Retinoblastoma/p107/p130 pocket proteins: protein dynamics and interactions with target gene promoters.
ABSTRACT The retinoblastoma (RB) tumor suppressor and its family members, p107 and p130, function by repressing E2F transcription factor activity to limit the expression of genes required for cell cycle progression. Traditionally, it is thought that the RB family proteins repress E2F target gene expression through complexing with E2F at gene promoters. However, whereas chromatin immunoprecipitation experiments have demonstrated p107 and p130 at E2F-responsive promoters, RB chromatin association has not been reliably observed. Here we used green fluorescent protein-tagged proteins to rigorously explore the mechanism of RB-mediated transcriptional repression relative to its p107 and p130 family members. The use of live cell fluorescent imaging demonstrated that RB, p107, and p130 exhibit similar nuclear dynamics. Although these findings suggest a similar engagement with nuclear structures, chromatin immunoprecipitation approaches with multiple independent antibodies failed to detect the association of RB with target gene promoters. However, by employing antibodies directed against green fluorescent protein, we could utilize the same antibody to assess RB, p107, and p130 engagement. This approach demonstrated RB association with target gene promoters in a fashion analogous to p107 and p130. Extension of this technology demonstrated that direct RB phosphorylation disrupts promoter association to regulate transcription. Thus, RB is associated with promoters in a manner similar to p107/p130 and that association is modulated by phosphorylation during cell cycle progression.
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ABSTRACT: Background MicroRNAs are short (∼22 nt) non-coding regulatory RNAs that control gene expression at the post-transcriptional level. Here the functional impact of microRNAs on cell cycle arrest during neuronal lineage differentiation of unrestricted somatic stem cells from human cord blood (USSC) was analyzed.Methodology/Principal FindingsExpression profiling revealed downregulation of microRNAs miR-17, -20a, and -106b in USSC differentiated into neuronal lineage but not in USSC differentiated into osteogenic lineage. Transfection experiments followed by Ki67 immunostainings demonstrated that each of these microRNAs was able to promote proliferation of native USSC and to prevent in part cell cycle arrest during neuronal lineage differentiation of USSC. Bioinformatic target gene predictions followed by experimental target gene validations revealed that miR-17, -20a, and -106b act in a common manner by downregulating an overlapping set of target genes mostly involved in regulation and execution of G1/S transition. Pro-proliferative target genes cyclinD1 (CCND1) and E2F1 as well as anti-proliferative targets CDKN1A (p21), PTEN, RB1, RBL1 (p107), RBL2 (p130) were shown as common targets for miR-17, -20a, and -106b. Furthermore, these microRNAs also downregulate WEE1 which is involved in G2/M transition. Most strikingly, miR-17, -20a, and -106b were found to promote cell proliferation by increasing the intracellular activity of E2F transcription factors, despite the fact that miR-17, -20a, and -106b directly target the transcripts that encode for this protein family.Conclusions/SignificanceMir-17, -20a, and -106b downregulate a common set of pro- and anti-proliferative target genes to impact cell cycle progression of USSC and increase intracellular activity of E2F transcription factors to govern G1/S transition.PLoS ONE 01/2011; 6(1). · 3.53 Impact Factor
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ABSTRACT: Carboxypeptidase D (CPD), a membrane-bound metallocarboxypeptidase that functions as a docking receptor for duck hepatitis B virus, is frequently overexpressed in human cancers. We have explored its expression pattern, clinical significance, and biological function of CPD in hepatocellular carcinoma (HCC). CPD expression was markedly elevated in HCCs relative to adjacent non-tumor liver tissues, as determined by quantitative real-time polymerase chain reaction (qPCR) and Western blot analysis. Immunohistochemistry showed that 164 of 400 (41%) HCCs had high expression of CPD. CPD overexpression was significantly associated with serum levels of hepatitis B surface antigen and hepatitis B e antigen, liver cirrhosis, pathological grade, and intrahepatic metastasis. Knockdown of endogenous CPD expression in Huh7 HCC cells by RNA interference reduced cell proliferation, blocked the cell cycle at G1 phase, and increased apoptosis. Many genes implicated in cell-cycle regulation, including P21waf1, P27 Kip1, SKP2, and CDC2, were deregulated by CPD downregulation. Thus CPD is frequently upregulated in HCC, and targeting CPD inhibits HCC cell proliferation through induction of G1 cell-cycle arrest and apoptosis, thereby providing a potential therapeutic target for this malignancy.Cell Biology International 09/2013; 37(9). DOI:10.1002/cbin.10113 · 1.64 Impact Factor
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ABSTRACT: The retinoblastoma protein (pRb) is required for cell-cycle exit of embryonic mammalian hair cells but is not required for hair cell fate determination and early differentiation, and this provides a strategy for hair cell regeneration by manipulating the pRb pathway. To reveal the mechanism of pRb functional modification in the inner ear, we compared the effects of attenuated pRb phosphorylation by an inhibitor of the Mitogen-Activated Protein (MAP) kinase pathway and an inhibitor of the Rb-Raf-1 interaction on cultured chicken otocysts. We demonstrated that the activity of pRb is correlated with its phosphorylation state, which is regulated by a newly established cell cycle-independent pathway mediated by the physical interaction between Raf-1 and pRb. The phosphorylation of pRb plays an important role during the early stage of inner ear development, and attenuated phosphorylation in progenitor cells leads to cell cycle arrest and increased apoptosis along with a global down-regulation of the genes involved in cell cycle progression. Our study provides novel routes to modulate pRb function for hair cell regeneration.PLoS ONE 12/2013; 8(12):e83726. DOI:10.1371/journal.pone.0083726 · 3.53 Impact Factor