Differential Regulation of Telomerase in Endothelial Cells by Fibroblast Growth Factor-2 and Vascular Endothelial Growth Factor-A: Association with Replicative Life Span

Cellular Mechanisms of FGF-Stimulated Tissue Repair

Growth factors belonging to the FGF family play important roles in tissue and organ repair after trauma. In this review, I discuss the regulation by FGFs of the aspects of cellular behavior important for reparative processes. In particular, I focus on the FGF-dependent regulation of cell proliferation, cell stemness, de-differentiation, inflammation, angiogenesis, cell senescence, cell death, and the production of proteases. In addition, I review the available literature on the enhancement of FGF expression and secretion in damaged tissues resulting in the increased FGF supply required for tissue repair.

Telomerase and telomeres in aging theory and chronographic aging theory (Review)

The current review focuses on the connection of telomerase and telomeres with aging. In this review, we describe the changes in telomerase and telomere length (TEL) during development, their role in carcinogenesis processes, and the consequences of reduced telomerase activity. More specifically, the connection of TEL in peripheral blood cells with the development of aging‑associated diseases is discussed. The review provides systematic data on the role of telomerase in mitochondria, the biology of telomeres in stem cells, as well as the consequences of the forced expression of telomerase (telomerization) in human cells. Additionally, it presents the effects of chronic stress exposure on telomerase activity, the effect of TEL on fertility, and the effect of nutraceutical supplements on TEL. Finally, a comparative review of the chronographic theory of aging, presented by Olovnikov is provided based on currently available scientific research on telomere, telomerase activity, and the nature of aging by multicellular organisms.

Telomere Biology and Thoracic Aortic Aneurysm

Ascending aortic aneurysms are mostly asymptomatic and present a great risk of aortic dissection or perforation. Consequently, ascending aortic aneurysms are a source of lethality with increased age. Biological aging results in progressive attrition of telomeres, which are the repetitive DNA sequences at the end of chromosomes. These telomeres play an important role in protection of genomic DNA from end-to-end fusions. Telomere maintenance and telomere attrition-associated senescence of endothelial and smooth muscle cells have been indicated to be part of the pathogenesis of degenerative vascular diseases. This systematic review provides an overview of telomeres, telomere-associated proteins and telomerase to the formation and progression of aneurysms of the thoracic ascending aorta. A better understanding of telomere regulation in the vascular pathology might provide new therapeutic approaches. Measurements of telomere length and telomerase activity could be potential prognostic biomarkers for increased risk of death in elderly patients suffering from an aortic aneurysm.

Molecular pathways governing development of vascular endothelial cells from ES/iPS cells

Assembly of complex vascular networks occurs in numerous biological systems through morphogenetic processes such as vasculogenesis, angiogenesis and vascular remodeling. Pluripotent stem cells such as embryonic stem (ES) and induced pluripotent stem (iPS) cells can differentiate into any cell type, including endothelial cells (ECs), and have been extensively used as in vitro models to analyze molecular mechanisms underlying EC generation and differentiation. The emergence of these promising new approaches suggests that ECs could be used in clinical therapy. Much evidence suggests that ES/iPS cell differentiation into ECs in vitro mimics the in vivo vascular morphogenic process. Through sequential steps of maturation, ECs derived from ES/iPS cells can be further differentiated into arterial, venous, capillary and lymphatic ECs, as well as smooth muscle cells. Here, we review EC development from ES/iPS cells with special attention to molecular pathways functioning in EC specification.

Transplantation of mesenchymal cells rejuvenated by the overexpression of telomerase and myocardin promotes revascularization and tissue repair in a murine model of hindlimb ischemia

RATIONALE

The number and function of stem cells decline with aging, reducing the ability of stem cells to contribute to endogenous repair processes. The repair capacity of stem cells in older individuals may be improved by genetically reprogramming the stem cells to exhibit delayed senescence and enhanced regenerative properties.

