Resolvin D2-G-Protein Coupled Receptor 18 Enhances Bone Marrow Function and Limits Steatosis and Hepatic Collagen Accumulation in Aging.

Aging is associated with nonresolving inflammation and tissue dysfunction. Resolvin D2 (RvD2) is a proresolving ligand that acts through the G-protein-coupled receptor called GPR18. Unbiased RNA sequencing revealed increased Gpr18 expression in macrophages from old mice, and in livers from elderly humans, which was associated with increased steatosis and fibrosis in middle-aged (MA) and old mice. MA mice that lacked GPR18 on myeloid cells had exacerbated steatosis and hepatic fibrosis, which was associated with a decline in Mac2+ macrophages. Treatment of MA mice with RvD2 reduced steatosis and decreased hepatic fibrosis, correlating with increased Mac2+ macrophages, increased monocyte-derived macrophages, and elevated numbers of monocytes in the liver, blood, and bone marrow. RvD2 acted directly on the bone marrow to increase monocyte-macrophage progenitors. A transplantation assay further demonstrated that bone marrow from old mice facilitated hepatic collagen accumulation in young mice. Transient RvD2 treatment to mice transplanted with bone marrow from old mice prevented hepatic collagen accumulation. Together, this study demonstrates that RvD2-GPR18 signaling controls steatosis and fibrosis and provides a mechanistic-based therapy for promoting liver repair in aging.

The Resolvin D2-GPR18 Axis Enhances Bone Marrow Function and Limits Hepatic Fibrosis in Aging.

Aging is associated with non-resolving inflammation and tissue dysfunction. Resolvin D2 (RvD2) is a pro-resolving ligand that acts through the G-protein coupled receptor (GPCR) called GRP18. Using an unbiased screen, we report increased Gpr18 expression in macrophages from old mice and in livers from elderly humans that is associated with increased steatosis and fibrosis in middle-aged (MA) and old mice. MA mice that lack GPR18 on myeloid cells had exacerbated steatosis and hepatic fibrosis, which was associated with a decline in Mac2+ macrophages. Treatment of MA mice with RvD2 reduced steatosis and decreased hepatic fibrosis, correlating with increased Mac2+ macrophages, monocyte-derived macrophages and elevated numbers of monocytes in the liver, blood, and bone marrow. RvD2 acted directly upon the bone marrow to increase monocyte-macrophage progenitors. Using a transplantation assay we further demonstrated that bone marrow from old mice facilitated hepatic collagen accumulation in young mice, and transient RvD2 treatment to mice transplanted with bone marrow from old mice prevented hepatic collagen accumulation. Together, our study demonstrates that RvD2-GPR18 signaling controls steatosis and fibrosis and provides a mechanistic-based therapy for promoting liver repair in aging.

Bioactive lipids and metabolic syndrome-a symposium report.

Recent research has shed light on the cellular and molecular functions of bioactive lipids that go far beyond what was known about their role as dietary lipids. Bioactive lipids regulate inflammation and its resolution as signaling molecules. Genetic studies have identified key factors that can increase the risk of cardiovascular diseases and metabolic syndrome through their effects on lipogenesis. Lipid scientists have explored how these signaling pathways affect lipid metabolism in the liver, adipose tissue, and macrophages by utilizing a variety of techniques in both humans and animal models, including novel lipidomics approaches and molecular dynamics models. Dissecting out these lipid pathways can help identify mechanisms that can be targeted to prevent or treat cardiometabolic conditions. Continued investigation of the multitude of functions mediated by bioactive lipids may reveal additional components of these pathways that can provide a greater understanding of metabolic homeostasis.

Radiation-Induced Macrophage Senescence Impairs Resolution Programs and Drives Cardiovascular Inflammation.

