Thromboxane

Last updated
Thromboxane A2 Thromboxane A2.png
Thromboxane A2
Thromboxane B2 Thromboxane B2.svg
Thromboxane B2

Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring.

Contents

Thromboxane is named for its role in clot formation (thrombosis).

Production

Enzymes and substrates associated with thromoboxane and prostacyclin synthesis. Thromboxane synthesis.png
Enzymes and substrates associated with thromoboxane and prostacyclin synthesis.
Eicosanoid synthesis. Eicosanoid synthesis.svg
Eicosanoid synthesis.

Thromboxane-A synthase, an enzyme found in platelets, converts the arachidonic acid derivative prostaglandin H2 to thromboxane.

Mechanism

Thromboxane acts by binding to any of the thromboxane receptors, G-protein-coupled receptors coupled to the G protein Gq. [1]

Functions

Thromboxane is a vasoconstrictor and a potent hypertensive agent, and it facilitates platelet aggregation.

It is in homeostatic balance in the circulatory system with prostacyclin, a related compound. The mechanism of secretion of thromboxanes from platelets is still unclear. They act in the formation of blood clots and reduce blood flow to the site of a clot.

If the cap of a vulnerable plaque erodes or ruptures, as in myocardial infarction, platelets stick to the damaged lining of the vessel and to each other within seconds and form a plug. These "Sticky platelets" secrete several chemicals, including thromboxane A2 that stimulate vasoconstriction, reducing blood flow at the site.

Role of A2 in platelet aggregation

Thromboxane A2 (TXA2), produced by activated platelets, has prothrombotic properties, stimulating activation of new platelets as well as increasing platelet aggregation.

Platelet aggregation is achieved by mediating expression of the glycoprotein complex GP IIb/IIIa in the cell membrane of platelets. Circulating fibrinogen binds these receptors on adjacent platelets, further strengthening the clot.

Pathology

It is believed that the vasoconstriction caused by thromboxanes plays a role in Prinzmetal's angina. Omega-3 fatty acids are metabolized to produce higher levels of TxA,3 which is relatively less potent than TxA2 and PGI3; therefore, there is a balance shift toward inhibition of vasoconstriction and platelet aggregation. It is believed that this shift in balance lowers the incidence of myocardial infarction (heart attack) and stroke. Vasoconstriction and, perhaps, various proinflammatory effects exerted by TxA on tissue microvasculature, is probable reason why the TxA is pathogenic in various diseases, such as ischemia-reperfusion injury., [2] hepatic inflammatory processes, [3] acute hepatotoxicity [4] etc. TxB2, a stable degradation product of TxA2, plays a role in acute hepatoxicity induced by acetaminophen. [5] [6]

Inhibitors

Thromboxane inhibitors are broadly classified as either those that inhibit the synthesis of thromboxane, or those that inhibit the target effect of it.

Thromboxane synthesis inhibitors, in turn, can be classified regarding which step in the synthesis they inhibit:

The inhibitors of the target effects of thromboxane are the thromboxane receptor antagonist, including terutroban.

Picotamide has activity both as a thromboxane synthase inhibitor and as a thromboxane receptor antagonist. [11]

Ridogrel is another example.

Related Research Articles

Aspirin Medication used to reduce pain and inflammation.

Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation. Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever. Aspirin given shortly after a heart attack decreases the risk of death. Aspirin is also used long-term to help prevent further heart attacks, ischaemic strokes, and blood clots in people at high risk. It may also decrease the risk of certain types of cancer, particularly colorectal cancer. For pain or fever, effects typically begin within 30 minutes. Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) and works similarly to other NSAIDs but also suppresses the normal functioning of platelets.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are members of a drug class that reduces pain, decreases fever, prevents blood clots, and in higher doses, decreases inflammation. Side effects depend on the specific drug but largely include an increased risk of gastrointestinal ulcers and bleeds, heart attack, and kidney disease.

