Imaging tumor angiogenesis Kristy Red-Horse, Napoleone Ferrara J Clin Invest. 2006;116(10):2585-2587. https://doi.org/10.1172/JCI30058. Commentary Since the discovery of vascular-specific growth factors with angiogenic activity, there has been a significant effort to develop cancer drugs that restrict tumorigenesis by targeting the blood supply. In this issue of the JCI, Mancuso et al. use mouse models to better understand the plasticity of the tumor vasculature in the face of antiangiogenic therapy (see the related article beginning on page 2610). They describe a rapid regrowth of the tumor vasculature following withdrawal of VEGFR inhibitors, emphasizing the importance of fully understanding the function of these and similar treatments used in the clinic at the cellular and molecular level. Find the latest version: https://jci.me/30058/pdf commentaries Imaging tumor angiogenesis Kristy Red-Horse and Napoleone Ferrara Genentech Inc., South San Francisco, California, USA. Since the discovery of vascular-specific growth factors with angiogenic activ- in 1 week (Figure 1). In accordance with ity, there has been a significant effort to develop cancer drugs that restrict previous findings (4, 6), posttreatment tumorigenesis by targeting the blood supply. In this issue of the JCI, Mancuso angiogenic growth correlated with the et al. use mouse models to better understand the plasticity of the tumor vas- disappearance of empty basement mem- culature in the face of antiangiogenic therapy (see the related article begin- brane sleeves, suggesting that regrowth ning on page 2610). They describe a rapid regrowth of the tumor vasculature occurred along these structures (Figure 1). following withdrawal of VEGFR inhibitors, emphasizing the importance of Attempts to prevent regrowth by block- fully understanding the function of these and similar treatments used in the ing MMP activity with the MMP inhibitor clinic at the cellular and molecular level. AG3340 or blocking a cryptic site within type IV collagen with the monoclonal Inhibiting tumor growth by limiting growth. However, not all vessels are elimi- antibody HUIV26 were unsuccessful. angiogenesis can, in combination with nated. A subset of VEGF-independent ves- However, this does not rule out the poten- chemotherapy, extend life expectancy in sels expressing lower levels of VEGFR2 (4) tial of regulating tumor angiogenesis by patients with certain malignancies and remain, resulting in a more “normalized” targeting these vascular relics. is being heavily pursued as a promising vascular bed, no longer exhibiting the tor- cancer therapy (1). There are several FDA- tuosity and dysfunction frequently asso- Role of basement membranes in the approved antiangiogenenic drugs current- ciated with tumor blood vessels. These context of normal angiogenesis ly used in the clinic to treat cancer and neo- remaining vessels could provide a source In the adult, the majority of angiogen- vascular age-related macular degeneration from which new vessels may bud follow- esis is associated with wound healing, the (bevacizumab, pegaptanib, ranibizumab, ing renewed VEGF activity. In addition, reproductive tract, and inflammation. At sunitinib, and sorafenib) and many others vascular regression leaves behind base- the cellular level, this process occurs by under development. These agents inhibit ment membrane sleeves — once associ- several different mechanisms, including a angiogenesis by targeting the signaling ated with the endothelial cell layer — that phenomenon termed sprouting (9), which pathways mediated by VEGF-A and poten- can persist for up to 21 days (5). In pre- is a major method by which tumors recruit tially other VEGF family members, such as vious work, McDonald and colleagues the preexisting vasculature. Sprouting is placental growth factor (PlGF), VEGF-B, established that, in normal vascular beds stimulated in response to local molecular VEGF-C, and VEGF-D. However, VEGF-A such as the trachea and thyroid, regrowth cues, such as hypoxia- or inflammation- is probably the major positive regulator of following drug withdrawal is rapid and induced VEGF-A production. Thought blood vessel formation. Given the success appears to be aided by the presence of to be a stepwise process, sprouting begins of anti-VEGF therapy in multiple mouse basement membrane sleeves, identifying with local increases in vascular perme- models of cancer and in clinical trials, it is these structures as potential antiangio- ability, followed by basement membrane important to fully understand the mech- genic targets (4, 6). and ECM degradation. Subsequently, anisms by which anti-VEGF treatment In their current report (2), the authors endothelial cells sometimes called tip cells affects tumor angiogenesis at the