Expression of the angiopoietins and their receptor Tie2 in human renal clear cell carcinomas; regulation by the von Hippel-Lindau gene and hypoxia

Regulation of angiogenesis by hypoxia and hypoxia-inducible factors

Maintenance of oxygen homeostasis is critical for the survival of multicellular organs. As a result, both invertebrates and vertebrates have developed highly specialized mechanisms to sense changes in oxygen levels and to mount adequate cellular and systemic responses to these changes. Hypoxia, or low oxygen tension, occurs in physiological situations such as during embryonic development, as well as in pathological conditions such as ischemia, wound healing, and cancer. A primary effector of the adaptive response to hypoxia in mammals is the hypoxia-inducible factor (HIF) family of transcription regulators. These proteins activate the expression of a broad range of genes that mediate many of the responses to decreased oxygen concentration, including enhanced glucose uptake, increased red blood cell production, and the formation of new blood vessels via angiogenesis. This latter process is dynamic and results in the establishment of a mature vascular system that is indispensable for proper delivery of oxygen and nutrients to all cells in both normal tissue and hypoxic regions. Angiogenesis is essential for normal development and neoplastic disease as tumors must develop mechanisms to stimulate vascularization to meet increasing metabolic demands. The link between hypoxia and the regulation of angiogenesis is an area of intense research and the molecular details of this connection are still being elaborated. This chapter will provide an overview of current knowledge and highlight new insights into the importance of HIF and hypoxia in angiogenesis in both physiological and pathophysiological conditions.

Expression and prognostic significance of angiopoietin in colorectal carcinoma

BACKGROUND AND OBJECTIVES

Growth and metastasis of malignant tumors depend on the angiogenesis. The aim of this study was to elucidate the prognostic significance of angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) expression in advanced colorectal carcinoma.

METHODS

Totally, 101 patients with surgically resected advanced colorectal carcinomas were enrolled. The tumor expressions of Ang-1 and Ang-2 were evaluated immunohistochemically, and their relationships with clinicopathological factors and prognosis were investigated. Tumor microvessel density (MVD) was also calculated and correlated with angiopoietin expression.

RESULTS

Ang-1 and Ang-2 were detected in 26 (25.7%) and 45 (44.6%), respectively, of 101 cancerous lesions. Overexpression of Ang-1 was correlated with high MVD. Overexpression of Ang-2 was correlated with lymph node metastasis, venous invasion, preoperative carcinoembryonic antigen levels, and high MVD (P < or = 0.05). MVD was not significantly upregulated by Ang-1 expression, but was significantly upregulated by Ang-2 expression (P < or = 0.01). However, only patients with Ang-2 overexpression showed a significantly worse prognosis than those without Ang-2 overexpression. Multivariate analysis with logistic regression for 5-year survival revealed that cancerous stage and Ang-2 overexpression were independent prognostic indicators.

CONCLUSIONS

The Ang-1 expression correlated with MVD. However, Ang-2 expression was a useful prognostic marker in the management of patients with advanced colorectal carcinoma.

Id1 gene transfer confers angiogenic property on fully differentiated endothelial cells and contributes to therapeutic angiogenesis

BACKGROUND

Transplantation of endothelial progenitor cells has been proposed as a potential strategy for therapeutic revascularization. However, the limited endogenous cell pool and the related technical difficulties constitute clinically important disadvantages to autologous transplantation. In this study we investigated whether fully differentiated endothelial cells (ECs) modified with gene transfer of Id1, a helix-loop-helix transcription factor involved in angiogenesis, have the potential to contribute to therapeutic angiogenesis.

METHODS AND RESULTS

The Id1 gene was transferred into human umbilical vein ECs (HUVECs) via a Sendai virus vector. Id1 stimulated migration, proliferation, and capillary-like tube/cord formation of HUVECs. In addition, Id1 reduced serum deprivation-induced HUVEC apoptosis, as shown by FACS analysis with annexin V and TUNEL staining. Transplantation of Id1-overexpressing HUVECs accelerated recovery of blood flow as evaluated by laser-Doppler perfusion imaging, increased capillary density, and improved the rate of limb salvage compared with the transplantation of control HUVECs. Histochemical analysis revealed that the regenerated vascular networks of limbs transplanted with Id1-overexpressing HUVECs contained numerous HUVECs, some of which were in a proliferative state. Untransfected HUVECs were also incorporated with Id1-transfected HUVECs, suggesting the noncell autonomous effect of Id1. Finally, angiopoietin-1 was upregulated in Id1-overexpressing HUVECs and functionally contributed to the in vitro angiogenic effect of Id1.

