document: the generation of a humanized monoclonal antibody ( mab ) that inhibits the interaction of vegf - a with its receptors vegfr-1 and vegfr-2 and the development of small - molecule protein tyrosine kinase ( ptk ) inhibitors to the cytoplasmic regions of the vegf receptors have provided avenues to explore the efficacy of targeting the vegf - a signaling pathway in cancer . 
   extensive mouse tumor models developed through the 1990s led to the success of the vegf - a mab known as avastin ( or bevacizumab ) in treating a range of human cancers including colorectal cancer , lung cancer , and renal cell carcinoma . 
   however , the recent withdrawal of avastin for treating breast cancer in the united states has highlighted the limitations of anti - angiogenic treatments , as well as the need for a careful cost - benefit analysis to ensure that scarce healthcare resources are well spent in the treatment of cancer . while avastin is clearly beneficial in a subset of cancer patients , the development of resistance to avastin and 
   the pre - existing refractoriness of some tumors suggest the role of other growth factors in sustaining tumor angiogenesis . 
   alternative ligands ( other than vegf - a ) for the angiogenic receptor vegfr-2 ( i.e. , vegf - c and vegf - d ; see below ) and other angiogenic factors from distinct growth factor families [ e.g. , fibroblast growth factor ( fgf ) ] are logical candidates for promoting resistance to avastin . in addition , 
   other mechanisms , such as vascular mimicry , the involvement of endothelial precursor cells ( epcs ) or altered tumor metabolism leading to reduced oxygen requirement , may also contribute to the development of resistance . 
   pigf is a ligand for vegfr-1 , which has been reported to play a dual role in cancer as a stimulator of angiogenesis and as an autocrine or paracrine factor promoting proliferation of tumor cells expressing vegfr-11 , . 
   it has also been proposed that pigf may modulate the effect of vegf - a by displacing vegf - a from vegfr-1 expressed on endothelial cells . 
   a range of animal tumor models have provided supportive or contradictory data on the role of pigf in driving tumor angiogenesis and tumor growth , . 
   these findings suggest a context - dependent activity for pigf in cancer , either due to the presence of other vegf family members or the diverse expression of vegf receptors on various cell types including endothelial cells , tumor cells , or associated immune cells , . 
   while vegf - b , a ligand for vegfr-1 , has been shown to be expressed in human cancer , its functions in arteriogenesis and in fatty acid uptake by cells have highlighted its role in other pathologies , predominantly cardiovascular disease . 
   recently , vegf - b was shown to regulate endothelial cell uptake and transport of fatty acids in muscle , leading to the hypothesis that vegf - b antagonists could be used as novel pharmacologic agents in the treatment of type 2 diabetes . 
   although inhibitory antibodies against vegf - b have been developed , so far there is little data on the role of this growth factor in tumor biology ; it was reported , however , that overexpression of vegf - b in a mouse model of pancreatic cancer suppressed tumor growth , highlighting the differences in biological functions between the vegf family members . 
   vegf - c and vegf - d form a subfamily within the vegf family , consisting of highly related polypeptides that require post - translational cleavage of the n- and c - terminal propeptides to generate mature forms with enhanced binding affinities for vegfr-2 and vegfr-3,. notably , vegfr-3 is expressed on lymphatic endothelium , where it can promote lymphangiogenesis , and it is also expressed on angiogenic blood vessels . 
   vegf - c is expressed in a range of human cancers , including solid tumors such as nasopharyngeal , liver and gastric cancers , with a preponderance in asian populations . 
   mouse tumor studies have displayed the ability of vegf - c to promote tumor angiogenesis and lymphangiogenesis in vivo and to drive tumor growth and metastatic spread. given that the mature form of vegf - c is a high affinity ligand for vegfr-2 and that vegf - c is expressed in many human cancers , vegf - c is likely to be an alternative ligand to vegf - a for vegfr-2binding , which could in turn promote tumor angiogenesis . 
   therefore , vegf - c , in combination with anti - angiogenic drugs such as avastin , may be a viable target for anti - cancer therapy . 
   the growth factor vegf - d is closely related in structure to vegf - c , and it includes a central vegf homology domain ( vhd ) related to other vegf family members , with n- and c - terminal propeptides that can be proteolytically cleaved by enzymes such as proprotein convertases and plasm in , , . 
   vegf - d is expressed in a range of human cancers , , and has been associated with poor patient outcome in some tumor types , . 
   importantly , animal models of cancer have demonstrated that vegf - d can promote tumor angiogenesis and lymphangiogenesis , solid tumor growth , dilation of collecting lymphatic vessels , and lymphatic and distant organ metastasis. the proteolytic processing of vegf - d is required for promoting tumor growth and spread . 
   opportunities for targeting vegf - d signaling in cancer could involve mabs specific to the vhd of vegf - d that are capable of inhibiting binding to vegfr-2 and vegfr-3 , , . 
   alternatively , ptk inhibitors that block vegfr-2 and vegfr-3 signaling would interfere with vegf - d - mediated signal transduction . 
   further , mabs to vegfr-2 and vegfr-3 , that would prevent the binding of vegf - d , or a soluble form of vegfr-3 that could sequester both vegf - c and vegf - d , could be employed . 
