Harnessing retinal phagocytes to combat pathological neovascularization in ischemic retinopathies?

Ischemic retinopathies (IR) are vision-threatening diseases that affect a substantial amount of people across all age groups worldwide. The current treatment options of photocoagulation and anti-VEGF therapy have side effects and are occasionally unable to prevent disease progression. It is therefore worthwhile to consider other molecular targets for the development of novel treatment strategies that could be safer and more efficient. During the manifestation of IR, the retina, normally an immune privileged tissue, encounters enhanced levels of cellular stress and inflammation that attract mononuclear phagocytes (MPs) from the blood stream and activate resident MPs (microglia). Activated MPs have a multitude of effects within the retinal tissue and have the potential to both counter and exacerbate the harmful tissue microenvironment. The present review discusses the current knowledge about the role of inflammation and activated retinal MPs in the major IRs: retinopathy of prematurity and diabetic retinopathy. We focus particularly on MPs and their secreted factors and cell-cell-based interactions between MPs and endothelial cells. We conclude that activated MPs play a major role in the manifestation and progression of IRs and could therefore become a promising new target for novel pharmacological intervention strategies in these diseases.

Hematopoietic hypoxia-inducible factor 2α deficiency ameliorates pathological retinal neovascularization via modulation of endothelial cell apoptosis.

A hallmark of proliferative retinopathies, such as retinopathy of prematurity (ROP), is a pathological neovascularization orchestrated by hypoxia and the resulting hypoxia-inducible factor (HIF)-dependent response. We studied the role of Hif2α in hematopoietic cells for pathological retina neovascularization in the murine model of ROP, the oxygen-induced retinopathy (OIR) model. Hematopoietic-specific deficiency of Hif2α ameliorated pathological neovascularization in the OIR model, which was accompanied by enhanced endothelial cell apoptosis. That latter finding was associated with up-regulation of the apoptosis-inducer FasL in Hif2α-deficient microglia. Consistently, pharmacological inhibition of the FasL reversed the reduced pathological neovascularization from hematopoietic-specific Hif2α deficiency. Our study found that the hematopoietic cell Hif2α contributes to pathological retina angiogenesis. Our findings not only provide novel insights regarding the complex interplay between immune cells and endothelial cells in hypoxia-driven retina neovascularization but also may have therapeutic implications for proliferative retinopathies.-Korovina, I., Neuwirth, A., Sprott, D., Weber, S., Sardar Pasha, S. P. B., Gercken, B., Breier, G., El-Armouche, A., Deussen, A., Karl, M. O., Wielockx, B., Chavakis, T., Klotzsche-von Ameln, A. Hematopoietic hypoxia-inducible factor 2α deficiency ameliorates pathological retinal neovascularization via modulation of endothelial cell apoptosis.

Endothelial-Specific Deficiency of ATG5 (Autophagy Protein 5) Attenuates Ischemia-Related Angiogenesis.

Objective- Pathological angiogenesis, such as exuberant retinal neovascularization during proliferative retinopathies, involves endothelial responses to ischemia/hypoxia and oxidative stress. Autophagy is a clearance system enabling bulk degradation of intracellular components and is implicated in cellular adaptation to stressful conditions. Here, we addressed the role of the ATG5 (autophagy-related protein 5) in endothelial cells in the context of pathological ischemia-related neovascularization in the murine model of retinopathy of prematurity. Approach and Results- Autophagic vesicles accumulated in neovascular tufts of the retina of retinopathy of prematurity mice. Endothelium-specific Atg5 deletion reduced pathological neovascularization in the retinopathy of prematurity model. In contrast, no alterations in physiological retina vascularization were observed in endothelial-specific ATG5 deficiency, suggesting a specific role of endothelial ATG5 in pathological hypoxia/reoxygenation-related angiogenesis. Consistently, in an aortic ring angiogenesis assay, endothelial ATG5 deficiency resulted in impaired angiogenesis under hypoxia/reoxygenation conditions. As compared to ATG5-sufficient endothelial cells, ATG5-deficient cells displayed impaired mitochondrial respiratory activity, diminished production of mitochondrial reactive oxygen species and decreased phosphorylation of the VEGFR2 (vascular endothelial growth factor receptor 2). Consistently, ATG5-deficient endothelial cells displayed decreased oxidative inactivation of PTPs (phospho-tyrosine phosphatases), likely due to the reduced reactive oxygen species levels resulting from ATG5 deficiency. Conclusions- Our data suggest that endothelial ATG5 supports mitochondrial function and proangiogenic signaling in endothelial cells in the context of pathological hypoxia/reoxygenation-related neovascularization. Endothelial ATG5, therefore, represents a potential target for the treatment of pathological neovascularization-associated diseases, such as retinopathies.

