Growing tumor vessels: more than one way to skin a cat - implications for angiogenesis targeted cancer therapies

VEGFR-2 expression in malignant tumours of the canine mammary gland: a prospective survival study

Vascular endothelial growth factor receptor-2 (VEGFR-2) is the main receptor activated by vascular endothelial growth factor -A (VEGF-A) to promote tumour angiogenesis. Its clinical prognostic value has not been studied in canine mammary tumours (CMTs). Dogs with mammary cancer were enrolled in a survival study and the immunohistochemical expressions of VEGFR-2 and VEGF-A were analysed and associated with clinicopathological features. VEGFR-2 expression was associated with VEGF immunoreactivity in cancer cells, supporting the presence of an autocrine loop that may be involved in CMTs growth and survival. VEGFR-2 was also expressed by endothelial cells from tumour vasculature and positively associated with stromal matrix metalloproteinase-9 (MMP-9), suggesting the existence of a link between endothelial cells activation and up-regulation of matrix degrading proteins. Carcinosarcomas exhibited high VEGFR-2 expression suggesting that it may be one of the activated molecular pathways in this aggressive histological type and that VEGFR-2 inhibitors may constitute a potential treatment to improve the prognosis of these patients. Both VEGF and VEGFR-2 immunoreactivities were independent of patients' overall survival (OS) and disease-free survival (DFS).

Anti-angiogenic therapy for cancer: current progress, unresolved questions and future directions

Tumours require a vascular supply to grow and can achieve this via the expression of pro-angiogenic growth factors, including members of the vascular endothelial growth factor (VEGF) family of ligands. Since one or more of the VEGF ligand family is overexpressed in most solid cancers, there was great optimism that inhibition of the VEGF pathway would represent an effective anti-angiogenic therapy for most tumour types. Encouragingly, VEGF pathway targeted drugs such as bevacizumab, sunitinib and aflibercept have shown activity in certain settings. However, inhibition of VEGF signalling is not effective in all cancers, prompting the need to further understand how the vasculature can be effectively targeted in tumours. Here we present a succinct review of the progress with VEGF-targeted therapy and the unresolved questions that exist in the field: including its use in different disease stages (metastatic, adjuvant, neoadjuvant), interactions with chemotherapy, duration and scheduling of therapy, potential predictive biomarkers and proposed mechanisms of resistance, including paradoxical effects such as enhanced tumour aggressiveness. In terms of future directions, we discuss the need to delineate further the complexities of tumour vascularisation if we are to develop more effective and personalised anti-angiogenic therapies.

The tumor microenvironment: characterization, redox considerations, and novel approaches for reactive oxygen species-targeted gene therapy

The tumor microenvironment is a complex system that involves the interaction between malignant and neighbor stromal cells embedded in a mesh of extracellular matrix (ECM) components. Stromal cells (fibroblasts, endothelial, and inflammatory cells) are co-opted at different stages to help malignant cells invade the surrounding ECM and disseminate. Malignant cells have developed adaptive mechanisms to survive under the extreme conditions of the tumor microenvironment such as restricted oxygen supply (hypoxia), nutrient deprivation, and a prooxidant state among others. These conditions could be eventually used to target drugs that will be activated specifically in this microenvironment. Preclinical studies have shown that modulating cellular/tissue redox state by different gene therapy (GT) approaches was able to control tumor growth. In this review, we describe the most relevant features of the tumor microenvironment, addressing reactive oxygen species-generating sources that promote a prooxidative microenvironment inside the tumor mass. We describe different GT approaches that promote either a decreased or exacerbated prooxidative microenvironment, and those that make use of the differential levels of ROS between cancer and normal cells to achieve tumor growth inhibition.

Changes in tumour vessel density upon treatment with anti-angiogenic agents: relationship with response and resistance to therapy

Background:

We examine how changes in a surrogate marker of tumour vessel density correlate with response and resistance to anti-angiogenic therapy.

Methods:

In metastatic renal cancer patients treated with anti-angiogenic tyrosine kinase inhibitors, arterial phase contrast-enhanced computed tomography was used to simultaneously measure changes in: (a) tumour size, and (b) tumour enhancement (a surrogate marker of tumour vessel density) within individual lesions.

