Eicosanoids in carcinogenesis

-- In ﬂ ammation is the body ’ s reaction to pathogenic (biological or chemical) stimuli and covers a burgeoning list of compounds and pathways that act in concert to maintain the health of the organism. Eicosanoids and related fatty acid derivatives can be formed from arachidonic acid and other polyenoic fatty acids via the cyclooxygenase and lipoxygenase pathways generating a variety of pro-and anti-in ﬂ ammatory mediators, such as prostaglandins, leukotrienes, lipoxins, resolvins and others. The cytochrome P450 pathway leads to the formation of hydroxy fatty acids, such as 20-hydroxyeicosatetraenoic acid, and epoxy eicosanoids. Free radical reactions induced by reactive oxygen and/or nitrogen free radical species lead to oxygenated lipids such as isoprostanes or isolevuglandins which also exhibit pro-in ﬂ ammatory activities. Eicosanoids and their metabolites play fundamental endocrine, autocrine and paracrine roles in both physiological and pathological signaling in various diseases. These molecules induce various unsaturated fatty acid dependent signaling pathways that in ﬂ uence crosstalk, alter cell – cell interactions, and result in a wide spectrum of cellular dysfunctions including those of the tissue microenvironment. Although the complete role of eicosanoids, including that of the recently elucidated anti-in ﬂ ammatory specialized pro-resolving lipid mediators (SPMs), e.g. lipoxins, resolvins, protectins and maresins, is not completely understood, the result of unremitting chronic in ﬂ ammation is fostering early stages of carcinogenesis. Chronic in ﬂ ammation facilitates the transition from a normal cell to a cancerous one. The disruption of homeostasis across a wide, but identi ﬁ able, swath of diverse molecular pathways creates a micromilieu which constitutes an early and necessary step in the 6-step sequence of carcinogenesis for the vast majority of cancers, termed “ sporadic cancers ” .


Introduction
The term eicosanoids comes from the Ancient Greek term 'εἴkosi' (eíkosi) meaning twenty.A chain of 20 polyunsaturated fatty acids (PUFA) is found in every mammalian cell with 20 carbon atoms and 4 double bonds in the cis-position also known as arachidonic acids (AA) and named all-cis-5,8,11,14-eicosatetraenoic acid [1].C 20 H 32 O 2 , 20:4(v-6) is the chemical formular for omega-6 (v-6) PUFA in which 20:4 refers to its 20 carbon atom chain with four double bonds and v-6 refers to the position of the first double bond from the last, omega carbon atom.AA are found in animal and human tissue, in plants, and in food [1][2][3][4][5][6].AA are esterified to membrane phospholipids.We now recognize their importance "for normal cellular membrane fluidity, but also as a substrate for numerous enzymatic transformations that form biologically active lipid mediators, such as prostaglandins, leukotrienes, epoxyeicosatetraenoic acids, and endocannabinoids" [5].
It was observed that the application of human seminal fluid induced uterine contractions [21 reviewed in 22], which later was identified as due to lipophilic compounds [23].Von Euler thought that such compounds were produced by the prostate and therefore named these as prostaglandins (PGs) [24].There is an alternative reason for the name of PGs, which is the 5-membered prostane ring in its structure.It was Karl Sune Detlof Bergström (1916Bergström ( -2004) ) and Jan Sjövall who showed that PGs come from seminal vesicles and isolated PG E (PGE) and PG F2alpha (PGF2a) [25].Two independent groups showed that PGs are derived from essential fatty acids [26,27].Sir John Robert Vane (1927Vane ( -2004) ) showed that PG synthesis could be inhibited by aspirin and aspirin-like drugs [28].In 1982, Vane and Bergström received the Nobel Prize for the discoveries of PGs, and Samuelsson for the discoveries of leukotrienes.
Due to their complexity, the following discussion of eicosanoids contains their pathways of cyclooxygenases (Cox), lipoxygenase (ALOX), cytochrome P450 (CYP) and reactive oxygen species (ROS) and/or reactive nitrogen species (RNS), followed by unsaturated fatty acids and the recently discovered specialized antiinflammatory lipid mediators (SPMs).
For some time, eicosanoids have been known to be associated with diarrhea [52] and inflammation [53], and it was known that exogenous AA resulted in PGE synthesis [54].Furthermore, that PGE was produced by mast cells [55,56], polymorphonuclear leukocytes [57], and macrophages [58].Once the inflammatory effect of PGs were identified [59,60], contradictory findings were reported which suggested that PGs act as a turn-on/off mediator relevant for homoestasis [61,62] and that PGs were not accumulated but instead were newly synthesized [56].
One key checkpoint of AA metabolism is phospholipase A2 (PLA2); this enzyme was first isolated and purified from the venom of Vipera berus in 1971 [63].Later the purification, sequencing and cloning of the first human non-pancreatic form obtained from synovial arthritic knee fluid (a point of inflammation) was identified [64,65].A PLA2 superfamily has since been recognized [66].
