Adverse effects of air pollution-derived fine particulate matter on cardiovascular homeostasis and disease

Abstract

Air pollution is a rapidly growing major health concern around the world. Atmospheric particulate matter that has a diameter of less than 2.5 μm (PM_2.5) refers to an air pollutant composed of particles and chemical compounds that originate from various sources. While epidemiological studies have established the association between PM_2.5 exposure and cardiovascular diseases, the precise cellular and molecular mechanisms by which PM_2.5 promotes cardiovascular complications are yet to be fully elucidated. In this review, we summarize the various sources of PM_2.5, its components, and the concentrations of ambient PM_2.5 in various settings. We discuss the experimental findings to date that evaluate the potential adverse effects of PM_2.5 on cardiovascular homeostasis and function, and the possible therapeutic options that may alleviate PM_2.5-driven cardiovascular damage.

Introduction

Air pollution has risen as a major risk factor for global mortality. Despite governmental measures to decrease pollutant levels over the last several decades, air pollution continues to pose a global threat to public health [1]. Ambient air pollution contributes to more deaths than all other known environmental diseases combined [2]. Out of the 7 million premature deaths caused by air pollution, approximately 3 million are attributed to cardiovascular diseases [3].

Particulate matter (PM) is commonly categorized as PM₁₀, PM_2.5, and PM_0.1, which are PM less than 10, 2.5, and 0.1 μm in diameter, respectively. Atmospheric particulate matter with a diameter of less than 2.5 μm (PM_2.5) is an air pollutant of special concern and, according to a recent review [4], it can be considered the most important environmental threat to the global disease burden. PM_2.5-attributable premature deaths range from 3–125 deaths per 100,000 people in urban areas worldwide where more than half of these deaths occur as a result of increased mortality from cardiovascular diseases (CVDs), including ischemic heart disease, heart failure, cardiac arrhythmias, and hypertension [3–6]. In particular, chronic PM_2.5 exposure increases these risks in subpopulations with greater susceptibilities, such as individuals with pre-existing conditions (e.g. cardiopulmonary complications, asthma, chronic obstructive pulmonary disease, and diabetes mellitus), lower socioeconomic status communities, underrepresented racial and ethnic minority, and aging populations [7–9]. This is concerning since out of the 250 most populous cities worldwide only 8% have population-weighted mean concentrations below the WHO guideline for annual average PM_2.5 [5].

The different size categories of particulate matter (PM) determine their transport and depth of penetration into the respiratory and cardiovascular system. While larger particles, such as PM₁₀ mostly affect the upper respiratory tract, PM_2.5 can penetrate the upper and lower respiratory system and enter the circulatory system leading to direct health effects and increased disease risk [10,11]. Furthermore, the size of the PM along with the shape, density, airway geometry, and breathing patterns determines the carrying capacity of toxic compounds in the lungs, with Brownian diffusion, inertial impaction, interception, and gravitational settling as dominant processes of particle deposition [12,13].

Inside the respiratory airways and lung alveoli, PM_2.5 stimulates macrophages and epithelial cells to release proinflammatory cytokines, causing vascular damage and activating systemic inflammation [3,14–16]. Although the association between PM_2.5 exposure and CVD risk has been epidemiologically established, the molecular and cellular basis of PM_2.5-associated cardiovascular dysfunction is yet to be fully understood. Suggested mechanisms to date include: (i) local inflammation and oxidative stress in the lungs leading to subsequent systemic inflammation, which contributes to cardiovascular stress, endothelial dysfunction, thrombosis, and atherosclerosis; (ii) activation of platelets in the bloodstream leading to increased risk of thrombosis, myocardial infarction (MI), and ischemic stroke; (iii) autonomic nervous system alterations leading to arrhythmias; (iv) direct damage to tissues and cells within the cardiovascular system; and (v) exacerbation of arrhythmogenic phenotypes leading to arrhythmias [3,6,8].

Recently published reviews have comprehensively discussed the epidemiological effects of air pollution on the cardiovascular system [3,17,18]. In this article, we focus on the molecular and cellular toxicity of fine particulate matter, originating from a wide range of sources and composed of various constituents, on the cardiovascular cell types and summarize the relevant findings from empirical studies using small animal models or tissue culture systems. Thorough understanding of the cellular and molecular mechanisms underlying particulate matter-induced cardiovascular damage is undoubtedly critical for development of effective therapeutic options in combatting this significant health risk.

Characteristics of PM_2.5

Sources of PM_2.5.

PM_2.5 imposes health risk in combined effects of ambient (outdoor) and household (indoor) air pollution and can originate as a result from anthropogenic (man-made) or natural activities. In urban areas, the main sources of outdoor PM_2.5 include vehicles, diesel exhaust, power generation, building heating systems, industrial emissions, and domestic combustion. The main indoor sources of PM_2.5 include cooking, heating, and particle resuspension, particularly from burning fuels such as wood and coal in inefficient stoves or open hearth furnaces [19,20]. [19,20]. In rural areas, emission related to farming and biomass burning are the main contributing anthropogenic sources while natural emission sources include those from vegetation, seas and oceans, soil, volcanic eruptions, and wildfires, each its own chemical composition and contribution [21].

Wildfires have also been an increasingly large source of PM_2.5 in less urbanized environments, for example during summertime in the Western United States and Australia [22]. In addition, more individuals are directly susceptible to fire conditions after an influx of new homes and development within the wildland urban interface, with their structures and furnishings contributing to the diversity of constituents within PM_2.5 [23]. Wildfire events may significantly increase ground-level PM_2.5 concentration, exceeding the established levels for protecting the environment and human health [24]. Because fine particles settle out of the atmosphere more slowly than coarse particles, fine particles disperse farther from the source [25]. Therefore, these particles can cross geographical boundaries and negatively affect human health across a wide range of areas, even where no domestic fires or wildfires are active, increasing the disease burden associated with air pollution [3,26].

