By Pari Rapti, Endocrinologist
What is striking about COVID-19 is that right from the onset of the infection, the progression and pathophysiology of the disease appear to be almost predetermined. The binding of the coronavirus spike glycoprotein (S‐protein) to the receptor of the renin-angiotensin-converting enzyme (ACE2), which is found (at a rate of 83% of the total ACE2) on the surface of alveolar epithelial cells, conditionally to the existence of serine protease that makes this binding possible, seems to be nearly inevitable, thus forming a complex whose function resembles that of the “Bermuda Triangle”.
As is well known, TMPRSS2 serine protease, which is necessary to form a complex with coronavirus spike proteins, is androgen-dependent; it is interesting to consider whether it would be possible to reduce or inactivate it so that to inhibit the creation of this “Spike S-ACE2” complex. To date, there is no reference in international literature as to whether this is possible, considering that it seems to be a key factor in disease progression.
As a result of this binding and due to the depletion of the transmembrane ACE2 receptor on the surface of alveolar epithelial cells, following the formation and entrance of the “Spike S-ACE2” complex into cytoplasm, endocrine and metabolic pathways are activated or inhibited accordingly.
In addition, in COVID-19 and in some other incidents, it appears that the production of various pro-inflammatory agents, growth factors and other substances is promoted by specific mechanisms, some of which are known, which determine the progression and severity of the disease, with the involvement of multiple systems and organs. The course, however, is probably determined by genetic factors that can affect all stages of the disease. This is because various intermediates produced could possibly modify the gene expression of multiple other intermediate factors.
The complexity of biological processes, considering that disease expression differs between the sexes and age groups, may also be determined/influenced by gene expression, gene polymorphism and possible genetic modification at different levels of biological processes and disease progression, such as e.g. the expression of cytokine genes and their association with the TSs transcriptional factor, since cytokines are also regulated by their binding to this factor.
They are also likely to be affected by gene expression involving ACE, ACE2, angiotensin II, etc. Therefore, the “investigation” of genome at multiple levels during different biological phases in COVID-19 progression should probably be further studied. The modifiers of these genes’ expression should also be studied.
As mentioned, cytokines are polypeptides affecting the regulation of the human immune response. It is also already known that human cytokines, which range from 132 to 261 genes, hormones, growth factors or other chemokines affect their expression, raising the question as to their possible role in immune response.
Just as important is the cytokines gene expression and reference after they bind to TS, transcriptional factor TSs, which controls the expression of cytokines in different cell types of the body. The question is whether this could possibly deregulate the secretion and action of the cytokines and chemokines involved.
Angiotensin II, a hormone with multiple actions at different levels, is a key vasoconstrictor hormone, which regulates aldosterone secretion, blood pressure and the cardiovascular system, and is also gene-determined.
The gene controlling the AT1 receptor has the most important cardiovascular effects. So, genes and their expression probably play a very important role, through all these points of interaction, as regards the expression, progression and severity of COVID-19.
The activation of endocrine mechanisms and metabolic and endocrine pathways may have a prominent role in the pathophysiology of the disease, which is activated by infection with the new coronavirus and possibly involves neuro-immune-endocrine mechanisms.
We are going to look into some of these mechanisms that may be relevant to COVID-19.
The hypothalamic–pituitary–adrenal axis has an extremely important function, concerning the production of neurotransmitters, mainly corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH), and consequently the production of cortisol by the adrenal glands and the interactions of this system with the stress and sleep-wake cycle mechanisms, and dominant effect on human survival and quality of life. This axis is affected, among other things, by the immune system and is involved in inflammatory processes.
We are aware that cells of the immune system, such as monocytes, macrophages and helper T-cells, produce inflammatory cytokines – for example, monocytes produce Interleukin-1α, Interleukin-1β, Interleukin-6 and TNFα, which are pro‑inflammatory factors that could possibly have an interesting involvement in the mechanisms of the new COVID-19 disease.
In COVID-19 the presence of the Spike (S) protein of the virus seems to bind to converting enzyme ACE-2 receptor, with the presence of the serine protease TMPRSS2 that is prerequisite for this complex and its activation. The presence of the Spike (S) protein seems to play an essential role for the onset of a dramatic response or cytokine storm, in some patients, because cytokines trigger activation of multiple inflammation-associated intracellular pathways through this protein and the protein’s binding to membrane receptors. For example, Nfkb, production and hypersecretion of interleukin 6, TNFα, overexpression of TLR (toll like receptors), which appear to complicate the inflammation through autocrine, paracrine and endocrine mechanisms along with the involvement of pro-inflammatory agents, cytokines, growth factors, hormones and chemokines.
