Immortalization of normal human embryonic fibroblasts by introduction of either the human papillomavirus type 16 E6 or E7 gene alone

The other side of the coin: mesenchymal stromal cell immortalization beyond evasion of senescence

Mesenchymal stromal cells (MSC) are promising options to cellular therapy to several clinical disorders, mainly because of its ability to immunomodulate and differentiate into different cell types. Even though MSC can be isolated from different sources, a major challenge to understanding the biological effects is that the primary cells undergo replicative senescence after a limited number of cell divisions in culture, requiring time-consuming and technically challenging approaches to get a sufficient cell number for clinical applications. Therefore, a new isolation, characterization, and expansion is necessary every time, which increases the variability and is time-consuming. Immortalization is a strategy that can overcome these challenges. Therefore, here, we review the different methodologies available to cellular immortalization, and discuss the literature regarding MSC immortalization and the broader biological consequences that extend beyond the mere increase in proliferation potential.

E6/E7 from Beta-2-HPVs 122, 38b, and 107 possess transforming properties in a fibroblast model in vitro

Beta-2 Human papillomaviruses 38b, 107, and 122 have been frequently found in cervical cancer samples in western Mexico. Because their E6/E7 genes functions are not fully elucidated, we deepen into their transformation capabilities. To achieve this goal, primary human fibroblasts (FB) were transduced with E6/E7 genotype-specific viral particles. Additionally, E6/E7 from HPVs 16 and 18 were included as controls. All E6/E7-cell models increased their lifespan; however, it is important to highlight that FB-E6/E7-122 showed growth as accelerated as FB-E6/E7-16 and 18. Furthermore, both FB-E6/E7-38b and 122 exhibited abilities to migrate, and FB-E6/E7-122 presented high invasive capacity. On the other hand, ΔNp73 expression was found in all cell models, except for FB-pLVX (empty-vector). Finally, RNAseq found differentially expressed genes enriched in signaling pathways related to cell cycle, epithelial-mesenchymal transition, and cancer, among others. This study shows for the first time, the great transformative potential that genotypes of the Beta-2 also possess, especially HPV122. These Beta-2 HPVs can modulate some of the genes that are well known to be regulated by Alpha-HPVs, however, they also possess alternative strategies to modulate diverse signaling pathways. These data support the idea that Beta-2 HPVs should play an important role in co-infections with Alpha-HPV during carcinogenesis.

Generation and characterization of human Fetal membrane and Decidual cell lines for reproductive biology experiments†

Human fetal membrane and maternal decidua parietalis form one of the major feto-maternal interfaces during pregnancy. Studies on this feto-maternal interface is limited as several investigators have limited access to the placenta, and experience difficulties to isolate and maintain primary cells. Many cell lines that are currently available do not have the characteristics or properties of their primary cells of origin. Therefore, we created, characterized the immortalized cells from primary isolates from fetal membrane-derived amnion epithelial cells, amnion and chorion mesenchymal cells, chorion trophoblast cells and maternal decidua parietalis cells. Primary cells were isolated from a healthy full-term, not in labor placenta. Primary cells were immortalized using either a HPV16E6E7 retroviral or a SV40T lentiviral system. The immortalized cells were characterized for the morphology, cell type-specific markers, and cell signalling pathway activation. Genomic stability of these cells was tested using RNA seq, karyotyping, and short tandem repeats DNA analysis. Immortalized cells show their characteristic morphology, and express respective epithelial, mesenchymal and decidual markers similar to that of primary cells. Gene expression of immortalized and primary cells were highly correlated (R = 0.798 to R = 0.974). Short tandem repeats DNA analysis showed in the late passage number (>P30) of cell lines matched 84-100% to the early passage number (<P10) of the cell lines revealing there were no genetic drift over the passages. Karyotyping also revealed no chromosomal anomalies. Creation of these cell lines can standardize experimental approaches, eliminate subject to subject variabilities, and benefit the reproductive biological studies on pregnancies by using these cells.

