The retinoblastoma-related Rb2/p130 gene is an effector downstream of AP-2 during neural differentiation

Hepatitis C virus core protein modulates pRb2/p130 expression in human hepatocellular carcinoma cell lines through promoter methylation

Background

Hepatitis C Virus (HCV) infection is associated with chronically evolving disease and development of hepatocellular carcinoma (HCC), albeit the mechanism of HCC induction by HCV is still controversial. The nucleocapsid (core) protein of HCV has been shown to be directly implicated in cellular transformation and immortalization, enhancing the effect of oncogenes and decreasing the one of tumor suppressor genes, as RB1 and its protein product pRB. With the aim of identifying novel molecular mechanisms of hepatocyte transformation by HCV, we examined the effect of HCV core protein on the expression of the whole Retinoblastoma (RB) family of tumor and growth suppressor factors, i.e. pRb, p107 and pRb2/p130.

Methods

We used a model system consisting of the HuH-7, HCV-free, human hepatocellular carcinoma cell line and of the HuH-7-CORE cells derived from the former and constitutively expressing the HCV core protein. We determined pRb, p107 and pRb2/p130 protein and mRNA amount of the respective genes RB1, RBL1 and RBL2, RBL2 promoter activity and methylation as well as DNA methyltransferase 1 (DNMT1) and 3b (DNMT3b) expression level. The effect of pRb2/p130 over-expression on the HCV core-expressing HuH-7-CORE cells was also evaluated.

Results

We found that the HCV core protein expression down-regulated pRb2/p130 protein and mRNA levels in HuH-7-CORE cells by inducing promoter hyper-methylation with the concomitant up-regulation of DNMT1 and DNMT3b expression. When pRb2/p130 expression was artificially re-established in HuH-7-CORE cells, cell cycle analysis outlined an accumulation in the G0/G1 phase, as expected.

Conclusions

HCV core appears indeed able to significantly down-regulate the expression and the function of two out of three RB family tumor and growth suppressor factors, i.e. pRb and pRb2/p130. The functional consequences at the level of cell cycle regulation, and possibly of more complex cell homeostatic processes, may represent a plausible molecular mechanism involved in liver transformation by HCV.

Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina

BACKGROUND

We have previously shown that the transcription factor AP-2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina-specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP-2β and AP-2γ, showed expression patterns similar to AP-2α.

RESULTS

Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP-2α and AP-2β are also co-expressed in developing horizontal cells. AP-2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP-2α/β-immunopositive population. To address possible redundant roles for AP-2α and AP-2β during retinogenesis, Tcfap2a/b-deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone.

CONCLUSIONS

These studies have uncovered critical roles for AP-2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development.

Identification of genes differentially expressed as result of adenovirus type 5- and adenovirus type 12-transformation

BACKGROUND

Cells transformed by human adenoviruses (Ad) exhibit differential capacities to induce tumours in immunocompetent rodents; for example, Ad12-transformed rodent cells are oncogenic whereas Ad5-transformed cells are not. The E1A gene determines oncogenic phenotype, is a transcriptional regulator and dysregulates host cell gene expression, a key factor in both cellular transformation and oncogenesis. To reveal differences in gene expression between cells transformed with oncogenic and non-oncogenic adenoviruses we have performed comparative analysis of transcript profiles with the aim of identifying candidate genes involved in the process of neoplastic transformation.

RESULTS

Analysis of microarray data revealed that a total of 232 genes were differentially expressed in Ad12 E1- or Ad5 E1-transformed BRK cells compared to untransformed baby rat kidney (BRK) cells. Gene information was available for 193 transcripts and using gene ontology (GO) classifications and literature searches it was possible to assign known or suggested functions to 166 of these identified genes. A subset of differentially-expressed genes from the microarray was further examined by real-time PCR and Western blotting using BRK cells immortalised by Ad12 E1A or Ad5 E1A in addition to Ad12 E1- or Ad5 E1-transformed BRK cells. Up-regulation of RelA and significant dysregulation of collagen type I mRNA transcripts and proteins were found in Ad-transformed cells.

CONCLUSION

These results suggest that a complex web of cellular pathways become altered in Ad-transformed cells and that Ad E1A is sufficient for the observed dysregulation. Further work will focus on investigating which splice variant of Ad E1A is responsible for the observed dysregulation at the pathway level, and the mechanisms of E1A-mediated transcriptional regulation.

