Down-regulation and decreased activity of cyclin-dependent kinase 2 in H2O2-induced premature senescence

Senolytics and Senomorphics Targeting p38MAPK/NF-κB Pathway Protect Endothelial Cells from Oxidative Stress-Mediated Premature Senescence

Oxidative stress is a prominent causal factor in the premature senescence of microvascular endothelial cells and the ensuing blood-brain barrier (BBB) dysfunction. Through the exposure of an in vitro model of human BBB, composed of brain microvascular endothelial cells (BMECs), astrocytes, and pericytes to H2O2, this study examined whether a specific targeting of the p38MAPK/NF-κB pathway and/or senescent cells could delay oxidative stress-mediated EC senescence and protect the BBB. Enlarged BMECs, displaying higher β-galactosidase activity, γH2AX staining, p16 expression, and impaired tubulogenic capacity, were regarded as senescent. The BBB established with senescent BMECs had reduced transendothelial electrical resistance and increased paracellular flux, which are markers of BBB integrity and function, respectively. Premature senescence disrupted plasma-membrane localization of the tight junction protein, zonula occludens-1, and elevated basement membrane-degrading matrix metalloproteinase-2 activity and pro-inflammatory cytokine release. Inhibition of p38MAPK by BIRB796 and NF-κB by QNZ and the elimination of senescent cells by a combination of dasatinib and quercetin attenuated the effects of H2O2 on senescence markers; suppressed release of the pro-inflammatory cytokines interleukin-8, monocyte chemoattractant protein-1, and intercellular adhesion molecule-1; restored tight junctional unity; and improved BBB function. In conclusion, therapeutic approaches that mitigate p38MAPK/NF-κB activity and senescent cell accumulation in the cerebrovasculature may successfully protect BBB from oxidative stress-induced BBB dysfunction.

Cellular and molecular mechanisms of xenobiotics-induced premature senescence

Premature senescence, which share common features with replicative senescence such as morphology, senescence-associated galactosidase (SA-β-gal) activity, cell cycle regulation, and gene expression, can be triggered by the exposure of various xenobiotics including environmental pollutant, peroxides, and anticancer drugs. The exact mechanisms underlying the senescence onset and stabilization are still obscure. In this review, we summarized the possible cellular and molecular mechanisms of xenobiotics-induced premature senescence, including induction of reactive oxygen species (ROS), tumor suppressors, and DNA damage; disequilibrium of calcium homeostasis; activation of transforming growth factor-β (TGF-β); and blockage of aryl hydrocarbon receptor (AHR) pathway. The deeper understanding of the molecular mechanisms underlying xenobiotics-induced senescence may shed light on new therapeutic strategies for age-related pathologies and extend healthy lifespan.

Diperoxovanadate can substitute for H(2)O(2) at much lower concentration in inducing features of premature cellular senescence in mouse fibroblasts (NIH3T3)

Stress induced premature senescence (SIPS) in mammalian cells is an accelerated ageing response and experimentally obtained on treatment of cells with high concentrations of H(2)O(2), albeit at sub-lethal doses, because H(2)O(2) gets depleted by abundant cellular catalase. In the present study diperoxovanadate (DPV) was used as it is known to be stable at physiological pH, to be catalase-resistant and to substitute for H(2)O(2) in its activities at concentrations order of magnitudes lower. On treating NIH3T3 cells with DPV, SIPS-like morphology was observed along with an immediate response of rounding of the cells by disruption of actin cytoskeleton and transient G2/M arrest. DPV could bring about growth arrest and senescence associated features at 25 μM dose, which were not seen with similar doses of either H(2)O(2) or vanadate. A minimal dose of 150 μM of H(2)O(2) was required to induce similar affects as 25 μM DPV. Increase in senescent associated markers such as p21, HMGA2 and PAI-1 was more prominent in DPV treated cells compared to similar dose of H(2)O(2). DPV-treated cells showed marked relocalization of Cyclin D1 from nucleus to cytoplasm. These results indicate that DPV, stable inorganic peroxide, is more efficient in inducing SIPS at lower concentrations compared to H(2)O(2).

p38(MAPK) in the senescence of human and murine fibroblasts

Oncogenic and environmental stresses, such as reactive oxygen species, UV radiation etc, can induce premature cellular senescence without critical telomere shortening. The role of the Ras/Raf/ERK signal transduction cascade in this process has been previously established, but recent evidence also indicates a critical role of the p38 MAP kinases pathway. Oncogenic and environmental stresses impinge upon the p38(MAPK) pathway, suggesting a major role of this pathway in senescence induced by stresses. Prematurely senescent cells are most likely to appear in several age-relatedpathologies associated with a stressful environment and/or the release of pro-inflammatory cytokines.

