Prostanoids: Prostaglandins, Prostacyclins
The prostanoids are part of the oxylipin family of biologically active lipids derived from the action of cyclooxygenases or prostaglandin synthases upon the twenty-carbon essential fatty acids or eicosanoids, primarily arachidonic acid. They can be further subdivided by structure into two main groups, the prostacyclopentanes, comprising the prostaglandins and prostacyclins, and the thromboxanes with a 6-membered ether-containing ring, each of which is involved in some aspect of signalling and especially the inflammatory response. The prostaglandins were first isolated from semen and named from the prostate gland, thought to be their source, as long ago as the 1930s, but it was the 1960s before the biosynthetic relationship to specific essential fatty acids was described and intensive research into their biological properties began. The Nobel Prize for Medicine for 1982 was awarded to Professors Bengt Samuelsson, John Vane and Sune Bergström for their discoveries in this field (see Samuelsson, B., 2012; DOI). In general, prostaglandins occur at very low levels in tissues, of the order of nanomolar concentrations, but they have profound biological activities as short-lived autocrine and paracrine signalling molecules. While most studies have been concerned with their occurrence and function in mammals, they have also been detected in birds, ray-finned fishes, marine invertebrates, trypanosomes, blood flukes, and some algae and yeasts.
1. Nomenclature and Structures of Prostanoids
In structure, prostanoids are best considered as derivatives of a C20 saturated fatty acid, prostanoic acid, which does not itself occur in nature. A key feature is a five-membered ring encompassing carbons 8 to 12, as illustrated below. The thromboxanes are similar but have heterocyclic oxane structures. They are all synthesised by specific enzymes, which confer stereospecificity and chirality on every functional group, and are thus distinct from the isoprostanes, which are produced by non-enzymic means and have their own web page.
In the approved nomenclature, each prostaglandin is named using the prefix 'PG' followed by a letter A to K depending on the nature and position of the substituents on the ring. Thus PGA to PGE and PGJ have a keto group in various positions on the ring, and are further distinguished by the presence or absence of double bonds or hydroxyl groups in various positions in the ring. PGF has two hydroxyl groups while PGK has two keto substituents on the ring. PGG and PGH are bicyclic endoperoxides. An oxygen bridge between carbons 6 and 9 distinguishes prostacyclin (PGI). Thromboxane A (TXA) contains an unstable bicyclic oxygenated ring structure, while thromboxane B (TXB) has a stable oxane ring. In addition, all prostaglandins have a hydroxyl group in the S-configuration on carbon 15 and a trans-double bond at carbon 13 of the alkyl substituent (R2).
Further, a numerical subscript (1 to 3) is used to denote the total number of double bonds in the alkyl substituents, and a Greek subscript (α or β) is used with prostaglandins of the PGF series to describe the stereochemistry of the hydroxyl group on carbon 9. This is illustrated for prostaglandins PGE and PGFα of the 1, 2 and 3 series below, as examples.
The number of double bonds depends on the nature of the fatty acid precursor. Thus, the prostaglandins PGE1, PGE2 and PGE3 are derived from 8c,11c,14c-eicosatrienoic (dihomo-γ-linolenic), 5c,8c,11c,14c-eicosatetraenoic (arachidonic) and 5c,8c,11c,14c,17c-eicosapentaenoic acids, respectively. Of these, PGE2 is the most actively produced, and it is involved in innumerable physiological processes. Dihomo-prostaglandins derived from adrenic acid (22:4(n-6) have also been detected in cell preparations, but no such compounds are produced from docosahexaenoic acid (DHA).
Prostanoids can also be classified somewhat simplistically according to their main physiological functions, i.e. prostaglandins with an involvement in pro-inflammatory processes mainly, prostacyclins functioning in the resolution of inflammation, and thromboxanes with the related roles of platelet aggregation and vasoconstriction.
2. Biosynthesis of Prostaglandins
Cyclooxygenases: Eicosanoids, including the prostanoids, are not stored within cells but are synthesised as required in response to hormonal stimuli. The prostaglandins PGE2 and PGF2α were first isolated and characterized from human seminal fluid in 1963 by Samuelsson, but prostaglandins and other eicosanoids are now known to be produced in a highly selective manner by most cell types, depending on the activation state and the physiological condition of the tissues in which they occur. The first step in their synthesis is the release of the substrate fatty acid, such as arachidonic acid, from the cellular phospholipids by the action of the enzyme phospholipase A2, and this is discussed in the Introductory document to this series.
Next, the free acids are acted upon by one of two related enzymes, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2), more correctly termed prostaglandin endoperoxide H synthases-1 and 2 (PGHS-1 and PGHS-2), respectively (or the prostaglandin G/H synthases, PTGS1 and PTGS2). These are key heme-containing enzymes that possess both oxygenase and peroxidase activities and catalyse the first committed steps in the synthesis of prostanoids from fatty acid precursors; COX-1 is always present in tissues, while COX-2 is induced by appropriate physiological stimuli (cytokines, tumor promoters and growth factors). The two iso-enzymes have approximately 90% sequence identity, and they are very similar in structure, but an important difference is that COX-2 has a larger pocket at the active site because of an isoleucine to valine substitution. Although arachidonic acid is the preferred substrate for both enzymes, one result is that COX-2 is more permissive in that it is able to utilize fatty acids such as dihomo-γ-linolenic and eicosapentaenoic acids (and even linoleic and α-linolenic acids in vitro at least).
In humans, COX-1 and COX-2 are homodimers of 576 and 581 amino acids, respectively, and each has three highly conserved mannose-containing oligosaccharides N-linked to it at asparagine residues, one of which facilitates protein folding. A fourth oligosaccharide is found only in COX-2 and regulates its degradation. Although the enzymes are sequence homodimers, they are believed to be conformational heterodimers because one monomer functions in catalysis while the other is an allosteric regulator; this quaternary structure is necessary for enzymatic activity. Each subunit of the dimer consists of three domains: an epidermal growth factor-like (EGF) domain, a membrane binding domain and a catalytic domain. The EGF-like domain is located at the interface of the dimer and may facilitate dimerization and perhaps membrane binding, while the membrane binding domain has four amphipathic α-helices that insert into one face of the bilayer. The substantial catalytic domain contains separate oxygenase and peroxidase sites on opposite sides of the heme prosthetic group. They are integral membrane proteins of the endoplasmic reticulum, where they are located on the lumenal side only of the bilayer, and of the inner and outer nuclear membranes (COX-2 localizes to the Golgi in cancer cell lines).
