Simultaneous blockade of VEGFR-1 and VEGFR-2 activation is necessary to efficiently inhibit experimental melanoma growth and metastasis formation

Metastasis continues to be the major cause of morbidity and mortality in malignant melanoma. In our study, we explored whether inhibition of VEGFR-1 or VEGFR-2 signaling conveys distinct suppressive effects on B16 melanoma subcutaneous growth and metastasis formation. The inhibition of VEGFR-1 or -2 alone had no significant influence on both melanoma growth and metastasis formation. In contrast, simultaneous blockade of VEGFR-1 and -2 signaling strongly suppressed progression in both B16 tumor models. There was no expression of VEGFR-1 or -2 detectable on the B16 cells used, excluding the melanoma cells as direct therapeutic targets. Analyzing the contribution of progenitor-like cells during melanoma metastasis formation, we observed an enhanced proliferation and mobilization of VEGFR-1+ myeloid and VEGFR-2+ endothelial cells with progenitor potential by the induction of melanoma lung metastasis, which was not influenced by interference with VEGFR signaling. These results indicate that the antimetastatic effects exerted by combined inhibition of VEGFR-1 and -2 signaling were mediated via targeting cell populations other than progenitors only. Sole inhibition of VEGFR-1 signaling led to a strong reduction of the CD45-positive inflammatory infiltrate in the tumor tissue. However, the formation of lung metastasis was not affected, indicating that inhibition of the inflammatory response was not sufficient to efficiently block B16 melanoma metastasis development. Taken together, our data suggest that in the utilized B16 tumor models the blockade of both the inflammatory and the VEGFR-2-dependent angiogenic response are necessary to effectively inhibit solid tumor growth and formation of lung metastasis by B16 melanoma cells.

Endothelial P-selectin as a target of heparin action in experimental melanoma lung metastasis

Spontaneous and experimental metastasis can be effectively inhibited by the widely used anticoagulant heparin in different tumor models. At the cellular level, many of the antimetastatic effects of heparin in vivo are due to its action on P-selectin-mediated binding. Whereas previous attention has focused on P-selectin-dependent tumor-cell-platelet interactions in blood-borne metastasis, we sought to address the potential contribution of endothelial P-selectin expression to adhesive events between the microvasculature and melanoma cells in vivo. Transplantation of bone marrow from P-selectin-deficient into wild-type mice conveyed inhibition of ex-perimental melanoma metastasis. However, the extent to which bone marrow-conferred lack of platelet P-selectin expression attenuated melanoma lung metastasis was significantly less than that seen in P-selectin-deficient mice, suggesting that endothelial P-selectin expression may additionally contribute to formation of hematogenous metastases. This assumption was supported by our intravital microscopy studies, in which a significant proportion of melanoma cells were capable of directly interacting with postcapillary venules of the murine ear in a P-selectin-dependent manner. Heparin not only inhibits P-selectin-mediated melanoma cell rolling but also attenuates melanoma metastasis formation in vivo, further supporting the concept that endothelial P-selectin expression may represent an additional target of heparin action in experimental melanoma lung metastasis.