Loss of p21 promoted tumorigenesis in the background of telomere dysfunctions induced by TRF2 and Wrn deficiency

The dual role of the DREAM/G2M pathway in non-tumorigenic immortalization of senescent cells

Anti-aging and tumorigenesis share common genes and pathways, and thus targeting these genes as part of anti-aging interventions carries the risk of tumorigenesis. It is essential to understand the gene signatures that balance tumorigenesis and aging. To achieve this goal, we analyzed RNA-sequencing data from three non-tumorigenic immortalized cell lines that spontaneously escaped from senescence. By single sample gene set enrichment assay (ssGSEA) and GSEA analysis, we found that both cell growth signaling (E2F targets, MYC targets) and tumor surveillance mechanisms (DNA repair, G2M checkpoint, mitotic spindle) were up-regulated in all three cell lines, suggesting that these genes are potential signatures for non-tumorigenic immortalization. Further analysis revealed that the 182 commonly up-regulated genes in these three cell lines overlapped with the DREAM/G2M pathway, which is known to be the upstream regulator of E2F, Myc targets, DNA repair, G2M checkpoint and mitotic spindle pathways in its cell cycle activation or inhibitory form. By western blotting, quantitative PCR and co-immunoprecipitation, we verified that both forms of the DREAM pathway are up-regulated in all three cell lines; this pathway facilitates control of cell cycle progression, supporting a new mechanism for non-tumorigenic immortalization. Thus, we propose that the DREAM/G2M pathway plays important dual roles with respect to preventing tumorigenesis in the process of immortalization. Our data might serve as the basis for the identification of new signature pathways or gene biomarkers for non-tumorigenic immortalization, and may aid in the discovery of new targets for tumor-free anti-aging drug screening.

The spatiotemporal expression of TERT and telomere repeat binding proteins in the postnatal mouse testes

Telomeres consist of repetitive DNA sequences and telomere-associated proteins. Telomeres located at the ends of eukaryotic chromosomes undergo shortening due to DNA replication, genotoxic factors and reactive oxygen species. The short telomeres are elongated by the enzyme telomerase expressed in the germ line, embryonic and stem cells. Telomerase is in the structure of ribonucleoprotein composed of telomerase reverse transcriptase (TERT), telomerase RNA component (Terc) and other components. Among telomere-associated proteins, telomeric repeat binding factor 1 (TRF1) and 2 (TRF2) exclusively bind to the double-stranded telomeric DNA to regulate its length. However, protection of telomeres 1 (POT1) interacts with the single-stranded telomeric DNA to protect from DNA damage response. Herein, we characterised the spatial and temporal expression of the TERT, TRF1, TRF2 and POT1 proteins in the postnatal mouse testes at the ages of 6, 8, 16, 20, 29, 32 and 88 days by using immunohistochemistry. Significant differences in the spatiotemporal expression patterns and levels of these proteins were determined in the postnatal testes (p < .05). These findings indicate that TERT and telomere repeat binding proteins seem to be required for maintaining the length and structural integrity of telomeres in the spermatogenic cells from newborn to adult terms.

The Distinct Function of p21Waf1/Cip1 With p16Ink4a in Modulating Aging Phenotypes of Werner Syndrome by Affecting Tissue Homeostasis

