Midkine systemically induces neo-lymphangiogenesis via mTOR signaling in patients with melanoma.
Major finding: Midkine systemically induces neo-lymphangiogenesis via mTOR signaling in patients with melanoma.
Approach: Whole-body imaging of Vegfrluc reporter mouse models characterized melanoma-induced neolymphangiogenesis.
Impact: The Vegfrluc mouse model may enhance the discovery of metastasis drivers and inhibitors in other cancers.
In patients with melanoma, the detection of metastases in sentinel lymph nodes, which is preceded by lymphangiogenesis induced by melanoma-secreted lymphangiogenic factors such as VEGFC, is a prognostic parameter. However, given the minimal impact on overall survival engendered by sentinel lymph node removal and the development of dysfunctional intratumor lymphatics, the role of lymphangiogenesis in melanoma metastasis is uncertain. To identify lymphangiogenic drivers of metastasis, Olmeda and colleagues generated VEGFR3-luciferase (Vegfr3luc) reporter mouse models to monitor human and mouse melanoma. Whole-body imaging of human melanoma cell line xenografts in immunocompromised reporter mice identified four major patterns of tumor-driven neolymphangiogenesis; although patterns I and II exhibited no or tumor-restricted lymphangiogenesis, pattern III exhibited both local and distant lymphangiogenesis, pattern IV exhibited systemic lymphangiogenesis with or without highly delayed primary tumor lymphangiogenesis, and both patterns III and IV exhibited efficient nodal metastasis. Proteomic analysis revealed that the heparin-binding factor midkine (MDK), which has previously been suggested to promote metastasis via lymphovascular-independent functions, was the most upregulated exosomal protein in lymphangiogenic cells compared to nonlymphangiogenic cells. Similarly, human benign nevi and melanoma tumors exhibited MDK expression, and patients with high MDK nodal expression had significantly worse prognosis than patients with low MDK nodal expression. Consistent with these findings, MDK depletion resulted in decreased lymphangiogenesis and systemic metastasis but did not affect tumor angiogenesis, and expression of MDK induced systemic metastasis in nonlymphangiogenic melanoma. Pathway analysis of differentially expressed genes between lymph endothelial cells grown in media from MDK-depleted or overexpressed cells identified the mTOR pathway as highly upregulated by MDK, and media from MDK-overexpressed cells induced the phosphorylation of the mTOR target RPS6, VEGFR3 expression, and lymph endothelial cell sprouting. Taken together, these results identify midkine as a driver of melanoma lymphangiogenesis and describe a mouse lymphoreporter model that provides insight into the establishment of premetastatic niches.