Over past few years, there has been a dramatic increase in studying
physiological mechanisms of the activity of various signaling low-molecular
molecules that directly or indirectly initiate adaptive changes in the
cardiovascular system cells (CVSC) to hypoxia. These molecules include
biologically active endogenous gases or gasotransmitters (H
Historically mitochondria have been considered as molecular “power stations” that produce and store energy in the form of the high-energy bonds of ATP. This energy is used by cells to sustain their functions, including signaling and adaptation to the effects of negative environmental factors. In the cardiovascular system (CVS), most ATP molecules are generated by oxidative phosphorylation that occurs in mitochondria and supports energy-consuming and long-running biochemical processes underlying myocardial automatism and muscle contraction. About two thirds of the energy consumption by smooth muscle and endothelial cells of blood vessels are covered by anaerobic glycolysis, which makes these cells less vulnerable to oxygen deficiency . Nevertheless, mitochondria both of cardiomyocytes (CM) and blood vessel cells (BVC) remain highly sensitive to the impact of the negative environmental factors and undergo metabolic adaptation in response to changes in environmental conditions. When this adaptation is impaired, a progressive decline in the mitochondrial function contributes to the development of CVS diseases . On the other hand, mitochondria house proteins of signal transduction pathways that regulate activity and adaptation of the CVS cells during hypoxia and development of hypoxia-related chronic diseases.
Over past few years, there has been a dramatic increase in studying
physiological mechanisms of the activity of various signaling molecules or
messengers that directly or indirectly initiate adaptive changes in the CVS cells
to hypoxia and ischemia/reperfusion. These messengers include biologically active
endogenous gas molecules (NO, H
The gas messengers are able to modulate mitochondrial function by targeting
mainly the ATP-dependent mitochondrial transport, electron transport chain (ETC),
and functions of ATP synthase, mitoK
Influence of gasotransmitters (H
An increased sensitivity to the opening of MPTP has been known to result in mitochondrial dysfunction and apoptosis in CVS diseases. Hence, there is an urgent need for clarification of the gasotransmitters role in the mitochondrial mechanisms related to the regulation and formation of MPTP.
Understanding the role of gas transmitters in regulation of the mitochondrial functions and cell signaling that initiate protective mechanisms of the CVS cells may contribute to the development of new antihypoxic drugs aimed at preventing and treating a broad range of pathologies, including ischemic cardiomyopathy and cardiac ischemia-reperfusion injury.
The main function of mitochondria is to generate energy in the form of ATP. The ATP is produced by substrate phosphorylation (glycolysis) and mitochondrial aerobic respiration (oxidative phosphorylation). The oxidative phosphorylation occurs in the inner mitochondrial membrane (IMM) and uses the electrochemical proton gradient to generate ATP. The CVS cells rely on both glycolysis and oxidative phosphorylation to sustain their function. Herewith, the uniqueness of the mitochondria as energy-producing organelles precisely defines the second pathway of the ATP generation. In fact, this is a sequential transformation of the chemical energy of the reducing NADH equivalents into the electrochemical proton gradient across the IMM that activates the membrane-bound ATP synthase and results in the formation of the high-energy bonds of ATP .
In light of contemporary insights about energy producing mechanisms in cellular systems, the whole energy production process in mitochondria can be divided into four main stages. The first two, conversion of substrates to acetyl-CoA and its’ oxidation to NADH in the Krebs cycle, occur in the mitochondrial matrix. The last two, electron transfer from NADH to oxygen through the respiratory chain and formation of ATP by ATP synthase complex, occur in the internal membranes of mitochondrial cristae . The electron transfers and ATP synthase activities are membrane potential-dependent processes. Therefore, maintaining a stable mitochondrial membrane potential (MMP) is one of the vital conditions to support healthy mitochondrial function and oxidative phosphorylation. A decrease or, opposite, excessive increase in the MMP that happen during the development of CVS pathology serve as a pharmacological target for treatment of a various CVS diseases associated with the mitochondrial dysfunction and circulatory hypoxia.
Because of the high importance of generation and maintenance of the MMP, the CVS
cells (CM and VSMCs) developed special mechanisms intended to support a proper
ETC functioning in hypoxic conditions. One of these mechanisms include a
rearrangement of the substrate region of the respiratory chain by prioritizing
FAD-dependent over NAD-dependent substrates and transferring electrons to the ETC
complexes II–IV bypassing complex I. Activation of the alternative metabolic
pathways allows maintaining the electron flow to the cytochrome c region
without disrupting the electron transport function of complexes III, IV, and V.
This process is called succinate-oxidase pathway, which is more effective
energetically in hypoxic conditions. Thereby, a decrease in the rate of oxidative
transformations is compensated by this process, “also, a metabolic acidosis as a
consequence of hypoxia is eliminated and, as a result, the resistance of the
heart muscle to oxygen deficiency is increased” . Since the activation of
complex II determines Ca
Over the past years, there has been a significant increase in attention to the
metabolic regulation of the ETC complexes activity and finding endogenous
substances that can increase the activity of the complexes in hypoxia.
