Therapeutic potential of manipulating VEGF splice isoforms in oncology

Bevacizumab May Differentially Improve Prognosis of Advanced Ovarian Cancer Patients with Low Expression of VEGF-A165b, an Antiangiogenic VEGF-A Splice Variant

PURPOSE

The identification of a robust IHC marker to predict the response to antiangiogenic bevacizumab in ovarian cancer is of high clinical interest. VEGF-A, the molecular target of bevacizumab, is expressed as multiple isoforms with pro- or antiangiogenic properties, of which VEGF-A165b is the most dominant antiangiogenic isoform. The balance of VEGF-A isoforms is closely related to the angiogenic capacity of a tumor and may define its vulnerability to antiangiogenic therapy. We investigated whether the expression of VEGF-A165b could be related to the effect of bevacizumab in advanced ovarian cancer patients.

EXPERIMENTAL DESIGN

Formalin-fixed paraffin-embedded tissues from 413 patients of the ICON7 multicenter phase III trial, treated with standard platinum-based chemotherapy with or without bevacizumab, were probed for VEGF-A165b expression by IHC.

RESULTS

In patients with low VEGF-A165b expression, the addition of bevacizumab to standard platinum-based chemotherapy significantly improved progression-free (HR: 0.727; 95% CI, 0.538-0.984; P = 0.039) and overall survival (HR: 0.662; 95% CI, 0.458-0.958; P = 0.029). Multivariate analysis showed that the addition of bevacizumab in low VEGF-A165b-expressing patients conferred significant improvements in progression-free survival (HR: 0.610; 95% CI, 0.446-0.834; P = 0.002) and overall survival (HR: 0.527; 95% CI, 0.359-0.775; P = 0.001), independently from established risk factors.

CONCLUSIONS

We demonstrate for the first time that bevacizumab may differentially improve the prognosis of advanced ovarian cancer patients with low expression of VEGF-A165b, an antiangiogenic VEGF-A splice variant. We envision that this novel biomarker could be implemented into routine diagnostics and may have direct clinical implications for guiding bevacizumab-related treatment decisions in advanced ovarian cancer patients.

How VEGF-A and its splice variants affect breast cancer development - clinical implications

BACKGROUND

Altered expression levels and structural variations in the vascular endothelial growth factor (VEGF) have been found to play important roles in cancer development and to be associated with the overall survival and therapy response of cancer patients. Particularly VEGF-A and its splice variants have been found to affect physiological and pathological angiogenic processes, including tumor angiogenesis, correlating with tumor progression, mostly caused by overexpression. This review focuses on the expression and impact of VEGF-A splice variants under physiologic conditions and in tumors and, in particular, the distribution and role of isoform VEGF165b in breast cancer.

CONCLUSIONS AND PERSPECTIVES

Many publications already highlighted the importance of VEGF-A and its splice variants in tumor therapy, especially in breast cancer, which are summarized in this review. Furthermore, we were able to demonstrate that cytoplasmatic VEGFA/165b expression is higher in invasive breast cancer tumor cells than in normal tissues or stroma. These examples show that the detection of VEGF splice variants can be performed also on the protein level in formalin fixed tissues. Although no quantitative conclusions can be drawn, these results may be the starting point for further studies at a quantitative level, which can be a major step towards the design of targeted antibody-based (breast) cancer therapies.

VEGF-A165b levels are reduced in breast cancer patients at primary diagnosis but increase after completion of cancer treatment

The antiangiogenic splice variant VEGF-A165b is downregulated in a variety of cancer entities, but little is known so far about circulating plasma levels. The present analysis addresses this question and examines circulating VEGF-A/VEGF-A165b levels in a collective of female high-risk breast cancer patients over the course of treatment. Within the SUCCES-A trial 205 patients were recruited after having received primary breast surgery. Using ELISA VEGF-A/VEGF-A165b concentrations were determined and correlated to clinical characteristics (1) before adjuvant chemotherapy, (2) four weeks and (3) two years after therapy and compared to healthy controls (n = 107). VEGF_165b levels were significantly elevated after completion of chemotherapy. Within the breast cancer cohort, VEGF-A165b levels increased two years after completion of chemotherapy. VEGF-A plasma concentrations were significantly elevated in the breast cancer cohort at all examined time points and decreased after treatment. VEGF-A levels two years after chemotherapy correlated with increased cancer related mortality, no such correlation could be found between VEGF-A165b and the examined clinical characteristics. Compared to controls, VEGF-A/VEGF-A165b ratios were decreased in patients before and after chemotherapy. Our data suggests that circulating VEGF-A165b is significantly reduced in women with primary breast cancer at time of diagnosis; furthermore, levels change during adjuvant treatment.

