Pathways mediating VEGF-independent tumor angiogenesis

Cytokine & Growth Factor Reviews 21 (2010) 21–26

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Pathways mediating VEGF-independent tumor angiogenesis Napoleone Ferrara * Genentech, Inc., 1 DNA Way, South San Francisco, CA 94080, USA

A R T I C L E I N F O

A B S T R A C T

Article history: Available online 11 December 2009

FDA approval of several inhibitors of the VEGF pathway has enabled significant advances in the therapy of cancer and neovascular age-related macular degeneration. However, similar to other therapies, inherent/acquired resistance to anti-angiogenic drugs may occur in patients, leading to disease progression. So far the lack of predictive biomarkers has precluded identification of patients most likely to respond to such treatments. Recent suggest that both tumor and non-tumor (stromal) cell types are involved in the reduced responsiveness to the treatments. The present review examines the role of tumor- as well as stromal cell-derived pathways involved in tumor growth and in refractoriness to antiVEGF therapies. ß 2009 Elsevier Ltd. All rights reserved.

Keywords: Angiogenesis VEGF Bv8 Myeloid cells Fibroblasts

1. Introduction Angiogenesis is crucial for normal development. In embryonic life, the primary network of vascular endothelial cells is established by a process called vasculogenesis, followed by sprouting from preexisting endothelium and finally by remodeling of the network into mature vasculatures to create an efficient circulatory system [1,2]. It is also well established that angiogenesis is implicated in a number of pathological processes [3]. Several angiogenic activators including members of the VEGF and FGF gene families (reviewed in [4–6]) and various inhibitors of angiogenesis such as thrombospondin, endostatin and tumstatin have been described [7]. In steady-state conditions, the balance between angiogenic activators and inhibitors results in very limited new blood vessel growth in the majority of tissues. However, the balance tilts in favor of the angiogenic stimulators in a variety of proliferative processes. It is now generally accepted that angiogenesis is a rate-limiting process in tumor growth [8,9]. Without new blood vessels to supply nutrients and dispose of catabolic products, tumor cells could not sustain proliferation and thus are likely to remain dormant [10,11]. Furthermore, much evidence links neovascularization with intraocular diseases resulting in blindness such as the wet form of age-related macular degeneration [12]. VEGF and its receptors represent one of the best-validated signaling pathways in angiogenesis [13]. Indeed, the current FDA approved anti-angiogenic agents inhibit the VEGF pathway. These agents include bevacizumab, a humanized anti-VEGF-A monoclonal antibody [14], and two small molecule inhibitors targeting

* Tel.: +1 650 225 2968; fax: +1 650 225 4265. E-mail address: [email protected]. 1359-6101/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cytogfr.2009.11.003

VEGFR2 (in addition to other kinases), sorafenib and sunitinib [15– 18]. However, not all cancer patients benefit from such antiangiogenic therapies, and some that do benefit initially might become less responsive during the treatment as well as showing some adverse effects [16,18,19]. Hence, there is an urgent need to elucidate the mechanisms that mediate resistance to antiangiogenic agents. Tumor cells have been traditionally thought to be the major sources of angiogenic factors [20]. However, much evidence now supports the notion that the stroma also contributes to tumorigenesis not only through secretion of cytokines that stimulate tumor cell proliferation and angiogenesis, but also by modulation of the immune system (reviewed in [21–25]). This article will discuss our current understanding of both tumor- and stromal-derived molecular pathways mediating VEGFindependent tumor angiogenesis. In several cases, significant overlaps occur. 2. Tumor vessels are abnormal Blood vessel proliferation is an essential physiological process [5,26]. Sprouting is one of the major mechanisms of expansion in the network of vessels in the growing tumors through filopodia and endothelial stalk cells [27]. Tumor vessels are distinct in several respects relative to normal vasculature as they are disorganized and tortuous and their spatial distribution is significantly heterogeneous, resulting in uneven drug distribution in the tumors [28]. Tumor vessels do not follow the hierarchy of arterioles, capillaries and venules. In addition, tumor vessels are leakier than normal ones since tumor-associated endothelial cells are widened and loosely connected. Recent studies suggest that tumor endothelial cells have cytogenetical abnormalities including aneuploidy, multiple chromosomes and multiple centrosomes,

