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© 2002 American Society for Clinical Oncology
Vascular Permeability Factor/Vascular Endothelial Growth Factor: A Critical Cytokine in Tumor Angiogenesis and a Potential Target for Diagnosis and TherapyByFrom the Department of Pathology, Beth Israel Deaconess Medical Center, and Department of Pathology, Harvard Medical School, Boston, MA. Address reprint requests to Harold F. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215; email: hdvorak{at}caregroup.harvard.edu
ABSTRACT: Vascular endothelial growth factor A (VEGF-A), the founding member of the vascular permeability factor (VPF)/VEGF family of proteins, is an important angiogenic cytokine with critical roles in tumor angiogenesis. This article reviews the literature with regard to VEGF-A’s multiple functions, the mechanisms by which it induces angiogenesis, and its current and projected roles in clinical oncology. VEGF-A is a multifunctional cytokine that is widely expressed by tumor cells and that acts through receptors (VEGFR-1, VEGFR-2, and neuropilin) that are expressed on vascular endothelium and on some other cells. It increases microvascular permeability, induces endothelial cell migration and division, reprograms gene expression, promotes endothelial cell survival, prevents senescence, and induces angiogenesis. Recently, VEGF-A has also been shown to induce lymphangiogenesis. Measurements of circulating levels of VEGF-A may have value in estimating prognosis, and VEGF-A and its receptors are potential targets for therapy. Recognized as the single most important angiogenic cytokine, VEGF-A has a central role in tumor biology and will likely have an important role in future approaches designed to evaluate patient prognosis. It may also become an important target for cancer therapy.
VASCULAR PERMEABILITY factor (VPF)/vascular endothelial growth factor A (VEGF-A) is the founding member of a family of closely related cytokines that exert critical functions in vasculogenesis and in both pathologic and physiologic angiogenesis and lymphangiogenesis. Discovered in the late 1970s as a tumor-secreted protein that potently increased microvascular permeability to plasma proteins,1 VEGF-A is a multifunctional cytokine that exerts a variety of effects on vascular endothelial cells that together promote the formation of new blood vessels. Thus, in addition to rendering microvessels hyperpermeable, VEGF-A stimulates endothelial cells to migrate and divide, profoundly alters their pattern of gene expression, and protects them from apoptosis and senescence.2-5 Of course, there are many angiogenic factors, and the current interest in VEGF-A is attributable to several of its unique properties:
As noted above, VEGF-A is a member of a family of dimeric glycoproteins that belong to the platelet-derived growth factor (PDGF) superfamily of growth factors. Other members of the VEGF family include VEGF-B, VEGF-C, VEGF-D, and VEGF-E and placenta growth factor (PlGF). These have thus far been less well studied than VEGF-A, and the interested reader is referred to several recent articles for additional information.11,15-18 Only a few relevant facts concerning other members of the VEGF family are mentioned here. Receptors to which the several members of the VEGF family bind are summarized in Fig 1. PlGF is, as its name implies, strongly expressed in placenta and is thought to have an accessory role in pathologic angiogenesis, serving to potentiate the activity of VEGF-A.16 Mice lacking PlGF are apparently otherwise normal. VEGF-B knockout mice are viable but have small hearts, suggesting that this cytokine has a role in coronary artery development.19 VEGF-C binds to VEGFR-3 (Fig 1), expressed on lymphatic endothelium, and has been implicated in lymphangiogenesis. Like VEGF-C, to which it is structurally related, VEGF-D is an endothelial cell mitogen and interacts with VEGRFR-2 and VEGFR-3. VEGF-E, encoded by the ORF virus, induces angiogenesis through an interaction with VEGFR-2.20
