Abstract 3096: c-Myc regulates MAP4K1 expression and AP-1 activity in Pdcd4 knock-down cells

Abstract

Abstract Programmed Cell Death 4 (Pdcd4) is a novel tumor suppressor that frequently exhibits down-regulated expression in several types of cancer. In colon cancer tissues, expression level of Pdcd4 is continuously down-regulated in the order of normal-adenoma-carcinoma. Over-expression of Pdcd4 has been demonstrated to inhibit 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced transformation and tumor phenotype. In addition to cultured cells, Pdcd4 transgenic mice that over-express Pdcd4 in the epidermis show significant reductions in 7,12-dimethylbenz(a)anthracene (DMBA)/TPA induced skin papilloma formation and carcinoma incidence. Knock-out of Pdcd4 expression in mice resulted in induction of lymphomas with frequent metastases to the liver and kidneys, and an increase in DMBA/TPA induced skin papilloma formation and carcinoma incidence. These findings suggest that Pdcd4 suppresses carcinogenesis not only at an early stage (tumor promotion), but also at a late stage (tumor progression) in both cultured cells and transgenic mice. Recently, we demonstrated that over-expression of Pdcd4 inhibits invasion and MAP4K1 expression in colon tumor cells. Conversely, knock-down of Pdcd4 promotes invasion, activates AP-1 and β-catenin/Tcf dependent transcription, and up-regulates c-Myc expression in colon tumor cells. However, how MAP4K1 expression is regulated by Pdcd4 remains unknown. MAP4K1, a kinase upstream of JNK signaling pathway, is also known as HPK1 (haematopoietic progenitor kinase 1) whose function and expression regulation in solid tumors is not well studied. It has been known that activation of MAP4K1 results in activation of JNK, c-Jun and AP-1 dependent transcription. In the present study, we found that MAP4K1 expression is up-regulated by Pdcd4 knock-down. Concurrently, the phosphorylation of JNK and c-Jun is increased in Pdcd4 knock-down cells. c-Myc is a direct downstream target gene of β-catenin/Tcf dependent transcription and is up-regulated by Pdcd4 knock-down. To investigate the role of c-Myc in regulating MAP4K1 expression, we over-expressed c-Myc in HEK293 cells and found that over-expression of c-Myc induces MAP4K1 expression and activates c-Jun. Using chromatin immunoprecipitation, we demonstrate that c-Myc directly binds to MAP4K1 promoter in Pdcd4 knock-down cells. Down-regulation of c-Myc using c-myc RNAi reverses MAP4K1 expression and c-Jun activation in Pdcd4 knock-down cells. Thus, these results provide an insight into how Pdcd4 regulates MAP4K1 expression and AP-1 dependent transcription. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3096.