The Multilayered Regulation of the Oncogenic Protein YY1

The multifunctional protein Yin Yang 1 (YY1) plays critical roles in tumorigenesis. YY1 has been shown to be involved in the development, progression, resistance, and invasiveness of many types of cancers. Today, the value of YY1 as a prognostic marker and as a potential target in cancer therapy is being explored by multiple research groups around the world. Over the past 25 years, we have accumulated a wealth of information about the wide-ranging biological functions of YY1 at the molecular, cellular, and organismal levels. However, our knowledge of how YY1 is regulated and what regulates it has lagged behind. In the past few years, there has been a significant increase in the research addressing this issue. In this review, we summarize and analyze recent findings about the regulation of YY1 at multiple levels. We emphasize the necessity for deeper insights into these regulatory mechanisms if YY1 is to find its way to the clinical setting.

c-Abl phosphorylation of Yin Yang 1's conserved tyrosine 254 in the spacer region modulates its transcriptional activity

Yin Yang 1 (YY1) is a multifunctional transcription factor that can activate or repress transcription depending on the promotor and/or the co-factors recruited. YY1 is phosphorylated in various signaling pathways and is critical for different biological functions including embryogenesis, apoptosis, proliferation, cell-cycle regulation and tumorigenesis. Here we report that YY1 is a substrate for c-Abl kinase phosphorylation at conserved residue Y254 in the spacer region. Pharmacological inhibition of c-Abl kinase by imatinib, nilotinib and GZD824, knock-down of c-Abl using siRNA, and the use of c-Abl kinase-dead drastically reduces tyrosine phosphorylation of YY1. Both radioactive and non-radioactive in vitro kinase assays, as well as co-immunoprecipitation in different cell lines, show that the target of c-Abl phosphorylation is tyrosine residue 254. c-Abl phosphorylation has little effect on YY1 DNA binding ability or cellular localization in asynchronous cells. However, functional studies reveal that c-Abl mediated phosphorylation of YY1 regulates YY1's transcriptional ability in vivo. In conclusion, we demonstrate the novel role of c-Abl kinase in regulation of YY1's transcriptional activity, linking YY1 regulation with c-Abl tyrosine kinase signaling pathways.

Aurora A Phosphorylation of YY1 during Mitosis Inactivates its DNA Binding Activity

Successful execution of mitotic cell division requires the tight synchronisation of numerous biochemical pathways. The underlying mechanisms that govern chromosome segregation have been thoroughly investigated. However, the mechanisms that regulate transcription factors in coordination with mitotic progression remain poorly understood. In this report, we identify the transcription factor YY1 as a novel mitotic substrate for the Aurora A kinase, a key regulator of critical mitotic events, like centrosome maturation and spindle formation. Using in vitro kinase assays, we show that Aurora A directly phosphorylates YY1 at serine 365 in the DNA-binding domain. Using a new phospho-specific antibody, we show that YY1 phosphorylation at serine 365 occurs during mitosis, and that this phosphorylation is significantly reduced upon inhibition of Aurora A. Furthermore, we show, using electrophoretic mobility shift and chromatin immunoprecipitation assays, that phosphorylation of YY1 at this site abolishes its DNA binding activity in vitro and in vivo. In conformity with this loss of binding activity, phosphorylated YY1 also loses its transctivation ability as demonstrated by a luciferase reporter assay. These results uncover a novel mechanism that implicates Aurora A in the mitotic inactivation of transcription factors.

Identification of the oncogenic kinase TOPK/PBK as a master mitotic regulator of C2H2 zinc finger proteins

TOPK/PBK is an oncogenic kinase upregulated in most human cancers and its high expression correlates with poor prognosis. TOPK is known to be activated by Cdk1 and needed for mitotic cell division; however, its mitotic functions are not yet fully understood. In this study, we show that TOPK plays a global mitotic role by simultaneously regulating hundreds of DNA binding proteins. C2H2 zinc finger proteins (ZFPs) constitute the largest family of human proteins. All C2H2 ZFPs contain a highly conserved linker sequence joining their multi-zinc finger domains. We have previously shown that phosphorylation of this conserved motif serves as a global mechanism for the coordinate dissociation of C2H2 ZFPs from condensing chromatin, during mitosis. Here, using a panel of kinase inhibitors, we identified K252a as a potent inhibitor of mitotic ZFP linker phosphorylation. We generated a biotinylated form of K252a and used it to purify candidate kinases. From these candidates we identified TOPK/PBK, in vitro and in vivo, as the master ZFP linker kinase. Furthermore, we show precise temporal correlation between TOPK activating phosphorylation by Cdk1 and linker phosphorylation in mitosis. The identification of this fundamental role of TOPK underscores its significance as a promising novel target of cancer therapeutics.

