Goodman and Gilman Manual of Pharmacology and Therapeutics
Inflammation, Immunomodulation, and Hematopoiesis
Lipid-Derived Autacoids: Eicosanoids and Platelet-Activating Factor
Membrane lipids supply the substrate for the synthesis of eicosanoids and platelet-activating factor (PAF). Eicosanoids—arachidonate metabolites, including prostaglandins (PGs),prostacyclin (PGI2), thromboxane A2 (TxA2), leukotrienes (LTs), lipoxins, and hepoxilins—are not stored but are produced by most cells when a variety of physical, chemical, and hormonal stimuli activate acyl hydrolases that make arachidonate available. Membrane glycerophosphocholine derivatives can be modified enzymatically to produce PAF. PAF is formed by a smaller number of cell types, principally leukocytes, platelets, and endothelial cells. Eicosanoids and PAF lipids contribute to inflammation, smooth muscle tone, hemostasis, thrombosis, parturition, and gastrointestinal secretion. Several classes of drugs, most notably aspirin, the traditional nonsteroidal anti-inflammatory agents (tNSAIDs), and the specific inhibitors of cyclooxygenase-2 (COX-2), such as the coxibs, owe their principal therapeutic effects to blockade of eicosanoid formation.
PGs, LTs, and related compounds are called eicosanoids, from the Greek eikosi (“twenty”). Precursor essential fatty acids contain 20 carbons and 3, 4, or 5 double bonds. Arachidonic acid (AA; 5,8,11,14-eicosatetraenoic acid) is the most abundant precursor, derived from dietary linoleic acid (9,12-octadecadienoic acid) or ingested directly as a dietary constituent.
BIOSYNTHESIS. Biosynthesis of eicosanoids is limited by the availability of AA and depends primarily on the removal of esterified AA from membrane phospholipids or other complex lipids by acyl hydrolases, notably phospholipase A2 (PLA2). Once liberated, AA is metabolized rapidly to oxygenated products by cyclooxygenases (COXs), lipoxygenases(LOXs), and CYPs (Figure 33–1).
Figure 33–1 Metabolism of arachidonic acid (AA). The cyclooxygenase (COX) pathway is highlighted in gray. The lipoxygenase (LOX) pathways are expanded in Figure 33–2. Major degradation pathways are shown in Figure 33–3. Cyclic endoperoxides (PGG2 and PGH2) arise from the sequential cyclooxygenase and hydroperoxidase actions of COX-1 or COX-2 on AA released from membrane phospholipids. Subsequent products are generated by tissue-specific synthases and transduce their effects via membrane-bound receptors (blue boxes). Dashed lines indicate putative ligand-receptor interactions. Epoxyeicosatrienoic acids (EETs; shaded in blue) and isoprostanes are generated via CYP activity and non-enzymatic free radical attack, respectively. COX-2 can use modified arachidonoylglycerol, an endocannabinoid, to generate the glyceryl prostaglandins. Aspirin and tNSAIDs are nonselective inhibitors of COX-1 and COX-2 but do not affect LOX activity. Epilipoxins are generated by COX-2 following its acetylation by aspirin (see Figure 33–2). Dual 5-LOX-COX inhibitors interfere with both pathways. See the text for other abbreviations.
Chemical and physical stimuli activate the Ca2+-dependent translocation of group IVA cytosolic PLA2 (cPLA2) to the membrane, where it hydrolyzes the sn-2 ester bond of membrane phosphatidylcholine and phosphatidylethanolamine, releasing AA. Multiple additional PLA2 isoforms (secretory [s] and Ca2+-independent [i] forms) have been characterized. Under basal conditions, AA liberated by iPLA2 is reincorporated into cell membranes. The inducible sPLA2 contributes to AA release under conditions of sustained or intense stimulation of AA production.
PRODUCTS OF PROSTAGLANDIN G/H SYNTHASES (CYCLOOXYGENASES). PG endoperoxide G/H synthase also is called cyclooxygenase or COX. Products of this pathway are PGs, prostacyclin (PGI2), and thromboxanes (TX2), collectively termed prostanoids. The pathway is well described by Figure 33–1 and its legend.
Prostanoids are distinguished by substitutions on their cyclopentane rings the number of double bonds in their side chains, as indicated by numerical subscripts (dihomo-γ-linolenic acid is the precursor of series1, AA for series2, and EPA for series3). Prostanoids derived from AA carry the subscript 2 and are the major series in mammals.