Surprisingly, hydrogen sulfide (H
It was originally demonstrated that hydrogen sulfide at high doses causes
irreversible inhibition of cytochrome oxidase that prompts the ETC dysfunction,
uncoupling oxidative phosphorylation, and subsequent cell de-energization .
However, further studies revealed that H
Analysis of findings on the cytoprotective effect of H
In the experimental studies, the intraperitoneal administration of the saturated
Interestingly, both in vivo and vitro experiments have
revealed that H
Other targets of the H
|Main molecular targets in CVS||Effects on cells of CVS||Clinical significance and prospects for pharmacological use||References|
|Hydrogen sulfide (H|
|ETC enzymes||Activation and increase of ATP synthesis||Adaptive response in vessels during hypoxia||[23, 26]|
|Homocysteine||Restoring the level of CSE activities||Protection the myocardium from oxidative and ER stress induced by HHcy||[14, 15, 17]|
|Nrf2||Decreasing in generation of ROS||Reducing the risk of hypertension and myocardial infarction||[21, 22, 23]|
|NLRP3 inflammasome||Inhibiting both nuclear translocation of NF-kappaB and NLRP3 inflammasome activation||Inhibition the vicious cycle of oxidative stress and inflammation in hypertension||[45, 46]|
|KATP||Increasing of MMP,decreasing of mitochondrial Ca
||Protection of cells from ischemic and reperfusion damages. Control the ventilatory responses to hypoxia||[16, 20, 24, 25, 203]|
|Glu-receptors||Activation of NTS neurons||Ventilatory and cardiovascular control|||
|BKCa||Transient receptor potential vanilloid 4 (TRPV4) channel-mediated Ca
|ER stress-related proteins||The decrease of activity caspase 1/2, expression of glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP)||Suppressing of ER stress. Reducing of cytotoxicity||[48, 49]|
|Nitric oxide (NO)|
|mitoKATP||Attenuation of mitochondrial respiration caused by complex I substrates||The decrease of ROS production. Protection from IRI||[23, 27, 37, 56, 212]|
|Nitric oxide—releasing molecules (NO-RMs)||Stimulation of NO/cyclic guanosine 5’ monophosphate (cGMP) pathway||Regulation of vascular contractility||[57, 58, 59]|
|HIF-1alpha||Activation of HIF-1alpha and subsequent expressionof glycolysis genes, GLUT family (glucose transporter) genes, EPO and VEGF/R genes. Modulating redox signaling.omega-Alkynyl arachidonic aciddiminished HIF-1alpha binding to the HRE sequence in iNOS promoter||Reducing the inflammatory response in hypertension. Antioxidant effects. Vascular reconstruction and angiogenesis. Reducing infarct size||[27, 28, 29]|
|SIRT1||Suppressing of NF-kappaB signaling via eNOS expression||Amelioration of myocardial ischemia/reperfusion injury|||
|BH4||Remoting ischemic preconditioning by limiting cardiac eNOS uncoupling||Mitigation of myocardial IR injury. Reducing infarct size|||
|PKC||iNOS mediated activation of PKC and mitoKATP channel opening||Increasing of cardiac tolerance to ischemia and reperfusion||[32, 60]|
|CaM||CaM facilitates a conformational shift in NOS allowing for efficient electron transfer||Mitigation of myocardial IR injury|||
|ETC enzymes||Inhibition of mitochondrial respiration||Protection of cells from ischemic and reperfusion damages||[34, 35, 36]|
|Carbon monoxide (CO)|
|mitoKATP||Regulation of mitochondrial respiration and membrane potential||Protective response of cardiac muscle to oxidative stress. Vasodilatation||[23, 212]|
|mitoBKCa||The increase in the oxygen consumption rate in endothelial cells. Inhibition of glycolysis (extracellular acidification rate, and a decrease in ATP-turnoverenhanced non-mitochondrial respiration).||Mild uncoupling of mitochondrial respiration in endothelial cells induces adaptive response in vessels during hypoxia|||
|TASK-3||Regulation of mitochondrial respiration and membrane potential||Vasodilatation andreducing of cardiac hypertrophy||[23, 51]|
|Сarbon monoxide-releasing molecules (CO-RMs)||Stimulation of cGMP andNa/H exchange. Activation of BKCa through NO via the NOS and through the PKG, PKA, and S-nitrosylation pathways.||Regulation of vascular contractility; attenuation of coronary vasoconstriction and significantly reducing of acute hypertension||[38, 40, 43, 44, 52, 211]|
|Ntf2||Stimulation of HO-1 and subsequent expression of HSP32, sGC, p38MAP; the decrease of NFkappaB expression||Heme oxygenase suppresses markers of heart failure and ameliorates cardiomyopathy. Facilitating tissue regeneration/repair and the formation of new blood vessels||[39, 41, 42, 53, 212]|
|T-type Cav||Inhibition of T-type Cav via induction of HO-1||Control of cellproliferation (for example in hypertrophic cardiomyopathy and atherosclerosis)||[54, 210]|
|L-type CaV||Inhibition of pore-forming subunit CaV cardiac L-type Ca
||Protection of cells from ischemic and reperfusion damages||[55, 210]|
Previous physiological studies have demonstrated that H
Experiments with isolated mitochondria from rat hearts revealed that the 4CPI