Prostaglandin Group of physiologically active lipid compounds

The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derived enzymatically from the fatty acid arachidonic acid. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. They are a subclass of eicosanoids and of the prostanoid class of fatty acid derivatives.

An antiplatelet drug (antiaggregant), also known as a platelet agglutination inhibitor or platelet aggregation inhibitor, is a member of a class of pharmaceuticals that decrease platelet aggregation and inhibit thrombus formation. They are effective in the arterial circulation, where anticoagulants have little effect.

Cyclooxygenase class of enzymes

Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme that is responsible for formation of prostanoids, including thromboxane and prostaglandins such as prostacyclin, from arachidonic acid. A member of the animal-type heme peroxidase family, it is also known as prostaglandin G/H synthase. The specific reaction catalyzed is the conversion from arachidonic acid to Prostaglandin H2, via a short-living Prostaglandin G2 intermediate.

Prostacyclin chemical compound

Prostacyclin (also called prostaglandin I2 or PGI2) is a prostaglandin member of the eicosanoid family of lipid molecules. It inhibits platelet activation and is also an effective vasodilator.

Ticlopidine chemical compound

Ticlopidine is an antiplatelet drug in the thienopyridine family which is an adenosine diphosphate (ADP) receptor inhibitor. Research initially showed that it was useful for preventing strokes and coronary stent occlusions. However, because of its rare but serious side effects of neutropenia and thrombotic thrombocytopenic purpura it was primarily used in patients in whom aspirin was not tolerated, or in whom dual antiplatelet therapy was desirable. With the advent of newer and safer antiplatelet drugs such as clopidogrel and ticagrelor, its use remained limited.

Prasugrel drug that acts as a platelet inhibitor and is used to prevent formation of blood clots.

Prasugrel (trade name Effient in the US, Australia and India, and Efient in the EU) is a drug used to prevent formation of blood clots. It is a platelet inhibitor and an irreversible antagonist of P2Y12 ADP receptors and is of the thienopyridine drug class. It was developed by Daiichi Sankyo Co. and produced by Ube and currently marketed in the United States in cooperation with Eli Lilly and Company.

Thromboxane-A synthase mammalian protein found in Homo sapiens

Thromboxane A synthase 1 , also known as TBXAS1, is a cytochrome P450 enzyme that, in humans, is encoded by the TBXAS1 gene.

Thromboxane receptor mammalian protein found in Homo sapiens

The thromboxane receptor (TP) also known as the prostanoid TP receptor is a protein that in humans is encoded by the TBXA2R gene, The thromboxane receptor is one among the five classes of prostanoid receptors and was the first eicosanoid receptor cloned. The TP receptor derives its name from its preferred endogenous ligand thromboxane A2.

A prostaglandin antagonist is a hormone antagonist acting upon one or more prostaglandins, a subclass of eicosanoid compounds which function as signaling molecules in numerous types of animal tissues.

Thromboxane A2 chemical compound

Thromboxane A2 (TXA2) is a type of thromboxane that is produced by activated platelets during hemostasis and has prothrombotic properties: it stimulates activation of new platelets as well as increases platelet aggregation. This is achieved by activating the thromboxane receptor, which results in platelet-shape change, inside-out activation of integrins, and degranulation. Circulating fibrinogen binds these receptors on adjacent platelets, further strengthening the clot. Thromboxane A2 is also a known vasoconstrictor and is especially important during tissue injury and inflammation. It is also regarded as responsible for Prinzmetal's angina.

Prostaglandin H<sub>2</sub> chemical compound

Prostaglandin H2 is a type of prostaglandin and a precursor for many other biologically significant molecules. It is synthesized from arachidonic acid in a reaction catalyzed by a cyclooxygenase enzyme. The conversion from Arachidonic acid to Prostaglandin H2 is a two step process. First, COX-1 catalyzes the addition of two free oxygens to form the 1,2-Dioxane bridge and a peroxide functional group to form Prostaglandin G2. Second, COX-2 reduces the peroxide functional group to a Secondary alcohol, forming Prostaglandin H2. Other peroxidases like Hydroquinone have been observed to reduce PGG2 to PGH2. PGH2 is unstable at room temperature at room temperature with a half life of 90-100 seconds, so it is often converted into a different prostaglandin.

