Lipid-Derived Autocoids Eicosanoids and Platelet-Activating Factor

Lipid-Derived Autocoids Membrane lipids supply the substrate for the synthesis of eicosonoids and PAF. Eicosonoids-arachidonate metabolits, including PGs, PGI2, TxA2, LTs, lipoxins and hepoxilins, are not stored but are produced, by most cells, when a variety of physical, chemical, and hormonal stimuli activate acycl hydrolases that make arachidonate available.

Lipid-Derived Autocoids Membrane glycerophosphocoline derivatives can be modified enzymathically to produce PAF. PAF is formed by a smaller number of cell types, principally Leucocytes, Platelets, and endothelial cells.

Lipid-Derived Autocoids Eicosanoids and PAF lipids contribute to inflammation, smooth muscle tone, hemostasis, thrombosis, parturition, and GI secretion.

Lipid-Derived Autocoids Several classes of drugs, most notably Aspirin, the tNSAIDs and spicific inhibitor of COX-2, such as the Coxibs, owe their principle therapeutic effects to blockade of eicosanoid formation.

EICOSANOIDS History 1930 Kurzrok and Lieb 1935 von Euler Prostaglandin 1962 Samuelsson, Bergström PGE1 PGF1α 1964 Bergstrom and Van Dorp biosynthesis of PGE2 1971 Vane, Smith and Willis, Aspirin and NSAIDs

PGs, LTs, and related compounds are called eicosanoids, Precursor essential fatty acids contain 20 carbons and 3,4,5 double bonds: 8,11,14-eicosatrienoic acid (dihomo--linolenic acid) 5,8,11,14-eicosatetraenoic acid (AA) 5,8,11,14,17-eicosapentaenoic acid (EPA)

Eicosanoids Biosynthesis Phosphatidylcholine and phosphatidylethanolamine cPLA2 (cytosolic, Ca2+_dependent) sPLA2 (secratory) iPLA2 (Ca2+ _independent) COXs, Lipoxygenases (LOXs), and CYPs

Products of PG G/H synthases Complex microsomal enzymes COX and hydroperoxidase (HOX) activities Prostanoids (PGs, Prostacyclin, PGI2, and TxA2) COX-1 and COX-2

Products of Lipoxygenases (LOXs) HPETE → HETE These are five active human LOXs- 5(S)-LOX, 12(S)-LOX,12(R)-LOX; 15(S)-LOX-1, and 15(S)-LOX-2 5-LOX pathway leads to the synthesis of the LTs. FLAP LTC4, LTD4, and LTE4 (SRS-A)

Products of CYPs epoxyeicosatrienoic acids (EETs) endothelial cells (EDHFs)

Other Pathways non-enzymatic free radical catalyzed oxidation of AA Isoprostanes their formation is suppressed by antioxidants. endocannabinoids arachidonylethanolamide (anandamide) and 2-arachidonoylglycerol

Inhibitors of Eicosanoid Biosynthesis PLA2 - Glucocorticoids (annexins ) COX-2 – induced expression – Glucocorticoides COXs- Aspirin and tNSAIDs COX and 5-LOX – Licofelone COX-2 – Coxibs TX synthase LOXs - Zileuton

Eicosanoid Catabolism pulmonary circulation - PGE2 TXA2 – (t1/2 – 30 seconds) PGI2 – (t1/2 – 3 minutes) LTs – long acting

Pharmacological Properties of Eicosanoids Prostaglandin Receptors Cell Signaling Pathways and Expression Receptors for LTs Endogenous PGs, Txs, and LTs: - Functions in Physiological and Pathological Processes

Platelets Platelet aggregation leads to activation of membrane PLs → AA → eicosanoids In human platelets, TxA2 and 12-HETE Low dose Aspirin TxB2 – unstable angina, MI, and stroke PGI2 – Inhibition platelet aggregation and disaggregation performed clumps Vascular injury COX-2 inhibitors

Vascular Tone Prostanoids modulate vascular tone locally COX-2 derived PGI2 PGI2 – Pulmonary hypertension PGE2 – maintenance of renal blood flow PGI2 and PGE2 – septic shock PGs – maintanence of placental blood flow COX-2 derived PGE2 (DA) EETs - hypertension

Inflammatory Vascular Disease TxA2 - atherogenesis PGI2 - atheroprotective COX-2 - abdominal aortic aneurism formation LTs

Lung A complex mixture of autacoids is released when sensitized lung tissue is challenged by the appropriate antigen. COX-derived (PGE2, PGF2α, TxA2, PGD2 ,and PGI2). CysLTs probably dominate during allergic constriction of the airway. CysLT-receptor antagonists and 5-LOX inhibitors are effective in the treatment of human asthma. slow LT metabolism in lung.

Kidney Long-term use of all COX inhibitors is limited by the development of hypertension, edema, and congestive heart failure in a significant number of patients. COX-2 derived PGE2 and PGI2 (RBF and salt excretion). Biosynthesis of PGE2 and PGI2 is increased by factors that reduce renal blood flow. Bartter's syndrome inhibition of COX-2

Inflammatory and Immune Responses PGs and LTs are synthesized in response to a host of stimuli that elicit inflammatory and immune responses, and contribute significantly to inflammation and immunity. Prostanoids generally promote acute inflammation. LTs are potent mediators of inflammation. LTB4 in chemotaxis, adhesion, and recruitment of leukocytes. Increased vascular permeability

Heart PGI2 and PGE2 protect against oxidative injury in cardiac tissue.

Reproduction and Parturition PGF2α appears important for luteolysis. COX-2 generates prostanoids (PGF2α and TxA2) that are important in the final stages of parturition.