hydrogen sulfide donor caused a decrease in MMP and weakened the activity of
caspase-9. This effect was canceled by 5-hydroxydecanoate, a selective blocker of
Results of physiological experiments supporting the cardioprotective effect of
In addition, the protective effect of H
So far, H
Thus, the recent studies have demonstrated the protective effect of small
(physiological) doses of H
More recently, considerable research has been devoted to the role of nitric oxide (NO) in the development of adaptation to various pathological conditions, including ischemic heart disease and ischemia/reperfusion [68-72]. It has been shown that NO causes relaxation of vasculature, participates in protection of the myocardium against reperfusion injuries, and regulates apoptosis and proliferation of vascular smooth muscle cells [27,71,73]. Under physiological conditions, NO reacts with oxygen molecules and forms intermediate compounds, known as reactive nitrogen species (RNS). Formation of NO and RNS in cells is controlled by hormones, neurotransmitters, cytokines, and growth factors. With regard to the latter, NO and its derivatives act as secondary paracrine factors that transmit a signal from NO-producing cells to neighboring cells . Intracellular NO and RNS receptors, which include Src protein-tyrosine kinases, Ras family proteins, cytochrome oxidase, and soluble guanylate cyclase (sGC), are mainly proteins containing heme, active SH- and iron-sulfur groups. They are localized both on the surface of the plasma membrane and in the internal compartments of cells. Most of the NO receptors are key components of intracellular signaling systems that regulate transcription factors AP-1, HIF-1, NF-kappaB, FoxO and the expression of their subordinate genes [28,29,74]. A feature that distinguishes NO from other high molecular weight signaling molecules is that the change in the redox potential of the cells switches the redox-dependent NO receptor and modifies the action of NO. Depending on the ROS level in cells, NO activates different redox-dependent signaling systems (Fig. 1). This is important in induction and suppression of the cellular protective responses to hypoxia  (Table 1).
In the CVS, NO is derived from a spectrum of molecular structures integrated into a nitroxidergic system (NOES) that includes neuronal (neurons) and extraneuronal (endothelium, cardiomyocytes, macrophages, platelets, SMC, glia, etc.) cells . NO in the NOES is produced from L-arginine by three isoforms of NO synthases: neuronal (nNOS), endothelial (eNOS) and macrophage (mNOS) also known as NO synthase I, II, and III, respectively. The endothelial and neuronal NOS belong to a group of constitutive NOS (cNOS). The macrophagal NO synthase belongs to a group of inducible NOS (iNOS). The constitutive isoforms of NO synthase can generate NO in response to background receptor stimulation of the mechanical, neuronal, or humoral nature. The inducible enzyme isoforms are usually formed in response to excessive activation of cells by cytokines. To date, there is evidence of the existence of the inducible isoforms of NOS-I and NOS-II . These isoforms are widely represented in various types of cells including blood vessel epithelial and smooth muscle cells, CM, and skeletal muscle myocytes. They are activated under stress condition, hypoxia, and various pathologies [30,31,76]. A subcellular localization and activity of NOS is determined by myristoylation and palmitoylation of N-terminal sequence, while acetylation of N-terminal glycine residues to amide bonds defines a membrane fixation of the enzyme. Therefore, the neuronal and endothelial enzyme forms are usually associated with cell membranes, and the inducible macrophage NOS exists mainly in the dissolved state in the cytosol .
NOS phosphorylation by a number of protein kinases is an important mechanism regulating NO production [32,77]. The phosphorylation of the constitutive NOS leads to a decrease in the enzymatic activity, whereas dephosphorylation with the participation of phosphatases, in particular, calcineurin can increase the catalytic activity of the enzyme . The actual mechanism of regulation of NOS activity is far more diverse and complex than we reported here. The association of constitutive NOS isoforms with the cell membrane directly or indirectly through calmodulin or other specific membrane-associated proteins involves a coordinated modulation of the NOS activity through phosphorylation/dephosphorylation at Ser-1177 and Thr-495 (Find out more about this in seminal work of S. Dimmeler, I. Fleming and R Busse groups ).
It has been known that cNOS begin to synthesize NO in response to increase in
cytosolic calcium concentration. This makes calmodulin (CaM) to bind a 30-amino
acid peptide connecting oxygenase and reductase domains of the NOS subunits. A
mechanism of the calmodulin activation for the Ca
The mechanism of the eNOS and nNOS action is similar in the CVS system.
