Withanolides are a large group of steroidal
lactones found in Solanaceae plants that exhibit potential anticancer activities. We have previously demonstrated that a withanolide,
tubocapsenolide A, induced cycle arrest and apoptosis in human
breast cancer cells, which was associated with the inhibition of
heat shock protein 90 (Hsp90). To investigate whether other
withanolides are also capable of inhibiting Hsp90 and to analyze the structure-activity relationships, nine
withanolides with different structural properties were tested in human
breast cancer cells MDA-MB-231 and MCF-7 in the present study. Our data show that the 2,3-unsaturated double bond-containing
withanolides inhibited Hsp90 function, as evidenced by selective depletion of Hsp90 client
proteins and induction of Hsp70. The inhibitory effect of the
withanolides on Hsp90 chaperone activity was further confirmed using in vivo heat shock
luciferase activity recovery assays. Importantly, Hsp90 inhibition by the
withanolides was correlated with their ability to induce
cancer cell death. In addition, the
withanolides reduced constitutive NF-κB activation by depleting IκB
kinase complex (IKK) through inhibition of Hsp90. In
estrogen receptor (ER)-positive MCF-7 cells, the
withanolides also reduced the expression of ER, and this may be partly due to Hsp90 inhibition. Taken together, our results suggest that Hsp90 inhibition is a general feature of cytotoxic
withanolides and plays an important role in their anticancer activity.