CONCLUSIONS

Id1 gene transfer conferred HUVECs with an angiogenic property, contributing to neovascularization after transplantation into ischemic lesions. Transplantation of Id1-overexpressing mature ECs may serve as a novel and useful strategy for therapeutic angiogenesis.

Biological characterization of angiopoietin-3 and angiopoietin-4

The angiopoietin (Ang) family of growth factors includes Ang1, Ang2, Ang3, and Ang4, all of which bind to the endothelial receptor tyrosine kinase Tie2. Ang3 (mouse) and Ang4 (human) are interspecies orthologs. In experiments with human endothelial cell lines, Ang3 was identified as an antagonist of Tie2 and Ang4 was identified as an agonist of Tie2. However, the biological roles of Ang3 and Ang4 are unknown. We examined the biological effect of recombinant Ang3 and Ang4 proteins in primary cultured endothelial cells and in vivo in mice. Recombinant Ang3 and Ang4 formed disulfide-linked dimers. Ang4 (400 ng/mL) markedly increased Tie2 and Akt phosphorylation in primary cultured HUVECs whereas Ang3 (400 ng/mL) did not produce significant changes. Accordingly, Ang4, but not Ang3, induced survival and migration in primary cultured HUVECs. Unexpectedly, intravenously administered Ang3 (30 microg) was more potent than Ang4 (30 microg) in phosphorylating the Tie2 receptor in lung tissue from mice in vivo. Accordingly, Ang3 was more potent than Ang4 in phosphorylating Akt in primary cultured mouse lung microvascular endothelial cells. Ang3 and Ang4 both produced potent corneal angiogenesis extending from the limbus across the mouse cornea in vivo. Thus, Ang3 and Ang4 are agonists of Tie2, but mouse Ang3 has strong activity only on endothelial cells of its own species.

Expression of Angiopoietin-2 and Matrix metalloproteinase-9 in gastric carcinoma

AIM: To investigate the expression of angiopoietin-2 (Ang-2) and matrix metalloproteinase-9 (MMP-9) in gastric carcinoma.

METHODS: The expression of Ang-2 and MMP-9 was detected in tissues of gastric cancer (n = 50) and non-cancerous gastric mucosa (n = 10) by immunohistochemistry.

RESULTS: The positive rates of Ang-2 and MMP-9 in gastric cancer were 56% and 60%, which were significantly higher than those in non-cancerous mucosa. The level of Ang-2 and MMP-9 expression had a negative correlation with the differentiation degree (25% vs 71%, χ² = 9.18, P<0.01; 31% vs 74%, χ² = 8.10, P<0.01), but a positive correlation with TNM stage (30% vs 78%, P<0.01; 35% vs 81%, P<0.01), lymphatic metastasis (39% vs 70%, P<0.05; 43% vs 74%, P<0.05), or microvascular density (Ang-2: 74.911.8 vs 65.611.5, P<0.05). In addition, the level of MMP-9 expression was correlated with the invasion depth (33% vs 79%, P<0.01). Also, there was a significant correlation between the expression of Ang-2 and MMP-9 (χ² = 13.0, P<0.01).

CONCLUSION: Ang-2 and MMP-9 play a synergistic role in the invasion and metastasis of gastric carcinoma. Their colocalization may be a useful indicator for the prognosis of the disease.

Housekeeping proteins: A preliminary study illustrating some limitations as useful references in protein expression studies

A primary objective of many protein expression studies is to define expression patterns that can distinguish between normal and diseased states, enabling a better understanding of molecular events associated with disease development and progression and ultimately potentially finding novel markers or therapeutic targets. Exploration and confirmation of many proteins is often done using Western blotting with normalization against "housekeeping proteins", such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), beta-actin, or beta-tubulin, to correct for protein loading and factors, such as transfer efficiency. Increasingly, in studies examining gene transcript levels, it has been shown that some of the commonly used housekeeping genes may be unsuitable due to the influence of various physiological and pathological factors on their expression. This has not been examined to any great extent for proteins, however. This study examines the degree of variability of three commonly used "housekeeping" proteins (GAPDH, beta-actin, and beta-tubulin) together with class I beta-tubulin, with comparisons being made between a number of different established renal cancer cell lines, matched pairs of renal tumor and normal kidney lysates as well as nine different human tissues and highlights some of the problems encountered.