Components of ambient fine particulate matter.

PM_2.5 is the most relevant for public health concerns because of its ability to dissipate over large geographic distances before settling down. When inhaled, PM_2.5 is capable of penetrating beyond the alveolar regions and entering circulation [1]. Hence, it is important to consider not only the quantitative aspects of the PM_2.5 concentration, but also its qualitative composition. PM_2.5 from combustion sources has been shown to contain metallic elements (including iron, copper, chromium, and nickel), organic molecules (such as polycyclic aromatic hydrocarbons and other volatile organic compounds), and other major contributors (such as nitrate, sulfate, and mineral dust) (Table 1). Liu et al. showed that the main components of PM_2.5 in six urban and six background sites in China were elemental carbon (EC), nitrate (NO₃⁻), sulfate (SO₄²⁻), ammonium (NH₄⁺), chlorine (Cl⁻), and organic matter (OM) [27]. Furthermore, Alves et al. offers an analysis of PM collection filters by scanning electron microscopy and energy dispersive X-ray spectroscopy that highlights differences between particles of biological origin, soot and salt particles, and soil resuspension of possible anthropic sources of emissions [21]. These findings indicate that PM is made of a carbonaceous core linked with various metallic elements and organic molecules [1].

Table 1. Components of PM2.5.

The adverse effects of PM_2.5 on cardiovascular health vary according to different source and pollutant composition. The components of PM_2.5 can be largely divided into two categories: primary aerosols, which include metals and other elements, and secondary aerosols, which are produced by the gas-to-aerosol chemical conversions [4]. This table was adapted and modified from Al-Kindi et al. [4] and Brown et al. [28].

Aerosol type	Element / compound	Source
Primary aerosols (metals and elements)	Potassium (K)	Biomass; refuse incineration; refuse incineration; soil
	Sodium (Na)	Sea salt
	Calcium (Ca)	Cement; soil and road dust
	Aluminum (Al)	Coal burning; soil and road dust
	Selenium (Se)	Coal burning
	Cobalt (Co)	Coal burning
	Arsenic (As)	Coal burning
	Iron (Fe)	Industries; oil burning; soil
	Zinc (Zn)	Industries; refuse incineration; auto-related
	Copper (Cu)	Industries; auto-related
	Lead (Pb)	Industries; refuse incineration
	Silicon (Si)	Soil and road dust
	Vanadium (V)	Oil burning
	Nickel (Ni)	Oil burning
	Manganese (Mn)	Oil burning
	Titanium (Ti)	Soil
	Elemental carbon (EC)	Biomass, diesel and petrol; auto-related
	Organic carbon (OC)	Biomass, diesel and petrol; auto-related
Secondary aerosols (gas-to-aerosol chemical conversions)	Sulfates (SO4) from O2 emissions	Diesel and coal combustion
	Nitrates (NO3) from NOx emissions	High-temperature combustion
	Ammonium (NH4) from NH3 emissions	Fertilizer usage and animal husbandry
	Organic aerosol from volatile organic compound (VOC) emissions	Biomass, diesel, petrol and gas combustion

The components and compositions of PM_2.5 vary by area and season, which makes it important to study the individual and combinatorial impact of PM_2.5 compounds [10,28,29]. PM_2.5 mass concentrations were higher and greater in variability in the warm season compared to those in the cool season for most indoor and outdoor microenvironments. However, seasonal differences were not evident inside vehicles. This was likely because PM_2.5 that stays inside automobiles reacts with sunlight and warmer temperature to form secondary species, more so than the outdoor particles in the cold ambience (Table 1). With a few exceptions, PM_2.5 concentrations of elements found in the soil (Al, Ca, Fe, K and Ti) and automobile-related pollutants (elemental carbon, organic carbon, Zn and Cu) were higher inside cars than those measured in ambience in both warm and cool seasons [28]. Compared to the composition of ambient PM_2.5 without the presence of wildfire smoke, wildfire PM_2.5 in the Western United States showed significant increase in the fraction of organic carbon (OC) by 20 percentage points (95% confidence interval (CI): 17, 23) and an increase in the fraction of EC by 0.99 percentage points (95% CI: 0.43, 1.6), whereas a decrease in the fraction of sulfate by 9.5 percentage points (95% CI: −12, −7.4), sulfur by 3.6 percentage points (95% CI: −4.4, −2.8), silicon by 2.0 percentage points (95% CI: −2.8, −1.2), aluminum by 0.74 percentage points (95% CI: −1.1, −0.43), calcium by 0.55 percentage points (95% CI: −0.75, −0.35), and iron by 0.53 percentage points (95% CI: −0.74, −0.32) [30]. Although PM composition did influence biomarker levels, PM consistently demonstrated oxidative potential, and it was concluded that the metallic components of PM induce pro-thrombotic and oxidative biomarkers [31,32].

Use of particulate matter in cardiovascular research

In vitro administration of PM_2.5.

In vitro experiments investigate the adverse health effects of PM_2.5 on various cell types. The types of cells that have been used for in vitro PM_2.5 studies include mice cardiomyocytes, human cardiomyocytes, including induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMs), and human umbilical vein endothelial cells (Table 2). The traditional model of PM_2.5 and CVD for in vitro studies involved immersion in cell culture in PM_2.5 dilutions (Figure 1). To establish an effective in vitro platform for investigating the precise molecular and cellular effects of PM_2.5, it is important to incorporate the cellular interactions among different cell types [33]. However, in studies involving monolayer cells and individual cell types, the main shortcoming in such setups do not fully recapitulate the complex biological conditions of human organisms. One of the methods that addressed such limitations of in vitro studies was air liquid interface cell culture, which emulates the pseudostratified epithelium by having basal stem cells grown in contact with the culture medium and the apical surface exposed to air, is also an option for in vitro administration of PM_2.5, but it has been primarily used for respiratory epithelial cells rather than cardiomyocytes or endothelial cells [34,35].