Vasodilator agents (acetylcholine, adenosine, 5-hydroxytryptamine, glutamate,
bradykinin, histamine, etc.) increase cytosolic Ca
The endothelial and neuronal NO
synthases are involved in such processes as conductance of nerve impulses,
peristalsis provision and practically instantaneous regulation of blood pressure.
For example, factors like acetylcholine and bradykinin activate the
phosphoinositide signaling pathway in endothelial cells, resulting in
In the CVS cells, the NOS level frequently increases under conditions of oxygen deficiency, since NO formed by NO synthases is a trigger of mobilization processes aimed at maintaining cell viability during hypoxia. Herewith, the mitochondrial synthase of nitric oxide (mtNOS) plays the most important role. Its function is closely interrelated with other regulatory mitochondrial factors and signaling pathways and involved in implementing adaptive cellular responses to hypoxia . mtNOS has been recognized as a constitutive form of the nNOS and it was first discovered in the mitochondria of the excitable brain and heart cells [80,81]. The mtNOS is involved in cytochrome oxidase reversible inhibition and functionally associated with complex I of the ETC [70,81]. At the same time, mtNOS reminds more inducible isoform rather than constitutive enzyme isoform by its main characteristics. Unlike the constructive enzyme isoform that is compartmentalized in the cytosol, mtNOS is localized in the inner mitochondrial membrane (IMM) .
The question whether mtNOS is a separate isoform of the enzyme or its post-translational modification remains open and is interpreted in a different manner [34,82-84]. Regardless, discovering NOS in mitochondria has opened up new possible ways to study the roles of mtNOS and mitochondrial NO in mechanisms of the cellular adaptation and cardioprotection.
The mtNOS is directly related to cell functioning in hypoxic and ischemic
conditions [35,85]. On the one hand, tissue hypoxia significantly slows down the
NOS-dependent synthesis of NO due to the lack of O
NO synthesized in mitochondria during hypoxia modulates the mitoK
A transcription rate of genes regulating mtNOS, content of mtNOS substrates
(NADPH, L-arginine) and its cofactors (FAD, FMN, tetrahydrobiopterin (BH
During hypoxia, when a significant Ca
As discussed earlier, ROS overproduction during many pathological conditions,
including hypoxia and ischemia, leads to the interaction of oxygen free radicals
with NO and production of the highly reactive peroxynitrite oxidant (ONOO
The effect of NO is multifunctional due to its multidirectional action on cell function that results in different cell responses to the same stimulus . Such effect of NO is determined by the ratio of various NOS isoforms and location of NO production in cell compartments. In this regard, it is important to note that increase in the activity of the NOS and NO production does not always impair cells and lead to the programmed cell death (see below) but also has a positive role [81,96,97]. Signaling pathways that involve both pro- and anti-apoptotic proteins are activated in the cells accordingly [97,98]. The effect of mtNOS and mitochondrial NO on the cells is determined by the ratio of stress factors and cell survival factors that direct NO in one way or another [82,99].
At low concentrations, NO reversibly binds the electron transport chain cytochrome oxidase and blocks MPTP, therefore, contributing to the cell survival in hypoxia. At high concentrations, it causes S-nitrosylation of the thiol groups of the mitochondrial proteins and inhibits ATP synthesis that results vice versa in MPTP opening, release of apoptogenic factors and cytochrome c into the cytosol, and triggering the mitochondrial apoptotic pathway [100,101]. The toxic NO influence is associated both with the direct effect on cellular iron-containing enzymes and formation of the highly reactive and permeable to membranes peroxynitrite. This results in not only mitochondrial dysfunction and reduction of ATP production but also in damage of cell nuclear apparatus as a consecquence of DNA deamination and ribonucleotide reductase inhibition .
It is obvious that the pharmacological regulation of the mtNOS activity is of great scientific and practical importance and involves development of selective drugs blocking mtNOS that could be used specifically for myocardial ischemia-reperfusion treatment. Increased mtNOS activity has been demonstrated in experiments with IHD (the severe hypoxia case especially) and right ventricular hypertrophy in hypoxia-induced pulmonary hypertension. The inhibition of mtNOS has been shown to lead to myocardial contractility increase in cardiomyopathy .
In addition to inhibitors, there is a search for new inductors of NOS. Currently, cNOS inductors are of great importance in modern medicine. They can be effective cardioprotectors in hypoxic conditions since the cNOS-produced NO initiates the activity of hypoxia-induced factor HIF-1 . This factor is transcriptional and regulates the expression of redox-dependent genes that allow cells to adapt to oxygen deficiency. These include genes of glycolysis (aldolase, lactate dehydrogenase, phosphofructokinase genes), glucose transport (GLUT family glucose transporter genes), angiogenesis (erythropoietin (EPO) genes), vascular reconstruction (vascular endothelial growth factor (VEGF) gene, and VEGF receptor 1 (VEGF1)) . In addition, HIF-1 activates vasomotor genes that are important for vascular response to hypoxia .