Novel Antiangiogenic Therapies for Renal Cell Cancer

Renal cell cancer remains a disease for which highly effective therapy for the majority of patients with metastatic disease is lacking. The biology of clear cell carcinomas and their association with mutations of the von Hippel-Lindau gene and its resultant increased expression of vascular endothelial growth factor (VEGF) make angiogenesis a potentially pathophysiologic mechanism for tumor development. As a result, the use of antiangiogenic therapy is an intriguing concept for the treatment of renal cell cancer. Various agents, aside from the inhibitors of VEGF, have been studied, including thalidomide, low-dose interferon, and novel antiangiogenic agents such as the thrombospondin-1 mimetics. Use of these agents has been associated with some degree of objective response or prolonged stabilization of disease, and their true value needs to be assessed in ongoing prospective studies. Combinations of antiangiogenic agents either with other similarly acting drugs or as a component of a "cocktail" with other noncytotoxic therapies should be explored in this patient population.

Renal cell carcinoma: relevance of pathology

PURPOSE OF REVIEW

Although pathological findings including tumour grade and tumour-node-metastasis stage provide prognostic information, the outcome for patients with renal cell carcinoma is occasionally unpredictable. Many studies have been undertaken to improve prediction of the prognosis of renal cell carcinoma, along with efforts to clarify the pathogenesis of renal cell carcinoma at the molecular level. This article reviews the advances in renal cell carcinoma research relevant to pathology published between 1 May 2002 and 30 April 2003.

RECENT FINDINGS

The establishment of a strict tumour grading system or adding microscopic venous invasion or tumour necrosis is potentially useful for predicting patient outcomes. The contribution of the von Hippel-Lindau-related pathway in early carcinogenesis has been partly revealed with regard to angiogenesis and cell cycle control. Tumour development in renal cell carcinoma can be considered as accumulating heterogenous molecular events related to cytokine production, cell cycle control, anti-apoptotic signal machinery and angiogenesis.

SUMMARY

Finding the target genes related to the von Hippel-Lindau pathway may explain the resistance of chemotherapy, and also provide a clue to finding a novel therapeutic option. Molecular targeting therapy can be applicable after more general molecules leading to tumour development and the metastasis of renal cell carcinoma have been identified.

A mathematical model of tumour angiogenesis, regulated by vascular endothelial growth factor and the angiopoietins

Angiogenesis--the growth of new blood vessels from existing ones--is a prerequisite for the growth of solid tumours beyond a diameter of approximately 2 mm. In recent years, the angiopoietins have emerged as important regulators of angiogenesis. They mediate a delicate balance between vascular quiescence, regression and new growth, but their mechanism of action is not fully understood. This work attempts to provide a mathematical description of the role of the angiopoietins in angiogenesis. The model is formulated within the framework of reinforced random walks, which allows easy transition between the continuum (macroscopic) and discrete (microscopic) forms. Model predictions are in qualitative agreement with experimental observations, and may have implications for anti-cancer therapies based on the prevention of angiogenesis.

Expression of angiopoietins in renal epithelial and clear cell carcinoma cells: regulation by hypoxia and participation in angiogenesis

The hereditary von Hippel-Lindau (VHL) syndrome predisposes sufferers to highly vascularized tumors such as renal clear cell carcinoma (RCC) and central nervous system hemangioblastoma. In RCC4 and RCC786-0 VHL- cells with VHL mutations, the protein of hypoxia-inducible factor-1alpha (HIF-1alpha) is constitutively stabilized and the mRNA levels of HIF target genes, including vascular endothelial growth factor (VEGF), are elevated. However, the expression of angiopoietins in these cells and their involvement in angiogenesis are not well known. In this study, we compared the mRNA levels of angiopoietins in human kidney proximal tubule epithelial (RPTE) and RCC4 and RCC786-0 VHL- cells. In RPTE cells, angiopoietin-4 (Ang-4) expression was selectively induced by hypoxia or by expression of a hybrid form of HIF-1alpha. Under normoxic conditions, the mRNA levels of Ang-4 were higher in RCC4 and RCC786-0 VHL- than RPTE cells. Angiopoietin-1 expression was detectable in RCC4 and RCC786-0 VHL- cells but not RPTE cells. In RCC786-0 VHL+ cells, which were stably transfected with a wild-type copy of VHL, the mRNA levels of VEGF and Ang-4 were suppressed and the hypoxic response was restored. We also demonstrated that stimulation of endothelial tube formation by conditioned medium harvested from RCC4 cells was inhibited by a soluble Tie-2 receptor. These results suggest that the angiopoietin/Tie-2 system may participate in the angiogenic response to hypoxia in renal tissues and in tumor angiogenesis in renal carcinoma.