Nerve growth factor regulates endothelial cell survival and pathological retinal angiogenesis.

The mechanism underlying vasoproliferative retinopathies like retinopathy of prematurity (ROP) is hypoxia-triggered neovascularisation. Nerve growth factor (NGF), a neurotrophin supporting survival and differentiation of neuronal cells may also regulate endothelial cell functions. Here we studied the role of NGF in pathological retinal angiogenesis in the course of the ROP mouse model. Topical application of NGF enhanced while intraocular injections of anti-NGF neutralizing antibody reduced pathological retinal vascularization in mice subjected to the ROP model. The pro-angiogenic effect of NGF in the retina was mediated by inhibition of retinal endothelial cell apoptosis. In vitro, NGF decreased the intrinsic (mitochondria-dependent) apoptosis in hypoxia-treated human retinal microvascular endothelial cells and preserved the mitochondrial membrane potential. The anti-apoptotic effect of NGF was associated with increased BCL2 and reduced BAX, as well as with enhanced ERK and AKT phosphorylation, and was abolished by inhibition of the AKT pathway. Our findings reveal an anti-apoptotic role of NGF in the hypoxic retinal endothelium, which is involved in promoting pathological retinal vascularization, thereby pointing to NGF as a potential target for proliferative retinopathies.

Blocking CD40-TRAF6 signaling is a therapeutic target in obesity-associated insulin resistance.

The immune system plays an instrumental role in obesity and insulin resistance. Here, we unravel the role of the costimulatory molecule CD40 and its signaling intermediates, TNF receptor-associated factors (TRAFs), in diet-induced obesity (DIO). Although not exhibiting increased weight gain, male CD40(-/-) mice in DIO displayed worsened insulin resistance, compared with wild-type mice. This worsening was associated with excessive inflammation of adipose tissue (AT), characterized by increased accumulation of CD8(+) T cells and M1 macrophages, and enhanced hepatosteatosis. Mice with deficient CD40-TRAF2/3/5 signaling in MHCII(+) cells exhibited a similar phenotype in DIO as CD40(-/-) mice. In contrast, mice with deficient CD40-TRAF6 signaling in MHCII(+) cells displayed no insulin resistance and showed a reduction in both AT inflammation and hepatosteatosis in DIO. To prove the therapeutic potential of inhibition of CD40-TRAF6 in obesity, DIO mice were treated with a small-molecule inhibitor that we designed to specifically block CD40-TRAF6 interactions; this compound improved insulin sensitivity, reduced AT inflammation, and decreased hepatosteatosis. Our study reveals that the CD40-TRAF2/3/5 signaling pathway in MHCII(+) cells protects against AT inflammation and metabolic complications associated with obesity whereas CD40-TRAF6 interactions in MHCII(+) cells aggravate these complications. Inhibition of CD40-TRAF6 signaling by our compound may provide a therapeutic option in obesity-associated insulin resistance.

Dual role of B7 costimulation in obesity-related nonalcoholic steatohepatitis and metabolic dysregulation.

