Kari Alitalo
    Academy Professor, M.D., Dr. Med.Sci.

    Translational Cancer Biology Research Program
    Institute of Biomedicine
    Biomedicum Helsinki

    Director
    Wihuri Research Institute

    P.O. Box 63 (Haartmaninkatu 8)
    FI-00014 University of Helsinki
    Finland

    Tel. +358 2 941 25511
    Fax +358 2 941 25510

    firstname.surname@helsinki.fi

Affiliations

Alma-intranet Flammaintra

Targeting tumor stem cells and vascular signaling in cancer

A major goal of our research is to define the roles of specific receptor tyrosine kinases and their growth factor ligands, which we have cloned, in the growth and differentiation of endothelial cells and their roles in tumor angiogenesis and lymphatic metastasis in various neoplasia. We also want to understand the relationship of the cancer stem cell niche provided by tumor vessels. Vascular endothelial growth factors (VEGFs), angiopoietins (Ang) and their receptors play crucial roles in the formation of blood vessels and lymphatic vessels in embryogenesis and during several pathological conditions, such as tumor progression and metastasis.

The importance of the growth of new blood vessels, or angiogenesis, for the growth of tumors has been realized decades ago and the first anti-angiogenic agents have recently been approved for clinical use. Although these treatments have been highly successful in the treatment of several types of solid tumors, most patients are either refractory or eventually acquire resistance to current anti-angiogenic therapeutics by a variety of mechanisms. Therefore additional treatment modalities to effectively and safely target tumor vessels are required to complement existing therapies. We believe that a combination of angiogenesis inhibitors based on the major interacting angiogenesis signaling pathways can be used to significantly advance the efficacy of tumor therapy, and we aim to provide proof of principle of this based on new knowledge of the biology behind the angiogenesis process.

Lymphatic vessels maintain tissue fluid homeostasis and collect interstitial fluid and associated antigens permitting downstream immune monitoring by sentinel lymph nodes, with entry for antigens and antigen-presenting cells required to mount immune responses. The growth of lymphatic vessels, lymphangiogenesis, is actively involved in a number of pathological processes including tissue inflammation and tumor dissemination but is insufficient in patients suffering from lymphedema, which often is a complication of cancer surgery. Our recent studies on the molecular mechanisms governing lymphangiogenesis provides new possibilities for the treatment of lymphatic metastasis.

Growth factor mechanisms operating in angiogenesis and lymphangiogenesis. Members of the VEGF family, currently comprising five mammalian proteins, are major regulators of blood and lymphatic vessel development and growth. VEGFs stimulate angiogenesis and lymphangiogenesis by activating VEGF receptor (VEGFR) tyrosine kinases and neuropilin (NRP) coreceptors in endothelial cells.

VEGF promotes angiogenesis

VEGF, is induced in tissue hypoxia, for example in embryos and in tumors and binds VEGFR-1 and VEGFR-2 in nearby endothelial cells. This initiates the process of new vessel growth. Antibodies that block VEGF have been successfully used in tumor therapy.

VEGF-C and VEGF-D promote lymphangiogenesis.

In adults, the third VEGF receptor, VEGFR-3, is primarily expressed in the lymphatic vasculature, where it acts as an essential regulator of lymphatic endothelial cell migration, proliferation and survival. We have shown that VEGF-C and VEGF-D can directly induce lymphangiogenesis by activating VEGFR-3 in lymphatic endothelial cells. Tumors that overexpress either growth factor show enhanced metastasis to the lymph nodes. Conversely, we have shown that blocking VEGFR-3 suppresses lymphatic metastasis by 50-70% in preclinical animal models. Despite the considerable decrease in the rate of metastasis obtained with the VEGFR-3 neutralizing agents, new and more effective tools are needed for inhibition of metastasis.

Importantly, VEGFR-3 is also highly expressed in angiogenic blood vessels in embryos, tumors and wounds, but becomes downregulated as the blood vessels mature. Our recent results indicate that specific blocking of VEGFR-3 signaling with monoclonal antibodies reduces the number of blood vessels in tumors and suppresses the rate of tumor growth. Interestingly, combining anti-VEGFR-3 treatment with the  “golden standard” anti-angiogenic therapy (anti-VEGF) provided an additional reduction in the tumor growth rate. More detailed studies showed that VEGFR-3 is induced at the onset of angiogenesis in endothelial tip cells that lead the formation of new vessel sprouts, and, once induced, can sustain angiogenesis even in the presence of VEGFR-2 blocking agents. Further studies also elucidated that VEGFR-3 expression is regulated by Delta-like ligand 4 (Dll4)/Notch signaling in endothelial cells.

Angiopoietins mediate endothelial cell quiescence vs. angiogenesis

Besides the VEGFs, the angiopoietins (Ang1, Ang2, Ang3/4) are critical in tumor angiogenesis. Angiopoietins bind to one of the two Tie receptor tyrosine kinases (Tie2); of which we cloned the first one (Tie1). Activated endothelial cells in tumor blood vessels produce Ang2, promoting the so-called angiogenic switch. In contrast, the tumor vessel normalization that occurs e.g. during VEGF blockage requires Ang1. Although Ang2 inhibitors have already provided promising results, the full exploitation of the angiopoietins and Tie receptors as therapeutic targets requires a better understanding of their signaling functions.

Our objective is the discovery and characterization of novel growth factor-receptor signaling network interactions, and their use in the design of effective anti-angiogenic treatments for cancer. In particular, we want to explore the relationship of cancer stem cells, tumor vasculature and angiogenesis. As a convenient model, we use transgenic mice and cancer stem-cell based organoid cultures from colorectal cancer patients.