Nuclear localization of leukotriene A4 hydrolase in type II alveolar epithelial cells in normal and fibrotic lung

Alpha-Lipoic Acid-Mediated Inhibition of LTB4 Synthesis Suppresses Epithelial-Mesenchymal Transition, Modulating Functional and Tumorigenic Capacities in Non-Small Cell Lung Cancer A549 Cells

Background

Leukotriene B4 (LTB4) plays a crucial role in carcinogenesis by inducing epithelial-mesenchymal transition (EMT), a process associated with tumor progression. The synthesis of LTB4 is mediated by leukotriene A4 hydrolase (LTA4H), and it binds to the receptors BLT1 and BLT2. Dysregulation in LTB4 production is linked to the development of various pathologies. Therefore, the identification or design of inhibitors of LTB4 synthesis or receptor antagonists represents an ongoing challenge. In this context, our laboratory previously demonstrated that alpha-lipoic acid (ALA) inhibits LTA4H. The objective of this study was to evaluate the effect of ALA on the expression of canonical EMT markers and the functional and tumorigenic capacities induced by LTB4 in A549 cells.

Methods

The expression of cPLA2, 5LOX, FLAP, LTA4H, BLT1, and LTB4 production in human adenocarcinomic alveolar basal epithelial A549 cells was assessed using Western blot, RT-qPCR, and ELISA, respectively. Subsequently, the expression of canonical EMT markers was evaluated by Western blot. Functional assays were performed to assess cell viability, proliferation, invasion, migration, and clonogenicity using MTT, Western blot, Transwell assays, and colony formation assays, respectively. Results were expressed as median with interquartile range (n≥3) and analyzed using the Kruskal-Wallis or Tukey multiple comparisons tests.

Results

A549 cells express key proteins involved in LTB4 synthesis and receptor binding, including LTA4H and BLT1, and ALA inhibits the production of LTB4. Additionally, LTA4H and BLT1 were detected in lung adenocarcinoma tissue samples. LTB4 was found to induce EMT, whereas ALA treatment enhanced the expression of epithelial markers and reduced the expression of mesenchymal markers. Furthermore, ALA treatment resulted in a decrease in LTB4 levels and attenuated the functional and tumorigenic capacities of A549 cells, including their viability, migration, invasion, and clonogenic potential.

Conclusions

These findings suggest that ALA may offer therapeutic potential in the context of lung cancer, as it could be integrated into conventional pharmacological therapies to enhance treatment efficacy and mitigate the adverse effects associated with chemotherapy. Further studies are warranted to confirm the clinical applicability of ALA as an adjunctive treatment in lung cancer.

Activatable Fluorescence and Bio/Chemiluminescence Probes for Aminopeptidases: From Design to Biomedical Applications

Aminopeptidases are exopeptidases that catalyze the cleavage of amino acid residues from the N-terminal fragment of protein or peptide substrates. Owing to their function, they play important roles in protein maturation, signal transduction, cell-cycle control, and various disease mechanisms, notably in cancer pathology. To gain better insights into their function, molecular imaging assisted by fluorescence and bio/chemiluminescence probes has become an indispensable method to their superiorities, including excellent sensitivity, selectivity, and real-time and noninvasive imaging. Numerous efforts are made to develop activatable probes that can effectively enhance efficiency and accuracy as well as minimize the side effects. This review is classified according to the type of aminopeptidases, summarizing some recent works on the design, work mechanism, and sensing, imaging, and theranostic performance of their activatable probe. Finally, the current challenges are outlined in developing activatable probes for aminopeptidases and provide possible solutions for future advancements.

