Wild-type p53 triggers a rapid senescence program in human tumor cells lacking functional p53

Elucidating Cellular Senescence-related Genes in Benign Prostatic Hyperplasia Through Mendelian Randomization and Single-cell RNA Sequencing

BACKGROUND

Benign prostatic hyperplasia (BPH) is a widely observed disorder in older men, with substantial evidence indicating that cellular senescence serves a pivotal function in its progression. This investigation seeks to pinpoint cellular senescence-related genes causally connected with BPH and to examine their expression and regulatory networks across distinct prostate cells.

METHODS

Using exposure data from the eQTLGen database and outcome data from both FinnGen Consortium and UKB database, Mendelian randomization was utilized to determine cell senescence genes that are causally linked to BPH. These associations were further validated through colocalization analysis. Expression patterns of these genes in different prostate cells were assessed via single-cell RNA sequencing, and changes along pseudotime were tracked. Regulatory networks were evaluated using single-cell regulatory network inference and clustering to identify key transcription factors involved.

RESULTS

Six cell senescence genes causally linked to BPH were identified through Mendelian randomization. ATM, ATRAID, MAP2K1, and TP53 were identified as protective factors, whereas ITPR1 and SENP7 were associated with increased risk. Colocalization analysis suggested that ATM and TP53 are likely to share the same variant implicated in BPH. MAP2K1 expression demonstrated a steady decline along inferred pseudotime across fibroblasts, macrophages, T cells, and epithelial cells, while the remaining 5 genes exhibited an opposite trend. ATF3, EGR1, and FOS were pinpointed as the core transcription factors regulating these genes.

CONCLUSIONS

These observations emphasize consistent expression patterns among different prostate cell types and suggest a highly interconnected regulatory network that underpins BPH pathology, thereby providing fresh perspectives on the molecular mechanisms underlying the disease.

Oncogene-like addiction to aneuploidy in human cancers

Most cancers exhibit aneuploidy, but its functional significance in tumor development is controversial. Here, we describe ReDACT (Restoring Disomy in Aneuploid cells using CRISPR Targeting), a set of chromosome engineering tools that allow us to eliminate specific aneuploidies from cancer genomes. Using ReDACT, we created a panel of isogenic cells that have or lack common aneuploidies, and we demonstrate that trisomy of chromosome 1q is required for malignant growth in cancers harboring this alteration. Mechanistically, gaining chromosome 1q increases the expression of MDM4 and suppresses p53 signaling, and we show that TP53 mutations are mutually exclusive with 1q aneuploidy in human cancers. Thus, tumor cells can be dependent on specific aneuploidies, raising the possibility that these "aneuploidy addictions" could be targeted as a therapeutic strategy.

p53 Regulates Mitochondrial Dynamics in Vascular Smooth Muscle Cell Calcification

Arterial calcification is an important characteristic of cardiovascular disease. It has key parallels with skeletal mineralization; however, the underlying cellular mechanisms responsible are not fully understood. Mitochondrial dynamics regulate both bone and vascular function. In this study, we therefore examined mitochondrial function in vascular smooth muscle cell (VSMC) calcification. Phosphate (Pi)-induced VSMC calcification was associated with elongated mitochondria (1.6-fold increase, p < 0.001), increased mitochondrial reactive oxygen species (ROS) production (1.83-fold increase, p < 0.001) and reduced mitophagy (9.6-fold decrease, p < 0.01). An increase in protein expression of optic atrophy protein 1 (OPA1; 2.1-fold increase, p < 0.05) and a converse decrease in expression of dynamin-related protein 1 (DRP1; 1.5-fold decrease, p < 0.05), two crucial proteins required for the mitochondrial fusion and fission process, respectively, were noted. Furthermore, the phosphorylation of DRP1 Ser637 was increased in the cytoplasm of calcified VSMCs (5.50-fold increase), suppressing mitochondrial translocation of DRP1. Additionally, calcified VSMCs showed enhanced expression of p53 (2.5-fold increase, p < 0.05) and β-galactosidase activity (1.8-fold increase, p < 0.001), the cellular senescence markers. siRNA-mediated p53 knockdown reduced calcium deposition (8.1-fold decrease, p < 0.01), mitochondrial length (3.0-fold decrease, p < 0.001) and β-galactosidase activity (2.6-fold decrease, p < 0.001), with concomitant mitophagy induction (3.1-fold increase, p < 0.05). Reduced OPA1 (4.1-fold decrease, p < 0.05) and increased DRP1 protein expression (2.6-fold increase, p < 0.05) with decreased phosphorylation of DRP1 Ser637 (3.20-fold decrease, p < 0.001) was also observed upon p53 knockdown in calcifying VSMCs. In summary, we demonstrate that VSMC calcification promotes notable mitochondrial elongation and cellular senescence via DRP1 phosphorylation. Furthermore, our work indicates that p53-induced mitochondrial fusion underpins cellular senescence by reducing mitochondrial function.

Oncogene-like addiction to aneuploidy in human cancers

Most cancers exhibit aneuploidy, but its functional significance in tumor development is controversial. Here, we describe ReDACT (Restoring Disomy in Aneuploid cells using CRISPR Targeting), a set of chromosome engineering tools that allow us to eliminate specific aneuploidies from cancer genomes. Using ReDACT, we created a panel of isogenic cells that have or lack common aneuploidies, and we demonstrate that trisomy of chromosome 1q is required for malignant growth in cancers harboring this alteration. Mechanistically, gaining chromosome 1q increases the expression of MDM4 and suppresses TP53 signaling, and we show that TP53 mutations are mutually-exclusive with 1q aneuploidy in human cancers. Thus, specific aneuploidies play essential roles in tumorigenesis, raising the possibility that targeting these "aneuploidy addictions" could represent a novel approach for cancer treatment.

mTOR inhibition attenuates chemosensitivity through the induction of chemotherapy resistant persisters

Chemotherapy can eradicate a majority of cancer cells. However, a small population of tumor cells often survives drug treatments through genetic and/or non-genetic mechanisms, leading to tumor recurrence. Here we report a reversible chemoresistance phenotype regulated by the mTOR pathway. Through a genome-wide CRISPR knockout library screen in pancreatic cancer cells treated with chemotherapeutic agents, we have identified the mTOR pathway as a prominent determinant of chemosensitivity. Pharmacological suppression of mTOR activity in cancer cells from diverse tissue origins leads to the persistence of a reversibly resistant population, which is otherwise eliminated by chemotherapeutic agents. Conversely, activation of the mTOR pathway increases chemosensitivity in vitro and in vivo and predicts better survival among various human cancers. Persister cells display a senescence phenotype. Inhibition of mTOR does not induce cellular senescence per se, but rather promotes the survival of senescent cells through regulation of autophagy and G2/M cell cycle arrest, as revealed by a small-molecule chemical library screen. Thus, mTOR plays a causal yet paradoxical role in regulating chemotherapeutic response; inhibition of the mTOR pathway, while suppressing tumor expansion, facilitates the development of a reversible drug-tolerant senescence state.

