Telomerized human microvasculature is functional in vivo

Therapeutic targeting of telomerase ameliorates experimental choroidal neovascularization

Choroidal neovascularization (CNV) is the principal driver of blindness in neovascular age-related macular degeneration (nvAMD). Increased activity of telomerase, has been associated with endothelial cell proliferation, survival, migration, and invasion in the context of tumor angiogenesis. Expanding on this knowledge, we investigated the role of telomerase in the development of CNV in mouse model. We observed increased gene expression and activity of telomerase in mouse CNV. Genetic deficiency of the telomerase components, telomerase reverse transcriptase (Tert) and telomerase RNA component (Terc) suppressed laser-induced CNV in mice. Similarly, a small molecule inhibitor of TERT (BIBR 1532), and antisense oligonucleotides (ASOs) targeting Tert and Terc reduced CNV growth. Bone marrow chimera studies suggested that telomerase activity in non-bone marrow-derived cells is crucial for the development of CNV. Comparison of BIBR 1532 with VEGF neutralizing therapeutic strategy in mouse revealed a comparable level of angiosuppressive activity. However, when BIBR and anti-VEGF antibodies were administered as a combination at sub-therapeutic doses, a statistically significant suppression of CNV was observed. These findings underscore the potential benefits of combining sub-therapeutic doses of BIBR and anti-VEGF antibodies for developing newer therapeutic strategies for NV-AMD. Telomerase inhibition with BIBR 1532 suppressed induction of multiple cytokines and growth factors critical for neovascularization. In conclusion, our study identifies telomerase as a promising therapeutic target for treating neovascular disease of the eye and thus provides a proof of principle for further exploration of telomerase inhibition as a novel treatment strategy for nvAMD.

Human-Based New Approach Methodologies in Developmental Toxicity Testing: A Step Ahead from the State of the Art with a Feto-Placental Organ-on-Chip Platform

Developmental toxicity testing urgently requires the implementation of human-relevant new approach methodologies (NAMs) that better recapitulate the peculiar nature of human physiology during pregnancy, especially the placenta and the maternal/fetal interface, which represent a key stage for human lifelong health. Fit-for-purpose NAMs for the placental-fetal interface are desirable to improve the biological knowledge of environmental exposure at the molecular level and to reduce the high cost, time and ethical impact of animal studies. This article reviews the state of the art on the available in vitro (placental, fetal and amniotic cell-based systems) and in silico NAMs of human relevance for developmental toxicity testing purposes; in addition, we considered available Adverse Outcome Pathways related to developmental toxicity. The OECD TG 414 for the identification and assessment of deleterious effects of prenatal exposure to chemicals on developing organisms will be discussed to delineate the regulatory context and to better debate what is missing and needed in the context of the Developmental Origins of Health and Disease hypothesis to significantly improve this sector. Starting from this analysis, the development of a novel human feto-placental organ-on-chip platform will be introduced as an innovative future alternative tool for developmental toxicity testing, considering possible implementation and validation strategies to overcome the limitation of the current animal studies and NAMs available in regulatory toxicology and in the biomedical field.

Ross J, Scott Herron G. Genetic and Epigenetic Influences on Cutaneous Cellular Senescence

Skin is the largest human organ system, and its protective function is critical to survival. The epithelial, dermal, and subcutaneous compartments are heterogeneous mixtures of cell types, yet they all display age-related skin dysfunction through the accumulation of an altered phenotypic cellular state called senescence. Cellular senescence is triggered by complex and dynamic genetic and epigenetic processes. A senescence steady state is achieved in different cell types under various and overlapping conditions of chronological age, toxic injury, oxidative stress, replicative exhaustion, DNA damage, metabolic dysfunction, and chromosomal structural changes. These inputs lead to outputs of cell-cycle withdrawal and the appearance of a senescence-associated secretory phenotype, both of which accumulate as tissue pathology observed clinically in aged skin. This review details the influence of genetic and epigenetic factors that converge on normal cutaneous cellular processes to create the senescent state, thereby dictating the response of the skin to the forces of both intrinsic and extrinsic aging. From this work, it is clear that no single biomarker or process leads to senescence, but that it is a convergence of factors resulting in an overt aging phenotype.

Bone Tissue Regeneration in the Oral and Maxillofacial Region: A Review on the Application of Stem Cells and New Strategies to Improve Vascularization

Bone tissue engineering techniques are a promising alternative for the use of autologous bone grafts to reconstruct bone defects in the oral and maxillofacial region. However, for successful bone regeneration, adequate vascularization is a prerequisite. This review presents and discusses the application of stem cells and new strategies to improve vascularization, which may lead to feasible clinical applications. Multiple sources of stem cells have been investigated for bone tissue engineering. The stromal vascular fraction (SVF) of human adipose tissue is considered a promising single source for a heterogeneous population of essential cells with, amongst others, osteogenic and angiogenic potential. Enhanced vascularization of tissue-engineered grafts can be achieved by different mechanisms: vascular ingrowth directed from the surrounding host tissue to the implanted graft, vice versa, or concomitantly. Vascular ingrowth into the implanted graft can be enhanced by (i) optimizing the material properties of scaffolds and (ii) their bioactivation by incorporation of growth factors or cell seeding. Vascular ingrowth directed from the implanted graft towards the host tissue can be achieved by incorporating the graft with either (i) preformed microvascular networks or (ii) microvascular fragments (MF). The latter may have stimulating actions on both vascular ingrowth and outgrowth, since they contain angiogenic stem cells like SVF, as well as vascularized matrix fragments. Both adipose tissue-derived SVF and MF are cell sources with clinical feasibility due to their large quantities that can be harvested and applied in a one-step surgical procedure. During the past years, important advancements of stem cell application and vascularization in bone tissue regeneration have been made. The development of engineered in vitro 3D models mimicking the bone defect environment would facilitate new strategies in bone tissue engineering. Successful clinical application requires innovative future investigations enhancing vascularization.

