Review

Nature Clinical Practice Urology (2006) 3, 157-169
doi:10.1038/ncpuro0434  
Received 16 August 2005 | Accepted 24 January 2006

Mechanisms of Disease: angiogenesis in urologic malignancies

Philip JS Charlesworth* and Adrian L Harris  About the authors

Correspondence *Angiogenesis and Growth Factors Group, Wetherall Institute of Molecular Medicine and Oxford Radcliffe Hospitals Medical Oncology Department, John Radcliffe and Churchill Hospitals, Oxford OX3 9DS, UK

Email philip.charlesworth@cancer.org.uk

Summary

Angiogenesis is critical for growth of tumors and their metastasis. In this article we review the literature on studies of angiogenesis pathways and markers for renal cancer, prostate cancer and bladder cancer. Overall, there is clear evidence that markers of angiogenesis and expression of angiogenic factors are associated with adverse outcomes in each of these tumor types. Relatively few angiogenic pathways have been investigated so far, although over 50 factors are known to be involved, and little has been studied on the antiangiogenic pathways and their suppression. The failing in many of the studies is small size and lack of suitable statistical analysis. Nevertheless, this review demonstrates the importance of these pathways and the need to develop selection criteria for patients who are candidates for antiangiogenic therapies. On the basis of the expression profiles reported so far, therapies that target vascular endothelial growth factor should be considered for the treatment of renal, prostate and bladder cancers. As most tumors express factors that are involved in multiple angiogenic pathways, further research is needed to determine which are coregulated and what the most common patterns are.

Review criteria

A literature review of PubMed records using the search terms below (or their standard abbreviations) was performed on papers published between January 1965 and August 2005. Around 550 relevant articles were examined, but only those including clinical studies with more than 45 patients and using multivariate analyses are cited here. Search terms were "angiogenesis", "bladder", "kidney", "prostate", "renal", "cancer", "angiogenic factors", "angiogenic inhibitors", "angiogenin", "angiostatin", "basic fibroblast growth factor", "cyclo-oxygenase 2", "epidermal growth factor", "epidermal growth factor receptor", "endostatin", "fibroblast growth factor mutations", "hypoxia-inducible factor", "hypoxia", "matrix metalloprotease", "microvessel density", "p53", "prostate-specific antigen", "transforming growth factor-alpha", "transforming growth factor-beta", "thrombospondin", "vascular endothelial growth factor" and "von Hippel–Lindau".

Keywords:

angiogenesis, bladder cancer, prostate cancer, renal cancer

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Introduction

Angiogenesis is the development of new branching vessels from existing vasculature. Although angiogenesis is a hallmark of cancer that is essential for tumor development and metastasis, it is also a normal physiologic process seen in fetal development, wound healing and endometrial hyperplasia associated with the menstrual cycle.

There are a number of different mechanisms involved, including vessel sprouting and bridge formation. These processes depend on endothelial cell migration and proliferation. Circulating endothelial progenitor cells derived from bone marrow are also recruited to sites of active angiogenesis by tumor-derived growth factors such as VASCULAR ENDOTHELIAL GROWTH FACTOR (VEGF).1

Other mechanisms are involved in vascularization of a tumor: intussusception, where interstitial tissue columns expand and insert into the lumen of an existing vessel; vasculogenesis, where major vessels are formed de novo from mesenchymal tissues (as in the developing embryo); vascular co-option, where tumor cells invade surrounding tissue and make use of existing blood vessels; and vascular mimicry, where pseudovascular channels are formed by tumor cells self-organizing into tubular structures without an endothelial lining.2