There are 2 distinct COX isoforms, COX-1 and COX-2. COX-1, expressed constitutively in most cells, is the dominant source of prostanoids for housekeeping functions. COX-2, in contrast, is upregulated by cytokines, shear stress, and growth factors and is the principal source of prostanoid formation in inflammation and cancer. However, this distinction is not absolute; both enzymes may contribute to the generation of prostanoids of some physiologic and pathophysiologic processes. These enzymes are expressed in a relatively cell-specific fashion. For example, COX-1-derived TxA2 is the dominant product in platelets, whereas COX-2-derived PGE2 and TxA2 dominate in activated macrophages. Prostanoids are released from cells predominantly by facilitated transport through the PG transporter and possibly other transporters.
LIPOXYGENASE (LOX) PRODUCTS. Products of the LOX pathways are hydroxy fatty acid derivatives (HETEs), LTs, and lipoxins (LXs) (Figure 33–2). LTs play a major role in the development and persistence of the inflammatory response.
Figure 33–2 Lipoxygenase pathways of arachidonic acid metabolism. 5-LOX-activating protein (FLAP) presents arachidonic acid to 5-LOX, leading to the generation of the LTs. CysLTs are shaded in gray. Lipoxins (shaded in orange) are products of cellular interaction via a 5-LOX-12-LOX pathway or via a 15-LOX-5-LOX pathway. Biological effects are transduced via membrane-bound receptors (blue boxes). The dashed line indicates putative ligand–receptor interactions. Zileuton inhibits 5-LOX but not the COX pathways (expanded in Figure 33–1). Dual 5-LOX-COX inhibitors interfere with both pathways. CysLT antagonists prevent activation of the CysLT1 receptor. See the text for abbreviations.
LOXs are a family of non-heme iron–containing enzymes that catalyze the oxygenation of polyenic fatty acids to corresponding lipid hydroperoxides. The enzymes require a fatty acid substrate with 2 cis double bonds separated by a methylene group. AA is metabolized to hydroperoxy eicosatetraenoic acids (HPETEs). HPETEs are converted to HETEs and leukotrienes.
The 5-LOX pathway leads to the synthesis of the LTs. When eosinophils, mast cells, polymorphonuclear leukocytes, or monocytes are activated, 5-LOX translocates to the nuclear membrane and associates with 5-LOX-activating protein (FLAP), an integral membrane protein that facilitates AA to 5-LOX interaction. Drugs that inhibit FLAP block LT production. A 2-step reaction is catalyzed by 5-LOX: oxygenation of AA to form 5-HPETE, followed by dehydration of 5-HPETE to an unstable epoxide, LTA4. LTA4 is transformed by LTA4 hydrolase to LTB4; or is conjugated with GSH by LTC4 synthase to form LTC4. Extracellular metabolism of the peptide moiety of LTC4 and the removal of glutamic acid and subsequent cleavage of glycine generates LTD4 and LTE4, respectively. LTC4, LTD4, and LTE4 are the cysteinyl leukotrienes (CysLTs). LTB4 and LTC4 are actively transported out of the cell. LTA4, the primary product of the 5-LOX pathway, is metabolized by 12-LOX to form the lipoxins LXA4 and LXB4. These mediators also can arise through 5-LOX metabolism of 15-HETE.
PRODUCTS OF CYPs. AA is metabolized to epoxyeicosatrienoic acids (EETs) by CYP epoxygenases, primarily CYP2C and CYP2J. EETs are synthesized in endothelial cells, where they function as endothelium-derived hyperpolarizing factors (EDHFs), particularly in the coronary circulation. EETs biosynthesis can be altered by pharmacological, nutritional, and genetic factors that affect CYP expression.
Other Pathways. The isoeicosanoids, a family of eicosanoid isomers, are generated by nonenzymatic free radical catalyzed oxidation of AA. Unlike PGs, these compounds are initially formed esterified in phospholipids and released by phospholipases; the isoeicosanoids then circulate and are metabolized and excreted into urine. Their production is not inhibited in vivo by inhibitors of COX-1 or COX-2, but their formation is suppressed by antioxidants. Isoprostanes correlate with cardiovascular risk factors and increased levels are found in a large number of clinical conditions.
INHIBITORS OF EICOSANOID BIOSYNTHESIS. Inhibition of PLA2 decreases the release of the precursor fatty acid and the synthesis of all its metabolites. PLA2 may be inhibited by drugs that reduce the availability of Ca2+. Glucocorticoids inhibit PLA2 indirectly by inducing the synthesis of a group of proteins termed annexins that modulate PLA2 activity. Glucocorticoids also downregulate induced expression of COX-2 but not of COX-1 (see Chapter 42). Aspirin and tNSAIDs inhibit the COX, but not the hydroperoxidase (HOX), moieties of both PG G/H synthases, and thus the formation of their downstream prostanoid products. In addition, these drugs do not inhibit LOXs and may cause increased formation of LTs by shunting of substrate to the LOX pathway. LTs may contribute to the GI side effects associated with NSAIDs.
