How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor

Macrophage migration inhibitory factor (MIF) is a ubiquitously expressed pro-inflammatory mediator that has also been implicated in the process of oncogenic transformation and tumor progression. We used a genetic approach to show that deletion of the MIF gene in mice has several major consequences for the proliferative and transforming properties of cells. MIF-deficient cells exhibit increased resistance to oncogenic transformation. The transformation defects associated with MIF deficiency can be overcome through concomitant inactivation of the p53 and Rb/E2F tumor suppressor pathways. We have produced compelling evidence that the effects of MIF on cell survival and tumorigenesis are mediated through overlapping pathways, wherein MIF and p53 functionally antagonize each other in the cell. However, the involvement of MIF in p53 function is secondary to p53-independent mechanisms controlling protein stability, DNA damage checkpoints, and the integrity of the genome. Given the broad spectrum of cell types that normally express MIF and its elevated levels at sites of chronic inflammation, this pathway may be generic for many early stage tumors.     

Rb function is required for E1A-induced S-phase checkpoint activation

It is widely accepted that adenoviral E1A exerts its influence on recipient cells through binding to the retinoblastoma (Rb) family proteins, followed by a global release of E2F factors from pocket-protein control. Our study challenges this simple paradigm by demonstrating previously unappreciated complexity. We show that E1A-expressing primary and transformed cells are characterized by the persistence of Rb-E2F1 complexes. We provide evidence that E1A causes Rb stabilization by interfering with its proteasomal degradation. Functional experiments supported by biochemical data reveal not only a dramatic increase in Rb and E2F1 protein levels in E1A-expressing cells but also demonstrate their activation throughout the cell cycle. We further show that E1A activates an Rb- and E2F1-dependent S-phase checkpoint that attenuates the growth of cells that became hyperploid through errors in mitosis and supports the fidelity DNA replication even in the absence of E2F complexes with other Rb family proteins, thereby functionally substituting for the loss of p53. Our results support the essential role of Rb and E2F1 in the regulation of genomic stability and DNA damage checkpoints.     

Reintroduction of a normal retinoblastoma gene into retinoblastoma and osteosarcoma cells inhibits the replication associated function of SV40 large T antigen

The product of the retinoblastoma (Rb) gene can form complexes with the transforming proteins of small DNA tumor viruses, including SV40 large T antigen (Tag), adenovirus E1A, and the human papilloma virus E7. The strong correlation between their ability to transform and their ability to bind Rb protein suggests that these oncoproteins exert their effect through blocking the Rb function. SV40 Tag causes oncogenic cell transformation of rodent cells, and it is also required for viral DNA replication. In this paper, we investigated the effect of the Rb protein on the SV40 replication associated function of Tag. We present evidence suggesting that the complex formation between Rb and Tag interferes with the viral DNA replication. In Y79 retinoblastoma and Saos-2 osteosarcoma cells, which lack functional Rb protein, a SV40 based plasmid vector, pSVEpR4, replicates well. In the same cells reconstituted for Rb expression with an intact Rb gene introduced by retroviral mediated gene transfer, pSVEpR4 replicates to a considerably lower level. The inhibitory effect of Rb protein was surmounted by increasing the intracellular level of Tag. Increasing amounts of Tag in wild-type Rb negative Y79 cells had virtually no effect on SV40 replication. Furthermore, the overexpression of Tag in Rb reconstituted Y79 cells did not alter the growth rate of the cells. These data suggest that Rb protein interacts with Tag and modulates its ability to promote SV40 DNA replication.