Epigenetic perturbations and cancer: innovative therapeutic strategies against cancer

A complex system of molecular milestones ensures labelling of the genome, driving its organization and functions. These milestones correspond to particular marks associated to active and repressed genes, as well as to non-coding regions or those containing repetitive sequences. Most of these marks are chemical modifications of DNA, corresponding to cytosine methylation, or various posttranslational modifications of histones, the proteins which package the genome. These chemical modifications of DNA or histones are reversible and are catalysed and removed by enzymatic activities associated with factors ensuring critical cellular functions. Indeed, these enzymes are directly connected with signalling pathways, sensing extra- and intracellular environments. Altogether these mechanisms globally control the expression status of genes in each cell, meaning that certain genes are kept active, while most of the genome remains silent. Subtle metabolic changes or intra and extracellular modifications, by altering the marking associated to genes, can have long-term consequences on their expression status. Genes coding for essential regulators of cellular proliferation and differentiation could be among these genes, such as tumor suppressor genes for instance. Hence the knowledge of all these so-called "epigenetic" mechanisms will shed new light on the environmental impact on the control of gene expression and associated diseases, including malignant transformation. The understanding of these mechanisms will also pave the way for innovative therapeutic strategies to fight cancer. This review is aiming to give an overview to the reader of the relevance of epigenetic mechanisms for the understanding and treatment of cancer.     

Depletion of the human Nα-terminal acetyltransferase A induces p53-dependent apoptosis and p53-independent growth inhibition

The human protein N(α)-terminal acetyltransferase A complex (hNatA), composed of the catalytic hNaa10p (hArd1) and auxiliary hNaa15p (hNat1/NATH/Tubedown) subunits, was reported to be important for cell survival and growth of various types of cancer. However, little is known about the mechanisms mediating growth inhibition and apoptosis following loss of hNatA function. Here, we have screened 11 different thyroid cell lines for hNAA10 RNAi phenotypes and observed mostly growth inhibition, which was independent of TP53 functional status and developed by several different mechanisms involving (i) downregulation of cyclin D1, (ii) increase in p27/Kip1 and (iii) inactivation of Rb/E2F pathway. hNatA depletion in aggressive thyroid cancer cell lines (8305C, CAL-62 and FTC-133) with mutated TP53 increased sensitivity to drug-induced cytotoxicity, but in a cell type specific manner: 8305C (TRAIL), CAL-62 (daunorubicin) and FTC-133 (troglitazone). Cells harboring wild-type TP53 were also prone to apoptosis via the p53 pathway after hNatA downregulation. Importantly, in hNatA-depleted cells DNA-damage signaling was activated in the absence of exogenous DNA damage independent on TP53 status. Our findings indicate that several mechanisms of growth inhibition and apoptosis may be induced by hNatA knockdown and that hNatA knockdown could be exploited for use in combinatorial chemotherapy.     

辛二酰苯胺异羟肟酸对结肠癌HCT116和SW480细胞增殖、周期和凋亡的影响

目的:研究辛二酰苯胺异羟肟酸(suberoylanilide hydroxamic acid,SAHA)对结肠癌细胞系HCT116和SW480增殖、周期和凋亡的影响,并对其分子作用机制进行初步探讨.方法:将不同浓度SAHA分别处理结肠癌HCT116和SW480细胞后,MTT法检测SAHA对HCT116和SW480细胞增殖的影响,流式细胞仪检测HCT116和SW480细胞周期和细胞凋亡率,罗丹明(rhodamine) 123和二氯二氢荧光素二乙酸酯(DCFH-DA)法检测HCT116和SW480细胞线粒体跨膜电位(Aψm)和活性氧(ROS)水平,Real-time PCR和Western blotting法检测乙酰化组蛋白3(Ac-H3)、p21、CyclinD1、Bax和Bcl-2的mRNA和蛋白的表达水平.结果:SAHA作用于HCT116和SW480细胞48 h后,细胞增殖被抑制、细胞周期G1期比率升高、凋亡率升高(均P＜0.05),线粒体跨膜电位显著下降、细胞内ROS产生增多(均P＜0.05).与对照组比较,SAHA处理组p21和Bax mRNA增多、Cyclin D1和Bcl-2 mRNA表达量减少(均P＜0.05),相关蛋白Ac-H3、p21和Bax增多,CyclinD1和Bcl-2减少(均P＜0.05).结论:SAHA抑制结肠癌HCT116和SW480细胞增殖、阻滞细胞周期并诱导细胞凋亡,其机可能与调节p21、CyclinD1和Bcl-2家族基因的表达、促进组蛋白乙酰化有关.     

