It has been already clear that lipid peroxidation plays a role in my case, however the way we and doctors consider this process is partly false. Of course many studies touch on this matter, so it would seem unlikely that something so “commonplace” could be the cause of such a particular and rare disease as POIS. What makes POIS different however is not quality, but rather quantity, which is also accompanied by a qualitative transformation making it kind of a next level disease. During sexual activity and especially at ejaculation an excessive amount of ROS is produced. There could be numerous reasons for this and the root may be different between POISers. However the important thing is that excessive ROS establishes a junction point in this process and downstream from this it practically explains everything about the peculiarities of POIS and its symptoms at least from my point of view. The next step after ROS production is lipid peroxidation and this is the crucial step that leads to symptoms. During lipid peroxidation numerous lipid peroxidation products (LPOs) are released and these are emerging as disease biomarkers as they are involved in practically every kind of disease (e.g. diabetes, neurodegenerative diseases, cancer, infections, etc.). LPOs are clearly elevated in POIS as well, even if none have bothered to measure it so far.
However elevation is only a mild term for what actually happens in POIS as LPOs are produced in such a brutally excessive manner that they transform from simple biomarkers to actual toxic agents that profoundly poison our bodies giving rise to a sepsis-like state. Probably numerous LPOs are involved in POIS, but I chose 4-Hydroxynonenal (4-HNE or HNE) as a model example as it is one of the most researched one along with malondialdehyde (MDA). From my point of view probably the most important fact is that many LPOs activate TRPV1 and TRPA1, which serve as biological alarms. This practically explains the burning pain I get, which is the most persistent indicator for my POIS. As I feel this burning pain, even if a bit reduced, in the chronic phase of POIS, it must mean that ROS release after ejaculation is exorbitant and thus production of LPOs as well in the following cascade of events.
4-HNE as an inflammatory mediator plays a role in many diseases. 4-HNE is partly responsible for lethal covid-19 and clearly must be involved in postcovid as well. It drives the development of senescence and autoimmunity by forming protein adducts and thus impairing many biological processes. 4-HNE can modulate many factors that have been discussed in relation to POIS. Even just from a theoretical view it is crystal clear that excessive lipid peroxidation can explain the entirety of the POIS phenomenon. The following quotes are meant to serve as partial evidence for this theory. The first few studies are especially noteworthy as they are practically discussing the probable root (ROS) causes of POIS.
Oxidative stress (OS) in pathologies of male reproductive disorders
ROS, the toxic derivates of oxygen metabolism, mediate some important male reproductive functions such as sperm capacitation, hyperactivation, and acrosome reactions. However, excess generation of ROS impairs redox balance in the reproductive tract, leading to cellular and molecular damage.
Germ cells are a significant source of ROS that are critical for the sperm maturation process alongside other signaling functions. Following spermatogenesis, ROS are important in epididymal transport and maturation associated mostly with motility. ROS further mediate postejaculation maturation in the female reproductive tract, in which case redox switches are required for capacitation, acrosome reaction, sperm-zona binding, and oocyte fusion for successful fertilization.
ROS are generated from spermatids and mature spermatozoa directly through a mechanism of ROS-induced ROS generation.
Sources of ROS in the male reproductive tract:
- Leukocytes
Seminal plasma contains a substantial quantity of peroxidase-positive leukocytes (polymorphonuclear leukocytes, chiefly neutrophils, 50%-60%) as well as activated macrophages (20%-30%). If the number of leukocytes in the seminal plasma exceeds 1 x 10^6 WBC/mL of semen, the condition is referred to as leukocytospermia. In a state of inflammation, leukocytes generate 100-times greater ROS than that produced in normal state. This response is triggered as a defense mechanism that also induces nicotinamide adenine dinucleotide phosphate (NADPH) production via the hexose monophosphate (HMP) shunt. Leukocyte-mediated immune response against male urogenital inflammation involves ROS generation as the prime mechanism and is thus responsible for the induction of OS. The concomitant increase in levels of proinflammatory mediators (IL-8, IL-6, and TNFa) together with a decrease in antioxidant capacity during inflammation can trigger respiratory burst, leading to OS.
