Alox15 and its encoded 12-LOX is expressed in the intestinal epithelium
[77][28]. Indeed, high levels of 12-HETE have been found in colonic mucosal tissue from patients with inflammatory bowel disease by thin-layer chromatography and high-performance liquid chromatography
[76,78][27][29]. The neutrophil chemoattractant hepoxilin A3 (HXA
3) is a downstream metabolite of 12-HPETE, and HXA
3 functions in polymorphonuclear leukocyte (PMN) recruitment to sites of mucosal inflammation. Using a model that has been described to study basolateral to apical PMN transepithelial migration
[79][30], it has been demonstrated that HXA
3 promotes the final step of PMN recruitment to sites of inflammation by establishing a gradient across the epithelial tight junction. Inhibition of 12-LOXs by treatment with the small molecule inhibitor baicalein lead to blockage of HXA
3 generation and inhibition of PMN transmigration stimulated by
Salmonella typhimurium infection
[80][31]. Furthermore, 12-LOXs have been shown to play a pathophysiological role in an animal model of IBD. In a dextran sodium sulfate (DSS) induced colitis model that was restricted to female mice,
Alox15−/− mice were robustly protected from colitis and weight loss by a mechanism involving sustained epithelial tight junction protein expression
[81][32]. Interestingly, expression of
Alox15 was not seen in healthy mouse colon but was significantly upregulated in the inflamed colon after 8 days of DSS induced colitis. Expression was restricted to the stromal cells, which represent invading leukocytes. Inflammatory marker analysis revealed that
Alox15 deficient mice exhibited less colonic macrophage infiltration assessed by F4/80 staining and mRNA analysis of distal colon revealed increased iNOS and TNF-α expression. Colon permeability studies revealed that
Alox15 deficient mice had significantly reduced permeability and higher functional ZO-1 expression compared to the DSS-treated control mice.
Thus, products of 12-LOXs appear to play a dual role in gut inflammatory disease by affecting gut epithelial cell integrity and by promoting polymorphonuclear leukocyte migration.
4. 12-LOXs and Cardiovascular Disease
Vascular remodeling is an active process in which the vessel wall thickens owing to vascular smooth muscle cell (VSMC) migration, thrombosis, and proliferation, leading to the formation of a thickened neointima layer in response to elevated shear stress, pressure, or arterial injury
[82][33]. 12-LOXs have been implicated in VSMC migration, proliferation and apoptosis
[83][34]. The metabolites 15-HETE and 12-HETE have both been found to act as mitogens in a MAPK dependent pathway in vascular endothelial cells and VSMCs
[83,84][34][35]. Both 12-HETE and 15-HETE have been shown to promote VSMC migration via cAMP-response element-binding protein (CREB) mediated IL-6 expression
[85][36].
Atherosclerotic lesions develop at sites of endothelial dysfunction and represent a chronic inflammatory process characterized by macrophage chemotaxis, accumulation, and foam cell formation
[86][37]. Because of its ability to oxidize biomembranes and lipoproteins, 12-LOXs have been studied in the setting of atherogenesis. Human atherosclerotic lesions contain
ALOX15 mRNA and 12-LOX products
[87,88][38][39]. 12-HETE has been shown to directly induce monocyte binding to human aortic endothelial cells
[55[5][40],
89], promote endothelial wall disruption
[90][41], and to directly oxidize LDL, which contributes to foam cell formation
[91,92][42][43]. In this regard, studies in which mice have been crossed with athero-susceptible backgrounds have shown protection from the development of atherosclerosis
[93,94,95][44][45][46].