The substrate for Cox, ALOX and CYP to generate eicosanoids are the AA.PLA2 in the cytosol is a key control checkpoint for inflammation and acts by hydrolyzing AA from membrane phospholipids [67].The crystalline structure of human pancreatic PLA2 was determined [68] including its anti-bacterial and anti-viral functions [69,70].PLA2s are expressed by macrophages, monocytes, T cells, mast cells, and neutrophils [71] and we know that the PLA2 in the cytoplasm responsible for the production of lipid mediators in human macrophages is cytosolic PLA2-alpha [72 reviewed in 66].PLA2 knockdown in mice results in less PG synthesis, decreased inflammation, and decreased cancer development [73].The anti-inflammatory effect of zinc is thought to be mediated by inhibition of PLA2 [74].
RAF proto-oncogene serine/mitogen-activated protein kinase kinase 1,2/threonine-protein kinase/extracellular signal-regulated kinase (Raf1/MEK1,2/Erk1,2) is activated by protein kinase C (PKC) activators such as phorbol esters but after stimulation of Erk2 phosphorylation, e.g., endotoxin.This pathway is only partially inhibited showing that there is a PKC independent pathway involved in E2k2 phosphorylation.The authors showed that using the selective inhibitor bis-indolylmaleimide (BIM) inhibited arachidonate metabolites from activated macrophages and that PKC-dependent Erk2-phosphorylation by the benzamide derivative JM34, which inhibits tumor necrosis factor alpha (TNFa) from macrophages, and is based on its inhibitory effect on PLA2 activation [75].
There is an association of cyclic adenosine 3 0 ,5 0monophosphate (cAMP) and PGs.cAMP was shown to be activated or abnormally regulated in neoplasia [90,91] but its impact in cancer is controversial as it was suggested cAMP has an inverse correlation with tumor growth [92] or is involved in hormone-induced tumor growth arrest [93], compared to the interrelationship between cAMP and 3 0 ,5 0 -cyclic guanosine monophosphate (cGMP) [94].
Mouse-derived fibroblasts transformed by the oncogenic polyomavirus, simian virus (SV40), resulted in constant cAMP activity.On the other hand, SV40transformed rat fibroblasts show decreased cAMP levels.Stimulating cAMP with PGs revealed that this phenomenon depends on prostaglandin E2 (PGE2).PGF2a and prostaglandin B1 (PGB1) were more effective as compared to prostaglandin A2 (PGA2) [95].These experiments are important in that they reveal that PGs have different effects and that the same experiment in different species may produce different results.
PGs can be produced by tumors [96,97] and it was shown that PGs activate cAMP [98].It has long been suspected that PGs and cAMP are involved in cell growth and cancer development [99] but this seems to be dependent on which specific PG is involved.PGF1alpha (PGF1a) and PGF2alpha (PGF2a) do not increase cAMP but 2-acetylaminofluorene (AAF)-induced cancer is associated with increased prostaglandin E1 (PGE1) [100].
Cox-2 mRNA and protein suppression by calcitriol leads to an increase of the PG catabolyzing enzyme, 15-hydroxyprostaglandin dehydrogenase, which in turn decreases levels of PGs and inflammatory cytokines [106].The bioxygenase activity of Cox catalyzes the addition of two oxygens to AA producing an unstable cyclic hydroperoxide, prostaglandin G2 (PGG2), which is then reduced by Cox peroxidase activity to an endoperoxide, prostaglandin H2 (PGH2).PGH2 is converted by various routes to prostaglandins (e.g.PGE2), prostacyclin, and thromboxane A2 (TXA2) [107,108 reviewed in 101].
Another Cox isoform, Cox-3, is under investigation and not completely elucidated.It was first disovered in 1989 and found primarily to be expressed in the cerebral cortex and the heart [117].We now know that Cox-3 is also found in the kidney and aorta [118].Cox-3 is encoded by the same gene as Cox-1 with a difference of one intron and is under investigation [119]: Cox-3 derives from Cox-1 and is a smaller protein which is why it is also named partial Cox-1 (pCox-1), pCox-1 protein or Cox variant Cox-1V1.It is selectively inhibited by acetaminophen, phenacetin, antipyrine, and dipyrone, as well as by some non-steroidal anti-inflammatory drugs (NSAIDs) and useful in treating pain and fever.It shares all the catalytic features and important structural features of Cox-1 and À2 and its inhibition results in decreases of PGE2.Cox-3 may be involved in the regulation of body temperature [120][121][122].Cox-3 assays using an anti-Cox-3 polyclonal sera pCox-1a was shown to have a little lower weight compared to Cox-3 (65kD) [Fig. 1 of 123].NSAIDs inhibit PG synthesis and thereby function as anti-inflammatories and analgesics [124].
