RB-E2F1

The retinoblastoma protein (RB) is the product encoded by the first tumor suppressor gene identified, and exerts a pleitropic spectrum of functions. Although RB interacts with hundreds of proteins, one of the key factors to understand many of its functions is the interaction with the E2F family of transcription factors. For instance, the physical interaction between RB and E2F1, a pro-apoptotic member of the E2F family, is a master gatekeeper for cell cycle progression. We have recently reported that the RB-E2F1 axis has an important role in the regulation of autophagy. Our studies show that the transfer of Rb to Rb-null cells results in the induction of autophagy, whereas transfer of E2F1 results in the induction of apoptosis. Rb and E2F1 proteins are part of two families of proteins whose members may play agonistic or antagonistic roles. It would be very interesting to ascertain whether other members of the RB family, p107 and p130, are also regulators of autophagy, as it could be logically expected, and whether there is overlapping in the regulation of autophagy by RB and the RB-homologous proteins.

Our observations are in agreement with previous reports showing that other tumor suppressor proteins, including PTEN and p53, positively regulate autophagy. Further supporting our data is the observation that upstream regulators of RB, including the cyclin-dependent kinase inhibitors p16^INK4a and p27^kip1, also induce autophagy. While in the case of PTEN, which inhibits mTOR by blocking AKT activity and p53, which activates DRAM, the mechanisms for regulation of autophagy are partially described, in the case of RB these mechanisms remain unknown. In our report, we speculated that the inhibition of E2F1 activity could be in part responsible for the trigger of autophagy, and thus we propose that RB and E2F1 are on the two opposite sides of the regulation of autophagy/apoptosis pathways (Fig. 1). In this regard, the anti-apoptotic protein Bcl-2 is also involved in the suppression of autophagy by interfering with the functions of Beclin 1 and PI3KC3 proteins. We have previously reported that E2F1 transactivates Bcl-2, and we recently observed an ectopic RB protein repress Bcl-2 expression in cells undergoing autophagy. Our observations are consistent with previous data from Levine's laboratory, suggesting that Bcl-2 plays a role in the suppression of autophagy.

Figure 1.

A schematic model of the RB-E2F1 pathway in the regulation of autophagy/apoptosis. Depending on the status of the RB-E2F1 pathway, cells deal with stress through the induction of autophagy or apoptosis. Bcl-2, an inhibitor of autophagy, is a downstream effector of the pathway that is upregulated by E2F1 activity and downregulated by RB activity.

The fact that RB regulates autophagy may have important overtones for the therapy of cancer. Autophagy, for instance, could be the underlying cause of the RB-mediated chemo- and radio-resistance. Thus, autophagy pathways could offer suitable targets for the development of molecular strategies leading to improvements in the chemo- and radio-sensitivity of RB-expressing cancer cells.

Lately, it was reported that autophagy might represent one of the effector mechanisms of senescence, a process also regulated by RB, and thus it would be important to ascertain whether RB-mediated autophagy is required for cellular senescence.

Currently, two reports shed some light on the regulation of autophagy by the RB-E2F pathway. One report describes RB blocking autophagy under hypoxia conditions by inhibiting the expression of the Bcl-2-related protein, BNIP3. Together with our data, this study suggests that the role of RB is different in cells growing under normoxia and hypoxia conditions. In addition, another report revealed that E2F1 is capable of transactivating autophagy-related genes. Due to the dual role E2F1 plays as regulator of cell cycle progression and inducer of apoptosis, the results of the study suggest the possibility that the role of E2F1 in the regulation of autophagy is contextual in nature.

In summary, there is preliminary evidence suggesting that RB-E2F1 regulates autophagy. This is important, not only because it shows the basis for a new function of the RB and E2F1 proteins, but also because it implies the existence of common key mechanisms underlying the regulation of cell cycle, apoptosis and autophagy.

In the future, further studies should clarify whether cell cycle regulation and the induction of autophagy are two separate or interdependent functions of the RB protein. A better understanding of these connections should enrich our knowledge of the role of growth arrest during cellular starvation and the survival of normal and cancer cells under genotoxic stress. In addition, because RB protein plays a role in development, it would be important to determine to what extent RB-mediated autophagy is involved in the Rb-mediated regulation of homeostasis in developing organs. It is clear that RB knockout mice show increased cell death that is at least in part due to E2F1-mediated apoptosis. Are the Rb^-/- embryos also defective for autophagy? Is there any interplay between RB-induced autophagy and E2F1-mediated apoptosis? Answers to these questions will expand our understanding of the mechanisms through which tumor suppressors regulate homeostasis and development and exert cancer suppression.

Punctum to: Jiang H, Martin V, Gomez-Manzano C, Johnson DG, Alonso M, White E, Xu J, McDonnell TJ, Shinojima N, Fueyo J. The RB-E2F1 pathway regulates autophagy. Cancer Res. 2010;70:7882–7893. doi: 10.1158/0008-5472.CA.