Deficiency of bone marrow beta3-integrin enhances non-functional neovascularization

Integrins as therapeutic targets in the organ-specific metastasis of human malignant melanoma

Integrins are a large family of adhesion molecules that mediate cell-cell and cell-extracellular matrix interactions. Among the 24 integrin isoforms, many have been found to be associated with tumor angiogenesis, tumor cell migration and proliferation, and metastasis. Integrins, especially αvβ3, αvβ5 and α5β1, participate in mediating tumor angiogenesis by interacting with the vascular endothelial growth factor and angiopoietin-Tie signaling pathways. Melanoma patients have a poor prognosis when the primary tumor has generated distant metastases, and the melanoma metastatic site is an independent predictor of the survival of these patients. Different integrins on the melanoma cell surface preferentially direct circulating melanoma cells to different organs and promote the development of metastases at specific organ sites. For instance, melanoma cells expressing integrin β3 tend to metastasize to the lungs, whereas those expressing integrin β1 preferentially generate lymph node metastases. Moreover, tumor cell-derived exosomes which contain different integrins may prepare a pre-metastatic niche in specific organs and promote organ-specific metastases. Because of the important role that integrins play in tumor angiogenesis and metastasis, they have become promising targets for the treatment of advanced cancer. In this paper, we review the integrin isoforms responsible for angiogenesis and organ-specific metastasis in malignant melanoma and the inhibitors that have been considered for the future treatment of metastatic disease.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Vein graft failure: from pathophysiology to clinical outcomes

Occlusive arterial disease is a leading cause of morbidity and mortality worldwide. Aside from balloon angioplasty, bypass graft surgery is the most commonly performed revascularization technique for occlusive arterial disease. Coronary artery bypass graft surgery is performed in patients with left main coronary artery disease and three-vessel coronary disease, whereas peripheral artery bypass graft surgery is used to treat patients with late-stage peripheral artery occlusive disease. The great saphenous veins are commonly used conduits for surgical revascularization; however, they are associated with a high failure rate. Therefore, preservation of vein graft patency is essential for long-term surgical success. With the exception of 'no-touch' techniques and lipid-lowering and antiplatelet (aspirin) therapy, no intervention has hitherto unequivocally proven to be clinically effective in preventing vein graft failure. In this Review, we describe both preclinical and clinical studies evaluating the pathophysiology underlying vein graft failure, and the latest therapeutic options to improve patency for both coronary and peripheral grafts.

Endothelial progenitor cells in tumor angiogenesis: another brick in the wall

Until 15 years ago, vasculogenesis, the formation of new blood vessels from undifferentiated cells, was thought to occur only during embryonic development. The discovery of circulating cells that are able to promote vascular regeneration and repair—the so-called endothelial progenitor cells (EPCs)—changed that, and EPCs have since been studied extensively. It is already known that EPCs include many subtypes of cells that play a variety of roles in promoting vascular growth. Some EPCs are destined to differentiate into endothelial cells, whereas others are capable of promoting and sustaining angiogenesis through paracrine mechanisms. Vasculogenesis and angiogenesis might constitute complementary mechanisms for postnatal neovascularization, and EPCs could be at the core of this process. Although the formation of new blood vessels from preexisting vasculature plays a beneficial role in many physiological processes, such as wound healing, it also contributes to tumor growth and metastasis. However, many aspects of the role played by EPCs in tumor angiogenesis remain unclear. This review aims to address the main aspects of EPCs differentiation and certain characteristics of their main function, especially in tumor angiogenesis, as well as the potential clinical applications.

Impaired integrin β3 delays endothelial cell regeneration and contributes to arteriovenous graft failure in mice

OBJECTIVE

Neointima formation is associated with stenosis and subsequent thrombosis in arteriovenous grafts (AVGs). A role of integrin β3 in the neointima formation of AVGs remains poorly understood.