Picotamide chemical compound

Picotamide is a platelet aggregation inhibitor. It works as a thromboxane synthase inhibitor and a thromboxane receptor inhibitor, the latter by modifying cellular responses to activation of the thromboxane receptor. Picotamide is licensed in Italy for the treatment of clinical arterial thrombosis and peripheral artery disease.

Mechanism of action of aspirin

Aspirin causes several different effects in the body, mainly the reduction of inflammation, analgesia, the prevention of clotting, and the reduction of fever. Much of this is believed to be due to decreased production of prostaglandins and TXA2. Aspirin's ability to suppress the production of prostaglandins and thromboxanes is due to its irreversible inactivation of the cyclooxygenase (COX) enzyme. Cyclooxygenase is required for prostaglandin and thromboxane synthesis. Aspirin acts as an acetylating agent where an acetyl group is covalently attached to a serine residue in the active site of the COX enzyme. This makes aspirin different from other NSAIDs, which are reversible inhibitors. However, other effects of aspirin, such as uncoupling oxidative phosphorylation in mitochondria, and the modulation of signaling through NF-κB, are also being investigated. Some of its effects are like those of salicylic acid, which is not an acetylating agent.

Seratrodast Chemical used in the treatment of asthma

Seratrodast (development name, AA-2414; marketed originally as Bronica) is a thromboxane A2 (TXA2) receptor (TP receptor) antagonist used primarily in the treatment of asthma. It was the first TP receptor antagonist that was developed as an anti-asthmatic drug and received marketing approval in Japan in 1997. As of 2017 seratrodast was marketed as Bronica in Japan, and as Changnuo, Mai Xu Jia, Quan Kang Nuo in China.

Adenosine diphosphate (ADP) receptor inhibitors are a drug class of antiplatelet agents, used in the treatment of acute coronary syndrome (ACS) or in preventive treatment for patients who are in risk of thromboembolism, myocardial infarction or a stroke. These drugs antagonize the P2Y12 platelet receptors and therefore prevent the binding of ADP to the P2Y12 receptor. This leads to a decrease in aggregation of platelets, prohibiting thrombus formation. The P2Y12 receptor is a surface bound protein found on blood platelets. They belong to G protein-coupled purinergic receptors (GPCR) and are chemoreceptors for ADP.

Management of acute coronary syndrome is targeted against the effects of reduced blood flow to the afflicted area of the heart muscle, usually because of a blood clot in one of the coronary arteries, the vessels that supply oxygenated blood to the myocardium. This is achieved with urgent hospitalization and medical therapy, including drugs that relieve chest pain and reduce the size of the infarct, and drugs that inhibit clot formation; for a subset of patients invasive measures are also employed. Basic principles of management are the same for all types of acute coronary syndrome. However, some important aspects of treatment depend on the presence or absence of elevation of the ST segment on the electrocardiogram, which classifies cases upon presentation to either ST segment elevation myocardial infarction (STEMI) or non-ST elevation acute coronary syndrome (NST-ACS); the latter includes unstable angina and non-ST elevation myocardial infarction (NSTEMI). Treatment is generally more aggressive for STEMI patients, and reperfusion therapy is more often reserved for them. Long term therapy is necessary for prevention of recurrent events and complications.

Thromboregulation is the series of mechanisms in how a primary clot is regulated. These mechanisms include, competitive inhibition or negative feedback. It includes primary hemostasis, which is the process of how blood platelets adhere to the endothelium of an injured blood vessel. Platelet aggregation is fundamental to repair vascular damage and the initiation of the blood thrombus formation. The elimination of clots is also part of thromboregulation. Failure in platelet clot regulation may cause hemorrhage or thrombosis. Substances called thromboregulators control every part of these events.

