Telomere loss has been proposed as a mechanism for counting cell divisions during aging in normal somatic cells. How such a mitotic clock initiates the intracellular signalling events that culminate in G1 cell cycle arrest and senescence to restrict the lifespan of normal human cells is not known. We investigated the possibility that critically short telomere length activates a DNA damage response pathway involving p53 and p21(WAF1) in aging cells.
The yeast Sir2 protein mediates chromatin silencing through an intrinsic NAD-dependent histone deacetylase activity. Sir2 is a conserved protein and was recently shown to regulate lifespan extension both in budding yeast and worms. Here, we show that SIRT1, the human Sir2 homolog, is recruited to the promyelocytic leukemia protein (PML) nuclear bodies of mammalian cells upon overexpression of either PML or oncogenic Ras (Ha-rasV12). SIRT1 binds and deacetylates p53, a component of PML nuclear bodies, and it can repress p53-mediated transactivation.
The vast majority of breast cancers are carcinomas that arise from mammary epithelial cells (MECs). One of the key early events in tumorigenic transformation is the ability of cells to overcome replicative senescence. However, the precise genetic changes that are responsible for this event in MECs is largely unknown. Here, we report that Bmi-1, originally identified as a c-Myc cooperating oncoprotein, can bypass senescence, extend the replicative life span, and immortalize MECs. Furthermore, Bmi-1 was overexpressed in immortal MECs and several breast cancer cell lines.
Calorie restriction extends lifespan in organisms ranging from yeast to mammals. In yeast, the SIR2 gene mediates the life-extending effects of calorie restriction. Here we show that the mammalian SIR2 orthologue, Sirt1 (sirtuin 1), activates a critical component of calorie restriction in mammals; that is, fat mobilization in white adipocytes. Upon food withdrawal Sirt1 protein binds to and represses genes controlled by the fat regulator PPAR-gamma (peroxisome proliferator-activated receptor-gamma), including genes mediating fat storage.
Fetal cardiomyocytes have been proposed as a potential source of cell-based therapy for heart failure. This study examined cellular senescence in cultured human fetal ventricular cardiomyocytes (HFCs). HFCs were isolated and identified by immunocytochemistry and RT-PCR. Cells were found to senesce after 20-25 population doublings, as determined by growth arrest, morphological changes and senescence-associated beta-galactosidase activity. Using the telomeric repeat amplification protocol assay, telomerase activity was undetectable in primary HFCs.
In lower organisms, increased expression of the NAD-dependent deacetylase Sir2 augments lifespan. The mechanism through which this life extension is mediated remains incompletely understood. Here we have examined the cellular effects of overexpression of SIRT1, the closest mammalian ortholog of Sir2. In PC12 cells, increased expression of the NAD-dependent deacetylase SIRT1 reduces cellular oxygen consumption by approximately 25%. We further demonstrate that SIRT1 expression can alter the transcriptional activity of the mitochondrial biogenesis coactivator PGC-1alpha.
Proceedings of the National Academy of Sciences of the United States of America
In investigating the role of metal ions in the pathogenesis of Huntington's disease, we examined the effects of clioquinol, a metal-binding compound currently in clinical trials for Alzheimer's disease treatment, on mutant huntingtin-expressing cells. We found that PC12 cells expressing polyglutamine-expanded huntingtin exon 1 accumulated less mutant protein and showed decreased cell death when treated with clioquinol. This effect was polyglutamine-length-specific and did not alter mRNA levels or protein degradation rates.
C57BL/6J mice carrying the Min allele of Adenomatous polyposis coli (Apc) develop numerous adenomas along the entire length of the intestine and consequently die at an early age. This short lifespan would prevent the accumulation of somatic genetic mutations or epigenetic alterations necessary for tumor progression. To overcome this limitation, we generated F(1) Apc(Min/+) hybrids by crossing C57BR/cdcJ and SWR/J females to C57BL/6J Apc(Min/+) males. These hybrids developed few intestinal tumors and often lived longer than 1 year.
The heat shock factor (HSF), a protein evolutionarily conserved from yeasts to human, regulates the expression of a set of proteins called heat shock proteins (HSPs), many of which function as molecular chaperones. In Saccharomyces cerevisiae, the HSF binds to the 5' upstream region of YGR146C and activates its transcription. YGR146C encodes a functional homolog of ecl1 (+), ecl2 (+), and ecl3 (+) of Schizosaccharomyces pombe. At present, these Ecl1 family genes, which are extenders of chronological lifespan, have been identified only in fungi groups.
Human LMNA gene mutations result in laminopathies that include Emery-Dreifuss muscular dystrophy (AD-EDMD) and Hutchinson-Gilford progeria, the premature aging syndrome (HGPS). The Lmna null (Lmna(-/-)) and progeroid Lmna?9 mutant mice are models for AD-EDMD and HGPS, respectively. Both animals develop severe tissue pathologies with abbreviated life spans. Like HGPS cells, Lmna(-/-) and Lmna?9 fibroblasts have typically misshapen nuclei.