Modulation of the G₂ Cell Cycle Checkpoint by Sesquiterpene Lactones Psilostachyins A and C Isolated from the Common Ragweed Ambrosia artemisiifolia

Abstract

A phenotypic cell-based assay for inhibitors of the G₂ DNA damage checkpoint was used to screen plant extracts from the US National Cancer Institute Natural Products Repository. It revealed activity in a methanol extract from the common ragweed Ambrosia artemisiifolia. Assay-guided fractionation led to the identification of the sesquiterpene lactones psilostachyins A and C as novel checkpoint inhibitors. Elimination of their α,β-unsaturated carbonyl group caused a loss of activity, suggesting that the compounds can bind covalently to target proteins through Michael addition. Psilostachyins A and C also blocked cells in mitosis and caused the formation of aberrant microtubule spindles. However, the compounds did not interfere with microtubule polymerization in vitro. The related sesquiterpene lactones psilostachyin B, paulitin and isopaulitin were also isolated from the same extract but showed no checkpoint inhibition. The identification of the target(s) of psilostachyins A and C may provide further insight into the signalling pathways involved in cell cycle arrest and mitotic progression.

Abbreviations

ATM:ataxia telangiectasia mutated

ATR:ATM- and Rad3-related

Chk:checkpoint kinase

CDC:cell division cycle

CDK:cyclin-dependent kinase

DBH:debromohymenialdisine

IGR:isogranulatimide

OZ:13-hydroxy-15-oxozoapatlin

Key words

Ambrosia artemisiifolia - cancer - cell cycle - checkpoint - G₂ phase - mitosis

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