COX-1 and COX-2 differ in their sensitivity to inhibition by certain anti-inflammatory drugs. This led to the development of selective inhibitors of COX-2, including the coxibs (seeChapter 34). These drugs were hypothesized to offer therapeutic advantages over tNSAIDs (many of which are nonselective COX inhibitors) because COX-2 is the predominant COX at sites of inflammation, whereas COX-1 is the major source of cytoprotective PGs in the GI tract. There now is compelling evidence that COX-2 inhibitors confer a spectrum of cardiovascular hazards (myocardial infarction, stroke, systemic and pulmonary hypertension, congestive heart failure, and sudden cardiac death). The hazards can be explained by suppression of cardioprotective COX-2-derived PGs, especially PGI2, and the unrestrained effects of endogenous stimuli, such as platelet COX-1-derived TxA2, for platelet activation, vascular proliferation and remodeling, hypertension, and atherogenesis.
Because LTs mediate inflammation, efforts have focused on development of LT-receptor antagonists and selective inhibitors of the LOXs. Zileuton, an inhibitor of 5-LOX, and selective CysLT-receptor antagonists (zafirlukast, pranlukast, and montelukast) have established efficacy in the treatment of mild to moderate asthma (see Chapter 36). A common polymorphism in the gene for LTC4 synthase that correlates with increased LTC4 generation is associated with aspirin-intolerant asthma and with the efficacy of anti-LT therapy. Interestingly, although polymorphisms in the genes encoding 5-LOX or FLAP do not appear to be linked to asthma, studies have demonstrated an association of these genes with myocardial infarction, stroke, and atherosclerosis; thus, inhibition of LT biosynthesis may be useful in the prevention of cardiovascular disease.
EICOSANOID DEGRADATION (FIGURE 33–3). Most eicosanoids are efficiently and rapidly inactivated. The enzymatic catabolic reactions are of 2 types: a rapid initial step, catalyzed by widely distributed PG-specific enzymes, wherein PGs lose most of their biological activity; and a second step in which these metabolites are oxidized, probably by enzymes identical to those responsible for the β and ω oxidation of fatty acids. The lung, kidney, and liver play prominent roles in the enzymatically catalyzed reactions. PGI2 and TxA2 undergo spontaneous hydrolysis as a first degradative step.
Figure 33–3 Major pathways of prostanoid degradation. Active metabolites are shaded in gray. Major urinary metabolites are shaded in orange. The red dashed lines indicate reactions that use common enzymatic processes. M, metabolite. See the text for other abbreviations.
The eicosanoids function through activation of specific GPCRs that couple to intracellular second-messenger systems to modulate cellular activity (Table 33–1 and Figure 33–4).
Figure 33–4 Prostanoid receptors and their primary signaling pathways. Prostanoid receptors are heptaspanning GPCRs. The terms “relaxant,” “contractile,” and “inhibitory” refer to the phylogenetic characterization of their primary effects. **All EP3 isoforms couple through Gi; some can also activate Gs or G12/13 pathways. See the text for additional details.
Prostaglandin Receptors. PGs activate membrane receptors locally near their sites of formation. Eicosanoid receptors interact with Gs, Gi, and Gq to modulate the activities of adenylyl cyclase and phospholipase C (see Chapter 3). Single gene products have been identified for the receptors for PGI2 (the IP), PGF2α (the FP), and TxA2 (the TP). Four distinct PGE2 receptors (EP1-4) and 2 PGD2 receptors (DP1 and DP2—also known as CRTH2) have been cloned. Additional isoforms of the TP (α and β), FP (A and B), and EP3 (I-VI, e, f) receptors can arise through differential mRNA splicing.
The prostanoid receptors appear to derive from an ancestral EP receptor and share high homology. Phylogenetic comparison of this receptor family reveals 3 subclusters:
• The relaxant receptors EP2, EP4, IP, and DP1, which increase cellular cyclic AMP generation
• The contractile receptors EP1, FP, and TP, which increase cytosolic levels of Ca2+
• EP3, which can couple to both elevation of cytosolic [Ca2+] and inhibition of adenylyl cyclase
The DP2 receptor is an exception and is unrelated to the other prostanoid receptors; rather, it is a member of the formyl-methionyl-leucyl-phenylalanine (fMLP)-receptor superfamily.
