Targeting Angiogenesis via Resolution of Inflammation

Abstract

Angiogenesis, the growth of new blood vessels, plays a critical role in tissue repair and regeneration, as well as in cancer. A paradigm shift is emerging in our understanding of the resolution of inflammation as an active biochemical process with the discovery of novel endogenous specialized pro-resolving mediators (SPMs), including resolvins. Angiogenesis and the resolution of inflammation are critical interdependent processes. Disrupted inflammation resolution can accelerate tumor growth, which is angiogenesis-dependent. SPMs, including resolvins and lipoxins, inhibit physiologic and pathological angiogenesis at nanogram concentrations. The failure of resolution of inflammation is an emerging hallmark of angiogenesis-dependent diseases including arthritis, psoriasis, diabetic retinopathy, age-related macular degeneration, inflammatory bowel disease, atherosclerosis, endometriosis, Alzheimer's disease, and cancer. Whereas therapeutic angiogenesis repairs tissue damage (e.g., limb ischemia), inhibition of pathological angiogenesis suppresses tumor growth and other non-neoplastic diseases such as retinopathies. Stimulation of resolution of inflammation via pro-resolving lipid mediators promotes the repair of tissue damage and wound healing, accelerates tissue regeneration, and inhibits cancer. Here we provide an overview of the mechanisms of cross talk between angiogenesis and inflammation resolution in chronic inflammation-driven diseases. Stimulating the resolution of inflammation via pro-resolving lipid mediators has emerged as a promising new field to treat angiogenic diseases.

Angiogenesis is an organizing principle of many diseases, as demonstrated by the seminal studies of Dr. Judah Folkman and colleagues (Folkman 2007). Angiogenesis, the growth of new blood vessels, occurs throughout the human lifetime. Pathological angiogenesis, distinct from physiological angiogenesis in development, reproduction, organ regeneration, and wound repair, is a persistent process that can last years (Folkman 2007; Panigrahy et al. 2013). Prolonged angiogenesis can result in vascular leakage, tissue damage/destruction, and bleeding, all of which are implicated in the pathogenesis of angiogenesis-dependent diseases (Folkman 1995, 2007). Angiogenesis can be promoted via chronic inflammation, especially in diseases such as rheumatoid arthritis, endometriosis, obesity, and cancer (Folkman 1995; Lin et al. 2006; Marginean and Sharma-Walia 2015; Kolb et al. 2019; Samimi et al. 2019). The Serhan laboratory has discovered a family of pro-resolving lipid mediators with potent antiangiogenic activity (Fierro et al. 2002; Connor et al. 2007; Jin et al. 2009; Serhan 2014), suggesting a critical role for stimulating the resolution of inflammation in the treatment of angiogenic diseases.

ANGIOGENESIS AND INFLAMMATION ARE INTERDEPENDENT

While angiogenesis and inflammation are two distinct processes, they are interdependent, critically linked, and can potentiate each other. The failure of resolution of inflammation and pathological angiogenesis characterize many diseases (Ashraf et al. 2010; Ardelean et al. 2014; Peter et al. 2014; Serhan 2014). Angiogenesis promotes chronic inflammation (Ashraf et al. 2010), which can contribute to pathological angiogenesis via multiple mechanisms including vascular endothelial growth factor (VEGF) signaling, hypoxia-induced factor (HIF) signaling, and Toll-like-receptor (TLR) pathway signaling (Cramer et al. 2003; Pinhal-Enfield et al. 2003; Facciabene et al. 2011; Bartels et al. 2013; Imazeki et al. 2021; Liotti et al. 2021).

VEGF/vascular permeability factor (VPF) and its role in pathologic as well as physiological angiogenesis have been well-characterized (Senger et al. 1983; D'Amore 1994; Bielenberg et al. 1999; Carmeliet 2005; Melincovici et al. 2018). VEGF is a potent proangiogenic factor and is implicated in the development and progression of angiogenic diseases (Ferrara 1999; Dvorak 2000; Folkman 2007). VEGF is a diffusible protein that is secreted extracellularly from a multitude of cell types (Unemori et al. 1992). This potent angiogenic factor promotes angiogenesis through activation of several pathways that regulate endothelial cell (EC) proliferation, migration, and overall survival (Hicklin and Ellis 2005). VEGF can also increase vascular permeability, further promoting angiogenesis (Dvorak 2000; Hicklin and Ellis 2005). Importantly, VEGF-neutralizing therapies reduce inflammation and angiogenesis in animal models (Ardelean et al. 2014; Imazeki et al. 2021), suggesting interdependence between these distinct processes.

Angiopoietin 2 (Ang2) is a proangiogenic protein that also directly mediates inflammation pathways (Davis et al. 1996; Parikh 2017). Ang2 binds to the tyrosine kinase receptor Tie2/Tek, which is expressed primarily on ECs and, when activated, acts to maintain endothelial stability and promote vessel maturation (Maisonpierre et al. 1997; Gavard et al. 2008). Ang2 competes with the antiangiogenic vessel-stabilizing protein angiopoietin 1 (Ang1) to bind Tie2/Tek: while Ang1 activates the receptor, Ang2 antagonizes it, thus promoting endothelial instability, permeability, and angiogenesis (Maisonpierre et al. 1997; Parikh 2017). While the proangiogenic role of Ang2 has been well-characterized, recent studies have shifted the focus to the potential role of Ang2 in promoting inflammation. In an experimental study that involved treating healthy human volunteers with low levels of endotoxin, Ang2 levels rose significantly following treatment (Kumpers et al. 2009). Rising levels of Ang2 were associated with an increase of cytokines associated with inflammation, suggesting that Ang2 may regulate the human inflammatory response (Kumpers et al. 2009). In a proinflammatory environment, Ang2 antagonizes Tie2, thereby promoting angiogenesis to regulate inflammation (Parikh 2017). This mechanism provides an example of how inflammation can disrupt the balance of antiangiogenic and proangiogenic mediators, increasing the levels of the proangiogenic mediator Ang2 to induce angiogenesis. The complementary activity of Ang2 and Tie2 demonstrates how inflammation and angiogenesis are interdependent processes that regulate each other.

Hypoxia, a condition in which insufficient oxygen is available, may provide a critical link between angiogenesis and inflammation, as it is implicated in exacerbating both processes (Krock et al. 2011; Mamlouk and Wielockx 2013). Hypoxia directly promotes angiogenesis through inducing VEGF secretion (Schweiki et al. 1992). For example, breast cancer cells secrete both Ang2 and VEGF in response to hypoxia (Balamurugan 2016). Additionally, hypoxia and inflammation have been described as “two sides of the same coin,” in reference to the direct link between hypoxic conditions and inflammation (Bartels et al. 2013). In human cancer, hypoxia is considered to be a frequent component of the tumor microenvironment that acts to promote tumorigenesis (Balamurugan 2016).

