[The negative regulation of ribosome biogenesis: a new Arf-dependent pathway controlling cell proliferation?]

[Ribosomes synthesis at the heart of cell proliferation]

Ribosomes are central to gene expression. Their assembly is a complex and an energy consuming process. Many controls exist to make it possible a fine-tuning of ribosome production adapted to cell needs. In this review, we describe recent advances in the characterisation of the links occurring between ribosome synthesis and cell proliferation control. Defects in ribosome biogenesis directly impede cellular cycle and slow-down proliferation. Among the different factors involved, we could define the 5S particle, a ribosome sub-complex, as a key-regulator of p53 and other tumour suppressors such as pRB. This cross-talk between ribosome neogenesis defects and proliferation and cellular cycle also involves other cell cycle controls such as p14^ARF, SRSF1 or PRAS40 pathways. These data place ribosome synthesis at the heart of cell proliferation and offer new therapeutic strategies against cancer.

Housekeeping genes in prostate tumorigenesis

Housekeeping (HK) genes are involved in basic cellular functions and tend to be constitutively expressed across various tissues and conditions. A number of studies have analyzed the value of HK genes as an internal standard for assessing gene expression, but the role of HK genes in cancer development has never been specifically addressed. In this study, we sought to evaluate the expression of HK genes during prostate tumorigenesis. We performed a meta-analysis of gene expression during the transition from normal prostate (NP) to localized prostate cancer (LPC) (i.e., NP > LPC) and from localized to metastatic prostate cancer (MPC) (i.e., LPC > MPC). We found that HK genes are more likely to be differentially expressed during prostate tumorigenesis than is the average gene in the human genome, suggesting that prostate tumorigenesis is driven by modulation of the expression of HK genes. Cell-cycle genes and proliferation markers were up-regulated in both NP > LPC and LPC > MPC transitions. We also found that the genes encoding ribosomal proteins were up-regulated in the NP > LPC and down-regulated in the LPC > MPC transition. The expression of heat shock proteins was up-regulated during the LPC > MPC transition, suggesting that in its advanced stages, prostate tumor is under cellular stress. The results of these analyses suggest that during prostate tumorigenesis, there is a period when the tumor is under cellular stress and, therefore, may be the most vulnerable and responsive to treatment.

[Diamond-Blackfan anemia reveals the dark side of ribosome biogenesis]

Diamond-Blackfan anemia (DBA), a rare congenital erythroblastopenia, has recently become a paradigm for a growing set of genetic diseases linked to mutations in genes encoding ribosomal proteins or factors involved in ribosome biogenesis. Recent studies of the structure and the function of ribosomal proteins affected in DBA indicate that their mutation in DBA primarily impacts ribosome biogenesis. Accordingly, cells from DBA patients display anomalies in the maturation of ribosomal RNAs. The explanation of this unexpected link between ribosome biogenesis, a ubiquitous process, and a disease mostly affecting erythroid differentiation may stem in part from the emerging concept of ribosomal stress response, a signaling pathway triggering cell cycle arrest in response to a defect in ribosome synthesis. Future studies of DBA and other diseases related to defects in ribosome biogenesis are likely to rapidly provide important insights into the regulatory mechanisms linking cell cycle progression to this major metabolic pathway.

Progressive silencing of p14ARF in oesophageal adenocarcinoma

The frequency of oesophageal adenocarcinoma is increasing in Western countries for unknown reasons, and correlates with a corresponding increase in the pre-malignant condition, Barrett's Oesophagus, which raises the risk of adenocarcinoma by some 40- to 125-fold. We have examined how disease progression correlates with changes in expression of the p14ARF (ARF) tumour suppressor, a key regulator of the p53 tumour suppressor pathway that is silenced in some 30% of cancers overall, but for which a role in oesophageal cancer is unclear. We have used quantitative PCR, RT-PCR, methylation-specific PCR and chromatin-immunoprecipitation to examine the regulation and function of ARF in oesophageal adenocarcinoma tissue specimens and cell lines. We find highly significant reductions (P< 0.001) in ARF expression during disease progression from normal oesophageal epithelium to Barrett's Oesophagus to adenocarcinoma, with 57/76 (75%) adenocarcinomas displaying undetectable levels of ARF expression. Retention of ARF expression in adenocarcinoma is a highly significant indicator of increased survival (P< 0.001) and outperforms all clinical variables in a multivariate model. CpG methylation as well as histone H3 methylation of lysines 9 and 27 contribute independently to ARF gene silencing in adenocarcinoma cell lines and can be reversed by 5-aza-2′-deoxycytidine. The results suggest that silencing of ARF is involved in the pathogenesis of oesophageal adenocarcinoma and show that either DNA or histone methylation can provide the primary mechanism for ARF gene silencing. Silencing of ARF could provide a useful marker for increased risk of progression and poor prognosis.