Acetaminophen (Paracetamol ® ) was synthesized 1878 [125] and is included as a NSAID.It is interesting that it was reported not to reduce inflammation or inhibit platelet aggregation and does not prolong bleeding time nor does it induce bleeding or ulceration [126] but it inhibits Cox-3 with a decrease in body temperature [122].Cox-3 is highly expressed in cerebral endothelial cells and decreases PGE2 [127].Cox-3 inhibition in rats increases PGE2, prostaglandin I2 (PGI2, prostacyclin), and TXA2 levels in rats in both inflammed and non-inflammed tissues suggesting that Cox-1 and Cox-2 are more specific and effective in terms of prostanoid synthesis of PGE2, PGI2, and TXA2 as compared to Cox-3.Furthermore, Cox-3 results in the augmentation of the vasodilatory activity of bradykinin by modifying Cox-1 and Cox-2 [128].
Dipyrone (Metamizole ® ) inhibits Cox-3 more effectively than Cox-1 and Cox-2 and this effect is concentration dependent in that high levels (IC50 >1000 mM) inhibit all three isoforms, moderate levels (IC50: 350 mM) inhibt Cox-1 and Cox-3 and low levels (IC50: 52 mM) inhibit only Cox-3 [120 reviewed in 129,130].It was suggested "that Cox inhibition achieved with dipyrone may be responsible for the augmentation of the smooth-muscle relaxing effects of the angiotensin-converting enzymes (ACEs) inhibitor losartan or lisinopril" while the combination of dipyrone with losartan inhibited phenylephrine (Phe), potassium chloride (KCl), and angiotensin II (Ang II) induced contractions compared to combined dipyrone with lisinopril inhibiting Phe and Ang II-induced contractions [130].
All Cox enzymes (Cox-1, Cox-2 and Cox-3) have been found to be expressed in glioblastoma and normal brain tissues but Cox-3 expression was significant higher in cancerous versus normal tissues [131].Inhibiting Cox-3 with acetaminophen decreased glioblastoma tumor size by 71% in a rat model and by 43% using indomethacin.The exact molecular mechanisms still are unresolved.However, evidence such as epidemiology, molecular, genetic, polymorphisms, epigenetics, or proteomic Cox-3 research is warranted as information abouts its catalytic activity and signaling influence and crosstalk remains largely unknown.
Aspirin has long been known to be anti-inflammatory, anti-pyretic, and analgesic but this goes back to the knowledge of the salicytes as the medicine of the early ages used in a powder from the bark of the willow tree to treat fevers and pain [132].Aspirin was produced in 1853 by the French chemist, Charles Frédéric Gerhardt (1816-1856) [133] and the isolation of acetylsalicyclic acid (ASA) in pure form occurred in 1897 by Felix Hoffmann .The laboratory manager, Arthur Eichengrün (1867-1949), claimed that he had advised his co-worker Hoffmann to produce esters of salicyclic acid [134], but Eichengrün was a German Jew and was arrested by the Nazis in 1943 and imprisoned in the Konzentrationslager Theresienstadt; after Eichengrün survived the KZ, he was confident enough to publish his claim in 1949 which was at first ignored.The British chemist, Walter Sneader, investigated the case during the 90s and came to the conclusion that attributing the discovery and first isolation of aspirin to the German chemist Felix Hoffmann (1868-1946) in 1897 had been wrong which was denied by Bayer [132,[135][136][137].
Aspirin affects the PG pathway by suppressing the production of PGs and TXAs through acetylation of a serine-rest of Cox-1 through prostaglandine-H2-synthasis-1 (PTGS1) and Cox-2 through PTGS2, discovered by British pharmacologist, Sir John Robert Vane (1927Vane ( -2004) ) in 1971 [28].For this he was awarded the Nobel Prize in 1982.
However, the mechanistic signaling pathways of aspirin are still under investigation [171].Aspirin is absorbed in high-pH environments (pH of proximal colon 5-8.0) [172,173].
A meta-analysis of 5648 patients from 29 studies revealed that an overexpression of Cox-2 correlated with recurrence and survival in CRC.The analysis was judged to have contradictory findings in terms of survival and recurrence and, therefore, it was not recommended until recently to use Cox-2 as a prognostic biomarker for CRC patients [149].For completeness, some authors advise caution with regard to bleeding complications [159].
Additional insights on the role of Cox pathways in cancer come from the following findings: (1) when TGF-b1 was applied to human lung cancer A549 cells, it induced a downregulation of Cox-2 which resulted in a decrease of PGE2; (2) the researchers administered PGE2 or PGE2 receptor agonists, and found that it suppressed TGF-b1induced actin remodeling [174] and PGE-2 inhibited the transition of a normal cell to a cancerous cell [175].On the other hand, the continuous activation of macrophages led to interleukin 6 (IL-6)-induced increases in Cox-2 expression and serum PGE2 levels (Cox-2/PGE2 pathway), thus facilitating cell transition and metastasis in lung cancer [176].The fact that increased levels of TNFa and IL-6 in patients in various stages of chronic kidney disease reflects an ongoing chronic inflammatory state [177] and highlights the importance of maintaining Cox homeostasis.This observation may explain why a continuous activation of T-cells with triggering of neutrophils, macrophages, and their cytokine release may be of greater consequence than previously thought.Excellent reviews about the different opinions on Cox isoenzymes, in vivo PGs, PGE2, PGI2, prostaglandin D2 (PGD2) and PGF2a, and the diversity of receptor subtypes (EP1-EP4) are available [178,179].

