Hsp90 in Cancer: Beyond the Usual Suspects

Gaurav Garg, … Brian S.J. Blagg1, in Advances in Cancer Research, 2016
Withaferin A, a Disruptor of Hsp90–Cdc37 Interaction
Withaferin A (WA, Fig. 13) is a withanolide isolated from the Indian medicinal plant of Withania somnifera (commonly known as “Ashwagandha” or “Indian Winter Cherry” in Ayurvedic medicine) and possesses diverse biological activities, such as anti-inflammatory (Kaileh et al., 2007), antistress, antioxidant, immunomodulatory (Mishra, Singh, & Dagenais, 2000), antiangiogenesis (Mohan et al., 2004), and anticancer activities (Yang, Shi, & Dou, 2007). Since its discovery in the late 1960s, withaferin A has been extensively studied for its anticancer activity, and numerous mechanisms and molecular targets proposed (Falsey et al., 2006; Kaileh et al., 2007; Shohat, Gitter, Abraham, & Lavie, 1967; Srinivasan, Ranga, Burikhanov, Han, & Chendil, 2007; Yang et al., 2007; Yokota, Bargagna-Mohan, Ravindranath, Kim, & Mohan, 2006). It has been reported that withaferin A inhibits nuclear factor-κB (NF-κB) activation of IκB kinase via a thioalkylation-sensitive redox mechanism (Kaileh et al., 2007) induces apoptosis in prostate cancer cells through Par-4 induction (Srinivasan et al., 2007), targets β5 subunit of tumor proteasome (Yang et al., 2007), and covalently binds to Annexin II to alter cytoskeletal architecture (Falsey et al., 2006). In 2010, Yu and coworkers demonstrated that withaferin A exhibits antiproliferative activity and inhibits Hsp90 in pancreatic cells where it was reported to deplete cellular levels of Hsp90-dependent client proteins (Akt, Cdk4, and GR) (Yu et al., 2010). In addition, it was observed that withaferin A induces Hsp70 expression, without affecting Hsp90 levels. Moreover, these researchers found that withaferin A binds Hsp90 and halts the Hsp90 chaperone cycle through a novel ATP-independent mechanism. To identify the domain to which withaferin A binds in Hsp90, a pull-down assay using WA-biotin was used, which suggested interaction with the chaperone’s C-terminal domain (Yu et al., 2010). Coimmunoprecipitation studies showed that withaferin A disrupts formation of the Hsp90/Cdc37 complex in pancreatic cancer cells (Yu et al., 2010). Structure–activity relationship studies have identified a pharmacophore of WA that involves the 4-hydroxy-5,6-epoxy-22-en-1-one moiety and its unsaturated lactone as critical for cytotoxic activity (Mohan et al., 2004; Yousuf et al., 2011). Recent studies with the withanolides indicate that the 5,6-epoxide may react with reactive cysteine residues in Hsp90 and induce aggregation, leading to disruption of Hsp90 function (Gu et al., 2014).