Radiation is associated with tissue damage and increased risk of atherosclerosis, but there are currently no treatments and a very limited mechanistic understanding of how radiation impacts tissue repair mechanisms. We uncovered that radiation significantly delayed temporal resolution programs that were associated with decreased efferocytosis in vivo. Resolvin D1 (RvD1), a known proresolving ligand, promoted swift resolution and restored efferocytosis in sublethally irradiated mice. Irradiated macrophages exhibited several features of senescence, including increased expression of p16INK4A and p21, heightened levels of SA-ß-gal, COX-2, several proinflammatory cytokines/chemokines, and oxidative stress (OS) in vitro, and when transferred to mice, they exacerbated inflammation in vivo. Mechanistically, heightened OS in senescent macrophages led to impairment in their ability to carry out efficient efferocytosis, and treatment with RvD1 reduced OS and improved efferocytosis. Sublethally irradiated Ldlr -/- mice exhibited increased plaque necrosis, p16INK4A cells, and decreased lesional collagen compared with nonirradiated controls, and treatment with RvD1 significantly reduced necrosis and increased lesional collagen. Removal of p16INK4A hematopoietic cells during advanced atherosclerosis with p16-3MR mice reduced plaque necrosis and increased production of key intraplaque-resolving mediators. Our results demonstrate that sublethal radiation drives macrophage senescence and efferocytosis defects and suggest that RvD1 may be a new therapeutic strategy to limit radiation-induced tissue damage.

Endothelial SOCS3 maintains homeostasis and promotes survival in endotoxemic mice.

SOCS3 is the main inhibitor of the JAK/STAT3 pathway. This pathway is activated by interleukin 6 (IL-6), a major mediator of the cytokine storm during shock. To determine its role in the vascular response to shock, we challenged mice lacking SOCS3 in the adult endothelium (SOCS3iEKO) with a nonlethal dose of lipopolysaccharide (LPS). SOCS3iEKO mice died 16-24 hours postinjection after severe kidney failure. Loss of SOCS3 led to an LPS-induced type I IFN-like program and high expression of prothrombotic and proadhesive genes. Consistently, we observed intraluminal leukocyte adhesion and neutrophil extracellular trap-osis (NETosis), as well as retinal venular leukoembolization. Notably, heterozygous mice displayed an intermediate phenotype, suggesting a gene dose effect. In vitro studies were performed to study the role of SOCS3 protein levels in the regulation of the inflammatory response. In human umbilical vein endothelial cells, pulse-chase experiments showed that SOCS3 protein had a half-life less than 20 minutes. Inhibition of SOCS3 ubiquitination and proteasomal degradation led to protein accumulation and a stronger inhibition of IL-6 signaling and barrier function loss. Together, our data demonstrate that the regulation of SOCS3 protein levels is critical to inhibit IL-6-mediated endotheliopathy during shock and provide a promising therapeutic avenue to prevent multiorgan dysfunction through stabilization of endothelial SOCS3.

Pro-Resolving Ligands Orchestrate Phagocytosis.

The resolution of inflammation is a tissue protective program that is governed by several factors including specialized pro-resolving mediators (SPMs), proteins, gasses and nucleotides. Pro-resolving mediators activate counterregulatory programs to quell inflammation and promote tissue repair in a manner that does not compromise host defense. Phagocytes like neutrophils and macrophages play key roles in the resolution of inflammation because of their ability to remove debris, microbes and dead cells through processes including phagocytosis and efferocytosis. Emerging evidence suggests that failed resolution of inflammation and defective phagocytosis or efferocytosis underpins several prevalent human diseases. Therefore, understanding factors and mechanisms associated with enhancing these processes is a critical need. SPMs enhance phagocytosis and efferocytosis and this review will highlight mechanisms associated with their actions.

Resolvin D1 Enhances Necroptotic Cell Clearance Through Promoting Macrophage Fatty Acid Oxidation and Oxidative Phosphorylation.