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raising the possibility that such instability may contribute to resistance to antiangiogenenic therapies [29]. Pericytes in tumor vessels are abnormal since they may not be in close association with ECs and their cytoplasmic processes might extend to tumor parenchyma [30]. The basement membrane is also different in the tumor vasculature as it is loosely attached to ECs [31]. There is significant debate regarding the origin of the cells contributing to tumor vasculature. Initial studies indicated a role for endothelial progenitor cells (EPCs), originating from the bone marrow, in tumor growth [32]. However, subsequent studies concluded that EPCs are primarily recruited during the early stages of tumor growth but are replaced by locally derived endothelial cells in later stages of tumorigenesis [33]. In addition, circulating endothelial progenitor cells (CEPs) are abundant in tumor rims and their mobilization and recruitment are induced by treatment with vascular disrupting agents (VDA) [34]. Very recently, G-CSF upregulation has been implicated in such events [35]. However, other studies suggest that the contribution of EPC in tumor growth is very limited, if not absent. For example, Gothert et al. did not observe any participation of EPCs in the tumor vasculature using a genetic model of an endothelial-specific inducible gene [36]. Purhonen et al. also failed to document any contribution of EPC to the tumor vasculature [37]. Therefore, further studies are needed to elucidate the contribution of EPC in pathological angiogenesis. 3. Role of VEGF-A in angiogenesis VEGF-A is one of the major regulators of both physiological and pathological angiogenesis [38]. VEGF is a member of a gene family that also includes VEGF-B, VEGF-C, VEGF-D and PlGF [1,13,39]. VEGF-A binds two tyrosine kinase receptors, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1) [13]. The importance of VEGF-A in the development of the vascular system is underscored by early embryonic lethality following inactivation of a single VEGF-A allele [40]. Several studies indicated that VEGF is highly expressed in a variety of human tumors (reviewed in [18,20]). VEGF is a hypoxiaregulated gene due to a binding site for HIF in the promoter [41]. Rapid proliferation of tumor cells and poor blood flow suggest a hypoxia-conducive environment in different areas of tumors [41], resulting in upregulation of VEGF. Expression of VEGF-A was initially detected in a variety of tumor cell lines, while VEGFR-1 and VEGFR-2 were found to be predominantly expressed in endothelial cells [4]. However, more recent studies have shown that VEGF receptors are, in some circumstances, expressed also in tumor cells (reviewed in [20]). Treatment with anti-VEGF antibodies significantly inhibited growth of several tumor cell lines, suggesting that blockade of VEGF alone may substantially suppress tumor growth through inhibition of angiogenesis [42,43]. Bevacizumab, a humanized variant of an anti-VEGF-A neutralizing monoclonal antibody (Mab) [44], is the first antiangiogenic agent to be approved by the FDA for combinatorial treatment with chemotherapy for metastatic colorectal cancer [45], non-small-cell lung cancer [46], metastatic breast cancer [47] and most recently also for glioblastoma multiforme and renal cell carcinoma. However, not all trials have been positive; for example combining bevacizumab to chemotherapy in pancreatic cancer did not result in increased patient survival relative to chemotherapy alone (reviewed in [6]). In addition to inhibiting VEGF-A, strategies aimed at blocking VEGF receptors also result in inhibition of tumor growth [6,18,20]. The VEGF-Trap (Aflibercept), a chimeric soluble receptor containing structural elements from VEGFR1 and VEGFR2, binds not only VEGF-A but also PlGF and VEGF-B and has been shown to inhibit growth in several tumor models [48]. This agent is currently in late-stage clinical trials in several tumors. Most recently, a phase III study in pancreatic cancer patients, testing the VEGF-Trap in