- Immature spermatozoa
Abnormalities in spermiogenesis often result in excess cytoplasm retention in the mid-piece of matured spermatozoa, referred to as excess residual cytoplasm. Sperm with excess cytoplasm, or teratozoospermic sperm, generate greater amounts of ROS than what is produced by morphologically normal spermatozoa. The key enzyme for ROS production in sperm, NADPH oxidase, differs from those in leukocytes, the former being calcium dependent, while in leukocyte it is protein kinase-C (PKC) dependent.
- Autoimmune/inflammatory conditions
Non-infective or non-bacterial chronic prostatitis is a very common prostate pathology resulting in high seminal OS. This accounts for more than 90% of all prostatitis cases, affecting 10% of all men worldwide. These conditions generally trigger autoimmune responses against self-antigens of a prostate or seminal origin. These exaggerated responses induce proinflammatory mediators and trigger increased seminal leukocytes, thereby generating excessive ROS and leading to OS. A speculated mechanism that evokes this autoimmune response is probably the polymorphism of interleukin 10 (IL-10), which is an antiinflammatory TH-2 cytokine. A decrease in IL-10 may initiate the TH-1 dependent immune responses that activate T-lymphocytes against prostate antigens. Inflammatory cytokines like TNF-a, IFN-g, and IL-1B initiate chemotaxis and leukocytes activation. These leukocytes serve as major sources of seminal OS, leading to severe disruption of sperm membrane integrity and significantly reduces semen quality.
- Cryptorchidism and varicocele
- Hyperthermia and oxygen insufficiency
- Spermatic cord torsions, testicular ischemia
Adequate erection requires nitric oxide (NO) through transcription of nitric oxide synthase (NOS) for vasodilation and penile engorgement with blood. MPO-dependent oxidized low density lipoprotein (LDL) has been shown to reduce endothelial NOS mRNA expression and is correlated with poor International Index of Erectile Function (IIEF) scores, alongside increased inflammatory markers including IL8.
Sperm plasma membrane is rich in PUFAs with unconjugated double bonds within their methylene groups. Unregulated increase in sperm intracellular ROS aids a progressive reaction cascade inducing LPO. As this is a self-propagating autocatalytic reaction, it accounts for 60% of sperm membrane fatty acids damage and, thus, sperm membrane fluidity is compromised. Induction of LPO follows a progressive destructive chain of reactions till it terminates. “Initiation” of oxidative damage is via hydrogen atom isolation from the loosened bond with carbon. This is followed by further free radicals (FRs) and lipid radical production. The FRs interact with oxygen giving rise to peroxyl radicals that further take hydrogen atoms from membrane lipids. This autocatalytic reaction continues in the “propagation” phase of oxidative damage, rapidly damaging cellular components. The existing FRs keep attacking the succeeding lipids by yielding toxic aldehydes through degradation of hydroperoxide. Cytotoxic peroxyl and alkyl radicals are repeatedly produced till formation of a stable end product, malondialdehyde (MDA), that marks the “termination” of the peroxidation chain. Hydrophilic 4-hydroxynonenal is also a crucial LPO product that affects proteomics and genomics of spermatozoa.
LPO can be assessed by MDA, thiobarbituric acid assays (TBARS), or 4-hydroxynonenal (4-HNE).https://sci-hub.st/https://www.sciencedirect.com/science/article/abs/pii/B9780128159729000020Seminal leukocyte contamination can contribute to oxidative stress in the ejaculate while, in the germ line, the dysregulation of electron transport in the sperm mitochondria, elevated NADPH oxidase activity, or the excessive stimulation of amino acid oxidase action are all potential contributors to oxidative stress.
They were the first to point out the vulnerability of mammalian spermatozoa to peroxidative damage because of their high cellular content of polyunsaturated fatty acids, and the first to identify the particular vulnerability of plasmalogen to this process.
In addition to H2O2 and O2•-, spermatozoa also generate another free radical, nitric oxide (NO), as well as other powerful oxidants such as peroxynitrite (ONOO-) or hypochlorous acid (HOCl), as well as peroxyl radicals (ROO•), alkoxyl radicals (RO•), and organic hydroperoxides (ROOH), any, and all of which, can be damaging to spermatozoa when generated in excess.
