Inhibition of heat shock protein 90 (hsp90) in proliferating endothelial cells uncouples endothelial nitric oxide synthase activity

Dual increases in nitric oxide (•NO) and superoxide anion (O2•−) production are one of the hallmarks of endothelial cell proliferation. Increased expression of endothelial nitric oxide synthase (eNOS) has been shown to play an important role in maintaining high levels of •NO generation to offset the increase in O2•− that occurs during proliferation. Although recent reports indicate that heat shock protein 90 (hsp90) associates with eNOS to increase •NO generation, the role of hsp90 association with eNOS during endothelial cell proliferation remains unknown. In this report, we examine the effects of endothelial cell proliferation on eNOS expression, hsp90 association with eNOS, and the mechanisms governing eNOS generation of •NO and O2•−. Western analysis revealed that endothelial cells not only increased eNOS expression during proliferation but also hsp90 interactions with the enzyme. Pretreatment of cultures with radicicol (RAD, 20 μM), a specific inhibitor that does not redox cycle, decreased A23187-stimulated •NO production and increased Lω-nitroargininemethylester (L-NAME)-inhibitable O2•− generation. In contrast, A23187 stimulation of controls in the presence of L-NAME increased O2•− generation, confirming that during proliferation eNOS generates •NO. Our findings demonstrate that hsp90 plays an important role in maintaining •NO generation during proliferation. Inhibition of hsp90 in vascular endothelium provides a convenient mechanism for uncoupling eNOS activity to inhibit •NO production. This study provides new understanding of the mechanisms by which ansamycin antibiotics inhibit endothelial cell proliferation. Such information may be useful in the development and design of new antineoplastic agents in the future.