A turn on and a turn off: BLT1 and BLT2 mechanisms in the lung

Abstract

Leukotriene B₄ (LTB₄), a potent lipid mediator of inflammation derived from arachidonic acid through the action of 5-lipoxygenase, has been implicated in the pathophysiology of several inflammatory diseases, including asthma and chronic obstructive pulmonary disease. A high-affinity LTB₄ receptor BLT1 has been shown to exert proinflammatory roles. A cyclooxygenase metabolite, 12(S)-hydroxyheptadeca-5Z, 8E, 10E-trienoic acid (12-HHT), is an endogenous ligand for BLT2, a low-affinity LTB₄ receptor. The recent study indicated that BLT2 has a protective role in allergic airway inflammation, suggesting different functions between BLT1 and BLT2 in the pathogenesis of asthma. Selective BLT1 antagonists may have a potential therapeutic application in patients with asthma, and BLT2 may represent a novel therapeutic target for lung diseases.

Keywords::

• BLT1
• BLT2
• LTB₄
• lung disease

BLT1/BLT2

Leukotriene B₄ (LTB₄) is a potent lipid mediator of inflammation derived from arachidonic acid by the sequential enzymatic actions of 5-lipoxygenase (5-LO), 5-LO-activating protein and leukotriene A4 hydrolase. LTB₄ mediates multiple biological functions through its interaction with two distinct receptors, LTB₄ receptor 1 (BLT1) and LTB₄ receptor 2 (BLT2), which were cDNA-cloned by Yokomizo et al. [1,2]. Both receptors are members of the G-protein-coupled seven-transmembrane domain receptor superfamily, whose coding genes are located in very close proximity to each other in human or mouse genomes. The receptors have a similar structure with 45.2% amino acid identity, but their affinity, specificity for LTB₄ and expression pattern are quite different [3].

BLT1, a high-affinity receptor for LTB₄, is expressed in inflammatory and immune cells, including granulocytes, monocytes/macrophages, dendritic cells, mast cells and effector T cells in both humans and mice. The potent leukocyte chemotactic activity of LTB₄ through BLT1 signaling has been well established [3]. In addition, functional BLT1 is now known to be expressed in nonmyeloid cells such as vascular smooth muscle cells, neural stem cells and endothelial cells. In contrast to the specific activation of BLT1 by LTB₄, BLT2 is activated by several 12- and 15-lipoxygenase products in addition to LTB₄. Furthermore, a cyclooxygenase metabolite 12-hydroxypeptadecatrienoic acid (12-HHT) binds to and activates BLT2 [4]. 12-HHT, which does not bind to BLT1, activates BLT2 at a 10-fold lower concentration than does LTB₄, suggesting that 12-HHT is a specific and high-affinity ligand for BLT2 [4]. In humans, BLT2 is widely distributed across multiple tissues. Mouse BLT2 is expressed predominantly in the small intestine, followed by skin, with low expression in the colon and spleen. BLT2 on mast cells mediates recruitment and accumulation of the cells in response to LTB₄ production at the sites of inflammation. Functional BLT2 is also expressed on antigen-specific effector memory CD4⁺ T cells [5].

LTB₄/BLT1 pathway in lung diseases

The LTB₄/BLT1 pathway has been implicated in the pathophysiology of bronchial asthma and chronic obstructive pulmonary disease (COPD). Increased LTB₄ levels have been reported in sputum, plasma, bronchoalveolar lavage (BAL) fluid, and breath condensate from patients with asthma and COPD [6,7]. LTB₄ via BLT1 activation has been shown to be the major neutrophil chemoattractant in sputum and exhaled breath condensate from COPD patients [8] and promotes the survival of neutrophils [9]. In addition to granulocytes, BLT1 is expressed in differentiated T cells and mediates the recruitment of CD4⁺ and CD8⁺ T cells in allergic airway inflammation. BLT1-deficient mice exhibited decreased airway hyperresponsiveness, pulmonary inflammation and mucus secretion after allergen sensitization and challenge as compared to wild-type mice [10,11]. Increased numbers of BLT1-expressing CD8⁺ T cells have been reported in BAL fluid of asthmatics compared to healthy subjects [12].