Trends and Challenges in Tumor Anti-Angiogenic Therapies

Excessive abnormal angiogenesis plays a pivotal role in tumor progression and is a hallmark of solid tumors. This process is driven by an imbalance between pro- and anti-angiogenic factors dominated by the tissue hypoxia-triggered overproduction of vascular endothelial growth factor (VEGF). VEGF-mediated signaling has quickly become one of the most promising anti-angiogenic therapeutic targets in oncology. Nevertheless, the clinical efficacy of this approach is severely limited in certain tumor types or shows only transient efficacy in patients. Acquired or intrinsic therapy resistance associated with anti-VEGF monotherapeutic approaches indicates the necessity of a paradigm change when targeting neoangiogenesis in solid tumors. In this context, the elaboration of the conceptual framework of “vessel normalization” might be a promising approach to increase the efficacy of anti-angiogenic therapies and the survival rates of patients. Indeed, the promotion of vessel maturation instead of regressing tumors by vaso-obliteration could result in reduced tumor hypoxia and improved drug delivery. The implementation of such anti-angiogenic strategies, however, faces several pitfalls due to the potential involvement of multiple pro-angiogenic factors and modulatory effects of the innate and adaptive immune system. Thus, effective treatments bypassing relapses associated with anti-VEGF monotherapies or breaking the intrinsic therapy resistance of solid tumors might use combination therapies or agents with a multimodal mode of action. This review enumerates some of the current approaches and possible future directions of treating solid tumors by targeting neovascularization.

Targeting VEGF pathway to normalize the vasculature: an emerging insight in cancer therapy

Vascular normalization is a new concept of targeting angiogenesis to restore vessel structure and function and to increase blood perfusion and delivery of drugs. It has been confirmed that vascular normalization can decrease relapse and benefit other cancer therapy, including chemotherapy, radiotherapy, and immune cell therapy. The key point of this therapy is to inhibit pro-angiogenic factors and make it be balanced with anti-angiogenic factors, resulting in a mature and normal vessel characteristic. Vascular endothelial growth factor (VEGF) is a key player in the process of tumor angiogenesis, and inhibiting VEGF is a primary approach to tumor vessel normalization. Herein, we review newly uncovered mechanisms governing angiogenesis and vascular normalization of cancer and place emphasis on targeting VEGF pathway to normalize the vasculature. Also, important methods to depress VEGF pathway and make tumor vascular are discussed.

Dynamics and implications of circulating anti-angiogenic VEGF-A165b isoform in patients with ST-elevation myocardial infarction

Angiogenesis is crucial to restore microvascular perfusion in the jeopardized myocardium in the weeks following reperfused ST-segment elevation myocardial infarction (STEMI). (VEGF)-A₁₆₅b, an anti-angiogenic factor, has been identified as a regulator of vascularization; however, it has not been previously implicated in acute myocardial infarction. We sought to investigate the dynamics of circulating VEGF-A₁₆₅b and its association with cardiac magnetic resonance-derived infarct size and left ventricular ejection fraction (LVEF). 50 STEMI patients and 23 controls were included. Compared with control individuals, serum VEGF-A₁₆₅b was elevated in STEMI patients prior to primary percutaneous coronary intervention (PCI). Following PCI, serum VEGF-A₁₆₅b increased further, reaching a maximum level at 24 h and decreased one month after reperfusion. VEGF-A₁₆₅b levels at 24 h were associated with a large infarct size and inversely related to LVEF. VEGF-A₁₆₅b expression was increased in myocardial infarct areas from patients with previous history of AMI. An ex vivo assay using serum from STEMI patients showed that neutralization of VEGF-A₁₆₅b increased tubulogenesis. Overall, the study suggests that VEGF-A₁₆₅b might play a deleterious role after AMI as an inhibitor of angiogenesis in the myocardium. Accordingly, neutralization of VEGF-A₁₆₅b could represent a novel pro-angiogenic therapy for reperfusion of myocardium in STEMI.

