Melanoma cell adhesion can be blocked by heparin in vitro: Suggestion of VLA-4 as a novel target for antimetastatic approaches

The clinical benefit of heparin in cancer patients to prolong survival can be attributed to non-anticoagulant mechanisms. Since adhesion molecules are crucially involved in tumour cell metastasis, their inhibition offers an attractive approach for interfering with the metastatic cascade. Heparin is known to attenuate metastasis in a selectin-dependent manner and possesses a variety of additional effects that are thought to influence tumour cell dissemination, proliferation, and angiogenesis. We investigated the adhesion behaviour of BI6F10 melanoma cells in vitro regarding selectin- and VLA-4/VCAM-1-mediated binding to get an insight into underlying mechanisms of melanoma cell metastasis. We show that BI6F10 cells display binding ability to P- and L-selectin as well as to isolated platelets. In contrast, BI6F10 cells did not adhere to immobilized P-selectin under flow. This contributes to recent findings that elucidate a major role of platelet P-selectin for microemboli formation and thus, facilitating metastasis. In contrast, BI6F10 cells adhered to endothelial cells under flow, which could partly be inhibited by a function-blocking anti-VCAM-1 mAb. To emphasize VCAM-1 function, we analyzed cell adhesion at immobilized VCAM-I and observed an integrin dependency. Inhibition experiments reveal that heparin influences VLA-4-mediated binding pathways. By a combination of different techniques we prove that the site of heparin action is rather VLA-4 than VCAM-1. To our knowledge,this is the first time that heparin is shown to interfere with the VLA-4/VCAM-1 interaction leading to the suggestion of a novel heparin target. Our results may contribute to the understanding of how heparin exerts its anti-metastatic activity.