5934

RBL2

P130|Rb2;retinoblastoma-like 2;RBL2;retinoblastoma-like 2

130 kDa retinoblastoma-associated protein|PRB2|RBR-2|retinoblastoma-like protein 2|retinoblastoma-related protein 2

16q12.2

protein-coding

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.

Ski interacting protein (Skip) plays an important role in the transforming activity of both v-Ski and EBNA2 (Epstein-Barr virus encoded latency protein) and is involved in EBNA2 and NotchIC activation of CBF1-repressed promoters. We have previously shown that Skip acts as a transcriptional co-activator on a number of cellular and viral promoters. Here, we report that Skip also interacts with pRb and, in cooperation with Ski, can overcome pRb-induced transcriptional repression. We show a strong and direct interaction between pRb and Skip, and we map the site of interaction to amino acid residues 171-353 of the evolutionarily conserved SNW domain of Skip. Furthermore, the combination of Skip and Ski can successfully overcome the G1 arrest and flat cell phenotype induced by pRb. Taken together, these studies suggest that one potential function of the Skip-Ski complex is to overcome the growth-suppressive activities of pRb.

PURPOSE: RB2/p130, a member of the retinoblastoma gene family, maps to human chromosome 16q12.2, a region in which deletions have been found in several human neoplasms including breast, prostatic, and ovarian carcinoma. We sought to evaluate pRb2/p130 protein expression and function in ovarian carcinoma. EXPERIMENTAL DESIGN: pRb2/p130 expression was detected by immunohistochemical and Western blot analyses in 45 primary ovarian carcinoma samples. RESULTS: Immunohistochemical analysis revealed loss or decrease of pRb2/p130 expression in 18 cases (40%). pRb2/p130 expression was mostly nuclear and inversely correlated to the tumor grade (P < 0.05). Western blot analysis correlated with immunohistochemical expression. Reverse transcription-PCR followed by Southern blot analysis was performed on a representative set of 20 ovarian carcinomas. RB2/p130 mRNA levels were consistent with protein expression. We found a significant increase in the percentage of G(1)-phase-arrested cells in CAOV3 and A2780 ovarian carcinoma cell lines after transduction with an adenovirus carrying the RB2/p130 gene (Ad-CMV-RB2/p130). CONCLUSIONS: These data indicate that loss or decrease of pRb2/p130 expression is a frequent event in ovarian carcinoma and is regulated mostly at the transcriptional level. Moreover, pRb2/p130 overexpression is able to arrest cell growth in ovarian carcinoma cells, suggesting the putative role of pRb2/p130 as a tumor suppressor in this malignancy.

The retinoblastoma gene family consists of three genes: RB, p107, and p130. While loss of pRB causes retinoblastoma in humans and pituitary gland tumors in mice, tumorigenesis in other tissues may be suppressed by p107 and p130. To test this hypothesis, we have generated chimeric mice from embryonic stem cells carrying compound loss-of-function mutations in the Rb gene family. We found that Rb/p107- and Rb/p130-deficient mice were highly cancer prone. We conclude that in a variety of tissues tumor development by loss of pRB is suppressed by its homologs p107 and p130. The redundancy of the retinoblastoma proteins in vivo is reflected by the behavior of Rb-family-defective mouse embryonic fibroblasts in vitro.

BACKGROUND: The RB1 gene was the first tumor suppressor gene cloned from humans by studying genetic lesions in families with retinoblastoma. Children who inherit one defective copy of the RB1 gene have an increased susceptibility to retinoblastoma. Several years after the identification of the human RB1 gene, a targeted deletion of Rb was generated in mice. Mice with one defective copy of the Rb gene do not develop retinoblastoma. In this manuscript, we explore the different roles of the Rb family in human and mouse retinal development in order to better understand the species-specific difference in retinoblastoma susceptibility. RESULTS: We found that the Rb family of proteins (Rb, p107 and p130) are expressed in a dynamic manner during mouse retinal development. The primary Rb family member expressed in proliferating embryonic retinal progenitor cells in mice is p107, which is required for appropriate cell cycle exit during retinogenesis. The primary Rb family member expressed in proliferating postnatal retinal progenitor cells is Rb. p130 protein is expressed redundantly with Rb in postmitotic cells of the inner nuclear layer and the ganglion cell layer of the mouse retina. When Rb is inactivated in an acute or chronic manner during mouse retinal development, p107 is upregulated in a compensatory manner. Similarly, when p107 is inactivated in the mouse retina, Rb is upregulated. No changes in p130 expression were seen when p107, Rb or both were inactivated in the developing mouse retina. In the human retina, RB1 was the primary family member expressed throughout development. There was very little if any p107 expressed in the developing human retina. In contrast to the developing mouse retina, when RB1 was acutely inactivated in the developing human fetal retina, p107 was not upregulated in a compensatory manner. CONCLUSION: We propose that intrinsic genetic compensation between Rb and p107 prevents retinoblastoma in Rb- or p107-deficient mice, but this compensation does not occur in humans. Together, these data suggest a model that explains why humans are susceptible to retinoblastoma following RB1 loss, but mice require both Rb and p107 gene inactivation.

