1. The Fusarium toxin Enniatin exerts p53- dependent cytostatic and p53-independent cytotoxic activities against human cancer cells R. Dornetshuber a, P. Heffeter b, M. Kamyar a, W. Berger b, R. Lemmens-Gruber a a Department of Pharmacology and Toxicology, University of Vienna b Institute of Cancer Research, Department of Medicine I, Medical University of Vienna The tumor-suppressor gene p53, the “guardian of the genome“, is mediating cell cycle regulating- as well as apoptosis-inducing properties. The major mechanism to induce apoptosis is believed to work transcriptionally via p53 binding to the promoter region of bax, a proapoptotic member of the bcl-2 protein family. But there also exists a less common, p53-mediated way of inducing apoptosis, believed to be independent of transcription regulation. Enniatin (ENN) is a cyclic hexadepsipeptide, produced by the genus Fusarium, which is well known as an inhibitor of mammalian cholesterol acyl-transferase. ENN possesses antibiotic, immunomodulatory, and ionophoric activities. Here we demonstrate that ENN exerts profound cytotoxic activity against several human tumor cells. Consequently, we further investigated the mechanisms underlying ENN-induced cell death with a focus on apoptosis- and cell cycle-regulating proteins. HCT116 cells with p53, p21 or bax genes disrupted by targeted homologous recombination were used to study ENN-induced cytotoxicity. In MTT assays, no significant influences of these proteins were detected, resulting for all HCT116 subclones at IC 50 values in the low µM range at a 72 h drug exposure. p21 ß-actin bax p53 p53 kop53 (+/+) Co 1µM 2.5µM 0.5µM 5µM 10µM In Western blot analysis, induction of p53 was, as expected, only detectable in p53 wild- type cells, whereas bax activation was evident in p53 wild-type cells as well as in cells with disrupted p53. Cell line IC 50 (µM) Cell line IC 50 (µM) Melanoma Mean  SD Osteosarcoma Mean  SD GUBSA3.190.85U2-OS1.770.24 PNJC2.670.08OS-93.550.77 RIMA1.750.15OS-102.100.15 WUBI1.750.15SAOS2.130.07 TPCK2.720.11 Human lung cancer Glioblastoma A5494.081.04 GBL12.650.30A4271.610.14 GBL22.290.05 Diverse carcinomas GBL32.550.14KB-3-11.950.12 LB2.330.30MDA-MB-2311.450.49 MGC3.040.58SW4804.001.12 U3734.880.09CaCo21.990.09  We demonstrate that ENN exerts profound cytotoxic activity against several tested human tumor cell lines.  Assays on HCT116 cells with disrupted p53, suggest that the cytostatic effects of ENN are mediated by p53-dependent mechanisms.  But we also show an uncommon p53-independent induction of bax accompanied by apoptotic cell death. So further studies are underway to clarify the molecular mechanisms underlying the p53-independent cytotoxic activity of ENN. DNA synthesis Cell cycle distribution Accordingly, PI-staining revealed a more potent cell cycle arrest in G O /G 1 phase following ENN treatment. 3 H-thymidine incorporation revealed a significantly more efficient block of DNA synthesis by ENN in p53 wild- type as compared to corresponding knock-out cells. Measured by MTT assays ENN showed profound cytotoxic activity against several human tumor cell lines. The IC 50 values were calculated from whole dose response curves and given as means ± SD from at least two independent experiments performed in triplicates. Summary