Acetyl-11-keto-beta-boswellic acid induces apoptosis in HL-60 and CCRF-CEMcells and inhibits topoisomerase I

Antiproliferative action of different pentacyclic triterpenes has repeatedl y been reported, and some lipoxygenase inhibitors have been shown to induce cell death in various cell systems. Acetyl-11-keto-beta-boswellic acid (AK BA) is a pentacyclic triterpene that inhibits 5-lipoxygenase in a selective , enzyme-directed, nonredox, and noncompetitive manner. To investigate a po ssible effect of AKBA on leukemic cell growth, proliferation of HL-60 and C CRF-CEM cells was assayed in the presence of AKBA and a structural analog w ithout effect on 5-lipoxygenase, amyrin. Cell counts and [H-3]thymidine inc orporation were significantly reduced in a dose-dependent manner in the pre sence of AKBA (IC50 = 30 mu M) but not amyrin. An additive effect of AKBA w ith the crosslinking of the CD95 receptor was also observed. Flow cytometri c analysis of propidium iodide-stained cells indicated that the cells under went apoptosis. This was confirmed by flow cytometric detection of sub-G(1) peaks in AKBA-treated cells and by DNA laddering. However, because HL-60 a nd CCRF-CEM do not express 5-lipoxygenase mRNA constitutively, a mechanism distinct from inhibition of 5-lipoxygenase must account for the effect of A KBA. In a DNA relaxation assay with phi X174RF DNA, AKBA inhibited topoisom erase I from calf thymus at concentrations of greater than or equal to 10 m u M. A semiquantitative cDNA polymerase chain reaction approach was used to estimate the relative level of expression of topoisomerases in both cell l ines. The data suggest that induction of apoptosis in HL-60 and CCRF-CEM by AKBA may be due to inhibition of topoisomerase I in these cells.