Engineered exosomes targeting MYC reverse the proneural-mesenchymal transition and extend survival of glioblastoma

Dysregulated Myc signaling is a key oncogenic pathway in glioblastoma multiforme (GBM). Yet, effective therapeutic targeting of Myc continues to be challenging. Here, we demonstrate that exosomes generated from human bone marrow mesenchymal stem cells (MSCs) engineered to encapsulate siRNAs targeting Myc (iExo-Myc) localize to orthotopic GBM tumors in mice. Treatment of late stage GBM tumors with iExo-Myc inhibits proliferation and angiogenesis, suppresses tumor growth, and extends survival. Transcriptional profiling of tumors reveals that the mesenchymal transition and estrogen receptor signaling pathways are impacted by Myc inhibition. Single nuclei RNA sequencing (snRNA-seq) shows that iExo-Myc treatment induces transcriptional repression of multiple growth factor and interleukin signaling pathways, triggering a mesenchymal to proneural transition and shifting the cellular landscape of the tumor. These data confirm that Myc is an effective anti-glioma target and that iExo-Myc offers a feasible, readily translational strategy to inhibit challenging oncogene targets for the treatment of brain tumors.

Unique somatic variants in DNA from urine exosomes of individuals with bladder cancer

Bladder cancer (BC), a heterogeneous disease characterized by high recurrence rates, is diagnosed and monitored by cystoscopy. Accurate clinical staging based on biopsy remains a challenge, and additional, objective diagnostic tools are needed urgently. We used exosomal DNA (exoDNA) as an analyte to examine cancer-associated mutations and compared the diagnostic utility of exoDNA from urine and serum of individuals with BC. In contrast to urine exosomes from healthy individuals, urine exosomes from individuals with BC contained significant amounts of DNA. Whole-exome sequencing of DNA from matched urine and serum exosomes, bladder tumors, and normal tissue (peripheral blood mononuclear cells) identified exonic and 3' UTR variants in frequently mutated genes in BC, detectable in urine exoDNA and matched tumor samples. Further analyses identified somatic variants in driver genes, unique to urine exoDNA, possibly because of the inherent intra-tumoral heterogeneity of BC, which is not fully represented in random small biopsies. Multiple variants were also found in untranslated portions of the genome, such as microRNA (miRNA)-binding regions of the KRAS gene. Gene network analyses revealed that exoDNA is associated with cancer, inflammation, and immunity in BC exosomes. Our findings show utility of exoDNA as an objective, non-invasive strategy to identify novel biomarkers and targets for BC.

Novel anti-angiogenic PEDF-derived small peptides mitigate choroidal neovascularization

Abnormal migration and proliferation of endothelial cells (EC) drive neovascular retinopathies. While anti-VEGF treatment slows progression, pathology is often supported by decrease in intraocular pigment epithelium-derived factor (PEDF), an endogenous inhibitor of angiogenesis. A surface helical 34-mer peptide of PEDF, comprising this activity, is efficacious in animal models of neovascular retina disease but remains impractically large for therapeutic use. We sought smaller fragments within this sequence that mitigate choroidal neovascularization (CNV). Expecting rapid intravitreal (IVT) clearance, we also developed a method to reversibly attach peptides to nano-carriers for extended delivery. Synthetic fragments of 34-mer yielded smaller anti-angiogenic peptides, and N-terminal capping with dicarboxylic acids did not diminish activity. Charge restoration via substitution of an internal aspartate by asparagine improved potency, achieving low nM apoptotic response in VEGF-activated EC. Two optimized peptides (PEDF 335, 8-mer and PEDF 336, 9-mer) were tested in a mouse model of laser-induced CNV. IVT injection of either peptide, 2-5 days before laser treatment, gave significant CNV decrease at day +14 post laser treatment. The 8-mer also decreased CNV, when administered as eye drops. Also examined was a nanoparticle-conjugate (NPC) prodrug of the 9-mer, having positive zeta potential, expected to display longer intraocular residence. This NPC showed extended efficacy, even when injected 14 days before laser treatment. Neither inflammatory cells nor other histopathologic abnormalities were seen in rabbit eyes harvested 14 days following IVT injection of PEDF 336 (>200 μg). No rabbit or mouse eye irritation was observed over 12-17 days of PEDF 335 eye drops (10 mM). Viability was unaffected in 3 retinal and 2 choroidal cell types by PEDF 335 up to 100 μM, PEDF 336 (100 μM) gave slight growth inhibition only in choroidal EC. A small anti-angiogenic PEDF epitope (G-Y-D-L-Y-R-V) was identified, variants (adipic-Sar-Y-N-L-Y-R-V) mitigate CNV, with clinical potential in treating neovascular retinopathy. Their shared active motif, Y - - - R, is found in laminin (Ln) peptide YIGSR, which binds Ln receptor 67LR, a known high-affinity ligand of PEDF 34-mer.

Apigenin Inhibits UVB-Induced Skin Carcinogenesis: The Role of Thrombospondin-1 as an Anti-Inflammatory Factor

We have previously demonstrated that apigenin promotes the expression of antiangiogenic protein thrombospondin-1 (TSP1) via a mechanism driven by mRNA-binding protein HuR. Here, we generated a novel mouse model with whole-body THBS-1 gene knockout on SKH-1 genetic background, which allows studies of UVB-induced acute skin damage and carcinogenesis and tests TSP1 involvement in apigenin's anticancer effects. Apigenin significantly inhibited UVB-induced carcinogenesis in the wild-type (WT) animals but not in TSP1 KO (TKO) mice, suggesting that TSP1 is a critical component of apigenin's chemopreventive function in UVB-induced skin cancer. Importantly, TKO mice presented with the elevated cutaneous inflammation at baseline, which was manifested by increased inflammatory infiltrates (neutrophils and macrophages) and elevated levels of the two key inflammatory cytokines, IL-6 and IL-12. In agreement, maintaining normal TSP1 expression in the UVB-irradiated skin of WT mice using topical apigenin application caused a marked decrease of circulating inflammatory cytokines. Finally, TKO mice showed an altered population dynamics of the bone marrow myeloid progenitor cells (CD11b+), with dramatic expansion of the population of neutrophil progenitors (Ly6ClowLy6Ghigh) compared to the WT control. Our results indicate that the cutaneous tumor suppressor TSP1 is a critical mediator of the in vivo anticancer effect of apigenin in skin, specifically of its anti-inflammatory action.

Abstract 3133: Reprogramming of tumor-associated macrophages by a short synthetic peptide eradicates ovarian cancer

Purpose: Ovarian cancer is the deadliest gynecologic malignancy with limited treatment options and novel therapies urgently needed. Immunosuppressive microenvironment is critical for tumor progression and immune checkpoint inhibitors, which enable T-cell anticancer immunity revolutionized the outcomes in multiple cancer types. However, this approach had limited success in ovarian cancer. Our small therapeutic peptides, derived from an endogenous type 2 tumor suppressor, Pigment Epithelium-Derived Factor (PEDF), act through an alternative immune mechanism, repolarization of tumor-associated macrophages (TAMs) to the tumor-suppressive phenotype.Experimental Design: Short peptides based on the PEDF's active domain were modified for improved stability and efficacy. Two peptides (PMD-427, PMD-336) were tested in preclinical ovarian cancer models using the human chemoresistant cell line, OvCar-3, and transformed mouse cell line ID8. We also performed mechanistic analysis of the peptides' anti-tumor action, including effects on macrophages cytotoxic, cytokine secretion and migratory activity in vitro and in vivo. Results: PEDF peptide PMD-427 caused > 20-fold reduction in tumor burden. PMD-427 induced selective apoptosis in ovarian cancer cells but not in normal ovarian epithelium. This selectivity was based on context-specific modulation of extrinsic death cascades, Fas and FasL. More importantly, PMD-427 peptides also stimulated macrophage polarization from M2 to M1 phenotype as was evidenced by the shift in cytokine profile (decreased IL-10 and increased IL-12 expression), altered morphology (increased number of dendrite-like-processes) and other changes in M2 markers (attenuated PD-L1 expression). M2/M1 macrophage polarization was also evident by tumor immunostaining. Critically, PMD peptides ovarian cancer cell killing by macrophages as was determined in co-culture studies; this fratricidal activity was reliant on the expression of TRAIL by the macrophages and of its cognate receptor, DR5 by ovarian cancer cells, respectively. Combined with enhanced macrophage motility as observed by time-lapse micropscopy, these changes resulted in increased macrophage recruitment to the tumors and enhance killing of the cancer cells in vivo. The key role of macrophages in the anti-cancer effects of PMD peptides was confirmed by depletion of macrophages in ovarian tumor bearing mice using clodronate liposomes. Conclusions: We have generated a first-in-class multi-targeted peptide drug, which promotes macrophage polarization that results in eradication of ovarian tumors in mice.

