|Year : 2020 | Volume
| Issue : 4 | Page : 112-119
Diesel exhaust particles induced oxidative stress, autophagy, and apoptosis in human umbilical vein endothelial cells
Yang Guo1, Longfei Guan2, Yu Ji3, Hangil Lee4, Wenjing Wei5, Changya Peng6, Xiaokun Geng7, Yuchuan Ding6
1 Department of Neurology, Beijing Luhe Hospital, Capital Medical University, Beijing, China
2 Department of Neurology; China-America Institute of Neuroscience, Beijing Luhe Hospital, Capital Medical University, Beijing, China; Department of Neurosurgery, Wayne State University School of Medicine; Department of Research and Development Center, John D. Dingell VA Medical Center, Detroit, MI, USA
3 Department of General Surgery, Beijing Luhe Hospital, Capital Medical University, Beijing, China
4 Department of Neurosurgery, Wayne State University School of Medicine, Detroit, MI, USA
5 Department of Neurology, China-America Institute of Neuroscience, Xuanwu Hospital, Capital Medical University, Beijing, China
6 Department of Neurosurgery, Wayne State University School of Medicine; Department of Research and Development Center, John D. Dingell VA Medical Center, Detroit, MI, USA
7 Department of Neurology; China-America Institute of Neuroscience, Beijing Luhe Hospital, Capital Medical University, Beijing, China; Department of Neurosurgery, Wayne State University School of Medicine, Detroit, MI, USA
|Date of Submission||08-Dec-2020|
|Date of Decision||13-Dec-2020|
|Date of Acceptance||14-Dec-2020|
|Date of Web Publication||31-Dec-2020|
Dr. Xiaokun Geng
Department of Neurology, Beijing Luhe Hospital, Capital Medical University, No. 82 Xinhua South Road, Tongzhou District, Beijing 101149
Source of Support: None, Conflict of Interest: None
Background: Air pollution is one of the greatest public health concerns worldwide. In order to understand its mechanism of harm, we investigated the effects of diesel exhaust particles (DEPs), one of the major constituents of ambient air pollutants, on reactive cell viability, oxygen stress, autophagy, and apoptosis.
Materials and Methods: In in vitro human umbilical vein endothelial cell (HUVEC) model, cells were exposed to freshly dispersed DEP preparations at 0, 12.5, 25, 50, 100, or 200 µg/mL for 24 h or at 50 µg/mL DEP for 1, 3, 6, 12, or 24 h. Cell survival and oxidative stress were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX), and generation of reactive oxygen species (ROS). Protein expressions of autophagy (Beclin-1, p62, and light chain 3 [LC3]-II) and apoptosis (Bcl2 and Bax) were assayed by Western blotting.
Results: DEP induced a significant dose-dependent and temporal decrease in cell viability and increase in ROS generation and NOX activity, in association with decreased or increased protein levels of p62 or Beclin-1, as well as conversion of the LC3 in a dose-dependent manner. DEP increased pro-apoptotic protein Bax and decreased anti-apoptotic protein Bcl2.
Conclusions: These results demonstrated that DEP exposure induced cytotoxicity, oxidative stress, autophagy, and apoptosis in HUVECs. Novel insight into the mechanisms of cardiovascular diseases caused by air pollution may be provided through these findings.
Keywords: Apoptosis, autophagy, diesel exhaust particles, reactive oxygen species, toxicology
|How to cite this article:|
Guo Y, Guan L, Ji Y, Lee H, Wei W, Peng C, Geng X, Ding Y. Diesel exhaust particles induced oxidative stress, autophagy, and apoptosis in human umbilical vein endothelial cells. Environ Dis 2020;5:112-9
|How to cite this URL:|
Guo Y, Guan L, Ji Y, Lee H, Wei W, Peng C, Geng X, Ding Y. Diesel exhaust particles induced oxidative stress, autophagy, and apoptosis in human umbilical vein endothelial cells. Environ Dis [serial online] 2020 [cited 2023 Mar 28];5:112-9. Available from: http://www.environmentmed.org/text.asp?2020/5/4/112/305712
| Introduction|| |
Air pollution has become one of the greatest public health concerns due to the rapid industrialization and motorization of the world. Accumulating evidence demonstrated that airborne particulate matter (PM) is one of the most harmful pollutants. Diesel exhaust particles (DEPs), derived from motor vehicle engines' diesel oil combustion and other industries, are an important source of fine and ultrafine PM and are ubiquitously present in urban ambient air. DEP is a highly complex mixture containing thousands of compounds, including halogenated aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and redox-active quinones;, it has been classified as a human carcinogen (Group 1).,
Epidemiological studies show that DEP inhalation causes adverse pulmonary and cardiovascular health effects., Vascular endothelial cells line the inner surface of blood vessels and are essential for maintaining the structural and biological barrier of vascular function and homeostasis. Endothelial cells have also been used to elucidate the mechanisms that underlie microvascular dysfunction and various tissue injuries. Accumulating evidence in vivo studies have been demonstrated that PM exposure enhances microvascular dysfunction and atherosclerosis through reactive oxygen species (ROS)-dependent pathways., In our previous work, we have demonstrated that real-world PM2.5 exposure induced intracranial endothelial atherosclerosis in rats., In the present study, we further determined the detrimental effects of DEP air pollution in vitro, which may be related to cerebro- or cardiovascular diseases.