The Molecular and Cellular Strategies of Glioblastoma and Non-Small-Cell Lung Cancer Cells Conferring Radioresistance

Ionizing radiation (IR) has been shown to play a crucial role in the treatment of glioblastoma (GBM; grade IV) and non-small-cell lung cancer (NSCLC). Nevertheless, recent studies have indicated that radiotherapy can offer only palliation owing to the radioresistance of GBM and NSCLC. Therefore, delineating the major radioresistance mechanisms may provide novel therapeutic approaches to sensitize these diseases to IR and improve patient outcomes. This review provides insights into the molecular and cellular mechanisms underlying GBM and NSCLC radioresistance, where it sheds light on the role played by cancer stem cells (CSCs), as well as discusses comprehensively how the cellular dormancy/non-proliferating state and polyploidy impact on their survival and relapse post-IR exposure.

Identification and Analysis of Senescence-Related Genes in Head and Neck Squamous Cell Carcinoma by a Comprehensive Bioinformatics Approach

Head and neck cancer is the sixth most frequent cancer all over the world, with the majority of subtypes of head and neck squamous cell carcinoma (HNSCC). Cellular senescence-associated genes have been confirmed to play a critical role in cancer and have the potential to be prognostic biomarkers for cancer. Clinical information of HNSCC samples and expression data were acquired from public databases. Expression profiles of genes related to cellular senescence were used to identify molecular subtypes by consensus clustering. To screen differentially expressed genes (DEGs) between different subtypes, differential analysis was performed. We used the univariate Cox regression to identify prognostic DEGs and performed least absolute shrinkage and selection operator (LASSO) to optimize and construct a prognostic model. CIBERSORT, ESTIMATE, and TIDE tools were applied to estimate immune characteristics. Four molecular subtypes were established based on cellular senescence-associated genes. Differential prognosis was observed among different subtypes with C4 having the longest overall survival and C1 having the worst prognosis. C4 subtype also showed the highest immune infiltration. We screened a total of eight cellular senescence prognosis-related genes and established a cellular senescence-related signature score (CSRS.Score) that could stratify samples into high-CSRS.Score and low-CSRS.Score groups. The high-CSRS.Score group had worse prognosis, lower immune infiltration, and lower response to immunotherapy. We further improved the prognostic model and survival prediction by combining CSRS.Score with clinicopathological features using a decision tree model, which had high predictive accuracy and survival prediction. This study demonstrated an important role of cellular senescence in HNSCC. The identified eight cellular senescence-associated genes have the potential to provide ideas for adjuvant treatment and personalized treatment of HNSCC patients.

Senescence and cancer - role and therapeutic opportunities

Cellular senescence is a state of stable, terminal cell cycle arrest associated with various macromolecular changes and a hypersecretory, pro-inflammatory phenotype. Entry of cells into senescence can act as a barrier to tumorigenesis and, thus, could in principle constitute a desired outcome for any anticancer therapy. Paradoxically, studies published in the past decade have demonstrated that, in certain conditions and contexts, malignant and non-malignant cells with lastingly persistent senescence can acquire pro-tumorigenic properties. In this Review, we first discuss the major mechanisms involved in the antitumorigenic functions of senescent cells and then consider the cell-intrinsic and cell-extrinsic factors that participate in their switch towards a tumour-promoting role, providing an overview of major translational and emerging clinical findings. Finally, we comprehensively describe various senolytic and senomorphic therapies and their potential to benefit patients with cancer.

The entry of cells into senescence can act as a barrier to tumorigenesis; however, in certain contexts senescent malignant and non-malignant cells can acquire pro-tumorigenic properties. The authors of this Review discuss the cell-intrinsic and cell-extrinsic mechanisms involved in both the antitumorigenic and tumour-promoting roles of senescent cells, and describe the potential of various senolytic and senomorphic therapeutic approaches in oncology.

Cellular senescence is a natural barrier to tumorigenesis; senescent cells are widely detected in premalignant lesions from patients with cancer.

Cellular senescence is induced by anticancer therapy and can contribute to some treatment-related adverse events (TRAEs).

Senescent cells exert both protumorigenic and antitumorigenic effects via cell-autonomous and paracrine mechanisms.

Pharmacological modulation of senescence-associated phenotypes has the potential to improve therapy efficacy and reduce the incidence of TRAEs.

Of the many cellular responses activated by TP53, which ones are critical for tumour suppression?

The tumour suppressor TP53 is a master regulator of several cellular processes that collectively suppress tumorigenesis. The TP53 gene is mutated in ~50% of human cancers and these defects usually confer poor responses to therapy. The TP53 protein functions as a homo-tetrameric transcription factor, directly regulating the expression of ~500 target genes, some of them involved in cell death, cell cycling, cell senescence, DNA repair and metabolism. Originally, it was thought that the induction of apoptotic cell death was the principal mechanism by which TP53 prevents the development of tumours. However, gene targeted mice lacking the critical effectors of TP53-induced apoptosis (PUMA and NOXA) do not spontaneously develop tumours. Indeed, even mice lacking the critical mediators for TP53-induced apoptosis, G1/S cell cycle arrest and cell senescence, namely PUMA, NOXA and p21, do not spontaneously develop tumours. This suggests that TP53 must activate additional cellular responses to mediate tumour suppression. In this review, we will discuss the processes by which TP53 regulates cell death, cell cycling/cell senescence, DNA damage repair and metabolic adaptation, and place this in context of current understanding of TP53-mediated tumour suppression.

Evolutionarily conserved transcription factors as regulators of longevity and targets for geroprotection

Aging is the single largest risk factor for many debilitating conditions, including heart diseases, stroke, cancer, diabetes, and neurodegenerative disorders. While far from understood in its full complexity, it is scientifically well-established that aging is influenced by genetic and environmental factors, and can be modulated by various interventions. One of aging's early hallmarks are aberrations in transcriptional networks, controlling for example metabolic homeostasis or the response to stress. Evidence in different model organisms abounds that a number of evolutionarily conserved transcription factors, which control such networks, can affect lifespan and healthspan across species. These transcription factors thus potentially represent conserved regulators of longevity and are emerging as important targets in the challenging quest to develop treatments to mitigate age-related diseases, and possibly even to slow aging itself. This review provides an overview of evolutionarily conserved transcription factors that impact longevity or age-related diseases in at least one multicellular model organism (nematodes, flies, or mice), and/or are tentatively linked to human aging. Discussed is the general evidence for transcriptional regulation of aging and disease, followed by a more detailed look at selected transcription factor families, the common metabolic pathways involved, and the targeting of transcription factors as a strategy for geroprotective interventions.

Cooperative stabilisation of 14-3-3σ protein-protein interactions via covalent protein modification

14-3-3 proteins are an important family of hub proteins that play important roles in many cellular processes via a large network of interactions with partner proteins. Many of these protein-protein interactions (PPI) are implicated in human diseases such as cancer and neurodegeneration. The stabilisation of selected 14-3-3 PPIs using drug-like 'molecular glues' is a novel therapeutic strategy with high potential. However, the examples reported to date have a number of drawbacks in terms of selectivity and potency. Here, we report that WR-1065, the active species of the approved drug amifostine, covalently modifies 14-3-3σ at an isoform-unique cysteine residue, Cys38. This modification leads to isoform-specific stabilisation of two 14-3-3σ PPIs in a manner that is cooperative with a well characterised molecular glue, fusicoccin A. Our findings reveal a novel stabilisation mechanism for 14-3-3σ, an isoform with particular involvement in cancer pathways. This mechanism can be exploited to harness the enhanced potency conveyed by covalent drug molecules and dual ligand cooperativity. This is demonstrated in two cancer cell lines whereby the cooperative behaviour of fusicoccin A and WR-1065 leads to enhanced efficacy for inducing cell death and attenuating cell growth.