VEGF-A is overexpressed by the vast majority of important solid human cancers (reviewed in2-4) and, increasingly, has been found to be of importance in lymphomas and a variety of hematologic malignancies.21-27 VEGF-A is overexpressed not only by invasive cancer cells but also by at least some premalignant lesions (eg, precursor lesions of breast, cervix, and colon cancers); furthermore, expression levels increased in parallel with malignant progression.28-30 An association between VEGF-A expression and benign tumors is less well established, in part because benign tumors have been less carefully studied in this regard. Pituitary adenomas31 and benign hemangiomas rarely overexpress VEGF-A, whereas more malignant vascular tumors and leiomyomas of the uterus do so.32,33 Finally, although the malignant cells themselves are primarily responsible for VEGF-A expression in tumors, stromal cells and even vascular endothelium may also express VEGF-A, although in lesser amounts, especially at sites of hypoxia.28,34-37
VEGF-A is a highly conserved, disulfide-bonded dimeric glycoprotein of 34 to 45 kd.2-5 It shares low but significant sequence homology with PDGF and, as with PDGF, cysteines form interchain and intrachain bonds that are integral to structure. The two chains that constitute VEGF-A are arranged in antiparallel fashion and the crystal structure and identification of receptor-binding sites (two per molecule) have recently been solved.37 On reduction, VEGF-A separates into major bands of approximately 17 to 23 kd and loses all of its biologic activity. The VEGF-A gene is located on the short arm of chromosome 6 and is differentially spliced to yield several different isoforms, the three most prominent of which encode polypeptides of 189, 165, and 121 amino acids in human cells (corresponding murine proteins are one amino acid shorter).2,5,38,39 The coding region of VEGF-A is composed of eight exons, the first of which encodes a hydrophobic leader sequence, typical of secreted proteins. The 189–amino acid VEGF-A isomer is encoded by all eight exons. The shorter isoforms include the first five exons as well as exon 8. VEGF-A165, most often the predominant isoform, lacks the residues encoded by exon 6 and VEGF-A121 lacks those encoded by both exons 6 and 7. Very recently, an additional form of VEGF-A has been described that acts selectively on endocrine vascular endothelium.40 The several VEGF-A isoforms encoded by alternative splicing have distinct physical properties. VEGF-A121 is freely soluble and does not bind heparin. By contrast, isoforms 165 and 189 have increasing basic charge and bind heparin with increasing affinity; in fact, VEGF-A165 was originally purified on the basis of its affinity for heparin.9,41 Heparin, heparan sulfate, and heparinase all displace the larger VEGF-A isoforms from proteoglycan-binding sites; plasmin cleavage also activates these isoforms by freeing them from cells or matrix.42 Despite these physical differences, the several VEGF-A isoforms apparently have identical biologic activities when free in solution. Evidence supporting the activation of VEGF-A by freeing it from the bound state comes from recent studies of pancreatic islet cell tumors that develop under the control of the SV40 T antigen (so-called RIP-tag tumors). Invasive tumor cells and normal islet cells made roughly equivalent amounts of VEGF-A; however, angiogenesis was initiated only at a stage when islet cells expressed metalloproteases that liberated VEGF-A from matrix.43 Therefore, in this and likely other instances, proteases are required to "pull the angiogenic switch."
VEGF-A mediates its effects by interacting with two high-affinity transmembrane tyrosine kinase receptors that are selectively though not exclusively expressed by vascular endothelium (Fig 1). Both VEGFR-1 (Flt-1) and VEGFR-2 (KDR, flk-1) are overexpressed in the vasculature of tumors that express VEGF-A (Fig 2C and 2D) 44 and have seven immunoglobulin-like extracellular domains and a kinase insert sequence.45,46 In various cultured endothelial cells, VEGFR-1 has a Kd of 1 to 20 pmol/L and a frequency of approximately 3,000 copies per cell; VEGFR-2 binds with a Kd of 50 to 770 pmol/L with approximately 40,000 copies per cell. Receptor frequency may vary in different tissues in vivo. A truncated soluble form of VEGFR-1 that results from alternative splicing is found in serum and retains VEGF-A–binding activity.47,48 Mice null for either VEGFR-1 or VEGFR-2 are embryonic lethals.49,50 Unlike VEGF-C and VEGF-D, VEGF-A does not bind to VEGFR-3 (Fig 1).