Results:

No correlation between baseline tumour enhancement and lesion shrinkage was observed, but a reduction in tumour enhancement on treatment was strongly correlated with reduction in lesion size (r=0.654, P<0.0001). However, close examination of individual metastases revealed different types of response: (1) good vascular response with significant tumour shrinkage, (2) good vascular response with stabilisation of disease, (3) poor vascular response with stabilisation of disease and (4) poor vascular response with progression. Moreover, contrasting responses between different lesions within the same patient were observed. We also assessed rebound vascularisation in tumours that acquired resistance to treatment. The amplitude of rebound vascularisation was greater in lesions that had a better initial response to therapy (P=0.008).

Interpretation:

Changes in a surrogate marker of tumour vessel density correlate with response and resistance to anti-angiogenic therapy. The data provide insight into the mechanisms that underlie response and resistance to this class of agent.

Endoglin is necessary for angiogenesis in human ovarian carcinoma-derived primary endothelial cells

Endoglin (CD105, END) is upregulated in proliferating endothelial cells, suggesting potential therapeutic properties. However, it is not clear whether endoglin mediates an enhanced proliferative rate or may be upregulated as part of a negative feedback loop. To gain insights into context-dependent and cell type-dependent regulatory effects of endoglin, we studied its role properties in human ovarian carcinoma-derived endothelial cells (ODMECs). We isolated and cultured primary ODMECs from epithelial ovarian carcinoma tissue. ODMECs had higher expression of endoglin and VEGFR-2, and also exhibited enhanced spontaneous formation of vessel-like structures in vitro. Transfection of siRNA targeting endoglin in ODMECs cells resulted in the reduction of the proliferation and tube formation. These results indicate that a subset of ODMECs display abnormal angiogenic properties and this phenotype was blocked by decreasing endoglin levels, suggesting endoglin is essential for stimulating angiogenesis, and targeting it may be an attractive approach to anti-angiogenesis therapy for ovarian carcinoma.

Quantitative mass spectrometry-based proteomics in angiogenesis

The process of new blood vessel formation from pre-existing ones is called angiogenesis. Beyond playing a critical role in the physiological development of the vascular system, angiogenesis is a well-recognised hallmark of cancer. Unbiased system-wide approaches are required to complement the current knowledge, and intimately understand the molecular mechanisms regulating this process in physiological and pathological conditions. In this review we describe the cellular and molecular dynamics regulating the physiological growth of vessels and their deregulation in cancer, survey in vitro and in vivo models currently exploited to investigate various aspects of angiogenesis and describe state-of-the-art and most widespread methods and technologies in MS shotgun proteomics. Finally, we focus on current applications of MS to better understand endothelial cell behaviour and propose how modern proteomics can impact on angiogenesis research.

The "cord of life" serving antiangiogenic therapy

In this issue of Blood, Seano et al describe a new method to screen drugs that starve tumors by using umbilical cords, which allow nourishment and waste product exchange between mother and embryo.(1)

Vessel co-option in primary human tumors and metastases: an obstacle to effective anti-angiogenic treatment?

Angiogenesis has been regarded as essential for tumor growth and progression. Studies of many human tumors, however, suggest that their microcirculation may be provided by nonsprouting vessels and that a variety of tumors can grow and metastasize without angiogenesis. Vessel co-option, where tumor cells migrate along the preexisting vessels of the host organ, is regarded as an alternative tumor blood supply. Vessel co-option may occur in many malignancies, but so far mostly reported in highly vascularized tissues such as brain, lung, and liver. In primary and metastatic lung cancer and liver metastasis from different primary origins, as much as 10–30% of the tumors are reported to use this alternative blood supply. In addition, vessel co-option is introduced as a potential explanation of antiangiogenic drug resistance, although the impact of vessel co-option in this clinical setting is still to be further explored. In this review we discuss tumor vessel co-option with specific examples of vessel co-option in primary and secondary tumors and a consideration of the clinical implications of this alternative tumor blood supply.

Both primary and metastatic tumors use preexisting host tissue vessels as their blood supply. Tumors may grow to a clinically detectable size without angiogenesis and makes them less likely to respond to drugs designed to target the abnormal vasculature produced by angiogenesis, but further studies to explore the biological and clinical implication of these co-opted vessels is needed.