14-3-3 protein targets misfolded chaperone-associated proteins to aggresomes

The aggresome is a key cytoplasmic organelle for sequestration and clearance of toxic protein aggregates. Although loading misfolded proteins cargos to dynein motors has been recognized as an important step in the aggresome formation process, the molecular machinery that mediates the association of cargos with the dynein motor is poorly understood. Here, we report a new aggresome-targeting pathway that involves isoforms of 14-3-3, a family of conserved regulatory proteins. 14-3-3 interacts with both the dynein-intermediate chain (DIC) and an Hsp70 co-chaperone Bcl-2-associated athanogene 3 (BAG3), thereby recruiting chaperone-associated protein cargos to dynein motors for their transport to aggresomes. This molecular cascade entails functional dimerization of 14-3-3, which we show to be crucial for the formation of aggresomes in both yeast and mammalian cells. These results suggest that 14-3-3 functions as a molecular adaptor to promote aggresomal targeting of misfolded protein aggregates and may link such complexes to inclusion bodies observed in various neurodegenerative diseases.

Slk19 clusters kinetochores and facilitates chromosome bipolar attachment

Yeast kinetochore protein Slk19 is required for kinetochore clustering, and nocodazole exposure to slk19 mutant cells causes impaired kinetochore capture and delayed chromosome bipolar attachment after nocodazole washout.

In all eukaryotic cells, DNA is packaged into multiple chromosomes that are linked to microtubules through a large protein complex called a kinetochore. Previous data show that the kinetochores are clustered together during most of the cell cycle, but the mechanism and the biological significance of kinetochore clustering are unknown. As a kinetochore protein in budding yeast, the role of Slk19 in the stability of the anaphase spindle has been well studied, but its function in chromosome segregation has remained elusive. Here we show that Slk19 is required for kinetochore clustering when yeast cells are treated with the microtubule-depolymerizing agent nocodazole. We further find that slk19Δ mutant cells exhibit delayed kinetochore capture and chromosome bipolar attachment after the disruption of the kinetochore–microtubule interaction by nocodazole, which is likely attributed to defective kinetochore clustering. In addition, we show that Slk19 interacts with itself, suggesting that the dimerization of Slk19 may mediate the interaction between kinetochores for clustering. Therefore Slk19 likely acts as kinetochore glue that clusters kinetochores to facilitate efficient and faithful chromosome segregation.

The transcription factor YY1 is a novel substrate for Aurora B kinase at G2/M transition of the cell cycle

Yin Yang 1 (YY1) is a ubiquitously expressed and highly conserved multifunctional transcription factor that is involved in a variety of cellular processes. Many YY1-regulated genes have crucial roles in cell proliferation, differentiation, apoptosis, and cell cycle regulation. Numerous mechanisms have been shown to regulate the function of YY1, such as DNA binding affinity, subcellular localization, and posttranslational modification including phosphorylation. Polo-like kinase 1(Plk1) and Casein kinase 2α (CK2 α) were the first two kinases identified to phosphorylate YY1. In this study, we identify a third kinase. We report that YY1 is a novel substrate of the Aurora B kinase both in vitro and in vivo. Serine 184 phosphorylation of YY1 by Aurora B is cell cycle regulated and peaks at G2/M and is rapidly dephosphorylated, likely by protein phosphatase 1 (PP1) as the cells enter G1. Aurora A and Aurora C can also phosphorylate YY1 in vitro, but at serine/threonine residues other than serine 184. We present evidence that phosphorylation of YY1 in the central glycine/alanine (G/A)-rich region is important for DNA binding activity, with a potential phosphorylation/acetylation interplay regulating YY1 function. Given their importance in mitosis and overexpression in human cancers, Aurora kinases have been identified as promising therapeutic targets. Increasing our understanding of Aurora substrates will add to the understanding of their signaling pathways.