Recently, a nonkinase receptor, neuropilin (NRP-1), has been found to potentiate VEGF-A’s binding to VEGFR-2; previously, neuropilin had been shown to mediate guidance of neurite growth cones.14 Neuropilin is less selectively expressed on vascular endothelium than VEGFR-1 and VEGFR-2, and its role in tumor angiogenesis is only now being investigated.51,52
On binding to its receptors, VEGF-A initiates a cascade of signaling events that begins with autophosphorylation of both receptor tyrosine kinases, followed by activation of numerous downstream proteins including phospholipase C
Earliest Activity In Vivo: Increased Microvascular Permeability VEGF-A was originally discovered because of its ability to increase the permeability of microvessels, primarily postcapillary venules and small veins, to circulating macromolecules. The ability to enhance microvascular permeability remains one of VEGF-A’s most important properties, and the nearly universal hyperpermeability of tumor blood vessels to plasma proteins is largely attributable to tumor cell expression of VEGF-A (Fig 3a). In fact, VEGF-A is one of the most potent vascular permeabilizing agents known, acting at concentrations below 1 nmol/L, and with a potency some 50,000 times that of histamine.1,9,57 VEGF-C, also expressed by some tumors, does enhance microvascular permeability, but less potently than VEGF-A.58,59
Increased microvascular permeability is VEGF-A’s first and most distinctive biologic activity in vivo, becoming evident within 1 minute of injection. Induction of vascular permeability is likely an essential first step in angiogenesis, as all examples of angiogenesis thus far studied, both physiologic and pathologic, are characterized by increased microvascular permeability.2-4,57,60 Increased microvascular permeability results in leakage of plasma proteins, including fibrinogen and other clotting proteins. The clotting system is rapidly activated by means of the tissue factor pathway61,62 and results in deposition of extravascular fibrin in tumor stroma (Fig 3b). Fibrin is a water-retaining gel whose deposition in tissues has at least two consequences: (1) fibrin transforms the normally antiangiogenic stroma of normal tissues into provisional stroma that is strongly proangiogenic8,60,62,63; and (2) coupled with the apparent lack of lymphatics in tumors, fibrin retards clearance of edema fluid, resulting in increased interstitial pressure, a characteristic feature of most solid tumors.64 Although relatively innocuous in some locations, increased intracranial pressure may be disastrous, leading to brain herniation and death.65,66 Tumor-secreted VEGF-A causes microvascular permeability and plasma leakage in the peritoneal and other body cavities, leading to ascites accumulation that can reach massive proportions (many liters). Such accumulations of fluid are not uncommon in patients with primary ovarian carcinoma or metastatic breast cancer; consequences such as impaired breathing and electrolyte imbalance are well known. Immunoreactive and bioactive VEGF-A protein is commonly found in these malignant fluid accumulations, sometimes in high concentrations.67,68 Although there is now general agreement as to VEGF-A’s potency in enhancing microvascular permeability, there is debate as to the pathways that plasma proteins and other circulating macromolecules follow in extravasating from vessels. Work from our laboratory has consistently indicated that, in response to VEGF-A, macromolecules cross the endothelium predominantly by means of a transendothelial cell pathway that involves vesiculovacuolar organelles (VVOs); this is true both in tumor vessels and in the venules of several normal tissues.57,69-71 VVOs are interconnected chains of uncoated cytoplasmic vesicles and vacuoles that are abundant in the endothelial cells of normal venules and of many tumor vessels and that span the entire thickness of endothelial cell cytoplasm from lumen to ablumen (Fig 4a).72 The individual vesicles and vacuoles that constitute VVOs interconnect with each other and with the endothelial cell plasma membrane by stomata