RNA-Sequencing data supports the existence of novel VEGFA splicing events but not of VEGFAxxxb isoforms

Vascular endothelial growth factor (VEGFA), a pivotal regulator of angiogenesis and valuable therapeutic target, is characterised by alternative splicing which generates three principal isoforms, VEGFA₁₂₁, VEGFA₁₆₅ and VEGFA₁₈₉. A second set of anti-angiogenic isoforms termed VEGFA_xxxb that utilise an alternative splice site in the final exon have been widely reported, with mRNA detection based principally upon RT-PCR assays. We sought confirmation of the existence of the VEGFA_xxxb isoforms within the abundant RNA sequencing data available publicly. Whilst sequences derived specifically from each of the canonical VEGFA isoforms were present in many tissues, there were no sequences derived from VEGFA_xxxb isoforms. Sequencing of approximately 50,000 RT-PCR products spanning the exon 7–8 junction in 10 tissues did not identify any VEGFA_xxxb transcripts. The absence or extremely low expression of these transcripts in vivo indicates that VEGFA_xxxb isoforms are unlikely to play a role in normal physiology. Our analyses also revealed multiple novel splicing events supported by more reads than previously reported for VEGFA₁₄₅ and VEGFA₁₄₈ isoforms, including three from novel first exons consistent with existing transcription start site data. These novel VEGFA isoforms may play significant roles in specific cell types.

Biology and therapeutic implications of VEGF-A splice isoforms and single-nucleotide polymorphisms in colorectal cancer

Tumor growth, dissemination and metastasis are dependent on angiogenesis. The predominant vascular endothelial growth factor (VEGF) isoform that plays a major role in angiogenesis is VEGF-A. Indeed, VEGF-A is implicated in promoting angiogenesis of numerous solid malignancies, including colorectal cancer (CRC). A large body of preclinical and clinical evidence indicates that the expression of specific VEGF-A isoforms represents a predominant pro-angiogenic factor, which is associated with formation of metastases and poor prognosis in CRC patients. Different isoforms of human VEGF-A have been identified, all of which arise from alternative splicing of the primary transcript of a single gene. Notably, it has been recently demonstrated that expression of type 3 isoform pattern is significantly correlated with venous involvement in CRC as well as in progression to metastatic colorectal cancer (mCRC), although it remains unclear what proportion of CRC tumors express these isoforms. This review highlights the importance of investigating the genetic and the epigenetic variations in VEGF-A pathways in CRC, the functions of different VEGF-A isoforms and their potential application as prognostic markers and/or therapeutic targets. Better understanding of the mechanisms controlling angiogenesis in liver metastases is necessary to address the limitations of current anti-angiogenic therapies.

Novel mechanisms of resistance to vemurafenib in melanoma - V600E B-Raf reversion and switching VEGF-A splice isoform expression

Targeting activating mutations in the proto-oncogene B-Raf, in melanoma, has led to increases in progression free survival. Treatment with vemurafenib, which inhibits the most common activating-mutated form of B-Raf (B-Raf(V600E)), eventually results in resistance to therapy. VEGF-A is the principal driver of angiogenesis in primary and metastatic lesions. The bioactivity of VEGF-A is dependent upon alternative RNA splicing and pro-angiogenic isoforms of VEGF-A are upregulated in many disease states dependent upon angiogenesis, including cancers. Using techniques including RT-PCR, Western blotting, ELISA and luciferase reporter assays, the effect of vemurafenib on proliferation, ERK1/2 phosphorylation and the levels of pro- and anti-angiogenic VEGF-A isoforms was investigated in melanoma cell types expressing either wild-type B-Raf or B-Raf(V600E), including a primary melanoma culture derived from a highly vascularised and active nodule taken from a patient with a V600E mutant melanoma. The primary melanoma culture was characterised and found to have reverted to wild-type B-Raf. In B-Raf(V600E) A375 cells ERK1/2 phosphorylation, pro-angiogenic VEGF-A mRNA, total VEGF-A protein expression and VEGF-A 3'UTR activity were all decreased in a concentration-dependent manner by vemurafenib. Conversely vemurafenib treatment of wild-type B-Raf cells significantly increased ERK1/2 phosphorylation, pro-angiogenic VEGF-A mRNA and total VEGF-A expression in a concentration-dependent manner. A switch to pro-angiogenic VEGF-A isoforms, with a concomitant upregulation of expression by increasing VEGF-A mRNA stability, may be an additional oncogenic and pathological mechanism in B-Raf(V600E) melanomas, which promotes tumor-associated angiogenesis and melanoma-genesis. We have also identified the genetic reversal of B-Raf(V600E) to wild-type in an active melanoma nodule taken from a V600E-positive patient and continued vemurafenib treatment for this patient is likely to have had a detrimental effect by promoting B-Raf(WT) activity.