Targeted clearance of senescent cells using an antibody-drug conjugate against a specific membrane marker

A wide range of diseases have been shown to be influenced by the accumulation of senescent cells, from fibrosis to diabetes, cancer, Alzheimer's and other age-related pathologies. Consistent with this, clearance of senescent cells can prolong healthspan and lifespan in in vivo models. This provided a rationale for developing a new class of drugs, called senolytics, designed to selectively eliminate senescent cells in human tissues. The senolytics tested so far lack specificity and have significant off-target effects, suggesting that a targeted approach could be more clinically relevant. Here, we propose to use an extracellular epitope of B2M, a recently identified membrane marker of senescence, as a target for the specific delivery of toxic drugs into senescent cells. We show that an antibody-drug conjugate (ADC) against B2M clears senescent cells by releasing duocarmycin into them, while an isotype control ADC was not toxic for these cells. This effect was dependent on p53 expression and therefore more evident in stress-induced senescence. Non-senescent cells were not affected by either antibody, confirming the specificity of the treatment. Our results provide a proof-of-principle assessment of a novel approach for the specific elimination of senescent cells using a second generation targeted senolytic against proteins of their surfaceome, which could have clinical applications in pathological ageing and associated diseases.

Metformin Prevents H₂O₂-Induced Senescence in Human Lens Epithelial B3 Cells

Background

The primary purpose of this study was to investigate the protective effect of metformin against hydrogen peroxide (H₂O₂)-induced cellular senescence and to explore the underlying molecular mechanism of lens epithelial cell senescence.

Material/Methods

We used H₂O₂ to establish senescence in human lens epithelial B3 cells. The cells were exposed to H₂O₂ for different numbers of days to mimic aging. Senescence was assessed by senescence-associated β-galactosidase staining, and the molecular mechanism was assessed by real-time polymerase chain reaction (RT-PCR) and western blot analysis. The cultured cells were exposed to 150 μM H₂O₂ for 7 days with or without metformin to detect the underlying molecular mechanism of lens epithelial cell senescence.

Results

The lens epithelial cells exposed to 150 μM H₂O₂ for 7 days exhibited senescence. The expression levels of senescence-related markers were increased in H₂O₂-treated cells. Metformin prevented H₂O₂-induced cellular senescence in human lens epithelial B3 cells.

Conclusions

These findings suggest that senescence marker expression is increased in the cells exposed to H₂O₂. Metformin protects human lens epithelial B3 cells from H₂O₂-induced senescence.

p53 regulates mitochondrial dynamics by inhibiting Drp1 translocation into mitochondria during cellular senescence

Cellular senescence acts as an important barrier to tumorigenesis by eliminating precancerous cells. Previous studies have shown an essential role of the tumor suppressor p53 in cellular senescence, but how p53 induces cellular senescence is not fully understood. We found that p53 promoted the formation of highly interconnected and elongated mitochondria prior to the onset of cellular senescence. The inhibition of mitochondrial elongation upon p53 expression suppressed cellular senescence, suggesting that mitochondrial elongation is required for the induction of p53-dependent senescence. p53-induced mitochondrial elongation resulted in mitochondrial dysfunction and subsequent increases in intracellular reactive oxygen species (ROS) levels, an important mediator of cellular senescence. Mechanistically, the inhibitory phosphorylation of Drp1 Ser637 increased upon p53 expression, suppressing the translocation of Drp1 into mitochondria. The transcriptional function of p53 was crucial for controlling the inhibitory phosphorylation of Drp1, whereas p21 was nonessential. Protein kinase A (PKA) activity was responsible for p53-mediated Drp1 Ser637 phosphorylation and mitochondrial dysfunction. Taken together, these results suggest that p53 regulates mitochondrial dynamics through the PKA-Drp1 pathway to induce cellular senescence.

Effect of Pectin on the Expression of Proteins Associated with Mitochondrial Biogenesis and Cell Senescence in HT29-Human Colorectal Adenocarcinoma Cells

Mitochondria dynamic is regulated by different proteins, maintaining a balance between fission and fusion. An imbalance towards mitochondrial fission has been associated with tumor cell proliferation. The aim of this study was to analyze whether pectin modifies the viability of human colon cancer cells and the expression of proteins involved in mitochondrial fusion and fission. The human colon carcinoma cell line HT29 cells was growth in 10% fetal bovine serum in the absence and presence of pectin. Pectin reduced HT29 cell viability in a concentration-dependent manner, reaching a plateau at 150~300 μmol/L pectin. The presence of 200 μmol/L pectin reduced the expression of dynamin-related protein-1 and increased expression of the mitochondrial fusion-associated proteins mitofusin-1 and 2. Expression of cyclin B1, a protein involved in G2/M transition, was found decreased in pectin-incubated HT29 cells. Moreover, expression of p53 protein, the amount of p53 in the nucleous and β-galactosidase activity, which are all biomarkers for cellular senescence, were significantly higher in pectin-incubated HT29 cells than in HT29 cells incubated without pectin. Expression of the protein B-cell lymphoma 2 (Bcl-2) homologous antagonist/killer was increased in response to incubation with pectin. However, incubation with pectin did not affect expression of Bcl-2-associated X protein or Bcl-2, or the caspase-3 activity. Overall, we concluded that pectin reduces the viability of human HT29 colon cancer cells, which is accompanied with a shift in the expression of proteins associated with mitochondrial dynamics towards mitochondrial fusion. Moreover, incubation with pectin favors cellular senescence over apoptosis in HT29 cells.

PRMT5 as a druggable target for glioblastoma therapy

Background

In spite of standard multimodal therapy consisting of surgical resection followed by radiation and concurrent chemotherapy, prognosis for glioblastoma (GBM) patients remains poor. The identification of both differentiated and undifferentiated "stem cell like" populations in the tumor highlights the significance of finding novel targets that affect the heterogeneous tumor cell population. Protein arginine methyltransferase 5 (PRMT5) is one such candidate gene whose nuclear expression correlates with poor survival and has been reported to be required for survival of differentiated GBM cells and self-renewal of undifferentiated GBM cells. In the current study we screened the specificity and efficacy of 4 novel PRMT5 inhibitors in the treatment of GBM.

Methods

Efficacies of these inhibitors were screened using an in vitro GBM neurosphere model and an in vivo intracranial zebrafish model of glioma. Standard molecular biology methods were employed to investigate changes in cell cycle, growth, and senescence.

Results

In vitro and in vivo studies revealed that among the 4 PRMT5 inhibitors, treatment of GBM cells with compound 5 (CMP5) mirrored the effects of PRMT5 knockdown wherein it led to apoptosis of differentiated GBM cells and drove undifferentiated primary patient derived GBM cells into a nonreplicative senescent state.

Conclusion

In vivo antitumor efficacy combined with the specificity of CMP5 underscores the importance of developing it for translation.

