No abstract

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.

Posttranslational modifications of core histones are central to the regulation of gene expression. Histone deacetylases (HDACs) repress transcription by deacetylating histones, and class I HDACs have a crucial role in mouse, Xenopus laevis, zebra fish, and Caenorhabditis elegans development. The role of individual class I HDACs in tumor cell proliferation was investigated using RNA interference-mediated protein knockdown. We show here that in the absence of HDAC1 cells can arrest either at the G 1 phase of the cell cycle or at the G 2 /M transition, resulting in the loss of mitotic cells, cell growth inhibition, and an increase in the percentage of apoptotic cells. On the contrary, HDAC2 knockdown showed no effect on cell proliferation unless we concurrently knocked down HDAC1. Using gene expression profiling analysis, we found that inactivation of HDAC1 affected the transcription of specific target genes involved in proliferation and apoptosis. Furthermore, HDAC2 downregulation did not cause significant changes compared to control cells, while inactivation of HDAC1, HDAC1 plus HDAC2, or HDAC3 resulted in more distinct clusters. Loss of these HDACs might impair cell cycle progression by affecting not only the transcription of specific target genes but also other biological processes. Our data support the idea that a drug targeting specific HDACs could be highly beneficial in the treatment of cancer.

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.

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.The tumor suppressor p53 can induce cell cycle arrest or apoptosis in response to a variety of stress signals, such as DNA damage, oncogenic stimuli, or hypoxia (reviewed in reference 49). Activation of p53 occurs by several mechanisms including protein stabilization and modification of the protein by phosphorylation and acetylation. p53 is a transcription factor that recognizes specific binding sites within numerous target genes including mdm2, cyclin G, bax, and p21/WAF1/CIP1 (for reviews see references 5 and 12). While multiple downstream targets are involved in the mediation of apoptotic effects, the main target for p53-induced cell cycle arrest seems to be the p21 gene. p21 has been identified by virtue of its activation by p53 (13), its association with cyclin/cyclin-dependent kinase (CDK) complexes (23, 66), and its up-regulation during senescence (47). Furthermore, the p21 protein was shown previously to interact with the proliferating cell nuclear antigen (PCNA), thereby preventing DNA replication (10). Induction of p21 expression by genotoxic stress and its role during terminal differentiation of various cell types have been investigated intensively. While p21 is activated by p53-dependent mechanisms in response to DNA damage to ensure cell cycle arrest and repair, a variety of agents that promote differentiation, like phorbol ester or okadaic acid, can up-regulate p21 independently of p53 (for a review see reference 16). Similarly, the p21 gene can be activated by transforming growth factor ␤, Ca 2ϩ , lovastatin, or nerve growth factor (16).Recently, a number of reports demonstrated the induction of p21 by inhibitors of histone deacetylases (HDACs), such as sodium butyrate (46), trichostatin A (TSA) (56), suberoylanilide hydroxamic a...

The human papillomavirus (HPV) family includes more than 170 different types of virus that infect stratified epithelium. High-risk HPV is well established as the primary cause of cervical cancer, but in recent years, a clear role for this virus in other malignancies is also emerging. Indeed, HPV plays a pathogenic role in a subset of head and neck cancers-mostly cancers of the oropharynx-with distinct epidemiological, clinical and molecular characteristics compared with head and neck cancers not caused by HPV. This review summarises our current understanding of HPV in these cancers, specifically detailing HPV infection in head and neck cancers within different racial/ethnic subpopulations, and the differences in various aspects of these diseases between women and men. Finally, we provide an outlook for this disease, in terms of clinical management, and consider the issues of 'diagnostic biomarkers' and targeted therapies.

The human papillomavirus (HPV) family comprises more than 170 different types that preferentially infect the mucosa of the genitals, upper-respiratory tract, or the skin. The ‘high-risk HPV type’, a sub-group of mucosal HPVs, is the cause of approximately 5% of all human cancers, which corresponds to one-third of all virus-induced tumours. Within the high-risk group, HPV16 is the most oncogenic type, being responsible for approximatively 50% of all worldwide cervical cancers. Many studies suggest that, in addition to the high-risk mucosal HPV types, certain cutaneous HPVs also have a role in the development of non-melanoma skin cancer (NMSC).Functional studies on the HPV early gene products showed that E6 and E7 play a key role in carcinogenesis. These two proteins use multiple mechanisms to evade host immune surveillance, allowing viral persistence, and to deregulate cell cycle and apoptosis control, thus facilitating the accumulation of DNA damage and ultimately cellular transformation.The demonstration that high-risk HPV types are the etiological agents of cervical cancer allowed the implementation in the clinical routine of novel screening strategies for cervical lesions, as well as the development of a very efficient prophylactic vaccine. Because of these remarkable achievements, there is no doubt that in the coming decades we will witness a dramatic reduction of cervical cancer incidence worldwide.

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 (NPM͞ 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 (E2͞Ubc9), 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. Gam1 ͉ Ubc9T he mouse Ink4a-Arf locus encodes two tumor-suppressor proteins, p16 Ink4a and p19 Arf (p14 ARF in humans) that increase the growth-suppressive activities of the retinoblastomafamily proteins (Rb, p107, and p130) and the p53 transcription factor, respectively (1-3). By inhibiting Cdk4 and Cdk6, p16 Ink4a helps to maintain Rb-family proteins in their active hypophosphorylated state, thereby inhibiting an E2F-dependent transcriptional program that is required for cellular DNA replication. In contrast, p19 Arf antagonizes the p53 negative regulator Mdm2 (Hdm2 in humans) to trigger a p53 transcriptional response that can lead to proliferative arrest or apoptosis (4). Expression of Ink4a and Arf is regulated by distinct promoters upstream of alternative first exons whose products are spliced to a common second exon translated in alternative reading frames (from which Arf gets its name) (2). The two genes are induced by different stress signals and can be separately mutated or silenced in tumor cells (4).Arf is activated by abnormally elevated and sustained mitogenic signals triggered by oncogenes but not by physiologic signaling thresholds conveyed by their appropriately regulated protooncogenic counterparts (4). For example, Arf is not induced by Myc or Ras during normal cell-cycle progression, but it is transcribed when such signals are constitutively enforced through Myc translocation or oncogenic Ras mu...