No abstract

This model provides a conceptual framework and a reproducible in vivo system to study unresolved central questions in cancer biology regarding the initiation, reversibility, and molecular regulation of the timing of the angiogenic switch.

Clinical trials with antiangiogenic agents have not been able to validate plasma or serum levels of angiogenesis regulators as reliable markers of cancer presence or therapeutic response. We recently reported that platelets contain numerous proteins that regulate angiogenesis. We now show that accumulation of angiogenesis regulators in platelets of animals bearing malignant tumors exceeds significantly their concentration in plasma or serum, as well as their levels in platelets from non-tumor-bearing animals. This process is selective, as platelets do not take up a proportional amount of other plasma proteins (eg, albumin), even though these may be present at higher concentrations. We also find that VEGFenriched Matrigel pellets implanted subcutaneously into mice or the minute quantities of VEGF secreted by microscopic subcutaneous tumors (0.5-1 mm 3 ) result in an elevation of VEGF levels in platelets, without any changes in its plasma levels. The profile of other angiogenesis regulatory proteins (eg, platelet-derived growth factor, basic fibroblast growth factor) sequestered by platelets also reflects the presence of tumors in vivo before they can be macroscopically evident. The ability of platelets to selectively take up angiogenesis regulators in cancerbearing hosts may have implications for the diagnosis and management of many angiogenesis-related diseases and provide a guide for antiangiogenic therapies. IntroductionPlatelets play a major role in hemostasis, as well as in tissue repair, maintenance of endothelium, and vascular tone. They may also facilitate delivery of angiogenesis regulators and other growth factors to sites of pathologic angiogenesis. 1,2 Correlative studies suggest that increasing platelet counts may be linked to tumor progression. 3,4 We and others have reported previously that platelets contain several proteins that regulate angiogenesis. [5][6][7][8] We have now discovered that the platelet concentrations of angiogenesis regulatory proteins, although relatively constant and stable under physiologic conditions, are modified by and reflect the presence of a tumor. In the presence of microscopic (Ͻ 1.0 mm) tumors in a mouse, circulating platelets sequester increased concentrations of angiogenesis regulatory proteins, without a corresponding elevation in their plasma levels. The uptake of angiogenesis regulatory proteins is selective, as platelets do not take up other plasma proteins. For example, although albumin is present in plasma at much higher concentrations than, for example, vascular endothelial growth factor (VEGF), albumin levels in platelets do not differ in the presence or absence of tumors.In this study, we used a high-throughput surface-enhanced laser desorption/ionization-time-of-flight mass spectrometry (SELDIToF MS), which permitted a rapid analysis of a large number of samples in a highly efficient and reproducible manner. 9,10 In this open-ended proteomic comparison of platelets from tumor-bearing and non-tumor-bearing animals, the majority of identified differentiall...

Tumor dormancy has important implications for early detection and treatment of cancer. Lack of experimental models and limited clinical accessibility constitute major obstacles to the molecular characterization of dormant tumors. We have developed models in which human tumors remain dormant for a prolonged period of time (>120 days) until they switch to rapid growth and become strongly angiogenic. These angiogenic tumors retain their ability to grow fast once injected in new mice. We hypothesized that dormant tumors undergo a stable genetic reprogramming during their switch to the fast-growing phenotype. Genomewide transcriptional analysis was done to dissect the molecular mechanisms underlying the switch of dormant breast carcinoma, glioblastoma, osteosarcoma, and liposarcoma tumors. A consensus expression signature distinguishing all four dormant versus switched fast-growing tumors was generated. In alignment with our phenotypic observation, the angiogenesis process was the most significantly affected functional gene category. The switch of dormant tumors was associated with down-regulation of angiogenesis inhibitor thrombospondin and decreased sensitivity of angiogenic tumors to angiostatin. The conversion of dormant tumors to exponentially growing tumors was also correlated with regulation and activation of pathways not hitherto linked to tumor dormancy process, such as endothelial cell-specific molecule-1, 5 ¶-ecto-nucleotidase, tissue inhibitor of metalloproteinase-3, epidermal growth factor receptor, insulin-like growth factor receptor, and phosphatidylinositol 3-kinase signaling. Further, novel dormancy-specific biomarkers such as H2BK and Eph receptor A5 (EphA5) were discovered. EphA5 plasma levels in mice and mRNA levels in tumor specimens of glioma patients correlated with diseases stage. These data will be instrumental in identifying novel early cancer biomarkers and could provide a rationale for development of dormancy-promoting tumor therapy strategies. [Cancer Res 2009;69(3):836-44]