Excessive milk production during breast-feeding prior to breast cancer diagnosis is associated with increased risk for early events

Breast-feeding is a known protective factor against breast cancer. Breast-feeding duration is influenced by hormone levels, milk production, and lifestyle factors. The aims were to investigate how breast-feeding duration and milk production affected tumor characteristics and risk for early breast cancer events in primary breast cancer patients. Between 2002 and 2008, 634 breast cancer patients in Lund, Sweden, took part in an ongoing prospective cohort study. Data were extracted from questionnaires, pathology reports, and patients’ charts from 592 patients without preoperative treatment. Breast-feeding duration ≤12 months of the first child was associated with higher frequency of ER+/PgR+ tumors (P=0.02). Median follow-up time was 4.9 years. Higher risk for early events was observed for breast-feeding duration of first child >12 months (LogRank P=0.001), total breast-feeding duration >12 months (LogRank P=0.008), as well as ‘excessive milk production’ during breast-feeding of the first child (LogRank P=0.001). Patients with ‘almost no milk production’ had no events. In a multivariable model including both ‘excessive milk production’ and breast-feeding duration of the first child >12 months, both were associated with a two-fold risk for early events, adjusted HRs 2.33 (95% CI: 1.25-4.36) and 2.39 (0.97-5.85), respectively, while total breast-feeding duration was not. ‘Excessive milk production’ was associated with a two-fold risk of early distant metastases, adjusted HR 2.59 (1.13-5.94), but not duration. In conclusion, ‘excessive milk production’ during breast-feeding was associated with higher risk for early events independent of tumor characteristics, stressing the need to consider host factors in the evaluation of prognostic markers.

Tumor stroma: a complexity dictated by the hypoxic tumor microenvironment

A lot of effort has been done to study how cancer cells react to low-oxygen tension, a condition known as hypoxia. Indeed, abnormal and dysfunctional blood vessels in the tumor are incapable to restore oxygenation, therefore perpetuating hypoxia, which, in turn, will fuel tumor progression, metastasis and resistance to antitumor therapies. Nevertheless, how stromal components including blood and lymphatic endothelial cells, pericytes and fibroblasts, as well as hematopoietic cells, respond to low-oxygen tension in comparison with their normoxic counterparts has been a matter of investigation in the last few years only and, to date, this field of research remains poorly understood. In general, opposing phenotypes can arise from the same stromal component when embedded in different tumor microenvironments, and, vice versa, different stromal components can have opposite reaction to the same tumor microenvironment. In this article, we will discuss the emerging link between tumor stroma and hypoxia, and how this complexity is translated at the molecular level.

Mechanisms of resistance to anti-angiogenesis therapies

Angiogenesis, the formation of new blood vessels from preexisting ones, provides oxygen and nutrients to actively proliferating tumor cells. Hence, it represents a critical aspect of tumor progression and metastasis. Because inhibition of angiogenesis represents a major approach to cancer treatment, the development of inhibitors of angiogenesis is a major challenge. The first FDA approved anti-angiogenic drug bevacizumab, a humanized monoclonal antibody directed against the Vascular Endothelial Growth Factor (VEGF), has been approved for the treatment of metastatic colorectal, lung, breast, and kidney cancers. The encouraging results have lead to the development, in the past few years, of other agents targeting angiogenic pathways as potent anti-cancer drugs and a number of them have been approved for metastatic breast, lung, kidney, and central nervous system cancers. Despite a statistically significant increase in progression free survival, which has accelerated FDA approval, no major benefit to overall survival was described and patients inevitably relapsed due to acquired resistance. However, while progression free survival was increased by only a few months for the majority of the patients, some clearly benefited from the treatment with a real increase in life span. The objective of this review is to present an overview of the different treatments targeting angiogenesis, their efficacy and the mechanisms of resistance that have been identified in different cancer types. It is essential to understand how resistance (primary or acquired over time) develops and how it may be overcome.

