ synthesis comes from increased vasoconstriction in response to Ang II and loss of endothelium-dependent vasorelaxation.³⁵ In other in vitro studies, Ang II has been shown to enhance the uptake and oxidation of LDL by monocytes and macrophages.^{18 19} In this study, we demonstrate that Ang II increases uptake of ox-LDL by HCAECs in a concentration-dependent manner. Thus, LDL upregulates AT₁ expression in the blood vessels and Ang II enhances LDL uptake in monocytes and ox-LDL uptake in endothelial cells. This interaction (cross talk) suggests a critically important role for the renin-angiotensin system and abnormal lipid levels in blood. Indeed, several clinical studies have shown that inhibition of the renin-angiotensin system and reduction in LDL-cholesterol levels independently, but similarly, improve markers of atherosclerosis and endothelial dysfunction and reduce the number of cardiac events.^{36 37} Another study¹⁹ showed that Ang II administration to mice enhanced ox-LDL uptake by macrophages via its stimulatory effect on cellular proteoglycan content, and this process can lead to foam cell formation and atherosclerosis. An experimental study³⁸ showed that angiotensin-converting enzyme inhibitors significantly attenuate the toxic effect of ox-LDL in aortas of rats.

Upregulation of LOX-1 by Ang II
In the present study, we demonstrate that Ang II upregulates LOX-1 gene and protein expression in cultured HCAECs. These effects of Ang II were completely blocked by losartan, a specific AT₁ blocker, but not by PD123319, a specific AT₂ blocker. We also show that the enhanced expression of LOX-1 is the basis of increased uptake of ox-LDL by endothelial cells in the presence of Ang II. The blockade of LOX-1 upregulation may be the basis of potent beneficial effect of AT₁ blockers in reducing acute cardiac events.³⁶ On the basis of the present study, it is likely that AT₁ blockade decreases the uptake of ox-LDL by human coronary arterial tissues by blocking expression of LOX-1. In view of the upregulation of AT₁ gene and protein expression in hyperlipidemia, the present observation provides definitive information as to how ox-LDL uptake is increased by AT₁ activation. This cross talk between ox-LDL and Ang II may also be clinically relevant, as competitive blockers of AT₁ have the potential to block LOX-1 expression and ox-LDL uptake by endothelial cells.

In a recent study,³⁹ LOX-1 expression was found to be upregulated in spontaneously hypertensive rats, an animal model with increased Ang II expression and activity.⁴⁰ This observation further supports the contention of an interaction between Ang II and ox-LDL.

Cell Injury and the Interaction Between Ang II and ox-LDL
It is widely appreciated that Ang II and ox-LDL are important factors in inducing endothelial dysfunction and injury. Work from our laboratory has shown that Ang II¹⁴ and ox-LDL²⁰ decrease nitric oxide generation and increase lipid peroxidation and LDH release in cultured HCAECs. Furthermore, Ang II and ox-LDL enhance anoxia-reoxygenation–mediated HCAEC injury. Work from other laboratories^{41 42} also suggests that Ang II and ox-LDL causes injury to endothelial cells. In the present study, we demonstrate that the presence of Ang II enhanced ox-LDL–mediated cell injury, as indicated by a decrease in HCAEC viability and increase in LDH release. The mechanism of enhancement of cell injury may be related to the upregulation of LOX-1 mediated by AT₁ activation, given that losartan, but not PD123319, inhibited the stimulatory effect of Ang II on ox-LDL–mediated cell injury. These data also provide a basis for suggesting that AT₁ blockers may inhibit ox-LDL–mediated injury in clinical disease states.

In summary, this study shows that HCAECs possess abundant endothelial receptors for ox-LDL (LOX-1) as well as Ang II. Ang II upregulates LOX-1 mRNA and protein expression via activation of AT₁. These observations may have important implications with regard to propagation of atherosclerosis and its therapy.

	Acknowledgments

 
This work was supported by a Grant-in-Aid from the American Heart Association, Florida Affiliate, a Merit Review Award from the Department of Veterans Affairs central office, and an NIH (MERIT) award.

Received September 29, 1998; accepted February 24, 1999.

	References

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