The most important finding in the present
The most important finding in the present study is that the EKR1/2-Egr-1 signaling pathway might be involve in the mechanisms underlying CysLT2 receptor-mediated IL-8 production. The signaling profile can be described by the following sequential processes: the agonists (LTC4 and LTD4) activates CysLT2 receptors and results in [Ca2+]i elevation, then EKR1/2 is phosphorylated to activate Egr-1, and Egr-1 finally regulates IL-8 production (Fig. 10). This may be an alternative mechanism in addition to the reported PKCδ/AP-1 and PKCɛ/NF-κB pathways (Thompson et al., 2008).
In this signal pathway, as the first step of the signaling, our results confirmed the agonist-induced elevation of [Ca2+]i, a post-activation signaling of CysLT1 and CysLT2 receptors (Back et al., 2011). The next step, ERK1/2 phosphorylation, may resulted from the initial response of [Ca2+]i elevation since ERK1/2 phosphorylation is a calcium-dependent process in other cell types (Carmona et al., 2010, Huang et al., 2012, Trevisi et al., 2010). Then, the activated ERK1/2 regulates LTC4-induced Egr-1 activation. As the supporting findings, we found that LTC4 enhanced ERK1/2 phosphorylation (Fig. 8B), and the ERK1/2 inhibitor U01216 (but not JUN and p38 inhibitors) inhibited LTC4-induced Egr-1 expression, especially in the nucleus, and IL-8 release (Fig. 7). This has also been reported in human intestinal epithelial cells (Moon et al., 2007a, Moon et al., 2007b). Finally, we demonstrated that Egr-1 regulates LTC4-induced IL-8 production because down-regulation of Egr-1 by RNA interference with siRNA inhibited IL-8 release (Fig. 9). Similarly, in human epithelial intestine 407 cells, Egr-1 regulates IL-8 secretion after exposure to ribotoxin deoxynivalenol (Moon et al., 2007a). The present study provided a line of evidence to reveal one of the signaling pathways for CysLT2 receptor-mediated production of the inflammatory mediator IL-8.
In summary, we found that CysLTs can activate CysLT2 receptor to induce IL-8 production which is mediated by the ERK1/2-Egr-1 pathway in HEK293 cells stably expressing CysLT2 receptors. Our study highlights the molecular mechanisms of the CysLT2 receptor in inflammation. The ERK1/2-Egr-1 pathway will contribute to understanding the role of CysLT2 receptors in inflammation development and pharmacological management. The implications of this signal pathway remain to be elucidated.
Introduction The leukotrienes are a group of major pro-inflammatory lipid mediators derived from the lipoxygenase pathway of the arachidonic L189 mg metabolism and were so named, because of the salient feature that they can be produced by leukocytes and they have a common conjugated triene in their structure. The leukotrienes consist of cysteinyl leukotrienes (cysLTs) and leukotriene B4 (LTB4). The various subtypes known for CysLTs are leukotriene C4 (LTC4), leukotriene D4 (LTD4) and leukotriene E4 (LTE4) (also shown in Fig. 1). CysLTs and LTB4 mediates their biological activity through G protein-coupled receptors (GPCR) i.e. cysLT receptors (cysLT-1 and cysLT-2) and BLT receptors (BLT1 and BLT2) respectively (Ghosh et al., 2016). The cysLTs are characterized by the presence of a cysteine ring whereas LTB4 is a non-cysteine containing dihydroxy-leukotriene (Capra, 2004). The biological properties of leukotrienes suggest that cysLTs in particular, play an important role in the pathogenesis of allergic and severe asthma (Capra et al., 2007). During the inflammatory activation, these leukotrienes are synthesized and become functional. CysLTs induce smooth muscle contraction, vascular leakage, eosinophil recruitment, mucus production and chemotaxis, whereas LTB4 induces leukocyte chemoattraction, particularly of granulocytes and T cells, rapid invasion and recruitment of these cells to the plasma membrane of endothelial cells and production of reactive oxygen species. Studies have shown weak expression of cysLT receptors in healthy brain but a significant increase is observed during pathological conditions (Fang et al., 2006, Fang et al., 2007). CysLT-1 and cysLT-2 receptor contain 336 and 345 amino acid residues respectively. The cyslt-1 receptor was found to be localized on X chromosome (Xq13–Xq21) while and cysLT-2 receptor was found on chromosome 13q14 (Singh et al., 2010). CysLT-1 receptor shows a higher affinity towards LTD4 but lower affinity towards LTC4 and LTE4 while cysLT-2 receptor shows an equal affinity for both LTC4 and LTD4 but lower affinity for LTE4 (Gelosa et al., 2017). Interestingly, studies have reported that astrocytic proliferation is mainly mediated through the activation of cysLT-1 receptors (Huang et al., 2008, Huang et al., 2012) while microglial activation was mediated through the activation of CysLT-2 receptors (Chu et al., 2006, Shi et al., 2015). Three cysLT-1 antagonists are already in the market viz. pranlukast, zafirlukast, montelukast. The specific LTB4 modulators, BLT antagonists, on the other hand, are still in the preliminary stages of clinical development (Montuschi et al., 2007). Although leukotrienes are traditionally known for their role in allergic asthma, allergic rhinitis; the recent literature has highlighted the role of these inflammatory mediators in a broader range of diseases such as in the inflammation associated with the central nervous system (CNS) disorders, vascular inflammation (atherosclerosis), and in cancer (Gelosa et al., 2017, Ghosh et al., 2016). Among the CNS diseases, cysteinyl leukotrienes along with their synthesis precursor enzyme 5-lipoxygenase (5-LOX) and their receptors have been shown to be associated with brain injury, multiple sclerosis, Alzheimer’s disease (AD), Parkinson’s disease, brain ischemia, epilepsy, Huntington’s disease and depression (Ghosh et al., 2016). However, a lot more remains to be elucidated as the research in these areas have emerging interest and only a little has been known so far. Herein, through this review, we have provided current and emerging information on the role of cysLTs and their receptors in the progression and development of neuromodulatory complications associated with the pathophysiology of AD, with an insight on the future perspectives.