Constitutive overexpression of human telomerase reverse transcriptase but not c-myc blocks terminal differentiation in human HaCaT skin keratinocytes

RETRACTED: MicroRNA138 regulates keratin 17 protein expression to affect HaCaT cell proliferation and apoptosis by targeting hTERT in psoriasis vulgaris

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor-in-Chief. An Expression of Concern for this article was previously published while an investigation was conducted (see related editorial: https://doi.org/10.1016/j.biopha.2022.113812). This retraction notice supersedes the Expression of Concern published earlier. Concern was raised about the reliability of the flow cytometry data in Figure 9, with suspected image similarities reported between the ‘miR-138 mimic’ and ‘siRNA hTERT’ groups, as detailed here: https://pubpeer.com/publications/983120063E7603EBA705B85BBF77F5; and here: https://docs.google.com/spreadsheets/d/1r0MyIYpagBc58BRF9c3luWNlCX8VUvUuPyYYXzxWvgY/edit#gid=262337249. Independent analysis confirmed this finding and identified additional suspected image duplications within Figure 9. The journal requested the corresponding author comment on these concerns and provide the associated raw data. The authors did not respond to this request and therefore the Editor-in-Chief decided to retract the article.

The New Immortalized Uroepithelial Cell Line HBLAK Contains Defined Genetic Aberrations Typical of Early Stage Urothelial Tumors

Background: Cell culture models of normal urothelial cells are important for studying differentiation, disease mechanisms and anticancer drug development. Beyond primary cultures with their limitations in lifespan, interindividual heterogeneity and supply, few conditionally immortalized cell lines with limited applicability due to partial transformation or impaired differentiation capacity are available. We describe characteristics of the new spontaneously immortalized cell line HBLAK derived from a primary culture of uroepithelial cells.

Objective: To characterize utility and limitations of HBLAK cells as an urothelial cell culture model.

Methods: Differentiation markers were investigated by immunofluorescence and RT-PCR, genetic changes by standard karyotyping, array-CGH, PCR, RT-PCR and exome sequencing; expression of p53 and p21 by Western blotting.

Results: HBLAK cells proliferated for >50 passages without senescing. They expressed cytokeratins of basal urothelial cells. Terminal differentiation markers appeared only after induction of differentiation by specific protocols. The karyotype was stable, with few chromosomal changes, especially gains of chromosomes 5 and 20 and a chromosome 9p21 deletion resulting in p16^INK4A loss. A C228T TERT promoter mutation was present, but no other mutation typical of urothelial carcinoma. TP53 was wild-type and the cell cycle was arrested in response to genomic stress.

Conclusions: HBLAK cells retain some differentiation potential and respond to cytotoxic agents similar to normal urothelial cells, but contain genetic changes contributing to immortalization in urothelial tumors. HBLAK may be valuable for evaluating the tumor specificity of novel cancer drugs, but may also be applied as an urothelial in vitro carcinogenesis model.

Contribution of Sp1 to Telomerase Expression and Activity in Skin Keratinocytes Cultured With a Feeder Layer

The growth of primary keratinocytes is improved by culturing them with a feeder layer. The aim of this study was to assess whether the feeder layer increases the lifespan of cultured epithelial cells by maintaining or improving telomerase activity and expression. The addition of an irradiated fibroblast feeder layer of either human or mouse origin (i3T3) helped maintain telomerase activity as well as expression of the transcription factor Sp1 in cultured keratinocytes. In contrast, senescence occurred earlier, together with a reduction of Sp1 expression and telomerase activity, in keratinocytes cultured without a feeder layer. Telomerase activity was consistently higher in keratinocytes grown on the three different feeder layers tested relative to cells grown without them. Suppression of Sp1 expression by RNA inhibition (RNAi) reduced both telomerase expression and activity in keratinocytes and also abolished their long-term growth capacity suggesting that Sp1 is a key regulator of both telomerase gene expression and cell cycle progression of primary cultured human skin keratinocytes. The results of the present study therefore suggest that the beneficial influence of the feeder layer relies on its ability to preserve telomerase activity in cultured human keratinocytes through the maintenance of stable levels of Sp1 expression.

The telomerase activator TA-65 elongates short telomeres and increases health span of adult/old mice without increasing cancer incidence

Here, we show that a small-molecule activator of telomerase (TA-65) purified from the root of Astragalus membranaceus is capable of increasing average telomere length and decreasing the percentage of critically short telomeres and of DNA damage in haploinsufficient mouse embryonic fibroblasts (MEFs) that harbor critically short telomeres and a single copy of the telomerase RNA Terc gene (G3 Terc(+/-) MEFs). Importantly, TA-65 does not cause telomere elongation or rescue DNA damage in similarly treated telomerase-deficient G3 Terc(-/-) littermate MEFs. These results indicate that TA-65 treatment results in telomerase-dependent elongation of short telomeres and rescue of associated DNA damage, thus demonstrating that TA-65 mechanism of action is through the telomerase pathway. In addition, we demonstrate that TA-65 is capable of increasing mouse telomerase reverse transcriptase levels in some mouse tissues and elongating critically short telomeres when supplemented as part of a standard diet in mice. Finally, TA-65 dietary supplementation in female mice leads to an improvement of certain health-span indicators including glucose tolerance, osteoporosis and skin fitness, without significantly increasing global cancer incidence.