The most widely used mouse models of atherosclerosis are the
ApoE−/− and Ldlr−/− mice, which both develop hypercholesterolemia
[96][47]. It has been demonstrated that
Alox15−/− deficient mice on the
ApoE null background developed significantly reduced atherosclerotic lesions even at 1 year of age. This observation was likely due to a reduction in oxidized LDL given the reductions in plasma autoantibody titers to oxidized LDL epitopes in double knockout mice compared to controls
[95][46]. In another study, deletion of
Alox15 in the
Ldlr−/− background also lead to a significant reduction in plaque formation at 3, 9, 12, and 18 weeks of high-fat diet feeding, while the cellular content of macrophages and T cells within the plaques did not change
[93][44]. Both studies showed that
Alox15 deficiency did not influence lipid profiles. However, a study in which
Alox15−/−;Ldlr−/− mice were fed a polyunsaturated fatty acid-enriched diet (10% calories as safflower oil) in which 12-LOX products are enhanced, atherosclerosis was reduced, but levels of cholesterol and triglyceride also decreased with improvement in hepatic steatosis compared to controls
[97][48]. Therefore, 12-LOXs are involved in several steps in the pathogenesis of atherosclerosis, specifically through the promotion of LDL oxidation and induction of a pro-inflammatory state which promotes macrophage metabolic activity. Whether 12-LOXs products affect lipid profiles relevant to human disease has not been demonstrated.
5. 12-LOXs in Neuroinflammation and Neurodegenerative Disease
Neuroinflammation is an underlying cause of neuronal damage and brain disorders, including Alzheimer’s disease (AD) and Parkinson’s disease (PD)
[98,99,100][49][50][51]. One of the main features of AD is the presence of senile plaques containing beta-amyloid peptide. 12-LOXs and 12-HETE are highly expressed in neurons of human brains
[101][52], and 12-LOXs have been implicated in promoting neuroinflammation in both humans and mice
[79][30]. Different mechanisms have been proposed for 12-LOXs mediated neuroinflammation, including its role in increasing oxidative stress in the neurons
[101,102][52][53]. The transcription factor c-Jun is associated with neuronal apoptosis and has been shown to be active following exposure of neurons to the beta-amyloid peptide found in AD
[103][54]. However, inhibition of 12-LOXs using anti-sense oligonucleotide strategy leads to a disruption in c-Jun dependent beta-amyloid-induced apoptosis in cortical cells
[104][55]. Post-mortem analyses of brain sections from patients with AD show increased levels of 12-LOXs in the temporal and frontal lobes, with increased levels of both 12-HETE and 15-HETE
[101][52]. Another study reported increased levels of 12-HETE in the cerebrospinal fluid of AD patients
[105][56]. These studies implicate a role for 12-LOXs in the formation of beta-amyloid deposits. In a transgenic mouse model of AD-like amyloidosis in which mice develop amyloid deposits and cognitive impairment, mice were found to have a significant reduction in beta-amyloid production and deposition, as well as improvement in memory in the absence of
Alox15 [106][57].
Neuroinflammation is also considered as an underlying cause of the pathogenesis of Parkinson’s disease
[107][58]. Although the pathogenesis of PD has focused on the presence of Lewy bodies in dopaminergic neurons, more recent studies have implicated oxidative stress and altered protein metabolism in precipitating the risk for PD development
[107,108][58][59]. The 12-LOX pathway has been implicated with the progression of Parkinson’s disease through its role in oxidative and ER stress. An early finding of PD is a reduction in the level of the anti-oxidant glutathione
[109][60], and its reduced levels lead to increased levels of nitric oxide (NO) in the neurons, which is neurotoxic
[110][61]. An in vitro study with rat midbrain cultures showed that the NO-mediated neurotoxicity is reduced with the inhibition of 12-LOX with baicalein
[111][62]. Moreover, studies in vitro with murine neurons show that a reduction in glutathione levels is associated with the upregulation of 12-LOX protein levels as well as 12-HETE
[112][63].
12-LOXs and their pro-inflammatory mediators have also been implicated in nerve cell death (94) and brain ischemia
[113,114][64][65]. In a murine model of transient middle cerebral artery occlusion, levels of 12-LOX were increased in the peri-infarct region of the neurons, and expression levels of 12-HETE also increased after brain ischemia in gerbil forebrains
[115][66]. In a mouse study, when 12-LOX was inhibited by baicalein or genetically inactivated (
Alox15−/−), mice were protected against transient focal ischemia
[113][64]. A recent study showed that subarachnoid hemorrhage led to increased 12-LOX protein levels in murine brain macrophages and promoted neuronal death. When 12-LOX was inhibited by ML351 or by genetic inactivation (
Alox15−/−), neuronal death was reduced, resulting in protection from brain edema and improved behavioral outcomes
[116][67]. Treatment of mice with a novel inhibitor of 12-LOX, LOXBlock-1 reduced infarct sizes at both 24 h and 14 days post-stroke, with improved behavioral parameters. LOXBlock-1 also reduced oxidative stress in the cultured murine neuronal HT22 cells. It was observed that 12-LOX co-localized with lipids MDA2. This co-localization was also detected in the brain of two human stroke patients
[114][65]. Indeed, plasma levels of 12-HETE were found to be elevated up to 7 days after stroke in a biomarker study that involved over 60 stroke patients
[117][68]. Together, these data point to a critical role of 12-LOX in oxidative stress-related glutathione depletion in neuronal cell death relevant to human disease.
