Role of the tumor stroma in resistance to anti-angiogenic therapy

Targeting Tumor Vascular CD99 Inhibits Tumor Growth

CD99 (MIC2; single-chain type-1 glycoprotein) is a heavily O-glycosylated transmembrane protein (32 kDa) present on leukocytes and activated endothelium. Expression of CD99 on endothelium is important in lymphocyte diapedesis. CD99 is a diagnostic marker for Ewing's Sarcoma (EWS), as it is highly expressed by these tumors. It has been reported that CD99 can affect the migration, invasion and metastasis of tumor cells. Our results show that CD99 is also highly expressed in the tumor vasculature of most solid tumors. Furthermore, we found that in vitro CD99 expression in cultured endothelial cells is induced by starvation. Targeting of murine CD99 by a conjugate vaccine, which induced antibodies against CD99 in mice, resulted in inhibition of tumor growth in both a tumor model with high CD99 (Os-P0109 osteosarcoma) and low CD99 (CT26 colon carcinoma) expression. We demonstrated that vaccination against CD99 is safe, since no toxicity was observed in mice with high antibody titers against CD99 in their sera during a period of almost 11 months. Targeting of CD99 in humans is more complicated due to the fact that the human and mouse CD99 protein are not identical. We are the first to show that growth factor activated endothelial cells express a distinct human CD99 isoform. We conclude that our observations provide an opportunity for specific targeting of CD99 isoforms in human tumor vasculature.

The Role of Platelets in the Tumor-Microenvironment and the Drug Resistance of Cancer Cells

Besides the critical functions in hemostasis, thrombosis and the wounding process, platelets have been increasingly identified as active players in various processes in tumorigenesis, including angiogenesis and metastasis. Once activated, platelets can release bioactive contents such as lipids, microRNAs, and growth factors into the bloodstream, subsequently enhancing the platelet⁻cancer interaction and stimulating cancer metastasis and angiogenesis. The mechanisms of treatment failure of chemotherapeutic drugs have been investigated to be associated with platelets. Therefore, understanding how platelets contribute to the tumor microenvironment may potentially identify strategies to suppress cancer angiogenesis, metastasis, and drug resistance. Herein, we present a review of recent investigations on the role of platelets in the tumor-microenvironment including angiogenesis, and metastasis, as well as targeting platelets for cancer treatment, especially in drug resistance.

Cancer-associated fibroblast regulation of tumor neo-angiogenesis as a therapeutic target in cancer

Adequate blood supply is essential for tumor survival, growth and metastasis. The tumor microenvironment (TME) is dynamic and complex, comprising cancer cells, cancer-associated stromal cells and their extracellular products. The TME serves an important role in tumor progression. Cancer-associated fibroblasts (CAFs) are the principal component of stromal cells within the TME, and contribute to tumor neo-angiogenesis by altering the proteome and degradome. The present paper reviews previous studies of the molecular signaling pathways by which CAFs promote tumor neo-angiogenesis and highlights therapeutic response targets. Also discussed are potential strategies for antitumor neo-angiogenesis to improve tumor treatment efficacy.

Résistance aux inhibiteurs des tyrosines kinases dans le cancer du rein

FOCUS ON RESISTANCE TO TYROSINE KINASE INHIBITORS IN RENAL CANCER: The last decade has seen significant advances in understanding the biology and genetics of kidney cancer, some of which have radically changed treatment standards, including the emergence of targeted therapies. TKIs have significantly improved outcome in patients with metastatic disease. Nevertheless, a subset of patients (approximately 25 %) does not show any clinical benefit from targeted therapies. In many cases, patients initially respond to therapy but resistance to targeted agents has been shown to develop after a median of 6-12 months of treatment. Two general models of tumor resistance to anti-angiogenic agents targeting the VEGF pathway have been proposed: an adaptive (evasive) resistance, which occurs after a prolonged application of a drug (providing a period of tumor control), or intrinsic one (preexisting) non-responsiveness despite the presence of an active agent, showing no therapeutic benefit. Intrinsic resistance is related to tumor redundancy of pro-angiogenic pathways. Acquired resistance is associated with activation of alternative pathways either by upregulation of the VEGF pathway or by recruitment of alternative angiogenic factors responsible for tumor revascularization. Because different combinations and sequences of TKI are tested in clinical trials and immunotherapy (alone or in combination) radically alters patient management in its metastatic disease, the current effort aims at identifying resistance processes, evaluating their importance and proposing rational therapeutic approaches in order to obtain an additional clinical benefit. Our article summarizes the different mechanisms of resistance described in the literature.

