Activation of caspases and poly (ADP-ribose) polymerase cleavage to induce apoptosis in leukemia HL-60 cells by Inula racemosa

Inula racemosa Hook.f. commonly known as Pushkarmula (Compositae) has been used as a traditional drug in India, China and Europe. In the present study, 95% ethanolic extract of roots and its fractions (n-hexane, chloroform, n-butanol and aqueous) were evaluated for in vitro cytotoxicity against cancer cell lines of colon, ovary, prostate, lung, CNS and leukemia. The n-hexane fraction containing alantolactone and isoalantolactone as its major constituents was further studied for its mode of action in HL-60 cells. The lowest IC₅₀ value of n-hexane fraction was 10.25 μg/ml for Colo-205, a colon cancer cell line whereas, 17.86 μg/ml was the highest IC₅₀ value observed against CNS cancer cell line SF-295. Further studies on HL-60 cells treated with n-hexane fraction at 10, 25 and 50 μg/ml for 6 h, revealed that it induces apoptosis through intrinsic as well as extrinsic pathways by generating reactive oxygen species (ROS) intermediates. Mitochondrial dysfunction prompted the release of cytochrome c, translocation of pro-apoptotic protein (Bax), activation of caspase cascade, resulting in the cleavage of some specific substrates for caspase-3 such as poly (ADP-ribose) polymerase (PARP), which eventually leads to apoptosis. The results of present study strongly support further research and development of bioactive constituents from Inula racemosa as potential anticancer agent with possible therapeutic implication.