Studies on leukotriene B4 and alarmins in inflammatory responses

Abstract

Leukotriene B4 (LTB4) is a potent proinflammatory lipid mediator that is involved in host defense and inflammatory diseases, such as atherosclerosis. LL-37 and heparin binding protein (HBP) are cationic antimicrobial polypeptides, which belong to the alarmin family known to promote innate and adaptive immune reactions in response to tissue infection or injury. In the present thesis, the aims were to investigate the expression profile of enzymes and receptors for LT biosynthesis in atherosclerotic lesions and study the mechanisms of LTB4/LL-37 and LTB4/HBP interactions, the functions of these interactions and how anti-inflammatory lipids may interfere with the LTB4/LL-37 circuit.

We found that mRNA levels of 5-lipoxygenase (5-LO), 5-lipoxygenase-activating protein (FLAP) and leukotriene A4 hydrolase (LTA4H), are significantly increased in human atherosclerotic plaques. Immunostaining confirmed abundant expression of these enzymes, colocalized in macrophages of intimal lesions. Furthermore, we have shown that in lesions of human tissues arachidonic acid may be converted into LTB4, which is blocked by a selective LTA4H inhibitor. In addition, expression of 5-LO and LTA4H, but not FLAP, is increased in patients with recent or ongoing symptoms of plaque instability.

In search for the mechanisms by which LTB4 could exert a proinflammatory action within the vascular wall, we have found that LTB4 strongly promotes the release of LL-37 and HBP from human polymorphonuclear neutrophils (PMNs) in a time- and dosedependent manner. The induced release of LL-37 and HBP by LTB4 stimulation is mediated by the BLT1 receptor. Furthermore, protein phosphatase-1 (PP-1) inhibits the release by suppressing the BLT1-mediated exocytosis of PMN granules.

LL-37 does not only exhibit potent antimicrobial activities, but the stimulation of PMNs with LL-37 also induces intracellular calcium ([Ca2+]i) mobilization in a dosedependent manner resulting in cPLA2 phosphorylation and translocation of 5-LO from the cytosol to the perinuclear membrane. Thus, LL-37 promotes the synthesis and release of LTB4 in intact or primed PMNs. This response is mediated by formyl peptide receptor like-1 (FPRL-1). Apparently, in human PMNs, positive feedback circuits exist between LTB4 and LL-37. Furthermore, this LTB4/LL-37 feedback loop is extended to functional responses, such as phagocytosis. We have also found that the two antiinflammatory lipids, resolvin E1 (RvE1) and lipoxin A4 (LXA4) inhibit the release of LL-37 by LTB4 stimulation and the production of LTB4 by LL-37 induction, respectively. These compounds may serve as negative brake signals for the positive LTB4/LL-37 feedback circuit.

Moreover, we have shown that postsecretory supernatants from LTB4-stimulated PMNs induce [Ca2+]i mobilization in endothelial cells in vitro and enhance vascular permeability in vivo by employing a mouse model of pleurisy. Selective removal of HBP from the supernatants significantly reduces these activities, attributing a key role to HBP in LTB4-induced increase in vascular permeability.

Taken together, we have provided indirect evidence that LTB4 plays a role in plaque instability and this mediator may act in synergy with LL-37 and HBP to promote vascular inflammation. These lipid-petide interactions may be regulated by endogenous anti-inflammatory lipids and offer novel opportunities for pharmacological intervention in inflammation.