Antiangiogenic actions of vascular endothelial growth factor-A165b, an inhibitory isoform of vascular endothelial growth factor-A, in human obesity

BACKGROUND

Experimental studies suggest that visceral adiposity and adipose tissue dysfunction play a central role in obesity-related cardiometabolic complications. Impaired angiogenesis in fat has been implicated in the development of adipose tissue hypoxia, capillary rarefaction, inflammation, and metabolic dysregulation, but pathophysiological mechanisms remain unknown. In this study, we examined the role of a novel antiangiogenic isoform of vascular endothelial growth factor-A (VEGF-A), VEGF-A165b, in human obesity.

METHODS AND RESULTS

We biopsied paired subcutaneous and visceral adipose tissue in 40 obese subjects (body mass index, 45±8 kg/m(2); age, 45±11 years) during bariatric surgery and characterized depot-specific adipose tissue angiogenic capacity using an established ex vivo assay. Visceral adipose tissue exhibited significantly blunted angiogenic growth compared with subcutaneous fat (P<0.001) that was associated with marked tissue upregulation of VEGF-A165b (P=0.004). The extent of VEGF-A165b expression correlated negatively with angiogenic growth (r=-0.6, P=0.006). Although recombinant VEGF-A165b significantly impaired angiogenesis, targeted inhibition of VEGF-A165b with neutralizing antibody stimulated fat pad neovascularization and restored VEGF receptor activation. Blood levels of VEGF-A165b were significantly higher in obese subjects compared with lean control subjects (P=0.02), and surgical weight loss induced a marked decline in serumVEGF-A165b (P=0.003).

CONCLUSIONS

We demonstrate that impaired adipose tissue angiogenesis is associated with overexpression of a novel antiangiogenic factor, VEGF-A165b, that may play a pathogenic role in human adiposopathy. Moreover, systemic upregulation of VEGF-A165b in circulating blood may have wider-ranging implications beyond the adipose milieu. VEGF-A165b may represent a novel area of investigation to gain further understanding of mechanisms that modulate the cardiometabolic consequences of obesity.

Alternative RNA splicing and cancer

Alternative splicing of pre-messenger RNA (mRNA) is a fundamental mechanism by which a gene can give rise to multiple distinct mRNA transcripts, yielding protein isoforms with different, even opposing, functions. With the recognition that alternative splicing occurs in nearly all human genes, its relationship with cancer-associated pathways has emerged as a rapidly growing field. In this review, we summarize recent findings that have implicated the critical role of alternative splicing in cancer and discuss current understandings of the mechanisms underlying dysregulated alternative splicing in cancer cells.

Compartment model predicts VEGF secretion and investigates the effects of VEGF trap in tumor-bearing mice