Endogenous CO imaging in bacterial pneumonia with a NIR fluorescent probe

Bacterial pneumonia is a serious respiratory illness that poses a great threat to human life. Rapid and precise diagnosis of bacterial pneumonia is crucial for symptomatic clinical treatment. Endogenous carbon monoxide (CO) is regarded as a significant indicator of bacterial pneumonia; herein, we developed a near-infrared (NIR) probe for fluorescence and photoacoustic (PA) dual-mode imaging of endogenous CO in bacterial pneumonia. NO2-BODIPY could rapidly and specifically react with CO to produce strong NIR fluorescence as well as ratiometric PA signals. NO2-BODIPY has outstanding features including fast response, fluorescence/PA dual mode signals, good specificity, and a low limit of detection (LOD = 20.3 nM), which enables it to image endogenous CO in cells and bacterial pneumonia mice with high sensitivity and high contrast ratio. In particular, NO2-BODIPY has two-photon excited (1340 nm, σ1 = 1671 GM) NIR fluorescence and has been utilized to image endogenous CO in bacterial pneumonia mice with deep tissue penetration. NO2-BODIPY has been demonstrated a good capability of fluorescence/PA dual-mode imaging of CO in bacterial pneumonia mice, providing a precise manner to diagnose bacterial pneumonia.

An enzyme activated fluorescent probe for LTA4H activity sensing and its application in cancer screening

Early diagnosis of cancer is an efficient strategy to prevent tumor progression and improve the survival rate of patients. However, to discovery of reliable tumor-specific biomarkers remains a great challenge. Leukotriene A4 hydrolase (LTA₄H) is a bifunctional zinc metalloenzyme with epoxide hydrolase activity and aminopeptidase activity, which plays important roles in allergic and inflammatory reactions and showed strong relevance with carcinoma progression. We thus sought to investigate the possibility of application LTA₄H activity detection in cancer diagnosis. To achieve this, we herein develop an enzyme activated fluorescent probe for LTA₄H activity sensing by incorporating the specific recognition unit of LTA₄H with a red-emitting fluorophore. The acquired probe (named as ADMAB) showed high sensitivity and specificity toward LTA₄H in vitro. By further application of ADMAB in living cells, significantly elevated LTA₄H activity in cancer cell lines was observed when compared with normal cell lines and in vivo tracing A549 tumor in nude mice was also realized by ADMAB. Meanwhile, the wound-healing assay further revealed the importance of LTA₄H in tumor metastasis. Moreover, the LTA₄H activity in human serum sample was successfully detected by ADMAB and significantly elevated LTA₄H activity in patients diagnosed with cancer was firstly found, which demonstrated ADMAB to be a useful tool for cancer diagnosis and LTA₄H related biological study.

Substrate-dependent modulation of the leukotriene A4 hydrolase aminopeptidase activity and effect in a murine model of acute lung inflammation

The aminopeptidase activity (AP) of the leukotriene A4 hydrolase (LTA4H) enzyme has emerged as a therapeutic target to modulate host immunity. Initial reports focused on the benefits of augmenting the LTA4H AP activity and clearing its putative pro-inflammatory substrate Pro-Gly-Pro (PGP). However, recent reports have introduced substantial complexity disconnecting the LTA4H modulator 4-methoxydiphenylmethane (4MDM) from PGP as follows: (1) 4MDM inhibits PGP hydrolysis and subsequently inhibition of LTA4H AP activity, and (2) 4MDM activates the same enzyme target in the presence of alternative substrates. Differential modulation of LTA4H by 4MDM was probed in a murine model of acute lung inflammation, which showed that 4MDM modulates the host neutrophilic response independent of clearing PGP. X-ray crystallography showed that 4MDM and PGP bind at the zinc binding pocket and no allosteric binding was observed. We then determined that 4MDM modulation is not dependent on the allosteric binding of the ligand, but on the N-terminal side chain of the peptide. In conclusion, our study revealed that a peptidase therapeutic target can interact with its substrate and ligand in complex biochemical mechanisms. This raises an important consideration when ligands are designed to explain some of the unpredictable outcomes observed in therapeutic discovery targeting LTA4H.