Every single human semen sample is contaminated with leukocytes, comprised largely of peroxidase-positive eosinophils and neutrophils, macrophages, and a lesser number of B- and T- lymphocytes.
Interleukin-8 (IL-8) has been found to correlate with sperm DNA damage and sperm vitality in certain studies, but no other cytokine. Alternatively, other studies found significantly increased levels of IL-2, IL-4, IL-6, IFN-g, and, particularly, TNF-a in human semen in association with leukocytospermia. These, and other seminal cytokine-like molecules such as resistin, are clearly indicative of a proinflammatory state and are intimately associated with the induction of leucocyte infiltration.
So why does leukocytic infiltration occur? Presumably, a variety of pro-inflammatory states generated by such conditions as varicocele, spinal cord injury, cigarette smoking, obesity, and infection, including COVID-19, promote the local expression of cytokines such as IL-8 that, in turn, stimulate an influx of leucocytes into the male reproductive tract. If, as is generally assumed, these cells enter the ejaculate via the secondary sexual glands, then the first time that leucocytes will come into contact with the spermatozoa will be at the moment of ejaculation, when the spermatozoa will be protected by a pool of powerful and diverse antioxidants in the seminal plasma. Alternatively, if the leukocytic infiltration is secondary to orchitis or epididymitis, then the implications for sperm function are significantly greater since the leukocytes will have had more time to overwhelm the local antioxidant protection offered by the male reproductive tract and damage the spermatozoa.
Once the lipid peroxidation process has been initiated, the aldehydes generated as a result of this process will form adducts with proteins, including several that are involved in the orchestration of mitochondrial electron transport. This leads to electron leakage, O2•- generation, and the precipitation of yet more lipoperoxidative damage in a self-perpetuating spiral. Spermatozoa are particularly vulnerable to this process because they are so enriched in the polyunsaturated fatty acids that drive the lipid peroxidation process.
The ability of 4-HNE to adduct these proteins and stimulate mitochondrial ROS generation is also reinforced by the tendency of this aldehyde to form adducts with aldehyde dehydrogenase (ALDH), a key enzyme in the protection of spermatozoa against oxidative stress.
Similarly, the parent estrogen (estradiol-17B) has been found to induce mitochondrial ROS generation in high doses, possibly after being converted to catechol estrogens that are much more redox active than estradiol-17B and constitute powerful inducers of mitochondrial ROS generation by human spermatozoa, as well as other cell types.
Many of the antioxidant polyphenols that are actually known to be beneficial for sperm function induce ROS generation at complex III, from whence the toxic oxygen metabolites are released into the intramembranous space and, ultimately, the cytoplasm, with minimal induction of cellular damage. Damage occurs when ROS are released into the mitochondrial matrix, whereupon they induce lipid peroxidation and the total disruption of sperm function due to the formation of toxic electrophiles, such as 4-HNE, that form adducts with both DNA and a range of functionally important proteins.
Another group of compounds that are involved in the stimulation of mitochondrial ROS generation by human spermatozoa are polyunsaturated fatty acids (PUFA). Exposure of these cells to free, unesterified, unsaturated fatty acids elicits a powerful mitochondrial ROS response in human spermatozoa; the more unsaturated the fatty acid, the more reactive oxygen metabolites are generated. Thus, the major PUFAs in human spermatozoa, arachidonic and docosahexaenoic acids, are capable of inducing significant mitochondrial ROS formation in these cells, promoting peroxidative damage and generating both a loss of sperm motility and an increase in oxidative DNA damage.
Mitochondrial ROS generation is also a distinctive feature of spermatozoa as they enter the intrinsic apoptotic pathway. Spermatozoa are capable of undergoing a truncated form of apoptosis characterized by rapid motility loss, mitochondrial ROS generation, caspase activation in the cytosol, cytoplasmic vacuolization, and oxidative DNA damage. In many ways, death via the intrinsic apoptotic cascade is the default destiny for this cell type as all ejaculated spermatozoa are preordained to become senescent and die except, of course, the cell that successfully achieves fertilization, which are rewarded with a certain measure of immortality.