Under hypoxic conditions, HIF interacts with the NF-κB pathway to promote inflammation (Bartels et al. 2013). The two subunits of HIF-1, HIF-1α and HIF-1β, bind to hypoxia response elements (HREs) of various genes, initiating transcription of proinflammatory NF-κB and TLR genes (Bartels et al. 2013). TLRs are a large family of pathogen pattern-recognition receptors that can bind both exogenous and endogenous ligands. TLR4 exhibits a potential role in inflammation-induced angiogenesis (Murad 2014). TLR4, along with other TLRs such as TLR-7 and TLR-9, signals in conjunction with adenosine receptor 2A (A_2A) to increase VEGF production by macrophages (Leibovich et al. 2002; Murad 2014). Notably, this synergistic signaling increases transcription of HIF, reinforcing hypoxia as a critical link between inflammation and angiogenesis (Ramanathan et al. 2007). NF-κB also activates transcription of proinflammatory genes including HIF, creating a positive feedback loop (Bartels et al. 2013). Importantly, resolution metabolomes are also activated by the hypoxic environments (Norris et al. 2019) such as bone marrow and spleen as well as sites of inflammation, stimulating specialized pro-resolving mediator (SPM)-biosynthetic circuits that induce the resolution of inflammation and clearance of senescent erythrocytes and apoptotic neutrophils (Norris et al. 2019).

HYPOXIA AND ANGIOGENESIS IN THE TUMOR MICROENVIRONMENT

Angiogenesis is required for tumor growth (Folkman 1971, 1990). Late-stage tumors of many cancer types are characterized by high levels of blood vessel permeability, continuous pathological angiogenesis, and elaborate abnormal vascularization (Hanahan and Folkman 1996). Magnetic resonance (MR) imaging of tumor vascular permeability following antiangiogenic treatment revealed that vascular permeability correlates with tumor growth (Raatschen et al. 2008). These results suggest that tumor vascular permeability is an important biomarker of tumor progression and support angiogenesis as a necessity for tumor growth (Raatschen et al. 2008). Additionally, blood vessel density has been reported to predict prognosis of patient survival in various cancers (Hanahan and Folkman 1996; Folkman 2007). During angiogenesis, the basement membrane of the EC tube is degraded, allowing adjacent ECs to migrate and invade the surrounding environment, creating a migrating column of proliferating cells (Hanahan and Folkman 1996). Without the development of functioning microvasculature, solid tumors cannot grow beyond 1 mm³; thus, angiogenesis is rate-limiting for tumor growth and expansion (Folkman 1971, 2007; Hanahan and Folkman 1996).

Tumor angiogenesis is determined by a balance between angiogenesis promotors (e.g., VEGF and fibroblast growth factors [FGFs]) and inhibitors (e.g., angiostatin and thrombospondin 1) (Folkman and Klagsbrun 1987; O'Reilly et al. 1994; Hanahan and Folkman 1996; Lawler and Lawler 2012). Tumor progression from an in situ state is driven by the “angiogenic switch” in which that balance is shifted toward angiogenesis stimulation (O'Reilly et al. 1994; Hanahan and Folkman 1996; Carmeliet 2005; Yeo et al. 2014). Consequently, the most vulnerable stage of tumor progression is the period before vascularization (Folkman 1971). Tumor-associated angiogenesis is directly dependent on secretion of angiogenic growth factors (e.g., VEGF).

Tumor angiogenesis can be initiated by hypoxic conditions (Lugano et al. 2020) as it stimulates various signaling pathways in cancer cells, including the HIF, PI3K, MAPK, and NF-κB pathways, resulting in dysfunctional vascularization, epithelial-to-mesenchymal transition (EMT), and resistance to treatment for metastasis by inducing cell quiescence (Muz et al. 2015). For example, the HIF pathway is a master regulator of angiogenesis, as hypoxia stimulates angiogenesis via the promotion of several proangiogenic pathways (Krock et al. 2011). VEGF mRNA levels have been reported to be increased by up to 13-fold in cell culture when oxygen is depleted from the environment (Shweiki et al. 1992), and reoxygenation of the environment leads to a rapid reduction of VEGF mRNA to normal prehypoxic levels (Shweiki et al. 1992). Capillary bundles tend to cluster in areas with a high density of cells expressing VEGF, further suggesting the critical role of VEGF in hypoxia-initiated angiogenesis. The vasculature that develops in solid tumors is vastly different than normal vasculature in healthy tissue (Carmeliet 2005; Jain 2005) in that this is characterized by vessels that are disorganized, structurally weak, and leaky (Carmeliet 2005; Jain 2005). These blood vessels bring oxygen to some areas of the tumor, but their abnormal structure prevents them from effectively supplying enough oxygen and other nutrients to all areas of the tumor tissue (Carmeliet 2005), further aggravating hypoxia and promoting more VEGF production (Carmeliet 2005). Tumor hypoxia elevates levels of CCL-28, a proangiogenic chemotactic factor (Facciabene et al. 2011; Huang et al. 2016). Through these mechanisms, hypoxia promotes angiogenesis in the tumor microenvironment, contributing to tumor growth and metastasis.

MACROPHAGES: CRITICAL REGULATORS OF ANGIOGENESIS AND INFLAMMATION RESOLUTION

Macrophages act to coordinate developmental angiogenesis, but their influence, via control of angiogenesis, extends to cancer and other angiogenic diseases (Noy and Pollard 2014). In murine models of cancer, macrophages act to promote cancer progression by inducing angiogenesis, increasing tumor cell invasion, and suppressing antitumor immunity (Cassetta and Pollard 2018). Infiltration of macrophages is associated with a poor prognosis and chemotherapy resistance (Cassetta and Pollard 2018). In a mouse model of breast cancer, macrophages have been shown to regulate the angiogenic switch (Lin et al. 2006; Jia and Zhou 2020). In these elegant studies, inhibition of macrophage infiltration into the tumor delayed the angiogenic switch and tumor progression, whereas restoration of the macrophage population rescued the stable vessel phenotype in these tumors (Lin et al. 2006). Other studies from the Pollard laboratory demonstrated a critical role of myeloid WNT7B in tumor progression via angiogenesis, invasion, and metastasis (Yeo et al. 2014). Neutralization of myeloid WNT7B inhibits lung metastasis and disruption of the angiogenic switch via reduced macrophage-induced tumor cell invasions and decreased VEGF (Yeo et al. 2014). Throughout the human life span, macrophages interact continuously with ECs to modulate the maintenance, maturation, and survival of blood vessels (Fantin et al. 2010). Macrophages can display a variety of different phenotypes and have been categorized into two main types: M1 macrophages and M2 macrophages (Martin and Gurevich 2021). M1 macrophages are generally considered proinflammatory, as they secrete proinflammatory cytokines such as interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)-α, and nitric oxide (NO) (Martin and Gurevich 2021), whereas M2 macrophages secrete anti-inflammatory cytokines such as IL-10 along with growth factors like TGF-β and EGF. However, the M1/M2 classification paradigm does not fully define the wide variety of macrophage phenotypes and their subsequent functions (Houser et al. 2011). Several subtypes of M2 macrophages have various functions: M2d macrophages (also known as tumor-associated macrophages [TAMs]) are the primary subtype involved in tumor infiltration and growth (Shapouri-Moghaddam et al. 2018).