Novel p63 target genes involved in paracrine signaling and keratinocyte differentiation

The transcription factor p63 is required for proper epidermal barrier formation and maintenance. Herein, we used chromatin immunoprecipitation coupled with DNA sequencing to identify novel p63 target genes involved in normal human epidermal keratinocyte (NHEKs) growth and differentiation. We identified over 2000 genomic sites bound by p63, of which 82 were also transcriptionally regulated by p63 in NHEKs. Through the discovery of interleukin-1-α as a p63 target gene, we identified that p63 is a regulator of epithelial–mesenchymal crosstalk. Further, three-dimensional organotypic co-cultures revealed TCF7L1, another novel p63 target gene, as a regulator of epidermal proliferation and differentiation, providing a mechanism by which p63 maintains the proliferative potential of basal epidermal cells. The discovery of new target genes links p63 to diverse signaling pathways required for epidermal development, including regulation of paracrine signaling to proliferative potential. Further mechanistic insight into p63 regulation of epidermal cell growth and differentiation is provided by the identification of a number of novel p63 target genes in this study.

A decisive function of transforming growth factor-β/Smad signaling in tissue morphogenesis and differentiation of human HaCaT keratinocytes

By interfering with the TGFβ/Smad pathway in the human HaCaT keratinocytes, this study provides novel insights into the role of Smad signaling for regular tissue homeostasis and demonstrates its crucial role in terminal epidermal differentiation and in the decision between alternative epithelial differentiation programs.

 The mechanism by which transforming growth factor-β (TGFβ) regulates differentiation in human epidermal keratinocytes is still poorly understood. To assess the role of Smad signaling, we engineered human HaCaT keratinocytes either expressing small interfering RNA against Smads2, 3, and 4 or overexpressing Smad7 and verified impaired Smad signaling as decreased Smad phosphorylation, aberrant nuclear translocation, and altered target gene expression. Besides abrogation of TGFβ-dependent growth inhibition in conventional cultures, epidermal morphogenesis and differentiation in organotypic cultures were disturbed, resulting in altered tissue homeostasis with suprabasal proliferation and hyperplasia upon TGFβ treatment. Neutralizing antibodies against TGFβ, similar to blocking the actions of EGF-receptor or keratinocyte growth factor, caused significant growth reduction of Smad7-overexpressing cells, thereby demonstrating that epithelial hyperplasia was attributed to TGFβ-induced “dermis”-derived growth promoting factors. Furthermore impaired Smad signaling not only blocked the epidermal differentiation process or caused epidermal-to-mesenchymal transition but induced a switch to a complex alternative differentiation program, best characterized as mucous/intestinal-type epithelial differentiation. As the same alternative phenotype evolved from both modes of Smad-pathway interference, and reduction of Smad7-overexpression caused reversion to epidermal differentiation, our data suggest that functional TGFβ/Smad signaling, besides regulating epidermal tissue homeostasis, is not only essential for terminal epidermal differentiation but crucial in programming different epithelial differentiation routes.

Copy number gain and oncogenic activity of YWHAZ/14-3-3zeta in head and neck squamous cell carcinoma

Gene amplification, a common mechanism for oncogene activation in cancers, has been used in the discovery of novel oncogenes. Low-level copy number gains are frequently observed in head and neck squamous cell carcinomas (HNSCCs) where numerous amplification events and potential oncogenes have already been reported. Recently, we applied restriction landmark genome scanning to study gene amplifications in HNSCC and located novel and uncharacterized regions in primary tumor samples. Gain on chromosome 8q22.3, the location of YWHAZ (14-3-3zeta), is found in 30-40% HNSCC cases. Data obtained from fluorescence in situ hybridization and immunohistochemistry on HNSCC tissue microarrays confirmed frequent low-level YWHAZ copy number gain and protein overexpression. YWHAZ mRNA was frequently upregulated in patients' tumor tissues. Furthermore, YWHAZ RNAi significantly suppressed the growth rate of HNSCC cell lines, and overexpression of YWHAZ in HaCaT immortalized human skin keratinocytes promotes overgrowth, as well as morphological changes. Reduced YWHAZ levels increased the G1/G0-phase proportion, decreased the S-phase proportion and the rate of DNA synthesis. Based on this evidence, we suggest that YWHAZ is a candidate proto-oncogene and deserves further investigation into its role in HNSCC carcinogenesis.