RB and RB2/P130 genes cooperate with extrinsic signals to promote differentiation of rat neural stem cells

Mechanisms governing commitment and differentiation of the cells of the nervous system begin to be elucidated: how extrinsic and intrinsic components are related remains poorly understood. To investigate this issue, we overexpressed genes of the retinoblastoma (Rb) family RB and RB2/p130, which play an important role during nerve cell maturation, in rat neural stem cells (NSCs). Immunostaining of neurons, astrocytes and oligodendrocytes in cultures overexpressing pRb or pRb2/p130 revealed that these genes affect lineage specification of differentiating NSCs. We observed modifications in percentage of differentiated cells indicating a shift towards the phenotype induced by culture conditions. Results were confirmed by detection of the expression levels of differentiation markers by RT-PCR. Analysis of BrdU incorporation and detection of an early marker of apoptosis suggest that the effect of pRb and pRb2/p130 overexpression is not dependent on the inhibition of cell proliferation, nor does it rely on the regulation of cell survival. Our findings suggest that Rb family genes are involved in fate determination of the cells of the nervous system. However, their role seems subsidiary to that of the extrinsic signals that promote lineage specification and appear to be mediated by a direct effect on the acquisition of a specific phenotype.

AP-2gamma induces p21 expression, arrests cell cycle, and inhibits the tumor growth of human carcinoma cells

Activating enhancer-binding protein 2gamma (AP-2gamma) is a member of the developmentally regulated AP-2 transcription factor family that regulates the expression of many downstream genes. Whereas the effects of AP-2alpha overexpression on cell growth are fairly well established, the cellular effects of AP-2gamma overexpression are less well studied. Our new findings show that AP-2gamma significantly upregulates p21 mRNA and proteins, inhibits cell growth, and decreases clonogenic survival. Cell cycle analysis revealed that forced AP-2gamma expression induced G1-phase arrest, decreased DNA synthesis, and decreased the fraction of cells in S phase. AP-2gamma expression also led to cyclin D1 repression, decreased Rb phosphorylation, and decreased E2F activity in breast carcinoma cells. AP-2gamma binding to the p21 promoter was observed in vivo, and the absence of growth inhibition in response to AP-2gamma expression in p21(-/-) cells demonstrated that p21 caused, at least in part, AP-2-induced cell cycle arrest. Finally, the tumor growth of human breast carcinoma cells in vivo was inhibited by the expression of AP-2gamma relative to empty vector-infected cells, suggesting that AP-2gamma acts as a tumor suppressor. In summary, expression of either AP-2gamma or AP-2alpha inhibited breast carcinoma cell growth; thus, these genes may be therapeutic targets for breast cancer.

AP-2alpha and AP-2gamma are transcriptional targets of p53 in human breast carcinoma cells

Activating enhancer-binding protein 2alpha (AP-2alpha) and activating enhancer-binding protein 2gamma (AP-2gamma) are transcription factors that bind GC-rich consensus sequences and regulate the expression of many downstream genes. AP-2alpha and AP-2gamma interact with p53 both physically and functionally. Expression microarray results in human breast carcinoma cells with forced p53 expression revealed AP-2gamma as a putative transcriptional target of p53. To confirm and extend these findings we measured the effects of forced p53 expression in human breast carcinoma cells by real-time reverse transcription-PCR, Western blotting, electrophoretic gel mobility shift assays, promoter reporter, chromatin immunoprecipitation and chromatin accessibility assays. Wild-type p53 expression rapidly induced not only AP-2gamma but also AP-2alpha mRNA. The subsequent increase in these proteins led to increased AP-2 DNA-binding and transactivating activity. Candidate p53-binding sites were identified in the AP-2alpha and AP-2gamma promoters. p53 binding to these cis-elements in vivo was also observed, together with a relaxation of chromatin structure in these regions. Finally, expression of either AP-2alpha or gamma inhibited growth of human breast carcinoma cells in vitro. Taken together, our findings indicate that these AP-2 genes are targets for transcriptional activation by p53 and suggest that AP-2 proteins may mediate some of the downstream effects of p53 expression such as inhibition of proliferation.

Gene expression profiling of cerebellar development with high-throughput functional analysis

We measured the expression levels of 450 genes during mouse postnatal cerebellar development by quantitative PCR using RNA purified from layers of the cerebellar cortex. Principal component analysis of the data matrix demonstrated that the first and second components corresponded to general levels of gene expression and gene expression patterns, respectively. We introduced 288 of the 450 genes into PC12 cells using a high-throughput transfection assay based on atelocollagen and determined the ability of each gene to promote neurite outgrowth or cell proliferation. Five genes induced neurite outgrowth, and seven genes enhanced proliferation. Evaluation of the functional data and gene expression patterns showed that none of these genes exhibited elevated expression at maturation, suggesting that genes characteristic of mature neurons are not likely to participate in neuronal development. These results demonstrate that functional data can facilitate interpretation of expression profiles and identification of new molecules that participate in biological processes.