6. 12-LOXs in Pulmonary Inflammation and Disease
Pulmonary inflammation can be caused by infectious or non-infectious agents. Acute pulmonary inflammation leads to immune cell infiltration, mucus production, vascular leak into the airways, and epithelial cell damage. Unregulated inflammation is an underlying cause of many chronic pulmonary diseases such as asthma, chronic obstructive pulmonary disease, and fibrosis
[118][69].
Eicosanoid levels are known to increase in response to inflammatory stimuli in the lungs
[119][70], and PMN infiltration into the pulmonary space is a hallmark feature of pneumococcal pneumonia. While PMN activity is imperative to innate immunity, uncontrolled inflammation can result in tissue destruction and lung disease. In a model of
Streptococcus pneumoniae infection, 12-LOXs play a major role in PMN migration to the site of pneumococcal infection. Indeed, inhibition of 12-LOX with baicalein prevented the transepithelial infiltration of PMN cells and reduced pulmonary infiltration. Furthermore, depletion of 12-LOX activity in
Alox15−/− mice led to reduced bacteremia and increased survival as compared to controls where the pulmonary challenge with
S. pneumoniae was lethal
[120][71]. In an acute lung injury (ALI) mouse model, mouse inhalation of LPS led to induction of inflammation, increased vascular permeability, and upregulation of 12-LOXs. This study also revealed that depletion of 12-LOX lead to significantly reduced vascular permeability upon LPS treatment along with improved gas exchange and increased survival compared to the control littermates
[121][72]. The same group further demonstrated that inflammation in this ALI model is mediated in part via recruitment of neutrophils, as depletion of 12-LOX significantly reduced neutrophil infiltration and prevented edema formation
[122][73]. Therefore, these data demonstrate that 12-LOXs inflammatory activity is crucial in pulmonary infection pathology.
Non-epithelial lung cells appear to exhibit 12-LOX activity in relation to allergic airway inflammation
[123,124][74][75]. A study showed that intranasal administration of 12-LOX to healthy Balb/c mice leads to airway epithelial injury that promotes airway hyper-responsiveness as seen in asthma. Production of 12-LOX in alveolar macrophages and fibroblasts leads to bronchial epithelial injury via 12-HETE in an IL-13 dependent mechanism
[125][76]. In an allergen-induced lung inflammation model, depletion of 12-LOX (
Alox15−/−) reduced airway inflammation as seen by reduced bronchoalveolar lavage fluid leukocytes (eosinophils, lymphocytes, and macrophages), decreased cytokines (IL-4, IL-5, and IL-13), and reduced luminal mucus secretions in
Alox15−/− mice compared to wt controls
[126][77]. Recently, it was also shown that deficiency of
Alox15 impairs the granulopoiesis of neutrophils and prevents inflammatory responses to fungal
Aspergillus fumigatus infection in the lungs
[127][78]. In humans, a hypomethylation of
ALOX12 is associated with asthma in children
[128][79]. Therefore, 12-LOXs participate in lung disease first by affecting infectious inflammation through PMN recruitment and second through promoting increased leukocyte cytokine production in airway hyper-responsiveness.
With respect to human disease, 12-LOXs and 12-HETE have been implicated in inflammatory lung disorders. Lungs of patients exposed to sulfur mustard toxin showed obstructive and restrictive lung disease and an increase in 12-LOX expression compared to control patients
[129][80]. Furthermore, in tuberculosis, the expression of
ALOX12 increases, and it is positively correlated with neutrophil count and bacterial load in the airway
[130][81]. Together these results implicate the 12-LOX pathway in airway epithelial injury relevant to human disease.