Angiogenesis

About: Angiogenesis is a research topic. Over the lifetime, 58248 publications have been published within this topic receiving 3290129 citations. The topic is also known as: blood vessel formation from pre-existing blood vessels & GO:0001525.

Role of Dicer and Drosha for endothelial microRNA expression and angiogenesis.

Abstract: MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression by binding to the cellular transcript leading to translational repression or degradation of the target mRNA. Dicer and Drosha are the miRNA processing enzymes that are required for the maturation of miRNAs. Here, we investigated the role of Dicer and Drosha for angiogenesis. Endothelial cells were transfected with siRNA against Dicer and Drosha to inhibit miRNA biogenesis. Genetic silencing of Dicer and Drosha significantly reduced capillary sprouting of endothelial cells and tube forming activity. Migration of endothelial cells was significantly decreased in Dicer siRNA-transfected cells, whereas Drosha siRNA had no effect. Silencing of Dicer but not of Drosha reduced angiogenesis in vivo. Next, we attempted to identify miRNAs expressed in endothelial cells. A screening analysis of 168 human miRNAs using real-time PCR revealed that members of the let-7 family, mir-21, mir-126, mir-221, and mir-222 are highly expressed in endothelial cells. Dicer and Drosha siRNA reduced lef-7f and mir-27b expression. Inhibitors against let-7f and mir-27b also reduced sprout formation indicating that let-7f and mir-27b promote angiogenesis by targeting antiangiogenic genes. In silico analysis of predicted targets for let-7 cluster identified the endogenous angiogenesis inhibitor thrombospondin-1. Indeed, Dicer and Drosha siRNA significantly increased the expression of thrombospondin-1. Taken together, transient reduction of the miRNA-regulating enzyme Dicer impairs angiogenesis in vitro and in vivo, whereas Drosha siRNA induced a minor antiangiogenic effect in vitro and was not effective in vivo. The let-7 family and mir-27b appear to be attractive targets for modulating angiogenesis.

Less is more, regularly: metronomic dosing of cytotoxic drugs can target tumor angiogenesis in mice

Abstract: Chemotherapeutic drugs, long the mainstay of cancer treatment, cause DNA damage and disrupt DNA replication in proliferating cells. Drug regimens have been designed to kill as many tumor cells as possible by treating with “maximum tolerated doses” (MTDs) of these cytotoxic agents. Side effects such as neurotoxicity and damage to proliferating cells in healthy tissues pose serious constraints on the use of chemotherapy. In an effort to balance toxicity with efficacy, a conventional dosing schedule calls for episodic application of a cytotoxic drug at or near the MTD, followed by periods of rest to allow normal tissues to recover. Many such chemotherapy regimens are initially efficacious, resulting in tumor regression or stabilization and prolonged survival. In rare cases, cures are achieved. In general, however, responses are short-lived, with relapses often marked by aggressive cancers that are resistant to the cytotoxic drug. Furthermore, the standard MTD regimen as a rule seriously impairs quality of life. The harsh side effects and the ultimate failures of most chemotherapies have fueled broad investigation of alternatives, including drugs that target not the transformed tumor cells themselves, but rather a genetically stable constituent cell type of tumors, the endothelial cells that form blood vessels. Angiogenesis, the process by which new blood vessels are formed, is a hallmark capability of cancer (1); a compelling body of evidence argues that tumor growth depends on the vasculature, and, in particular, on continuing angiogenesis (2, 3). More than two dozen new drugs that are in or soon to enter clinical trials appear to interfere with tumor angiogenesis (3, 4); there is considerable anticipation about their benefits in treating cancer. Now, two studies suggest a potentially complementary strategy of rescheduling the administration of classical cytotoxic drugs in order to target tumor endothelial cells. It is well established that tumor-associated endothelial cells proliferate during chronic angiogenesis in tumors, albeit at lower frequencies than the tumor cells themselves. Apparently because of their lower rate of cell division, replication of these endothelial cells is only weakly disrupted by the episodic regimens of standard chemotherapeutic protocols. In these two new studies, however, cytotoxic drugs were administered routinely, to target the slowly proliferating tumor endothelial cells and abrogate their apparent capability to repair and recover during the usual rest periods. Both groups worked with mice bearing subcutaneous tumors, and each presents data suggesting that “metronomic” dosing regimens—either continuous infusion or frequent administration without extended rest periods—could have real value in the clinic. Both also demonstrated combinatorial effects of such altered cytotoxic drug regimens with newer, more specific angiogenesis inhibitors.

