Toward a new anti-inflammatory and analgesic agent.

I a comparative study on normal and B1-receptor deficient (B1-KO) mice, Pesquero et al. (1) provide evidence in this issue of PNAS in favor of crucial roles played by the kinin B1 receptor in toxic shock, inflammation, and nociception. B1 receptors are generally silent or absent in healthy states and are induced or activated by pathological challenges. When they are absent, as in B1-KO mice, tissue reactions to microbial toxins, local inflammatory agents, and painful stimulations are reduced without any apparent drawback. The B1 receptor appears therefore to favor the appearance and worsening of pathological lesions and symptoms. From this paper, the B1 receptor emerges as a new target of great potential for the development of agents directed to reduce the generally excessive responses that are used by the body to counteract noxious stimuli. These agents should be antagonists, possibly specific and selective for the B1 receptor. In the late ’70s, two different types of receptors for bradykinin and related kinins were identified, using pharmacological in vitro assays on rabbit isolated vessels, the aorta for the B1 and the jugular vein for the B2 receptor (2). The B1 receptor was characterized earlier than the B2, thanks to the discovery of selective and specific antagonists. It soon became evident that kinins [bradykinin (BK) and kallidin (LysBK)] are the natural ligands for the B2, and their desArg9 metabolites (desArg9 BK and Lys desArg9 BK) are the ligands of the B1 receptor (Fig. 1). Indeed, treatment of rabbit aorta with Mergetpa (a carboxypeptidase M inhibitor) prevented the myotropic effect of BK, while leaving intact the activity of desArg9 BK, which indicated that activation of the B1 receptor by BK requires the conversion of the nonapeptide to desArg9 BK by intramural carboxypeptidases (3). This appears today to be the general pathway by which B1 receptor ligands are generated in tissues in pathological conditions, when the B1 receptor is made to express itself and needs the ligands for its activation (4). In fact, BK and LysBK are the only species that are released when plasma kallikrein acts on high molecular weight kininogen (to release BK) and tissue kallikreins hydrolyze the low molecular weight kininogen to release LysBK (5); the B1 selective agonists can derive only from the kinins, which are therefore to be considered as biologically active agents as well as precursors of other species (Fig. 1). In the early ’90s, B1 and B2 receptors were cloned in animals and humans (4, 6) and found to lie close to each other in human chromosome no. 14. No genetic or pharmacological evidence has been reported until now in favor of the existence of other types of kinin receptors (4, 6, 7) in mammals. B1 and B2 receptors differ, however, in their sequence, expression, and function: they are different pharmacological entities and their physiopathological roles are certainly distinct. In fact, sequence homology between B1 and B2 receptors is only 36%, approximately the same as between B1 and AT1, the functional receptor for angiotensin II. B2 receptors are constitutive structures that are expressed by a variety of cells under physiological conditions, whereas B1 are generally absent from healthy tissues, but rapidly appear after injuries of various nature (e.g., injection of lipopolysaccharide or IL-1B), eventually in parallel with cyclooxygenase 2 and carboxypeptidases or other functional tissue components. Interestingly, a reduction of NO synthase activity is associated to the blunted expression of the B1 receptor in B1-KO mice. Thus, the B1 receptor up-regulation appears to be part of a more generalized response that includes the local coexpression (eventually up-regulation) of enzymes, receptors, and autacoids that notoriously play key roles in the early and late responses of tissues to various types of injury. Worthy of mention is that studies on B1 receptors have been important for demonstrating differences between responses of normal or pathological tissues to the same agents, opening the way to modern pathopharmacology. An eloquent demonstration of these facts is provided by the results presented by Pesquero et al. (1), who first cloned the mouse B1 receptor (7) and then disrupted its gene by targeting technology to obtain B1-KO mice. When lipopolysaccharide is injected to induced toxic shock, the control mice respond with a marked fall of blood pressure, whereas little, if any, change is observed in the B1-KO. The blood pressure fall is not immediate and progresses over 30–40 min, as expected by the timing needed for lipopolysaccharide to release interleukins (IL-1B, according to ref. 4) and the subsequent induction of the B1 receptor, which is known to require new protein synthesis (4). The acute blood pressure fall is attributed in large part to the release of NO (from the endothelium). A reduced level of NO synthase activity may contribute to the blunted blood pressure response to lipopolysaccharide, observed in B1-KO mice, because it may reduce the massive and rapid production of NO that worsen toxic shock. In normal conditions, reduction of NO synthase activity could interfere, however, with the protective role of NO on endothelia, a role that may account for part of the beneficial effects of angiotensin-converting enzyme (ACE) inhibitors (8) in hypertension, heart failures, and diabetes. Induction of these diseases in the B1-KO mice and the subsequent treatment with ACE inhibitors could help to assess the possible implication of the B1 receptor, both in some cardiovascular disorders and in the therapeutic efficacy of ACE inhibitors. Much remains to be done with B1-KO mice. Other findings by Pesquero et al. (1) concern inflammation, namely pleurisy