OBJECTIVE: Plaque necrosis is a key feature of defective resolution in atherosclerosis. Recent evidence suggests that necroptosis promotes plaque necrosis; therefore, we sought to determine how necroptotic cells (NCs) impact resolution programs in plaques. Approach and Results: To investigate the role(s) of necroptosis in advanced atherosclerosis, we used mice deficient of Mlkl, an effector of necroptosis. Mlkl-/- mice that were injected with a gain-of-function mutant PCSK9 (AAV8-gof-PCSK9) and fed a Western diet for 16 weeks, showed significantly less plaque necrosis, increased fibrous caps and improved efferocytosis compared with AAV8-gof-PCSK9 injected wt controls. Additionally, hypercholesterolemic Mlkl-/- mice had a significant increase in proresolving mediators including resolvin D1 (RvD1) and a decrease in prostanoids including thromboxane in plaques and in vitro. We found that exuberant thromboxane released by NCs impaired the clearance of both apoptotic cells and NCs through disruption of oxidative phosphorylation in macrophages. Moreover, we found that NCs did not readily synthesize RvD1 and that exogenous administration of RvD1 to macrophages rescued NC-induced defective efferocytosis. RvD1 also enhanced the uptake of NCs via the activation of p-AMPK (AMP-activated protein kinase), increased fatty acid oxidation, and enhanced oxidative phosphorylation in macrophages. CONCLUSIONS: These results suggest that NCs derange resolution by limiting key SPMs and impairing the efferocytic repertoire of macrophages. Moreover, these findings provide a molecular mechanism for RvD1 in directing proresolving metabolic programs in macrophages and further suggests RvD1 as a potential therapeutic strategy to limit NCs in tissues. Graphic Abstract: A graphic abstract is available for this article.

Resolvin D1 promotes efferocytosis in aging by limiting senescent cell-induced MerTK cleavage.

Inflammation-resolution is mediated by the balance between specialized pro-resolving mediators (SPMs) like resolvin D1 (RvD1) and pro-inflammatory factors, like leukotriene B4 (LTB4). A key cellular process of inflammation-resolution is efferocytosis. Aging is associated with defective inflammation-resolution and the accumulation of pro-inflammatory senescent cells (SCs). Therefore, understanding mechanism(s) that underpin this impairment is a critical gap. Here, using a model of hind limb ischemia-reperfusion (I/R) remote lung injury, we present evidence that aging is associated with heightened inflammation, impaired SPM:LT ratio, defective efferocytosis, and a decrease in MerTK levels in injured lungs. Treatment with RvD1 mitigated I/R lung injury in aging, promoted efferocytosis, and prevented the decrease of MerTK in injured lungs from old mice. Old MerTK cleavage-resistant mice (MerTKCR) exhibited less neutrophils or polymorpho nuclear cells infiltration and had improved efferocytosis compared with old WT controls. Mechanistically, macrophages that were treated with conditioned media (CM) from senescent cells had increased MerTK cleavage, impaired efferocytosis, and a defective RvD1:LTB4 ratio. Macrophages from MerTKCR mice were resistant to CM-induced efferocytosis defects and had an improved RvD1:LTB4 ratio. RvD1-stimulated macrophages prevented CM-induced MerTK cleavage and promoted efferocytosis. Together, these data suggest a new mechanism and a potential therapy to promote inflammation-resolution and efferocytosis in aging.

Resolvin D1 promotes the targeting and clearance of necroptotic cells.

Inflammation-resolution is a protective response that is mediated by specialized pro-resolving mediators (SPMs). The clearance of dead cells or efferocytosis is a critical cellular program of inflammation-resolution. Impaired efferocytosis can lead to tissue damage in prevalent human diseases, like atherosclerosis. Therefore understanding mechanisms associated with swift clearance of dead cells is of utmost clinical importance. Recently, the accumulation of necroptotic cells (NCs) was observed in human plaques and we postulated that this is due to defective clearance programs. Here we present evidence that NCs are inefficiently taken up by macrophages because they have increased surface expression of a well-known "don't eat me" signal called CD47. High levels of CD47 on NCs stimulated RhoA-pMLC signaling in macrophages that promoted "nibbling", rather than whole-cell engulfment of NCs. Anti-CD47 blocking antibodies limited RhoA-p-MLC signaling and promoted whole-cell NC engulfment. Treatment with anti-CD47 blocking antibodies to Ldlr-/- mice with established atherosclerosis decreased necrotic cores, limited the accumulation of plaque NCs and increased lesional SPMs, including Resolvin D1 (RvD1) compared with IgG controls. Mechanistically, RvD1 promoted whole-cell engulfment of NCs by decreasing RhoA signaling and activating CDC42. RvD1 specifically targeted NCs for engulfment by facilitating the release of the well-known "eat me signal" called calreticulin from macrophages in a CDC42 dependent manner. Lastly, RvD1 enhanced the clearance of NCs in advanced murine plaques. Together, these results suggest new molecules and signaling associated with the clearance of NCs, provide a new paradigm for the regulation of inflammation-resolution, and offer a potential treatment strategy for diseases where NCs underpin the pathology.