Role of telomerase in the tumour microenvironment

Telomeres are specialized genomic structures that protect chromosomal ends to maintain genomic stability. Telomeric length is primarily regulated by the telomerase complex, essentially consisting of an RNA template (TERC), an enzymatic subunit (telomerase reverse transcriptase, TERT). In humans, telomerase activity is repressed during embryonic differentiation and is absent in most somatic cells. However, it is upregulated or reactivated in 80%-90% of the primary tumours in humans. The human TERT (hTERT) plays a pivotal role in cellular immortality and tumourigenesis. However, the molecular mechanisms of telomerase functioning in cancer have not been fully understood beyond the telomere maintenance. Several research groups, including ours, have demonstrated that hTERT possesses vital functions independent of its telomere maintenance, including angiogenesis, inflammation, cancer cell stemness, and epithelial-mesenchymal transformation (EMT). All these telomere-independent activities of hTERT may contribute to the regulation of the dynamics and homeostasis of the tumour microenvironment (TME), thereby promoting tumour growth and development. Cancer progression and metastasis largely depend upon the interactions between cancer cells and their microenvironment. In this review, the involvement of TERT in the tumour microenvironment and the underlying implications in cancer therapeutics have been summarized.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

Expansion of functional personalized cells with specific transgene combinations

Fundamental research and drug development for personalized medicine necessitates cell cultures from defined genetic backgrounds. However, providing sufficient numbers of authentic cells from individuals poses a challenge. Here, we present a new strategy for rapid cell expansion that overcomes current limitations. Using a small gene library, we expanded primary cells from different tissues, donors, and species. Cell-type-specific regimens that allow the reproducible creation of cell lines were identified. In depth characterization of a series of endothelial and hepatocytic cell lines confirmed phenotypic stability and functionality. Applying this technology enables rapid, efficient, and reliable production of unlimited numbers of personalized cells. As such, these cell systems support mechanistic studies, epidemiological research, and tailored drug development.

Personalised medicine requires cell cultures from defined genetic backgrounds, but providing sufficient numbers of cells is a challenge. Here the authors develop gene cocktails to expand primary cells from a variety of different tissues and species, and show that expanded endothelial and hepatic cells retain properties of the differentiated phenotype.

Proliferation status defines functional properties of endothelial cells

Homeostasis of solid tissue is characterized by a low proliferative activity of differentiated cells while special conditions like tissue damage induce regeneration and proliferation. For some cell types it has been shown that various tissue-specific functions are missing in the proliferating state, raising the possibility that their proliferation is not compatible with a fully differentiated state. While endothelial cells are important players in regenerating tissue as well as in the vascularization of tumors, the impact of proliferation on their features remains elusive. To examine cell features in dependence of proliferation, we established human endothelial cell lines in which proliferation is tightly controlled by a doxycycline-dependent, synthetic regulatory unit. We observed that uptake of macromolecules and establishment of cell-cell contacts was more pronounced in the growth-arrested state. Tube-like structures were formed in vitro in both proliferating and non-proliferating conditions. However, functional vessel formation upon transplantation into immune-compromised mice was restricted to the proliferative state. Kaposi's sarcoma-associated herpes virus (KSHV) infection resulted in reduced expression of endothelial markers. Upon transplantation of infected cells, drastic differences were observed: proliferation arrested cells acquired a high migratory activity while the proliferating counterparts established a tumor-like phenotype, similar to Kaposi Sarcoma lesions. The study gives evidence that proliferation governs endothelial functions. This suggests that several endothelial functions are differentially expressed during angiogenesis. Moreover, since proliferation defines the functional properties of cells upon infection with KSHV, this process crucially affects the fate of virus-infected cells.

Friend or foe? Telomerase as a pharmacological target in cancer and cardiovascular disease

Aging, cancer, and chronic disease have remained at the forefront of basic biological research for decades. Within this context, significant attention has been paid to the role of telomerase, the enzyme responsible for lengthening telomeres, the nucleotide sequences located at the end of chromosomes found in the nucleus. Alterations in telomere length and telomerase activity are a common denominator to the underlying pathology of these diseases. While nuclear-specific, telomere-lengthening effects of telomerase impact cellular/organismal aging and cancer development, non-canonical, extra-nuclear, and non-telomere-lengthening contributions of telomerase have only recently been described and their exact physiological implications are ill defined. Although the mechanism remains unclear, recent reports reveal that the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), regulates levels of mitochondrial-derived reactive oxygen species (mtROS), independent of its established role in the nucleus. Telomerase inhibition has been the target of chemotherapy (directed or indirectly) for over a decade now, yet no telomerase inhibitor is FDA approved and few are currently in late-stage clinical trials, possibly due to underappreciation of the distinct extra-nuclear functions of telomerase. Moreover, evaluation of telomerase-specific therapies is largely limited to the context of chemotherapy, despite reports of the beneficial effects of telomerase activation in the cardiovascular system in relation to such processes as endothelial dysfunction and myocardial infarction. Thus, there is a need for better understanding of telomerase-focused cell and organism physiology, as well as development of telomerase-specific therapies in relation to cancer and extension of these therapies to cardiovascular pathologies. This review will detail findings related to telomerase and evaluate its potential to serve as a therapeutic target.

