Mercury and cadmium-induced inflammatory cytokines activation and its effect on the risk of preeclampsia: a review

Introduction

Exposure to heavy metals in the form of Hg and Cd has become a special concern studied by WHO. Exposure to Hg and Cd itself is obtained mainly from contaminated food and water [1]. Heavy metal pollution of Hg and Cd can come from the disposal of industrial waste, gold mining especially artisanal small-scale gold mining (ASGM), excessive application of chemical fertilizers and chemical pesticides, and the disposal of household waste into rivers [2]. Marine biota contaminated with heavy metals can migrate into the waters and pose a serious threat to people with high consumption of seafood, such as coastal residents, especially pregnant women [3].

Hg and Cd can undergo a biological accumulation process (bioaccumulation), a biological transfer process (bio transfer), and biological enlargement (biomagnification) that occurs naturally [1]. Marine organisms that live in polluted water can be a source of accumulation of heavy metals in high concentrations and furthermore, it can cause poisoning in humans who consume it, especially in long-term exposure [1], 3].

Exposure to heavy metals Hg and Cd is dangerous because these metal elements penetrate the blood-brain barrier, which can cause neurotoxicity, and the placental barrier, a high risk of triggering blood vessel disorders, and in the long term can accumulate in the body, especially the liver and kidneys, causing renal damage [4], 5]. The main mechanism of exposure to heavy metals is it can trigger the formation of reactive oxygen species (ROS) and an immune response [6].

The impact of exposure to Hg and Cd, especially in pregnant women, can increase the risk of preeclampsia [7]. Based on research by El-Badry et al., it was found that pregnant women who were exposed to Hg had a higher frequency of spontaneous abortion and preeclampsia compared to the group of pregnant women who were not exposed to Hg and had a higher risk of developing preeclampsia [7]. Several previous studies also obtained significant results between increased levels of Cd and the incidence of preeclampsia in pregnant women [4], 8].

Preeclampsia is a pregnancy-specific condition characterized by placental dysfunction and maternal response to systemic inflammation with endothelial activation and coagulation [9]. The diagnosis of preeclampsia is based on the presence of systolic blood pressure greater than or equal to 140 mmHg or diastolic blood pressure greater than or equal to 90 mmHg in a pregnant woman at 20 or more weeks of gestation, with normal blood pressures before pregnancy and have one or more of the following manifestations appeared recently such as proteinuria, organ dysfunction, hepatic compromise, neurological complications, hematological disorders, and uterus-placental dysfunction [10]. Preeclampsia can affect the mother during pregnancy causing pulmonary edema, HELLP syndrome, eclampsia, hepatic rupture, and acute kidney injury, and causes postpartum problems among mothers, such as the risk of hypertension, ischemic heart disease, stroke, and venous thromboembolism, and affect the fetus such as fetal growth restriction and fetal distress as well [9], [10], [11]. Decreased placental perfusion in extreme cases can lead to oligohydramnios to intrauterine fetal death [9].

Preeclampsia becomes a concern in a pregnancy condition because it affects 5–7 % of all pregnant women, but is responsible for over 70,000 maternal deaths and 500,000 fetal deaths worldwide every year, 9–26 % of maternal deaths in low-income countries, and 16 % in high-income countries [11], 12]. The incidence of preeclampsia can be influenced by parity, race, genetic and environmental factors [4]. The pathophysiology of preeclampsia is not known for certain, but one of the main causes is the mechanism of oxidative stress which then affects inflammatory factors, impaired immunological balance, and endothelial dysfunction [13].

Placental abnormalities in preeclampsia develop in 2 stages: (1) abnormal placentation in the early first trimester, followed by (2) maternal syndrome in the second and late third trimesters which is characterized by excessive presence of antiangiogenic factors that lead to clinical manifestations that can trigger preeclampsia, hypertension, multi-organ failure, heart and kidney disease, and obesity [9]. The early stage of placental dysfunction, it is associated with several theories, namely the theory due to oxidative stress, abnormal NK cells, genetics, and environmental factors [14]. Excessive oxidative stress triggers the release of placental factors into the maternal circulation which increases secretion of some cytokines (IL-6, IL-17, IFN-γ, TNF-α, etc.), then causes a systemic inflammatory response and endothelial dysfunction which are prominent components of preeclampsia [14], 15].