Angiogenic vessels show several differences from mature vessels. They have a disorganized and irregular structure, the interactions between cells, endothelial cells and pericytes are altered, and the blood flow is abnormal. This is in contrast to the organized, regular structure and normal blood flow seen in mature vessels. Angiogenic vessels are leaky, a feature that aids extracellular matrix signaling and metabolism, as well as contributing to tumor cell invasion and metastasis. Angiogenic endothelial cells exhibit altered surface markers and cell adhesion molecules that reflect their increased proliferation, protein expression and secretion. These changes can be detected by in vivo phage display,3 proteomic studies on ex vivo endothelial membranes,4 serial analysis of gene expression in microdissected endothelium and bioinformatics approaches. Stromal cells also have an important role in angiogenesis, especially TUMOR-ASSOCIATED MACROPHAGES, which are drawn into the tumor at an early stage. Tumor-associated macrophages are attracted by hypoxia-induced cytokines. The macrophages respond to hypoxia by switching on hypoxia-regulated genes.5

There are particular reasons to investigate angiogenesis in urologic cancers, because each tumor type represents an important paradigm: clear-cell renal cancer is caused by a mutation in the regulatory pathway controlling a key transcription factor that regulates angiogenesis; prostate cancer can exhibit tumor dormancy that is regulated by angiogenesis; and bladder cancer has clearly defined superficial and invasive stages, with the advantage of direct accessibility to serial tumor samples and secreted proteins in the urine. In this review, angiogenesis in prostate, bladder and clear-cell renal cancer is discussed.

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The angiogenic switch

For angiogenesis to occur there must be a switch from the physiologic quintessence of endothelial cells to an 'angiogenic' phenotype. This might occur as a result of metabolic stresses such as hypoxia, acidosis, inflammation or immune-cell activation. Genetic mutations might also directly affect the equilibrium of these metabolic factors, through the effects of oncogenes and tumor-suppressor genes (such as HRAS, ERBB2 and TP53).6 These mutations have been the subject of recent comprehensive reviews, and only those studied in human urologic cancers, rather than cell lines, are reviewed here. Some of the most intensively investigated angiogenic pathways in human urologic cancers are described below, but reviews encompassing all of the known pathways have also been published.7 Table 1 lists the different stimulators and inhibitors of angiogenesis involved in urologic cancers.


Hypoxic regulation

Tumor neovascularization often lags behind tumor growth, leaving areas of hypoxia. The decrease in oxygen tension stimulates further angiogenesis through various signaling pathways, via production of numerous transcriptional factors.8 The most important of these are hypoxia-inducible factor (HIF)-1 and HIF-2.

The HIF active moiety is a heterodimer of alpha and beta subunits. When oxygen levels are sufficient to meet cellular demand (normoxia), HIF-alpha subunits are subjected to oxygen-dependent prolyl hydroxylation, which promotes an interaction with von Hippel–Lindau (VHL) protein, an E3 ubiquitin ligase. Ubiquitinated HIF-alpha is rapidly destroyed by 26S proteasomes. Conversely, when oxygen levels are insufficient (hypoxia), prolyl hydroxylation and subsequent HIF-alpha destruction are avoided, allowing HIF-alpha to bind to HIF-beta at the nuclear hypoxia response elements and, therefore, activate expression in numerous genes (Figure 1). In addition to this degradation pathway, hydroxylation of an asparagine residue in the C-terminal transactivation domain of HIF-alpha prevents transcriptional activation. This hydroxylation is catalyzed by an enzyme called factor-inhibiting HIF, which also requires oxygen for its activity.

Figure 1 In normoxia, interaction between hypoxia-inducible factor alpha subunits and von Hippel–Lindau complex is promoted by oxygen-dependent prolyl hydroxylation of hypoxia-inducible factor alpha subunits.
Figure 1 : In normoxia, interaction between hypoxia-inducible factor alpha subunits and von Hippel|[ndash]|Lindau complex is promoted by oxygen-dependent prolyl hydroxylation of hypoxia-inducible factor alpha subunits. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

The von Hippel–Lindau complex forms part of a larger complex with elongin-B, elongin-C and cullin-2, which is later destroyed by proteosomes. In hypoxia, hypoxia-inducible factor alpha binds to hypoxia-inducible factor beta at the nuclear hypoxia response elements. In the absence of oxygen this is made possible because hypoxia-inducible factor alpha does not undergo prolyl hydroxylation. Adapted with permission from reference 8 © (2002) Adrian L Harris.