Oncofoetal insulin receptor isoform A marks the tumour endothelium; an underestimated pathway during tumour angiogenesis and angiostatic treatment

Background

In a genomic screen for determinants of the tumour vasculature, we identified insulin receptor (INSR) to mark the tumour endothelium. As a functional role for insulin/INSR in cancer has been suggested and markers of the tumour endothelium may be attractive therapeutic targets, we investigated the role of INSR in angiogenesis.

Methods

In a genomic screen for determinants of the tumour vasculature we identified insulin receptor to mark the tumour endothelium.

Results

The current report demonstrates the following: (i) the heavy overexpression of INSR on angiogenic vasculature in human tumours and the correlation to short survival, (ii) that INSR expression in the tumour vasculature is mainly representing the short oncofoetal and non-metabolic isoform INSR-A, (iii) the angiogenic activity of insulin on endothelial cells (EC) in vitro and in vivo, (iv) suppression of proliferation and sprouting of EC in vitro after antibody targeting or siRNA knockdown, and (v) inhibition of in vivo angiogenesis in the chicken chorioallantoic membrane (CAM) by anti-INSR antibodies. We additionally show, using preclinical mouse as well as patient data, that treatment with the inhibitor sunitinib significantly reduces the expression of INSR-A.

Conclusions

The current study underscores the oncogenic impact of INSR and suggests that targeting the INSR-A isoform should be considered in therapeutic settings.

Targeting PDGF-mediated recruitment of pericytes blocks vascular mimicry and tumor growth

Aggressive tumor cells can adopt an endothelial cell-like phenotype and contribute to the formation of a tumor vasculature, independent of tumor angiogenesis. This adoptive mechanism is referred to as vascular mimicry and it is associated with poor survival in cancer patients. To what extent tumor cells capable of vascular mimicry phenocopy the angiogenic cascade is still poorly explored. Here, we identify pericytes as important players in vascular mimicry. We found that pericytes are recruited by vascular mimicry-positive tumor cells in order to facilitate sprouting and to provide structural support of the vascular-like networks. The pericyte recruitment is mediated through platelet-derived growth factor (PDGF)-B. Consequently, preventing PDGF-B signaling by blocking the PDGF receptors with either the small tyrosine kinase inhibitor imatinib or blocking antibodies inhibits vascular mimicry and tumor growth. Collectively, the current study identifies an important role for pericytes in the formation of vascular-like structures by tumor cells. Moreover, the mechanism that controls the pericyte recruitment provides therapeutic opportunities for patients with aggressive vascular mimicry-positive cancer types. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

PDGFRα expression as a novel therapeutic marker in well-differentiated neuroendocrine tumors

AIMS

To evaluate the biological significance of dense vascular networks associated with low-grade NENs, we assessed the impact of PDGFRα tissue expression in 77 GEP/NEN patients, associating PDGFRα expression with the morphological characterization in low-grade tumors.

METHODS AND RESULTS

Paraffin-embedded specimens of 77 GEP- NEN tissues, collected from January 2006 to March 2018, were evaluated for PDGFRα tissue expression and correlations with clinicopathological characteristics. PDGFRα tissue expression was significantly correlated with grade and the NEN growth pattern (p < 0.001) but not with gender, primary site or lymph nodes metastatic status. PDGFRα staining was mainly localized in the vascular pole of the neuroendocrine cells and in Enterochromaffin (EC) cells. In particular PDGFRα tissue expression was significantly more expressed in G2 (p < 0.001) than G1 and G3 cases (p 0.004; p < 0.0002;) and correlated with an insular growth pattern. PDGFRα tissue expression was associated with the Ki67 index and we found a significant negative trend of association with the Ki67 proliferation index (P < 0.001): thus PDGFRα expression is referred to morphological and not to proliferative data.

