Cancer metastasis enables cancer cells to spread from the primary tumor and establish themselves in other tissues. Lymphatic circulation is a common route for cancer metastasis; with the thin walls and few tight junctions in the lymphatic vessels offering good permeability. Moreover, the lack of basal lamina and associated pericytes means that lymphatic capillaries have an easy opening; for the uptake of macromolecular cancer cells into the lymphatic vessels.
Lymphangiogenesis, the process by which new lymphatic vessels grow out of pre-existing vessels; enables lymphatic endothelial cells (LECs) to proliferate and migrate through lymphatic vessels; surrounding the tumors. Lymphangiogenic factors, including vascular endothelial growth factor (VEGF)-C or D, predominantly bind to VEGF receptor-3 (VEGFR-3) and promote several downstream signaling pathways for regulating lymphangiogenesis.
Migration and tube formation
During the lymphangiogenic process, LEC survival, proliferation, migration and tube formation depends upon the activation of the VEGF-C/VEGFR-3 axis. In various experimental tumor models, overexpression of VEGF-C can induce lymphangiogenesis and disseminate metastatic tumor cells to lymph nodes, and the use of neutralizing antibodies against VEGF-C and VEGFR-3 can prevent tumor lymphangiogenesis and lymphatic metastasis.
Therefore lymphangiogenesis is an important biological process with cancer metastasis. The development of new drugs that block lymphangiogenesis represents a promising therapeutic strategy. Marine fungus-derived compound phomaketide A, isolated from the fermented broth of Phoma sp. NTOU4195 has reported exhibiting anti-angiogenic and anti-inflammatory effects.
However, its anti-lymphangiogenic activity has not clarified to date. In this study; they showed that phomaketide A inhibited cell growth, migration, and tube formation of lymphatic endothelial cells (LECs); without evidence of cytotoxicity. Mechanistic investigations revealed that phomaketide A reduced LECs-induced lymphangiogenesis via vascular endothelial growth factor receptor-3 (VEGFR-3), protein kinase Cδ (PKCδ), and endothelial nitric oxide synthase (eNOS) signalings.
Tissue inhibitor matrix
Furthermore, human proteome array analysis indicated that phomaketide A significantly enhanced the protein levels of various protease inhibitors, including cystatin A, serpin B6, tissue factor pathway inhibitor (TFPI), and tissue inhibitor matrix metalloproteinase 1 (TIMP-1). Importantly, phomaketide A impeded tumor growth and lymphangiogenesis by decreasing the expression of LYVE-1, a specific marker for lymphatic vessels, in tumor xenograft animal model.
These results suggest that phomaketide A may impair lymphangiogenesis by suppressing VEGFR-3, PKCδ, and eNOS signaling cascades, while simultaneously activating protease inhibitors in human LECs. We document for the first time that phomaketide A inhibits lymphangiogenesis both in vitro and in vivo; which suggests that this natural product could potentially treat cancer metastasis.
In conclusion, this report discloses a novel mechanism by which phomaketide A reduces LECs lymphangiogenesis in vitro and in vivo. They demonstrate that phomaketide A antagonizes lymphangiogenesis by decreasing VEGFR-3 and its downstream PKCδ and eNOS signaling pathways; as well as increasing protease inhibitors in human LECs