The expression of HIF-1 gene itself and the level of its’ protein product depend on the concentration and partial pressure of oxygen (pO2) in the blood. The HIF-1 activity is increased during hypoxia. In this regard, a special attention is paid to the search for new substances that mediate the induction of HIF-1 in the CVS cells.
Currently, some of the NO donors (S-nitroso-N-acetyl-D, L-penicillamine; S-nitrosoglutathione) have been demonstrated to induce an increase in the HIF-1 activity . This process is independent of cGMP, however, associated with activation of the redox-sensitive PI3K/AKT/ mTOR signaling pathway that controls the key cell functions . NO can bind iron in the HIF hydroxylases and block their binding with oxygen, thereby, inhibiting the hydroxylation reaction of the adaptation factor to hypoxia . This makes relevant the search of the constitutive NOS inducers as well as compounds that enable to prolong the effect of NO and support its transport to various organs and tissues.
Findings from experiments with application of activators and inhibitors of mito-
and sarcolemmal K
Since the majority of the NO effects are mediated via cGMP-activated signaling
pathways, a study has been conducted to determine the direct effect of 8Br-cGMP,
a non-hydrolyzable cGMP analogue, on the activation of the mitoK
On the other hand, NO can regulate mitoK
In summary, we affirm that NO is an important signaling molecule in the CVS and
modulator of mitochondrial respiration, ATP synthesis, activity of mitoK
Carbon monoxide (CO), also known as a “silent killer”, is one of the most toxic substances that have harmful effect on all eukaryotic organisms. It is a frequent cause of morbidity and mortality as a result of poisoning [115-118]. Symptoms and signs of CO poisoning and death result from tissue hypoxia due to its high affinity for hemoglobin. Carbon monoxide has approximately 210–250 times higher affinity for hemoglobin than oxygen at normal atmospheric pressure . Binding of CO with heme of hemoglobin molecule causes allosteric conformational change of hemoglobin, resulting in the formation of carboxyhemoglobin (COHb), a strong compound where hemoglobin bound to CO is unable to transport oxygen to tissues of the body [120-122]. Hypoxia induced by oxygen displacement from hemoglobin, known as a carbon monoxide hypoxia, leads to fatal inhibition of ATP synthase, mitochondrial dysfunction, intracellular accumulation of superoxide and cell death [123-125]. However, recent studies have shown that CO at micromolar concentrations can participate in the regulation of physiological functions and even act as a cytoprotector during development of a number of pathological conditions [118,126,127].
Endogenous CO is a product of heme catabolism to carbon monoxide, biliverdin, iron, and controlled heme oxygenases. Degradation of heme occurs in the presence of certain enzymatic systems among which heme oxygenase (HO) and biliverdin reductase are directly involved into the oxidative conversion of heme. Heme oxygenase cleaves the tetrapyrrole ring in the heme to form CO and biliverdin .
The key enzyme in the heme oxygenase reaction is heme oxygenase. Until recently, HO was assumed to be expressed mainly in brain, liver and spleen cells. However, it has been now established that HO is widely distributed in the cells of the CVS [38,127,129]. Three isoforms of heme oxygenases are known. Among them, HO-1 is a stress-induced form and known as a heat shock protein 32 (HSP32), and HO-2/3 are constitutive forms . The HO-1 plays an important role in the mechanisms of cell adaptation to various pathological processes, including hypoxia . Initially, HO-1 was considered as a microsomal protein, mainly localized in the endoplasmic reticulum, however, later the enzyme was found in the cytoplasm, nuclear matrix, peroxisomes, and mitochondria of the spleen and liver .
In this context, it is important to note that HO-1 is also expressed in the CVS cells, including CM, endothelial and vascular smooth muscle cells, thereby controlling the formation of CO [131-135]. The HO-1 is activated by various oxidant species, including endogenous prooxidants, such as heme and its derivatives [39,136]. It is known that “free” heme at high concentrations is a prooxidant and direct participant in the processes of free radical oxidation. In this regard, induction of HO-1 is primarily aimed at preventing from the development of oxidative stress and cytotoxic effects of byproducts of heme protein degradation on the CVS cells [136,137]. Inhibition of HO-1 has been demonstrated to increase oxidative stress and reperfusion injury of the cells [40-42,127].