OBJECTIVES: To identify the relationships between key components of the proliferative and apoptotic pathways in bladder tumors. METHODS: A tissue array of 88 bladder tumors was assembled. Immunohistochemical analyses were used to investigate the relationship between nine different parameters: stage, proliferation (Ki67), apoptosis (in situ DNA nick end labeling), the anti-apoptotic protein Bcl-2, tumor suppressors p53 and retinoblastoma protein (Rb), the Rb-related protein p130, cyclin E, and the cyclin-dependent kinase inhibitor p27. The protein expression in each tumor was reported as the percentage of positively staining cells. RESULTS: The analysis focused on Stage 1 to 3 tumors. Analysis found that p53 expression increased progressively with stage, and Rb and p27 decreased with increasing stage. Overall, the cyclin E levels correlated with the proliferative index. Cyclin E levels were low in Stage 1 tumors and elevated in Stage 2 tumors, but were decreased in Stage 3 tumors. Multivariate analysis uncovered a correlation between cyclin E and proliferation (Ki67) and a weak correlation between p53 and Bcl-2 and between p27 and Rb. A strong correlation was found between the expression of p53 and p130, which was apparent in Stages 1 and 3, but not in Stage 2. Furthermore, high levels of p130 protein were detected primarily in the cytoplasm. CONCLUSIONS: These results suggest a novel p53/p130 axis in bladder tumors.

Adipogenesis involves cell proliferation and differentiation, both of which have been shown to be regulated by micro (mi)RNA. During mouse preadipocyte 3T3L1 cell differentiation, we found that miR-17-92, a miRNA cluster that promotes cell proliferation in various cancers, was significantly up-regulated at the clonal expansion stage of adipocyte differentiation. Stable transfection of 3T3L1 cells with miR-17-92 resulted in accelerated differentiation and increased triglyceride accumulation after hormonal stimulation. By using a luciferase reporter assay, we demonstrated that miR-17-92 directly targeted the 3 UTR region of Rb2/p130, accounting for subsequently reduced Rb2/p130 mRNA and protein quantities at the stage of clonal expansion. siRNA-mediated knock-down of Rb2/p130 at the same stage of clonal expansion recapitulated the phenotype of overexpression of miR-17-92 in the stably transfected 3T3L1 cells. These data indicate that miR-17-92 promotes adipocyte differentiation by targeting and negatively regulating Rb2/p130.

Rb2/p130 tumor suppressor protein regulates cell cycle progression primarily through interactions with members of the E2F family of transcription factors and repression of the transactivation of E2F target genes. In ATRA sensitive ovarian carcinoma CA-OV3 cells, a dramatic increase in Rb2/p130 protein mediates growth arrest at G0/G1. However, although Rb2/p130 is expressed at high levels in SK-OV3 cells, they fail to growth arrest in response to ATRA treatment. We show that the functional activity of Rb2/p130 in SK-OV3 cells is reduced when compared to CA-OV3 cells. To determine the basis for the reduced functional activity, we characterized the Rb2/p130 protein in SK-OV3 cells and investigated the possible role of alterations to this molecule in mediating resistance to ATRA growth suppression. Direct sequencing of Rb2/p130 cDNA cloned from SK-OV3 cells revealed the presence of two homozygous missense mutations (T178C and C259G) which result in amino acid changes Ser60 to Pro60 and Pro87 to Ala87 respectively. Unfortunately the same missense mutations were observed in Rb2/p130 cDNA cloned from ATRA sensitive CA-OV3 cells. We next investigated differences in Rb2/p130 protein subcellular localization. While Rb2/p130 was localized in the nucleus in both cell lines, we observed regions of intense staining within the nucleus of SK-OV3 cells. This is suggestive of aggregation and/or subnuclear sequestration of the Rb2/p130 protein. Finally, the PAGE migration pattern of Rb2/p130 suggested that a hyperphosphorylated form of Rb2/p130 accumulated in SK-OV3 cells but not in CA-OV3 cells. It is possible that this hyperphosphorylated form can be responsible for the decreased Rb2/p130 functional activity observed in SK-OV3 cells and may contribute to the resistance of these cells to ATRA mediated growth suppression.

Utx is a candidate tumor suppressor gene that encodes histone H3 lysine 27 (H3K27) demethylase. In this study, we found that ectopic expression of Utx enhanced the expression of retinoblastoma tumor suppressor gene Rb and its related gene Rbl2. This activation was dependent on the demethylase activity of Utx, and was suggested to contribute to the decreased cell proliferation induced by Utx. A chromatin immunoprecipitation assay showed that over-expressed Utx was associated with the promoter regions of Rb and Rbl2 resulting in the removal of repressive H3K27 tri-methylation and the increase in active H3K4 tri-methylation. Furthermore, siRNA-mediated knockdown of Utx revealed the recruitment of endogenous Utx protein on the promoters of Rb and Rbl2 genes. These results indicate that Rb and Rbl2 are downstream target genes of Utx and may play important roles in Utx-mediated cell growth control.