Angiogenesis – recent developments in molecular pathogenesis, morphological markers, and therapeutic applications
Angiogenesis is of fundamental importance in health and disease (1). In 1971, Folkman suggested that the growth of malignant tumours is dependent on the angiogenic process (2). Since then, extensive experimental data have focused on various angiogenic factors, their receptors, and the molecular pathogenesis and balanced regulation of the angiogenic response. Recent research has revealed multiple cross-talking molecules and interacting cell types as active partners in this complex process, e.g. tumour cells, endothelial cells, stromal cells, inflammatory cells, and even circulating endothelial progenitor cells originating from the bone marrow (3). Whereas the initial stages of malignant tumours might be independent of active angiogenesis, further growth and spread depend on the so-called angiogenic switch (4), the onset of neovascularization. Current studies within this field have had two important aims: to detect the onset of this switch as early as possible (diagnosis), and to prevent this switch from occurring (therapy). A detailed understanding of neoangiogenesis, and the mechanisms and timing of the angiogenic switch, is necessary to achieve this.
Some of the findings from experimental models have been further explored in translational studies to establish the clinical importance of angiogenesis in various diseases and to detect possible targets for novel therapy. Today, multiple trials of promising antiangiogenic treatment have been initiated for some benign conditions and for different types of malignant tumours. The clinical effect of anti-VEGF treatment in metastatic colorectal cancer was recently reported (5), and the use of this treatment was approved by the US Food and Drug Administration (FDA) in 2004. Nevertheless, more data are needed on how to design and combine various antiangiogenic treatment modalities. In addition, the practical value of angiogenic markers when examined in pathology specimens submitted for diagnostic and prognostic evaluation needs to be further studied, especially to assess how they might be more efficiently used to predict or monitor the response to treatment. In particular, the limitations of microvessel density, which has been widely used in prognostic studies, have been discussed recently (6), and a more detailed profiling of the angiogenic and lymphangiogenic response of human tumours in vivo is needed.
In this special issue on angiogenesis, some updates on recent developments are presented, especially related to tumour-associated angiogenesis. The reviews include data on basic mechanisms, discussions of morphological markers, and views and perspectives on clinical applications and novel therapy.
In their review, Costa and co-workers (pp. 402–12) focus on the complex roles of VEGF, including the presence of VEGF-receptors on tumour cells and other cell types. The presence of autocrine stimulation might be important for tumour growth and expansion, also suggesting that the inhibitory effect of anti-VEGFR-2 blocking agents may be explained both by inhibiting angiogenesis and by direct targeting of cancer cells. In addition, the authors discuss the role of circulating endothelial progenitor cells for active angiogenesis in tumours as well as other diseases.
More details of the VEGF subtypes, different isoforms, VEGF receptors, and other angiogenic factors are given in the paper by McColl et al. (pp. 463–80). The role of proteolysis and hypoxia is focused upon and seems to be of particular interest in the development of therapeutic possibilities for various diseases where such mechanisms are involved. The authors also describe the importance of various growth factors for lymphangiogenesis.
Rosenkilde & Schwartz (pp. 481–95) present a review of receptor-dependent and -independent mechanisms by which multiple members of the chemokine system influence angiogenesis, with the focus on wound healing and tumour development. The interesting phenomenon of molecular piracy of host-encoded genes by a certain group of herpes viruses is discussed.
Sund et al. (pp. 450–62) highlight the contribution of extracellular matrix and vascular basement membrane in the regulation of angiogenesis and tumour progression, especially with respect to the dynamic interaction between endothelial cells and pericytes. They report on a new concept where the vascular basement membrane influences endothelial cell and pericyte interactions by constituting an extracellular microenvironment sensor which affects cell growth, differentiation and apoptosis. Furthermore, they discuss the role of endogeneous angiogenesis inhibitors as parts of the vascular basement membrane system.
How the angiogenic process can be quantified or graded has been discussed for several years. Following early proposals in 1972 (7), later studies by Weidner et al. have established criteria for microvessel counts (8). Other features of the tumour-associated angiogenic phenotype, such as glomeruloid microvascular proliferation (9) and vascular maturation (10), have also been reported. In this issue, Fox & Harris (pp. 413–30) discuss various aspects of the histological quantification of tumour angiogenesis. Considering the complex nature of vessel formation, it is not well established how this process can be most reliably estimated in tissue samples, for instance in malignant tumours. Moreover, the practical value of angiogenic markers with reference to prognostic impact or prediction of therapeutic response to antiangiogenic therapy is still being discussed (6). The authors describe different antibodies that can be used, methods for quantification (Chalkley counts, image analysis), evaluation of vascular proliferation and maturity, and other issues. Importantly, they discuss the problem of intra- and interobserver variation when trying to standardize the assessment of angiogenesis in tissue sections.
Another aspect of evaluating tumour angiogenesis is discussed in the paper by Giatromanolaki et al. (pp. 431–40). The authors suggest that combining vessel counts at the edge of the tumour with the inner tumour area might give a better overall picture of the angiogenic activity and of vascular survival. These vascular patterns might also provide prognostic information.
Hoffmann (pp. 441–9) has developed three elegant in vivo mouse models to visualize angiogenesis using fluorescent proteins. Either the tumour implants or the recipient mice are genetically manipulated to express green or red fluorescent proteins in vivo, thus enabling dynamic studies of many different aspects of angiogenesis using real-time imaging, including tumour-stroma interactions and tumour-induced angiogenesis. The possibilities and applications of this powerful technique seem numerous, not least for studies of targeted therapy.
Recently, several basic and clinical studies have concentrated on the regulation and role of lymphangiogenesis in various diseases, including malignant tumours (11). In this issue, Jackson (pp. 526–38) discusses the biochemistry and biology of LYVE-1, which has been considered a sensitive and specific marker for lymphatic endothelium, and anti-LYVE-1 has been used as a molecular marker on tissue sections in multiple translational studies. The role of LYVE-1 in the trafficking of cells within lymphatic vessels and lymph nodes is considered.
Stacker et al. (pp. 539–49) discuss the complex process of tumour metastases and focus on members of the VEGF family, which in many solid tumours play a key role in promoting angiogenesis and lymphangiogenesis and also metastatic spread. They especially focus on VEGF-C and VEGF-D, signalling via VEGFR-2 and VEGFR-3.
Folberg & Maniotis (pp. 508–25) discuss the challenging concept of vasculogenic mimicry, which describes the formation of fluid-conducting channels by highly invasive tumour cells. Others have suggested that endothelial cells and tumour cells might form a “mosaic” within the vessel walls (12). The authors discuss two subtypes, and present the morphology and proposed molecular pathogenesis behind these observed patterns, which might be relevant for the development of antivascular therapy in a broader context (13).
Against the background of our present knowledge, is it possible to translate the available information into the efficient treatment of individual patients? In a comprehensive and “state of the art” review by Folkman (pp. 496–507), who founded the field of modern angiogenesis research more than 30 years ago, a detailed discussion of present treatment options is given. Folkman describes the three main groups of angiogenesis inhibitors, with especial attention to the role of endogenous inhibitors, such as angiostatin, endostatin and tumstatin. A combination of more than one type of inhibitor might be more efficient than single-drug regimens.
In conclusion, the number of studies and publications in the field of angiogenesis has increased exponentially during recent years, and at present there are more than 20,000 hits in the PubMed database of biomedical research. Extensive data have been reported, and the challenge ahead will be to increase the sensitivity and specificity of angiogenesis-related diagnostic markers and to design increasingly effective antiangiogenic treatment protocols.