Tannic acid alleviates experimental pulmonary fibrosis in mice by inhibiting inflammatory response and fibrotic process

Pulmonary fibrosis (PF) is a chronic and irreversible scarring disease in the lung with limited treatment options. Therefore, it is critical to identify new therapeutic options. This study was undertaken to identify the effects of tannic acid (TA), a naturally occurring dietary polyphenol, in a mouse model of PF. Bleomycin (BLM) was intratracheally administered to induce PF. Administration of TA significantly reduced BLM-induced histological alterations, inflammatory cell infiltration and the levels of various inflammatory mediators (nitric oxide, leukotriene B₄ and cytokines). Additionally, treatment with TA also impaired BLM-mediated increases in pro-fibrotic (transforming growth factor-β1) and fibrotic markers (alpha-smooth muscle actin, vimentin, collagen 1 alpha and fibronectin) expression. Further investigation indicated that BLM-induced phosphorylation of Erk1/2 (extracellular signal-regulated kinases 1 and 2) in lungs was suppressed by TA treatment. Findings of this study suggest that TA has the potential to mitigate PF through inhibiting the inflammatory response and fibrotic process in lungs and that TA might be useful for the treatment of PF in clinical practice.

Leukotriene A4 hydrolase: an emerging target of natural products for cancer chemoprevention and chemotherapy

Cancer is the second leading cause of death worldwide and has become a global burden. It has long been known that inflammation is related to cancer, as inflammatory components have been identified in the tumor microenvironment and support tumor progression. Among the key inflammatory mediators, leukotrienes were found to be involved in cancer development. In particular, leukotriene B4, which is converted from leukotriene A4 by leukotriene A4 hydrolase (LTA4H), has been implicated in several types of cancer. In addition, LTA4H has attracted attention because of purported roles in inflammation and cancer development. Herein, we review the history of LTA4H, its emerging roles in cancer development, and the development of LTA4H inhibitors in cancer prevention and therapy.

Leukotriene B4 Activates Pulmonary Artery Adventitial Fibroblasts in Pulmonary Hypertension

A recent study demonstrated a significant role for leukotriene B4 (LTB4) causing pulmonary vascular remodeling in pulmonary arterial hypertension. LTB4 was found to directly injure luminal endothelial cells and promote growth of the smooth muscle cell layer of pulmonary arterioles. The purpose of this study was to determine the effects of LTB4 on the pulmonary adventitial layer, largely composed of fibroblasts. Here, we demonstrate that LTB4 enhanced human pulmonary artery adventitial fibroblast proliferation, migration, and differentiation in a dose-dependent manner through its cognate G-protein-coupled receptor, BLT1. LTB4 activated human pulmonary artery adventitial fibroblast by upregulating p38 mitogen-activated protein kinase as well as Nox4-signaling pathways. In an autoimmune model of pulmonary hypertension, inhibition of these pathways blocked perivascular inflammation, decreased Nox4 expression, reduced reactive oxygen species production, reversed arteriolar adventitial fibroblast activation, and attenuated pulmonary hypertension development. This study uncovers a novel mechanism by which LTB4 further promotes pulmonary arterial hypertension pathogenesis, beyond its established effects on endothelial and smooth muscle cells, by activating adventitial fibroblasts.