CONCLUSIONS

PDGFRα represents an effective target for new anti-angiogenic treatment in WD- GEP-NENs, in particular in G2 cases, and in G3 cases only when there is a mixed insular-acinar pattern. In this context, it is important to carefully delineate those tumors that might better respond to this type of treatment alone or in combination. Further investigation of the relationship between PD-L1 and PDGFRa is warranted, and may contribute to optimize the therapeutic approach in patients with GEP-NENs.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Predictive potential of tumour-stroma ratio on benefit from adjuvant bevacizumab in high-risk stage II and stage III colon cancer

Background

The tumour–stroma ratio (TSR) has proven to be an independent prognostic factor in colon cancer.

methods

Haematoxylin eosin tissue slides of patients from the AVANT trial were microscopically scored for TSR and categorised as stroma -low or stroma -high. Scores were correlated to the primary and secondary endpoint disease-free survival (DFS) and overall survival (OS).

Results

Patients with stroma-high tumours (N = 339, 28%) had a significantly shorter DFS (p < 0.001) compared to stroma-low tumours (N = 824, 68%). In the bevacizumab-FOLFOX-4 arm, DFS was significantly shorter compared to FOLFOX-4 in stroma-low tumours, with a hazard ratio (HR) of 1.94 (95% CI 1.24–3.04; p = 0.004). In stroma-high tumours a trend for better DFS was seen in bevacizumab-FOLFOX-4 vs. FOLFOX-4 (HR 0.61 (95% CI 0.35–1.07; p = 0.08)). For bevacizumab-XELOX vs. FOLFOX-4, this was not seen (stroma-low HR 1.07 (95% CI 0.64–1.77; p = 0.80); stroma-high HR 0.78 (95% CI 0.47–1.30; p = 0.35)). OS showed the same pattern for bevacizumab-FOLFOX-4 vs. FOLFOX-4 with a HR of 2.53 (95% CI 1.36–4.71; p = 0.003) for stroma-low and HR 0.50 (95% CI 0.22–1.14; p = 0.10) for stroma-high tumours. For bevacizumab-XELOX vs. FOLFOX-4, HR 1.13 (95% CI 0.55–2.31; p = 0.74) for stroma-low tumours and HR 0.74 (95% CI 0.37–1.51; p = 0.41) for stroma-high tumours.

Conclusions

This exploratory analysis suggests a significantly shorter DFS and OS in stroma-low tumours with addition of bevacizumab to intravenous oxaliplatin-based chemotherapy, contrary to stroma-high tumours, where a beneficial trend is observed.

Apelin: A putative novel predictive biomarker for bevacizumab response in colorectal cancer

Bevacizumab (bvz) is currently employed as an anti-angiogenic therapy across several cancer indications. Bvz response heterogeneity has been well documented, with only 10-15% of colorectal cancer (CRC) patients benefitting in general. For other patients, clinical efficacy is limited and side effects are significant. This reinforces the need for a robust predictive biomarker of response. To identify such a biomarker, we performed a DNA microarray-based transcriptional profiling screen with primary endothelial cells (ECs) isolated from normal and tumour colon tissues. Thirteen separate populations of tumour-associated ECs and 10 of normal ECs were isolated using fluorescence-activated cell sorting. We hypothesised that VEGF-induced genes were overexpressed in tumour ECs; these genes could relate to bvz response and serve as potential predictive biomarkers. Transcriptional profiling revealed a total of 2,610 differentially expressed genes when tumour and normal ECs were compared. To explore their relation to bvz response, the mRNA expression levels of top-ranked genes were examined using quantitative PCR in 30 independent tumour tissues from CRC patients that received bvz in the adjuvant setting. These analyses revealed that the expression of MMP12 and APLN mRNA was significantly higher in bvz non-responders compared to responders. At the protein level, high APLN expression was correlated with poor progression-free survival in bvz-treated patients. Thus, high APLN expression may represent a novel predictive biomarker for bvz unresponsiveness.

Engineering of a Nanosized Biocatalyst for Combined Tumor Starvation and Low-Temperature Photothermal Therapy

Tumor hypoxia is one of the major challenges for the treatment of tumors, as it may negatively affect the efficacy of various anticancer modalities. In this study, a tumor-targeted redox-responsive composite biocatalyst is designed and fabricated, which may combine tumor starvation therapy and low-temperature photothermal therapy for the treatment of oxygen-deprived tumors. The nanosystem was prepared by loading porous hollow Prussian Blue nanoparticles (PHPBNs) with glucose oxidase (GOx) and then coating their surface with hyaluronic acid (HA) via redox-cleavable linkage, therefore allowing the nanocarrier to bind specifically with CD44-overexpressing tumor cells while also exerting control over the cargo release profile. The nanocarriers are designed to enhance the efficacy of the hypoxia-suppressed GOx-mediated starvation therapy by catalyzing the decomposition of intratumoral hydroperoxide into oxygen with PHPBNs, and the enhanced glucose depletion by the two complementary biocatalysts may consequently suppress the expression of heat shock proteins (HSPs) after photothermal treatment to reduce their resistance to the PHPBN-mediated low-temperature photothermal therapies.

