E1A 12S and 13S of the transformation-defective adenovirus type 12 strain CS-1 inactivate proteins of the RB family, permitting transactivation of the E2F-dependent promoter

Retinoblastoma gene family expression in lymphoid tissues

It appears more and more clear that retinoblastoma (RB) family of proteins represents key molecules in tumour suppression. This family consists of pRb/p105, p107 and pRb2/p130, which participate in a gene regulatory network that governs the cellular response to antimitogenic signals, and whose deregulation constitutes one of the hallmarks of cancer. Irrespective of their structural and biochemical similarities, RB proteins carry out different functional tasks. The expression of RB gene family in the reactive lymphoid tissues again confirms the different role of each member in cell cycle control and differentiation of normal cells. These different functional properties appear to be maintained in tumours lymphoid tissues, where alterations of the RB/p105 gene appear to be relatively rare. In this review, we will summarize the current knowledge about the role of the RB proteins in reactive and neoplastic lymphoid tissue.

Rb2/p130 and protein phosphatase 2A: key mediators of ovarian carcinoma cell growth suppression by all-trans retinoic acid

Despite a number of attempts to improve treatment of ovarian cancer, it remains the most common cause of death from gynecological cancers. Thus, it is very important to identify more effective drugs for treatment and prevention of ovarian cancer. All-trans-retinoic acid (ATRA) has been shown to arrest the growth of ovarian carcinoma cells in G0/G1 and to significantly elevate levels of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. As ATRA treatment leads to a significant increase in the amount of Rb2/p130 protein but not mRNA, the elevated levels of Rb2/p130 protein is likely the result of increased stability. In studies to elucidate the mechanism by which ATRA alters Rb2/p130 stability in ovarian cancer cells, it was determined that PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130. Dephosphorylated Rb2/p130 exhibits decreased ubiquitination and thus is not degraded by the proteasome. The sites at which PP2A catalytic subunit (PP2Ac) interacts with Rb2/p130 have been localized to the NLS in the C-terminus of Rb2/p130. These sites are also involved in the interaction of Rb/p130 with importin beta and importin alpha, members of the nuclear transport machinery. It is known that importin alpha recognizes a NLS on a target protein and importin beta binds the nuclear pore complex. Moreover, it has been shown that the binding of importin alpha to NLS significantly decreases with phosphorylation of NLS. In ATRA-treated ovarian carcinoma cells, PP2A binds to Rb2/p130 and dephosphorylates the NLS of Rb2/p130 leading to the interaction of importin alpha with Rb2/p130. Importin beta then binds to the importin alpha-Rb2/p130 complex, leading to the translocation of the Rb2/p130 to the nucleus where it acts to arrest ovarian cancer cells in G1 and suppress proliferation.

Oncolytic adenoviruses as antiglioma agents

The treatment for malignant gliomas is suboptimal. Oncolytic adenoviruses hold the promise of being effective agents for the treatment of solid tumors. Importantly, the first oncolytic viral therapy has just been approved for use in combination with chemotherapy for late-stage refractory nasopharyngeal cancer by the Chinese State FDA, following a successful Phase III randomized clinical trial. The concept underlying treatment with oncolytic adenoviruses is based on cancer selectivity by confining viral replication and infectivity to cancer cells. For this purpose, the main strategies used currently to modify the viruses include: functional deletions in essential viral genes; tumor- or tissue-specific promoters used to control the expression of these viral genes; and tropism modification to redirect adenovirus to the cancer cell surface. In the near future, oncolytic adenoviruses need to be optimized to fully realize their potential as critical anticancer tools and, thus, improve the prognosis for patients with malignant gliomas.

HMGA2 induces pituitary tumorigenesis by enhancing E2F1 activity

HMGA2 gene amplification and overexpression in human prolactinomas and the development of pituitary adenomas in HMGA2 transgenic mice showed that HMGA2 plays a crucial role in pituitary tumorigenesis. We have explored the pRB/E2F1 pathway to investigate the mechanism by which HMGA2 acts. Here we show that HMGA2 interacts with pRB and induces E2F1 activity in mouse pituitary adenomas by displacing HDAC1 from the pRB/E2F1 complex-a process that results in E2F1 acetylation. We found that loss of E2F1 function (obtained by mating HMGA2 and E2F1(-/-) mice) suppressed pituitary tumorigenesis in HMGA2 mice. Thus, HMGA2-mediated E2F1 activation is a crucial event in the onset of these tumors in transgenic mice and probably also in human prolactinomas.