Table 2. Studies investigating the effects of PM_2.5 in cardiovascular homeostasis and function.

This table summarizes in vitro and in vivo studies published to date investigating the physiological effects of PM_2.5 in cardiovascular system, which show association between PM_2.5 intoxication and CVDs. PM; particulate matter. NIST; National Institutes of Standards and Technology. eNOS; endothelial nitric oxide synthase.

Disease type	Phenotype	Species	Species or cell lines used for investigation	Exp. Type	Treatment method	Treatment dosage & duration	Source of PM2.5	Ref.
Cardiac dysfunction	Cardiac ATP synthesis abnormality	Rat	H9c2 (rat cardiomyocyte cell line)	In vitro	Submerged cell culture	1–10 μg/cm2 PM2.5 for 24 h	Taiyuan, China	[29]
Cardiac dysfunction	Functional cardiotoxicities, mitochondrial dysfunction, DNA damage	Rat	Neonatal rat cardiomyocyte (NRCM)	In vitro	Submerged cell culture	25, 50, 75, 100, 200 and 400 μg/ml for 48 h	Taiyuan, China	[55]
Cardiac dysfunction	Contractile dysfunction, reduced calcium handling ability	Rat	Sprague Dawley (SD) rat cardiomyocytes	In vitro	Submerged cell culture	0.25, 0.50, 1.0, and 25 μg/ml for 1h	Diesel exhaust particle (DEP), NIST Standard Reference Matter (SRM) 1650b	[64]
Cardiac dysfunction	Exacerbates High Glucose-Induced Cardiomyocyte Dysfunction through ROS Generation	Rat	Sprague Dawley (SD) rat cardiomyocytes	In vitro	Submerged cell culture	0.1 μg/ml for 24h	Diesel exhaust particle (DEP), NIST Standard Reference Matter (SRM) 1650b	[54]
Cardiac dysfunction	Mitochondria-mediated apoptosis	Human	AC16	In vitro	Submerged cell culture	25, 50, and 100 μg/ml for 24h	Beijing, China	[58]
Cardiac dysfunction	Cardiac ATP synthesis abnormality; functional decrement of myocardium	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	summer PM2.5 groups (0.2, 0.6, 1.5 mg/kg, and winter PM2.5 group (0.3, 1.5, 2.7 mg/kg), once every 3 days for 2 months	Taiyuan, China	[29]
Cardiac dysfunction	Cardiac fibrosis	Mouse	C57BL/6 mice	In vivo	Oropharyngeal aspiration	3 mg/kg every other day for 4 weeks	Taiyuan, China	[36]
Cardiac dysfunction	Myocardial Ischemia/Reperfusion Injury Through Farnesoid-X-Receptor-Induced Autophagy	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	6.67 mg/ml for 24 h	Lanzhou, China	[60]
Cardiac dysfunction	Autophagy; decreased myocardial layers in the heart	Zebrafis h	Zebrafish embryos; wildtype zebrafish (AB strain) and transgenic zebrafish (Tg:zlyz-enhanced green fluorescent protein (EGFP))	In vivo	Submerged cell culture	200, 300, 400, 500, 600, and 800 μg/ml for 120 h	Jinan, China	[56]
Cardiac dysfunction	Fibrosis, myocardial remodeling, inflammatory response, oxidative stress	Rat	Wistar rats	In vivo	Exposure chamber	daily average of between 600–800 μg/m3, 5 days a week, for 4 weeks, or 7 weeks	São Paulo, Brazil	[52]
Cardiac dysfunction	Fibrosis, myocardial remodeling, inflammatory response, oxidative stress	Mouse	BALB/c mice	In vivo	Intratracheal instillation	4.0 mg/kg, once per day for 5 consecutive days	China	[49]
Cardiac dysfunction	Cardiomyopathy confers susceptibility to particulate matter-induced oxidative stress, vagal dominance, arrhythmia, and pulmonary inflammation	Rat	Spontaneously Hypertensive Heart Failure (SHHF) rats	In vivo	24-port nose-only flow-by inhalation chambers	580 μg/m3 for 4h	Synthesized by combining metal compounds in molar ratios comparable to those in the historic residual oil fly ash (ROFA) sample collected as a postcontrol fugitive stack emission at a Florida Power and Light plant burning #6 grade residual oil containing 1% sulfur	[67]
Cardiac dysfunction	Cardiac and mitochondrial dysfunction	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	300 μl of vehicle (5% FBS in PBS) with 300 μg of PM for 24 h	Collected within one mile of an active mountaintop removal mines (MTM) site	[59]
Cardiac dysfunction	Exacerbation of viral myocarditis	Mouse	BALB/c mice	In vivo	Intratracheal instillation	10 mg/kg for 7d	Non-industrial district in Shanghai, China	[41]
Cardiac dysfunction	Exacerbation of viral myocarditis	Mouse	BALB/c mice	In vivo	Intratracheal instillation	10 mg/kg for 7d	Non-industrial district in Shanghai, China	[42]
Cardiac dysfunction	Systemic oxidative stress	Mouse	ApoE−/− mice, C57BL/6 mice	In vivo	Intratracheal instillation	3, 10, or 30 mg/kg for 3d	Shanghai, China	[69]
Cardiac dysfunction	Inflammation, oxidative stress, hypercoagulation, cardiomyocyte apoptosis	Rat	Wistar rats	In vivo	Intratracheal instillation	0, 4, 40 mg/kg once every 2d for 3 times	Beijing, China	[53]
Cardiac dysfunction	Inflammation, imbalance of oxidative stress, the altered Ca2+ channel related proteins and the increased intracellular free Ca2+, heart impairment	Mouse	BALB/c mice	In vivo	Intratracheal instillation	0.01 mg/μl for 2d	Hebei, China	[46]