Therapeutic Targeting of Telomerase

Telomere length and cell function can be preserved by the human reverse transcriptase telomerase (hTERT), which synthesizes the new telomeric DNA from a RNA template, but is normally restricted to cells needing a high proliferative capacity, such as stem cells. Consequently, telomerase-based therapies to elongate short telomeres are developed, some of which have successfully reached the stage I in clinical trials. Telomerase is also permissive for tumorigenesis and 90% of all malignant tumors use telomerase to obtain immortality. Thus, reversal of telomerase upregulation in tumor cells is a potential strategy to treat cancer. Natural and small-molecule telomerase inhibitors, immunotherapeutic approaches, oligonucleotide inhibitors, and telomerase-directed gene therapy are useful treatment strategies. Telomerase is more widely expressed than any other tumor marker. The low expression in normal tissues, together with the longer telomeres in normal stem cells versus cancer cells, provides some degree of specificity with low risk of toxicity. However, long term telomerase inhibition may elicit negative effects in highly-proliferative cells which need telomerase for survival, and it may interfere with telomere-independent physiological functions. Moreover, only a few hTERT molecules are required to overcome senescence in cancer cells, and telomerase inhibition requires proliferating cells over a sufficient number of population doublings to induce tumor suppressive senescence. These limitations may explain the moderate success rates in many clinical studies. Despite extensive studies, only one vaccine and one telomerase antagonist are routinely used in clinical work. For complete eradication of all subpopulations of cancer cells a simultaneous targeting of several mechanisms will likely be needed. Possible technical improvements have been proposed including the development of more specific inhibitors, methods to increase the efficacy of vaccination methods, and personalized approaches. Telomerase activation and cell rejuvenation is successfully used in regenerative medicine for tissue engineering and reconstructive surgery. However, there are also a number of pitfalls in the treatment with telomerase activating procedures for the whole organism and for longer periods of time. Extended cell lifespan may accumulate rare genetic and epigenetic aberrations that can contribute to malignant transformation. Therefore, novel vector systems have been developed for a 'mild' integration of telomerase into the host genome and loss of the vector in rapidly-proliferating cells. It is currently unclear if this technique can also be used in human beings to treat chronic diseases, such as atherosclerosis.

Targeting prion-like protein doppel selectively suppresses tumor angiogenesis

Controlled and site-specific regulation of growth factor signaling remains a major challenge for current antiangiogenic therapies, as these antiangiogenic agents target normal vasculature as well tumor vasculature. In this article, we identified the prion-like protein doppel as a potential therapeutic target for tumor angiogenesis. We investigated the interactions between doppel and VEGFR2 and evaluated whether blocking the doppel/VEGFR2 axis suppresses the process of angiogenesis. We discovered that tumor endothelial cells (TECs), but not normal ECs, express doppel; tumors from patients and mouse xenografts expressed doppel in their vasculatures. Induced doppel overexpression in ECs enhanced vascularization, whereas doppel constitutively colocalized and complexed with VEGFR2 in TECs. Doppel inhibition depleted VEGFR2 from the cell membrane, subsequently inducing the internalization and degradation of VEGFR2 and thereby attenuating VEGFR2 signaling. We also synthesized an orally active glycosaminoglycan (LHbisD4) that specifically binds with doppel. We determined that LHbisD4 concentrates over the tumor site and that genetic loss of doppel in TECs decreases LHbisD4 binding and targeting both in vitro and in vivo. Moreover, LHbisD4 eliminated VEGFR2 from the cell membrane, prevented VEGF binding in TECs, and suppressed tumor growth. Together, our results demonstrate that blocking doppel can control VEGF signaling in TECs and selectively inhibit tumor angiogenesis.

Prevascularization in tissue engineering: Current concepts and future directions

The survival of engineered tissue constructs during the initial phase after their implantation depends on the rapid development of an adequate vascularization. This, in turn, is a major prerequisite for the constructs' long-term function. 'Prevascularization' has emerged as a promising concept in tissue engineering, aiming at the generation of a preformed microvasculature in tissue constructs prior to their implantation. This should shorten the time period during which the constructs are avascular and suffer hypoxic conditions. Herein, we provide an overview of current strategies for the generation of preformed microvascular networks within tissue constructs. In vitro approaches use cell seeding, spheroid formation or cell sheet technologies. In situ approaches use the body as a natural bioreactor to induce vascularization by angiogenic ingrowth or flap and arteriovenous (AV)-loop techniques. In future, these strategies may be supplemented by the transplantation of adipose tissue-derived microvascular fragments or the in vitro generation of highly organized microvascular networks by means of sophisticated microscale technologies and microfluidic systems. The further advancement of these prevascularization concepts and their adaptation to individual therapeutic interventions will markedly contribute to a broad implementation of tissue engineering applications into clinical practice.

Poly(ethylene glycol) Hydrogel Scaffolds Containing Cell-Adhesive and Protease-Sensitive Peptides Support Microvessel Formation by Endothelial Progenitor Cells

The development of stable, functional microvessels remains an important obstacle to overcome for tissue engineered organs and treatment of ischemia. Endothelial progenitor cells (EPCs) are a promising cell source for vascular tissue engineering as they are readily obtainable and carry the potential to differentiate towards all endothelial phenotypes. The aim of this study was to investigate the ability of human umbilical cord blood-derived EPCs to form vessel-like structures within a tissue engineering scaffold material, a cell-adhesive and proteolytically degradable poly(ethylene glycol) (PEG) hydrogel. EPCs in co-culture with angiogenic mural cells were encapsulated in hydrogel scaffolds by mixing with polymeric precursors and using a mild photocrosslinking process to form hydrogels with homogeneously dispersed cells. EPCs formed 3D microvessels networks that were stable for at least 30 days in culture, without the need for supplemental angiogenic growth factors. These 3D EPC microvessels displayed aspects of physiological microvasculature with lumen formation, expression of endothelial cell proteins (connexin 32, VE-cadherin, eNOS), basement membrane formation with collagen IV and laminin, perivascular investment of PDGFR-β and α-SMA positive cells, and EPC quiescence (<1% proliferating cells) by 2 weeks of co-culture. Our findings demonstrate the development of a novel, reductionist system that is well-defined and reproducible for studying progenitor cell-driven microvessel formation.

