Biomedical subjects

Susanna Chiocca

Publications and source records attributed to Susanna Chiocca.

9 recordsLinked to original sources

PubMed2006

Negative and positive regulation of gene expression by mouse histone deacetylase 1.

Histone deacetylases (HDACs) catalyze the removal of acetyl groups from core histones. Because of their capacity to induce local condensation of chromatin, HDACs are generally considered repressors of transcription. In this report, we analyzed the role of the class I histone deacetylase HDAC1 as a transcriptional regulator by comparing the expression profiles of wild-type and HDAC1-deficient embryonic stem cells. A specific subset of mouse genes (7%) was deregulated in the absence of HDAC1. We identified several putative tumor suppressors (JunB, Prss11, and Plagl1) and imprinted genes (Igf2, H19, and p57) as novel HDAC1 targets. The majority of HDAC1 target genes showed reduced expression accompanied by recruitment of HDAC1 and local reduction in histone acetylation at regulatory regions. At some target genes, the related deacetylase HDAC2 partially masks the loss of HDAC1. A second group of genes was found to be downregulated in HDAC1-deficient cells, predominantly by additional recruitment of HDAC2 in the absence of HDAC1. Finally, a small set of genes (Gja1, Irf1, and Gbp2) was found to require HDAC activity and recruitment of HDAC1 for their transcriptional activation. Our study reveals a regulatory cross talk between HDAC1 and HDAC2 and a novel function for HDAC1 as a transcriptional coactivator.

Animals↗

PubMed2006

Viruses and sumoylation: recent highlights.

Since its discovery in 1997, SUMO (small ubiquitin-like modifier) has been implicated in a range of activities, indicating that this protein is as important in the cell as ubiquitin is. Although it can function throughout the cell, it appears to be involved more in nuclear functions. The growing list of substrates that are covalently modified by SUMO includes many viral proteins; SUMO appears to facilitate viral infection of cells, making it a possible target for antiviral therapies. It therefore is important to understand how viruses manipulate the cellular sumoylation system and how sumoylation affects viral functions.

Animals↗

PubMed2005

Sumoylation induced by the Arf tumor suppressor: a p53-independent function.

The mouse p19(Arf) protein has both p53-dependent and p53-independent tumor-suppressive activities. Arf triggers sumoylation of many cellular proteins, including Mdm2 and nucleophosmin (NPM/B23), with which p19(Arf) physically interacts in vivo, and this occurs equally well in cells expressing or lacking functional p53. In an Arf-null NIH 3T3 cell derivative (MT-Arf cells) engineered to reexpress an Arf transgene driven by a zinc-inducible metallothionein promoter, sumoylation of endogenous Mdm2 and NPM proteins was initiated as p19(Arf) was induced and was observed before p53-dependent cell cycle arrest. Predominately nucleoplasmic molecules visualized by immunofluorescence with antibodies to small ubiquitin-like modifier (SUMO) 1 localized to nucleoli as p19(Arf) accumulated there. Two Arf mutants, one of which binds to Mdm2 and NPM but is excluded from nucleoli and the other of which enters nucleoli but is handicapped in binding to Mdm2 and NPM, were defective in inducing sumoylation of these two target proteins and did not localize bulk sumoylated molecules to nucleoli. The CELO adenovirus protein, Gam1, which inhibits the SUMO activating enzyme (E1) and leads to down-regulation of the SUMO conjugating enzyme (E2/Ubc9), had no overt effect on the ability of p19(Arf) to activate p53 or the p53-responsive genes encoding Mdm2 and p21(Cip1), despite the fact that Arf-induced sumoylation of Mdm2 was blocked. Reduction of Ubc9 levels with short hairpin RNAs rendered similar results. We suggest that Arf's p53-independent effects on gene expression and tumor suppression might depend on Arf-induced sumoylation.

Animals↗

PubMed2005

Gam1 and the SUMO pathway.

Post-translational modifications of proteins have critical roles in many cellular processes because they can cause rapid changes in the functions of preexisting proteins, multiprotein complexes and subcellular structures. Sumoylation, a ubiquitin-like dynamic and reversible post-translational modification system, is an enzymatic cascade leading to the covalent attachment of SUMO to it target proteins. This modification involves three steps and different enzymes: SUMO-activating enzyme E1 (SAE1/SAE2), SUMO-conjugating enzyme E2 (UBC9), SUMO ligases E3s, and SUMO cleaving enzymes. Although the identification of SUMO-modified substrates has progressed rapidly, the biological function of SUMO and regulation of SUMO conjugation are still not well understood. Some viral proteins have been identified as substrates for SUMO modification as well as altering the sumoylation status of host cell proteins. We have been studying an unusual adenoviral protein, Gam1, a strong and global transcriptional activator of both viral and cellular genes that inactivates HDAC1. We have recently expanded the known functions of Gam1 by demonstrating that Gam1 also inhibits the SUMO pathway by interfering with the activity of E1 heterodimer (SAE1/SAE2), leading to the accumulation of SUMO-unmodified substrates. Our data provides a clear example of the effects of a viral infection on host sumoylation and supports the idea that viruses have multifunctional protein that can target essential biochemical pathways.

Adenosine Triphosphatases↗

PubMed2004

A mechanism for inhibiting the SUMO pathway.