The role of tumor microenvironment in resistance to anti-angiogenic therapy

Anti-angiogenic therapy has been demonstrated to increase progression-free survival in patients with many different solid cancers. Unfortunately, the benefit in overall survival is modest and the rapid emergence of drug resistance is a significant clinical problem. Over the last decade, several mechanisms have been identified to decipher the emergence of resistance. There is a multitude of changes within the tumor microenvironment (TME) in response to anti-angiogenic therapy that offers new therapeutic opportunities. In this review, we compile results from contemporary studies related to adaptive changes in the TME in the development of resistance to anti-angiogenic therapy. These include preclinical models of emerging resistance, dynamic changes in hypoxia signaling and stromal cells during treatment, and novel strategies to overcome resistance by targeting the TME.

Senescent stromal cell-induced divergence and therapeutic resistance in T cell acute lymphoblastic leukemia/lymphoma

T cell Acute Lymphoblastic Leukemia/Lymphoma (T-ALL/LBL) is a precursor T cell leukemia/lymphoma that represents approximately 15% of all childhood and 25% of adult acute lymphoblastic leukemia. Although a high cure rate is observed in children, therapy resistance is often observed in adults and mechanisms leading to this resistance remain elusive. Utilizing public gene expression datasets, a fibrotic signature was detected in T-LBL but not T-ALL biopsies. Further, using a T-ALL cell line, CCRF-CEM (CEM) cells, we show that CEM cells induce pulmonary remodeling in immunocompromised mice, suggesting potential interaction between these cells and lung fibroblasts. Co-culture studies suggested that fibroblasts-induced phenotypic and genotypic divergence in co-cultured CEM cells leading to diminished therapeutic responses in vitro. Senescent rather than proliferating stromal cells induced these effects in CEM cells, due, in part, to the enhanced production of oxidative radicals and exosomes containing miRNAs targeting BRCA1 and components of the Mismatch Repair pathway (MMR). Collectively, our studies demonstrate that there may be bidirectional interaction between leukemic cells and stroma, where leukemic cells induce stromal development in vivo and senescent stromal cells generates genomic alterations in the leukemic cells rendering them therapeutic resistant. Thus, targeting senescent stroma might prove beneficial in T-ALL/LBL patients.

CYP4X1 Inhibition by Flavonoid CH625 Normalizes Glioma Vasculature through Reprogramming TAMs via CB2 and EGFR-STAT3 Axis

Tumor-associated macrophages (TAMs) are pivotal effector cells in angiogenesis. Here, we tested whether CYP4X1 inhibition in TAMs by flavonoid CH625 prolongs survival and normalizes glioma vasculature. CH625 was selected against the CYP4X1 3D model by virtual screening and showed inhibitory activity on the CYP4X1 catalytic production of 14,15-EET-EA in the M2-polarized human peripheral blood mononuclear cells (IC₅₀ = 16.5 μM). CH625 improved survival and reduced tumor burden in the C6 and GL261 glioma intracranial and subcutaneous model. In addition, CH625 normalized vasculature (evidenced by a decrease in microvessel density and HIF-1α expression and an increase in tumor perfusion, pericyte coverage, and efficacy of temozolomide therapy) accompanied with the decreased secretion of 14,15-EET-EA, VEGF, and TGF-β in the TAMs. Furthermore, CH625 attenuated vascular abnormalization and immunosuppression induced by coimplantation of GL261 cells with CYP4X1^high macrophages. In vitro TAM polarization away from the M2 phenotype by CH625 inhibited proliferation and migration of endothelial cells, enhanced pericyte migration and T cell proliferation, and decreased VEGF and TGF-β production accompanied with the downregulation of CB2 and EGFR-dependent downstream STAT3 expression. These effects were reversed by overexpression of CYP4X1 and STAT3 or exogenous addition of 14,15-EET-EA, VEGF, TGF-β, EGF, and CB2 inhibitor AM630. These results suggest that CYP4X1 inhibition in TAMs by CH625 prolongs survival and normalizes tumor vasculature in glioma via CB2 and EGFR-STAT3 axis and may serve as a novel therapeutic strategy for human glioma.