Exogenous DNA Loading into Extracellular Vesicles via Electroporation is Size-Dependent and Enables Limited Gene Delivery

Extracellular vesicles (EVs) hold immense promise for utilization as biotherapeutics and drug delivery vehicles due to their nature as biological nanoparticles that facilitate intercellular molecular transport. Specifically, EVs have been identified as natural carriers of nucleic acids, sparking interest in their use for gene therapy and RNA interference applications. So far, small RNAs (siRNA and miRNA) have been successfully loaded into EVs for a variety of delivery applications, but the potential use of EVs for DNA delivery has scarcely been explored. Here, we report that exogenous linear DNA can be associated with EVs via electroporation in quantities sufficient to yield an average of hundreds of DNA molecules per vesicle. We determined that loading efficiency and capacity of DNA in EVs is dependent on DNA size, with linear DNA molecules less than 1000 bp in length being more efficiently associated with EVs compared to larger linear DNAs and plasmid DNAs using this approach. We further showed that EV size is also determinant with regard to DNA loading, as larger microvesicles encapsulated more linear and plasmid DNA than smaller, exosome-like EVs. Additionally, we confirmed the ability of EVs to transfer foreign DNA loaded via electroporation into recipient cells, although functional gene delivery was not observed. These results establish critical parameters that inform the potential use of EVs for gene therapy and, in agreement with other recent results, suggest that substantial barriers must be overcome to establish EVs as broadly applicable DNA delivery vehicles.

ADAM17 Promotes Motility, Invasion, and Sprouting of Lymphatic Endothelial Cells

Tumor-associated lymphatic vessels actively participate in tumor progression and dissemination. ADAM17, a sheddase for numerous growth factors, cytokines, receptors, and cell adhesion molecules, is believed to promote tumor development, facilitating both tumor cell proliferation and migration, as well as tumor angiogenesis. In this work we addressed the issue of whether ADAM17 may also promote tumor lymphangiogenesis. First, we found that ADAM17 is important for the migratory potential of immortalized human dermal lymphatic endothelial cells (LEC). When ADAM17 was stably silenced in LEC, their proliferation was not affected, but: (i) single-cell motility, (ii) cell migration through a 3D Matrigel/collagen type I matrix, and (iii) their ability to form sprouts in a 3D matrix were significantly diminished. The differences in the cell motility between ADAM17-proficient and ADAM17-silenced cells were eliminated by inhibitors of EGFR and HER2, indicating that ADAM17-mediated shedding of growth factors accounts for LEC migratory potential. Interestingly, ADAM17 depletion affected the integrin surface expression/functionality in LEC. ADAM17-silenced cells adhered to plastic, type I collagen, and fibronectin faster than their ADAM17-proficient counterparts. The difference in adhesion to fibronectin was abolished by a cyclic RGD peptide, emphasizing the involvement of integrins in the process. Using a soluble receptor array, we identified BIG-H3 among several candidate proteins involved in the phenotypic and behavioral changes of LEC upon ADAM17 silencing. In additional assays, we confirmed the increased expression of BIG-H3, as well as TGFβ2 in ADAM17-silenced LEC. The antilymphangiogenic effects of ADAM17 silencing in lymphatic endothelial cells suggest further relevance of ADAM17 as a potential target in cancer therapy.

MicroRNAs in vascular tissue engineering and post-ischemic neovascularization

Increasing numbers of paediatric patients with congenital heart defects are surviving to adulthood, albeit with continuing clinical needs. Hence, there is still scope for revolutionary new strategies to correct vascular anatomical defects. Adult patients are also surviving longer with the adverse consequences of ischemic vascular disease, especially after acute coronary syndromes brought on by plaque erosion and rupture. Vascular tissue engineering and therapeutic angiogenesis provide new hope for these patients. Both approaches have shown promise in laboratory studies, but have not yet been able to deliver clear evidence of clinical success. More research into biomaterials, molecular medicine and cell and molecular therapies is necessary. This review article focuses on the new opportunities offered by targeting microRNAs for the improved production and greater empowerment of vascular cells for use in vascular tissue engineering or for increasing blood perfusion of ischemic tissues by amplifying the resident microvascular network.

Emerging roles for extracellular vesicles in tissue engineering and regenerative medicine

Extracellular vesicles (EVs)-comprising a heterogeneous population of cell-derived lipid vesicles including exosomes, microvesicles, and others-have recently emerged as both mediators of intercellular information transfer in numerous biological systems and vehicles for drug delivery. In both roles, EVs have immense potential to impact tissue engineering and regenerative medicine applications. For example, the therapeutic effects of several progenitor and stem cell-based therapies have been attributed primarily to EVs secreted by these cells, and EVs have been recently reported to play direct roles in injury-induced tissue regeneration processes in multiple physiological systems. In addition, EVs have been utilized for targeted drug delivery in regenerative applications and possess unique potential to be harnessed as patient-derived drug delivery vehicles for personalized medicine. This review discusses EVs in the context of tissue repair and regeneration, including their utilization as drug carriers and their crucial role in cell-based therapies. Furthermore, the article highlights the growing need for bioengineers to understand, consider, and ultimately design and specifically control the activity of EVs to maximize the efficacy of tissue engineering and regenerative therapies.

hTERT- and hCTLA4Ig-expressing human bone marrow-derived mesenchymal stem cells: in vitro and in vivo characterization and osteogenic differentiation

