Epsins in vascular development, function and disease

Abstract

Epsins are a family of adaptor proteins involved in clathrin-dependent endocytosis. In the vasculature, epsins 1 and 2 are functionally redundant members of this family that are expressed in the endothelial cells of blood vessels and the lymphatic system throughout development and adulthood. These proteins contain a number of peptide motifs that allow them to interact with lipid moieties and a variety of proteins. These interactions facilitate the regulation of a wide range of cell signaling pathways. In this review, we focus on the involvement of epsins 1 and 2 in controlling vascular endothelial growth factor receptor signaling in angiogenesis and lymphangiogenesis. We also discuss the therapeutic implications of understanding the molecular mechanisms of epsin-mediated regulation in diseases such as atherosclerosis and diabetes.

Introduction

Epsins are evolutionarily conserved proteins that are essential for fundamental cellular processes in embryos [1–3] and adults [4–6]. While working in Pietro De Camilli’s laboratory at the Yale University School of Medicine, one of us (Hong Chen) discovered that rat epsin 1 interacted with the endocytic accessory protein Eps15 [7] and helped to identify the epsin 2 isoform [8]. Epsins 1 and 2 are universally expressed and enriched in the brain, whereas epsin 3 expression is more restricted [9]. Structurally, epsins contain a conserved NH₂-terminal homology (ENTH) domain, which anchors them to the plasma membrane, and ubiquitin-interacting motifs (UIMs), which interact with ubiquitinated cargo. This region also contains less -structured clathrin-binding motifs and a number of amino acid repeats that are upstream from the carboxyl terminus. Together, these domains help deliver cargo to coated vesicle formation sites for internalization and intracellular trafficking (Fig. 1) [7, 10].

Fig. 1.

Structure and function of epsin endocytic adaptors. Linear depiction of epsin functional regions. The epsin N-terminus homology (ENTH) domain interacts with the plasma membrane and the UIM associates with ubiquitinated proteins. Endosomal proteins bind to DPW (Asp–Pro–Trp) repeats, while Eps15-homology (EH) domain proteins bind to NPF (Asn–Pro–Phe) repeats during clathrin-coated pit formation

The ENTH domain is approximately 150 amino acids in length, highly conserved, and responsible for binding inositol phospholipids such as phosphatidylinositol-4,5-bisphosphate (PI[4,5]P2) and proteins. This domain contributes to the nucleation and formation of clathrin coatings on membranes [11]. The UIM is located downstream of the ENTH and non-covalently binds ubiquitinated proteins during endocytosis [12, 13]. Epsins also contain several clathrin-binding motifs in addition to a series of DPW/F (aspartate–proline–tryptophan/phenylalanine) and NPF (asparagine–proline–phenylalanine) tripeptide repeats that bind to proteins involved in endocytosis. As a result, these proteins act as endocytic adaptors that can directly bind ubiquitinated cargo and interact with accessory proteins such as clathrin and AP2. These diverse functions make epsins essential for the activation of a number of signal transduction pathways including Notch [14], Rho GTPase [6], VEGFR2 [15], and VEGFR3 [16].

The formation of new blood vessels is a prerequisite for the growth and maintenance of organs. This fundamental process can result from both vasculogenesis and angiogenesis. Angiogenesis refers to the growth of new capillaries from existing blood vessels, while vasculogenesis occurs when mesodermal precursor cells called angioblasts differentiate into endothelial cells to form a primitive network of blood vessels de novo. In the embryo, these distinct processes are not mutually exclusive. The formation of a primitive vascular plexus through the differentiation of angioblasts is often followed by angiogenesis, which remodels and expands this vascular network. Although vasculogenesis is predominantly associated with embryonic and fetal development, it also occurs in adults, whereas angiogenesis is largely responsible for blood vessel growth during development and in diseases throughout life [17].

The British surgeon John Hunter first coined the term ‘angiogenesis’ in 1787. Nearly two centuries later, the American surgeon Judah Folkman hypothesized that the growth of tumors depended on this phenomenon [18]. Dr. Folkman also described the process of sprouting angiogenesis [19], and an abundance of subsequent research provided much greater clarity about the fundamental molecular aspects of this process as well as another mechanism of blood vessel growth called intussusceptive angiogenesis, where a pre-existing capillary wall extends into the vascular lumen to divide a single vessel into two in the absence of sprouting [20–22]. A common feature of both forms of angiogenesis, as well as vasculogenesis, is a reliance on the interaction of a family of growth factors with cognate receptor tyrosine kinases in endothelial cells to stimulate blood vessel formation.