UNLABELLED: The low-grade inflammatory state present in obesity contributes to obesity-related metabolic dysregulation, including nonalcoholic steatohepatitis (NASH) and insulin resistance. Intercellular interactions between immune cells or between immune cells and hepatic parenchymal cells contribute to the exacerbation of liver inflammation and steatosis in obesity. The costimulatory molecules, B7.1 and B7.2, are important regulators of cell-cell interactions in several immune processes; however, the role of B7 costimulation in obesity-related liver inflammation is unknown. Here, diet-induced obesity (DIO) studies in mice with genetic inactivation of both B7.1 and B7.2 (double knockout; DKO) revealed aggravated obesity-related metabolic dysregulation, reduced insulin signalling in the liver and adipose tissue (AT), glucose intolerance, and enhanced progression to steatohepatitis resulting from B7.1/B7.2 double deficiency. The metabolic phenotype of B7.1/B7.2 double deficiency upon DIO was accompanied by increased hepatic and AT inflammation, associated with largely reduced numbers of regulatory T cells (Tregs) in these organs. In order to assess the role of B7 costimulation in DIO in a non-Treg-lacking environment, we performed antibody (Ab)-mediated inhibition of B7 molecules in wild-type mice in DIO. Antibody-blockade of both B7.1 and B7.2 improved the metabolic phenotype of DIO mice, which was linked to amelioration of hepatic steatosis and reduced inflammation in liver and AT. CONCLUSION: Our study demonstrates a dual role of B7 costimulation in the course of obesity-related sequelae, particularly NASH. The genetic inactivation of B7.1/B7.2 deteriorates obesity-related liver steatosis and metabolic dysregulation, likely a result of the intrinsic absence of Tregs in these mice, rendering DKO mice a novel murine model of NASH. In contrast, inhibition of B7 costimulation under conditions where Tregs are present may provide a novel therapeutic approach for obesity-related metabolic dysregulation and, especially, NASH.

The complement anaphylatoxin C5a receptor contributes to obese adipose tissue inflammation and insulin resistance.

Obese adipose tissue (AT) inflammation contributes critically to development of insulin resistance. The complement anaphylatoxin C5a receptor (C5aR) has been implicated in inflammatory processes and as regulator of macrophage activation and polarization. However, the role of C5aR in obesity and AT inflammation has not been addressed. We engaged the model of diet-induced obesity and found that expression of C5aR was significantly upregulated in the obese AT, compared with lean AT. In addition, C5a was present in obese AT in the proximity of macrophage-rich crownlike structures. C5aR-sufficient and -deficient mice were fed a high-fat diet (HFD) or a normal diet (ND). C5aR deficiency was associated with increased AT weight upon ND feeding in males, but not in females, and with increased adipocyte size upon ND and HFD conditions in males. However, obese C5aR(-/-) mice displayed improved systemic and AT insulin sensitivity. Improved AT insulin sensitivity in C5aR(-/-) mice was associated with reduced accumulation of total and proinflammatory M1 macrophages in the obese AT, increased expression of IL-10, and decreased AT fibrosis. In contrast, no difference in ß cell mass was observed owing to C5aR deficiency under an HFD. These results suggest that C5aR contributes to macrophage accumulation and M1 polarization in the obese AT and thereby to AT dysfunction and development of AT insulin resistance.

Characterization of tryptophan-containing dipeptides for anti-angiogenic effects.

AIMS: In the pathogenesis of several diseases, neo-angiogenesis is increased (e.g. tumour growth). The peptide L-glutamyl-L-tryptophan (EW/IM862) has been claimed to exhibit inhibitory effects on tumour growth in vivo. However, the potential role of natural peptides with respect to anti-angiogenic properties is unsettled. The current study explores anti-angiogenic effects of the dipeptides WL, EW, IW and WE. METHODS AND RESULTS: Using a bottom-up strategy, we first evaluated the effects of the peptides on VEGFR-2 signalling and quantified their effects in different angiogenesis assays. WL consistently had the strongest effects on phosphorylation of VEGFR-2 and downstream signalling. Therefore, this peptide was chosen in comparison with EW to further assess anti-angiogenic properties. However, sprout formation in three-dimensional (3D) fibrin gel bead assay was significantly inhibited by EW only. Furthermore, vessel sprouting in the mouse aortic ring assay was decreased by the presence of WL and EW compared to control. Results from a chorioallantoic membrane assay showed that under vascular endothelial growth factor (VEGF) stimulation WL and EW decreased the number of blood vessels versus control. These results were in line with those obtained in a matrigel plug assay. The VEGF-induced increase in the haemoglobin content was nearly abolished when treatment was combined with either WL or EW application. In the murine model of oxygen-induced retinopathy, WL exhibited a small albeit significant anti-angiogenic effect. CONCLUSION: Comprehensive screening of WL suggests an anti-angiogenic effect, demonstrated in in vitro, ex vivo and in vivo models. Thus, WL is a dipeptide with potential anti-angiogenic effects and is worthy for further exploration.

Myeloid SOCS3 Deficiency Regulates Angiogenesis via Enhanced Apoptotic Endothelial Cell Engulfment.