Exposure to Hg and Cd can increase the secretion of pro-inflammatory cytokines and suppress anti-inflammatory cytokines production [16], 17]. This inflammatory condition is also related to its role in the pathophysiological mechanism of preeclampsia [18]. This review suggests that heavy metals exposure, especially mercury and cadmium can induce inflammatory cytokines activation related to the risk of preeclampsia.

Forms of mercury and cadmium

Hg is a metallic element commonly known as mercury. It is a chemical element with atomic number 80, an atomic weight of 200.59 g/mol, a freezing point of −39 °C, and a boiling point of 356.5 °C [7]. Currently, mercury is known as a heavy metal that has toxic properties and pollutes the air, water, and soil environment through natural processes and human activities [3], 19].

Hg has three main forms, namely elemental (Hg₀), inorganic (Hg²⁺), and organic mercury (MeHg) [3]. Elemental Hg in the gaseous state is found in the atmosphere [3]. The elemental form has no charge and is relatively insoluble in water [3]. Inorganic mercury is obtained from elemental mercury that undergoes an oxidation process after entering the atmosphere and is commonly found in the form of salts, for example, HgCl₂ [19]. Meanwhile, methylmercury (MeHg) is the most common organic form of mercury [20]. Organic Hg is the dominant form of mercury in biota. Its compounds can be alkyl or phenyl. Ethyl and methyl are part of alkyl mercury which are commonly found in the environment [20].

Hg naturally occurs in the earth’s crust with a concentration of 0.08 ppm [1]. Usually found in marine fish or shellfish naturally ± 0.1 mg/kg [1]. The Hg can spread through natural and anthropogenic processes, by human activities, to the air, water, and soil environment [1]. Hg is present in the atmosphere through natural processes such as volcanic eruptions, the release of natural gas, evaporation of soil sediments, forest fires, and erosion [17]. Meanwhile, human activities such as mining, industry, and the intentional burning of forests produce mercury vapor and enter the atmosphere [20].

Cd is a chemical metal element that has an atomic number of 48, an atomic weight of 112.4 g/mol, a melting point of 321 °C, and a boiling point of 765 °C. Cadmium has the characteristics of being a flexible metal, silver-white in color, soft, resistant to pressure, soluble in acids, shiny, easy to react, and when heated, it can form cadmium oxide. Cd in nature has several forms, mostly Cd sulfide (CdS) and Cd chloride (CdCl₂) [21].

Mercury and cadmium exposures

Sea water is a component related to the terrestrial environment, where waste disposal from the land will end up in the sea. In addition, seawater is also a place for receiving pollutants (pollutants) that fall from the atmosphere [22]. Heavy metals can enter the body of marine biota through three mechanisms, which are through the food chain, gills, and diffusion through the skin surface. Most of it is through the food chain, and only a few are taken directly from the water [4].

Heavy metals Hg and Cd which are in sediment and water will be absorbed by marine biota and then concentrated up to 100–1,000 times more than the amount in the environment [1]. This accumulation process is called bioaccumulation. In the body of marine biota, the amount of accumulated metal will continue to increase with the biomagnification process, where the level of biota in the food chain system also determines the amount of metal that accumulates [1]. Higher strata of biota will find higher levels of mercury and cadmium in their bodies. However, the highest metal content is generally found in invertebrates, such as clams and oysters [3].

If the concentration of heavy metals in the water is high, then there is a possibility that the concentration of heavy metals in the sediment is also high, so the concentration of heavy metals in animal tissues on the seabed is also high [22]. Sources of exposure to Hg can come from industrial activities with Hg waste, mining amalgam activities, marine biota contaminated by Hg contamination in waters, dental amalgam, insecticides, cosmetics, nail polish, and hair dyes [23].

Hg can enter the human body through absorption of air containing mercury vapor, or when consuming food, especially marine biota contaminated with mercury [20]. Ding et al. show that Hg exposure may trigger apoptosis through the mitogen-activated protein kinase MAPK/p38 pathway, which activates the c-Jun N-terminal kinase (JNK) in response to stress or injury [24]. Organic Hg in the form of MeHg can have a neurotoxicity effect and other function disturbances, and will cause various symptoms such as irritability, trembling, memory difficulties, ataxia, concentric constriction of visual fields, sensory and auditory disturbances, and others [25], 26].