胆囊癌组织中蛋白赖氨酸乙酰化的变化及临床意义

目的:探讨蛋白赖氨酸乙酰化在胆囊癌组织中的变化情况。方法:采用Western 免疫印迹法,分别检测30例胆囊癌组织和20例慢性胆囊炎组织中蛋白赖氨酸乙酰化的变化。结果:与慢性胆囊炎组织相比,胆囊癌组织中的蛋白赖氨酸乙酰化显著增加（P < 0.01）,乙酰化的显著增加主要发生在相对分子质量为10 000~36 000的蛋白。结论:利用赖氨酸乙酰化单克隆抗体特异性能够检测到胆囊组织蛋白的赖氨酸乙酰化,且相对分子质量为10 000~36 000蛋白的赖氨酸乙酰化增加显著。     

蛋白质乙酰化对HBV复制的影响

目的探讨宿主细胞蛋白质乙酰化修饰对HBV复制的影响。方法采用去乙酰化酶抑制剂制滴菌素A(TSA)和尼克酰胺(NAM)刺激HBV复制细胞模型Hep G2.2.15和Hep AD38,并检测细胞内的HBV复制指标。采用Western Blot检测广谱乙酰化蛋白和乙酰化组蛋白H3的表达水平。结果 TSA和NAM刺激细胞会引起细胞内蛋白质的乙酰化水平升高,且乙酰化修饰水平增高呈时间和浓度依赖的变化趋势。两种细胞受TSA和NAM刺激的影响,培养上清中的HBs Ag水平降低,而HBV DNA水平升高,且呈时间和浓度依赖关系,而两种细胞培养上清中的HBe Ag和细胞内核心颗粒DNA的表达水平变化趋势不明显。结论宿主蛋白质的乙酰化可能参与和影响了细胞内HBV的复制过程,进一步深入分析和鉴定影响HBV胞内复制的宿主乙酰化蛋白质将有助于加深对HBV复制调控的认识,为开发特异性抗病毒策略提供新思路。     

Molecular models for post-meiotic male genome reprogramming

The molecular basis of post-meiotic male genome reorganization and compaction constitutes one of the last black boxes in modern biology. Although the successive transitions in DNA packaging have been well described, the molecular factors driving these near genome-wide reorganizations remain obscure. We have used a combination of different approaches aiming at the discovery of critical factors capable of directing the post-meiotic male genome reprogramming, which is now shedding new light on the nature of the fundamental mechanisms controlling post-meiotic histone replacement and genome compaction. Here we present a summary of these findings. The identification of the first factor capable of reading a precise combination of histone acetylation marks, BRDT, allowed highlighting a critical role for the genome-wide histone hyperacetylation that occurs before generalized histone replacement. In this context, the recent identification of a group of new histone variants capable of forming novel DNA packaging structures on specific regions during late spermatogenesis, when hyperacetylated histones are massively replaced in spermatids, also revealed the occurrence of a post-meiotic region-specific genome reprogramming. Additionally, the functional characterization of other molecular actors and chaperones in action in post-meiotic cells now allows one to describe the first general traits of the mechanisms underlying the structural transitions taking place during the post-meiotic reorganization and epigenetic reprogramming of the male genome.     

CHROMATIN REMODELING IN SPERMATIDS: A SENSITIVE STEP FOR THE GENETIC INTEGRITY OF THE MALE GAMETE

Several causes of male infertility remain idiopathic. Recently, the condensed state of the sperm head has been demonstrated as a discriminating parameter for the assessment of male fertility. Altered DNA condensation is associated with an increase in DNA strand breakage so the genetic integrity of the male gamete is threatened. The origin of the DNA strand breaks is unknown. However, transient DNA strand breaks appear in the whole population of elongating spermatids during mid-spermiogenesis steps. Most likely, these transient breaks are required to support the change in DNA topology associated with chromatin remodeling at these steps. Histones hyperacetylation is also coincident with the DNA strand breakage steps. Hyperacetylation of histones may represent a necessary condition for strand breakages to form allowing access to the yet unknown enzymatic activity involved in the removal of DNA supercoils. A better characterization of this enzyme activity at these steps is necessary as this may represent a very sensitive process where alterations in the genetic integrity of the male gamete may arise and persist up to the mature spermatozoa. During the chromatin remodeling in spermatids, the combined DNA-condensing activities provided by the basic transition proteins and protamines may optimize the strand repair process emphasizing the link between altered sperm DNA condensation and DNA fragmentation. The mutagenic potential of these events may have been overlooked as it may result in fertility and/or developmental problems.     

Icariin Improves Cognitive Impairment after Traumatic Brain Injury by Enhancing Hippocampal Acetylation

Objective

To examine the effect of icariin (ICA) on the cognitive impairment induced by traumatic brain injury (TBI) in mice and the underlying mechanisms related to changes in hippocampal acetylation level.

Methods

The modifified free-fall method was used to establish the TBI mouse model. Mice with post-TBI cognitive impairment were randomly divided into 3 groups using the randomised block method (n=7): TBI (vehicle-treated), low-dose (75 mg/kg) and high-dose (150 mg/kg) of ICA groups. An additional sham-operated group (vehicle-treated) was employed. The vehicle or ICA was administrated by gavage for 28 consecutive days. The Morris water maze (MWM) test was conducted. Acetylcholine (ACh) content, mRNA and protein levels of choline acetyltransferase (ChAT), and protein levels of acetylated H3 (Ac-H3) and Ac-H4 were detected in the hippocampus.

Results

Compared with the sham-operated group, the MWM performance, hippocampal ACh content, mRNA and protein levels of ChAT, and protein levels of Ac-H3 and Ac-H4 were signifificantly decreased in the TBI group (P<0.05). High-dose of ICA signifificantly ameliorated the TBI-induced weak MWM performance, increased hippocampal ACh content, and mRNA and protein levels of ChAT, as well as Ac-H3 protein level compared with the TBI group (P<0.05).

Conclusion

ICA improved post-TBI cognitive impairment in mice by enhancing hippocampal acetylation, which improved hippocampal cholinergic function and ultimately improved cognition.