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Prevalent dysregulation of epigenetic modifications plays a pivotal role in cancer. Targeting epigenetic abnormality is a new strategy for cancer therapy. Understanding how conventional oncogenic factors cause epigenetic abnormality is of great basic and translational value. O-GlcNAcylation is a protein modification which affects physiology and pathophysiology. In mammals, O-GlcNAcylation is catalyzed by one single enzyme OGT and removed by one single enzyme OGA. O-GlcNAcylation is affected by the availability of the donor, UDP-GlcNAc, generated by the serial enzymatic reactions in the hexoamine biogenesis pathway (HBP). O-GlcNAcylation regulates a wide spectrum of substrates including many proteins involved in epigenetic modification. Like epigenetic modifications, abnormality of O-GlcNAcylation is also common in cancer. Studies have revealed substantial impact on HBP enzymes and OGT/OGA by oncogenic signals. In this review, we will first summarize how oncogenic signals regulate HBP enzymes, OGT and OGA in cancer. We will then integrate this knowledge with the up to date understanding how O-GlcNAcylation regulates epigenetic machinery. With this, we propose a signal axis from oncogenic signals through O-GlcNAcylation dysregulation to epigenetic abnormality in cancer. Further elucidation of this axis will not only advance our understanding of cancer biology but also provide new revenues towards cancer therapy.

m6A, a conserved and abundant modification on RNA, regulates RNA processing and function. RNA m6A machinery, including writers, erasers, and readers of m6A, is indispensable for m6A installation and function. Intriguingly, recent studies have revealed that m6A machinery can be recruited to chromatin by pleiotropic factors, including nascent RNA, transcription factors, regulatory RNA, histone modifications, and epigenetic machinery. Consequently, recruitment of m6A machinery can directly regulate chromatin biology, such as transcription, DNA damage repair, and DNA recombination beyond installation of m6A on nascent mRNA. Here, we discuss recent evidence showing that m6A machinery is targeted to chromatin and the direct biological consequences along with the underlying mechanisms.

The p53 tumor suppressor is a sequence-specific DNA binding protein that activates gene transcription to regulate cell survival and proliferation. Dynamic control of p53 degradation and DNA binding in response to stress signals are critical for tumor suppression. The p53 N terminus (NT) contains two transactivation domains (TAD1 and TAD2), a proline-rich region (PRR), and multiple phosphorylation sites. Previous work revealed the p53 NT reduced DNA binding in vitro. Here, we show that TAD2 and the PRR inhibit DNA binding by directly interacting with the sequence-specific DNA binding domain (DBD). NMR spectroscopy revealed that TAD2 and the PRR interact with the DBD at or near the DNA binding surface, possibly acting as a nucleic acid mimetic to competitively block DNA binding. In vitro and in vivo DNA binding analyses showed that the NT reduced p53 DNA binding affinity but improved the ability of p53 to distinguish between specific and nonspecific sequences. MDMX inhibits p53 binding to specific target promoters but stimulates binding to nonspecific chromatin sites. The results suggest that the p53 NT regulates the affinity and specificity of DNA binding by the DBD. The p53 NT-interacting proteins and posttranslational modifications may regulate DNA binding, partly by modulating the NT-DBD interaction.

Summary: Histone lysine methylation functions at the interface of the extracellular environment and intracellular gene expression. DOT1L is a versatile histone H3K79 methyltransferase with a prominent role in MLL-fusion leukemia, yet little is known about how DOT1L responds to extracellular stimuli. Here, we report that DOT1L protein stability is regulated by the extracellular glucose level through the hexosamine biosynthetic pathway (HBP). Mechanistically, DOT1L is O-GlcNAcylated at evolutionarily conserved S1511 in its C terminus. We identify UBE3C as a DOT1L E3 ubiquitin ligase promoting DOT1L degradation whose interaction with DOT1L is susceptible to O-GlcNAcylation. Consequently, HBP enhances H3K79 methylation and expression of critical DOT1L target genes such as HOXA9/MEIS1, promoting cell proliferation in MLL-fusion leukemia. Inhibiting HBP or O-GlcNAc transferase (OGT) increases cellular sensitivity to DOT1L inhibitor. Overall, our work uncovers O-GlcNAcylation and UBE3C as critical determinants of DOT1L protein abundance, revealing a mechanism by which glucose metabolism affects malignancy progression through histone methylation.