Multipotent mesenchymal stem cells (MSCs) are commonly used as seed cells in studies of tissue engineering and regenerative medicine but their clinical application is limited, due to insufficient numbers of autogeneic MSCs, immune rejection of allogeneic MSCs and replicative senescence. We constructed two gene expression vectors for transfection of the human telomerase reverse transcriptase (hTERT) and cytotoxic T lymphocyte-associated antigen 4-Ig (CTLA4Ig) genes into human bone marrow-derived stem cells (hBMSCs). Successful transfection of both genes generated hTERT-CTLA4Ig hBMSCs that expressed both telomerase (shown by immunohistochemistry and a TRAPeze assay) and CTLA4Ig (demonstrated by immunocytochemistry and western blotting) without apparent mutual interference. Both hTERT BMSCs (92 population doublings) and hTERT-CTLA4Ig hBMSCs (60 population doublings) had an extended lifespan compared with hBMSCs (18 population doublings). Cell cycle analysis revealed that, compared with hBMSCs, a lower proportion of hTERT hBMSCs were in G₀ /G₁ phase but a higher proportion were in S phase; compared with hTERT hBMSCs, a higher proportion of hTERT-CTLA4Ig hBMSCs were in G₀ /G₁ phase, while a lower proportion were in S and G₂ /M phases. hTERT-CTLA4Ig hBMSCs retained their capacity for osteogenic differentiation in vitro, shown by the detection of hydroxyapatite mineral deposition (labelled tetracycline fluorescence staining), calcareous nodules (alizarin red S staining), alkaline phosphatase (calcium-cobalt method) and osteocalcin (immunocytochemistry). Furthermore, subcutaneous transplantation of hTERT-CTLA4Ig hBMSCs in a rat xenotransplantation model resulted in the successful generation of bone-like tissue, confirmed using radiography and histological assessment. We propose that allogeneic hTERT-CTLA4Ig hBMSCs may be ideal seed cells for bone tissue engineering. Copyright © 2014 John Wiley & Sons, Ltd.

Role of angiogenesis in bone repair

Bone vasculature plays a vital role in bone development, remodeling and homeostasis. New blood vessel formation is crucial during both primary bone development as well as fracture repair in adults. Both bone repair and bone remodeling involve the activation and complex interaction between angiogenic and osteogenic pathways. Interestingly studies have demonstrated that angiogenesis precedes the onset of osteogenesis. Indeed reduced or inadequate blood flow has been linked to impaired fracture healing and old age related low bone mass disorders such as osteoporosis. Similarly the slow penetration of host blood vessels in large engineered bone tissue grafts has been cited as one of the major hurdle still impeding current bone construction engineering strategies. This article reviews the current knowledge elaborating the importance of vascularization during bone healing and remodeling, and the current therapeutic strategies being adapted to promote and improve angiogenesis.

Modeling human endothelial cell transformation in vascular neoplasias

Endothelial cell (EC)-derived neoplasias range from benign hemangioma to aggressive metastatic angiosarcoma, which responds poorly to current treatments and has a very high mortality rate. The development of treatments that are more effective for these disorders will be expedited by insight into the processes that promote abnormal proliferation and malignant transformation of human ECs. The study of primary endothelial malignancy has been limited by the rarity of the disease; however, there is potential for carefully characterized EC lines and animal models to play a central role in the discovery, development and testing of molecular targeted therapies for vascular neoplasias. This review describes molecular alterations that have been identified in EC-derived neoplasias, as well as the processes that underpin the immortalization and tumorigenic conversion of ECs. Human EC lines, established through the introduction of defined genetic elements or by culture of primary tumor tissue, are catalogued and discussed in relation to their relevance as models of vascular neoplasia.

Effects of aging on angiogenesis

Aging is a dominant risk factor for most forms of cardiovascular disease. Impaired angiogenesis and endothelial dysfunction likely contribute to the increased prevalence of both cardiovascular diseases and their adverse sequelae in the elderly. Angiogenesis is both an essential adaptive response to physiological stress and an endogenous repair mechanism after ischemic injury. In addition, induction of angiogenesis is a promising therapeutic approach for ischemic diseases. For these reasons, understanding the basis of age-related impairment of angiogenesis and endothelial function has important implications for understanding and managing cardiovascular disease. In this review, we discuss the molecular mechanisms that contribute to impaired angiogenesis in the elderly and potential therapeutic approaches to improving vascular function and angiogenesis in aging patients.

Smooth muscle and other cell sources for human blood vessel engineering

Despite substantial progress in the field of vascular tissue engineering over the past decades, transition to human models has been rather challenging. The limited replicative life spans of human adult vascular cells, and their slow rate of collagenous matrix production in vitro, have posed important hurdles in the development of mechanically robust and biologically functional engineered grafts. With the more recent advances in the field of stem cells, investigators now have access to a plethora of new cell source alternatives for vascular engineering. In this paper, we review various alternative cell sources made available more recently for blood vessel engineering and also present some recent data on the derivation of smooth muscle cells from human induced pluripotent stem cells.