Vascular endothelial growth factors (VEGFs) regulate vasculogenesis and angiogenesis. There are five members of these secreted factors in mammals; namely, VEFG-A, VEGF-B, VEGF-C, VEGF-D, and placental growth factor (PlGF) [23–27]. VEGF-A was the first of these to be identified and is composed of multiple isoforms that result from alternative mRNA splicing [28]. Together, these glycoproteins interact with various tyrosine kinase receptors on the endothelial cell surface such as VEGFR1 (also known as Flt-1), VEGFR2 (Flk-1 in mice and KDR in humans), and VEGFR3 (Flt-4) [29]. Both VEGFR2 and VEGFR3 play pivotal role during vasculogenesis and angiogenesis, in addition to lymphangiogenesis, in embryonic development as well as throughout life [30–32]. For example, VEGF-A stimulates VEGFR2 during sprouting angiogenesis and filopodia formation in endothelial tip cells, and VEGFR2 blockade causes defects in sprouting [33, 34]. Although intussusceptive angiogenesis is less well understood, both forms of angiogenesis are thought to occur in every organ throughout development and adulthood; however, it is not clear whether the same holds true for vasculogenesis.

VEGF binding to VEGF receptors induces receptor dimerization, which results in the activation of their tyrosine kinase domains and the autophosphorylation of intracellular tyrosine residues. The transduction of these signals is greatly influenced by the availability of VEGF ligands, the quantity and location of cell surface receptors, the presence of co-receptors, the internalization of activated receptors, and the extent of their degradation or recycling to the plasma membrane. Following internalization, VEGFRs are either directed to lysosomal compartments for their eventual degradation or directed back to the cell membrane by means of recycling endosomes. Consequently, the specificity, duration, and amplitude of VEGF signaling is precisely regulated by endocytosis and intracellular trafficking of activated receptors [35, 36].

Endocytosis of VEGFRs takes place in a clathrin-dependent manner. Clathrin-mediated endocytosis is caused by the formation of clathrin-coated pits (CCPs), where clathrin is recruited by binding to various adapters, such as adapter protein 2 (AP2) and clathrin-associated sorting proteins (CLASPs), which includes AP180 and the epsin proteins [37]. During the initiation of endocytosis, multi-domain scaffold proteins like the Intersectins and different adaptor proteins such as AP-2, FCHO1/2, Eps15, Eps15R, and CALM bind the plasma membrane. This binding is facilitated by cargo proteins or membrane lipids such as PI[4,5]P2 [38]. AP-2 plays crucial role in target selection for endocytosis by direct binding with cargo proteins or helper proteins [39]. The α-subunit of AP-2 facilitates weak binding with PI[4,5]P2, whereas phosphorylation of a threonine residue in the μ2-subunit increases binding affinity [40]. Other adaptor proteins like CLAM help to form a clathrin lattice, which initiates the formation of CCP [41]. Following clathrin polymerization, membrane invagination occurs, which involves BAR-domain containing proteins such Endophilin [42] and Amphiphysin [43]. Fusion of the vesicle requires the termination of clathrin-mediated endocytosis and disassembly of the clathrin coat. Axilin, which is a homologue of HSP40, helps in the process of clathrin removal by recruiting HSC70 chaperon proteins [44, 45].

After fusing with early endosomes, clathrin-coated vesicles proceed through a series of steps that can result in transport back to the plasma membrane through either fast or slow recycling pathways [46] or targeting them for degradation by lysosomal fusion via the Ras-related small GTPase protein 7 (Rab7) pathway [47]. The decision between these two fates largely depends on the adaptor proteins that link VEGF receptors to the endocytic machinery by recognizing cytosolic sorting determinants within the cargo proteins. These adaptor proteins are transiently recruited to the site of endocytosis and cluster with other coat components through interacting domains [48].

The degradation pathway of this cargo is governed by intraluminal vesicles (ILV) of early endosomes [49]. ILV biogenesis and cargo sorting are characteristics of the maturation of early endosomes into late endosomes [50]. Most late endosomes or multi-vesicular bodies fuse with lysosomes to form endolysosomes. Formation of endolysosomes triggers the degradation of cargo proteins in the vesicles by acid hydrolases [51]. In contrast, recycling cargo escapes from ILVs through a cargo retrieval mechanism [52]. Upon retrieval, the cargo returns to the cell surface through the fast or slow recycling pathways [53]. Ubiquitination of lysine residues of activated growth factors triggers lysosomal degradation [54]. Ubiquitination is detected by the family of endosomal sorting complexes required for transport (ESCRT) proteins such as ESCRT-0, ESCRT-I, and ESCRT-II [55–57]. These ESCRT proteins play a crucial role in orchestrating cargo fate.