Blocking VEGFR-3 suppresses angiogenic sprouting and vascular network formation

Abstract: Angiogenesis, the growth of new blood vessels from pre-existing vasculature, is a key process in several pathological conditions, including tumour growth and age-related macular degeneration. Vascular endothelial growth factors (VEGFs) stimulate angiogenesis and lymphangiogenesis by activating VEGF receptor (VEGFR) tyrosine kinases in endothelial cells. VEGFR-3 (also known as FLT-4) is present in all endothelia during development, and in the adult it becomes restricted to the lymphatic endothelium. However, VEGFR-3 is upregulated in the microvasculature of tumours and wounds. Here we demonstrate that VEGFR-3 is highly expressed in angiogenic sprouts, and genetic targeting of VEGFR-3 or blocking of VEGFR-3 signalling with monoclonal antibodies results in decreased sprouting, vascular density, vessel branching and endothelial cell proliferation in mouse angiogenesis models. Stimulation of VEGFR-3 augmented VEGF-induced angiogenesis and sustained angiogenesis even in the presence of VEGFR-2 (also known as KDR or FLK-1) inhibitors, whereas antibodies against VEGFR-3 and VEGFR-2 in combination resulted in additive inhibition of angiogenesis and tumour growth. Furthermore, genetic or pharmacological disruption of the Notch signalling pathway led to widespread endothelial VEGFR-3 expression and excessive sprouting, which was inhibited by blocking VEGFR-3 signals. Our results implicate VEGFR-3 as a regulator of vascular network formation. Targeting VEGFR-3 may provide additional efficacy for anti-angiogenic therapies, especially towards vessels that are resistant to VEGF or VEGFR-2 inhibitors.

flk-1, an flt-related receptor tyrosine kinase is an early marker for endothelial cell precursors

Abstract: We have used RT-PCR to screen pluripotent murine embryonic stem cells to identify receptor tyrosine kinases (RTKs) potentially involved in the determination or differentiation of cell lineages during early mouse development. Fourteen different tyrosine kinase sequences were identified. The expression patterns of four RTKs have been examined and all are expressed in the mouse embryo during, or shortly after, gastrulation. We report here the detailed expression pattern of one such RTK, the flt-related gene flk-1. In situ hybridization analysis of the late primitive streak stage embryo revealed that flk-1 was expressed in the proximal-lateral embryonic mesoderm; tissue fated to become heart. By headfold stages, staining was confined to the endocardial cells of the heart primordia as well as to the blood islands of the visceral yolk sac and the developing allantois. Patchy, speckled staining was detected in the endothelium of all the major embryonic and extraembryonic blood vessels as they formed. During early organogenesis, expression was detected in the blood vessels of highly vascularized tissues such as the brain, liver, lungs and placenta. Since flk-1 was expressed in early mesodermal cells prior to any morphological evidence for endothelial cell differentiation (vasculogenesis), as well as in cells that form blood vessels from preexisting ones (angiogenesis), it appears to be a very early marker of endothelial cell precursors. We have previously reported that another novel RTK, designated tek, was expressed in differentiating endothelial cells. We show here that flk-1 transcripts are expressed one full embryonic day earlier than the first tek transcripts. The expression of these two RTKs appear to correlate with the specification and early differentiation of the endothelial cell lineage respectively, and therefore may play important roles in the establishment of this lineage.

Pericytes in the microvasculature

Abstract: Pericytes, also known as Rouget cells or mural cells, are associated abluminally with all vascular capillaries and post-capillary venules. Differences in pericyte morphology and distribution among vascular beds suggest tissue-specific functions. Based on their location and their complement of muscle cytoskeletal proteins, pericytes have been proposed to play a role in the regulation of blood flow. In vitro studies demonstrating the contractile ability of pericytes support this concept. Pericytes have also been suggested to be oligopotential and have been reported to differentiate into adipocytes, osteoblasts and phagocytes. The mechanisms involved in vessel formation have yet to be elucidated but observations indicate that the primordial endothelium can recruit undifferentiated mesenchymal cells and direct their differentiation into pericytes in microvessels, and smooth muscle cells in large vessels. Communication between endothelial cells and pericytes, or their precursors, may take many forms. Soluble factors such as platelet-derived growth factor and transforming growth factors-beta are likely to be involved. In addition, physical contact mediated by cell adhesion molecules, integrins and gap junctions appear to contribute to the control of vascular growth and function. Development of culture methods has allowed some functions of pericytes to be directly examined. Co-culture of pericytes with endothelial cells leads to the activation of transforming growth factor-beta, which in turn influences the growth and differentiation of the vascular cells. Finally, the pericyte has been implicated in the development of a variety of pathologies including hypertension, multiple sclerosis, diabetic microangiopathy and tumor vascularization.