Citation Format: Reshma Bhowmick, Elena Vinokour, Michael Paul Plebanek, Marisol Villanueva, Victor Shifrin, Jack Henkin, Ignacio Melgar-Asensio, James Petrik, Raghu Kallurie, Olga V. Volpert. Reprogramming of tumor-associated macrophages by a short synthetic peptide eradicates ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3133.

Pre-metastatic cancer exosomes induce immune surveillance by patrolling monocytes at the metastatic niche

Metastatic cancers produce exosomes that condition pre-metastatic niches in remote microenvironments to favor metastasis. In contrast, here we show that exosomes from poorly metastatic melanoma cells can potently inhibit metastasis to the lung. These "non-metastatic" exosomes stimulate an innate immune response through the expansion of Ly6Clow patrolling monocytes (PMo) in the bone marrow, which then cause cancer cell clearance at the pre-metastatic niche, via the recruitment of NK cells and TRAIL-dependent killing of melanoma cells by macrophages. These events require the induction of the Nr4a1 transcription factor and are dependent on pigment epithelium-derived factor (PEDF) on the outer surface of exosomes. Importantly, exosomes isolated from patients with non-metastatic primary melanomas have a similar ability to suppress lung metastasis. This study thus demonstrates that pre-metastatic tumors produce exosomes, which elicit a broad range of PMo-reliant innate immune responses via trigger(s) of immune surveillance, causing cancer cell clearance at the pre-metastatic niche.

PEDF increases the tumoricidal activity of macrophages towards prostate cancer cells in vitro

Background

Although inflammation and prostate cancer (PCa) have been linked, the molecular interactions between macrophages and PCa cells are poorly explored. Pigment Epithelium-Derived Factor (PEDF) is an anti-angiogenic and anti-tumor factor. We previously showed that PEDF induces macrophages recruitment in vitro, correlates with macrophages density in human prostate, and stimulates macrophages polarization towards the classically activated pathway. Here, we demonstrate that PEDF modulates the interaction between macrophages and PCa cells through a bidirectional signalling leading to tumor cell apoptosis and phagocytosis.

Methods

RAW 264.7 and THP-1 cells, and BMDMs were grown in vitro as mono- or co-cultures with PC3 or CL1 tumor cells. The effects of PEDF and its derived P18 peptide were measured on macrophages differentiation, migration, and superoxide production, and tumor cell apoptosis and phagocytosis. PEDF receptors (ATP5B, PNPLA2, and LRP6) and CD47 mRNA and protein expression were quantified in macrophages and tumor cells by quantitative RT-PCR, western blot, immunofluorescence and flow cytometry.

Results

We found that PEDF induced the migration of macrophages towards tumor 3D spheroids and 2D cultures. In co-culture, PEDF increased PCa cells phagocytosis through an indirect apoptosis-dependent mechanism. Moreover, PEDF stimulated the production of superoxide by macrophages. Conditioned media from macrophages exposed to PEDF induced tumor cells apoptosis in contrast to control conditioned media suggesting that ROS may be involved in tumor cells apoptosis. ATP5B and PNPLA2 PEDF receptors on macrophages and CD47 on tumor cells were respectively up- and down-regulated by PEDF. As PEDF, blocking CD47 induced phagocytosis. Inhibiting ATP5B reduced phagocytosis. Inversely, PNPLA2 inhibition blocks differentiation but maintains phagocytosis. CD47-induced phagocytosis was partially reverted by ATP5B inhibition suggesting a complementary action. Similar effects were observed with P18 PEDF-derived peptide.

Conclusions

These data established that modulating the molecular interactions between macrophages and PCa cells using PEDF may be a promising strategy for PCa treatment.

Pathways for Modulating Exosome Lipids Identified By High-Density Lipoprotein-Like Nanoparticle Binding to Scavenger Receptor Type B-1

Exosomes are produced by cells to mediate intercellular communication, and have been shown to perpetuate diseases, including cancer. New tools are needed to understand exosome biology, detect exosomes from specific cell types in complex biological media, and to modify exosomes. Our data demonstrate a cellular pathway whereby membrane-bound scavenger receptor type B-1 (SR-B1) in parent cells becomes incorporated into exosomes. We tailored synthetic HDL-like nanoparticles (HDL NP), high-affinity ligands for SR-B1, to carry a fluorescently labeled phospholipid. Data show SR-B1-dependent transfer of the fluorescent phospholipid from HDL NPs to exosomes. Modified exosomes are stable in serum and can be directly detected using flow cytometry. As proof-of-concept, human serum exosomes were found to express SR-B1, and HDL NPs can be used to label and isolate them. Ultimately, we discovered a natural cellular pathway and nanoparticle-receptor pair that enables exosome modulation, detection, and isolation.

Melanoma Cells Block PEDF Production in Fibroblasts to Induce the Tumor-Promoting Phenotype of Cancer-Associated Fibroblasts

Loss of pigment epithelium-derived factor (PEDF, SERPINF1) in cancer cells is associated with poor prognosis and metastasis, but the contribution of stromal PEDF to cancer evolution is poorly understood. Therefore, we investigated the role of fibroblast-derived PEDF in melanoma progression. We demonstrate that normal dermal fibroblasts expressing high PEDF levels attenuated melanoma growth and angiogenesis in vivo, whereas PEDF-depleted fibroblasts exerted tumor-promoting effects. Accordingly, mice with global PEDF knockout were more susceptible to melanoma metastasis. We also demonstrate that normal fibroblasts in close contact with PEDF-null melanoma cells lost PEDF expression and tumor-suppressive properties. Further mechanistic investigations underlying the crosstalk between tumor and stromal cells revealed that melanoma cells produced PDGF-BB and TGFβ, which blocked PEDF production in fibroblasts. Notably, cancer-associated fibroblasts (CAF) isolated from patient-derived tumors expressed markedly low levels of PEDF. Treatment of patient CAF and TGFβ-treated normal fibroblasts with exogenous PEDF decreased the expression of CAF markers and restored PEDF expression. Finally, expression profiling of PEDF-depleted fibroblasts revealed induction of IL8, SERPINB2, hyaluronan synthase-2, and other genes associated with tumor promotion and metastasis. Collectively, our results demonstrate that PEDF maintains tumor-suppressive functions in fibroblasts to prevent CAF conversion and illustrate the mechanisms by which melanoma cells silence stromal PEDF to promote malignancy. Cancer Res; 76(8); 2265-76. ©2016 AACR.

