"Loss of function" alterations in CCAAT/Enhancer Binding Proteindelta (C/EBPdelta) have been reported in a number of human cancers including breast, prostate and cervical cancer, hepatocellular carcinoma and acute myeloid leukemia. C/EBPdelta gene transcription is induced during cellular quiescence and repressed during active cell cycle progression. C/EBPdelta exhibits tumor suppressor gene properties including reduced expression in cancer cell lines and tumors and promoter methylation silencing. We previously reported that C/EBPdelta expression is inversely correlated with c-Myc (Myc) expression. Aberrant Myc expression is common in cancer and transcriptional repression is a major mechanism of Myc oncogenesis. A number of tumor suppressor genes are targets of Myc transcriptional repression including C/EBPalpha, p15INK4, p21CIP1, p27KIP1 and p57 KIP2 . This study investigated the mechanisms underlying Myc repression of C/EBPdelta expression.

Myc represses C/EBPdelta promoter activity in nontransformed mammary epithelial cells in a dose-dependent manner that requires Myc Box II, Basic Region and HLH/LZ domains. Chromatin Immunoprecipitation (ChIP) assays demonstrate that Myc, Miz1 and Max are associated with the C/EBPdelta promoter in proliferating cells, when C/EBPdelta expression is repressed. EMSAs demonstrate that Miz1 binds to a 30 bp region (-100 to -70) of the C/EBP promoter which contains a putative transcription initiator (Inr) element. Miz1 functions exclusively as a repressor of C/EBPdelta promoter activity. Miz1 siRNA expression or expression of a Miz1 binding deficient Myc (MycV394D) construct reduces Myc repression of C/EBPdelta promoter activity. Max siRNA expression, or expression of a Myc construct lacking the HLH/LZ (Max interacting) region, also reduces Myc repression of C/EBPdelta promoter activity. Miz1 and Max siRNA treatments attenuate Myc repression of endogenous C/EBPdelta expression. Myc Box II interacting proteins RuvBl1 (Pontin, TIP49) and RuvBl2 (Reptin, TIP48) enhances Myc repression of C/EBPdelta promoter activity.

Myc represses C/EBPdelta expression by associating with the C/EBPdelta proximal promoter as a transient component of a repressive complex that includes Max and Miz1. RuvBl1 and RuvBl2 enhance Myc repression of C/EBPdelta promoter activity. These results identify protein interactions that mediate Myc repression of C/EBPdelta, and possibly other tumor suppressor genes, and suggest new therapeutic targets to block Myc transcriptional repression and oncogenic function.

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