Expression of Rb2/p130 protein correlates with the degree of malignancy in gliomas

It has been reported that there is an inverse correlation between the immunohistochemical expression of Rb2/p130, a member of the retinoblastoma gene family, and the degree of malignancy in at least some histological types. In order to investigate the expression of this protein in gliomas, we evaluated 58 samples from patients with resected gliomas. We focused on the relationship between the degree of malignancy of the glioma and the immunohistochemical detection of Rb2/p130. Expression of Rb2/p130 was observed in 38 glioma specimens (65.5%), including a high expression level in low-grade glioma specimens (> 30% positive cells in 84% of tumors) and a low expression level in high-grade glioma specimens (> 30% positive cells in 12% of tumors). The most aggressive of the gliomas exhibited very low to undetectable levels of Rb2/p130. Moreover, we observed an inverse correlation between Rb2/p130 expression and the degree of malignancy. These findings suggest that the differentiation of gliomas might be partially mediated by the Rb2/p130 gene, and that Rb2/ p130 expression can additionally be an indicator of a better prognosis in patients with gliomas.

Negative regulation of chondrocyte differentiation by transcription factor AP-2alpha

This study investigated the role of transcription factor AP-2alpha in chondrocyte differentiation in vitro. AP-2alpha mRNA declined during differentiation, and overexpression of AP-2alpha inhibited differentiation. The results demonstrated that AP-2alpha plays a negative role in chondrocyte differentiation.

INTRODUCTION

Transcription factor AP-2alpha has been detected in growth plate and articular chondrocytes and has been shown to regulate cartilage matrix gene expression in vitro. However, the precise functional role of AP-2alpha in chondrocyte differentiation is not known. In this study, we assessed the expression and the function of AP-2alpha in chondrocyte differentiation of ATDC5 cells.

MATERIALS AND METHODS

Chondrocyte differentiation of ATDC5 cells was induced with insulin or transforming growth factor beta (TGF-beta). Proteoglycan production was assessed by alcian blue staining, and expression levels of chondrocyte marker genes and AP-2 gene family were determined by quantitative real time reverse transcriptase-polymerase chain reaction (RT-PCR). Overexpression of AP-2alpha in ATDC5 cells was accomplished by retroviral infection. Infected cells were selected for G418 resistance and pooled for further analysis.

RESULTS AND CONCLUSIONS

Quantitative real time RT-PCR analysis showed that among the four members of the AP-2 gene family, AP-2alpha mRNA was the most abundant. AP-2alpha mRNA levels progressively declined during the differentiation induced by either insulin or TGF-beta treatment. Retroviral expression of AP-2alpha in ATDC5 cells prevented the formation of cartilage nodules, suppressed the proteoglycan production, and inhibited the expression of type II collagen, aggrecan, and type X collagen. Expression profile analysis of key transcription factors involved in chondrogenesis showed that overexpression of AP-2alpha maintained the expression of Sox9 but suppressed the expression of SoxS and Sox6. Taken together, we provide, for the first time, molecular and cellular evidence suggesting that AP-2alpha is a negative regulator of chondrocyte differentiation.

Reduced transcription of the RB2/p130 gene in human lung cancer

Reduced expression of the retinoblastoma gene (RB)2/p130 protein, as well as mutation of exons 19, 20, 21, and 22 of the same gene, has been reported in primary lung cancer. However, it has been suggested by other investigators that mutational inactivation and loss of the RB2/p130 gene and protein, respectively, are rare events in lung cancer. In order to determine the contribution and mechanisms of RB2/p130 gene inactivation to lung cancer development and progression, we quantified RB2/p130 mRNA expression levels in a range of human lung cancer cell lines (n = 13) by real-time reverse transcription (RT)-polymerase chain reaction (PCR) analysis. In comparison to normal lung tissue, reduced transcription of the RB2/p130 gene was found in all small cell lung cancer cell lines examined, along with six out of the eight nonsmall cell lung cancers tested, most of which had inactivation of RB/p16 pathway. On the basis of Western blot analysis, the expression of RB2/p130 protein was consistent with RNA expression levels in all lung cancer cell lines examined. In addition, the mutational status of the RB2/p130 gene (specifically, exons 19, 20, 21, and 22) was determined in 30 primary lung cancers (from patients with distant metastasis) and 30 lung cancer cell lines by PCR-single strand conformation polymorphism (SSCP) analysis and direct DNA sequencing. There was no evidence of somatic mutations within the RB2/p130 gene in the 60 lung cancer samples (both cell lines and tumors) assessed, including the 11 lung cancer cell lines that displayed reduced expression of the gene. Furthermore, hypermethylation of the RB2/p130 promoter was not found in any of the above-mentioned 11 cell lines, as determined by a DNA methylation assay, combined bisulfite restriction analysis (COBRA). The results of the present study suggest that the reduced RB2/p130 expression seen in lung cancer may be in part transcriptionally mediated, albeit not likely via a mechanism involving hypermethylation of the RB2/p130 promoter. The observed reduction in RB2/p130 gene expression may be due to histone deacetylation, altered mRNA stability, and/or other forms of transcriptional regulation.