Both enzymes catalyse the same two reactions. Thus, each carries out a cyclooxygenase reaction in which two molecules of oxygen are added to arachidonic acid to form a bicyclic endoperoxide with a further hydroperoxy group in position 15, i.e. to form prostaglandin PGG2. The first reaction occurs at a hydrophobic channel in the centre of the enzyme, before the hydroperoxide intermediate is transferred to across the heme-containing site for reduction by a peroxidase to form prostaglandin PGH2.
Although the reactions occur at different sites, they are functionally coupled. The combined reactions are initiated by both enzymes by the oxidation of the heme group involved in the peroxidase reaction by traces of endogenous hydroperoxides with formation of a tyrosyl radical. With COX-1, this abstracts the 13‑pro‑S hydrogen from arachidonic acid and initiates the cyclooxygenase reaction by formation of a carbon-centered radical at C-11; attack of molecular oxygen at this position leads to the intramolecular rearrangement with formation of a bicyclic endoperoxide and a further carbon-centered radical at C-15. This radical reacts with a further molecule of oxygen to form a hydroperoxide, which is reduced to form PGG2 and thence PGH2 is formed via the peroxidase activity by each enzyme by comparable mechanisms. During the reduction step the tyrosyl radical is regenerated so that activated COX can carry out multiple turnovers without a need to repeat the activation step. The other precursor polyunsaturated fatty acids interact with the enzymes in similar ways. As the catalytic tyrosyl radical can be transferred to an adjacent tyrosyl residue and become inactive after about 300 turnovers, the enzymes must be re‑expressed constantly to generate metabolites. PGH2 is highly reactive and is the starting point for the biosynthesis of most other prostanoids. Most steps in the reaction of COX-2 proceed in a similar way, although there are differences in the point of attack of the radical ions.
The requirement for two distinct cyclooxygenases is not fully understood. In spite of the structural homology, separate genes encode COX-1 (on chromosome 9 in humans) and COX-2 (on chromosome 1) and they are regulated independently by different systems. The enzymes differ in their subcellular localization, substrate specificity and the manner in which they are coupled to upstream and downstream enzymes. In addition, the catalytic domains differ in structure, so that the susceptibilities to some inhibitors are not the same. It is now apparent that the two enzymes have different functional roles. Although it is almost certainly an over-simplification, it is usually suggested that COX-1 is used for ‘housekeeping’ (homeostatic) purposes, responding rapidly to circulating hormones, which require constant monitoring and regulation. It is a constitutive enzyme that produces prostaglandins in the endoplasmic reticulum, which exit cells and signal through G-protein-linked receptors at the cell surface. However, there are also suggestions that it functions only at relatively high concentrations of arachidonic acid, for example during platelet aggregation, cell injury or acute inflammation. In those tissues where prostaglandins have specialized signalling functions, such as kidney, stomach, vascular endothelium, and especially blood platelets, COX-1 is expressed at higher concentrations, i.e. where the enzyme provides precursors for thromboxane synthesis.
In contrast, COX-2 is an inducible enzyme that is not present in unactivated tissues other than lung, kidney and brain (where COX-2 is constitutive in neurons and radial glia, but not other cell types). It is expressed under the control of the pro-inflammatory transcription factor NF‑κB in response to a wide range of extracellular and intracellular stimuli, such as cytokines, growth factors and tumor promoters, and produces prostanoids that are primarily pathophysiological or that function during defined stages of cellular development. It is able to utilize much lower concentrations of arachidonic acid and substrates other than the free acid. COX-2 expression is inducible by bacterial lipopolysaccharides and is especially important in cells that are involved in inflammation, such as macrophages and monocytes, and it is believed to be the form of the enzyme that has the main responsibility for the synthesis of those prostanoids involved in most severe inflammatory states, including cancer, rheumatoid arthritis, Alzheimer's disease and respiratory disorders, although COX-1 is also important in this context. On the other hand, COX-2 provides the substrate for synthesis of prostacyclin, which opposes the actions of thromboxanes (see below). Some COX-2 products may modulate the transcription of certain genes in the cell nucleus. COX-2 is activated by hydroperoxide concentrations that are approximately tenfold lower than those that activate COX-1, raising the possibility that under limiting concentrations of peroxide, COX-2 may be fully active while COX-1 is not. Induction of COX-2 expression is also regulated by sphingosine-1-phosphate, a further effect of sphingolipids on prostanoid biosynthesis.
Other prostaglandin synthases: As research has expanded on these oxylipins, the naming of the various enzymes has changed. I tend to use the older more common names here as opposed to those that may now be considered more correct from an academic standpoint. PGH2 produced by the COX enzymes is an unstable intermediate from which all other prostanoids are derived by a variety of different enzymic reactions. Some of these are illustrated next for arachidonate as the primary precursor. The nature and proportions of the various enzymes and of the prostanoids produced differ according to cell type. Indeed different forms of some of the enzymes exist in cells that may be functionally similar, but differ in amino acid sequence, structure and co-factor requirements.
Thus, PGH2 is converted to PGE2 by prostaglandin E synthases, of which at least three forms exist that are structurally and biologically distinct; two membrane-bound forms require glutathione while a cytosolic form is glutathione independent. The most important of these is the cytosolic enzyme (cPGES), which is expressed constitutively in many different types of cell and is linked functionally to COX-1 to promote immediate PGE2 production. A second membrane-bound PGE synthase (mPGES-1) is induced by inflammatory stimuli and functions in concert with the inducible COX-2; it is over-expressed in various tumors. Membrane-bound mPGES-2 is a bifunctional enzyme that may be involved in the pathogenesis of liver diseases.
PGD2 is formed in a similar way through an intramolecular rearrangement from PGH2 by the action of prostaglandin D synthases, which exist in two forms that are evolutionarily distinct but convergent in their functions; one is located in the central nervous system and heart (glutathione independent) and the other in peripheral tissues (glutathione dependent). In rat peritoneal macrophages, PGD synthase and COX-1 appear to be functionally coupled.