Leukotriene and Lipoxin Receptors. Two receptors exist for both LTB4 (BLT1 and BLT2) and the cysteinyl leukotrienes (CysLT1 and CysLT2). A receptor that binds lipoxin, ALX, is identical to the fMLP-1 receptor; the nomenclature now reflects LXA4 as a natural and potent ligand. All are GPCRs and couple with Gq and other G proteins, depending on the cellular context. BLT1 is expressed predominantly in leukocytes, thymus, and spleen, whereas BLT2, the low-affinity receptor for LTB4, is found in spleen, leukocytes, ovary, liver, and intestine. CysLT1 binds LTD4 with higher affinity than LTC4, while CysLT2 shows equal affinity for both LTs. Both receptors bind LTE4 with low affinity. CysLT1 is expressed in lung and intestinal smooth muscle, spleen, and peripheral blood leukocytes, whereas CysLT2 is found in heart, spleen, peripheral blood leukocytes, adrenal medulla, and brain. Responses to ALX-receptor activation vary with cell type. The ALX receptor is expressed in lung, peripheral blood leukocytes, and spleen.
Other Agents. Other AA metabolites (e.g., isoprostanes, epoxyeicosatrienoic acids, hepoxilins) have potent biological activities, and there is evidence for distinct receptors for some of these substances. Specific receptors for the HETEs and EETs have been proposed but not yet isolated.
CARDIOVASCULAR SYSTEM. In most vascular beds, PGE2, PGI2, and PGD2 elicit vasodilation and a drop in blood pressure; physiologically, these responses are quite local, since endogenous prostanoids are paracrine mediators that do not circulate. Responses to PGF2α vary with vascular bed; it is a potent constrictor of both pulmonary arteries and veins; however, it does not alter blood pressure in humans. TxA2 is a potent vasoconstrictor and a mitogen in smooth muscle cells.
PGE2 can cause vasoconstriction through activation of the EP1 and EP3 receptors. Infusion of PGD2 in humans results in flushing, nasal stuffiness, and hypotension. Local subcutaneous release of PGD2 contributes to dilation of the vasculature in the skin, which causes facial flushing associated with niacin treatment in humans. Subsequent formation of F-ring metabolites from PGD2 may result in hypertension. PGI2 relaxes vascular smooth muscle, causing hypotension and reflex tachycardia on intravenous administration. LTC4 and LTD4 can constrict or relax isolated vascular smooth muscle preparations, depending on the concentrations used and the vascular bed. The renal vasculature is resistant to this constrictor action, but the mesenteric vasculature is not. LTC4 and LTD4 act in the microvasculature to increase permeability of postcapillary venules; they are ~1,000-fold more potent than histamine in this regard. At higher concentrations, LTC4 and LTD4 can constrict arterioles and reduce exudation of plasma. EETs cause vasodilation in a number of vascular beds by activating the large conductance Ca2+-activated K+ channels of smooth muscle cells, thereby hyperpolarizing the smooth muscle and causing relaxation. EETs likely also function as EDHFs. Isoprostanes usually are vasoconstrictors, although there are examples of vasodilation in preconstricted vessels.
PLATELETS. Mature platelets express only COX-1. TxA2, the major product of COX-1 in platelets, induces platelet aggregation and amplifies the signal for other, more potent platelet agonists, such as thrombin and ADP. Low concentrations of PGE2, via the EP3, enhance platelet aggregation. In contrast, higher concentrations of PGE2, acting via the IP or possibly EP2 or EP4 receptors, inhibit platelet aggregation. Both PGI2 and PGD2 inhibit the aggregation of platelets. PGI2 limits platelet activation by TxA2, and disaggregates preformed clumps.
TxA2 induces platelet shape change, through G12/G13-mediated Rho/Rho-kinase-dependent regulation of myosin light-chain phosphorylation, and aggregation through Gq-dependent activation of PKC. The actions of TxA2 on platelets are restrained by its short t1/2 (~30 sec), by rapid TP desensitization, and by endogenous inhibitors of platelet function, including NO and PGI2.
INFLAMMATION AND IMMUNITY. Eicosanoids play a major role in the inflammatory and immune responses. LTs generally are pro-inflammatory and lipoxins anti-inflammatory. Prostanoids can exert both kinds of activity. COX-2 is the major source of prostanoids formed during and after an inflammatory response.