Aging and replicative senescence have related effects on human stem and progenitor cells

The regenerative potential diminishes with age and this has been ascribed to functional impairments of adult stem cells. Cells in culture undergo senescence after a certain number of cell divisions whereby the cells enlarge and finally stop proliferation. This observation of replicative senescence has been extrapolated to somatic stem cells in vivo and might reflect the aging process of the whole organism. In this study we have analyzed the effect of aging on gene expression profiles of human mesenchymal stromal cells (MSC) and human hematopoietic progenitor cells (HPC). MSC were isolated from bone marrow of donors between 21 and 92 years old. 67 genes were age-induced and 60 were age-repressed. HPC were isolated from cord blood or from mobilized peripheral blood of donors between 27 and 73 years and 432 genes were age-induced and 495 were age-repressed. The overlap of age-associated differential gene expression in HPC and MSC was moderate. However, it was striking that several age-related gene expression changes in both MSC and HPC were also differentially expressed upon replicative senescence of MSC in vitro. Especially genes involved in genomic integrity and regulation of transcription were age-repressed. Although telomerase activity and telomere length varied in HPC particularly from older donors, an age-dependent decline was not significant arguing against telomere exhaustion as being causal for the aging phenotype. These studies have demonstrated that aging causes gene expression changes in human MSC and HPC that vary between the two different cell types. Changes upon aging of MSC and HPC are related to those of replicative senescence of MSC in vitro and this indicates that our stem and progenitor cells undergo a similar process also in vivo.

Tissue context-activated telomerase in human epidermis correlates with little age-dependent telomere loss

Telomerase- and telomere length regulation in normal human tissues is still poorly understood. We show here that telomerase is expressed in the epidermis in situ independent of age but was repressed upon the passaging of keratinocytes in monolayer culture. However, when keratinocytes were grown in organotypic cultures (OTCs), telomerase was re-established, indicating that telomerase activity is not merely proliferation-associated but is regulated in a tissue context-dependent manner in human keratinocytes. While not inducible by growth factors, treatment with the histone deacetylation inhibitor FK228 restored telomerase activity in keratinocytes grown in monolayer cultures. Accordingly, CHIP analyses demonstrated an acetylated, active hTERT promoter in the epidermis in situ and in the epidermis of OTCs but a deacetylated, silenced hTERT promoter with subsequent propagation in monolayer culture suggesting that histone acetylation is part of the regulatory program to guarantee hTERT expression/telomerase activity in the epidermis. In agreement with the loss of telomerase activity, telomeres shortened during continuous propagation in monolayer culture by an average of approximately 70 base pairs (bp) per population doubling (pd). However, telomere erosion varied strongly between different keratinocyte strains and even between individual cells within the same culture, thereby arguing against a defined rate of telomere loss per replication cycle. In the epidermis in situ, as determined from early-passage keratinocytes and tissue sections from different age donors, we calculated a telomere loss of only approximately 25 bp per year. Since we determined the same rate for the non-regenerating melanocytes and dermal fibroblasts, our data suggest that in human epidermis telomerase is a protective mechanism against excessive telomere loss during the life-long regeneration.

Telomerase regulation in hematological cancers: a matter of stemness?

Human telomerase is a nuclear ribonucleoprotein enzyme complex that catalyzes the synthesis and extension of telomeric DNA. This enzyme is highly expressed and active in most malignant tumors while it is usually not or transiently detectable in normal somatic cells, suggesting that it plays an important role in cellular immortalization and tumorigenesis. As most leukemic cells are generally telomerase-positive and have often shortened telomeres, our understanding of how telomerase is deregulated in these diseases could help to define novel therapies targeting the telomere/telomerase complex. Nonetheless, considering that normal hematopoietic stem cells and some of their progeny do express a functional telomerase, it is tempting to consider such an activity in leukemias as a sustained stemness feature and important to understand how telomere length and telomerase activity are regulated in the various forms of leukemias.

Stromal control of oncogenic traits expressed in response to the overexpression of GLI2, a pleiotropic oncogene

Hedgehog signaling is often activated in tumors, yet it remains unclear how GLI2, a transcription factor activated by this pathway, acts as an oncogene. We show that GLI2 is a pleiotropic oncogene. Overexpression induces genomic instability and blocks differentiation, likely mediated in part by enhanced expression of the stem cell gene SOX2. GLI2 also induces TGFβ dependent transdifferentiation of foreskin and tongue, but not gingival fibroblasts into myofibroblasts, creating an environment permissive for invasion by keratinocytes, which are in various stages of differentiation having down regulated GLI2. Thus, up-regulated GLI2 expression is sufficient to induce a number of the acquired characteristics of tumor cells; however the stroma, in a tissue specific manner, determines whether certain GLI2 oncogenic traits are expressed.

