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Actions of Aspirin via Lipid Modulators of Inflammation
Aspirin is the acetylated form of salicylic acid. Salicylate is a common constituent of numerous medicinal plants which have been used for thousands of years to treat pain and rheumatic fever. Ancient Egyptians used the leaves of the Myrtle tree to treat rheumatic pain and Hippocrates treated eye infections with extracts from poplar trees and used extracts from willow bark in treating the pain and fever associated with childbirth. Salicylate was first chemically synthesized in 1859 and it entered into widespread use as an anti-inflammatory in 1876. Salicylate has an extremely bitter taste and causes gastric irritation so researchers set out to develop analogs that would have the same pharmacological benefits but be easier to tolerate upon ingestion. In 1897 Felix Hoffman, at the Bayer company, discovered the mechanism to acetylate salicylate giving rise to the advent of aspirin (acetylsalicylic acid).
Even though aspirin was in use for over 70 years its mode of action remained unknown. In 1960 H.O Collier and colleagues determined that aspirin worked, in part, through modulation of the activation of the pathways involved in the synthesis of the prostaglandins (PGs: see the Eicosanoids page for details). In the 1970s it was determined that aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) all exerted their effects through the inhibition of PG synthesis via the inhibition of the bifunctional enzyme called cyclooxygenase (there are two forms COX-1 and COX-2). However, this did not explain all of the actions that were being described for aspirin, in particular the ability of aspirin to limit leukocyte migration into sites of inflammation, thereby dampening host inflammatory responses.
Among the NSAIDs, aspirin is unique in that it not only has analgesic (pain), antipyretic (fever) and anti-inflammatory effects (exerted at the level of the PG and TX synthesis) but it also exerts beneficial effects on the cardiovascular system via anti-inflammatory pathways distinct from PG and TX inhibition. At high doses aspirin functions to block the PG and TX synthesizing activity of COX-1 which results in inhibition of the primary pro-inflammatory, pyretic and pain-inducing action of these eicosanoids. In addition, aspirin is an important inhibitor of platelet activation by reducing the production of thromboxane A2 (TXA2). Aspirin also reduces endothelial cell production of prostacyclin (PGI2), an inhibitor of platelet aggregation and a vasodilator. Localized to the site of coagulation is a balance between the levels of platelet derived TXA2 and endothelial cell derived PGI2. This allows for platelet aggregation and clot formation but preventing excessive accumulation of the clot, thus maintaining blood flow around the site of the clot. Endothelial cells regenerate active COX faster than platelets because mature platelets cannot synthesize the enzyme, requiring new platelets to enter the circulation (platelet half-life is approximately 4 days). Therefore, PGI2 synthesis is greater than that of TXA2. The net effect of aspirin is more in favor of endothelial cell-mediated inhibition of the coagulation cascade.
Part of the cardiovascular benefits of aspirin are related to its dose-dependent differential effects on inflammatory events. Only at low doses (e.g. 81mg) will aspirin elicit its most important anti-inflammatory benefits. The low dose anti-inflammatory effects of aspirin are due to its ability to trigger the synthesis of the lipoxins (LXs: LXA4 and LXB4). Higher doses of aspirin have no significant effect on LX synthesis. The lipoxins are anti-inflammatory eicosanoids synthesized through lipoxygenase interactions (hence the derivation of the name: see Figure below). The anti-inflammatory actions of the LXs are, in part, a function of their ability to inhibit the actions of the leukotrienes. The finding that aspirin could induce the synthesis of LXs was one of the most exciting discoveries in pharmocology and was deduced in 1995 by Charles Serhan and colleagues. Aspirin was shown to trigger the synthesis of stereoisomers (epimers) of LXA4 and LXB4 identified as 15 epi-LXA4 and 15 epi-LXB4 (these compounds are also referred to as aspirin-triggered lipoxins (ATLs).
It was known for many years that aspirin inhibited the action of COX-1 and COX-2 by causing the acetylation of the enzyme. However, in endothelial and epithelial cells the aspirin-induced acetylation of COX-2 alters the enzyme such that it now converts arachidonic acid to 15R hydroxyeicosatetraenoic acid (15R-HETE). This latter compound is then rapidly metabolised to the epi-LXs in monocytes and leukocytes through the action of 5-lipoxygenase (5-LOX), the enzyme also responsible for initiation of leukotriene synthesis. There are two additional "classic" pathways for LX synthesis that are not triggered by aspirin as shown in the Figure below.
Synthesis of the lipoxins from arachidonic acid via transcellular interactions. Three pathways exist for the synthesis of the lipoxins. The "classic" pathway involves 5-LOX activity in leukocytes followed by 12-LOX action in platelets. The action of 15-LOX in epithelial cell (such as in the airway) followed by 5-LOX action in leukocytes is the second major lipoxin synthesis pathway. The action of aspirin on COX-2 in epithelial, or endothelial cells as wells as in monocytes results in the eventual production of the 15 epi-lipoxins (also referred to as aspirin-triggered lipoxins, ATLs).
Structure images are only available in the main Medical Biochemistry pages.
Activities of the Lipoxins
As indicated in the Figure above, the two "classical" pathways for the synthesis of the lipoxins are the result of the concerted actions of 15-LOX (acting on arachidonic acid in epithelial cells, such as in the airway) and 5-LOX in leukocytes or through the actions of 5-LOX in leukocytes followed by 12-LOX action in platelets. There is also an aspirin-triggered pathway leading to the synthesis of the epimeric lipoxins, 15 epi-LXA4 and 15 epi-LXB4 (also called aspirin-triggered lipoxins, ATLs). As described above, this latter pathway is initiated as a consequence of the aspirin-induced acetylation of COX-2 which leads to 15R-HETE synthesis from arachidonic acid which is then converted to the epi-LXs. The aspirin-triggered lipoxin synthesis pathway is initiated when activated circulating leukocytes (primarily neutrophils) adhere to the vascular endothelium. Activated leukocytes that adhere to epithelial cells as a consequence inflammation (such as gastrointestinal, airway or kidney epithelia) also result in the production of LXs. An additional stimulus that leads to LX production is epithelial cell conversion of LTA4 that is released from airway epithelia.
