Our experiments have demonstrated that peripheral
Our experiments have demonstrated that peripheral blood basophils express mRNA for cysLT1 and cysLT2, detectable levels of cysLT1 and cysLT2 protein, and functional responses to LTD4. We have shown that despite the variable level of cysLT1 on basophils, there is indeed a functionally active receptor. Because 2 identified functional effects, increase of cytosolic Ca++ and decreased expression of CD95, were fully reversed by the presence of a cysLT1 antagonist, we conclude that we have not identified a functional role for the cysLT2 receptor, despite its equivalent level of protein expression, as shown by means of flow cytometry. Whether the observed level of response can account for the accumulation of basophils in allergic tissue remains to be seen.
Introduction Cysteinyl–leukotrienes (Cys-LTs), namely leukotriene C4, D4 and E4, are synthesized de novo by transformation of arachidonic SIN-1 chloride synthesis via the activity of the 5-lipoxygenase enzyme (Samuelsson, 1983). Experimental research characterized Cys-LTs as potent stimulators of smooth muscle contraction, their effects being of particular relevance in the respiratory system, where they can cause severe bronchoconstriction (Hanna et al., 1981, Griffin et al., 1983). In normal subjects, inhalation of leukotriene D4 (LTD4) produces the same degree of airway obstruction as 10,000 times more concentrated inhaled solution of histamine. Moreover, increased synthesis of leukotrienes has been demonstrated in vivo following antigen challenge of allergic subjects (Creticos et al., 1984). Early studies pertaining to Cys-LTs effects have been centered on their contractile activity whereas, more recently, several investigators demonstrated their contribution to airway hyperresponsiveness, mucus plug formation, epithelial cell damage, and airway remodeling (reviewed in Holgate et al., 2003). It is now clear that cysteinyl–leukotrienes are endowed with a variety of proinflammatory actions, and it has been therefore speculated that these mediators might play a pivotal role in the pathogenesis of asthma. Recent pharmaceutical research in the field of respiratory diseases has sought to make available new agents that specifically and selectively antagonize the actions of cysteinyl–leukotrienes. Such an effort has resulted in the development of two classes of drugs, effective in counteracting Cys-LTs pathway, that are now offered for the treatment of asthma: LTD4 receptor antagonists (montelukast, pranlukast, and zafirlukast) and leukotriene synthesis inhibitors (zileuton; Brooks and Summers, 1996). Cysteinyl–leukotrienes act via two different G-protein-coupled receptors, leukotrienes CysLT1 and leukotriene CysLT2 (Gorenne et al., 1996, Heise et al., 2000). The former appears to be the primary regulator of the deleterious effects that cysteinyl–leukotrienes exert on human airways because selective antagonism of leukotriene CysLT1 receptor, by means of the previously cited compounds, has been shown to improve both asthma symptoms (Suissa et al., 1997) and the underlying inflammatory response (Nakamura et al., 1998). In the clinical setting, leukotriene CysLT1 receptor antagonists were demonstrated to be effective for the treatment of mild to moderate asthma, particularly that associated with inhaled steroid and β-adrenergic agonist therapies (Liu et al., 1996). Moreover, they are highly efficacious in the prevention of exercise-induced bronchoconstriction, and can be used in aspirin-intolerant asthmatic subjects (Manning et al., 1990, Dahlen et al., 1993).