A key role for telomerase reverse transcriptase unit in modulating human embryonic stem cell proliferation, cell cycle dynamics, and in vitro differentiation

Embryonic stem cells (ESC) are a unique cell population with the ability to self-renew and differentiate into all three germ layers. Human ESC express the telomerase reverse transcriptase (TERT) gene and the telomerase RNA (TR) and show telomerase activity, but TERT, TR, and telomerase are all downregulated during the differentiation process. To examine the role of telomerase in human ESC self-renewal and differentiation, we modulated the expression of TERT. Upregulation of TERT and increased telomerase activity enhanced the proliferation and colony-forming ability of human ESC, as well as increasing the S phase of the cell cycle at the expense of a reduced G1 phase. Upregulation of TERT expression was associated with increases in CYCLIN D1 and CDC6 expression, as well as hyperphosphorylation of RB. The differentiated progeny of control ESC showed shortening of telomeric DNA as a result of loss of telomerase activity. In contrast, the differentiated cells from TERT-overexpressing ESC maintained high telomerase activity and accumulated lower concentrations of peroxides than wild-type cells, implying greater resistance to oxidative stress. Although the TERT-overexpressing human ESC are able to form teratoma composed of three germ layers in vivo, their in vitro differentiation to all primitive and embryonic lineages was suppressed. In contrast, downregulation of TERT resulted in reduced ESC proliferation, increased G1, and reduced S phase. Most importantly, downregulation of TERT caused loss of pluripotency and human ESC differentiation to extraembryonic and embryonic lineages. Our results indicate for the first time an important role for TERT in the maintenance of human ESC pluripotency, cell cycle regulation, and in vitro differentiation capacity.

The multi-step process of human skin carcinogenesis: A role for p53, cyclin D1, hTERT, p16, and TSP-1

As proposed by Hanahan and Weinberg (2000. Cell 100, 57-70) carcinogenesis requires crucial events such as (i) genomic instability, (ii) cell cycle deregulation, (iii) induction of a telomere length maintenance mechanism, and (iv) an angiogenic switch. By comparing the expression of p53, cyclin D1, p16, hTERT, and TSP-1 in spontaneously regressing keratoacanthoma (KA) as a paradigm of early neoplasia, with malignant invasive cutaneous squamous cell carcinoma (SCC) as a paradigm of advanced tumour development, we are now able to assign the changes in the expression of these proteins to specific stages and allocate them to defined roles in the multi-step process of skin carcinogenesis. We show that mutational inactivation of the p53 gene, and with that the onset of genomic instability is the earliest event. Individual p53-positive cells are already seen in "normal" skin, and 3/5 actinic keratoses (AKs), 5/22 KAs, and 13/23 SCCs contain p53-positive patches. Cell cycle deregulation was indicated by the overexpression of the cell cycle regulator cyclin D1, as well as by the loss of the cell cycle inhibitor p16. Interestingly, overexpression of cyclin D1 - observed in 80% of KAs and SCCs, respectively - showed a cell cycle-independent function in HaCaT cell transplants on nude mice. Cyclin D1 overexpression was associated with a massive inflammatory response, finally leading to tissue destruction. Loss of the cell cycle inhibitor p16, on the other hand, correlated with SCCs. Thus, it is tempting to suggest that overexpression of cyclin D1 is an early change that in addition to growth stimulation leads to an altered epithelial-mesenchymal interaction, while functional p16 is able to control this deregulated growth and needs to be eliminated for malignant progression. Another requirement for uncontrolled growth is the inhibition of telomere erosion by up-regulating telomerase activity. As measured by hTERT protein expression, all of the KAs and SCCs studied were positive, with a similar distribution of the protein in both groups and an expression pattern resembling that of normal epidermis. Thus, telomerase may not need to be increased significantly in skin carcinomas. Finally, we show that the angiogenesis inhibitor TSP-1 is strongly expressed in most KAs, and mainly by the tumour cells, while in SCCs the generally weak expression is restricted to the tumour-stroma. Furthermore, we provide evidence that the loss of a copy of chromosome 15 is responsible for reduced TSP-1 expression and thereby this aberration contributes to tumour vascularisation (i.e. the angiogenic switch) required for malignant growth.

Improved alkaline comet assay protocol for adherent HaCaT keratinocytes to study UVA-induced DNA damage

The comet assay is one of the well-accepted tests to measure radiation-induced DNA damage. The most commonly used protocols require single-cell suspensions that are embedded in agarose in order to perform electrophoresis. For adherently growing cells such as human HaCaT skin keratinocytes this method bears several problems. We show that trypsinization required for maintaining single-cell suspensions is prolonged after UV radiation and thereby reduces cell viability and allows partial repair, with the consequence of reduced damage detection after irradiation. Therefore, we here introduce a modified version of the comet assay where HaCaT cells are seeded onto comet slides 24h before the assay and overlaid with agarose immediately after irradiation. Using this modification we are now able to reproducibly measure high DNA-damage levels (13-fold increase compared with controls) following irradiation with 60J/cm(2) UVA as well as a dose-dependent increase of DNA damage after 10, 20 and 60J/cm(2) UVA. Thus, by maintaining the cells in their natural configuration, i.e. adherently growing, we exclude several artefacts that are likely to influence the damage responses. These include: (i) trypsinization-dependent changes in cell morphology and polarity (clear lateral, i.e. adherent, and apical side of keratinocytes) which are likely of consequence for the gene-expression pattern, (ii) trypsin- and dislodgement-induced damage reducing cell viability, and (iii) the time delay between damage induction and damage evaluation to unpredictable results due to partial repair. Since these advantages pertain to all adherently growing cells, this improved protocol is not restricted to HaCaT cells but offers great potential also with all non-haematopoietic cells for obtaining accurate results and for studying repair processes in a highly reproducible manner.

