Response to Ricciardolo: The functional significance of arginase in asthma is supported by gene expression

Abstract

We are grateful to Ricciardolo [1] for his supportive comments regarding our recent Opinion article in TiPS [2]. We fully agree that understanding the complex interactions between multiple biochemical events involved in nitric oxide (NO) homeostasis at distinct microenvironmental levels in the airway wall will be necessary to fully appreciate its role in the pathogenesis of asthma and to develop new therapeutic strategies. In our TiPS article, we hypothesized that increased arginase activity and consequently reduced L-arginine availability to constitutive nitric oxide synthase (cNOS) and inducible nitric oxide synthase (iNOS) has a major role in the pathogenesis of asthma. This hypothesis was based primarily on our biochemical and pharmacological investigations in a guinea-pig model of allergic asthma. These investigations demonstrated that allergen challenge causes a considerable increase in arginase activity in the airways, which results in: (i) a relative deficiency of both cNOSand iNOS-derived NO; (ii) iNOS-derived superoxide (O2 ) production; and, as a consequence, (iii) airway hyperresponsiveness [2–4]. In this regard, the recent article by Zimmermann et al. [5], which is cited by Ricciardolo and which appeared after the final processing of our TiPS article, is of considerable interest. In two mouse models of allergic asthma, induced by sensitization with ovalbumin or Aspergillus fumigatus, it was confirmed that arginase activity is increased after allergen challenge. By microarray analysis of gene expression this finding was extended by the observation that among the 291 common genes induced in these models, genes related to the metabolism of L-arginine, specifically arginase I, arginase II and the cationic amino acid transporter 2, were prominently overexpressed. Moreover, enhanced mRNA expression of arginase I was also observed in human asthmatic lung tissue, particularly in inflammatory cells and in the airway epithelium. This strongly indicates that increased arginase activity, presumably caused by T helper 2 (Th2) cell-derived cytokine-induced expression of the enzyme [5], could indeed be of clinical importance for the development of airway hyperresponsiveness in asthmatic patients. Furthermore, as discussed in our TiPS article and mentioned by Ricciardolo, increased production of arginasederived L-ornithine as a precursor of proline and polyamines could have an important role in airway remodelling in the asthmatic lung. However, the role of arginase in airway remodelling remains to be established. As illustrated by Ricciardolo and previously proposed by Vercelli [6], polymorphisms of the human arginase genes could be of functional importance for the NO-dependent and the abovementioned NO-independent effects of these enzymes in the pathogenesis of asthma, although these studies have not been performed. Of interest, various mutations causing reduced arginase activity have been identified in the coding region of arginase I from patients with liver arginase deficiency and hyperargininemia [7]. With regard to NO-induced counter-regulatory mechanisms of NOS activity, the finding by Ricciardolo and colleagues of a negative post-transcriptional regulation of iNOS by bradykinin-activated cNOS in allergic asthma seems very interesting. However, this finding is difficult to reconcile with the observed allergen-induced deficiency of cNOS-derived NO [3,8]. In this regard, the recent finding that iNOS expression is under translational control of L-arginine by regulation of the phosphorylation status of the eukaryotic initiation factor eIF2a [9] might be of great relevance. It indicates that reduced cellular L-arginine levels induced by increased arginase activity or inhibition of cationic amino acid yþ transporters by inflammationderived cationic proteins [10,11] could regulate NO production not only by limiting the availability of substrate for NOS, but also by altering NOS protein expression.