Myc-induced anchorage of the rDNA IGS region to nucleolar matrix modulates growth-stimulated changes in higher-order rDNA architecture.

Chromatin domain organization and the compartmentalized distribution of chromosomal regions are essential for packaging of deoxyribonucleic acid (DNA) in the eukaryotic nucleus as well as regulated gene expression. Nucleoli are the most prominent morphological structures of cell nuclei and nucleolar organization is coupled to cell growth. It has been shown that nuclear scaffold/matrix attachment regions often define the base of looped chromosomal domains in vivo and that they are thereby critical for correct chromosome architecture and gene expression. Here, we show regulated organization of mammalian ribosomal ribonucleic acid genes into distinct chromatin loops by tethering to nucleolar matrix via the non-transcribed inter-genic spacer region of the ribosomal DNA (rDNA). The rDNA gene loop structures are induced specifically upon growth stimulation and are dependent on the activity of the c-Myc protein. Matrix-attached rDNA genes are hypomethylated at the promoter and are thus available for transcriptional activation. rDNA genes silenced by methylation are not recruited to the matrix. c-Myc, which has been shown to induce rDNA transcription directly, is physically associated with rDNA gene looping structures and the intergenic spacer sequence in growing cells. Such a role of Myc proteins in gene activation has not been reported previously.

c-Myc associates with ribosomal DNA and activates RNA polymerase I transcription.

The c-Myc oncoprotein regulates transcription of genes that are associated with cell growth, proliferation and apoptosis. c-Myc levels are modulated by ubiquitin/proteasome-mediated degradation. Proteasome inhibition leads to c-Myc accumulation within nucleoli, indicating that c-Myc might have a nucleolar function. Here we show that the proteins c-Myc and Max interact in nucleoli and are associated with ribosomal DNA. This association is increased upon activation of quiescent cells and is followed by recruitment of the Myc cofactor TRRAP, enhanced histone acetylation, recruitment of RNA polymerase I (Pol I), and activation of rDNA transcription. Using small interfering RNAs (siRNAs) against c-Myc and an inhibitor of Myc-Max interactions, we demonstrate that c-Myc is required for activating rDNA transcription in response to mitogenic signals. Furthermore, using the ligand-activated MycER (ER, oestrogen receptor) system, we show that c-Myc can activate Pol I transcription in the absence of Pol II transcription. These results suggest that c-Myc coordinates the activity of all three nuclear RNA polymerases, and thereby plays a key role in regulating ribosome biogenesis and cell growth.

PKCδ mediates mitochondrial ROS generation and oxidation of HSP60 to relieve RKIP inhibition on MAPK pathway for HCC progression.

Both protein kinase C (PKC) and reactive oxygen species (ROS) are well-known signaling messengers cross-talking with each other to activate mitogen-activated protein kinases (MAPKs) for progression of hepatocellular carcinoma (HCC). However, the underlying mechanisms are not well elucidated. Especially, whether mitochondrial ROS (mtROS) is involved and how it triggers MAPK signaling are intriguing. In this study, we found mtROS generation and phosphorylation of MAPKs were mediated by PKCδ in HCCs treated with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Heat shock protein 60 (HSP60), one of the chaperones in mitochondria was the major protein oxidized in TPA-treated HCCs. Moreover, depletion of HSP60 or expression of HSP60 cysteine mutant prevented TPA-induced phosphorylation of MAPKs. To delineate how HSP60 mediated MAPK activation, the role of Raf kinase inhibitor protein (RKIP), a negative regulator of MAPK, was investigated. TPA dissociated RKIP from HSP60 in both mitochondria and cytosol, concurrently with translocation of HSP60 and MAPK from mitochondria to cytosol, which was associated with robust phosphorylation of MAPKs in the cytosol. Moreover, TPA induced opposite phenotypical changes of HCCs, G1 cell cycle arrest, and cell migration, which were prevented by mtROS scavengers and depletion of PKCδ and HSP60. Consistently, TPA increased the migration-related genes, hydrogen peroxide inducible clone5, matrix metalloproteinase-1/3, lamininγ2, and suppressed the cell cycle regulator cyclin E1 (CCNE1) via PKCδ/mtROS/HSP60/MAPK-axis. Finally, c-jun and c-fos were required for TPA-induced expression of the migration-related genes and a novel microRNA, miR-6134, was responsible for TPA-induced suppression of CCNE1. In conclusion, PKCδ cross-talked with mtROS to trigger HSP60 oxidation for release of RKIP to activate MAPK, regulating gene expression for migration, and G1 cell cycle arrest in HCC. Targeted therapy aiming at key players like PKCδ, RKIP, and HSP60 is promising for preventing HCC progression.