Cigarette smoke components inhibited intercellular communication and differentiation in human pancreatic ductal epithelial cells

Smoking is a well-documented risk factor for the development of pancreatic adenocarcinoma. Although the most abundant polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke are methylated anthracenes and phenanthrenes, the epigenetic toxicity of these compounds has not been extensively studied. We previously showed that methylanthracenes, which possess a bay-like structure, affect epigenetic events such as an induced release of arachidonic acid, inhibition of gap junctional intercellular communication (GJIC) and induction of mitogen-activated protein kinases in a pluripotent rat liver epithelial stem cell line. Anthracenes with no bay-like structures were inactive. These biological effects are all molecular events associated with the promotional phase of cancer. A human immortalized, nontumorigenic pancreatic ductal epithelial cell line, H6c7, was examined to study the epigenetic toxicity of PAHs related to pancreatic cancer by using scrape-loading dye transfer, immunostaining, RT-PCR and telomerase assay methods. H6c7 cells were GJIC-incompetent and exhibited high telomerase activity when grown in growth factor and hormone-supplemented medium. In the presence of the cAMP elevating drugs (forskolin and IBMX) the cells became GJIC competent and expressed connexins. Telomerase activity was also decreased by cAMP elevating drug treatment. After induction of cAMP, 1-methylanthracene with bay-like structures inhibited GJIC, whereas the 2-methylanthracene lacking a bay-like structure had no effect on GJIC. Telomerase activity remained high in 1-methylanthracene treatment but not with 2-methylanthracene. These results indicate that a prominent component of cigarette smoke, namely methylanthracenes with distinct structural configurations, could be a potential etiological agent contributing to the epigenetic events of pancreatic cancer.

A Novel Regulator of Telomerase

Recently we reported a differentiation-dependent inhibition of telomerase activity in human epidermis. Consistent with this observation we found that in keratinocyte cultures calcium-induced differentiation correlates with a decline in telomerase activity. To get further support for a role of calcium in the regulation of telomerase and to elucidate the underlying molecular mechanisms we investigated the effect of calcium on telomerase in the human epidermal keratinocyte line HaCaT. Treatment with thapsigargin, which increases intracellular calcium concentrations, inhibited telomerase activity without down-regulating the expression of hTERT (human telomerase reverse transcriptase). This observation together with the fact that increasing calcium reduced telomerase activity in cell-free extracts suggests that calcium directly interacts with the telomerase complex. This interaction could be mediated by the calcium-binding protein S100A8 as indicated by its ability to mimic the inhibitory effect of calcium. S100A8-induced reduction in telomerase activity was abrogated by S100A9. The ratio of both proteins remained constant in cells treated with thapsigargin, but their interactions were altered similarly in intact cells after thapsigargin treatment and in cell-free extracts in response to calcium. We hypothesize that calcium binds to S100A8/S100A9 complexes and alters their composition, thus enabling S100A8 to interact with the telomerase complex and inhibit its activity.

Telomeres rather than telomerase a key target for anti-cancer therapy?

It was in the 1930s that telomeres (from the Greek telos = end and meros = part) were first recognized as essential structures at the ends of the chromosomes and were shown to be important for chromosomal stability (Muller HJ: The remaking of chromosomes. The Collecting Net-Woods Hole 1938: 13: 181-198, McClintock B, The stability of broken ends of chromosomes in Zea mays. Genetics 1041: 26: 234-282). However, it was only in 1978 that the first telomeric sequence was identified -- in the protocoa Tetrahymena, a single cell organism that at a certain stage of development has many identical minichromosomes with twice as many telomeres (Blackburn EH and Gall JG. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J. Mol. Biol. 1978: 120: 33-53.). Today we know that telomeres form specialized, three-dimensional DNA-protein structures and fulfil important capping functions. Besides, telomeric DNA is essential as ''access DNA'' for those cells that are not able to counteract loss of DNA during replication because they do not express telomerase, the enzyme responsible for telomere length maintenance. Since telomerase is mostly found in tumor cells and inhibition correlates with telomere shortening and finally growth inhibition, telomerase and lately also the telomeres themselves have become attractive targets for anti-cancer therapy. This review aims to critically throw light on different therapeutical approaches and comes to the conclusion that telomeres may be the better targets for cancer therapeutics.

