In addition to effects on tumor cell proliferation and apoptosis, microtubule-binding agents are potent inhibitors of angiogenesis. The cancer chemotherapeutic drug Taxotere (docetaxel) inhibited vascular endothelial growth factor (VEGF)–induced human umbilical vein endothelial cell (HUVEC) migration in vitro at concentrations substantially lower than required to cause cell cycle arrest or apoptosis. Here, we show that Taxotere caused the ubiquitination and subsequent proteasomal degradation of heat shock protein 90 (Hsp90) in HUVEC. This prevented signaling from the focal adhesions and VEGF receptors and inhibited integrin activation. Taxotere prevented the VEGF-induced phosphorylation of focal adhesion kinase, Akt, and endothelial nitric oxide synthase (eNOS), all of which are Hsp90 client proteins. Taxotere completely blocked the VEGF-induced increase in eNOS activity, and the addition of a NO donor reversed the inhibitory effect of Taxotere on VEGF-induced migration. A similar reversal occurred with a proteasomal inhibitor of Hsp90 degradation. Furthermore, overexpression of Hsp90 rescued HUVEC from the inhibition of VEGF-induced migration by Taxotere. Previous studies have suggested that tubulin is also a client protein of Hsp90, and immunocytochemical analysis showed that Taxotere caused the dissociation of Hsp90 from tubulin. We suggest that uncomplexed Hsp90 is more susceptible to ubiquitination and subsequent proteasomal degradation than the bound form. Although inhibitors of Hsp90 are currently under clinical investigation as antitumor agents, this seems to be the first account of a drug that reduces Hsp90 function by enhancing its proteasomal degradation. Further, the loss of Hsp90 and the inactivation of Hsp90 client proteins are previously undescribed actions of Taxotere that may contribute to its antiangiogenic activity. (Cancer Res 2006; 66(16): 8192-9)