Protective Effects of Lipoxin A4 and B4 Signaling on the Inner Retina in a Mouse Model of Experimental Glaucoma

Glaucoma is a common neurodegenerative disease characterized by progressive degeneration of retinal ganglion cells (RGCs) and the retinal nerve fiber layer (RNFL), resulting in a gradual decline of vision. A recent study by our groups indicated that the levels of lipoxins A4 (LXA4) and B4 (LXB4) in the retina and optic nerve decrease following acute injury, and that restoring their function is neuroprotective. Lipoxins are members of the specialized pro-resolving mediator (SPM) family and play key roles to mitigate and resolve chronic inflammation and tissue damage. Yet, knowledge about lipoxin neuroprotective activity remains limited. Here we investigate the in vivo efficacy of exogenous LXA4 and LXB4 administration on the inner retina in a mouse model of chronic experimental glaucoma. To investigate the contribution of LXA4 signaling we used transgenic knockout (KO) mice lacking the two mouse LXA4 receptors (Fpr2/Fpr3-/-). Functional and structural changes of inner retinal neurons were assessed longitudinally using electroretinogram (ERG) and optical coherence tomography (OCT). At the end of the experiment, retinal samples were harvested for immunohistological assessment. While both lipoxins generated protective trends, only LXB4 treatment was significant, and consistently more efficacious than LXA4 in all endpoints. Both lipoxins also appeared to dramatically reduce Müller glial reactivity following injury. In comparison, Fpr2/Fpr3 deletion significantly worsened inner retinal injury and function, consistent with an essential protective role for endogenous LXA4. Together, these results support further exploration of lipoxin signaling as a treatment for glaucomatous neurodegeneration.

Dysregulation of Neuroprotective Lipoxin Pathway in Astrocytes in Response to Cytokines and Ocular Hypertension

Glaucoma leads to vision loss due to retinal ganglion cell death. Astrocyte reactivity contributes to neurodegeneration. Our recent study found that lipoxin B4 (LXB4), produced by retinal astrocytes, has direct neuroprotective actions on retinal ganglion cells. In this study, we aimed to investigate how the autacoid LXB4 influences astrocyte activity in the retina under inflammatory cytokine-induced activation and during ocular hypertension. The protective activity of LXB4 was investigated in vivo using the mouse silicone-oil model of chronic ocular hypertension (n=40). By employing a range of analytical techniques, including bulk RNA-seq, RNAscope in-situ hybridization, qPCR, and lipidomic analyses, we discovered the formation of lipoxins and expression of the lipoxin pathway in rodents (including the retina and optic nerve), primates (optic nerve), and human brain astrocytes, indicating the presence of this neuroprotective pathway across various species. Findings in the mouse retina identified significant dysregulation of the lipoxin pathway in response to chronic ocular hypertension, leading to an increase in 5-lipoxygenase (5-LOX) activity and a decrease in 15-LOX activity. This dysregulation was coincident with a marked upregulation of astrocyte reactivity. Reactive human brain astrocytes also showed a significant increase in 5-LOX. Treatment with LXB4 amplified the lipoxin biosynthetic pathway by restoring and amplifying the generation of another member of the lipoxin family, LXA4, and mitigated astrocyte reactivity in mouse retinas and human brain astrocytes. In conclusion, the lipoxin pathway is functionally expressed in rodents, primates, and human astrocytes, and is a resident neuroprotective pathway that is downregulated in reactive astrocytes. Novel cellular targets for LXB4's neuroprotective action are inhibition of astrocyte reactivity and restoration of lipoxin generation. Amplifying the lipoxin pathway is a potential target to disrupt or prevent astrocyte reactivity in neurodegenerative diseases, including retinal ganglion cell death in glaucoma.

Quantitative profiling of inflammatory and pro-resolving lipid mediators in human adolescents and mouse plasma using UHPLC-MS/MS

OBJECTIVES

Lipid mediators are bioactive lipids which help regulate inflammation. We aimed to develop an ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method to quantify 58 pro-inflammatory and pro-resolving lipid mediators in plasma, determine preliminary reference ranges for adolescents, and investigate how total parenteral nutrition (TPN) containing omega-3 polyunsaturated fatty acid (n-3 PUFA) or n-6 PUFA based lipid emulsions influence lipid mediator concentrations in plasma.

METHODS

Lipid mediators were extracted from plasma using SPE and measured using UHPLC-MS/MS. EDTA plasma was collected from healthy adolescents between 13 and 17 years of age to determine preliminary reference ranges and from mice given intravenous TPN for seven days containing either an n-3 PUFA or n-6 PUFA based lipid emulsion.

RESULTS

We successfully quantified 43 lipid mediators in human plasma with good precision and recovery including several leukotrienes, prostaglandins, resolvins, protectins, maresins, and lipoxins. We found that the addition of methanol to human plasma after blood separation reduces post blood draw increases in 12-hydroxyeicosatetraenoic acid (12-HETE), 12-hydroxyeicosapentaenoic acid (12-HEPE), 12S-hydroxyeicosatrienoic acid (12S-HETrE), 14-hydroxydocosahexaenoic acid (14-HDHA) and thromboxane B2 (TXB2). Compared to the n-6 PUFA based TPN, the n-3 PUFA based TPN increased specialized pro-resolving mediators such as maresin 1 (MaR1), MaR2, protectin D1 (PD1), PDX, and resolvin D5 (RvD5), and decreased inflammatory lipid mediators such as leukotriene B4 (LTB4) and prostaglandin D2 (PGD2).

CONCLUSIONS

Our method provides an accurate and sensitive quantification of 58 lipid mediators from plasma samples, which we used to establish a preliminary reference range for lipid mediators in plasma samples of adolescents; and to show that n-3 PUFA, compared to n-6 PUFA rich TPN, leads to a less inflammatory lipid mediator profile in mice.

Formyl peptide receptor 1 suppresses gastric cancer angiogenesis and growth by exploiting inflammation resolution pathways

Chronic inflammation can result from inadequate engagement of resolution mechanisms, mainly accomplished by specialized pro-resolving mediators (SPMs) arising from the metabolic activity of lipoxygenases (ALOX5/15) on ω-6 or ω-3 essential polyunsaturated fatty acids (PUFA). We previously demonstrated that formyl peptide receptor 1 (FPR1) suppresses gastric cancer (GC) by inhibiting its inflammatory/angiogenic potential. In this study, we asked whether FPR1 exploits inflammation resolution pathways to suppress GC angiogenesis and growth. Here, we demonstrate that genetic or pharmacologic modulation of FPR1 in GC cells regulated ALOX5/15 expression and production of the SPMs Resolvin D1 (RvD1) and Lipoxin B4 (LXB4). SPM treatment of GC cells abated their angiogenic potential. Genetic deletion of ALOX15 or of the RvD1 receptor GPR32 increased the angiogenic and tumorigenic activity of GC cells thereby mimicking FPR1 loss. Deletion/inhibition of ALOX5/15 or GPR32 blocked FPR1-mediated anti-angiogenic activities, indicating that ALOX5/15 and GPR32 are required for FPR1's pro-resolving action. An ω-3- or ω-6-enriched diet enforced SPM endogenous production in mice and inhibited growth of shFPR1 GC xenografts by suppressing their angiogenic activity. These data implicate that FPR1 and/or pro-resolving pathway components might be used as risk/prognostic markers for GC; ω-6/3-enriched diets, and targeting FPR1 or SPM machinery may be exploited for GC management.