Thiolutin, an inhibitor of HUVEC adhesion to vitronectin, reduces paxillin in HUVECS and suppresses tumor cell-induced angiogenesis

Recent studies have shown that integrin alphav beta3, a receptor for vitronectin, plays an important role in tumor-induced angiogenesis and tumor growth and that antagonists of alphav beta3 inhibit angiogenic processes including endothelial cell adhesion and migration. On the other hand, most inhibitors of integrin alphav beta3 are peptide antagonists that include the ArgGly-Asp (RGD) motif We therefore reasoned that non-peptide inhibitors of endothelial cell adhesion to vitronectin might be useful for inhibition of tumor angiogenesis in vivo. We screened for low-molecular-weight natural products able to inhibit adhesion of human umbilical vein endothelial cells (HUVECs) to vitronectin, and pyrrothine group compounds including aureothricin, thioaurin and thiolutin were isolated from microbial culture broths, Of these compounds, thiolutin inhibited adhesion of HUVECs to vitronectin the most effectively (IC50, 0.83 muM) In vivo experiments showed that thiolutin significantly suppressed angiogenesis induced by tumor cells (S-180), a pathological form of neovascularization, in a mouse dorsal air sac assay system, To explore the mechanism of inhibition of HUVEC adhesion to vitronectin by thiolutin, we examined the effect of this agent on intracellular cell adhesion signaling. We found that the amount of paxillin in HUVECs war significantly reduced by thiolutin treatment, while those of other focal adhesion proteins including vinculin and focal adhesion kinase (FAK) were not. were Metabolic labeling experiments showed that thiolutin enhanced degradation of paxillin in HUVECs. Protease inhibitors (MG 1 1 5 and E64-D) decreased the rate of degradation of the paxillin induced by thiolutin and partially restored thiolutin-induced inhibition of HUVEC adhesion to vitronectin, Based on these findings, we concluded that thiolutin, an inhibitor of HUVEC adhesion to vitronectin, reduces the paxillin level in HUVECs and suppresses tumor cell-induced angiogenesis in vivo. (C) 2001 Wiley-Liss, Inc.