Homocysteine is a sulphur amino acid that originates from the metabolism of methionine. Methionine, an essential food-derived amino acid, plays a vital role in cellular processes through the donation of methyl groups
[10]. The first metabolite originating from methyl transfer is S-adenosyl methionine, which is subsequently converted to S-adenosyl homocysteine, the immediate precursor of homocysteine. The latter can be ‘recycled’ by taking the methylation route, resulting in the regeneration of methionine, or alternatively, it can be eliminated renally via the transulfuration route, leading to the formation of cysteine. Both processes are mediated by enzymes whose cofactors are vitamin B12, folic acid, and vitamin B6
[11][12]. Under physiological conditions, there is a balance between homocysteine formation and elimination
[12]. If homocysteine can accumulate in the body, the biochemical transformation process fails, leading to a serum level increase
[12]. Serum homocysteine values between 5 and 15 micromol/L are considered normal while mild hyperhomocysteinemia is defined as values between 15 and 30 micromol/L; moderate, between 30 and 100 micromol/L; and severe, if greater than 100 micromol/L
[13]. In the healthy population, blood levels of homocysteine do not appear to be significantly influenced by dietary intake
[14]. Hyperhomocysteinemia has many causes, with genetic profiles playing a dominant role: several genetic polymorphisms have been recognized
[15] as responsible for the deficiency of enzymes involved in homocysteine metabolism
[16]. The most frequent polymorphisms involve the gene coding for methylenetetrahydrofolate reductase and the one coding for cystathionine beta synthase
[17]. Other causes include vitamin B12, B6 and folic acid deficiency
[18]; advanced age; male sex; menopause; lifestyle habits, such as alcohol abuse and smoking
[19]; and certain diseases, including cancers
[15], chronic kidney disease
[20], hypothyroidism
[21], and inflammatory bowel disease
[22]. Mention should be made of drugs that may interfere with the metabolism of homocysteine or its enzymatic cofactors: these include methotrexate, carbamazepine, nitrates, fibrates, and metformin
[23].
In light of these basic findings, several clinical studies have investigated whether the treatment of hyperhomocysteinemia might result in cardiovascular benefits in terms of cardio- and cerebrovascular event reduction with conflicting results.
Uric acid (UA) is the final product of purine metabolism. The increase in its blood levels may depend either on an increased production or on a reduced elimination
[60]. If hyperuricemia develops, urate crystals accumulation may occurs in the joints leading to the clinical manifestations of gout, subsequently also affecting the renal parenchyma and the excretory tracts with the picture of gouty nephropathy and nephro/urolithiasis
[61]. Beyond this known effect, several other clinical studies have also investigated the relationship between high blood levels of UA and the development of CVDs
[62] and, as for homocysteine, with conflicting results. The Framingham Heart Study did not indicate hyperuricemia as an independent risk factor for coronary artery disease, cardiovascular death, and death from all causes
[63][64]. Some epidemiological studies have described a J- or U-shaped relationship between UA levels and cardiovascular risk, meaning that patients with either very low or very high UA values have an increased cardiovascular risk
[65]. More recently, clinical studies seem to support the role of hyperuricemia in atherosclerosis, systemic arterial hypertension, atrial fibrillation, and chronic kidney disease as the pathophysiological processes promoted by UA, such as oxidative stress and inflammation that are the basis of endothelial dysfunction, which may contribute to atherothrombotic events. An increase in the activity of the enzyme xanthine oxidase, which regulates the synthesis of UA and which uses molecular oxygen as an electron acceptor for its function, determines the formation of reactive oxygen species (ROS)
[66]. ROS are responsible for the lipid oxidation and the reduction of the nitric oxide concentration, which causes the loss of the physiological vasodilating effect of the endothelium and determines a prothrombotic phenotype. UA also favors an increase in the deposition of low-density lipoproteins at the endothelial level and their uptake by macrophages, which are transformed into foam cells, thus starting the process of atherosclerosis
[67]. More recently, it has been highlighted how endothelial cells (ECs) may acquire a prothrombotic phenotype by expressing functional tissue factor (TF) once exposed to increasing doses of UA that can be reversed by the preincubation with an uricosuric agent
[68]. Moreover, the endothelial dysfunction induced by hyperuricemia also favors the expression on the cell surface of the adhesion molecules (CAMs) involved in the initiation of the atherosclerosis process. This mechanism appears to be regulated by a modulation of the NF-kappaB pathway, leading to the upregulation of TF on cell surface and downregulation of its natural inhibitor, the Tissue Factor Pathway inhibitor (TFPI)
[69]. Furthermore, the inflammasome
[70] seems also to be involved with an increase in caspase-1 function, which would promote a particular type of endothelial cell apoptosis, known as pyroptosis, and the release of TNF-alpha
[71]. A summary of the possible mechanisms by which UA is involved in CVD is provided in
Figure 2.
