The cysteinyl leukotrienes (LT), LTC 4 , LTD 4 , and LTE 4 , are oxygenated metabolites of arachidonic acid derived from the 5-lipoxygenase (5-LO) pathway (Figure 1) (1). The term leukotriene was originally proposed by Samuelsson as a trivial name to describe these compounds that are derived from leukocytes and are conjugated trienes (2). 5-LO is an iron-containing enzyme most often found free in the cytosol of cells of myeloid origin (3). With cellular activation, this enzyme translocates to the nuclear envelope (4). Once translocated, it is responsible for the first committed steps of the 5-LO pathway in association with the 5-LO–activating protein (FLAP). The precise role of FLAP remains obscure; however, it is required for appreciable 5-LO activity in intact inflammatory cells (5). 5-LO inserts molecular oxygen at the carbon 5 position of arachidonic acid forming the hydroperoxy-intermediate, 5-hydroperoxyeicosatetraenoic acid. This intermediate is further metabolized by 5-LO via a dehydration reaction to form the epoxy intermediate, LTA 4 . LTA 4 can be further metabolized to LTB 4 by the enzyme LTA 4 hydrolase, an epoxide hydrolase enzyme with a ubiquitous distribution (6). The role of LTB 4 in asthma remains to be clarified. LTA 4 can also be metabolized to LTC 4 by the enzyme LTC 4 synthase (7). This enzyme is a microsomal glutathioneS -transferase that belongs to a larger family of membrane-bound proteins involved in eicosanoid and glutathione metabolism (8). LTC 4 synthase appears to have a more restricted distribution than 5-LO and appears to be limited to eosinophils, mast cells, basophils, mononuclear phagocytes, and some leukemic cells lines such as THP-1 (9) and KG-1 (10). The concept that lipid soluble mediators may play a role in asthma is not new. Van Leeuwen and Zeydner demonstrated that alcohol extracts of blood obtained from subjects with asthma, but not healthy individuals, contained a substance that caused contraction of smooth muscle (11). Moreover, Harkavy reported in 1930 that alcohol extracts of sputum obtained from asthmatics during exacerbation also contained a substance that could induce sustained smooth muscle contraction, which he termed “spasm-producing substance” (12). In studies performed by Feldberg and Kellaway (13, 14), and subsequently by Kellaway and Trethewie (15), a substance that could induced sustained smooth muscle contraction was present in the blood of antigen-challenged guinea pigs. These authors coined the term “slow-reacting smooth-muscle stimulating substance in anaphylaxis,” which ultimately was refined to “slow-reacting substance of anaphylaxis” (SRS-A) by Brocklehurst (16). The subsequent forty years of research and major efforts of many investigators culminated in the characterization of SRS-A as the cysteinyl leukotrienes C 4 , D 4 , and E 4 (17). These compounds mediate sustained smooth muscle contraction, mucus hypersecretion, airway edema (18), and eosinophil recruitment (19). Since 1979 a substantial body of evidence has accumulated demonstrating that the cysteinyl LTs are increased in the sputum (20), lung lining fluid (21), blood, and urine (22) of asthmatics when compared with healthy subjects. Aspirin-intolerant asthma (AIA) is a syndrome of aspirin sensitivity, asthma, and nasal polyposis (23). Individuals with this syndrome often have profound systemic reactions when challenged with aspirin. Reactions to aspirin may include cutaneous and gastrointestinal manifestations, in addition to potentially life-threatening acute airway obstruction. The onset of AIA is usually beyond the third decade, and these individuals commonly have severe asthma, which may require systemic corticosteroids. We now recognize that this is not an allergically mediated phenomenon, but is instead a disorder that can occur with any nonsteroidal anti-inflammatory drug that acts via inhibition of cyclo-oxygenase. However, the molecular mechanism(s) involved in the pathogenesis of AIA remains unclear (23). The initial hypothesis suggested that AIA was due to a shunt of arachidonic acid from the cyclo-oxygenase pathway to the lipoxygenase pathway (24). We now understand that the pathogenesis of AIA is more complex. Utilization of arachidonic acid within the cell is a highly ordered process with multiple regulatory steps, and simple shunting of substrate does not appear to play a major role. Additional hypotheses have been put forth to explain AIA. For example, studies suggest that a failure of cyclooxygenase-2 expression in the nasal mucosa of patients with AIA may be a feature (25). The largest body of evidence, however, supports a central role for cysteinyl LT synthesis and an enzyme responsible for this synthesis, LTC 4 synthase, in AIA. AIA patients have markedly elevated cysteinyl LT levels at baseline that increase dramatically with aspirin challenge (26). Moreover, studies have demonstrated that LTC 4 synthase is strikingly increased in eosinophils within the airways of aspirin-intolerant asth( Received in original form July 10, 2000 )
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May 1, 2013
Abby Rynko + 2
Open Access