MeHg will be absorbed more completely through the gastrointestinal (GI) tract because it is easily soluble in fat and binds to sulfhydryl groups, or forms bonds with l-cysteine, which is called MeHg–cysteine complex, mimicking the structure of essential amino acid methionine, and crosses cell membranes including placental barrier and blood-brain barrier, through large and neutral amino acid carriers [20], 27]. The alkyl form is highly soluble in fat and distributed in the body, accumulating especially in the brain, and other organs such as kidneys, liver, hair, and skin [23], 28].

Hg has a lower affinity for thiol groups and a higher affinity for selenium-containing groups [29]. Selenium is suggested may provide a protective role in Hg toxicity such as facilitating the demethylation of organic mercury to inorganic mercury, binding to inorganic mercury and forming an insoluble, stable, and inert Hg: Se complex, reducing Hg absorption from the GI tract, restoration of target selenoprotein activity and restoring the intracellular redox environment, etc. [29]. Hg can stay around 44–80 days inside the human body before it is excreted, but based on its physical condition, after crossing the BBB, the MeHg stays trapped within the CNS, then causes the neurotoxicity effect [30]. The excretion of MeHg is related to kidney and gastrointestinal function through urine, feces, hair, and breast milk [1].

Cd and its compounds also can pollute the environment, mainly as a result of industrial activities involving cadmium in its industrial operational processes, burning household waste, burning fossil fuels, the use of artificial phosphate fertilizers, cigarettes, nail polish, disposal of industrial waste, mining waste and deposits from the atmosphere [4], 13], 21].

Cd can enter the human body in several ways, for example from cigarette smoke and coal burning smoke, containers or places to eat and drink that are coated with cadmium, contamination of waters, aquatic products contaminated with cadmium, and food chains [4].

The pathophysiology of Cd toxicity can go through various pathways. Once in the bloodstream, Cd binds to alpha-2-macroglobulin and albumin and gets distributed to the target organs [31]. Inside the cell, Cd can cause mitochondrial damage via enzyme degradation and protein destruction, then makes the cell more susceptible to oxidative stress, affecting cell adhesion and calcium transport that can lead to cell dysfunction and cell death [31]. It also can modulate Ca²⁺ levels in the cellular and the activities of caspases and nitrogen-activated protein kinases (MRPKs), in which these processes cause apoptosis indirectly [5]. Cd also can bind the S (sulfur) and COOH groups of protein molecules (amino acids and amides) and stimulate the production of ROS such as superoxide ions, hydrogen peroxide, and hydroxyl radicals [5]. However, higher zinc levels in the body are related to a protective effect of Cd toxicity [5]. This is thought to be because zinc stimulates the metallothionein production [4]. Metallothionein is a low-molecular-weight protein, 6,500 Da, consist of zinc-concentrating protein that contains 33 % cysteine, which also can act as a free-radical scavenger and can bind Cd, which plays a major role in the kinetics and metabolism of this metal [5], 31], 32].

Cd can cross BBB and placental barriers by the zinc transporter [33]. Cd exposure can increase the permeability of the BBB by disrupting the integrity of the hCMEC/D3 monolayer and suppressing the cAMP/PKA/Sp1 pathway, altering the thiol redox status which affects the antioxidant defenses and then leading to BBB dan placental barrier dysfunction, resulting in metal entering the brain and placenta [33], [34], [35].

Cd has a strong affinity for the kidney and liver [4]. In general, about 50–75 % of Cd in the body is found in these two organs [4]. Other organs, including the spleen, heart, lungs, bones, and thymus, also store Cd [4]. Cd exposure can cause various problems in the human body such as osteoporotic bone caused by skeletal demineralization, calcium loss, and proteinuria because of tubular lesions causing renal damage, blood vessel disorder in the form of endothelial dysfunction causing cardiovascular disease, and others [5]. In humans, most of the Cd is excreted in the urine, while in animals most of the Cd is excreted in the feces. The half-life of Cd in the body ranges from 10–30 years [4]. Hg and Cd cannot pass BBB and placental barrier in an inorganic state, and, some studies show that it is concentration dependent [30], [35], [36], [37].

Mercury-induced inflammation

In humans, Hg and Cd are heavy metals that are harmful to health. It can be distributed in the body, and then disrupt the balance of various mechanisms in the body, one of which is the immune mechanism through inflammatory mechanisms [38]. Prolonged inflammation can play a role in the emergence of chronic diseases, including cardiovascular disease, cancer, diabetes, neurodegenerative, and autoimmune diseases [38].

The mechanism of inflammation involves the innate and adaptive immune systems, which require synergistic coordination with blood vessels, immune cells, and molecular mediators in the development and balance of the inflammatory process [38]. Cytokines are a group of secreted proteins that are secreted by immune cells, with diverse structures and functions, and have a function to regulate and coordinate the various activities in the immune system [39]. In the inflammatory reaction, cytokines that play a role in stimulating inflammation are known as pro-inflammatory cytokines such as IFN-γ and IL-6 [39]. While cytokines that act as inflammatory inhibiting factors are called anti-inflammatory cytokines such as IL-10 [39]. Meanwhile, in the process of pregnancy, IL-6 has an important role. One of them is the attachment of the blastocyst and its invasion into the maternal endometrium [40]. This process is maintained by several inflammatory cytokines, namely IL-6 and IL-1. IL-6 is also known to play a role in the stages of the labor process [40]. IFN-γ is also one of the factors that play an important function in the continuity of a pregnancy [41]. It is naturally produced by healthy placentas and is thought to play a strong role in the development and survival of cytotrophoblasts by controlling the rate of apoptosis of these cells [41]. It was found that high levels of IFN-γ are associated with harmful conditions to the fetus and indicate disturbances in placental development that can lead to complications in pregnancy [41]. IFN-γ is thought to be one of the potential agents that play a role in fetal susceptibility and pregnancy complications through mechanisms related to the immune system in angiogenesis in the placentation process [41].

Exposures of Hg can cause the activation of the innate immune system via cell death, engagement of pattern recognition receptors, migration of antigen-presenting cells to secondary lymphoid organs, and subsequent activation of the adaptive immune system including inflammatory cytokines such as IFN-γ, IL-1β, 1L-4, IL-6, IL-10, IL-12, IL-17, IL-18, and TNF-α, also decrease anti-inflammatory cytokines such as IL-1Ra and IL-10 [19], [42], [43], [44]. Through the mechanism of oxidative stress, Hg can trigger inflammatory reactions [16]. MeHg can cause an increase in the production of ROS, then affect the decrease in glutathione (GSH) levels which act as antioxidants [16].

Based on animal studies, mercury exposure can disrupt the Th1/Th2 balance, potentially a cause of autoimmunity, and dysregulation of the immune response to infection [17]. Mercury exposure induces cell death, modifies self-antigen, then when its particle is presented to CD4+ T Helper cells by monocytes, macrophages, and dendritic cells, resulting in the production of autoantibodies [17]. Mercury can activate the toll-like receptor (TLR)-4, then trigger the activation of MAPKs and NF-kB in the mechanism of regulation of pro-inflammatory responses [45].

Cadmium-induced inflammation

Cd has been listed as one of the most toxic substances that affect many tissues or organs, including the immune system [4]. Cd can interact directly with immune system cells through changes in the status and its function [4]. The direct impact can be in the form of cell death and disruption of signaling pathways resulting in changes in cytokine production, expression of cell surface markers, cell activation, and differentiation which can eventually lead to immunosuppression or immunostimulant states [46].

The impact of Cd on innate immunity was seen primarily in phagocytic cells, using both rodent and human cells. Studies in previous studies have shown that Cd causes important changes in the activation, migration, and activity of immune cells in peripheral tissues. Administration of a single moderate dose of Cd (1 mg/kg i.p.) in rats increased the number of neutrophil leukocytes in the blood and triggered oxidative activity (production of ROS and NO), effector enzymes (myeloperoxidase), and expression of CD11b/CD18, an adhesion molecule that is an important component in the process of PMN attachment to the vascular endothelium [47]. The increase in circulating PMN in rats exposed to Cd is part of systemic inflammation, which is characterized by increased plasma levels of TNF-α and IL-6 [47].

Cd also might affect adaptive immunity by interfering with lymphocyte development, subset distribution, activity, and representation in lymphoid tissue and blood [4]. Based on Goyal et al., it was found that there was an increase in the levels of IL-4, IL-6, and TNF-α and a decrease in the levels of IL-2 and IL-10 which indicated the dominance of the secretion of pro-inflammatory cytokines due to exposure to Cd [21] (Table 1). Cd may directly affect the activity of B and T cells through the activation and proliferation of antigen-specific cells, and indirectly through its effects on innate immune cells such as macrophages and dendritic cells which are required as APCs to respond to B and T cells. Cd influences the development of a subset of Th cells and regulatory T cells [4].

Changes in the immune response induced by heavy metals can harm humans due to disruption of the balance in the immune response mechanism [38]. Initially, in response to exposure to heavy metals that enter the body, the immune function will be enhanced as a protective mechanism in response to danger signals originating from endogenous and exogenous molecules after acute exposure to occupational and environmental pollutants [38]. However, long-term exposure to heavy metals, either low doses or high doses, can cause immunosuppression and directly impair immune system sensitization to antigens. Continuous exposure to heavy metals can impair immune function, which in turn results in disease [38].

Inflammation effect of mercury and cadmium-induced on the risk of preeclampsia

In normal pregnancy, the vascular invasion progresses deep into the spiral arteries to reach the myometrium, which causes extensive remodeling of the maternal spiral arteries, which then undergo vasodilatation and modification to become high-capacity, high-flow vessels to meet the needs of the fetus [40].

In preeclampsia, the cytotrophoblast fails to transform from a proliferative epithelial subtype to an invasive endothelial subtype leading to incomplete remodeling of the spiral arteries [18]. Inadequate remodeling of the spiral arteries leads to constriction of maternal blood vessels and relative placental ischemia [18]. These findings indicate that abnormalities in the trophoblast can result in shallow attachment of the placenta and incomplete transformation of the spiral arteries, resulting in placental ischemia and the clinical syndrome of preeclampsia, namely maternal systemic hypertension and organ failure [18]. Cd can interfere with the work of enzymes in the antioxidant system of cells and induce oxidative stress which then triggers preeclampsia. It is also known that the accumulation of Cd in the kidney can lead to proteinuria and renal dysfunction [4], 8].

In the pathogenesis of preeclampsia, inadequate invasion of trophoblasts in the spiral arteries leads to inadequate perfusion of the placenta, which is the central pathology of preeclampsia [48]. This inadequate perfusion causes ischemia or hypoxia in the placenta which in turn stimulates the release of several factors into the maternal circulation [48]. These factors include antiangiogenic factors such as sFlt, sEng, inflammatory mediators such as TNF-α, IL-6, etc., and immune cells such as neutrophils, monocytes, NK cells, T cells, and angiotensin-1 autoantibodies [49]. These molecules in the maternal circulation cause an exaggerated inflammatory response and increase levels of oxidative stress, by the presence of ROS such as superoxide, and RNS such as peroxynitrite, in the blood vessels [48].

It is reported that some cytokines increase in preeclampsia (Table 2). The secretion of cytokines such as IL-1, TNF-α, IFN-γ, and also other substances like granulocyte-macrophage colony factor (GM-CSF), and stimulant colony factor-1 (CSF-1) can affect blastocyte attachment and trophoblast implantation, proliferation, and invasion [18], 50]. Substances such as prostaglandin E2, TGF-β, GM-CSF, and IL-10 also play a role in the immunoendocrine circuit to maintain the pregnancy [50]. IL-10 potentially has 2 mechanisms that can inhibit the immune system, directly acting as a cytokine synthesis inhibitor factor and indirectly as a trigger for trophoblast invasion into the spiral artery [15]. Trophoblast invasion will trigger leukocyte activity resulting in an inflammatory reaction and then lead to an increase in the pro-inflammatory cytokine IL-6 which affects endothelial cells such as increased permeability, stimulation of growth factor synthesis from platelets, and cessation of prostacyclin synthesis [50]. Oxygen free radicals have been known to trigger the formation of endothelial IL-6 cytokines [50]. IFN-γ levels are also increased in preeclampsia. It is associated with ICAM-1 expression, endothelial cell, and IL-6 synthesis, which may be an additional explanation for the increased levels of IL-6 in preeclampsia [41]. Based on previous studies, it was shown that levels of IFN-γ in mothers with preeclampsia were higher than levels in mothers with normal pregnancies. Increased IFN-γ can affect the progression of preeclampsia by interfering with trophoblast invasion, spiral artery remodeling, apoptosis of trophoblast cells, and placental angiogenesis [41].