Recombinant attenuated Salmonella typhimurium carrying a plasmid co-expressing ENDO-VEGI151 and survivin siRNA inhibits the growth of breast cancer in vivo

The aim of this study was to investigate the antitumor effect of a plasmid co-expressing ENDO-VEGI151 and survivin siRNA on breast cancer in nude mice, and to explore the feasibility of attenuated Salmonella typhimurium (S. typhimurium) as a delivery vector for cancer gene therapy in vivo. Three recombinant expression plasmids pENDO‑VEGI151 (pEV), pSurvivin-siRNA (psi-survivin) and co-expressing plasmid pENDO-VEGI151/survivin‑siRNA (pEV/si-survivin), were transferred into the attenuated S. typhimurium strain SL7207, respectively. MDA-MB-231 cells were infected with these recombinants in vitro, and the expression of ENDO-VEGI151 and survivin was detected. In order to detect S. typhimurium distribution and gene delivery efficiency in vivo, the plasmid pEGFP-N1 which encodes green fluorescent protein was transferred into SL7207, and the recombinant known as SL-pEGFP was orally administered to tumor-bearing nude mice. The gene transfer efficiency, distribution and survival time of the SL-pEGFP in vivo were evaluated by detection of GFP fluorescence. SL-pEGFP not only infected the cancer cells effectively, but also allowed the survival and expression of specific genes mainly in the xenografts of nude mice. To further identify the anticancer effects of these recombinants in vivo, mice burdened with xenografts were randomly divided into 6 groups, which were subjected to intragastric administration of vehicle, SL7207, SL-pcDNA3.1, SL-pEV, SL-psi-survivin and SL-pEV/si-survivin, respectively. Eight weeks after implantation, tumor size, weight, inhibition rate, intratumoral microvessel density (MVD), apoptotic index (AI), ENDO‑VEGI151 and survivin expression were evaluated. Compared with the SL-pEV or SL-psi-survivin-treated groups, the growth of tumors was significantly reduced in the SL-pEV/si-survivin group with an inhibition rate of 90.28 vs. 69.12 and 65.61%, respectively. MVD and the expression of survivin were decreased significantly in the SL-pEV/si-survivin-treated group, while AI increased significantly in the SL-pEV/si-survivin-treated group. These results indicated that attenuated S. typhimurium carrying the dual function plasmid pEV/si-survivin cannot only be specifically enriched in the tumor tissue, but also showed a synergistic antitumor effect in vivo.

VEGFA and tumour angiogenesis

In this review we summarize the current understanding of signal transduction downstream of vascular endothelial growth factor A (VEGFA) and its receptor VEGFR2, and the relationship between these signal transduction pathways and the hallmark responses of VEGFA, angiogenesis and vascular permeability. These physiological responses involve a number of effectors, including extracellular signal-regulated kinases (ERKs), Src, phosphoinositide 3 kinase (PI3K)/Akt, focal adhesion kinase (FAK), Rho family GTPases, endothelial NO and p38 mitogen-activated protein kinase (MAPK). Several of these factors are involved in the regulation of both angiogenesis and vascular permeability. Tumour angiogenesis primarily relies on VEGFA-driven responses, which to a large extent result in a dysfunctional vasculature. The reason for this remains unclear, although it appears that certain aspects of the VEGFA-stimulated angiogenic milieu (high level of microvascular density and permeability) promote tumour expansion. The high degree of redundancy and complexity of VEGFA-driven tumour angiogenesis may explain why tumours commonly develop resistance to anti-angiogenic therapy targeting VEGFA signal transduction.

Targeted therapies in colorectal cancer-an integrative view by PPPM

In developed countries, colorectal cancer (CRC) is the third most common malignancy, but it is the second most frequent cause of cancer-related death. Clinicians are still faced with numerous challenges in the treatment of this disease, and future approaches which target the molecular features of the disorder will be critical for success in this disease setting. Genetic analyses of many solid tumours have shown that up to 100 protein-encoding genes are mutated. Within CRC, numerous genetic alterations have been identified in a number of pathways. Therefore, understanding the molecular pathology of CRC may present information on potential routes for treatment and may also provide valuable prognostic information. This will be particularly pertinent for molecularly targeted treatments, such as anti-vascular endothelial growth factor therapies and anti-epidermal growth factor receptor (EGFR) monoclonal antibody therapy. KRAS and BRAF mutations have been shown to predict response to anti-EGFR therapy. As EGFR can also signal via the phosphatidylinositol 3-kinase (PI3K) kinase pathway, there is considerable interest in the potential roles of members of this pathway (such as PI3K and PTEN) in predicting treatment response. Therefore, a combined approach of new techniques that allow identification of these biomarkers alongside interdisciplinary approaches to the treatment of advanced CRC will aid in the treatment decision-making process and may also serve to guide future therapeutic approaches.