YY1 associates with the macrosatellite DXZ4 on the inactive X chromosome and binds with CTCF to a hypomethylated form in some male carcinomas

DXZ4 is an X-linked macrosatellite composed of 12–100 tandemly arranged 3-kb repeat units. In females, it adopts opposite chromatin arrangements at the two alleles in response to X-chromosome inactivation. In males and on the active X chromosome, it is packaged into heterochromatin, but on the inactive X chromosome (Xi), it adopts a euchromatic conformation bound by CTCF. Here we report that the ubiquitous transcription factor YY1 associates with the euchromatic form of DXZ4 on the Xi. The binding of YY1 close to CTCF is reminiscent of that at other epigenetically regulated sequences, including sites of genomic imprinting, and at the X-inactivation centre, suggesting a common mode of action in this arrangement. As with CTCF, binding of YY1 to DXZ4 in vitro is not blocked by CpG methylation, yet in vivo both proteins are restricted to the hypomethylated form. In several male carcinoma cell lines, DXZ4 can adopt a Xi-like conformation in response to cellular transformation, characterized by CpG hypomethylation and binding of YY1 and CTCF. Analysis of a male melanoma cell line and normal skin cells from the same individual confirmed that a transition in chromatin state occurred in response to transformation.

Global mitotic phosphorylation of C2H2 zinc finger protein linker peptides

Cessation of transcriptional activity is a hallmark of cell division. Many biochemical pathways have been shown and proposed over the past few decades to explain the silence of this phase. In particular, many individual transcription factors have been shown to be inactivated by phosphorylation. In this report, we show the simultaneous phosphorylation and mitotic redistribution of a whole class of modified transcription factors. C(2)H(2) zinc finger proteins (ZFPs) represent the largest group of gene expression regulators in the human genome. Despite their diversity, C(2)H(2) ZFPs display striking conservation of small linker peptides joining their adjacent zinc finger modules. These linkers are critical for DNA binding activity. It has been proposed that conserved phosphorylation of these linker peptides could be a common mechanism for the inactivation of the DNA binding activity of C(2)H(2) ZFPs, during mitosis. Using a novel antibody, raised against the phosphorylated form of the most conserved linker peptide sequence, we are able to visualize the massive and simultaneous mitotic phosphorylation of hundreds of these proteins. We show that this wave of phosphorylation is tightly synchronized, starting in mid-prophase right after DNA condensation and before the breakdown of the nuclear envelope. This global phosphorylation is completely reversed in telophase. In addition, the exclusion of the phospho-linker signal from condensed DNA clearly demonstrates a common mechanism for the mitotic inactivation of C(2)H(2) ZFPs.

The transcription factor YY1 is a substrate for Polo-like kinase 1 at the G2/M transition of the cell cycle

Yin-Yang 1 (YY1) is an essential multifunctional zinc-finger protein. It has been shown over the past two decades to be a critical regulator of a vast array of biological processes, including development, cell proliferation and differentiation, DNA repair, and apoptosis. YY1 exerts its functions primarily as a transcription factor that can activate or repress gene expression, dependent on its spatial and temporal context. YY1 regulates a large number of genes involved in cell cycle transitions, many of which are oncogenes and tumor-suppressor genes. YY1 itself has been classified as an oncogene and was found to be upregulated in many cancer types. Unfortunately, our knowledge of what regulates YY1 is very minimal. Although YY1 has been shown to be a phosphoprotein, no kinase has ever been identified for the phosphorylation of YY1. Polo-like kinase 1 (Plk1) has emerged in the past few years as a major cell cycle regulator, particularly for cell division. Plk1 has been shown to play important roles in the G/M transition into mitosis and for the proper execution of cytokinesis, processes that YY1 has been shown to regulate also. Here, we present evidence that Plk1 directly phosphorylates YY1 in vitro and in vivo at threonine 39 in the activation domain. We show that this phosphorylation is cell cycle regulated and peaks at G2/M. This is the first report identifying a kinase for which YY1 is a substrate.