cellular use elegant microscopic imaging tech- send out projections and initiate migration and molecular level. This will be particu- niques to extend these findings to angio- along newly deposited ECM tracts. Finally, larly important when designing combina- genesis associated with 2 tumor models, lumen-containing vessels are formed and torial treatments aimed at circumventing a transgenic mouse model of insulinoma integrated into the circulation. Recent evi- drug resistance and tailoring patient- (RIP-Tag2 tumors) and mice implanted dence using real-time imaging techniques specific regimens. with Lewis lung carcinoma xenografts. in zebrafish demonstrates that the lat- In this issue of the JCI, Mancuso et al. Two different small molecule tyrosine ter steps involve, at least in some vessels, further our understanding of how anti- kinase inhibitors were used to target endothelial pinocytosis followed by coales- VEGF therapy affects blood vessels by VEGFR activity: AG-013736 (7), which cence of intracellular vacuoles (10). describing the dynamics of tumor vas- also blocks other receptor tyrosine kinas- At the molecular level, sprouting angio- culature regrowth after drug withdrawal es including PDGFR-β and c-kit, and genesis requires multiple factors, includ- (2). Work from multiple laboratories (3) AG-028262, which has been reported to ing MMPs such as MMP-2 (11) and has established that inhibiting the VEGF be a more selective inhibitor of VEGFR2 MMP-9 (12), which function in ECM pathway rapidly reduces the vascularity of (8). Similar results were obtained using remodeling and in the liberation and solid tumors, leading to a decline in tumor both drugs, suggesting that they function modification of angiogenic factors such primarily through VEGF inhibition. The as VEGF-A and FGF (13). Integrins are authors demonstrate that cessation of a also involved, as they bind sites within the Conflict of interest: The authors are employed by Genentech Inc., manufacturer of bevacizumab. 7-day therapy regimen is followed by rapid ECM and are critical for endothelial cell Citation for this article: J. Clin. Invest. 116:2585–2587 vascular regrowth, whereby the vessel den- proliferation, migration, and survival (14). (2006). doi:10.1172/JCI30058. sity returned to pretreatment levels with- In some cases, these 2 factors function in The Journal of Clinical Investigation http://www.jci.org Volume 116 Number 10 October 2006 2585 commentaries Figure 1 The effects of VEGF inhibition on the tumor vasculature are reversible. (A) Tumor growth stimulates angiogenesis, producing an abnormal vas- cular bed with disorganized branching and increased permeability. In many cases, this is due to increased VEGF production. (B) Antiangiogenic therapy that inhibits VEGF activity decreases tumor vascularity. Vascular regression often leaves behind a pericyte layer and empty basement membrane sleeves that can persist for up to 21 days. The pericytes were, however, observed to have reduced immunoreactivity to α–smooth muscle actin. (C) Cessation of anti-VEGF therapy following a 7-day treatment regimen results in rapid vascular regrowth. Vessel density returns to pretreatment levels within 7 days. the same pathway where MMPs cleave the experimental models and clinical settings, efficacy of chemotherapeutics and radia- ECM to expose cryptic angiogenenic inte- treatment is given regularly for a sus- tion (18–20). The quantitative imaging grin-binding sites (15, 16). Thus, blocking tained period of time, somewhat longer techniques well established by the authors certain MMP and integrin family mem- than that used in the current study (2). In should be helpful in addressing these and bers can inhibit angiogenesis (12, 16). addition, antiangiogenic therapy is com- other issues by defining the tumor vascu- Mancuso et al. (2) similarly target MMPs bined with treatment with other cytotox- lature during critical stages of anti-VEGF and an integrin-binding site on type IV ic agents. Thus, to answer this question, treatment. Similarly, it may be informative collagen in their system, but there is no future work should focus on assessing to assess vasculature growth at the precise effect on tumor revascularization. The the dynamics of the tumor vasculature time when tumors begin to acquire resis- difference between their model and oth- following prolonged VEGF inhibition as tance to antiangiogenic drugs. Resistance ers may be the presence of preassembled well as after combination therapy. The is associated with reestablishment of the basement membrane sleeves that facilitate imaging analysis described in this manu- tumor vasculature due to compensatory sprouting. In support of this theory, the script could also be extended to human upregulation of other angiogenic factors