Inhibition of mTOR by apigenin in UVB-irradiated keratinocytes: A new implication of skin cancer prevention

Ultraviolet B (UVB) radiation is the major environmental risk factor for developing skin cancer, the most common cancer worldwide, which is characterized by aberrant activation of Akt/mTOR (mammalian target of rapamycin). Importantly, the link between UV irradiation and mTOR signaling has not been fully established. Apigenin is a naturally occurring flavonoid that has been shown to inhibit UV-induced skin cancer. Previously, we have demonstrated that apigenin activates AMP-activated protein kinase (AMPK), which leads to suppression of basal mTOR activity in cultured keratinocytes. Here, we demonstrated that apigenin inhibited UVB-induced mTOR activation, cell proliferation and cell cycle progression in mouse skin and in mouse epidermal keratinocytes. Interestingly, UVB induced mTOR signaling via PI3K/Akt pathway, however, the inhibition of UVB-induced mTOR signaling by apigenin was not Akt-dependent. Instead, it was driven by AMPK activation. In addition, mTOR inhibition by apigenin in keratinocytes enhanced autophagy, which was responsible, at least in part, for the decreased proliferation in keratinocytes. In contrast, apigenin did not alter UVB-induced apoptosis. Taken together, our results indicate the important role of mTOR inhibition in UVB protection by apigenin, and provide a new target and strategy for better prevention of UV-induced skin cancer.

Therapeutic manipulation of angiogenesis with miR-27b

BACKGROUND

Multiple studies demonstrated pro-angiogenic effects of microRNA (miR)-27b. Its targets include Notch ligand Dll4, Sprouty (Spry)-2, PPARγ and Semaphorin (SEMA) 6A. miR-27 effects in the heart are context-dependent: although it is necessary for ventricular maturation, targeted overexpression in cardiomyocytes causes hypertrophy and dysfunction during development. Despite significant recent advances, therapeutic potential of miR-27b in cardiovascular disease and its effects in adult heart remain unexplored. Here, we assessed the therapeutic potential of miR-27b mimics and inhibitors in rodent models of ischemic disease and cancer.

METHODS

We have used a number of models to demonstrate the effects of miR-27b mimicry and inhibition in vivo, including subcutaneous Matrigel plug assay, mouse models of hind limb ischemia and myocardial infarction and subcutaneous Lewis Lung carcinoma.

RESULTS

Using mouse model of myocardial infarction due to the coronary artery ligation, we showed that miR-27b mimic had overall beneficial effects, including increased vascularization, decreased fibrosis and increased ejection fraction. In mouse model of critical limb ischemia, miR-27b mimic also improved tissue re-vascularization and perfusion. In both models, miR-27b mimic clearly decreased macrophage recruitment to the site of hypoxic injury. In contrast, miR-27b increased the recruitment of bone marrow derived cells to the neovasculature, as was shown using mice reconstituted with fluorescence-tagged bone marrow. These effects were due, at least in part, to the decreased expression of Dll4, PPARγ and IL10. In contrast, blocking miR-27b significantly decreased vascularization and reduced growth of subcutaneous tumors and decreased BMDCs recruitment to the tumor vasculature.

CONCLUSIONS

Our study demonstrates the utility of manipulating miR-27b levels in the treatment of cardiovascular disease and cancer.

Chemopreventive apigenin controls UVB-induced cutaneous proliferation and angiogenesis through HuR and thrombospondin-1

Plant flavonoid apigenin prevents and inhibits UVB-induced carcinogenesis in the skin and has strong anti-proliferative and anti-angiogenic properties. Here we identify mechanisms, by which apigenin controls these oncogenic events. We show that apigenin acts, at least in part, via endogenous angiogenesis inhibitor, thrombospondin-1 (TSP1). TSP1 expression by the epidermal keratinocytes is potently inhibited by UVB. It inhibits cutaneous angiogenesis and UVB-induced carcinogenesis. We show that apigenin restores TSP1 in epidermal keratinocytes subjected to UVB and normalizes proliferation and angiogenesis in UVB-exposed skin. Importantly, reconstituting TSP1 anti-angiogenic function in UVB-irradiated skin with a short bioactive peptide mimetic representing exclusively its anti-angiogenic domain reproduced the anti-proliferative and anti-angiogenic effects of apigenin. Cox-2 and HIF-1α are important mediators of angiogenesis. Both apigenin and TSP1 peptide mimetic attenuated their induction by UVB. Finally we identified the molecular mechanism, whereby apigenin did not affect TSP1 mRNA, but increased de novo protein synthesis. Knockdown studies implicated the RNA-binding protein HuR, which controls mRNA stability and translation. Apigenin increased HuR cytoplasmic localization and physical association with TSP1 mRNA causing de novo TSP1 synthesis. HuR cytoplasmic localization was, in turn, dependent on CHK2 kinase. Together, our data provide a new mechanism, by which apigenin controls UVB-induced carcinogenesis.

High Density Lipoprotein Nanoparticles Deliver RNAi to Endothelial Cells to Inhibit Angiogenesis

Systemic delivery of therapeutic nucleic acids to target cells and tissues outside of the liver remains a major challenge. We synthesized a biomimetic high density lipoprotein nanoparticle (HDL NP) for delivery of a cholesteryl modified therapeutic nucleic acid (RNAi) to vascular endothelial cells, a cell type naturally targeted by HDL. HDL NPs adsorb cholesteryl modified oligonucleotides and protect them from nuclease degradation. As proof of principle, we delivered RNAi targeting vascular endothelial growth factor receptor 2 (VEGFR2) to endothelial cells to effectively silence target mRNA and protein expression in vitro. In addition, data show that treatment strongly attenuated in vivo neovascularization measured using a standard angiogenesis assay and in hypervascular tumor allografts where a striking reduction in tumor growth was observed. For effective delivery, HDL NPs required the expression of the cell surface protein scavenger receptor type-B1 (SR-B1). No toxicity of HDL NPs was measured in vitro or after in vivo administration. Thus, by using a biomimetic approach to nucleic acid delivery, data demonstrate that systemically administered RNAi-HDL NPs target SR-B1 expressing endothelial cells to deliver functional anti-angiogenic RNAi as a potential treatment of cancer and other neo-vascular diseases.

The many facets of PEDF in drug discovery and disease: a diamond in the rough or split personality disorder?

INTRODUCTION

Pigment epithelium-derived factor (PEDF) was discovered as a neurotrophic factor secreted by retinal pigment epithelial cells. A decade later, it re-emerged as a powerful angiogenesis inhibitor guarding ocular function. Since then, significant advances were made identifying PEDF's mechanisms, targets and biomedical applications.

AREAS COVERED

The authors review several methodologies that have generated significant new information about the potential of PEDF as a drug. Furthermore, the authors review and discuss mechanistic and structure-function analyses combined with the functional mapping of active fragments, which have yielded several short bioactive PEDF peptides. Additionally, the authors present functional studies in knockout animals and human correlates that have provided important information about conditions amenable to PEDF-based therapies.

EXPERT OPINION

Through its four known receptors, PEDF causes a wide range of cellular events vitally important for the organism, which include survival and differentiation, migration and invasion, lipid metabolism and stem cell maintenance. These processes are deregulated in multiple pathological conditions, including cancer, metabolic and cardiovascular disease. PEDF has been successfully used in countless preclinical models of these conditions and human correlates suggest a wide utility of PEDF-based drugs. The most significant clinical application of PEDF, to date, is its potential therapeutic use for age-related macular degeneration. Moreover, PEDF-based gene therapy has advanced to early stage clinical trials. PEDF active fragments have been mapped and used to design short peptide mimetics conferring distinct functions of PEDF, which may address specific clinical problems and become prototype drugs.