Accumulation of the ROS, in turn, induces transcriptional activity of
HO-1 gene that plays a significant role during hypoxia and oxidative
stress. The HO-1 knockout mice developed hypertrophy of pulmonary artery and
hypertension during hypoxia, while overexpression of HO-1 was
accompanied by a decrease in proinflammatory cytokine production and
vasoconstriction under the same condition . These protective mechanisms are
caused mainly by products of the heme oxygenase reaction, such as ferritin that
The induction of HO-1 often occurs when the NOS is stimulated by donors of NO
and its derivatives, and during S-nitrosotiols and S-nitrosoglutathione
formation. Along with the redox-dependent regulation of HO-1 expression,
The constitutive isoform HO-2 (36 kDa) found in many tissues determines
the degradation rate of heme in physiological conditions. It is abundantly
expressed in the cardiovascular and nervous systems [40,136]. The HO-2
is a Ca
Numerous studies indicate that CO and its donors are involved in regulation of myogenic vascular tone by causing SMC relaxation [38,44,139,140] (Table 1), and also cause anti-inflammatory and aniapoptotic effects . In this regard, it seems to be rational applying the CO positive effects for correcting hypoxia-induced pathological conditions and reducing a course of chronic cardiovascular diseases. Cardiovascular diseases have the utmost potential for therapeutic application of the CO. However, many mechanisms underlying the CO effects on CVS cells are still not well known. Considering the importance of the perspectives of CO use as an endogenous regulator and cytoprotector in the CVS, we will focus on common mechanisms underlying its vasodilating and anti-apoptotic effects (Fig. 1).
To date, it has been known that the vasorelaxing effect of CO is mainly related
to its ability to regulate the ion permeability of cell membranes through an
increase in the activity of soluble guanylate cyclase and modulation of various
types of ion channels . Moreover, activation of BK
Under physiological conditions, BK
The vasorelaxing activity of CO is mediated by its binding to the alpha-subunit
of the BK
Since the suppression of Ca
In turn, activation of the BK
On the other hand, the relaxing effect of CO on SMC is conditioned by activation
of the soluble guanylate cyclase and an increase in intracellular cGMP
concentration [159-161]. The cGMP-dependent protein kinase G (PKG) is a key
participant in the mechanism of cGMP-mediated CO effect on vascular SMC. The
kinase induces re-uptake of Ca
Although further studies are needed to determine the more precise effect of CO on the molecular structures of the cells during vasorelaxation, CO donors can already be used in practical medicine to reduce blood pressure in hypertensive patients. In addition, the endogenous CO induction might be one of the ways to reduce a stage of ischemic injury caused by circulatory disorders associated with pathological vasoconstriction during acute coronary syndrome and angina pectoris.
To date, the CO activation of the K
Most of the cGMP effects are mediated via cGMP-dependent PKG, which phosphorylates a wide range of regulatory target proteins in the CVS cells, and thereby modulates the functional activity of these cells. Inhibition of the cGMP synthesis or the kinase itself causes a weakening of the contractile effects of CO on various SMC types [39,167]. In the organism, vessels are often influenced by two gas transmitters (CO and NO), and the CO effect is enhanced in the presence of NO . These effects are associated with sGC stimulation. In vitro experiments have shown that NO is 30–100 times more potent sGC stimulator than CO , and this explains why the NO-induced vasorelaxation is significantly more pronounced than the CO-modulated one.
Along with existing information of CO as a vasodilator, during the oxidative stress CO can exhibit a constrictive effect and promote ROS formation in mitochondria [140,168]. In turn, the CO-induced ROS production  is a prerequisite for activation of antioxidant enzymes and redox-dependent expression of corresponding genes. Modulating various signaling cascades, including PI3K/Akt , NF-kappaB, HIF-1alpha , p38 MAPK , JNK1/2 , sGC/cGMP [170-174], CO is able to exert a protective anti-apoptotic, anti-inflammatory, and anti-proliferative effect.
This chapter will focus on the protective mechanisms that underlie the CO,
The main mechanism by which CO mediates anti-apoptotic effect in the internal mitochondrial pathway is preventing association of Bid and Bax proteins, which are pro-apoptotic members of the Bcl-2 family, on the surface of the external mitochondrial membrane. The CO inhibits caspase-8, whose function is to activate the pro-apoptotic protein Bax by cleaving it to the tBid active fragment [176,179]. The activated tBid is translocated into the mitochondria, where it binds Bax protein, whose oligomeric form causes permeabilization of the outer mitochondrial membrane, release of cytochrome c and other pro-apoptotic proteins from the mitochondrial intramembrane space, apoptosome formation and ultimately cell death [180-185].
As for the external receptor-dependent apoptotic pathway, CO inhibits formation and movement of the death-inducing signaling complex DISC from the Golgi apparatus to the plasma membrane. This signaling pathway is initiated by the FasL (Fas cell death ligand) that interacts with its receptor (Fas-R) localized to the cell membrane . The FAS activation induces oligomerization and rapid recruitment of an adapter protein (FADD) that interacts with the death domain of the Fas receptor (Fas-associated death domain FADD) and caspase-8 that form DISC. Inside the signal complex, auto-proteolytic generation of caspase-8 occurs from procaspase-8 . Although exact mechanisms underlying the DISC formation and translocation of Fas, FADD, and caspase-8 have not been fully characterized, the DISC assembly has been demonstrated to occur in the Golgi apparatus and its activation happens in the plasma membrane [176,188,189]. Activated in the apoptosome, the caspase-8 cleaves Bid to the active tBid fragment, which transfers from the cytosol into the mitochondrial membrane, where it promotes activation of Bax, a main molecule of the internal mitochondrial apoptotic pathway .
