Agext and Its Active Compounds as Potential Therapeutic Prevention against Atherosclerosis and Cardiovascular Disease

Research Article

J Immun Res. 2022; 8(1): 1045.

Agext and Its Active Compounds as Potential Therapeutic Prevention against Atherosclerosis and Cardiovascular Disease

Liu K1, Mi Y2, Bao Y3, Luan Q1, Liu Y4, Zhang M4, Ammar AB5, Ali K6, Elosta A7, Lin M4, Ahmed N7, Slevin M7,8, Liu F4* and Liu D7*

1The Affiliated Hospital of Weifang Medical University, Shandong, China

2Department of Pharmacy, The First Affiliated Hospital, Weifang Medical University, Shandong, China

3No.4 People’s Hospital of Zibo, Shandong, China

4Faculty of Medical Images, Weifang Medical University, Shandong, China

5College of Applied Medical Science, University of Hail, Saudi Arabia

6Yanbu College of Applied Medical Sciences, Taibah University, Saudi Arabia

7Department of Life Sciences, Manchester Metropolitan University, Manchester, M1 5GD, UK

8The University of Medicine and Pharmacy of Targu Mures, 541039, Romania

*Corresponding author: Donghui Liu, Department of Life Sciences, Manchester Metropolitan University, Manchester, M1 5GD, UK

Feng Liu, Faculty of Medical Images, Weifang Medical University, Shandong, 261053, China

Received: April 12, 2022; Accepted: May 07, 2022; Published: May 14, 2022

Abstract

Advanced Glycation End-Products (AGEs) have been implicated in the chronic complications of Diabetes Mellitus (DM) and play an important role in the pathogenesis of atherosclerosis in human arteries. The main active compounds found in Aged Garlic Extract (AGExt) are S-Allyl Cysteine (SAC), N-Acetylcysteine (NAC) and S-allyl Mercaptocysteine (SAMC) and have powerful protective effects against oxidative stress and inhibit cellular damage by AGEs.

Aims: To explore the effect of AGExt on the development of atherosclerosis also investigated the endothelial protective effects of AGExt, using Human Coronary Artery-Derived Endothelial Cells (HCAEC). The use of an optimised mixture of the above compounds required to obtain and provide maximal beneficial effects against oxidative stress, cell apoptosis and protein glycation. Results: AGExt has protective effects against the cellular damage of HCAEC. In addition, the in vivo study accessed the activity of SAC and NAC to reduce the severity of atherosclerosis formation in Apolipoprotein (ApoE-/-) DM model mouse. The compounds of AGExt in vitro had strong protective actions against the AGEs induced damage to ECs even at low concentrations (100 ng/ml). Such low concentrations may have therapeutic usefulness in patients with diabetes.

Keywords: AGExt; AGEs; Atherosclerosis; Diabetes mellitus; Glycation; Oxidative stress

Introduction

Hyperglycaemia has an important role in the development of long-term complications of Diabetes Mellitus (DM) such as retinopathy, nephropathy, neuropathy, cataract, impaired wound healing and atherosclerosis [1,2]. However, the relationship between tissue damage and hyperglycaemia is not fully understood [3,4]. Several mechanisms have been proposed but the most interesting one is the role of protein glycation and the formation and accumulation of Advanced Glycation End-Products (AGEs) [5,6]. During chronic hyperglycaemia, body proteins undergo a process called glycation where covalent binding between carbonyl groups from reducing sugars and free amino groups from proteins forms a labile Schiff base [6,7]. This unstable Schiff base rearranges to a more stable compound called an Amadori product. These glycated proteins undergo further oxidative reactions, involving dicarbonyl intermediates, forming AGEs. The process of protein glycation is usually accompanied by the generation of Reactive Oxygen Species (ROS) [8,9]. Furthermore, glycation-derived free radicals can cause protein oxidation and the fragmentation of lipids and nucleic acids [10,11].

It is commonly accepted that the accumulation of tissue AGEs and associated oxidative stress are responsible for the molecular and cellular damage which have been implicated in diabetic complications [12-16]. The accumulation of AGEs has also been implicated in the development of insulin resistance as well as in the pathogenesis of diabetic complication [17-19]. AGEs play important role in the pathogenesis of atherosclerosis in human arteries as well as cancer, chronic kidney disease, cardiovascular and various neurodegenerative diseases including Alzheimer’s disease [20-23].

Cardiovascular disease remains the foremost cause of death worldwide though a steady decline in mortality and morbidity has been recognised over the past few decades [24,25]. Prolonged exposure to hyperglycaemia is now considered as a major factor in the pathogenesis of atherosclerosis in patients with DM [26].