Snf1-related kinase inhibits colon cancer cell proliferation through calcyclin-binding protein-dependent reduction of β-catenin

Sucrose nonfermenting 1 (Snf1)-related kinase (SNRK) is a serine/threonine kinase with sequence similarity to AMP-activated protein kinase (AMPK); however, its function is not well characterized. We conducted a gene array to determine which genes are regulated by SNRK. The array demonstrated that SNRK overexpression increased the levels of genes involved in cell proliferation, including calcyclin-binding protein (CacyBP), a member of the ubiquitin ligase complex that targets nonphosphorylated β-catenin for degradation. We confirmed that SNRK increased CacyBP mRNA and protein, and decreased β-catenin protein in HCT116 and RKO colon cancer cells. Furthermore, SNRK inhibited colon cancer cell proliferation, and CacyBP down-regulation reversed the SNRK-mediated decrease in proliferation and β-catenin. SNRK overexpression also decreased β-catenin nuclear localization and target gene transcription, and β-catenin down-regulation reversed the effects of SNRK knockdown on proliferation. SNRK transcript levels were reduced in human colon tumors compared to normal tissue by 35.82%, and stable knockdown of SNRK increased colon cancer cell tumorigenicity. Our results demonstrate that SNRK is down-regulated in colon cancer and inhibits colon cancer cell proliferation through CacyBP up-regulation and β-catenin degradation, resulting in reduced proliferation signaling. These findings reveal a novel function for SNRK in the regulation of colon cancer cell proliferation and β-catenin signaling.

Therapies using anti-angiogenic peptide mimetics of thrombospondin-1

INTRODUCTION

The role of hrombospondin-1 (TSP1) as a major endogenous angiogenesis inhibitor has been confirmed by numerous studies and subsequent mechanistic discoveries. It has yielded a new class of potential drugs against cancer and other angiogenesis-driven diseases.

AREAS COVERED

An overview of TSP1 functions and molecular mechanisms, including regulation and signaling. Functions in endothelial and non-endothelial cells, with emphasis on the role of TSP1 in the regulation of angiogenesis and inflammation. The utility of duplicating these activities for drug discovery. Past and current literature on endogenous TSP1 and its role in the progression of cancer and non-cancerous pathological conditions is summarized, as well as the research undertaken to identify and optimize short bioactive peptides derived from the two TSP1 anti-angiogenic domains, which bind CD47 and CD36 cell surface receptors. Lastly, there is an overview of the efficacy of some of these peptides in pre-clinical and clinical models of angiogenesis-dependent disease.

EXPERT OPINION

It is concluded that TSP1-derived peptides and peptide mimetics hold great promise as future agents for the treatment of cancer and other diseases driven by excessive angiogenesis. They may fulfill unmet medical needs including neovascular ocular disease and the diseases of the female reproductive tract including ovarian cancer.

p53-responsive miR-194 inhibits thrombospondin-1 and promotes angiogenesis in colon cancers

Thrombospondin-1 (TSP-1) is an endogenous inhibitor of angiogenesis encoded by the THBS1 gene, whose promoter is activated by p53. In advanced colorectal cancers (CRC), its expression is sustained or even slightly increased despite frequent loss of p53. Here, we determined that in HCT116 CRC cells, p53 activates the THBS1 primary transcript, but fails to boost THBS1 mRNA or protein levels, implying posttranscriptional regulation by microRNAs (miRNA). In a global miRNA gain-of-function screen done in the Dicer-deficient HCT116 variant, several miRNAs negatively regulated THBS1 mRNA and protein levels, one of them being miR-194. Notably, in agreement with published data, p53 upregulated miR-194 expression in THBS1 retrovirus-transduced HCT116 cells, leading to decreased TSP-1 levels. This negative effect was mediated by a single miR-194 complementary site in the THBS1 3'-untranslated region, and its elimination resulted in TSP-1 reactivation, impaired angiogenesis in Matrigel plugs, and reduced growth of HCT116 xenografts. Conversely, transient overexpression of miR-194 in HCT116/THBS1 cells boosted Matrigel angiogenesis, and its stable overexpression in Ras-induced murine colon carcinomas increased microvascular densities and vessel sizes. Although the overall contribution of miR-194 to neoplastic growth is context dependent, p53-induced activation of this GI tract-specific miRNA during ischemia could promote angiogenesis and facilitate tissue repair.

Sonic hedgehog induces angiogenesis via Rho kinase-dependent signaling in endothelial cells

The morphogen Sonic hedgehog (Shh) promotes neovascularization in adults by inducing pro-angiogenic cytokine expression in fibroblasts; however, the direct effects of Shh on endothelial cell (EC) function during angiogenesis are unknown. Our findings indicate that Shh promotes capillary morphogenesis (tube length on Matrigel increased to 271+/-50% of the length in untreated cells, p=0.00003), induces EC migration (modified Boyden chamber assay, 191+/-35% of migration in untreated cells, p=0.00009), and increases EC expression of matrix metalloproteinase 9 (MMP-9) and osteopontin (OPN) mRNA (real-time RT-PCR), which are essential for Shh-induced angiogenesis both in vitro and in vivo. Shh activity in ECs is mediated by Rho, rather than through the "classic" Shh signaling pathway, which involves the Gli transcription factors. The Rho dependence of Shh-induced EC angiogenic activity was documented both in vitro, with dominant-negative RhoA and Rho kinase (ROCK) constructs, and in vivo, with the ROCK inhibitor Y27632 in the mouse corneal angiogenesis model. Finally, experiments performed in MMP-9- and OPN-knockout mice confirmed the roles of the ROCK downstream targets MMP-9 and OPN in Shh-induced angiogenesis. Collectively, our results identify a "nonclassical" pathway by which Shh directly modulates EC phenotype and angiogenic activity.

Natural angiogenesis inhibitor signals through Erk5 activation of peroxisome proliferator-activated receptor gamma (PPARgamma)

Erk-5, a member of the MAPK superfamily, has a catalytic domain similar to Erk1/2 and a unique C-terminal domain enabling binding with transcription factors. Aberrant vascularization in the Erk5-null mice suggested a link to angiogenesis. Ectopic expression of constitutively active Erk5 blocks endothelial cell morphogenesis and causes HIF1-alpha destabilization/degradation. However the mechanisms by which endogenous Erk5 regulates angiogenesis remain unknown. We show that Erk5 and its activating kinase MEK5 are the upstream mediators of the anti-angiogenic signal by the natural angiogenesis inhibitor, pigment epithelial-derived factor (PEDF). We demonstrate that Erk5 phosphorylation allows activation of PPARgamma transcription factor by displacement of SMRT co-repressor. PPARgamma, in turn is critical for NFkappaB activation, PEDF-dependent apoptosis, and anti-angiogenesis. The dominant negative MEK5 mutant and Erk5 shRNA diminished PEDF-dependent apoptosis, inhibition of the endothelial cell chemotaxis, and angiogenesis. This is the first evidence of Erk5-dependent transduction of signals by endogenous angiogenesis inhibitors.

Short pigment epithelial-derived factor-derived peptide inhibits angiogenesis and tumor growth

PURPOSE

Pigment epithelial-derived factor (PEDF) is a potent angiogenesis inhibitor with multiple other functions, some of which enhance tumor growth. Our previous studies mapped PEDF antiangiogenic and prosurvival activities to distinct epitopes. This study was aimed to determine the minimal fragment of PEDF, which maintains antiangiogenic and antitumor efficacy.

EXPERIMENTAL DESIGN

We analyzed antigenicity, hydrophilicity, and charge distribution of the angioinhibitory epitope (the 34-mer) and designed three peptides covering its COOH terminus, P14, P18, and P23. We analyzed their ability to block endothelial cell chemotaxis and induce apoptosis in vitro and their antiangiogenic activity in vivo. The selected peptide was tested for the antitumor activity against mildly aggressive xenografted prostate carcinoma and highly aggressive renal cell carcinoma. To verify that P18 acts in the same manner as PEDF, we used immunohistochemistry to measure PEDF targets, vascular endothelial growth factor receptor 2, and CD95 ligand expression in P18-treated vasculature.