PGE2 and PGI2 are the predominant pro-inflammatory prostanoids, as a result of increased vascular permeability and blood flow in the inflamed region. TxA2 can increase platelet–leukocyte interaction. Prostanoids, especially PGD2, also contribute to resolution of inflammation. PGs generally inhibit lymphocyte function and proliferation, suppressing the immune response. PGE2 depresses the humoral antibody response by inhibiting the differentiation of B lymphocytes into antibody-secreting plasma cells. PGE2 acts on T lymphocytes to inhibit mitogen-stimulated proliferation and lymphokine release by sensitized cells. PGE2 and TxA2 also may play a role in T lymphocyte development by regulating apoptosis of immature thymocytes. PGD2 is a potent leukocyte chemoattractant primarily through the DP2.
LTB4 is a potent chemotactic agent for neutrophils, T lymphocytes, eosinophils, monocytes, dendritic cells, and possibly also mast cells. LTB4 stimulates the aggregation of eosinophils and promotes degranulation and the generation of superoxide. LTB4 promotes adhesion of neutrophils to vascular endothelial cells and their transendothelial migration and stimulates synthesis of pro-inflammatory cytokines from macrophages and lymphocytes. The CysLTs are chemotaxins for eosinophils and monocytes. They also induce cytokine generation in eosinophils, mast cells, and dendritic cells. At higher concentrations, these LTs also promote eosinophil adherence, degranulation, cytokine or chemokine release, and oxygen radical formation. In addition, CysLTs contribute to inflammation by increasing endothelial permeability, thus promoting migration of inflammatory cells to the site of inflammation. Lipoxins A and B inhibit natural killer cell cytotoxicity.
BRONCHIAL AND TRACHEAL MUSCLE. In general, TxA2, PGF2α, and PGD2 contract, and PGE2 and PGI2 relax, bronchial and tracheal muscle. PGD2 appear to be the primary bronchoconstrictor prostanoid of relevance in humans.
Roughly 10% of people given aspirin or tNSAIDs develop bronchospasm. This appears attributable to a shift in AA metabolism to LT formation. This substrate diversion appears to involve COX-1, not COX-2. CysLTs are bronchoconstrictors that act principally on smooth muscle in the airways and are a thousand times more potent than histamine. They also stimulate bronchial mucus secretion and cause mucosal edema. PGI2 causes bronchodilation in most species; human bronchial tissue is particularly sensitive, and PGI2 antagonizes bronchoconstriction induced by other agents.
UTERUS. Strips of nonpregnant human uterus are contracted by PGF2α and TxA2 but are relaxed by PGEs. PGE2, together with oxytocin, is essential for the onset of parturition. PGI2 and high concentrations of PGE2 produce relaxation. The intravenous infusion of low concentrations of PGE2 or PGF2α to pregnant women produces a dose-dependent increase in uterine tone and in the frequency and intensity of rhythmic uterine contractions. PGEs and PGFs are used to terminate pregnancy.
GI SMOOTH MUSCLE. PGEs and PGFs stimulate contraction of the main longitudinal muscle from stomach to colon. Circular muscle generally relaxes in response to PGE2 and contracts in response to PGF2α. The LTs have potent contractile effects. Diarrhea, cramps, and reflux of bile have been noted in response to oral PGE. PGEs and PGFs stimulate the movement of water and electrolytes into the intestinal lumen. PGE2 appears to contribute to the water and electrolyte loss in cholera, a disease that is somewhat responsive to therapy with tNSAIDs.
GASTRIC AND INTESTINAL SECRETIONS. In the stomach, PGE2 and PGI2 contribute to increased mucus secretion (cytoprotection), reduced acid secretion, and reduced pepsin content. PGE2 and its analogs also inhibit gastric damage caused by a variety of ulcerogenic agents and promote healing of duodenal and gastric ulcers (see Chapter 45). CysLTs, by constricting gastric blood vessels and enhancing production of pro-inflammatory cytokines, may contribute to the gastric damage.
KIDNEY. COX-2-derived PGE2 and PGI2 increase medullary blood flow and inhibit tubular sodium reabsorption. Expression of medullary COX-2 is increased during high salt intake. COX-1-derived products promote salt excretion in the collecting ducts. Cortical COX-2-derived PGE2 and PGI2 increase renal blood flow and glomerular filtration through their local vasodilating effects. There is an added layer of complexity: low dietary salt intake increases expression of cortical COX-2 expression. Through the action of PGE2, and also possibly PGI2, this results in increased renin release, leading to sodium retention and elevated blood pressure.