Human Werner syndrome (WS) is an autosomal recessive progeria disease. A mouse model of WS manifests the disease through telomere dysfunction-induced aging phenotypes, which might result from cell cycle control and cellular senescence. Both p21^Waf1/Cip1 (p21, encoded by the Cdkn1a gene) and p16^Ink4a (p16, encoded by the Ink4a gene) are cell cycle inhibitors and are involved in regulating two key pathways of cellular senescence. To test the effect of p21 and p16 deficiencies in WS, we crossed WS mice (DKO) with p21^–/– or p16^–/– mice to construct triple knockout (p21-TKO or p16-TKO) mice. By studying the survival curve, bone density, regenerative tissue (testis), and stem cell capacity (intestine), we surprisingly found that p21-TKO mice displayed accelerated premature aging compared with DKO mice, while p16-TKO mice showed attenuation of the aging phenotypes. The incidence of apoptosis and cellular senescence were upregulated in p21-TKO mice tissue and downregulated in p16-TKO mice. Surprisingly, cellular proliferation in p21-TKO mice tissue was also upregulated, and the p21-TKO mice did not show telomere shortening compared with age-matched DKO mice, although p16-TKO mice displayed obvious enhancement of telomere lengthening. Consistent with these phenotypes, the SIRT1-PGC1 pathway was upregulated in p16-TKO but downregulated in p21-TKO compared with DKO mouse embryo fibroblasts (MEFs). However, the DNA damage response pathway was highly activated in p21-TKO, but rescued in p16-TKO, compared with DKO MEFs. These data suggest that p21 protected the stem cell reservoir by regulating cellular proliferation and turnover at a proper rate and that p21 loss in WS activated fairly severe DNA damage responses (DDR), which might cause an abnormal increase in tissue homeostasis. On the other hand, p16 promoted cellular senescence by inhibiting cellular proliferation, and p16 deficiency released this barrier signal without causing severe DDR.

Quercetin has a protective effect on atherosclerosis via enhancement of autophagy in ApoE-/- mice

The present study examined the involvement of autophagy as a mechanism in the protective effect of quercetin (QUE) on atherosclerosis (AS) in ApoE^−/− mice. An AS model was established by feeding ApoE^−/− mice a high-fat diet (HFD). Mice were divided into four experimental groups: The model, QUE, 3-methyladenine (3-MA) and QUE + 3-MA groups. Additionally, age-matched wild-type C57BL/6 mice were used as a Control group. Autophagosomes in the aorta were examined using a transmission electron microscope. Aorta pathology, serum lipid accumulation and collagen deposition were determined by hematoxylin and eosin, Oil Red O and Masson staining, respectively. The levels of cytokines, including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-18 (IL-18) were measured using ELISA assays. Protein levels of mTOR, microtubule associated protein 1 light chain 3a (LC3), P53 and cyclin dependent kinase inhibitor 1A (P21) in the aorta were analyzed using western blotting. ApoE^−/− mice which were fed HFD exhibited substantial AS pathology, no autophagosomes, higher levels of TNF-α, IL-1β, IL-18 and mTOR and lower ratios of LC3 II/I. All these alterations were ameliorated and aggravated by QUE and 3-MA treatment, respectively. The inhibition of AS by QUE may be associated with the enhancement of autophagy and upregulation of P21 and P53 expression.

Mouse models in modeling aging and cancer

Mouse models have been widely used in the research of human diseases. Aging, just as cancer, is influenced by the interaction of various genetic and environmental factors. Currently, aging could be induced by many mechanism, including telomere dysfunction, oxidase stress, DNA damage and epigenetic changes. Many of these genetic pathways are also shared by aging and cancer. The mouse models generated to study these pathways might manifest either aging or cancer phenotypes, sometimes both, which in deed has worked as a good model system in understanding the correlation between aging and cancer. Here, we reviewed these mouse models that were generated to model aging or cancer. These mouse models might help us put those related pathways in context and discover essential interactions in cancer and aging regulation.

Telomere length and telomerase activity during folliculogenesis in mammals

Telomeres are repetitive non-coding DNA sequences located at the ends of chromosomes in eukaryotic cells. Their most important function is to protect chromosome ends from being recognized as DNA damage. They are also implicated in meiosis and synapse formation. The length of telomeres inevitably shortens at the end of each round of DNA replication and, also, as a consequence of the exposure to oxidative stress and/or genotoxic agents. The enzyme telomerase contributes to telomere lengthening. It has been reported that telomerase is exclusively expressed in germ cells, granulosa cells, early embryos, stem cells, and various types of cancerous cells. Granulosa cells undergo many mitotic divisions and either granulosa cells or oocytes are exposed to a variety of genotoxic agents throughout folliculogenesis; thus, telomerase plays an important role in the maintenance of telomere length. In this review article, we have comprehensively evaluated the studies focusing on the regulation of telomerase expression and activity, as well as telomere length, during folliculogenesis from primordial to antral follicles, in several mammalian species including mice, bovines, and humans. Also, the possible relationships between female infertility caused by follicular development defects and alterations in the telomeres and/or telomerase activity are discussed.