Angiopoietin: a TIE(d) balance in tumor angiogenesis

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

Angiopoietin-2: development of inhibitors for cancer therapy

Angiopoietin-2 (Ang2) is a member of the Ang family, which plays an important role in angiogenesis during the development and growth of human cancers. Ang2's role in angiogenesis generally is considered as an antagonist for Ang1, inhibiting Ang1-promoted Tie2 signaling, which is critical for blood vessel maturation and stabilization. Ang2 modulates angiogenesis in a cooperative manner with another important angiogenic factor, vascular endothelial growth factor A. Genetic studies have revealed that Ang2 also is critical in lymphangiogenesis during development. However, new evidence suggests more complicated roles for Ang2 in angiogenesis in physiologic processes and invasive phenotypes of cancer cells during progression of human cancers. This article discusses recent studies of Ang2 in angiogenesis and the implication of Ang2 as a therapeutic target as well as a potential inhibitor for antiangiogenesis treatment for cancer patients.

Serum angiopoietin-1 as a prognostic marker in resected early stage lung cancer

PURPOSE

We evaluated the clinical significance of angiopoietins and vascular endothelial growth factor (VEGF) in patients with resected early stage lung cancer.

PATIENTS AND METHODS

The study enrolled 101 patients with completely resected non-small cell lung cancer (NSCLC) of stage I or II, along with 70 healthy volunteers. Serum concentrations of angiopoietin-1, angiopoietin-2, and VEGF were measured with an ELISA. Immunohistochemical expression of angiopoietin-1 was compared with the microvessel density on the lung cancer tissues.

RESULTS

The patients had lower serum angiopoietin-1 (32.1+/-9.9 ng/mL vs. 39.0+/-10.8 ng/mL, p<0.001), higher angiopoietin-2 (1949.2+/-1099.4 pg/mL vs. 1498.6+/-650.0 pg/mL, p<0.01), and higher VEGF (565.1+/-406.3 pg/mL vs. 404.6+/-254.8 pg/mL, p<0.01) levels than the controls. The angiopoietin-2 level was higher in stage II than in stage I patients (p<0.05). The levels of angiopoietin-1 (r=0.28) and angiopoietin-2 (r=0.36) each correlated with the VEGF level. Patients with a higher level of angiopoietin-1 (> or =31.2 ng/mL) had better disease-specific and relapse-free survival than those with a lower angiopoietin-1 level (<31.2 ng/mL). Angiopoietin-1 expression negatively correlated with the microvessel density.

CONCLUSION

Serum angiopoietin-1 is a potential marker for predicting postoperative survival and recurrence in patients with early stage NSCLC.

The angiopoietin/Tie2 axis mediates malignant pleural effusion formation

PURPOSE

Angiopoietins and their receptor, Tie2, participate in angiogenesis, regulation of vascular permeability, and inflammation, all central to the pathogenesis of malignant pleural effusions (MPEs). In the present study, we aimed to examine the role of the angiopoietin/Tie2 axis in MPE pathogenesis.

EXPERIMENTAL DESIGN

MPE was induced by intrapleural injection of murine adenocarcinoma cells in C57BL/6 mice. Animals were given twice-weekly intraperitoneal injections of 40 mg/kg MuTekdeltaFc or vehicle. MuTekdeltaFc is a soluble Tie2 (sTie2) receptor that binds murine angiopoietins thereby disrupting their interaction with Tie2 receptors expressed on tissues. Animals were killed on day 14.

RESULTS

Angiopoietin/Tie2 axis blockade significantly reduced pleural fluid volume and pleural tumor foci. The mean +/- SEM pleural fluid volumes were 617 +/- 48 microl and 316 +/- 62 microl for the control and treated groups, respectively (P = .001), whereas the mean +/- SEM tumor foci were 7.3 +/- 1.0 and 3.0 +/- 0.52 for the control and treated groups, respectively (P = .001). In addition, tumor-associated cachexia, tumor angiogenesis, pleural vascular permeability, recruitment of inflammatory cells to the pleural cavity, and local elaboration of vascular endothelial growth factor and interleukin 6 were also downregulated, and tumor cell apoptosis was induced in animals treated with the inhibitor.

CONCLUSIONS

Our results indicate that the angiopoietin/Tie2 axis is an important component of MPE pathogenesis. Further studies are required to determine whether therapeutic interventions targeting this pathway could be beneficial for patients with MPE.

Integrin and growth factor receptor alliance in angiogenesis

A sequence of events in vascular and stromal cells maintained in a highly coordinated manner regulates angiogenesis and tissue remodeling. These processes are mediated by the ability of cells to respond to environmental cues and activate surface integrins. Physiological and pathological processes in vascular biology are dependent on the specificity of important signaling mechanisms that are activated through the association between growth factors, their receptors, integrins, and their specific extracellular matrix ligands. A large body of evidence from in vitro and in vivo models demonstrates the importance of coordination of signals from the extracellular environment that activates specific tyrosine kinase receptors and integrins in order to regulate angiogenic processes in vivo. In addition to complex formation between growth factor receptors and integrins, growth factors and cytokines also directly interact with integrins, depending upon their concentration levels in the environment, and differentially regulate integrin-related processes. Recent studies from a number of laboratories including ours have provided important novel insights into the involvement of many signaling events that improve our existing knowledge on the cross-talk between growth factor receptors and integrins in the regulation of angiogenesis. In this review, our focus will be on updating the recent developments in the field of integrin-growth factor receptor associations and their implications in the vascular processes.

