Slit2-Robo4 Pathway Modulates Lipopolysaccharide-Induced Endothelial Inflammation and Its Expression Is Dysregulated during Endotoxemia

The secretory protein Slit2 and its receptors Robo1 and Robo4 are considered to regulate mobility and permeability of endothelial cells and other cell types. However, the roles of Slit2 and its two receptors in endothelial inflammatory responses remain to be clarified. In this study, we show that, in primary HUVECs, Slit2 represses LPS-induced secretion of certain inflammatory cytokines/chemokines, cell adhesion molecule ICAM-1 upregulation, and monocyte adhesion. Slit2's anti-inflammatory effect is mediated by its dominant endothelial-specific receptor Robo4. However, the minor receptor Robo1 has proinflammatory properties and is downregulated by Slit2 via targeting of miR-218. Elucidation of molecular mechanism reveals that Slit2 represses inflammatory responses by inhibiting the Pyk2-NF-kappa B pathway downstream of LPS-TLR4. Further studies reveal that LPS enhances endothelial inflammation by downregulating the anti-inflammatory Slit2 and Robo4 in HUVECs in vitro, as well as in arterial endothelial cells and liver in vivo during endotoxemia. These results suggest that Slit2-Robo4 signaling is important in regulating LPS-induced endothelial inflammation, and LPS, in turn, enhances inflammation by interfering with the expression of the anti-inflammatory Slit2-Robo4 during the disease state. This implies that Slit2-Robo4 is a key regulator of endothelial inflammation, and its dysregulation during endotoxemia is a novel mechanism for LPS-induced vascular pathogenesis.