Missense p53 mutants often accumulate in tumors and drive progression through gain of function. MDM2 efficiently degrades wild-type p53 but fails to degrade mutant p53 in tumor cells. Previous studies revealed that mutant p53 inhibits MDM2 autoubiquitination, suggesting that the interaction inhibits MDM2 E3 activity. Recent work showed that MDM2 E3 activity is stimulated by intramolecular interaction between the RING and acidic domains. Here, we show that in the mutant p53-MDM2 complex, the mutant p53 core domain binds to the MDM2 acidic domain with significantly higher avidity than wild-type p53. The mutant p53-MDM2 complex is deficient in catalyzing ubiquitin release from the activated E2 conjugating enzyme. An MDM2 construct with extra copies of the acidic domain is resistant to inhibition by mutant p53 and efficiently promotes mutant p53 ubiquitination and degradation. The results suggest that mutant p53 interferes with the intramolecular autoactivation mechanism of MDM2, contributing to reduced ubiquitination and increased accumulation in tumor cells.

Nucleomethylin (NML), a novel nucleolar protein, is important for mediating the assembly of the energy-dependent nucleolar silencing complex (eNoSC), which also contains SirT1 and SUV39H1. eNoSC represses rRNA transcription during nutrient deprivation, thus reducing energy expenditure and improving cell survival. We found that NML is an RNA binding protein that copurifies with 5S, 5.8S, and 28S rRNA. The SirT1 and RNA binding regions on NML showed partial overlap, and the NML-SirT1 interaction was competitively inhibited by rRNA. Nutrient deprivation triggered downregulation of rRNA transcription, reduced the level of NML-associated rRNA, and stimulated NML-SirT1 binding. Assembly of eNoSC facilitated repression of pre-rRNA transcription. These results suggest that nascent rRNA generates a positive-feedback signal by suppressing the assembly of eNoSC and protecting active ribosomal DNA units from heterochromatin formation. This RNA-mediated mechanism enables the eNoSC to amplify the effects of upstream nutrient-responsive regulators.

The MDMX oncoprotein is an important regulator of tumor suppressor p53 activity during embryonic development. Despite sequence homology to the ubiquitin E3 ligase MDM2, MDMX depletion activates p53 without significant increase in p53 level, implicating a degradation-independent mechanism. We present evidence that MDMX inhibits the sequence-specific DNA binding activity of p53. This function requires the cooperation between MDMX and CK1α, and phosphorylation of S289 on MDMX. Depletion of MDMX or CK1α increases p53 DNA binding without stabilization of p53. A proteolytic fragment release assay revealed that in the MDMX-p53 complex, the MDMX acidic domain and RING domain interact stably with the p53 DNA binding domain. These interactions are referred to as secondary interactions because they only occur after the canonical-specific binding between the MDMX and p53 N termini, but exhibit significant binding stability in the mature complex. CK1α cooperates with MDMX to inhibit p53 DNA binding by further stabilizing the MDMX acidic domain and p53 core domain interaction. These results suggest that secondary intermolecular interaction is important in p53 regulation by MDMX, which may represent a common phenomenon in complexes containing multidomain proteins.

Tumor cells with defective apoptosis pathways often respond to chemotherapy by entering irreversible cell cycle arrest with features of senescence. However, rare cells can bypass entry to senescence, or re-enter cell cycle from a senescent state. Deficiency in senescence induction and maintenance may contribute to treatment resistance and early relapse after therapy. Senescence involves epigenetic silencing of cell cycle genes and reduced rRNA transcription. We found that senescence-inducing treatments such as DNA damage and RNA polymerase I inhibition stimulate the binding between the nucleolar protein NML (nucleomethylin) and SirT1. The NML complex promotes rDNA heterochromatin formation and represses rRNA transcription. Depletion of NML reduced the levels of H3K9Me3 and H3K27Me3 heterochromatin markers on rDNA and E2F1 target promoters in senescent cells, increased rRNA transcription, and increased the frequency of cell cycle re-entry. Depletion of the nucleolar transcription repressor factor TIP5 also promoted escape from senescence. Furthermore, tumor tissue staining showed that breast tumors without detectable nucleolar NML expression had poor survival. The results suggest that efficient regulation of nucleolar rDNA transcription facilitates the maintenance of irreversible cell cycle arrest in senescent cells. Deficiency in nucleolar transcription repression may accelerate tumor relapse after chemotherapy.