combination with gemcitabine, was stopped following the determination that such treatment would not increase survival relative to gemcitabine alone: (http://newsroom.regeneron.com/ releasedetail.cfm?ReleaseID=408744). These findings suggest that the additional inhibition of PlGF and VEGF-B does not result in enhanced efficacy over VEGF-A blockade, at least in this circumstance. A variety of small molecule receptor tyrosine kinases (RTKs) inhibitors targeting the VEGF signaling pathway is undergoing clinical development: (http://www.cancer.gov/cancertopics/factsheet/Therapy/angiogenesis-inhibitors). Two of such molecules have been approved by the FDA for cancer therapy: sorafenib and sunitinib. Sorafenib is a raf kinase inhibitor that also inhibits VEGFR-2 and -3, PDGFR-b, Flt-3 and c-kit [49]. Sorafenib has been approved by FDA for advanced renal cell carcinoma (RCC) [50] and hepatocellular carcinoma [51]. Similarly, Sutent inhibits several RTKs including VEGFRs, PDGFR, c-kit and Flt-3 and has shown efficacy in advanced RCC [17] and in imatinib-resistant gastrointestinal stromal tumors [52]. 4. Inherent/acquired resistance to anti-VEGF treatment 4.1. Resistance to anti-angiogenic therapies In principle, at least some of the cellular and molecular mechanisms of resistance to anti-angiogenic compounds may be similar to those associated with cytotoxic agents [53]. For example, resistance to cytotoxic or anti-angiogenic agents may arise from a reduction in their bioavailability in the tumors or through upregulation of anti-apoptotic factors. Several mechanisms of inherent refractoriness or acquired resistance to anti-angiogenic agents have been identified in pre-clinical models (reviewed in [54]). Tumor vessels can become less sensitive to anti-angiogenic agents and sustained tumor angiogenesis might occur via VEGFindependent mechanisms [16,18,54,55]. In some cases the tumor, at least initially, coopts existing vessels and thus obviates the need to promote neovascularization [56,57]. Selection of hypoxiaresistant clones that outgrow sensitive clones is thought to be some one of the mechanisms underlying reduced responsiveness to anti-VEGF treatment. For example, lack of p53 in the tumor cells reduced their response to anti-angiogenic combination therapy (anti-VEGFR2 Mab plus vinblastine) while p53+/+ tumor cells were quite sensitive to the same treatment [58]. However, the efficacy of bevacizumab in treating patients with colorectal cancer does not significantly correlate with mutations of k-ras, b-raf, or p53 [59]. Another possibility is that improvements in the potency/ binding affinity of monoclonal antibodies (Mab) blocking VEGF-A may result in greater anti-tumor efficacy. However, increasing binding affinity beyond an optimal level did not significantly increase the efficacy of anti-VEGF-A Mabs in a human VEGF-A knock-in mouse model, but resulted in greater toxicity, especially nephrotoxicity [60]. While VEGF pathway inhibitors have been shown to result in suppression of both primary and metastatic tumor growth in many models (reviewed in [61]), recently the possibility that VEGF inhibition may results in an invasive phenotype, resulting in metastasis, has received considerable attention. Sunitinib and the anti-VEGFR2 antibody DC101 were recently reported to stimulate the invasive behavior of tumor cells despite their inhibition of primary tumor growth and increased overall survival in some cases [62,63]. It is conceivable that increased invasion may result from enhanced expression of various cytokines induced by the treatment [64] or from hypoxia-driven effects, including transcriptional activation of the hepatocyte growth factor receptor, cMet [62,65]. In the case of sunitinib, it is conceivable that disruption of vascular integrity due to pericyte detachment

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mediated by PDGFRbeta inhibition facilitates intravasation of tumor cells [66]. However, pro-metastatic effects of sunitinib were seen when tumor cells were injected intravenously but not in orthotopic models, raising the possibility that the effects may be model- or site-dependent [63]. In this context, a study in an orthotopic genetic model of lung cancer reported that sunitinib treatment does not increase the incidence of metastasis and results in increased overall survival [67]. Recent clinical trial results address the consequences of VEGF inhibition on metastatic disease. Wolmark et al. recently presented the results of a Phase III study testing whether adjuvant mFOLFOX6 plus bevacizumab, administered respectively for six months and one year after surgery, would prolong disease-free survival compared to mFOLFOX6 alone in stage II or III carcinoma of the colon [68]. Although the trial did not meet its primary endpoint of increasing disease-free survival at 36 months, a significant benefit was observed during the interval in which bevacizumab was administered (hazard ratio: 0.6). Importantly, at the 36-month follow-up there were no differences between groups in second primary cancers, 2-year survival after recurrence, or the proportion of patients who had recurrence at multiple sites. These findings appear to argue against the possibility that at least blocking VEGF-A alone promotes metastatic disease in humans. 5. Alternative angiogenic pathways potentially involved in resistance to anti-VEGF tharapy