Leukotrienes
Under the influence of mast cells, the expression of ALOX in immune cells such as leukocytes, eosinophils, basophils, neutrophils, macrophages, and platelets is activated resulting into the release of leukotrienes (LTs) from these cell types [183].LTs are brokers of the inflammatory immune response and are found in body fluids.Cysteinyl LTs (CysLTs) that involve the amino acid cysteine in their structure include LTC4, LTD4, LTE4 and LTF4, LTB4, LTG4 and leukotriene 5 (LTB5) [184].LTB5 is the LTB4 equivalent formed from eicosapentaenoc acid and its chemotactic activity is several orders of magnitude lower than that of LTB4 [185].
The LT receptor slow reacting substance (SRS) of anaphylaxis antagonist FPL-55712 was found to be much more effective in inhibiting LTE4 and LTF4 compared to LTC4 and LTD4 [186] which raises the question if there are different bioactivities and if the concentrations in biological samples as well as bioactivity might be poorly understood.
The association of various leukotrienes and cancers such as lung, esophageal and prostate cancer had been reported in detail [reviewed in 182].However, the cysteinyl leukotrienes LTC4, LTD4 and LTE4 induce various effects, such as cell recruitment, muscle contraction and vessel dilatation and permeability, and 5-LO signaling can The disruption of homeostatic mechanisms by LTs alters the tumor microenvironment and facilitates the progression of cancer [182] but it is important to note that immune cells can play dual roles both producing immunosuppressive and inflammatory regulatory mediators [199].Especially the interaction of the immune systems with its various cell types needs to be further elucidated as Cox-2 inhibition can result into LTB4 increase [reviewed in 182].The homeostasis of Cox-2 versus 5-LO seems to be of importance if cell proliferation is effectively or ineffectively inhibited.For this, the various ALOX enzymes are likely to be important.
Increased expression of Cox-2 and ALOX5 are reported in lung cancer and knocking-out 5-LOX resulted into progression [216].In this regard 5-LOX blockade resulted in an increase of apoptosis [200].5-LOX was shown to be higher expressed in HCC versus normal liver tissues and inhibiting 5-LOX induces apoptosis and blocks cancer progression [217].A 22-fold elevated expression of 15-lipoxygenase-2 in ovarian cancer compared to normal ovarian tissue was observed and 15-lipoxygenase-2 was augmented [218].Increased 5-LOX metabolites enhanced TNFa and heparin-binding epidermal growth factor-like growth factor (HB-EGF) through upregulation of matrixmetalloproteinase 7 (MMP-7) which was associated with increased tumor-associated macrophages infiltration [219].5-LOX was also increased in brain cancer [210], thyroid cancer together with promotion of metalloproteinase 9 (MMP-9) [220].
Chronic cystitis showed slight increases of 5-LOX and 12-LOX versus marked increase in bladder cancer tissues and inhibition of lipoxygenase resulted into "chromatin condensation, cellular shrinkage, small membrane bound bodies (apoptotic bodies) and cytoplasmic condensation" [224].This effect was also shown in bladder cancer cells in vitro [225].In canine osteosarcoma cells 5-LOX was upregulated in about 65% in the cytoplasm, cell culture and xenograft model and application of the canine 5-lipoxygenase inhibitor tepoxalin diminished xenograft tumor growth together with cell proliferation in mice [226].
Applying the thromboxane synthetase and 5-lipoxygenase inhibitor ketoconazole versus placebo into melanoma cell incubated mice showed significantly reduced incidence of metastasis and tumor mass with better survival in the ketoconazole-treated mice compared to placebo [227].Recently CarbORev-5901 as a new carborane-based inhibitor of the 5-LOX was reported to be more stable and effective in melanoma and colon cancer cell lines was reported [228], but investigations in various cancers so far missing.It seems that 5-LOX is also constitutively highly expressed in patients with idiopathic pulmonary fibrosis [229].