Mononuclear phagocytes, such as macrophages and microglia, are key regulators of organ homeostasis including vascularization processes. Here, we investigated the role of the suppressor of cytokine signaling 3 (SOCS3) in myeloid cells as a regulator of mononuclear phagocyte function and their interaction with endothelial cells in the context of sprouting angiogenesis. As compared to SOCS3-sufficient counterparts, SOCS3-deficient microglia and macrophages displayed an increased phagocytic activity toward primary apoptotic endothelial cells, which was associated with an enhanced expression of the opsonin growth arrest-specific 6 (Gas6), a major prophagocytic molecule. Furthermore, we found that myeloid SOCS3 deficiency significantly reduced angiogenesis in an ex vivo mouse aortic ring assay, which could be reversed by the inhibition of the Gas6 receptor Mer. Together, SOCS3 in myeloid cells regulates the Gas6/Mer-dependent phagocytosis of endothelial cells, and thereby angiogenesis-related processes. Our findings provide novel insights into the complex crosstalk between mononuclear phagocytes and endothelial cells, and may therefore provide a new platform for the development of new antiangiogenic therapies.

53BP1 Deficiency Promotes Pathological Neovascularization in Proliferative Retinopathy.

The replication stress inflicted on retinal endothelial cells (ECs) in the context of hypoxia-induced pathological neovascularization during proliferative retinopathy is linked with activation of the deoxyribonucleic acid (DNA) repair response. Here, we studied the effect of deficiency of the DNA damage response adaptor 53BP1, which is an antagonist of homologous recombination (HR), in the context of proliferative retinopathy. In the model of retinopathy of prematurity (ROP), 53BP1-deficient mice displayed increased hypoxia-driven pathological neovascularization and tuft formation, accompanied by increased EC proliferation and reduced EC apoptosis, as compared with 53BP1-sufficient mice. In contrast, physiological retina angiogenesis was not affected by 53BP1 deficiency. Knockdown of 53BP1 in ECs in vitro also resulted in enhanced proliferation and reduced apoptosis of the cells under hypoxic conditions. Additionally, upon 53BP1 knockdown, ECs displayed increased HR rate in hypoxia. Consistently, treatment with an HR inhibitor reversed the hyper-proliferative angiogenic phenotype associated with 53BP1 deficiency in ROP. Thus, by unleashing HR, 53BP1 deletion increases pathological EC proliferation and neovascularization in the context of ROP. Our data shed light to a previously unknown interaction between the DNA repair response and pathological neovascularization in the retina.

Endogenous developmental endothelial locus-1 limits ischaemia-related angiogenesis by blocking inflammation.

We have recently identified endothelial cell-secreted developmental endothelial locus-1 (Del-1) as an endogenous inhibitor of ß2-integrin-dependent leukocyte infiltration. Del-1 was previously also implicated in angiogenesis. Here, we addressed the role of endogenously produced Del-1 in ischaemia-related angiogenesis. Intriguingly, Del-1-deficient mice displayed increased neovascularisation in two independent ischaemic models (retinopathy of prematurity and hind-limb ischaemia), as compared to Del-1-proficient mice. On the contrary, angiogenic sprouting in vitro or ex vivo (aortic ring assay) and physiological developmental retina angiogenesis were not affected by Del-1 deficiency. Mechanistically, the enhanced ischaemic neovascularisation in Del-1-deficiency was linked to higher infiltration of the ischaemic tissue by CD45+ haematopoietic and immune cells. Moreover, Del-1-deficiency promoted ß2-integrin-dependent adhesion of haematopoietic cells to endothelial cells in vitro, and the homing of hematopoietic progenitor cells and of immune cell populations to ischaemic muscles in vivo. Consistently, the increased hind limb ischaemia-related angiogenesis in Del-1 deficiency was completely reversed in mice lacking both Del-1 and the ß2-integrin LFA-1. Additionally, enhanced retinopathy-associated neovascularisation in Del-1-deficient mice was reversed by LFA-1 blockade. Our data reveal a hitherto unrecognised function of endogenous Del-1 as a local inhibitor of ischaemia-induced angiogenesis by restraining LFA-1-dependent homing of pro-angiogenic haematopoietic cells to ischaemic tissues. Our findings are relevant for the optimisation of therapeutic approaches in the context of ischaemic diseases.