Fluctuations in p53 Signaling Allow Escape from Cell-Cycle Arrest

Biological signals need to be robust and filter small fluctuations yet maintain sensitivity to signals across a wide range of magnitudes. Here, we studied how fluctuations in DNA damage signaling relate to maintenance of long-term cell-cycle arrest. Using live-cell imaging, we quantified division profiles of individual human cells in the course of 1 week after irradiation. We found a subset of cells that initially establish cell-cycle arrest and then sporadically escape and divide. Using fluorescent reporters and mathematical modeling, we determined that fluctuations in the oscillatory pattern of the tumor suppressor p53 trigger a sharp switch between p21 and CDK2, leading to escape from arrest. Transient perturbation of p53 stability mimicked the noise in individual cells and was sufficient to trigger escape from arrest. Our results show that the self-reinforcing circuitry that mediates cell-cycle transitions can translate small fluctuations in p53 signaling into large phenotypic changes.

Acute HSF1 depletion induces cellular senescence through the MDM2-p53-p21 pathway in human diploid fibroblasts

Heat shock transcription factor 1 (HSF1) regulates the expression of a wide array of genes, control of the expression of heat shock proteins (HSPs) and cell growth. Although acute depletion of HSF1 induces cellular senescence, the underlying mechanisms are poorly understood. Here, we report that HSF1 depletion-induced senescence (HDIS) of human diploid fibroblasts (HDFs) was independent of HSP-mediated proteostasis but dependent on activation of the p53-p21 pathway, partly because of the increased expression of dehydrogenase/reductase 2 (DHRS2), a putative MDM2 inhibitor. We observed that HDIS occurred without decreased levels of major HSPs or increased proteotoxic stress in HDFs. Additionally, an inhibitor of HSP70 family proteins increased proteotoxicity and suppressed cell growth, but failed to induce senescence. Importantly, we found that activation of the p53-p21 pathway due to reduced MDM2-dependent p53 degradation was required for HDIS. Furthermore, we provide evidence that increased DHRS2 expression contributes to p53 stabilization and HDIS. Collectively, our observations uncovered a molecular pathway in which HSF1 depletion-induced DHRS2 expression leads to activation of the MDM2-p53-p21 pathway required for HDIS.

Epithelial cell senescence: an adaptive response to pre-carcinogenic stresses?

Senescence is a cell state occurring in vitro and in vivo after successive replication cycles and/or upon exposition to various stressors. It is characterized by a strong cell cycle arrest associated with several molecular, metabolic and morphologic changes. The accumulation of senescent cells in tissues and organs with time plays a role in organismal aging and in several age-associated disorders and pathologies. Moreover, several therapeutic interventions are able to prematurely induce senescence. It is, therefore, tremendously important to characterize in-depth, the mechanisms by which senescence is induced, as well as the precise properties of senescent cells. For historical reasons, senescence is often studied with fibroblast models. Other cell types, however, much more relevant regarding the structure and function of vital organs and/or regarding pathologies, are regrettably often neglected. In this article, we will clarify what is known on senescence of epithelial cells and highlight what distinguishes it from, and what makes it like, replicative senescence of fibroblasts taken as a standard.

Cooperation between p21 and Akt is required for p53-dependent cellular senescence

Cellular senescence has been implicated in normal aging, tissue homeostasis, and tumor suppression. Although p53 has been shown to be a central mediator of cellular senescence, the signaling pathway by which it induces senescence remains incompletely understood. In this study, we have shown that both Akt and p21 are required to induce cellular senescence in response to p53 expression. In a p53-induced senescence model, we found that Akt activation was essential for inducing a cellular senescence phenotype. Surprisingly, Akt inhibition did not abolish p53-induced cell cycle arrest, but it suppressed the increase in intracellular reactive oxygen species (ROS) levels. The results of the cell cycle and morphological analysis suggest that p53 induced quiescence, not senescence, following Akt inhibition. Conversely, the inhibition of p21 induction abolished cell cycle arrest but did not affect the p53-induced increase in ROS levels. Additionally, p21 and Akt separately controlled cell cycle arrest and ROS levels, respectively, during H-Ras-induced senescence in human normal fibroblasts. The mechanistic analysis revealed that Akt increased ROS levels through NOX4 induction, and increased Akt-dependent NF-κB binding to the NOX4 promoter is responsible for NOX4 induction upon p53 expression. We further showed that Akt activation upon p53 expression is mediated by mammalian target of rapamycin complex 2. In addition, p53-mediated IL6 and IL8 induction was abrogated by Akt inhibition, suggesting that Akt activation is also required for the senescence-associated secretory phenotype. Collectively, these results suggest that p53 simultaneously controls multiple pathways to induce cellular senescence through p21 and Akt.

Caveolin-1 deficiency induces premature senescence with mitochondrial dysfunction

Paradoxical observations have been made regarding the role of caveolin-1 (Cav-1) during cellular senescence. For example, caveolin-1 deficiency prevents reactive oxygen species-induced cellular senescence despite mitochondrial dysfunction, which leads to senescence. To resolve this paradox, we re-addressed the role of caveolin-1 in cellular senescence in human diploid fibroblasts, A549, HCT116, and Cav-1^-/- mouse embryonic fibroblasts. Cav-1 deficiency (knockout or knockdown) induced cellular senescence via a p53-p21-dependent pathway, downregulating the expression level of the cardiolipin biosynthesis enzymes and then reducing the content of cardiolipin, a critical lipid for mitochondrial respiration. Our results showed that Cav-1 deficiency decreased mitochondrial respiration, reduced the activity of oxidative phosphorylation complex I (CI), inactivated SIRT1, and decreased the NAD⁺ /NADH ratio. From these results, we concluded that Cav-1 deficiency induces premature senescence via mitochondrial dysfunction and silent information regulator 2 homologue 1 (SIRT1) inactivation.

Wild-type p53 controls the level of fibronectin expression in breast cancer cells

Aberrant fibronectin (FN) expression is associated with poor prognosis, cell adhesion, and cell motility in a variety of cancer cells. In this study, we investigated the relationship between p53 and FN expression in breast cancer cells. Basal FN expression was significantly decreased by treatment with the p53 activator III, RITA, in MCF7 breast cancer cells with wild-type p53. In addition, overexpression of wild-type p53 markedly decreased the level of FN expression in p53-mutant breast cancer cells. To examine the mechanism underlying the relationship between p53 and FN expression, we treated MCF7 breast cancer cells with the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). Our results showed that basal FN expression was increased by TPA treatment in a time-dependent manner. In contrast, the level of p53 expression was decreased by TPA treatment. However, the expression of FN and p53 was not altered by TPA in p53-mutant breast cancer cells. Furthermore, the alterations in FN and p53 expression in response to TPA were prevented by a specific MEK inhibitor, UO126. Finally, we demonstrated that TPA triggers degradation of p53 through the proteasomal pathway in MCF7 cells. TPA-induced FN expression was decreased by the proteasome inhibitor MG132. Under the same condition, p53 protein expression, but not mRNA expression, was reversed by MG132. Taken together, our data demonstrate that the level of FN expression is associated with the status and expression of p53 in breast cancer cells.

