Oliver Bischof

Functional interaction between PML and SATB1 regulates chromatin-loop architecture and transcription of the MHC class I locus

The function of the subnuclear structure the promyelocytic leukaemia (PML) body is unclear largely because of the functional heterogeneity of its constituents. Here, we provide the evidence for a direct link between PML, higher-order chromatin organization and gene regulation. We show that PML physically and functionally interacts with the matrix attachment region (MAR)-binding protein, special AT-rich sequence binding protein 1 (SATB1) to organize the major histocompatibility complex (MHC) class I locus into distinct higher-order chromatin-loop structures. Interferon γ (IFNγ) treatment and silencing of either SATB1 or PML dynamically alter chromatin architecture, thus affecting the expression profile of a subset of MHC class I genes. Our studies identify PML and SATB1 as a regulatory complex that governs transcription by orchestrating dynamic chromatin-loop architecture.

Deconstructing PML-induced premature senescence.

In this study, we investigated the subcellular and molecular mechanisms underlying promyelocytic leu kemia (PML)‐induced premature senescence. We demonstrate that intact PML nuclear bodies are not required for the induction of senescence. We have determined further that of seven known PML isoforms, only PML IV is capable of causing premature senescence, providing the first evidence for functional differences among these isoforms. Of interest is the fact that in contrast to PML+/+ fibroblasts, PML−/− cells are resistant to PML IV‐induced senescence. This suggests that although PML IV is necessary for this process to occur, it is not sufficient and requires other components for activity. Finally, we provide evidence that PML IV‐induced senescence involves stabilization and activation of p53 through phosphorylation at Ser46 and acetylation at Lys382, and that it occurs independently of telomerase and differs from that elicited by oncogenic Ras. Taken together, our data assign a specific pro‐senescent activity to an individual PML isoform that involves p53 activation and is independent from PML nuclear bodies.

C-terminal modifications regulate MDM2 dissociation and nuclear export of p53

p53 functions to prevent malignant progression, in part by inhibiting proliferation or inducing the death of potential tumour cells. One of the most important regulators of p53 is MDM2, a RING domain E3 ligase that ubiquitinates p53, leading to both proteasomal degradation and relocation of p53 from the nucleus to the cytoplasm. Previous studies have suggested that although polyubiquitination is required for degradation, monoubiquitination of p53 is sufficient for nuclear export. Using a p53–ubiquitin fusion protein we show that ubiquitination contributes to two steps before export: exposure of a carboxy-terminal nuclear export sequence (NES), and dissociation of MDM2. Monoubiquitination can directly promote further modifications of p53 with ubiquitin-like proteins and MDM2 promotes the interaction of the SUMO E3 ligase PIASy with p53, enhancing both sumoylation and nuclear export. Our results suggest that modifications such as sumoylation can regulate the strength of the p53–MDM2 interaction and participate in driving the export of p53.

Senescence is an endogenous trigger for microRNA-directed transcriptional gene silencing in human cells

Cellular senescence is a tumour-suppressor mechanism that is triggered by cancer-initiating or promoting events in mammalian cells. The molecular underpinnings for this stable arrest involve transcriptional repression of proliferation-promoting genes regulated by the retinoblastoma (RB1)/E2F repressor complex. Here, we demonstrate that AGO2, RB1 and microRNAs (miRNAs), as exemplified here by let-7, physically and functionally interact to repress RB1/E2F-target genes in senescence, a process that we call senescence-associated transcriptional gene silencing (SA-TGS). Herein, AGO2 acts as the effector protein for let-7-directed implementation of silent-state chromatin modifications at target promoters, and inhibition of the let-7/AGO2 effector complex perturbs the timely execution of senescence. Thus, we identify cellular senescence as the an endogenous signal of miRNA/AGO2-mediated TGS in human cells. Our results suggest that miRNA/AGO2-mediated SA-TGS may contribute to tumour suppression by stably repressing proliferation-promoting genes in premalignant cancer cells.

Protein-rRNA binding features and their structural and functional implications in ribosomes as determined by cross- linking studies

We have investigated protein‐rRNA cross‐links formed in 30S and 50S ribosomal subunits of Escherichia coli and Bacillus stearothermophilus at the molecular level using UV and 2‐iminothiolane as cross‐linking agents. We identified amino acids cross‐linked to rRNA for 13 ribosomal proteins from these organisms, namely derived from S3, S4, S7, S14, S17, L2, L4, L6, L14, L27, L28, L29 and L36. Several other peptide stretches cross‐linked to rRNA have been sequenced in which no direct cross‐linked amino acid could be detected. The cross‐linked amino acids are positioned within loop domains carrying RNA binding features such as conserved basic and aromatic residues. One of the cross‐linked peptides in ribosomal protein S3 shows a common primary sequence motif–the KH motif–directly involved in interaction with rRNA, and the cross‐linked amino acid in ribosomal protein L36 lies within the zinc finger‐like motif of this protein. The cross‐linked amino acids in ribosomal proteins S17 and L6 prove the proposed RNA interacting site derived from three‐dimensional models. A comparison of our structural data with mutations in ribosomal proteins that lead to antibiotic resistance, and with those from protein‐antibiotic cross‐linking experiments, reveals functional implications for ribosomal proteins that interact with rRNA.