A key molecular mechanism of PM-mediated cytotoxicity has been recognized as oxidative stress., Oxidative stress can be defined as the imbalance between cellular oxidant species production and antioxidant capability.,,, Although there are many potential sources of these pathogenic ROS, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) has been heavily implicated as a main contributor., Not surprisingly, DEP is also known to induce free radical generation, which leads to cellular oxidative stress, and has been shown to cause significant damages in cell cultures and animal models.,,,
Apoptosis is a mechanism of programmed cell death and is an essential process for maintaining homeostasis by removing excessive, unwanted, and harmful cells. Apoptotic cells undergo characteristic changes in cell morphology: cell rounding, plasma membrane blebbing, and nuclear fragmentation. In the present study, we determined the effect of DEP on cellular apoptotic injury. We further determined here whether the DEP-induced apoptosis was related to autophagy, as previous studies reported that autophagy plays important roles in cell survival and maintenance and is also involved in PM-mediated cytotoxicity.,,,,
In the present study, using human umbilical vein endothelial cells (HUVECs) as an in vitro model, we modeled adverse cardiovascular effects induced by DEP and determined cytotoxicity of air pollution on oxidative stress, apoptosis, and autophagy.
| Materials and methods|| |
HUVEC line and F-12K medium were obtained from the American Type Culture Collection (Manassas, VA, USA). Dimethyl sulfoxide (DMSO), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, methyl tetrazolium), 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA), and heparin were purchased from Sigma-Aldrich (St. Louis, MO, USA). Fetal bovine serum (FBS), 0.25% trypsin, penicillin, and streptomycin were purchased from GIBCO (Grand Island, NY, USA). Endothelial cell growth supplement (ECGS) was purchased from BD Biosciences (San Jose, CA, USA). 6-well, 96-well plates and cell culture flasks were obtained from Costar Cambridge (MA, USA). DEPs (SRM 2975) were purchased from NIST (Gaithersburg, MD, USA).
Cell culture and diesel exhaust particle treatment
Complete growth medium for HUVECs was F-12K base medium supplemented with 10% FBS, 1% ECGS, 1% heparin, and 1% penicillin and streptomycin. HUVECs were cultured in F-12K complete growth mediums at 37°C, humidified by 5% CO2. All cell exposure experiments were performed at cell confluence of 80%–90% with ≥90% viability, determined by the trypan blue staining. Cells were harvested using 0.25% trypsin and subcultured into 6-well plates or 96-well plates according to the selection of experiments. Prior to treatment, cells were seeded in triplicate for 24 h. Then, the culture medium was replaced with serum-free medium, and the cells were incubated with freshly dispersed DEP for the indicated concentration and duration, respectively.
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay
MTT assay was used to assess cell viability. 96-well plates were used to plate the HUVECs at a density of 1.0 × 104 cells per well to be cultured for 24 h in 100 µl of medium. HUVECs were then treated with either 0, 12.5, 25, 50, 100, or 200 µg/ml DEP for 24 h or with 50 µg/ml DEP for 1, 3, 6, 12, or 24 h. After exposure, 10 µl of MTT was added to each well and incubated for 1 h at 37°C. Cells were then treated with 100 µl of DMSO. A microplate reader was used to quantify the absorbance (Thermo MK3, MA, USA) at a wavelength of 492 nm. The viability of the treated cells was expressed as a percentage of untreated cells, which was assumed to be 100%.
Reactive oxygen species assay
The level of intracellular ROS in HUVECs was determined by measuring the oxidative conversion of DCFH-DA to fluorescent compound dichlorofluorescein. Briefly, HUVECs (2 × 105 cells) were treated with either 0, 12.5, 25, 50, 100, or 200 µg/ml DEP for 24 h or with 50 µg/ml DEP for 1, 3, 6, 12, or 24 h. After exposure, DCFH-DA, dissolved in ethanol, was added to the cell culture at a final concentration of 40 µM for 30 min at 37°C. The fluorescence intensity with excitation and emission wavelengths of 485 nm and 525 nm was detected in a fluorescence multi-well plate reader (BioTek Gen5, Winooski, VT, USA). Results were measured as ratios of fluorescence intensity change relative to untreated cells.