Hypophosphatemic rickets associated with epidermal nevus syndrome and giant hairy nevus.

The association of hypophosphatemic rickets and epidermal nevus or giant hairy nevus is rare. We report two patients with hypophosphatemic rickets, one associated with epidermal nevus syndrome and the other with giant hairy nevus, and describe their clinical features and variable response to treatment. The abnormal nevus tissue may have contributed to the pathogenesis of hypophosphatemic rickets. We did not find a PHEX gene mutation in these two patients, and the mechanism for their rickets may be different from that in X-linked hypophosphatemic rickets.

AP-1-mediated chromatin looping regulates ZEB2 transcription: new insights into TNFα-induced epithelial-mesenchymal transition in triple-negative breast cancer.

The molecular determinants of malignant cell behaviour in triple-negative breast cancer (TNBC) are poorly understood. Recent studies have shown that regulators of epithelial-mesenchymal transition (EMT) are potential therapeutic targets for TNBC. In this study, we demonstrate that the inflammatory cytokine TNFα induces EMT in TNBC cells via activation of AP-1 signaling and subsequently induces expression of the EMT regulator ZEB2. We also show that TNFα activates both the PI3K/Akt and MAPK/ERK pathways, which act upstream of AP-1. We further investigated in detail AP-1 regulation of ZEB2 expression. We show that two ZEB2 transcripts derived from distinct promoters are both expressed in breast cancer cell lines and breast tumor samples. Using the chromosome conformation capture assay, we demonstrate that AP-1, when activated by TNFα, binds to a site in promoter 1b of the ZEB2 gene where it regulates the expression of both promoter 1b and 1a, the latter via mediating long range chromatin interactions. Overall, this work provides a plausible mechanism for inflammation-induced metastatic potential in TNBC, involving a novel regulatory mechanism governing ZEB2 isoform expression.

Cytogenetic surveillance of mentally-retarded school children in southern Taiwan.

BACKGROUND AND PURPOSE: Mental retardation (MR), defined as having an IQ of less than 70, is present in approximately 2 to 3% of the population. Data on chromosomal abnormalities, an important cause of MR, are limited in the Taiwanese literature. This study evaluated the frequency and pattern of chromosomal abnormalities in school children with MR in southern Taiwan. METHODS: Peripheral blood samples of 419 children were collected from November 1999 to January 2003. Those with Prader-Willi syndrome (PWS), Angelman syndrome (AS) or fragile-X syndrome were excluded from the study. Metaphase chromosome preparations were obtained from peripheral blood cultures, and trypsin-giemsa (GTG) banded chromosomes were examined at the level of 500 to 600 bands. Fluorescence in situ hybridization was done for cases whose karyotypes could not be determined by conventional cytogenetic analysis. RESULTS: Of the 419 enrollees with MR, 10 had mild MR, while most had moderate to profound MR. Chromosomal abnormalities were found in 22.43% of the cases, with trisomy 21 being the major chromosomal abnormality, occurring in 77 cases (18.38%, 77/419 cases). Sex chromosome aneuploidies were found in 3 cases (0.72%, 3/419 cases). Structural abnormalities of autosomes were found in 13 cases (3.10%, 13/419 cases), including deletion, markers, unbalanced translocations, and inversions. One subject was found to have monosomy 20 mosaicism (0.24%, 1/419). CONCLUSIONS: Chromosomal abnormalities occurred in a high proportion of mentally retarded school children from southern Taiwan, with trisomy 21 being the most prevalent. These findings indicate the need for increased attention to prenatal, perinatal and postnatal screening in this population.

Nucleolar organization, growth control and cancer.

The nucleolus is a dynamic region of the nucleus that is disassembled and reformed each cell cycle and whose size is correlated with cell growth rate. Nucleolar size is a prognostic measure of cancer disease severity and increasing evidence suggests a causative role of nucleolar lesions in many cancers. In recent work (Shiue et al. Oncogene 28, 1833-42, 2009) we showed that the c-Myc oncoprotein induces changes in the higher order structure of rDNA chromatin in the nucleolus of growth stimulated quiescent rat cells. Here we show that c-Myc induces similar changes in human cells, that c-Myc plays a role in the overall structural integrity of the nucleolus and that c-Myc and its antagonistic partner Mad1 interact to program the epigenetic status of rDNA chromatin. These changes are discussed in relation to current knowledge about nucleolar structure as well as the organization of chromosomes and transcription factories in nuclear regions outside the nucleolus.