It is assumed that CO is also involved in other cytoprotective mechanisms during activation of the external apoptotic pathway, particularly through activation of the p38 MAP kinase signaling pathway and regulation of the transcription factor NF-kappaB activity. The interaction of the signaling proteins with CO results in activation of the FADD-like ICE-inhibitory protein, which inhibits the TNF-alpha/Act-D-induced caspase-8 cleavage [191,192].
The anti-apoptotic CO effects can be useful for practical medical applications in cases when improving cell survival is essential to protect against acute stress or chronic destructive changes. For example, ischemic stroke and acute coronary syndrome are representative diseases when ischemic injury is caused by failure of circulation. Treatment of these diseases is associated with repeated vascularization (blood flow restoration in the damaged area), which causes additional ischemic-reperfusion injury (IRI). In such conditions, CO by exerting an anti-apoptotic effect on cells can reduce tissue damage caused both by the IRI and initial ischemia.
In addition, favorable CO effects on the CVS can include its antiproliferative
effects on VSMC and mitochondrial respiration modulation associated with mild
uncoupling of the oxidative phosphorylation and preconditioning [193,194]. The
CO directly regulates the expression both of cyclin D1, a key regulator of cell
cycle progression in the G
In addition, CO contributes to the uncoupling of the mitochondrial respiration and modulates the production of ROS. During the mitochondrial oxidative phosphorylation, 1–3% of consumed oxygen is not completely reduced to the superoxide produced by the ETC and form primary moderately reactive oxygen derivatives that contribute to the formation of more reactive or secondary oxygen derivatives even under physiological conditions [177,199]. In pathological conditions, reverse of the electron flow can lead to a persistent and enhanced ROS generation. Thus, the mild mitochondrial uncoupling is an integral cellular mechanism for limiting ROS overproduction and oxidative stress . Uncoupling of the mitochondrial respiration by CO via stimulation of mitochondrial uncouplers and/or the ATP/ADP translocase plays an important role in the uncoupling of the oxidative phosphorylation at low CO levels .
At the same time, CO partially can inhibit electron transfer along the ETC, which results in the preconditioning at the cellular level (ATP production increase and mitochondrial respiration stimulation) [177,193]. These CO-mediated preconditioning effects have a positive effect on the survival of the CVS cells during ischemia/reperfusion .
Recent studies have shown that some pharmacological drugs, which increase the
endogenous synthesis of H
Molecular studies have shown that the protective preconditioning effect of
As noted earlier, the effects of NO on CVS cells depend on its concentration. Higher NO concentrations depress CM function, mediate inflammatory processes following IR, impair MMP, mitochondrial respiration, IMM permeability, and finally inducing apoptosis or necrosis in CM.
Lower concentration of NO or its donor SNAP (2
In addition, there is information that
In addition, the authors evaluated the expressions of NOS isoforms after MI, as
well as the role they played in the cardioprotective effects of
It was also found that mRNA expression of nNOS was significantly increased in
the MI+BRL group compared to MI group. These results showed that the modulation
Various studies have shown that the effect of the studied gas transmitters
Nitric oxide refers to compounds that have a polyfunctional effect and can have
both physiological and toxic effects. The toxic effect of NO is primarily
manifested in the inhibition of mitochondrial respiratory chain enzymes, which
leads to a decrease in the production of ATP, as well as enzymes involved in DNA
replication. In addition, excessive NO production leads to hyperactivation of the
NMDA subtype of Glu receptors and increase of [Ca
The participation of NO has been also demonstrated in the development of insulin-dependent diabetes although the direct target of the action of NO and other free radicals is the DNA of the pancreatic beta-cells in the islets of Langerhans . Furthermore, excessive production of NO by the iNOS is an important link in the pathogenesis of acute circulatory failure in thermal, cardiogenic, septic and other types of shock .
Multiple factors such as low-density lipoproteins, high glucose concentrations, and ischemia can cause a decrease in NO production, both by inhibiting of NOS and by reducing their expression. Low levels of NO may lead to increased vascular tone, blood clotting and reduced immunity, thereby contributing to the development of hypertension, atherosclerosis, thrombosis, coronary heart disease, infectious diseases, and tumor growth [217-219].
Schematic representation of NO synthesis from L-arginine.
The formation of excessive amounts of NO is mainly caused by the activation of
iNOS, localized in the cytosol of cells (mainly macrophages) and expressed under
the influence of cytokines and bacterial polysaccharides. The inducible NOS
produces NO hundreds and thousands of times more than the constitutive isoforms
of the enzyme. It has been recently shown that iNOS is synthesized not only by
macrophages, but many other cells under certain external stimuli, mainly during
pathological conditions. Interaction of NO with the oxygen radical O
Nitric oxide is involved in various functional processes via interaction with
regulatory molecules. One of the most studied functions is the relaxation of SMC.