Role of leukotriene A4 hydrolase aminopeptidase in the pathogenesis of emphysema

The leukotriene A4 hydrolase (LTA4H) is a bifunctional enzyme with epoxy hydrolase and aminopeptidase activities. We hypothesize that the LTA4H aminopeptidase activity alleviates neutrophilic inflammation, which contributes to cigarette smoke (CS)-induced emphysema by clearing proline-glycine-proline (PGP), a triamino acid chemokine known to induce chemotaxis of neutrophils. To investigate the biological contributions made by the LTA4H aminopeptidase activity in CS-induced emphysema, we exposed wild-type mice to CS over 5 mo while treating them with a vehicle or a pharmaceutical agent (4MDM) that selectively augments the LTA4H aminopeptidase without affecting the bioproduction of leukotriene B4. Emphysematous phenotypes were assessed by premortem lung physiology with a small animal ventilator and by postmortem histologic morphometry. CS exposure acidified the airspaces and induced localization of the LTA4H protein into the nuclei of the epithelial cells. This resulted in accumulation of PGP in the airspaces by suppressing the LTA4H aminopeptidase activity. When the LTA4H aminopeptidase activity was selectively augmented by 4MDM, the levels of PGP in the bronchoalveolar lavage fluid and infiltration of neutrophils into the lungs were significantly reduced without affecting the levels of leukotriene B4. This protected murine lungs from CS-induced emphysematous alveolar remodeling. In conclusion, CS exposure promotes the development of CS-induced emphysema by suppressing the enzymatic activities of the LTA4H aminopeptidase in lung tissues and accumulating PGP and neutrophils in the airspaces. However, restoring the leukotriene A4 aminopeptidase activity with a pharmaceutical agent protected murine lungs from developing CS-induced emphysema.

5-Lipoxygenase contributes to PPARγ activation in macrophages in response to apoptotic cells

Macrophage polarization to an anti-inflammatory phenotype upon contact with apoptotic cells is a contributing hallmark to immune suppression during the late phase of sepsis. Although the peroxisome proliferator-activated receptor γ (PPARγ) supports this macrophage phenotype switch, it remains elusive how apoptotic cells activate PPARγ. Assuming that a molecule causing PPARγ activation in macrophages originates in the cell membrane of apoptotic cells we analyzed lipid rafts from apoptotic, necrotic, and living human Jurkat T cells which showed the presence of 5-lipoxygenase (5-LO) in lipid rafts of apoptotic cells only. Incubating macrophages with lipid rafts of apoptotic, but not necrotic or living cells, induced PPAR responsive element (PPRE)-driven mRuby reporter gene expression in RAW 264.7 macrophages stably transduced with a 4xPPRE containing vector. Experiments with lipid rafts of apoptotic murine EL4 T cells revealed similar results. To verify the involvement of 5-LO in activating PPARγ in macrophages, Jurkat T cells were incubated with the 5-LO inhibitor MK-866 prior to induction of apoptosis, which failed to induce mRuby expression. Similar results were obtained with lipid rafts of apoptotic EL4 T cells preexposed to the 5-LO inhibitors zileuton and CJ-13610. Interestingly, Jurkat T cells overexpressing 5-LO failed to activate PPARγ in macrophages, while their 5-LO overexpressing apoptotic counterparts did. Our results suggest that during apoptosis 5-LO gets associated with lipid rafts and synthesizes ligands that in turn stimulate PPARγ in macrophages.

Location, location, location: compartmentalization of early events in leukotriene biosynthesis

Leukotrienes (LTs), derived from arachidonic acid (AA) released from the membrane by the action of phospholipase A(2), are potent lipid mediators of the inflammatory response. In 1983, Dahlén et al. demonstrated that LTC(4), LTD(4), and LTE(4) mediate antigen-induced constriction of bronchi in tissue obtained from subjects with asthma (Dahlén, S. E., Hansson, G., Hedqvist, P., Björck, T., Granström, E., and Dahlén, B. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1712-1716). Over the last 25+ years, substantial progress has been made in understanding how LTs exert their effects, and a broader appreciation for the numerous biological processes they mediate has emerged. LT biosynthesis is initiated by the action of 5-lipoxygenase (5-LOX), which catalyzes the transformation of AA to LTA(4) in a two-step reaction. Ca(2+) targets 5-LOX to the nuclear membrane, where it co-localizes with the 5-LOX-activating protein FLAP and, when present, the downstream enzyme LTC(4) synthase, both transmembrane proteins. Crystal structures of the AA-metabolizing LOXs, LTC(4) synthase, and FLAP combined with biochemical data provide a framework for understanding how subcellular organizations optimize the biosynthesis of these labile hydrophobic signaling compounds, which must navigate pathways that include both membrane and soluble enzymes. The insights these structures afford and the questions they engender are discussed in this minireview.