IGF-1 rescues human intervertebral annulus cells from in vitro stress-induced premature senescence

The aging human intervertebral disc contains a sizeable population of senescent cells. Since senescent cells cannot divide, senescence reduces the disc's ability to generate new cells to replace existing ones lost to necrosis or apoptosis. The objectives of the present work were: (1) to develop a reliable in vitro model for stress-induced premature senescence in human annulus cells, and (2) to investigate the potential for insulin-like growth factor-1 (IGF-1) to prevent or ameliorate senescence in vitro. The developed experimental model employs a 2 h exposure to 50 microM hydrogen peroxide; immunocytochemical localization of senescence associated-beta-galactosidase at pH 6.0 was used as the marker for senescent cells, and the percentage of senescent cells quantified after 3 days of culture. Nine sets of annulus cells were obtained from eight human surgical disc specimens; cells were tested with 0, 50, 100 or 500 ng/ml IGF-1. Although 50 or 100 ng/ml IGF-1 did not significantly alter the percentage of senescent cells, a significant reduction was present following exposure to 500 ng/ml IGF-1 (control, 56.3% +/- 8.5 (9); mean +/- SEM, (n) vs. treated, 39.6% +/- 6.6 (9), p = 0.0009). These novel findings point to the value of continued research towards development of future biologic therapies designed to reduce cell senescence in degenerating human discs.

Establishment of H2O2-Induced Premature Senescence in Human Fibroblasts Concomitant with Increased Cellular Production of H2O2

Premature senescence of human fibroblasts is established after exposure to an acute sublethal concentration of H(2)O(2). Overexpression of transforming growth factor-beta1 (TGF-beta1) was shown to be responsible for the appearance of the biomarkers of senescence in these conditions. Other studies have shown that incubation of human fibroblasts with TGF-beta1 leads to overexpression of H(2)O(2). In this work, we show an increased production of H(2)O(2) by human fibroblasts as premature senescence is established after an initial exposure to H(2)O(2).

Phosphorylation of human oxoguanine DNA glycosylase (alpha-OGG1) modulates its function

Oxoguanine DNA glycosylase (OGG1) initiates the repair of 8-oxoguanine (8-oxoG), a major oxidative DNA base modification that has been directly implicated in cancer and aging. OGG1 functions in the base excision repair pathway, for which a molecular hand-off mechanism has been proposed. To date, only one functional and a few physical protein interactions have been reported for OGG1. Using the yeast two-hybrid system and a protein array membrane, we identified two novel protein interactions of OGG1, with two different protein kinases: Cdk4, a serine-threonine kinase, and c-Abl, a tyrosine kinase. We confirmed these interactions in vitro using recombinant proteins and in vivo by co-immunoprecipitation from whole cell extracts. OGG1 is phosphorylated in vitro by Cdk4, resulting in a 2.5-fold increase in the 8-oxoG/C incision activity of OGG1. C-Abl tyrosine phosphorylates OGG1 in vitro; however, this phosphorylation event does not affect OGG1 8-oxoG/C incision activity. These results provide the first evidence that a post-translational modification of OGG1 can affect its catalytic activity. The distinct functional outcomes from serine/threonine or tyrosine phosphorylation may indicate that activation of different signal transduction pathways modulate OGG1 activity in different ways.

Gene expression and regulation in H2O2-induced premature senescence of human foreskin fibroblasts expressing or not telomerase

We compared the DNA-binding activity of transcription factors and gene expression patterns in BJ human diploid fibroblasts (HDFs) expressing or not telomerase (hTERT) in stress-induced premature senescence (SIPS). Senescent BJ cells were also studied. Hydrogen peroxide (H2O2)-induced SIPS modulated gene expression in both BJ and hTERT-BJ1 cells. Increased p21(WAF-1) mRNA level was amongst the common gene expression changes in BJ and hTERT-BJ1 cells induced by SIPS. Telomerase expression markedly changed gene expression in non-stressful conditions. Expression patterns of senescent BJ cells partially overlapped those of BJ and hTERT-BJ1 cells in SIPS. The basal levels of DNA-binding activity of NF-kappaB and phosphorylated ATF-2 were different in BJ and hTERT-BJ1 cells. Both cell lines displayed a higher DNA-binding activity of p53 and HIF-1 72 h after H2O2 exposure. Our results indicate that similar mechanisms involving p21(WAF-1) and probably p53 are at work in BJ and hTERT-BJ1 HDFs under H2O2-induced SIPS, suggesting that generalized DNA damage rather than telomere length/telomerase plays a crucial role in H2O2induced SIPS. We propose that H2O2-induced SIPS involves a rearrangement of proliferative and apoptotic pathways. The marked changes in gene expression induced by telomerase suggest that apart from immortalization of HDFs, telomerase also alters the normal cellular functions but does not protect against SIPS.

Differential gene expression of early and late passage retinal pigment epithelial cells

We examined the gene expression profiles of retinal pigment epithelial (RPE) cells which were aged in vitro by repeated passage. RPE cells from human eyes were cultured to passage 3-5 (early passage) or 19-21 (late passage) and used to study gene expression profiles by cDNA microarray. Results from microarray analysis were further confirmed by real-time PCR. Microarray analysis showed gene expression changes among 588 known genes. The expression levels of 15 genes (2.6%) increased in late passage RPE cells, while 43 genes (7.3%) decreased using a two-fold criterion. These differentially expressed genes encompassed many functional classes. A small number of stress genes, such as clusterin, replication protein A and Ku80, were up-regulated. The down-regulated genes included many enzymes of energy and biomolecule metabolism as well as cell cycle proteins and cell adhesion proteins. Results from real-time PCR were generally consistent with microarray findings. The expression levels of the examined angiogenic factors were either unchanged or down-regulated. Comparing early (p=3-5) and late (p=9-12) passage RPE cells, several categories of differentially expressed genes were identified. However, there was no enhanced expression of known angiogenic factors.