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.
Humans are genetically unable to produce the sialic acid N-glycolylneuraminic acid (Neu5Gc), because of a mutation that occurred after our last common ancestor with great apes. Although Neu5Gc is presumed absent from normal humans, small amounts have been claimed to exist in human tumors and fetal meconium. We have generated an antibody with high specificity and avidity for Neu5Gc. Fetal tissues, normal adult tissues, and breast carcinomas from humans showed reactivity to this antibody, primarily within secretory epithelia and blood vessels. The presence of small amounts of Neu5Gc was confirmed by MS. Absent any known alternate pathway for its synthesis, we reasoned that these small amounts of Neu5Gc might originate from exogenous sources. Indeed, human cells fed with Neu5Gc incorporated it into endogenous glycoproteins. When normal human volunteers ingested Neu5Gc, a portion was absorbed and eliminated in urine, and small quantities were incorporated into newly synthesized glycoproteins. Neu5Gc has never been reported in plants or microbes to our knowledge. We found that Neu5Gc is rare in poultry and fish, common in milk products, and enriched in red meats. Furthermore, normal humans have variable amounts of circulating IgA, IgM, and IgG antibodies against Neu5Gc, with the highest levels comparable to those of the previously known anti-␣-galactose xenoreactive antibodies. This finding represents an instance wherein humans absorb and metabolically incorporate a nonhuman dietary component enriched in foods of mammalian origin, even while generating xenoreactive, and potentially autoreactive, antibodies against the same molecule. Potential implications for human diseases are briefly discussed.
Genetic analysis in Caenorhabditis elegans has uncovered essential roles for DAF-16 in longevity, metabolism, and reproduction. The mammalian orthologs of DAF-16, the closely-related FOXO subclass of forkhead transcription factors (FKHR͞FOXO1, FKHRL1͞FOXO3a, and AFX͞FOXO4), also have important roles in cell cycle arrest, apoptosis and stress responses in vitro, but their in vivo physiological roles are largely unknown. To elucidate their role in normal development and physiology, we disrupted each of the Foxo genes in mice. Foxo1-null embryos died on embryonic day 10.5 as a consequence of incomplete vascular development. Foxo1-null embryonic and yolk sac vessels were not well developed at embryonic day 9.5, and Foxo1 expression was found in a variety of embryonic vessels, suggesting a crucial role of this transcription factor in vascular formation. On the other hand, both Foxo3a-and Foxo4-null mice were viable and grossly indistinguishable from their littermate controls, indicating dispensability of these two members of the Foxo transcription factor family for normal vascular development. Foxo3a-null females showed age-dependent infertility and had abnormal ovarian follicular development. In contrast, histological analyses of Foxo4-null mice did not identify any consistent abnormalities. These results demonstrate that the physiological roles of Foxo genes are functionally diverse in mammals.
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...
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