2-Acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino) phenyl carbamoylsulfanyl] propionic acid, a glutathione reductase inhibitor, induces G2/M cell cycle arrest through generation of thiol oxidative stress in human esophageal cancer cells

Esophageal squamous cell carcinoma (ESCC) is a highly malignant cancer with poor response to both of chemotherapy and radiotherapy. 2-Acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino) phenyl carbamoylsulfanyl] propionic acid (2-AAPA), an irreversible inhibitor of glutathione reductase (GR), is able to induce intracellular oxidative stress, and has shown anticancer activity in many cancer cell lines. In this study, we investigated the effects of 2-AAPA on the cell proliferation, cell cycle and apoptosis and aimed to explore its mechanism of action in human esophageal cancer TE-13 cells. It was found that 2-AAPA inhibited growth of ESCC cells in a dose-dependent manner and it did not deplete reduced glutathione (GSH), but significantly increased the oxidized form glutathione (GSSG), resulting in decreased GSH/GSSG ratio. In consequence, significant reactive oxygen species (ROS) production was observed. The flow cytometric analysis revealed that 2-AAPA inhibited growth of esophageal cancer cells through arresting cell cycle in G₂/M phase, but apoptosis-independent mechanism. The G₂/M arrest was partially contributed by down-regulation of protein expression of Cdc-25c and up-regulation of phosphorylated Cdc-2 (Tyr15), Cyclin B1 (Ser147) and p53. Meanwhile, 2-AAPA-induced thiol oxidative stress led to increased protein S-glutathionylation, which resulted in α-tubulin S-glutathionylation-dependent depolymerization of microtubule in the TE-13 cells. In conclusion, we identified that 2-AAPA as an effective thiol oxidative stress inducer and proliferation of TE-13 cells were suppressed by G₂/M phase cell cycle arrest, mainly, through α-tubulin S-glutathionylation-mediated microtubule depolymerization. Our results may introduce new target and approach for esophageal cancer therapy through generation of GR-mediated thiol oxidative stress.

Shining the Light on Senescence Associated LncRNAs

Cellular senescence can be described as a complex stress response that leads to irreversible cell cycle arrest. This process was originally described as an event that primary cells go through after many passages of cells during cell culture. More recently, cellular senescence is viewed as a programmed process by which the cell displays a senescence phenotype when exposed to a variety of stresses. Cellular senescence has been implicated in tumor suppression and aging such that senescence may contribute to both tumor progression and normal tissue repair. Here, we review different forms of cellular senescence, as well as current biomarkers used to identify senescent cells in vitro and in vivo. Additionally, we highlight the role of senescence-associated long noncoding RNAs (lncRNAs).

Function of the SIRT3 mitochondrial deacetylase in cellular physiology, cancer, and neurodegenerative disease

In mammals, seven members of the sirtuin protein family known as class III histone deacetylase have been identified for their characteristic features. These distinguished characteristics include the tissues where they are distributed or located, enzymatic activities, molecular functions, and involvement in diseases. Among the sirtuin members, SIRT3 has received much attention for its role in cancer genetics, aging, neurodegenerative disease, and stress resistance. SIRT3 controls energy demand during stress conditions such as fasting and exercise as well as metabolism through the deacetylation and acetylation of mitochondrial enzymes. SIRT3 is well known for its ability to eliminate reactive oxygen species and to prevent the development of cancerous cells or apoptosis. This review article provides a comprehensive review on numerous (noteworthy) molecular functions of SIRT3 and its effect on cancer cells and various diseases including Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease.

A single non-synonymous NCOA5 variation in type 2 diabetic patients with hepatocellular carcinoma impairs the function of NCOA5 in cell cycle regulation

Type 2 Diabetes (T2D) is a risk factor for hepatocellular carcinoma (HCC). We have previously described that haploinsufficiency of nuclear receptor coactivator 5 (NCOA5) is a genetic defect linking glucose intolerance to HCC. Here we report identification and characterization of a single nucleotide variation (T445A) in NCOA5, causing an amino acid Thr to Ala substitution, in adjacent non-tumorous liver tissues derived from patients with concurrent HCC and T2D. By using Tet-On inducible expression cells, we show that ectopic expression of NCOA5wt suppressed proliferation of HCC cells via induction of G2/M arrest, while ectopic expression of NCOA5T445A had a significantly lesser effect compared to ectopic expression of NCOA5wt. Furthermore, ectopic expression of NCOA5wt increased the occurrence of DNA damage and cell senescence, whereas expression of NCOA5T445A partly lost this activity. Xenograft tumor model analysis demonstrated that ectopic NCOA5wt expression reduced HCC tumor growth and the T445A variation impairs its tumor growth inhibitory function. Collectively, our data show that the T445A variation impairs the ability of NCOA5 to inhibit growth of HCC, suggesting that this variation may have potential to increase susceptibility to HCC comorbid with T2D.

Detection of Senescent Cells by Extracellular Markers Using a Flow Cytometry-Based Approach

Senescence is a cellular process that is thought to have prognostic and therapeutic relevance in conditions such as cancer, aging, and fibrosis. However, current protocols for identifying senescent cells in vitro and in vivo have several drawbacks. Most markers used lack sufficient specificity and false positives and negatives in common. In addition, classical staining techniques often require lengthy protocols and do not offer objective quantification. Recently, several novel markers of senescence associated with the plasma membrane have been identified. Here, we propose to take advantage of these markers to define a customizable FACS-based protocol to detect senescent cells using antibodies tagged with fluorescence dyes. This method has the advantage of being fast and allowing quantitation. Furthermore, its specificity is increased using several markers simultaneously.

The Role of Sumoylation in Senescence

Cellular senescence is a program initiated by many stress signals including aberrant activation of oncogenes, DNA damage, oxidative lesions and telomere attrition. Once engaged senescence irreversibly limits cellular proliferation and potently prevents tumor formation in vivo. The precise mechanisms driving the onset of senescence are still not completely defined, although the pRb and p53 tumor suppressor pathways converge with the SUMO cascade to regulate cellular senescence. Sumoylation translocates p53 to PML nuclear bodies where it can co-operate with many sumoylated co-factors in a program that activates pRb and favors senescence. Once activated pRb integrates various proteins, many of them sumoylated, into a repressor complex that inhibits the transcription of proliferation-promoting genes and initiates chromatin condensation. Sumoylation is required for heterochromatin formation during senescence and may act as a scaffold to stabilize the pRb repressor complex. Thus, SUMO is a critical component of a tumor-suppressor network that limits aberrant cell proliferation and tumorigenesis.

Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemness

Since its discovery nearly 40 years ago, p53 has ascended to the forefront of investigated genes and proteins across diverse research disciplines and is recognized most exclusively for its role in cancer as a tumor suppressor. Levine and Oren (2009) reviewed the evolution of p53 detailing the significant discoveries of each decade since its first report in 1979. In this review, we will highlight the emerging non-canonical functions and regulation of p53 in stem cells. We will focus on general themes shared among p53's functions in non-malignant stem cells and cancer stem-like cells (CSCs) and the influence of p53 on the microenvironment and CSC niche. We will also examine p53 gain of function (GOF) roles in stemness. Mutant p53 (mutp53) GOFs that lead to survival, drug resistance and colonization are reviewed in the context of the acquisition of advantageous transformation processes, such as differentiation and dedifferentiation, epithelial-to-mesenchymal transition (EMT) and stem cell senescence and quiescence. Finally, we will conclude with therapeutic strategies that restore wild-type p53 (wtp53) function in cancer and CSCs, including RING finger E3 ligases and CSC maintenance. The mechanisms by which wtp53 and mutp53 influence stemness in non-malignant stem cells and CSCs or tumor-initiating cells (TICs) are poorly understood thus far. Further elucidation of p53's effects on stemness could lead to novel therapeutic strategies in cancer research.

