Prodigiosin (2-methyl-3-pentyl-6-methoxyprodigiosene) is a bacterial metabolite that has anticancer and antimetastatic properties. However, the molecular mechanisms responsible for these abilities are not fully understood. Gene expression profiling of the human breast cancer cell line MCF-7 treated with prodigiosin was analyzed by cDNA array technology. The majority of the significantly modified genes were related to apoptosis, cell cycle, cellular adhesion, or transcription regulation. The dramatic increase of the nonsteroidal anti-inflammatory drugactivated gene 1 (NAG-1) made this gene an interesting candidate regarding the possible mechanism by which prodigiosin induces cytotoxicity in MCF-7 cells. Our results show that prodigiosin triggers accumulation of the DNAdamage response tumor-suppressor protein p53 but that NAG-1 induction was independent of p53 accumulation. Moreover, prodigiosin caused AKT dephosphorylation and glycogen synthase kinase-3B (GSK-3B) activation, which correlated with NAG-1 expression. Prodigiosin-induced apoptosis was recovered by inhibiting GSK-3B, which might be due, at least in part, to the blockade of the GSK-3B -dependent up-regulation of death receptors 4 and 5 expression. These findings suggest that prodigiosinmediated GSK-3B activation is a key event in regulating the molecular pathways that trigger the apoptosis induced by this anticancer agent. [Mol Cancer Ther 2007;6(1):362 -9]