Research highlights

Tumour Suppressors: Multi-tasking

Nature Reviews Cancer 9, 6 (01 June 2009) | doi:10.1038/nrc2660

James McQuat

RB can induce apoptosis in response to genotoxic stress, but how it achieves this has not been established. Jackie Lees, Alberto Gulino and colleagues used a variety of techniques to address this question. They found that DNA damaging agents, such as doxorubicin, induced formation of a RB–E2F1 complex in both proliferating and non-proliferating cells. Although RB can bind to both pro-apoptotic genes and cell cycle regulatory genes after treatment with doxorubicin, RB repressed the cell cycle genes through the recruitment of histone deacetylase 1 and induced transcription of pro-apoptotic genes such as CASP7 and TP73 as a result of recruitment of E2F1, RNA polymerase II and the histone acetyltransferase PCAF. Indeed, a partial knockdown of RB1 reduced the fraction of cells undergoing apoptosis in response to either doxorubicin or etoposidein vitro. Similar results were found in vivo, with Rb1-null or heterozygous epithelial cells in the small intestine showing reduced levels of apoptosis in response to doxorubicin compared with wild type. Thus, changes in the level of RB expression in vivo can alter the response to DNA damaging agents owing to reduced levels of pro-apoptotic proteins.

A second publication indicates that the maintenance of contact inhibition by Rb family members might also be a novel tumour suppressor function. Douglas Dean and colleagues found that mouse embryonic fibroblasts (MEFs) lacking expression of RB, RBL1 (p107) and RBL2 (p130) (triple knockout (TKO)) are not contact inhibited and form spheres in culture. Although Rb1-null MEFs undergo contact inhibition and do not naturally form these spheres, they can be forced to. Under these conditions theydevelop a stem cell-like phenotype, as do the TKO MEFs, and this remains evident once these cells are replated, suggesting that growth in spheroid cultures leads to the reprogramming of these cells. Further characterization of Rb1-null sphere cells revealed that a minority of the cells had characteristics of side population cells, which are thought to be stem cells. Rb1-null side population cells isolated from the spheres expressed the embryonic stem cell-associated genes Oct4 (Pou5f1) and Nanog. Significantly, injection of the Rb1-null side population cells into nude mice produced spindle cell sarcomas, and small clusters of cells throughout the tumours retained expression of OCT4 and NANOG. These and other data indicate that these side population cells are possible cancer stem cells and these results imply that, by maintaining contact inhibition, Rb family members can prevent cell–cell interactions that promote a cancer stem cell phenotype.

Chiaki Takahashi and colleagues have examined the pathways that cells activate to limit cell proliferation and transformation when RB function is compromised. Rb1 heterozygous mice, which develop Rb1-null thyroid C cell adenomas, only develop C cell adenocarcinomas following biallelic loss of Nras. Why is NRAS loss required for tumour progression? These authors show that Rb1 loss in C cell adenomas results in E2f-mediated aberrant expression of several proteins involved in isoprenylation. This results in increased isoprenylation and activation of NRAS, which triggers senescence through a RBL2-dependent pathway. Moreover, loss of genes involved in senescence such as Arf or Ink4a also induce C cell adenocarcinoma formation in Rb1-heterozygous mice. These findings indicate that carcinogenesis induced by Rb1 loss is restricted by senescence and that RB limits cell transformation in part by restricting activation of NRAS.

All three papers show that the days when RB was thought of as a simple brake on cell cycle progression are long gone.

Author: Nicola McCarthy