Influence of Human p53 on Plant Development

Mammalian p53 is a super tumor suppressor and plays a key role in guarding genome from DNA damage. However, p53 has not been found in plants which do not bear cancer although they constantly expose to ionizing radiation of ultraviolet light. Here we introduced p53 into the model plant Arabidopsis and examined p53-conferred phenotype in plant. Most strikingly, p53 caused early senescence and fasciation. In plants, fasciation has been shown as a result of the elevated homologous DNA recombination. Consistently, a reporter with overlapping segments of the GUS gene (1445) showed that the frequency of homologous recombination was highly induced in p53-transgenic plants. In contrast to p53, SUPPRESSOR OF NPR1-1 INDUCIBLE 1 (SNI1), as a negative regulator of homologous recombination in plants, is not present in mammals. Comet assay and clonogenic survival assay demonstrated that SNI1 inhibited DNA damage repair caused by either ionizing radiation or hydroxyurea in human osteosarcoma U2OS cancer cells. RAD51D is a recombinase in homologous recombination and functions downstream of SNI1 in plants. Interestingly, p53 rendered the sni1 mutants madly branching of inflorescence, a phenotype of fasciation, whereas rad51d mutant fully suppressed the p53-induced phenotype, indicating that human p53 action in plant is mediated by the SNI1-RAD51D signaling pathway. The reciprocal species-swap tests of p53 and SNI1 in human and Arabidopsis manifest that these species-specific proteins play a common role in homologous recombination across kingdoms of animals and plants.

BTK Modulates p53 Activity to Enhance Apoptotic and Senescent Responses

p53 is a tumor suppressor that prevents the emergence of transformed cells by inducing apoptosis or senescence, among other responses. Its functions are regulated tightly by posttranslational modifications. Here we show that Bruton's tyrosine kinase (BTK) is a novel modulator of p53. We found that BTK is induced in response to DNA damage and p53 activation. BTK induction leads to p53 phosphorylation, which constitutes a positive feedback loop that increases p53 protein levels and enhances the transactivation of its target genes in response to stress. Inhibiting BTK reduced both p53-dependent senescence and apoptosis. Further, BTK expression also upregulated DNA damage signals and apoptosis. We conclude that despite being involved in oncogenic signals in blood malignancies, BTK has antineoplastic properties in other contexts, such as the enhancement of p53's tumor suppressor responses. Along with evidence that BTK expression correlates with good prognosis in some epithelial tumors, our findings may encourage a reevaluation of the clinical uses of BTK inhibitors in cancer therapy. Cancer Res; 76(18); 5405-14. ©2016 AACR.

Rif1 and Exo1 regulate the genomic instability following telomere losses

Telomere attrition is linked to cancer, diabetes, cardiovascular disease and aging. This is because telomere losses trigger further genomic modifications, culminating with loss of cell function and malignant transformation. However, factors regulating the transition from cells with short telomeres, to cells with profoundly altered genomes, are little understood. Here, we use budding yeast engineered to lack telomerase and other forms of telomere maintenance, to screen for such factors. We show that initially, different DNA damage checkpoint proteins act together with Exo1 and Mre11 nucleases, to inhibit proliferation of cells undergoing telomere attrition. However, this situation changes when survivors lacking telomeres emerge. Intriguingly, checkpoint pathways become tolerant to loss of telomeres in survivors, yet still alert to new DNA damage. We show that Rif1 is responsible for the checkpoint tolerance and proliferation of these survivors, and that is also important for proliferation of cells with a broken chromosome. In contrast, Exo1 drives extensive genomic modifications in survivors. Thus, the conserved proteins Rif1 and Exo1 are critical for survival and evolution of cells with lost telomeres.

Shifting p53-induced senescence to cell death by TIS21(/BTG2/Pc3) gene through posttranslational modification of p53 protein

Cellular senescence and apoptosis can be regulated by p53 activity, although the underlying mechanism of the switch between the two events remains largely unknown. Cells exposed to cancer chemotherapy can escape to senescence phenotype rather than undergoing apoptosis. By employing adenoviral transduction of p53 or TIS21 genes, we observed shifting of p53 induced-senescence to apoptosis in EJ bladder cancer cells, which express H-RasV12 and mutant p53; transduction of p53 increased H-RasV12 expression along with senescence phenotypes, whereas coexpression with TIS21 (p53+TIS21) induced cell death rather than senescence. The TIS21-mediated switch of senescence to apoptosis was accompanied by nuclear translocation of p53 protein and its modifications on Ser-15 and Ser-46 phosphorylation and acetylations on Lys-120, -320, -373 and -382 residues. Mechanistically, TIS21(/BTG2) regulated posttranslational modification of p53 via enhancing miR34a and Bax expressions as opposed to inhibiting SIRT1 and Bcl2 expression. At the same time, TIS21 increased APAF-1 and p53AIP1 expressions, but inhibited the interaction of p53 with iASPP. In vitro tumorigenicity was significantly reduced in the p53+TIS21 expresser through inhibiting micro-colony proliferation by TIS21. Effect of TIS21 on the regulation of p53 activity was confirmed by knockdown of TIS21 expression by RNA interference. Therefore, we suggest TIS21 expression as an endogenous cell death inducer at the downstream of p53 gene, which might be useful for intractable cancer chemotherapy.

Reactivating p53 functions by suppressing its novel inhibitor iASPP: a potential therapeutic opportunity in p53 wild-type tumors

Although mutational inactivation of p53 is found in 50% of all human tumors, a subset of tumors display defective p53 function, but retain wild-type (WT) p53. Here, direct and indirect mechanisms leading to the loss of WT p53 activities are discussed. We summarize the oncogenic roles of iASPP, an inhibitor of WT p53, in promoting proliferation, invasion, drug or radiation-resistance and metastasis. From the therapeutic view, we highlight promising perspectives of microRNA-124, peptide and small molecules that reduce or block iASPP for the treatment of cancer. High iASPP expression enhances proliferation, aggressive behavior, the resistance to radiation/chemotherapy and correlates with poor prognosis in a range of human tumors. Overexpression of iASPP accelerates tumorigenesis and invasion through p53-dependent and p53-independent mechanisms. MicroRNA-124 directly targets iASPP and represses the growth and invasiveness of cancer cells. The disruption of iASPP-p53 interaction by a p53-derived peptide A34 restores p53 function in cancer cells. The inhibition of iASPP phosphorylation with small molecules induces p53-dependent apoptosis and growth suppression. The mechanisms underlying aberrant expression of iASPP in human tumors should be further investigated. Reactivating WT p53 functions by targeting its novel inhibitor iASPP holds promise for potential therapeutic interventions in the treatment of WT p53-containing tumors.

