The retinoblastoma tumor-suppressor gene, the exception that proves the rule

Abstract

The retinoblastoma tumor-suppressor gene (Rb1) is centrally important in cancer research. Mutational inactivation of Rb1 causes the pediatric cancer retinoblastoma, while deregulation of the pathway in which it functions is common in most types of human cancer. The Rb1-encoded protein (pRb) is well known as a general cell cycle regulator, and this activity is critical for pRb-mediated tumor suppression. The main focus of this review, however, is on more recent evidence demonstrating the existence of additional, cell type-specific pRb functions in cellular differentiation and survival. These additional functions are relevant to carcinogenesis suggesting that the net effect of Rb1 loss on the behavior of resulting tumors is highly dependent on biological context. The molecular mechanisms underlying pRb functions are based on the cellular proteins it interacts with and the functional consequences of those interactions. Better insight into pRb-mediated tumor suppression and clinical exploitation of pRb as a therapeutic target will require a global view of the complex, interdependent network of pocket protein complexes that function simultaneously within given tissues.

Figure 1. Models for explaining the effects of pRb on cellular differentiation and survival

During development, pRb is a wide spread regulator of the cell cycle. However, loss of pRb elicits tissue specific defects in cellular differentiation and survival as well as in cellular proliferation. The effects of pRb loss on differentiation and survival may be an indirect consequence of the inability to exit the cell cycle (A). This model is based on the frequent observation that cellular differentiation and apoptotis is coupled to the cell cycle. The model predicts that the molecular mechanisms underlying Rb1 associated differentiation and survival defects will be similar to those involved in pRb-mediated cell cycle control, primarily negative regulation of E2F transcription factors. Rb1-dependent differentiation and survival may also be mediated by direct mechanisms (B). In this model, pRb is proposed to utilize additional, cell cycle independent mechanisms to directly influence cell differentiation and survival. These mechanisms may or may not involve E2F transcription factors. This model predicts that the effects of pRb on proliferation, differentiation, and apoptosis should be genetically separable, at least in some cases. The models are not mutually exclusive. It is conceivable that both models are operable, and the relative contribution of each to the phenotypes associated with pRb loss may be dependent on biological context.

Figure 2. A model for a direct role for pRb in repair of trapped TopII DNA lesions

Rb1 protein is suggested to serve as an adaptor protein to recruit processing and repair factors to trapped TOP2 cleavage complexes. BRCA1/BARD1 E3 ubiquitin ligase activity facilitates degradation of TOP2 to reveal free DSBs that are then repaired by BRCA1 associated double stranded DNA break repair factors. The model is consistent with the observed requirement for an E2F- and cell cycle-independent pRb activity in the efficient processing and repair of etoposide induced DNA lesions.

Figure 3. The pocket protein interaction network

While the pRb/E2F complex is central to pRb mediated cell cycle control and contributes to pRb-dependent differentiation and cell survival, more than 150 additional pRb protein binding partners have been described in the literature. The other pocket proteins, p107 and p130, interact with an overlapping subset of these cellular proteins. The model pictured attempts to illustrate some of the functional implications of this interdependent protein interaction network. Cell A and cell B represent two different cell types that express different constellations pocket protein complexes. Hence the function of pRb in these two cells is different. In cell A, p107 and p130 are either absent or they are unable to interact with the pRb-associated proteins shown. In this simple case, Rb1 loss results in the loss of all functions mediated by the indicated protein interactions. Cell B expresses a unique set of cellular proteins that interact with each of the pocket proteins with differing affinities. Based on these differing affinities and simple stoichiometric considerations, loss of Rb1 causes a reshuffling of pocket protein complexes resulting in loss of some functions (W,Z), gain of other functions (Y), or no significant change in function (E2F-functional compensation). Even more complicated scenarios can be envisioned upon partial inactivation of pocket proteins by phosphorylation.

Figure 4. Mechanisms of pRb inactivation in human cancer

Three main mechanisms of pRb inactivation are observed in human cancer, and these mechanisms are not randomly distributed. Rb1 mutation is common in some cancers, but rare in others. Likewise, inactivation by deregulated pRb phosphorylation is common in a distinct subset of human cancers, but Rb1 mutation is rarely observed in this subset. The model proposes that each mechanism of inactivation has different molecular and functional consequences. Deregulated pocket protein phosphorylation is proposed to abrogate some protein interactions but not others. Coupled with considerations of the pocket protein interaction network outlined in Figure 3, different cell types may select for different mechanisms of pRb inactivation because the molecular consequences are more favorable for carcinogenesis.