Histone deacetylation is directly involved in desilencing the expression of the catalytic subunit of telomerase in normal lung fibroblast

Histone Modification in NSCLC: Molecular Mechanisms and Therapeutic Targets

Lung cancer is the leading cause of cancer mortality in both genders, with non-small cell lung cancer (NSCLC) accounting for about 85% of all lung cancers. At the time of diagnosis, the tumour is usually locally advanced or metastatic, shaping a poor disease outcome. NSCLC includes adenocarcinoma, squamous cell carcinoma, and large cell lung carcinoma. Searching for novel therapeutic targets is mandated due to the modest effect of platinum-based therapy as well as the targeted therapies developed in the last decade. The latter is mainly due to the lack of mutation detection in around half of all NSCLC cases. New therapeutic modalities are also required to enhance the effect of immunotherapy in NSCLC. Identifying the molecular signature of NSCLC subtypes, including genetics and epigenetic variation, is crucial for selecting the appropriate therapy or combination of therapies. Epigenetic dysregulation has a key role in the tumourigenicity, tumour heterogeneity, and tumour resistance to conventional anti-cancer therapy. Epigenomic modulation is a potential therapeutic strategy in NSCLC that was suggested a long time ago and recently starting to attract further attention. Histone acetylation and deacetylation are the most frequently studied patterns of epigenetic modification. Several histone deacetylase (HDAC) inhibitors (HDIs), such as vorinostat and panobinostat, have shown promise in preclinical and clinical investigations on NSCLC. However, further research on HDIs in NSCLC is needed to assess their anti-tumour impact. Another modification, histone methylation, is one of the most well recognized patterns of histone modification. It can either promote or inhibit transcription at different gene loci, thus playing a rather complex role in lung cancer. Some histone methylation modifiers have demonstrated altered activities, suggesting their oncogenic or tumour-suppressive roles. In this review, patterns of histone modifications in NSCLC will be discussed, focusing on the molecular mechanisms of epigenetic modifications in tumour progression and metastasis, as well as in developing drug resistance. Then, we will explore the therapeutic targets emerging from studying the NSCLC epigenome, referring to the completed and ongoing clinical trials on those medications.

Histone deacetylase inhibitor AR42 regulates telomerase activity in human glioma cells via an Akt-dependent mechanism

Epigenetic regulation via abnormal activation of histone deacetylases (HDACs) is a mechanism that leads to cancer initiation and promotion. Activation of HDACs results in transcriptional upregulation of human telomerase reverse transcriptase (hTERT) and increases telomerase activity during cellular immortalization and tumorigenesis. However, the effects of HDAC inhibitors on the transcription of hTERT vary in different cancer cells. Here, we studied the effects of a novel HDAC inhibitor, AR42, on telomerase activity in a PTEN-null U87MG glioma cell line. AR42 increased hTERT mRNA in U87MG glioma cells, but suppressed total telomerase activity in a dose-dependent manner. Further analyses suggested that AR42 decreases the phosphorylation of hTERT via an Akt-dependent mechanism. Suppression of Akt phosphorylation and telomerase activity was also observed with PI3K inhibitor LY294002 further supporting the hypothesis that Akt signaling is involved in suppression of AR42-induced inhibition of telomerase activity. Finally, ectopic expression of a constitutive active form of Akt restored telomerase activity in AR42-treated cells. Taken together, our results demonstrate that the novel HDAC inhibitor AR42 can suppress telomerase activity by inhibiting Akt-mediated hTERT phosphorylation, indicating that the PI3K/Akt pathway plays an important role in the regulation of telomerase activity in response to this HDAC inhibitor.

The regulation of telomerase in oncogenesis

The influence that the expression of the human (glial-derived neurotrophic factor (GDNF)) neurotrophic factor has on the morphology and proliferative activity of embryonic stem cells (SC) of a mouse with R1 lineage, as well as their ability to form embroid bodies (EB), has been studied. Before that, using a PCR (polymerase chain reaction) coupled with reverse transcription, it was shown that, in this very lineage of the embryonic SC, the expression of the receptors' genes is being fulfilled for the neurotropfic RET and GFRα1 glia factor. The mouse's embryonic SC lineage has been obtained, transfected by the human GDNF gene, and has been fused with the "green" fluorescent protein (GFP) gene. The presence of the expression of the human GDNF gene in the cells was shown by northern hybridization and the synthesis of its albuminous product by immunocitochemical coloration with the use of specific antibodies. The reliable slowing-down of the embriod-body formation by the embryonic SC transfected by the GDNF gene has been shown. No significant influence of the expression of the GDNF gene on the morphology and the proliferative activity of the transfected embryonic SCs has been found when compared with the control ones.

