Abstract B59: Zhangfei/CREBZF arrests the growth of osteosarcoma cells by displacing Mdm2 and stabilizing p53.

Abstract Osteosarcomas (OS) are the most common malignant bone tumors in humans and dogs. Although there has been dramatic progress in treating OS in both species by surgery and chemotherapy, these therapies often fail, leading to recurrence of the tumor and metastatic spread. Our previous studies showed that Zhangfei/CREBZF, a basic region-leucine zipper (bLZip) transcription factor, is a potent suppressor of growth in some cancer cell lines, including the canine osteosarcoma cell line D-17. However, the mechanisms underlying the suppressive effects of Zhangfei have not been fully elucidated. Furthermore, the effects of Zhangfei are not universal and it has no obvious effects on untransformed cells and some cancer cell lines, suggesting that Zhangfei may act through an intermediary that is either not induced or is defective in cells that it does not affect. The objective of this study was to identify such an intermediary and to determine how Zhangfei exerts its effect. By monitoring the mRNA levels, protein, localization, and transcription activity of tumor suppressor protein p53 and its target genes in D-17 cells infected with adenovirus vectors expressing Zhangfei, we found Zhangfei stabilizes p53 and co-localizes with it in cellular nuclei and that the basic-leucine zipper domain of Zhangfei is required for its profound effects on cell growth and interaction with p53. Suppression of p53 by siRNA at least partially inhibited the effects of Zhangfei on the growth of cells. The effects of Zhangfei on dog osteosarcoma cells (D-17 cells) is mirrored by its effects on the p53 expressing human osteosarcoma cell line U2OS while Zhangfei has no effect on the p53-null osteosarcoma cell line MG63. In U2OS cells Zhangfei displaces the ubiquitin ligase, MDM2, from its association with p53 suggesting a mechanism for the effects of Zhangfei on p53. Overall, we identified that Zhangfei suppressed cell growth in osteosarcoma cells through direct interaction with tumor suppressor protein p53. We demonstrated that Zhangfei stabilized p53 and promoted its nuclear retention by displacing the E3 ubiquitin ligase, Mdm2. Our findings reveal a novel mechanism by which Zhangfei may inhibit tumor growth and metastasis. This may provide a potential application to gene therapy of certain types of osteosarcoma, and perhaps other tumors containing functional p53. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B59. Citation Format: Rui Zhang, Vikram Misra. Zhangfei/CREBZF arrests the growth of osteosarcoma cells by displacing Mdm2 and stabilizing p53. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B59.