Atherosclerosis is a disease characterised by dysfunction and inflammations of arterial endothelial cells [27-29]. However, atherosclerosis is a complex and multifactorial process, the key initiating process in atherogenesis is the subendothelial cholesterol retention which is both necessary and sufficient to provoke lesion initiation [30]. It has been shown that cellular lipidosis is the principal event in the pathogenesis of the atherosclerotic lesion [31,32]. Retention of cholesterol, transported by Low-Density Lipoprotein (LDL) in subendothelial space of arterial wall, is an absolute requirement for lesion development. According to Tabas et al. [33], the molecular basis of lipoprotein retention is associated with the interaction of lipoprotein and extracellular matrix molecules. Local responses to these retained lipoproteins include an inflammatory response with subsequent lesion development [34]. Specific focus is placed on the potential of these innate immune targets for therapeutic interventions to retard the progression of atherosclerosis or to induce its regression [34,35]. The response-to-retention model considers only the retention of cholesterol on the extracellular matrix, completely ignoring the retention of intracellular cholesterol [36].

Natural plant products have been claimed to possess beneficial effects for the prevention of various aspects of cardiovascular disease [37]. AGExt is one of these products which is formulated by soaking sliced raw garlic cloves in 15 - 20% aqueous ethanolic solution for up to 20 months at room temperature. The extract is then filtered and concentrated under pressure at low temperature. The ageing process of garlic converts the odorous and harsh irritating compounds to odourless and non-irritating water-soluble organic sulphur compounds, such as SAC, NAC, SAMC, allicin and selenium [38]. These compounds are exceptionally powerful antioxidant phytochemicals with increasing antioxidant activities [39]. On the other hand, Black-AGExt (b-AGExt) is another processed food that is produced by heating raw garlic at highly controlled temperature and humidity for approximately one month. It contains proteins, high amounts of polysaccharides, melanoidins, phenolic and sulphur compounds. The antioxidant activities of b-AGExt are mainly related to presence of high amounts of polyphenols as well as the transformation of unstable compounds in raw garlic into more stable organosulphur compounds [40].

There is growing evidence that AGExt has moderate cholesterollowering and blood pressure-reducing effects [41] and has significant vascular protective effects against the AGEs induced damage to the cardio/cerebrovascular system [17,42-44]. Several studies have shown the protective effects of AGExt against atherosclerosis by preventing hypertension, decreasing serum cholesterol and triglycerides levels, and inhibiting platelet aggregation and LDL oxidation [45-50]. Currently, a novel approach is under phase 1 and phase 2 clinical trial in diabetes type 1 and type 2 patients (our and Don et al’s unpublished data).

The present study investigates and compares the ability of potent cysteine-derived compounds found in AGExt or b-AGExt, to provide potentially anti-atherogenic/endothelial protective effects and provide justification for their consideration as main-line additional supplements or therapeutics for patients at risk of cardio/ cerebrovascular disease. In vitro studies were also carried out to assess their endothelial protective effects using Human Coronary Artery- Derived Endothelial Cell (HCAEC) and produce an optimised mixture providing maximal beneficial effects against oxidative stress, cell apoptosis (induction by hypoxia and re-oxygenation) and protein glycation (induction by exposure to high glucose levels and methylglyoxal). This would allow us to optimise the prevention of Endothelial Cell (EC) damage (key to the initiation and progression of atherosclerosis) and EC activation (associated with inflammation and plaque progression). Furthermore, the effects of AGExt, b-AGExt and their active components on EC’s vascularisation, and the EC angiogenesis in the presence of AGEs following pre-incubation of the EC cells with varying AGExt components was also investigated. In addition, the combination of SAC and NAC has been conducted in vitro and has also been assessed in the Apolipoprotein (ApoE-/-) DM model mouse in vivo.

Materials and Methods

Reagents

Unless stated, otherwise, all chemicals were purchased from Sigma-Aldrich at the highest purity. AGExt and other bio-active components, such as SAC, NAC and SAMC were kindly provided by the Wakunaga Pharmaceutical Company, Japan). B-AGExt was supplied by SINO BNP COM (Qingdao, Shandong, China). The compounds were tested alone and in combinations and varying concentrations to identify the maximal potential therapeutic effects.