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mFlt(1–3)-IgG, a high affinity chimeric soluble VEGFR1 variant which blocks VEGF-A, VEGF-B, and PlGF was not more efficacious than anti-VEGF antibodies alone in slowing the growth of EL4 or LLC tumors, which are relatively refractory to anti-VEGF-A therapy [89]. Similarly, an anti-VEGFR-1 antibody did not inhibit tumor angiogenesis or tumor growth in Rip-Tag mice as a single agent nor did it add any benefit when combined with antiVEGFR2 antibodies [90]. Further studies are necessary to investigate the role of PlGF in tumor angiogenesis. VEGF-C and VEGF-D have been primarily characterized for their role in lympangiogenesis by virtue of their interaction with VEGFR3 [91]. However, recent studies have shown that VEGFR-3 is expressed not only in lymphatic vessels but also in the tumor vasculature [92]. Also, antibodies against VEGFR-3 enhanced the anti-tumor activity of anti-VEGFR-2 antibodies, suggesting the possibility that targeting VEGFR-3 may provide additional efficacy for anti-angiogenic therapies targeting VEGF or VEGFR-2 [92]. Neuropilin (NRP)-1 and NRP-2, two molecules initially characterized for their role in axon guidance, have been shown to modulate the VEGF pathway [93]. NRP-1 binds heparin-binding isoforms of VEGF-A and PlGF, resulting in more effective presentation of these ligands to their signaling receptors [93]. Blocking NRP-1 with a blocking antibody was additive to antiVEGF-A antibodies in reducing tumor growth [94]. Also, earlier studies implicated NRP-2 in modulating lymphangiogenesis [95] and subsequently it was shown that VEGF-C and VEGF-D interact with this receptor [96]. Recent studies show that the of targeting NRP-2 inhibits lymphatic metastasis [97].

5.1. Members of the VEGF family 5.2. Fibroblast growth factors Various members of the VEGF gene family have been implicated in incomplete response to VEGF-A blockers. Placenta growth factor (PlGF), a member of VEGF family that binds specifically to VEGFR1 has received recently significant attention [69,70]. VEGFR-1 is expressed not only in endothelial cells but also in monocytes/ macrophages, sub-populations of bone marrow progenitors, and even some tumor cells [71,72]. While abundant evidence supports the notion that most biological effects of VEGF-A are mediated by VEGFR-2 activation [13,73,74], the role of VEGFR-1 in angiogenesis and VEGF-A signaling is incompletely understood. Even though vegfr-1 null mice die in utero [75], mice that express a VEGFR-1 variant lacking the tyrosine kinase domain are viable and have no obvious vascular defects. Thus, it has been postulated that VEGFR1 tyrosine kinase activity is not required for developmental angiogenesis and that, at least in this context, VEGFR-1 acts primarily as a ‘‘decoy’’ receptor, regulating the availability of VEGFA for VEGFR-2 [70,76]. Mapping the ligand binding site in VEGFR-1 to the second Ig-like domain [77] led to the development of high affinity soluble receptors, able to potently antagonize VEGF activities [48,78,79]. However, much recent data indicates that, in a number of circumstances, VEGFR-1 tyrosine kinase signaling plays a role in metastasis and also in pathological angiogenesis [80–84], although not all studies support such conclusions [85]. An important question is the identity of the ligand/s that activate VEGFR-1 signaling in these specific contexts. Upregulation of VEGF and PlGF has been reported in patients treated with anti-angiogenic agents targeting the VEGF pathway [86,87]. This observation suggested the possibility that PlGF may be involved in escape to anti-VEGF therapy. Fischer et al. reported that PlGF inhibition with a neutralizing antibody inhibits primary tumor growth and inhibits metastasis in murine models [88]. Moreover, anti-PlGF antibody treatment was additive with anti-VEGFR2 antibodies in slowing the growth of CT26 tumors, which were found to be poorly responsive to antiVEGFR2 antibodies. However, the role of PlGF in tumor angiogenesis and growth has been unclear. For example,