RESULTS

P14 and P18 blocked endothelial cell chemotaxis; P18 and P23 induced apoptosis. P18 showed the highest IC50 and blocked angiogenesis in vivo: P23 was inactive and P14 was proangiogenic. P18 increased the production of CD95 ligand and reduced the expression of vascular endothelial growth factor receptor 2 by the endothelial cells in vivo. In tumor studies, P18 was more effective in blocking the angiogenesis and growth of the prostate cancer than parental 34-mer; in the renal cell carcinoma, P18 strongly decreased angiogenesis and halted the progression of established tumors.

CONCLUSIONS

P18 is a novel and potent antiangiogenic biotherapeutic agent that has potential to be developed for the treatment of prostate and renal cancer.

Pigment epithelium-derived factor and interleukin-6 control prostate neuroendocrine differentiation via feed-forward mechanism

PURPOSE

PEDF (pigment epithelium-derived factor) promotes the differentiation and survival of neuronal cells, and expands the adult neuronal stem cell niche. In the prostate PEDF is suppressed by androgen with unclear physiological consequences. We report that PEDF induced the neuroendocrine differentiation of prostate cancer cells, which was accompanied by neurite outgrowth and chromogranin A expression.

MATERIALS AND METHODS

We performed neuroendocrine differentiation assay, Western blot analysis, immunostaining and reverse transcriptase-polymerase chain reaction in the human prostate cancer cell lines LNCaP, PC-3 and DU145, and the prostate epithelial strain RWPE-1 (ATCC).

RESULTS

Ectopic and endogenous PEDF caused neuroendocrine differentiation of prostate cancer cells, as manifested by neurite-like outgrowths and chromogranin A expression. The transdifferentiated cells expressed axonal and dendritic markers, as ascertained by immunoblotting for specific markers. Neuroendocrine cells formed multiple synaptophysin positive protrusions resembling dendritic spines and vesicles containing serotonin, pointing to possible synapse formation. The known transdifferentiating agent interleukin-6 induced PEDF secretion. Moreover, PEDF neutralizing antibodies abolished the transdifferentiation of interleukin-6 treated cells, suggesting an autocrine loop. Neurogenic events were independent of cyclic adenosine monophosphate. Instead, PEDF activated in this order RhoA, nuclear factor kappaB and Stat3. Inhibitors of the Rho, nuclear factor kappaB and STAT pathways abolished differentiation and synapse formation. Additionally, nuclear factor kappaB activation caused interleukin-6 expression.

CONCLUSIONS

We discovered that nuclear factor kappaB controls the formation of neuronal communications in the prostate due to PEDF. We defined a feed-forward loop, in which nuclear factor kappaB induction elicits Stat3 activation and pro-differentiating interleukin-6 expression causes the further expansion of neuroendocrine communications. Our findings point to the role of nuclear factor kappaB and PEDF in coordinated prostate development.

Androgen receptor targets NFkappaB and TSP1 to suppress prostate tumor growth in vivo

The androgen role in the maintenance of prostate epithelium is subject to conflicting opinions. While androgen ablation drives the regression of normal and cancerous prostate, testosterone may cause both proliferation and apoptosis. Several investigators note decreased proliferation and stronger response to chemotherapy of the prostate cancer cells stably expressing androgen receptor (AR), however no mechanistic explanation was offered. In this paper we demonstrate in vivo anti-tumor effect of the AR on prostate cancer growth and identify its molecular mediators. We analyzed the effect of AR on the tumorigenicity of prostate cancer cells. Unexpectedly, the AR-expressing cells formed tumors in male mice at a much lower rate than the AR-negative controls. Moreover, the AR-expressing tumors showed decreased vascularity and massive apoptosis. AR expression lowered the angiogenic potential of cancer cells, by increasing secretion of an anti-angiogenic protein, thrombospondin-1. AR activation caused a decrease in RelA, a subunit of the pro-survival transcription factor NFkappaB, reduced its nuclear localization and transcriptional activity. This, in turn, diminished the expression of its anti-apoptotic targets, Bcl-2 and IL-6. Increased apoptosis within AR-expressing tumors was likely due to the NFkappaB suppression, since it was restricted to the cells lacking nuclear (active) NFkappaB. Thus we for the first time identified combined decrease of NFkappaB and increased TSP1 as molecular events underlying the AR anti-tumor activity in vivo. Our data indicate that intermittent androgen ablation is preferable to continuous withdrawal, a standard treatment for early-stage prostate cancer. (c) 2007 Wiley-Liss, Inc.

Pigment epithelium-derived factor as a multifunctional antitumor factor

The design of new therapeutic strategies for cancer treatment is based on the combination of drugs directed against different tumor compartments, including the tumor cells themselves and components of the stroma, such as the tumor vasculature. Indeed, several antiangiogenic compounds have entered clinical trials for use alone or in combination with conventional cytotoxic drugs. Pigment epithelium-derived factor (PEDF) is a multifunctional natural peptide with complex neurotrophic, neuroprotective, antiangiogenic, and proapoptotic biological activities, any of which could potentially be exploited for therapeutic purposes. This review summarizes recent studies that reveal the antitumor potential of PEDF based on its antiangiogenic properties and its newly discovered direct antitumor effects, which involve the induction of differentiation or apoptosis in tumor cells. We also discuss possible therapeutic applications of PEDF, based on these mechanistic insights and on the identification of functional domains that retain specific biological activities.

MCT-1 oncogene contributes to increased in vivo tumorigenicity of MCF7 cells by promotion of angiogenesis and inhibition of apoptosis

Overexpression of a novel oncogene MCT-1 (multiple copies in a T cell malignancy) causes malignant transformation of murine fibroblasts. To establish its role in the pathogenesis of breast cancer in humans, we generated stable transfectants of MCF7 breast cancer cells negative for endogenous MCT-1 (MCF7-MCT-1). Overexpression of MCT-1 in these cells resulted in a slight elevation of estrogen receptor-alpha, and higher rates of DNA synthesis and growth in response to estradiol compared with the empty vector control (MCF7-EV). The pure antiestrogen fulvestrant inhibited the estradiol-stimulated proliferation of MCF7-MCT-1 cells. The MCF7-MCT-1 clones showed increased invasiveness in the presence of 50% serum compared with the MCF7-EV. In a tumor xenograft model, MCT-1-overexpressing cells showed higher take rates and formed significantly larger tumors than MCF7-EV controls. When we examined angiogenic phenotype and molecular mediators of angiogenesis in MCF7-MCT-1 tumors in vivo, we found greater microvascular density and lower apoptosis in the MCF7-MCT-1 tumors compared with MCF7-EV controls accompanied by a dramatic decline in the levels of angiogenesis inhibitor, thrombospondin-1 (TSP1). In vitro, blocking TSP1 in the medium conditioned by MCT-1-negative cells restored its angiogenic potential to that of the MCF7-MCT-1 cells. Conversely, despite an increase in mRNA encoding vascular endothelial growth factor upon MCT-1 overexpression, vascular endothelial growth factor protein levels have not been notably altered. Taken together, our results suggest that MCT-1 may contribute to the pathogenesis and progression of human breast cancer via at least two routes: promotion of angiogenesis through the decline of TSP1 and inhibition of apoptosis.

In vivo upregulation of CD95 and CD95L causes synergistic inhibition of angiogenesis by TSP1 peptide and metronomic doxorubicin treatment

Antiangiogenic thrombospondin-1 (TSP1) induces endothelial cell death via a CD95-mediated cascade. We used this signaling pathway, where CD95/Fas is a rate-limiting intermediate, as a target to optimize the efficacy of TSP1 active peptide, DI-TSP. Like TSP1, DI-TSP upregulated endothelial CD95L in vivo. To modulate CD95 levels, we chose chemotherapy agent doxorubicin (DXR). DXR caused sustained upregulation of CD95 in the activated endothelium at 1/100 of the maximal tolerated dose. DI-TSP and DXR synergistically induced endothelial apoptosis in vitro, and in vivo, in developing murine vessels. Fas decoy, TSP1 receptor antibody and Pifithrin, a p53 inhibitor, severely decreased apoptosis and restored angiogenesis by DXR-DI-TSP combination, evidencing critical roles of CD95 and TSP1. Combined therapy synergistically blocked neovascularization and progression of the bladder and prostate carcinoma. Such informed design of a complex antiangiogenic therapy based on the rate-limiting molecular targets is a novel concept, which may yield new approaches to cancer treatment.