Angiogenesis, the formation of new blood vessels from existing vasculature, is important in tumor growth and metastasis. A key regulator of angiogenesis is vascular endothelial growth factor (VEGF), which has been targeted in numerous anti-angiogenic therapies aimed at inhibiting tumor angiogenesis. Systems biology approaches, including computational modeling, are useful for understanding this complex biological process and can aid in the development of novel and effective therapeutics that target the VEGF family of proteins and receptors. We have developed a computational model of VEGF transport and kinetics in the tumor-bearing mouse, which includes three-compartments: normal tissue, blood, and tumor. The model simulates human tumor xenografts and includes human (VEGF121 and VEGF165) and mouse (VEGF120 and VEGF164) isoforms. The model incorporates molecular interactions between these VEGF isoforms and receptors (VEGFR1 and VEGFR2), as well as co-receptors (NRP1 and NRP2). We also include important soluble factors: soluble VEGFR1 (sFlt-1) and α-2-macroglobulin. The model accounts for transport via macromolecular transendothelial permeability, lymphatic flow, and plasma clearance. We have fit the model to available in vivo experimental data on the plasma concentration of free VEGF Trap and VEGF Trap bound to mouse and human VEGF in order to estimate the rates at which parenchymal cells (myocytes and tumor cells) and endothelial cells secrete VEGF. Interestingly, the predicted tumor VEGF secretion rates are significantly lower (0.007-0.023 molecules/cell/s, depending on the tumor microenvironment) than most reported in vitro measurements (0.03-2.65 molecules/cell/s). The optimized model is used to investigate the interstitial and plasma VEGF concentrations and the effect of the VEGF-neutralizing agent, VEGF Trap (aflibercept). This work complements experimental studies performed in mice and provides a framework with which to examine the effects of anti-VEGF agents, aiding in the optimization of such anti-angiogenic therapeutics as well as analysis of clinical data. The model predictions also have implications for biomarker discovery with anti-angiogenic therapies.

Effect of tumor microenvironment on tumor VEGF during anti-VEGF treatment: systems biology predictions

BACKGROUND

Vascular endothelial growth factor (VEGF) is known to be a potent promoter of angiogenesis under both physiological and pathological conditions. Given its role in regulating tumor vascularization, VEGF has been targeted in various cancer treatments, and anti-VEGF therapy has been used clinically for treatment of several types of cancer. Systems biology approaches, particularly computational models, provide insight into the complexity of tumor angiogenesis. These models complement experimental studies and aid in the development of effective therapies targeting angiogenesis.

METHODS

We developed an experiment-based, molecular-detailed compartment model of VEGF kinetics and transport to investigate the distribution of two major VEGF isoforms (VEGF121 and VEGF165) in the body. The model is applied to predict the dynamics of tumor VEGF and, importantly, to gain insight into how tumor VEGF responds to an intravenous injection of an anti-VEGF agent.

RESULTS

The model predicts that free VEGF in the tumor interstitium is seven to 13 times higher than plasma VEGF and is predominantly in the form of VEGF121 (>70%), predictions that are validated by experimental data. The model also predicts that tumor VEGF can increase or decrease with anti-VEGF treatment depending on tumor microenvironment, pointing to the importance of personalized medicine.

CONCLUSIONS

This computational study suggests that the rate of VEGF secretion by tumor cells may serve as a biomarker to predict the patient population that is likely to respond to anti-VEGF treatment. Thus, the model predictions have important clinical relevance and may aid clinicians and clinical researchers seeking interpretation of pharmacokinetic and pharmacodynamic observations and optimization of anti-VEGF therapies.

Splice isoforms as therapeutic targets for colorectal cancer

Alternative pre-mRNA splicing allows exons of pre-mRNA to be spliced in different arrangements to produce functionally distinct mRNAs. More than 95% of human genes encode splice isoforms, some of which exert antagonistic functions. Recent studies revealed that alterations of the splicing machinery can cause the development of neoplasms, and understanding the splicing machinery is crucial for developing novel therapeutic strategies for malignancies. Colorectal cancer patients need novel strategies not only to enhance the efficacy of the currently available agents but also to utilize newly identified therapeutic targets. This review summarizes the current knowledge about the splice isoforms of VEGFA, UGT1A, PXR, cyclin D1, BIRC5 (survivin), DPD, K-RAS, SOX9, SLC39A14 and other genes, which may be possible therapeutic targets for colorectal cancer. Among them, the VEGFA splice isoforms are classified into VEGFAxxx and VEGFAxxxb, which have proangiogenic and antiangiogenic properties, respectively; UGT1A is alternatively spliced into UGT1A1 and other isoforms, which are regulated by pregnane X receptor isoforms and undergo further splicing modifications. Recently, the splicing machinery has been extensively investigated and novel discoveries in this research field are being reported at a rapid pace. The information contained in this review also provides suggestions for how therapeutic strategies targeting alternative splicing can be further developed.