The most common stereochemical form of prostaglandin F2α (PGF2α) is synthesised by two main routes. For example, PGF2α per se is synthesised from PGE2 by the action of a cytosolic enzyme prostaglandin E 9-ketoreductase (carbonyl reductase 1), which is an NADPH-dependent oxidoreductase with a wide range of substrate specificity and tissue expression. It can also be produced directly from PGH2 by the action of prostaglandin H-endoperoxide reductase, requiring NADPH. Interestingly, this enzyme can also utilize PGD2 as a substrate for the synthesis of the second of the four stereochemical forms of PGF2α, i.e. 9α,11β-PGF2α, in the uterus to promote uterine contractions.
Levuglandins, such as LGE2 (also termed ‘isoketals’), are formed from PGH2 by a non-enzymic rearrangement. They have a very short half-life and react more rapidly than most lipid oxidation products with the free primary amine groups of proteins and phosphatidylethanolamine (see below) to form covalent adducts. Indeed, the reaction is so rapid that the free levuglandins have never been isolated.
The cyclopentenone prostaglandins A and J, with reactive α,β-unsaturated keto groups and high biological activity, are produced by spontaneous dehydration reactions (non-enzymatic) from PGE and PGD, respectively, and further modifications can then occur. For example, PGA2 isomerizes to form the highly unstable PGC2, which rapidly undergoes a secondary isomerization to produce PGB2. Similarly, in the presence of human serum albumin in vitro, it has been demonstrated that PGD2 is transformed into three dehydration products, i.e. 15‑deoxy-PGD2, Δ12-PGJ2 and 15‑deoxy-Δ12,14-PGJ2 (the last two via the intermediate PGJ2).
PGE2 is the major product of prostaglandin biosynthesis pathways following activation by pro-inflammatory substances and with its metabolite PGF2α, it is involved in a positive feedback loop to regulate COX-2 expression. Before they can function, prostanoids that have been newly synthesised must be transported from the cytosol and cross various membranes by means of active transporter systems.
In a minor side reaction, the dioxygenase activity of COX-2 can produce a lipoxygenase-type reaction, in which one dioxygen molecule is introduced but no endoperoxide formation occurs. After reduction, this can lead to the formation of 11R-hydroxy-eicosatetraenoic acid (11R-HETE), 15R-HETE and 15S‑HETE, from which it is able to produce diHETE metabolites (and from 5-HETE, i.e. 5S,15R-diHETE). In addition, COX-2 can utilize 2-arachidonoyl-lysophospholipids to generate the oxylipin-containing lysophospholipids, including PGE2- and 11-HETE-lysophosphatidylcholine, with potential biological activity (see our web page on HETE).
The Role of Aspirin: Both COX iso-enzymes and thence prostaglandin synthesis are inhibited by non-steroidal anti-inflammatory drugs ('NSAIDs'), such as aspirin (acetylsalicylic acid - one of the earliest known and most widely used of all pharmaceuticals) and ibuprofen. Aspirin exerts this inhibition by binding to the cyclooxygenase site and transferring its acetyl group irreversibly to a specific serine residue (Ser-530), which then protrudes into the active site and prevents the very first step of creating the tyrosyl radical that starts the cyclooxygenase reactiion. Because of differences in the structures of the binding sites, COX-1 is completely inhibited by this means, but aspirin acetylation of COX-2 results in a shift in reaction specificity, converting the enzyme activity from that of a cyclooxygenase to a lipoxygenase, and resulting in the generation of 15(R)-hydroxy-5,8,11,13-eicosatetraenoic acid (15(R)‑HETE), i.e. with the opposite chirality to that produced in the lipoxygenase reaction. In addition, a small amount of PGD2, but not PGE2, may be formed - again with the 15(R)‑configuration. In human mast cells, both enantiomers of 15-HETE are produced by COX-1 but the 15(S)-isomer is selectively depleted by reaction with 15-hydroxyprostaglandin dehydrogenase.
The specific inhibition by aspirin is the reason for its well-known analgesic, anti-pyretic and anti-inflammatory effects as a pharmaceutical. Via its effect on cyclooxygenases, it inhibits thromboxane synthesis and thence platelet aggregation, and it is now recommended in cardiovascular therapy. However, this does not fully explain aspirin's repertoire of anti-inflammatory effects, and it is now known to be intimately involved, through an action with COX-2, in the generation of oxygenated lipid mediators such as the ‘aspirin-triggered’ protectins (resolvins) and the epi-lipoxins, as well as the eicosanoids with the (R)‑configuration, which all exert profound anti-inflammatory effects. This may be the reason for some of the clinical benefits of aspirin, especially in neuro-inflammation. In contrast, ibuprofen and all other drugs of this type exert their effects by reversible binding and competition with arachidonic acid for the active sites.
Synthesis of COX-2 is inhibited by steroidal anti-inflammatory drugs at the level of transcription. As the active site of COX-2 is smaller than that of COX-1, it has proved possible to develop a number of drugs that specifically inhibit the action of COX-2. As well as having analgesic and anti-inflammatory effects, these are used clinically to prevent cancer of the colon. However, some COX-2 selective inhibitors have been associated with an increased risk of cardiovascular disease and have been withdrawn from the market.
Endocannabinoid metabolism: There is a significant difference in the substrate requirements of the two iso-enzymes. While both utilize unesterified arachidonic acid as substrate, COX-2 can also metabolize dihomo-γ-linolenic and eicosapentaenoic acids. COX-1 can only utilize free fatty acids, but COX-2 can react with the endocannabinoid 2-arachidonoylglycerol to form esterified 2‑prostanoylglycerol derivatives, i.e. hydroxy endoperoxides analogous to PGH2, which can be further metabolized by downstream prostaglandin synthases, such as mPGES-1. Similarly, COX-2 is involved in conversion of anandamide (arachidonoylethanolamine) and arachidonoylglycine to biologically active ‘prostamides’, though with lower efficiency. While these may simply serve as precursors of free prostanoids through hydrolysis, there is increasing evidence that they are new classes of lipid mediators with distinct biological properties of their own. The amide derivatives especially are relatively long-lived in plasma, and amides of PGF2α are available as drugs to lower ocular pressure and treat glaucoma. There is evidence for effects of 2-prostanoylglycerol on calcium mobilization through distinct and novel receptors as well as activation of the PPARδ receptor. It is subject to hydrolysis by esterases present in blood and some tissues, and especially the lysophospholipid lipase LYPLA2, which releases the prostaglandin in free form.