Developmental endothelial locus-1 modulates platelet-monocyte interactions and instant blood-mediated inflammatory reaction in islet transplantation.

Platelet-monocyte interactions are strongly implicated in thrombo-inflammatory injury by actively contributing to intravascular inflammation, leukocyte recruitment to inflamed sites, and the amplification of the procoagulant response. Instant blood-mediated inflammatory reaction (IBMIR) represents thrombo-inflammatory injury elicited upon pancreatic islet transplantation (islet-Tx), thereby dramatically affecting transplant survival and function. Developmental endothelial locus-1 (Del-1) is a functionally versatile endothelial cell-derived homeostatic factor with anti-inflammatory properties, but its potential role in IBMIR has not been previously addressed. Here, we establish Del-1 as a novel inhibitor of IBMIR using a whole blood-islet model and a syngeneic murine transplantation model. Indeed, Del-1 pre-treatment of blood before addition of islets diminished coagulation activation and islet damage as assessed by C-peptide release. Consistently, intraportal islet-Tx in transgenic mice with endothelial cell-specific overexpression of Del-1 resulted in a marked decrease of monocytes and platelet-monocyte aggregates in the transplanted tissues, relative to those in wild-type recipients. Mechanistically, Del-1 decreased platelet-monocyte aggregate formation, by specifically blocking the interaction between monocyte Mac-1-integrin and platelet GPIb. Our findings reveal a hitherto unknown role of Del-1 in the regulation of platelet-monocyte interplay and the subsequent heterotypic aggregate formation in the context of IBMIR. Therefore, Del-1 may represent a novel approach to prevent or mitigate the adverse reactions mediated through thrombo-inflammatory pathways in islet-Tx and perhaps other inflammatory disorders involving platelet-leukocyte aggregate formation.

Endothelial-specific deficiency of Junctional Adhesion Molecule-C promotes vessel normalisation in proliferative retinopathy.

In proliferative retinopathies, like proliferative diabetic retinopathy and retinopathy of prematurity (ROP), the hypoxia response is sustained by the failure of the retina to revascularise its ischaemic areas. Non-resolving retina ischaemia/hypoxia results in upregulation of pro-angiogenic factors and pathologic neovascularisation with ectopic, fragile neovessels. Promoting revascularisation of the retinal avascular area could interfere with this vicious cycle and lead to vessel normalisation. Here, we examined the function of endothelial junctional adhesion molecule-C (JAM-C) in the context of ROP. Endothelial-specific JAM-C-deficient (EC-JAM-C KO) mice and littermate JAM-C-proficient (EC-JAM-C WT) mice were subjected to the ROP model. An increase in total retinal vascularisation was found at p17 owing to endothelial JAM-C deficiency, which was the result of enhanced revascularisation and vessel normalisation, thereby leading to significantly reduced avascular area in EC-JAM-C KO mice. In contrast, pathologic neovessel formation was not affected by endothelial JAM-C deficiency. Consistent with improved vessel normalisation, tip cell formation at the interface between vascular and avascular area was higher in EC-JAM-C KO mice, as compared to their littermate controls. Consistently, JAM-C inactivation in endothelial cells resulted in increased spreading on fibronectin and enhanced sprouting in vitro in a manner dependent on ß1-integrin and on the activation of the small GTPase RAP1. Together, endothelial deletion of JAM-C promoted endothelial cell sprouting, and consequently vessel normalisation and revascularisation of the hypoxic retina without altering pathologic neovascularisation. Thus, targeting endothelial JAM-C may provide a novel therapeutic strategy for promoting revascularisation and vessel normalisation in the treatment of proliferative retinopathies.

Interleukin-3 amplifies acute inflammation and is a potential therapeutic target in sepsis.

Sepsis is a frequently fatal condition characterized by an uncontrolled and harmful host reaction to microbial infection. Despite the prevalence and severity of sepsis, we lack a fundamental grasp of its pathophysiology. Here we report that the cytokine interleukin-3 (IL-3) potentiates inflammation in sepsis. Using a mouse model of abdominal sepsis, we showed that innate response activator B cells produce IL-3, which induces myelopoiesis of Ly-6C(high) monocytes and neutrophils and fuels a cytokine storm. IL-3 deficiency protects mice against sepsis. In humans with sepsis, high plasma IL-3 levels are associated with high mortality even after adjusting for prognostic indicators. This study deepens our understanding of immune activation, identifies IL-3 as an orchestrator of emergency myelopoiesis, and reveals a new therapeutic target for treating sepsis.

