Loss-of-function mutations in the retinoblastoma gene product (RB) or its signaling network are considered requisite for cancer development; hence, the roles and regulation of RB have been intensively studied [reviewed in (Burkhart and Sage, 2008
; Rowland and Bernards, 2006
)]. The best-characterized RB activity relates to its ability to control the G1-S transition, where it negatively regulates the E2F family of transcription factors. Cyclin-dependent kinases (CDKs) activated in response to mitogenic stimuli phosphorylate and inactivate RB, allowing the released E2F to transcriptionally activate genes required for cell cycle progression. Certain viral oncoproteins bind RB and release E2F, leading to forced S phase entry. Since spontaneous mutations in RB
may produce similar effects, the ability of RB to halt cell cycle transitions is considered central to its tumor suppressor function. Nevertheless, RB binds other proteins besides E2F and can regulate processes such as apoptosis, quiescence, differentiation, and senescence. How these proteins and processes contribute to the tumor suppressor activities of RB is poorly understood.
is a member of a multigene family consisting also of RBL1
(p107) and RBL2
(p130) (Burkhart and Sage, 2008
). Studies using both biochemical and genetic approaches have identified distinct and overlapping functions of each family member (Classon and Harlow, 2002
). Like RB, both p107 and p130 bind E2F proteins and are substrates for phosphorylation by active cyclin/CDKs (Classon and Harlow, 2002
). Furthermore, p107 and p130 also associate with DNA tumor virus oncoproteins and can induce cell cycle arrest when over-expressed (Mulligan and Jacks, 1998
). Yet, despite the similarities among the RB proteins in structure and function, somatic mutations affecting p107 or p130 are rare in human cancers (Burkhart and Sage, 2008
In contrast to their action in cell cycle control, less is known about how RB proteins influence cellular senescence. Senescent cells exit the cycle irreversibly, acquire a large and flat morphology, accumulate a senescence-associated β-galactosidase (SA-β-gal) and undergo changes in gene expression linked to cell cycle inhibition and inflammation (Campisi and d’Adda di Fagagna, 2007
). In cultured cells, senescence can be triggered by replicative exhaustion, or in response to activated oncogenes, DNA damage, or oxidative stress (Courtois-Cox et al., 2008
)., Accordingly, the senescence program acts as a general anti-proliferative stress response and is considered a potent tumor suppressive mechanism in vivo [reviewed in (Narita and Lowe, 2005
) (Prieur and Peeper, 2008
)]. Indeed, senescent cells accumulate in benign tumors in mice expressing activated oncogenes and in these settings co-disruption of genes controlling senescence regulators lead to malignant progression. Moreover, certain DNA damaging chemotherapeutic agents can induce senescence in tumors, and the integrity of the senescence program contributes to the anti-tumor effect of these agents.
The regulation of cellular senescence involves interplay between the p53 and RB tumor suppressor networks (Courtois-Cox et al., 2008
). For example, DNA tumor virus oncoproteins that target p53 and RB bypass senescence in cultured cells (Shay et al., 1991
). Although these oncoproteins bind all three RB family members, acute inactivation of RB is sufficient to promote proliferation in senescent mouse embryo fibroblasts (MEFs) (Sage et al., 2003
) and prevents SAHF accumulation and cooperates with p53 loss to bypass senescence in human diploid fibroblasts (Narita et al., 2003
; Voorhoeve and Agami, 2003
). Based on these observations, we hypothesized that RB must have targets in senescence that differ from those controlled by p107 and p130, and that these targets might highlight processes that mediate its tumor suppressive effects.