Normalization of the vasculature for treatment of cancer and other diseases

New vessel formation (angiogenesis) is an essential physiological process for embryologic development, normal growth, and tissue repair. Angiogenesis is tightly regulated at the molecular level. Dysregulation of angiogenesis occurs in various pathologies and is one of the hallmarks of cancer. The imbalance of pro- and anti-angiogenic signaling within tumors creates an abnormal vascular network that is characterized by dilated, tortuous, and hyperpermeable vessels. The physiological consequences of these vascular abnormalities include temporal and spatial heterogeneity in tumor blood flow and oxygenation and increased tumor interstitial fluid pressure. These abnormalities and the resultant microenvironment fuel tumor progression, and also lead to a reduction in the efficacy of chemotherapy, radiotherapy, and immunotherapy. With the discovery of vascular endothelial growth factor (VEGF) as a major driver of tumor angiogenesis, efforts have focused on novel therapeutics aimed at inhibiting VEGF activity, with the goal of regressing tumors by starvation. Unfortunately, clinical trials of anti-VEGF monotherapy in patients with solid tumors have been largely negative. Intriguingly, the combination of anti-VEGF therapy with conventional chemotherapy has improved survival in cancer patients compared with chemotherapy alone. These seemingly paradoxical results could be explained by a "normalization" of the tumor vasculature by anti-VEGF therapy. Preclinical studies have shown that anti-VEGF therapy changes tumor vasculature towards a more "mature" or "normal" phenotype. This "vascular normalization" is characterized by attenuation of hyperpermeability, increased vascular pericyte coverage, a more normal basement membrane, and a resultant reduction in tumor hypoxia and interstitial fluid pressure. These in turn can lead to an improvement in the metabolic profile of the tumor microenvironment, the delivery and efficacy of exogenously administered therapeutics, the efficacy of radiotherapy and of effector immune cells, and a reduction in number of metastatic cells shed by tumors into circulation in mice. These findings are consistent with data from clinical trials of anti-VEGF agents in patients with various solid tumors. More recently, genetic and pharmacological approaches have begun to unravel some other key regulators of vascular normalization such as proteins that regulate tissue oxygen sensing (PHD2) and vessel maturation (PDGFRβ, RGS5, Ang1/2, TGF-β). Here, we review the pathophysiology of tumor angiogenesis, the molecular underpinnings and functional consequences of vascular normalization, and the implications for treatment of cancer and nonmalignant diseases.

The use of hTERT-immortalized cells in tissue engineering

The use of human telomerase reverse transcriptase (hTERT)-immortalized cells in tissue engineering protocols is a potentially important application of telomere biology. Several human cell types have been created that overexpress the hTERT gene with enhanced telomerase activity, extended life span and maintained or even improved functional activities. Furthermore, some studies have employed the telomerized cells in tissue engineering protocols with very good results. However, high telomerase activity allows extensive cell proliferation that may be associated with genomic instability and risk for cell transformation. Thus, safety issues should be studied carefully before using the telomerized tissues in the clinic. Alternatively, the development of conditional or intermittent telomerase activation protocols is needed.

Baculoviral expression of telomerase in primary human fibroblasts to rejuvenate cells for tissue engineering

Tissue engineering involves the use of synthetic or natural materials as a scaffold to support the growth of replacement tissue or organs. The use of autologous cells to populate the scaffold avoids problems associated with rejection; however, a major limitation of this approach is the finite lifespan of primary cells in culture. This finite lifespan is due to the shortening of telomeres, short repetitive sequences of DNA located at the ends of eukaryotic chromosomes. Ectopic expression of telomerase reverse transcriptase (hTERT) is able to reconstitute telomerase activity and maintain the length of telomeres. This study investigated an alternative gene delivery vector, baculovirus, for the expression of hTERT in primary human cells. A recombinant baculovirus was used to efficiently deliver the hTERT gene to primary fibroblasts and the telomerase enzyme was found to be active. Although no increase in telomere length was detected, expression of hTERT in primary fibroblasts resulted in a significant extension of replicative lifespan. To our knowledge this is a novel attempt to use a recombinant baculovirus for the extension of cellular lifespan by exogenous expression of telomerase.

Fibroblast growth factor 9 delivery during angiogenesis produces durable, vasoresponsive microvessels wrapped by smooth muscle cells

The therapeutic potential of angiogenic growth factors has not been realized. This may be because formation of endothelial sprouts is not followed by their muscularization into vasoreactive arteries. Using microarray expression analysis, we discovered that fibroblast growth factor 9 (FGF9) was highly upregulated as human vascular smooth muscle cells (SMCs) assemble into layered cords. FGF9 was not angiogenic when mixed with tissue implants or delivered to the ischemic mouse hind limb, but instead orchestrated wrapping of SMCs around neovessels. SMC wrapping in implants was driven by sonic hedgehog-mediated upregulation of PDGFRβ. Computed tomography microangiography and intravital microscopy revealed that microvessels formed in the presence of FGF9 had enhanced capacity to receive flow and were vasoreactive. Moreover, the vessels persisted beyond 1 year, remodeling into multilayered arteries paired with peripheral nerves. This mature physiological competency was attained by targeting mesenchymal cells rather than endothelial cells, a finding that could inform strategies for therapeutic angiogenesis and tissue engineering.

The therapeutic potential of engineered human neovessels for cell-based gene therapy

IMPORTANCE OF THE FIELD

Several works have shown the feasibility of engineering functional blood vessels in vivo using human endothelial cells and mural cells. In this context, the genetic modification of endothelial cells would ensure the secretion of a therapeutic protein into the systemic circulation for a prolonged period of time.

AREAS COVERED IN THIS REVIEW

We discuss the different strategies aimed at the formation of long-lasting neovessels in vivo, using human endothelial and mural cells. The main focus is the potential of these constructs in gene therapy strategies for the in vivo production of therapeutic proteins.

WHAT THE READER WILL GAIN

The reader will have an outline of the different types of cells that have been used for microvessel engineering in vivo, as well as scaffolds employed to seed these cells. We provide a critical review of their advantages and drawbacks, along with examples of their potential in cell-based gene therapy strategies.

TAKE HOME MESSAGE

There is a real potential for neovessels derived from human endothelial and mural cells to be incorporated in clinical interventions, either as a cell-based gene therapy to produce a therapeutic protein or as a component of engineered tissue constructs in regenerative medicine.

Role of cellular senescence in lifestyle-related disease

Epidemiological studies have shown that age is the chief risk factor for lifestyle-related diseases such as cardiovascular disease and diabetes, but the molecular mechanisms that underlie the increase in the risk of such diseases conferred by aging remain unclear. Recently, genetic analyses using various animal models have identified molecules that are crucial for aging. These include components of the DNA repair system, the tumor suppressor pathway, the telomere maintenance system, the insulin/Akt pathway, and other metabolic pathways. Interestingly, most of the molecules that influence the phenotypic changes of aging also regulate cellular senescence, suggesting a causative link between cellular senescence and aging. This review examines the hypothesis that cellular senescence might contribute to lifestyle-related disease.