Myrtucommulone, a natural acylphloroglucinol, inhibits microsomal prostaglandin E(2) synthase-1

BACKGROUND AND PURPOSE

The selective inhibition of prostaglandin (PG)E(2) formation via interference with microsomal PGE(2) synthase (mPGES)-1 could have advantages in the treatment of PGE(2)-associated diseases, such as inflammation, fever and pain, compared with a general suppression of all PG biosynthesis, provided by inhibition of cyclooxygenase (COX)-1 and 2. Here, we addressed whether the naturally occurring acylphloroglucinol myrtucommulone (MC) from Myrtus communis L. (myrtle) affected mPGES-1.

EXPERIMENTAL APPROACH

The effect of MC on PGE(2) formation was investigated in a cell-free assay by using microsomal preparations of interleukin-1beta-stimulated A549 cells as the source of mPGES-1, in intact A549 cells, and in lipopolysaccharide-stimulated human whole blood. Inhibition of COX-1 and COX-2 activity in cellular and cell-free assays was assessed by measuring 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid and 6-oxo PGF(1alpha) formation.

KEY RESULTS

MC concentration-dependently inhibited cell-free mPGES-1-mediated conversion of PGH(2) to PGE(2) (IC(50) = 1 micromol x L(-1)). PGE(2) formation was also diminished in intact A549 cells as well as in human whole blood at low micromolar concentrations. Neither COX-2 activity in A549 cells nor isolated human recombinant COX-2 was significantly affected by MC up to 30 micromol x L(-1), and only moderate inhibition of cellular or cell-free COX-1 was evident (IC(50) > 15 micromol x L(-1)).

CONCLUSIONS AND IMPLICATIONS

MC is the first natural product to inhibit mPGES-1 that efficiently suppresses PGE(2) formation without significant inhibition of the COX enzymes. This provides an interesting pharmacological profile suitable for interventions in inflammatory disorders, without the typical side effects of coxibs and non-steroidal anti-inflammatory drugs.

Aminopeptidase O contains a functional nucleolar localization signal and is implicated in vascular biology

We have identified a gene trap integration into Aminopeptidase O, the gene encoding a member of the M1 family of metalloproteases. Using the betagal reporter of the gene trap vector, we have revealed that at least some ApO isoforms are expressed predominantly in embryonic and adult blood vessels leading us to propose that ApO plays a role in vascular cell biology. The protein produced from an engineered Gfp-ApO fusion cDNA localises to the nucleolus in transfected COS7 cells. We confirm that indeed the APO protein contains a functional nucleolar localisation domain by demonstrating that GFP-APO fusion proteins that lack the predicted nucleolar localisation signal are retained in the cytoplasm. We report the existence of multiple alternatively spliced Apo isoforms that differ with respect to the presence of exons encoding important functional domains. Alternative splicing predictably produces protein products with or without the catalytic domain and/or a nucleolar localisation signal and therefore likely represents an important mechanism in regulating the biological activity of APO that has been reported to cleave one of the peptides of the renin angiotensin pathway.

The potential link between atherosclerosis and the 5-lipoxygenase pathway: investigational agents with new implications for the cardiovascular field

The 5-lipoxygenase pathway is responsible for the production of leukotrienes--inflammatory lipid mediators that have a role in innate immunity, but that can also have pathological effects in inflammatory diseases. Recently, a potential link between leukotriene production and atherosclerosis has been proposed. The expression of leukotriene biosynthetic enzymes and leukotriene receptors has been identified in coronary and carotid atherosclerotic plaques, and the levels of biosynthetic enzymes have been correlated with the clinical symptoms of unstable plaques. Genetic variants in 5-lipoxygenase pathway genes have also been associated with a relative risk of developing myocardial infarction and stroke. On the basis of these discoveries, antileukotriene compounds are now being evaluated for the treatment of cardiovascular disease. Several tool compounds have been shown to limit the progression of lesion development in preclinical models of atherosclerosis, and three compounds, including two drugs previously developed for asthma, are undergoing clinical trials in patients with acute coronary syndromes.