Conditional reprogramming: next generation cell culture

Long-term primary culture of mammalian cells has been always difficult due to unavoidable senescence. Conventional methods for generating immortalized cell lines usually require manipulation of genome which leads to change of important biological and genetic characteristics. Recently, conditional reprogramming (CR) emerges as a novel next generation tool for long-term culture of primary epithelium cells derived from almost all origins without alteration of genetic background of primary cells. CR co-cultures primary cells with inactivated mouse 3T3-J2 fibroblasts in the presence of RHO-related protein kinase (ROCK) inhibitor Y-27632, enabling primary cells to acquire stem-like characteristics while retain their ability to fully differentiate. With only a few years' development, CR shows broad prospects in applications in varied areas including disease modeling, regenerative medicine, drug evaluation, drug discovery as well as precision medicine. This review is thus to comprehensively summarize and assess current progress in understanding mechanism of CR and its wide applications, highlighting the value of CR in both basic and translational researches and discussing the challenges faced with CR.

Cellular models for human cardiomyopathy: What is the best option?

The genetic cardiomyopathies are a group of disorders related by abnormal myocardial structure and function. Although individually rare, these diseases collectively represent a significant health burden since they usually develop early in life and are a major cause of morbidity and mortality amongst affected children. The heterogeneity and rarity of these disorders requires the use of an appropriate model system in order to characterize the mechanism of disease and develop useful therapeutics since standard drug trials are infeasible. A common approach to study human disease involves the use of animal models, especially rodents, but due to important biological and physiological differences, this model system may not recapitulate human disease. An alternative approach for studying the metabolic cardiomyopathies relies on the use of cellular models which have most frequently been immortalized cell lines or patient-derived fibroblasts. However, the recent introduction of induced pluripotent stem cells (iPSCs), which have the ability to differentiate into any cell type in the body, is of great interest and has the potential to revolutionize the study of rare diseases. In this paper we review the advantages and disadvantages of each model system by comparing their utility for the study of mitochondrial cardiomyopathy with a particular focus on the use of iPSCs in cardiovascular biology for the modeling of rare genetic or metabolic diseases.

Promoter methylation status of the tumor suppressor genes p16 and cadherin 1 in cervical intraepithelial neoplasia

Cervical cancer is the second most common female cancer worldwide. DNA methylation is one of a number of epigenetic regulation mechanisms leading to gene silencing in neoplastic cells. Aberrant methylation results in the silencing of tumor suppressor gene expression, and has been detected in a high percentage of human cancers. In the present study, the methylation status of three tumor suppressor genes, retinoic acid receptor β (RARβ), p16 and cadherin 1 (CDH1), and the inflammatory-associated cyclooxygenase-2 (COX-2) gene, was examined at distinct stages of cervical intraepithelial neoplasia (CIN). The results of the present study revealed that the COX-2 gene was unmethylated between CIN I and carcinoma specimens. The RARβ gene exhibited a minimal change in methylation frequency, whereas the CDH1 methylation level was increased <2-fold between CIN I and carcinoma. Notably, the methylation frequency of p16 was 13.2% in normal specimens; 18.2% in CIN I; 35.7% in CIN II; 31.6% in CIN III; and 15.4% in carcinoma. By contrast, the methylation frequency of p16 increased between CIN I and carcinoma in the absence of high-risk group papillomaviruses. The results of bisulfite sequencing indicated that the 10 CpG sites were all methylated in p16 gene methylation-positive individuals. The results of the present study demonstrate that the methylation frequency of p16 and CDH1 was progressively increased during the development of malignant stages in CIN, and may be an additional tool for current cytomorphology-based screening of cervical cell specimens.

Extremely stringent activation of p16INK4a prevents immortalization of uterine cervical epithelial cells without human papillomavirus oncogene expression

Cervical epithelial cell immortalization with defined genetic factors without viral oncogenes has never been reported. Here we report that HPV-negative cervical epithelial cells failed to be immortalized by telomerase activation or the combination of p53 knockdown and telomerase activation. Under those conditions, p16^INK4a expression was always elevated during the late stage of limited cell lifespan, suggesting that cervical epithelial cells possess an intrinsic property of uniquely stringent activation of p16^INK4a, which may offer an explanation for the rarity of HPV-negative cervical cancer. Combining p16^INK4a knockdown with telomerase activation resulted in efficient immortalization of HPV-negative cervical epithelial cells under ordinary culture conditions. Compared with the HPV16-E6E7-immortalized cell lines derived from the same primary cell sources, the novel HPV-negative immortalized cell lines had lower degrees of chromosomal instability, maintained more sensitive p53/p21 response to DNA damage, exhibited more stringent G2 checkpoint function, and were more resistant to replication-stress-induced genomic instability. The newly immortalized HPV-negative cervical epithelial cell lines were non-tumorigenic in nude mice. The cell lines can be used not only as much-needed HPV-negative non-malignant cell models but also as starting models that can be genetically manipulated in a stepwise fashion to investigate the roles of defined genetic alterations in the development of HPV-negative cervical cancer.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

Low-molecular-weight inhibitors of cell differentiation enable efficient growth of mouse iPS cells under feeder-free conditions

Embryonic stem cells and induced pluripotent stem (iPS) cells are usually maintained on feeder cells derived from mouse embryonic fibroblasts (MEFs). In recent years, the cell culture of iPS cells under serum- and feeder-free conditions is gaining attention in overcoming the biosafety issues for clinical applications. In this study, we report on the use of multiple small-molecular inhibitors (i.e., CHIR99021, PD0325901, and Thiazovivin) to efficiently cultivate mouse iPS cells without feeder cells in a chemically-defined and serum-free condition. In this condition, we showed that mouse iPS cells are expressing the Nanog, Oct3/4, and SSEA-1 pluripotent markers, indicating that the culture condition is optimized to maintain the pluripotent status of iPS cells. Without these small-molecular inhibitors, mouse iPS cells required the adaptation period to start the stable cell proliferation. The application of these inhibitors enabled us the shortcut culture method for the cellular adaptation. This study will be useful to efficiently establish mouse iPS cell lines without MEF-derived feeder cells.

Tumor viruses and replicative immortality--avoiding the telomere hurdle

Tumor viruses promote cell proliferation in order to gain access to an environment suitable for persistence and replication. The expression of viral products that promote growth transformation is often accompanied by the induction of multiple signs of telomere dysfunction, including telomere shortening, damage of telomeric DNA and chromosome instability. Long-term survival and progression to full malignancy require the bypassing of senescence programs that are triggered by the damaged telomeres. Here we review different strategies by which tumor viruses interfere with telomere homeostasis during cell transformation. This frequently involves the activation of telomerase, which assures both the integrity and functionality of telomeres. In addition, recent evidence suggests that oncogenic viruses may activate a recombination-based mechanism for telomere elongation known as Alternative Lengthening of Telomeres (ALT). This error-prone strategy promotes genomic instability and could play an important role in viral oncogenesis.

A unique host defense pathway: TRIF mediates both antiviral and antibacterial immune responses

Both anti-viral and anti-bacterial host defense mechanisms involve TRIF signaling. TRIF provides early clearance of pathogens and coordination of a local inflammatory ensemble through an interferon cascade, while it may trigger organ damage. The multipotentiality of TRIF-mediated immune machinery may direct the fate of our continuous battle with microbes.

A postulated role of p130 in telomere maintenance by human papillomavirus oncoprotein E7

High-risk human papillomaviruses (HR-HPVs) infections is highly associated with the development of cervical cancer. It is now recognized that telomere length maintenance or extension is indispensable for carcinogenesis. The early oncoproteins E6 and E7 are the main malignant transformation factors of HR-HPVs and they maintain telomeres by different mechanisms, of which E6 protein activating telomerase is well documented. Reports showed that E7 protein utilized an alternative lengthen of telomere (ALT) mechanism to restore telomere length, yet the underlying molecular basis remains largely unknown. We propose that degradation of tumor suppressor pRb family member p130 plays an essential role in E7-regulated telomere extension by ALT. ALT is a mechanism based on homologous recombination (HR) between telomere sister chromatids, and a number of proteins involved in the HR pathway, such as MRN [MRE11 (meiotic recombination 11)-Rad50-NBS1 (Nijmegen breakage syndrome 1)] complex are required for the ALT pathway. Rb family member p130 could inhibit ALT by interacting with Rad50, while HPV E7 could activate ALT by degrading p130. We will make E7 mutants which are defective in p130 degradation to test whether these cells have a limited life span. Besides, immunofluorescence assay will show an ALT-related promyelocytic leukemia (PML) body (APBs) in E7-expressing cells. Although cervical cancer usually has high telomerase activities since the expressing of HPV E6, the anti-telomerase therapy will be unavailable for cervical cancer since it may activate E7-induced ALT. Our hypothesis not only enrich the knowledge of the regulation of ALT, but also indicate that p130 may serve as a potential suppressor of ALT, and gene therapy of p130 may be used in cervical cancers.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Assaying and investigating Alternative Lengthening of Telomeres activity in human cells and cancers

Alternative Lengthening of Telomeres (ALT) activity can be deduced from the presence of telomere length maintenance in the absence of telomerase activity. More convenient assays for ALT utilize phenotypic markers of ALT activity, but only a few of these assays are potentially definitive. Here we assess each of the current ALT assays and their implications for understanding the ALT mechanism. We also review the clinical situations where availability of an ALT activity assay would be advantageous. The prevalence of ALT ranges from 25% to 60% in sarcomas and 5% to 15% in carcinomas. Patients with many of these types of ALT[+] tumors have a poor prognosis.

