Limits to lifespan

Epstein-Barr virus LMP1 blocks p16INK4a-RB pathway by promoting nuclear export of E2F4/5

The p16INK4a-RB pathway plays a critical role in preventing inappropriate cell proliferation and is often targeted by viral oncoproteins during immortalization. Latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) is often present in EBV-associated proliferative diseases and is critical for the immortalizing and transforming activity of EBV. Unlike other DNA tumor virus oncoproteins, which possess immortalizing activity, LMP1 does not bind to retinoblastoma tumor suppressor protein, but instead blocks the expression of p16INK4a tumor suppressor gene. However, it has been unclear how LMP1 represses the p16INK4a gene expression. Here, we report that LMP1 promotes the CRM1-dependent nuclear export of Ets2, which is an important transcription factor for p16INK4a gene expression, thereby reducing the level of p16INK4a expression. We further demonstrate that LMP1 also blocks the function of E2F4 and E2F5 (E2F4/5) transcription factors through promoting their nuclear export in a CRM1-dependent manner. As E2F4/5 are essential downstream mediators for a p16INK4a-induced cell cycle arrest, these results indicate that the action of LMP1 on nuclear export has two effects on the p16INK4a-RB pathway: (1) repression of p16INK4a expression and (2) blocking the downstream mediator of the p16INK4a-RB pathway. These results reveal a novel activity of LMP1 and increase an understanding of how viral oncoproteins perturb the p16INK4a-RB pathway.

Transcriptome signature of irreversible senescence in human papillomavirus-positive cervical cancer cells

A frequent characteristic of human papillomavirus (HPV)-positive cervical cancers is the loss of viral E2 gene expression in HPV-infected cervical epithelial cells as a consequence of viral DNA integration into the cellular genome. The expression of E2 in HPV-positive cancer cells results in the repression of the viral E6/E7 oncogenes, activation of the p53 and pRB pathways, and a G1 cell cycle arrest, followed by induction of cellular senescence. The transcriptional consequences of E2-mediated cell cycle arrest that lead to senescence currently are unknown. Using conditional senescence induction in HeLa cells and microarray analysis, we describe here the expression profile of cells irreversibly committed to senescence. Our results provide insight into the molecular anatomy of senescence pathways and its regulation by HPV on-coproteins. These include the induction of the RAB vesicular transport machinery and a general down-regulation of chromatin regulatory molecules. The repression of tumor-specific G antigens during E2 senescence supports a reversal of the tumorigenic phenotype by E2 and the potential approach of tumor-specific G antigen-specific immunotherapy for cervical cancer.

p53 and ageing: too much of a good thing?

A recent report by Tyner et al.(1) suggests that p53 is bad for longevity. Heterozygotic mice carrying a p53 mutation that apparently enhances the stability of the wild-type protein showed shorter lifespans and faster ageing while also developing fewer tumours. This fits with the idea that cellular ageing is the price paid for better protection against unlimited proliferation of cancer cells. But other work shows that there is a strong positive association between DNA repair-mediated protection against cancer and ageing. So what are we to make of the new data with regard to overall understanding of the mechanisms of ageing?