TxA2, generated at low levels in the normal kidney, has potent vasoconstrictor effects that reduce renal blood flow and glomerular filtration rate. Infusion of PGF2α causes both natriuresis and diuresis. Conversely, PGF2α may activate the renin–angiotensin system, contributing to elevated blood pressure. There is substantial evidence for a role of the CYP epoxygenase products in regulating renal function, although their exact role in the human kidney remains unclear. Both 20-HETE and the EETs are generated in renal tissue. 20-HETE constricts the renal arteries, while EETs mediate vasodilation and natriuresis.
EYE. PGF2α induces constriction of the iris sphincter muscle, but its overall effect in the eye is to decrease intraocular pressure (IOP) by increasing the aqueous humor outflow. A variety of FP receptor agonists have proven effective in the treatment of open-angle glaucoma, a condition associated with the loss of COX-2 expression in the pigmented epithelium of the ciliary body (see Chapter 64).
CNS. PGE2 induces fever.
The hypothalamus regulates the body temperature set point, which is elevated by endogenous pyrogens. The response is mediated by coordinate induction of COX-2 and mPGE synthase-1 in the endothelium of blood vessels in the preoptic hypothalamic area to form PGE2. PGE2 acts on EP3, and perhaps EP1, on thermosensitive neurons. This triggers the hypothalamus to elevate body temperature. Exogenous PGF2α and PGI2 induce fever but do not contribute to the pyretic response. PGD2 and TxA2 do not induce fever. PGD2 also appears to act on arachnoid trabecular cells in the basal forebrain to mediate an increase in extracellular adenosine that, in turn, facilitates induction of sleep. COX-2-derived prostanoids also have been implicated in several CNS degenerative disorders (e.g., Alzheimer disease, Parkinson disease; see Chapter 22).
PAIN. Inflammatory mediators, including LTs and PGs, increase the sensitivity of nociceptors and potentiate pain perception.
Centrally, both COX-1 and COX-2 are expressed in the spinal cord under basal conditions and release PGs in response to peripheral pain stimuli. PGE2, and perhaps PGD2, PGI2, and PGF2α, can increase excitability in pain transmission neuronal pathways in the spinal cord, causing hyperalgesia and allodynia. Hyperalgesia also is produced by LTB4. The role of PGE2 and PGI2 in inflammatory pain is discussed in more detail in Chapter 34.
ENDOCRINE SYSTEM. The systemic administration of PGE2 increases circulating concentrations of adrenocorticotropic hormone (ACTH), growth hormone, prolactin, and gonadotropins. Other effects include stimulation of steroid production by the adrenals, stimulation of insulin release, and thyrotropin-like effects on the thyroid. The critical role of PGF2α in parturition relies on its ability to induce an oxytocin-dependent decline in progesterone levels. PGE2 works as part of a positive-feedback loop to induce oocyte maturation required for fertilization during and after ovulation. LOX metabolites also have endocrine effects. 12-HETE stimulates the release of aldosterone from the adrenal cortex and mediates a portion of the aldosterone release stimulated by AngII, but not that which occurs in response to ACTH.
BONE. PGs are strong modulators of bone metabolism. COX-1 is expressed in normal bone, while COX-2 is upregulated in settings such as inflammation and during mechanical stress. PGE2 stimulates bone formation by increasing osteoblastogenesis and bone resorption via activation of osteoclasts.
INHIBITORS AND ANTAGONISTS. The nonselective tNSAIDs, and those with selective COX-2 inhibition, are used widely as anti-inflammatory drugs, whereas low-dose aspirin is employed frequently for cardioprotection. LT antagonists are useful clinically in the treatment of asthma, and FP agonists are used in the treatment of open-angle glaucoma (seeChapter 64). EP agonists are used to induce labor and to ameliorate gastric irritation owing to tNSAIDs. DP1 antagonists may be useful in offsetting the facial flushing associated with niacin. Orally active antagonists of LTC4 and D4, which block the CysLT1 receptor, are used in the treatment of mild to moderately severe asthma (see Chapter 36). Their effectiveness in patients with aspirin-induced asthma also has been shown. Prostanoids have a short t1/2 in the circulation and their systemic administration produces significant adverse effects. Nonetheless, several prostanoids are of clinical utility in the following situations.
Therapeutic Abortion. PGEs, PGFs, and their analogs, are used to induce labor and terminate pregnancy at any stage by promoting uterine contractions. Dinoprostone, a synthetic preparation of PGE2, is approved for inducing abortion in the second trimester of pregnancy, for missed abortion, for cervical ripening prior to induction of labor, and for managing benign hydatidiform moles. Systemic or intravaginal administration of the PGE1 analog misoprostol in combination with mifepristone (RU486) or methotrexate is highly effective in the termination of early pregnancy. An analog of PGF2α, carboprost tromethamine, is used to induce second-trimester abortions and to control postpartum hemorrhage that is not responding to conventional methods.
