Platelets have been hypothesized to contribute to
tumor cell
metastasis, but the underlying mechanism(s) remain unknown. We demonstrate here that one mechanism whereby platelets may facilitate
metastasis is by potentiating
tumor cell-induced endothelial cell retraction, a prerequisite for the extravasation of most
tumor cell types. The integrity of cultured microvascular endothelial cell (CD3 cells) monolayers was perturbed by
12[S]-hydroxyeicosatetraenoic acid (12(S)-HETE), a
lipoxygenase metabolite of
arachidonic acid, as well as by
tumor cells (i.e.,
Lewis lung carcinoma cells or 3LL). 3LL cells induced a concentration- and time-dependent retraction of the CD3 monolayers, as assessed by quantitative binding assays as well as by phase-contrast microscopy. In contrast, normal murine fibroblasts (3T3) did not induce endothelial cell retraction. 3LL cell-induced endothelial cell retraction was potentiated, in a dose- and time-dependent manner, by homologous murine platelets while platelets alone did not induce endothelial cell retraction. Platelet-enhanced,
tumor cell-induced endothelial cell retraction was inhibited by treating either
tumor cells or platelets with the
lipoxygenase inhibitors nordihydroguaiaretic acid or
N-benzyl-N-hydroxy-5-phenylpentanamide (
BHPP) as well as by PGI2 or its analogs
iloprost and ZK96.480 (
cicaprost), but not by the
cyclooxygenase inhibitor aspirin (ASA).
Tumor cells, upon adhesion to endothelium, initiated 12(S)-HETE biosynthesis, which was inhibited by pretreating
tumor cells with
BHPP but not with ASA. Additionally, 12(S)-HETE biosynthesis during
tumor cell-endothelial cell adhesion was significantly enhanced by the addition of homologous platelets. Collectively, these results suggest that
tumor cell-platelet-endothelial cell interactions lead to enhanced biosynthesis of 12(S)-HETE by
tumor cells and/or platelets, which in turn induces endothelial cell retraction, thus facilitating
tumor cell extravasation and
metastasis.