Do anti-angiogenic VEGF (VEGFxxxb) isoforms exist? A cautionary tale

Splicing of the human vascular endothelial growth factor-A (VEGF-A) gene has been reported to generate angiogenic (VEGFxxx) and anti-angiogenic (VEGFxxxb) isoforms. Corresponding VEGFxxxb isoforms have also been reported in rat and mouse. We examined VEGFxxxb expression in mouse fibrosarcoma cell lines expressing all or individual VEGF isoforms (VEGF120, 164 or 188), grown in vitro and in vivo, and compared results with those from normal mouse and human tissues. Importantly, genetic construction of VEGF164 and VEGF188 expressing fibrosarcomas, in which exon 7 is fused to the conventional exon 8, precludes VEGFxxxb splicing from occurring. Thus, these two fibrosarcoma cell lines provided endogenous negative controls. Using RT-PCR we show that primers designed to simultaneously amplify VEGFxxx and VEGFxxxb isoforms amplified only VEGFxxx variants in both species. Moreover, only VEGFxxx species were generated when mouse podocytes were treated with TGFβ-1, a reported activator of VEGFxxxb splice selection in human podocytes. A VEGF164/120 heteroduplex species was identified as a PCR artefact, specifically in mouse. VEGFxxxb isoform-specific PCR did amplify putative VEGFxxxb species in mouse and human tissues, but unexpectedly also in VEGF188 and VEGF164 fibrosarcoma cells and tumours, where splicing to produce true VEGFxxxb isoforms cannot occur. Moreover, these products were only consistently generated using reverse primers spanning more than 5 bases across the 8b/7 or 8b/5 splice junctions. Primer annealing to VEGFxxx transcripts and amplification of exon 8b primer ‘tails’ explained the artefactual generation of VEGFxxxb products, since the same products were generated when the PCR reactions were performed with cDNA from VEGF164/VEGF188 ‘knock-in’ vectors used in the generation of single VEGF isoform-expressing transgenic mice from which the fibrosarcoma lines were developed. Collectively, our results highlight important pitfalls in data interpretation associated with detecting VEGFxxxb isoforms using current methods, and demonstrate that anti-angiogenic isoforms are not commonly expressed in mouse or human tissues.

Predicting the effects of anti-angiogenic agents targeting specific VEGF isoforms

Vascular endothelial growth factor (VEGF) is a key mediator of angiogenesis, whose effect on cancer growth and development is well characterized. Alternative splicing of VEGF leads to several different isoforms, which are differentially expressed in various tumor types and have distinct functions in tumor blood vessel formation. Many cancer therapies aim to inhibit angiogenesis by targeting VEGF and preventing intracellular signaling leading to tumor vascularization; however, the effects of targeting specific VEGF isoforms have received little attention in the clinical setting. In this work, we investigate the effects of selectively targeting a single VEGF isoform, as compared with inhibiting all isoforms. We utilize a molecular-detailed whole-body compartment model of VEGF transport and kinetics in the presence of breast tumor. The model includes two major VEGF isoforms, VEGF(121) and VEGF(165), receptors VEGFR1 and VEGFR2, and co-receptors Neuropilin-1 and Neuropilin-2. We utilize the model to predict the concentrations of free VEGF, the number of VEGF/VEGFR2 complexes (considered to be pro-angiogenic), and the receptor occupancy profiles following inhibition of VEGF using isoform-specific anti-VEGF agents. We predict that targeting VEGF(121) leads to a 54% and 84% reduction in free VEGF in tumors that secrete both VEGF isoforms or tumors that overexpress VEGF(121), respectively. Additionally, 21% of the VEGFR2 molecules in the blood are ligated following inhibition of VEGF(121), compared with 88% when both isoforms are targeted. Targeting VEGF(121) reduces tumor free VEGF and is an effective treatment strategy. Our results provide a basis for clinical investigation of isoform-specific anti-VEGF agents.

Development of a multiplexed PCR-coupled liquid bead array assay for vascular endothelial growth factor (VEGF) splice variants

OBJECTIVES

To develop a multiplex PCR-coupled liquid bead array assay for the expression of VEGF splice variants.