Cardiac dysfunction	Cardiomyopathy, oxidative stress, fibrosis, inflammation, mitochondrial disorder	Mouse	C57BL/6J mice; wild type (Nrf2+/+) and Nrf2 knockout (Nrf2−/−) mice	In vivo	Exposure chamber	50.1 ± 2.5 μg/m3, flow rate of 65 L/min for 6 h/day, 5 times a week, for 24 weeks	Yuquan Road, Beijing, China	[47]
Cardiac dysfunction	Cardiac hypertrophy, fibrosis, metabolic alterations	Mouse	C57BL/6J mice	In vivo	Exposure chamber	100 μg/m3, flow rate 3.5-4.0 L/min, exposed for 28 consecutive days (6 h/day), with 2 days of break every 5 days	Taipei Main Station, Taiwan	[10]
Cardiac dysfunction	Increased glycemic homeostasis, inflammation, myocarditis, aortic medial thickness	Rat	Sprague Dawley (SD) rats	In vivo	Exposure chamber	PM2.5 at ambient concentration for 24 h/day, 7 days/week, for a total of 16 weeks	Taipei City, Taiwan	[43]
Cardiac dysfunction	Impaired oxygen metabolism and contractile function	Mouse	Swiss mice	In vivo	Intranasal instillation	1 mg/kg	Mystic, Connecticut, USA	[91]
Cardiac dysfunction	Systemic adverse effects, alterations in lung and cardiac gene expression	Mouse	BALB/c mice	In vivo	Intratracheal instillation	100 μg of PM2.5 in 100 μl of isotonic saline solution on d0, d3, d6	Milan, Italy	[50]
Cardiac dysfunction	Systemic and pulmonary inflammation, decreased left ventricular ejection fraction, pulmonary and myocardial fibrosis and oxidative stress	Mouse, human	C57BL/6J mice and AMPKα2−/− mice for in vivo, human bronchial epithelial BEAS-2B cells and rat cardio myoblast H9C2 cells for in vitro	In vivo, in vitro	Intratracheal instillation, Submerged cell culture	10 mg/kg PM2.5 in 10 μl PBS every other day for 4w in vivo, 24h in vitro	China	[88]
Vascular dysfunction	Vascular inflammation, decreased viability, migration and angiogenesis	Human	Human umbilical vein endothelial cells (HUVECs), human microvascular endothelial cells (HMEC-1)	In vitro	Submerged cell culture	0–800 μg/ml for 6h	NIST Standard Reference Matter (SRM)	[77]
Vascular dysfunction	Autophagy, FHL2 upregulation, IL-6 production, activation of NF-κB pathway	Mouse	Mouse aortic endothelial cells (MAECs)	In vitro	Submerged cell culture	100 μg/ml for 24 h	Beijing, China	[63]
Vascular dysfunction	Oxidative stress and reduced the PAI-1 production of endothelial cells	Rat	Rat heart microvessel endothelial (RHMVE) cells	In vitro	Submerged cell culture	0, 5, 10, and 25 μg/ml for 12h	Saitama, Japan	[78]
Vascular dysfunction	Vascular remodeling, exacerbated transition from left ventricular failure to right ventricular hypertrophy	Mouse	BALB/c mice	In vivo	Exposure chamber	Local PM2.5 for 10h each day for 3w	Beijing, China	[66]
Vascular dysfunction	Atherosclerosis progression, activation of circulating leukocytes, platelets and associated inflammatory factors	Mouse	ApoE−/− mice	In vivo	Intratracheal instillation	30 mg/kg each day for 8w	Shijiazhuang, China	[80]
Vascular dysfunction	Disseminated intravascular coagulation, inflammatory response, vascular endothelial injury and prothrombotic state	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	0, 1.8, 5.4 and 16.2 mg/kg every 3 days for 30d	Beijing, China	[84]
Vascular dysfunction	Autophagy, VEGF induction, inflammation	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	1.5 mg/kg every 2d for 3 times	Wuhan, China	[86]
Vascular dysfunction	Intracraneal atherosclerosis	Rat	Sprague Dawley (SD) rats	In vivo	Exposure chamber	Ambient PM2.5 for 6w or 12w	Beijing, China	[89]
Vascular dysfunction	Endoplasmic reticulum instability, ANGII-dependent endothelial dysfunction	Rat	Sprague Dawley (SD) rats	In vivo	Intratracheal instillation	1.5 mg/kg every 2d for 3 times	Wuhan, China	[83]
Vascular dysfunction	Endothelial dysfunction, decreased endothelium-dependent relaxation and eNOS expression on pulmonary arteries associated with local inflammation	Rat	Wistar rats	In vivo	Exposure chamber	Ambient PM2.5 (approx. 600 μg/m3) for 2w	São Paulo, Brazil	[82]
Cardiac dysfunction	Heart failure, acute inflammatory response, chronic matrix remodeling, electrical remodeling, epigenetic changes	Mouse	FVB mice	In utero	Exposure chamber	average concentration of 73.61 μg/m3, for 6h/day, 7 days/week	Columbus, Ohio, USA	[51]
Cardiac dysfunction	Neonatal cardiac dysfunction	Mouse	FVB mice	In utero	Exposure chamber	average concentration of 91.87 μg/m3, for 6h/day, 5 days/week throughout gestation period (20 days)	Columbus, Ohio, USA	[65]
Cardiac dysfunction	Cardiac dysfunction	Mouse	FVB mice	In utero	Exposure chamber	average concentration of 38.58 μg/m3 for 6 hours/day, 5 days/week for 3 months	Columbus, Ohio, USA	[48]

Figure 1. Preparation and administration of PM_2.5 for in vitro and in vivo studies.

