Leukotriene A<sub>4</sub> Hydrolase: The Janus Enzyme Shows Its Ugly Side in Smokers
Authors: Robert Snelgrove, Farrah Kheradmand
Journal: American Journal of Respiratory and Critical Care Medicine (2014)

Many uninvited immune cells find their way into the lungs of smokers with chronic obstructive pulmonary disease (COPD). Although neutrophils and macrophages are among the first cells to arrive in response to cigarette smoke, animal models of smoke-induced emphysema have confirmed the pathogenic role of activated T and B cells, as well as antigen-presenting cells that act upstream of lymphocyte activation (1). Similarly, in smokers with emphysema, the acquired immune responses persist long after smoking cessation, and autoreactive T cells could predict loss of lung function (2). Although memory T and B cell responses are reactive to specific self- or neo- (modified-self) antigens, how innate immune cells, such as neutrophils that lack recall responses, continue their omnipresence in the lungs of former smokers with COPD has remained a mystery.

ELR+ CXC chemokines (e.g., IL-8) bind to CXCR1 and CXCR2 receptors expressed on neutrophils, and attract them to sites of inflammation. Ensuing degradation of collagen by neutrophil-derived proteases (Figure 1) generates many tripeptide proline-glycine-proline (PGP) peptides that mimic key sequences found in ELR+ CXC chemokines, and act as a potent neutrophil chemoattractant (3). PGP can subsequently undergo N-terminal acetylation to produce AcPGP, which exhibits enhanced chemotactic activity, and both peptides have been implicated in the pathophysiology of COPD: significant concentrations of PGP/AcPGP have been detected in bronchoalveolar lavage (BAL) fluid, sputum, and plasma of patients with COPD, spiking immediately preceding an exacerbation (3–5); repeated intratracheal administration to mice of AcPGP alone drives emphysema (3); cigarette smoke exposure of mice drives AcPGP accumulation, and neutralization of this peptide ameliorates neutrophilic inflammation and emphysema (6).



Figure 1.

Cigarette smoke disrupts leukotriene (LT) A4 hydrolase (LTA4H)–mediated resolution of pulmonary neutrophilic inflammation. (1) In response to infection or injury, resident cells within the lung will release chemoattractants that will promote neutrophil ...



Extracellular leukotriene (LT) A4 hydrolase (LTA4H) degrades PGP and resolves neutrophilic inflammation (7), whereas intracellular epoxide hydrolase function of the same enzyme converts LTA4 to LTB4. LTB4 is a proinflammatory lipid mediator capable of recruiting and activating an array of cells, including neutrophils. Thus, LTA4H exhibits opposing pro- and anti-inflammatory roles that govern neutrophil recruitment. Although efficient clearance of PGP occurs in acute neutrophilic inflammation, this system is perturbed by cigarette smoke in two ways. First, cigarette smoke chemically acetylates PGP, enhancing its chemotactic activity and protecting it from degradation by LTA4H. Second, biochemical and preliminary murine studies suggest that cigarette smoke can selectively abrogate the peptidase activity of LTA4H, with minimal effect on the hydrolase activity (7). Thus, cigarette smoke pushes LTA4H toward a uniquely proinflammatory phenotype, whereby LTB4 and PGP together induce increased pulmonary neutrophilic inflammation.

In this issue of the Journal, the article by Wells and colleagues (pp. 51–61) extends these latter studies in a cigarette smoke–induced emphysema model and translates them into two human smoking/COPD cohorts (8). Cigarette smoke–exposed mice displayed elevated BAL fluid concentrations of PGP and a profound loss of LTA4H peptidase activity, while maintaining LTA4H hydrolase activity and LTB4 production. Consistently, healthy smokers displayed elevated LTA4H expression and LTB4 concentration, but with selective inhibition of the enzyme’s peptidase activity with ensuing PGP accumulation. This phenotype persisted in patients with COPD, and was intriguingly maintained despite smoking cessation—hinting at a mechanism that may underlie failure of neutrophilic inflammation to resolve.

Although elevated concentrations of PGP have been previously reported in patients with COPD (4), it is noteworthy in this article that healthy smokers also displayed elevated PGP owing to reduced LTA4H peptidase activity. Concentrations of PGP in the BAL fluid of healthy smokers and patients with COPD were found to be indistinguishable; however, AcPGP was only elevated in those with disease, potentially pointing to acetylation as a key process in disease progression. Despite diminished LTA4H peptidase activity persisting in ex-smokers with COPD AcPGP levels are seemingly reduced, most likely due to a reduction in PGP-generating enzymes. Given these findings, it will be intriguing to elucidate if infection could induce COPD exacerbation by driving a further spike in PGP-generating enzymes.

The authors show that acrolein in cigarette smoke in part inhibits LTA4H peptidase activity. Thus, acrolein perturbs the PGP–LTA4H pathway in two ways: selective inhibition of the peptidase function presented here, and through the acetylation and protection of the peptide previously demonstrated (9). Acrolein is a component of cigarette smoke, but can also be generated physiologically during inflammation. A precedent exists for sulphydryl blocking agents selectively inhibiting the peptidase activity of LTA4H, and this could represent a means by which acrolein selectively abrogates this important function (10). Interestingly, acrolein scavengers, such as N-acetylcysteine and carbocysteine, have shown some clinical benefit in COPD trials (11, 12), which could feasibly now be in part attributable to a reduction in PGP/AcPGP.

There is now a compelling case for a role of PGP/AcPGP in the pathogenesis of COPD. The concept of a protease imbalance and matrikine-driven, self-sustaining cycle of inflammation in COPD progression is appealing, but does PGP instigate inflammation/neutrophilia, or, rather, is it a biomarker of the disease? A study investigating the use of macrolides in COPD demonstrated amelioration of symptoms, and reduced neutrophilia and PGP concentration (5), but in which order do these events occur? Interestingly, neutralization of IL-8 and LTB4 in COPD sputum/BAL fluid partially inhibits its capacity to drive neutrophilic inflammation, implying that there are other key mediators, of which PGP is an obvious candidate (13).

If PGP is a key player in COPD development, then what is the best way to target it for pharmaceutical intervention? Targeting PGP-generating enzymes may be problematic, owing to the pleiotropic roles of matrix metalloproteinases and prolyl endopeptidase. Complementary peptides that neutralize PGP (arginine-threonine-arginine) (14) may offer therapeutic potential, as would CXCR1 and/or CXCR2 antagonists. The use of N-acetylcysteine or carbocysteine, which scavenge acrolein, may prove beneficial either alone or in combination with a novel class of LTA4H peptidase activators (15). Furthermore, the studies described by Wells and colleagues highlight the risk of limiting PGP degradation by LTA4H, with clear repercussions for current therapeutic strategies that target LTA4H, given how such inhibitors may fail to distinguish between the opposing activities of the enzyme, and could inadvertently lead to persistent neutrophilia. Going forward, it will be important to more fully characterize and ascertain the significance of the PGP pathway in these and other chronic neutrophilic diseases of the lung and extrapulmonary sites, as it is clearly one that represents an attractive, novel therapeutic strategy.