Insects: The phospholipids in most insects tend to contain very little arachidonic acid, so the starting point in the biosynthesis of prostaglandins is the release of linoleic acid by the action of phospholipase A2; mosquitos are an exception and require arachidonic acid as a nutrient. Linoleic acid serves as the precursor for arachidonate biosynthesis, and this is acted upon by a specific peroxidase termed peroxinectin (Pxt), and not by cyclooxygenases, to produce PGH2, which is then converted to PGE2 and PGD2 by PGE2 and PGD2 synthases, respectively. Insects have distinct catabolic enzymes also.
Related enzyme activities: The nematode Caenorhabditis elegans lacks cyclooxygenase enzymes, but it is able to produce a molecule that appears identical to prostaglandin F2α by some as yet unknown mechanism. Certain pathogenic fungi and yeasts produce 3-hydroxy-eicosanoids from host arachidonic acid and they can hijack the host’s COX-2 enzymes to produce 3-hydroxy-prostaglandins from these that are as active biologically as the normal compounds. Other fungi produce enzymes with significant homologies to mammalian cyclooxygenases COX-1 and COX-2 and termed Ppo proteins that synthesise PGH2. In addition, the yeast Candida albicans and related pathogenic fungi produce PGE2 and other prostanoids in vitro from exogenous arachidonate, but the enzymes with COX-like activities have not yet been characterized. Indeed, whole genome sequencing has revealed that fungi have no homologues for the mammalian enzymes, suggesting that they have evolved alternative mechanisms for the synthesis of eicosanoids. A prostaglandin H synthase isolated from the red alga Gracilaria vertniculophylla is very different in structure from its animal counterparts, but it appears to function in a similar way, although it is not inhibited by non-steroidal anti-inflammatory drugs. SImilarly, some pathogenic protozoa including the Chaga's disease agent Trypanosoma cruzi produce COX-like proteins that are distant evolutionarily from mammalian COX; thromboxane A2 is the main prostanoid found with a little PGF2α.
3. Prostacyclin and Thromboxane Biosynthesis
Prostacyclin (PGI2) and thromboxanes are also synthesised directly from PGH2 as illustrated below. Thus, a prostacyclin synthase (CYP8A1) is constitutively expressed in endothelial cells and in neurons and glial cells and converts PGH2 (synthesised by COX-2) to PGI2 (half-life 42 seconds). A thromboxane A2 synthase (CYP5A1) catalyses the production of thromboxane TXA2 from PGH2 (synthesised by COX-1). These enzymes are related to the cytochrome P450 super-family of proteins and are located on the cytosolic face of the endoplasmic reticulum, so the precursor PGH must cross the membrane. PGI and TXA are the main prostanoids formed in endothelial and smooth muscle cells and in platelets and lung, respectively. In addition, PGI2 and some other prostanoids can be produced by cell-cell interactions by using enzymes in adjacent cells, i.e. PGH2 of platelet origin is converted to PGI2 in the vascular epithelium. Subsequently, PGI2 can be released by endothelial cells to function through a signalling cascade with G-protein coupled receptors on nearby platelets. Similarly, prostacyclin production by erythrocytes is at least in part dependent on PGH2 from lymphocytes. COX-2 is the enzyme that provides PGH2 required for prostacyclin synthesis.
While platelets are able to synthesise thromboxane TXA2 from endothelial PGH2 in vitro, this is not believed to be a major pathway in vivo. In rat peritoneal macrophages, thromboxane A synthase and COX-1 appear to be functionally coupled in the endoplasmic reticulum. The thromboxane A2 synthase also produces 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT) from PGH2 by a rearrangement of the cyclopentane-endoperoxide structure with malondialdehyde (MDA) production in epithelial cells in various tissues but especially the intestine and skin; relatively large amounts are produced in activated platelets during skin injury and may contribute to wound healing. This oxylipin is of special relevance to leukotriene function as an endogenous ligand for the leukotriene receptor BLT2. It can be oxidized by 15-hydroxyprostaglandin dehydrogenase to the 12-keto metabolite (12-KHT) with some functions that oppose those of thromboxane A2.
Diepoxide: Thrombin-activated human platelets generate an eicosanoid in nanogram amounts that has been identified as 8,9‑11,12‑diepoxy-13-hydroxyeicosadienoic acid (8,9-11,12-DiEp-13-HEDE or DiEpHEDE), which both stimulates and primes the expression of human neutrophil integrin (it was first erroneously identified as 8-hydroxy-9,10-dioxolane A3 (DXA3)). It is believed to have a role in innate immunity and acute inflammation. COX-1 is the key enzyme involved in its biosynthesis from unesterified arachidonic acid. After synthesis, it is rapidly esterified to position sn-2 of phosphatidylethanolamine in which position sn-1 is occupied by a 16:0, 18:0 or 18:1 vinyl ether or an 18:0 fatty acid; the intact phospholipid remains in the membrane and has similar biological activity to the free eicosanoid. PGE2/D2 formed in platelets via COX-1 are esterified in the same way. Similar endoperoxides may be formed in tissues via the co-occurrence of LOX and cytochrome P450 or peroxygenase enzymes in tissues.