Two methods exist for preparation of PM_2.5 for experimental usage. (A) One method is to collect ambient PM_2.5 on ultra-fine filters using a high-volume air sampler; cut the filters adhering PM_2.5 into small pieces; immerse the pieces in sterile distilled water and perform ultrasonic sonication; vacuum-freeze dry the particles to obtain powder from; and then perform chromatography and spectrometry to analyze the components and composition. (B) Alternatively, PM products are available for purchase from the National Institute of Standards and Technology (NIST) Standard Reference Materials (SRM). Products NIST SRM 1650B, 2876, and 2975 fit the definition of PM_2.5 (particulate matter with diameter of less than 2.5 μm).

In vivo administration of PM_2.5.

To date, rodent models for PM_2.5-induced CVDs have been used to investigate systemic and physiological effects of PM_2.5. Rather than using in vitro platforms, small animal models represent multi-dimensional organisms with cell-to-cell interactions. The drawback is that the findings from these models may not necessarily translate to human pathophysiology of PM_2.5-induced cardiovascular dysfunction. Intratracheal instillation, intranasal instillation, oropharyngeal aspiration, and inhalation exposure via exposure chambers have been the main methods used for in vivo administration of PM_2.5 (Figure 1), with the first and last being the most used.

Notably, intratracheal instillation involves administering the PM_2.5 directly to the trachea and delivers a high dose of particles to the lung rather than repeated inhalation at lower doses. This may not recapitulate the naturally occurring air pollution exposure compared to exposure chambers [36]. Moreover, a major limitation of intratracheal instillation in comparison to inhalation exposure is that it is invasive and non-physiological in nature, involves a difficult operation needing experienced in vivo animal trainers, bypasses the upper respiratory tract, and possesses confounding variables due to anesthesia and delivery vehicle (e.g. saline) [33,37]. Nevertheless, intratracheal is still commonly used because it provides greater control in administering varying concentration of PM and of target delivery of administration. It is also less expensive than the inhalation exposure approach, which involves expensive exposure chambers that also require technical experience [33,38]. Another method is to expose the mice to concentrated fine particulate matter using in vivo administration techniques mentioned above, and then isolate the aorta to be culture in standard cell culture conditions before performing assays for vascular inflammation analyses [39,40]. Different model systems that have been used for in vivo PM_2.5 studies include mice, rats, and zebrafish embryos (Table 2).

PM_2.5-induced effects on the heart

Mechanical complications:

Inflammation and oxidative stress.

Inflammation is a commonly suggested mechanism for PM_2.5-induced cardiac dysfunction. Several studies have implicated cardiac inflammation resulting either from direct effects of translocated nanoparticles on the heart or from indirect effects of pulmonary oxidative stress and inflammatory responses leading to systemic inflammation [2–4]. PM_2.5-induced inflammation in the myocardium has been suggested to exacerbate viral myocarditis, possibly through Treg responses and increased viral replication [41–43]. It has been shown that viruses, including avian influenza virus and SARS-CoV-2, are carried by fine particles and may use PM as vectors. Oxidative stress, a cellular reaction closely related to inflammation, has also been associated with PM_2.5 exposure. PM-induced oxidative stress may activate proteases, which play an essential role in cleaving the viral membrane protein, and hence facilitate viral entry into the host cells. Cardiovascular impairment of COVID-19 includes myopericarditis, vasculitis, and systemic inflammation, showing inflammation markers such as antiphospholipid antibodies, fibrinogen, and D-dimer [44,45].

To date, multiple studies using mouse models have unveiled the capacity of PM_2.5 to induce inflammation and oxidative stress in the adult mammalian heart (Figure 2). Upregulated cardiac inflammatory responses upon chronic or acute PM_2.5 exposure are indicated by the elevated expression of: transforming growth factor β1 (TGF-β1) [36], TNF-α [46–48], IL-6 [36,46–49], IL-18 [46,49], IL-15 [48], heat shock protein 70 (Hsp-70) [50], myeloperoxidase (MPO) [50], CXCL1 [49], C-reactive protein (CRP) [48], CD26E [48], C26P [48], intercellular adhesion molecule 1 (ICAM-1) [48], NF-κB [47–49], NLRP3 [49], RIPK3 [47] and monocyte chemoattractant protein-1 (MCP-1) [48]. Interestingly, expression of NLRP12, a gene previously suggested to form an inflammasome with ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain) but recently shown to be a negative regulator of inflammation via inhibiting NF-κB signaling, was decreased after PM_2.5 exposure. It is proposed that NLRP12 may play a role in PM_2.5-induced cardiac inflammation by regulating NF-κB-p52/p100 and NF-κB-p65 [49].

Figure 2. Adverse cardiovascular effects of PM_2.5.

Plausible pathophysiological mechanisms linking PM_2.5 exposure and cardiovascular disorders, categorized into cardiac and vascular disorders, include pathological fibrosis; cardiac autonomic dysfunction; contractile dysfunction and electrophysiological abnormalities; inflammation; oxidative stress; autophagy; and atherosclerosis, thrombosis, and vascular remodeling.

Feng and colleagues showed that the mRNA expression levels of IL-1β and IL-10 in PM_2.5-treated mice did not show significant difference from those of the control [49], whereas Xu and colleagues showed that PM_2.5 exposure increased IL-1β, TNF-α, and IL-6 levels in serum and heart, leading to systolic dysfunction and accelerated cardiac fibrosis and mitochondrial damage, phenotypes of which were further pronounced in mice deficient of transcription factor Nrf2, a master regulator and activator of various cytoprotective genes with an antioxidant-response element [47].