Nucleoside diphosphate kinases in mammalian signal transduction systems: recent development and perspective

The role of nucleoside diphosphate (NDP) kinase with special reference to mammalian signal transduction systems was described. The interaction between NDP kinases and G proteins was reevaluated in view of their protein structural information and its significance was extended further on the basis of recent findings obtained with small molecular weight G proteins such as Rad, menin, and Rac. Meanwhile, observations suggesting involvement of NDP kinases in the regulation of cell growth and differentiation led to the realization that NDP kinases may play a crucial role in receptor tyrosine kinase signal transduction systems. In fact, a number of experimental results, particularly obtained with PC12 cells, implicate that NDP kinases appear to regulate differentiation marker proteins and cell-cycle-associated proteins cooperatively. Consequently, we propose a hypothesis that NDP kinases might act like a molecular switch to determine the cell fate toward proliferation or differentiation in response to environmental signals.

Simian immunodeficiency virus Nef protein delays the progression of CD4+ T cells through G1/S phase of the cell cycle

Human and simian immunodeficiency virus (HIV and SIV, respectively) infections are characterized by gradual depletion of CD4+ T cells. The underlying mechanisms of CD4+ T-cell depletion and HIV and SIV persistence are not fully determined. The Nef protein is expressed early in infection and is necessary for pathogenesis. Nef can cause T-cell activation and downmodulates cell surface signaling molecules. However, the effect of Nef on the cell cycle has not been well characterized. To determine the role of Nef in the cell cycle, we investigated whether the SIV Nef protein can modulate cell proliferation and apoptosis in CD4+ Jurkat T cells. We developed a CD4+ Jurkat T-cell line that stably expresses SIV Nef under the control of an inducible promoter. Alterations in cell proliferation were determined by flow cytometry using stable intracytoplasmic fluorescent dye 5- and 6-carboxyfluorescein diacetate succinimidyl ester and bromodeoxyuridine incorporation. Apoptotic cell death was measured by annexin V and propidium iodide staining. Our results demonstrated that SIV Nef inhibited Fas-induced apoptosis in these cells and that the mechanism involved upregulation of the Bcl-2 protein. SIV Nef suppressed CD4+ T-cell proliferation by inhibiting the progression of cells into S phase of the cell cycle. Suppression involved an upregulation of cyclin-dependent kinase inhibitors p21 and p27 and the downregulation of cyclin D1 and cyclin A. In summary, inhibition of apoptosis by Nef can lead to persistence of infected cells and can support viral replication. In addition, a Nef-mediated delay in cell cycle progression may contribute to CD4+ T-cell anergy/depletion seen in HIV and SIV disease.

PC3 potentiates NGF-induced differentiation and protects neurons from apoptosis

PC3TIS21/BTG2 is member of a novel family of antiproliferative genes (BTG1, ANA/BTG3, PC3B, TOB, and TOB2) that play a role in cellular differentiation. We have previously shown that PC3TIS21/BTG2 is induced by nerve growth factor (NGF) at the onset of neuronal differentiation in the neural crest-derived PC12 cell line, and is a marker for neuronal birth. We now observe that PC3TIS21/BTG2 ectopically expressed in PC12 cells synergises with NGF, similarly to the cyclin-dependent kinase inhibitor p21, potentiating the induction of the neuronal markers tyrosine hydroxylase and neurofilament 160 kDa. Furthermore, PC3TIS21/BTG2 protects from apoptosis elicited by NGF deprivation in terminally differentiated PC12 cultures. Such effects might be a consequence of the arrest of cell cycle exerted by PC3TIS21/BTG2, or expression of a sensitizing (neurogenic) property of the molecule.

Interplay between the antimetastatic nm23 and the retinoblastoma-related Rb2/p130 genes in promoting neuronal differentiation of PC12 cells

Increasing evidence indicates that the nm23 genes, initially documented as suppressors of metastasis progression, are involved in normal development and differentiation. We have shown previously that the murine nm23 gene enhances pheochromocytoma PC12 cells responsiveness to NGF by accelerating cell growth arrest and neurite outgrowth. The present study was aimed at elucidating the mechanisms by which nm23 controls cell proliferation and promotes neuronal differentiation. We demonstrated that nm23 modulates the expression of the Rb2/p130 gene, a negative regulator of cell cycle progression also implicated in the maintenance of the differentiated state. Furthermore, we showed that nm23-H1 mutants, defective in inhibiting the invasive phenotype, downregulate Rb2/p130 expression and inhibit NGF-induced PC12 cell differentiation. In synthesis, our results provide first evidence of interplay between the nm23 and the Rb2/p130 genes in driving PC12 cells neuronal differentiation and suggest that the antimetastatic and the differentiative nm23 functions can have similar features.