To Örebro University

Endothelial cells lining the luminal side of blood vessels creates a barrier between the circulating blood and the extracellular matrix. Endothelial cells have important functions in regulation of vessel tension and inflammation. Furthermore, endothelium-derived vasodilators prevent our smallest blood cells, platelets, to aggregate in the circulation. The main physiological role of platelets is to protect us from bleeding by creating aggregates at sites of injury. Platelets is also increasingly recognised as mediators in acute inflammation. The focus of this thesis has been to study the impact of inflammatory cytokines in the interleukin (IL)-6 family on endothelial cells and platelets. IL-6 has pleiotropic effects where IL-6 trans-signalling via the soluble IL-6 receptor (IL-6R) is associated to more pro-inflammatory outcomes than classic signalling via the membrane bound IL-6R. Both classic and trans-signalling need the ubiquitously expressed glycoprotein (gp)130 to induce intracellular signalling. Since the IL-6R is expressed on a restricted number of cell types, transsignalling exerts a broader IL-6 response. Paper I reveal that endothelial cells express IL-6R which facilitates both classic and trans-signalling. IL-6 trans-signalling activates more signalling pathways and results in proinflammatory responses in contrast to classic signalling. Paper II show that IL-6 trans-signalling, but not classic signalling occurs in platelets and results in inhibition of epinephrine-induced platelet aggregation. Paper III reveal inter-individual differences in platelet reactivity towards activators and the inhibitor nitric oxide (NO). Individuals with more NOsensitive platelets showed greater capacity of vasodilation, indicating a connection between endothelial function and platelet inhibition. In Paper IV, the impact of various gp130 signalling cytokines on endothelial cells revealed differences and similarities in intracellular signalling, gene expression and protein release. In summary, this thesis investigates the impact of the IL-6 family cytokines on endothelial cells and platelets in regards of intracellular signalling and functional responses.

Interleukin-6 is pleotropic cytokine produced by several types of cells including endothelial cells (ECs). IL-6 acts on target cells via two major signaling mechanisms known as classic signaling and trans-signaling. Whileactivation of IL-6 classic signaling is associated with homeostatic and tissue regeneration functions, the trans-signaling is linked to pro-inflammatory effects. Our studies reveal that ECs respond to both IL-6 classic- aswell as trans-singling pathways in distinct but also overlapping manner.While IL-6 classic-signaling activated JAK/STAT3 pathway, the trans-signaling additionally engaged PI3K/AKT and MAPK/ERK pathways. Further, IL-6 trans-singling, but not classic signaling, led to secretion of proinflammatory chemokine MCP-1 mainly via JAK/STAT3 and PI3K/AKTpathways. In addition, IL-6 trans-signaling regulate expression of angiogenesis related genes to subsequently impair endothelial tube formationability. Autocrine IL-6 classic-signaling, however, was vital to maintainthe angiogenic response of ECs. Further proteomic analyses showed thatIL-6 trans-signaling in ECs regulates secretion of several inflammatoryproteins and also shifts laminin secretion from LAMA4 to LAMA5, whichmight collectively favor binding and trans-endothelial migration of mononuclear cells. In human atherosclerotic plaques, we found that expression of LAMA4 and LAMA5 is altered compared to healthy vessels, andthat the alteration appears to be associated with immune cell content andstability of the plaque. Using plasma IL-6 binary complex, a novel biomarker, we showed a strong association between IL-6 trans-signalingand risk of future myocardial infarction (MI). In addition, we showed thatelevated plasma IL-6 binary complex mediates the association betweentraditional risk factors (hypertension and smoking) and MI, suggestingthat elevated plasma IL-6 binary complex concentration could partly explain the increased risk of MI in smokers and hypertensive participants.