All the putative cardiovascular mechanisms associated with vitamin D are provided in Figure 3.
3.1. Obstructive Sleep Apnea Syndrome: The Diving Board to CVDs
Obstructive sleep apnea (OSA) syndrome is a clinical condition characterized by cyclical episodes of total (apnea) or partial (hypopnea) collapse of the upper airways, occurring during sleep, with the persistence of thoracoabdominal movements. At the end of the events, arousal occurs with transient hypoxemia, autonomic alterations, and sleep fragmentation
[92].
Apnea is defined as a reduction in airflow of at least 90% compared to the basal one, lasting at least 10 s while hypopnea is defined as a reduction in airflow of at least 30%, for no less than 10 s, associated with a reduction of at least 3% in oxygen saturation (SaO
2)
[93].
The severity of OSA is based on the number of events/hour, and it is defined as AHI index (apnea/hypopnea index). Specifically, <5 events/hour define a normal respiratory pattern, 5–14 events/hour a mild apnea, 15–29 events/hour a moderate apnea, and from 30 events/h a severe apnea
[93]. The gold standard for the diagnosis of OSA is represented by polysomnography (PSG)
[93].
A diagnosis of OSA is made based on nocturnal breathing disorders (snoring, breathing pauses in sleep, restless sleep, awakening choking) and/or daytime sleepiness symptoms associated with an AHI > 5; on the contrary, if the AHI index is greater than 15, OSA can be diagnosed in the absence of symptoms
[94].
In general population, OSA prevalence is approximately 34% in men and 17% in women
[92][95] while in CVD populations, it ranges from 40% to 60%
[96][97].
During sleep, a failure of the neuromuscular reflex that preserves the patency of the airways occurs, resulting in hypoxemia and hypercapnia, determining an increase in the respiratory effort and an awakening of a few seconds, which restores patency of the upper airways, thanks to a series of reflex mechanisms. When sleep resumes, the cycle repeats
[98].
OSA represents an independent risk factor for CVDs, such as HTN, AF and other arrhythmias, HF, CAD, stroke, pulmonary hypertension, metabolic syndrome, and diabetes as shown in Figure 4. The involved mechanisms are multiple and probably interconnected.
Figure 4. Pathophysiological pathways OSA related leading to CVD.
During the apneic phase, by stimulating peripheral and central chemoreceptors
[99], hypoxia and hypercapnia determine the activation of the sympathetic nervous system with consequent peripheral vasoconstriction and an increase in vascular resistance and heart rate
[100]. This results in an increase in left ventricular afterload and cardiac work. In addition, there is an overall increase in left ventricular transmural pressure (that is the difference between ventricular systolic pressure and intrathoracic pressure) with increased wall stress
[101][102]. The cycle repeats many times every night; therefore, the cardiovascular system is chronically exposed to neuro-hormonal stress, and the hyperactivity of the autonomic nervous system also extends to the daytime hours over time
[100][103].
Intermittent hypoxia is also responsible for an increase in oxidative stress
[104]: during the hypoxic phase, the cells adapt to an environment with low oxygen content, and with the reoxygenation phase, there is a sudden increase of oxygen with ROS formation, leading to cellular damage in the ischemic tissue
[105][106].
Furthermore, a reduction in the levels of circulating NO has also been highlighted during OSA
[107], and this could be implicated in endothelial dysfunction
[108].
OSA is present in up to 30–50% of HTN patients, and 80% of patients with resistant HTN have OSA
[94][109], representing an independent risk factor
[92]. In patients with OSA, due to the overactivity of the sympathetic nervous system, the physiological reduction in blood pressure during the night (which configures the “dipper” profile) does not occur
[110][111]. Therefore, there seems to be a correlation between sleep apnea and the non-dipper profile of essential HTN
[112][113]. Furthermore, several randomized trials and meta-analysis have shown a reduction in blood pressure in patients with sleep apnea treated with CPAP
[92][114].
OSA is associated with heart rhythm disturbances and sudden death; pauses and bradycardia are common in patients with OSA
[94].
