The Alarmin Interleukin-33 is a Notch Target in Quiescent Endothelial Cells (174.6)

The molecular mechanisms that induce and maintain expression of the alarming interleukin-33 (IL-33) in endothelial cells are unknown. Based on the observation that IL-33 is also a marker of endothelial cell quiescence, we hypothesized that the Notch signaling pathway might be involved in regulation of IL-33 expression. Here we show that activation of Notch1 by immobilized Notch ligands was sufficient to induce nuclear IL-33 expression in human umbilical vein endothelial cells (HUVECs). Furthermore, IL-33 expression in confluent HUVECs was inhibited by the γ-secretase inhibitor DAPT or by blocking antibodies to Dll4, Jagged1 or Notch1, as well as by knockdown of the transcription factor RBP-Jκ. In vivo, expression of Dll4 but not Jagged1 was generally observed in all segments of quiescent vessels in healthy organs in both human and rat tissue, and therefore well correlated with expression of IL-33. Moreover, subcutaneous injections of DAPT markedly reduced expression of vascular IL-33, compatible with a role for Notch signaling in driving vascular IL-33 expression in vivo. Taken together, our data demonstrate that the alarmin IL-33 is a target of Notch and serve to further establish Notch signaling in innate immune defense.

Molecular interplay between endostatin, integrins, and heparan sulfate

Endostatin is an endogenous inhibitor of angiogenesis. Although several endothelial cell surface molecules have been reported to interact with endostatin, its molecular mechanism of action is not fully elucidated. We used surface plasmon resonance assays to characterize interactions between endostatin, integrins, and heparin/heparan sulfate. alpha5beta1 and alphavbeta3 integrins form stable complexes with immobilized endostatin (KD=approximately 1.8x10(-8) M, two-state model). Two arginine residues (Arg27 and Arg139) are crucial for the binding of endostatin to integrins and to heparin/heparan sulfate, suggesting that endostatin would not bind simultaneously to integrins and to heparan sulfate. Experimental data and molecular modeling support endostatin binding to the headpiece of the alphavbeta3 integrin at the interface between the beta-propeller domain of the alphav subunit and the betaA domain of the beta3 subunit. In addition, we report that alpha5beta1 and alphavbeta3 integrins bind to heparin/heparan sulfate. The ectodomain of the alpha5beta1 integrin binds to haparin with high affinity (KD=15.5 nM). The direct binding between integrins and heparin/heparan sulfate might explain why both heparan sulfate and alpha5beta1 integrin are required for the localization of endostatin in endothelial cell lipid rafts.