Gastric Cytoprotection. Several PG analogs are used to suppress gastric ulceration. Misoprostol (CYTOTEC), a PGE1 analog, is approved for prevention of NSAID-induced gastric ulcers.
Impotence. PGE1 (alprostadil), given as an intracavernous injection (CAVERJECT, EDEX) or urethral suppository (MUSE) is a second-line treatment of erectile dysfunction, PDE5 inhibitors being preferred (see Chapters 27 and 28).
Maintenance of Patent Ductus Arteriosus. The ductus arteriosus in neonates is highly sensitive to vasodilation by PGE1. PGE1 (alprostadil, PROSTIN VR PEDIATRIC) is highly effective for palliative therapy to maintain temporary patency until surgery can be performed.
Pulmonary Hypertension. Long-term therapy with PGI2 (prostacyclin; epoprostenol, FLOLAN), via continuous intravenous infusion, improves symptoms and can delay or preclude the need for lung or heart-lung transplantation in a number of patients. Several PGI2 analogs with longer t1/2 have been used clinically. Iloprost can be inhaled (VENTAVIS) or delivered by intravenous administration (not available in the U.S.). Treprostinil (REMODULIN) (t1/2 ~4 h) may be delivered by continuous subcutaneous or intravenous infusion.
Glaucoma. Latanoprost, a PGF2α derivative, was the first prostanoid used for glaucoma. Similar prostanoids with ocular hypotensive effects include bimatoprost and travoprost. These drugs act as agonists at the FP receptor and are administered as ophthalmic drops (see Chapter 64).
PAF is 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. PAF represents a family of phospholipids because the alkyl group at position 1 can vary in length from 12-18 carbon atoms. In human neutrophils, PAF consists predominantly of a mixture of the 16- and 18-carbon ethers, but its composition may change when cells are stimulated. PAF is not stored in cells but is synthesized from an acyl precursor in response to stimulation by a 2-step process (Figure 33–5).
Figure 33–5 Synthesis and degradation of platelet-activating factor (PAF). PAF synthesis occurs in 2 steps. In the first step, activated PLA2 cleaves membrane 1-O-alkyl-2-acyl-glycerophosphocholine to form lyso-PAF and a free fatty acid (usually AA that may be metabolized to eicosanoids). In the rate-limiting second step, PAF is formed from lyso-PAF by an acetyl CoA-lyso-PAF acetyltransferase. The synthesis of PAF may be stimulated during antigen–antibody reactions or by a variety of agents, including chemotactic peptides, thrombin, collagen, and other autacoids; PAF also can stimulate its own formation. PAF synthesis is regulated by the availability of Ca2?. PAF is degraded by the reversal of the synthetic steps, de-acetylation by acetylhydrolases (AHs) followed by acylation at the 2 position to regenerate a 1-O-alkyl-2-acyl-glycerophosphocholine. PAF synthesis also can occur de novo: a phosphocholine substituent is transferred to alkyl acetyl glycerol by a distinct lyso-glycerophosphate acetyl-CoA transferase. CoA, coenzyme A.
PAF is synthesized by platelets, neutrophils, monocytes, mast cells, eosinophils, renal mesangial cells, renal medullary cells, and vascular endothelial cells. Depending on cell type, PAF can either remain cell-associated or be secreted. For example, PAF is released from monocytes but retained by leukocytes and endothelial cells. In endothelial cells, PAF is displayed on the surface for juxtacrine signaling and stimulates adherent leukocytes. PAF-like molecules can be formed from the oxidative fragmentation of membrane phospholipids (oxPLs). These compounds are increased in settings of oxidant stress, such as cigarette smoking, and differ structurally from PAF in that they contain a fatty acid at the sn-1 position of glycerol joined through an ester bond and various short-chain acyl groups at the sn-2 position. oxPLs mimic the structure of PAF closely enough to bind to its receptor and elicit the same responses. Increased levels of plasma PAF acyl hydrolase (PAF-AH) have been associated with colon cancer, cardiovascular disease, and stroke.
MECHANISM OF ACTION OF PAF. Extracellular PAF exerts its actions by stimulating a specific GPCR. The PAF receptor couples to Gq (to activate the PLC-IP3–Ca2+ pathway) and to Gi (to inhibit adenylyl cyclase). Consequent activation of phospholipases A2, C, and D gives rise to myriad messengers, including AA-derived PGs, TxA2, or LTs, which may function as mediators of the effects of PAF.