Improving cell therapy--experiments using transplanted telomerase-immortalized cells in immunodeficient mice

Cell therapy is the use of stem cells and other types of cells in various therapies for age-related diseases. Two issues that must be addressed before cell therapy could be used routinely in medicine are improved efficacy of the transplanted cells and demonstrated long-term safety. Desirable genetic modifications that could be made to cells to be used for cell therapy include immortalization with human telomerase reverse transcriptase (hTERT). We have used a model for cell therapy in which transplantation of adrenocortical cells restores glucocorticoid and mineralocorticoid hormone levels in adrenalectomized immunodeficient mice. In this model, clones of cells that had been immortalized with hTERT were shown to be able to replace the function of the animals' adrenal glands by forming vascularized tissue structures when cells were transplanted beneath the capsule of the kidney. hTERT-modified cells showed no tendency for neoplastic changes. Moreover, a series of experiments showed that hTERT does not cooperate with known oncoproteins in tumorigenesis either in adrenocortical cells or in human fibroblasts. Nevertheless, hTERT was required for tumorigenesis when cells were implanted subcutaneously rather than in the subrenal capsule space. Changes in gene expression make hTERT-modified cells more robust. Understanding these changes is important so as to be able to separately control immortalization and other desirable properties of cells that could be used in cell therapy. Alternatively, desirable properties of transplants might be provided by co-transplanted mesenchymal cells: mesenchymal cell-assisted cell therapy. For both hTERT modification and mesenchymal cell-assisted cell therapy, genomics approaches will be needed to define what genetic modifications are desirable and safe in cells used in cell therapy.

Epidermal homeostasis in long-term scaffold-enforced skin equivalents

Epidermal homeostasis is understood as the maintenance of epidermal tissue structure and function by a fine tuned regulatory mechanism balancing proliferation and cell loss by desquamation and apoptosis. The lack of appropriate experimental models has largely prevented a better understanding of the regulatory mechanisms controlling epidermal tissue homeostasis in human skin. Keratinocyte culture studies had revealed a strict dependency of regular epidermal differentiation on dermal interactions only accomplishable in three-dimensional skin models. As major drawbacks, conventional models, employing collagen hydrogels as dermal equivalents (DEs) exhibit a rather poor stability and limited lifespan. Here, we present an improved stabilized in vitro-model for long-term growth and differentiation of keratinocytes providing the basis for tissue homeostasis. Keratinocytes were grown on DEs reinforced by modified hyaluronic acid fibers (Hyalograft-3D) and colonized with skin fibroblasts, producing genuine dermis-type matrix. These skin equivalents (SEs) develop superior epidermal architecture with regular differentiation and ultrastructure. Critical aspects of differentiation, still unbalanced in early stages, are renormalized, most strikingly the coexpression of keratins K1/K10, downregulation of regeneration-associated keratins (K16), and restriction of K15 to the basal layer. The strict localization of integrins to basal cells underlining restored tissue polarity, the drop of keratinocyte growth rates towards physiological levels and the rapid formation of a mature basement membrane with abundant anchoring fibrils are altogether features fulfilling the criteria of tissue homeostasis. Therefore, these scaffold-based SEs not only allow for studying homeostasis control but also for the first time provide proper experimental conditions for establishing a stem cell niche in vitro.

Gain of 11q/cyclin D1 overexpression is an essential early step in skin cancer development and causes abnormal tissue organization and differentiation

Non-melanoma skin cancers, in particular keratoacanthomas (KAs) and squamous cell carcinomas (SCCs), have become highly frequent tumor types especially in immune-suppressed transplant patients. Nevertheless, little is known about essential genetic changes. As a paradigm of 'early' changes, that is, changes still compatible with tumor regression, we studied KAs by comparative genomic hybridization and show that gain of chromosome 11q is not only one of the most frequent aberration (8/18), but in four tumors also the only aberration. Furthermore, 11q gain correlated with amplification of the cyclin D1 locus (10/14), as determined by fluorescence in situ hybridization, and overexpression of cyclin D1 protein (25/31), as detected by immunohistochemistry. For unraveling the functional consequence, we overexpressed cyclin D1 in HaCaT skin keratinocytes. These cells only gained little growth advantage in conventional and in organotypic co-cultures. However, although the control vector-transfected cells formed a well-stratified and orderly differentiated epidermis-like epithelium, they showed deregulation of tissue architecture with an altered localization of proliferation and impaired differentiation. The most severe phenotype was seen in a clone that additionally upregulated cdk4 and p21. These cells lacked terminal differentiation, exhibited a more autonomous growth in vitro and in vivo and even formed tumors in two injection sites with a growth pattern resembling that of human KAs. Thus, our results identify 11q13 gain/cyclin D1 overexpression as an important step in KA formation and point to a function that exceeds its known role in proliferation by disrupting tissue organization and thereby allowing abnormal growth.