Macrophages are effector cells (considered the big “eaters”) that stimulate phagocytosis of pathogenic debris (efferocytosis), a critical process in the resolution of inflammation (Serhan 2011, 2014). Whereas macrophages are typically characterized as CD11b^high, recent studies have characterized a new set of pro-resolving (“debris-clearing”) macrophages, identified and classified as CD11b^low macrophages (Schif-Zuck et al. 2011; Serhan 2014). Pro-resolving, CD11b^low macrophages were characterized in vivo during the resolution of inflammation-driven diseases such as peritonitis (Schif-Zuck et al. 2011). Pro-resolving lipid mediators such as resolvin E1 and D1, and the glucocorticoid dexamethasone, regulate pro-resolving macrophage functions in vivo (Schif-Zuck et al. 2011). These macrophages are distinct from the majority of peritoneal macrophages in terms of their functional protein expression profile, as well as pro-resolving properties, such as apoptotic leukocyte engulfment (Schif-Zuck et al. 2011).

Macrophages can be converted to the pro-resolving phenotype following phagocytosis of apoptotic cells: coincubation of CD11b^high peritoneal macrophages with apoptotic leukocytes leads to their conversion into CD11b^low macrophages, which are not phagocytic (Schif-Zuck et al. 2011). These pro-resolving macrophages secrete soluble mediators that attenuate tube formation in human umbilical vein endothelial cells (HUVECs), suggesting an antiangiogenic role. Additionally, media conditioned by CD11b^low macrophages contain lower levels of proangiogenic factors (VEGF, CXCL16, CCL2, osteopontin, and HGF) and higher levels of antiangiogenic inhibitors (PEDF, thrombospondin 2, and endostatin) as compared to conditioned media from CD11b^high macrophages (Michaeli et al. 2018). Moreover, CD11b^low macrophages can inhibit HUVEC proliferation and motility, thereby limiting their angiogenic potential. Additionally, CD11b^low macrophages inhibit EC differentiation into vessel-like networks; tubule formation is impaired in the presence of CD11b^low macrophages and those tubules that do form are unstable (Michaeli et al. 2018). Finally, treatment with conditioned media from CD11b^low macrophages attenuates neovascularization as measured via angiogenic sprouting from a segmented aorta ex vivo in the rat aorta ring assay (Michaeli et al. 2018) and HUVECs treated with CD11b^low macrophage-conditioned media showed decreased VEGFR2 phosphorylation (Michaeli et al. 2018). Absence of this phosphorylation (typically induced by VEGF) suggests that CD11b^low macrophages may inhibit tube formation in a VEGF-dependent manner (Michaeli et al. 2018). Thus, macrophages are a key cell type in the regulation of inflammation via angiogenesis.

UNRESOLVED INFLAMMATION IN DISEASE

In healthy individuals, acute inflammatory responses are normally self-limited and divided into initiation and resolution phases (Serhan and Levy 2018). During inflammation initiation, polymorphonuclear neutrophils (PMNs) infiltrate affected tissues (Mortaz et al. 2018). Two families of bioactive lipids, prostaglandins and leukotrienes, facilitate PMN chemotaxis and infiltration, governing venules and chemotaxis, respectively (Flower 2006; Serhan 2014). As such, prostaglandins and leukotrienes are recognized as proinflammatory lipid mediators (Samuelsson 1983; Flower 2006). Following the infiltration of PMNs, prostaglandin D2 (PGD₂) and prostaglandin E2 (PGE₂) trigger a lipid autacoid “class switching” to anti-inflammatory and pro-resolving lipid mediators such as resolvins, lipoxins, and protectins (Levy et al. 2001; Serhan 2014). SPMs synthesized in pro-resolving exudates promote a shift from the proinflammatory M1 phenotype to the anti-inflammatory M2 phenotype, enhancing phagocytosis of apoptotic neutrophils (Serhan 2014). Lipoxins and resolvins block neutrophil infiltration into the inflammatory site, reducing inflammation and promoting a return to homeostasis via the resolution of inflammation (Serhan 2014). Humans exhibit a differential resolution response to inflammatory insults (Morris et al. 2010).

Historically, the resolution of inflammation was considered to be a passive process (Serhan and Savill 2005), so the approach to inhibit inflammation over the past century has been to block proinflammatory mediators, enzymes, and pathways via small-molecule inhibitors, receptor antagonists, or neutralizing antibodies (Abrams 2008; Wang and Dubois 2010; Greene et al. 2011). However, this strategy resulted in only transient anti-inflammatory and antitumor activity (Wang and Dubois 2010; Greene et al. 2011; Sulciner et al. 2018a). Current anti-inflammatories also exhibit multiple toxicities including osteoporosis, fungal infections, and immunosuppression (steroids); bleeding, cardiovascular toxicity, and kidney toxicity (NSAIDs); thrombosis and heart attacks (COX-2 inhibitors); and increased infection and cancer (e.g., lymphoma) risk (cytokine inhibitors) (Wang and Dubois 2010; Pitter et al. 2016; Fishbein et al. 2021). There is an emerging paradigm shift in the understanding of the resolution of inflammation as an active biochemical process, orchestrated by a superfamily of SPMs (Serhan 2014), endogenously produced from omega-3 fatty acids (e.g. docosahexaenoic acid [DHA], n-3DPA, and eicosapentaenoic acid [EPA]) by ECs and innate immune cells (Mas et al. 2012; Serhan 2014, 2017; Norris et al. 2017, 2018b; Lumelsky et al. 2018; Norris and Serhan 2018; Serhan and Levy 2018). The stereochemistry and biosynthesis of each SPM by human leukocytes has been established (Arita et al. 2005a; Serhan et al. 2006, 2012; Sun et al. 2007; Oh et al. 2011; Serhan and Petasis 2011; Chiang et al. 2012; Tungen et al. 2014; English et al. 2017; Hansen et al. 2017). Pro-resolving lipid mediators actively dampen host inflammatory responses by stimulating macrophage phagocytosis of debris, resulting in reduced localized inflammatory cytokines without being immunosuppressive (Serhan 2014; Serhan and Levy 2018).