APPROACH AND RESULTS

In integrin β3(-/-) mice, we found significantly accelerated occlusion of AVGs compared with the wild-type mice. This is caused by the development of neointima and lack of endothelial regeneration. The latter is a direct consequence of impaired functions of circulating angiogenic cells (CACs) and platelets in integrin β3(-/-) mice. Evidence suggests the involvement of platelet regulating CAC homing to and differentiation at graft sites via transforming growth factor-β1 and Notch signaling pathway. First, CACs deficient of integrin β3 impaired adhesion activity toward exposed subendothelium. Second, platelets from integrin β3(-/-) mice failed to sufficiently stimulate CACs to differentiate into mature endothelial cells. Finally, we found that transforming growth factor-β1 level was increased in platelets from integrin β3(-/-) mice and resulted in enhanced Notch1 activation in CACs in AVGs. These results demonstrate that integrin β3 is critical for endothelial cell homing and differentiation. The increased transforming growth factor-β1 and Notch1 signaling mediates integrin β3(-/-)-induced AVG occlusion. This accelerated occlusion of AVGs was reversed in integrin β3(-/-) mice transplanted with the bone marrow from wild-type mice.

CONCLUSIONS

Our results suggest that boosting integrin β3 function in the endothelial cells and platelets could prevent neointima and thrombosis in AVGs.

Redefining the role(s) of endothelial αvβ3-integrin in angiogenesis

For nearly two decades now, the RGD (Arg-Gly-Asp)-binding αvβ3-integrin has been a focus of anti-angiogenic drug design. These inhibitors are well-tolerated, but have shown only limited success in patients. Over the years, studies in β3-integrin-knockout mice have shed some light on possible explanations for disappointing clinical outcomes. However, studying angiogenesis in β3-integrin-knockout mice is a blunt tool to investigate β3-integrin's role in pathological angiogenesis. Since establishing our laboratory at University of East Anglia (UEA), we have adopted more refined models of genetically manipulating the expression of the β3-integrin subunit. The present review will highlight some of our findings from these models and describe how data from them have forced us to rethink how targeting αvβ3-integrin expression affects tumour angiogenesis and cancer progression. Revisiting the fundamental biology behind how this integrin regulates tumour growth and angiogenesis, we believe, is the key not only to understanding how angiogenesis is normally co-ordinated, but also in success with drugs directed against it.

Inhibition of tumor-associated αvβ3 integrin regulates the angiogenic switch by enhancing expression of IGFBP-4 leading to reduced melanoma growth and angiogenesis in vivo

A more complete understanding of the mechanisms that regulate the angiogenic switch, which contributes to the conversion of small dormant tumors to actively growing malignancies, is important for the development of more effective anti-angiogenic strategies for cancer therapy. While significant progress has been made in understanding the complex mechanisms by which integrin αvβ3 expressed in endothelial cells governs angiogenesis, less is known concerning the ability of αvβ3 expressed within the tumor cell compartment to modulate the angiogenic output of a tumor. Here we provide evidence that αvβ3 expressed in melanoma cells may contribute to the suppression of IGFBP-4, an important negative regulator of IGF-1 signaling. Given the multiple context-dependent roles for αvβ3 in angiogenesis and tumor progression, our novel findings provide additional molecular insight into how αvβ3 may govern the angiogenic switch by a mechanism associated with a p38 MAPK and matrix metalloproteinases-dependent regulation of the endogenous angiogenesis inhibitor IGFBP-4.

Angiopoietin 2 induces pericyte apoptosis via α3β1 integrin signaling in diabetic retinopathy

Pericyte loss is an early characteristic change in diabetic retinopathy (DR). Despite accumulating evidence that hyperglycemia-induced angiopoietin 2 (Ang2) has a central role in pericyte loss, the precise molecular mechanism has not been elucidated. This study investigated the role of Ang2 in pericyte loss in DR. We demonstrated that pericyte loss occurred with Ang2 increase in the diabetic mouse retina and that the source of Ang2 could be the endothelial cell. Ang2 induced pericyte apoptosis via the p53 pathway under high glucose, whereas Ang2 alone did not induce apoptosis. Integrin, not Tie-2 receptor, was involved for Ang2-induced pericyte apoptosis under high glucose as an Ang2 receptor. High glucose changed the integrin expression pattern, which increased integrin α3 and β1 in the pericyte. Furthermore, Ang2-induced pericyte apoptosis in vitro was effectively attenuated via p53 suppression by blocking integrin α3 and β1. Although intravitreal injection of Ang2 induced pericyte loss in C57BL/6J mice retina in vivo, intravitreal injection of anti-integrin α3 and β1 antibodies attenuated Ang2-induced pericyte loss. Taken together, Ang2 induced pericyte apoptosis under high glucose via α3β1 integrin. Glycemic control or blocking Ang2/integrin signaling could be a potential therapeutic target to prevent pericyte loss in early DR.

