Phenotypic alterations in senescent large-vessel and microvascular endothelial cells

Stretch-induced damage in endothelial monolayers

Endothelial cells are constantly exposed to mechanical stimuli, of which mechanical stretch has shown various beneficial or deleterious effects depending on whether loads are within physiological or pathological levels, respectively. Vascular properties change with age, and on a cell-scale, senescence elicits changes in endothelial cell mechanical properties that together can impair its response to stretch. Here, high-rate uniaxial stretch experiments were performed to quantify and compare the stretch-induced damage of monolayers consisting of young, senescent, and aged endothelial populations. The aged and senescent phenotypes were more fragile to stretch-induced damage. Prominent damage was detected by immunofluorescence and scanning electron microscopy as intercellular and intracellular void formation. Damage increased proportionally to the applied level of deformation and, for the aged and senescent phenotype, induced significant detachment of cells at lower levels of stretch compared to the young counterpart. Based on the phenotypic difference in cell-substrate adhesion of senescent cells indicating more mature focal adhesions, a discrete network model of endothelial cells being stretched was developed. The model showed that the more affine deformation of senescent cells increased their intracellular energy, thus enhancing the tendency for cellular damage and impending detachment. Next to quantifying for the first-time critical levels of endothelial stretch, the present results indicate that young cells are more resilient to deformation and that the fragility of senescent cells may be associated with their stronger adhesion to the substrate.

Vascular Endothelial Growth Factor Receptor-1 Modulates Hypoxia-Mediated Endothelial Senescence and Cellular Membrane Stiffness via YAP-1 Pathways

Hypoxia-induced endothelial cell (EC) dysfunction has been implicated as potential initiators of different pathogenesis, including Alzheimer’s disease and vascular dementia. However, in-depth structural, mechanical, and molecular mechanisms leading to EC dysfunction and pathology need to be revealed. Here, we show that ECs exposed to hypoxic conditions readily enter a senescence phenotype. As expected, hypoxia upregulated the expression of vascular endothelial growth factor (VEGFs) and its receptors (VEGFRs) in the ECs. Interestingly, Knockdown of VEGFR-1 expression prior to hypoxia exposure prevented EC senescence, suggesting an important role of VEGFR-1 expression in the induction of EC senescence. Using atomic force microscopy, we showed that senescent ECs had a flattened cell morphology, decreased membrane ruffling, and increased membrane stiffness, demonstrating unique morphological and nanomechanical signatures. Furthermore, we show that hypoxia inhibited the Hippo pathway Yes-associated protein (YAP-1) expression and knockdown of YAP-1 induced senescence in the ECs, supporting a key role of YAP-1 expression in the induction of EC senescence. And importantly, VEGFR-1 Knockdown in the ECs modulated YAP-1 expression, suggesting a novel VEGFR-1-YAP-1 axis in the induction of hypoxia-mediated EC senescence. In conclusion, VEGFR-1 is overexpressed in ECs undergoing hypoxia-mediated senescence, and the knockdown of VEGFR-1 restores cellular structural and nanomechanical integrity by recovering YAP-1 expression.

Epigenetics and Vascular Senescence-Potential New Therapeutic Targets?

Epigenetics is defined as the heritable alterations of gene expression without changes to the coding sequence of DNA. These alterations are mediated by processes including DNA methylation, histone modifications, and non-coding RNAs mechanisms. Vascular aging consists of both structural and functional changes in the vasculature including pathological processes that drive progression such as vascular cell senescence, inflammation, oxidation stress, and calcification. As humans age, these pathological conditions gradually accumulate, driven by epigenetic alterations, and are linked to various aging-related diseases. The development of drugs targeting a spectrum of epigenetic processes therefore offers novel treatment strategies for the targeting of age-related diseases. In our previous studies, we identified HDAC4, JMJD3, Fra-1, and GATA4 as potential pharmacological targets for regulating vascular inflammation, injury, and senescence.

Novel Contributors and Mechanisms of Cellular Senescence in Hypertension-Associated Premature Vascular Aging

Hypertension has been described as a condition of premature vascular aging, relative to actual chronological age. In fact, many factors that contribute to the deterioration of vascular function as we age are accelerated in hypertension. Nonetheless, the precise mechanisms that underlie the aged phenotype of arteries from hypertensive patients and animals remain elusive. Cellular senescence is an age-related physiologic process in which cells undergo irreversible growth arrest. Although controlled senescence negatively regulates cell proliferation and promotes tissue regeneration, uncontrolled senescence can contribute to disease pathogenesis by presenting the senescence-associated secretory phenotype, in which molecules such as proinflammatory cytokines, matrix metalloproteases, and reactive oxygen species are released into tissue microenvironments. This review will address and critically evaluate the current literature on the role of cellular senescence in hypertension, with particular emphasis on cells types that mediate and modulate vascular function and structure.

Age-Related Impairment of Structure and Function of Iliac Artery Endothelium in Rats Is Improved by Elevated Fluid Shear Stress

Background

Aging plays an important role in endothelial dysfunction. Fluid shear stress (FSS) can activate endothelial cells (ECs). Herein, we tested the hypothesis that this endothelial impairment could be improved by elevated FSS (EFSS) in aged rats.

Material/Methods

EFSS was created through ligation of the unilateral common iliac artery in 20-−month-old rats, evaluated by measuring blood flow velocity with Doppler spectrum. The effect of FSS on aged ECs was examined by senescence-associated β-galactosidase (SA-β-Gal) staining, ultrastructural observation, and immunostaining and qPCR analysis of eNOS and SIRT1 expression on both the mRNA and protein levels.

Results

(1) FSS was significantly increased in the right common iliac artery (RCIA) in rats with the ligation of the left common iliac artery (LCIA). (2) SA-β-Gal staining was significantly attenuated by EFSS in the RCIA of aged rats. (3) Ultrastructural observation showed that ECs in the RCIA of normal aged rats became irregular and enlarged, with increasingly polypoid nuclei and fewer mitochondria, whereas ECs in the RCIA of aged rats with LCIA ligation became more prominent and contained more mitochondria. (4) eNOS and SIRT1 expression in the RCIA of aged rats with LCIA ligation was significantly upregulated compared with that in control group rats.

Conclusions

The present study for the first time shows that EFSS has the ability to improve age-related impairment of endothelial structure and functions.

Induced pluripotent stem cell-conditional medium inhibits H9C2 cardiomyocytes apoptosis via autophagy flux and Wnt/β-catenin pathway

Induced pluripotent stem cell‐derived conditioned medium (iPS‐CM) could improve cell viability in many types of cells and may be a better alternative for the treatment of myocardial infarction. This study aimed to examine the influence of iPS‐CM on anti‐apoptosis and the proliferation of H9C2 cardiomyocytes and investigate the underlying mechanisms. H9C2 cardiomyocytes were exposed to 200 μmol/L hydrogen peroxide (H₂O₂) for 24 hours with or without pre‐treatment with iPS‐CM. The ratio of apoptotic cells, the loss of mitochondrial membrane potential (△Ψm) and the levels of intracellular reactive oxygen species were analysed by flow cytometric analysis. The expression levels of BCL‐2 and BAX proteins were analysed by Western blot. Cell proliferation was assessed using cell cycle and EdU staining assays. To study cell senescence, senescence‐associated β‐galactosidase (SA‐β‐gal) staining was conducted. The levels of malondialdehyde, superoxide dismutase and glutathione were also quantified using commercially available enzymatic kits. The results showed that iPS‐CM containing basic fibroblast growth factor significantly reduced H₂O₂‐induced H9C2 cardiomyocyte apoptosis by activating the autophagy flux pathway, promoted cardiomyocyte proliferation by up‐regulating the Wnt/β‐catenin pathway and inhibited oxidative stress and cell senescence. In conclusion, iPS‐CM effectively enhanced the cell viability of H9C2 cardiomyocytes and could potentially be used to inhibit cardiomyocytes apoptosis to treat myocardial infarction in the future.

Kallistatin attenuates endothelial senescence by modulating Let-7g-mediated miR-34a-SIRT1-eNOS pathway

Kallistatin, a plasma protein, protects against vascular and organ injury. This study is aimed to investigate the role and mechanism of kallistatin in endothelial senescence. Kallistatin inhibited H2 O2 -induced senescence in human endothelial cells, as indicated by reduced senescence-associated-β-galactosidase activity, p16INK4a and plasminogen activator inhibitor-1 expression, and elevated telomerase activity. Kallistatin blocked H2 O2 -induced superoxide formation, NADPH oxidase levels and VCAM-1, ICAM-1, IL-6 and miR-34a synthesis. Kallistatin reversed H2 O2 -mediated inhibition of endothelial nitric oxide synthase (eNOS), SIRT1, catalase and superoxide dismutase (SOD)-2 expression, and kallistatin alone stimulated the synthesis of these antioxidant enzymes. Moreover, kallistatin's anti-senescence and anti-oxidant effects were attributed to SIRT1-mediated eNOS pathway. Kallistatin, via interaction with tyrosine kinase, up-regulated Let-7g, whereas Let-7g inhibitor abolished kallistatin's effects on miR-34a and SIRT1/eNOS synthesis, leading to inhibition of senescence, oxidative stress and inflammation. Furthermore, lung endothelial cells isolated from endothelium-specific kallistatin knockout mice displayed marked reduction in mouse kallistatin levels. Kallistatin deficiency in mouse endothelial cells exacerbated senescence, oxidative stress and inflammation compared to wild-type mouse endothelial cells, and H2 O2 treatment further magnified these effects. Kallistatin deficiency caused marked reduction in Let-7g, SIRT1, eNOS, catalase and SOD-1 mRNA levels, and elevated miR-34a synthesis in mouse endothelial cells. These findings indicate that endogenous kallistatin through novel mechanisms protects against endothelial senescence by modulating Let-7g-mediated miR-34a-SIRT1-eNOS pathway.

Age-related Impairment of Vascular Structure and Functions

Among age-related diseases, cardiovascular and cerebrovascular diseases are major causes of death. Vascular dysfunction is a key characteristic of these diseases wherein age is an independent and essential risk factor. The present work will review morphological alterations of aging vessels in-depth, which includes the discussion of age-related microvessel loss and changes to vasculature involving the capillary basement membrane, intima, media, and adventitia as well as the accompanying vascular dysfunctions arising from these alterations.

Endovascular biopsy: Strategy for analyzing gene expression profiles of individual endothelial cells obtained from human vessels✩

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The combination of guide wire sampling, FACS and high throughput microfluidic single-cell quantitative RT-PCR, is an effective strategy for analyzing molecular changes of ECs in vascular lesions.

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Although heterogeneous, the ECs in normal iliac artery fall into two classes.

Purpose

To develop a strategy of achieving targeted collection of endothelial cells (ECs) by endovascular methods and analyzing the gene expression profiles of collected single ECs.

Methods and results

134 ECs and 37 leukocytes were collected from four patients' intra-iliac artery endovascular guide wires by fluorescence activated cell sorting (FACS) and analyzed by single-cell quantitative RT-PCR for expression profile of 48 genes. Compared to CD45⁺ leukocytes, the ECs expressed higher levels (p < 0.05) of EC surface markers used on FACS and other EC related genes. The gene expression profile showed that these isolated ECs fell into two clusters, A and B, that differentially expressed 19 genes related to angiogenesis, inflammation and extracellular matrix remodeling, with cluster B ECs have demonstrating similarities to senescent or aging ECs.

Conclusion

Combination of endovascular device sampling, FACS and single-cell quantitative RT-PCR is a feasible method for analyzing EC gene expression profile in vascular lesions.

Central Arterial Aging and Angiotensin II Signaling

Arterial remodeling over time is a cornerstone of normal systemic aging. The age-associated arterial structural and functional changes in the intima, the media, and the adventitia are closely linked to angiotensin II (Ang II) signaling. A growing line of evidence indicates that essential elements of Ang II signaling, which encompasses milk fat globule epidermal growth factor-8, calpain-1, transforming growth factor-β1, matrix metalloproteinase-2/9, monocyte chemoattractant protein-1, nicotinamide adenine dinucleotide phosphate-oxidase, and reactive oxygen species, are upregulated within the central arterial wall in rats, nonhuman primates, and humans during aging. In vitro studies show that the elevation of Ang II signaling induces the accumulation of collagen and advanced glycated end-products, the degradation of elastin, and the increased cell cycle disorder, invasion, and hypertrophy of endothelial and vascular smooth muscle cells. Further, in vivo studies demonstrate that increased Ang II signaling accelerates arterial aging. Conversely, attenuating Ang II signaling via an inhibition of angiotensin conversing enzyme or a blockade of AT1 activation retards age-associated arterial remodeling. This review attempts to integrate complex facts of Ang II signaling within the aged central arterial wall and may shed light on new therapeutic targets for arterial aging.

Cellular senescence, ageing and disease

Cellular senescence is the irreversible growth arrest of individual mitotic cells, which as a consequence display a radically altered phenotype that is thought to impair tissue function and predispose tissues to disease development and/or progression as they gradually accumulate. However, in the past, research into mechanisms of ageing has commonly been researched and treated separately from disease development. This may partly be due to the lack of understanding concerning mechanisms of ageing and the difficulty in implementing what was known into models of disease development. Only in the last 10 years, with increasing knowledge of the senescent phenotype and the ability to detect senescent cells in human tissues, have biologists been able to investigate the relationship between cellular senescence and disease. This review therefore brings together and discusses recent findings which suggest that cellular senescence does contribute to ageing and the development/progression of disease.

Cellular senescence: molecular mechanisms, in vivo significance, and redox considerations

Cellular senescence is recognized as a critical cellular response to prolonged rounds of replication and environmental stresses. Its defining characteristics are arrested cell-cycle progression and the development of aberrant gene expression with proinflammatory behavior. Whereas the mechanistic events associated with senescence are generally well understood at the molecular level, the impact of senescence in vivo remains to be fully determined. In addition to the role of senescence as an antitumor mechanism, this review examines cellular senescence as a factor in organismal aging and age-related diseases, with particular emphasis on aberrant gene expression and abnormal paracrine signaling. Senescence as an emerging factor in tissue remodeling, wound repair, and infection is considered. In addition, the role of oxidative stress as a major mediator of senescence and the role of NAD(P)H oxidases and changes to intracellular GSH/GSSG status are reviewed. Recent findings indicate that senescence and the behavior of senescent cells are amenable to therapeutic intervention. As the in vivo significance of senescence becomes clearer, the challenge will be to modulate the adverse effects of senescence without increasing the risks of other diseases, such as cancer. The uncoupled relation between cell-cycle arrest and the senescent phenotype suggests that this is an achievable outcome.

Donor age intensifies the early immune response after transplantation

Increasing donor age is associated with reduced graft function. We wondered if donor age may not only affect intrinsic function but also alter the immune response of the recipient. Kidneys from young and old F-344 rats (3 vs 18 months) were transplanted into bilaterally nephrectomized young Lewis recipients and compared with age-matched controls (follow-up: 6 months). Renal function and structural changes were assessed serially in both native kidneys and allografts. Host alloreactivity, graft-infiltrating cells, and their inflammatory products were determined at intervals to examine the correlation of immune response and donor age. Functional and structural deterioration had advanced significantly in older allografts compared with age-matched native controls, whereas differences between young allografts and native controls of similar age were only minor. Changes in grafts from elderly rats were associated with a more intense host immune response early post-transplant (up to 1 month) reflected by significantly higher numbers of peripheral T and B cells, increased T-cell alloreactivity and modified cytokine patterns associated with elevated frequencies of intragraft dendritic cells, B cells, and CD31+ cells. By 6 months, recipients of young donor grafts produced comparable or more intense alloantigen-specific immune responses. Older donor grafts elicit a stronger immune response in the early period after transplantation.

Protein expression dynamics during replicative senescence of endothelial cells studied by 2-D difference in-gel electrophoresis

Endothelial senescence contributes to endothelium dysfunctionality and is thereby linked to vascular aging. A dynamic proteomic study on human umbilical vein endothelial cells, isolated from three umbilical cords, was performed. The cells were cultured towards replicative senescence and whole cell lysates were subjected to 2-D difference gel electrophoresis (DIGE). Despite the biological variability of the three independent isolations, a set of proteins was found that showed senescence-dependent expression patterns in all isolations. We focused on those proteins that showed significant changes, with a paired analysis of variance (RM-ANOVA) p-value of < or =0.05. Thirty-five proteins were identified with LC-Fourier transform MS, and functional annotation revealed that endothelial replicative senescence is accompanied by increased cellular stress, protein biosynthesis and reduction in DNA repair and maintenance. Nuclear integrity becomes affected and cytoskeletal structure is also changed. Such important changes in the cell infrastructure might accelerate endothelium dysfunctionality. This study provides biological information that will initiate studies to further unravel endothelial senescence and gain more knowledge about the consequences of this process in the in vivo situation.

Contribution of p16INK4aand p21CIP1pathways to induction of premature senescence of human endothelial cells: permissive role of p53

We have previously found that nonenzymatically glycated collagen I (GC), mimicking diabetic microenvironment, can induce senescent phenotype in early passage human umbilical vein endothelial cells (HUVECs). In the present study, we explored the functional involvement of cell cycle checkpoint pathways in initiating GC-induced premature endothelial cell senescence. When compared with native collagen, early passage HUVECs showed increased p53, p21CIP1(p21), and p16INK4a(p16) mRNA expression after exposure to GC. Twenty-four hours after transfection of p16, p21, and p53-enhanced green fluorescent protein (EGFP) recombinant plasmids, HUVECs entered G1-phase cell cycle arrest. By days 3 and 5, HUVECs transfected with p16-EGFP showed an increased proportion of senescent cells, and this increase was more prominent in the GFP-positive cell population, which exhibited 68% of senescent cells. Transfection of p21 also induced senescence but only by day 5. Cotransfection of p16 and p21 showed no additive effect. Transfection of p21 or p53 induced apoptosis in HUVECs. Next, we suppressed endogenous p53, p21, p16, or retinoblastoma (Rb) gene expression through small interference RNA strategy and investigated their influence in p16- and p21-initiated endothelial cell senescence. Analysis indicated that suppression of p53 expression can abolish senescence induced by p16 overexpression. Paradoxically, this effect was not observed when p21 was suppressed. On the other hand, suppression of Rb eliminated senescence initiated by either p16 or p21 overexpression. In summary, the p53/p21 pathway is mainly responsible for GC-induced apoptosis, but the coordinated activation of the p53/p21 and p16 pathway is responsible for GC-induced endothelial cell senescence through a Rb-dependent mechanism.

Differential expression of EDF-1 and endothelial nitric oxide synthase by proliferating, quiescent and senescent microvascular endothelial cells

Endothelial Differentiation-related Factor (EDF)-1 is a low molecular weight polypeptide downregulated in endothelial cells exposed to HIV-1-Tat or the phorbol ester TPA. EDF-1 acts in the cytosol as a calmodulin binding protein, and in the nucleus as a transcriptional coactivator. Here, we show that EDF-1 is downregulated in non-proliferating microvascular endothelial cells. Indeed, both quiescence and senescence reduce the levels of EDF-1 and this is due to protein degradation through the proteasome. We also describe a different subcellular localization of EDF-1 which is mainly nuclear in senescent 1G11 cells. Since (i) endothelial nitric oxide (NO) seems to play a role in endothelial proliferation and (ii) NO is an important mediator involved in the control of vascular tone, inflammatory responses and angiogenesis, it is noteworthy that senescence downregulates the expression and the activity of endothelial nitric oxide synthase (eNOS) in microvascular endothelial cells. On the contrary, quiescence does not affect NOS expression and activity. The modulation of EDF-1 in microvascular endothelial cells might offer new insights into the molecular events involved in angiogenesis and in microvascular dysfunctions in the elderly.

Molecular mechanisms of replicative senescence in endothelial cells

As human somatic cells age, they stop replicating and enter an irreversible state of growth arrest known as replicative senescence. Senescent cells are viable, metabolically active, and display altered gene and protein expression compared to proliferating cells. Endothelial cells, both in vitro and in vivo, are known to undergo senescence. As endothelial cells are a critical component of the vasculature, senescence of these cells can have a significant impact of vascular integrity, function, and overall homeostasis. This review will summarize recent work to understand the molecular mechanisms of endothelial cell senescence and the resulting alterations in gene/protein expression in these cells. Endothelial cell senescence will then be discussed in the context of disease development with a focus on atherosclerosis, an important age-associated disease of the vasculature.

Correlation of age with in vivo expression of endothelial markers

The importance of endothelial senescence as a pathogenetic factor in age-related vascular alterations has almost exclusively been studied in vitro. However, the in vitro-findings have rarely been compared with histomorphological changes in aging human tissue or in age-related degenerative diseases. Therefore, we compared the expression of the endothelial marker CD34 and the procoagulant protein von Willebrand factor (vWf) in lung endothelium by conventional immunohistochemistry and confocal laser scan microscopy (CLSM), taking the age of the patients into consideration. The staining reactions were statistically analysed by covariance analysis. With age the endothelial staining intensity of CD34 increased in arteries and veins, but decreased in arterioles, capillaries and venules. For vWf, on the contrary, the endothelial staining intensity increased with age in all types of vessels. CLSM confirmed a mosaic staining pattern. This study demonstrates age-associated phenotypical alterations of CD34 and vWf. Whether the down-regulation of CD34 correlates with an age-associated reduction of the angiogenetic properties of EC or an age-related over-expression of vWf as a relevant cofactor for the raised coagulatory activity and the increase in thrombotic diseases resp coronary heart disease in older patients, remains subject to debate.

Enhanced apoptosis in prolonged cultures of senescent porcine pulmonary artery endothelial cells

Senescent or aged endothelial cells in culture remain metabolically active after cessation of division, and are generally believed to eventually die. However, mechanisms underlying the terminal aging of cultured cells, i.e. from senescence to death, are poorly understood. Here, we report that culturing of replicative senescent endothelial cells for a prolonged period of time without passaging leads to enhanced programmed cell death or apoptosis. Senescent (passage 45) and young (passage 3) porcine pulmonary artery endothelial cells (PAEC) were cultured for 0-42 days post confluence. The cells attached to culture dishes and floating in medium were collected at 0, 7, 14, 21, 28, 35 and 42 days post confluence and were assessed for markers of apoptosis. Morphology studies showed that ratios between senescent and young cells attached to dishes declined to 45% after 42 days postconfluence. Apoptotic cells in prolonged cultures of senescent PAEC increased from 5 to 35% as determined by protein mass, DNA breakage, and caspase-3 activation. Steady state levels of Bcl-2, an anti-apoptotic protein, in senescent prolonged cultures decreased to less than 20% for all time points compared with young cells. Relative levels of Bad, a pro-apoptotic protein, in senescent cells were elevated from 60 to 130% during prolonged culturing. These results indicate that terminal cellular aging enhances apoptosis and the levels of Bcl-2/Bad may be associated with the apoptotic process in porcine lung endothelial cells.