Boswellic
acids are the effective components of gum resin of Boswellia serrata, which has anti-inflammatory properties. Recent studies on
brain tumors and leukemic cells indicate that boswellic
acids may have antiproliferative and apoptotic effects with the mechanisms being not studied in detail. We studied their antiproliferative and apoptotic effects on
colon cancer cells and the pathway leading to apoptosis. HT-29 cells were treated with
beta-boswellic acid (BA), keto-
beta-boswellic acid (K-BA) and acetyl-keto-
beta-boswellic acid (AK-BA), respectively. Apoptosis was determined by flow cytometry, by cytoplasmic
DNA-
histone complex and the activity of
caspase-3. The cleavage of
poly-(ADP-ribose)-polymerase (PARP) and expression of Fas were examined by western blot. Specific
caspase inhibitors, polyclonal Fas antibody, and antagonistic Fas antibody ZB4 were employed to elucidate apoptotic pathways.
DNA synthesis and cell viability were examined. Both K-BA and AK-BA increased cytoplasmic
DNA-
histone complex dose-dependently and increased pre-G(1) peak in flow cytometer analysis, with the effects of AK-BA being stronger than K-BA. BA only increased the formation of
DNA-
histone complex at a high concentration. K-BA and AK-BA increased
caspase-8,
caspase-9 and
caspase-3 activities accompanied by cleavage of PARP. The effects of AK-BA on formation of cytoplasmic
DNA histone and on
caspase-3 activation were 3.7- and 3.4-fold, respectively, more effective than those induced by
camptothecin. The apoptosis induced by AK-BA was inhibited completely by
caspase-3 or
caspase-8 inhibitor and partially by
caspase-9 inhibitor. ZB4 blocked exogenous
Fas ligand-induced apoptosis, but had no effect on AK-BA-induced apoptosis. AK-BA had no significant effect on expression of Fas. Apart from apoptotic effect, these
acids also inhibited [(3)H]
thymidine incorporation and cell viability to different extent. In conclusion, boswellic
acids, particularly AK-BA and K-BA have antiproliferative and apoptotic effects in human HT-29 cells. The apoptotic effect is mediated via a pathway dependent on
caspase-8 activation but independent of Fas/FasL interaction.