Oxidative stress was exhibited in mice exposed to PM_2.5 via increased levels of malondialdehyde (MDA) [36,49], heme oxygenase-1 (HO-1) [46,50], NOS2 [48], and Nrf2 [48]. Superoxide dismutase (SOD), an antioxidant enzyme that inhibits reactive oxygen species (ROS) and resists oxidative stress, was reported to be both increased [48] and decreased [46,49] after PM_2.5 exposure. This divergence may be due to differences in the PM_2.5 exposure methods, where preconception exposure led to an increase in SOD [48] and a more direct intratracheal instillation [46,49] led to protein damage and thus a decrease in SOD. Interestingly, parental preconception exposure to PM_2.5 was also shown to lead to cardiac dysfunction in the offspring via oxidative stress and inflammatory pathways in mice [48,51].

Studies using rat models similarly demonstrated inflammatory and oxidative stress in response to PM_2.5 exposure. PM_2.5 increased the expression of TGF-β, TNF-α, and INF-γ genes and increased the concentration of glutathione (GSH) [52]. High sensitivity (hs)-CRP, creatine kinase (CK), CK-MB, LDH, and cTnI levels also increased after exposure, more so in hyperlipidemic rats than in normal rats, whereas the SOD level was significantly lowered [53]. When ventricular myocytes isolated from male adult Sprague-Dawley (SD) rats were exposed overnight to diesel exhaust particles (DEP), an important component of outdoor PM_2.5, the myocardial dysfunction was exacerbated in isolated cardiomyocytes, potentially mediated through ROS generation pathways [54]. The cardiotoxicity of PM_2.5 was also observed in neonatal rat cardiomyocytes (NRCMs), where PM_2.5 induced cell membrane damage and increased the ROS level at concentrations higher than 50 μg/mL [55].

In zebrafish embryos at 4 hours post-fertilization, upregulation of genes associated with inflammation (TGF-β and COX2), endoplasmic reticulum stress (HSPA5, CHOP, IRE1, XBP1s, and ATF6), and autophagy (LC3, Beclin3, and ATG3) were observed after exposure to PM_2.5 for 120 hours, indicating a role of IRE1-XBP1 and ATF6 pathways in PM_2.5-induced inflammation and autophagy responses [56].

Pathological cardiac fibrosis.

Closely related to cardiac inflammation, cardiac fibrosis is an essential component of tissue repair and thus another marker of cardiac injury commonly found in several cardiac diseases, including myocardial infarction and heart failure [36,57]. Fibrosis is characterized by the adverse formation of collagen and other extracellular matrix proteins [36]. Studies have shown that exposure to PM_2.5 induces cardiac fibrosis in mice, demonstrated histologically by increased collagen accumulation and myocardial fibrosis in mouse [10,36,47] or rat [52] hearts. PM_2.5 exposure also has been shown to induce the expression of cardiac fibrosis markers, such as Col1a1 [36,47], Col3a1 [36,48], TGF-β1 [36,47,52], α-SMA [47], FN [47], p-Smad2 [47], p-Smad3 [47], CREB [10], and GSK-3β [10] in mouse and rat models.

PM_2.5-induced cardiac fibrosis occurs primarily from inflammation and oxidative stress triggered by PM exposure. This results in irreversible loss of cardiomyocytes and subsequent replacement by extracellular matrix components including collagen fibers, leading to pathological fibrosis [52]. One signaling pathway heavily involved is the TGF-β1 axis, where the activation of Smad2 and Smad3 stimulate matrix-component synthesis. Particularly, PM_2.5-induced cardiac fibrosis has been shown to be associated with NOX4-ROS-TGF-β1-Smad signaling; this is suggested by increased NOX-4 and TGF-β1 protein levels and increased phosphorylation of Smad3 in juvenile and old mice hearts after 4 weeks of PM_2.5 exposure [36]. Another plausible pathway is the CREB/GSK3β/SOS1 pathway, where CREB is a protein associated with fibrosis (16).

Autophagy.

Autophagy is a dynamic process that removes damaged cellular components through a lysosome-associated degradation process, contributing to cellular homeostasis by regulating cell survival and death pathways, tissue specialization and differentiation, and organogenesis [56,58]. In addition to having functional benefits, autophagy can also increase cell death, causing adverse effects on the cardiovascular system. Furthermore, a recent study showed that PM_2.5 exposure leads to excessive autophagy, which induces endothelial dysfunction, apoptosis, and tissue damage [58].

Long-term exposure to PM_2.5 causes higher expression of proteins involved in autophagy regulation, including Beclin1, Vps34, LC3B-II, and ATG5 in cardiac tissues of mice [47]. In human cardiomyocytes, PM_2.5 exposure activates the mitochondria-mediated apoptotic pathway, where the apoptotic proteins, Caspase-3, Caspase-9, and Bax, were upregulated, whereas the anti-apoptotic protein, Bcl-2, was downregulated. PM_2.5 also induces the activation of LC3I and LC3II and the formation of autophagosomes. Moreover, it was shown that PM_2.5 exposure upregulates Farnesoid-X-receptor (FXR), which plays a role in autophagy regulation, to stimulate autophagy after myocardial ischemia/reperfusion injury (MI/RI) and enhance tissue damage [58–60].