In vitro glycation of protein

Lysozyme (10 mg/ml) was glycated by methylglyoxal (0.1 M) with and/or without AGExt components in 0.1 M sodium phosphate buffer, pH 7.4, containing 3 mM sodium azide, at 37°C in dark for one week. After incubation, unreacted sugars were removed by extensive dialysis against distilled water for 2 days at 4°C. The endotoxin content of all protein solutions was below the detection limit (<0.125 EU/ml). The aliquoted samples were stored at -20°C until needed for analysis.

Measurement of cross-linked AGEs

Cross-linked AGEs were assessed by using 12% Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis (SDS–PAGE). The gels were stained with Coomassie Brilliant Blue, de-stained, photographed and analysed using Image-Lab software (Bio-Rad, Watford, UK). The integrated density was measured to analyse the cross-linked AGEs on one-dimensional electrophoretic gels and the relative intensity after the addition of AGExt, b-AGExt and their components SAC, NAC and SAMC. Furthermore, the effects of combinations of SAC+NAC, SAC+SAMC and NAC+SAMC were also tested.

Cell culture

Human Coronary Artery-Derived Endothelial Cells (HCAECs) were purchased from Health Protection Agency Culture Collections (HPACC, UK), and maintained in a T-25 flask and in MV endothelial cell growth medium (HPACC, UK) supplemented with 10% decomplemented Foetal Bovine Serum (FBS) under water humidified 5% CO2 air at 37°C. The media were changed every 3 days and used between passage 2 and 5. Cell viability was optimised by the Alamar blue assay [51,52]. Cells were seeded in 96 well plates (1.6 - 1.8 × 104 cells per well) for 72 hours, with or without tested compounds. The absorbance of each well was measured at 490 nm using a microplate reader (Multiskan Ascent, Thermo Life Sciences, Hampshire, UK).

Cell migration assay

HCAECs migration assay was performed using our standard wound scratch assay as described previously [53] and the HCAECs were tested in the presence and absence of AGEs. Briefly, HCAECs were seeded on 1 cm × 1 cm glass coverslips at a concentration of 4×105 cells/ml in 1 ml of completed MV medium in each well of 12-well plates and incubated in a water humidified and 5% CO2 air incubator at 37°C. When cells reached about 90% confluence, the growth medium was replaced with Serum Poor Medium [SPM, basal growth medium containing 0.5% (v/v) FBS] and incubated for a further 24 hours. After the 24 hours incubation, each well of the 12- well plates was washed with warm Phosphate Buffer Saline (PBS) three times and the adherent cells were then scratched in one single continuous line across the glass coverslip using a razor blade and the wells were then washed carefully with warm PBS three times to remove any floating cells. Then the AGExt and its components were added to the wells (at 0.1 μg/ml) and SPM [basal medium contains 0.5% (v/v) FBS] was applied to the cells, and cells were incubated under the same conditions as mentioned above for 24 hours. HCAECs incubated in SPM were used as control. Fibroblast Growth Factor-2 (FGF-2) at a concentration of 25 ng/ml acted as a positive control.

After the 24 hours incubation, 100 μl of 4% (w/v) of Paraformaldehyde (PFA) was added to each well at Room Temperature (RT) for 15 min to fix the cells. The medium was then removed, and the wells were washed with PBS, followed by 200 μl of 100% ethanol fixation for 5 min. The wells were left to dry before staining the cells with methylene blue for 5 min. The stain was removed, and the wells were washed with distilled water (dH2O). Finally, the cell migration was assessed by phase-contrast microscopy and images were taken using a digital camera (ZEISS, Fisher Scientific, Loughborough, UK). Cells were treated in triplicate and pictures of five areas of each well were taken. The image analysis was performed using Image-J. Both migration distance and the number of migrated cells were measured and the mean ± SD were calculated. Each experiment was performed at least twice and a representative example is given in Figure 3.

MatrigelTM endothelial cell tube formation assay

The preparation of HCAECs was performed as described above. Briefly, 50 μl of MatrigelTM basement membrane matrix reduced in growth factors (BD Bioscience, Berkshire, UK) was added to each well in 96-well plates and incubated at 37°C for 1 hour to let the MatrigelTM polymerise. After that, 9×103/50 μl cells were added to each well including those compounds in low serum (0.5% FBS) MV media with 0.1μg/ml or without the test compounds. Both the test and controls were incubated for 24 hours at 37°C. After the 24 hoursincubation, HCAECs migrated and aligned to form tubes (defined by the enclosure of circumscribed areas). The counts of closed tube-like areas were made in five fields by microscopy (ZEISS, Fisher Scientific, Loughborough, UK) using the ×10 objective. FGF-2 (25 ng/ml) were used as positive control. All the experiments were done in triplicate, repeated three times, and results shown as mean ± SD.