4. Prostanoid Catabolism
Prostanoids function close to the site of synthesis, and they are often deactivated before they are exported into the circulation as inactive metabolites. Some, such as PGI and TXA, are deactivated spontaneously, but active enzyme systems also operate, and these function primarily by reaction with the 15(S)-hydroxyl group as discussed in the Introductory web page. Prostanoids are first transported from the extracellular fluid to the cytoplasm by the prostaglandin transport protein (PGT) where, for example, prostaglandin PGE2 is oxidized to 15-keto-PGE2, which was long thought to be biologically inactive. It is now recognized that inactivation of PGE2 by 15‑hydroxyprostaglandin dehydrogenase is a vital step in halting tumor cell proliferation, and that the product 15-keto-PGE2 is an electrophilic molecule that functions in association with PPARγ (see below) and other proteins to inhibit cell proliferation, but 13,14-dihydro-15-keto-prostaglandin-E2 is inactive. Further oxidation of prostanoids eventually yields metabolites such as 11α‑hydroxy-9,15-dioxo-2,3,4,5-tetranor-prostane-1,20-dioic acid or PGE-M, which is the main metabolite of PGE2 found in urine. Many more catabolic products can be formed, and for example, when PGD2 is injected into humans, it is metabolized to 25 identifiable metabolites. Some PGE2 and PGF2α excreted in human urine is from the kidney rather than the general circulation.
The vinyl ether moiety in prostacyclin is unstable below pH 8.0, and PGI2 is rapidly deactivated non-enzymatically by a hydrolysis reaction to form 6‑keto-PGF1α. Similarly, TXA2 contains an unstable ether linkage with a half-life of 30 seconds, and it is deactivated by non-enzymatic hydrolysis to open the bicyclic oxygenated ring and form inert thromboxane B2 (TXB2). A significant portion of TXB2 then undergoes dehydrogenation at C-11 by an 11‑dehydroxythromboxane B2 dehydrogenase to form 11‑dehydro-TXB2, a metabolite found in human blood plasma and urine, which can be monitored to assess COX-1 activity and its responses to drug treatments.
5. The Functions of Prostanoids
Prostanoids are ubiquitous lipids in animal tissues that coordinate a multitude of physiological and pathological processes at concentrations down to 10-9g per g of tissue, either within the cells in which they are formed (autocrine) or in closely adjacent cells (paracrine) (they are deactivated too readily to be transported far) in response to specific stimuli. They are transported out of cells mainly by members of the ABC transporter superfamily. Under normal physiological conditions, they have essential homeostatic functions in the cytoprotection of gastric mucosa, renal physiology, gestation, and parturition, but they are also implicated in a number of pathological conditions, such as inflammation, cardiovascular disease and cancer. Different prostanoids can have complementary or opposing functions depending on tissue or physiological conditions and the correct balance between them can often be crucial. Such is the complexity of these interactions that an outline only of some of the more important can be presented here.
Receptors: Prostanoids are sometimes described as local hormones that act in an autocrine fashion close to the site of their synthesis to coordinate the effects of other hormones in the circulation, although some can undergo facilitated transport from the cell via specific transporters to exert paracrine actions. In order to express their activity, they interact with specific cell-surface G-protein-linked receptors (GPCRs) mainly. These comprise a large protein family with seven trans-membrane domains that sense molecules outside the cell and activate signal transduction pathways inside the cell and thence the cellular responses. When a ligand binds to a GPCR, it causes a conformational change, which allows it to act as a guanine nucleotide exchange factor to activate an associated G protein.
Five classes (and several sub-classes) of GPCR have been identified in the mouse and man that interact with prostanoids, and these are specific for PGE2 (designated EP or four subclasses EP1 to EP4), PGD2 (DP or two subclasses DP1 and DP2), PGF2α (FP), PGI2 (IP) and TXA2 (TP, in two isoforms - TPα and TPβ). The immediate result is an increase or decrease in the rate of generation of cytosolic second messengers (cAMP or Ca2+), a change in membrane potential or activation of a specific protein kinase. The different receptors characterized from diverse cell types tend to have high, but not absolute, specificity for particular prostanoids with characteristic functions in each cell.
Certain of the cyclopentanone prostanoids (PGA and PGJ series) interact at the cell nucleus with peroxisome proliferator-activated receptors (PPARs) of which there are three, but in this context PPARγ is especially important. This is a nuclear hormone receptor or ligand-activated transcription factor regulating the expression of genes involved in adipogenesis, glucose homeostasis and lipid metabolism. All PPARs heterodimerize with the retinoid X receptor (RXR), which must itself be activated by binding to 9-cis-retinoic acid, and bind to specific regions on the DNA of target genes.
The picture of prostanoid actions is complicated by the fact that a given prostanoid can have a number of different biological functions, sometimes opposing, according to the cell type, the nature of the stimulatory response and the type of receptor. For example, PGE2 can have either pro- or anti-inflammatory effects depending on its interactions with one of four receptors in different cell types. The relative activities of the two iso-enzymes COX-1 and COX-2 are also essential to an understanding of the activity of prostanoids in any given circumstance. However, the complexity of the various interactions can only be hinted at here.
Inflammation and immune responses: Arguably the best known of the functions of prostaglandins and thromboxanes in cells is that they modify the inflammatory response, affecting symptoms, such as pain, fever and swelling. It should be recognized that inflammation is an intrinsically beneficial event that leads to removal of offending molecules and restoration of tissue structure and function. The main cause for concern is when acute inflammation fails to resolve and causes excessive tissue damage as occurs in sepsis, in which a prolonged hyperactive immune response is followed by an immunosuppressive stage often leading to high mortality rates. In the early days of prostaglandin research, it was evident that prostaglandins injected into tissues could induce all the symptoms of inflammation. However, it is now recognized that the interactions are complex, and prostanoids can act both in a pro- and anti-inflammatory manner according to the nature of the inflammatory stimulus and the specific prostanoid produced, together with the profile of prostanoid receptors in a given type of cell. For example, EP3 receptors are involved in the development of fever, while EP2 and EP4 function in allergy and bone resorption. Receptor-specific actions of prostaglandins heighten neuronal excitability and so generate and transmit pain signals.
Under normal conditions, prostanoid levels in cells are low, but during inflammation both the nature and concentration of prostanoids can change dramatically. For example, macrophages produce both PGE2 and TXA2, but the ratio changes to an excess of PGE2 with an inflammatory stimulus, for example by enhancement of the release cascade of pro-inflammatory cytokines, such as interleukin (IL)-1, IL-2, and tumor necrosis factor‑α (TNF‑α). In these actions, prostanoids are best viewed as part of complex regulatory networks that modulate the actions of immune cells. PGE2 in particular has potent pro-inflammatory effects and is involved in all the processes leading to the classic signs of inflammation, including inducing fever and enhancing pain. It can cause the transition to chronic inflammation by acting as a cytokine amplifier. PGE2 signalling through its EP2 receptor in mice promotes an energy-deficient state in the brain that drives pro-inflammatory responses leading to cognitive decline, effects that can be reversed by inhibition of myeloid EP2 signalling. There is a particular interest in findings that in its pro-inflammatory role, PGE2 promotes the growth of colorectal tumors (see below), and it is also involved in the pathology of rheumatoid arthritis and in respiratory diseases; with the latter, it can have both positive and negative effects depending upon circumstances.