Nicotinamide adenine dinucleotide phosphate oxidase activity
NOX activity was determined as described by us previously. HUVECs (2.0 × 105 cells) were treated with 0, 12.5, 25, 50, 100, or 200 µg/ml DEP for 24 h. Then cells were harvested using trypsin. Moreover, cell samples containing phenylmethylsulfonyl fluoride and a protease inhibitor cocktail (Thermo Fisher Scientific; 20 µl) were added to a 96-well luminescence plate containing 6.25 µmol/L of lucigenin. The reaction was initiated by the addition of NADPH (100 µmol/L). NOX activity was calculated by the change in luminescence recorded by the DTX880 Multimode Detector (Beckman Coulter, Fullerton, CA, USA).
After exposure to DEP, HUVECs were separated from the culture medium and lysed in RIPA buffer. Cell lysates were collected using centrifugation at 12,000 g for 5 min at 4°C. Protein concentrations were determined using a bicinchoninic acid protein assay kit (Pierce Biotechnology, Inc., Rockford, IL, USA). Proteins were subjected to 10% or 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis and then transferred to a PVDF membrane. The PVDF membranes were then blocked in 5% nonfat milk at room temperature for 1 h. They were subsequently incubated first with primary antibodies diluted (1:1,000) in Tris-buffered saline/Tween-20 (TBST) containing 5% bovine serum albumin for overnight at 4°C, then with secondary antibodies diluted (1:1,000) at room temperature for 1 h. The membranes were washed with TBST four times for 5 min each. Immunoreactive bands were detected with ECL reagents (Millipore, Billerica, MA, USA) according to the manufacturer's instructions. β-actin was used as a loading control for the total protein content and showed no differences between the groups.
GraphPad Prism 8.0.2 software (San Diego, CA, USA) was used for the statistical analysis. All results obtained were expressed as mean ± standard deviation of three independent experiments. The data were analyzed using one-way analysis of variance followed by post hoc comparisons using the Tukey's multiple paired comparison test. In all cases, P < 0.05 was considered statistically significant.
| Results|| |
Effects of diesel exhaust particle on cell viability
To investigate the potential dose-dependent and temporal effect of DEP toxicity, HUVECs were exposed to either 0, 25, 50, 100, or 200 µg/ml DEP for 24 h or to 50 µg/ml DEP for 1, 3, 6, 12, or 24 h. As shown in [Figure 1]a, DEP induced a decrease in cell viability by inducing cell damage with concentrations ≥25 µg/ml (P < 0.05). The greatest cell damage was observed with 200 µg/ml (P < 0.001). Furthermore, as shown in [Figure 1]b, DEP induced a further decrease in cell viability with increasing duration of time after exposure, with significant decreases in viability observed after 3 h.
|Figure 1: Effect of diesel exhaust particle exposure on cell viability in human umbilical vein endothelial cells. The human umbilical vein endothelial cells were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml diesel exhaust particle for 24 h (a) or 50 µg/ml diesel exhaust particle for 1, 3, 6, 12, and 24 h (b). Cell viability was then assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Values are represented as mean ± standard deviation of three independent experiments. **P < 0.05, ***P < 0.001 versus the untreated control cells|
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Effects of diesel exhaust particle on oxidative stress
DCFH-DA intensity was used to evaluate the potential of DEP to induce intracellular ROS generation in HUVECs. As shown in [Figure 2]a, DEP induced a dose-dependent increase in ROS generation, with significant increases from 50 µg/ml for 24 h and exposure. Furthermore, 50 µg/ml DEP induced a temporal increase in ROS level, with significant increases between 3 and 24 h [Figure 2]b.
|Figure 2: Effect of diesel exhaust particle exposure on reactive oxygen species generation in human umbilical vein endothelial cells. The human umbilical vein endothelial cells were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml diesel exhaust particle for 24 h (a) or 50 µg/ml diesel exhaust particle for 1, 3, 6, 12, and 24 h (b). The reactive oxygen species levels were determined by measuring the oxidative conversion of 2',7'-dichlorodihydrofluorescein diacetate to dichlorofluorescein. Values are represented as mean ± standard deviation of three independent experiments. *P < 0.05, ***P < 0.001 versus the untreated control cells|
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Next, we examined the NOX activity and levels of gp91, a NOX subunit, to determine the source of ROS in the context of DEP exposure. As shown in [Figure 3]a, a progressive increase in NOX activity was observed with increasing concentrations of DEP. While NOX activity did not significantly increase with 12.5 or 25 µg/ml, it did significantly increase with 50, 100, and 200 µg/ml relative to the control. The increased expression of gp91 followed the same trend as NOX activity. DEP induced a dose-dependent increase in gp91 expression with significant increases from 50 µg/ml [Figure 3]b, [Figure 3]c].
