Induction of heat‐shock response and alterations of protein phosphorylation by a novel topoisomerase ii inhibitor, withangulatin A, in 9L rat brain tumor cells

Withangulatin A is a newly identified in vitro topoisomerase II inhibitor isolated from the Chinese antitumor herb Physalis angulata. In vivo, it was found to be cytotoxic, capable of suppressing general protein synthesis and of inducing the synthesis of a small set of proteins including those generated by heat‐shock treatment. The 70 kDa protein generated by withangulatin A was unequivocally identified as the heat‐shock protein70 (HSP70) since both proteins migrated to the same position on two‐dimensional polyacrylamide gels, could be recognized by a monoclonal antibody to human HSP70, and exhibited identical peptide maps. The induction of protein synthesis by withangulatin A was regulated at the transcriptional level since it was aborted in cells pre‐treated with actinomycin D. However, the initiation of this process did not require de novo protein synthesis since it was not affected by cycloheximide. Other cellular effect of withangulatin A was alterations of protein phosphorylation including an enhancement of phosphorylation of a 32 kDa protein which was also detected in the heat‐shocked cells. Morevoer, this process was observed within 7.5 min after the initial heat treatment which is much faster than the onset of HSP synthesis. Therefore, increased phosphorylation of the 65 kDa protein may represent on of the earliest signals generated by both heat‐shock and withangulatin A and may be involved in the upstream regulation of heat‐shock response in cells.