Transcriptional regulation and transformation by Myc proteins

c-Myc substitutes for Notch1-CBF1 functions in cooperative transformation with papillomavirus oncogenes

Infection by high-risk human papillomaviruses (HPV) and persistent expression of the viral oncogenes E6 and E7 are causally linked to the development of cervical cancers. These oncogenes require additional events for the complete transformation of human epithelial cells. Although exaggerated levels of c-Myc are observed in many cases of cervical cancer, the actual function of c-Myc in the process of HPV-mediated transformation is unclear. Here, we show that analogous to activated alleles of Notch1, c-Myc can cooperate with E6/E7 in epithelial transformation and can substitute for CBF1-dependent signals generated by Notch1. In addition, dominant-negative forms of c-Myc block transformation by activated Notch1, E6 and E7, suggesting that c-Myc is required for HPV16-mediated transformation.

The ubiquitin-specific protease USP10 modulates androgen receptor function

The role of the ubiquitin/proteasome system in degrading nuclear hormone receptors and regulating their transcriptional function has emerged in the last few years. We identified the ubiquitin-specific protease USP10 as part of DNA-bound androgen receptor (AR) complexes purified from nuclear extracts of PC-3 cells stably expressing the AR. The interaction between USP10 and the AR was confirmed by GST pull-down assays. Fluorescence microscopy documented that USP10 was localised in the nucleus and the cytoplasm. Cell-based transactivation assays in PC-3/AR cells revealed that overexpression of wild-type USP10, but not of an enzymatically inactive form, stimulated AR activity mediated by reporter constructs harbouring selective androgen response elements (AREs), non-selective steroid response elements (SREs) or the mouse mammary tumour virus (MMTV) promoter. Conversely, USP10 expression knock-down by siRNAs impaired the MMTV response to androgen. In summary, the data indicate that USP10 is a new cofactor that binds to the AR and stimulates the androgen response of target promoters. This finding underlines the role of the ubiquitin/proteasome system in modulating the AR function.

The ubiquitin ligase HectH9 regulates transcriptional activation by Myc and is essential for tumor cell proliferation

The Myc oncoprotein forms a binary activating complex with its partner protein, Max, and a ternary repressive complex that, in addition to Max, contains the zinc finger protein Miz1. Here we show that the E3 ubiquitin ligase HectH9 ubiquitinates Myc in vivo and in vitro, forming a lysine 63-linked polyubiquitin chain. Miz1 inhibits this ubiquitination. HectH9-mediated ubiquitination of Myc is required for transactivation of multiple target genes, recruitment of the coactivator p300, and induction of cell proliferation by Myc. HectH9 is overexpressed in multiple human tumors and is essential for proliferation of a subset of tumor cells. Our results suggest that site-specific ubiquitination regulates the switch between an activating and a repressive state of the Myc protein, and they suggest a strategy to interfere with Myc function in vivo.

Transcriptional silencing of EphB6 receptor tyrosine kinase in invasive breast carcinoma cells and detection of methylated promoter by methylation specific PCR

The receptor tyrosine kinase EphB6 is expressed at reasonable levels in normal breast cells. It shows decreased abundance in non-invasive breast carcinoma cells and is transcriptionally silenced in invasive breast carcinoma cells. We have characterized EphB6 promoter and correlated the expression of EphB6 transcript to differential methylation of the promoter region. The demethylation of promoter sequence in vivo by growth in media containing 5-aza-2'-deoxycytidine restores the expression of EphB6 to normal levels in breast carcinoma cells, and the ability of the promoter to initiate transcription of a reporter gene is lost after methylation of the promoter sequence. The promoter region has binding sites for various factors such as SP1 and p300. The specific methylation of CpG dinucleotides has allowed us to design primers that can selectively amplify the methylated promoter and thus facilitate identification of normal, non-invasive, and invasive breast cells. The potential significance of EphB6 to serve as a diagnostic and prognostic indicator is discussed.

The great MYC escape in tumorigenesis

Increased wild-type MYC expression occurs frequently in human cancers, except in Burkitt's lymphoma, where the translocated MYC allele is frequently mutated at several hotspots, including a major one at threonine-58. Acute MYC expression increases p53 or ARF levels and induces apoptosis, and previous transgenic animal studies revealed frequent inactivating mutations of p53 or p19ARF in transgenic Myc-induced lymphomas. Lowe and coworkers (Hemann et al., 2005) demonstrate that wild-type MYC can also trigger apoptosis by inducing Bim, which neutralizes Bcl-2. In contrast, the MYC point mutants failed to induce Bim, promoting murine lymphomas that escaped both wild-type p53 and p19ARF, and in doing so, evaded apoptosis.