Xanthohumol Induces Apoptosis in B-CLL Mediated by Caspase 9 Following Downregulation of bcl-xL and mcl-1.

Introduction

Xanthohumol is a hop-derived prenylated chalcone with structural similarities to flavopiridol. It has documented impact on breast cancer cell growth and invasiveness in vitro. Based on these observations, lymphocytes from patients with 20 B-CLL were cultured in the presence of xanthohumol in vitro.

Methods

Xanthohumol was dissolved in DMSO and used at concentrations up to 50 μM. B-CLL cells were cultured in RPMI 1640–10% FBS in vitro. Apoptosis, cell cycle analysis and bcl-2 expression was assessed by flow cytometry. PARP, Erk1/2, p38, Akt, JNK, p70^S6K, and P-p38, P-Akt, P-JNK, and P-p70^S6K, caspase 3, 8 and 9, and bcl-XL and mcl-1 were assessed by Western Blot.

Results

After 24h, xanthohumol induced dose-dependent killing of B-CLL cells at a LD_{50 (24h)} of 24.4 ± 6.6 μM, independently of known adverse prognostic factors such as Ig mutation status, ZAP-70, and cytogenetic abnormalities including 17p-loss of p53. Cell death was associated with PARP cleavage and Annexin V positivity, suggestive of an apoptotic mechanism. Apoptosis was accompanied by cleavage of procaspase-9 but not of procaspase-8. Among bcl-2 family members tested, there was a decrease in bcl-2, bcl-xL and mcl-1 expression. The effects of xanthohumol on the major signal transduction pathways showed no effect on Jnk, Akt, p38 and Erk phosphorylation, but stimulation of p70^S6K phosphorylation which could not be inhibited by rapamycin, a specific inhibitor of mTOR.

Conclusion

Xanthohumol has antitumor activity on B-CLL cells in vitro that appears to be independent both of DNA-damage and of Zap-70 effects on signal transduction.