Stimulating the resolution of inflammation via SPMs, such as resolvins, is protective during hyperinflammatory diseases including infection, sepsis, and cancer (Spite et al. 2009; Chiang et al. 2012; Dalli et al. 2015; Norris et al. 2018a; Sulciner et al. 2018b). Resolvins (e.g., RvD1, RvE1, and RvD2) prevent cytokine storms and decrease septic mediators (e.g., high mobility group box 1 [HMGB1]) and cell death at nanogram concentrations in many experimental models of hyperinflammatory conditions, including asthma, colitis, pneumonia, arthritis, ocular diseases, acute lung injury (e.g., from cigarette smoke), acute respiratory distress syndrome (ARDS), sciatica, neuroinflammatory diseases (e.g., Alzheimer's), sepsis, and cardiac diseases (e.g., myocardial infarction) (Aoki et al. 2008; Haworth et al. 2008; Jin et al. 2009; Ishida et al. 2010; Seki et al. 2010; Bento et al. 2011; Lima-Garcia et al. 2011; Rajasagi et al. 2011, 2017; Eickmeier et al. 2013; Hsiao et al. 2013; Sun et al. 2014, 2019; Wang et al. 2014, 2018; Croasdell et al. 2015; Hiram et al. 2015; Rossi et al. 2015; Khaddaj-Mallat et al. 2016; Liu et al. 2016, 2019; Rey et al. 2016; Yin et al. 2017; Kantarci et al. 2018; Zhuo et al. 2018; Benabdoun et al. 2019; Kain and Halade 2019; Xia et al. 2019; Zhang et al. 2019, 2020; Isopi et al. 2020). The production of SPMs is critical to enable the active resolution of inflammation (Serhan 2014; Serhan and Levy 2018). Conventional anti-inflammatory drugs such as steroids, NSAIDs, and cytokine antagonists do not clear debris and can be “resolution toxic,” as they indiscriminately inhibit eicosanoid pathways that produce pro-resolution mediators and thereby prevent complete resolution (Gilroy et al. 1999; Serhan 2014; Serhan and Levy 2018). By contrast, SPMs terminate self-sustaining inflammatory processes by counterregulating proinflammatory cytokines and prostaglandins promoting a return to tissue homeostasis (Serhan 2014). Moreover, resolvins are active at more than 10,000-fold lower concentrations (pico to nanogram range) than omega-3 fatty acids, NSAIDs, and aspirin (Sulciner et al. 2018b; Gilligan et al. 2019; Panigrahy et al. 2019). SPMs may alleviate the hyperinflammatory response by reducing endoplasmic reticulum (ER) stress and down-regulating NF-κB (Serhan 2014), the central regulator of eicosanoid-induced cytokine storms. Additionally, resolvins reduce the systemic inflammatory markers C-reactive protein (CRP) and IL-1β in inflammatory diseases, including periodontitis, obesity, and acute kidney injury (Hasturk et al. 2007; Hellmann et al. 2011; Chen et al. 2014).

Lipid autacoid mediators are biosynthesized from polyunsaturated fatty acid (PUFA) precursors and are critical regulators of both the initiation and resolution of inflammation (Serhan 2014; Serhan and Levy 2018). Acute inflammation initiates the release of proinflammatory eicosanoids, that, when uncontrolled, lead to an “eicosanoid storm” that drives proinflammatory cytokine production (Serhan 2014; Dennis and Norris 2015). Prostaglandins (e.g., PGE₂ and PGD₂) induce a pivotal lipid mediator class-switching event from production of proinflammatory eicosanoids via the lipoxygenase pathways to production of SPMs (e.g., resolvins, maresins, and lipoxins) (Serhan et al. 2000, 2002, 2008; Levy et al. 2001; Hong et al. 2003). The resolution of inflammation is regulated by endogenous mechanisms via an active switch from proinflammatory to pro-resolving lipid mediators that are separate and distinct from anti-inflammatory processes (Serhan 2014).

Three families of SPMs (lipoxins, D-series resolvins, and E-series resolvins) are biosynthesized from PUFAs (Serhan 2014). Lipoxins are biosynthesized via arachidonic acid via lipoxygenases such as 15-lipoxygenase (5-LOX), whereas resolvins of the D-series (RvDs) are biosynthesized from DHA and RvEs from EPA (Serhan et al. 2002; Serhan 2017). SPMs are actively generated during a defined period of inflammation resolution and maintain as well as restore tissue homeostasis (Serhan and Levy 2018). SPMs contribute to resolution of inflammation by a variety of mechanisms, including through modifying leukocyte phenotype. During resolution of inflammation, PGE₂ and PGD₂ signal for production of 15-lipoxygenase, enabling leukocytes to acquire a pro-resolving phenotype and initiate lipoxin production (Serhan 2014). Resolvin E1 (RvE1) reduces inflammation and blocks neutrophil migration, counterregulating proinflammatory signals to promote resolution (Serhan 2010, 2014). RvE1 antagonizes the leukotriene B₄ (LTB₄) receptor BLT1 on the surface of neutrophils, reducing the proinflammatory signals induced by LTB₄ binding (Arita et al. 2007). The Serhan laboratory recently discovered T-series resolvins that stimulate macrophage clearance of neutrophil extracellular traps (NETs) and limit neutrophil infiltration, accelerating resolution of inflammation (Chiang et al. 2022). This has important implications in cancer, as NETs produced during inflammation have been reported to awaken dormant cancer cells in mice (Albrengues et al. 2018).

Despite many approaches to block inflammation, there are no current FDA-approved therapies designed to stimulate the active resolution of inflammation in cancer patients (Panigrahy et al. 2021). SPMs enhance the resolution of inflammation in cancer models via macrophage phagocytosis of tumor-promoting cancer cell debris as well as counterregulation of proinflammatory eicosanoid-driven cytokines (Sulciner et al. 2018b; Gilligan et al. 2019; Panigrahy et al. 2021). Resolvins inhibit the growth of debris-stimulated tumors in murine cancer models via inhibition of tumor angiogenesis as evidenced by a reduction in CD31⁺-positive ECs colocalized with vessel structures (Sulciner et al. 2018b). Cell death, which leads to the accumulation of apoptotic cells, necrotic cells, and cell fragments, and is referred to as debris, generated by cytotoxic cancer therapy or carcinogens accelerates tumor progression by stimulating a macrophage-derived eicosanoid and cytokine storm (Sulciner et al. 2018b; Chang et al. 2019; Gartung et al. 2019; Fishbein et al. 2020, 2021; Haak et al. 2021; Panigrahy et al. 2021). The activity of released cytokines/chemokines can cause massive cell death that provokes a cascade of host responses, including the production of macrophage-derived eicosanoids that potentiates a vicious cycle of eicosanoid and cytokine storms (Serhan 2014; Gartung et al. 2019; Fishbein et al. 2021). Thus, targeting a single proinflammatory mediator via conventional anti-inflammatory agents that do not stimulate clearance of debris is unlikely to prevent tumor progression, and there is an urgent unmet need to stimulate the resolution of inflammation to prevent carcinogenesis (Fishbein et al. 2021).

