Methicillin-resistant Staphylococcus aureus enhances alveolar epithelial cell permeability through vascular endothelial growth factor and cytoskeletal disruption

Background

Methicillin-resistant Staphylococcus aureus (MRSA) is now a common infection encountered in hospitals and communities. We have previously shown that MRSA causes reactive oxygen and nitrogen (ROS/RNS)-dependent increased vascular permeability, multiorgan system failure and death in our ovine model. Using type II alveolar epithelial cells (A549), we hypothesized that MRSA increases expression of vascular endothelial growth factor (VEGF), a regulator of vascular permeability to water and proteins, and disrupts barrier function by disrupting cytoskeletal integrity.

Methods

A549 cells were challenged with 10⁵ colony-forming units MRSA over a time course of 24 hours and were visualized for markers of ROS/RNS formation (2,7-dichlorodihydrofluorescein), as well as VEGF and actin expression by confocal imaging and western blot analyses. Cellular permeability was measured by quantifying FITC-dextran flow through a monolayer of A549 cells.

Results

MRSA caused a significant 7.4-fold increase in 2,7-dichloro-dihydrofluorescein fluorescence over unchallenged controls. L-NAME, an inhibitor of nitric oxide formation, blocked this response. Western blot analyses confirmed the confocal observations that MRSA caused an 8.15-fold increase in VEGF expression, versus cells that were pre-incubated with L-NAME (3.4-fold). MRSA also induced formation of actin stress fibers and subsequent cellular contraction. In support of these observations, MRSA caused a 405% increase in cellular permeability to FITC-dextran. However, pre-incubation with L-NAME had no effect on MRSA-induced barrier dysfunction.

Conclusion

MRSA induces VEGF expression in a ROS/RNS-dependent manner. MRSA also causes alveolar epithelial cell barrier dysfunction by disrupting the actin cytoskeleton independent of nitric oxide synthase activity. Together, the data suggest that MRSA-increased vascular permeability in the lung may be due, in part, to disruption of the cytoskeletal integrity and increased expression of VEGF, but the overall mechanism involves multiple pathways and requires further study.