Retinoids induce cellular senescence in breast cancer cells by RAR-β dependent and independent pathways: Potential clinical implications (Review)

Most studies on cellular senescence (CS) have been performed in vitro by employing cytotoxic agents, irradiation, chromatin and telomerase modulators or by activating certain oncogenes. All these approaches usually lead to DNA damage, gene instability and/or chromatin alterations that primarily affect p53-p21 signaling. Little is known on whether retinoids and rexinoids, which are cell differentiation agents, can also induce CS in vitro and in vivo, and which molecular mechanisms are involved in promoting the senescent phenotype. We reviewed the recent publications on CS induced by retinoids and rexinoids in ER⁺ and ER⁻ breast cancer cell lines and in corresponding animal models of mammary carcinogenesis which simulate those of human breast cancer. The role of retinoic acid receptors β2 and 5 (RARβ2 and RARβ5) and of receptor independent genes involved in mediating the senescence program of retinoids and rexinoids in ER⁺ and ER⁻ breast cancer cells is discussed. Potential strategists for clinical implication of CS as biomarker of prognosis and of response to treatment with retinoids, rexinoids and with other cell differentiation and antitumor agents are outlined.

The role of DNA damage responses in p53 biology

The tumour suppressor p53 is a central player in cellular DNA damage responses. P53 is upregulated and activated by genotoxic stress and induces a transcriptional programme with effectors promoting apoptosis, cell cycle arrest, senescence and DNA repair. For the best part of the last three decades, these DNA damage-related programmes triggered by p53 were unequivocally regarded as the major if not sole mechanism by which p53 exerts its tumour suppressor function. However, this interpretation has been challenged by a number of recent in vivo studies, demonstrating that mice which are defective in inducing p53-dependent apoptosis, cell cycle arrest and senescence suppress thymic lymphoma as well as wild-type p53 expressing animals. Consequently, the importance of DNA damage responses for p53-mediated tumour suppression has been questioned. In this review, I summarize current knowledge on p53-controlled DNA damage responses and argue that these activities, while their role has certainly changed, remain an important feature of p53 biology with relevance for cancer therapy and tumour suppression.

p53 protein-mediated up-regulation of MAP kinase phosphatase 3 (MKP-3) contributes to the establishment of the cellular senescent phenotype through dephosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2)

Growth arrest is one of the essential features of cellular senescence. At present, the precise mechanisms responsible for the establishment of the senescence-associated arrested phenotype are still incompletely understood. Given that ERK1/2 is one of the major kinases controlling cell growth and proliferation, we examined the possible implication of ERK1/2. Exposure of normal rat epithelial cells to etoposide caused cellular senescence, as manifested by enlarged cell size, a flattened cell body, reduced cell proliferation, enhanced β-galactosidase activity, and elevated p53 and p21. Senescent cells displayed a blunted response to growth factor-induced cell proliferation, which was preceded by impaired ERK1/2 activation. Further analysis revealed that senescent cells expressed a significantly higher level of mitogen-activated protein phosphatase 3 (MKP-3, a cytosolic ERK1/2-targeted phosphatase), which was suppressed by blocking the transcriptional activity of the tumor suppressor p53 with pifithrin-α. Inhibition of MKP-3 activity with a specific inhibitor or siRNA enhanced basal ERK1/2 phosphorylation and promoted cell proliferation. Apart from its role in growth arrest, impairment of ERK1/2 also contributed to the resistance of senescent cells to oxidant-elicited cell injury. These results therefore indicate that p53-mediated up-regulation of MKP-3 contributes to the establishment of the senescent cellular phenotype through dephosphorylating ERK1/2. Impairment of ERK1/2 activation could be an important mechanism by which p53 controls cellular senescence.

Genotoxic stress/p53-induced DNAJB9 inhibits the pro-apoptotic function of p53

DNAJB9 is a recently isolated member of the molecular chaperone gene family, whose precise function is largely unknown. In the present study, we have identified DNAJB9 as an inducible gene of the tumor suppressor p53. DNAJB9 expression was induced by p53 or genotoxic stress in a p53-dependent manner, which was mediated by the Ras/Raf/ERK pathway. In addition, depletion of DNAJB9 by using siRNAs greatly increased genotoxic stress/p53-induced apoptosis, suggesting that DNAJB9 inhibits the pro-apoptotic function of p53. We also found that DNAJB9 physically interacts with p53 through its J domain, through which it inhibits the pro-apoptotic function of p53. Moreover, DNAJB9 colocalized with p53 in both cytoplasm and nucleus under genotoxic conditions. Together, these results demonstrate that DNAJB9 is a downstream target of p53 that belongs to the group of negative feedback regulators of p53.

Ash2L enables P53-dependent apoptosis by favoring stable transcription pre-initiation complex formation on its pro-apoptotic target promoters

Chromatin conformation plays a major role in all cellular decisions. We showed previously that P53 pro-apoptotic target promoters are enriched with H3K9me3 mark and induction of P53 abrogates this repressive chromatin conformation by down-regulating SUV39H1, the writer of this mark present on these promoters. In the present study, we demonstrate that in response to P53 stabilization, its pro-apoptotic target promoters become enriched with the H3K4me3 epigenetic mark as well as its readers, Wdr5, RbBP5 and Ash2L, which were not observed in response to SUV39H1 down-regulation alone. Overexpression of Ash2L enhanced P53–dependent apoptosis in response to chemotherapy, associated with increased P53 pro–apoptotic gene promoter occupancy and target gene expression. In contrast, pre–silencing of Ash2L abrogated P53's ability to induce the expression of these transcriptional targets, without affecting P53 or RNAP II recruitment. However, Ash2L pre–silencing, under the same conditions, resulted in reduced RNAP II ser5–CTD phosphorylation on these same pro-apoptotic target promoters, which correlated with reduced promoter occupancy of TFIIB as well as TFIIF (RAP74). Based on these findings, we propose that Ash2L acts in concert with P53 promoter occupancy to activate RNAP II by aiding formation of a stable transcription pre–initiation complex required for its activation.

Molecular Basis for the Cellular Senescence Program and Its Application to Anticancer Therapy

Although dysfunctional telomeres and oncogenic or stressful stimuli are known to trigger cellular senescence in normal human diploid cells, the molecules and signaling network involved in the cellular senescence program are not fully understood. We have been trying to identify cellular senescence-inducing factors by various means. First, we screened for an extrinsic signal that can induce cellular senescence in human lung adenocarcinoma cell line A549, and identified transforming growth factor-beta (TGF-beta) as the cellular senescence-inducing factor. Cancer cells senesced by treatment with TGF-beta impaired tumorigenicity both in vitro and in vivo, suggesting that cellular senescence functions as a tumor suppression mechanism. Next, we identified 86 independent senescence-associated genes by subtractive screening using A549-derived cell lines. Thirdly, we established novel cell lines (AST cells) from A549 cells exposed to mild oxidative stress. AST cells demonstrated functional impairment of telomerase due to perturbed subcellular localization of human telomerase reverse transcriptase, suggesting that mild oxidative stress might affect the cell fate of cancer cells. These results should provide insight into the molecular basis of the cellular senescence program.