Another class of enzymes thought to be involved in the production of ROS by human spermatozoa are the NADPH oxidases (NOX). NOX5 is unusual in that it possesses EF-hand motifs that render the enzyme responsive to calcium. This feature explains why reagents that elevate intracellular calcium levels, such as divalent cation ionophores (A23187 and ionomycin) and progesterone, have been found to be capable of triggering ROS generation by human spermatozoa via mechanisms that can be blocked by DPI (diphenylene iodonium), SOD (superoxide dismutase), zinc, and the calcium chelator BAPTA. It is possible that NOX5, and the ROS generation it triggers, are key regulators of sperm function, shutting down tyrosine phosphatase activity, orchestrating cholesterol efflux from the plasma membrane, enhancing cAMP generation, and precipitating the cytoplasmic alkalinization needed to drive calcium influx via Catsper.
Depletion of Antioxidants: It is always difficult to differentiate the cause and effect in such circumstances - did the lack of antioxidant protection induce a state of oxidative stress or did the excessive generation of ROS by spermatozoa, infiltrating leukocytes, or, in cases of obesity, adipocytes lead to the depletion of antioxidant equivalents from the semen?
3. Capacity of Spermatozoa to Repair Oxidative Damage
We have already mentioned the particular importance of aldehyde dehydrogenase. Phospholipase A2 (PLA2) is also critical because this enzyme cleaves oxidized fatty acids from the second position of membrane phospholipids so that they can be neutralized by glutathione.
Another molecule that is known to possess both PLA2 and peroxidase activities is peroxiredoxin 6. Spermatozoa address the formation of oxidatively-damaged DNA with a severely truncated base excision repair pathway which is initiated by OGG1 (8-oxoguanine glycosylase).https://www.mdpi.com/2076-3921/11/2/306?trk=public_post_main-feed-card-textLipid peroxidation (LPO) is an autocatalytic process involving three main steps resulting in formation of end products, namely nonreactive malondialdehyde and 4-hydroxynonenal (4-HNE), which are disastrous for the genome and proteome as they can cause double-stranded DNA breaks.
It is well known that sperm, while in the testes are protected from immune system via the blood-testis barrier. Once they enter epididymis and move along the duct, the sperm are protected by antioxidant enzymes secreted by the epididymal epithelium into the lumen. Once ejaculation occurs, while located in the urethra sperm might come into contact with activated phagocytic leukocytes producing free radicals as a result of an infection. Inflammatory process affecting prostate or seminal vesicles such as prostatitis can trigger peroxidase-positive leucocytes and they can produce exorbitant level of ROS. This condition is described as leukocytospermia and often requires pharmacotherapy. As a result of such inflammation an increase in proinflammatory cytokines, such as interleukin (IL)-8 occurs in tandem with a decrease in the enzymatic antioxidant SOD that leads to production of high levels of ROS. Correlation between impaired sperm function and seminal plasma with elevated levels of ROS, TNF-a, IL-6 and IL-8 was found to result in an increased LPO of sperm membrane.
Another marker is hyperviscosity of seminal plasma associated with increased levels of MDA and impaired antioxidant status. Urobacteria infections that affect prostate and seminal vesicles can also contribute to increased seminal plasma viscosity and an increase in ROS production. The presence of a large number of round cells imply possible oxidative stress caused by leukocytospermia or immature spermatozoa. To distinguish leukocytes from germ cells a peroxidase test is required, CD45 (leukocyte common antigen) immunostaining or measurement of seminal elastase. Visualization of excessive residual cytoplasm in abnormal sperm is indicative for high levels of ROS.https://www.researchgate.net/profile/Ekaterina-Pavlova-2/publication/372703662_Spermatozoa_under_Oxidative_Stress_Risk_or_Benefit/links/64ff30e825ee6b7564e11e2d/Spermatozoa-under-Oxidative-Stress-Risk-or-Benefit.pdfWith less than 1% of the immune system present in the epididymis, spermatozoa appear to use reactive oxygen species (ROS), including H2O2, as a defense mechanism against pathogens. Second, spermatozoa appear to produce low levels of H2O2 to drive a signal transduction pathway known as capacitation, a process essential for fertilization. In particular, low levels of ROS are beneficial for the cell, however, higher, uncontrolled levels are detrimental.
Given that sperm motility is severally affected in infertile men, it has been speculated that oxidative stress results in lipid peroxidation in the sperm plasma membrane, which then leads to decreased motility.
