CINCINNATI—Gene microarray technology has revealed up-regulation of the enzyme arginase in two distinct models of experimental asthma—and in the lungs of asthma patients.[1] Up-regulation of arginase occurs in response not only to allergens but also to interleukin 4 (IL-4) and IL-13, cytokines thought to be central to development of the asthma phenotype. The findings point to new targets for asthma therapy.

In an effort to determine which genes are active in asthma, principal author Marc E. Rothenberg, MD, PhD, and coworkers used DNA microarray analysis to detect genes up-regulated in the lungs in two murine models of asthma. “The asthma genome is different depending upon which asthma model you examine,” observed Dr. Rothenberg, Professor of Pediatrics at the University of Cincinnati and Director of the Division of Allergy and Immunology at Children’s Hospital Medical Center in Cincinnati. This suggests that “patients with very different causes for asthma may have significant differences in the genes and pathways involved,” he said.

Nevertheless, he reported, “We have identified a set of asthma-associated genes—291 so-called signature genes” common to both models. Dr. Rothenberg added, “We also identified a potentially critical role for arginase” in asthma etiology. Because this enzyme is dramatically up-regulated in both types of experimental asthma, the data suggest that “arginase may be involved in asthma … independent of the underlying cause.”

In one asthma model, mice were sensitized with intraperitoneal injections of ovalbumin combined with an alum adjuvant. They then received two separate intranasal ovalbumin challenges. In the second, mice were sensitized to Aspergillus antigen. Both experimental models yielded eosinophilic inflammation, mucus production, and airway hyperresponsiveness—hallmarks of asthma.

Microarray analysis showed induction of 496 and 527 genes in the ovalbumin and Aspergillus models, respectively. While some genes were specific to each sensitization protocol, most overlapped. The core group of 291 genes induced in both experimental asthma models probably reflects pathways of asthma pathogenesis. Among the transcripts up-regulated most dramatically were three genes involved in arginine metabolism: arginase I, arginase II, and the cationic amino acid transporter 2. Arginase I was up-regulated in perivascular and peribronchial pockets of inflammation within the lungs of asthmatic mice; specifically, arginase I was expressed in alveolar macrophages.

Because experimental expression of IL-4 can trigger features of asthma, the researchers examined arginase induction in mice overexpressing this cytokine in the lung. The mice showed increased expression of arginase I. However, if the mice lacked STAT6, a downstream signaling protein known to mediate responses to IL-4 and IL-13, arginase I was not induced. Because intranasal application of IL-13 can produce eosinophilic inflammation, chemokine induction, mucus production, and airway hyperresponsiveness, the researchers tested its effects on arginase expression. IL-13 administration induced arginase I and, to a lesser extent, arginase II within 12 hours—paralleling the development of IL-13–induced airway hyperresponsiveness but preceding IL-13–induced leukocyte recruitment.

To test the clinical relevance of these findings, the researchers examined bronchoalveolar lavage and bronchial biopsy samples from patients with or without asthma. The lavage samples from asthma patients contained a higher proportion of cells expressing arginase I than did the other samples. These arginase-expressing cells appeared to be mononuclear macrophages. Similarly, arginase I expression was strong in biopsy samples from asthma patients but barely detectable in nonasthmatic samples. Expression was observed in submucosal inflammatory cell infiltrates as well as in patches of epithelial cells within the lung.

WHAT DOES ARGINASE DO?

Increased arginase activity could enhance production of polyamines, small signaling molecules that are important in cell growth and differentiation. Effects on cell growth could influence smooth muscle, fibroblasts, and lymphocytes; effects on cell differentiation could increase mucus production. Polyamines are also linked with increased smooth muscle contractility, which could worsen bronchoconstriction.

Another potential effect of increased arginase activity stems from the fact that the enzyme shares a substrate with nitric oxide synthase (NOS): arginine (which is illustrated on the cover). “Arginase and NOS compete for arginine, and therefore, induction of one indirectly affects the other,” Dr. Rothenberg noted. “Thus, elevated arginase likely lowers NO production—and its bronchodilatory action.” Reduced NOS activity is a phenomenon implicated in enhanced airway hyperresponsiveness in experimental asthma.

Although specific inhibitors of arginase I activity exist, the enzyme’s importance in the liver and other organs might be problematic. Theoretically, said Dr. Rothenberg, an arginase inhibitor “would be helpful … if you deliver the drug directly to the lung and [do] not affect liver arginase.” As an alternative, however, blocking elements upstream of arginase I induction might be more fruitful targets for therapy. For example, “IL-13 appears to be important, but it synergizes with IL-4,” he noted. “Both require STAT6 for arginase induction.”

—Mimi Zucker, PhD