Characterization of neuronal Src kinase purified from a bacterial expression system

Neuronal Src (n-Src) is an alternative isoform of Src kinase containing a 6-amino acid insert in the SH3 domain that is highly expressed in neurons of the central nervous system (CNS). To investigate the function of n-Src, wild-type n-Src, constitutively active n-Src in which the C-tail tyrosine 535 was mutated to phenylalanine (n-Src/Y535F) and inactive n-Src in which the lysine 303 was mutated to arginine in addition to the mutation of Y535F (n-Src/K303R/Y535F), were expressed and purified from Escherichia coli BL21(DE3) cells. We found that all three types of n-Src constructs expressed at very high yields (∼500 mg/L) at 37°C, but formed inclusion bodies. In the presence of 8M urea these proteins could be solubilized, purified under denaturing conditions, and subsequently refolded in the presence of arginine (0.5M). These Src proteins were enzymatically active except for the n-Src/K303R/Y535F mutant. n-Src proteins expressed at 18°C were soluble, albeit at lower yields (∼10-20 mg/L). The lowest yields were for n-Src/Y535F (∼10 mg/L) and the highest for n-Src/K303R/Y535F (∼20 mg/L). We characterized the purified n-Src proteins expressed at 18°C. We found that altering n-Src enzyme activity either pharmacologically (e.g., application of ATP or a Src inhibitor) or genetically (mutation of Y535 or K303) was consistently associated with changes in n-Src stability: an increase in n-Src activity was coupled with a decrease in n-Src stability and vice versa. These findings, therefore, indicate that n-Src activity and stability are interdependent. Finally, the successful production of functionally active n-Src in this study indicates that the bacterial expression system may be a useful protein source in future investigations of n-Src regulation and function.

Regulation of the transcription factor YY1 in mitosis through phosphorylation of its DNA-binding domain

Yin-Yang 1 (YY1) is a ubiquitously expressed zinc finger transcription factor. It regulates a vast array of genes playing critical roles in development, differentiation, and cell cycle. Very little is known about the mechanisms that regulate the functions of YY1. It has long been proposed that YY1 is a phosphoprotein; however, a direct link between phosphorylation and the function of YY1 has never been proven. Investigation of the localization of YY1 during mitosis shows that it is distributed to the cytoplasm during prophase and remains excluded from DNA until early telophase. Immunostaining studies show that YY1 is distributed equally between daughter cells and rapidly associates with decondensing chromosomes in telophase, suggesting a role for YY1 in early marking of active and repressed genes. The exclusion of YY1 from DNA in prometaphase HeLa cells correlated with an increase in the phosphorylation of YY1 and loss of DNA-binding activity that can be reversed by dephosphorylation. We have mapped three phosphorylation sites on YY1 during mitosis and show that phosphorylation of two of these sites can abolish the DNA-binding activity of YY1. These results demonstrate a novel mechanism for the inactivation of YY1 through phosphorylation of its DNA-binding domain.

Hepatoprotective activity of Centaurium erythraea on acetaminophen-induced hepatotoxicity in rats

The methanol extract of the leaves of Centaurium erythraea L. (Gentianaceae) was evaluated for hepatoprotective activity against acetaminophen-induced liver toxicity in rats. An oral dose of 300 mg/kg/day for 6 days or a single dose of 900 mg/kg for 1 day exhibited a significant protective effect by lowering serum glutamate oxaloacetate transaminase (SGOT), glutamate pyruvate transaminase (SGPT) and lactate dehydrogenase (LDH). The activity of the extract was supported by histopathological examination of liver sections.

Intra-articular injuries of the elbow: pitfalls of diagnosis and treatment

Poor results in treating fractures and dislocations about the elbow may be avoided if the surgeon is aware of the possible injuries, examines good radiographs of both elbows, and treats the injury promptly and appropriately. A displaced fracture of the lateral or medial condyle of the humerus should be suspected if there is a flake fracture of the adjoining metaphysis; open reduction and internal fixation give better results than closed reduction. A shear fracture of the capitulum humeri can only be seen on a lateral radiograph; excision of the fragment, followed by mobilization, is sufficient for a good functional result. Dislocation of the elbow in a child may avulse the medial epicondyle, which sometimes lodges in the joint; it is essential to recognize this and remove the fragment without delay to avoid early degenerative arthritis. An apparently isolated fracture of the ulna should alert the surgeon to the possibility of a dislocation of the radial head; the dislocation and the fracture must be reduced and stabilized to conserve elbow function.