CBP, cyclic AMP response-element binding protein (CREB)-binding protein; CUL-2, cullin-2; E1, ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; HIF-alpha, hypoxia-inducible factor alpha; HIF-1beta, hypoxia-inducible factor-1beta; HRE, hypoxia response element; O2, oxygen; OH, hydroxyl ions; p300, a histone acetyltransferase closely related to CBP; Poll II, RNA polymerase II; Rbx1, really interesting new gene (RING)-box protein 1 (or regulator of cullins 1); Ub, ubiquitin; VHL, von Hippel–Lindau protein.

Full figure and legend (51K)Figures & Tables indexDownload PowerPoint slide (253K)

Many of the genes induced by HIF are beneficial to the tumor, including those involved in angiogenesis (VEGF; FLT1, also known as VEGFR1; ANGPT2), glucose metabolism (GLUCOSE TRANSPORTER 1), proliferation (IGF2; insulin-like growth factor 2) and pH regulation (CA9; CARBONIC ANHYDRASE 9). The loss of VHL protein has been demonstrated as a characteristic finding in clear-cell carcinoma of the kidney, and the key genetic variant in the pathogenesis of VHL syndrome.

Major angiogenic stimulators

Mediation of the physiological and pathological stimulation that causes a change in cellular phenotype is enacted by a variety of proangiogenic factors, which include secreted proteins, transcription factors, cytokines, oncogenes, trace elements, proteases, enzymes and low-molecular-weight mediators. A detailed list of all these stimulators relevant in urologic malignancies is provided in Table 1.

Vascular endothelial growth factor

VEGF, the most prominent of the angiogenic stimulators, exists in four main forms of 121, 165, 189 and 206 amino acids in length. Each has a variety of functions, including recruitment and mitogenic stimulation of endothelial cells. These effects are mediated through tyrosine kinase receptors, including VEGF receptor 1 (now known as fms-like tyrosine kinase 1, or Flt-1) and VEGF receptor 2 (also known as kinase insert domain receptor, which is the product of the KDR gene). Other VEGF receptors include neuropilin-1 and neuropilin-2.

Thymidine phosphorylase

Thymidine phosphorylase (TP) is also known as platelet-derived endothelial growth factor. TP catalyses phosphorylation of thymidine to 2-deoxy-D-ribose-1-phosphate and its angiogenic action seems to be mediated by extracellular 2-deoxy-D-ribose (a powerful reducing sugar).9 Phosphorylation of thymidine occurs after dephosphorylation of 2-deoxy-D-ribose-1-phosphate. The mechanism of action of TP is poorly understood, but it might have a role in allowing solid tumors to withstand hypoxia, and could enhance tumor invasion and metastasis through inducing expression of VEGF, interleukins (potent endothelial-cell chemoattractants) and MATRIX METALLOPROTEINASES (MMPS).10

Matrix metalloproteinases

MMPs are a multifarious family of proteolytic enzymes involved in the breakdown of extracellular matrix. They are activated by proenzymes, hypoxia and acidosis and, through their degradation of the extracellular matrix, they release many proangiogenic factors such as BASIC FIBROBLAST GROWTH FACTOR (bFGF). They therefore have a role in the acidotic stimulation of angiogenesis.11

Carbonic anhydrase 9

Carbonic anhydrase 9 (CAIX) is an enzyme that catalyzes the rapid conversion of carbon dioxide and water into carbonic acid, protons and bicarbonate ions. It has a role in the control of intracellular pH and, therefore, protects the tumor cell from hypoxia-induced apoptosis.12

Cyclo-oxygenase 2

CYCLO-OXYGENASE (COX) is a key enzyme in the prostaglandin biosynthesis pathway that converts arachidonic acid to prostaglandin. It has two isoforms, COX1 and COX2. COX2 is proinflammatory, and is usually produced by macrophages and monocytes in response to tissue hypoxia or injury. The proangiogenic effects of COX2 might relate to increased expression of proangiogenic factors such as VEGF and bFGF, or to inhibition of endothelial-cell apoptosis, via stimulation of intracellular antiapoptotic pathways. Alternatively, its proangiogenic effects could be the result of the prostaglandin products of COX, such as PGE2 and PGI2, which directly stimulate angiogenesis, possibly via activation of tyrosine kinase receptors.13