Multiple molecules such as acetylcholine, histamine, bradykinin, serotonin,
adenine nucleotides, and some others are called “endothelium-dependent
vasodilators”. Under physiological conditions, stimulation of the endothelium by
these molecules leads to the NO synthesis. In turn, NO diffuses to SMC and
stimulates GC, resulting in formation of cGMP. In the SMC of the internal organs,
cGMP reduces the [Ca
One of the most important and well-studied target organs for NO is heart. In myocardium, NO becomes one of the cardioprotective regulatory factors. Nitric oxide is synthesized in the coronary endothelium, endocardium, and cardiomyocytes. It enhances ventricular relaxation and contributes to diastolic heart function by increasing the intracellular concentration of cGMP. Under experimental conditions, NO has been demonstrated to have a pronounced effect on heart and hemodynamics by causing a decrease in heart rate, stroke volume, an increase in the duration of the PQ interval and period of expulsion.
In some cases, an increase in the level of NO can be protective. For example, it reduces mortality in patients with moderate hypercholesterolemia and atherogenic stenosis of the internal carotid artery .
In extreme conditions, changes in NO level might be considered as an indicator
that reflects the ability of the organism to provide an adequate regional
perfusion. A decrease in the level of NO metabolites in patients with ischemic
heart disease is a poor prognostic sign for a long-term ischemia. The results of
numerous studies justify the need for use of nitrates in the treatment of various
forms of IHD in patients with reduced levels of NO metabolites . Thus, NO
donor drugs or drugs that stimulate the release of NO from endothelium have a
therapeutic interest. The donors of NO include traditional heart drugs such as
nitroglycerin and other organic nitrates, which serve as exogenous sources of NO.
They have a strong side effect caused by a sudden drop in blood pressure due to
NO hyperproduction. In this regard, more attention has been currently given to
the development of new drugs for clinical use that have modulating properties
without significant side effects. Such modulators include drugs of nitrate-like
action molsidomine, sodium nitroprusside that stimulate the activity of guanylate
cyclase and NOS. Nebivolol is another potent option to patients with newly
diagnosed or poorly controlled hypertension. This drug, a representative of the
latest group of
At low concentrations of NO, other endogenous gasotransmitters (H
Hydrogen sulfide is involved in the regulation of many physiological processes,
including homeostasis, immunity, and transmission of nerve impulses in the cells
of the central and peripheral nervous system. It also plays a vital role in
vasodilation and reducing blood pressure. The discovery of these properties of
The endogenous source of H
Sulfur metabolism and H
Clinical studies have shown that the level of H
In vitro, H
Hydrogen sulfide reduces myocardial contractility both in vitro and in vivo[66,228,229]. This effect is partially
related to the activation of K
Given the importance of H
Right now, sodium hydrosulfide (NaHS) and sodium sulfide Na
Li Ling and co-authors  obtained a new H
Another direction in the development of H
Furthermore, inhibitors of enzymes H
Carbon monoxide (CO) is formed during the oxidative cleavage of protoheme IX by heme oxygenase-1 (HO-1) . In turn, protoheme IX is formed in the process of heme catabolism from hemoglobin and myoglobin as well as other hemeproteins (Fig. 4).
Schematic representation of CO formation as a result of heme catabolism.
During the reaction, heme is converted to biliverdin by the enzyme heme oxygenase, CO is produced, and the iron is released from the heme as the ferrous ion. Biliverdin then is converted to bilirubin by the biliverdin reductase. All three products of the heme oxygenase reaction are biologically active.
Studies on the role of endogenous CO as an anti-inflammatory agent and cytoprotector have been conducted in numerous laboratories around the world . These properties of endogenous CO make it an interesting therapeutic target for the treatment of such pathological conditions as tissue injury caused by ischemia and subsequent reperfusion (for example, myocardial infarction, ischemic stroke), graft rejection, vascular atherosclerosis, severe sepsis, severe malaria, and autoimmune diseases. Human clinical trials have also been conducted, but the results have not been published yet .
Experimental approaches of cancer therapies include the use of free CO donors
In summary, the adaptive changes in the CVS cells in chronic diseases and
response to hypoxia are closely associated with the participation of Ca
IS generated the idea of this review, collected materials and wrote the main text; VN participated in the formulation of the research problem; LE participated in the writing an article; SK collected literary materials and participated in data analysis. All authors participated in the drafting, writing and approval of the final version of this review.
We would like to thank our colleagues I. Vozny and N. Stepanova for their help in the design of the manuscript. The work was carried out within the state assignment of FASO of Russia (theme No. АААА-А18-118012290142-9).
This research received no external funding.
The authors declare no conflict of interest.