EMP2 regulates angiogenesis in endometrial cancer cells through induction of VEGF

Understanding tumor-induced angiogenesis is a challenging problem with important consequences for the diagnosis and treatment of cancer. In this study, we define a novel function for epithelial membrane protein-2 (EMP2) in the control of angiogenesis. EMP2 functions as an oncogene in endometrial cancer, and its expression has been linked to decreased survival. Using endometrial cancer xenografts, modulation of EMP2 expression resulted in profound changes to the tumor microvasculature. Under hypoxic conditions, upregulation of EMP2 promoted vascular endothelial growth factors (VEGF) expression through a HIF-1α-dependent pathway and resulted in successful capillary-like tube formation. In contrast, reduction of EMP2 correlated with reduced HIF-1α and VEGF expression with the net consequence of poorly vascularized tumors in vivo. We have previously shown that targeting of EMP2 using diabodies in endometrial cancer resulted in a reduction of tumor load, and since then we have constructed a fully human EMP2 IgG1. Treatment of endometrial cancer cells with EMP2-IgG1 reduced tumor load with a significant improvement in survival. These results support the role of EMP2 in the control of the tumor microenvironment and confirm the cytotoxic effects observed by EMP2 treatment in vivo.

Establishing a peritoneal dissemination xenograft mouse model for survival outcome assessment of experimental gastric cancer

BACKGROUND

Peritoneal dissemination of gastric cancer is a common reason for unresectability, a frequent recurrence mechanism, and a common cause for death. The present study was performed to test peritoneal dissemination gastric cancer xenografts mouse models that would support survival outcome analyses.

MATERIALS AND METHODS

Human gastric cancer cell lines AGS, NCI-N87, and SNU-16 were intraperitoneally injected into nude mice and severe combined immunodeficiency (SCID) mice. The peritoneal tumor formation and mouse survival were compared among different groups. Mice were treated with oxaliplatin (5 mg/kg) and NVP-BEZ235 (10 mg/kg).

RESULTS

The formation rate of peritoneal cancer after intraperitoneal injection of 5 × 10(6) SNU16, NCI-N87, and AGS cells was 2/8, 6/8, and 0/8 in nude mice, and 6/6, 6/6, and 0/6 in SCID mice, respectively. Median animal survival with peritoneal dissemination was 74 d for NCI-N87 cells (10 × 10(6)), 95 d for SNU16 cells (10 × 10(6)), 78 d for SNU16 cells (20 × 10(6)), and 44 d for SNU16 cells (40 × 10(6)). In a therapeutic experiment with 40 × 10(6) SNU16 cells, animal survival was significantly improved by oxaliplatin treatment compared with the control group (58.5 d versus 45 d, P < 0.001), but not by NVP-BEZ235 (48 d versus 45 d, P = 0.249) treatment. In the accompanying subcutaneous SNU16 mouse model, relative tumor volume compared with controls was not significantly decreased by oxaliplatin treatment (P = 0.151) but by NVP-BEZ235 therapy (P = 0.008).

CONCLUSIONS

Peritoneal gastric cancer xenografts were successfully established after intraperitoneal injection NCI-N87 and SNU16 cells. These findings provide a useful survival outcome assessment model for experimental gastric cancer research.

