withanolides and Sarcoma--Ewing

Withanolides constitute a valuable class of bioactive natural products because some members of the class are known to exhibit cytotoxic activity and also induce a cytoprotective heat-shock response. In order to understand the relationship between their structures and these dual bioactivities of the withanolide scaffold, we obtained 25 analogues of withaferin A (WA) and withanolide D (WD) including 17 new compounds by semisynthesis involving chemical and microbial transformations. Hitherto unknown 16β-hydroxy analogues of WA and WD were prepared by their reaction with triphenylphosphine/iodine, providing unexpected 5β-hydroxy-6α-iodo analogues (iodohydrins) followed by microbial biotransformation with Cunninghamella echinulata and base-catalyzed cyclization of the resulting 16β-hydroxy iodohydrins. Evaluation of these 25 withanolide analogues for their cytotoxicity and heat-shock-inducing activity (HSA) confirmed the known structure-activity relationships for WA-type withanolides and revealed that WD analogues were less active in both assays compared to their corresponding WA analogues. The 5β,6β-epoxide moiety of withanolides contributed to their cytotoxicity but not HSA. Introduction of a 16β-OAc group to 4,27-di- O-acetyl-WA enhanced cytotoxicity and decreased HSA, whereas introduction of the same group to 4- O-acetyl-WD decreased both activities.

Topics: Biological Products; Cell Line; Cell Line, Tumor; Cytotoxins; Heat-Shock Response; HEK293 Cells; Humans; Iodine; Organophosphorus Compounds; Sarcoma, Ewing; Structure-Activity Relationship; Withanolides

To understand the relationship between the structure and the remarkably diverse bioactivities reported for withanolides, we obtained withaferin A (WA; 1) and 36 analogues (2-37) and compared their cytotoxicity to cytoprotective heat-shock-inducing activity (HSA). By analyzing structure-activity relationships for the series, we found that the ring A enone is essential for both bioactivities. Acetylation of 27-OH of 4-epi-WA (28) to 33 enhanced both activities, whereas introduction of β-OH to WA at C-12 (29) and C-15 (30) decreased both activities. Introduction of β-OAc to 4,27-diacetyl-WA (16) at C-15 (37) decreased HSA without affecting cytotoxicity, but at C-12 (36), it had minimal effect. Importantly, acetylation of 27-OH, yielding 15 from 1, 16 from 14, and 35 from 34, enhanced HSA without increasing cytotoxicity. Our findings demonstrate that the withanolide scaffold can be modified to enhance HSA selectively, thereby assisting development of natural product-inspired drugs to combat protein aggregation-associated diseases by stimulating cellular defense mechanisms.

Topics: Antineoplastic Agents; Apoptosis; Bone Neoplasms; Cell Proliferation; Heat-Shock Response; Humans; Molecular Structure; Sarcoma, Ewing; Structure-Activity Relationship; Sulfhydryl Compounds; Tumor Cells, Cultured; Withania; Withanolides