Contribution of HMSE-1 to surfactant conversion under acute inflammatory conditions

Introduction

A reduced content of biophysically active large surfactant aggregates is a common finding in acute inflammatory lung disease. Cyclic surface area changes and a carboxylesterase activity (surfactant convertase) are thought to mediate this subtype conversion. However, data concerning regulation of surfactant convertase are scarce. We therefore investigated the expression and activity of lung surfactant convertase and HMSE-1, a potential macrophage-derived human convertase, under normal and acute inflammatory conditions.

Methods

Convertase activity in lavage fluid (BALF) was assessed using the in vitro cycling assay. The relative large surfactant aggregate content was determined by phospholipid quantification in the pellet following centrifugation at 48,000 × g. Esterase activity was assessed by means of a chromogenic substrate assay. Expression of both convertase and HMSE upon LPS challenge was assessed by real-time (TaqMan) PCR in murine alveolar macrophages, murine primary type II cells, and the human monocytic cell line U937, respectively.

Results

Lavage fluid from ARDS patients displayed an increased esterase activity when compared with BALF from healthy controls. In addition, a pronounced large to small aggregate conversion was observed for BALF from LPS-challenged mice or BALF from ARDS patients. Incubation with LPS resulted in a significant increase in convertase gene expression in primary mouse type II cells as well as in HMSE-1 gene expression in U937 cells and monocytes from peripheral blood. No convertase expression was found in cultured murine alveolar macrophages.

Conclusion

An increased convertase activity was found under acute inflammatory conditions of the alveolar compartment, and type II cells seem to be a relevant source of this increased convertase activity. However, leakage of esterase activity from the vascular space and other inflammatory cells cannot be ruled out.