The SUMO pathway parallels the classical ubiquitinylation pathway with three discrete steps: activation involving the enzyme E1, conjugation involving the E2 enzyme UBC9, and substrate modification through the cooperative association of UBC9 and E3 ligases. We report here that the adenoviral protein Gam1 inhibits the SUMO pathway by interfering with the activity of E1 (SAE1/SAE2). In vivo, Gam1 expression leads to SAE1/SAE2 inactivation, both SAE1/SAE2 and UBC9 disappearance, and overall inhibition of protein sumoylation. This results in transcriptional activation of some promoters and is directly linked to inhibition of sumoylation of the transcriptional activators involved. Our results identify a mechanism for interfering with the SUMO pathway and with transcription that could have an impact in the design of novel pharmaceutical agents. They also point out once again to the extraordinary ability of eukaryotic viruses to interfere with the biology of host cells by targeting fundamental biochemical processes.

Blotting, Western↗

PubMed2003

Modulation of p120E4F transcriptional activity by the Gam1 adenoviral early protein.

Gam1, an early adenoviral CELO protein, is required for viral replication. Consistent with its ability to inhibit histone deacetylation by HDAC1, Gam1 activates transcription. In this report, we identify the cellular transcription factor p120(E4F) as a Gam1 interaction partner. p120(E4F) is a low-abundance transcription factor that represses the adenovirus E4 promoter. Here we demonstrate that p120(E4F) interacts with HDAC1 in vivo and in vitro, and that E4F-mediated transcriptional repression is alleviated by the HDAC inhibitor trichostatin A or by overexpressing Gam1. A mutant E4 promoter unresponsive to E4F-mediated transcriptional repression is also not stimulated by Gam1. Moreover, our cofractionation experiments demonstrate that p120(E4F), HDAC1 and Gam1 may be concomitantly present in protein complexes. We conclude that Gam1 activates E4-dependent transcription possibly by inactivating HDAC1.

Acylation↗

PubMed2003

The tumor suppressor p53 and histone deacetylase 1 are antagonistic regulators of the cyclin-dependent kinase inhibitor p21/WAF1/CIP1 gene.

The cyclin-dependent kinase inhibitor p21/WAF1/CIP1 is an important regulator of cell cycle progression, senescence, and differentiation. Genotoxic stress leads to activation of the tumor suppressor p53 and subsequently to induction of p21 expression. Here we show that the tumor suppressor p53 cooperates with the transcription factor Sp1 in the activation of the p21 promoter, whereas histone deacetylase 1 (HDAC1) counteracts p53-induced transcription from the p21 gene. The p53 protein binds directly to the C terminus of Sp1, a domain which was previously shown to be required for the interaction with HDAC1. Induction of p53 in response to DNA-damaging agents resulted in the formation of p53-Sp1 complexes and simultaneous dissociation of HDAC1 from the C terminus of Sp1. Chromatin immunoprecipitation experiments demonstrated the association of HDAC1 with the p21 gene in proliferating cells. Genotoxic stress led to recruitment of p53, reduced binding of HDAC1, and hyperacetylation of core histones at the p21 promoter. Our findings show that the deacetylase HDAC1 acts as an antagonist of the tumor suppressor p53 in the regulation of the cyclin-dependent kinase inhibitor p21 and provide a basis for understanding the function of histone deacetylase inhibitors as antitumor drugs.

Acetylation↗

PubMed2002

The adenovirus protein Gam1 interferes with sumoylation of histone deacetylase 1.

The adenovirus early gene product Gam1 is crucial for virus replication and induces certain cellular genes by inactivating histone deacetylase 1 (HDAC1). We demonstrate that Gam1 (i) destroys promyelocitic leukemia nuclear bodies, (ii) delocalizes SUMO-1 into the cytoplasm and (iii) influences the SUMO-1 pathway. In addition, we show that Gam1 counteracts HDAC1 sumoylation both in vivo and in vitro. Sumoylation of HDAC1 does not seem to be absolutely required for HDAC1 biological activity but is part of a complex regulatory circuit that also includes phosphorylation of the deacetylase. Our data demonstrate that Gam1 is a viral protein that can affect simultaneously two signaling pathways: sumoylation and acetylation.

3T3 Cells↗

PubMed2002

Histone deacetylase 1 inactivation by an adenovirus early gene product.

Gam1 is an early gene product of the avian adenovirus CELO and is essential for viral replication. Gam1 has no homology to any known proteins; however, its early expression and nuclear localization suggest that the protein functions to influence transcription in the infected cell. A determinant of eukaryotic gene expression is the acetylation state of chromosomal histones and other nuclear proteins. We find that Gam1 expression increases the level of transcription from a variety of eukaryotic promoters, similar to the effect of treating cells with the histone deacetylase (HDAC) inhibitor trichostatin A (TSA ). We show that Gam1 can effectively inhibit histone deacetylation by HDAC1 and that Gam1 binds to HDAC1 both in vitro and in vivo. A CELO virus lacking Gam1 (CELOdG) is replication defective, but the defect can be overcome by either expressing an interfering HDAC1 mutant or by treating infected cells with TSA. The identification of a viral early gene product having the specific function of binding and inactivating HDAC suggests that deacetylase complexes play an important role in limiting early gene expression from invading viruses.

Animals↗