12-Hydroxyheptadecatrienoic acid Chemical compound

12-Hydroxyheptadecatrenoic acid is a 17 carbon metabolite of the 20 carbon polyunsaturated fatty acid, arachidonic acid. It was first detected and structurally defined by P. Wlodawer, Bengt I. Samuelsson, and M. Hamberg as a product of arachidonic acid metabolism made by microsomes isolated from sheep seminal vesicle glands and by intact human platelets. 12-HHT is less ambiguously termed 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid to indicate the S stereoisomerism of its 12-hydroxyl residue and the Z, E, and E cis-trans isomerism of its three double bonds. The metabolite was for many years thought to be merely a biologically inactive byproduct of prostaglandin synthesis. More recent studies, however, have attached potentially important activity to it.

References

  1. Rat kidney thromboxane receptor: molecular cloning, signal ...
  2. Ito Y (2003). "Effects of selective cyclooxygenase inhibitors on ischemia/reperfusion-induced hepatic microcirculatory dysfunction in mice". Eur Surg Res. 35 (5): 408–16. doi:10.1159/000072174. PMID   12928598.
  3. Katagiri H (2004). "Role of thromboxane derived from COX-1 and -2 in hepatic microcirculatory dysfunction during endotoxemia in mice". Hepatology. 39 (1): 139–150. doi:10.1002/hep.20000. PMID   14752832.
  4. Yokoyama Y (2005). "Role of thromboxane in producing hepatic injury during a hepatic stress disorder". Arch. Surg. 140 (8): 801–7. doi: 10.1001/archsurg.140.8.801 . PMID   16103291.
  5. Cavar I (2011). "Anti-thromboxane B2 antibodies protect against acetaminophen-induced liver injury in mice". Journal of Xenobiotics. 1 (1): 38–44. doi: 10.4081/xeno.2011.e8 .
  6. Cavar I (2010). "The role of prostaglandin E2 in acute acetaminophen hepatotoxicity in mice". Histol Histopathol. 25 (7): 819–830. PMID   20503171.
  7. American Heart Association: Aspirin in Heart Attack and Stroke Prevention "The American Heart Association recommends aspirin use for patients who've had a myocardial infarction (heart attack), unstable angina, ischemic stroke (caused by blood clot) or transient ischemic attacks (TIAs or "little strokes"), if not contraindicated. This recommendation is based on sound evidence from clinical trials showing that aspirin helps prevent the recurrence of such events as heart attack, hospitalization for recurrent angina, second strokes, etc. (secondary prevention). Studies show aspirin also helps prevent these events from occurring in people at high risk (primary prevention)."
  8. Tohgi, H; S Konno; K Tamura; B Kimura; K Kawano (1992). "Effects of low-to-high doses of aspirin on platelet aggregability and metabolites of thromboxane A2 and prostacyclin". Stroke. 23 (10): 1400–1403. doi: 10.1161/01.STR.23.10.1400 . PMID   1412574.
  9. Dockens, RC; Santone, KS; Mitroka, JG; Morrison, RA; Jemal, M; Greene, DS; Barbhaiya, RH (Aug 2000). "Disposition of radiolabeled ifetroban in rats, dogs, monkeys, and humans". Drug Metabolism and Disposition. 28 (8): 973–80. PMID   10901709.
  10. Coxib and traditional NSAID Trialists' (CNT) Collaboration (30 May 2013). "Vascular and upper gastrointestinal effects of non-steroidal anti-inflammatory drugs: meta-analyses of individual participant data from randomised trials". Lancet. 382 (9894): 769–79. doi:10.1016/S0140-6736(13)60900-9. PMC   3778977 . PMID   23726390.
  11. Ratti, S; Quarato, P; Casagrande, C; Fumagalli, R; Corsini, A (1998). "Picotamide, an antithromboxane agent, inhibits the migration and proliferation of arterial myocytes". European Journal of Pharmacology. 355 (1): 77–83. doi:10.1016/S0014-2999(98)00467-1. PMID   9754941.