Acute Depletion of Endothelial β3-Integrin Transiently Inhibits Tumor Growth and Angiogenesis in Mice

Rationale:

The dramatic upregulation of αvβ3-integrin that occurs in the vasculature during tumor growth has long suggested that the endothelial expression of this molecule is an ideal target for antiangiogenic therapy to treat cancer. This discovery led to the development of small-molecule inhibitors directed against αvβ3-integrin that are currently in clinical trials. In 2002, we reported that β3-integrin−knockout mice exhibit enhanced tumor growth and angiogenesis. However, as β3-integrin is expressed by a wide variety of cells, endothelial cell–specific contributions to tumor angiogenesis are muddied by the use of a global knockout of β3-integrin function.

Objective:

Our aim was to examine the endothelial-specific contribution β3-integrin makes to tumor growth and angiogenesis.

Methods and Results:

We have crossed β3-integrin–floxed (β3-floxed) mice to 2 endothelial-specific Cre models and examined angiogenic responses in vivo, ex vivo, and in vitro. We show that acute depletion of endothelial β3-integrin inhibits tumor growth and angiogenesis preventatively, but not in already established tumors. However, the effects are transient, and long-term depletion of the molecule is ineffective. Furthermore, long-term depletion of the molecule correlates with many molecular changes, such as reduced levels of focal adhesion kinase expression and a misbalance in focal adhesion kinase phosphorylation, which may lead to a release from the inhibitory effects of decreased endothelial β3-integrin expression.

Conclusions:

Our findings imply that timing and length of inhibition are critical factors that need to be considered when targeting the endothelial expression of β3-integrin to inhibit tumor growth and angiogenesis.

Cell trafficking of endothelial progenitor cells in tumor progression

Blood vessel formation plays an essential role in many physiologic and pathologic processes, including normal tissue growth and healing, as well as tumor progression. Endothelial progenitor cells (EPC) are a subtype of stem cells with high proliferative potential that are capable of differentiating into mature endothelial cells, thus contributing to neovascularization in tumors. In response to tumor-secreted cytokines, EPCs mobilize from the bone marrow to the peripheral blood, home to the tumor site, and differentiate to mature endothelial cells and secrete proangiogenic factors to facilitate vascularization of tumors. In this review, we summarize the expression of surface markers, cytokines, receptors, adhesion molecules, proteases, and cell signaling mechanisms involved in the different steps (mobilization, homing, and differentiation) of EPC trafficking from the bone marrow to the tumor site. Understanding the biologic mechanisms of EPC cell trafficking opens a window for new therapeutic targets in cancer.

Endothelial progenitor cells and integrins: adhesive needs

In the last decade there have been multiple studies concerning the contribution of endothelial progenitor cells (EPCs) to new vessel formation in different physiological and pathological settings. The process by which EPCs contribute to new vessel formation in adults is termed postnatal vasculogenesis and occurs via four inter-related steps. They must respond to chemoattractant signals and mobilize from the bone marrow to the peripheral blood; home in on sites of new vessel formation; invade and migrate at the same sites; and differentiate into mature endothelial cells (ECs) and/or regulate pre-existing ECs via paracrine or juxtacrine signals. During these four steps, EPCs interact with different physiological compartments, namely bone marrow, peripheral blood, blood vessels and homing tissues. The success of each step depends on the ability of EPCs to interact, adapt and respond to multiple molecular cues. The present review summarizes the interactions between integrins expressed by EPCs and their ligands: extracellular matrix components and cell surface proteins present at sites of postnatal vasculogenesis. The data summarized here indicate that integrins represent a major molecular determinant of EPC function, with different integrin subunits regulating different steps of EPC biology. Specifically, integrin α4β1 is a key regulator of EPC retention and/or mobilization from the bone marrow, while integrins α5β1, α6β1, αvβ3 and αvβ5 are major determinants of EPC homing, invasion, differentiation and paracrine factor production. β2 integrins are the major regulators of EPC transendothelial migration. The relevance of integrins in EPC biology is also demonstrated by many studies that use extracellular matrix-based scaffolds as a clinical tool to improve the vasculogenic functions of EPCs. We propose that targeted and tissue-specific manipulation of EPC integrin-mediated interactions may be crucial to further improve the usage of this cell population as a relevant clinical agent.