Fibrocytes: A Novel Stromal Cells to Regulate Resistance to Anti-Angiogenic Therapy and Cancer Progression

An adequate blood supply is essential for cancer cells to survive and grow; thus, the concept of inhibiting tumor angiogenesis has been applied to cancer therapy, and several drugs are already in clinical use. It has been shown that treatment with those anti-angiogenic drugs improved the response rate and prolonged the survival of patients with various types of cancer; however, it is also true that the effect was mostly limited. Currently, the disappointing clinical results are explained by the existence of intrinsic or acquired resistance to the therapy mediated by both tumor cells and stromal cells. This article reviews the mechanisms of resistance mediated by stromal cells such as endothelial cells, pericytes, fibroblasts and myeloid cells, with an emphasis on fibrocytes, which were recently identified as the cell type responsible for regulating acquired resistance to anti-angiogenic therapy. In addition, the other emerging role of fibrocytes as mediator-producing cells in tumor progression is discussed.

Collateral Damage Intended-Cancer-Associated Fibroblasts and Vasculature Are Potential Targets in Cancer Therapy

After oncogenic transformation, tumor cells rewire their metabolism to obtain sufficient energy and biochemical building blocks for cell proliferation, even under hypoxic conditions. Glucose and glutamine become their major limiting nutritional demands. Instead of being autonomous, tumor cells change their immediate environment not only by their metabolites but also by mediators, such as juxtacrine cell contacts, chemokines and other cytokines. Thus, the tumor cells shape their microenvironment as well as induce resident cells, such as fibroblasts and endothelial cells (ECs), to support them. Fibroblasts differentiate into cancer-associated fibroblasts (CAFs), which produce a qualitatively and quantitatively different extracellular matrix (ECM). By their contractile power, they exert tensile forces onto this ECM, leading to increased intratumoral pressure. Moreover, along with enhanced cross-linkage of the ECM components, CAFs thus stiffen the ECM. Attracted by tumor cell- and CAF-secreted vascular endothelial growth factor (VEGF), ECs sprout from pre-existing blood vessels during tumor-induced angiogenesis. Tumor vessels are distinct from EC-lined vessels, because tumor cells integrate into the endothelium or even mimic and replace it in vasculogenic mimicry (VM) vessels. Not only the VM vessels but also the characteristically malformed EC-lined tumor vessels are typical for tumor tissue and may represent promising targets in cancer therapy.

Revisiting ovarian cancer microenvironment: a friend or a foe?

Development of ovarian cancer involves the co-evolution of neoplastic cells together with the adjacent microenvironment. Steps of malignant progression including primary tumor outgrowth, therapeutic resistance, and distant metastasis are not determined solely by genetic alterations in ovarian cancer cells, but considerably shaped by the fitness advantage conferred by benign components in the ovarian stroma. As the dynamic cancer topography varies drastically during disease progression, heterologous cell types within the tumor microenvironment (TME) can actively determine the pathological track of ovarian cancer. Resembling many other solid tumor types, ovarian malignancy is nurtured by a TME whose dark side may have been overlooked, rather than overestimated. Further, harnessing breakthrough and targeting cures in human ovarian cancer requires insightful understanding of the merits and drawbacks of current treatment modalities, which mainly target transformed cells. Thus, designing novel and precise strategies that both eliminate cancer cells and manipulate the TME is increasingly recognized as a rational avenue to improve therapeutic outcome and prevent disease deterioration of ovarian cancer patients.