Major angiogenic inhibitors

Endogenous angiogenic inhibitors have attracted much attention, because of their potential uses as therapeutic agents. Their expression is often increased or decreased by tumor-suppressor genes and oncogenes, via the same signaling pathways as stimulators of angiogenesis. Their action has been recently reviewed by Folkman14 and a list of those involved in urologic cancers is shown in Table 1.

Angiostatin and endostatin

Angiostatin (a fragment of plasminogen) and endostatin (a fragment of collagen XVIII) were the first two endogenous inhibitors to be characterized. Many more inhibitors have subsequently been discovered (Table 1). Often, they are fragments of larger natural precursors.

p53 and thrombospondin-1

p53 exerts an antiangiogenic effect through upregulation of THROMBOSPONDIN 1,15 a 430 kDa glycoprotein produced by normal urothelial cells that is strongly antiangiogenic. p53 also downregulates VEGF,16 bFGF,17 and HIF-1, thereby inhibiting angiogenesis further.18

Microvessel density

Tumor angiogenesis can be assessed by using an antibody to endothelial cells, to identify tumor vasculature. Microvessel density (MVD) can then be measured either as an average of counts over a number of randomly selected areas, termed the mean MVD, or quantified in the densest areas of neovascularization, termed hotspots. A variety of immunohistochemical techniques have been used to assess MVD, based on antibodies to CD31, CD34 or factor-VIII-related antigen. The inconsistency between results from different investigators has often been partly blamed on variation in the methodology of measuring MVD.

There have been a number of reviews on MVD that summarize the growing body of evidence supporting MVD as a method of assessing urologic malignancies, but that also show the pitfalls.14, 19, 20, 21

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Angiogenesis and bladder cancer

High MVD was initially shown to be an independent predictor of survival by Dickinson et al. in 1994.22 Increasing MVD has subsequently been shown to be associated with increasing incidence of tumor recurrence, stage progression, lymph-node metastases and the presence of vascular invasion.23, 24

VEGF and bFGF are the primary pro-angiogenic agents that have been studied clinically. They are both independent prognostic factors that have a positive correlation with tumor recurrence.25 High serum levels of VEGF (>400 pg/ml) have shown a high specificity and sensitivity for predicting metastatic disease,26 and high levels of VEGF measured in urine correlate with tumor recurrence.27 Elevated levels of bFGF in urine are directly associated with high-grade, high-stage tumors and recurrent tumors. Mutations in the tyrosine kinase receptor (FGFR-3) of bFGF are very common (present in 70% of bladder cancers), and are associated with low-grade and low-stage tumors.28 Mutations in FGFR-3 are distinct from p53 mutations, which have a positive correlation with high-grade, high-stage tumors with a high level of angiogenesis (quantified using MVD).28 This increased angiogenesis might be due to the effects of p53 upregulating VEGF and inhibiting thrombospondin.29 The mutations in FGFR-3 and p53 are mutually exclusive, suggesting two discrete pathways.30

Hypoxic regulation of bladder cancer is dominated by HIF-1alpha and its expression is an independent prognostic factor in patients with bladder cancer. Overexpression of HIF-1alpha combined with a mutation in p53 indicates a more aggressive cancer phenotype.31 CAIX and glucose transporter type 1, are both regulated by HIF-1alpha, and have been shown to be independent predictors of poor survival.32 HIF-2 is predominantly expressed in tumor-associated macrophages, which are often located adjacent to areas of necrosis within the tumor, and its expression correlates positively with angiogenesis and cancer progression.33

Thymidine phosphorylase is an independent predictor of survival in patients with transitional-cell carcinoma of the bladder.34 Its expression has a positive correlation with invasion and malignant potential in bladder cancer, and it upregulates VEGF, interleukins and MMPs.