Differential expression of epigenetic modulators during human embryonic stem cell differentiation

Although the progression of aging and the diseases associated with it are extensively studied, little is known about the initiation of the aging process. Telomerase is down-regulated early in embryonic differentiation, thereby contributing to telomeric attrition and aging. The mechanisms underlying this inhibition remain elusive, but epigenetic studies in differentiating human embryonic stem (hES) cells could give clues about how and when DNA methylation and histone deacetylation work together to contribute to the inactivation of hTERT, the catalytic subunit of telomerase, at the onset of the aging process. We have confirmed the differentiation status of cultured hES colonies with morphological assessment and immunohistochemical stainings for pluripotent stem cells. In hES cells with varying degrees of differentiation, we have shown a stronger association between hES differentiation and expression of the epigenetic regulators DNMT3A and DNMT3B than between genetic modulators of differentiation such as c-MYC. We also propose a new model system for analyses of stem cell regions, which are differentially down-regulating the expression of hTERT and the actions of epigenetic modulators such as the DNMTs and histone methyltransferases.

Role of melatonin in the epigenetic regulation of breast cancer

The oncostatic properties of melatonin as they directly or indirectly involve epigenetic mechanisms of cancer are reviewed with a special focus on breast cancer. Five lines of evidence suggest that melatonin works via epigenetic processes: (1) melatonin influences transcriptional activity of nuclear receptors (ERalpha, GR and RAR) involved in the regulation of breast cancer cell growth; (2) melatonin down-regulates the expression of genes responsible for the local synthesis or activation of estrogens including aromatase, an effect which may be mediated by methylation of the CYP19 gene or deacetylation of CYP19 histones; (3) melatonin inhibits telomerase activity and expression induced by either natural estrogens or xenoestrogens; (4) melatonin modulates the cell cycle through the inhibition of cyclin D1 expression; (5) melatonin influences circadian rhythm disturbances dependent on alterations of the light/dark cycle (i.e., light at night) with the subsequent deregulation of PER2 which acts as a tumor suppressor gene.

Transcriptional and epigenetic status of protamine 1 and 2 genes following round spermatids injection into mouse oocytes

The use of round spermatids that are fully active at the transcriptional level to create zygotes (i.e. round spermatid injection; ROSI) raises the question regarding the downregulation of all specific genes that are transcribed from the paternal genome at fertilization. In this study, we show that protamine 1 and 2 mRNAs, which are specific to the round spermatid stage, are repressed at the two-pronuclei (6 h) and two-cell (30 h) stages postfertilization, respectively, in ROSI embryos, by distinct mechanisms. Both genes are fully methylated in round spermatids and sperm but unmethylated in oocytes. At 6 h postfertilization, the protamine 1 and 2 genes are actively demethylated, but the demethylation process happens more rapidly in ROSI than in sperm zygotes. Treatment of zygotes with trichostatin A, a histone deacetylase (HDAC) inhibitor, maintained the protamine 2 mRNAs expression up to 30 h postfertilization while the DNA methylation status of the gene is not affected. Thus, HDACs are involved in the clearance of protamine 2 mRNAs in ROSI two-cell embryos independently of the methylation status of the repressed gene. Contrastingly, HDACs are not directly involved in protamine 1 regulation since trichostatin A does not reverse the silencing of the gene in ROSI embryos at 6 h. The protamine 1 CpG island located in the coding region is actively demethylated in ROSI one-cell embryos where the gene is repressed and may contribute to the regulation of protamine 1 gene expression. The comparison with gene reprogramming occurring during nuclear transfer makes ROSI embryos an attractive model to study the mechanisms involved in gene silencing elicited by the oocyte.

Downregulation of multiple stress defense mechanisms during differentiation of human embryonic stem cells

Evolutionary theory predicts that cellular maintenance, stress defense, and DNA repair mechanisms should be most active in germ line cells, including embryonic stem cells that can differentiate into germ line cells, whereas it would be energetically unfavorable to keep these up in mortal somatic cells. We tested this hypothesis by examining telomere maintenance, oxidative stress generation, and genes involved in antioxidant defense and DNA repair during spontaneous differentiation of two human embryonic stem cell lines. Telomerase activity was quickly downregulated during differentiation, probably due to deacetylation of histones H3 and H4 at the hTERT promoter and deacetylation of histone H3 at hTR promoter. Telomere length decreased accordingly. Mitochondrial superoxide production and cellular levels of reactive oxygen species increased as result of increased mitochondrial biogenesis. The expression of major antioxidant genes was downregulated despite this increased oxidative stress. DNA damage levels increased during differentiation, whereas expression of genes involved in different types of DNA repair decreased. These results confirm earlier data obtained during mouse embryonic stem cell differentiation and are in accordance with evolutionary predictions.

