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Hydrolysis of Surfactant Phospholipids Catalyzed by Phospholipase A₂ and Eosinophil Lysophospholipases Causes Surfactant Dysfunction^*

A Mechanism for Small Airway Closure in Asthma

^* From the University of Illinois at Chicago (Dr. Ackerman, Mr. Kwatia and Ms. Doyle), Chicago, IL; and the State University of New York at Buffalo (Dr. Enhorning), Buffalo, NY.

Correspondence to: Steven J. Ackerman, PhD, Department of Biochemistry and Molecular Biology (MC536), A-312 College of Medicine West, 1819 West Polk St, Chicago, IL 60612-7334; e-mail: sackerma{at}uic.edu

During asthma exacerbations, there is increased release of phospholipase A₂ (PLA₂) from inflammatory cells into the airway. PLA₂ has the capacity to hydrolyze the principal component of pulmonary surfactant, phosphatidylcholine (PC), which is responsible for maintaining airway patency in terminal bronchioles and alveoli. The products of PLA₂ hydrolysis of PC include fatty acids such as palmitic acid (PA) and lysophosphatidylcholine (LPC). The LPC then may serve as a substrate for eosinophil lysophospholipases (LPLases). LPLases are expressed by eosinophils in the lung in murine models of asthma, and surfactant dysfunction recently has been identified in patients with asthma exacerbations, as well as allergic subjects undergoing experimental segmental lung allergen challenges.

We first determined by Western blot analysis and reverse transcriptase polymerase chain reaction that eosinophils express a number of LPLases, including a 75-kd enzyme that is identical to pancreatic LPLase, and a 25-kd enzyme that was first cloned from the human brain. These LPLases may catalyze the hydrolysis of PLA₂-generated LPC in surfactant into additional PA and glycerophosphocholine, products lacking the ability to form an air-liquid interface film with sufficiently high surface pressure to keep small airways open. A capillary surfactometer was used to determine how the combined actions of PLA₂ and eosinophil LPLases might adversely affect the ability of the surfactant to maintain airway patency. The enzyme-induced changes in surfactant activity were correlated with the changes that occurred in surfactant phospholipid, LPC, and fatty acid composition as determined by thin layer chromatography (TLC). Calf lung surfactant extract (CLSE) was incubated alone, or was hydrolyzed with PLA₂ or a purified recombinant LPLase (25-kd), tested individually or in combination. CLSE was hydrolyzed for periods of 1 to 96 h, followed by capillary surfactometer measurements of surfactant activity. PLA₂ alone had essentially no effect on surfactant function, even though TLC showed that a significant amount of PC was converted into LPC and free fatty acid, mainly PA. The 25-kd LPLase caused only a minimal decrease in surfactant activity alone, but did degrade the small amount of LPC present in the CLSE.

In contrast, the combined action of the two enzymes resulted in a highly significant dose-dependent and time-dependent loss of surfactant function, concomitant with PC and LPC hydrolysis and the generation of free PA. PLA₂ alone was insufficient to generate a loss of surfactant function, possibly because the LPC product functionally replaces the lost PC. However, with the hydrolysis catalyzed by the eosinophil LPLase, the LPC concentration diminished significantly, likely leading to the loss of surfactant activity. Thus, the critical enzyme in this system is the 25-kd eosinophil LPLase. Equivalent results were obtained using whole-cell lysates of an eosinophil cell line (AML14.3D10) in the absence of exogenously added PLA₂, suggesting that the eosinophil expresses all of the necessary lipolytic activities. The inhibition of the LPLase activity in AML14.3D10 eosinophil extracts by a non-selective, sulfhydryl-reactive, ser/thr protease inhibitor, N-ethylmaleimide, reduced its ability to decrease surfactant activity.

These findings provide initial confirmation of our proposed hypothesis that eosinophil-derived phospholipases and LPLases may contribute to surfactant dysfunction in the asthmatic lung during periods of asthma exacerbations related to acute or chronic airway inflammation.

	Footnotes

 
Abbreviations: CLSE = calf lung surfactant extract; LPC = lyso-phosphatidylcholine; LPLase = lysophospholipase; PA = palmitic acid; PC = phosphatidylcholine; PLA₂ = phospholipase A₂; TLC = thin layer chromatography

This research was supported by National Institutes of Health grant AI25230 (SJA) and by a grant From ONY, Inc (GE).

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