OSA is also an independent risk factor for AF with several pathophysiological mechanisms implicated. In particular, sudden changes in intrathoracic pressure can cause atrial remodeling and atrial fibrosis with consequent electrophysiological alterations
[115]. Moreover, the sudden increase in sympathetic activity during apneas can lead to the activation of catecholamine-sensitive atrial on channels, thus determining focal discharges from which AF can be originated
[116]. OSA is also associated with an increase in systemic inflammation, which may contribute to the genesis of AF
[117].
Sleep apnea also increases the risk of CAD by favoring atherosclerotic process via oxidative stress, endothelial dysfunction, inflammatory state, and autonomic dysfunction. It has been reported that in OSA patients, myocardial infarction occurs more frequently during the night hours
[118], and a higher pro-inflammatory profile is present
[119] with an effective reduction of the latter if CPAP therapy is used
[119]. This study, therefore, suggests that OSA could activate vascular inflammation with non-traditional pathogenetic mechanisms.
OSA is also a risk factor for incident strokes, stroke recurrence
[120], and functional and cognitive outcomes
[121].
Pulmonary hypertension is closely related to OSA. Hypoxia and hypercapnia induce arteriolar vasoconstriction in the short term and vascular remodeling in the long term that could lead to an irreversible increase in pulmonary vascular resistance and the development of pulmonary hypertension
[122].
Sleep apnea, mainly the central form (CSA), is highly prevalent in HF patients as well, ranging from 40% to 60% of symptomatic patients
[123].
OSA is also linked to obesity and metabolic syndrome since chronic intermittent hypoxemia and sleep loss is associated to higher plasma leptin levels
[124], glucose metabolism impairment, and insulin resistance
[125].
At least, there is a reciprocal interaction between obesity and OSA where they both reinforce their progression and their severity in a vicious circle. It is believed that the deposition of fat in the upper airways and the functional alteration of the airways themselves are the mechanisms involved in the pathogenesis of OSA in the obese subjects
[126]. On the other hand, daytime sleepiness and decreased physical activity together with hyperleptinemia are the mechanisms probably implicated in weight gain in OSA.
3.2. Air Pollution: Health Breath as Part of Prevention
Air pollution is the contamination of the environment, indoor or outdoor, by a mixture of chemical, physical, or biological agents that change the characteristics of the atmosphere and even at low concentrations cause damage to human health, other living organisms and the environment
[127]. According to the Global Burden of Disease (GBD) report, air pollution was responsible for 6.7 million deaths in 2019 alone
[127][128]. Globally, nearly 20% of CVD deaths are attributable to air pollution
[128]. The main components of this mixture of pollutants are Total Suspended Particulate Matter (PM), gaseous compounds including ozone (O
3), nitrogen dioxide (NO
2), carbon monoxide (CO), sulfur dioxide (SO
2), and volatile organic compounds including benzene
[127]. According to the World Health Organization, 99% of the world’s population breathes air that contains annual average levels of air pollutants that exceed guideline recommendations. Particularly high exposures have been documented in cities in Asia, western sub-Saharan Africa, and Latin America
[127]. The most consistent evidence on health damage is attributed to PM, i.e., the set of airborne particles, ranging in diameter from 0.1 to 100 mm, capable of remaining in suspension in the air even for long periods
[129][130]. Short- and long-term exposure to PM is associated with increased morbidity and mortality, impacting the progression of atherosclerosis
[131], ischemic heart disease
[132][133][134], stroke
[135], and lung disease as well as the course of pregnancy and the health of newborns
[136]. PM
10 (particles between 2.5 and 10 mm in diameter) and, largely, PM
2.5 (diameter < 2.5 mm), are the most linked to CVD and affecting global public health
[137][138]. Lung inflammation and oxidative stress pathway is the primary response to air pollution exposure
[139], contributing to the development of a systemic pro-inflammatory state and activation of secondary effector pathways that result in endothelial dysfunction, increased atherosclerotic plaque vulnerability, and the activation of a prothrombotic and proarrhythmic state
[132][140][141]. Experimental animal models seem indeed to support this hypothesis
[142]. Moreover, human exposure to pollutant nanoparticles causes their translocation into the systemic circulation through the alveolus-capillary membrane, interacting with the endothelium, accumulating at sites of vascular inflammation, thus favoring atherosclerotic process
[143][144][145], with effects similar to those observed in the lungs
[146] and thrombotic complications
[147]. A relevant change in platelet function toward increased prothrombotic tendency has been confirmed in diabetic patients after recent (within two hours) exposure to PM
[148]. In addition to these mechanisms, short-term PM
2.5 exposure in animal models is associated with sympathetic nervous system activation and hypertension, probably mediated by neuroinflammation