It has also been indicated that autophagy is promoted by PM_2.5-triggered inflammation [56]. Proinflammatory cytokines, such as COX2, can induce endoplasmic reticulum (ER) stress by inhibiting the ER calcium pump and upregulating the iNOS expression [61]. This calcium leak in the ER can then induce mitochondrial ROS production, which affects downstream signaling pathways and makes cells prone to autophagy [62]. On the other hand, ER stress also activates three major UPR signaling pathways and thus stimulates the expression of inflammatory cytokines and NF-κB signaling. Thus, PM_2.5-induced ER stress is closely intertwined with autophagy and inflammation [56]. Activation of NF-κB signaling has also been observed in PM_2.5-exposed mouse aortic endothelial cells in a study by Xia et al., where they suggested a connection between autophagy-induced FHL2 upregulation and IL-6 production under PM_2.5 treatment [63]. After 120-hour treatment of PM_2.5 on zebrafish embryos, Zhang et al. observed histological results showing that zebrafish heart exhibited abnormal changes and an increase in cellular autophagic accumulation. The PM_2.5-treated zebrafish also showed an increase in the expression of genes associated with inflammation, ER stress, and autophagy pathways, suggesting that PM_2.5 caused inflammation and induced ER stress and autophagy through the activation of the IRE1-XBP1 and ATF6 pathways, which are genes associated with the ERS pathway [56]. These findings open a possibility to further research on these cross-talks between these PM_2.5-induced stress responses.

Systolic and Diastolic function.

In a recent study, PM_2.5 exposure has been shown to induce aging-associated cardiac abnormalities, such as cardiac diastolic and systolic dysfunction and elevated heart rate and blood pressure in mice [36]. Impaired contractile function (as evidenced by contractile velocity, increased relaxation time, and reduced percent sarcomere shortening), prolongated TR90 and tau (Ʈ), reduced calcium transient amplitude, and reduced peak shortening was observed, demonstrating the adverse effects of PM_2.5 on intracellular calcium handling of the heart and diastolic dysfunction [64,65]. Ventricular hypertrophy and QT prolongation were evident in PM_2.5-exposed subjects, suggesting progression towards cardiac sudden death [10].

Plausible explanations for contractile dysfunctions induced by PM_2.5 include ROS generation, metabolic disorders, and alterations in the cardiac hypertrophy pathway via eNOS and CREB-related signaling [10,54]. Potential mechanisms for PM_2.5-induced hypertrophy are alterations in the glycogen synthase kinase-3 (GSK3), particularly GSK3β/CREB protein-associated pathways and TLR4/MyD88 signal pathways [10]. Transverse aortic constriction (TAC)-induced heart failure was shown to be exacerbated by PM_2.5 through pathways related to lung oxidative stress, inflammation, vascular remodeling, and RV hypertrophy [66]. Treatment with antioxidants, such as fucoidan, was shown to mitigate PM_2.5-induced cardiac contractile abnormalities [10,64].

Electrical complications:

Cardiac autonomic dysfunction.

Although the underlying mechanisms need to be further investigated, PM_2.5-induced acute adverse cardiovascular events are associated with physiological disturbances related to modulation of the autonomic nervous system (ANS) [67]. In most cases, PM_2.5 has been shown to increase heart rate variability (HRV), a marker of imbalance in the cardiac autonomic nervous system. However, it is important to note that factors such as differences in the composition of PM particles have led to inconsistencies among previous studies on HRV [68]. After exposure to medium and high-dose PM_2.5 in mice, frequency-domain measurements showed a decrease in low frequency (LF) and an increase in high frequency (HF), which thus resulted in a decrease in the ratio of low frequency to high frequency (LF/HF). Regarding HRV, certain frequency ranges are associated with certain physiological activities. HF (the frequency activity in the 0.15 – 0.40 Hz range) reflects the cardiac parasympathetic activity, LF (the frequency activity in the 0.04 – 0.15 Hz range) reflects the cardiac sympathetic and parasympathetic activities, and LF/HF reflects the balance of cardiac autonomic nervous control. Thus, PM_2.5-induced changes to these measures suggested that PM_2.5 was involved in causing an imbalance in autonomic control, particularly through modulating neural input to the heart and shifting the balance to parasympathetic dominance. Atherosclerotic-susceptible mice models showed an even greater increase in HF compared to their wildtype counterparts, implicating a greater shift of autonomic balance to parasympathetic dominance [67,69].

On a genetic level, toll-like receptor 2 (TLR2) methylation has been suggested to increase susceptibility to adverse cardiac autonomic effects of PM_2.5 exposure in older individuals. Dietary modulation by higher flavonoid and methyl nutrients intake has been shown to mitigate the PM_2.5-induced adverse effects and the individuals’ susceptibility to HRV, possibly by decreasing the TLR2 methylation [70].

Repolarization abnormalities and conduction block.

Exposure to PM_2.5 has been shown to exacerbate electrophysiological abnormalities where those with pre-existing conditions, such as coronary artery disease and heart failure, experienced greater elevated risks of cardiac dysfunction [66,69]. Abnormal ECG types associated with PM_2.5 exposure included ST-segment depression and conduction block, which are symptoms related to ischemia and hypoxia in the myocardium [69].

Arrhythmias and sudden cardiac death.

PM_2.5 exposure is also associated with cardiac arrhythmia [71]. Both atrial and ventricular arrhythmias are augmented in the setting of air pollution [72,73]. Air pollution and PM_2.5 are associated with increased hospitalization and mortality due to arrhythmia. Air pollution is also associated with out-of-hospital cardiac arrest [74]. Arrhythmias causing sudden cardiac death are temporally associated with acute exposures to elevated levels of PM_2.5. Case reports suggest that individuals might be more susceptible to the effects of air pollution causing arrhythmia and sudden cardiac death [75].

Vascular complications:

Atherosclerosis, thrombosis, and vascular remodeling.