OBJECTIVE

We examined whether the overexpression of myocardin (MYOCD) and telomerase reverse transcriptase (TERT) enhanced the survival, growth, and myogenic differentiation of mesenchymal stromal cells (MSCs) isolated from adipose or bone marrow tissues of aged mice. We also examined the therapeutic efficacy of transplanted MSCs overexpressing MYOCD and TERT in a murine model of hindlimb ischemia.

METHODS AND RESULTS

MSCs from adipose or bone marrow tissues of young (1 month old) and aged (12 months old) male C57BL/6 and apolipoprotein E-null mice were transiently transduced with lentiviral vectors encoding TERT, MYOCD, or both TERT and MYOCD. Flow cytometry and bromodeoxyuridine cell proliferation assays showed that transduction with TERT and, to a lesser extent, MYOCD, increased MSC viability and proliferation. In colony-forming assays, MSCs overexpressing TERT and MYOCD were more clonogenic than mock-transduced MSCs. Fas-induced apoptosis was inhibited in MSCs overexpressing MYOCD or TERT. When compared with aged mock-transduced MSCs, aged MSCs overexpressing TERT, MYOCD, or both TERT and MYOCD increased myogenic marker expression, blood flow, and arteriogenesis when transplanted into the ischemic hindlimbs of apolipoprotein E-null mice.

CONCLUSIONS

The delivery of the TERT and MYOCD genes into MSCs may have therapeutic applications for restoring, or rejuvenating, aged MSCs from adipose and bone marrow tissues.

Effects of PEMF on microcirculation and angiogenesis in a model of acute hindlimb ischemia in diabetic rats

Hindlimb ischemia is a major complication of diabetic patients due to poor neovascularization. Therapy with pulsed electromagnetic fields (PEMF) can promote angiogenesis in ischemic lesions. However, the efficacy and therapeutic mechanisms of PEMF in diabetes-related hindlimb ischemia are unclear. Sprague-Dawley rats were injected with streptozocin to induce diabetes, and 10 weeks later diabetic rats were subjected to surgical induction of acute hindlimb ischemia. The rats were randomized and treated with PEMF, and the blood perfusion of individual rats was determined longitudinally by laser Doppler perfusion imaging (LDPI). The neovascular density was examined using immunofluorescent analysis of CD31 expression and alkaline phosphatase (AP) staining. The levels of VEGF, VEGFR, FGF-2, and FGFR1 expression, and ERK 1/2 and P38 phosphorylation in the muscles were characterized using enzyme-linked immunosorbent assay (ELISA) and Western blot assays. The values of LDPI in the PEMF-treated rats at 14 and 28 days post surgery were significantly greater than those in the controls, accompanied by significantly elevated levels of anti-CD31 and AP staining. The relative levels of FGF-2 and FGFR1, but not VEGF and VEGFR expression, and ERK1/2, but not P38 phosphorylation, in the muscles of the PEMF-treated rats were significantly higher than those in the controls. Our data indicated that PEMF enhanced acute hindlimb ischemia-related perfusion and angiogenesis, associated with up-regulating FGF-2 expression and activating the ERK1/2 pathway in diabetic rats. Therefore, PEMF may be valuable for the treatment of diabetic patients with ischemic injury.

Roles of FGF signaling in stem cell self-renewal, senescence and aging

The aging process decreases tissue function and regenerative capacity, which has been associated with cellular senescence and a decline in adult or somatic stem cell numbers and self-renewal within multiple tissues. The potential therapeutic application of stem cells to reduce the burden of aging and stimulate tissue regeneration after trauma is very promising. Much research is currently ongoing to identify the factors and molecular mediators of stem cell self-renewal to reach these goals. Over the last two decades, fibroblast growth factors (FGFs) and their receptors (FGFRs) have stood up as major players in both embryonic development and tissue repair. Moreover, many studies point to somatic stem cells as major targets of FGF signaling in both tissue homeostasis and repair. FGFs appear to promote self-renewing proliferation and inhibit cellular senescence in nearly all tissues tested to date. Here we review the role of FGFs and FGFRs in stem cell self-renewal, cellular senescence, and aging.

Dedifferentiation of human terminally differentiating keratinocytes into their precursor cells induced by basic fibroblast growth factor

Reprogramming differentiated cells toward stem cells may have long-term applications in stem-cell research and regenerative medicine. Here we report on the dedifferentiation of human epidermal keratinocytes into their precursor cells in vitro with basic fibroblast growth factor (bFGF) but not external gene intervention. After incubation of human terminally differentiating keratinocytes, some of the surviving keratinocytes reverted from a differentiated to a dedifferentiated state, as evidenced by re-expression of biological markers of native keratinocyte stem cells (nKSCs), including β(1)-integrin, CK19 and CK14. Moreover, these dedifferentiation-derived KSCs (dKSCs) showed an ability for high colony formation correlated with cell cycle analysis showing a marked accumulation in S phases, acquired a similar regional distribution of both α(6)-integrin and CD71 expression at the ultrastructural level, and had a increased proliferative capacity by releasing telomerase from nucleolar sites to nucleoplasmic distribution. However, on comparing dKSCs with nKSCs, 2 points seem noteworthy: (1) the proportion of transit amplifying cells in dKSCs treated with bFGF is much higher than that in nKSCs and (2) regional differences exist in the subcellular localization of telomerase in nKSCs and dKSCs. Most nKSCs showed a prominent nucleolar concentration of human telomerase reverse transcriptase expression, whereas most dKSCs showed a more diffuse intranuclear distribution of telomerase or even signal depletion at nucleoli relative to the general nucleoplasm. These results indicate that bFGF could induce the terminally differentiating epidermal keratinocytes to convert into their precursor cells, which offers a new approach for generating residual healthy stem cells for wound repair and regeneration.

Expression profile and regulation of telomerase reverse transcriptase on oxygen-induced retinal neovascularization

PURPOSE

Telomerase is critical for the control of replicative capacity, which plays a major role in proliferative retinal neovascularization. In this study, we investigated the expression profiles of telomerase reverse transcriptase (Tert) in a mouse model of oxygen-induced retinal neovascularization and explored the possibility of inhibiting a retinal Tert expression with small interfering RNAs (SiRNA) as a novel potential approach to suppress proliferative retinopathy.

METHODS

The mouse oxygen-induced retinal neovascularization model was used to examine expression profiles in different developmental phases and to assess the anti-angiogenic activity of Tert-SiRNA. Recombinant SiRNA plasmids were injected intravitreously into mice with or without pathological retinal neovascularization. Fluorescein angiography, vessel counting, and the expression levels of Tert mRNA and protein were used to evaluate the anti-angiogenic effects.

RESULTS

Retinal Tert expression, as assessed by both mRNA and protein levels, was significantly up-regulated during the proliferative phase of oxygen-induced retinal neovascularization. Intravitreous injection of Tert-SiRNA effectively suppressed the expression of Tert mRNA and proteins and inhibited retinal neovascularization, as confirmed by retinal flat angiography and vessel counting.

CONCLUSIONS

The expression of Tert was up-regulated during the development of oxygen-induced retinal neovascularization. Inhibiting Tert expression with SiRNA is effective in suppressing retinal neovascularization, suggesting that telomerase may be a potential therapeutic target for treating proliferative retinopathy.

Adipose tissue endothelial cells from obese human subjects: differences among depots in angiogenic, metabolic, and inflammatory gene expression and cellular senescence

OBJECTIVE

Regional differences among adipose depots in capacities for fatty acid storage, susceptibility to hypoxia, and inflammation likely contribute to complications of obesity. We defined the properties of endothelial cells (EC) isolated from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) biopsied in parallel from obese subjects.

RESEARCH DESIGN AND METHODS

The architecture and properties of the fat tissue capillary network were analyzed using immunohistochemistry and flow cytometry. CD34(+)/CD31(+) EC were isolated by immunoselection/depletion. Expression of chemokines, adhesion molecules, angiogenic factor receptors, as well as lipogenic and senescence-related genes were assayed by real-time PCR. Fat cell size and expression of hypoxia-dependent genes were determined in adipocytes from both fat depots.

RESULTS

Hypoxia-related genes were more highly expressed in VAT than SAT adipocytes. VAT adipocytes were smaller than SAT adipocytes. Vascular density and EC abundance were higher in VAT. VAT-EC exhibited a marked angiogenic and inflammatory state with decreased expression of metabolism-related genes, including endothelial lipase, GPIHBP1, and PPAR gamma. VAT-EC had enhanced expression of the cellular senescence markers, IGFBP3 and γ-H2AX, and decreased expression of SIRT1. Exposure to VAT adipocytes caused more EC senescence-associated β-galactosidase activity than SAT adipocytes, an effect reduced in the presence of vascular endothelial growth factor A (VEGFA) neutralizing antibodies.

CONCLUSIONS

VAT-EC exhibit a more marked angiogenic and proinflammatory state than SAT-EC. This phenotype may be related to premature EC senescence. VAT-EC may contribute to hypoxia and inflammation in VAT.

Regulation of telomerase activity by apparently opposing elements

Telomeres, the ends of chromosomes, undergo frequent remodeling events that are important in cell development, proliferation and differentiation, and neoplastic immortalization. It is not known how the cellular environment influences telomere remodeling, stability, and lengthening or shortening. Telomerase is a ribonucleoprotein complex that maintains and lengthens telomeres in the majority of cancers. Recent studies indicate that a number of factors, including hormones, cytokines, ligands of nuclear receptor, vitamins and herbal extracts have significantly influence telomerase activity and, in some instances, the remodeling of telomeres. This review summarizes the advances in understanding of the positive and negative regulation by extracellular factors of telomerase activity in cancer, stem cells and other systems in mammals.

Endothelial dysfunction and aging: an update

Aging is an important risk factor for the development of many cardiovascular diseases as atherosclerosis and hypertension with a common underlying circumstance: the progressive decline of endothelial function. Vascular endothelial dysfunction occurs during the human aging process and is accompanied by deterioration in the balance between vasodilator and vasoconstriction substances produced by the endothelium. This imbalance is mainly characterized by a progressive reduction of the bioavailability of nitric oxide (NO) and an increase in the production of cyclooxygenase (COX)-derived vasoconstrictor factors. Both circumstances are in turn related to an increased production of reactive oxygen and nitrogen species. The aim of this review is to describe the pathophysiological mechanisms involved in the endothelial function declination that accompanies the multifactorial aging process, including alterations related to oxidative stress and pro-inflammatory cytokines, senescence of endothelial cells and genetic factors.

Vascular endothelial senescence: from mechanisms to pathophysiology

Most mitotically competent mammalian cell types can react to stress by undergoing a phenotypically distinctive and permanent form of growth arrest called "cellular senescence." This response has been extensively characterized in cell culture and more recently it has been found to occur also in vivo in a number of tissues. In this review I will present the case for the occurrence of senescence in the vascular endothelium. I will also discuss the mechanisms and factors that modulate endothelial cell replicative capacity and the onset of senescence. Finally, I will examine the senescent phenotype and its possible consequences for the development and progression of vascular diseases.

Mechanisms of endothelial senescence

When endothelial cells from different vascular beds are grown in culture they show a limited capacity to divide, eventually entering into a permanent and phenotypically distinctive non-dividing state referred to as 'replicative senescence'. Replicative senescence is thought to result from progressive shortening of telomeric DNA and consequent telomere dysfunction. More recently, it has been realised that senescence can also be induced by a variety of insults, including those causing intracellular oxidative stress. In this report, we review evidence for the occurrence of endothelial cell senescence in vivo. We will also examine the causes, mechanisms and regulation of this process as they emerge from our studies in cell culture, focusing in particular on the roles of oxidative stress, telomerase, growth factors and nitric oxide.

Hypertrophy and fibrosis in the cardiomyopathy of uremia--beyond coronary heart disease

Cardiac disease is the leading cause of death in uremic patients. In contrast to previous opinion, coronary events account for a relatively small proportion of cardiac deaths, the most common causes being sudden death and heart failure. Against this background the current text will discuss noncoronary cardiac pathology, specifically the pathogenesis and the morphological findings caused by (pathological) cardiac hypertrophy, cardiac interstitial fibrosis and microvascular disease.

Differentiation of endothelial cells derived from mouse embryoid bodies: a possible in vitro vasculogenesis model

Mouse embryonic stem cells (mES cells), which are pluripotent and self-renewal cells, are derived from the inner cell mass of mouse blastocysts. The objective of this study was to construct more efficient mES cell-derived embryoid bodies (EBs) for use as a vasculogenesis model and as an in vitro vascular toxicity testing model. EBs were formed for 3 days using hanging drop cultures and plated on gelatin-coated plates in endothelial growth medium-2 (EGM-2) to promote vascular development. The differentiation of mES cell-derived EBs was confirmed by reverse transcription-polymerase chain reaction (RT-PCR), immunocytochemistry, and flow cytometry within 7 days after plating EBs. The mRNA and protein expressions of vascular endothelial growth factor receptors-2 (FLK-1), platelet endothelial cell adhesion molecule (PECAM), and vascular endothelial-cadherin (VE-cadherin) were observed in differentiated mES cells. When placed in matrigel, mES cell-derived endothelial like cells formed networks similar to vascular structures. mES cells were also exposed to 5-fluorouracil (5-FU), a strong inhibitor of vessel formation, and its cytotoxicity was determined using MTT assays. The inhibitory concentrations (IC50) of 5-FU for mES cells and C166 cells were 0.72 microM and 1.04 microM, respectively. These results demonstrate that mES cells can be used to study vasculogenesis and for cytotoxicity screening.

Immortalization of neural precursors when telomerase is overexpressed in embryonal carcinomas and stem cells

The DNA repair enzyme telomerase maintains chromosome stability by ensuring that telomeres regenerate each time the cell divides, protecting chromosome ends. During onset of neuroectodermal differentiation in P19 embryonal carcinoma (EC) cells three independent techniques (Southern blotting, Q-FISH, and Q-PCR) revealed a catastrophic reduction in telomere length in nestin-expressing neuronal precursors even though telomerase activity remained high. Overexpressing telomerase protein (mTERT) prevented telomere collapse and the neuroepithelial precursors produced continued to divide, but deaggregated and died. Addition of FGF-2 prevented deaggregation, protected the precursors from the apoptotic event that normally accompanies onset of terminal neuronal differentiation, allowed them to evade senescence, and enabled completion of morphological differentiation. Similarly, primary embryonic stem (ES) cells overexpressing mTERT also initiated neuroectodermal differentiation efficiently, acquiring markers of neuronal precursors and mature neurons. ES precursors are normally cultured with FGF-2, and overexpression of mTERT alone was sufficient to allow them to evade senescence. However, when FGF-2 was removed in order for differentiation to be completed most neural precursors underwent apoptosis indicating that in ES cells mTERT is not sufficient allow terminal differentiation of ES neural precursors in vitro. The results demonstrate that telomerase can potentiate the transition between pluripotent stem cell and committed neuron in both EC and ES cells.

Electronegative LDL impairs vascular endothelial cell integrity in diabetes by disrupting fibroblast growth factor 2 (FGF2) autoregulation

OBJECTIVE

L5, a circulating electronegative LDL identified in patients with hypercholesterolemia or type 2 diabetes, induces endothelial cell (EC) apoptosis by suppressing fibroblast growth factor (FGF)2 expression. FGF2 plays a pivotal role in endothelial regeneration and compensatory arteriogenesis. It is likely that vasculopathy and poor collateralization in diabetes is a result of FGF2 dysregulation.

RESEARCH DESIGN AND METHODS

To investigate this mechanism, we isolated L5 from type 2 diabetic patients. In cultured bovine aortic ECs (BAECs), L5 inhibited FGF2 transcription and induced apoptosis. Because FGF2 stimulates the phosphatidylinositol 3-kinase (PI3K)-Akt pathway, we examined whether FGF2 transcription is regulated by Akt through a feedback mechanism.

RESULTS

Diabetic L5 reduced FGF2 release to the medium but enhanced caspase-3 activity, with resultant apoptosis. Inhibition of PI3K with wortmannin or suppression of Akt activation with dominant-negative Akt inhibited FGF2 expression. Transfection of BAECs with FGF2 antisense cDNA depleted endogenous FGF2 protein. In these cells, not only was Akt phosphorylation inhibited, but FGF2 transcription was also critically impaired. In contrast, transfecting BAECs with FGF2 sense cDNA augmented Akt phosphorylation. Treatment with constitutively active Akt enhanced FGF2 expression. Augmentation of either FGF2 transcription or Akt phosphorylation rendered BAECs resistant to L5.

CONCLUSIONS

These findings suggest that FGF2 is the primary initiator of its own expression, which is autoregulated through a novel FGF2-PI3K-Akt loop. Thus, by disrupting FGF2 autoregulation in vascular ECs, L5 may impair reendothelialization and collateralization in diabetes.

JNK signaling pathway is required for bFGF-mediated surface cadherin downregulation on HUVEC

Angiogenesis, the process of new blood vessel formation, is important in wound healing, inflammation, tumorigenesis and metastases. During this process, it is a critical step of the loosening of cellular interactions between endothelial cells, which are dependent on the architecture of adherens junction constructed by homophilic interactions of cell surface cadherins. Several studies suggested that the dynamic changes of cadherins are necessary during angiogenesis. However, the mechanism of cadherins regulation on endothelial cells requires further delineation. Here, we showed that basic fibroblast growth factor (bFGF), a pivotal pro-angiogenic factor, can downregulate typical cadherins (E-, N-, P- and VE-cadherin) expression on the surface of human umbilical vein endothelial cells (HUVECs) via FGF receptor 1 (FGFR1) signaling. The bFGF-mediated surface cadherin downregulation was significantly reversed only when the HUVECs were treated with JNK inhibitor (SP600125), but not ERK (PD98059) or p38 inhibitor (SB203580). Infecting HUVECs with a dominant negative H-Ras mutant (Ras(S17N)) interferes bFGF-mediated cadherin downregulation, and the result suggests that bFGF attenuates surface cadherin expression on HUVECs via FGFR1 and intracellular Ras-JNK signaling. However, after growth factors withdrawal, FGFR1 blockade or JNK inhibition for 16 h, cadherins were re-expressed on cell surface of HUVECs. But the mRNA or total protein of cadherins had no significant change, suggesting that the effect of bFGF on cadherin expression may work through a post-translational control. Our data first suggest that JNK participates in bFGF-mediated surface cadherin downregulation. Loss of surface cadherins may affect the cell-cell interaction between endothelial cells and facilitate angiogenesis.

VEGF- and LPA-induced telomerase in human ovarian cancer cells is Sp1-dependent

OBJECTIVE

Both vascular endothelial growth factor (VEGF) and lysophosphatidic acid (LPA) are secreted by ovarian cancer cells and are known to promote cancer cell growth though the exact mechanism(s) are not completely understood. Since telomerase, a ribonucleprotein expressed in 95% of ovarian cancers, plays an important role in cellular immortalization, growth, and tumor progression, we examined whether telomerase is a molecular target of LPA and VEGF in ovarian cancer.

METHODS

Telomerase-positive ovarian carcinoma cell lines PA-1, SW 626, and one telomerase-negative, non-tumorigenic SV40 large-T antigen-transfected human ovarian surface epithelial (IOSE) cell line, FHIOSE 118, derived from normal ovarian surface epithelium were cultured with and without VEGF and LPA for 4 h and 24 h, respectively. Telomerase PCR-ELISA, RT-PCR, VEGF ELISA and luciferase assays were performed to determine the effect of VEGF and LPA on telomerase activity in ovarian cancer cells. Western blot analyses were used to examine the signaling pathway involved in telomerase regulation by VEGF and LPA.

RESULTS

We report that: (1) both VEGF and LPA upregulate telomerase activity; (2) LPA induction of telomerase activity is VEGF-dependent; (3) VEGF and LPA induction of telomerase activity is ERK 1/2-dependent; and (4) Sp1 binding sites within the proximal 976- to 378-bp regions of the hTERT promoter are essential for VEGF- and LPA-induced hTERT promoter activity.

CONCLUSION

Consequently, these data show the novel finding that VEGF can regulate telomerase activity in non-endothelial cells and that telomerase appears to be a novel molecular target of LPA.

Telomere biology and cardiovascular disease

Accumulation of cellular damage with advancing age leads to atherothrombosis and associated cardiovascular disease. Ageing is also characterized by shortening of the DNA component of telomeres, the specialized genetic segments located at the end of eukaryotic chromosomes that protect them from end-to-end fusions. By inducing genomic instability, replicative senescence and apoptosis, shortening of the telomeric DNA is thought to contribute to organismal ageing. In this Review, we discuss experimental and human studies that have linked telomeres and associated proteins to several factors which influence cardiovascular risk (eg, estrogens, oxidative stress, hypertension, diabetes, and psychological stress), as well as to neovascularization and the pathogenesis of atherosclerosis and heart disease. Two chief questions that remain unanswered are whether telomere shortening is cause or consequence of cardiovascular disease, and whether therapies targeting the telomere may find application in treating these disorders (eg, cell "telomerization" to engineer blood vessels of clinical value for bypass surgery, and to facilitate cell-based myocardial regeneration strategies). Given that most research to date has focused on the role of telomerase, it is also of up most importance to investigate whether alterations in additional telomere-associated proteins may contribute to the pathogenesis of cardiovascular disease.

Evidence against the involvement of nitric oxide in the modulation of telomerase activity or replicative capacity of human endothelial cells

Telomerase, a reverse transcriptase involved in the maintenance of telomere function and cellular replicative capacity, is thought to be regulated by nitric oxide (NO). Here, we have used pharmacological tools and RNA interference to re-assess the role of NO in the regulation of telomerase and senescence of human umbilical vein endothelial cells. Acute or chronic treatment of these cells with the NO donors diethylenetriamine/NO (DETA-NO) or S-nitroso-N-acetylpenicillamine (SNAP) at concentrations which generated NO in the 1-300 nM range did not modulate telomerase activity. Similarly these agents did not affect cellular replicative capacity during long-term sub-cultivation. The NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (1 mM) reduced basal levels of c-GMP by 50% but had no effect on telomerase activity or replicative capacity. Withdrawal of ascorbic acid increased the intracellular pro-oxidant capacity, reduced telomerase activity and increased the accumulation of senescent cells upon serial passage in culture. However, this shift to a more oxidative redox state did not unmask the putative capacity of NO to modulate telomerase or senescence. Infection of cells with a lentiviral vector expressing a small hairpin RNA targeted against endothelial NOS inhibited endogenous NO production completely but failed to affect the decrease of telomerase activity or the accumulation of senescent cells observed with passage in culture. Our findings suggest that physiological concentrations of NO do not modulate telomerase levels or replicative capacity of endothelial cells, regardless of their cellular oxidative status.

Endothelial cell senescence

The wear and tear processes that are thought to contribute to human ageing may play an important role in the development of vascular diseases. One such process is cellular senescence. In endothelial cells the senescent phenotype can be induced by a number of factors, including telomere damage, oxidative stress and sustained mitogenic stimulation. Several lines of evidence indicate that endothelial cell senescence maybe relevant to vascular disease. In this chapter we examine the causes, mechanisms and regulation of endothelial cell senescence as they emerge from studies in cell culture. We also describe the senescent phenotype and discuss its pathophysiological implications. We review the evidence for the occurrence of endothelial cell senescence in vivo and examine findings in animal models of ageing and human genetic disorders that argue for and against a role of endothelial cell senescence in age-related vascular pathology. Finally, we address the particular case of endothelial progenitor cell senescence and discuss the relevance of this phenomenon for angiogenesis and vascular repair.

Establishment and characterization of conditionally immortalized endothelial cell lines from the thoracic duct and inferior vena cava of tsA58/EGFP double-transgenic rats

The basic biology of blood vascular endothelial cells has been well documented. However, little is known about that of lymphatic endothelial cells, despite their importance under normal and pathological conditions. The lack of a lymphatic endothelial cell line has hampered progress in this field. The objective of this study has been to establish and characterize lymphatic and venous endothelial cell lines derived from newly developed tsA58/EGFP transgenic rats harboring the temperature-sensitive simian virus 40 (SV40) large T-antigen and enhanced green fluorescent protein (EGFP). Endothelial cells were isolated from the transgenic rats by intraluminal enzymatic digestion. The cloned cell lines were named TR-LE (temperature-sensitive rat lymphatic endothelial cells from thoracic duct) and TR-BE (temperature-sensitive rat blood-vessel endothelial cells from inferior vena cava), respectively, and cultured on fibronectin-coated dishes in HuMedia-EG2 supplemented with 20% fetal bovine serum and Endothelial Mitogen at a permissive temperature, 33 degrees C. A temperature shift to 37 degrees C resulted in a decrease in proliferation with degradation of the large T-antigen and cleavage of poly (ADP-ribose) polymerase. TR-LE cells expressed lymphatic endothelial markers VEGFR-3 (vascular endothelial growth factor receptor), LYVE-1 (a lymphatic endothelial receptor), Prox-1 (a homeobox gene product), and podoplanin (a glomerular podocyte membrane mucoprotein), together with endothelial markers CD31, Tie-2, and VEGFR-2, whereas TR-BE cells expressed CD31, Tie-2, and VEGFR-2, but no lymphatic endothelial markers. Thus, these conditionally immortalized and EGFP-expressing lymphatic and vascular endothelial cell lines might represent an important tool for the study of endothelial cell functions in vitro.

Telomerase mediates vascular endothelial growth factor-dependent responsiveness in a rat model of hind limb ischemia

Telomere dysfunction contributes to reduced cell viability, altered differentiation, and impaired regenerative/proliferative responses. Recent advances indicate that telomerase activity confers a pro-angiogenic phenotype to endothelial cells and their precursors. We have investigated whether telomerase contributes to tissue regeneration following hind limb ischemia and vascular endothelial growth factor 165 (VEGF(165)) treatment. VEGF delivery induced angiogenesis and increased expression of the telomerase reverse transcriptase (TERT) and telomerase activity in skeletal muscles and satellite and endothelial cells. Adenovirus-mediated transfer of wild type TERT but not of a dominant negative mutant, TERTdn, significantly induced capillary but not arteriole formation. However, when co-delivered with VEGF, TERTdn abrogated VEGF-dependent angiogenesis, arteriogenesis, and blood flow increase. This effect was paralleled by in vitro evidence that telomerase inhibition by 3'-azido-3'-deoxythymidine in VEGF-treated endothelial cells strongly reduced capillary density and promoted apoptosis in the absence of serum. Similar results were obtained with adenovirus-mediated expression of TERTdn and AKTdn, both reducing endogenous TERT activity and angiogenesis on Matrigel. Mechanistically, neo-angiogenesis in our system involved: (i) VEGF-dependent activation of telomerase through the nitric oxide pathway and (ii) telomerase-dependent activation of endothelial cell differentiation and protection from apoptosis. Furthermore, detection of TERT in activated satellite cells identified them as VEGF targets during muscle regeneration. Because TERT behaves as an angiogenic factor and a downstream effector of VEGF signaling, telomerase activity appears required for VEGF-dependent remodeling of ischemic tissue at the capillaries and arterioles level.