Independent studies indicate that expression of sialylated fucosylated mucins by human carcinomas portends a poor prognosis because of enhanced metastatic spread of tumor cells, that carcinoma metastasis in mice is facilitated by formation of tumor cell complexes with blood platelets, and that metastasis can be attenuated by a background of P-selectin deficiency or by treatment with heparin. The effects of heparin are not primarily due to its anticoagulant action. Other explanations have been suggested but not proven. Here, we bring together all these unexplained and seemingly disparate observations, showing that heparin treatment attenuates tumor metastasis in mice by inhibiting P-selectin-mediated interactions of platelets with carcinoma cell-surface mucin ligands. Selective removal of tumor mucin P-selectin ligands, a single heparin dose, or a background of P-selectin deficiency each reduces tumor cell-platelet interactions in vitro and in vivo . Although each of these maneuvers reduced the in vivo interactions for only a few hours, all markedly reduce long-term organ colonization by tumor cells. Three-dimensional reconstructions by using volume-rendering software show that each situation interferes with formation of the platelet “cloak” around tumor cells while permitting an increased interaction of monocytes (macrophage precursors) with the malignant cells. Finally, we show that human P-selectin is even more sensitive to heparin than mouse P-selectin, giving significant inhibition at concentrations that are in the clinically acceptable range. We suggest that heparin therapy for metastasis prevention in humans be revisited, with these mechanistic paradigms in mind.

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, and͞or NK cells has a role in facilitating metastasis, acting beyond the early time points wherein P-selectin mediates interactions of platelet with tumor cells.cancer ͉ heparin ͉ glycolipids ͉ platelets H ematogeneous metastasis occurs when tumor cells enter the bloodstream, interact with host blood cells, and eventually colonize a distant tissue. Much evidence indicates that hematogenously disseminating tumor cells interact with platelets and leukocytes, forming microemboli that facilitate their arrest in the vasculature (1-4). Although the contribution of platelets to metastasis has been confirmed in several experimental settings (1, 2, 5-10), the mechanisms and significance of leukocyte participation in tumor microemboli remain largely unknown.Altered cell surface glycosylation is a prominent feature of tumor cells (11)(12)(13)(14)(15). In particular, the expression of sialylated fucosylated glycans like sialyl Lewis x/a correlate with a poor prognosis because of tumor progression and a high rate of metastasis (for examples, see refs. 16-18). Selectins are vascular cell adhesion molecules that recognize certain sialyl Lewis x/a carrying mucin-type glycoproteins found on normal leukocytes and endothelium (11,(19)(20)(21)(22)(23). Carcinoma cell surface molecules carrying sialyl Lewis x/a also can be recognized by all three selectins (11-15, 17, 24-27), mediating tumor cell interactions with platelets, leukocytes, and endothelium in vitro (10,25,27,28). Based on such data, an in vivo role for E-and P-selectins in metastatic spread has been suggested. The hypothesis that E-selectin on activated endothelium might facilitate cancer cell seeding (14, 29) is supported by transgenic over-expression of E-selectin in the mouse liver, which redirected metastases to this organ (30), and a system where tumor cell expression of sialylated fucosylated glycans was increased by transfecting a fucosyltransferase (31). Carcinoma lines expressing E-selectin ligands also displayed an increased metastatic capacity in cytokine-pretreated mice (32). However, E-selectin is not constitutively expressed in vivo but must be transcriptionally activated by certain stimuli. We have previously shown that platelets adhere to some human carcinoma cells mostly via P-selectin and that inhibition of th...

Cell adhesion molecules play a significant role in cancer progression and metastasis. Cell-cell interactions of cancer cells with endothelium determine the metastatic spread. In addition, direct tumor cell interactions with platelets, leukocytes, and soluble components significantly contribute to cancer cell adhesion, extravasation, and the establishment of metastatic lesions. Clinical evidence indicates that heparin, commonly used for treatment of thromboembolic events in cancer patients, is beneficial for their survival. Preclinical studies confirm that heparin possesses antimetastatic activities that lead to attenuation of metastasis in various animal models. Heparin contains several biological activities that may affect several steps in metastatic cascade. Here we focus on the role of cellular adhesion receptors in the metastatic cascade and discuss evidence for heparin as an inhibitor of cell adhesion. While P- and L-selectin facilitation of cellular contacts during hematogenous metastasis is being accepted as a potential target of heparin, here we propose that heparin may also interfere with integrin activity and thereby affect cancer progression. This review summarizes recent findings about potential mechanisms of tumor cell interactions in the vasculature and antimetastatic activities of heparin.