We have recently demonstrated that excessive ROS generation within spermatozoa leads to the production of the toxic lipid-derived compound 4-hydroxynonenal (4-HNE). During oxidative stress, levels of 4-HNE rise significantly such that it can accumulate in the membrane at concentrations up to 5 mM. Not surprisingly, high 4-HNE levels have been implicated in the onset of many oxidative-related pathologies such as cardiovascular and neurodegenerative diseases. We have recently demonstrated that high doses of 4-HNE (between 200 and 400 uM) induce oxidative stress and inhibit sperm motility within 3 h.
Furthermore, it was recognized that the effect of ROS was downstream from the action of cAMP.
The Epigenetic Effects of 4-HNE
Perhaps one of the most significant findings of this study has been the identification of a 4-HNE adduct on histone-lysine N-methyltransferase. This enzyme plays a key role in the addition of a methyl group to either lysine or arginine on histones.https://academic.oup.com/biolreprod/article/92/4/108,%201-10/2434086These findings indicate androgen-deprivation induces OS in the rat ventral prostate (VP) through elevation of ROS anabolism and diminution of antioxidant detoxification. Androgen replacement partially reduces OS in rat VP to precastration levels. Expression of Noxs remained high amid a broad-based recovery of antioxidant defense mechanism(s).
Induction of high levels of ROS subjects the cell to a state of oxidative stress (OS), which may damage cellular DNA, proteins, and lipids and result in cell-cycle arrest, cellular senescence, and cell death.
Enzymes such as glutathione reductase (GR), glucose-6-phosphate dehydrogenase (G6PDH), gamma-glutamyl transpeptidase (GGTP), and glutathione synthetase (GS) play major interactive roles in the replenishment of cellular reducing power. Other antioxidant molecules include thioredoxin (Txn), a protein thiol that directly detoxifies ROS and serves as an electron donor to other antioxidant enzymes, including peroxiredoxin 5 Prdx5.
OS in prostate was demonstrated by immunohistochemical detection of 8-OHdG and 4-HNE protein adducts, both of which are biomarkers for OS.
In castrated animals treated with testosterone for 7 days, the amount of 4-HNE protein adducts was significantly reduced, and they presented a patchy distribution in the glandular epithelium with diffuse and heterogeneous cytoplasmic staining.
However, in the castrates with testosterone replacement, Nox1 exhibited only a trend of reduction in expression as compared with values in castrates but remained onefold elevated over levels in intact controls.
It is currently unknown why ductal epithelia express such a high level of OS constitutively. One possible explanation may be related to previously reported retrograde transport of genotoxic substances or pathogens from the urethra to prostatic ducts, a mechanism implicated in prostatitis of the human gland.
It has been demonstrated that protein kinase C mediates the regulation of gene expression for NAD(P)H oxidases by angiotensin II in vascular smooth muscle cells and by zinc in neuronal cells.
Our results clearly demonstrate that expression of NAD(P)H oxidases in rat VP is dependent on the androgen status of the animal.
Ectopic expression of Nox1 in fibroblasts induces proliferation and transformation, but that expression of Nox4 induces senescence. In human prostate cancer cells, overexpression of Nox1 increases tumorigenicity and anti-sense inhibition of Nox5 suppresses proliferation and triggers apoptosis.
The first group of VP genes includes SOD2, Gpx1, Txn, and Prdx5. These genes are most likely regulated by androgen directly; their expression declines on androgen withdrawal and rebounds to precastration levels on testosterone replacement.
The second set of VP genes, including catalase, GR, GGTP, and GS, are insensitive to androgen withdrawal but are significantly up-regulated on testosterone replacement in castrated animals. Intriguingly, this set of genes, except for catalase, are more distantly related to the removal of ROS but closely associated with replenishment of GSH.
Degeneration and constriction of the vascular system in the prostate were found to be early events after castration, leading to reduced oxygen tension in prostate epithelial cells and activation of hypoxia- and stress-signaling pathways. Similarly, oxygen deprivation alone and combined oxygen and glucose deprivation induce ROS production in microvasculature and neurons, respectively. It is therefore logical to speculate that castration induces a hypoxic environment in the regressing prostate that leads directly or indirectly to elevation of OS in this gland.https://www.sciencedirect.com/science/article/pii/S0002944010636061Low levels of reactive oxygen species (ROS) and calcium are necessary for sperm function. NADPH oxidase 5 (NOX5) is a membrane enzyme which produces ROS. This enzyme is dependent on calcium for its activity.