Epigenetic programming of mesenchymal stem cells from human adipose tissue

Stromal stem cells identified in various adult mesenchymal tissues (commonly called mesenchymal stem cells [MSCs]) have in past years received more attention as a result of their potential interest as replacement cells in regenerative medicine. An abundant and easily accessible source of adult human MSCs are stem cells harvested from liposuction material. Similarly to bone marrow-derived MSCs, human adipose tissue-derived stem cells (ASCs) can give rise to a variety of cell types in vitro and in vivo; however, they have a propensity to differentiate into primarily mesodermal lineages. Even so, their capacity to differentiate into nonadipogenic mesodermal pathways seems to be restricted. Emerging DNA methylation profiles at adipogenic and nonadipogenic gene promoters in freshly isolated, cultured, or differentiated ASCs aim to provide an epigenetic explanation for this restrictive differentiation potential. A review of these studies indicates that human ASCs are epigenetically marked by mosaic hypomethylation of adipogenic promoters, whereas nonadipogenic lineage-specific promoters are hypermethylated. Surprisingly, in vitro differentiation toward various pathways maintains the overall methylation profiles of undifferentiated cells, raising the hypothesis that ASCs are at least epigenetically preprogrammed for adipogenesis. Novel attempts at reprogramming the epigenome of MSCs have been initiated to enhance the differentiation capacity of these cells.

Inhibition of telomerase activity and human telomerase reverse transcriptase gene expression by histone deacetylase inhibitor in human brain cancer cells

The aim of the present study is to investigate the effect of histone deacetylase inhibitor, trichostatin A (TSA) on the cell growth, apoptosis, genomic DNA damage and the expression of telomerase and associated factors in human normal and brain cancer cells. Here, human normal un-transformed fibroblasts (MRC-5), human normal hTERT-immortalised fibroblasts (hTERT-BJ1) and human brain cancer cell lines (glioblastoma cell line, A-172 and medulloblastoma cell line, ONS-76) were treated with 0.5-3.0microM TSA for 24h. Exposure to TSA resulted in apoptosis in a dose-dependent manner in the brain cancer cells. Glioblastoma cell line (A-172) displayed higher sensitivity to TSA-induced cell killing effect and apoptosis than the medulloblastoma cell line (ONS-76). The brain cancer cell lines and hTERT-BJ1 cell line displayed significant inhibition in telomerase activity and hTERT mRNA level after 2microM TSA treatment. Elevated expressions of p53 and p21 with a decrease in cyclin-D level supported the observation on cell cycle arrest following TSA treatment. Upregulation of Bax and cytochrome c correlated with the apoptotic events in TSA-treated cells. This study suggests that telomerase and hTERT might be the primary targets of TSA which may have the potential to be used as a telomerase inhibitor in cancer therapy.

Gene regulation by chromatin structure: paradigms established in Drosophila melanogaster

Studies in Drosophila melanogaster have revealed paradigms for regulating gene expression through chromatin structure, including mechanisms of gene activation and silencing. Regulation occurs at the level of individual genes, chromosomal domains, and entire chromosomes. The chromatin state is dynamic, allowing for changes in gene expression in response to cellular signals and/or environmental cues. Changes in chromatin result from the action of ATP-dependent chromatin-remodeling complexes, reversible epigenetic histone modifications, and the incorporation of histone variants. Many of the chromatin-based transcriptional regulatory mechanisms discovered in D. melanogaster are evolutionarily conserved and therefore serve as a foundation for studies in other organisms.

Epigenetics of lung cancer

Epigenetics is the study of heritable changes in gene expression that occur without changes in DNA sequence. It has a role in determining when and where a gene is expressed during development. Perhaps the most well known epigenetic mechanism is DNA methylation whereby cytosines at position 5 in CpG dinucleotides are methylated. Histone modification is another form of epigenetic control, which is quite complex and diverse. Histones and DNA make up the nucleosome which is the structural unit of chromatin which are involved in packaging DNA. Apart from the crucial role epigenetics plays in embryonic development, transcription, chromatin structure, X chromosome inactivation and genomic imprinting, its role in an increasing number of human diseases is more and more recognized. These diseases include cancer, and lung cancer in particular has been increasingly studied for the potential biological role of epigenetic changes with the promise of better and novel diagnostic and therapeutic tools.

Evolution of cancer stem cells

Cancer as a disease driven by cancer stem cells is a concept that has emerged over the last few years. However, several issues relating to this phenomenon as yet remain unaddressed. A fundamental question is one relating to the identification of events leading to transformation of a normal tissue stem cell to a cancer stem cell. Complete knowledge of this evolutionary process may be crucial for the development of novel effective therapies that influence patient prognosis. The scope of this review is to discuss reports that have begun to elucidate stem cell transformation either as an isolated event or as a progression as an attempt towards understanding some of the critical events involved in the process.