Epidermal growth factor receptor (EGFR) exerts angiogenic effects via stimulation of VEGF and bFGF. EGFR expression is common in tumors, and directly correlates with tumor grade and stage, as well as patient survival.35 Clinical trials in which EGFR is inhibited by monoclonal antibodies or tyrosine kinase inhibitors are ongoing.

COX2 is not expressed in tissues under normal conditions, but is expressed at progressively higher levels with increasing severity of benign inflammatory conditions of the bladder, and in superficial and invasive bladder cancer.36 Other promoters of angiogenesis, such as epidermal growth factor and TRANSFORMING GROWTH FACTOR-alpha (TGF-alpha), which are EGFR's primary ligands,37 angiogenin38 and interleukin (IL) 8 (regulated by nuclear factor kappaB)39 have also been shown to be important (Table 2).


Thrombospondin exerts its effects via inhibition of VEGF and bFGF, and is the most important antiangiogenic factor in bladder cancer. Loss of thrombospondin is a key event in the angiogenic switch, and muscle-invasive bladder tumors that express low levels of thrombospondin show increased tumor recurrence and are associated with reduced patient survival.40 Effects of other inhibitors of angiogenesis, such as angiostatin, endostatin, IL-1 and IL-12, have yet to be demonstrated in clinical studies.

MMP-1 is not excreted in the urine of healthy individuals, but has been positively correlated with higher-grade tumors, more-invasive cancer and cancer-related deaths in patients with bladder cancer.41 Increased expression of MMP-1, MMP-2 and MMP-9, as well as heparanase (another degradative enzyme), are independent predictors of bladder cancer survival,42 and the ratio of these enzymes to their regulatory inhibitors (the tissue inhibitors of matrix metalloproteinases, TIMPs) can predict risk of tumor recurrence.43 Regulation of MMP expression (in particular MMP-2) has been shown to be associated with osteonectin (also known as SECRETED PROTEIN, ACIDIC AND RICH IN CYSTEINE or BM40). Osteonectin has a positive correlation with bladder cancer progression and with increasing grade and stage in a number of tumor types, including bladder cancer.44

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Angiogenesis and kidney cancer

Angiogenesis, as quantified by MVD, has a significant role in renal-cell carcinoma (RCC). It is an independent predictor of survival in patients with RCC, is directly associated with the development of metastases and is closely related to the expression of VEGF.45 The different markers of angiogenesis in kidney cancer are summarized in Table 3.


Mutation in the VHL gene, or loss of VHL expression, occurs in over 80% of clear-cell renal tumors,46 and leads to increased expression of HIF and VEGF. HIF regulates many angiogenic pathways in addition to VEGF8 and, therefore, the strong associations with VEGF might reflect these additional pathways.47 This could explain why the role of VEGF is better validated in RCC compared to other urologic malignancies.48 VEGF is not expressed in renal tissue from patients with benign renal disease, but its expression is increased in patients with conventional, papillary and chromophobe RCC,49 particularly in clear-cell tumors. High VEGF expression is a predictor of unfavorable prognosis, and its expression level is proportional to both the malignant potential of a tumor and to tumor progression, indicating that VEGF has a role as a prognostic marker in RCC.

HIF-1 expression is a predictor of favorable prognosis in patients with RCC.50 This might be because clear-cell cancers express both HIF-1 and HIF-2,51 and because proapoptotic pathways induced by HIF-1 can be downregulated by HIF-2, resulting in more effective functioning of the proangiogenic pathways in tumor growth.52

CAIX expression has also been correlated with VHL mutation, and has a role in the survival of cells under hypoxia.53 HIF-1 upregulates CAIX, which might help to explain why low CAIX expression is a predictor of poor survival in RCC patients.54 High levels of CAIX expression in RCC patients has been correlated with complete response to IL-2 therapy.55 An opposite effect is seen with VEGF, where high levels correlate with IL-2 resistance,56 which might be explained by the immunosuppressive role of VEGF. This highlights the interactions between angiogenesis and potential targets for RCC therapies. EGFR, epidermal growth factor, and TGF-alpha are overexpressed in RCC, upregulated by the HIF pathway and associated with high tumor grade. A phase II study investigated the efficacy of the anti-EGFR antibody cetuximab in 55 patients with metastatic RCC. Results were disappointing, as no patients achieved either a partial or complete response to the treatment.57

