Transcriptional inhibition by the retinoblastoma protein

Phosphorylation-induced conformational changes in the retinoblastoma protein inhibit E2F transactivation domain binding

Inactivation of the retinoblastoma protein (Rb) through phosphorylation is an important step in promoting cell cycle progression, and hyperphosphorylated Rb is commonly found in tumors. Rb phosphorylation prevents its association with the E2F transcription factor; however, the molecular basis for complex inhibition has not been established. We identify here the key phosphorylation events and conformational changes that occur in Rb to inhibit the specific association between the E2F transactivation domain (E2F(TD)) and the Rb pocket domain. Calorimetry assays demonstrate that phosphorylation of Rb reduces the affinity of E2F(TD) binding approximately 250-fold and that phosphorylation at Ser(608)/Ser(612) and Thr(356)/Thr(373) is necessary and sufficient for this effect. An NMR assay identifies phosphorylation-driven conformational changes in Rb that directly inhibit E2F(TD) binding. We find that phosphorylation at Ser(608)/Ser(612) promotes an intramolecular association between a conserved sequence in the flexible pocket linker and the pocket domain of Rb that occludes the E2F(TD) binding site. We also find that phosphorylation of Thr(356)/Thr(373) inhibits E2F(TD) binding in a manner that requires the Rb N-terminal domain. Taken together, our results suggest two distinct mechanisms for how phosphorylation of Rb modulates association between E2F(TD) and the Rb pocket and describe for the first time a function for the structured N-terminal domain in Rb inactivation.

Role of APC and DNA mismatch repair genes in the development of colorectal cancers

Colorectal cancer is the third most common cause of cancer-related death in both men and women in the western hemisphere. According to the American Cancer Society, an estimated 105,500 new cases of colon cancer with 57,100 deaths will occur in the U.S. in 2003, accounting for about 10% of cancer deaths. Among the colon cancer patients, hereditary risk contributes approximately 20%. The main inherited colorectal cancers are the familial adenomatous polyposis (FAP) and the hereditary nonpolyposis colorectal cancers (HNPCC). The FAP and HNPCC are caused due to mutations in the adenomatous polyposis coli (APC) and DNA mismatch repair (MMR) genes. The focus of this review is to summarize the functions of APC and MMR gene products in the development of colorectal cancers.

The retinoblastoma tumour suppressor in development and cancer

Since its discovery, the retinoblastoma (RB) tumour-suppressor protein has been a focal point of cancer research. Accumulating evidence indicates a complex role for RB in cell proliferation, differentiation and survival. To further complicate matters, proteins that are related to RB have redundant as well as antagonistic functions. Recent studies of knockout mice and cells that lack one or more of these proteins have begun to clarify their various context-specific functions and the unique activity of this tumour suppressor.

The E2F transcription factors: key regulators of cell proliferation

Ever since its discovery, the RB-1 gene and the corresponding protein, pRB, have been a focal point of cancer research. The isolation of E2F transcription factors provided the key to our current understanding of RB-1 function in the regulation of the cell cycle and in tumor suppression. It is becoming more and more evident that the regulatory circuits governing the cell cycle are very complex and highly interlinked. Certain aspects of RB-1 function, for instance its role in differentiation, cannot be easily explained by the current models of pRB-E2F interaction. One reason is that pRB has targets different from E2F, molecules like MyoD for instance. Another reason may be that we have not completely understood the full complexity of E2F function, itself. In this review, we will try to illuminate the role of E2F in pRB- and p53-mediated tumor suppression pathways with particular emphasis on the aspect of E2F-mediated transcriptional regulation. We conclude that E2F can mediate transcriptional activation as well as transcriptional repression of E2F target genes. The net effect of E2F on the transcriptional activity of a particular gene may be the result of as yet poorly understood protein-protein interactions of E2F with other components of the transcriptional machinery, as well as it may reflect the readout of the different ways of regulating E2F activity, itself. We will discuss the relevance of a thorough understanding of E2F function for cancer therapy.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Interaction between a cellular protein that binds to the C-terminal region of adenovirus E1A (CtBP) and a novel cellular protein is disrupted by E1A through a conserved PLDLS motif

Adenovirus E1A proteins immortalize primary animal cells and cooperate with several other oncogenes in oncogenic transformation. These activities are primarily determined by the N-terminal half (exon 1) of E1A. Although the C-terminal half (exon 2) is also essential for some of these activities, it is dispensable for cooperative transformation with the activated T24 ras oncogene. Exon 2 negatively modulates in vitro cooperative transformation with T24 ras as well as the tumorigenic and metastatic potentials of transformed cells. A short C-terminal sequence of E1A governs the oncogenesis-restraining activity of exon 2. This region of E1A binds with a cellular phosphoprotein, CtBP, through a 5-amino acid motif, PLDLS, conserved among the E1A proteins of human adenoviruses. To understand the mechanism by which interaction between E1A and CtBP results in tumorigenesis-restraining activity, we searched for cellular proteins that complex with CtBP. Here, we report the cloning and characterization of a 125-kDa protein, CtIP, that binds with CtBP through the PLDLS motif. E1A exon 2 peptides that contain the PLDLS motif disrupt the CtBP-CtIP complex. Our results suggest that the tumorigenesis-restraining activity of E1A exon 2 may be related to the disruption of the CtBP-CtIP complex through the PLDLS motif.

Role of c-fos and E2F in the induction of cyclin A transcription and vascular smooth muscle cell proliferation

Excessive proliferation of vascular smooth muscle cells (VSMCs) contributes to vessel renarrowing after angioplasty. Here we investigated the transcriptional regulation of the cyclin A gene, a key positive regulator of S phase that is induced after angioplasty. We show that Ras-dependent mitogenic signaling is essential for the normal stimulation of cyclin A promoter activity and DNA synthesis in VSMCs. Overexpression of the AP-1 transcription factor c-fos can circumvent this requirement via interaction with the cAMP-responsive element (CRE) in the cyclin A promoter. Moreover, c-fos overexpression in serum-starved VSMCs results in the induction of cyclin A promoter activity in a CRE-dependent manner, and increased binding of endogenous c-fos protein to the cyclin A CRE precedes the onset of DNA replication in VSMCs induced by serum in vitro and by angioplasty in vivo. We also show that E2F function is essential for both serum- and c-fos-dependent induction of cyclin A expression. Taken together, these findings suggest that c-fos and E2F are important components of the signaling cascade that link Ras activity to cyclin A transcription in VSMCs. These studies illustrate a novel link between the transcriptional and cell cycle machinery that may be relevant to the pathogenesis of vascular proliferative disorders.

Abnormalities of retinoblastoma gene expression in hematological malignancies

The human retinoblastoma gene product which is involved in cell cycle control and also acts as a transcriptional repressor of genes involved in growth control, is constitutively expressed as a phosphoprotein in normal hemopoietic cells. Abnormalities of the retinoblastoma gene expression leading to loss of protein expression either due to structural changes, mutations or transcriptional abnormalities have been found in a variety of hematological malignancies. There is evidence that loss of Rb protein expression is particularly associated with tumour progression and an adverse response to therapy which may be linked to the biological effect of Rb protein loss on the growth characteristics of tumour cells.