Cellular debris is assumed to be inert or even to be inhibitory to tumor growth because of the stimulation of tumor immunity (Bonavita et al. 2018; Sulciner et al. 2018b; Haak et al. 2021). However, spontaneous apoptotic cell death is elevated in tumors of cancer patients and correlates with poor prognosis (Wyllie 1985; Kornbluth 1994; de Jong et al. 2000; Naresh et al. 2001; Jalalinadoushan et al. 2004; Sun et al. 2006; Gregory and Pound 2011; Alcaide et al. 2013; Ichim and Tait 2016). Further, coinjection of cellular debris with live tumor cells reduces the number of tumor cells needed to produce tumors in rodents (Révész phenomenon) (Révész 1956; Huang et al. 2011; Chaurio et al. 2013; Ford et al. 2015; da Silva et al. 2017). Cellular debris can also stimulate the escape from tumor dormancy via failure to resolve inflammation (Sulciner et al. 2018b; Fishbein et al. 2021; Panigrahy et al. 2021).

The SPMs are potent “stop” signals that actively limit leukocyte infiltration, counterregulate the cytokine storm, and are highly effective in treating experimental pathologies driven by uncontrolled inflammation at nanogram concentrations in vivo (logs lower than aspirin or dexamethasone) (Serhan et al. 2000, 2002, 2012; Serhan 2014; Serhan and Levy 2018). Stimulation of inflammation resolution inhibits experimental ovarian, liver, lung, prostate, and pancreatic cancer via debris-clearance mechanisms and counterregulation of proangiogenic debris-induced eicosanoid and cytokine “storms” (Sulciner et al. 2018b; Gartung et al. 2019; Gilligan et al. 2019; Fishbein et al. 2020; Deng et al. 2021; Panigrahy et al. 2021).

SPMs AS BIOMARKERS IN HYPERINFLAMMATORY DISEASES

SPMs can function as biomarkers of inflammation resolution in human disease. Chronic inflammation characterizes a number of human diseases and is associated with the loss or dysregulation of SPMs (e.g., decreased resolvin/eicosanoid ratio [RvD1/LTB₄]), including atherosclerosis, sepsis, COVID-19, periodontitis, sickle cell anemia, asthma, colon cancer, and cystic fibrosis (Stenke et al. 1991; Serhan 2014; Croasdell et al. 2015; Dalli et al. 2017; Serhan and Levy 2018; Zhuang et al. 2018; Matte et al. 2019; Schwarz et al. 2020). Eicosanoids, SPMs, and SPM/eicosanoid ratios in blood have been identified as serological biomarkers to monitor disease and treatment efficacy in various hyperinflammatory diseases (Colas et al. 2014; Dalli et al. 2017; Chhonker et al. 2018). Dysregulated levels of SPMs with elevated eicosanoid patterns are detected in human patients with various inflammatory diseases, including sepsis, COPD, colon cancer, and leukemia (Stenke et al. 1991; Pillai et al. 2012; Croasdell et al. 2015; Fredman et al. 2016; Dalli et al. 2017; Thul et al. 2017; Zhuang et al. 2018; Matte et al. 2019; Wang et al. 2019; Isopi et al. 2020).

In sepsis patients, the Serhan laboratory identified elevated inflammation-initiating eicosanoids, leukotrienes, and certain pro-resolving lipid mediators, which predicted ARDS survival, progression, and respiratory failure (Dalli et al. 2017). In contrast, other traditional clinical indices were not predictive of ARDS progression or clinical outcomes (Dalli et al. 2017). In angioplasty, leukotrienes and lipoxins are released in response to inflammation (Brezinski et al. 1992). The ratio between RvD1 and LTB₄ has been recently targeted in the clinic as a marker of the systemic inflammatory response and inflammation resolution, with a lower ratio (low levels of RvD1 and high levels of LTB₄), indicating increased systemic levels of inflammation. A lowered RvD1/LTB₄ ratio is observed in atherosclerotic plaques, accompanied by low levels of pro-resolving lipoxin A₄ (LXA₄) in the circulation (Fredman et al. 2016). The RvD1/LTB₄ ratio is also reduced in preeclampsia, as is the ratio of maresin 1 to LTB₄ (Oliveira Perucci et al. 2020). Neuroinflammatory diseases such as neuromyelitis also exhibit a decreased RvD1/LTB₄ ratio in cerebrospinal fluid (CSF) (Wang et al. 2019). A study of patients with tuberculosis meningitis found that increased production of inflammatory eicosanoids and decreased production of SPMs resulted in worsened disease severity, and CSF of surviving patients showed elevated levels of pro-resolving resolvin T (RvT)2, RvT4, and LXB₄ (Colas et al. 2019). Additionally, the wide range of diseases characterized by an altered ratio of proinflammatory and pro-resolving mediators suggests an important clinical application for SPMs as biomarkers of inflammatory diseases including COVID-19 and cancer characterized by “failure of resolution” (Panigrahy et al. 2019, 2020, 2021).

PRO-RESOLVING LIPID MEDIATORS INHIBIT ANGIOGENESIS-DEPENDENT DISEASES

Chronic and persistent inflammation is a defining feature of many angiogenesis-dependent diseases (Fierro et al. 2002). Pro-resolving lipid mediators inhibit disease progression and alleviate symptoms of several angiogenesis-dependent diseases (Table 1).

Table 1.

Role of specialized pro-resolving mediators (SPMs) in alleviating angiogenesis-dependent diseases

Disease	Symptoms	Potential role(s) of SPMs	References
Arthritis	Joint pain, swelling, and immobility	Resolvin (Rv)D1: slow disease progression, inhibit angiogenesis	Sun et al. 2020
Atherosclerosis	Chest pain and dyspnea	RvE1: inhibit plaque formation	Hasturk et al. 2015; Salic et al. 2016
Inflammatory bowel disease	Chronic intestinal inflammation, diarrhea, abdominal pain	RvD1, RvD2, PD1, RvD5: reduce colitis, slow disease progression, reduce inflammation	Bento et al. 2011; Gobbetti et al. 2017
Diabetic retinopathy	Vision loss	RvD1 and RvE1: reduce inflammation and inhibit angiogenesis	Connor et al. 2007
Psoriasis	Rash, itching, pain, flaky skin	RvD3: reduce inflammation and itching	Lee et al. 2020

Arthritis

Rheumatoid arthritis is an autoimmune disease characterized by persistent inflammation and angiogenesis in joints. Animal models have demonstrated elevated levels of proinflammatory cytokines and reactive oxygen species (Yang et al. 2018). RvD1 alleviates disease progression through inhibition of angiogenesis in human rheumatoid arthritis (Sun et al. 2020).