It was suggested that Cox-2 is responsible for the regulation of the lipid metabolism [230].This goes in line with the findings that inhibiting 5-LOX and Cox-2 blocks colon cancer proliferation, migration, as well as invasion in vitro [231].This may explain why NSAID intake might result in some 50% reduction of the relative CRC risk [232,233] as the combined Cox-1 and Cox-2 activity is increased in CRC [234].The non-enteroendocrine "tuft cells" which are referred to chemosensory cells showed mainly Cox-1 overexpression while Cox-2 was primarily found in absorptive cells and are "rather constitute a distinct entity with transcription factor requirements for differentiation that differ from those of enterocytes, enteroendocrine, Paneth, and goblet cells" [supplemental material Fig. S1A  trosamino-1-(3-pyridyl)-1-butanone suppresses carcinogenesis [238] and modulating LOX by clearly defining proand anti-carcinogenic effects depend on which metabolite is used and may be an option in anticancer treatment [181].Receptors for LTB4 are upregulated in gastric cancer but 5-LOX does not appear to be involved in gastric [239] or in colon carcinogenesis in rodents [240] which may explain why 5-LOX is not widely observed in carcinogenesis or it may well be that 5-LOX itself may have its own homeostasis maintenance pathway.Furthermore, anti-Helicobacter therapy by Tanshinone IIA from Salvia miltiorrhiza Bge resulted in lower chronic inflammation and 5-LOX [241].15-LOX suppresses colitis associated colon cancer by inhibiting IL-6/signal transducers and activators of transcription (STAT3) signaling [242].Therefore antagonizing 5-LOX and/or promoting 15-LOX is thought being effective as a future anti-cancer therapy.However, recent experiments with 15-LOX1 knockout mice suggested that this enzyme exhibits a proinflammatory role in the dextrane sodium sulfate induced mouse colitis model.In fact, these knockout mice were strongly protected from inflammatory symptoms [243].
The idea of using EETs in anticancer therapy emerged in the late 1980s [254,255].An elevated CYP2J2 expression has been reported in esophageal, liver, breast, lung, and colorectal organs [250,256 reviewed in 253].Markers such as these eicosanoids could serve as targets in cancer therapy [257].The potential for such a use was demonstrated by the inhibition of sEH, which decreased NF-kB, TGF-b1/Smad3, and inflammatory signaling pathways, together with activating peroxisome proliferator-activated receptor (PPAR) isoforms with consequent treating effectively renal interstitial fibrogenesis in obstructive nephropathy in mice [258].
Elastase (ELA) is known to be induced by bacteria and regulates fibrosis and PG output; ELA-inhibition is assumed to reduce "mare endometrial fibrosis by stimulating the production of anti-fibrotic PGE2 and inhibiting pro-fibrotic PGF2 a" [266].In the rabbit, 5,6-EET stimulates endogenous PGE2 synthesis [267].
EETs are rapidly metabolized, short-lived signaling molecules produced by various cell types and investigated and reported as potential targets for treating inflammation and cancer [269].Lipid hydroperoxides (LOOHs), with Vitamin C as a regulator, seem to be necessary for EET formation [270].On the other hand, EETs have been seen as a double-edged sword in cardiovascular diseases and cancer concerning the use of anti-EET drugs [253].The anti-inflammatory effects of soluble sEH inhibitors appear to be independent of leukocyte recruitment [271].
Another bridge to chronic inflammation was recently shown.sEH plays a significant role in neurological diseases such as Parkinson or dementia [284] but also triggers obesity induced chronic colonic inflammation [285].The importance of the research of LOXs may be recognized as most recently endogeneous nitro-fatty acids (NFAs) were identified as potential future new well tolerated chemotherapeutic drug candidates [286].

Reactive oxygen and nitrogen species induced formation of EETs and oxygenated lipids
The free radical story [287,288] started by the Swedish student John Rhodin in 1954 who reported in his doctorate microbodies [289], which afterwards were "mistakenly suggested ….that they were precursors to mitochondria" [290 reviewed in [291][292][293].In 1966, these microbodies were described as peroxysomes by the British born Belgian biochemist Christian de Duve (1917-2013) [294].At that time De Duve already had discovered new organelles and had termed lysosomes and endosomes; he also discovered important processes, such as autophagy, endocytosis and exocytosis [293].
McCord and Fridovich created an inactive metal-free apoenzyme and adding copper resulted into some 80% recovery from dismutase activity; they reported superoxide dismutase (SOD) catalyzing "the dismutation or disproportionation of superoxide free radical anions" of the superoxide (O 2 − ) radical into both molecular oxygen (O 2) and hydrogen peroxide ( [295,296].Saito et al. investigated SOD in erythrocytes without finding a difference between young and elderly subjects [297].Peroxisomes from fungi and plants contain antibiotics [298], toxins [299], and signaling molecules [300 reviewed in 292], and liver peroxiosomes are involved in beta-oxidation of rare fatty acids [301][302][303][304]. Today, three SOD families are known in regard to the metal cofactor and protein: (1) copper-zinc-SOD (Cu-Zn-SOD) in eukaryotes (animals/humans, plants, fungus) found in cytosol, peroxisomes, or chloroplast, (2) iron-SOD (Fe-SOD) or manganese-SOD (Mn-SOD) which are found in peroxisomes and mitochondria in prokaryotes (archaea and bacteria) and plants (Fe-SOD) or in humans (Mn-SOD) and (3) nickel-SOD (Ni-SOD) in prokaryotes [305].There are not just intra-and intersubunit motions within the different subdomains of SOD known but there also seems to be an intersubunit information exchange [306].