Telomerase regulation of myofibroblast differentiation

Telomerase activity, which has wide expression in cancerous cells, is induced in lung proliferating fibroblasts. It is preferentially expressed in fibroblasts versus myofibroblasts. It is unknown whether regulation of telomerase expression is related to the process of fibroblast differentiation into myofibroblasts. The objective of this study was to clarify such a potential link between telomerase expression and myofibroblast differentiation. Telomerase inhibitor, 3'-azido-2',3'-dideoxythymidine, or antisense oligonucleotide to the telomerase RNA component was used to inhibit the induced fibroblast telomerase activity. The results showed that inhibition of induced telomerase increased alpha-smooth muscle actin expression, an indicator of myofibroblast differentiation. In contrast, induction of telomerase by basic fibroblast growth factor inhibited alpha-smooth muscle actin expression. These findings suggest that the loss of telomerase activity is closely associated with myofibroblast differentiation and possibly functions as a trigger for myofibroblast differentiation. Conversely, expression of telomerase suppresses myofibroblast differentiation.

Telomere-dependent chromosomal instability

Telomeres are specialized DNA-protein structures at the ends of the linear chromosomes. In mammalian cells, they are composed of multifold hexameric TTAGGG repeats and a number of associated proteins. The double-stranded telomeric DNA ends in a 3' single stranded overhang of 150 to 300 base pair (bp) which is believed to be required for a higher order structure (reviewed in (Blackburn, 2001)). One important model is that the telomeres form loop structures, the T-loops, and by invasion of the 3' overhang into the duplex region of the double stranded part protect the DNA against degradation and hinder the cellular machinery to recognize the ends as broken DNA, thus providing chromosomal integrity (Griffith et al, 1999). If telomeres become critically short they loose their capping function, become sticky, and are prone to illegitimate chromosome end-to-end fusions. The resulting dicentric chromosomes are highly unusable and because of bridge-fusion-breakage cycles they give rise to chromosomal translocations, deletions, and amplifications. Thus, critically short telomeres are thought to be responsible for the onset of genomic instability. In addition, we provide evidence that in a length-independent manner telomeres can confer to genomic instability by forming telomericaggregates which through chromosomal dys-locations contribute to chromosomal aberrations.

UV-induced Skin Cancer: Similarities - Variations. Durch UV-Strahlung induzierte Hauttumore: Gemeinsamkeiten - Unterschiede

Skin cancer, the most common cancer world wide, encompasses different tumor entities, the keratinocyte-derived basal cell carcinoma (BCC), and squamous cell carcinoma (SCC) as well as the neuroectodermal malignant melanoma (MM) and the neuroendocrine Merkel cell carcinomas (MCC). While knowledge is significantly increasing about genetic changes contributing to BCCs and MMs, our understanding for the development and progression of SCCs and MCCs is still fragmentary. This review, thus, aims, on the one hand to summarize the present knowledge without claiming completeness and, on the other hand, to provide information on the HaCaT in vitro skin carcinogenesis model that is used to evaluate the functional consequence of genetic aberrations believed to play a role in skin cancer development and progression.

Authentic fibroblast matrix in dermal equivalents normalises epidermal histogenesis and dermoepidermal junction in organotypic co-culture

Besides medical application as composite skin grafts, in vitro constructed skin equivalents (SEs) or organotypic co-cultures represent valuable tools for cutaneous biology. Major drawbacks of conventional models, employing collagen hydrogels as dermal equivalents (DEs), are a rather poor stability and limited life span, restricting studies to early phases of skin regeneration. Here we present an improved stabilised in vitro model actually providing the basis for skin-like homeostasis. Keratinocytes were grown on dermal equivalents (DEs) reinforced by modified hyaluronic acid fibres (Hyalograft-3D) and colonised with skin fibroblasts, producing genuine dermis-type matrix. These SEs developed a superior epidermal architecture with regular differentiation and ultrastructure, which occurred also faster than in SEs based on collagen-DEs. Critical aspects of differentiation, still unbalanced in early stages, were perfectly re-normalised, most strikingly the co-expression of keratins K1/K10 and downregulation of regeneration-associated keratins such as K16. The restriction of integrin and K15 distribution as well as keratinocyte proliferation to the basal layer underlined the restored tissue polarity, while the drop of growth rates towards physiological levels implied finally accomplishment of homeostasis. This correlated to faster basement membrane (BM) formation and ultrastructurally defined dermo-epidermal junction including abundant anchoring fibrils for strong tissue connection. Whereas the fibroblasts in the scaffold initially secreted a typical provisional regenerative matrix (fibronectin, tenascin), with time collagens of mature dermis (type I and III) were accumulating giving rise to an in vivo-like matrix with regularly organised bundles of striated collagen fibrils. In contrast to the more catabolic state in conventional DEs, the de novo reconstruction of genuine dermal tissue seemed to be a key element for maintaining prolonged normal keratinocyte proliferation (followed up to 8 wks), fulfilling the criteria of tissue-homeostasis, and possibly providing a stem cell niche.