The cells of the immune system, such as macrophages, have a dominant role in this process, which are important for the non-specific immune response. Natural immune response recognizes important components of the invader, such as lipopolysaccharides, viral RNA, etc.
These cytokines mentioned above are known to promote the release of CTH, stimulating the secretion of CRH as well as AVP. Therefore, it seems that the involvement of endocrine organs, mechanisms and substances in COVID-19, may affect different stages from the onset and throughout the progression of the disease, and this is probably evident by studying known mechanisms.
Another question that arises concerns the importance of the new coronavirus’ involvement with the RAAS system.
Coronavirus and the RAAS system
COVID-19 has been shown to affect not only the respiratory system but also other organs and systems. It is important to further study COVID-19 in people with obesity, metabolic syndrome (because adipose tissue is not a storehouse of fat, but an endocrine organ which produces a variety of prohormones, enzyme hormones, etc. the involvement of which has been examined in detail in other articles), as well as diabetes mellitus and hypertension. Patients with these underlying diseases should always be properly regulated, even more so in view of this new threat.
In COVID-19, thrombotic episodes, pulmonary embolism, myocarditis, myocardial infarctions and arrhythmias have been observed in some cases.
What are, however, the biochemical mechanisms involved in COVID-19 cases? This issue concerns the scientific community. In trying to understand them, we are going to refer to some of them.
Pulmonary lesions, acute respiratory distress syndrome, respiratory failure and various other systemic lesions, tissue lesions and fibrosis seem to be associated with already known pathophysiological mechanisms.
It is possible that the involvement of various substances, such as Interleukin 6, Interleukin 1-7, Interleukin 17 and TNF-α and others, as well as interactions with other factors and pathways, contribute in immune system overreaction mechanisms.
Furthermore, the stimulation of the adrenergic system of the hypothalamic-pituitary-adrenal axis and coagulation disorders, which have been observed in COVID-19 cases, may be the result of a “cross-talk” between protein and enzyme processes, and both immune and non-immune cells, which lead to a “storm”, with various pathways and systems, such as the complement system, the kinin-kallikrein system and the coagulation mechanisms being of paramount importance.
The formation of the Spike protein-ACE2 complex, provided the presence of TMPSS2 serine protein, the depletion of the transmembrane ACE2 receptor and the possible activation of alternative angiotensin II production through the stimulation of ACE alternatively to AT1 receptors, may be the reason for the activation of other pathways and multiple mechanisms that expand themselves, as if throwing a pebble into the sea. This happens because the protective effect of ACE2 appears to have diminished and there may be increased production of angiotensin II via an alternative pathway.
Furthermore, in COVID-19, immune response disturbances seem to be predominant, with hypersecretion of mostly Interleukin 6, initially by macrophages and monocytes, with the involvement of T lymphocytes (CD4, CD8).
The “key” starting point that could unlock these multiple biochemical processes may be the coronavirus spike (S) protein, which is involved in the renin-angiotensin-aldosterone system (which causes huge changes) since it binds to the ACE2 receptor.
As mentioned above, the result of this binding in COVID-19 is depletion of ACE2 levels. This ACE2 depletion alternatively results in overexpression of ACE. ACE is a receptor which is found and expressed in many tissues, not only in the lungs but also in the brain, kidneys, adipose tissue, cardiovascular system and gastrointestinal tract, reproduction system and other organs.
The effects of ACE2 depletion and ACE overexpression can be multiple and the mechanisms may be different. These mechanisms include vasoconstriction of smooth muscle fibers, increased vascular permeability, and increased expression of various proteins, expansion and production of other cytokines due to the interaction, and production of pro-inflammatory factors involved in immune response.
The action of angiotensin II and the expansion and effects of RAAS activation are multiple and follow other pathways as well. Angiotensin II is known to bind normally to two receptors, AT1 and AT2. Angiotensin II, in addition to causing vasoconstriction, stimulates the adrenal glands to make aldosterone.
It also contributes to sodium reabsorption and potassium excretion, but these are not the only effects of angiotensin II. There are many known effects, and others that are still to be investigated, such as the promotion of fibrosis and inflammation, and this seems to be occurring through AT1R receptors.
Why is this considered important?
Because the ACE2 enzyme determines the levels of angiotensin II, converting it to angiotensin 1-7, which binds to Mas receptors, exerting vasodilator and anti-proliferative activity. In other words, activation of the Mas receptor pathway seems to protect the body from the adverse effects of angiotensin II.