The astrocyte-expressed integrin αvβ8 governs blood vessel sprouting in the developing retina

The mouse retina is vascularized after birth when angiogenic blood vessels grow and sprout along a pre-formed latticework of astrocytes. How astrocyte-derived cues control patterns of blood vessel growth and sprouting, however, remains enigmatic. Here, we have used molecular genetic strategies in mice to demonstrate that αvβ8 integrin expressed in astrocytes is essential for neovascularization of the developing retina. Selective ablation of αv or β8 integrin gene expression in astrocytes leads to impaired blood vessel sprouting and intraretinal hemorrhage, particularly during formation of the secondary vascular plexus. These pathologies correlate, in part, with diminished αvβ8 integrin-mediated activation of extracellular matrix-bound latent transforming growth factor βs (TGFβs) and defective TGFβ signaling in vascular endothelial cells, but not astrocytes. Collectively, our data demonstrate that αvβ8 integrin is a component of a paracrine signaling axis that links astrocytes to blood vessels and is essential for proper regulation of retinal angiogenesis.

The role of β3-integrins in tumor angiogenesis: context is everything

Integrins are a family of cell-extracellular matrix adhesion molecules that play important roles in tumor angiogenesis. αvβ3-Integrin has received much attention as a potential anti-angiogenic target because it is upregulated in tumor-associated blood vessels. Agents targeting αvβ3-integrin are now showing some success in phase III clinical trails for the treatment of glioblastoma, but the exact function of this integrin in tumor angiogenesis is still relatively unknown. This review highlights some of the recent data illustrating that β3-integrins play both pro-angiogenic and anti-angiogenic roles in tumor angiogenesis depending on the context. Specifically we will discuss how the following differentially influence β3-integrin's role in tumor angiogenesis: first, cell-matrix interactions, second, β3-integrin inhibitor doses, third, cell type, and fourth, other interacting molecules.

Troubleshooting: Quantification of mobilization of progenitor cell subsets from bone marrow in vivo

INTRODUCTION

The molecular mechanisms that control the mobilization of specific stem cell subsets from the bone marrow are currently being intensely investigated. It is anticipated that boosting the mobilization of these stem cells via pharmacological intervention will not only produce more effective strategies for bone marrow transplant patients, but also provide novel therapeutic approaches for tissue regeneration.

METHODS

Measurement of stem cell mobilization by sampling peripheral blood is problematic because it is technically difficult to accurately determine absolute numbers of rare progenitor cells by blood sampling. Furthermore a rise in progenitors may be caused by release of stem cells from tissues other than the bone marrow (e.g. spleen and adipose), or indeed an inhibition of stem cell homing back to the bone marrow or other tissues. Finally it is not possible to distinguish whether the pharmacological agent is acting directly at the level of the bone marrow or mobilizing progenitors by a distinct indirect mechanism. To resolve these problems, we have developed a technique that allows perfusion of the vasculature of the hind limb bone marrow in situ in mice. In this system, the femoral artery and vein are cannulated in situ such that the femur and tibia bone marrow are perfused in isolation under anaesthesia. As such, pharmacological agents can be administered directly into the bone marrow vasculature. Mobilized cells are then collected via the femoral vein and colony assays performed in defined growth media to allow identification of haematopoietic, endothelial, and mesenchymal progenitor cells. We have used this system to determine the ability of a CXCR4 antagonist to mobilize these distinct types of progenitor cells from the bone marrow of mice pre-conditioned with either G-CSF or VEGF.

RESULTS AND CONCLUSION

This isolated hind limb perfusion system has allowed comparisons to be made between cytokines (G-CSF and VEGF) that act chronically, either alone or in combination with agents that act acutely on the bone marrow (CXCR4 antagonist) on their ability to directly mobilize specific populations of stem cells. Data obtained therefore gives a more accurate understanding of the efficacy of different mobilizing strategies compared to peripheral blood analysis.