Ectopic mTERT expression in mouse embryonic stem cells does not affect differentiation but confers resistance to differentiation- and stress-induced p53-dependent apoptosis

The fundamental role of telomerase is to protect telomere ends and to maintain telomere length during replication; hence, telomerase expression is high in stem cells but reduced upon differentiation. Recent studies indicate that telomerase might play other roles besides telomere maintenance. We have investigated the role of telomerase in cellular differentiation and death. Here, we show that ectopic expression of mouse telomerase catalytic subunit (mTERT) does not affect embryonic stem (ES) cell proliferation or differentiation in vitro, but protects ES cells against cell death during differentiation. Ectopic mTERT expression also confers resistance to apoptosis induced by oxidative stress and other genotoxic insults. This resistance depends on the catalytic activity of mTERT. Stress-signal-induced p53 accumulation and consequent p53-dependent apoptotic target gene expression was not affected by mTERT overexpression. However, although chemical inhibition of p53 by alpha-pifithrin reduced stress-induced apoptosis in vector-expressing cells, it did not significantly affect apoptosis in mTERT-expressing cells. Moreover, overexpression of mTERT in p53-/- ES cells did not confer further resistance to genotoxic insults, suggesting that mTERT might exert its protective effect by antagonizing the p53 pathway. Altogether, our findings indicate that ectopic mTERT expression in ES cells does not affect differentiation but confers resistance to apoptosis, and suggest that this strategy might be used in improving the efficiency of stem-cell therapies.

Myc-transformed epithelial cells down-regulate clusterin, which inhibits their growth in vitro and carcinogenesis in vivo

Effective treatment of malignant carcinomas requires identification of proteins regulating epithelial cell proliferation. To this end, we compared gene expression profiles in murine colonocytes and their c-Myc-transformed counterparts, which possess enhanced proliferative potential. A surprisingly short list of deregulated genes included the cDNA for clusterin, an extracellular glycoprotein without a firmly established function. We had previously demonstrated that in organs such as skin, clusterin expression is restricted to differentiating but not proliferating cell layers, suggesting a possible negative role in cell division. Indeed, its transient overexpression in Myc-transduced colonocytes decreased cell accumulation. Furthermore, clusterin was down-regulated in rapidly dividing human keratinocytes infected with a Myc-encoding adenovirus. Its knockdown via antisense RNA in neoplastic epidermoid cells enhanced proliferation. Finally, recombinant human clusterin suppressed, in a dose-dependent manner, DNA replication in keratinocytes and other cells of epithelial origin. Thus, clusterin appears to be an inhibitor of epithelial cell proliferation in vitro. To determine whether it also affects neoplastic growth in vivo, we compared wild-type and clusterin-null mice with respect to their sensitivity to 7, 12-dimethylbenz(a)anthracene /12-Otetradecanoylphorbol-13-acetate (DMBA/TPA)-induced skin carcinogenesis. We observed that the mean number of papillomas/mouse was higher in clusterin-null animals. Moreover, these papillomas did not regress as readily as in wild-type mice and persisted beyond week 35. The rate of progression toward squamous cell carcinoma was not altered, although those developing in clusterin-null mice were on average better differentiated. These data suggest that clusterin not only suppresses epithelial cell proliferation in vitro but also interferes with the promotion stage of skin carcinogenesis.

Mechanisms of ultraviolet (UV) B and UVA phototherapy

Ultraviolet (UV) radiation has been used for decades with great success and at a constantly increasing rate in the management of skin diseases, becoming an essential part of modern dermatologic therapy (Krutmann et al, 1999). For phototherapy, irradiation devices emitting either predominantly middlewave UV (UVB, 290-315 nm) or longwave UV (UVA, 315-400 nm) radiation are employed. In former years, patients were treated with broad-band UVB, broad-band UVA, or combination regimens. Broad-band UV phototherapy, however, is being replaced more frequently by the use of irradiation devices that allow treatment of patients' skin with selected emission spectra. Two such modalities which have their origin in European Photodermatology are 311 nm UVB phototherapy (which uses long-wave UVB radiation above 300 nm rather than broadband UVB) and high-dose UVA1 therapy (which selective employs long-wave UVA radiation above 340 nm). In Europe, 311 nm UVB phototherapy has almost replaced classical broad-band UVB phototherapy and has significantly improved therapeutic efficacy and safety of UVB phototherapy (van Welden et al, 1988; Krutmann et al, 1999). The constantly increasing use of UVA-1 phototherapy has not only improved UVA phototherapy for established indications such as atopic dermatitis (Krutmann et al, 1992a, 1998; Krutmann, 1996), but has also provided dermatologists with the opportunity to successfully treat previously untractable skin diseases, e.g., connective tissue diseases (Stege et al, 1997; Krutmann, 1997). These clinical developments have stimulated studies about the mechanisms by which UVB and UVA phototherapy work. The knowledge obtained from this work is an indispensable prerequisite to make treatment decisions on a rationale rather than an empirical basis. Modern dermatologic phototherapy has started to profit from this knowledge, and it is very likely that this development will continue and provide dermatologists with improved phototherapeutic modalities and regimens for established and new indications. This review aims to provide an overview about current concepts of the mode of action of dermatologic phototherapy. Special emphasis will be given on studies that have identified previously unrecognized immunosuppressive/anti-inflammatory principles of UV phototherapy.