The importance of endocytic adaptor proteins in VEGF signaling has been appreciated for over 20 years [7, 58, 59]. In this review, we will focus on the endocytic adaptors epsins 1, 2, and 3. Epsins 1 and 2 are ubiquitously expressed, while epsin 3 is primarily expressed in the stomach and epidermis [8, 9]. Below, we describe the role of endothelial epsins in angiogenesis and lymphangiogenesis, and focus on their relevance to diseases associated with VEGF receptor endocytosis.

Epsins in angiogenesis and lymphangiogenesis

Vasculogenesis and angiogenesis are most active during early embryogenesis. Under normal physiological conditions, new blood vessel formation in adults rarely occurs as the vasculature remains largely quiescent; however, endothelial cells can undergo sprouting, migration, and proliferation during tissue repair or in pathological conditions and VEGF plays a central role in regulating these angiogenic processes [60]. Endothelial cells form tip and stalk cells that coordinate the complex processes of this response. The formation of filopodia and the migration of endothelial tip cells is induced by VEGF gradients [34, 61–63] that stimulate proliferation and survival of endothelial cells [64, 65], as well as inducing vascular permeability and tubule formation [64, 66]. In addition, VEGF regulates some inflammatory responses by promoting immune cell migration during certain pathological conditions [60, 67]. While the migration of angioblasts and their differentiation into endothelial cells is also known to be regulated by VEGF, the process of vasculogenesis is less well understood [64–66].

Hypoxia is the strongest inducer of angiogenic responses and is coordinated by hypoxia inducible factor-1 (HIF-1) [68–70]. This heterodimeric protein complex activates the transcription of pro-angiogenic factors, including VEGF [71, 72]. VEGF subsequently binds to the extracellular domain of VEGF receptors (VEGFRs), which are tyrosine kinases [65, 73, 74]. The binding of VEGF to the extracellular domain of these receptors promotes their dimerization, activates their tyrosine kinase activity, and phosphorylates their intracellular domain tyrosine residues to activate downstream signaling pathways [65, 73, 74]. The resulting phosphotyrosine residues are required for the molecular docking of signaling molecules that act to induce downstream endothelial cell responses.

Downstream signaling of activated VEGFR2 promotes proliferation, migration, survival, and permeability of vascular endothelial cells [75]. Phosphorylation of different tyrosine residues permits binding to different intracellular signaling messenger molecules and, as a consequence, activates various cell signaling pathways. For instance, the major phosphorylation site Tyr951 of VEGFR2 promotes phosphorylation and internalization of VE-cadherin to regulate vascular permeabilization [76]. Another phosphotyrosine residue at Tyr1175 helps docking of PLCγ and triggers diacylglycerol (DAG) and inositol 1,4,5-triphosphate [IP₃] mediated signaling [77]. This leads to activation of ERK and calcium signaling pathways, which regulate the proliferation and migration of endothelial cells. There are also other phosphotyrosine sites with less well-defined activities [78] and some phosphorylation sites that are specific to VEGFR-1 that have a comparatively low activity and do not appear to induce endothelial cell migration or proliferation [79].

VEGF signaling is regulated by a variety of cellular mechanisms such as the endocytosis of activated VEGF receptors [36, 80]. For example, the sprouting of endothelial cells in the early postnatal retina coincides with a high rate of VEGFR endocytosis along with VEGF uptake [36]. Activated VEGFR2 is rapidly internalized in a clathrin- and dynamin-dependent fashion, and is trafficked to early antigen 1 (EEA1) positive vesicles via Rab5-endosomes [81]. VEGFR2 then moves from early endosomes into either Rab11- or Rab4-positive recycling endosomes or into multi-vesicular bodies (MVBs) and late endosomes (Rab7-positive) prior to recycling to the cell surface or transit toward degradative pathways, respectively (Fig. 2) [35, 82]. The intracellular trafficking of VEGF receptors involves numerous other proteins, which help carefully orchestrate angiogenic responses.

Fig. 2.