Reactive oxygen (ROS) and reactive nitrogen species (RNS)
Reactive oxygen species (ROS) "are a family of molecules that are continuously generated, transformed and consumed in all living organisms as a consequence of aerobic life" [101,307,308] meaning ROS are naturally a metabolism product relevant for homeostasis, physiology as well as for so-called oxidative stress, a term coined by Sies in 1985 [309 reviewed in 310,311].ROS include nonradical derivatives of singulet oxygen (1O2), H 2 O 2 and ozone (O 3 ), and oxygen radicals such as hydroxyl radical (OH), and superoxide (O2 • À).When ROS and RNS react with unsaturated lipids, lipid hydroperoxides are formated, which can damage biomembranes and or inactivate proteins.The major source of ROS in most mammalian cells is the respiratory chain of the mitochondria and about 1-2% of the daliy oxygen consumption is converted to ROS (incomplete reduction of oxygen during cellular respiration).1-2% does not really seems a lot but considering the fact thast a normal humen being consumes about 450 l of oxygen per day, a large number of ROS are normally produced.Luckily, most of these potentially damaging chemicals are readily detoxified by the antioxidative defense system.In addition, other ROS and RNS sources are the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) and xanthine oxidase (XO).Accumulation of ROS is prevented by SOD, which rapidly convert in the cytosol cytosolic and in the mitochondrial intermembrane space superoxide dismutase 1 (SOD1).Superoxide dismutase 2 (SOD2) is located in the mitochondrial matrix and contributes to detoxifying superoxide produced during cellular respiration [312][313][314].As pointed out, "accumulation of superoxide is more associated with oxidative stress than redox signaling" [315].
Reactive nitrogen species (RNS) are antimicrobial molecules; they are mainly generated by phagocytic cells and involve nitric oxide (NO), with its derivative peroxynitrite (ONOO À ), nitrogen dioxide (NO 2 − ), dinitrogen trioxide (N 2 O 3 ), nitrous acid (HNO 2 ), and others [316,317].Both, ROS and RNS can modulate regulatory proteins and are linked to each other in plants and animals/humans inducing consequent signaling [316,318].
ROS is also involved in autophagy and the major source regulating ROS is superoxide (O 2
ROS have various physiological and pathophysiological damaging properties next to evolution and are an essential consecutive part of signaling for various physiologies as cellular adhesion, signaling and migration, apoptosis, lipid metabolism, stem cell differentiation, immune response, and sport, and during aerobic and B.L.D.M. Brücher and I.S. Jamall: 4open 2019, 2, 9 anaerobic respiration as well as pathologies, such as e.g.acute and chronic inflammation, atherosclerosis, renal diseases, arthritis, cardiovascular and neurodegenerative diseases, ageing, cancer [reviewed in 315,323,324].ROS levels are increased by ultraviolet (UV) radiation, cigarette smoking, and alcohol consumption.They are also elevated in infections and after an ischemia-reperfusion (I/R) injury.It has been suggested that the homeostasis of ROS production versus the capacity to detoxify rapidly is what determines the degree of oxidative stress at any given time in a tissue or organ [101].In synovial fibroblasts, ROS promote the phosphorylation of mitogen-activated protein kinases (MAPKs) and NF-kB through the activation of transforming growth factor betaactivated kinase 1 (TAK1), and lead to an increased expression of Cox-2 and PGE2 [325].Further, even PGs induce Cox-2 expression [326].
Abnormal ROS levels can result in MAPK and PI3K signaling with STAT3 activation and phosphorylation of SNAIL with E-Cadherin suppression and a loss of cell polarity [327].A chronic increase in ROS by macrophages increases levels of C-X-C chemokine receptor type 4 (CXCR4) and trigger the transition of a normal cell to a cancer cell [328].It seems that under physiological conditions, a Cox-and ÀROS homeostasis exists.Both Cox-1 and Cox-2 are expressed in normal human gastric mucosa and in gastric ulcers; they are increased by an H. pylori infection, but Cox-1 alone can increase gastric PGE2 production [329].H. pyloriinfected gastric epithelium co-express gastrin, its receptors, cholecystokinin B receptor (CCK(B)-R), Cox-2, and prostaglandin [330].The continuous stimulation of Cox-2 and NF-kB signaling results in a persistent increase of inflammatory cytokines such as TNFa and IL-6, as well as ROS and nitrogen-free radical species [331].Skin samples from patients with chronologically aged and photoaged skin showed greater Cox-2 expression in keratinocytes and fibroblasts compared to that observed in younger individuals [332].
Since a multi-billion market grew promising the health effects of antioxidants and creating a global dietary supplements industry that is expected to grow in the U.S. alone to some $220 billion by 2022 [333].The detailed understanding of the ROS complexity may not be as easy as earning money through the sale of antioxidants.