[149][150]. In a meta-analysis of 33 studies, short-term exposure to PM
2.5 was associated with a significant decrease in heart rate variability (HRV)
[151]. Decreased HRV is an index of autonomic system dysfunction and predicts an increased risk of cardiovascular morbidity and mortality in patients with heart disease
[152]. Increased blood pressure and decreased HRV suggest an autonomic imbalance in favor of sympathetic tone and could further explain the rapid cardiovascular responses associated with air pollution, such as the initiation of fatal tachyarrhythmias and increased myocardial infarctions
[132], as confirmed by the available literature
[153]. High short-term exposure to PM
2.5 is associated with an increased risk of acute coronary event, acutely destabilizing and rupturing atherosclerotic plaque, in patients with clinically significant pre-existing CAD but not in those with uninjured coronary arteries
[154]. Moreover, short-term exposure to elevated levels of PM
2.5 and PM
10 is also associated with increased daily hospitalizations for STEMI and increased incidence of STEMI-related ventricular arrhythmias and cardiac death
[155]. The effect of long-term exposure to major air pollutants was assessed by the ESCAPE study that have evaluated the incidence of acute coronary events in 11 European cohorts. At a mean follow-up of 11.5 years, exposure to annual mean levels of PM
2.5 > 5 μg/m
3 and PM
10> 10 μg/m
3 was associated with a 13% and 12% increase in the risk of nonfatal acute coronary events, respectively, with no evidence of heterogeneity between cohorts
[156]. Other observational studies and meta-analyses have reported a positive correlation between long-term exposure to air pollution and the development and progression of subclinical atherosclerosis and calcium accumulation
[157] as well as increased carotid intima-media thickness
[158]. Based on the published data, no more doubts should exist on the role of air pollutants in CVD development. A schematic view of the relationship between air pollution and CVD is provided in
Figure 5.
Figure 5. Molecular mechanisms linked air pollution to CVD.
3.3. Climate Change: The Impact of Temperature
Temperature and its extreme variation is now recognized as a cardiovascular risk factor
[159][160][161]. A very recent analysis evaluating 32,000 cardiovascular deaths in 27 countries on 5 continents over 40 years support the role of extremely hot or cold temperatures in determining heart disease deaths
[161]. Mortality and morbidity induced by climate change are not exclusively due to hypothermia or hyperthermia, but also to indirect causes, such as respiratory diseases and CVDs, which can be undetected when the human body tries to adapt to climate changes
[162]. A relationship between mortality from CVD and temperature exists with a U-, V-, or J shaped curve
[163][164][165]. While the correlation between temperature and CVD has been established, the role of diurnal temperature range (DTR), defined as the difference between the maximum and minimum temperatures recorded in one day, in determining CV events needs to be better evaluated. Extreme cold weather conditions associated to climate change contributes to an increase in temperature variability that might increase clinical cardiovascular events
[160]. It is known that exposure to cold activates both the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS), which interact with each other, leading to HTN and myocardial damage
[166]. Skin blood flow (SBF) is reduced in response to cold due to vasoconstriction and increased urine output, thus inducing dehydration, hemoconcentration, and hyperviscosity
[167]. Furthermore, eNOS and adiponectin inhibition contributes to endothelial dysfunction and lipid deposition, thus favoring atherosclerosis and plaque instability. Cold exposure also triggers mitochondrial dysfunction with myocardial damage, cardiac hypertrophy, and cardiac dysfunction. The increase in cardiac work and peripheral resistance contributes to an increase in oxygen consumption and a reduction in the ischemic threshold
[166], which is clinically relevant, especially when the coronary circulation is already compromised.
On the contrary, exposure to heat leads to increased blood flow and sweating with loss of fluids and dehydration. The resulting hemoconcentration and hyperviscosity may cause thromboembolism, leading to increased risk of ischemic stroke
[168]. In the presence of heat stroke, the increase in core temperature redistributes the flow on the skin to facilitate heat loss. Intestinal blood flow is reduced, and this could cause increased permeability of the intestinal epithelium, allowing bacteria, their toxic cell wall component LPS, or HMBG1 to move from the intestinal lumen into the circulation. TLR4 recognizes these molecules, stimulating innate and adaptive immune responses and causing systemic inflammatory response syndrome (SIRS). Along with this, hyperthermia induces the occlusion of arterioles and capillaries (microcirculatory thrombosis) or excessive bleeding (consumptive coagulation), leading to multiorgan dysfunction. The putative mechanisms linking climate changes and CVD is provided in
Figure 6.
Figure 6. Correlation between climate changes and CVD: possible basic mechanisms. Several variables affect the response to temperature changes.