Epsins in angiogenesis. Schematic diagram showing the role of endothelial epsins on VEGFR2 signaling during angiogenesis. Epsins bind to activated VEGFR2 and facilitate its internalization and degradation to maintain normal angiogenic responses. In the absence of epsins, VEGFR2 degradation is reduced and recycling pathways are preferred, resulting in elevated cell surface VEGFR2 and aberrant angiogenesis

Endocytic adaptor proteins play a crucial role in VEGF signal intensity and amplitude by regulating the uptake, recycling, and degradation of VEGF receptors. Consequently, the loss of endothelial cell epsins 1 and 2 inhibits endocytosis and degradation of VEGFR2 causing excessive angiogenic responses [83]. For example, Tessneer et al. discovered aberrant angiogenesis in endothelial epsin 1 and 2 double knockout (iDKO) mice that could be rescued by the genetic reduction of VEGFR2, thereby restoring normal VEGF signaling [83]. In support of these findings, Pasula et al. demonstrated that tumor growth was suppressed in iDKO mice through enhanced VEGF signaling [84]. Mechanistically, it was shown that epsin-mediated VEGFR2 degradation required interaction of the epsin UIM and ubiquitinated VEGF receptors, as mutant UIM peptides failed to enhance VEGF-induced angiogenesis [15]. Loss of epsin is associated with increased VEGF signaling due to impaired VEGFR2 downregulation; however, signaling of other angiogenic receptors including platelet-derived growth factor receptor, fibroblast growth factor receptor, epidermal growth factor receptor, and transforming growth factor beta receptor are not affected by epsin loss [84]. Thus, epsins are specific adaptor proteins for regulation of endocytosis and degradation of VEGFR2 in endothelial cells. Rahman et al. showed that VEGFR2 specifically interacts with the UIM region of epsin [15]. Structural modeling revealed the unique residue in the UIM region that determines the interaction between epsin and the VEGFR2 kinase domain. In vitro angiogenesis was promoted using an epsin UIM inhibitory peptide. In addition, postnatal retinal angiogenesis and VEGF-mediated subcutaneous angiogenesis was retarded when the inhibitory peptide was administered to wild-type mice. The same effects were observed during dermal wound healing. Administration of the UIM peptide induced pro-angiogenic responses when compared to control peptides, while a mutant UIM peptide did not elicit these effects. Collectively, these studies demonstrate the specificity of the epsin–VEGFR2 interaction and the underlying molecular mechanism. Therefore, endothelial epsin 1 and 2, along with other endocytic adaptor proteins, act as critical determinants of the temporal and spatial coordination of VEGFR2 signaling (Fig. 2).

Similar to VEGFR2 regulation by epsins in blood vessel endothelial cells during VEGF-A-induced receptor internalization and degradation [83, 84], VEGFR3 activation by VEGF-C/D is dependent on epsin-mediated internalization and degradation in lymphatic endothelial cells [16, 85]. While VEGFR3 is initially expressed in blood vessel endothelial cells, its expression decreases after embryonic development [86, 87]. At the same time, VEGFR3 expression and activation of downstream signaling pathways is responsible for the proliferation and migration of lymphatic endothelial cells. VEGFR3 expression is tightly controlled during lymphangiogenesis and lymphatic vessel maturation as VEGFR3 expression is decreased after E16 in the trunk LECs of collecting lymphatic vessels within the lymphangion and is highly expressed in the lymphatic valve region [88]. VEGFR3 deletion induces lymphatic hypoplasia, while incomplete deletion during the postnatal stage results in hyperplasia [89], suggesting the importance of VEGFR3 homeostasis during lymphatic development and maintenance.

Epsins 1 and 2 also control VEGFR3 recycling and degradation in lymphatic endothelial cells to regulate lymphatic collecting vessel maturation, lymphangiogenesis, and lymphatic valve formation [16, 85]. They interact with VEGFR3 via their UIM [16], analogous to the findings for VEGFR2 in blood vessel endothelial cells [15, 84, 90]. Both epsin 1 and 2 expression are induced during late embryonic development and sustained postnatally to maintain low VEGFR3 levels in collecting lymphatic vessels. Therefore, epsins are important regulators of VEGFR3 signaling in physiological and pathological lymphangiogenesis throughout development and adulthood [91–95]. Understanding the role of epsins in impaired lymphangiogenesis and lymphatic function has the potential to provide insights into obesity and other metabolic diseases [96–102]. Stimulation of lymphatic vascular development and enhancement of lymphatic function may provide protection from the onset or progression of these disorders [102, 103].