Atherosclerosis

In atherosclerosis, inhibition of angiogenesis suppresses disease progression (Moulton et al. 2003; Folkman 2007). Treatment with RvE1 inhibits atherosclerotic plaque formation in a rabbit model of periodontitis-induced atherogenesis (Hasturk et al. 2015). Similar pro-resolving activity was observed in a mouse model of atherosclerosis: RvE1 at both low and high doses reduced formation of severe atherosclerotic plaques in ApoE*3Leiden mice (Salic et al. 2016).

Inflammatory Bowel Disease (IBD)

IBD, which encompasses both ulcerative colitis and Crohn's disease, is one of the most common inflammatory diseases worldwide. IBD is characterized by persistent intestinal inflammation and chronic angiogenesis (Zhang and Li 2014; Alkim et al. 2015). Anti-VEGF therapy reduces inflammation and attenuates disease progression in an Eng (+/–) murine model of chronic colitis through antiangiogenic activity (Ardelean et al. 2014). Dietary modification has been consistently proven to effect colitis symptoms and disease progression, with diets high in proinflammatory omega-6 fatty acids worsening colitis, and diets high in anti-inflammatory omega-3 fatty acids leading to its improvement (IBD in EPIC Study Investigators et al. 2009; Tian et al. 2013; Huang et al. 2017). Epidemiologic data indicate that increased omega-6 intake (such as that associated with the Western diet) elevates risk for colitis and subsequent colon cancer (Scaioli et al. 2017).

Levels of SPMs derived from omega-3 fatty acids have been shown to be lower in colonic biopsy samples from patients with ulcerative colitis than healthy patients (Ungaro et al. 2017). Accordingly, treatment with RvE1 reduced colitis in a mouse model of peritonitis (Arita et al. 2005b), and treatment with RvD1 and RvD2 inhibited disease progression in two different experimental mouse models of colitis (Bento et al. 2011). Treatment with protectin D1 and RvD5 was effective at attenuating intestinal inflammation and disease progression in a mouse model of colitis (Gobbetti et al. 2017). Taken together, the evidence suggests that SPMs are potent inhibitors of colitis-induced intestinal inflammation and angiogenesis with tremendous potential to alleviate the symptoms of inflammatory bowel disease.

Diabetic Retinopathy (DR)

Proliferative DR is characterized by excessive angiogenesis of the retina. Hypoxia-driven pathologic retinal angiogenesis can eventually lead to blindness (D'Amore 1994). Dietary supplementation with omega-3 PUFAs has been shown to reduce experimental hypoxia-driven retinal angiogenesis (Connor et al. 2007) and supplementation with RvD1 and RvE1, along with neuroprotectin D1, inhibits pathological angiogenesis in the retina. RvD1 attenuates inflammation in the rat model of streptozotocin-induced retinopathy (Yin et al. 2017). RvE1 induces microglia expressing ChemR23 in the retina to suppress production of proinflammatory cytokines that can stimulate angiogenesis. Based on this evidence, SPMs can alleviate progression of angiogenesis-dependent diseases (e.g., DR) through both direct inhibition of angiogenesis and stimulation of the resolution of inflammation.

SPECIALIZED PRO-RESOLVING MEDIATORS STIMULATE WOUND REPAIR AND TISSUE REGENERATION

Therapeutic angiogenesis aims to stimulate the growth of new vessels, to restore or augment circulatory perfusion of tissues, to reverse ischemia and/or to accelerate healing (Tímár et al. 2001) and is used clinically in various diseases including cases of heart and cerebrovascular diseases, critical limb ischemia, delayed wound healing, and peptic ulcer disease. In the context of physiologic angiogenesis, SPMs, such as D-series resolvins support vascularization in wound repair and tissue regeneration (Hellmann et al. 2018). Pro-resolution therapies, including resolvins (e.g., RvD1), can be used for the treatment of delayed healing of diabetic wounds (Tang et al. 2013).

Macrophages are critical regulators of wound healing, as they phagocytose proinflammatory neutrophils and secrete proangiogenic factors that support healing (Gurevich et al. 2018). Given the unique ability of SPMs to stimulate macrophage phagocytosis (Serhan 2014), the role of complex pro-resolving mediators in wound healing and associated vascularization has been investigated (Hellmann et al. 2012). Pro-resolving mediators play a critical role in regulating normal wound healing, but their mechanisms of action have been most thoroughly studied in the context of diabetic wound healing (Lu et al. 2010; Tian et al. 2011b; Hellmann et al. 2012). Diabetic wound healing is characterized by impaired clearance of apoptotic cells, reflecting a baseline malfunctioning of macrophage efferocytosis (Khanna et al. 2010). SPMs stimulate macrophage phagocytosis of apoptotic cells in wound healing, reducing apoptotic cell burden and promoting resolution of inflammation (Hellmann et al. 2011, 2012). Pro-resolving lipid mediators assist diabetic wound healing by increasing VEGF release and supporting phagocytosis and other proangiogenic mechanisms (Khanna et al. 2010; Tian et al. 2011a,b). RvD2 prevents secondary thrombosis and necrosis in a murine burn wound model (Bohr et al. 2013). A novel RvD6 stereoisomer induces corneal nerve regeneration and wound healing postinjury (Pham et al. 2020) and was shown to ameliorate ocular neuropathic pain and dry eye (Pham et al. 2020). Aspirin-triggered resolvin D1 (AT-RvD1) can induce inflammation resolution, enhancing vascular remodeling and a pro-regenerative shift in macrophage and dendritic cell phenotype, resulting in improved wound closure after skin transplantation (Turner et al. 2020).