PARP-1 transcriptional activity is regulated by sumoylation upon heat shock

Heat shock and other environmental stresses rapidly induce transcriptional responses subject to regulation by a variety of post‐translational modifications. Among these, poly(ADP‐ribosyl)ation and sumoylation have received growing attention. Here we show that the SUMO E3 ligase PIASy interacts with the poly(ADP‐ribose) polymerase PARP‐1, and that PIASy mediates heat shock‐induced poly‐sumoylation of PARP‐1. Furthermore, PIASy, and hence sumoylation, appears indispensable for full activation of the inducible HSP70.1 gene. Chromatin immunoprecipitation experiments show that PIASy, SUMO and the SUMO‐conjugating enzyme Ubc9 are rapidly recruited to the HSP70.1 promoter upon heat shock, and that they are subsequently released with kinetics similar to PARP‐1. Finally, we provide evidence that the SUMO‐targeted ubiquitin ligase RNF4 mediates heat‐shock‐inducible ubiquitination of PARP‐1, regulates the stability of PARP‐1, and, like PIASy, is a positive regulator of HSP70.1 gene activity. These results, thus, point to a novel mechanism for regulating PARP‐1 transcription function, and suggest crosstalk between sumoylation and RNF4‐mediated ubiquitination in regulating gene expression in response to heat shock.

Human Papillomavirus Oncoprotein E7 Targets the Promyelocytic Leukemia Protein and Circumvents Cellular Senescence via the Rb and p53 Tumor Suppressor Pathways

ABSTRACT Cellular senescence can be triggered by a variety of signals, including loss of telomeric integrity or intense oncogenic signaling, and is considered a potent, natural tumor suppressor mechanism. Previously, it was shown that the promyelocytic leukemia protein (PML) induces cellular senescence when overexpressed in primary human fibroblasts. The mechanism by which the PML IV isoform elicits this irreversible growth arrest is believed to involve activation of the tumor suppressor pathways p21/p53 and p16/Rb; however, a requirement for either pathway has not been demonstrated unequivocally. To investigate the individual contributions of p53 and Rb to PML-induced senescence, we used oncoproteins E6 and E7 from human papillomaviruses (HPVs), which predominantly target p53 and Rb. We show that E7, but not E6, circumvents PML-induced senescence. Using different E7 mutant proteins, dominant negative cyclin-dependent kinase 4, and p16 RNA interference, we demonstrate that Rb-related and Rb-independent mechanisms of E7 are necessary for subversion of PML-induced senescence and we identify PML as a novel target for E7. Interaction between E7 and a functional prosenescence complex composed of PML, p53, and CBP perturbs transcriptional activation of p53, thus highlighting a significant effect also on the p53 tumor suppressor pathway. Given the importance of HPV in the pathogenesis of cervical cancer, our results warrant a more detailed analyses of PML in HPV infections.

SUMO, the Three Rs and Cancer

SUMO modification (sumoylation) plays important roles in nucleo-cytoplasmic transport, maintenance of sub-nuclear architecture, the regulation of gene expression and in DNA replication, repair and recombination. Here we review recent evidence for SUMOs role in protecting genomic integrity at both the chromosomal and the DNA level. Furthermore, the involvement of sumoylation and of specific SUMO targets in cancer is discussed.

Sumoylation at chromatin governs coordinated repression of a transcriptional program essential for cell growth and proliferation

Despite numerous studies on specific sumoylated transcriptional regulators, the global role of SUMO on chromatin in relation to transcription regulation remains largely unknown. Here, we determined the genome-wide localization of SUMO1 and SUMO2/3, as well as of UBC9 (encoded by UBE2I) and PIASY (encoded by PIAS4), two markers for active sumoylation, along with Pol II and histone marks in proliferating versus senescent human fibroblasts together with gene expression profiling. We found that, whereas SUMO alone is widely distributed over the genome with strong association at active promoters, active sumoylation occurs most prominently at promoters of histone and protein biogenesis genes, as well as Pol I rRNAs and Pol III tRNAs. Remarkably, these four classes of genes are up-regulated by inhibition of sumoylation, indicating that SUMO normally acts to restrain their expression. In line with this finding, sumoylation-deficient cells show an increase in both cell size and global protein levels. Strikingly, we found that in senescent cells, the SUMO machinery is selectively retained at histone and tRNA gene clusters, whereas it is massively released from all other unique chromatin regions. These data, which reveal the highly dynamic nature of the SUMO landscape, suggest that maintenance of a repressive environment at histone and tRNA loci is a hallmark of the senescent state. The approach taken in our study thus permitted the identification of a common biological output and uncovered hitherto unknown functions for active sumoylation at chromatin as a key mechanism that, in dynamically marking chromatin by a simple modifier, orchestrates concerted transcriptional regulation of a network of genes essential for cell growth and proliferation.

SUMO is Growing Senescent

Cellular senescence is an irreversible cell cycle arrest that curbs unrestrained, aberrant proliferation of mammalian cells and is now recognized as a prominent tumor suppressive mechanism. We have shown recently that the E3 SUMO ligase PIASy actively contributes to execution of the senescence program, thus, providing the first evidence for a direct role of SUMO modification in this process. Here, we discuss some of the implications for this novel connection and future directions to study in more detail the importance of the SUMO pathway in senescence and tumorigenesis.