DESIGN AND METHODS

The assay was based on the combination of multiplex PCR with liquid bead array technology, and optimized and evaluated in terms of analytical sensitivity, specificity, and reproducibility using the MCF-7 cell line. Clinical performance was evaluated in 16 pairs of fresh frozen cancerous and corresponding noncancerous adjacent tissues from NSCLC patients.

RESULTS

The assay is highly sensitive, reproducible and can detect specifically VEGF splice variants in clinical samples. When applied in 32 clinical samples it gave comparable results to RT-qPCR (concordance of 81%, 75%, 88% and 81% for PBGD, VEGF(121), VEGF(165), and VEGF(189) respectively).

CONCLUSIONS

The developed assay can simultaneously detect three VEGF splice variants with high specificity and sensitivity and can be further used to evaluate the role of each individual VEGF splice variant in molecular therapies targeted against VEGF.

VEGF gene alternative splicing: pro- and anti-angiogenic isoforms in cancer

Tumor growth and progression depend on angiogenesis, a process of new blood vessels formation from a preexisting vascular endothelium. Tumors promote angiogenesis by secreting or activating angiogenic factors that stimulate endothelial proliferation and migration and capillary morphogenesis. The newly formed blood vessels provide nutrients and oxygen to the tumor, increasing its growth. Thus, angiogenesis plays a key role in cancer progression and development of metastases. An important growth factor that promotes angiogenesis and participates in a variety of physiological and pathological processes is the vascular endothelial growth factor (VEGF-A or VEGF). Overexpression of VEGF results in increased angiogenesis in normal and pathological conditions. The existence of an alternative site of splicing at the 3' untranslated region of the mRNA results in the expression of isoforms with a C-terminal region which are downregulated in tumors and may have differential inhibitory effects. This suggests that control of splicing can be an important regulatory mechanism of angiogenesis in cancer.

Overexpression of VEGF165b, an inhibitory splice variant of vascular endothelial growth factor, leads to insufficient angiogenesis in patients with systemic sclerosis

RATIONALE

Systemic sclerosis (SSc) is characterized by widespread microangiopathy, fibrosis, and autoimmunity. Despite the lack of angiogenesis, the expression of vascular endothelial growth factor A (VEGF) was shown to be upregulated in SSc skin and circulation; however, previous studies did not distinguish between proangiogenic VEGF(165) and antiangiogenic VEGF(165)b isoforms, which are generated by alternative splicing in the terminal exon of VEGF pre-RNA.

OBJECTIVE

We investigated whether VEGF isoform expression could be altered in skin and circulation of patients with SSc.

METHODS AND RESULTS

Here, we show that the endogenous antiangiogenic VEGF(165)b splice variant is selectively overexpressed at both the mRNA and protein levels in SSc skin. Elevated VEGF(165)b expression correlated with increased expression of profibrotic transforming growth factor-β1 and serine/arginine protein 55 splicing factor in keratinocytes, fibroblasts, endothelial cells, and perivascular inflammatory cells. Circulating levels of VEGF(165)b were significantly higher in patients with SSc than in control subjects. Microvascular endothelial cells (MVECs) isolated from SSc skin expressed and released higher levels of VEGF(165)b than healthy MVECs. Transforming growth factor-β1 upregulated the expression of VEGF(165)b and serine/arginine protein 55 in both SSc and healthy MVECs. In SSc MVECs, VEGF receptor-2 was overexpressed, but its phosphorylation was impaired. Recombinant VEGF(165)b and SSc-MVEC-conditioned medium inhibited VEGF(165)-mediated VEGF receptor-2 phosphorylation and capillary morphogenesis in healthy MVECs. The addition of anti-VEGF(165)b blocking antibodies abrogated the antiangiogenic effect of SSc-MVEC-conditioned medium. Capillary morphogenesis was severely impaired in SSc MVECs and could be ameliorated by treatment with recombinant VEGF(165) and anti-VEGF(165)b blocking antibodies.

CONCLUSIONS

In SSc, a switch from proangiogenic to antiangiogenic VEGF isoforms may have a crucial role in the insufficient angiogenic response to chronic ischemia.

