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Guaiane-type sesquiterpene lactones are naturally occurring compounds which have attracted attention due to their array of biological activities. In this study, chlorinated guaianolides <b>1</b>–<b>8</b>, isolated from plants of the genus <i>Centaurea</i>, were evaluated against the human leukemia cell lines HL-60, U-937, a specific U-937 cell line that overexpresses the anti-apoptotic Bcl-2 protein and the human melanoma cell line SK-MEL-1. This established the relevant structure-growth inhibition relationships. Chlorohyssopifolins A (<b>1</b>), C (<b>3</b>) and D (<b>4</b>) and linichlorin A (<b>6</b>) were the most potent compounds in terms of inducing growth inhibition in the four cell lines. IC₅₀ values were below 10 μM in all cases. Chlorohyssopifolins A (<b>1</b>) and D (<b>4</b>) and linichlorin A (<b>6</b>) were potent apoptotic inducers in human U-937 leukemia cells, as determined by fluorescent microscopy and flow cytometry, and their mechanism of action was associated with cytochrome <i>c</i> release, caspase activation and poly(ADP-ribose)polymerase cleavage. Overall this study shows that guaianolides induce cytotoxicity against human tumor cells and provides important insights into the cell death pathways that are involved.