Methionine aminopeptidase-2 as a selective target of myofibroblasts in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive, scarring lung disease characterized by fibroblast accumulation and deposition of collagen. Factors that promote growth and/or survival of fibroblasts are potential therapeutic targets. Methionine aminopeptidase 2 (MetAP2), a member of the aminopeptidase family of proteases, has been implicated in cell proliferation in a variety of cell types, but its expression and function in the lung is not known. By immunohistochemistry, MetAP2 was expressed in many cell types, including fibroblasts, in IPF lungs. Fumagillin, an irreversible inhibitor of the enzymatic activity of MetAP2, attenuated collagen deposition in the bleomycin model of acute lung injury in mice. Treatment with fumagillin caused a selective reduction in the numbers of bromodeoxyuridine (BrdU)-positive myofibroblasts, but not type II alveolar epithelial cells, macrophages, or B- and T-lymphocytes in the lungs of bleomycin-treated mice. Incubation of primary rat lung fibroblasts with either fumagillin or with short interfering RNA that targeted MetAP2 led to reduced proliferation, as assessed by incorporation of BrdU. The profibrotic growth factor, platelet-derived growth factor, increased expression of MetAP2 in rat lung fibroblasts. We propose that MetAP2 plays a role in the proliferation of fibroblasts and myofibroblasts in fibrotic lung diseases and may serve as a novel pharmacologic target in IPF.

Regulating leukotriene synthesis: the role of nuclear 5-lipoxygenase

Leukotrienes are lipid messengers involved in autocrine and paracrine cellular signaling. They are synthesized from arachidonic acid by the 5-lipoxygenase pathway. Current models of this enzymatic pathway recognize that a key step in initiating leukotriene synthesis is the calcium-mediated movement of enzymes, including 5-lipoxygenase, to intracellular membranes. However, 5-lipoxygenase can be imported into or exported from the nucleus before calcium activation. As a result, its subcellular localization will affect its ability to be activated by calcium, as well as the membrane to which it binds and its interaction with other enzymes. This commentary focuses on the role of 5-lipoxygenase compartmentation in determining its regulation and, ultimately, leukotriene synthesis.

Expression of 5-lipoxygenase in specialized epithelial cells of nasopharyngeal-associated lymphoid tissue

Leukotrienes are lipid mediators that are produced primarily by certain types of leukocytes. The synthesis of the leukotriene LTB(4) is initiated by the enzyme 5-lipoxygenase and completed by LTA(4) hydrolase. Epithelial cells constitutively express LTA(4) hydrolase but normally lack 5-lipoxygenase. In this study, we report that the stratified squamous epithelial cells from inflamed or hyperplastic tissues of palatine and pharyngeal tonsils (nasopharyngeal-associated lymphoid tissue) express 5-lipoxygenase protein. The localization of 5-lipoxygenase was indicated by immunohistochemical staining and presence confirmed by immunoblot. Positive staining for 5-lipoxygenase in infiltrating leukocytes in inflamed tissues served as internal positive controls for immunohistochemical staining. Staining for 5-lipoxygenase in appendix tissue was negative for epithelial cells while positive for polymorphonuclear leukocytes, indicating that 5-lipoxygenase expression is not a general feature of epithelial cells in mucosa-associated lymphoid tissue. In tonsils, 5-lipoxygenase staining was pronounced in broad regions but reduced or absent in others, suggesting regional regulation of expression. Epithelial cells of tonsils were also positive for 5-lipoxygenase activating protein and leukotriene A(4) hydrolase, indicating a capacity to produce LTB(4). Taken together, these results suggest that the specialized epithelial cells of the mucosa-associated lymphoid tissue of human tonsils can synthesize LTB(4). This lipid mediator may serve to modulate the function of cells within the lymphoid tissue as well as promote an inflammatory response.