Lymphangiogenesis and lymphatic metastasis

Abstract

The major cause of cancer mortality is metastasis, which relies on the growth of blood vessels (haemangiogenesis) and lymphatic vessels (lymphangiogenesis). Whereas the field of haemangiogenesis has been relatively thoroughly studied, little is known about the mechanisms regulating lymphangiogenesis. Recent research efforts in studying lymphangiogenesis have been focused on two members of the VEGF-family, VEGF-C and VEGF-D. However, it seems unlikely that these would be the sole factors regulating the formation and maintenance of the lymphatic system.

In this thesis work, we have identified several novel lymphangiogenic factors, including members of the PDGF-, IGF-, and VEGF families, and investigated the role of bone marrowderived circulating endothelial precursor cells (CEPCs) in promoting lymphangiogenesis.

It has previously been demonstrated that members of the PDGF family are potent haemangiogenic factors. In this thesis, we provide compelling evidence that PDGFs display direct lymphangiogenic activity. We have localized expression of the PDGF receptors on lymphatic endothelium and demonstrated direct stimulatory effects of PDGFs on primary lymphatic endothelial cells in vitro, as well as lymphangiogenic activities in vivo. Overexpression of PFGF-BB in murine fibrosarcomas stimulated tumoral lymphangiogenesis and promoted lymphatic metastasis. VEGF-A is another key angiogenic factor frequently utilized by tumors and other tissues to switch on their angiogenic phenotypes. This factor was previously thought to act as a specific haemangiogenic factor. However, we and others have identified VEGF-A as a novel lymphangiogenic factor in vivo.

We found that overexpression of VEGF-A in murine fibrosarcomas induced the accumulation of peritumoral lymphatic vessels and promoted metastasis to the regional lymph nodes. The mechanism by which VEGF-A induces lymphangiogenesis might involve both direct effects, through activation of VEG17R-2 expressed on lymphatic endothelium, and indirect effects, by recruiting inflammatory cells that secrete lymphangiogenic cytokines and growth factors. Members of the IGF family are widely expressed in various types of solid tumors. IGF-1R, the major receptor of the IGF family, may indirectly stimulate lymphangiogenesis by upregulating the expression of several known lymphangiogenic factors. However, we have demonstrated that IGF-1 and IGF-2 also can directly induce lymphangiogenesis by activating the cognate receptor expressed in lymphatic endothelium.

Our findings suggest that these factors may contribute to lymphatic metastasis. Bone marrow constitutes a rich reservoir of organ-specific pluripotent and committed stem cells. CEPCs have recently been shown to contribute to postnatal vasculogenesis. Although sprouting of new lymphatics from the pre-existing lymphatic network is a critical mechanism for postnatal lymphangiogenesis, it is possible that lymphvasculogenesis also occurs. In this thesis work we show that CEPCs were incorporated into newly formed lymphatic vessels at both physiological and pathological conditions, making CEPCs an interesting target in the development and evaluation of new therapeutic drugs and strategies for the treatment of lymphatic metastasis. Increased molecular understanding of regulators contributing to lymphangiogenesis will increase the possibility to prevent lymphatic, in addition to haematogenous, spread of tumors.