Mammalian cells can arrest and differentiate even in the complete absence of Rb family proteins, Wirt et al. reveal.
The retinoblastoma (Rb) tumor suppressor and its close relatives p107 and p130 arrest cells in G1 by binding and inactivating E2F transcription factors, blocking transcription of E2F target genes required for cell cycle progression. The Rb family is also thought to influence cell fate and promote terminal differentiation. To investigate the extent to which the Rb family is required for these processes during development, Wirt et al. generated mouse embryos lacking all three family members.
Triple knockout embryos died surprisingly late in embryogenesis, surviving until mid-gestation with normal-looking organs and embryonic patterning. Nevertheless, most wild-type cells are still proliferative at this point in embryogenesis, so Wirt et al. generated teratomas (embryonic stem cell tumors) to examine the Rb family's effect on cell cycle exit and terminal differentiation. Like wild-type teratomas, tumors lacking all three Rb proteins contained multiple different lineages and cells that had successfully exited the cell cycle. Triple knockout embryonic stem cells could also arrest and differentiate into specific cell types in vitro.
This suggests that Rb proteins have a regulatory rather than essential function in most differentiation pathways. Less clear is how some cells manage to arrest in the absence of the Rb family. Many E2F targets were still down-regulated in triple knockout cells, suggesting that atypical E2Fs, which repress genes independently of Rb, might control this alternative cell cycle exit route. If so, says author Julien Sage, activating atypical E2Fs might slow the growth of tumors carrying mutations in the Rb pathway.