Abstract
Apoptotic endothelial cells are an important component of the “response to injury” process. Several atherosclerosis risk factors such as hyperglycemia and oxidized low-density lipoproteins, and immune injuries, such as antibodies and complement, induce endothelial cell apoptosis. While endothelial cell apoptosis is known to affect neighboring vascular wall cell biology, its consequences on macrophage reprogramming are ill defined. In this study, we report that apoptosis of human and mouse endothelial cells triggers the release of milk fat globule-epidermal growth factor 8 (MFG-E8) and reprograms macrophages into an anti-inflammatory cells. We demonstrated that MFG-E8 is released by apoptotic endothelial cells in a caspase-3-dependent manner. When macrophages were exposed to conditioned media from serum-starved apoptotic endothelial cells, they adopt a high anti-inflammatory, low pro-inflammatory cytokine/chemokine secreting phenotype that is lost if MFG-E8 is absent from the media. Macrophage treatment with recombinant MFG-E8 recapitulates the effect of conditioned media. Finally, we showed that MFG-E8-mediated reprogramming of macrophages occurs through increased phosphorylation of signal transducer and activator of transcription-3 (STAT-3). Taken together, our study suggests a key role of MFG-E8 release from apoptotic endothelial cells in macrophage reprogramming and demonstrates the importance of the apoptotic microenvironment in anti-inflammatory macrophage responses.
Highlights
Apoptosis, an ubiquitous form of cell death, occurs during embryogenesis in normal tissues and during inflammation
Murine endothelial cells (EC) (MEC) and human umbilical vein endothelial cells (HUVEC) apoptosis was associated with milk fat globule-epidermal growth factor 8 (MFG-E8) release in the serum starved-conditioned media (SSC) (Figure 1b, upper panels), whereas the intracellular MFG-E8 content declined in EC, whilst simultaneously exhibiting heightened expression of active caspase-3 fragments (Figure 1b, lower panels)
MFG-E8 was absent from media conditioned by necrotic EC suggesting that this protein is not released passively as a consequence of cell membrane permeabilization (Figure 1d)
Summary
An ubiquitous form of cell death, occurs during embryogenesis in normal tissues and during inflammation. It has been classically associated with a silent form of cell dismissal [1]. Recent evidence suggests that apoptotic cells can modulate their microenvironment and neighboring cell biology. Mounting evidence indicates that the paracrine component of the apoptotic program is not limited to the regulation of leukocyte trafficking, and prepares the cellular microenvironment for remodeling after apoptotic cell deletion. The reprogramming consequences of this apoptotic microenvironment on macrophages have not yet been fully characterized
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