Metronomic Low-Dose Chemotherapy Boosts CD95-Dependent Antiangiogenic Effect of the Thrombospondin Peptide ABT-510: A Complementation Antiangiogenic Strategy

Blocking angiogenesis is a promising approach in cancer therapy. Natural inhibitors of angiogenesis and derivatives induce receptor-mediated signals, which often result in the endothelial cell death. Low-dose chemotherapy, given at short regular intervals with no prolonged breaks (metronomic chemotherapy), also targets angiogenesis by obliterating proliferating endothelial cells and circulating endothelial cell precursors. ABT-510, a peptide derivative of thrombospondin, kills endothelial cell by increasing CD95L, a ligand for the CD95 death receptor. However, CD95 expression itself is unaffected by ABT-510 and limits its efficacy. We found that multiple chemotherapy agents, cyclophosphamide (cytoxan), cisplatin, and docetaxel, induced endothelial CD95 in vitro and in vivo at low doses that failed to kill endothelial cells (cytoxan > cisplatin > docetaxel). Thus, we concluded that some of these agents might complement each other and together block angiogenesis with maximal efficacy. As a proof of principle, we designed an antiangiogenic cocktail combining ABT-510 with cytoxan or cisplatin. Cyclophosphamide and cisplatin synergistically increased in vivo endothelial cell apoptosis and angiosuppression by ABT-510. This synergy required CD95, as it was reversible with the CD95 decoy receptor. In a mouse model, ABT-510 and cytoxan, applied together at low doses, acted in synergy to delay tumor take, to stabilize the growth of established tumors, and to cause a long-term progression delay of PC-3 prostate carcinoma. These antitumor effects were accompanied by major decreases in microvascular density and concomitant increases of the vascular CD95, CD95L, and apoptosis. Thus, our study shows a "complementation" design of an optimal cancer treatment with the antiangiogenic peptide and a metronomic chemotherapy.

Two Functional Epitopes of Pigment Epithelial–Derived Factor Block Angiogenesis and Induce Differentiation in Prostate Cancer

Pigment epithelial-derived factor (PEDF), an angiogenesis inhibitor with neurotrophic properties, balances angiogenesis in the eye and blocks tumor progression. Its neurotrophic function and the ability to block vascular leakage is replicated by the PEDF 44-mer peptide (residues 58-101). We analyzed PEDFs' three-dimensional structure and identified a potential receptor-binding surface. Seeking PEDF-based antiangiogenic agents we generated and tested peptides representing the middle and lower regions of this surface. We identified previously unknown antiangiogenic epitopes consisting of the 34-mer (residues 24-57) and a shorter proximal peptide (TGA, residues 16-26) with the critical stretch L19VEEED24 and a fragment within the 44-mer (ERT, residues 78-94), which retained neurotrophic activity. The 34-mer and TGA, but not the 44-mer reproduced PEDF angioinhibitory signals hinged on c-jun-NH2-kinase-dependent nuclear factor of activated T cell deactivation and caused apoptosis. Conversely, the ERT, but not the 34-mer/TGA induced neuronal differentiation. For the 44-mer/ERT, we showed a novel ability to cause neuroendocrine differentiation in prostate cancer cells. PEDF and the peptides bound endothelial and PC-3 prostate cancer cells. Bound peptides were displaced by PEDF, but not by each other, suggesting multiple receptors. PEDF and its active fragments blocked tumor formation when conditionally expressed by PC-3 cells. The 34- and 44-mer used distinct mechanisms: the 34-mer acted on endothelial cells, blocked angiogenesis, and induced apoptosis whereas 44-mer prompted neuroendocrine differentiation in cancer cells. Our results map active regions for the two PEDF functions, signaling via distinct receptors, identify candidate peptides, and provide their mechanism of action for future development of PEDF-based tumor therapies.

Peroxisome proliferator-activated receptor gamma ligands improve the antitumor efficacy of thrombospondin peptide ABT510

An expanding capillary network is critical for several pathologic conditions. In cancer, the decrease of antiangiogenic thrombospondin-1 (TSP1) often enables an angiogenic switch, which can be reversed with exogenous TSP1 or its peptide derivative ABT510. TSP1 acts by inducing endothelial cell apoptosis via signaling cascade initiated at CD36, a TSP1 antiangiogenic receptor. Here, we show that the ligands of nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma), 15-deoxy-delta(12,14)-prostaglandin J2, troglitazone, and rosiglitazone increased PPARgamma and CD36 expression in endothelial cells and improved the efficacy of TSP1 and ABT510 in a CD36-dependent manner. The ABT510 and PPARgamma ligands cooperatively blocked angiogenic endothelial functions in vitro and neovascularization in vivo. In tumor xenografts, 15-deoxy-delta(12,14)-prostaglandin J2 and troglitazone synergistically improved antiangiogenic and antitumor effects of ABT510. Our data provide one mechanism for the in vivo angioinhibitory effect of PPARgamma ligands and show fine-tuning of the antiangiogenic efficacy via targeted up-regulation of the endothelial receptor.

Peroxisome Proliferator-Activated Receptor γ Ligands Improve the Antitumor Efficacy of Thrombospondin Peptide ABT510

An expanding capillary network is critical for several pathologic conditions. In cancer, the decrease of antiangiogenic thrombospondin-1 (TSP1) often enables an angiogenic switch, which can be reversed with exogenous TSP1 or its peptide derivative ABT510. TSP1 acts by inducing endothelial cell apoptosis via signaling cascade initiated at CD36, a TSP1 antiangiogenic receptor. Here, we show that the ligands of nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ), 15-deoxy-Δ12,14-prostaglandin J2, troglitazone, and rosiglitazone increased PPARγ and CD36 expression in endothelial cells and improved the efficacy of TSP1 and ABT510 in a CD36-dependent manner. The ABT510 and PPARγ ligands cooperatively blocked angiogenic endothelial functions in vitro and neovascularization in vivo. In tumor xenografts, 15-deoxy-Δ12,14-prostaglandin J2 and troglitazone synergistically improved antiangiogenic and antitumor effects of ABT510. Our data provide one mechanism for the in vivo angioinhibitory effect of PPARγ ligands and show fine-tuning of the antiangiogenic efficacy via targeted up-regulation of the endothelial receptor.

Nuclear factor of activated T cells balances angiogenesis activation and inhibition

It has been demonstrated that vascular endothelial cell growth factor (VEGF) induction of angiogenesis requires activation of the nuclear factor of activated T cells (NFAT). We show that NFATc2 is also activated by basic fibroblast growth factor and blocked by the inhibitor of angiogenesis pigment epithelial–derived factor (PEDF). This suggests a pivotal role for this transcription factor as a convergence point between stimulatory and inhibitory signals in the regulation of angiogenesis.

We identified c-Jun NH₂-terminal kinases (JNKs) as essential upstream regulators of NFAT activity in angiogenesis. We distinguished JNK-2 as responsible for NFATc2 cytoplasmic retention by PEDF and JNK-1 and JNK-2 as mediators of PEDF-driven NFAT nuclear export.

We identified a novel NFAT target, caspase-8 inhibitor cellular Fas-associated death domain–like interleukin 1β–converting enzyme inhibitory protein (c-FLIP), whose expression was coregulated by VEGF and PEDF. Chromatin immunoprecipitation showed VEGF-dependent increase of NFATc2 binding to the c-FLIP promoter in vivo, which was attenuated by PEDF. We propose that one possible mechanism of concerted angiogenesis regulation by activators and inhibitors may be modulation of the endothelial cell apoptosis via c-FLIP controlled by NFAT and its upstream regulator JNK.