|Figure 3: Effect of diesel exhaust particle exposure on nicotinamide adenine dinucleotide phosphate oxidase activity and protein expression of gp91. The human umbilical vein endothelial cells were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml diesel exhaust particle for 24 h. (a) Nicotinamide adenine dinucleotide phosphate oxidase activity was detected according to the manufacturer's instructions. (b) Quantification of gp91 expression was performed by Western blotting. (c) Representative Western blot of gp91 expression bands. Values are represented as mean ± standard deviation of three independent experiments. *P < 0.05, ***P < 0.001 versus the untreated control cells|
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Effects of diesel exhaust particle on autophagy
To detect whether DEP induced autophagy, the HUVECs were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml DEP for 24 h. Western blotting was used to detect autophagy-associated proteins (light chain 3 [LC3]-I, LC3-II, p62, and Beclin-1). As displayed in [Figure 4], DEP exposure induced a dose-dependent increase in LC3-I to LC3-II conversion [Figure 4]b and Beclin-1 expression [Figure 4]d, while a dose-dependent decrease was seen in p62 expression [Figure 4]c.
|Figure 4: Effect of diesel exhaust particle exposure on autophagy in human umbilical vein endothelial cells. The human umbilical vein endothelial cells were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml diesel exhaust particle for 24 h. Representative bands of p62, Beclin-1, and LC3-I/LC3-II, as determined by Western blotting (a). Quantification densitometric analysis of LC3-II:LC3-I ratio (b), p62 expression (c), and Beclin-1 expressions (d). Values are represented as mean ± standard deviation of three independent experiments. *P < 0.05, ***P < 0.001 versus the untreated control cells|
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Effects of diesel exhaust particle on apoptosis
We further examined the levels of key apoptotic proteins, Bcl2 and Bax, to reflect the role of DEP exposure on cell apoptosis. Cells were treated with 0, 12.5, 25, 50, 100, or 200 µg/ml DEP for 24 h. Western blot showed an increase in Bax and a decrease in Bcl2 protein expressions in increasing magnitudes with greater concentrations [Figure 5]. The expression of Bcl2 was significantly inhibited in the 200 µg/ml DEP-treated group [Figure 5]b, while Bcl2 expression was significantly increased in the 100 and 200 µg/ml DEP-treated groups [Figure 5]c. Bax/Bcl2 ratio was increased, showing the same trend as Bax [Figure 5]d.
|Figure 5: Effect of diesel exhaust particle exposure on apoptosis in human umbilical vein endothelial cells. The human umbilical vein endothelial cells were exposed to 0, 12.5, 25, 50, 100, or 200 µg/ml diesel exhaust particle for 24 h. Representative bands of Bcl2 and Bax, as determined by Western blotting (a). Quantification densitometric analysis of Bcl2 expression (b), Bax expression (c), and Bax:Bcl2 ratio (d). Values are represented as mean ± standard deviation of three independent experiments. *P < 0.05, ***P < 0.001 versus the untreated control cells|
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| Discussion|| |
In the present study, we used the HUVECs to investigate the effects of airborne DEP on cell injury. Our results demonstrate that DEP exposure results in a dose-dependent and temporal decrease in cell viability and increase in intracellular oxidative stress. Moreover, we demonstrated that autophagy was induced after 24-h DEP exposure as evidenced by upregulation of Beclin-1, LC3-II conversion, and reduced levels of p62 protein. We also found that 24-h DEP exposure induced an intrinsic cell apoptosis, as indicated by the increase in Bax protein expression and decrease in Bcl2 protein expression.
Epidemiologic studies have consistently demonstrated a link between DEP air pollution exposure and the increase in morbidity and mortality caused by various diseases, such as respiratory and cardiovascular diseases, which include ischemic heart disease, and cerebral vascular disease. Despite the recent efforts to reduce or even ban the diesel-fueled vehicles in many Western countries, DEP emissions remain a great concern for public health and may even worsen in the still growing megalopolises of developing countries.
