Synergistic effects of L- and P-selectin in facilitating tumor metastasis can involve non-mucin ligands and implicate leukocytes as enhancers of metastasis

P-selectin facilitates human carcinoma metastasis in immunodeficient mice by mediating early interactions of platelets with blood-borne tumor cells via their cell surface mucins, and this process can be blocked by heparin [Borsig, L., Wong, R., Feramisco, J., Nadeau, D. R., Varki, N. M. & Varki, A. (2001) Proc. Natl. Acad. Sci. USA 98, 3352-3357]. Here we show similar findings with a murine adenocarcinoma in syngeneic immunocompetent mice but involving a different P-selectin ligand, possibly a sulfated glycolipid. Thus, metastatic spread can be facilitated by tumor cell selectin ligands other than mucins. Surprisingly, L-selectin expressed on endogenous leukocytes also facilitates metastasis in both the syngeneic and xenogeneic (T and B lymphocyte deficient) systems. PL-selectin double deficient mice show that the two selectins work synergistically. Although heparin can block both P- and L-selectin in vitro, the in vivo effect of a single heparin dose given before tumor cells seems to be completely accounted for by blockade of P-selectin function. Thus, L-selectin on neutrophils, monocytes, and/or NK cells has a role in facilitating metastasis, acting beyond the early time points wherein P-selectin mediates interactions of platelet with tumor cells.