Captopril suppresses Post-transplantation angiogenic activity in rat allograft coronary vessels

BACKGROUND

The development of transplant coronary artery disease is associated with neovascularization in the thickened neointima. We previously reported that captopril inhibits neointimal proliferation in a rat allograft model. We postulated that angiogenic inducers are upregulated post-transplantation and captopril ameliorates transplant coronary artery disease by suppressing the angiogenic activity of coronaries.

METHODS

Animals received no treatment or captopril (50 mg/kg/day). Allograft hearts were analyzed at post-transplantation Days 0, 14, and 21 and angiogenic inducer, plasma platelet-activating factor, determined. The conditioned media from coronaries and myocardium were tested for vascular endothelial growth factor, thrombospondin-1 and angiogenic activity using an endothelial migration assay and rat corneal neovascularization assay.

RESULTS

The captopril-treated group had reduced plasma platelet-activating factor and coronary media revealed earlier upregulation of thrombospondin-1 secretion, diminished vascular endothelial growth factor and no angiogenic activity. At Day 0, the coronary and myocardial conditioned medium had inhibitory activity due to thrombospondin-1, and circulating levels of platelet-activating factor were negligible. By 21 days post-transplantation, plasma platelet-activating factor was elevated and the conditioned medium from untreated coronaries had significantly higher angiogenic activity due to increased vascular endothelial growth factor whereas the myocardium remained non-angiogenic.

CONCLUSIONS

After transplantation, coronary vessels switch to an angiogenic phenotype and vascular endothelial growth factor contributes to the high angiogenic activity, possibly exacerbated by high circulating levels of platelet-activating factor. The ability of captopril to modulate angiogenic mediators and maintain the allograft coronary to its normal anti-angiogenic phenotype may be one mechanism by which it suppresses transplant coronary artery disease.

α1-antitrypsin inhibits angiogenesis and tumor growth

Disturbances of the ratio between angiogenic inducers and inhibitors in tumor microenvironment are the driving force behind angiogenic switch critical for tumor progression. Angiogenic inhibitors may vary depending on organismal age and the tissue of origin. We showed that alpha(1)-antitrypsin (AAT), a serine protease inhibitor (serpin) is an inhibitor of angiogenesis, which induced apoptosis and inhibited chemotaxis of endothelial cells. S- and Z-type mutations that cause abnormal folding and defective serpin activity abrogated AAT antiangiogenic activity. Removal of the C-terminal reactive site loop had no effect on its angiostatic activity. Both native AAT and AAT truncated on C-terminus (AATDelta) inhibited neovascularization in the rat cornea and delayed the growth of subcutaneous tumors in mice. Treatment with native AAT and truncated AATDelta, but not control vehicle reduced tumor microvessel density, while increasing apoptosis within tumor endothelium. Comparative analysis of the human tumors and normal tissues of origin showed correlation between reduced local alpha(1)-antitrypsin expression and more aggressive tumor growth.

Hepatocyte growth factor/scatter factor mediates angiogenesis through positive VEGF and negative thrombospondin 1 regulation

Hepatocyte growth factor/scatter factor (HGF/SF), acting through the Met receptor, plays an important role in most human solid tumors, and inappropriate expression of this ligand-receptor pair is often associated with poor prognosis. The molecular basis for the malignant potential of the HGF/SF-Met signal in cancer cells has mostly been attributed to its mitogenic and invasive properties. However, HGF/SF also induces angiogenesis, but the signaling mechanism has not been fully explained, nor has this activity been directly associated with HGF/SF-Met-mediated tumorigenesis. It is known that HGF/SF induces in vitro expression of vascular endothelial growth factor (VEGF), a key agonist of tumor angiogenesis; by contrast, thrombospondin 1 (TSP-1) is a negative regulator of angiogenesis. Here, we show that, in the very same tumor cells, in addition to inducing VEGF expression, HGF/SF dramatically down-regulates TSP-1 expression. We show that TSP-1 shut-off plays an important, extrinsic role in HGF/SF-mediated tumor development, because ectopic expression of TSP-1 markedly inhibits tumor formation through the suppression of angiogenesis. Interestingly, although VEGF-induced expression is sensitive to inhibitors of several pathways, including mitogen-activated protein kinase, phosphoinositide 3-kinase, and signal transducer and activator of transcription 3, TSP-1 shut-off by HGF/SF is prevented solely by inhibiting mitogen-activated protein kinase activation. These studies identify HGF/SF as a key switch for turning on angiogenesis. They suggest that TSP-1 is a useful antagonist to tumor angiogenesis and that it may have therapeutic value when used in conjunction with inhibitors of VEGF.

Migration inhibitory factor mediates angiogenesis via mitogen-activated protein kinase and phosphatidylinositol kinase

In this study, we investigated the effects of migration inhibitory factor (rhMIF) on angiogenesis-related signaling cascades and apoptosis in human endothelial cells (ECs). We show that in vitro rhMIF induces migration and tube formation in Matrigel of human dermal microvascular endothelial cells (HMVECs), with potency comparable to that of basic fibroblast growth factor. In vivo, rhMIF induces angiogenesis in Matrigel plugs and in the corneal bioassay. Using panels of relatively specific kinase inhibitors, antisense oligonucleotides, and dominant-negative mutants, we show that mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) are critical for MIF-dependent HMVEC migration, whereas Src and p38 kinases are nonessential. Moreover, we demonstrate that rhMIF induces time-dependent increases in phosphorylation levels of MEK1/2, Erk1/2, and Elk-1, as well as PI3K, and its effector kinase, Akt, in HMVECs. Studies with dominant-negative mutants and antisense oligonucleotides corroborate these effects in HMVECs. Furthermore, we demonstrate that rhMIF-induced angiogenesis in the rat cornea in vivo and in the ex vivo endothelial cell morphogenesis assay is also MAPK- and PI3K-dependent. Our findings support a role for MIF as an angiogenic factor and provide a rationale for the use of MIF as a therapeutic inducer of neovascularization in the development of collateral circulation in coronary artery disease.

Wiring the angiogenic switch: Ras, Myc, and Thrombospondin-1

The formation of a blood supply is critical for tumor growth and metastasis; however, understanding the relationship of cellular transformation to tumor angiogenesis has been limited by the multifactorial nature of both processes. In this issue of Cancer Cell, Watnick and colleagues use a genetically defined tumor model system to determine the link between ras, myc, and angiogenesis and identify Thrombospondin-1 as being the critical regulator of angiogenesis in this system (Watnick et al., 2003).

Cyclooxygenase-2: a therapeutic target in angiogenesis

Angiogenesis has a role in the pathogenesis of several disorders, including cancer, chronic inflammatory diseases and retinopathies. Recent evidence demonstrates that the production of prostanoids by cyclooxygenase-2 (COX-2) promotes the expression of pro-angiogenic factors. Furthermore, inhibition of COX-2 by non-steroidal anti-inflammatory drugs leads to restricted angiogenesis and downregulated production of pro-angiogenic factors, such as vascular endothelial growth factor and basic fibroblast growth factor. These findings suggest that COX enzymes could be important therapeutic targets in the treatment of pathological angiogenesis.