Obesity is accompanied by an increase in both adipocyte number and size. The increase in adipocyte number is the result of recruitment to the adipocyte lineage of pluripotent stem cells present in the vascular stroma of adipose tissue. These pluripotent cells have the potential to undergo commitment and then differentiate into adipocytes, as well as myocytes, osteocytes, and chondrocytes. In this article, we show that both bone morphogenetic protein (BMP)2 and BMP4 can induce commitment of C3H10T1/2 pluripotent stem cells into adipocytes. After treatment of C3H10T1/2 stem cells with these BMPs during proliferation followed by exposure to differentiation inducers at growth arrest, nearly all cells enter the adipose development pathway, express specific adipocyte markers, and acquire the adipocyte phenotype. Overexpression of constitutively active BMP receptor (CA)-BMPr1A or CA-BMPr1B induces commitment in the absence of BMP2/4, whereas overexpression of a dominant-negative receptor dominant-negative-BMPr1A suppresses commitment induced by BMP. Also, knockdown of the expression of Smad4 (coregulator in the BMP/Smad signaling pathway) with RNAi disrupts commitment by the BMPs. However, knockdown of expression of p38 MAPK (an intermediary in the BMP/MAPK signaling pathway) with RNAi had little effect on BMP-induced commitment. Together, these findings indicate that the BMP/Smad signaling pathway has a dominant role in adipocyte lineage determination. Proteomic analysis identified lysyl oxidase (LOX), a bona fide downstream target gene of the BMP signaling pathway. Expression of LOX is induced by BMP2/4 during adipocyte lineage commitment, and knockdown of its expression disrupts the commitment process.

CHOP-10, a dominant-negative member of the C/EBP family of transcription factors, is initially expressed by growth-arrested preadipocytes and sequesters/inactivates C/EBPbeta through heterodimerization with its leucine zipper during 3T3-L1 preadipocyte differentiation. Our previous studies indicated that, FBS leads to the down-regulation of CHOP-10 expression after induction, and releasing C/EBPbeta from inhibitory constraint, allowing the transactivation of C/EBPalpha and PPARgamma genes, transcription factors required for terminal adipocyte differentiation. In the present study, we reported that FBS induced the expression of YY1, which bound to CHOP-10 promoter via two adjacent YY1-binding sites, suppressing its expression. The knock-down of YY1 expression with YY1 siRNA increased the expression of CHOP-10, inhibiting adipocyte differentiation. IGF-1, a growth factor present in greater concentration in FBS, independently induced the expression of YY1, and contributed to the down-regulation of CHOP-10 during the adipocyte differentiation program. Our studies suggested that YY1 can be a new adipocyte differentiation stimulator.

Adiponectin is an adipokine that predominantly synthesized and secreted from adipocytes mainly in the white adipose tissue. Here, we report that we have successfully expressed human gAdiponectin (the globular domain of adiponectin) in the methylotrophic yeast Pichia pastoris after codon optimization and established the purification procedure. The human gAdiponectin gene was designed and synthesized by PCR according to the P. pastoris preferred codons, and then inserted into the P. pastoris pPIC9K expression vector. The plasmid was electroporated into the P. pastoris strain GS115 and only the G418 resistance colonies could produce the gAdiponectin. After fermentation and purification, we could get 1.2g of recombinant gAdiponectin (purity is approximately 95%) from a 24 L culture media. The recombinant gAdiponectin is fully functional as evidenced by induction the phosphorylation of ACC in differentiated C2C12 myotubes, significantly lowering the blood glucose level and accelerating the clearance of free fatty acid in animal models.

Hormonal induction triggers a cascade leading to the expression of CCAAT/enhancer-binding protein(C/EBP)alpha and peroxisome proliferator-activated receptor (PPAR) gamma, C/EBPalpha, and PPARgamma turns on series of adipocyte genes that give rise to the adipocyte phenotype. Previous findings indicate that C/EBPbeta, a transcriptional activator of the C/EBPalpha and PPARgamma genes, is rapidly expressed after induction, but lacks DNA-binding activity and therefore cannot activate transcription of the C/EBPalpha and PPARgamma genes early in the differentiation program. Acquisition of DNA-binding activity of C/EBPbeta occurs when CHOP-10, a dominant-negative form of C/EBP family members, is down-regulated and becomes hyperphosphorylated as preadipocytes traverse the G1-S checkpoint of mitotic clonal expansion. Evidences are presented in this report that lactacystin, a proteasome inhibitor, up-regulated the CHOP-10 expression, blocked the DNA-binding activity of C/EBPbeta, and subsequently inhibited MCE as well as adipocyte differentiation.

PAI-1 is expressed and secreted by adipose tissue which may mediate the pathogenesis of obesity-associated cardiovascular complications. Evidence is presented in this report that PAI-1 is not expressed by preadipocyte, but significantly induced during 3T3-L1 adipocyte differentiation and the PAI-1 expression correlates with the induction of peroxisome proliferator-activated receptor gamma (PPARgamma). A peroxisome proliferator responsive element (PPRE)-like cis-element (-206TCCCCCATGCCCT-194) is identified in the mouse PAI-1 gene promoter by electrophoretic mobility shift assay (EMSA) combined with transient transfection experiments; the PPRE-like cis-element forms a specific DNA-protein complex only with adipocyte nuclear extracts, not with preadipocyte nuclear extracts; the DNA-protein complex can be totally competed away by non-labeled consensus PPRE, and can be supershifted with PPARgamma antibody. Mutation of this PPRE-like cis-element can abolish the transactivation of mouse PAI-1 promoter mediated by PPARgamma. Specific PPARgamma ligand Pioglitazone can significantly induce the PAI-1 expression, and stimulate the secretion of PAI-1 into medium.