PPARgamma and Agonists against Cancer: Rational Design of Complementation Treatments

PPARγ is a member of the ligand-activated nuclear receptor superfamily: its ligands act as insulin sensitizers and some are approved for the treatment of metabolic disorders in humans. PPARγ has pleiotropic effects on survival and proliferation of multiple cell types, including cancer cells, and is now subject of intensive preclinical cancer research. Studies of the recent decade highlighted PPARγ role as a potential modulator of angiogenesis in vitro and in vivo. These observations provide an additional facet to the PPARγ image as potential anticancer drug. Currently PPARγ is regarded as an important target for the therapies against angiogenesis-dependent pathological states including cancer and vascular complications of diabetes. Some of the studies, however, identify pro-angiogenic and tumor-promoting effects of PPARγ and its ligands pointing out the need for further studies. Below, we summarize current knowledge of PPARγ regulatory mechanisms and molecular targets, and discuss ways to maximize the beneficial activity of the PPARγ agonists.

Regulation of angiogenesis: apoptotic cues from the ECM

The extracellular matrix (ECM) acts both as a physical scaffold for cells and as a repository for growth factors. Moreover, ECM structure and physical-chemical properties convey precise information to cells that profoundly influences their biology by interactions with cell surface receptors termed integrins. During angiogenesis, the perivascular ECM plays a critical role in determining the proliferative, invasive and survival responses of the local vascular cells to the angiogenic growth factors. Dynamic changes in both the ECM and the local vascular cells act in concert to regulate new blood vessel growth. The digestion of ECM components by proteolysis is critical for the invasive capacity of endothelial cells, but also creates ECM fragments, which antagonize the mechanosensory function of integrins, and can be apoptogenic. Here, we discuss the roles of integrins in modulating cellular responses to a changing ECM, in particular the regulation of survival and invasion among invasive endothelial cells.

Angiopoietin-2 predicts disease-free survival after allogeneic stem cell transplantation in patients with high-risk myeloid malignancies

Emerging data suggest a critical role for bone marrow angiogenesis in hematologic malignancies. The angiopoietin/Tie ligand-receptor system is an essential regulator of this process. We evaluated whether circulating angiopoietin-2 (Ang-2) is a predictor for the probability of disease-free survival (DFS) in allogeneic hematopoietic stem cell transplantation (allo-HSCT) for high-risk acute myeloid leukemia or myelodysplastic syndrome. Ang-2 was measured by enzyme-linked immunosorbent assay in serum from 20 healthy controls and 90 patients with acute myeloid leukemia or myelodysplastic syndrome before conditioning for HSCT. Circulating Ang-2 was elevated in patients (median, 2.21 ng/mL; range, 0.18-48.84 ng/mL) compared with controls (median, 0.87 ng/mL; range, 0.27-4.51 ng/mL; P < .001). Multivariate analyses confirmed the independent prognostic impact of Ang-2 (hazard ratio [HR] = 2.46; 95% confidence interval [CI], 1.27-4.76, P = .005), percentage of bone marrow infiltration (HR = 1.14; 95% CI, 1.01-1.29, P = .033), and chemotherapy cycles before HSCT (HR = 1.38; 95% CI, 1.01-1.08, P = .048). Regression tree analysis detected optimal cutoff values for Ang-2 and recursively identified bone marrow blasts and Ang-2 as the best predictors for DFS. Because few predictors for DFS exist in the setting of allo-HSCT, Ang-2 may be used as a readily available powerful biomarker to pre-estimate DFS and may open new perspectives for risk-adapted treatment of high-risk myeloid malignancies.

Relationship between human tumour angiogenic profile and combretastatin-induced vascular shutdown: an exploratory study

Combretastatin-A4-phosphate (CA4P) acts most effectively against immature tumour vasculature. We investigated whether histological angiogenic profile can explain the differential sensitivity of human tumours to CA4P, by correlating the kinetic changes demonstrated by dynamic MRI (DCE-MRI) in response to CA4P, with tumour immunohistochemical angiogenic markers. Tissue was received from 24 patients (mean age 59, range 32–73, 18 women, 6 men). An angiogenic profile was performed using standard immunohistochemical techniques. Dynamic MRI data were obtained for the same patients before and 4 h after CA4P. Three patients showed a statistically significant fall in K^trans following CA4P, and one a statistically significant fall in IAUGC₆₀. No statistically significant correlations were seen between the continuous or categorical variables and the DCE-MRI kinetic parameters other than between ang-2 and K^trans (P=0.044). In conclusion, we found no strong relationships between changes in DCE-MRI kinetic variables following CA4P and the immunohistochemical angiogenic profile.

Oncolytic HSV-1 infection of tumors induces angiogenesis and upregulates CYR61

Oncolytic viral therapy is under evaluation for toxicity and efficacy in clinical trials relating to several different tumors. We report a significant increase in the angiogenic index of oncolytic virus (OV)-treated glioma-matrigel implants (2.83-fold, P < 0.02). In a rat intracranial glioma model, large tumors from OV-treated animals were significantly more angiogenic than the phosphate-buffered saline (PBS)-treated control tumors (OV: 101 +/- 21.6; PBS: 19.8 +/- 10; P = 0.0037). Transcript profiling of OV-treated tumors revealed dysregulation of several transcripts involved in glioma angiogenesis. OV-mediated induction of CYR61 gene expression (8.94-fold, P = 0.001) correlated significantly with the presence of OV in tumor tissue in vivo (R = 0.7, P < 0.001). Further, induction of CYR61 mRNA and protein were confirmed in multiple human cancer cell lines and primary human tumor-derived cells in vitro, and in tumor lysate and cerebrospinal fluid (CSF) in vivo. Finally, we show that treatment of glioma cells with Cilengitide, known to counter CYR61-induced integrin activation, significantly suppressed the proangiogenic effect of OV treatment of gliomas (P < 0.05).