Not all angiogenic pathways described in clear-cell cancer seem to be regulated by HIF; expression of bFGF and its receptor FGFR-1 have been positively correlated with higher tumor stage and grade, as well as development of metastases.58 bFGF can be detected in the serum of patients, and elevated levels of this marker are associated with larger tumors.59 A serum level of greater than 3.0 pg/ml of bFGF has been shown to be associated with a worse prognosis, compared with that of patients with a serum level of less than 3.0 pg/ml.58 PLACENTAL GROWTH FACTOR and TP have also been shown to be independent prognostic factors in patients with clear-cell cancer.60, 61

Mutant p53 does not correlate with MVD or VEGF;62 it is, however, a significant predictor of tumor recurrence63 and downregulates thrombospondin, which is thought to increase angiogenesis and decrease apoptosis.64

Other pathways interact with VEGF or are coexpressed, providing a complex angiogenic environment. This might be why single agents that inhibit only one pathway of angiogenesis do not seem to result in tumor regression. COX2 is overexpressed in RCC, but less dramatically when compared to bladder and prostate tumors.65 COX2 expression correlates linearly with MVD and MMP-2 expression, and is an independent risk factor for large renal tumor size (>7 cm).66 MMP-9 and VEGF have synergistic effects. MMP-2 and MMP-9, and their relationships with E-cadherin, TIMP-1 and TIMP-2, also have a significant role in the development and progression of RCC.67, 68

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Angiogenesis and prostate cancer

The significance of angiogenesis in prostate cancer is well established. Many studies have now demonstrated its direct correlation with Gleason score, tumor stage, progression, metastasis and survival.69, 70 Angiogenesis is not associated with serum PSA level, which might reflect the ability of PSA to convert plasminogen to angiostatin-like fragments, possibly contributing to the slow growth of prostate tumors.71

VEGF expression is low in normal prostate tissue, but markedly increased in tumor tissues, and has a positive association with MVD, tumor stage and grade, and disease-specific survival in patients with prostate cancer.72 The coexpression of VEGF with MMP-2 and MMP-9 further increases the malignant potential of prostate tumors,73 and levels in serum increase significantly from those seen in benign prostatic disease, through organ-confined prostate cancer, to metastatic disease.74 Although HIF-1 is a key mechanism for VEGF regulation, to date, expression of HIF-1 has not been well studied. It is, however, known that HIF-1 is upregulated in the majority of prostate tumor tissues and its expression is induced in prostate cancer in situ.75

Biopsy of the prostate, both before and after ablative endocrine therapy, is an important investigative method in prostate cancer. It has been shown (in human studies of prostate cancer) that complete androgen blockade downregulates VEGF expression (possibly via inhibition of HIF-1) with concomitant upregulation of thrombospondin and induction of endothelial-cell apoptosis.76, 77 High VEGF expression seen in neuroendocrine cells is independent of this downregulation and, therefore, might be important in the development of hormone-refractory disease.78

Meyer et al. reported that serum bFGF had a high sensitivity in the detection of carcinoma in patients with serum PSA levels below 4.0 ng/ml, and in differentiating between patients with local and advanced malignancy, but further study is needed.79 Serum bFGF and VEGF were, therefore, not helpful in differentiating between benign and malignant conditions affecting the prostate.80 In general, bFGF alone seems to have a less significant role in prostate cancer than in the other urologic malignancies,81 although its synergy with VEGF is reflected by its association with poorer prostate cancer survival.82

Tumor-associated macrophages aggregate in areas of hypoxia within tumors and have an important role in angiogenesis. They induce changes in the extracellular membrane as well as encouraging migration and proliferation of endothelial cells.83 Tumor-associated macrophages are present in markedly increasing numbers as the malignant and metastatic potential of tumors increases, and are directly associated with increased MVD.84 These macrophages are also linked to increased expression of TP, contributing to further angiogenesis and aggressive growth of tumors.85