SPARC is a key Schwannian-derived inhibitor controlling neuroblastoma tumor angiogenesis

Neuroblastoma (NB), a common pediatric neoplasm, consists of two main cell populations: neuroblastic/ganglionic cells and Schwann cells. NB tumors with abundant Schwannian stroma display a more benign clinical behavior than stroma-poor tumors. Recent studies suggest that Schwann cells influence NB tumor growth via secreted factors that induce differentiation, suppress proliferation, and inhibit angiogenesis. Two angiogenesis inhibitors, pigment epithelium-derived factor and tissue inhibitor of metalloproteinase-2, have been detected in Schwann cell secretions. Here, we isolated another Schwann cell-derived secreted inhibitor of angiogenesis, a 43-kDa protein identified as SPARC (secreted protein acidic and rich in cysteine), an extracellular matrix protein. We found SPARC to be critical for the antiangiogenic phenotype of cultured Schwann cells. We also show that purified SPARC potently inhibits angiogenesis and significantly impairs NB tumor growth in vivo. SPARC may be an effective candidate for the treatment of children with clinically aggressive, Schwannian stroma-poor NB tumors.

Id1 regulates angiogenesis through transcriptional repression of thrombospondin-1

Id proteins are helix-loop-helix transcription factors that regulate tumor angiogenesis. In order to identify downstream effectors of Id1 involved in the regulation of angiogenesis, we performed PCR-select subtractive hybridization on wild-type and Id1 knockout mouse embryo fibroblasts (MEFs). Here we demonstrate that thrombospondin-1 (TSP-1), a potent inhibitor of angiogenesis, is a target of transcriptional repression by Id1. We also show that Id1-null MEFs secrete an inhibitor of endothelial cell migration, which is completely inactivated by depletion of TSP-1. Furthermore, in vivo studies revealed decreased neovascularization in matrigel assays in Id1-null mice compared to their wild-type littermates. This decrease was completely reversed by a TSP-1 neutralizing antibody. We conclude that TSP-1 is a major target for Id1 effects on angiogenesis.

Inhibition of tumor growth by systemic treatment with thrombospondin-1 peptide mimetics

Many normal human cells produce thrombospondin-1 (TSP-1), a potent antiangiogenic protein that promotes vascular quiescence. In various organ systems, including the brain, breast and bladder and in fibroblasts, TSP-1 secretion is reduced during tumorigenesis, thereby allowing induction of the vigorous neovascularization required for tumor growth and metastasis. Full-length and short TSP-1-derived peptides inhibit angiogenesis by inducing endothelial cell apoptosis and thus disrupting the vasculature of the growing tumor. CD36 expressed on the surface of endothelial cells functions as the primary antiangiogenic receptor for TSP-1. A D-isoleucyl enantiomer of a TSP-1 heptapeptide specifically inhibits the proliferation and migration of capillary endothelial cells. DI-TSP, an approximately 1 kDa capped version of this peptide, is also antiangiogenic in vitro, with a specific activity approaching that of the 450 kDa parental molecule. Here, we show that DI-TSP delivered systemically dose-dependently inhibits the growth of murine melanoma metastases in syngeneic animals and that its more soluble isomer, DI-TSPa, similarly blocks the progression of primary human bladder tumors in an orthotopic model in immune-deficient mice. Like intact TSP-1, these peptide mimetics had no effect on cancer cells growing in vitro but markedly suppressed the growth of endothelial cells by inducing receptor-dependent apoptosis. Antibodies raised against CD36 blocked the ability of peptides to induce apoptosis in endothelial cells but had no effect on tumor necrosis factor-alpha-induced apoptosis. In vivo, the peptide mimetics were associated with a significantly reduced microvessel density and increased apoptotic indices in both the endothelial and tumor cell compartments. Such short peptides targeted to a specific antiangiogenic receptor, potent and easy to synthesize, show great promise as lead compounds in clinical antiangiogenic strategies.

Inducer-stimulated Fas targets activated endothelium for destruction by anti-angiogenic thrombospondin-1 and pigment epithelium-derived factor

Natural inhibitors of angiogenesis are able to block pathological neovascularization without harming the preexisting vasculature. Here we show that two such inhibitors, thrombospondin-1 and pigment epithelium-derived factor, derive specificity for remodeling vessels from their dependence on Fas/Fas ligand (FasL)-mediated apoptosis to block angiogenesis. Both inhibitors upregulated FasL on endothelial cells. Expression of the essential partner of FasL, Fas/CD95 receptor, was low on quiescent endothelial cells and vessels but greatly enhanced by inducers of angiogenesis, thereby specifically sensitizing the stimulated cells to apoptosis by inhibitor-generated FasL. The anti-angiogenic activity of thrombospondin-1 and pigment epithelium-derived factor both in vitro and in vivo was dependent on this dual induction of Fas and FasL and the resulting apoptosis. This example of cooperation between pro- and anti-angiogenic factors in the inhibition of angiogenesis provides one explanation for the ability of inhibitors to select remodeling capillaries for destruction.

Mechanistic insights on the inhibition of tumor angiogenesis

Angiogenesis, the growth of new vasculature, is an absolute requirement for the maintenance and progression of the overwhelming majority of the solid tumors. Unraveling the mechanisms that govern this complex biological process has become a central issue not only for understanding of the molecular basis of cancer but also for developing new therapeutic approaches that interfere with neovascularization of the tumor mass. Here we discuss the survival and apoptosis of endothelial cells in the context of vessel formation and regression in response to mediators of angiogenesis produced by tumors. It is the balance between proangiogenic and antiangiogenic molecules in the microenvironment of a vessel in vivo that determines whether the existing vasculature will expand, remain the same, or regress. Here we propose that the vascular endothelial cells themselves interpret and respond to these environmental cues by integrating the activities of the survival and apoptotic pathways within the cell. Thus it is the survival or death of the vulnerable cells that venture out to form new vessels that is the ultimate arbiter of whether neovascularization, as well as the growth of a malignancy that depends on it, succeeds or fails.

c-Jun N-terminal kinase activation is required for the inhibition of neovascularization by thrombospondin-1

Thrombospondin-1 (TSP-1) is a potent inhibitor of angiogenesis that acts directly on endothelial cells via the CD36 surface receptor molecule to halt their migration, proliferation, and morphogenesis in vitro and to block neovascularization in vivo. Here we show that inhibitory signals elicited by TSP-1 did not alter the ability of inducers of angiogenesis to activate p42 and p44 mitogen-activated protein kinase (MAPK). Rather, TSP-1 induced a rapid and transient activation of c-Jun N-terminal kinases (JNK). JNK activation by TSP-1 required engagement of CD36, as it was blocked by antagonistic CD36 antibodies and stimulated by short anti-angiogenic peptides derived from TSP-1 that act exclusively via CD36. TSP-1 inhibition of corneal neovascularization induced by bFGF was severely impaired in mice null for JNK-1, pointing to a critical role for this stress-activated kinase in the inhibition of neovascularization by TSP-1.

In vivo mechanisms by which tumors producing thrombospondin 1 bypass its inhibitory effects

Thrombospondin 1 (TSP1) is a multifunctional protein able to activate TGFbeta and to inhibit angiogenesis in vivo. Although usually thought of as an inhibitor of tumor growth, TSP1 may sometimes be present at high levels during tumor progression, suggesting that tumors can eventually overcome their anti-tumor effects. Using a tet-repressible expression system, we demonstrate that murine TSP1 delayed the onset of tumor growth when produced in the tumor bed by rat fibrosarcoma tumor cells or by stromal fibroblasts coinjected with unmodified C6 glioma tumor cells. Yet upon prolonged exposure to TSP1, tumors came to grow at the same rate in the presence as in the absence of TSP1 and transplantation experiments showed that they had become insensitive to inhibition by TSP1 in both syngeneic and immune compromised hosts. Tumor resistance to TSP1 developed as a result of the in vivo outgrowth of pre-existing tumor cell variants that (1) secreted increased amounts of angiogenic factors that counterbalanced the inhibitory effect of TSP1 on neovascularization and (2) grew more efficiently in the presence of TSP1-activated TGFbeta. These results indicate that prolonged and continuous local delivery of a single multifunctional angiogenesis inhibitor like TSP1 to fast-growing tumors can lead to tumor resistance in vivo by fostering the outgrowth of subpopulations that are a by-product of the genetic instability of the tumor cells themselves.