   targeting the vegf - d signaling pathway would likely have the merit of inhibiting both tumor angiogenesis and lymphangiogenesis , which could , in turn , restrict both solid tumor growth and metastatic spread . 
   what have the past 20 years taught us about targeting vegf - a signaling ? clearly , this period has provided biochemists , biologists , and clinicians the time to design and evaluate a variety of agents that modify or inhibit these signaling pathways . 
   the agents that were developed have included a broad range of molecules targeting different components of the pathways , including vegf - a itself , vegf receptors , vegf co - receptors , ptks and signaling intermediates , and transcription factors . during this period , 
   other vegf family members were also identified , enhancing the diversity of signaling induced by the vegf family of ligands . 
   avastin , a humanized mab to vegf - a , has been widely used in a range of prevalent human cancers over the past 8 years , typically in combination with cytotoxic chemotherapy . 
   although this agent has provided significant benefit to cancer patients , there is a need for other drugs that could be combined with avastin to deliver improved clinical outcomes . 
   use of agents targeting other vegf family members , in combination with avastin , may be a potential approach . 
   further , small - molecule ptk inhibitors of vegf receptors ( that are not highly specific ) have been employed although dose - limiting toxicity in combination with cytotoxic chemotherapy has restricted their widespread use . 
   further insights into the structure and function of the ptk domains of vegf receptors may allow development of more specific small - molecule ptk inhibitors . 
   further studies delving into the complex network of signaling cascades that drive angiogenesis , lymphangiogenesis , and resistance to anti - angiogenic drugs such as avastin are clearly required . 
   genome - wide functional approaches such as those using small interfering rna ( sirna ) technology , supported by bioinformatics , could help strategize effective targeting of growth factors , receptors , and ptks . 
   agents specifically targeting vegf family members and their receptors are currently in various stages of development ; mabs to vegf - b , vegf - c , vegf - d , pigf , vegfr-1 , vegfr-2 , and vegfr-3 are being evaluated by the biotechnology and pharmaceutic industries for their efficacy as anti - cancer agents . for these mabs , 
   a key challenge will be identifying specific cancer indications in which clinical benefit can be achieved . 
   given that many cancers are resistant to avastin or develop resistance over the course of treatment , these mabs may be tested in patients whose cancers are avastin - resistant and/or whose cancers express the appropriate target growth factor . 
   hence , assays that quantitate the levels of these growth factors or other relevant biomarkers will be required to identify the appropriate patients , although assessing these biomarkers may not be straightforward . 
   vegf family members are secreted proteins , which may easily allow testing in blood - drawn samples ; however , some family members ( e.g. , vegf - c and vegf - d ) are proteo - lytically processed to generate forms with a different bioactivity . 
   hence , it will be important to fully appreciate which forms are most bioactive and clinically relevant . 
   it is important to note that inhibiting signaling by vegf family members could potentially have pro - tumor effects . 
   recent studies have demonstrated that anti - angiogenic treatments could cause increased tumor metastasis through mechanisms such as the induction of hypoxia ( caused by vegf - a inhibition ) , resulting in increased tumor cell motility , . 
   nonetheless , these issues have been addressed in various tumor models and by comparing the effect of antibodies and small - molecule inhibitors to show that anti 
   clearly , other signaling pathways ( i.e. , not directly involving vegf family members ) such as those driven by members of the platelet - derived growth factor ( pdgf ) , insulin - like growth factor ( igf ) , and fgf growth factor families , exert control on tumor angiogenesis and may be important in promoting tumor growth . other alternative molecular targets , such as c - met , may also be able to affect multiple tumor properties such as metastasis , angiogenesis , tumor growth , and cellular motility upon hypoxia. although not the subject of this review , such pathways may provide valid therapeutic targets for inhibiting angiogenesis and/or lymphangio - genesis in cancer , thereby restricting tumor growth and spread . 
   furthermore , recent studies from our own laboratory have indicated the role of known inflammatory pathways in enhancing the spread of cancer , opening avenues for the use of existing anti - inflammatory drugs in cancer . this possibility is further exemplified by the recent studies published by rothwell et al . 
   , demonstrating the anti - cancer effect of non - steroidal anti- inflam matory drugs ( nsaids ) such as aspirin . while expensive drugs such as avastin allow the treatment of specific cancers in the western world , the capacity to employ relatively inexpensive  off - patent  small molecules ( such as aspirin ) with a defined chemistry and known biological effects could provide a cost - effective and population - based preventative approach to cancer in asian countries .
summary: the vascular endothelial growth factor ( vegf ) family of soluble protein growth factors consists of key mediators of angiogenesis and lymphangiogenesis in the context of tumor biology . 
   the members of the family , vegf - a ( also known as vegf ) , vegf - b , vegf - c , vegf - d , and placenta growth factor ( pigf ) , play important roles in vascular biology in both normal physiology and pathology . 
   the generation of a humanized neutralizing antibody to vegf - a ( bevacizumab , also known as avastin ) and the demonstration of its benefit in numerous human cancers have confirmed the merit of an anti - angiogenesis approach to cancer treatment and have validated the vegf - a signaling pathway as a therapeutic target . 
   other members of the vegf family are now being targeted , and their relevance to human cancer and the development of resistance to anti - vegf - a treatment are being evaluated in the clinic . here , we discuss the potential of targeting vegf family members in the diagnosis and treatment of cancer .