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

Expression of RB2/p130 tumor-suppressor gene in AIDS-related non-Hodgkin's lymphomas: implications for disease pathogenesis

In this study we examined 21 cases of AIDS-related lymphomas for genomic organization and expression of RB2/p130 oncosuppressor gene and compared the results with the proliferative features of these neoplasms. We found no mutations in the RB2/p130 gene and unusually high percentages of cells expressing nuclear pRb2/p130 in tumors with a high proliferative activity, such as AIDS-related lymphomas. These findings might suggest that a molecular mechanism usually observed in viral-linked oncogenesis could be involved. We performed in vitro and in vivo binding assays to investigate whether the human immunodeficiency virus (HIV) gene product Tat and Rb2/p130 could interact. The results of these assays revealed that the HIV-1 Tat protein binds specifically to pRb2/p130. This may result in the inactivation of its oncosuppressive properties and the induction of genes needed to proceed through the cell cycle including p107, cyclin A, and cyclin B. Using single-cell polymerase chain reaction (PCR) assay, we found HIV-1 DNA in the neoplastic cells of only 2 of the 21 cases examined, whereas PCR on whole tissue revealed HIV-1 DNA in all of the cases. Furthermore, a diffuse and nuclear stain was observed in tissue sections with anti-Tat monoclonal antibody. These findings are in accordance with the notion that soluble Tat protein could function as a biologically active extracellular protein released by infected cells and taken up readily by uninfected B cells. In conclusion, our results seem to suggest that pRb2/p130 oncosuppressor protein may be a target in the interaction between the HIV-1 gene products and host proteins.

Interactions of SV40 large T antigen and other viral proteins with retinoblastoma tumour suppressor

Simian virus 40 large T antigen, human papilloma virus E7 and adenovirus E1A are all potent oncoproteins that can induce several types of tumours. One of the major functions of these oncoproteins is to interact with the retinoblastoma tumour suppressor protein, Rb, a master switch of the mammalian cell cycle, and to inactivate its function. Rb promotes cell-cycle arrest by recruiting and regulating proteins involved in the transcription of cell proliferation genes. The binding of viral oncoproteins to Rb disrupts the Rb-E2F complex, a central component in the Rb-mediated cell-cycle network. The crystal structures of Rb pocket-viral oncoprotein complexes indicate that the viral proteins recognise a highly conserved region in the Rb pocket through a common motif, LxCxE, and through other unique regions within each viral protein. Although the mechanism of Rb inactivation by viral proteins is not fully understood, information at the atomic level about the interactions between the Rb pocket and viral proteins is providing some insights into how viral proteins dissociate E2F from Rb and thus how they deregulate the cell cycle.

Identification of the full-length huntingtin- interacting protein p231HBP/HYPB as a DNA-binding factor

Neurodegeneration in Huntington's disease (HD) is associated with an elongated glutamine tract in the widely expressed huntingtin protein. Although the pathogenic mechanisms are still unknown, the distinct physical properties of mutant huntingtin in the brain suggest that other factors including huntingtin-interacting proteins might play a specific role. We have previously identified a DNA-binding motif in the proximal E1A promoter of adenovirus serotype 12 as responsible for E1A autoregulation. Here, we identified the p231HBP protein as a DNA-binding factor, the C-terminal portion of which has recently been characterized as the huntingtin-interacting protein HYPB of unknown function. We have determined the full-length cDNA sequence, identified several domains supporting its gene regulatory functions, and mapped the HBP231 gene to chromosome 3p21.2-p21.3. Our results provide an interesting molecular link between huntingtin and a DNA-binding factor, implicating that this interaction might result in the alteration of cellular gene expression involved in HD pathogenesis.

Conserved region 2 of adenovirus E1A has a function distinct from pRb binding required to prevent cell cycle arrest by p16INK4a or p27Kip1

Ectopic expression of the CDK inhibitors (CKIs) p16INK4a and p27Kip1 in Rat1 fibroblasts induces dephosphorylation and activation of Retinoblastoma-family proteins (pRb, p107 and p130), their association with E2F proteins, and cell cycle arrest in G1. The growth-inhibitory action of p16, in particular, is believed to be mediated essentially via pRb activation. The 12S E1A protein of human Adenovirus 5 associates with pRb-family proteins via residues in its Conserved Regions (CR) 1 and 2, in particular through the motif LXCXE in CR2. These interactions are required for E1A to prevent G1 arrest upon co-expression of CKIs. We show here that mutating either of two conserved motifs adjacent to LXCXE in CR2, GFP and SDDEDEE, also impairs the ability of E1A to overcome G1 arrest by p16 or p27. Strikingly, however, these mutations affect neither the association of E1A with pRb, p07 and p130, nor its ability to derepress E2F-1 transcriptional activity in transient transfection assays. One of the EIA mutants, however, is defective in derepressing several endogenous E2F target genes in the presence of p16 or p27. Thus, CR2 possesses an essential function besides pRb-binding. We speculate that this function might be required for the full derepression of E2F-regulated genes in their natural chromatin context.