PGE2 has some anti-inflammatory properties, such as suppressing lymphocyte proliferation and inhibiting the production of certain interleukins and other cytokines. It inhibits the action of 5-lipoxygenase, which is involved in the synthesis of pro-inflammatory leukotrienes, and stimulates the activity of the anti-inflammatory lipoxins. In this manner, PGE2 has a role in initiating the inflammatory response and in its eventual resolution. By stimulating the production of endothelial progenitor cells, PGE2 promotes wound healing, but PGD2 has opposing effects; thromboxanes are also important in this context (see below). Prostaglandins with 15R‑stereochemistry are anti-inflammatory.
Prostaglandin PGF2α has an important pro-inflammatory function, especially in patients with chronic inflammatory diseases such as rheumatoid arthritis. In contrast to its protective role in cardiovascular disease (see below), PGI2 is an important mediator of the oedema and pain that accompany acute inflammation and it is produced rapidly following tissue injury or inflammation. For example, it is the most abundant prostanoid in synovial fluid in human arthritic knee joints. It may contribute to neuropathic pain also.
The high levels of prostanoids found in inflammation are presumed to be due to the recruitment of leukocytes and the induction of the COX-2 enzyme (COX-1 appears to have a minor role only), which then in many tissues produces the pro-inflammatory prostanoids mainly. This explains the interest in COX-2 inhibitors for treating arthritis and other chronic inflammatory diseases. Simplistically, it is believed that COX-2 is pro-inflammatory in the early stages of inflammation, but is beneficial at later stages by generating anti-inflammatory prostanoids. COX-1 derived prostanoids may sustain the inflammatory response. Inhibition of cyclooxygenases also explains the role of non-steroidal drugs, such as aspirin, in reducing the symptoms of fever. In the brain, COX-2 is present in neurons and has been implicated in the progression of Alzheimer's disease.
Immune responses are initiated and coordinated by T lymphocytes, which are influenced by prostanoids in a variety of ways; they appear to modify their development and maturation. Thus, PGE2 inhibits lymphocyte activation and proliferation, while TXA2 has opposing effects. PGE2 has beneficial functions on innate and adaptive immune systems also by regulating immunity and host defense against viral, fungal and bacterial pathogens, but prostanoids produced by such organisms can prolong the effects of infection. Again, the actions of COX-2 (and COX-1) may be the key to triggering antigen-specific inflammation.
Although PGD2 has some pro-inflammatory properties in allergic responses and in brain in the perception of pain, it is recognized to be a key anti-inflammatory prostanoid that may be involved in the resolution of inflammation. It is the principal ligand for two receptors, DP1 and DP2, but it is also an agonist of the thromboxane receptor, TP. Appreciable amounts are found only in the brain and in mast cells (eosinophils). PGD2 exerts significant anti-inflammatory effects in experimental colitis, and its synthesis is elevated rapidly in response to tissue injury to counter the pro-inflammatory effects of PGE2 especially, as well as other chemotaxins. For example, PGD2 decreases food intake, while PGE2 increases it; PGD2 may ameliorate ischemic injury. It has been suggested that because of the similarities in structure of these two prostanoids, they may act as partial agonists of each others' cognate receptors.
Similarly, PGJ2, with Δ12-PGJ2 and the short-lived 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2), the cyclopentenone-containing J‑series of prostaglandins produced by non-enzymatic dehydration of PGD2, are now well established as anti-inflammatory regulators, which function as agonists for PPARγ as discussed briefly above, although they also activate the DP2 receptor. PGA2 produced enzymatically must also be considered in this context. 15d-PGJ2 enters cells and binds to PPARγ, activating it to form heterodimers with retinoid X receptor alpha (RXRα), which bind to specific DNA sequences to activate expression of target genes with functions in lipid metabolism, inflammatory responses and immunity. The J-series of prostaglandins are involved in the immune response as they are produced in antigen-presenting cells such as activated T lymphocytes. For example, 15d-PGJ2 functions in the resolution of the inflammatory response by inducing apoptotic cell death of activated macrophages. As it contains an electrophilic α,β-unsaturated ketone moiety in its cyclopentenone ring, it can act as an endogenous electrophile, which can undergo Michael addition with key cellular nucleophiles such as the free cysteine residues of proteins; covalent modifications of this type may be one mechanism by which it induces many of its biological responses. Thus, cyclopentanone prostanoids are redox regulators of actin and can bind covalently on Cys374 of actin to induce morphological changes to the cytoskeleton of leukocyte, endothelial and muscle cells. Effects on cancer as an inhibitor of tumorigenesis are discussed below.
Polyunsaturated fatty acids of the omega-3 family are known to have anti-inflammatory properties. One explanation is that they inhibit the release of arachidonate from membrane phospholipids for eicosanoid production, or they may compete with arachidonate for the same enzymes of eicosanoid biosynthesis, for example to produce PGE3 from eicosapentaenoic acid (EPA). The 3-series prostanoids derived from EPA have very different biological activities from those of the 2-series, and tend to be much less inflammatory. The protectins, resolvins and maresins ('specialized pro-resolving mediators') must also be considered in this context as they can be effective in bringing about the resolution of inflammation. Similarly, prostaglandins derived from dihomo-γ-linolenic acid (20:3(n-6)), i.e. 1-series prostanoids, have properties distinct from those of the 2-series, and for example, PGE1 has been shown to suppress inflammation and promote vasodilation.