Apoptosis assays

Apoptosis and oxidative stress were generated using a hypoxia chamber (Stemcell Technologies, Cambridge, UK) and cells subjected to low oxygen levels (0.5%) for 24 hours. HCAEC cells were seeded in 48-well plates with complete growth media for overnight and then treated with AGExt, b-AGExt, SAC+NAC, SAC+SAMC, and NAC+SAMC at 0.1 μg/ml for 24 hours. The cells were then incubated in hypoxia chamber for further 24 hours. Then HCAECs were incubated with Propidium Iodide (PI) for one hour and fixed with 4% PFA for 15 minutes at room temperature. The HCAEC cells stained with PI were quantified with a fluorescence microscope (ZEISS, Fisher Scientific, Loughborough, UK). Untreated HCAEC cells and those in normoxia acted as control.

Nuclear membrane damage and generation of oxidative stress

Nuclear membrane damage and generation of oxidative stress were evaluated by nuclear inclusion of PI counterstained with Hoechst 33258 and by assessing the levels of active p53 (phospho-p53, Cell Signalling, London, UK) to determine the activation rate of apoptosis and protection of the selected active compounds. HCAECs cell lysates were prepared from HCAECs which have undergone apoptosis. Total proteins (30 μg/well) were applied to 10% SDS-PAGE followed by Western blotting.

The HCAECs stained with PI (nuclear positivity = apoptosis) were quantified using a fluorescence microscope. HCAECs untreated with AGExt, b-AGExt and other components were used as controls and the HCAECs with normoxia conditions also acted as a control.

Endothelial cell permeability assay

HCAEC cells permeability assay was performed by using an in vitro Vascular Permeability assay kit (Millipore, Burlington, USA). HCAECs were cultured to confluence (at about 90% confluence they formed an endothelial monolayer) on transwell collagen precoated permeable (pored) support in the insert well of a 24-well plate. Then the HCAEC cells monolayer was pre-incubated with AGExt, b-AGExt, and the mixed compounds (SAC+NAC, SAC+SAMC and NAC+SAMC) at 0.1 μg/ml for 24 hours. The monolayer was treated with AGEs (250.0 μg/ml) alone or together with SAC+NAC, SAC+SAMC and NAC+SAMC at 0.1 μg/ml for a further 24 hours. After treatment, a high molecular weight Fluorescein Isothiocyanate- Dextran (FITC-Dextran) was added on top of the cells (insert well), allowing the fluorescent molecules to pass through the ECs monolayer at a rate proportional to the monolayer’s permeability. The extents of permeability were determined by measuring the fluorescence as per the manufacture’s protocol of the receiver plate well solution.

Western-blot analysis

After cell culture, a general Radioimmunoprecipitation Assay Buffer (RIPA buffer) containing a protease and phosphatase inhibitor cocktail was used to prepare the cell lysates. These cell lysates were incubated on ice for 20 minutes, and then were sonicated for 20 sec and centrifuged for 10 min at 10×10³ ×g at 4°C. The supernatants containing protein were collected, their protein concentrations were estimated using the Bicinchoninic Acid (BCA) protein assay and were stored at -80°C for later use. Equal quantities of proteins (30 μg) were mixed with 2× Laemmli sample buffer, boiled in a water bath for 15 min and then centrifuged briefly. Samples were separated along with pre-stained molecular weight markers (32,000 - 200,000 kDa) by 10% SDS-PAGE. Proteins were electro-transferred (Hoefer, Bucks, UK) onto nitrocellulose filters for 1 hr (Whatman, Protran BA85, Germany) and the filters were blocked for 1 hour at room temperature in tris buffered saline with tween (TBS-Tween, pH 7.4) containing 1% Bovine Serum Albumin (BSA). Filters were stained with the primary antibodies diluted in the blocking buffer, overnight at 4°C on a rotating shaker. Primary antibody was applied at 1:1,000 dilution: p-p53 (Cell Signalling, London, UK). The following day, the filters were washed (5×5 minutes in TBS-Tween at room temperature), filters were stained with goat anti-rabbit HRP-conjugated secondary antibodies diluted in TBS-Tween containing 5% de-fatted milk (1:2,000 dilution for 1 hr at room temperature) with continuous mixing. After a further five washes in TBS-tween, proteins were visualized using enhanced chemiluminescence detection (ECL, Thermo Scientific, Cambridge, UK), and semi-quantitatively identified fold differences compared with house-keeping controls (α-tubulin, Abcam, Cambridge, UK) were determined using Image-Lab software (Bio-rad, Watford, UK). The Western blot experiments were repeated twice and a representative example is shown in Figure 6.