The lipoxins are potent anti-inflammatory eicosanoids and counteract the actions of the pro-inflammatory eicosanoids (primarily LTA4 but also PGE2 and TXA2). The lipoxins LXA4 and 15 epi-LXA4 elicit their effects by binding to a specific G protein-coupled receptor (GPCR) identified as ALXR. ALXR is also capable of interacting with several small peptide derived signaling molecules indicating that ALXR is a multi-recognition receptor involved in immune responses. In fact ALXR was originally identified as the formyl peptide receptor-like 1 (FPRL1) protein, a member of the formyl peptide receptor (FPR) family of receptors that bind N-formylated peptides derived by the degradation of bacteria or host cells. The original member of the FPR family recognizes the N-formylated bacterial peptide formyl-Met-Leu-Phe. The FPRs are involved in mediating immune responses to infection. The FPR family of receptors activates the MAP kinase and phospholipase C-γ (PLCγ)-mediated signaling pathways.
Both LXA4 and LXB4 have been shown to promote the relaxation of the vasculature (both aortic and pulmonary relaxation). Lipoxins and epi-LXs inhibit polymorphonuclear leukocyte (PMN) chemotaxis, PMN-mediated increases in vasopermeability, and PMN adhesion and migration through the endothelium. The LXs also stimulate phagocytosis of apoptotic PMNs by monocyte-derived macrophages. PMN phagocytosis represents the resolution phase of inflammatory events, thus aspirin promotes this process and increases the rate of return to the normal tissue state. The pro-resolving activity of aspirin is exerted not only through the induced synthesis of the lipoxins, but also via the induced synthesis of an additional class of anti-inflammatory lipid mediators known as the resolvins (Rvs)and the protectins (PDs).
Additional anti-inflammatory actions of the lipoxins and aspirin-triggered lipoxins include blocking expression of the IL-8 gene, a pro-inflammatory chemokine produced by macrophages and endothelial that stimulates neutrophil migration, inhibition of the release and actions of tumor necrosis factor-α (TNF-α), and stimulation of transforming growth factor-β (TGF-β) activity. By regulating the actions of histamine the lipoxins also lead to a reduction in swelling due to edema.
In addition, the actions of LXA4 in some tissues leads to the production of prostacyclin (PGI2) and nitric oxide (NO) both of which are vasodilators and may play roles in the anti-inflammatory properties of the aspirin-triggered lipoxins (ATLs) 15 epi-LXA4 and 15 epi-LXB4. The induction of NO by aspirin is correlated, in a dose-dependent manner, with a reduction in leukocyte accumulation at sites of inflammation. No other NSAID has been shown to exert this effect on NO production making aspirin unique among this class of drug. The induced production of NO by aspirin plays a significant role in the protective effects of aspirin on the cardiovascular system.
Actions of the Resolvins and Protectins
The resolvins (Rvs) have anti-inflammatory actions that lead to the resolution of the inflammatory cycle, hence the derivation of their names as resolvins. The resolvins are synthesized either from eicosapentaenoic acid (EPA, an essential omega-3 polyunsaturated fatty acid) or from docosahexaenoic (DHA, also an omega-3 fatty acid).
Structure of EPA
Structure of DHA
The D series resolvins are derived from DHA and the E series from EPA. An additional anti-inflammatory lipid derived from DHA is protectin D1 (PD1). Aspirin can trigger the synthesis of an epimeric protectin D1 compound as well as epimeric resolvin D series compounds. The abbreviation AT- refers to the aspirin-triggered compound.
Structure images for the resolvins and protectins are only available in the main Medical Biochemistry pages.
The E series resolvins, RvE1 and RvE2 reduce inflammation, regulate PMN infiltration by blocking transendothelial migration, reduce dendritic cell function (dendritic cells are potent antigen presenting cells which prime T cell mediated inflammatory responses), regulate IL-12 production and lead to resolution of the inflammatory responses. It was suspected that the actions of these lipid mediators were effected via the interactions with cell-surface receptors. Indeed, in a search for G-protein coupled receptors (GPCRs) that mediated the actions of the resolvins at the level of reduced TNF-α activity, an orphan receptor identified as ChemR23 was found that specifically bound RvE1. Evidence suggests that the main intracellular pathways initiated by RvE1 binding to ChemR23 involve phosphorylation events. This signaling mechanism is in contrast to those utilized by pro-inflammatory mediators which involves mobilization of intracellular calcium and increases in cAMP. A second high-affinity receptor for RvE1 was found to be the receptor for the leukotriene, LTB4, termed BLT1. The actions of low dose administration of the other E series resolvin, RvE2, indicate that it acts via different receptors than ChemR23 or BLT1 since under these conditions its effect is additive to RvE1.
The D series resolvins and protectin D1 are synthesized from DHA. In the presence of aspirin and DHA, COX-2 activity leads to the synthesis of the aspirin-triggered resolvins which are also members of the D series of resolvins. Protectin D1 and the aspirin-triggered resolvins block T cell and PMN migration, promote T cell apoptosis, decrease TNF-α and INF-γ secretion, reduce airway inflammation, and exert neuroprotective action during ischemia-reperfusion injury.
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Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu