The interplay between p16 serine phosphorylation and arginine methylation determines its function in modulating cellular apoptosis and senescence. Sci Rep. 2017;7:41390. [PMID: 28120917 PMCID: PMC5264599 DOI: 10.1038/srep41390]

Post-Translational Modifications of Proteins Orchestrate All Hallmarks of Cancer

Post-translational modifications (PTMs) of proteins dynamically build the buffering and adapting interface between oncogenic mutations and environmental stressors, on the one hand, and cancer cell structure, functioning, and behavior. Aberrant PTMs can be considered as enabling characteristics of cancer as long as they orchestrate all malignant modifications and variability in the proteome of cancer cells, cancer-associated cells, and tumor microenvironment (TME). On the other hand, PTMs of proteins can enhance anticancer mechanisms in the tumoral ecosystem or sustain the beneficial effects of oncologic therapies through degradation or inactivation of carcinogenic proteins or/and activation of tumor-suppressor proteins. In this review, we summarized and analyzed a wide spectrum of PTMs of proteins involved in all regulatory mechanisms that drive tumorigenesis, genetic instability, epigenetic reprogramming, all events of the metastatic cascade, cytoskeleton and extracellular matrix (ECM) remodeling, angiogenesis, immune response, tumor-associated microbiome, and metabolism rewiring as the most important hallmarks of cancer. All cancer hallmarks develop due to PTMs of proteins, which modulate gene transcription, intracellular and extracellular signaling, protein size, activity, stability and localization, trafficking, secretion, intracellular protein degradation or half-life, and protein-protein interactions (PPIs). PTMs associated with cancer can be exploited to better understand the underlying molecular mechanisms of this heterogeneous and chameleonic disease, find new biomarkers of cancer progression and prognosis, personalize oncotherapies, and discover new targets for drug development.

The Epigenetic Reader, Bromodomain Containing 2, Mediates Cholangiocyte Senescence via Interaction With ETS Proto-Oncogene 1

BACKGROUND & AIMS

We reported that cholangiocyte senescence, regulated by the transcription factor ETS proto-oncogene 1 (ETS1), is a pathogenic feature of primary sclerosing cholangitis (PSC). Furthermore, histone 3 lysine 27 is acetylated at senescence-associated loci. The epigenetic readers, bromodomain and extra-terminal domain (BET) proteins, bind acetylated histones, recruit transcription factors, and drive gene expression. Thus, we tested the hypothesis that BET proteins interact with ETS1 to drive gene expression and cholangiocyte senescence.

METHODS

We performed immunofluorescence for BET proteins (BRD2 and 4) in liver tissue from liver tissue from PSC patients and a mouse PSC model. Using normal human cholangiocytes (NHCs), NHCs experimentally induced to senescence (NHCsen), and PSC patient-derived cholangiocytes (PSCDCs), we assessed senescence, fibroinflammatory secretome, and apoptosis after BET inhibition or RNA interference depletion. We assessed BET interaction with ETS1 in NHCsen and tissues from PSC patient, and the effects of BET inhibitors on liver fibrosis, senescence, and inflammatory gene expression in mouse models.

RESULTS

Tissue from patients with PSC and a mouse PSC model exhibited increased cholangiocyte BRD2 and 4 protein (∼5×) compared with controls without disease. NHCsen exhibited increased BRD2 and 4 (∼2×), whereas PSCDCs exhibited increased BRD2 protein (∼2×) relative to NHC. BET inhibition in NHCsen and PSCDCs reduced senescence markers and inhibited the fibroinflammatory secretome. ETS1 interacted with BRD2 in NHCsen, and BRD2 depletion diminished NHCsen p21 expression. BET inhibitors reduced senescence, fibroinflammatory gene expression, and fibrosis in the 3,5-diethoxycarbonyl-1,4-dihydrocollidine-fed and Mdr2-/- mouse models.

CONCLUSION

Our data suggest that BRD2 is an essential mediator of the senescent cholangiocyte phenotype and is a potential therapeutic target for patients with PSC.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Skin-Aging Pigmentation: Who Is the Real Enemy?

Skin aging is induced and sustained by chronological aging and photoaging. Aging skin pigmentation such as mottled pigmentation (senile lentigo) and melasma are typical signs of photoaging. The skin, like other human organs, undergoes cellular senescence, and senescent cells in the skin increase with age. The crosstalk between melanocytes as pigmentary cells and other adjacent types of aged skin cells such as senescent fibroblasts play a role in skin-aging pigmentation. In this review, we provide an overview of cellular senescence during the skin-aging process. The discussion also includes cellular senescence related to skin-aging pigmentation and the therapeutic potential of regulating the senescence process.

Poly(ADP-ribose)polymerase-1 affects hydroquinone-induced aberrant cell cycle and apoptosis through activation of p16/pRb signaling pathway in TK6 cells

Hydroquinone (HQ), a key metabolite of benzene, affects cell cycle and apoptosis. Poly (ADP-ribose) polymerase-1 (PARP-1) plays an important role in DNA damage repair. To explore whether PARP-1 is involved in HQ-induced cell cycle and apoptosis, we assessed the effect of PARP-1 suppression and overexpression on induction of cell cycle and apoptosis analyzed by flow cytometry analysis. We observed that HQ induced aberrant cell cycle progression and apoptosis. We further confirmed that PARP-1 suppression accelerated the cell cycle progression and inhibited cell apoptosis via inhibiting p16/pRb signal pathway after acute HQ exposure, while overexpression of PARP-1 displayed the opposite results. Therefore, we concluded that HQ-induced cell cycle and apoptosis were regulated by PARP-1 through activation of p16/pRb signaling pathway.

p16 loss facilitate hydroquinone-induced malignant transformation of TK6 cells through promoting cell proliferation and accelerating the cell cycle progression

The p16^INK4A is a multifunction gene that includes regulation of the cell cycle, apoptosis, senescence and tumor development. However, the effects of p16 in hydroquinone-induced malignant transformation of TK6 cells remain unclear. The present study aimed to explore whether p16 loss facilitate malignant transformation in TK6 cells. The results demonstrated that p16/Rb signal pathway was suppressed in hydroquinone-induced malignant transformation of TK6 cells. We further confirmed that p16 loss stimulated cell proliferation, and accelerated cell cycle progression in vitro and in vivo. The immunoblotting analysis indicated that p16 regulated cell cycle progression via Rb and p53. Therefore, we conclude that p16 is involved in HQ-induced malignant transformation associated with suppressing Rb and p53 which resulting in accelerating the cell cycle progression.

PARP-1 via regulation of p53 and p16, is involved in the hydroquinone-induced malignant transformation of TK6 cells by decelerating the cell cycle

Poly(ADP-ribose)polymerase-1 (PARP-1) plays a crucial role in DNA damage repair and could be viewed as both a tumor promoter and tumor-suppressor gene. However, the effects of PARP-1 in hydroquinone-induced malignant transformation of TK6 cells remain to be further elucidated. The present research evaluated the potential mechanism of PARP-1 in hydroquinone-induced malignant transformation of TK6 cells. The results indicated that high PARP-1 inhibited TK6 cells malignant transformation after chronic exposure to HQ. We further confirmed that PARP-1 overexpression blocked cell proliferation, and decelerated cell cycle progression in vitro and in vivo. The immunoblotting analysis indicated that PARP-1 regulated cell cycle progression via p16/Rb and p53. Therefore, we conclude that PARP-1 is involved in HQ-induced malignant transformation associated with increasing p16/Rb and p53 which resulting in decelerating the cell cycle progression.

Molecular mapping of interstitial lung disease reveals a phenotypically distinct senescent basal epithelial cell population

Compromised regenerative capacity of lung epithelial cells can lead to cellular senescence, which may precipitate fibrosis. While increased markers of senescence have been reported in idiopathic pulmonary fibrosis (IPF), the origin and identity of these senescent cells remain unclear, and tools to characterize context-specific cellular senescence in human lung are lacking. We observed that the senescent marker p16 is predominantly localized to bronchiolized epithelial structures in scarred regions of IPF and systemic sclerosis-associated interstitial lung disease (SSc-ILD) lung tissue, overlapping with the basal epithelial markers Keratin 5 and Keratin 17. Using in vitro models, we derived transcriptional signatures of senescence programming specific to different types of lung epithelial cells and interrogated these signatures in a single-cell RNA-Seq data set derived from control, IPF, and SSc-ILD lung tissue. We identified a population of basal epithelial cells defined by, and enriched for, markers of cellular senescence and identified candidate markers specific to senescent basal epithelial cells in ILD that can enable future functional studies. Notably, gene expression of these cells significantly overlaps with terminally differentiating cells in stratified epithelia, where it is driven by p53 activation as part of the senescence program.

Effects of substrate modifications on the arginine dimethylation activities of PRMT1 and PRMT5

Histone arginine methylation is a prevalent posttranslational modification (PTM) in eukaryotic cells and contributes to the histone codes for epigenetic regulation of gene transcription. In this study, we determined how local changes on adjacent residues in the histone H4 substrate regulate arginine asymmetric dimethylation and symmetric dimethylation catalysed by the major protein arginine methyltransferase (PRMT) enzymes PRMT1 and PRMT5, respectively. We found that phosphorylation at histone H4 Ser-1 site (H4S1) was inhibitory to activities of PRMT1 and PRMT5 in both monomethylating and dimethylating H4R3. Also, a positively charged H4K5 was important for PRMT1 catalysis because acetylation of H4K5 or the loss of the H4K5 ε-amine had a similar effect in reducing the catalytic efficiency of asymmetric dimethylation of H4R3. An opposite effect was observed in that acetylation of H4K5 or the loss of the H4K5 ε-amine enhanced PRMT5-mediated symmetric dimethylation of H4R3. Furthermore, we observed that N-terminal acetylation of H4 modestly decreased asymmetric dimethylation of H4R3 by PRMT1 and symmetric dimethylation of H4R3 by PRMT5. This work highlights the significance of local chemical changes in the substrate to regulating PRMT activity and unravels the pattern complexities and subtleties of histone codes.

Comparison of several commonly used detection indicators of cell senescence

Cell senescence is the state of irreversible growth arrest that can be triggered by a variety of different cellular stresses. Currently, the commonly used detection indicators involved in the study of cell senescence include senescence-associated β-galactosidase, Clusterin, Telomeres/Telomerase, senescence-associated heterochromatin foci, senescence-associated secretory phenotype, senescence marker protein-30, tumor suppressor genes p53 and p16, and other indicators such as Ki67 and decoy receptor 2. These indicators are widely used in the study of cell senescence, each with its own characteristics, advantages, and disadvantages. This review summarizes several commonly used cell senescence indicators and compares their accuracy, credibility, specificity, and the scope of their potential application.

Regulation of Tumor Suppressor Gene CDKN2A and Encoded p16-INK4a Protein by Covalent Modifications

CDKN2A is one of the most studied tumor suppressor genes. It encodes the p16-INK4a protein that plays a critical role in the cell cycle progression, differentiation, senescence, and apoptosis. Mutations in CDKN2A or dysregulation of its functional activity are frequently associated with various types of human cancer. As a cyclin-dependent kinase inhibitor, p16-INK4a forms a complex with cyclin-dependent kinases 4/6 (CDK4/6) thereby competing with cyclin D. It is believed that the helix-turn-helix structures in the content of tandem ankyrin repeats in p16-INK4a are required for the protein interaction with CDK4. Until recently, the mechanisms considered to be involved in the regulation of p16-INK4a functions and cancer development have been mutations in DNA, homozygous or heterozygous gene loss, and methylation of CDKN2A promoter region. In this review, we discuss recent findings on the regulation of p16-INK4a by covalent modifications at both transcriptional and post-translational levels.

Cardiac specific PRMT1 ablation causes heart failure through CaMKII dysregulation

Dysregulation of Ca²⁺/calmodulin-dependent protein kinase (CaMK)II is closely linked with myocardial hypertrophy and heart failure. However, the mechanisms that regulate CaMKII activity are incompletely understood. Here we show that protein arginine methyltransferase 1 (PRMT1) is essential for preventing cardiac CaMKII hyperactivation. Mice null for cardiac PRMT1 exhibit a rapid progression to dilated cardiomyopathy and heart failure within 2 months, accompanied by cardiomyocyte hypertrophy and fibrosis. Consistently, PRMT1 is downregulated in heart failure patients. PRMT1 depletion in isolated cardiomyocytes evokes hypertrophic responses with elevated remodeling gene expression, while PRMT1 overexpression protects against pathological responses to neurohormones. The level of active CaMKII is significantly elevated in PRMT1-deficient hearts or cardiomyocytes. PRMT1 interacts with and methylates CaMKII at arginine residues 9 and 275, leading to its inhibition. Accordingly, pharmacological inhibition of CaMKII restores contractile function in PRMT1-deficient mice. Thus, our data suggest that PRMT1 is a critical regulator of CaMKII to maintain cardiac function.

The mechanisms that regulate the activity of Ca^2 +/calmodulin-dependent protein kinase II (CaMKII) in the context of heart failure are incompletely understood. Here the authors show that protein arginine methyltransferase 1 (PRMT1) prevents cardiac hyperactivation of CaMKII and heart failure development by methylating CaMKII at arginine residues 9 and 275.

Protein arginine methylation: an emerging regulator of the cell cycle

Protein arginine methylation is a common post-translational modification where a methyl group is added onto arginine residues of a protein to alter detection by its binding partners or regulate its activity. It is known to be involved in many biological processes, such as regulation of signal transduction, transcription, facilitation of protein–protein interactions, RNA splicing and transport. The enzymes responsible for arginine methylation, protein arginine methyltransferases (PRMTs), have been shown to methylate or associate with important regulatory proteins of the cell cycle and DNA damage repair pathways, such as cyclin D1, p53, p21 and the retinoblastoma protein. Overexpression of PRMTs resulting in aberrant methylation patterns in cancers often correlates with poor recovery prognosis. This indicates that protein arginine methylation is also an important regulator of the cell cycle, and consequently a target for cancer regulation. The effect of protein arginine methylation on the cell cycle and how this emerging key player of cell cycle regulation may be used in therapeutic strategies for cancer are the focus of this review.

Hallmarks of Cellular Senescence

Cellular senescence is a permanent state of cell cycle arrest that promotes tissue remodeling during development and after injury, but can also contribute to the decline of the regenerative potential and function of tissues, to inflammation, and to tumorigenesis in aged organisms. Therefore, the identification, characterization, and pharmacological elimination of senescent cells have gained attention in the field of aging research. However, the nonspecificity of current senescence markers and the existence of different senescence programs strongly limit these tasks. Here, we describe the molecular regulators of senescence phenotypes and how they are used for identifying senescent cells in vitro and in vivo. We also highlight the importance that these levels of regulations have in the development of therapeutic targets.

Snail suppresses cellular senescence and promotes fibroblast-led cancer cell invasion

Snail, a zinc finger transcription factor, induces an epithelial-mesenchymal transition (EMT) in various cancer and epithelial cells. We investigated the function of Snail (SNAI1) by downregulating its expression with short interfering RNA (siRNA). Suppression of Snail expression induced cellular senescence in several cancer cells and in normal fibroblast IMR90 cells. Cancer progression is facilitated by fibroblasts, so-called fibroblast-led cancer cell invasion. Snail-silenced cancer cells exhibited reduced motility, which was further decreased by cocultivation with Snail-silenced IMR90 cells. Our data suggest that cell motility and cellular senescence, which are regulated by Snail in cancer cells and fibroblasts, modulate fibroblast-led cancer cell invasion. Therefore, we propose that local suppression of Snail in cancer and the cancer microenvironment represents a potent therapeutic strategy.

肝星状细胞衰老与增殖、凋亡的关系

肝星状细胞(hepatic stellate cells, HSCs)活化、细胞外基质大量形成是肝纤维化发生发展的关键环节. 许多研究发现, 抑制HSCs增殖、促进HSCs凋亡可阻断肝纤维化进程; 同时研究发现, 促进活化HSCs衰老也可为肝纤维化的防治提供新的策略. 本文就HSCs衰老与增殖、凋亡的关系以及在肝纤维化中的作用相关研究进展予以综述.