Abstract:

Objective   To study the role of autophagy in the natural terpenoids and bisbibenzyls induced apoptosis in cancer cells. Methods   Human astrocytoma U87 cells stably expressing GFP-LC3 were treated with terpenoids ST52 and acetyl-11-keto-β-boswellic acid (AKBA), and bisbibenzyls including H50, F41, H48 and Riccardin D（RD）, which were able to inhibite the proliferation of cancer cells, GFP-LC3 punctate dots were observed with a fluorescence microscope. Western blot was used to analyze the expression levels of LC3, Atg5, Beclin1, p62 and cleaved-poly(ADP-ribose) polymerase(PARP) in human androgen-independent prostate cancer PC-3 cells exposed to chemicals. Autophagy inhibitors or Atg5-targeting siRNA was employed to examine the effect of chemicals on autophagy induction. Cellular apoptosis was analyzed by Flow Cytometry with PI staining, and cell viability was detected by MTT assays. Results   Compared with other compounds, bisbibenzyls such as RD induced more punctate dots in U87 cells. RD significantly induced the processing of LC3-I to LC3-II in PC-3 cells, and moderately up-regulated Atg5 and Beclin1 expressions, which in turn leading to reduction of p62. Blockage of autophagy resulted in enhancement of RD-induced apoptosis as evidenced by increased cleavage of PARP and reduced cell viability. However, the LC3-II protein expression induced by RD was not altered in the presence of an apoptosis inhibitor Z-VAD-FMK. Conclusion   These findings suggest that protetive autophagy is induced in PC-3 cells treated with RD and inhibition of autophagy could be an adjunctive strategy for enhancing antitumor activity.

Key words: Bisbibenzyls; Riccardin D; Autophagy; Apoptosis; U87 cells; PC-3 cells