Overall, progesterone is one of the most important stimulators of the human sperm. For instance, progesterone induces calcium entry into the human sperm through the CatSper channel and calcium is the main activator of the NOX5 enzyme.
Indeed, other studies have shown that adding either catalase or SOD will result in the loss of sperm fertilizing ability.
As was mentioned in the introduction, calcium activates the NOX5 enzyme. So it is likely that the CatSper channel affects sperm motility either directly or indirectly. For instance, it can be assumed that calcium stimulates NOX5 activity and ROS generation. Calcium and ROS can stimulate the signaling pathways that are involved in sperm motility. Therefore, we conclude that probably the actions of CatSper and NOX5 are somewhat linked together.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5775990/Are Sensory TRP Channels Biological Alarms for Lipid Peroxidation?
These ion channels are fundamentally detectors and signal converters for body-damaging environments such as heat and cold temperatures, mechanical attacks, and potentially toxic substances. When messages initiated by TRP activation arrive at the brain, we perceive pain, which results in our preparing defensive responses. Excessive activation of the sensory neuronal TRP channels upon prolonged stimulations sometimes deteriorates the inflammatory state of damaged tissues by promoting neuropeptide release from expresser neurons. These same paradigms may also work for pathologic changes in the internal lipid environment upon exposure to oxidative stress. Surprisingly, accumulating information about the identities of their chemical stimulators and their categorization suggests that those include many of LPO products.
LPO products that activate TRPA1, including 8-iso prostaglandins (PGs) and related cyclopentenone PGs also fall into this category. In addition, environmental pollutants including formaldehyde, acetaldehyde, industrial isothiocyanate, hypochlorite, and acrolein (which is also produced during endogenous LPO processes), have been shown to cause irritation, coughing, and pain by stimulating TRPA1 through the same covalent binding mechanism.
TRPV1 is not only activated by the pungent vanilloids, but also by noxiously high temperatures (>42 °C), hyperosmolalities, acidic pH (protons), a variety of lipidergic irritating chemicals, and polyamines, which indicates that TRPV1 is a polymodal detector sensitive to thermal, mechanical, and chemical insults.
In any case, TRPV1/TRPA1-double positive C-fibers are referred to as polymodal nociceptors because they can sense an extremely extensive number of damaging signals. Such polymodal nociceptors are found not only in somatosensory nerves but also in the visceral nerves.
3. Lipid Peroxidation (LPO) Products Activate TRPA1 and TRPV1
As discussed above, the two major nociceptive TRP ion channels, TRPA1 and TRPV1, act as warning alarms of damaging environments and molecules. TRPA1 and TRPV1 sense these signals, either directly or indirectly, and in turn transduce and relay them to the brain. The resultant pain perception prepares the body for defense. As listed above, the two TRPs are already known to detect a multitude of pathologically generated lipids. Furthermore, several reports have shown that TRPA1 and TRPV1 are also responsive to a certain reactive oxygen species generated during oxidative stress.
These primary compounds are chemically unstable and later decompose to more stable but still reactive secondary LPO compounds (4-hydroxy-2-nonenal (4-HNE), 4-oxo-2-nonenal (4-ONE), etc.). HpETEs can also be metabolized into hydroxy derivatives (hyroxyeicosatetraenoic acids: HETEs) by the glutathione (GSH)-utilizing antioxidant enzyme glutathione peroxidase.
When LPO products containing alpha,beta-unsaturated carbons are present in the cytosol, N-terminal lysine (epsilon-amino group), cysteine (sulfhydryl group), or histidine (imidazolyl group) of TRPA1 nucleophilically attacks those carbons to form covalent bonds (Michael addition).
In the case of some LPO products including 4-HNE, 4-ONE, hexanal, and methylglyoxal, Schiff base formation with the amines of lysine residues of a target protein is known to occur, and this reaction appears to be less reversible than Michael addition.