PHD4 stimulates tumor angiogenesis in osteosarcoma cells via TGF-α.

UNLABELLED: Solid tumor growth is intimately associated with angiogenesis, a process that is efficiently triggered by hypoxia. Therefore, oxygen-sensitive signaling pathways are thought to play a critical role in tumor angiogenesis and progression. Here, the function of prolyl hydroxylase-4 (PHD4), a relative of the prolyl hydroxylase domain proteins 1-3 that promote the degradation of hypoxia-inducible factors (HIF), was interrogated. To test the hypothesis that PHD4 might inhibit tumor angiogenesis, it was overexpressed in osteosarcoma cells, and unexpectedly, this manipulation led to increased tumor blood vessel density. However, the newly formed blood vessels were smaller than normal and appeared to be partially nonfunctional, as indicated by poor vessel perfusion. PHD4 overexpression in tumor cells stimulated the expression of TGF-α, which was necessary and sufficient to promote angiogenic sprouting of endothelial cells. On the other hand, PHD4 overexpression reduced HIF-2α protein levels, which in turn inhibited in vivo tumor growth. Combined, elevated PHD4 levels deregulate angiogenesis via increased TGF-α expression in vitro and in vivo. These data support the hypothesis that tumor growth can be uncoupled from vessel density and that the individual PHD family members exert distinct functions in tumors. IMPLICATIONS: PHD4 influences tumor growth and vascularization through discrete mechanisms and molecular pathways that likely have therapeutic potential.

HIF prolyl hydroxylase-2 inhibition diminishes tumor growth through matrix metalloproteinase-induced TGFß activation.

A right amount of oxygen and nutrients is essential for a tumor to develop. The role of oxygen dependent pathways and their regulators is therefore of utmost importance although little is known about the detailed impact they can have. Recently we have shown that inhibition of the oxygen sensor PHD2 in tumor cells blocks tumor growth due to the anti-proliferative activity of TGFß. In this study, we refined these results by comparing different shPHD2 sequences in depth in the early phase of tumor growth. Our findings also reveal an intriguing role for MMP2 and MT1MMP in these settings, as these activated proteases display an anti-proliferative characteristic through the activation of downstream TGFß targets. In conclusion, PHD2 inhibition is essential for the regulation of the anti-tumoral activity in mouse tumor cells and might bring some new insight in our understanding of tumor growth inhibition.

Proliferative and survival effects of PUMA promote angiogenesis.

The p53 upregulated modulator of apoptosis (PUMA) is known as an essential apoptosis inducer. Here, we report the seemingly paradoxical finding that PUMA is a proangiogenic factor critically required for the proliferation and survival of vascular and microglia cells. Strikingly, Puma deficiency by genetic deletion or small hairpin RNA knockdown inhibited developmental and pathological angiogenesis and reduced microglia numbers in vivo, whereas Puma gene delivery increased angiogenesis and cell survival. Mechanistically, we revealed that PUMA plays a critical role in regulating autophagy by modulating Erk activation and intracellular calcium level. Our findings revealed an unexpected function of PUMA in promoting angiogenesis and warrant more careful investigations into the therapeutic potential of PUMA in treating cancer and degenerative diseases.

Inhibition of HIF prolyl hydroxylase-2 blocks tumor growth in mice through the antiproliferative activity of TGFß.

Virtually all solid tumors are dependent on a vascular network to provide them with the right amount of nutrients and oxygen. In that sense, low oxygen tension or hypoxia leads to an adaptive response that is transcriptionally regulated by the hypoxia-inducible factors (HIF), which are tightly controlled by the HIF prolyl hydroxylases (PHD). In this study, we show that inhibition of the oxygen sensor PHD2 in tumor cells stimulates vessel formation but paradoxically results in a profound reduction of tumor growth. This effect relies on the antiproliferative nature of the TGFß signaling pathway, in a largely HIF-independent manner. Moreover, our findings reveal that PHD2 has an essential function in controlling the dual nature of TGFß during tumorigenesis and may offer an alternative opportunity for anticancer therapy.