Rationale: Tie2 is predominantly expressed by endothelial cells and is involved in vascular integrity control during sepsis. Changes in Tie2 expression during sepsis development may contribute to microvascular dysfunction. Understanding the kinetics and molecular basis of these changes may assist in the development of therapeutic intervention to counteract microvascular dysfunction.
Objective: To investigate the molecular mechanisms underlying the changes in Tie2 expression upon lipopolysaccharide (LPS) challenge.
Methods and results: Studies were performed in LPS and pro-inflammatory cytokine challenged mice as well as in mice subjected to hemorrhagic shock, primary endothelial cells were used for in vitro experiments in static and flow conditions. Eight hours after LPS challenge, Tie2 mRNA loss was observed in all major organs, while loss of Tie2 protein was predominantly observed in lungs and kidneys, in the capillaries. A similar loss could be induced by secondary cytokines TNF-α and IL-1β. Ang2 protein administration did not affect Tie2 protein expression nor was Tie2 protein rescued in LPS-challenged Ang2-deficient mice, excluding a major role for Ang2 in Tie2 down regulation. In vitro, endothelial loss of Tie2 was observed upon lowering of shear stress, not upon LPS and TNF-α stimulation, suggesting that inflammation related haemodynamic changes play a major role in loss of Tie2 in vivo, as also hemorrhagic shock induced Tie2 mRNA loss. In vitro, this loss was partially counteracted by pre-incubation with a pharmacologically NF-кB inhibitor (BAY11-7082), an effect further substantiated in vivo by pre-treatment of mice with the NF-кB inhibitor prior to the inflammatory challenge.
Conclusions: Microvascular bed specific loss of Tie2 mRNA and protein in vivo upon LPS, TNFα, IL-1β challenge, as well as in response to hemorrhagic shock, is likely an indirect effect caused by a change in endothelial shear stress. This loss of Tie2 mRNA, but not Tie2 protein, induced by TNFα exposure was shown to be controlled by NF-кB signaling. Drugs aiming at restoring vascular integrity in sepsis could focus on preventing the Tie2 loss.