Many studies showed that oxidative stress is a common pathway for PM-induced oxidative damage., Although the major composition of ROS produced in the cells exposed to DEP remains unexplored, these free radical species are very transient and cytotoxic.,, Exposure to DEP induces the generation of free radicals that leads to a state of cellular oxidative stress. This has been shown to cause significant damage in both cell cultures and animal models.,,[ In the present study, we found that DEP exposure induced a dose- and time-dependent increase in intracellular ROS level, which is consistent with previous studies. In vitro studies demonstrated that DEP upregulates antioxidant enzymes in various cell types, including bronchial and pulmonary epithelial cells,, macrophages, lymphocytes, and endothelial cells. Although there are many potential sources of these pathogenic ROS, NOX has been heavily implicated as the main contributor. NOX is an ROS-producing enzyme formed by the phosphorylation of several cytosolic subunits (including p47phox and p67phox) that, when activated, translocate to join membrane subunits (p22phox and gp91phox) to form a catalytic unit. NOX was commonly highly expressed in pathologic states and contributed to oxidative imbalances and thus caused oxidative stress within the body., Animal studies have shown increased oxidative stress and expression of vascular antioxidant genes after DEP PM exposure, which was accompanied by increased atherosclerotic plaque size and plasma lipid peroxidation, suggesting an increased atherosclerosis and cardiovascular disease.,
Autophagy is a highly conserved eukaryotic cellular recycling process and plays an important role in cell survival and maintenance. These cytoplasmic materials are sequestered in double-membrane vesicles known as “autophagosomes,” which fuse with lysosomes to form autolysosomes, and further degraded by lysosomal hydrolases. The formation of the autophagosomes involves various autophagy-related (Atg) genes, such as atg5, atg12, Beclin-1, and microtubule-associated proteins LC3. Moreover, when autophagy occurs, the p62 protein binds directly to LC3, serving as a mechanism for delivering selective autophagic cargo for degradation by autophagy. Therefore, the expressions of LC3-II/LC3-I and p62 are widely used as autophagy markers. Previous studies reported that autophagy was involved in PM-mediated cytotoxicity and many results were reported from PM-induced cytotoxicity in respiratory cells.,, In the present study, we found that DEP exposure induced autophagy in HUVECs, which was consistent with other studies.
Apoptosis is a mechanism of programmed cell death and is an essential process to remove excessive, unwanted, and harmful cells for maintaining homeostasis. Apoptotic cells undergo characteristic changes in cell morphology: cell rounding, plasma membrane blebbing, and nuclear fragmentation. It is generally accepted that there are two main apoptotic pathways: extrinsic signaling through death receptors that leads to the formation of the death-inducing signaling complex (DISC) and intrinsic signaling Bcl2-family (pro-/anti-apoptotic proteins (i.e., Bcl2, Bcl-XL, Bax, and Bid). Intrinsic signaling causes the mitochondrial outer membrane to become permeable and consequently form apoptosomes. Formation of DISC and apoptosomes activates initiator and common effector caspases, respectively, which execute the apoptosis process. PM2.5 induces Bim-mediated apoptosis by causing a reduction in the MMP and increasing caspase-9, caspase-3, and poly-ADP-ribose polymerase (PARP) in A549 cells. PM2.5 triggered-oxidative stress significantly reduces MMP, decreases expression of Bcl, and increases expression of Bax, resulting in the activation of caspase-9, caspase-7, and caspase-3, and PARP cleavage in A549 cells. DEP-induced mitochondrial superoxide anion generation leads to ATP depletion, followed by depolarization of actin cytoskeleton and prohibition of PI3K/Akt activity, which collectively contribute to endothelial apoptosis. These studies were consistent with our data as evidenced by a decrease in Bcl2 protein expression and an increase in Bax protein expression and Bax/Bcl2 ratio. ROS can regulate both cell survival and cell death signaling pathways. Previous studies showed that PM2.5-induced ROS leads to both autophagy and apoptosis in human lung epithelial cells. Our data showed that DEP-induced excessive ROS may lead to autophagy and apoptosis in HUVECs. The relation between autophagy and apoptosis induced by DEP needs to be elucidated.
| Conclusion|| |
In summary, the results of the present study provide evidence of DEP-induced oxidative stress, autophagy, and apoptosis in human endothelial cells. The information on DEP-induced reactive oxidative stress, inflammatory response, apoptosis, and autophagy pathways has tremendous potential to aid our understanding of toxicology and design of suitable pharmaceutical therapy and chemoprevention.
Financial support and sponsorship
This work was supported by the National Nature Science Foundation of China (71838, 81802231 and 21707095), Beijing Tongzhou District Financial Fund, and the Science and Technology Plan of Beijing Tongzhou District (KJ2018CX006, KJ2019CX012-38, KJ2019CX014-29).
Conflicts of interest
There are no conflicts of interest.
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