Diabetes mellitus mice model

Streptozotocin (STZ, Solarbio Co Ltd, Beijing, China) treatment was done as described [54,55]. Briefly, after 12-14 hours of fasting, diabetes in mice was induced by intraperitoneal injection of 140 mg/ kg STZ. Mice were considered diabetic when glucose levels exceeded 11.1 mmol/L (monitored after 24 hours of fasting) at 72 hours after STZ injection. Control mice were injected with saline only.

More than 95% of STZ (140 mg/kg) treated mice developed diabetes within 1 week, and the loss of body weight was relatively low (mean ± 0.3 g). Blood sugar was measured in tail blood samples using a glucometer (Siemens, Munich, Germany).

Healthy male mice (6 week old) were used for the study. Sixweek- old ApoE-/- mice were purchased from Nanjing Biomedical Research Institute of Nanjing University, Nanjing, China. They were maintained in a temperature-controlled room with a 12:12 hour light: dark schedule and provided with a standard mouse pellet diet (Biopike, Shanghai, China) and water ad libitum. They were kept in ventilated cages with free access to water and food for experimental use. No obvious adverse events were seen in any of the experimental group.

Drug intervention

The AGExt concentrations were optimised by our lab; we had found that 0.1 μg/ml was the lowest concentration that had any effect in vitro for cell culture. The compound dosages were defined at 200 mg/kg and were used in vivo in this study. The mice were divided into 6 groups: a: ApoE-/- mice fed with normal chow; b: ApoE-/- mice fed with high-fat chow diet; c: DM model mice fed with high-fat chow; d: ApoE-/- mice fed with normal chow plus SAC+NAC mixture; e: ApoE-/- mice fed with high-fat chow plus SAC+NAC mixture; f: DM model mice fed with high-fat chow plus SAC+NAC mixture.

Tissue preparation

Animals were anaesthetised with pentobarbital sodium (C11H17N2NaO3) solution (50 mg/kg; Xinhua Pharmaceutical Co Ltd, Shandong, China) and sacrificed by cervical dislocation. The blood samples were collected and the aortic artery was separated and removed immediately, and photographed. For histology study, diced 2mm tissues were resected quickly, fixed, and embedded in paraffin.

Histochemistry staining

Histological analysis was carried out on mice aortic artery following feeding with a high-fat diet with/without AGExt compounds according to standard histological protocols (n=5 mice were used; 10 sections from each analysed). Briefly, the paraffin-embedded tissues were deparaffinised by sequential washes in xylene (2×), rehydrated in descending ethanol from 100% (2×) to 70% (1×), then into tap water. Sections were stained followed by standard Haematoxylin & Eosin (H&E) staining procedure.

Image analysis

The stained slides were examined microscopically (ZEISS, Fisher Scientific, Loughborough, UK) and digital images of whole crosssections were taken. The Figure 10 shows representative images of the H&E-stained aortic artery.

Statistical analysis

All the in vitro data are presented as the mean ± SD for One-way ANOVA analysis from 3 independent experiments, each experiment was done in triplicate, and the data were analysed using GraphPad Prism software version 7.0 for Windows (GraphPad Software, USA). The values were compared using a paired Student’s t-test. The in vivo data were obtained from 5 mice in each group and are shown as mean±SD. The statistical differences: * (p<0.05), ** (p<0.01) and *** (p<0.001) were considered statistically significant as compared with the control.

Results

This study has examined the potential benefits of AGExt, b-AGExt and their active components to inhibit AGEs cross-links. As shown in Figure 1, lysozyme glycated by methylglyoxal for one week produces sufficient cross-linked AGEs that present as dimers with an approximate molecular weight of 28 kDa. Glycated lysozyme (Figure 1, lane 2) showed reduced electrophoretic mobility for the lysozyme monomer compared to native lysozyme (Figure 1, lane 1). AGEsinduced dimerization of lysozyme was inhibited by the addition of 0.1 μg/ml of AGExt, b-AGExt, SAC, NAC and SAMC (Figure 1, lanes 3-7) respectively. All inhibitors showed significant effects on the formation of AGEs. However, b-AGExt showed the most significant (p<0.001) effect on cross-linked AGEs formation. B-AGExt > AGExt > SAC > NAC > SAMC (Figure 1).