Applying the cytochrome c peroxidase assay to measure the rates of free H 2 O 2 with a cytosolic steadystate concentration and a rate of 90 nmol/l/min/wet weight of liver revealed that "some (40-80%) of the H202 generated in the peroxisomes is destroyed inside the organelle, and that the remaining 20-60% diffuses to the surrounding medium" [334].Half-life times (T1/2) of ROS are short contain in between < nanoseconds to seconds [335 reviewed in 336]: the very reactive OH radical has a T1/2 of approximately 10 À9 s [337] compared to O2 À and H 2 O 2 in between 10 À6 s and 10 À5 s [338] respectively [reviewed in 339].There is hope using low level magnetic field to modulate cellular produced H 2 O 2 [336].

Unsaturated fatty acids (PUFAs)
Unsaturated fatty acids are associated with the Cox and LOX pathways.PUFAs are essential and have to ingested by external sources (food).They are associated with inflammation and cancer; the main two PUFAs families are metabolized through 6D desaturase, elongase, 5D desaturase, and 4D desaturase; those deriving from a-linolenic acid (ALA) result into eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA) and are named family of omega-3 (n-3) PUFAs (v-3 PUFAs) and are less inflammatory.In contrast, those deriving from linoleic acid (LA) result in gamma-linolenic acid (GLA), di-homo-gamma-linolenic acid (DGLA), AA, docosatetraenoic and docosapentaenoic acid and are part of the family of omega-6 (n-6) PUFAs (v-6 PUFAs).The last (most far away from the carboxylic group) carbon-carbon double bond is found in the v-6 position.v-6 PUFAs are more pro-inflammatory while v-3 PUFAs with the carbon-carbon double bond in the v-3 position have more anti-inflammatory effects [340][341][342][343][344]. The third fatty acids family involves the nonessential monounsaturated fatt acid omega-9 (n-9), v-9 MUFAs with the carbon-carbon double bond in the v-9 position [345,346].v-9 MUFAs induce fatty acid oxidation and are associated with a greater anti-inflammatory effect in a sepsis model [347], but showed direct associations with mortality within the Cardiovascular Health Study [348].Furthermore, "high v-9:v-6 ratio and low v-6:v-3 ratio significantly reduced inflammatory response in rats submitted to dental extraction" [349].
As PUFAs are directly associated with various signaling pathways and crosstalk in chronic inflammation, obesity, and the microbiome, these topics are reviewed separately within this Special Issue.
Due to the above studies, the involvement of PGE2, and LTB4 involvement through Cox-1, Cox-2 and 5-LOX are thought to have a pro-carcinogenesis effects via inflammation [341,366,380].Cox-2, PGE2 and EP2 and EP4 receptors stimulating pancreatic cancer cell growth while v-3 PUFAs e.g.v-3 PUFAs downregulates MMP-9 [381] and PGE2 by competitive effect in the AA metabolism [382], and has anti-inflammatory effects [383].Granulocyte-macrophage CSF colony stimulating factor (CSF) (gm-CSF) induce PLA2 activity with PLA2 protein and consequent trigger rat alveolar macrophages (but not peritoneal macrophages or peripheral blood monocytes) to generate LTB4 as well as the 5-lipoxygenase products LTC, and 5-HETE [384 reviewed in 385].
Although not entirely clear at the present, the newly discovered anti-inflammatory and pro-resolving eicosanoids, such as lipoxins, resolvins, maresins and protectins could be one reason why multiple inflammatory changes in the tumor microenvironment directly results in a remodeled ECM.Furthermore, it could explain why the disruption of the homeostasis between anti-and proinflammatory mediators is needed, although there are additional pieces of missing information such as the period of time how long the disruption has to occur with regard to various pathogenic stimuli until a normal to cancer cell transition can occur.

Protectins (PDs)
Protectin D1 (PD1) is termed neuroprotection D1 (NPD1) when described in the central nerve system and will be here named NPD1 to avoid confusion; NPD1 derives from DHA lipooxygenation through 15-LOX metabolism and reported to be neuroprotective [426,427].Bcl-2 proteins such as Bcl-2 and BclxL are increased by NPD1 with a decrease of pro-apoptotic bcl-2associated X protein (Bax, bcl-2-like protein) and Bcl-2associated death promoter (BAD) expression together with inhibition of interleukin 1-beta (IL-1b) induced Cox-2 expression and it is thought that PLA2 releases a DHA precursor [426].NPD1 was found to be generated by LOXdependent T helper type 2-skewed peripheral blood mononuclear cells, and inhibits secretion of TNFa, interferon gamma (IFNg), promotes apoptosis and blocks even T cell migration [428].NPD1 "reduces infiltrating leukocytes and blocked TLR-mediated activation of macrophages" and alleviates kidney injury [429].
NPD1 protects against inflammation promoting wound healing [430] and is also found in neutrophils [413,431], and decreases inflammation and hyperresponsiveness in asthma [432].The dihydroxy-containing DHA derivative NPD1 decreases proaptotic protein and stimulates antiapoptotic Bcl-2 protein, protects against oxidative stress and inhibits mitochondrial cytochrome c activated caspase-3 and by this it's induced cleavage and inhibits inflammatory IL-1binduced Cox-2 expression [427].NPD1 selectively "enhances neuronal differentiation, an action not shared by eicosanoids, such as LTB4 or leukotriene C4" [433] and suppresses "Abeta42-triggered activation of proinflammatory genes while upregulating the antiapoptotic genes encoding Bcl-2, Bcl-xl, and Bfl-1(A1)" and is suggested to be effective in neurodegenerative diseases and spinal cord injuries [434].