Not just angiogenesis--wider roles for the angiopoietins

Since the discovery of the angiopoietins, much interest has been focused on their biological actions and their potential use as therapeutic targets. It is generally accepted that the angiopoietins play an important role in angiogenesis and hence are described as angiogenic factors. However, it is becoming increasingly clear that this is not their only role and it is likely that the angiopoietins have important roles in a wider range of biological and pathological functions.

Angiopoietin switching regulates angiogenesis and progression of human hepatocellular carcinoma

AIM

Angiopoietin 1 (Ang-1) and its antagonist, angiopoietin 2 (Ang-2), are novel ligands that regulate the Tie2 receptor. The Ang-2 gene is upregulated in the hypervascular type of human hepatocellular carcinoma (HCC). To gain a better understanding of the role of the Ang-Tie2 system in HCC the expression of these genes was investigated in a series of human HCCs.

METHODS

The expression of the angiopoietin and Tie2 proteins was investigated in nine normal liver tissues and 52 surgically resected HCCs. In addition, the effects of hypoxic stimuli on Ang-1, Ang-2, vascular endothelial growth factor (VEGF), and erythropoietin (EPO) expression was investigated in Hep3B cells.

RESULTS

Ang-1, rather than Ang-2, was more frequently expressed in the normal liver. Ang-1 was expressed in 68% of HCCs, whereas Ang-2 was expressed in 81%, and was significantly higher in poorly differentiated HCCs characterised by high vascularity (p = 0.02), and in tumours with a peliotic change (p = 0.02). Strong expression of Tie2 was seen in tumour vessels in accordance with Ang-2 expression. In Hep3B cells, hypoxic stimuli upregulated VEGF and EPO, but not Ang-1 or Ang-2.

CONCLUSIONS

These data support the evidence that the reversal of Ang-1 and Ang-2 expression plays an important role in the angiogenic and dedifferentiation processes in HCC. The hypoxic stimuli were not responsible for Ang-2 upregulation, unlike that of VEGF, in human HCC cells.

The enigmatic role of angiopoietin-1 in tumor angiogenesis

A tumor vasculature is highly unstable and immature, characterized by a high proliferation rate of endothelial cells, hyper-permeability, and chaotic blood flow. The dysfunctional vasculature gives rise to continual plasma leakage and hypoxia in the tumor, resulting in constant on-sets of inflammation and angiogenesis. Tumors are thus likened to wounds that will not heal. The lack of functional mural cells, including pericytes and vascular smooth muscle cells, in tumor vascular structure contributes significantly to the abnormality of tumor vessels. Angiopoietin-1 (Ang1) is a physiological angiogenesis promoter during embryonic development. The function of Ang1 is essential to endothelial cell survival, vascular branching, and pericyte recruitment. However, an increasing amount of experimental data suggest that Ang1-stimulated association of mural cells with endothelial cells lead to stabilization of newly formed blood vessels. This in turn may limit the otherwise continuous angiogenesis in the tumor, and consequently give rise to inhibition of tumor growth. We discuss the enigmatic role of Ang1 in tumor angiogenesis in this review.

Regulation of angiogenesis by hypoxia: role of the HIF system

The regulation of angiogenesis by hypoxia is an important component of homeostatic mechanisms that link vascular oxygen supply to metabolic demand. Molecular characterization of angiogenic pathways, identification of hypoxia-inducible factor (HIF) as a key transcriptional regulator of these molecules, and the definition of the HIF hydoxylases as a family of dioxygenases that regulate HIF in accordance with oxygen availability have provided new insights into this process. Here we review these findings, and the role of HIF in developmental, adaptive and neoplastic angiogenesis. We also discuss the implications of oncogenic activation of extensive, physiologically interconnected hypoxia pathways for the tumor phenotype.