SIRT1 contributes to telomere maintenance and augments global homologous recombination

SIRT6 is required for maintenance of telomere position effect in human cells

In Saccharomyces cerevisiae, the repressive chromatin environment at telomeres gives rise to telomere position effect (TPE), the epigenetic silencing of telomere-proximal genes. Chromatin-modifying factors that control TPE in yeast have been extensively studied, and among these, the lifespan regulator and silencing protein Sir2 plays a pivotal role. In contrast, the factors that generate and maintain silent telomeric chromatin in human cells remain largely unknown. Here we show that the Sir2 family member SIRT6 is required for maintenance of TPE in human cells. RNAi-mediated depletion of SIRT6 abrogates silencing of both an integrated telomeric transgene and an endogenous telomere-proximal gene. Moreover, enhanced telomeric silencing in response to telomere elongation is associated with increased repressive chromatin marks, and this heterochromatic milieu is lost in SIRT6-deficient cells. Together, these findings establish a new role for SIRT6 in regulating an aging-associated epigenetic silencing process and provide new mechanistic insight into chromatin silencing at telomeres.

Stem cell microRNAs in senescence and immortalization: novel players in cancer therapy

The molecular etiology of malignancy remains one of the most challenging disease processes under scientific investigation; therefore, improved approaches for their treatment are urgently needed. MicroRNAs are highly conserved nonprotein-coding RNAs that regulate gene expression. They are involved in important homeostatic processes, such as cellular proliferation, cell death and development, and affect many diseases, including cancer. High-throughput screenings based on microRNAs related to senescence/immortalization are potential tools for identifying novel proliferative microRNAs that might be involved in carcinogenesis. Recently, a subgroup of highly proliferative microRNAs, which belong to a cluster expressed exclusively in embryonic stem cells and their malignant derivatives (embryonic carcinoma cells), was revealed to play a role in senescence bypass, thereby providing immortalization to human cells. This finding supports the cancer stem cell theory and the relevance of microRNAs in human tumors. This article recapitulates the role of microRNAs that are associated with stem cell properties and their possible link in common pathways related to immortalization and cancer. Ultimately, cancer therapy that is based on the induction of a senescence response is proposed to be highly associated with the loss of stemness properties. Thus, it would be possible to "kill two birds with one stone": along with the inhibition of stemness properties in cancer stem cells, the senescence response could be induced to destroy the cancer stem cell population within a tumor.

Reduced dosage of the modifiers of epigenetic reprogramming Dnmt1, Dnmt3L, SmcHD1 and Foxo3a has no detectable effect on mouse telomere length in vivo

Studies carried out in cultured cells have implicated modifiers of epigenetic reprogramming in the regulation of telomere length, reporting elongation in cells that were null for DNA methyltransferase DNA methyltransferase 1 (Dnmt1), both de novo DNA methyltransferases, Dnmt3a and Dnmt3b or various histone methyltransferases. To investigate this further, we assayed telomere length in whole embryos or adult tissue from mice carrying mutations in four different modifiers of epigenetic reprogramming: Dnmt1, DNA methyltransferase 3-like, structural maintenance of chromosomes hinge domain containing 1, and forkhead box O3a. Terminal restriction fragment analysis was used to compare telomere length in homozygous mutants, heterozygous mutants and wild-type littermates. Contrary to expectation, we did not detect overall lengthening in the mutants, raising questions about the role of epigenetic processes in telomere length in vivo.

Sirtuin 1 is upregulated in a subset of hepatocellular carcinomas where it is essential for telomere maintenance and tumor cell growth

Hepatocellular carcinoma (HCC) is a highly malignant tumor with a poor prognosis. Treatment of HCC is complicated by the fact that the disease is often diagnosed at an advanced stage when it is no longer amenable to curative surgery, and current systemic chemotherapeutics are mostly inefficacious. Sirtuin 1 (SIRT1) is a class III histone deacetylase that is implicated in gene regulations and stress resistance. In this study, we found that SIRT1 is essential for the tumorigenesis of HCC. We showed that although SIRT1 was expressed at very low levels in normal livers, it was overexpressed in HCC cell lines and in a subset of HCC. Tissue microarray analysis of HCC and adjacent nontumoral liver tissues revealed a positive correlation between the expression levels of SIRT1 and advancement in tumor grades. Downregulation of SIRT1 consistently suppressed the proliferation of HCC cells via the induction of cellular senescence or apoptosis. SIRT1 silencing also caused telomere dysfunction-induced foci and nuclear abnormality that were clearly associated with reduced expressions of telomerase reverse transcriptase (TERT), and PTOP, which is a member of the shelter in complex. Ectopic expression of either TERT or PTOP in SIRT1-depleted cells significantly restored cell proliferation. There was also a positive correlation between the level of induction of SIRT1 and TERT [corrected] in human HCC. Finally, SIRT1-silencing sensitized HCC cells to doxorubicin treatment. Together, our findings reveal a novel function for SIRT1 in telomere maintenance of HCC, and they rationalize the clinical exploration of SIRT1 inhibitors for HCC therapy.