Furthermore, the expression of ADAM-17 metalloproteinase appears to be very important. Studies highlight the protective action of ADAM-17. It should be noted that angiotensin 1-7 has anti-oxidant and anti-fibrotic action, therefore it protects against lung damage.
The importance of the renin-angiotensin-aldosterone system (RAAS) has already been established through various studies worldwide many years ago, while new data on its importance are constantly emerging, which may be of paramount importance in COVID-19. It is a hormonal system, necessary for homeostasis of the body, but also of the cardiovascular system. It should be noted that the alternative-inappropriate activation of RAAS (possibly concerning COVID-19), in addition to its effects (such as vasoconstriction, sodium retention with increased blood pressure and myocardial contraction), promotes fibrosis, inflammation and proliferation of smooth muscle cell cells.
Renin, which is produced in the kidneys, initially acts on the liver to regulate the conversion of existing angiotensinogen to angiotensin I, which is biologically inactive, and which is converted to angiotensin II by the renin-angiotensin -converting enzyme ACE.
Angiotensin II produced through two different receptors, A1 and A2, leads to different results.
In the case of COVID-19, the binding of the coronavirus spike (S) protein to the ACE2 of the lung cells may have a major effect on the functioning of the RAAS system.
Several recent studies have highlighted the genetic involvement in the presence of ACE and ACE2 receptors and possibly their action may be competitive on the RAAS axis.
The action of ACE2 receptors by converting angiotensin II to angiotensin 1-7, through Mas receptors leads to the release of NO vasoactive peptides, bradykinin and prostaglandin and this appears to be protective. The enzymatic activity of ACE2 seems to decrease with age and is expressed differently in both sexes, being higher in women.
ACE2 and COVID-19
Understandably, the binding of ACE2 to Spike (S) protein leads to decreased expression on the surface of the lung cell membrane, decreasing the action of the metabolic pathway through ACE2, both at lung level (where 83% of ACE2 is found), as well as at myocardium level, as well as all organs where ACE2 is found.
These organs and systems are many (such as the CNS, endocrine glands, gastrointestinal track, hematopoietic system, vascular system, adrenal glands, liver, uterus, testes, spermatids, kidneys, retina), and the aforementioned reductions may promote inflammation in all these organs. The relationship and interaction of the ACE / ACE2 system is likely to affect the intensity, expression and severity of symptoms in patients with COVID-19.
Recent studies in lab animals have shown that acute respiratory disorder syndrome (ARDS) is associated with increased ACE activity and decreased ACE2 receptors, while the administration of angiotensin 1-7 improves the function of affected organs.
COVID-19 appears to disrupt the relationship between ACE and ACE2 receptors and causes overproduction of angiotensin II by an alternative pathway, i.e. by predominantly activating AT1 receptors, thus delivering all angiotensin II actions to the points of its biological activity, such as increased vascular permeability and tissue damage in various organs, but also damage to the endothelium, causing fibrosis and further adverse biochemical and tissue damage, some of which we are going to mention next.
The stages of COVID-19 appear to be the following:
- The binding of Spike (S) protein to ACE2 and the formation of a complex, subject to the presence of TMPRSS2 serine protease.
- This depletes ACE2 levels on cell membrane surface.
- Consequently, the potentially increased activity of ACE, mainly through the AT1 receptor, leads to an overproduction of angiotensin II with known harmful effects, due to non-protective conversion to angiotensin 1-7. Angiotensin 1-7 has anti-inflammatory, anti-fibrotic and depleting effects, as it reduces both locally and in general, the negative effects of angiotensin II, with known pro-inflammatory effects.
The formation of the above complex, following binding to ACE2, reduces the presence of this transmembrane receptor in Alveolar cells, while also reducing the protective action against infections. In that way, it acts as an angiotensin II overproduction mechanism, through activation of ACE and binding to the AT1 receptor via an alternative pathway, activating various pathways and affecting many organs, due to the action of angiotensin II and the “reduced production of angiotensin 1-7 with protective effects”.
ACE2 receptors provide the alternative angiotensin 1-7 production pathway which acts on mammals via Mas receptors.
At this point we intend to describe and highlight the possible occurrence of an unexpected link between the angiotensinogen and thrombin, which could play a significant role in COVID-19.
Since the renin-angiotensin-aldosterone system may be involved in the pathophysiological mechanisms of COVID-19, it is important to note that even the Sympathetic Nervous System regulates renin release.
In COVID-19, the effects of angiotensin II overproduction, alternatively via the AT1 receptor, since it appears that the ACE2/ACE1 balance is modified as a result of the ACE1 binding to the Spike (S) protein, necessitate that we further study in-depth the effects of angiotensin II, both on the whole body and on each organ separately.