There are other factors (such as IL-8) that have a further enhancing effect on angiogenesis, and which act partly by the modulation of VEGF in different cell types.86 Expression of IL-8 correlates directly with Gleason score, tumor stage and MVD,87 and its serum levels are elevated in patients with metastatic disease.88 Similarly, TISSUE FACTOR levels directly correlate with angiogenesis, as well as with preoperative serum levels of PSA.89 Endoglin, a receptor for TRANSFORMING GROWTH FACTOR-beta (TGF-beta) that is commonly expressed on newly formed blood vessels, has a direct association with MVD, Gleason score, tumor stage and metastatic spread of disease.90 Additional factors that are thought to enhance angiogenesis in prostate cancer include TUMOR NECROSIS FACTOR, insulin-like growth factor 1 and IL-6, but these have yet to be confirmed in clinical studies.91 The different markers of angiogenesis in prostate cancer are listed in Table 4.


Mutations in p53, or loss of p53 expression, lead to decreased thrombospondin-1 production, but do not affect VEGF expression in prostate tumors.92 Mutations in the TP53 gene are associated with increased MVD, and higher stage and grade of prostate cancer.93

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Conclusion

In this review we analyzed 568 papers to select those cited here. It is apparent that there are generic problems in translating new discoveries into clinical relevance, as exemplified here. Assays are often not validated (e.g. MVD, VEGF expression), or studies are too small, heterogeneous or without sufficient follow-up for statistical significance to be reached. The Cancer Diagnosis Program of the National Cancer Institute has proposed guidelines for future studies, which should facilitate collection of reliable data.94

Many studies have now been published on gene-expression arrays in urologic cancers that contain data on expression of proangiogenic and antiangiogenic factors. Further studies based on these data should become a rich resource to analyze issues that we could not cover in this review, such as the expression of multiple factors, pathophysiologic changes during tumor progression, multiple pathways and genes that might control them.95, 96, 97

It is apparent that only a few angiogenic pathways have been investigated to date, but already some have emerged as targets for anticancer therapy. VEGF blockade by the monoclonal antibody bevacizumab has shown a clear dose–response effect in renal cancer,98 and conventional antiandrogen therapy switches off this pathway in prostate cancer. These findings will provide the basis for combination therapies in the near future.99, 100, 101 This review shows that tumors express multiple angiogenic pathways, and some of the new-molecule inhibitors under investigation might have greater efficacy than those that are currently available.

A key issue in the future will be the identification of patients for whom a particular angiogenic pathway could be a therapeutic target; none of the angiogenesis studies so far reported can do this. Analysis of randomized trials with some of the markers described above, however, might help to individualize therapy, which is likely to be the only cost-effective way to use such agents.

Key points

  • Angiogenesis is essential for bladder, kidney and prostate cancer development

  • Switch to an angiogenic phenotype occurs early in tumor development, and is stimulated by hypoxia, acidosis, inflammation and immune-cell activation

  • Hypoxic stimulation leads to activation of many genes that promote tumor development through processes such as angiogenesis, glucose metabolism, cellular proliferation and pH regulation

  • There are many proangiogenic factors, which include secreted proteins, transcription factors, cytokines, oncogenes, trace elements, proteases, enzymes and low-molecular-weight mediators

  • There is clear evidence that the markers of angiogenesis (such as microvessel density) and expression of angiogenic factors (such as vascular endothelial growth factor) are related to adverse outcomes in bladder, kidney and prostate cancer

  • Investigation into the many angiogenic and antiangiogenic pathways has been limited, and further study is essential to define the importance of these pathways and to develop selection criteria for patients who are candidates for antiangiogenic therapies

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Acknowledgments

Research carried out by AL Harris was funded by Cancer Research UK.

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Competing interests

The authors declared no competing interests.

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Subject areas under which this article appears: Urologic oncology (nonprostatic) | Prostate cancer