Contrasting effects of sunitinib within in vivo models of metastasis

Sunitinib is a potent and clinically approved tyrosine kinase inhibitor that can suppress tumour growth by inhibiting angiogenesis. However, conflicting data exist regarding the effects of this drug on the growth of metastases in preclinical models. Here we use 4T1 and RENCA tumour cells, which both form lung metastases in Balb/c mice, to re-address the effects of sunitinib on the progression of metastatic disease in mice. We show that treatment of mice with sunitinib prior to intravenous injection of tumour cells can promote the seeding and growth of 4T1 lung metastases, but not RENCA lung metastases, showing that this effect is cell line dependent. However, increased metastasis occurred only upon administration of a very high sunitinib dose, but not when lower, clinically relevant doses were used. Mechanistically, high dose sunitinib led to a pericyte depletion effect in the lung vasculature that correlated with increased seeding of metastasis. By administering sunitinib to mice after intravenous injection of tumour cells, we demonstrate that while sunitinib does not inhibit the growth of 4T1 lung tumour nodules, it does block the growth of RENCA lung tumour nodules. This contrasting response was correlated with increased myeloid cell recruitment and persistent vascularisation in 4T1 tumours, whereas RENCA tumours recruited less myeloid cells and were more profoundly devascularised upon sunitinib treatment. Finally, we show that progression of 4T1 tumours in sunitinib treated mice results in increased hypoxia and increased glucose metabolism in these tumours and that this is associated with a poor outcome. Taken together, these data suggest that the effects of sunitinib on tumour progression are dose-dependent and tumour model-dependent. These findings have relevance for understanding how anti-angiogenic agents may influence disease progression when used in the adjuvant or metastatic setting in cancer patients.

Overcoming resistance to antiangiogenic therapies

The concept of targeting new blood vessel formation, or angiogenesis, in tumors is an important advancement in cancer therapy, resulting, in part, from the development of such biologic agents as bevacizumab, a monoclonal antibody directed against vascular endothelial growth factor (VEGF)-A. The rationale for antiangiogenic therapy is based on the hypothesis that if tumors are limited in their capacity to obtain a new blood supply, so too is their capacity for growth and metastasis. Additional evidence suggests that pruning and/or "normalization" of irregular tumor vasculature and reduction of hypoxia may facilitate greater access of cytotoxic chemotherapy (CT) to the tumor. Indeed, for metastatic colorectal cancer, bevacizumab in combination with established CT regimens has efficacy superior to that of CT alone. Despite ~2-month longer progression-free and overall survival times than with CT alone, patients still progress, possibly because of alternative angiogenic "escape" pathways that emerge independent of VEGF-A, or are driven by hypoxic stress on the tumor. Other VEGF family members may contribute to resistance, and many factors that contribute to the regulation of tumor angiogenesis function as part of a complex network, existing in different concentrations and spatiotemporal gradients and producing a wide range of biologic responses. Integrating these concepts into the design and evaluation of new antiangiogenic therapies may help overcome resistance mechanisms and allow for greater efficacy over longer treatment periods.

Angiopoietins/TIE2 system and VEGF are involved in ovarian function in a DHEA rat model of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrinological pathology among women of reproductive age. It is characterized by anovulation, oligo- or amenorrhea, hyperandrogenism, obesity, and insulin resistance. PCOS patients present with elevated levels of vascular endothelial growth factor (VEGF) in serum and follicular fluid. In this study, we examined the ovarian expression of angiopoietins (ANGPT) and their receptor tyrosine kinase receptor (TIE2), involved in the stabilization of blood vessels, in a rat model of dehydroepiandrosterone-induced PCOS. We also analyzed the effect of ovarian VEGF inhibition on ANGPT/TIE2, follicular development, and vascular stability. VEGF levels were increased in the PCOS ovaries, whereas the levels of its receptor fetal liver kinase-1 were decreased. In addition, the periendothelial cell area and the ANGPT1 to ANGPT2 ratio in the ovary were increased in the PCOS group. Percentage of primary follicles was increased and the percentage of preantral follicles and corpora lutea was decreased in the PCOS group. VEGF inhibition decreased the percentage of primary follicles close to control values. Interestingly, despite the presence of cysts in the ovaries from VEGF inhibitor-treated PCOS rats, its percentage was lower than the PCOS group without treatment. In summary, this study describes an alteration not only in the VEGF/fetal liver kinase-1 system but also in the ANGPT/TIE2 system in a dehydroepiandrosterone-induced PCOS rat model. This leads to an increase in periendothelial cell recruitment. We also demonstrated that ovarian VEGF inhibition can partially restore the accumulation of small follicles in PCOS rats and reduces cyst formation, improving ovulation and follicular development. Therefore, the inhibition of VEGF could be considered, in addition to other currently applied treatments, as a new strategy to be studied in PCOS patients to restore ovarian function.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