It may be of great importance to further investigate the potential effects locally, as well as the effects on vascular endothelium, the number and effects of different types of AT receptors, their gene expression, sex involvement or not, or even age involvement. Furthermore, it may be necessary to investigate other factors that may affect the expression of these receptors, as well as the importance of the renin-angiotensin-aldosterone axis, throughout the body, and its possible existence in different organs and cells (endothelium-muscle fibers), as well as its action locally.
Angiotensin II exerts its effects through at least four AT receptors, the AT1 receptor being the most important, as is known to date.
Stimulation of this receptor brings about various biological effects, through a variety of actions on various organs, such as the myocardium, increasing heart rate and myocardial contraction. In addition, angiotensin II through AT1 receptor regulates or stimulates aldosterone secretion, activates the sympathetic nervous system, and induces vasoconstriction.
It should be recalled that in COVID-19, the formation of clots inside blood vessels has been identified as a dominant expression in some cases. Assuming that angiotensin II is involved in this expression, we are going to make brief reference to findings from international literature.
The interior of vessels is lined with endothelial cells, which are not just a wall, but are involved in mechanisms having as a main goal of creating a protective film around clots, thus ensuring normal blood flow. These endothelial cells show intense activity, producing substances which are necessary for the protection of blood vessels in terms of vascular tone, hemostasis, hypertrophy or cell death, but they are also involved in immunological mechanisms, by regulating the migration of inflammatory cells.
What is the role of AT receptors locally (vessels) and what changes occur with the angiotensin II produced?
What are the effects of angiotensin II hypersecretion in general, but also locally, possibly on the endothelial-smooth muscle fibers of vessels.
Many of these questions have been answered in international literature and these findings are already being used to treat various diseases.
How could this knowledge be used in connection to the potentially expanded and modified stimulation of these axes in COVID-19?
Does angiotensin II affect the regulation and differentiation of immune helper T cells?
A brief reference is made next to the direct known vascular effects of angiotensin II in international literature.
It is emphasized that angiotensin II can act as a local mediator of vascular remodeling and lesion formation. Furthermore, through action on the endothelium it can modify the coagulation system, disrupting the balance between the coagulation system and the fibrinolytic system. By also acting as a growth factor, it can stimulate and increase the expression of other autocrine growth factors, such as insulin-like growth factor and platelet-derived growth factor, as well as other substances, in the endothelium and smooth muscle cells.
Vascular remodeling and vascular lesion formation are also extremely important because they affect smooth muscle viability, extracellular function, and vascular cell migration.
In addition, it may lead to the involvement of smooth muscle cells in the expression of pro-inflammatory conditions in vascular cells.
Angiotensin II can induce chemotactic monocyte expression of mRNA protein 1 in various cell types, such as monocytes, and vascular smooth muscle fibers.
The effect of vascular problems, which are due to endothelial dysfunction, is well studied. It is known to cause vasoconstriction, as well as modification and stimulation of local mediators, such as cytokines, adhesion molecules and chemokines, with a direct effect on the induction of inflammations.
It may also lead to thrombosis as a result of an imbalance between the tissue plasminogen activator and the plasminogen activator inhibitor type-1.
It is well known that angiotensin II is a major mediator of oxidative stress, therefore it is not necessary to analyze this further. Angiotensin II is likely to cause endothelial dysfunction and activate pro-inflammatory secretions in the smooth muscle fibers of vascular cells. Its pro-inflammatory action on blood vessel walls can contribute and even worsen disorders related to diabetes mellitus or dyslipidemia by interacting with other substances, thus exacerbating existing cardiovascular problems.
Another action of angiotensin II overproduction may be the disruption of balance between the coagulation system and the fibrinolytic system through its effect on the vascular endothelium. However, angiotensin II is also involved in vascular remodeling, by causing increased expression of certain growth factors, such as platelet growth factors, fibroblast growth factors, and can transform growth factor-β1, but also modify vascular cell migration.
These actions of angiotensin II, which appears to act as a local mediator of vascular remodeling and lesion formation, should lead us to further investigate the possible existence of a local RAAS system, on vessel wall and elsewhere. If present, it could through transform locally angiotensin II by means of overproduction, considering that this substance is a pleiotropic hormone. The investigation and its involvement in local damage/tissue effects and further investigation between endothelial dysfunction and vascular disease and the involvement of cytokines, chemokines, and adhesion molecules in the development of thrombotic effects appears to be important, since expanded production of angiotensin II may be present in COVID-19 via an alternative pathway.