Differential responses of human hepatocytes to the non-nucleoside HIV-1 reverse transcriptase inhibitor nevirapine

Nevirapine is a non-nucleoside reverse transcriptase (RT) inhibitor used for the treatment of AIDS and the prevention of mother-to-child transmission of HIV-1. Despite its therapeutic benefits, treatment with nevirapine has been associated with significant incidences of liver and dermal toxicity. The present study examined the effects of nevirapine on cell growth and death in human hepatocyte HepG2 cells and THLE2 cells and the possible pathways involved in these effects. The concentrations of nevirapine inhibiting 50% cell growth were similar for both cell lines. Nevirapine (0-250 µM) treatment caused a slight increase in the amount of lactate dehydrogenase released into the medium. Apoptotic cell death did not contribute to the decrease in viable cells. Exposing of HepG2 cells to nevirapine caused G2/M phase arrest, and the activity of senescence-associated β-galactosidase was not altered. In THLE2 cells, the percentage of cells in G1/G0 phase was increased and cellular senescence was induced in a concentration-dependent manner. Endogenous non-telomeric RT activity was not detected in either cell line. Western blot analysis indicated lower levels of p53 and phospho-p53 (ser15) in HepG2 cells as compared to THLE2 cells; no significant changes in p53 or phospho-p53 (ser15) were noted with nevirapine treatment. These data demonstrate that nevirapine inhibits cell growth, induces cell cycle arrest at different phases, and has different effects on cellular senescence in HepG2 cells and THLE2 cells. The differential responses appear to be related to differences in the basal levels of p53 in the HepG2 cells and THLE2 cells.

p53 as an intervention target for cancer and aging

p53 is well known for suppressing tumors but could also affect other aging processes not associated with tumor suppression. As a transcription factor, p53 responds to a variety of stresses to either induce apoptosis (cell death) or cell cycle arrest (cell preservation) to suppress tumor development. Yet, the effect p53 has on the non-cancer aspects of aging is complicated and not well understood. On one side, p53 could induce cellular senescence or apoptosis to suppress cancer but as an unintended consequence enhance the aging process especially if these responses diminish stem and progenitor cell populations. But on the flip side, p53 could reduce growth and growth-related stress to enable cell survival and ultimately delay the aging process. A better understanding of diverse functions of p53 is essential to elucidate its influences on the aging process and the possibility of targeting p53 or p53 transcriptional targets to treat cancer and ameliorate general aging.

p53-dependent gene repression through p21 is mediated by recruitment of E2F4 repression complexes

The p53 tumor suppressor protein is a major sensor of cellular stresses, and upon stabilization, activates or represses many genes that control cell fate decisions. While the mechanism of p53-mediated transactivation is well established, several mechanisms have been proposed for p53-mediated repression. Here, we demonstrate that the cyclin-dependent kinase inhibitor p21 is both necessary and sufficient for the downregulation of known p53-repression targets, including survivin, CDC25C, and CDC25B in response to p53 induction. These same targets are similarly repressed in response to p16 overexpression, implicating the involvement of the shared downstream retinoblastoma (RB)-E2F pathway. We further show that in response to either p53 or p21 induction, E2F4 complexes are specifically recruited onto the promoters of these p53-repression targets. Moreover, abrogation of E2F4 recruitment via the inactivation of RB pocket proteins, but not by RB loss of function alone, prevents the repression of these genes. Finally, our results indicate that E2F4 promoter occupancy is globally associated with p53-repression targets, but not with p53 activation targets, implicating E2F4 complexes as effectors of p21-dependent p53-mediated repression.

Expression of cyclin A, B1 and D1 after induction of cell cycle arrest in the Jurkat cell line exposed to doxorubicin

Jurkat human lymphoblastoid cells were incubated in increasing concentrations of doxorubicin (0.05, 0.1 and 0.15 μM) to induce cell death, and their expression of cyclin A, B1 and D1 was evaluated by flow cytometry (cell cycle progression, Annexin V assay, percentages and levels of each of the cyclins), transmission electron microscopy (ultrastructure) and confocal fluorescence microscopy (expression and intracellular localization of cyclins). After low-dose doxorubicin treatment, Jurkat cells responded mainly by G2/M arrest, which was related to increased cyclin B1, A and D1 levels, a low level of apoptosis and/or mitotic catastrophe. The influence of doxorubicin on levels and/or localization of selected cyclins was confirmed, which may in turn contribute to the G2/M arrest induced by the drug.

Stochastic and Deterministic Models of Cellular p53 Regulation

The protein p53 is a key regulator of cellular response to a wide variety of stressors. In cancer cells inhibitory regulators of p53 such as MDM2 and MDMX proteins are often overexpressed. We apply in silico techniques to better understand the role and interactions of these proteins in a cell cycle process. Furthermore we investigate the role of stochasticity in determining system behavior. We have found that stochasticity is able to affect system behavior profoundly. We also derive a general result for the way in which initially synchronized oscillating stochastic systems will fall out of synchronization with each other.

Stra6, a retinoic acid-responsive gene, participates in p53-induced apoptosis after DNA damage

Stra6 is the retinoic acid (RA)-inducible gene encoding the cellular receptor for holo-retinol binding protein. This transmembrane protein mediates the internalization of retinol, which then upregulates RA-responsive genes in target cells. Here, we show that Stra6 can be upregulated by DNA damage in a p53-dependent manner, and it has an important role in cell death responses. Stra6 expression induced significant amounts of apoptosis in normal and cancer cells, and it was also able to influence p53-mediated cell fate decisions by turning an initial arrest response into cell death. Moreover, inhibition of Stra6 severely compromised p53-induced apoptosis. We also found that Stra6 induced mitochondria depolarization and accumulation of reactive oxygen species, and that it was present not only at the cellular membrane but also in the cytosol. Finally, we show that these novel functions of Stra6 did not require downstream activation of RA signalling. Our results present a previously unknown link between the RA and p53 pathways and provide a rationale to use retinoids to upregulate Stra6, and thus enhance the tumour suppressor functions of p53. This may have implications for the role of vitamin A metabolites in cancer prevention and treatment.

Bexarotene induces cellular senescence in MMTV-Neu mouse model of mammary carcinogenesis

Previous studies have shown that retinoids and rexinoids can prevent breast cancer in animal models and in women with increased risk of developing the disease. The cellular effects of these vitamin A analogues have been primarily associated with induction of differentiation and inhibition of proliferation. In this study, we tested the hypothesis that bexarotene (LGD1069, Targretin), a rexinoid, can not only inhibit cell proliferation but also induce cellular senescence in mammary epithelial cells, premalignant lesions, and tumors of the MMTV-Neu model of mammary carcinogenesis, which develops estrogen receptor-negative tumors. Mice with palpable mammary tumors were treated for 4 weeks with bexarotene at 80 or 40 mg/kg body weight, and senescent cells were determined by SA-β-Gal assay. Bexarotene decreased in a dose-dependent manner the multiplicity of premalignant lesions and tumors, and this was associated with inhibition of cell proliferation and induction of cellular senescence and apoptosis. By double labeling of senescent cells, first by SA-β-Gal and then by antibodies against genes related to cellular senescence, we found that p21, p16, and RARβ, but not p53, were upregulated by bexarotene in mammary tumors and in breast cancer cell lines, suggesting involvement of multiple signaling pathways in mediating the senescence program of rexinoids. These findings indicate that, in addition to cell proliferation and apoptosis, cellular senescence could be used as a potential biomarker of response in breast cancer prevention and therapy studies with rexinoids and possibly with other antitumor agents.