Regulation of angiopoietin expression by bacterial lipopolysaccharide

Abstract

Angiopoietins are ligands for Tie-2 receptors and play important roles in angiogenesis and inflammation. While angiopoietin-1 (Ang-1) inhibits inflammatory responses, angiopoietin-2 (Ang-2) promotes cytokine production and vascular leakage. In this study, we evaluated in vivo and in vitro effects of Escherichia coli lipopolysaccharides (LPS) on angiopoietin expression. Wild-type C57/BL6 mice were injected with saline (control) or E. coli LPS (20 mg/ml ip) and killed 6, 12, and 24 h later. The diaphragm, lung, and liver were excised and assayed for mRNA and protein expression of Ang-1, Ang-2, and Tie-2 protein and tyrosine phosphorylation. LPS injection elicited a severalfold rise in Ang-2 mRNA and protein levels in the three organs. By comparison, both Ang-1 and Tie-2 levels in the diaphragm, liver, and lung were significantly attenuated by LPS administration. In addition, Tie-2 tyrosine phosphorylation in the lung was significantly reduced in response to LPS injection. In vitro exposure to E. coli LPS elicited cell-specific changes in Ang-1 expression, with significant induction in Ang-1 expression being observed in cultured human epithelial cells, whereas significant attenuation of Ang-1 expression was observed in response to E. coli LPS exposure in primary human skeletal myoblasts. In both cell types, E. coli LPS elicited substantial induction of Ang-2 mRNA, a response that was mediated in part through NF-κB. We conclude that in vivo endotoxemia triggers functional inhibition of the Ang-1/Tie-2 receptor pathway by reducing Ang-1 and Tie-2 expression and inducing Ang-2 levels and that this response may contribute to enhanced vascular leakage in sepsis.

Lipopolysaccharide (LPS)-mediated Angiopoietin-2-dependent Autocrine Angiogenesis Is Regulated by NADPH Oxidase 2 (Nox2) in Human Pulmonary Microvascular Endothelial Cells

“…The temporal increase in Ang2 and VEGF-A expression observed with LPS in HPMECs is consistent with rodent and human studies showing increased expression of Ang2 or VEGF-A in the whole lung or systemic circulation after LPS administration or systemic sepsis, respectively (12,(31)(32)(33). Our data indicate that pulmonary microvascular endothelial cells respond to bacterial ligands with robust Ang2, Tie2, and VEGF-A expression.…”

“…In the presence of VEGF, Ang2 induces migration, proliferation, and the sprouting of new blood vessels, whereas, in the absence of VEGF, endothelial cell death and vessel regression occur (7,11). Although systemic LPS administration has been shown to augment Ang2 expression in the liver, lung, and other tissues, the mechanisms involved in LPS-mediated Ang2 expression in lung endothelial cells remain unknown (12). Specifically, the involvement of redox signaling in the regulation of Ang2-dependent signaling and endothelial cell angiogenesis has not been studied.…”

Tie1 controls angiopoietin function in vascular remodeling and inflammation

“…Recombinant ANG1, inhibition of ANG2 or increasing Tie2 activity by therapeutic antibodies or a small-molecule phosphatase inhibitor have vascular protective effects in acute inflammation and in sepsis (32,33,34,62,72,73), consistent with an agonist action of ANG1 and antagonistic action of ANG2. Reduced Tie2 protein and Tie2 and Ang1 mRNA levels have been reported in murine sepsis and systemic inflammation (56,74). We confirmed these findings and additionally found reduced Tie1 mRNA in mice 3 to 12 hours after administration of endotoxin (LPS), a potent microbial mediator of the pathogenesis of Gram-negative bacterial sepsis and septic shock.…”

“…Decreased levels of Tie2 phosphorylation and protein have been observed in inflammatory diseases associated with increased ANG2 expression (56,57). In line with this, Tie2 staining in tracheal blood vessels decreased in a time-dependent manner from 1 to 16 hours after LPS challenge (Figure 8, A-C).…”

Angiopoietin-2 Enhances Survival in Experimental Sepsis Induced by Multidrug-ResistantPseudomonas aeruginosa

“…LPS is the most widely recognized bacterial PAMP that stimulates the production of Ang-2 from the vascular endothelium. As a consequence, increased tissue production has been documented in experimental mice and human volunteers (Kü mpers et al, 2008;Mofarrahi et al, 2008).…”