No abstract

The cellular proto-oncogene c-myc is involved in cell proliferation and transformation but is also implicated in the induction of programmed cell death (apoptosis). The same characteristics have been described for the tumor suppressor gene p53, the most commonly mutated gene in human cancer. In quiescent mouse fibroblasts expressing wild-type p53 protein, activation of c-Myc was found to induce apoptosis and cell cycle reentry, preceded by stabilization of p53. In contrast, in quiescent p53-null fibroblasts, activation of c-Myc induced cell cycle reentry but not apoptosis. These results suggest that p53 mediates apoptosis as a safeguard mechanism to prevent cell proliferation induced by oncogene activation.

Drugs for cancer therapy belong to different categories of chemical substances. The cellular targets for the therapeutic efficacy are often not unambiguously identified. Here, we describe the process of ribosome biogenesis as a target of a large variety of chemotherapeutic drugs. We determined the inhibitory concentration of 36 chemotherapeutic drugs for transcription and processing of ribosomal RNA by in vivo labeling experiments. Inhibitory drug concentrations were correlated to the loss of nucleolar integrity. The synergism of drugs inhibiting ribosomal RNA synthesis at different levels was studied. Drugs inhibited ribosomal RNA synthesis either at the level of (i) rRNA transcription (e.g. oxaliplatin, doxorubicin, mitoxantrone, methotrexate), (ii) early rRNA processing (e.g. camptothecin, flavopiridol, roscovitine), or (iii) late rRNA processing (e.g. 5-fluorouracil, MG-132, homoharringtonine). Blockage of rRNA transcription or early rRNA processing steps caused nucleolar disintegration, whereas blockage of late rRNA processing steps left the nucleolus intact. Flavopiridol and 5-fluorouracil showed a strong synergism for inhibition of rRNA processing. We conclude that inhibition of ribosome biogenesis by chemotherapeutic drugs potentially may contribute to the efficacy of therapeutic regimens.Chemotherapeutic drugs (hereinafter drugs) are used for the treatment of neoplastic diseases for more than 50 years. The mode of action and specifically the therapeutic relevant targets of many drugs, however, are often less defined. Recent studies revealed that some drugs like 5-fluorouracil (5-FU), 4 which were first assumed to interfere with DNA metabolism actually act mainly on RNA metabolism (1-9). In fact an increasing number of analyses identifies RNA metabolism as an important target of cancer drugs.In a hallmark study, Rubbi and Milner (10) showed that accumulation of the tumor suppressor p53 in UV-or drug-damaged cells occurs only if nucleolar functions are affected. Local, severe UV irradiation of the nucleoplasm could not stabilize p53 accumulation. In contrast, UV damage in the nucleolus induced a strong p53 response suggesting that the major sensor controlling the stability and degradation of p53 is located in the nucleolus, the place of ribosome biogenesis.The stability of the p53 protein is controlled by the ubiquitin ligase Mdm2, which targets p53 to the proteasome for degradation. Strikingly, several ribosomal proteins, including L5, L11, L23, and S7 proteins can bind and inactivate . Conditional knockdown of these ribosomal protein genes prevents Mdm2 inactivation and p53 stabilization in 5-FU-treated cells (15), consistent with the assumption that destruction of nucleolar functions by 5-FU inhibits ribosome biogenesis and results in liberation of ribosomal proteins followed by Mdm2 inactivation and p53 stabilization. The inhibition of rRNA transcription by knockout of the gene for the RNA polymerase I (Pol I) transcription factor TIF-1A (16), by blockage of the transcription factor UBF after microin...

SummaryPolycomb repressor complexes (PRCs) are important chromatin modifiers fundamentally implicated in pluripotency and cancer. Polycomb silencing in embryonic stem cells (ESCs) can be accompanied by active chromatin and primed RNA polymerase II (RNAPII), but the relationship between PRCs and RNAPII remains unclear genome-wide. We mapped PRC repression markers and four RNAPII states in ESCs using ChIP-seq, and found that PRC targets exhibit a range of RNAPII variants. First, developmental PRC targets are bound by unproductive RNAPII (S5p+S7p−S2p−) genome-wide. Sequential ChIP, Ring1B depletion, and genome-wide correlations show that PRCs and RNAPII-S5p physically bind to the same chromatin and functionally synergize. Second, we identify a cohort of genes marked by PRC and elongating RNAPII (S5p+S7p+S2p+); they produce mRNA and protein, and their expression increases upon PRC1 knockdown. We show that this group of PRC targets switches between active and PRC-repressed states within the ESC population, and that many have roles in metabolism.

RNA polymerase II is distinguished by its large carboxyl-terminal repeat domain (CTD), composed of repeats of the consensus heptapeptide Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Differential phosphorylation of serine-2 and serine-5 at the 5' and 3' regions of genes appears to coordinate the localization of transcription and RNA processing factors to the elongating polymerase complex. Using monoclonal antibodies, we reveal serine-7 phosphorylation on transcribed genes. This position does not appear to be phosphorylated in CTDs of less than 20 consensus repeats. The position of repeats where serine-7 is substituted influenced the appearance of distinct phosphorylated forms, suggesting functional differences between CTD regions. Our results indicate that restriction of serine-7 epitopes to the Linker-proximal region limits CTD phosphorylation patterns and is a requirement for optimal gene expression.