Cancer COX-2 - in models of colon, breast, lung, and other cancers. NSAIDs and cancers. PGE2 and TxA2 - pro-carcinogenic mediators. CysLTs and LTB4

Pharmacological Effects Cardiovascular System Eye Platelets CNS Inflammation and Immunity Endocrine system Smooth Muscle Bone Kidney

Cardiovascular effect local vascular tone and renal actions. PGE2, PGI2, and PGD2 elicit vasodilation and a drop in blood pressure . PGD2 (flushing, nasal stuffiness, and hypotension) TxA2 – potent vasoconstrictor LTC4 and LTD4 – Hypotension, permeability EET - EDHFs

Platelets LD of PGE2 - aggregation HD of PGE2 – Inhibition of aggregation PGI2 and PGD2 - Inhibition of aggregation COX-1 is dominant COX-2 - Megakaryocytes and immature platelet TxA2 the major product of COX-1 in platelets thrombin and ADP endogenous inhibitors, NO and PGI2

Inflammation and Immunity LTs - pro-inflammatory lipoxins - anti-inflammatory prostanoids (PGE2 and PGI2) both kinds of activity LOX-2 is dominant PGs – inhibit lymphocyte function and proliferation DP2 (most cells) – is a potent leukocyte chemoattractant LTB4 -is a potent activator and chemotactic agent for neutrophils, T lymphocytes, eosinophils, monocytes, dendritic cells, and mast cells LTB4 - aggregation of eosinophils and promotes degranulation and the generation of superoxide. LTB4 - promotes adhesion of neutrophils to vascular endothelial cells (transendothelial migration) LTB4 - stimulates synthesis of pro-inflammatory cytokines from macrophages and lymphocytes. Mast cell–generated LTB4 also may contribute significantly to T lymphocyte migration

Smooth Mucsle Bronchial and Tracheal muscle. TxA2, PGF2α, and PGD2 contract PGE2 and PGI2 relax CysLTs Bronchoconstrictors Bronchial mucus secretion (muscle edema)

Smooth Mucsle Uterus PGF2α, and PGE2 Together with Oxytocin is essential for the onset of parturition. Uterine responsiveness to PGs increases as pregnancy progresses but remains smaller than the response to Oxytocin.

Gastrointestinal Muscle PGEs and PGFs stimulate contraction of the main longitudinal muscle from stomach to colon PGs reduce transit time in the small intestine and colon. 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 (watery diarrhea). PGE2 appears to contribute to the water and electrolyte loss in cholera (therapy with tNSAIDs). The LTs have potent contractil effect.

Gastric and Intestinal Secretions PGE2 and PGI2 increased mucus secretion (cytoprotection), reduced acid secretion, and reduced pepsin content. PGE2 and PGI2 - vasodilatory properties and direct effects on secretory cells PGE2 and its analogs also inhibit gastric damage caused by a variety of ulcerogenic agents and promote healing of duodenal and gastric ulcers. COX-1 -dominant source (physiological conditions) COX-2 - predominates during ulcer healing. CysLTs – constrict blood vessels (gastric damage)

Kidney Both the renal medulla and cortex synthesize prostanoids. PGE2 and PGI2 – (COX-2 derived) 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 PGE2 and PGI2 increase renal BF and GF through their local vasodilating effects (marginally functioning kidneys and volume-contracted states). PGE2 and PGI2 - increase renin release

Eye PGF2α - decreases intraocular pressure (IOP) effective in the treatment of open-angle glaucoma

CNS The induction of fever by a range of endogenous and exogenous pyrogens appears to be mediated by PGE2 The body tempreture set points, is elevated by endogenous pyrogens (IL-1β, IL-6, TNF-α, and Interferone) The initial phase of thermoregulatory response to pyrogens is mediated by ceramide release in neurons of the preoptic area in the anterior hypothalamus. the late response in the endothelium of blood vessels in the preoptic hypothalamic area to form PGE2. PGE2 can cross the blood-brain barrier and acts on thermosensitive neurons. PD2 – increase extracellular adenosin. Facilitates induction of sleep. COX-2-derived prostanoids are implicated in several degenerative disorders (e.g. AD, PD).

Pain Inflammatory mediators, (LTs and PGs) increase the sensitivity of nociceptors and potentiate pain perception. Both PGE2 and PGI2 reduce the threshold to stimulation of nociceptors (peripheral sensitization). 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, PGD2, PGI2, and PGF2α, can increase excitability in pain transmission neuronal pathways in the spinal cord, (hyperalgesia and allodynia). LTB4- hyperalgesia Amplification system for the pain mechanism (in inflammatory process)

Endocrine System PGE2 increases ACTH, growth hormone, prolactin, and gonadotropins. Other effects (steroid production, insulin release, and thyroid-like effects). Critical role of PGF2α (induce an oxytocin-dependent decline in progesterone levels). PGE2 - positive-feedback loop to induce oocyte maturation required for fertilization during and after ovulation.

Bone PGs are strong modulators of bone metabolism. COX-1 is expressed in normal bone COX-2 is upregulated in certain settings (inflammation and mechanical stress). PGE2 - bone formation and Bone resorption

Therapeutic Uses Stable agonists, inhibiting eicosanoid formation, and antagonizing eicosanoid formation Therapeutic abortion (Dinoprostone, misoprostol, and carboprost) Gastric cytoprotection (misoprostol) Impotence (PGE1)- second line treatment of erectile dysfunction. Maintenance of PDA – congenital heart disease (PGE1) Pulmonary Hypertension – long term therapy with PGI2 (epoprostenol) Glaucoma – long term PGF2α derivative (latanoprost)