In addition, p38 MAP kinase is activated downstream of the PAF-receptor- Gq interaction, while ERK activation can occur via interaction of activated PAF receptor with Gq, Go, or Gβγ, or via transactivation of the EGF receptor, leading to NF-κB activation. PAF exerts many of its important pro-inflammatory actions without leaving its cell of origin. For example, PAF is synthesized in a regulated fashion by endothelial cells stimulated by inflammatory mediators. This PAF is presented on the surface of the endothelium, where it activates the PAF receptor on juxtaposed cells, including platelets, polymorphonuclear leukocytes, and monocytes, and acts cooperatively with P selectin to promote adhesion. This function of PAF is important for orchestrating the interaction of platelets and circulating inflammatory cells with the inflamed endothelium.
PHYSIOLOGICAL AND PATHOLOGICAL FUNCTIONS OF PAF
Inflammatory and Allergic Responses. The administration of PAF reproduces many of the signs and symptoms in anaphylactic shock. However, the effects of PAF antagonists in the treatment of inflammatory and allergic disorders have been disappointing. In patients with asthma, PAF antagonists partially inhibit the bronchoconstriction induced by antigen challenge but not by challenges by methacholine, exercise, or inhalation of cold air.
Cardiovascular System. PAF is a potent vasodilator in most vascular beds; when administered intravenously, it causes hypotension. PAF-induced vasodilation is independent of effects on sympathetic innervation, the renin–angiotensin system, or AA metabolism and likely results from a combination of direct and indirect actions. PAF may, alternatively, induce vasoconstriction depending on the concentration, vascular bed, and involvement of platelets or leukocytes. Intradermal injection of PAF causes an initial vasoconstriction followed by a typical wheal and flare. PAF increases vascular permeability and edema in the same manner as histamine and bradykinin. The increase in permeability is due to contraction of venular endothelial cells, but PAF is more potent than histamine or bradykinin by 3 orders of magnitude.
Platelets. The PAF receptor is constitutively expressed on the surface of platelets. PAF potently stimulates platelet aggregation in vitro and in vivo. Although this is accompanied by the release of TxA2 and the granular contents of the platelet, PAF does not require the presence of TxA2 or other aggregating agents to produce this effect. The intravenous injection of PAF causes formation of intravascular platelet aggregates and thrombocytopenia.
Leukocytes. PAF is a potent and common activator of inflammatory cells. PAF stimulates a variety of responses in polymorphonuclear leukocytes (eosinophils, neutrophils, and basophils). PAF stimulates PMNs to aggregate, degranulate, and generate free radicals and LTs. PAF is a potent chemotactic for eosinophils, neutrophils, and monocytes and promotes PMN-endothelial adhesion contributing, along with other adhesion molecular systems, to leukocyte rolling, tight adhesion, and migration through the endothelial monolayer. PAF also stimulates basophils to release histamine, activates mast cells, and induces cytokine release from monocytes. In addition, PAF promotes aggregation of monocytes and degranulation of eosinophils.
Smooth Muscle. PAF contracts GI, uterine, and pulmonary smooth muscle. PAF enhances the amplitude of spontaneous uterine contractions; these contractions are inhibited by inhibitors of PG synthesis. PAF does not affect tracheal smooth muscle but contracts airway smooth muscle. When given by aerosol, PAF increases airway resistance as well as the responsiveness to other bronchoconstrictors. PAF also increases mucus secretion and the permeability of pulmonary microvessels.
Stomach. PAF is the most potent known ulcerogen. When given intravenously, it causes hemorrhagic erosions of the gastric mucosa that extend into the submucosa.
Kidney. PAF decreases renal blood flow, glomerular filtration rate, urine volume, and excretion of Na+ without changes in systemic hemodynamics. PAF exerts a receptor-mediated biphasic effect on afferent arterioles, dilating them at low concentrations and constricting them at higher concentrations. The vasoconstrictor effect appears to be mediated, at least in part, by COX products, whereas vasodilation is a consequence of the stimulation of NO production by endothelium.
Other. PAF, a potent mediator of angiogenesis, has been implicated in breast and prostate cancer. PAF-AH deficiency has been associated with small increases in a range of cardiovascular and thrombotic diseases in some human populations.
PAF Receptor Antagonists. Several experimental PAF-receptor antagonists exist that selectively inhibit the actions of PAF in vivo and in vitro. None has proven clinically useful.