Inhibition of CYP4A by a novel flavonoid FLA-16 prolongs survival and normalizes tumor vasculature in glioma

Glioblastomas rapidly become refractory to anti-VEGF therapies. We previously showed that cytochrome P450 (CYP) 4A-derived 20-hydroxyeicosatetraenoic acid (20-HETE) promotes angiogenesis. Here, we tested whether a novel flavonoid (FLA-16) prolongs survival and normalizes tumor vasculature in glioma through CYP4A inhibition. FLA-16 improved survival, reduced tumor burden, and normalized vasculature, accompanied with the decreased secretion of 20-HETE, VEGF and TGF-β in tumor-associated macrophages (TAMs) and endothelial progenitor cells (EPCs) in C6 and U87 gliomas. FLA-16 attenuated vascular abnormalization induced by co-implantation of GL261 glioma cells with CYP4A10^high macrophages or EPCs. Mechanistically, the conditional medium from TAMs and EPCs treated with FLA-16 enhanced the migration of pericyte cells, and decreased the proliferation and migration of endothelial cells, which were reversed by CYP4A overexpression or exogenous addition of 20-HETE, VEGF and TGF-β. Furthermore, FLA-16 prevented crosstalk between TAMs and EPCs during angiogenesis. These results suggest that CYP4A inhibition by FLA-16 prolongs survival and normalizes vasculature in glioma through decreasing production of TAMs and EPCs-derived VEGF and TGF-β. This may represent a potential therapeutic strategy to overcome resistance to anti-VEGF treatment by effects on vessels and immune cells.

Development of a 3D angiogenesis model to study tumour - endothelial cell interactions and the effects of anti-angiogenic drugs

The tumour microenvironment and tumour angiogenesis play a critical role in the development and therapy of many cancers, but in vitro models reflecting these circumstances are rare. In this study, we describe the development of a novel tri-culture model, using non-small cell lung cancer (NSCLC) cell lines (A549 and Colo699) in combination with a fibroblast cell line (SV 80) and two different endothelial cell lines in a hanging drop technology. Endothelial cells aggregated either in small colonies in Colo699 containing microtissues or in tube like structures mainly in the stromal compartment of microtissues containing A549. An up-regulation of hypoxia and vimentin, ASMA and a downregulation of E-cadherin were observed in co- and tri-cultures compared to monocultures. Furthermore, a morphological alteration of A549 tumour cells resembling "signet ring cells" was observed in tri-cultures. The secretion of proangiogenic growth factors like vascular endothelial growth factor (VEGF) was measured in supernatants. Inhibition of these proangiogenic factors by using antiangiogenic drugs (bevacizumab and nindetanib) led to a significant decrease in migration of endothelial cells into microtissues. We demonstrate that our method is a promising tool for the generation of multicellular tumour microtissues and reflects in vivo conditions closer than 2D cell culture.

miRNAs: micro-managers of anticancer combination therapies

Angiogenesis is one of the hallmarks of cancer progression and as such has been considered a target of therapeutic interest. However, single targeted agents have not fully lived up to the initial promise of anti-angiogenic therapy. Therefore, it has been suggested that combining therapies and agents will be the way forward in the oncology field. In recent years, microRNAs (miRNAs) have received considerable attention as drivers of tumor development and progression, either acting as tumor suppressors or as oncogenes (so-called oncomiRs), as well as in the process of tumor angiogenesis (angiomiRs). Not only from a functional, but also from a therapeutic view, miRNAs are attractive tools. Thus far, several mimics and antagonists of miRNAs have entered clinical development. Here, we review the provenance and promise of miRNAs as targets as well as therapeutics to contribute to anti-angiogenesis-based (combination) treatment of cancer.

Anti-angiogenesis for cancer revisited: Is there a role for combinations with immunotherapy?

Angiogenesis is defined as the formation of new blood vessels from preexisting vessels and has been characterized as an essential process for tumor cell proliferation and viability. This has led to the development of pharmacological agents for anti-angiogenesis to disrupt the vascular supply and starve tumor of nutrients and oxygen, primarily through blockade of VEGF/VEGFR signaling. This effort has resulted in 11 anti-VEGF drugs approved for certain advanced cancers, alone or in combination with chemotherapy or other targeted therapies. But this success had only limited impact on overall survival of cancer patients and rarely resulted in durable responses. Given the recent success of immunotherapies, combinations of anti-angiogenics with immune checkpoint blockers have become an attractive strategy. However, implementing such combinations will require a better mechanistic understanding of their interaction. Due to overexpression of pro-angiogenic factors in tumors, their vasculature is often tortuous and disorganized, with excessively branched leaky vessels. This enhances vascular permeability, which in turn is associated with high interstitial fluid pressure, and a reduction in blood perfusion and oxygenation. Judicious dosing of anti-angiogenic treatment can transiently normalize the tumor vasculature by decreasing vascular permeability and improving tumor perfusion and blood flow, and synergize with immunotherapy in this time window. However, anti-angiogenics may also excessively prune tumor vessels in a dose and time-dependent manner, which induces hypoxia and immunosuppression, including increased expression of the immune checkpoint programmed death receptor ligand (PD-L1). This review focuses on revisiting the concept of anti-angiogenesis in combination with immunotherapy as a strategy for cancer treatment.