Cardiovascular effects: Two prostanoids are especially important and have essential but opposing functions in the maintenance of vascular homeostasis, i.e. thromboxane TXA2 and prostacyclin PGI2, although other prostaglandins, especially PGE2 and PGD2 are relevant. TXA2 is synthesised mainly in platelets (which express only COX-1), production being enhanced during platelet activation, and acting via the thromboxane receptor TPα, it promotes platelet aggregation, vasoconstriction, and smooth muscle proliferation even though it has a half-life of only 20-30 seconds. This is part of an essential repair mechanism for wound healing, including damaged vessel walls, and via TP receptor signalling is responsible for timely tissue regeneration. The thromboxane metabolite 12‑HHT is especially important for skin regeneration. However, when the damage is too great, blood clots can result with the potential to cause strokes or heart attacks.
In contrast, PGI2 is the main product of macro-vascular endothelial cells. It is produced as required, with a half-life of only about 42 seconds, and it exerts its effects as a potent vasodilator locally mainly through a specific IP receptor but also through the cytosolic nuclear receptor PPARβ. In addition, it inhibits platelet aggregation and smooth muscle cell proliferation and so contributes substantially to myocardial protection. Both TXA2 and PGI2 are therefore important mediators of pathological vascular events including thrombosis and atherogenesis, and it is evident that the correct balance between the two prostanoids is essential to good cardiovascular health.
The ratio of TXA2:PGI2 seems to be more important than the absolute amounts of these mediators that are produced in vivo. In platelets and certain other cells, PGI2 is believed to function by activating adenyl cyclase and elevating cAMP concentrations by stimulating the IP receptor, while TXA2 has the opposite effect. While prostacyclin can exert acute effects that are evident rapidly after adding prostacyclin to a system, it can also exert longer term genomic effects by directing gene transcription. Further relevant factors are increased expression and activation of the TP receptor (for TXA2) in atherosclerotic lesions, which can directly accelerate atherogenesis and plaque growth.
The cardio-protective effect of a low dose of aspirin (81 mg/d) that has been established by clinical trials is exerted by the irreversible long-term inhibition of platelet COX-1 and thence of TXA2 biosynthesis for the lifetime of a platelet in the circulation (aspirin has little effect on PGI synthesis). Indeed, aspirin appears to be the only COX inhibitor with proven cardio-protective activity and demonstrates the causality of eicosanoids in the development of cardiovascular pathophysiologies. It may be relevant that 15(R)-PGD2, produced by aspirin-treated COX-2, inhibits aggregation of human platelets. In contrast, there is some concern that some specific COX-2 inhibitors may have pro-thrombotic effects by inhibiting prostacyclin synthesis relative to that of thromboxanes. In clinical practice, such potential adverse effects of these drugs have to be balanced against positive effects in other tissues since only 1-2% of patients are believed to be at risk. Once more, polyunsaturated fatty acids of the omega-3 family are believed to have beneficial effects via the action of specialized pro-resolving mediators. As inflammation promotes atherogenesis and the associated thrombotic events, there is concern that inflammatory prostaglandins may exert deleterious effects, but PGE2 has been shown to be of benefit against the progression of atheroma plaques.
Lung: PGE2 produced by COX-2 in lung epithelial cells has robust anti-inflammatory and anti-asthmatic effects by activating the EP2 receptor. Similarly, PGI2 signalling through the IP receptor inhibits allergic airway inflammation, and PGI2 analogues are used to treat pulmonary arterial hypertension. The roles of PGD2 and thromboxane A2 are more complex, but they are believed to be mainly pro-inflammatory in relation to asthma; antagonists for the DP2 receptor are in phase III clinical trials for the treatment of asthma.
Gastrointestinal system: COX-1 is always present throughout the human gastrointestinal tract, and produces PGI2 and PGE2, which have protective effects on the gastrointestinal mucosa. Both of these prostanoids reduce acid secretion from parietal cells, while increasing blood flow and stimulating the secretion of mucus. In this instance, the non-steroidal anti-inflammatory drugs, such as aspirin, have negative effects, while the COX-2 inhibitors can be beneficial. On the other hand, these findings are challenged by studies showing that COX-2 is expressed in the intestinal mucosa, and is induced in ulceration, for example, when large amounts of prostaglandins are produced that assist in healing. PGD2 has beneficial effects, as discussed above.
Kidney function: Prostaglandins generated by both COX-1 and COX-2, especially PGE2, assist in the regulation of kidney function by maintaining vascular tone, blood flow, and salt and water excretion. PGE2 is required for the regulation of sodium re-absorption, while PGI2 (and possibly PGE2) increases potassium secretion. In addition, PGI2 with its well-known vasodilatory properties increases renal blood flow and the flow of fluids through the kidney. These actions are again mediated via specific receptors.
Reproductive system: Prostaglandins produced both by COX-1 and COX-2 are involved in many aspects of reproduction in females, from ovulation and fertilization through to labour. They are produced in the fetus and in the placenta as well as in other reproductive tissues. In particular, the synthesis of PGE2 and PGF2α is increased appreciably during labour, and these prostaglandins are in fact used as drugs to induce labour. PGF2α is used to induce ovulation in dairy cows and to induce abortions in women in midtrimester. Increased levels of placental thromboxanes are produced in patients with pre-eclampsia, a disease state during pregnancy that results in high blood pressure and often kidney failure, and the effects can often be ameliorated by the administration of aspirin.
Adipose tissue: Prostaglandins have diverse and opposing roles in adipogenesis and adipose tissue metabolism. For example PGE2 and PGF2α act together to inhibit the differentiation of pre-adipocytes, while PGD2 promotes adipogenesis by acting as a ligand for PPARγ and suppressing lipolysis via its receptor DP2. On the other hand, PGE2 stimulates thermogenesis in beige and brown adipocytes and so influences energy balance. The PGJ2 series activate PPARγ to up-regulate lipid accumulation in adipocytes, while PGI2 has broad effects upon the regulation of the life cycle of adipocytes and impacts upon terminal differentiation.