The fibroblast growth factor (FGF) family has been implicated in neurogenesis, organ development, branching morphogenesis, angiogenesis and various pathologic processes including cancer [98]. The FGF family includes 18 ligands. FGFs interact with 4 main receptors, FGFRs. Both FGFs and FGFRs can be temporally and spatially regulated [98]. While the prototype members, FGF-1 and FGF-2, are devoid of a signal peptide and thus are poorly secreted [99], most members of the family have a signal peptide and are efficiently secreted [98]. FGFs have been reported to promote angiogenesis independent of VEGF [100]. In the RIP-Tag model, FGF-1 and -2 were reported to be upregulated in tumors that relapsed from the anti-VEGFR antibody (DC101) treatment [90]. Treatment with a FGF-trap (FGFR-Fc fusion peptide) was initiated when tumors were at maximum response phase to VEGFR inhibitors. The combinational treatment resulted in slowed tumor growth and attenuated tumor angiogenesis. 5.3. Notch Delta-like ligand 4 Delta-like ligand 4 (DLL4) belongs to the Delta/Jagged family of transmembrane ligands that binds to Notch receptors [101]. Delta– Notch signaling mediates cell–cell communication and regulates cell fate determination. The Delta/Notch signaling is also critically important for proper vascular development [101]. One particular endothelial cell notch ligand, Dll4, was required for regulation of tip cell formation during angiogenesis [102–104]. Activation of Dll/ Notch pathway decreases tip cell numbers. Conversely, decreased Dll4 signaling increase tip cell formation [102–104]. Upregulation of Dll4 was also found in tumor vessels [105]. Two groups have demonstrated independently that inhibiting Dll4 leads to tumor growth suppression by deregulating angiogenesis, resulting in increased, but non-functional vessels [106,107]. Importantly, this strategy is also effective in slowing the growth of tumors that are relative resistant to anti-VEGF therapy and exhibit additive effects with anti-VEGF in slow resistant tumor growth [106,107].

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Tumor- or cancer-associated fibroblasts (TAFs or CAFs) are one of the major stromal elements implicated in tumor growth [23]. TAFs are enriched in PDGFR-a and are recruited to the tumors through gradient of PDGF-A and PDGF-C [108]. Previous studies showed that blocking only human VEGF-A in xenografted tumors is not sufficient to inhibit tumor growth since host VEGF-A, derived from various cell sources, including TAFs, compensates for the lack of tumor-derived VEGF-A [28,109]. A marked TAF infiltration has been reported in the human lung carcinoma CALU-6, resulting in relatively low anti-tumor efficacy of an antihuman VEGF-A monoclonal antibody, relative to reagents that block both human and mouse VEGF-A [110]. In addition, Orimo et al. provided evidence that stromal fibroblasts, isolated from invasive human breast carcinomas, can recruit hematopoietic cells through a gradient of SDF-1 [111]. TAFs need to be instructed by tumor cells to induce their growth and invasiveness. Olumi et al., showed that TAFs isolated from prostate cancers stimulated tumor growth in initiated prostate epithelial cancer cells but not in normal epithelial cells [112]. Finally, TAFs may also enhance the efficiency of chemotherapy by improving drug uptake in the tumors [113]. Recent studies addressed whether there are any tumorspecific activities of tumor-associated fibroblasts (TAFs) that modulate tumors response to anti-VEGF therapy. TAFs isolated from tumors that are either sensitive or resistant to anti-VEGF treatment were compared for their ability to promote tumor angiogenesis and growth [114]. Only TAFs from resistant tumors promoted tumor growth even when VEGF was inhibited. A variety of angiogenic genes including PDGF-C, Angptl2, and COX-2 were upregulated in TAF-resistant. Inhibiting platelet-derived growth factor (PDGF)-C activity with neutralizing antibodies reduced TAF-resistant induced angiogenesis and slowed the growth of resistant tumors. Recent studies indicate that stromal PDGF-C overexpression attenuates responsiveness to anti-VEGF therapy in a glioblastoma model [115]. The possibility that PDGF-C may be important in tumor growth and angiogenesis is also suggested by earlier studies showing that PDGF-C, together with PDGF-A, is upregulated in some tumor cells and is associated with recruitment of VEGF-producing fibroblasts [108,110]. Recent studies also confirm that PDGF-C can be upregulated in the tumor cells [116], whereas PDGFR-a and -b are upregulated in stromal fibroblast and pericytes [117].