Arginase inhibition suppresses lung metastasis in the 4T1 breast cancer model independently of the immunomodulatory and anti-metastatic effects of VEGFR-2 blockade

Tumor angiogenesis promotes tumor growth and metastasis. Anti-angiogenic therapy in combination with chemotherapy is used for the treatment of metastatic cancers, including breast cancer but therapeutic benefits are limited. Mobilization and accumulation of myeloid-derived suppressor cells (MDSC) during tumor progression and therapy have been implicated in metastasis formation and resistance to anti-angiogenic treatments. Here, we used the 4T1 orthotopic syngenic mouse model of mammary adenocarcinoma to investigate the effect of VEGF/VEGFR-2 axis inhibition on lung metastasis, MDSC and regulatory T cells (Tregs). We show that treatment with the anti-VEGFR-2 blocking antibody DC101 inhibits primary tumor growth, angiogenesis and lung metastasis. DC101 treatment had no effect on MDSC mobilization, but partially attenuated the inhibitory effect of mMDSC on T cell proliferation and decreased the frequency of Tregs in primary tumors and lung metastases. Strikingly, DC101 treatment induced the expression of the immune-suppressive molecule arginase I in mMDSC. Treatment with the arginase inhibitor N^ω-hydroxy-nor-Arginine (Nor-NOHA) reduced the inhibitory effect of MDSC on T cell proliferation and inhibited number and size of lung metastasis but had little or no additional effects in combination with DC101. In conclusion, DC101 treatment suppresses 4T1 tumor growth and metastasis, partially reverses the inhibitory effect of mMDSC on T cell proliferation, decreases Tregs in tumors and increases arginase I expression in mMDSC. Arginase inhibition suppresses lung metastasis independently of DC101 effects. These observations contribute to the further characterization of the immunomodulatory effect of anti-VEGF/VEGFR2 therapy and provide a rationale to pursue arginase inhibition as potential anti-metastatic therapy.

The Sabotaging Role of Myeloid Cells in Anti-Angiogenic Therapy: Coordination of Angiogenesis and Immune Suppression by Hypoxia

Tumor angiogenesis has become a promising target for anti-tumor therapy. Unfortunately, the somewhat inevitable occurrence of resistance has limited the efficacy of anti-angiogenic therapy. In addition to their well-established role in immune suppression, bone marrow-derived myeloid cells actively contribute to tumor angiogenesis. More importantly, myeloid cells constitute one of the major mechanisms of resistance to angiogenesis inhibition. As the most pervasive feature in tumor microenvironment, hypoxia is able to initiate both pro-angiogenic and immunosuppressive capacities of myeloid cells. Tumor adapts to hypoxic stress primarily through signaling mediated by hypoxic inducible factors (HIFs) and consequently utilizes hypoxia to its own advantage. In this regard, hypoxia orchestrates both angiogenesis and immune evasion to support tumor growth. In this article, we will review available information on the sabotaging role of myeloid cells in anti-angiogenic therapy. We will also discuss how hypoxia coordinates the dual-role cellular and molecular participants in microenvironment to maximize the efficiency of angiogenesis and immunosuppression to promote tumor progression. J. Cell. Physiol. 232: 2312-2322, 2017. © 2016 Wiley Periodicals, Inc.

Antiangiogenic drugs used with chemotherapy for patients with recurrent ovarian cancer: a meta-analysis

Objective

The value of antiangiogenic inhibitors for patients with recurrent ovarian cancer has not been completely affirmed. Therefore, we aimed to assess the effectiveness and toxicities of various antiangiogenic drugs for the treatment of recurrent ovarian cancer.

Methods

In this meta-analysis, we searched PubMed, EMBASE, and the Cochrane Central Register of Controlled Trials databases for complete randomized controlled trials. The searches were extended to May 15, 2016. The risk of bias of the included studies was evaluated via a Cochrane systematic evaluation, and the statistical analyses were performed using RevMan 5.2 software.

