Activation of the human thymidine kinase (TK) promoter by simian virus 40 large T antigen requires both the T antigen pRb family-binding domain and TK promoter sequences resembling E2F-binding sites

Novel insights into RB1 mutation

Since the discovery of the retinoblastoma susceptibility gene (RB1) decades ago, RB1 has been regarded as a prototype tumor suppressor gene providing a paradigm for tumor genetic research. Constant research has updated the understanding of RB1-related pathways and their impact on tumor and nontumor diseases. Mutation of RB1 gene has been observed in multiple types of malignant tumors including prostate cancer, lung cancer, breast cancer, and almost every familial and sporadic case of retinoblastoma. Even if well-known and long-investigated, the application potential of RB1 mutation has not been fully tapped. In this review, we focus on the mechanism underlying RB1 mutation during oncogenesis. Therapeutically, we have further discussed potential clinical strategies by targeting RB1-mutated cancers. The unsolved problems and prospects of RB1 mutation are also discussed.

Relevance of pRB Loss in Human Malignancies

The retinoblastoma tumor suppressor protein (pRB) is a known regulator of cell-cycle control; however, recent studies identified critical functions for pRB in regulating cancer-associated gene networks that influence the DNA damage response, apoptosis, and cell metabolism. Understanding the impact of these pRB functions on cancer development and progression in the clinical setting will be essential, given the prevalence of pRB loss of function across disease types. Moreover, the current state of evidence supports the concept that pRB loss results in pleiotropic effects distinct from tumor proliferation. Here, the implications of pRB loss (and resultant pathway deregulation) on disease progression and therapeutic response will be reviewed, based on clinical observation. Developing a better understanding of the pRB-regulated pathways that underpin the aggressive features of pRB-deficient tumors will be essential for further developing pRB as a biomarker of disease progression and for stratifying pRB-deficient tumors into more effective treatment regimens.

Molecular pathways: regulation of metabolism by RB

The discovery of the retinoblastoma (RB-1) gene as a tumor suppressor that is disrupted in a majority of human cancers either via direct or indirect genetic alterations has resulted in increased interest in its functions and downstream effectors. Although the canonical pathway that links this tumor suppressor to human cancers details its interaction with the E2F transcription factors and cell-cycle progression, recent studies have shown an essential role for RB-1 in the suppression of glycolytic and glutaminolytic metabolism. Characterization of the precise metabolic transporters and enzymes suppressed by the RB-E2F axis should enable the identification of small molecule antagonists that have selective and potent antitumor properties.

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.

Sp1 and NF-Y are necessary and sufficient for growth-dependent regulation of the hamster thymidine kinase promoter

Thymidine kinase (TK) genes from different species are growth- and cell cycle-regulated in a very similar manner; still, the promoter regions of these genes show little homology to each other. It was previously shown that the murine TK gene is growth-regulated by Sp1 and E2F. Here we have characterized cis-regulatory elements in the hamster promoter that are essential and sufficient to confer efficient and serum-responsive expression. The TK promoter was isolated from baby hamster kidney cells. DNase I protection experiments revealed a protected region from positions -24 to -99 relative to the transcription start site. Within this region, binding sites for the transcription factor Sp1 and a CCAAT box, which interacts with the transcription factor NF-Y, were identified. An E2F-like sequence was found not to bind protein, and its removal did not affect promoter activity. This was supported by the observation that cotransfection of a hamster TK reporter gene construct with E2F-1 does not lead to transactivation of the promoter. A 122-base pair region that contains a single Sp1 site, a CCAAT box, and a TATA element was found to be sufficient for serum-responsive expression of a reporter gene. Mutations that inactivate any one of these three elements caused a strong reduction or a loss of promoter activity.

A kinase activity associated with simian virus 40 large T antigen phosphorylates upstream binding factor (UBF) and promotes formation of a stable initiation complex between UBF and SL1

Simian virus 40 large T antigen is a multifunctional protein which has been shown to modulate the expression of genes transcribed by RNA polymerase I (Pol I), II, and III. In all three transcription systems, a key step in the activation process is the recruitment of large T antigen to the promoter by direct protein-protein interaction with the TATA binding protein (TBP)-TAF complexes, namely, SL1, TFIID, and TFIIIB. However, our previous studies on large T antigen stimulation of Pol I transcription also revealed that the binding to the TBP-TAFI complex SL1 is not sufficient to activate transcription. To further define the molecular mechanism involved in large T antigen-mediated Pol I activation, we examined whether the high-mobility group box-containing upstream binding factor (UBF) plays any role in this process. Here, using cell labeling experiments, we showed that large T antigen expression induces an increase in UBF phosphorylation. Further biochemical analysis demonstrated that UBF is phosphorylated by a kinase activity that is strongly associated with large T antigen, and that the carboxy-terminal activation domain of UBF is required for the phosphorylation to occur. Using in vitro reconstituted transcription assays, we demonstrated that the inability of alkaline phosphatase treated UBF to efficiently activate transcription can be rescued by large T antigen. Moreover, we showed that large T antigen-induced UBF phosphorylation promotes the formation of a stable UBF-SL1 complex. Together, these results provide strong evidence for an important role for the large T antigen-associated kinase in mediating the stimulation of RNA Pol I transcription.

Mechanisms of transcriptional regulation of cellular genes by SV40 large T- and small T-antigens

During the past decade a number of virus-encoded transcriptional trans-activators that regulate the expression of viral genes have been reported. These trans-activators may also affect the expression or activity of several cellular genes or gene products to create an optimal cellular environment that favors viral replication. Among the better-studied viral trans-activating proteins are the Simian virus 40 large T- and small t-antigens. During the last few years, mechanisms by which these two viral proteins influence cellular gene expression start to emerge. They are grouped provisionally and reflect the methods used to determine the effects of large T-antigen. Large T-antigen may influence cellular gene expression by: i. altering mRNA levels of cellular transcription factors; ii. interacting with and regulating the DNA-binding or transcriptional activity of specific transcription factors; iii. functionally substitution of eukaryotic transcription factors; iv. direct binding to DNA; or v. regulating components of signaling transduction pathways. Small t-ag seems to exert its effect mainly through inhibiting a cellular phosphatase, protein phosphatase 2A, thereby modulating components of signal transduction pathways and preventing dephosphorylation of several transcription factors. However, small t-ag may also control cellular gene expression by regulating mRNA levels of transcription factors or by interacting with other transcription factors.