Epsins as therapeutic targets in atherosclerosis

Atherosclerosis is a progressive disease characterized by a build-up of plaques on the luminal surface of arteries, which leads to a restriction in the flow of blood to organs. These plaques are primarily composed of fat, cholesterol, and calcium, as well as other substances found in the blood. In severe cases of this disease, these plaques can rupture, thereby triggering a blood clot [104]. Consequently, atherosclerosis is the primary cause of myocardial infarctions and strokes [105, 106]. Atherosclerosis is also characterized by chronic inflammation of the vascular wall, which is partially caused by immune and inflammatory cell infiltration, resulting in enhanced lipid accumulation in the endothelium [107, 108]. Macrophages play a critical role in atherogenesis by mediating lipid uptake and accumulation in arterial wall. On the other hand, inhibition of plaque neovascularization reduces macrophage accumulation and the progression of advanced atherosclerosis [108].

Because the accumulation of arterial cholesterol and fat is the most common phenotype in atherosclerosis [109], both apolipoprotein E-deficient (ApoE^−/−) and LDL-receptor (LDLr) knockout mice fed a high-fat/high-cholesterol ‘Western Diet’ have been established as valuable models for studying this disease [110, 111]. The Western Diet increased expression of epsin 1 and 2 in macrophages [112]. Brophy et al. demonstrated that myeloid cell epsins have an important role in atherosclerotic progression. Specifically, the myeloid-specific deletion of these proteins in double knockout (LysM-DKO) mice with an ApoE^−/− background was found to have a significant reduction in atherosclerotic lesion formation when compared to ApoE^−/− mice fed the same diet [112]. Mechanistically, this effect in macrophages was mediated through the prevention of LRP-1 (low-density lipoprotein receptor-related protein 1) degradation, which occurs by ubiquitination and binding to epsins. Amelioration of atherosclerosis in ApoE^−/−/LysM-DKO mice could be hindered by a reduction of LRP-1 expression in macrophages as was shown by crossing ApoE^−/−/LysM-DKO mice onto an LRP-1 heterozygous background [112]. Furthermore, minimal side effects were observed in the adult mice with an inducible epsin 1 and 2 knockout genotype [2]. For these reasons, designing molecules to target increased expression of macrophage epsins may represent a novel therapeutic strategy to treat atherosclerosis.

In addition to macrophages, endothelial cell epsins also play a role in atherosclerosis. Dong et al. have recently shown that oxidized LDL increased epsin 1 and 2 expression in endothelial cells [113]. Epsin 1 was found to interact with the inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) through a UIM domain in atherosclerosis. An epsin 1 construct lacking the UIM showed significantly reduced binding with IP3R1 suggesting a UIM-mediated interaction between these proteins. Moreover, the UIM of epsin interacted with the ubiquitinated IP3R1 suppressor domain (SD) to accelerate proteosomal degradation of IP3R1 and, thereby, modulate calcium release from the endoplasmic reticulum to fuel atherosclerosis [112, 113].

Epsins as therapeutic targets in diabetes

Diabetic patients with uncontrolled hyperglycemia often suffer from delayed peripheral limb wound healing and chronic lymphedema, which is, in part, due to impaired angiogenesis and lymphangiogenesis [114, 115]. Although these vascular complications are a hallmark of diabetes, the molecular mechanisms that underlie endothelial dysfunction still remain to be fully investigated. Recent studies have begun to provide a better understanding of the critical factors that influence the processes of wound healing in diabetes [116]. The normal wound-healing cascade is well coordinated and synchronized by different growth factors, cytokines, inflammatory cells, fibroblasts, and endothelial cells [116]. Growth factors play a critical role in initiating and sustaining the different phases of wound healing [117]. VEGF is one of the most potent growth factors, which mainly regulates the revascularization and permeability of the wound site, and participates in the formation of the granulation tissues. Decrease in VEGFR2 levels is the main reason for non-healing status of wounds [118]. In diabetic patients, reductions in growth factors and their receptors can cause delayed wound healing.

In endothelial cells, epsins 1 and 2 preferentially bind ubiquitinated VEGFR2 to mediate its lysosomal degradation without affecting other angiogenic receptors and downstream signaling pathways, including PDGFR, FGFR, EGFR, and TGF-βR. Furthermore, genetic reduction of VEGFR2 protects against the excessive angiogenesis caused by epsin 1 and 2 deletion [2, 7, 83, 84]. A recent study showed high glucose-induced ligand- and intrinsic kinase-independent VEGFR2 phosphorylation in the Golgi complex resulted in reduced receptor recycling to the cell surface, which implicated VEGFR2 in the promotion of reparative neovascularization during wound healing [119]. Together, these studies indicate that epsins play a critical role in VEGFR2-mediated diabetic wound healing.