Two synthesized diastereomers blocked neutrophil infiltration in an acute peritonitis model [440].MaR1 reduces neutrophil infiltration in peritonitis, reduces chemotaxis and enhances efferocytosis of apoptotic neutrophils with increased effectiveness compared to RvD.Investigating planaria flatworm infections (of the class Turbellaria) reported that MaR1 increased stimulation of tissue regeneration including shortening the regeneration time interval.MaR1 showed potent inflammatory analgesic effects with an IC50 of 0.49 ± 0.2 nM and by this reduced chemotherapy induced (vincristineinitiated) neuropathic pain [441]; an anti-neuropathic pain amelioration was recently reproduced [442].Further, 13S,14S-epoxy-maresin was reported as precursor of MaR1 converted by macrophages inhibiting LTA4 and 12-LOX promoting phenotype change of M1 into M2 macrophages [443].
Summary (Fig. 4) Eicosanoids "like PGs, lipoxins and leukotrienes play essential roles in maintenance of mucosal integrity" and "can become major drivers of inflammatory processes" [454].Otherwise the inevitable ageing process may be delayed by changing the homeostasis by calorie restriction, exercise, and parabiosis through growth differentiation factor-11 (GDF-11) and AA anti-inflammatory metabolites such as LXA4, RvDs, PvDs and MaRs increase NO, hydrogen sulfide (H 2 S), and carbon monoxide (CO) with suppressing NF-kB and alter mTOR [385].
Kaposi's sarcoma-associated herpesvirus (KSHV) decreased anti-inflammatory LXA4 in host cells; using de novo KSHV-infected endothelial cells in vitro Kaposi's sarcoma and primary effusion lymphoma models with lipoxin and epilipoxin decreased NF-kB, AKT, ERK1/2, Cox-2, and 5-LOX [455].Otherwise, Cox-2 is also "constitutively expressed, in the absence of overt inflammation, with a specific tissue distribution that includes the kidney, gastrointestinal tract, brain, and thymus" and this is "independent of commensal microorganisms and not associated with activity of the inflammatory transcription factor NF-kB " [456].
Influencing the disruption of homeostasis depends, amongst other things, on the local balance of eicosanoide pro-and anti-inflammation mediators: ionophore stimulated lymphocytes, monocytes, and basophils together with synthetic LTB4 induced in vitro chemotaxis of fibroblasts but applied LTB4 10 −8 concentrations result into optical migration while higher concentrations have an inhibitory effect [457].Triggering a fibrotic tissue process by the sulfidopeptide LT stimulated fibroblasts only when simultaneously PG synthesis was suppressed by indomethacin [458].Detailed knowledge of various pathways with its functional implications is needed and "…metabolic network by using systems biology approaches, should be strongly encouraged" [459].Furthermore, this needs to include reaction specifity information of regulatory mediators impacting the biological activity of various enzymes and poteins including its dynamic changes [460][461][462][463].
Furthermore, important consequences of homeostasis disruption influencing the preparation of the precancerous niche (PCN) in this Special Issue "Disruption of signaling homeostasis induced crosstalk in the carcinogenesis paradigm Epistemology of the origin of cancer" include undervalued ubiquitous proteins [464], various pathogenic stimulus evoking chronic inflammation [465], remodeled fibrosis by chronic inflammation [466], the microbiome and morbi obesity [467] and PCN induced chronic cellmatrix stress with normal to cancer cell transition [468].
Eicosanoids are hydrophobic hormone-like substances built from PUFAs that play important roles in maintaining physiological levels of inflammation and signaling pathways.Disruption of this delicate homeostasis can create not just persistent inflammation but, with ROS and RNS species, trigger fibrosis and cell transition leading to cancer.Where do we stand today?The available evidence in regard to eicosanoids has been provided.However, not everything is well understood.The data suggest that eicosanoids are not always deleterious as there are newly elucidated anti-inflammatory and pro-resolving eicosanoids (lipoxins, resolvins, maresins, and protectins) which counteract the inflammatory reaction that are important for homeostasis.Until now this mystery, especially with the detailed biological roles of such mediators in cancer and carcinogenesis and associated inflammation are not well understood [469][470][471][472].
Eicosanoids are derived from fatty acids which explain the necessity to discuss the microbiome and morbid obesity for understanding the "Disruption of signaling homeostasis induced crosstalk in the carcinogenesis paradigm Epistemology of the origin of cancer".The disruption of homeostasis across a wide, but identifiable, swath of diverse molecular pathways creates a micromilieu which constitutes an early and necessary step during the 6step sequence of carcinogenesis for the vast majority of cancers, termed "sporadic cancers" because their etiology is not understood [473,474].