Results

In total, we included 8 randomized controlled trials involving 3,211 patients and divided them into 3 groups, vascular endothelial growth factor receptor inhibitors (VEGFRIs), vascular endothelial growth factor (VEGF) inhibitors (bevacizumab), and angiopoietin inhibitors (trebananib). The progression-free survival improved significantly in all the groups being given antiangiogenic drugs (hazard ratio [HR]: 0.55, 95% confidence interval [CI]: 0.45–0.67, I²=0%, P<0.00001 for the VEGFRI group; HR: 0.53, 95% CI: 0.45–0.63, I²=51%, P<0.00001 for the VEGF inhibitor group; HR: 0.67, 95% CI: 0.58–0.77, I²=0%, P<0.00001 for the trebananib group). Overall survival was obviously prolonged in the VEGFRI (HR: 0.76, 95% CI: 0.59–0.97, I²=0%, P=0.03), the VEGF inhibitor (HR: 0.87, 95% CI: 0.77–0.99, I²=0%, P=0.03), and trebananib groups (HR: 0.81, 95% CI: 0.67–0.99, I²=0%, P=0.04). The incidence of grade 3/4 side effects was different among the 3 groups, for example, proteinuria, hypertension, gastrointestinal perforation, and arterial thromboembolism were presented in the VEGF inhibitor group. Increased incidences of fatigue, diarrhea, and hypertension were seen in the VEGFRI group, and the trebananib group had a higher incidence of hypokalemia.

Conclusion

This meta-analysis showed that antiangiogenic drugs improved the progression-free survival. The VEGFRI, bevacizumab, and trebananib groups showed increased overall survival. Adding antiangiogenic drugs to chemotherapy treatment resulted in a higher incidence of grade 3/4 side effects, but these were manageable.

Adaptive mechanisms of resistance to anti-neoplastic agents

Intrinsic and acquired resistance to conventional and targeted therapeutics is a fundamental reason for treatment failure in many cancer patients. Targeted approaches to overcome chemoresistance as well as resistance to targeted approaches require in depth understanding of the underlying molecular mechanisms. The anti-cancer activity of a drug can be limited by a broad variety of molecular events at different levels of drug action in a cell-autonomous and non-cell-autonomous manner. This review summarizes recent insights into the adaptive mechanisms used by tumours to resist therapy including cellular phenotypic plasticity, dynamic alterations of the tumour microenvironment, activation of redundant signal transduction pathways, modulation of drug target expression levels, and exploitation of pro-survival responses.

Insulin-like growth factor axis targeting in cancer and tumour angiogenesis - the missing link

Numerous molecular players in the process of tumour angiogenesis have been shown to offer potential for therapeutic targeting. Initially denoted to be involved in malignant transformation and tumour progression, the insulin-like growth factor (IGF) signalling axis has been subject to therapeutic interference, albeit with limited clinical success. More recently, IGFs and their receptors have received attention for their contribution to tumour angiogenesis, which offers novel therapeutic opportunities. Here we review the contribution of this signalling axis to tumour angiogenesis, the mechanisms of resistance to therapy and the interplay with other pro-angiogenic pathways, to offer insight in the renewed interest in the application of IGF axis targeting agents in anti-cancer combination therapies.

Targeting angiogenesis in gastrointestinal tumors: current challenges

Colorectal cancer (CRC) is one of the few cancers where screening modalities are standardized, but it still remains the third leading cause of cancer related mortality. For more than a decade now, the approval of anti-angiogenic therapy has led to an increase in the rate of overall survival (OS) of patients with advanced colon cancer. The drawback of the anti-angiogenic therapy is that their effect is short-lived and many patients progress through these therapies. Various mechanisms of resistance have been hypothesized, but overcoming this has been challenging. Also, there are no standardized predictive biomarkers that could aid in selecting patients who responds to the therapy upfront. This review focuses on the basis of angiogenesis, describing the approved anti-angiogenic therapies, discusses the challenges in terms of resistance to anti-angiogenic therapy and also the role of biomarkers. In the future, hopefully newer targeted therapies, immunotherapy, combination therapies and the standardization of biomarkers may result in improved outcomes and cure rates.