In other examples, intrathecal injection of RvD3 represents a promising therapeutic strategy to promote inflammation resolution, neuroprotection, and neurological functional recovery following spinal cord injury (Kim et al. 2021), and in a rat periodontitis model, RvD2 induces resolution of periapical inflammation and promotes healing of periapical lesions (Siddiqui et al. 2019). In these studies, RvD2 induces active resolution of inflammation with pulp-like dental tissue regeneration after root canal infection and thus may be suitable for treating periapical lesions (Siddiqui et al. 2019). Importantly, humanized nano-pro-resolving agents stimulate inflammation resolution and enhance wound healing (Norling et al. 2011). In a series of elegant studies, the Serhan laboratory constructed novel nanoparticles (NPs) containing AT-RvD1 or an LXA₄ analog. These NPs dramatically reduce polymorphonuclear cell influx in murine peritonitis, shorten resolution intervals, exhibit pro-resolving actions, and accelerate keratinocyte healing (Norling et al. 2011). Thus, NPs carrying mimetics of endogenous resolving agents possess potent beneficial bioactions and can reduce nanotoxicity (Norling et al. 2011). In addition to wound repair, SPMs also promote tissue regeneration in lung, muscle, and other tissues (Kim et al. 2016; Giannakis et al. 2019; Quiros et al. 2020). Resolvin conjugates in tissue regeneration are novel immunoresolvent agonists that accelerate resolution of inflammation, infection, and tissue regeneration (de la Rosa et al. 2018) and have been shown to accelerate planaria tissue regeneration and stimulate pro-resolving phagocyte functions in human tissues (de la Rosa et al. 2018).

Intriguingly, the Spite laboratory recently demonstrated that myeloid ALX/FPR2 regulates vascularization after tissue injury (Sansbury et al. 2020). In these studies, RvD1 induces a transcriptional program in macrophages that is characteristic of a pro-revascularization phenotype. Vascularization of ischemic skeletal muscle is impaired in mice with myeloid-specific Alx/Fpr2 deficiency (Sansbury et al. 2020). RvD1 treatment prevents tissue damage and promotes repair in lung tissue in a mouse model of chronic smoking exposure (Kim et al. 2016). Similarly, RvD2 treatment stimulates muscle regeneration and function by regulating macrophage phenotype (Giannakis et al. 2019) and RvE1 accelerates pulp repair by regulating inflammation and stimulating dentin regeneration in dental pulp stem cells (Chen et al. 2021). Additionally, RvE1 treatment promotes tissue regeneration and wound mucosal repair in the intestinal epithelium via in situ administration of targeted polymeric NPs into intestinal wounds (Quiros et al. 2020). RvE1 is locally produced in response to intestinal mucosal injury (Quiros et al. 2020). RvD1 stimulates epithelial wound repair via natural resolution pathways such as inhibiting TGF-β-induced EMT by reducing fibroproliferation and collagen production (Zheng et al. 2018). Topical application of RvD1 promotes corneal epithelial wound healing as well as the restoration of mechanical sensation in diabetic mice (Zhang et al. 2018).

LIPOXIN A₄ (LXA₄) SIGNALING INHIBITS ANGIOGENESIS

The anti-inflammatory and pro-resolving activity of the SPM LXA₄ in the tumor microenvironment makes it a compelling target to treat angiogenic diseases such as cancer. Inhibition of angiogenesis, which has been referred to as the “4th arm of cancer therapy” (Chen et al. 2010), is also regulated by SPMs such as LXA₄ (Table 2). LXA₄ has been shown to be a potent inhibitor of angiogenesis via regulation of VEGF: administration of exogenous LXA₄ to in vitro mouse hepatocarcinoma cells inhibited VEGF secretion in a dose-dependent manner (Chen et al. 2010). In vivo administration of the LXA₄ receptor agonist BML-111 also significantly reduced serum VEGF in hepatocarcinoma tumor-bearing mice (Chen et al. 2010). However, LXA₄ is rapidly metabolized, making it challenging for translation to the clinic (Chen et al. 2010). More stable synthetic compounds, such as ATL-1 and BML-111, are typically used for human translational studies (Jin et al. 2009).

Table 2.

Structures and antiangiogenic activities of pro-resolving lipid mediators

Molecule name	Description	Molecular structure	Antiangiogenic effects	References
Resolvin D1 (RvD1)	Specialized pro-resolving mediators (SPMs) derived from docosahexaenoic acid (DHA)		Suppresses cell migration and proliferation, reduces expression of vascular endothelial growth factor (VEGF)-A, VEGF-C, and VEGFR2	Sun et al. 2020
Resolvin E1 (RvE1)	SPM derived from eicosapentaenoic acid (EPA)		Reduces expression of VEGF-A, VEGF-C, and VEGFR2	Jin et al. 2009
Lipoxin A4 (LXA4)	SPM derived from arachidonic acid (AA)		Inhibits VEGF secretion	Chen et al. 2010; Leedom et al. 2010
ATL-1/ ATLa	Synthetic analog of aspirin-triggered LXA4		Reduces expression of VEGF-A, VEGF-C, and VEGFR2	Jin et al. 2009
BML-111	LXA4 receptor agonist		Inhibits tumor-derived endothelial cell (Td-EC) migration, suppresses COX-2 and hypoxia-induced factor (HIF) in Td-EC, and reduces VEGF levels	Chen et al. 2010

LXA₄ and BML-111 both inhibit tumor-derived-EC (Td-EC)-induced tube formation by HUVECs (Vieira et al. 2014; Lin et al. 2021). BML-111 also blocks migration and reduces tumor EC expression of COX-2 and HIF-1α, two genes that enable angiogenesis (Lin et al. 2021). In a murine hepatocarcinoma model, BML-111 administration significantly reduces levels of VEGF in serum and in tumor tissue, suggesting that BML-111 and LXA₄ may inhibit angiogenesis in a VEGF-dependent manner (Chen et al. 2010). In support of this concept, LXA₄ treatment inhibits VEGF secretion in a dose-dependent manner (Chen et al. 2010).

ATL-1, an LXA₄ mimetic, is a potent mediator of inflammation resolution, and other studies suggest that ATL-1 may regulate angiogenesis (Gil-Villa et al. 2012). ATL-1 inhibits EC proliferation and migration in cells stimulated with proinflammatory angiogenic factors such as VEGF and leukotriene D₄ (Fierro et al. 2002). Treatment with ATL-1 significantly reduces mRNA expression of the proangiogenic factors VEGF-A and VEGF-C (Jin et al. 2009). Additionally, ATL-1 treatment has been shown to reduce the endothelial permeability induced by VEGF, at least in part by inhibiting VEGF-induced phosphorylation of VE-cadherin (Vieira et al. 2014). ATL-1 treatment significantly reduced the migration of human MV3 melanocyte cells (Vieira et al. 2014).

ATL-1 regulates macrophage phenotype and activity (Simoes et al. 2017) via ALX, a G-protein-coupled receptor (Simoes et al. 2017). In vivo, ATL-1 inhibits tumor progression and modulates the M1/M2 marker expression profiles of TAMs (Simoes et al. 2017). In vitro, ATL-1 decreases M2-associated marker expression on TAM (Vieira et al. 2014), suggesting that ATL-1 reprograms TAM from the M2 phenotype to the M1 phenotype. Additionally, ATL-1 inhibits TAM release of IL-10, limiting the immunosuppressive potential of these macrophages and suppressing their protumorigenic activity (Vieira et al. 2014).