that are normally closed by thin diaphragms. In normal adult tissues, extravasation of circulating macromolecules from venules and small veins is quite limited but, to the extent that it occurs, takes place by way of VVOs. In tumor vessels, or in normal skin following local injection of VEGF-A, the stomata open to allow macromolecular tracers to pass through interconnecting vesicles and vacuoles and thereby cross the endothelial cell cytoplasm from the vascular lumen to the ablumen; thus, VVOs provide a pathway whereby plasma and plasma proteins may exit the circulation and enter the tissues. When the interconnections between VVO vesicles and vacuoles are fully opened, these structures form transendothelial cell pores.73-75 VEGF-A also induces endothelial fenestrations that provide an additional transcellular pathway for solute extravasation (Fig 4c).57,76 In contrast, not everyone accepts this formulation, arguing instead that macromolecules extravasate predominantly through an interendothelial cell pathway (ie, by an opening of the junctions between adjacent endothelial cells).77
Subsequent Biologic Activities on Vascular Endothelium VEGF-A is a multifunctional cytokine that exerts many effects on vascular endothelium that only become apparent over the course of hours to days and that critically influence the angiogenic response. These include striking changes in cell morphology and cytoskeleton accompanied by stimulation of endothelial cell migration and division. At a molecular level, VEGF-A reprograms endothelial cell gene expression, leading to increased expression of a number of different proteins, including the procoagulant tissue factor, proteins associated with the fibrinolytic pathway (urokinase, tissue-type plasminogen activator, type 1 plasminogen activator inhibitor, urokinase receptor), matrix metalloproteases, the GLUT-1 glucose transporter, nitric oxide synthase, numerous mitogens, and a number of antiapoptotic factors (eg, bcl-2, A1, survivin, XIAP).2-5,78 VEGF-A also serves as an endothelial cell survival factor, protecting endothelial cells against apoptosis,79 and it delays and may reverse endothelial cell senescence.80 Of considerable interest, tumors that overexpress VEGF-A are, so far without exception, supplied by microvessels whose lining endothelial cells overexpress both VEGFR-1 and VEGFR-2 (Fig 1C and 1D). 2,4,28,29,81-87 The mechanisms relating VEGF receptor overexpression to that of their ligand are largely unknown. However, there is evidence that chronic exposure to high levels of VEGF-A enhances the expression of both VEGF-A receptors on microvascular endothelial cells in vivo, as demonstrated in transgenic mice that overexpress VEGF-A in the skin under the control of a keratin promoter88 and also in mice injected in several different tissues with an adenovirus engineered to overexpress VEGF-A (see below).72
Effects of VEGF-A on Cells Other Than Vascular Endothelium VEGF-A protein is readily detected in tumors by immunohistochemistry.2,3,97 Of interest, antibodies to VEGF-A often stain not only tumor cells but also tumor-associated blood vessels, even though the latter do not themselves express VEGF-A.2,97,98 Indeed, tumor microvessel-associated VEGF-A has been localized to the endothelial cell plasma membrane and to intraendothelial cell VVOs by electron microscopic immunocytochemistry.99 Although these observations make sense in that tumor blood vessels are thought to be the primary target of tumor-secreted VEGF-A, it is nonetheless a surprising finding in that immunohistochemistry is a relatively insensitive detection method and indicates the presence of much larger amounts of VEGF-A than would be required to activate microvascular endothelium. The large VEGF-A accumulations found in tumor vessels likely indicate that these vessels provide a "sink" for binding and retaining VEGF-A locally, preventing the toxic side effects that would result if it were to disseminate widely.
Although constitutively expressed by many tumor cells and transformed cell lines, VEGF-A expression is subject to a number of control mechanisms.