Similar to human diabetic patients, wound healing in genetically diabetic (db/db) mice is impaired and exogenous VEGF-C administration can markedly improve this condition [120]. As mentioned above, the VEGF-C/VEGFR3 signaling axis is a well-studied central pathway for lymphangiogenesis [121, 122]. VEGFR3 is present in all endothelial cells during the early postnatal development period, but, in mature adult vessels, VEGFR3 becomes restricted to the lymphatic endothelium. High glucose-induced oxidative stress can cause the intrinsic phosphorylation of both VEGFR2 and VEGFR3 and subsequent trafficking to the lysosome for degradation [16, 85, 119]. In lymphatic endothelial cells, this effect diminishes lymphangiogenesis in diabetes through the generation of reactive oxygen species and c-Src-mediated activation of AP1.

Diabetes increases epsin 1 and 2 expression in lymphatic endothelial cells (LECs) [68]. Inducible LEC-specific deficiency of epsins 1 and 2 in mice significantly improved lymphatic sprouting and increased lymphatic vessel branching in non-diabetic LEC-iDKO and diabetic LEC-iDKO mice. Loss of epsins reversed the deficits in lymphatic function observed with hyperglycemia and led to the recovery from tail edema. Aside from these preclinical observations, epsins 1 and 2 are also known to be significantly upregulated in patients with diabetes mellitus [85]. In lymphatic endothelial cells, this effect diminished lymphangiogenesis through the generation of reactive oxygen species and c-Src-mediated activation of AP1. Consequently, this suggests a therapeutic benefit of targeting these proteins to treat type II diabetes [85, 123].

Epsins as therapeutic targets in cancer

Tumor growth depends on angiogenesis and the inhibition of angiogenesis can suppress this response by limiting the supply of oxygen and nutrients. Consequently, anti-angiogenic therapies targeting the VEGFR2 signaling pathway represent a promising strategy to prevent tumor growth, invasion, and metastatic dissemination [18, 124–127]. Because epsins are upregulated in a variety of cancers and their loss can prevent tumorigenesis [128–130], the role of these proteins in driving tumor angiogenesis through regulation of VEGFR2 signaling has been the focus of a number of studies [84, 131]. Recent studies indicated that a chemically synthesized tumor endothelium-targeting chimeric peptide (UPI), which contains the epsin UIM as well as plasma membrane targeting and tumor homing sequences, can retard the growth of a broad spectrum of tumors. These studies also determined the novel binding mechanism of the UPI to ubiquitinated VEGFR2 to impair VEGFR2 endocytosis and degradation during tumor angiogenesis [90]. The UPI peptide treatment augmented VEGFR2 signaling in vitro and in vivo. Furthermore, when combined with cytotoxic chemotherapeutics, including Doxorubicin (Dox), Taxol, or OKN-007, the UPI peptide showed great promise for cancer inhibition compared to an anti-VEGF therapy [90]. As a result, the regulation of VEGF-mediated ubiquitination of VEGR2 by epsins could represent a new therapeutic strategy to inhibit tumor growth [84, 132].

Summary and future directions

Here, we briefly described the function of epsin endocytic adaptor proteins in angiogenesis and lymphangiogenesis, and the role of epsins 1 and 2 in disease processes such as atherosclerosis, diabetes, and cancer (Fig. 3). Despite their multifaceted functions, epsins are highly specific with respect to the selection of their binding partners, which varies in pathophysiological states and in different cells. In atherosclerosis, epsin 1 and 2 expression is heightened in lesional endothelial cells and macrophages. While epsins interact with IP3R1 in endothelial cells, epsins bind to LRP-1 in macrophages. Consequently, cell-specific targeting of epsins has therapeutic potential. For example, targeting epsin–VEGFR2 binding in endothelial cells may provide a treatment for various cancers. Looking ahead, despite decades of work to elucidate the regulatory functions of epsins, there are still many intriguing questions to be addressed and preliminary findings to be confirmed. Continued studies on the fascinating proteins will increase our understanding of their role in health and disease.

Fig. 3.

Epsins are central players in various pathologies. Schematic representation of the role of epsin proteins in clinically relevant human diseases

Compliance with ethical standards

Conflict of interests

The authors have no competing interests.