Aciculatin Induces Apoptosis via p53 Pathway Figure 6. Aciculatin anti-cancer activity in the HCT116 xenograft models. HCT116 cells were injected subcutaneously into the flanks of 3 severe combined immunodeficient mice. The mice were divided into two groups (n = 5) and when the tumors reached the average volume (90 mm ) the treatment was initiated. The mice were sacrificed on day 12 thereafter. A, The curves show the mean (6SE) tumor sizes measured within each group. The differences in tumor size between the control and treated mice were statistically significant (*P,0.05). Bodyweight was measured every day from day 1 of administration. There were no significant differences after treatment in any of the groups (data not shown). B, Treated and untreated tumor slices were assessed by H&E (a, b) and IHC staining for p53 protein (c, d) and Ki-67 protein (e, f). Tumor samples were observed under 1006 (for H&E) and 2006 (for IHC) magnification. C, The tumor tissue slices were assessed by TUNEL assay kit and the bar charts indicates the fold change of sub-G1 phase. **P,0.01. doi:10.1371/journal.pone.0042192.g006 important role in aciculatin-induced p53 stabilization in HCT116 cells. We also found that aciculatin can still trigger cell death in p53- KO HCT116 and HT-29 (with mutant p53) (Table S1) cells via a p53-independent mechanism. Moreover, we proved that acicula- tin could also deplete the MDM2 mRNA level of p53-KO HCT116 cells (Fig. 4B), which may contribute to the p53- independent cell death. Several studies have indicated that MDM2 interacts not only with p53 but also with other pivotal proteins such as p73 and p21 in tumor cells without wild-type p53 [31,32,33]. Thus, these MDM2 related effects may contribute to p53-independent cell death. As demonstrated in our in vivo xenograft models, aciculatin effectively inhibited HCT116 tumor progression with no obvious PLOS ONE | www.plosone.org 9 August 2012 | Volume 7 | Issue 8 | e42192