Based on Table 3, it was found that women who were exposed to those heavy metals had a higher risk of preeclampsia compared to the group of women who were not exposed to Hg and Cd [7], 13], 51]. It is suggested that Hg induces mitochondrial dysfunction which occurs at the ubiquinone-cytochrome B region and with NADH dehydrogenase causing displacement of Fe²⁺ and Cu⁺ ions in the a3Cub center of cytochrome C, then this will increase oxidant stress and reduce oxidant defenses [52]. Those problems can affect immune function, one of them is the increase of TNF-α and IL-6, which causes endothelial dysfunction by reducing endothelial cell formation and migration, decreasing vascular endothelial repair, and decreasing nitric oxide which can lead to preeclampsia state [52]. Cd also may inhibit endothelial nitric oxide synthase and suppress acetylcholine-induced vascular relaxation, then may stimulate the production of cytokines and induce endothelial damage, resulting in hypertension [5].

The cytokines induced by Hg and Cd exposures and preeclampsia are in line with the study of exposure to Hg and Cd related to the risk of preeclampsia, so it is possible that Hg and Cd may trigger preeclampsia through the immune mechanism (Figure 1).

Figure 1:

Hg and Cd exposure affect the immune system by inducing NKT and Th0, increasing the response of Th1 that induces cytokines secretion such as IL-6, IFN-γ, and TNF-α, and reducing the response of Th2 cells, lowering the IL-10 and IL-4 secretion, which contribute to inflammation state. Uncontrolled inflammation can disrupt the trophoblast invasion process, causing shallow placentation. Abnormal placentation can cause the alteration of placental function and vasoconstriction of the spiral artery, which leads to the vasoconstriction of systemic arteries, resulting in preeclampsia. Hg, mercury; Cd, cadmium; NKT, natural killer T-cell; Th0, T-helper 0; Th1, T-helper 1; Th2, T-helper 2; IL-6, interleukine-6; IL-4, interleukine-4; IL-10, interleukine-10; IFN-γ, interferon-γ; TNF-α, tumor necrosis factor-α.

Because of the dangerous effect of Hg and Cd exposures, the U.S. Environmental Protection Agency (USEPA) and the Agency for Toxic Substances and Disease Registry (ATSDR) established a reference dose (RfD) for MeHg is 0.0001 mg/kg-day (0.0001 mg of MeHg per day for each kilogram of a person’s body mass) and for Cd is 0.001 mg/kg-day. EPA believes that exposures at or below the RfD are unlikely to be associated with an appreciable risk of adverse effects, especially for pregnant women [53], 54]. Based on the FDA (Food & Drug Administration) and EPA, those who are pregnant or breastfeeding can consume between 8 and 12 ounces per week, and for children 2–8 ounces per week based on a 2,000 calories diet of a variety of seafood from fish choices that are lower in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish [55], 56]. For Cd, based on The Panel on Contaminants in the Food Chain of the European Food Safety Authority (CONTAM Panel), the tolerable weekly intake (TWI) is 2.5 μg/kg body weight (b.w.) [57].

Conclusions

Exposure to Hg and Cd in the body can induce the secretion of inflammatory cytokines, resulting in an imbalance of the immune system, which affects the placental implantation process and endothelial dysfunction in pregnant women and becomes one of the causing factors of preeclampsia.

Therefore, this review also suggests that the countries that have a high incidence of preeclampsia should be aware of the environmental factors, especially heavy metal pollution such as Hg and Cd. The opportunity for collaborative research between obstetricians, toxicologists, and fisheries experts to investigate preventive action is still wide open.