From molecular biology to a causal treatment of lymphatic system disorders (abstract)

Research into the molecular basis of lymphangiogenesis in embryonic development and pathological processes has led to a rapid increase of our knowledge (Krebs and Jeltsch 2013A, 2013B). The molecular biology era of lymphatic research began with the discovery of the VEGF growth factors and their receptors 25 years ago, and these molecules are the focus of this overview.

After it had been shown in 1995 that VEGFR-3 is specifically expressed in the adult organism on lymphatic endothelium (Kaipainen et al. 1995), several laboratories tried to identify the ligands that would activate this receptor. The first discovery was made by Kari Alitalo in 1996 with VEGF-C (Joukov et al. 1996). Shortly thereafter, the group Alitalo could show that the primary target of VEGF-C growth factor is the lymphatic vasculature (Jeltsch et al 1997;. Oh et al., 1997) and also the details of the biosynthesis of VEGF-C were explored (Joukov et al., 1997). In 2000, it was published that mutations in the gene of its receptor VEGFR-3 can lead to hereditary lymphedema (Karkkainen et al. 2000). The fact that also mutations in VEGF-C itself can be a cause of lymphedema was discovered only twelve years later (Gordon et al. 2013). As early as in 2001, a pre-clinical study concluded that primary lymphedema could be successfully treated using VEGF-C (Karkkainen et al. 2001). Nevertheless, only this year clinical trials were scheduled, which (unlike the preclinical study) target not primary, but secondary lymphedema after breast cancer surgery. The planned trial will use VEGF-C-producing adenoviral vectors (brand name Lymfactin™) in order to increase the success rate of lymph node transplants. The clinical trials are performed by Herantis Pharma along with several European partners (Honkonen et al 2013; Herantis Pharma 2014).

The active role of VEGF-C on the lymphatic metastasis of primary tumors and thus the potential therapeutic efficacy of blocking the VEGF-C/VEGFR-3 signaling pathway was first described in 2001 by two research groups (Karpanen et al 2001; Mandriota et al. 2001). Meanwhile, two VEGF-C blocking biopharmaceuticals are in clinical trials, but not for metastasis inhibition, but as a supplement to the standard antiangiogenesis drug bevacizumab (Martell et al 2011; Falchook et al 2013).

Three of the recent, partly controversial (re)discoveries in lymphatic research, are worth mentioned here, all of which are related to the VEGF-C growth factor:

• The lymphatic capillaries of the skin contribute to osmoregulation and blood pressure regulation. This is regulated via the secretion of VEGF-C by salt-sensitive macrophages and the associated clearance of interstitial Na+ storages in the skin (Wiig et al. 2013).
• The Schlemm's canal develops from venous structures into a vessel expressing typical lymphatic molecular markers and characteristics (Aspelund et al., In press).
• The pro-lymphangiogenic CCBE1 protein works by enhancing ADAMTS3 activity and by concentrating inactive pro-VEGF-C on cell surfaces and extracellular structures, thereby promoting the VEGF-C activation by cell-surface associated ADAMTS3. When VEGF-C and CCBE1 are simultaneous applied a dramatic increase of the lymphangiogenic effect of VEGF-C can be observed in vivo, which suggests that CCBE1 could be used for pro-lymphangiogenic therapies (Jeltsch et al. 2014).

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Aspelund A, Tammela T, Antila S, Nurmi H, Leppänen V-M, Zarkada G, Stanczuk L, Francois M, Mäkinen T, Saharinen P, Immonen I, Alitalo K. The Schlemm’s canal is a VEGF-C/VEGFR-3 responsive lymphatic-like vessel. Journal of Clinical Investigation. In press.

Jeltsch M, Jha SK, Tvorogov D, Anisimov A, Leppänen V-M, Holopainen T, Kivelä R, Ortega S, Kärpanen T, Alitalo K. CCBE1 Enhances Lymphangiogenesis via A Disintegrin and Metalloprotease With Thrombospondin Motifs-3–Mediated Vascular Endothelial Growth Factor-C Activation. Circulation. 2014;129(19):1962–71. Available from: http://circ.ahajournals.org/content/129/19/1962