Oxygen Concentration The importance of hypoxic regulation of VEGF-A expression for tumor malignancy and tumor angiogenesis is subject to debate. Many tumors express VEGF-A at high levels even under normoxic conditions.2-4 Also, although it is true that many tumors express large amounts of VEGF-A immediately adjacent to zones of necrosis, the significance of such local expression is uncertain. Zones of tumor necrosis are generally distant from the nearest functioning blood vessels and therefore from potential sites of VEGF-A action; to exert an effect, VEGF-A would have to traverse significant distances, an unlikely possibility given the slow diffusion kinetics of macromolecules in solid tissues.106
Oncogenes and Tumor Suppressor Genes
Cytokines and Other Mediators
Hormones
Other Agents
Because tumors are complex entities that express many different growth factors and cytokines, it has been difficult to determine definitively which are specifically responsible for the growth of tumor blood vessels and, conversely, to determine the types of blood vessels that a single cytokine such as VEGF-A might induce on its own. Recent progress has been made with regard to the second of these questions in studies that made use of a nonreplicating adenoviral vector engineered to express VEGF-A.72 When injected into mouse tissues, the virus infected host cells and these expressed VEGF-A at steady-state levels for approximately 2 weeks. Secreted VEGF-A induced a highly reproducible, time-ordered sequence of events that was qualitatively similar in all tissues studied (Fig 5). Within hours of vector injection, local microvessels became hyperpermeable to plasma proteins, resulting in tissue edema and deposition of an extravascular fibrin gel. By 18 hours, enlarged, thin-walled, pericyte-poor, strongly VEGF receptor–positive "mother" vessels had formed within this gel. Mother vessels are a characteristic type of blood vessel that is found in many human and animal tumors (Fig 6). They form according to a three-step process: (1) proteolytic degradation of the noncompliant (nonelastic) vascular basement membrane, a step necessary to permit vessel enlargement; (2) detachment of pericytes from basement membrane; and (3) spreading and thinning of preexisting vascular endothelium over the expanded surface area that was generated by basement membrane degradation (Fig 4b). The resulting increased endothelial cell surface area is made possible by rapid incorporation of VVO membranes into the plasma membrane.
Mother vessels are transitional structures that generally persist as such for only a few days or weeks. As in tumors, mother vessels divided into smaller channels by a process that involved the projection of endothelial cell cytoplasmic bridges into and across mother vessel lumens; these transluminal "bridges" divided blood flow into smaller channels that over time became separated from each other, forming individual smaller caliber "daughter" capillaries.72,138 Mother vessels also evolved to form glomeruloid bodies, poorly organized conglomerates of endothelial cells and pericytes that, like mother vessels, are found in human tumors, particularly glioblastoma multiforme.139 Taken together, it is clear that VEGF-A can by itself, or through accessory molecules that it induces, generate the formation of at least two types of blood vessels that are characteristic of tumors, mother vessels, and glomeruloid bodies. There is every reason to think, therefore, that tumors induce these same vessel types by secreting VEGF-A and that VEGF-A is sufficient for their generation.
Tumor Diagnosis, Staging, and Prognosis The amount of VEGF-A expressed by cancer cells has been found to correlate with poor prognosis in many types of tumors, including carcinomas of the breast, kidney, colon, brain, ovary, cervix, thyroid, bladder, esophagus, and prostate, and in osteoid and soft tissue sarcomas and pediatric tumors.122,126,140-157 In all of these reports, the amount of VEGF-A (measured in different studies by immunohistochemistry, in situ hybridization, quantitative immunoassays, Western blotting, or reverse-transcriptase polymerase chain reaction) correlated with one or more of the following prognostic measures: tumor size, metastasis, and shorter tumor-free and overall survival. Also, VEGF-A mRNA levels have been found to correlate with vascular density in some (eg, carcinomas of the cervix, breast) but not all cancers.156,157 Although not as yet exhaustively investigated, it is clear that tumor metastases also overexpress VEGF-A much as do the primary tumors from which they arose.2,4,153 From the above, it is clear that the amount of VEGF-A expressed by tumors affects clinical outcome; however, none of the