Emerging paradigms in metastasis research.

Historically, therapy of metastatic disease has essentially been limited to using strategies that were identified and established to shrink primary tumors. The limited efficacy of such treatments on overall patient survival stems from diverging intrinsic and extrinsic characteristics of a primary tumor and metastases originating therefrom. To develop better therapeutic strategies to treat metastatic disease, there is an urgent need to shift the paradigm in preclinical metastasis research by conceptualizing metastatic dissemination, colonization, and growth as spatiotemporally dynamic processes and identifying rate-limiting vulnerabilities of the metastatic cascade. Clinically, while metastatic colonization remains the most attractive therapeutic avenue, comprehensive understanding of earlier steps may unravel novel metastasis-restricting therapies for presurgical neoadjuvant application. Moving beyond a primary tumor-centric view, this review adopts a holistic approach to understanding the spatial and temporal progression of metastasis. After reviewing recent developments in metastasis research, we highlight some of the grand challenges and propose a framework to expedite mechanism-based discovery research feeding the translational pipeline.

Beyond Angiogenesis: Exploiting Angiocrine Factors to Restrict Tumor Progression and Metastasis.

Looking beyond tumor angiogenesis, the past decade has witnessed a fundamental change of paradigm with the discovery that the vascular endothelium does not just respond to exogenous cytokines, but exerts active "angiocrine" gatekeeper roles, controlling their microenvironment in an instructive manner. While vascular niches host disseminated cancer cells and promote their stemness, endothelial cell-derived angiocrine signals orchestrate a favorable immune milieu to facilitate metastatic growth. Here, we discuss recent advances in the field of tumor microenvironment research and propose angiocrine signals as promising targets of future mechanism-driven antimetastatic therapies, which may prove useful to synergistically combine with chemotherapy and immunotherapy.

Endothelial cell spheroids as a versatile tool to study angiogenesis in vitro.

Given the need for robust and cost-efficient in vitro models to study angiogenesis and reproducibly analyze potential pro- and antiangiogenic compounds in preclinical studies, we developed a 3-dimensional in vitro angiogenesis assay that is based on collagen gel-embedded, size-defined spheroids generated from cultured human umbilical vein endothelial cells (HUVECs). Despite its wide distribution, limitations, sensitivity, robustness, and improvements, the capacity of this assay for functional screening purposes has not been elucidated thus far. By using time-lapse video microscopy, we show that tip cells lead the formation of capillary-like and partially lumenized sprouts originating from the spheroids. Angiogenic sprouting from spheroids generated from 5 different primary cultured human endothelial cell types was induced by physiologic concentrations of vascular endothelial cell growth factor 165. Based on this assay system, we determined the capacity of 880 approved drugs to interfere with or boost angiogenic sprouting, thereby assessing their putative angiogenesis-related side effects or novel applications. However, although this assay allowed for a rapid and reproducible determination of functional IC50 values of individual compounds, the sprouting results were partially affected by the HUVEC passage number and donor variability. To overcome this limitation, immortalized HUVECs (iHUVECs) showing a more homogenous response in terms of proliferation and sprouting over multiple population doublings were used in the course of this study. Collectively, the spheroid-based angiogenesis assay provides a sensitive and versatile tool to study the impact of pro- and antiangiogenic determinants on multiple steps of the angiogenic cascade. It is compatible with different endothelial cell types and allows use of iHUVECs to improve its overall robustness.

Mouse models of human cancer.

The Helmholtz Alliance Preclinical Comprehensive Cancer Center (PCCC; www.helmholtz-pccc.de) hosted the "1st International Kloster Seeon Meeting on Mouse Models of Human Cancer" in the Seeon monastery (Germany) from March 8 to 11, 2014. The meeting focused on the development and application of novel mouse models in tumor research and high-throughput technologies to overcome one of the most critical bottlenecks in translational bench-to-bedside tumor biology research. Moreover, the participants discussed basic molecular mechanisms underlying tumor initiation, progression, metastasis, and therapy resistance, which are the prerequisite for the development of novel treatment strategies and clinical applications in cancer therapy.

An inducible hepatocellular carcinoma model for preclinical evaluation of antiangiogenic therapy in adult mice.

The limited availability of experimental tumor models that faithfully mimic the progression of human tumors and their response to therapy remains a major bottleneck to the clinical translation and application of novel therapeutic principles. To address this challenge in hepatocellular carcinoma (HCC), one of the deadliest and most common cancers in the world, we developed and validated an inducible model of hepatocarcinogenesis in adult mice. Tumorigenesis was triggered by intravenous adenoviral delivery of Cre recombinase in transgenic mice expressing the hepatocyte-specific albumin promoter, a loxP-flanked stop cassette, and the SV40 large T-antigen (iAST). Cre recombinase-mediated excision of the stop cassette led to a transient viral hepatitis and resulted in multinodular tumorigenesis within 5 to 8 weeks. Tumor nodules with histologic characteristics of human HCC established a functional vasculature by cooption, remodeling, and angiogenic expansion of the preexisting sinusoidal liver vasculature with increasing signs of vascular immaturity during tumor progression. Treatment of mice with sorafenib rapidly resulted in the induction of vascular regression, inhibition of tumor growth, and enhanced overall survival. Vascular regression was characterized by loss of endothelial cells leaving behind avascular type IV collagen-positive empty sleeves with remaining pericytes. Sorafenib treatment led to transcriptional changes of Igf1, Id1, and cMet over time, which may reflect the emergence of potential escape mechanisms. Taken together, our results established the iAST model of inducible hepatocarcinogenesis as a robust and versatile preclinical model to study HCC progression and validate novel therapies.

EphB receptors and ephrinB ligands: regulators of vascular assembly and homeostasis.

Eph receptors comprise the largest family of receptor tyrosine kinases consisting of eight EphA receptors (with five corresponding glycosyl-phosphatidyl-inositol-anchored ephrinA ligands) and six EphB receptors (with three corresponding transmembrane ephrinB ligands). Originally identified as neuronal pathfinding molecules, genetic loss of function experiments have identified EphB receptors and ephrinB ligands as crucial regulators of vascular assembly, orchestrating arteriovenous differentiation and boundary formation. Despite these clearly defined rate-limiting roles of the EphB/ephrinB system for developmental angiogenesis, the mechanisms of the functions of EphB receptors and ephrinB ligands in the cells of the vascular system are poorly understood. Moreover, little evidence can be found in the recent literature regarding complementary EphB and ephrinB expression patterns that occur in the vascular system and that may bring cells into juxtapositional contact to allow bi-directional signaling between neighboring cells. This review summarizes the current knowledge of the role of EphB receptors and ephrinB ligands during embryonic vascular assembly and discusses recent findings on EphB/ephrinB-mediated cellular functions pointing to the crucial role of the Eph/ephrin system in controlling vascular homeostasis in the adult.