Down-regulation of the transcriptional mediator subunit Med1 contributes to the loss of expression of metastasis-associated dapk1 in human cancers and cancer cells.

DAPK1, a ca(+2)/calmodulin regulated serine/threonine kinase, is a major tumor suppressor, whose expression is lost in multiple tumor types. However, the mechanisms contributing to it are unclear. We have recently shown that CCAAT/Enhancer binding protein-beta (C/EBP-beta) is required for the basal and interferon gamma (IFN-gamma)-induced expression of dapk1 in many cell types. C/EBP-beta interacts with the transcriptional Mediator, a multisubunit complex that couples enhancer bound transcription factors to the basal transcriptional machinery in an IFN-gamma dependent manner for regulating dapk1 expression. Specifically, the Med1 (TRAP220/PBP/DRIP220/CRSP220) subunit associates with the enhancer bound C/EBP-beta at the CRE/ATF site of dapk1 in an IFN-gamma dependent manner for stimulating gene expression. Therefore, we investigated if the mechanism responsible for the loss of dapk1 expression in human cancers involves a failure to recruit C/EBP-beta and/or Med1 to the dapk1 promoter. We compared the relative occupancy of these factors at the dapk1 promoter at CRE/ATF sites in normal and cancer cell lines. A significantly lower binding of these factors to the CRE/ATF site of dapk1 promoter occurred in human cancer cell lines than in normal cells. We show that loss of Med1 expression correlates with a corresponding loss of dapk1 expression in a number of primary human lung carcinomas. Med1 levels were significantly lower in cancer cell lines than in normal controls. Importantly, we show that restoration of Med1 induces the expression of dapk1 in these cancer cells and also attenuates their metastatic potential in vivo. Our studies reveal a critical parameter limiting dapk1 expression in cancer cell lines.