LXA₄ can regulate pathological angiogenesis (Leedom et al. 2010). The cornea under normal conditions is an avascular site. The mouse cornea produces LXA₄, which has been suggested to play a role in maintaining its avascularity (Gronert et al. 2005). 15-LOX and 5-LOX are critical enzymes in the biosynthesis of SPMs (e.g., LXA₄) (Serhan 2014); genetic deletion of these precursor enzymes increased neovascularization and VEGF-A expression following chronic corneal injury (Leedom et al. 2010). Direct topical treatment of corneal injury with LXA₄ led to significantly lowered levels of VEGF-A and VEGFR3 mRNA (Leedom et al. 2010). Additionally, exogenous LXA₄ treatment down-regulated protective pathways normally activated in an inflammatory state, including heme oxygenase (HO)-1, 12-LOX, and 15-LOX (Leedom et al. 2010). Thus, the SPM LXA₄ plays an important role in regulating both physiological and pathological angiogenesis.

RESOLVINS INHIBIT ANGIOGENESIS

RvD1 can induce the resolution of inflammation and block angiogenesis in a murine collagen-induced arthritis (CIA) model (Sun et al. 2020). When treated with RvD1, CIA mice showed lower rates of joint inflammation and damage than control mice (Sun et al. 2020). RvD1 treatment down-regulated levels of proinflammatory cytokines and suppressed angiogenesis both in vivo in CIA mice and in vitro in HUVECs (Sun et al. 2020), suggesting that RvD1 can attenuate disease progression in RA (Sun et al. 2020). Additionally, RvD1 treatment suppressed cell migration and proliferation in CIA mice, indicating that it may slow RA progression, at least in part, through an angiogenesis-dependent mechanism (Sun et al. 2020). RvD1 treatment up-regulates miRNA-146-5P in RA fibroblast-like synoviocyte (FLS) cells (Sun et al. 2020). Interestingly, miRNA-146-5P down-regulates the NF-κB pathway, suggesting that the antiangiogenic activity of RvD1 may occur via NF-κB pathway inhibition (Sun et al. 2020).

RvD1 has been shown to be a potential mediator of angiogenesis in various cancers such as gastric cancer (Fig. 1). Formyl peptide receptor 1 (FPR1) activity inhibits angiogenesis in a murine model of gastric cancer, and genetic ablation of this receptor in tumor cells increases tumorigenesis and angiogenesis (Prevete et al. 2015). RvD1 binds to G-protein-coupled receptor 32 (GPR32) (Cash et al. 2014). The FPR1 pathway controls ALOX and GPR32 pathways, suggesting a key role for SPMs in modulating the antiangiogenic activity of FPR1 (Prevete et al. 2017). Treatment of FPR1-deficient gastric cancer cells with LXA₄ and RvD1 reversed their proangiogenic activity (Prevete et al. 2017). Further, genetic removal of GPR32 increases the angiogenic potential of gastric cancer cells, suggesting that RvD1 signaling through this receptor plays an important role in its antiangiogenic activity (Prevete et al. 2017). RvD1 also binds to LXA₄/formyl peptide receptor 2 (ALX/FPR2), inhibiting T-cell infiltration and angiogenesis in a corneal transplantation model (Hua et al. 2014).

Figure 1.

Mechanisms by which hypoxic conditions in the tumor microenvironment promote angiogenesis and counterregulation by specialized pro-resolving mediators (SPMs). (RvD1) Resolvin D1, (RvE1) resolvin E1, (LXA₄) lipoxin A₄, (VEGF) vascular endothelial growth factor, (Ang2) angiopoietin, (VEGFR2) VEGF receptor 2.

In DR, hyperglycemia triggers a cascade of events, resulting in retinal hypoxia, which induces the production and release of VEGF, leading to inflammation and angiogenesis, causing vascularization and vision impairment or blindness (Crawford et al. 2009). RvD1 inhibits inflammation in murine models of DR (Yin et al. 2017) and has been shown to inhibit angiogenesis through binding to receptor FPR2 (Maisto et al. 2020) by up-regulating levels of antiangiogenic miRNAs, miR-20a-3p, miR-20a-5p, miR-20b, and miR-106, and down-regulating VEGF (Maisto et al. 2020). While further mechanistic studies are needed, these results demonstrate that resolvins can regulate angiogenesis in a VEGF-dependent manner.

CONCLUSIONS AND OUTLOOK

As demonstrated in many angiogenesis-dependent diseases, promoting inflammation resolution clears inflammatory exudates and promotes a return to tissue homeostasis. Further, SPMs act at significantly lower doses compared with conventional anti-inflammatory agents and are not immunosuppressive. The current COVID-19 pandemic has made the need for pro-resolving treatments for inflammatory diseases even more urgent (Panigrahy et al. 2020). Vascular angiogenesis has been observed as a component of the pulmonary pathobiology of COVID-19 (Ackermann et al. 2020). Targeting endogenous lipid autacoid mediators, such as eicosanoids and SPMs, offers a new approach to SARS-CoV-2 infection and cancer therapy by targeting angiogenesis.

Pro-resolving lipid mediators enhance host defense to both viral and bacterial infections at nanomolar concentrations and lower the threshold for antibiotic therapy (Chiang et al. 2012; Morita et al. 2013). SPMs, including resolvins, lipoxins, and protectins, demonstrate direct antiviral activity in virally infected animal models, including infection with influenza (Rajasagi et al. 2011, 2013; Baillie and Digard 2013; Morita et al. 2013; Tam 2013; Tanner and Lee 2013; Imai 2015). Thus, SPMs may regulate endogenous inflammation resolution in COVID-19 (Panigrahy et al. 2020). Disrupted resolution mechanisms favor altered phagocyte response in COVID-19 (Koenis et al. 2021). Because SPM signaling plays a key role in attenuating hyperinflammatory disease conditions (Serhan 2014; Serhan and Levy 2018), stimulation of these pro-resolution lipid mediators, including resolvins, may “resolve” the inflammation in cancer and COVID-19 (Panigrahy et al. 2020). SPMs, including resolvins, lipoxins, and protectins, are currently in clinical development for the treatment of inflammatory diseases.

Resolvins are more active at more than 10,000-fold lower concentrations (pico to nanogram range) than omega-3 fatty acids, corticosteroids, NSAIDs, and aspirin in cancer models (Sulciner et al. 2018b; Gilligan et al. 2019; Panigrahy et al. 2019) and are naturally produced by the human body. Therefore, these lipid mediators hold great promise as a nontoxic therapeutic strategy (Serhan 2014; Serhan and Levy 2018; Panigrahy et al. 2020) for patients with angiogenic diseases including COVID-19 and cancer.