techniques used to quantitate tumor VEGF-A expression levels in solid tumors is routine and it is uncertain at present whether, if such studies were widely undertaken, they would be clinically useful or cost-effective for predicting outcomes in individual patients. VEGF-A levels can be more conveniently measured in body fluids than in tumor homogenates, and it is well documented that patients with large tumor burdens and widespread metastatic disease have increased levels of circulating VEGF-A as measured by enzyme-linked immunosorbent assay or other assays.68,126,158-166 In these and other studies, high serum VEGF-A levels in cancer patients, often multiples of those found in normal individuals, were generally associated with unfavorable clinical parameters such as disease progression, poor response to chemotherapy, and poor survival. Increasing serum VEGF-A levels may therefore be clinically useful in signifying increased growth, recurrence, or metastatic spread in individual patients. In contrast, VEGF-A serum measurements cannot be expected to be helpful in patients with minimal disease and therefore have no role as a screening tool. Unfortunately, measurements of VEGF-A in blood are not as straightforward as might be expected. Serum VEGF-A levels are difficult to interpret because platelets sequester this cytokine and because plasma alpha-2 macroglobulin binds it and makes it unavailable to at least some antibodies.126,163,166-169 Also, both megakaryocytes and leukocytes synthesize VEGF-A.166,167,170-173 Therefore, serum levels reflect not only VEGF-A of tumor origin but also that released from platelets and leukocytes during the course of blood clotting, making it difficult to establish a range of normal values. In one study,126 the mean VEGF-A plasma level in control subjects was 27 pg/mL, whereas in serum from the same individuals after blood clotting it was 172 pg/mL. Because of this complication, some authors have recommended that VEGF-A be measured in plasma.126,167 This also may not be entirely satisfactory because plasma VEGF-A levels represent equilibrium between free VEGF-A and that sequestered within platelets. Also, as the result of therapy or for other reasons, the blood cells of cancer patients may synthesize increased amounts of VEGF-A, and platelet activation is common in cancer and in other systemic disease states, potentially leading to increased levels of plasma VEGF-A unrelated to that synthesized by tumors. An additional factor to be considered is whether circulating VEGF-A is elevated in other diseases associated with angiogenesis and local overproduction of VEGF-A (eg, inflammatory diseases such as rheumatoid arthritis and psoriasis)2-4 (for a more complete discussion of VEGF-A measurements and their significance, see174). For the present, we must agree with George et al,163 who state that "definitive proof of the relationship of circulating VEGF levels to tumor angiogenesis, malignant potential, surgical cure, or responses to therapy has yet to be established in any study." Nonetheless, further work may demonstrate that circulating VEGF-A levels are of value in monitoring individual cancer patients who have undergone treatment or who are being considered for additional therapy. As noted above, VEGF-A has also been measured in other body fluids. VEGF-A levels are markedly elevated in malignant pleural effusions caused by ovarian, gastric, and colon cancers67,68,175,176 and in malignant as opposed to benign ovarian cysts.177 The sensitivity and specificity of urinary levels of VEGF-A for diagnosing primary or recurrent bladder cancer have been claimed to be superior to cytology.178
Potential Therapeutic Applications Related to VPF/VEGF VEGF-A’s importance in initiating tumor angiogenesis has made it a target of therapies designed to diminish the tumor vasculature. Human and animal tumors express and secrete large amounts of VEGF-A, and these accumulate on tumor blood vessels in amounts that apparently far exceed those needed for endothelial cell signaling.97,98,188 Moreover, systemically administered antibodies to VEGF-A accumulate selectively in tumor vessels in much higher concentrations than in normal tissues.189 There is strong evidence that neutralizing antibodies against VEGF-A, and antibodies that block VEGF-A receptors, effectively retard tumor growth and may reduce tumor size in mice, effects that are clearly mediated through inhibition of angiogenesis.184,190-198 More recently, antibodies have been developed that selectively recognize the complex that VEGF-A forms with one of its receptors (VEGFR-2) on vascular endothelium.199,200 These antibodies could be especially useful because they selectively target tumor vessels in