Pazopanib for the treatment of renal cancer

INTRODUCTION

Dramatic advances in the care of patients with advanced renal cell carcinoma (RCC) have occurred over the last 10 years. Insights into the molecular pathogenesis of this disease have elucidated the importance of signaling cascades related to angiogenesis in the management of RCC. Pazopanib is a novel, small-molecule tyrosine kinase inhibitor that targets vascular endothelial growth factor receptors (VEGFR)-1, -2, and -3; platelet-derived growth factor receptors (PDGFR)-α and -β; and c-kit tyrosine kinases. Pazopanib exhibits distinct pharmacokinetic and toxicity profiles compared with other agents in the class of VEGF signaling pathway inhibitors.

AREAS COVERED

This review discusses the scientific rationale for the development of pazopanib, as well as the preclinical and clinical trials that led to the approval of pazopanib for patients with advanced RCC. The most recent information, including data from the 2010 meeting of the American Society of Clinical Oncology and the design of ongoing Phase III trials, is discussed. Finally, an algorithm utilizing level I evidence for the treatment of patients with this disease is proposed.

EXPERT OPINION

The treatment of metastatic RCC has changed dramatically over the last 5 years. Six novel agents - sunitinib, sorafenib, temsirolimus, everolimus, bevacizumab (used in combination with interferon), and pazopanib (Votrient) - have been approved for the treatment of metastatic RCC. The clinical data to date clearly place pazopanib among the most active of the targeted therapies.

Refining orthologue groups at the transcript level

Background

Orthologues are genes in different species that are related through divergent evolution from a common ancestor and are expected to have similar functions. Many databases have been created to describe orthologous genes based on existing sequence data. However, alternative splicing (in eukaryotes) is usually disregarded in the determination of orthologue groups and the functional consequences of alternative splicing have not been considered. Most multi-exon genes can encode multiple protein isoforms which often have different functions and can be disease-related. Extending the definition of orthologue groups to take account of alternate splicing and the functional differences it causes requires further examination.

Results

A subset of the orthologous gene groups between human and mouse was selected from the InParanoid database for this study. Each orthologue group was divided into sub-clusters, at the transcript level, using a method based on the sequence similarity of the isoforms. Transcript based sub-clusters were verified by functional signatures of the cluster members in the InterPro database. Functional similarity was higher within than between transcript-based sub-clusters of a defined orthologous group. In certain cases, cancer-related isoforms of a gene could be distinguished from other isoforms of the gene. Predictions of intrinsic disorder in protein regions were also correlated with the isoform sub-clusters within an orthologue group.

Conclusions

Sub-clustering of orthologue groups at the transcript level is an important step to more accurately define functionally equivalent orthologue groups. This work appears to be the first effort to refine orthologous groupings of genes based on the consequences of alternative splicing on function. Further investigation and refinement of the methodology to classify and verify isoform sub-clusters is needed, particularly to extend the technique to more distantly related species.

Pazopanib for the treatment of patients with advanced renal cell carcinoma

Dramatic advances in the care of patients with advanced renal cell carcinoma have occurred over the last ten years, including insights into the molecular pathogenesis of this disease, that have now been translated into paradigm-changing therapeutic strategies. Elucidating the importance of signaling cascades related to angiogenesis is notable among these achievements. Pazopanib is a novel small molecule tyrosine kinase inhibitor that targets VEGFR-1, -2, and -3; PDGFR-α, PDGFR-β; and c-kit tyrosine kinases. This agent exhibits a distinct pharmacokinetic profile as well as toxicity profile compared to other agents in the class of VEGF signaling pathway inhibitors. This review will discuss the scientific rationale for the development of pazopanib, as well as preclinical and clinical trials that led to approval of pazopanib for patients with advanced renal cell carcinoma. The most recent information, including data from 2010 national meeting of the American Society of Clinical Oncology, and the design of ongoing Phase III trials, will be discussed. Finally, an algorithm utilizing Level I evidence for the treatment of patients with this disease will be proposed.

The mouse retina as an angiogenesis model

The mouse retina has been used extensively over the past decades to study both physiologic and pathologic angiogenesis. Over time, various mouse retina models have evolved into well-characterized and robust tools for in vivo angiogenesis research. This article is a review of the angiogenic development of the mouse retina and a discussion of some of the most widely used vascular disease models. From the multitude of studies performed in the mouse retina, a selection of representative works is discussed in more detail regarding their role in advancing the understanding of both the ocular and general mechanisms of angiogenesis.