IL-32–dependent effects of IL-1β on endothelial cell functions

Abstract

Increasing evidence demonstrates that interleukin (IL)–32 is a pro-inflammatory cytokine, inducing IL-1α, IL-1β, IL-6, tumor necrosis factor (TNF)–α, and chemokines via nuclear factor (NF)–κB, p38 mitogen-activated protein kinase (MAPK), and activating protein (AP)-1 activation. Here we report that IL-32 is expressed and is also functional in human vascular endothelial cells (EC) of various origins. Compared with primary blood monocytes, high levels of IL-32 are constitutively produced in human umbilical vein EC (HUVEC), aortic macrovascular EC, and cardiac as well as pulmonary microvascular EC. At concentrations as low as 0.1 ng/ml, IL-1β stimulated IL-32 up to 15-fold over constitutive levels, whereas 10 ng/ml of TNFα or 100 ng/ml of lipopolysaccharide (LPS) were required to induce similar quantities of IL-32. IL-1β–induced IL-32 was reduced by inhibition of the IκB kinase-β/NF-κB and ERK pathways. In addition to IL-1β, pro-coagulant concentrations of thrombin or fresh platelets increased IL-32 protein up to 6-fold. IL-1β and thrombin induced an isoform-switch in steady-state mRNA levels from IL-32α/γ to β/ε. Adult EC responded in a similar fashion. To prove functionality, we silenced endogenous IL-32 with siRNA, decreasing intracellular IL-32 protein levels by 86%. The knockdown of IL-32 resulted in reduction of constitutive as well as IL-1β–induced intercellular adhesion molecule–1 (ICAM-1) (of 55% and 54%, respectively), IL-1α (of 62% and 43%), IL-6 (of 53% and 43%), and IL-8 (of 46% and 42%). In contrast, the anti-inflammatory/anti-coagulant CD141/thrombomodulin increased markedly when IL-32 was silenced. This study introduces IL-32 as a critical regulator of endothelial function, expanding the properties of this cytokine relevant to coagulation, endothelial inflammation, and atherosclerosis.