Browsing by keyword "Nucleolus Organizer Region"
Now showing items 1-4 of 4
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Phenotypic transcription factors epigenetically mediate cell growth controlRibosomal RNA (rRNA) genes are down-regulated during osteogenesis, myogenesis, and adipogenesis, necessitating a mechanistic understanding of interrelationships between growth control and phenotype commitment. Here, we show that cell fate-determining factors [MyoD, myogenin (Mgn), Runx2, C/EBPbeta] occupy rDNA loci and suppress rRNA expression during lineage progression, concomitant with decreased rRNA expression and reciprocal loss of occupancy by c-Myc, a proliferation-specific activator of rRNA transcription. We find interaction of phenotypic factors with the polymerase I activator upstream binding factor UBF-1 at interphase nucleoli, and this interaction is epigenetically retained on mitotic chromosomes at nucleolar organizing regions. Ectopic expression and RNA interference establish that MyoD, Mgn, Runx2, and C/EBPbeta each functionally suppress rRNA genes and global protein synthesis. We conclude that epigenetic control of ribosomal biogenesis by lineage-specific differentiation factors is a general developmental mechanism for coordinate control of cell growth and phenotype.
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Signal recognition particle RNA localization within the nucleolus differs from the classical sites of ribosome synthesisThe nucleolus is the site of ribosome biosynthesis, but is now known to have other functions as well. In the present study we have investigated how the distribution of signal recognition particle (SRP) RNA within the nucleolus relates to the known sites of ribosomal RNA synthesis, processing, and nascent ribosome assembly (i.e., the fibrillar centers, the dense fibrillar component (DFC), and the granular component). Very little SRP RNA was detected in fibrillar centers or the DFC of the nucleolus, as defined by the RNA polymerase I-specific upstream binding factor and the protein fibrillarin, respectively. Some SRP RNA was present in the granular component, as marked by the protein B23, indicating a possible interaction with ribosomal subunits at a later stage of maturation. However, a substantial portion of SRP RNA was also detected in regions of the nucleolus where neither B23, UBF, or fibrillarin were concentrated. Dual probe in situ hybridization experiments confirmed that a significant fraction of nucleolar SRP RNA was not spatially coincident with 28S ribosomal RNA. These results demonstrate that SRP RNA concentrates in an intranucleolar location other than the classical stations of ribosome biosynthesis, suggesting that there may be nucleolar regions that are specialized for other functions.
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The leukemogenic t(8;21) fusion protein AML1-ETO controls rRNA genes and associates with nucleolar-organizing regions at mitotic chromosomesRUNX1/AML1 is required for definitive hematopoiesis and is frequently targeted by chromosomal translocations in acute myeloid leukemia (AML). The t(8;21)-related AML1-ETO fusion protein blocks differentiation of myeloid progenitors. Here, we show by immunofluorescence microscopy that during interphase, endogenous AML1-ETO localizes to nuclear microenvironments distinct from those containing native RUNX1/AML1 protein. At mitosis, we clearly detect binding of AML1-ETO to nucleolar-organizing regions in AML-derived Kasumi-1 cells and binding of RUNX1/AML1 to the same regions in Jurkat cells. Both RUNX1/AML1 and AML1-ETO occupy ribosomal DNA repeats during interphase, as well as interact with the endogenous RNA Pol I transcription factor UBF1. Promoter cytosine methylation analysis indicates that RUNX1/AML1 binds to rDNA repeats that are more highly CpG methylated than those bound by AML1-ETO. Downregulation by RNA interference reveals that RUNX1/AML1 negatively regulates rDNA transcription, whereas AML1-ETO is a positive regulator in Kasumi-1 cells. Taken together, our findings identify a novel role for the leukemia-related AML1-ETO protein in epigenetic control of cell growth through upregulation of ribosomal gene transcription mediated by RNA Pol I, consistent with the hyper-proliferative phenotype of myeloid cells in AML patients.
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Transcriptional corepressor TLE1 functions with Runx2 in epigenetic repression of ribosomal RNA genesEpigenetic control of ribosomal RNA (rRNA) gene transcription by cell type-specific regulators, such as the osteogenic transcription factor Runx2, conveys cellular memory of growth and differentiation to progeny cells during mitosis. Here, we examined whether coregulatory proteins contribute to epigenetic functions that are mitotically transmitted by Runx2 in osteoblastic cells. We show that the transcriptional corepressor Transducin Like Enhancer-1 (TLE1) associates with rRNA genes during mitosis and interphase through interaction with Runx2. Mechanistically, depletion of TLE1 relieves Runx2-mediated repression of rRNA genes transcription and selectively increases histone modifications linked to active transcription. Biologically, loss of TLE-dependent rRNA gene repression coincides with increased global protein synthesis and enhanced cell proliferation. Our findings reinforce the epigenetic marking target genes by phenotypic transcription factors in mitosis and demonstrate a requirement for retention of coregulatory factors to sustain physiological control of gene expression during proliferation of lineage committed cells.