Angiotensin II is probably overproduced in COVID-19, via ACE and AT1 receptors alternatively. Endothelial cells are known to produce substances and factors that regulate coagulation, cell growth and death, as well as vascular tone, and they control cell migration.
Does angiotensin II have a key effect on vascular endothelial function and involvement in thrombus formation? In our opinion, the investigation of a possible local renin-angiotensin-aldosterone axis in the cells of vessel wall is extremely important. In addition, the formation of atherosclerotic plaques with the involvement of AT1 and ACE receptors is further investigated.
It is noted that various factors, including endothelial cells, affect other vascular wall cells, such as smooth muscle cells, which produce and release cytokines and other growth regulators, and ultimately, affect the vascular cell phenotype, causing tissue damage and/or wall remodeling.
By stimulating AT1 receptors, the oxidative stress caused, the production of cytokines, growth factors and adhesive cells, the attraction of immune system cells and the differentiation of T-cells, may lead to complications.
How important is the action of angiotensin II in the progression of local inflammation at the level of blood vessels?
How is this done locally and in different organs of the body? And what are the effects on causing generalized and local inflammation, and how does this progress?
How important is gene expression in the progression of these biological processes? And what is the involvement of hormones and the neuro-endocrine-immune axis?
These are questions that necessitate further investigation.
It appears that the coronavirus Spike (S) protein, by binding to the renin-angiotensin converting enzyme receptor and in the presence of the serine protein, activates pathways and mechanisms and produces substances which are crucial for the progression of the disease, in a percentage of genetically determined patients.
Endocrinology and COVID-19
It should be noted that the link between endocrinology and COVID-19 is evident in many instances, e.g. when the kinase-serine receptors mediate the actions of activines, transforming growth factor beta, the Anti-Müllerian hormone (AMH) and bone morphogenetic proteins (BMPs).
It is noted that based on various investigation models, stressors, such as sepsis, affect the secretion of cortisol, as a result the body tries to suppress the immune response.
It is also possible that CRH, ACTH, IL, TNF, Platelet Activating Factor (PAF), and Bradykinin are involved.
Glucocorticoids are known to act on many levels, e.g. they inhibit the production and action of inflammatory mediators, lymphokines and prostaglandins, and inhibit the production and action of Interferon, via T cells, and the production of Interleukin 1 and 6 by macrophages.
Furthermore, they also inhibit the production of Interleukin-2 – T-cell growth factor – produced by lymphocytes, reverse the activation of macrophages, compete the action of the macrophage migration inhibitory factor (MIF), reduce attachment of macrophages to vascular endothelium and inhibit the production and anti-inflammatory effects of bradykinin, platelet-activating factor and serotonin.
Involvement of the endocrine system with the immune system has already been identified, as well as the fact that cytokines have been shown to play a key role in the immune system’s response to infections. The hypothalamic-pituitary-gonadal axis (HPG) and the hypothalamic-pituitary-adrenal (HPA) axis are affected by the action of cytokines. It is very important to understand how these pro-inflammatory and anti-inflammatory signals engage these axes in the body’s response to infections.
It has been suggested by various studies that the response to plasma and brain cytokine production is controlled by different pathways. In the presence of infection, the immune system appears to be stimulated, cytokines are produced and then released into circulation. They also appear to act locally, with some of them, such as Interleukin 1, Interleukin 6 and TNF-α, having the most significant modifying activity on the HPA axis.
Pro-inflammatory cytokines can be released by macrophages, T cells and B cells.
Many other hormones, such as sex hormones, have been implicated in the modification of immune response, since the receptors for these hormones appear to be present in immune cells. This means that even small periodic changes in sex hormone levels can affect the function of immune cells.
Estrogen receptors have been expressed intracellularly, in dendritic cells, monocytes, T and B cells, possibly affecting immune response. It is important to further investigate the pro-inflammatory effects of Interleukin 6 on the HPA axis.
It appears that there is a significant interaction between the immune system and the endocrine system, through the activity of cytokines, prohormones and hormones produced, on the vital HPA and HPG endocrine axes.
COVID-19, in addition to its complex sociological and economic implications, is a significant challenge when it comes to finding solutions and answers. Adapting to this new reality is a huge scientific challenge, which is vital for in-depth investigation and finding immediately answers and solutions.
At the moment, the most important thing is to follow the instructions of experts and the state.
Our next analysis is going to look into the action of AT1 and AT2 receptors and the action of the hypothalamic-pituitary-adrenal axis and the HPG.