However, treatment with the LTB₄ receptor antagonist, LTB019, had no effect on sputum neutrophil numbers or lung function in COPD patients [13]. In asthma, another LTB₄ receptor antagonist, LY293111, failed to attenuate late asthmatic response or early asthmatic response following allergen challenge, despite significant decrease in neutrophils in BAL fluid [14]. Thus, the absence of clinical effects of the LTB₄ receptor antagonists in these diseases has brought into question the functional role of LTB₄ in the airways and the potential therapeutic benefit of compounds that modulate the effects of LTB₄ in asthma and COPD.

Increased LTB₄ levels have also been reported in BAL fluid in patients with acute lung injury or acute respiratory distress syndrome [15]. Constitutive activation of the 5-LO pathway and elevated levels of LTB₄ in the lung tissue were found in patients with idiopathic pulmonary fibrosis. 5-LO-deficient mice and wild-type mice treated with 5-LO inhibitors were protected from lung inflammation in a post-sepsis lung injury model. Overexpression of BLT1 in mice leukocytes dramatically increased neutrophil trafficking to lungs after ischemia reperfusion, whereas mice deficient in 5-LO showed diminished neutrophil accumulation in reperfused lungs. These data suggest the role of 5-LO and BLT1 signaling in controlling inflammation in lung injury.

BLT2 in lung diseases

Although BLT2 has been considered as a proinflammatory receptor, we recently demonstrated that BLT2-deficient mice in an asthma model exhibit enhanced accumulation of eosinophils and increased IL-13 levels in the lungs without influencing IL-4 and IgE production [5]. IL-13-producing CD4⁺ T cells were increased in the lungs of BLT2-deficient mice after allergen challenges. Knockdown of BLT2 by siRNA enhanced IL-13 production in CD4⁺ T cells. Furthermore, expression of BLT2 mRNA in CD4⁺ T cells from asthmatics was reduced as compared to control subjects [5]. These data suggest that BLT2 protects the lungs from allergic airway inflammation and impaired BLT2 expression in CD4⁺ T cells may contribute to the development of asthma. The protective role of BLT2 against inflammation is consistent with the previous study showing that dextran sodium sulfate-induced colitis was exacerbated in BLT2-deficient mice [16]. BLT2 in intestinal epithelial cells maintains the mucosal barrier function and BLT2 deficiency possibly results in the invasion of intestinal flora into the colon tissues, causing severe inflammatory colitis. On the other hand, a previous study showed that either the BLT2 antagonist LY255283 or antisense BLT2 suppressed allergen-induced airway inflammation in a murine model [17]. However, LY255283 works as a potent antagonist for BLT1 in addition to BLT2 [5]. Considering that BLT1 plays an important role in the early recruitment of leukocytes into the airways in allergic asthma [11,18], the inhibition of BLT1 by LY255283 may lead to the anti-inflammatory effect in the experiments. Thus, we concluded that BLT2 in CD4⁺ T cells works as an anti-inflammatory receptor for asthma.

Immunohistochemical analysis of bronchial biopsy specimens from asthmatics revealed an increased expression of BLT2 in airway epithelial layers and microvascular endothelium [17]. Elevated expression of BLT2 mRNA in airway fibroblasts was also found in asthmatic subjects [19]. A recent study has shown that cigarette smoke extracts increase neutrophil adhesion to bronchial epithelial cells via BLT2 in vitro [20]. The precise role of BLT2 in airway structural cells in asthma and COPD remains to be investigated.

In addition to BLT1 and BLT2, LTB₄ at high concentrations binds and activates a nuclear receptor peroxisome proliferator-activated receptor α (PPARa) [21]. The interaction with PPARa induces the catabolic inactivation of LTB₄ and, in turn, limits LTB₄-related inflammation. Some LTB₄ receptor antagonists have also been reported to exhibit PPAR agonistic activity [22]. Thus, responses to LTB₄ could represent an integration of proinflammatory and anti-inflammatory effects.

In summary, our recent findings suggest different functions of BLT1 and BLT2 in the pathogenesis of asthma. Although LTB₄ receptor antagonists in asthma or COPD failed to show therapeutic benefit, more selective BLT1 antagonists may have a potential therapeutic application in patients with asthma and COPD. Moreover, BLT2 may represent a novel therapeutic target for lung diseases.

Acknowledgements

The authors thank Takehiko Yokomizo for critical reading and comments.

Financial & competing interests disclosure

This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (25293192, 24591172). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.