PPARs: Interference with Warburg' Effect and Clinical Anticancer Trials

The metabolic/cell signaling basis of Warburg's effect ("aerobic glycolysis") and the general metabolic phenotype adopted by cancer cells are first reviewed. Several bypasses are adopted to provide a panoramic integrated view of tumoral metabolism, by attributing a central signaling role to hypoxia-induced factor (HIF-1) in the expression of aerobic glycolysis. The cancer metabolic phenotype also results from alterations of other routes involving ras, myc, p53, and Akt signaling and the propensity of cancer cells to develop signaling aberrances (notably aberrant surface receptor expression) which, when present, offer unique opportunities for therapeutic interventions. The rationale for various emerging strategies for cancer treatment is presented along with mechanisms by which PPAR ligands might interfere directly with tumoral metabolism and promote anticancer activity. Clinical trials using PPAR ligands are reviewed and followed by concluding remarks and perspectives for future studies. A therapeutic need to associate PPAR ligands with other anticancer agents is perhaps an important lesson to be learned from the results of the clinical trials conducted to date.

Blood vessels as targets in tumor therapy

The landmark papers published by Judah Folkman in the early 1970s on tumor angiogenesis and therapeutic implications promoted the rapid development of a very dynamic field where basic scientists, oncologists, and pharmaceutical industry joined forces to determine the molecular mechanisms in blood vessel formation and find means to exploit this knowledge in suppressing tumor vascularization and growth. A wealth of information has been collected on angiogenic growth factors, and in 2004 the first specific blood vessel-targeted cancer therapy was introduced: a neutralizing antibody against vascular endothelial growth factor (VEGF). Now (2011) we know that suppression of tumor angiogenesis may be a double-edged sword and that the therapy needs to be further refined and individualized. This review describes the hallmarks of tumor vessels, how different angiogenic growth factors exert their function, and the perspectives for future development of anti-angiogenic therapy.

The biology of personalized cancer medicine: facing individual complexities underlying hallmark capabilities

It is a time of great promise and expectation for the applications of knowledge about mechanisms of cancer toward more effective and enduring therapies for human disease. Conceptualizations such as the hallmarks of cancer are providing an organizing principle with which to distill and rationalize the abject complexities of cancer phenotypes and genotypes across the spectrum of the human disease. A countervailing reality, however, involves the variable and often transitory responses to most mechanism-based targeted therapies, returning full circle to the complexity, arguing that the unique biology and genetics of a patient's tumor will in the future necessarily need to be incorporated into the decisions about optimal treatment strategies, the frontier of personalized cancer medicine. This perspective highlights considerations, metrics, and methods that may prove instrumental in charting the landscape of evaluating individual tumors so to better inform diagnosis, prognosis, and therapy. Integral to the consideration is remarkable heterogeneity and variability, evidently embedded in cancer cells, but likely also in the cell types composing the supportive and interactive stroma of the tumor microenvironment (e.g., leukocytes and fibroblasts), whose diversity in form, regulation, function, and abundance may prove to rival that of the cancer cells themselves. By comprehensively interrogating both parenchyma and stroma of patients' cancers with a suite of parametric tools, the promise of mechanism-based therapy may truly be realized.

In vivo time-course imaging of tumor angiogenesis in colorectal liver metastases in the same living mice using two-photon laser scanning microscopy

In vivo real-time visualization of the process of angiogenesis in secondary tumors in the same living animals presents a major challenge in metastasis research. We developed a technique for intravital imaging of colorectal liver metastasis development in live mice using two-photon laser scanning microscopy (TPLSM). We also developed time-series TPLSM in which intravital TPLSM procedures were performed several times over periods of days to months. Red fluorescent protein-expressing colorectal cancer cells were inoculated into the spleens of green fluorescent protein-expressing mice. First- and second-round intravital TPLSM allowed visualization of viable cancer cells (red) in hepatic sinusoids or the space of Disse. Third-round intravital TPLSM demonstrated liver metastatic colonies consisting of viable cancer cells and surrounding stroma with tumor vessels (green). In vivo time-course imaging of tumor angiogenesis in the same living mice using time-series TPLSM could be an ideal tool for antiangiogenic drug evaluation, reducing the effects of interindividual variation.