Cancer: COX-2 is over-expressed in many cancers, including those of the breast, colon and prostate. In particular, PGE2 produced by this enzyme together with the PGE synthase mPGES-1 occurs at much higher concentrations in tumor than in normal tissues, while urinary concentrations of its metabolite PGE-M are considered to be biomarkers for predicting the risk and prognosis for some cancer types. PGE2 has been shown to promote intestinal tumor initiation and growth by silencing certain tumor suppressor and DNA repair genes via DNA methylation. It promotes survival of tumor cells by inhibiting apoptosis and inducing proliferation, and by increasing cell motility and migration. In addition, via its effect on the immune system and inflammation, it has adverse effects in relation to the destruction of tumors. In consequence both the non-steroidal anti-inflammatory drugs, such as aspirin, and the COX-2 inhibitors have been found to have beneficial effects towards some types of cancer. Aspirin reduces the risk of gastrointestinal cancers, for example. Also, it is established that EP1 receptors are involved in chemically induced colon cancer. In contrast, both pro- and anti-tumorigenic activities have been demonstrated for PGD2 depending on the experimental model. Similarly, PGE3, derived from the n-3 eicosapentaenoic acid (EPA), has anti-proliferative activity in various cancers, possibly by interfering with PGE2 activity.
PGF2α appears to promote greater tumor progression and aggressiveness, while thromboxane TXA4 is a pro-carcinogenic mediator that affects a number of tumor cell survival pathways, including cell proliferation, apoptosis and metastasis, and its activity is again balanced by that of prostacyclin PGI2.
15-Deoxy-Δ12,14-PGJ2 (15d-PGJ2), a potent anti-inflammatory regulator that functions via its interaction with PPARγ, also regulates adipogenesis and tumorigenesis and is produced by a variety of cells. An active transport system may carry it to the cells where it is required, and thence it is transported into the nucleus, where it affects gene transcription. Unlike PGE2, 15d-PGJ2 is a potent anti-tumor agent, inhibiting tumor growth both in vitro and in vivo in many tissues. It appears to act in a number of ways, for example directly by inhibiting proliferation and stimulating apoptosis. Also, it can interact indirectly to inhibit migration of tumor cells, and it can affect surrounding cells to reduce the expression of key receptors. However, some experimental conditions have been identified in which it exerts contrary effects. In general, PGE2 and 15d-PGJ2 have profound but opposing effects on tumorigenesis. It is evident that the prostaglandin synthases that are responsible for their biosynthesis are likely to be key targets for the development of anticancer drugs.
Stem cells: Signalling by Wnt proteins, a family of proteolipids containing covalently esterified palmitoleic acid, controls the self-renewal of hematopoietic stem cells and bone marrow repopulation. Activation of this process requires prostaglandin E2, and it has been suggested that the PGE2/Wnt interaction is a master regulator of vertebrate regeneration and recovery in stem cells and other organ systems.
Protein metabolism: γ-Keto aldehydes such as the levuglandins (see above) and isolevuglandins, the latter produced in an analogous manner to the isoprostanes, have a remarkable reactivity towards proteins, forming adducts with greatly modified biological functions. Thus, these di-aldehydes react with lysyl residues on proteins to form first Schiff base adducts and thence pyrrole derivatives, which are able to form intra- and intermolecular protein-protein cross-links. Pyrrole adducts are in turn sensitive to oxygen and are further oxidized in vivo to stable lactam and hydroxylactam products. Protein adducts of this type are not at all easy to analyse, but those in brain have been correlated with the severity of Alzheimer’s disease, for example. Indeed, levuglandins and isolevuglandins are believed to be among the most potent neurotoxic products of lipid oxidation.
Levuglandins also react with phosphatidylethanolamine, which are readily oxidized to hydroxy-lactam derivatives that may be better markers of oxidative injury from a practical standpoint, as they are more easily analysed.
Insects: Prostaglandins have a wide range of downstream signalling functions in insects as in vertebrates, including hormone actions in the fat body and effects upon reproduction, fluid secretion, and the immune response, although little appears to be known of their receptors. As a means of increasing their virulence, some insect pathogens target phospholipase A2 to reduce prostaglandin biosynthesis and bring about immunosuppression.
Parasitic infections: It has been established that a number of parasitic organisms produce prostaglandins in the same way as their mammalian hosts, and by similar enzymic mechanisms. They may play a part in the pathogenesis of parasitic diseases.
6. Some Exotic Prostanoids
Marine invertebrates, including sponges, corals, and molluscs, contain a wide range of prostaglandins, many of which are of the conventional type such as PGE2, PGF2 and so forth. They are presumed to perform similar functions as in mammals, and are also involved in the regulation of oogenesis and spermatogenesis, in ion transport and perhaps as defence compounds. One species of coral (Plexaura homomalla) contains up to 8% of its dry mass as prostanoid esters. However, these have the 15-hydroxyl group in the (R)- rather than the S-configuration, so can have very different biological properties from the conventional prostanoids in that they are potent anti-inflammatory agents. In many marine invertebrates, the prostaglandins exist largely in esterified form or as lactones rather than as the free acids.
In addition, a number of novel prostanoids have been discovered in corals, some examples of which are illustrated above, which differ in stereochemistry from the typical prostanoids, or contain acetyl groups, or are substituted with halogen atoms, such as chlorine or bromine. Little is known of the biochemistry or function of the "clavulones, bromovulones or punaglandins" in marine organisms, but there is increasing interest in them because of reported anti-tumor activities.
It is perhaps more surprising that some red algae (seaweeds) such as Gracilaria species contain prostaglandins (PGE2, PGF2α and others) and are known to have a cyclooxygenase gene. In Gracilaria vermiculophylla, PGG2 is first synthesised from arachidonate by a cyclooxygenase, and this is converted to 15‑hydroperoxy-PGE2, which can then react either enzymatically or non-enzymatically to generate PGE2 or 15-keto-PGE2; a similar but rather minor pathway has since been discovered in animals. Why these organisms produce such oxylipins is not known, but they may have a defensive function as they are believed to be causative toxins for lethal food poisonings that have occurred in Japan.
7. Prostanoid Analysis
Analysis of prostanoids is not a simple task because they occur at such low levels in tissues and because of their high reactivity. Extraction must be carried out under mild conditions as rapidly as possible, and solid-phase extraction methods are now available that set the standard for isolation as a class. Subsequent analysis usually involves HPLC linked to mass spectrometry. Chiral chromatography can be used to distinguish between prostaglandins and isoprostanes. Immunoassays are available that may be suitable for some clinical applications, but they are not sensitive to minor differences in prostanoid structure.
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