PM_2.5 has been shown to induce endothelial injury and systemic inflammation [76]. ROS generation and inflammation were evident in vitro, indicating vascular and systemic inflammation [77,78]. Prolonged exposure to PM_2.5 impaired vascular cell viability; decreased angiogenesis; suppressed adhesion to endothelial extracellular matrix proteins; and decreased migration in a dosage-dependent manner [77,79]. Along with such symptoms, PM_2.5 exposure also elevated levels of circulating monocytes and T lymphocytes, platelets, and inflammatory cytokines which contributed to the pathogenic progression of atherogenesis and acute coronary events [79,80]. It also induced elevation of heavy metal components in the blood that was consistent with higher amounts of platelets. Moreover, elevated serum inflammatory factor levels; larger atherosclerotic plaques; higher numbers of lesion macrophages; exacerbated injury to endothelial layers with greater adherence to platelets and leukocytes; elevated levels of inflammatory factors, including NAD(P)H oxidase subunit p22phox and p47phox, and M1/M2 associated markers IL-6, TNF-α, iNOS, IL-12 and arginase-1, and CD206; all contributed to the progression of atherosclerosis, especially in atherosclerosis-prone apolipoprotein E-deficient mice [80].

Long-term exposure to PM_2.5 increases the risk of developing vascular dysfunction, such as atherosclerosis, stroke, hypertrophy, and vascular remodeling [66,81]. A potential mechanism involves increased vasoconstrictors in the system, including TNF-α, prostaglandin E2, CRP, IL-1β, and endothelin-1 [81,82]. PM_2.5-induced vasoconstriction was also associated with increased production and circulating level of ANGII, which was related to the activation of the IRE1α/XBP1s branch of unfolded protein response. This ablation of IRE1/XBP1/HIFα-dependent ACE/ANGII/AT1R axis activation was also shown to inhibit PM_2.5-induced oxidative stress and proinflammatory response in the vascular endothelial cells [83].

The PM_2.5-induced inflammatory response has been associated with the formation of deep venous thrombosis and disseminated intravascular coagulation (DIC), involving the JNK/P53 pathway [53,84]. After exposure to PM_2.5 in vivo, rats underwent Doppler ultrasound that showed that the pulmonary valve (PV) and aortic valve (AV) peak were decreased. Signs of vascular endothelial injury and prothrombotic state were observed, including the downregulation of thrombomodulin expression in the blood vessels, elevation of pro-inflammatory factors and adhesion molecules such as MCP-1, MIP-1α/β, IL-6, IL-1β, CRP, ICAM-1, VCAM-1, sICAM-1, and sVCAM-1, increase in tissue factor and coagulation factor, and elevated thrombin-antithrombin complex and fibrinolytic factor [79,84]. Other circulating inflammatory factors that were elevated include CD14⁺, CD16⁺, CD4⁺, and CD8⁺. PM_2.5 exposure was also associated with elevated levels of endothelial microparticles (annexin V⁺/CD4⁻/CD31⁺), changes which were accompanied by a decrease in the circulating levels of pro-angiogenic growth factors (EGF, sCD40L, PDGF, RANTES, GROα, and VEGF) and an increase in the levels of anti-angiogenic factors (TNFα and IP-10) [79].

Therapeutic options for PM_2.5-induced cardiovascular damage

While withdrawal from PM_2.5 exposure was shown to restore blood pressure, heart rate, cardiac function, expression of collagens, and MDA levels in hearts [36], suggesting the importance of primary prevention of PM_2.5 exposure, there are several other suggestions for potential therapeutic interventions. These suggestions can be largely categorized into three methods: (1) primary prevention, (2) secondary prevision, and (3) therapeutics that reverse adverse effects.

Secondary prevention targeting and altering genes that can alleviate PM_2.5-induced cardiovascular damage, which include: Nrf2 enhancement [47,85]; ablation of IRE1/XBP1/HIFα-dependent ACE/ANGII/AT1R axis activation; and blocking VEGF expression or autophagy induction by intervening in the ATR/CHK1/p53/DRAM1- and LKB1/p53/TIGAR-dependent autophagy pathways [86]. Additionally, human cardiomyocytes exposed to PM_2.5 showed an upregulation of TRPC3, which played an important role in causing abnormalities in the electrophysiology and calcium signaling of the cells. However, these adverse effects could be attenuated by pretreatment with Pyr3, an inhibitor for the TRPC3 channel, suggesting its potential therapeutic usage [87].

Therapeutics that have shown their potential to treat and reverse PM_2.5-induced cardiac impairments in experiments include: compound essential oil (CEO) [46], fucoidan [10], metformin [88], flavonoid [70], omega-3 fatty acids (O3FA) [89], vitamin B [90], Tiron and N-Acetyl-L-cysteine (NAC) [54], infliximab [91], and statins [92].

Concluding Remarks

PM_2.5 and its interaction with other environmental pollutants, such as other gaseous components, could contribute to the observed epidemiological effects of PM_2.5 exposure. This complex nature of the environmental interactions complicates air pollution research, leaving many research questions to be addressed by future experimental and clinical studies [48]. Experimental studies include initial analysis of the components of PM_2.5 with further characterization of the effects in vitro and in vivo. In vitro studies have focused on multiple human derived cell lines but still fail to capture the full complexity of the daily exposures. In vivo studies explore the effects on neurological and cardiovascular systems based on animal models and epidemiological studies.

The size of PM particles has been associated with cardiovascular mortality. Their toxicity increases with smaller size and hence larger surface area, leading to a stronger correlation between decreasing particle size and cardiovascular mortality. Ultrafine particles (UFP), otherwise called PM_0.1 have been shown to be associated with harmful effects that overlap with those of PM_2.5, such as systemic inflammation, electrocardiographic abnormalities, and heart failure. Coalesced PM_0.1 is also a major source of PM_2.5, and hence further studies on PM_0.1 can shed more light on PM_2.5 and its effects [93].

By discussing some important experimental studies performed related to PM_2.5 in this review, we expect to encourage further investigation on fine particulate matter and other environmental pollutants, and potential therapeutics to address the adverse cardiovascular effects.