Asthma and lower airway disease
Chitotriosidase is the primary active chitinase in the human lung and is modulated by genotype and smoking habit

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Background

Chitinolytic enzymes play important roles in the pathophysiology of allergic airway responses in mouse models of asthma. Acidic mammalian chitinase (AMCase) and chitotriosidase (CHIT1) have chitinolytic activity, but relatively little is known about their expression in human asthma.

Objective

We sought to determine the expression and activity of AMCase and CHIT1 in healthy subjects, subjects with asthma, and habitual smokers, taking account of the null 24-bp duplication in the CHIT1 gene.

Methods

We measured chitinase activity in bronchoalveolar lavage (BAL) fluid at multiple pHs by using a synthetic chitin substrate. We also determined AMCase and CHIT1 gene expression in epithelial brushings and BAL fluid macrophages by means of real time RT-PCR. Paired DNA samples were genotyped for the CHIT1 duplication.

Results

In all subgroups the pH profile of chitinase activity in BAL fluid matched that of CHIT1, but not AMCase, and chitinase activity was absent in subjects genetically deficient in active CHIT1. Although AMCase protein was detectable in lavage fluid, AMCase transcripts in macrophages were consistent with an isoform lacking enzymatic activity. Median chitinase activity in BAL fluid tended to be lower than normal in asthmatic subjects but was increased 7-fold in habitual smokers, where CHIT1 gene expression in macrophages was increased.

Conclusions

Chitinase activity in the lung is the result of CHIT1 activity. Although AMCase protein is detectable in the lung, our data indicate that it is inactive. Chitinase activity is not increased in subjects with asthma and in fact tends to be decreased. The high levels of chitinase activity in habitual smokers result from upregulation of CHIT1 gene expression, especially in macrophages.

Section snippets

Subjects and clinical samples

We studied biologic samples stored in the Airway Tissue Bank at the University of California, San Francisco, that had been collected during research bronchoscopy from 40 nonsmoking subjects with asthma, 25 habitual smokers without asthma, and 26 healthy nonsmoking control subjects (Table I).15 Asthmatic subjects had a prior physician's diagnosis of asthma, a PC20 methacholine value of less than 8 mg/mL, and were using only inhaled β-agonist medications for therapy (additional data on the asthma

Lung chitinase activity is modulated by pH and smoking habit

We measured total chitinase activity in the BAL fluid of 77 subjects, including 31 asthmatic subjects, 24 healthy subjects, and 22 habitual smokers, using a synthetic chitin substrate, 4-MU-(4-deoxy)chitobiose.21 This substrate is digested by both AMCase and CHIT1, but the pH profile for the activity of these 2 chitinolytic enzymes is distinct and can be used to infer whether CHIT1 or AMCase is the responsible enzyme for chitinase activity in a biologic sample. For example, previous studies

Discussion

We characterized the relative contribution of both active human chitinases, CHIT1 and AMCase, to chitinase activity in the lung from healthy subjects, asthmatic subjects, and habitual smokers. We determined CHIT1 to be the primary active chitinase in the lung, and we found that its expression is strongly dependent on genetics and on smoking habit.

We used a multifaceted approach to establish that CHIT1, but not AMCase, is the principal active chitinase in the human lung, including determination

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    Supported by National Institutes of Health grants AI077439 (J.V.F. and E.G.B.), HL080414 (J.V.F.), HL078885 (E.G.B.), and RR17002 (P.G.W.); the Sandler Asthma Basic Research Center (J.V.F. and E.G.B.); and the Sandler Program for Asthma Research (E.G.B.).

    Disclosure of potential conflict of interest: A. Innes has received research support from Genentech. P. G. Woodruff has received research support from Boehringer Ingelheim and Genentech. J. V. Fahy has received research support from Genentech, Roche, and Boehringer Ingelheim. The rest of the authors have declared that they have no conflict of interest.

    These authors contributed equally to this work.

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