Telomerase Inhibition and Human Telomeric G-Quadruplex DNA Stabilization by a ß-Carboline-Benzimidazole Derivative at Low Concentrations.

Guanine rich regions in DNA, which can form highly stable secondary structures, namely, G-quadruplex or G4 DNA structures, affect DNA replication and transcription. Molecules that stabilize G4 DNA have become important in recent years. In this study, G4 DNA stabilization, inhibition of telomerase, and anticancer activity of synthetic ß-carboline-benzimidazole derivatives (5a, 5d, 5h, and 5r) were studied. Among them, derivatives containing a 4-methoxyphenyl ring at C1 and a 6-methoxy-substituted benzimidazole at C3 (5a) were found to stabilize telomeric G-quadruplex DNA efficiently. The stoichiometry and interaction of a synthetic, ß-carboline-benzimidazole derivative, namely, 3-(6-methoxy-1H-benzo[d]imidazol-2-yl)-1-(4-methoxyphenyl)-9H-pyrido[3,4-b]indole (5a), with human intermolecular G-quadruplex DNA at low concentrations were examined using electrospray ionization mass spectrometry. Spectroscopy techniques indicate that 5a may intercalate between the two stacks of G-quadruplex DNA. This model is supported by docking studies. When cancer cells are treated with 5a, the cell cycle arrest occurs at the sub-G1 phase. In addition, an apoptosis assay and fluorescence microscopy studies using cancer cells indicate that 5a can induce apoptosis. Results of biochemical assays such as the polymerase chain reaction stop assay and telomerase activity assay indicate that 5a has the potential to stabilize G-quadruplex DNA, and thereby, it may interfere with in vitro DNA synthesis and decrease telomerase activity. The results of this study reveal that the ß-carboline-benzimidazole derivative (5a) is efficient in G-quadruplex DNA stabilization over double-stranded DNA, inhibits telomerase activity, and induces apoptosis in cancer cells.

Fine-tuning inflammation-resolution programs: focus on atherosclerosis.

PURPOSE OF REVIEW: Nonresolving inflammation is now considered the underpinning of several prevalent human diseases, including atherosclerosis. The resolution of inflammation is a highly coordinated program to counterbalance proinflammatory signals for a swift return to tissue homeostasis. This process is controlled in part by endogenous specialized proresolving lipid mediators (SPMs). Emerging evidence has revealed that the balance of SPMs and proinflammatory mediators during acute inflammation regulates the duration of the inflammatory response and the timing of tissue resolution. Moreover, an imbalance between SPMs and proinflammatory mediators has been linked to several prevalent chronic inflammatory diseases in humans, including atherosclerosis. RECENT FINDINGS: Lipid mediator imbalances have recently been linked to atherosclerotic plaque instability. Administration of key SPMs restored this imbalance and led to plaque stability. SPMs have also recently been shown to be protective in other cardiovascular disease models including myocardial infarction, stroke and neointimal hyperplasia. SUMMARY: The current review highlights recent work that supports the concept of dysregulated inflammation-resolution in atherosclerosis with a particular focus on mechanisms and therapeutic opportunities associated with SPM receptors and lipid mediator imbalances. This article is based on experimental studies.