responses, hence the denomination of myeloid derived suppressor cells [125]. Recent studies examined the mechanisms of refractoriness to treatment with an anti-VEGF Mab in syngeneic murine tumor models [89]. A significant infiltration of hematopoietic cells, particularly CD11b+Gr1+ myeloid cells was noted in refractory tumors compared to the sensitive ones [89]. Admixing myeloid cells isolated from mice bearing refractory tumors with sensitive tumors, showed that myeloid cells were sufficient to confer refractoriness to anti-VEGF treatment, possibly via induction of tumor angiogenesis [89]. Conversely, depletion of CD11b+Gr1+ using an anti-Gr1 Mab suppressed growth of refractory tumors when used in combination with anti-VEGF [89]. The combination treatment partially blocked tumor infiltration of myeloid cells and also inhibited tumor angiogenesis. Bv8 (also known as prokineticin-2) [127–129] was upregulated in CD11b+Gr1+ cells associated with resistant tumors. Both cytokines stimulated production and mobilization of granulocytic and monocytic cells [130,131]. Interestingly, G-CSF strongly induced the expression of Bv8 in CD11b+Gr1+ cells [132]. Functional blocking of Bv8 inhibits tumor angiogenesis and growth and exhibited additive effects with anti-VEGF antibodies in slowing the growth of anti-VEGF resistant tumors [132]. Furthermore, treatment of mice bearing human xenografts with anti-Bv8 Mab resulted in suppression of tumor growth through suppression of tumor angiogenesis and reduction of myeloid cell infiltration in the tumors [132]. Furthermore, cytotoxic chemotherapy resulted in upregulation of Bv8 in established tumors and combination of anti-Bv8 plus anti-VEGF and cytotoxic agent were most effective in inhibiting tumor growth [132]. Additional studies indicated that production of GCSF by tumor or stromal cells strongly correlated with refractoriness to anti-VEGF in mouse models [133]. Administration of antiG-CSF antibodies resulted in a dramatic reduction of circulating and tumor-associated myeloid cells in such models, suggesting that at least in these models G-CSF plays a dominant role in the mobilization of these cells [133]. Administration of anti-Bv8 antibodies significantly reduced neutrophil infiltration and angiogenic not only in transplantable models abut also in a genetic model of cancer progression, the RIPTag model [131]. This finding is consistent with previous studies showing that depletion of Gr1+ cells, using an anti-Gr1 Mab, results in inhibition of tumor growth during early tumorigenesis in RIP-Tag mice [134]. Very recent studies show that neutrophils infiltrating human tumors strongly express Bv8, raising the possibility that this protein might represent a therapeutic target for certain tumors [135].

6.2. Myeloid cells

7. Conclusion and perspectives

Neutrophils and other myeloid cells provide the first line of protection against pathogens [25]. Much evidence supports also the view that various bone marrow-derived cell types play important roles in regulating tumor angiogenesis and growth [22,24,118]. For example, macrophages have been long characterized as a highly plastic cell type capable of tumor suppressive or tumor promoting effects, depending on the polarization state [119]. A population of Tie2-expressing monocytes (TEM) has identified as a distinct proangiogenic myeloid cell population [120,121]. CD11b+Gr1+ myeloid cells are mixed populations of cells consisting primarily of neutrophils, but also including macrophages and dendritic cells [122]. Overproduction of CD11b+Gr1+ cells (or their functional equivalents) has been observed in tumorbearing mice and in cancer patients [123–126]. Moreover, they infiltrate tumor masses and promote tumor growth, invasion, and angiogenesis [54]. Furthermore, subsets of CD11b+Gr1+ cells have been implicated in the suppression of T-cell mediated immune

There is now clinical validation that therapies targeting the VEGF pathway are effective in slowing cancer progression and provide benefits to patients. However, tumors may be either intrinsically resistant or evolve to become resistant to such therapies. So far, no validated biomarkers predicting which patients are most likely to respond to the therapy have been identified. Preclinical studies indicate that refractoriness/resistance to anti-VEGF agents indeed may be due to multiple mechanisms. Several well-defined signaling pathways, such as the FGF, Dll4, PlGF/VEGFR1, and VEGF-C/VEGFR2, have been implicated and clinical trials are ongoing to test the hypothesis that inhibiting at least some of theses such pathways may augment the efficacy of VEGF inhibitors. Furthermore, signals from different stromal cell types have been shown to modulate tumor growth and their responsiveness to therapies in a variety of model, raising the possibility that drugs interfering with these pathways could provide additional therapeutic strategies.

6. Role of stromal cell types in tumor growth 6.1. Tumor-associated fibroblasts

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Dr. Ferrara obtained his M.D. degree from the University of Catania Medical School in Italy in 1981. He joined Genentech Inc. in 1988 after doing his postdoctoral research at the University of California at San Francisco. At present, he is the Genentech Fellow in the Genentech Research Organization. Dr. Ferrara’s main research interests are the biology of angiogenesis and the identification of its key regulators. His work on the isolation, molecular cloning and biological characterization of VEGF-A resulted in the development of bevacizumab, the first anti-angiogenic agent to be approved by the FDA for cancer therapy, and ranibizumab, which was FDA-approved for the treatment of neovascular agerelated macular degeneration. Dr. Ferrara is author or co-author of over 260 publications and is the recipient of several scientific awards.