Functional Significance of Tie2 Signaling in the Adult Vasculature

Abstract

Abundant data now demonstrate that the growth of new blood vessels, termed angiogenesis, plays both pathological and beneficial roles in human disease. Based on these data, a tremendous effort has been undertaken to understand the molecular mechanisms that drive blood vessel growth in adult tissues. Tie2 recently was identified as a receptor tyrosine kinase expressed principally on vascular endothelium. Disrupting Tie2 function in mice resulted in embryonic lethality with defects in embryonic vasculature, suggesting a role in blood vessel maturation and maintenance. Based on these studies, we undertook a series of studies to probe the function of Tie2 in adult vasculature that will form the focus of this chapter. Consistent with a role in blood vessel growth in adult vasculature, Tie2 was upregulated and activated in the endothelium of rat ovary and in healing rat skin wounds, both areas of active angiogenesis. Moreover, Tie2 was upregulated in the endothelium of vascular "hot spots" in human breast cancer specimens. Surprisingly, Tie2 also was expressed and activated in the endothelium of all normal rat tissues examined, suggesting a role in maintenance of adult vasculature. To determine the functional role of Tie2 in tumor vasculature, a soluble Tie2 extracellular domain (ExTek) was designed that blocked the activation of Tie2 by its activating ligand, angiopoietin 1 (Ang1). Administration of recombinant ExTek protein or an ExTek adenovirus inhibited tumor growth and metastasis in rodent tumor models, demonstrating a functional role for Tie2 in pathological angiogenesis in adult tissues. To begin to understand the endothelial signaling pathways and cellular responses that mediate Tie2 function, we identified signaling molecules that are recruited to the activated, autophosphorylated Tie2 kinase domain. Two of these molecules, SHP2 and GRB2, are part of the pathway upstream of mitogen-activated protein kinase (MAPK) activation, a pathway that may be responsible for morphogenetic effects of Tie2 on endothelial cells. Another signaling molecule, p85, is responsible for recruitment of phosphatidylinositol 3 kinase (PI3-K) and activation of the Akt/PI3-K pathway. Akt/PI3-K has emerged as a critical pathway downstream of Tie2 that is necessary for cell survival effects as well as for chemotaxis, activation of endothelial nitric oxide synthase, and perhaps for anti-inflammatory effects of Tie2 activation. Taken together, these studies and many others demonstrate that the Tie2 pathway has important functions in adult tissues, in both quiescent vasculature and during angiogenesis, and help to validate the Tie2 pathway as a therapeutic target.

Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response

“…Other angiopoeitins also have a role [104][105][106][107][108][109][110][111][112][113][114][115] . The 'angiogenic switch' describes a collection of circumstances under which angiogenesis can be initiated, which is determined by a balance of proand anti-angiogenic factors.…”

Marrow angiogenesis-associated factors as prognostic biomarkers in patients with acute myelogenous leukaemia

“…The receptor tyrosine kinase Tie-2 is expressed in the ECs, early HSCs, and the so-called leukaemic cells (Arai et al, 2004;Arai et al, 2005). Ang-1, but not Ang-2, induces autophosphorylation of the Tie-2 receptor upon binding (Korpelainen and Alitalo, 1998;Jones, 2003;Thurston, 2003;Peters et al, 2004). Therefore, Ang-1 and Ang-2 are believed to function as naturally occurring antagonists.…”

Functional implication of BMP4 expression on angiogenesis in malignant melanoma

“…Furthermore, it was demonstrated by Valdimarsdottir et al (2002) that ectopic Id-1 expression was sufficient to induce tube formation of bovine aortic endothelial cells. In addition to these molecules, several groups showed that the endothelial cell-specific tyrosine kinase receptor (TEK/Tie-2) is a key factor for the subsequent remodelling and stabilization of the developing vasculature and seems to be involved in tumor angiogenesis (Cavallaro and Christofori, 2000;Ward and Dumont, 2002;Peters et al, 2004).…”