which VEGF-A is receptor-bound and avoid side effects that might result from inactivation of free VEGF-A.201,202 Other approaches for inhibiting VEGF-A activity provide further proof of principle. These include withdrawal of androgen from mice bearing tumors that are dependent on this hormone for stimulating VEGF-A production and downregulation of VEGF-A transcription by administration of tetracycline to mice whose tumors have been engineered to repress VEGF-A transcription when this antibiotic is present.79,203,204 An approach that may ultimately be of great clinical importance is that afforded by small, orally available molecules that selectively block or prevent activation of VEGF-A’s receptor tyrosine kinases. Several of these with activity against VEGFR-2 have been found to inhibit the growth of murine tumors.205-207 Other types of small molecules may also have value in inhibiting VEGF-A–mediated tumor angiogenesis. Recently, it was found, quite unexpectedly, that dopamine and other related catecholamine neurotransmitters that interact with the dopamine D2 receptor selectively inhibit VEGF-A–induced angiogenesis and inhibit the growth of tumors that express VEGF-A.208 Dopamine is best known clinically for its use in the treatment of Parkinson’s disease, but recently dopamine receptors, including the D2 receptor, have been found on vascular endothelium. At nontoxic doses, dopamine inhibited all VEGF-A activities tested including stimulation of microvascular permeability and endothelial cell proliferation and migration. These results are of interest for a second reason in that they reveal a new link between the nervous system and angiogenesis. Despite the current wave of enthusiasm for therapies designed to inhibit tumor angiogenesis or to destroy existing tumor blood vessels, there are a number of reasons for caution. First, even if therapies directed against VEGF-A are effective initially, tumors may escape from inhibition after a time as they mutate to express other angiogenic growth factors.204 Second, strategies directed against the tumor vasculature, although successful in reducing tumor bulk, typically leave behind a viable rim of tumor cells at the tumor-host interface, a zone that is supplied by the normal host vasculature.209 This rim of viable tumor must be dealt with to prevent regrowth. Third, recent evidence suggests that p53-deficient tumors may be less responsive to antiangiogenic therapy than tumors in which p53 is functioning because the former are less dependent on the generation of a new vasculature than their genetically normal counterparts.210 p53 is of course mutated in a large fraction of human cancers. Finally, the goal is not to cure mouse cancer but human cancer. Despite the spectacular successes reported in the treatment of mouse tumors, clinical trials to date have for the most part been discouragingly negative. This is disappointing but perhaps not surprising in that most of the patients treated thus far have had advanced disease and had already failed conventional treatments. Also, antiangiogenesis therapy differs fundamentally from chemotherapy, and it may be some time before physicians learn how best to implement it optimally. On a more hopeful note, phase I and II studies with the humanized Genentech (South San Francisco, CA) monoclonal antibody against VEGF-A have shown promise in the treatment of several human cancers.211 However, this encouraging news must be tempered by side effects that occur in some patients, including hypertension, thrombosis, proteinuria, and fatal hemorrhage.212 Taken together, antiangiogenesis, antitumor vasculature and, more specifically, anti–VEGF-A therapies are in their infancy and it is too early to assess their ultimate potential for treating human cancer. Although it seems at present unlikely that these therapies will be effective as standalone therapies, they may well have great value as adjuvant therapies, allowing reduced dosage of toxic conventional treatments such as radiation and chemotherapy.186,187,196,213,214 Further results with antibodies, tyrosine kinase inhibitors, and other small molecules now in development or in early stages of testing are awaited with considerable interest.
Supported by United States Public Health Service, National Institutes of Health grant nos. CA-50453, CA-58845, and HL-54519; by the BIH Pathology Foundation, Inc; and under terms of a contract from the National Foundation for Cancer Research.
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ABSTRACT
INTRODUCTION
, PI3-K, GAP, the Ras GTPase-activating protein, MAPK, and others (reviewed in
), is degraded rapidly under normoxic conditions by the ubiquitin pathway; however, when stabilized by hypoxia, HIF-1