Acrolein is an environmental pollutant created by combustion of organic matters and is also generated endogenously in metal-catalyzed oxidation of PUFAs such as linoleic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid. Acrolein is among the most reactive LPO products (~100 times more reactive than 4-HNE in terms of electrophilicity).
Lipid hydroperoxides generated from PUFAs may undergo complex non-enzymatic LPO processes, which are in general constituted by alkoxyl radical formation, further oxygenation, and fragmentation. Consequently, 2-alkenals with diverse carbon lengths are formed, among which the best examples in TRP channel studies have been 4-HNE, 4-ONE, and 4-hydroxy-2-hexenal (4-HHE). 4-HNE and 4-ONE are generated from omega-6 PUFAs including arachidonic acid and linoleic acid while 4-HHE is formed from omega-3 PUFAs such as docoshexaenoic acid, eicosapentaenoic acid, and linolenic acid. Compared to lipid hydroperoxides, these fragmented end products are more chemically stable and diffusible through cell membranes and thus can travel to locations outside of those where they were originally produced. GSH conjugation (which is also a Michael addition) seems to be the major step for 2-alkenal scavenging, and this reaction can be both spontaneous and catalyzed by glutathione-S-transferases. However, because this reaction is reversible, alkenals occasionally travel as far as a GSH-bound form, dissociate again, and then covalently bind to local proteins, which may contribute to spreading of oxidative disease regions. Physiological 4-HNE levels are variable (0.3–100 uM, the plasma level is generally lower than tissue levels). Under disease states during oxidative insults, two to more than 100-fold increases have been detected (5 uM–5 mM). 4-ONE was once reported to achieve a comparable level with that of 4-HNE in vitro. Modest TRPV1 activation by 4-ONE was also observed in the hundred-micromolar range. At such concentrations, which can be reached under pathological conditions, TRPV1 might also take part in raising the alarm to elevated levels of 4-HNE.
Certain alkanals such as pentanal and hexanal have been shown to be increased during LDL peroxidation. Only pentanal was tested at millimolar ranges, and it was found to induce intracellular Ca2+ increases in nociceptive populations of rodent trigeminal neurons.
Lysophosphatidylcholine (LPC) may be enzymatically released from oxidized phospholipids in oxLDLs. In vivo animal injection of LPC leads to cold hypersensitivity, and thus LPC is among nociceptive endogenous molecules that exert their effect via TRPM8 activation.
Malondialdehyde might be interesting since it is present in larger amounts (about 80-fold) than HNEs in the body, but is a relatively weak electrophile and seldom targets cysteine, different from other reactive secondary products.
Lipofuscin is a granular pigment progressively formed in various tissues during aging. The accumulation of lipofuscin is closely associated with diseases related to aging such as macular degeneration and Alzheimer disease. It is a collection of multi-type LPO products including malondialdehyde and other lipids described above, as well as their protein adducts, and exposure to extreme oxidative stress may exacerbate lipofuscin accumulation.
HNEs are among the LPO products known to modify phosphatidyl ethanolamine.
TRPA1 and TRPV1 are central sensory alarms for acute pain to avoid potentially harmful damage. In a chronic disease stage, however, those are often turned into important aggravating factors amplifying neurogenic inflammation. For LPO processes as well, it should be cautiously considered whether tuning on those two alert signals simply indicates a DEFCON-type warning or contributes to a deteriorating state of war that has already begun. Indeed, TRPA1-mediated neuronal excitation by 4-HNE, isoprostanes, acrolein, and crotonaldehyde causes the release of neuropeptides such as CGRP or substance P, which are key amplifiers of neurogenic inflammation signaling in the periphery and potentiators for nociceptive synaptic strength in the central synapse. Cortical spreading depression, which is considered to be a mechanism for migraine aura, appears to be associated with increased levels of reactive oxygen species and LPO products, and their interactions with TRPA1 and the resulting CGRP release may exacerbate migraine pathology. If TRPA1/TRPV1 are important players for pathological pain development during oxidative stress, it might be conceivable that LPO levels in the serum or urine, can be utilized as indices for quantitatively determining how pain shifts towards chronic and pathological states. Similar concepts are becoming more and more recognized in other diseases including colon cancer and atherosclerosis.https://www.mdpi.com/1422-0067/15/9/16430