The RING domain ubiquitin E3 ligase MDM2 is a key regulator of p53 degradation and a mediator of signals that stabilize p53. The current understanding of the mechanisms by which MDM2 posttranslational modifications and protein binding cause p53 stabilization remains incomplete. Here we present evidence that the MDM2 central acidic region is critical for activating RING domain E3 ligase activity. A 30-amino-acid minimal region of the acidic domain binds to the RING domain through intramolecular interactions and stimulates the catalytic function of the RING domain in promoting ubiquitin release from charged E2. The minimal activation sequence is also the binding site for the ARF tumor suppressor, which inhibits ubiquitination of p53. The acidic domain-RING domain intramolecular interaction is modulated by ATM-mediated phosphorylation near the RING domain or by binding of ARF. These results suggest that MDM2 phosphorylation and association with protein regulators share a mechanism in inhibiting the E3 ligase function and stabilizing p53 and suggest that targeting the MDM2 autoactivation mechanism may be useful for therapeutic modulation of p53 levels.

The chromosomal region encoding the nuclear NAD(+) synthesis enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT1) is frequently deleted in human cancer. We describe evidence that NMNAT1 interacts with the nucleolar repressor protein nucleomethylin and is involved in regulating rRNA transcription. NMNAT1 binds to nucleomethylin and is recruited into a ternary complex containing the NAD(+)-dependent deacetylase SirT1. NMNAT1 expression stimulates the deacetylase function of SirT1. Knockdown of NMNAT1 enhances rRNA transcription and promotes cell death after nutrient deprivation. Furthermore, NMNAT1 expression is induced by DNA damage and plays a role in preventing cell death after damage. Heterozygous deletion of NMNAT1 in lung tumor cell lines correlates with low expression level and increased sensitivity to DNA damage. These results suggest that NMNAT1 deletion in tumors may contribute to transformation by increasing rRNA synthesis, but may also increase sensitivity to nutrient stress and DNA damage.

Distinct lysine methylation marks on histones create dynamic signatures deciphered by the "effector" modules, although the underlying mechanisms remain unclear. We identified the plant homeodomain- and Jumonji C domain-containing protein PHF2 as a novel histone H3K9 demethylase. We show in biochemical and crystallographic analyses that PHF2 recognizes histone H3K4 trimethylation through its plant homeodomain finger and that this interaction is essential for PHF2 occupancy and H3K9 demethylation at rDNA promoters. Our study provides molecular insights into the mechanism by which distinct effector domains within a protein cooperatively modulate the "cross-talk" of histone modifications.

The developmental pathway that gives rise to mature adipocytes involves two distinct stages: commitment and terminal differentiation. Although the important proteins/factors contributing to terminal adipocyte differentiation have been well defined, the proteins/factors in the commitment of mesenchymal stem cells to the adipocyte lineage cells have not. In this study, we applied proteomics analysis profiling to characterize differences between uncommitted C3H10T1/2 pluripotent stem cells and those that have been committed to the adipocyte lineage by BMP4 or BMP2 with the goal to identify such proteins/factors and to understand the molecular mechanisms that govern the earliest stages of adipocyte lineage commitment. Eight proteins were found to be up-regulated by BMP2, and 27 proteins were up-regulated by BMP4, whereas five unique proteins were up-regulated at least 10-fold by both BMP2/4, including three cytoskeleton-associated proteins (i.e. lysyl oxidase (LOX), translationally controlled tumor protein 1 (TPT1), and αB-crystallin). Western blotting further confirmed the induction of the expression of these cytoskeleton-associated proteins in the committed C3H10T1/2 induced by BMP2/4. Importantly, knockdown of LOX expression totally prevented the commitment, whereas knockdown of TPT1 and αB-crystallin expression partially inhibited the commitment. Several published reports suggest that cell shape can influence the differentiation of partially committed precursors of adipocytes, osteoblasts, and chondrocytes. We observed a dramatic change of cell shape during the commitment process, and we showed that knockdown of these cytoskeleton-associated proteins prevented the cell shape change and restored F-actin organization into stress fibers and inhibited the commitment to the adipocyte lineage. Our studies indicate that these differentially expressed cytoskeleton-associate proteins might determine the fate of mesenchymal stem cells to commit to the adipocyte lineage through cell shape regulation.