Abrogation of ionizing radiation-induced G₂ checkpoint and inhibition of nuclear export by Cryptocarya pyrones

G₂ checkpoint inhibitors can force cells arrested in G ₂ phase by DNA damage to enter mitosis. In this manner, several G₂ checkpoint inhibitors can enhance killing of cancer cells by ionizing radiation and DNA-damaging chemotherapeutic agents, particularly in cells lacking p53 function. All G₂ checkpoint inhibitors identified to date target protein phosphorylation by inhibiting checkpoint kinases or phosphatases. Using a phenotypic cell-based assay for G₂ checkpoint inhibitors, we have screened a large collection of plant extracts and identified Z-Cryptofolione and Cryptomoscatone D2 as highly efficacious inhibitors of the G₂ checkpoint. These compounds and related pyrones also inhibit nuclear export. Leptomycin B, a potent inhibitor of Crm1-mediated nuclear export, is also a very potent G₂ checkpoint inhibitor. These compounds possess a reactive Michael acceptor site and do not appear promising as a radiosensitizing agents because they are toxic to unirradiated cells at checkpoint inhibitory concentrations. Nevertheless, the results show that inhibition of nuclear export is an alternative to checkpoint kinase inhibition for abrogating the G₂ checkpoint and they should stimulate the search for less toxic nuclear export inhibitors.