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Macrophage Migration Inhibitory Factor Governs Endothelial Cell Sensitivity to LPS-Induced Apoptosis

¹ Department of Medicine, Division of Pulmonary and Critical Care Medicine, and ² The Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University, Baltimore, Maryland

Correspondence and requests for reprints should be addressed to Michael T. Crow, Ph.D., Department of Medicine, Division of Pulmonary and Critical Care Medicine, Johns Hopkins University, Johns Hopkins Asthma and Allergy Center, Rm 5A.58, 5501 Hopkins Bayview Circle, Baltimore, MD 21224. E-mail: mcrow1{at}jhmi.edu

Human endothelial cells (EC) are typically resistant to the apoptotic effects of stimuli associated with lung disease. The determinants of this resistance remain incompletely understood. Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine produced by human pulmonary artery EC (HPAEC). Its expression increases in response to various death-inducing stimuli, including lipopolysaccharide (LPS). We show here that silencing MIF expression by RNA interference (MIF siRNA) dramatically reduces MIF mRNA expression and the LPS-induced increase in MIF protein levels, thereby sensitizing HPAECs to LPS-induced cell death. Addition of recombinant human MIF (rhMIF) protein prevents the death-sensitizing effect of MIF siRNA. A common mediator of apoptosis resistance in ECs is the death effector domain (DED)-containing protein, FLIP (FLICE-like inhibitory protein). We show that LPS induces a transcription-independent increase in the short isoform of FLIP (FLIP_s). This increase is blocked by MIF siRNA but restored with the addition of recombinant MIF protein (rHMIF). While FLIP_s siRNA also sensitizes HPAECs to LPS-induced death, the addition of rhMIF does not affect this sensitization, placing MIF upstream of FLIP_s in preventing HPAEC death. These studies demonstrate that MIF is an endogenous pro-survival factor in HPAECs and identify a novel mechanism for its role in apoptosis resistance through the regulation of FLIP_s. These results show that MIF can protect vascular endothelial cells from inflammation-associated cell damage.

Key Words: endothelial cells • macrophage migration inhibitory factor • FLICE-like inhibitory protein • apoptosis

CLINICAL RELEVANCE Our study shows that macrophage migration inhibitory factor acts to protect vascular endothelial cells from sepsis-associated cell damage through its novel regulation of the short isoform of FLICE-like inhibitory protein.