Synthetic gene regulation circuits for control of cell expansion

A major drawback in the analysis of primary cells and in regenerative sciences concerns the limited number and homogeneity of cells. This limitation could be overcome by in vitro cell expansion that retains the properties of the cell types of interest. However, for most primary differentiated cells the proliferation capacity is finite and/or proliferation is associated with dedifferentiation of cells. We have developed a flexible cell expansion strategy that allows strict and reliable control of cell proliferation. This system relies on synthetic gene modules that employ positive feedback loops based on Tetracycline control. These gene modules were constructed and transduced by lentiviral vectors. We succeeded in the generation of murine and importantly also of human endothelial cell lines. The key feature of the established cell lines is that their proliferation status can be strictly controlled while the expression of relevant markers is maintained. This strict control of proliferation was observed in cell clones and in cell pools and was even maintained when two independent immortalizing genes were simultaneously employed. Thus, this strategy is flexible, easy to handle, and reliable. Most importantly, it allows expansion of human cells with a primary-like phenotype.

Rapid anastomosis of endothelial progenitor cell-derived vessels with host vasculature is promoted by a high density of cotransplanted fibroblasts

To ensure survival of engineered implantable tissues thicker than approximately 2-3 mm, convection of nutrients and waste products to enhance the rate of transport will be required. Creating a network of vessels in vitro, before implantation (prevascularization), is one potential strategy to achieve this aim. In this study, we developed three-dimensional engineered vessel networks in vitro by coculture of endothelial cells (ECs) and fibroblasts in a fibrin gel for 7 days. Vessels formed by cord blood endothelial progenitor cell-derived ECs (EPC-ECs) in the presence of a high density of fibroblasts created an interconnected tubular network within 4 days, compared with 5-7 days in the presence of a low density of fibroblasts. Vessels derived from human umbilical vein ECs (HUVECs) in vitro showed similar kinetics. Implantation of the prevascularized tissues into immune-compromised mice, however, revealed a dramatic difference in the ability of EPC-ECs and HUVECs to form anastomoses with the host vasculature. Vascular beds derived from EPC-ECs were perfused within 1 day of implantation, whereas no HUVEC vessels were perfused at day 1. Further, while almost 90% of EPC-EC-derived vascular beds were perfused at day 3, only one-third of HUVEC-derived vascular beds were perfused. In both cases, a high density of fibroblasts accelerated anastomosis by 2-3 days. We conclude that both EPC-ECs and a high density of fibroblasts significantly accelerate the rate of functional anastomosis, and that prevascularizing an engineered tissue may be an effective strategy to enhance convective transport of nutrients in vivo.

Strategies in cardiac tissue engineering

In heart failure, post-myocardial infarction and some congenital cardiac anomalies, organ transplantation is the only effective cure. Shortage of organ donors and complications of orthotopic heart transplant remain major challenges to the modern field of transplantation. Tissue engineering using cell-based strategies presents itself as a new way of generating functional myocardium. Engineering functional myocardium de novo requires an abundant source of cells that can form cardiomyocytes. These cells may be used with biocompatible scaffold materials to generate a contractile myocardium. Lastly, to sustain the high metabolism of the construct, a functional vasculature needs to be developed with the forming cardiac tissue. This review provides an update on the progress of stem cell research in the context of cardiac tissue development, types of biomaterials used in cardiac tissue engineering (CTE) and currently employed strategies for vascularization in CTE. In addition, a brief overview of strategies utilized in CTE is provided.

Bone regeneration: the stem/progenitor cells point of view

After bone injuries, several molecular mechanisms establish bone repair from stem/progenitor cells. Inflammation factors attract regenerative cells which expand and differentiate in order to build up a bone highly similar to that before injury. Bone marrow (BM) mesenchymal stem cells (MSCs) as skeletal stem cells and endothelial progenitors (EPCs) are at the origin of such reparation mechanisms. However, discrepancies exist about their identities. Although cultured MSCs are extensively described, their in vivo native forms are poorly known. In addition, recent experiments show that several types of EPC exist. We therefore review up-to-date data on the characterization of such stem/progenitor cells and propose a new point of view of their function in bone regeneration.

Prevascularization of a fibrin-based tissue construct accelerates the formation of functional anastomosis with host vasculature

One critical obstacle facing tissue engineering is the formation of functional vascular networks that can support tissue survival in vivo. We hypothesized that prevascularizing a tissue construct with networks of well-formed capillaries would accelerate functional anastomosis with the host upon implantation. Fibrin-based tissues were prevascularized with capillary networks by coculturing human umbilical vein endothelial cells (HUVECs) and fibroblasts in fibrin gels for 1 week. The prevascularized tissue and nonprevascularized controls were implanted subcutaneously onto the dorsal surface of immune-deficient mice and retrieved at days 3, 5, 7 and 14. HUVEC-lined vessels containing red blood cells were evident in the prevascularized tissue by day 5, significantly earlier than nonprevascularized tissues (14 days). Analysis of the HUVEC-lined vessels demonstrated that the number and area of perfused lumens in the prevascularized tissue were significantly larger compared to controls. In addition, collagen deposition and a larger number of proliferating cells were evident in the prevascularized tissue at day 14. Our results demonstrate that prevascularizing a fibrin-based tissue with well-formed capillaries accelerates anastomosis with the host vasculature, and promotes cellular activity consistent with tissue remodeling. Our prevascularization strategy may be useful to design large three-dimensional engineered tissues.

Immortalization and Characterization of Porcine Ventricular Endocardial Endothelial Cells

Endocardial endothelial cells (EECs), which form the inner lining of the cavities of the heart, are a distinct cell population whose dysfunction can be critical in pathological conditions of heart. Insights into the role and organization of these cells in pathological states of the heart are limited mainly due to a dearth of experimental models. To date no endocardial endothelial cell line is available. The authors attempted to immortalize porcine ventricular EECs by transfecting the cells with human telomerase reverse transcriptase (hTERT). EECs immortalized by ectopic expression of hTERT exhibit phenotypic and functional characteristics similar to primary EECs. The EE cell line could be useful for the study of mechanisms involved in the interaction of EECs with the underlying myocardium and cardiac interstitium and as useful tools in understanding their role in diseased states of heart.

The telomerase tale in vascular aging: regulation by estrogens and nitric oxide signaling

Hormones and nitric oxide (NO), a free radical, are ancestral molecules, conserved through evolution, that modulate many aspects of the physiology and pathophysiology of living organisms by regulating transcription of genes involved in development, metabolism, and differentiation. Of interest, both estrogen and NO signaling, specifically through the estrogen receptor-α (ERα) and the endothelial isoform of the nitric oxide synthase (eNOS), have been shown to counteract endothelial senescence through a shared downstream effector, the catalytic subunit of human telomerase (hTERT), a key molecule in the aging process. Since aging is the first and most relevant risk factor in cardiovascular diseases, it is tempting to speculate that hTERT may be at the cross point between the NO and estrogen pathways. The present review will focus on the evolutionary and molecular aspects linking eNOS, ERs, and hTERT in counteracting the process of endothelial cell aging.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Synthetic tissue biology: tissue engineering meets synthetic biology

We propose the term "synthetic tissue biology" to describe the use of engineered tissues to form biological systems with metazoan-like complexity. The increasing maturity of tissue engineering is beginning to render this goal attainable. As in other synthetic biology approaches, the perspective is bottom-up; here, the premise is that complex functional phenotypes (on par with those in whole metazoan organisms) can be effected by engineering biology at the tissue level. To be successful, current efforts to understand and engineer multicellular systems must continue, and new efforts to integrate different tissues into a coherent structure will need to emerge. The fruits of this research may include improved understanding of how tissue systems can be integrated, as well as useful biomedical technologies not traditionally considered in tissue engineering, such as autonomous devices, sensors, and manufacturing.

Peritubular endothelium: The Achilles heel of the kidney?

The development of renal ischemia has been postulated to be a main cause of the progressive nature of kidney diseases. In recent years, it has become clear that inappropriate and sustained activation of the endothelium could mediate this phenomenon. Endothelial activation will result in leucostasis and can compromise peritubular flow. The associated sustained redox signaling will also accelerate the development of endothelial senescence. In addition, risk factors for renal disease progression can reduce endothelial repair. In the course of these events, loss of capillary structure and rarefaction develops, which drives the further development of nephron loss. In this mini review, the evidence for this pathophysiological concept as well as the possibility to detect such endothelial activation in the clinical arena is summarized.

Vascular progenitor cells isolated from human embryonic stem cells give rise to endothelial and smooth muscle like cells and form vascular networks in vivo

We report that human embryonic stem cells contain a population of vascular progenitor cells that have the ability to differentiate into endothelial-like and smooth muscle (SM)-like cells. Vascular progenitor cells were isolated from EBs grown in suspension for 10 days and were characterized by expression of the endothelial/hematopoietic marker CD34 (CD34+ cells). When these cells are subsequently cultured in EGM-2 (endothelial growth medium) supplemented with vascular endothelial growth factor-165 (50 ng/mL), they give rise to endothelial-like cells characterized by a cobblestone cell morphology, expression of endothelial markers (platelet endothelial cell-adhesion molecule-1, CD34, KDR/Flk-1, vascular endothelial cadherin, von Willebrand factor), incorporation of acetylated low-density lipoprotein, and formation of capillary-like structures when placed in Matrigel. In contrast, when CD34+ cells are cultured in EGM-2 supplemented with platelet-derived growth factor-BB (50 ng/mL), they give rise to SM-like cells characterized by spindle-shape morphology, expression of SM cell markers (alpha-SM actin, SM myosin heavy chain, calponin, caldesmon, SM alpha-22), and the ability to contract and relax in response to common pharmacological agents such as carbachol and atropine but rarely form capillary-like structures when placed in Matrigel. Implantation studies in nude mice show that both cell types contribute to the formation of human microvasculature. Some microvessels contained mouse blood cells, which indicates functional integration with host vasculature. Therefore, the vascular progenitors isolated from human embryonic stem cells using methods established in the present study could provide a means to examine the mechanisms of endothelial and SM cell development, and they could also provide a potential source of cells for vascular tissue engineering.

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.

Spontaneous immortalization of human epidermal cells with naturally elevated telomerase

This work explores spontaneous immortalization in keratinocytes, derived from two skin samples, that display naturally elevated telomerase activity. Serially passaged with 3T3 feeder layer support, the keratinocytes were examined for colony-forming ability, telomerase activity, telomere length, and finally gene expression using Affymetrix DNA microarrays. The cells initially exhibited normal karyotypes and low colony-forming efficiencies typical of normal epidermal cells, but after 40 passages (approximately 400 generations) colony-forming ability increased markedly, yielding immortalized lines exhibiting a small number of chromosomal aberrations and functionally normal p53. An improved protocol for analysis of microarray data permitted detection of 707 transcriptional changes accompanying immortalization including reduced p16(INK4A) mRNA. Telomerase activity was clearly elevated in cells even at low passage from both samples, and telomerase catalytic subunit mRNA was greatly elevated in those with elevated colony-forming ability. The data raise the possibility of an unusual natural phenotype in which aberrant telomerase regulation extends keratinocyte lifespan until rare variants evade senescence. In addition to revealing a potential tumor-prone syndrome, the findings emphasize the desirability of carefully minimizing the degree or timing of elevated expression of telomerase used to immortalize cells for therapeutic purposes.