Effects of hsp90 binding inhibitors on sGC-mediated vascular relaxation

Vascular soluble guanylate cyclase (sGC) exists in multimeric complexes with endothelial nitric oxide (NO) synthase (eNOS) and heat shock protein 90 (hsp90). Whereas disruption of hsp90-eNOS complexes clearly attenuates eNOS-dependent vascular relaxation, the contribution of sGC-hsp90 complexes to eNOS- or NO donor-dependent relaxations remains unclear. Isolated rat thoracic aortic rings were preincubated with structurally diverse hsp90 binding inhibitors, radicicol (RA) or geldanamycin (GA), or vehicle for 0.5, 1, or 15 h. Preconstricted vessels were exposed to ACh, 8-bromo-cGMP (8-BrcGMP), forskolin, or one of three NO donors: nitroglycerin (NTG), sodium nitroprusside, or spermine NONOate (SNN). Both RA and GA inhibited endothelium-dependent relaxations dose dependently. Indomethacin or the antioxidant tiron did not affect the inhibition of ACh-induced relaxations by GA. Long-term (15 h) exposure to RA inhibited all NO donor-induced relaxations; however, GA inhibited SNN-induced relaxation only. The effects of GA and RA appeared to be selective because 15-h treatment with either agent did not affect forskolin-induced relaxations and only slightly decreased 8-BrcGMP-induced relaxations. Similarly to their effects on NO-donor-induced relaxation, 15-h exposure to RA, but not to GA, decreased hsp90-bound sGC protein expression and NTG-stimulated cGMP formation in aortic rings, whereas RA more than GA reduced SNN-stimulated cGMP formation. We conclude that RA, much more so than GA, selectively inhibits sGC-dependent relaxations of aortic rings by reducing sGC expression, disrupting sGC-hsp90 complex formation and decreasing cGMP formation. These studies suggest that hsp90 regulates both eNOS- and sGC-dependent relaxations.