Identification and characterization of WSG, a fusion gene associated with the proliferation of the WI-38 VA13 cells

It is well known that human fibroblasts can be immortalized using simian virus 40 (SV40) T antigen. However, the mechanisms of the SV40-immortalization processes remain unclear. In the present study, the authors identified and characterized a fusion gene, WSG (WI-38 VA13 Specific Gene), which has an integrated sequence of SV40 and chromosome 16p13. WSG is only detectable in WI-38 VA13 cells and not in other human cell lines or tissues. Transient transfection of the constructed pEGFP-WSG certified the WSG localization at the nuclear of HeLa cells. The growth assays and the knockdown experiment indicate that WSG is involved in the WI-38 VA13 cell proliferation. These results support potential capacities of WSG to be a candidate gene involved in proliferation of the WI-38 VA13 cells.

The human papillomavirus type 58 E7 oncoprotein modulates cell cycle regulatory proteins and abrogates cell cycle checkpoints

HPV type 58 (HPV-58) is the third most common HPV type in cervical cancer from Eastern Asia, yet little is known about how it promotes carcinogenesis. In this study, we demonstrate that HPV-58 E7 significantly promoted the proliferation and extended the lifespan of primary human keratinocytes (PHKs). HPV-58 E7 abrogated the G1 and the postmitotic checkpoints, although less efficiently than HPV-16 E7. Consistent with these observations, HPV-58 E7 down-regulated the cellular tumor suppressor pRb to a lesser extent than HPV-16 E7. Similar to HPV-16 E7 expressing PHKs, Cdk2 remained active in HPV-58 E7 expressing PHKs despite the presence of elevated levels of p53 and p21. Interestingly, HPV-58 E7 down-regulated p130 more efficiently than HPV-16 E7. Our study demonstrates a correlation between the ability of down-regulating pRb/p130 and abrogating cell cycle checkpoints by HPV-58 E7, which also correlates with the biological risks of cervical cancer progression associated with HPV-58 infection.

The epithelial-mesenchymal transition induced by keratinocyte growth conditions is overcome by E6 and E7 from HPV16, but not HPV8 and HPV38: characterization of global transcription profiles

The aim of this study was to evaluate the growth properties of primary human keratinocytes expressing E6 and E7 proteins, which are from either the beta- or alpha-genotypes, under different culture conditions. We demonstrated that keratinocytes expressing E6 and E7, from both HPV8 and 38, irreversibly underwent the epithelial-mesenchymal transition (EMT) when grown on plastic with FAD medium (F12/DMEM/5%FBS). Expression of E6/E7 from HPV16 was capable of fully overcoming the FAD-induced EMT. Immortalization was only observed in HPV16-transduced cell lines, while the more proliferating phenotype of both KerHPV8 and 38 was mainly related to FAD-induced EMT. Microarray analysis of exponentially growing cells identified 146 cellular genes that were differentially regulated in HPV16 compared to HPV8- and 38-transduced cells. A large accumulation of transcripts associated with epidermal development and differentiation was observed in HPV16-transduced cells, whereas transcripts of genes involved in the extracellular matrix, multicellular organismal processes, and inflammatory response were affected in HPV8 and 38-transduced cells.

HPV-16 E7 reveals a link between DNA replication stress, fanconi anemia D2 protein, and alternative lengthening of telomere-associated promyelocytic leukemia bodies

Expression of the high-risk human papillomavirus (HPV-16) E7 oncoprotein extends the life span of primary human keratinocytes and partially restores telomere length in the absence of telomerase. The molecular basis of this activity is incompletely understood. Here, we show that HPV-16 E7 induces an increased formation of alternative lengthening of telomeres (ALT)-associated promyelocytic leukemia bodies (APBs) in early passage primary human keratinocytes as well as HPV-negative tumor cells. This activity was found to require sequences of HPV-16 E7 involved in degradation of the retinoblastoma tumor suppressor protein as well as regions in the COOH terminus. HPV-16 E7-induced APBs contained ssDNA and several proteins that are involved in the response to DNA replication stress, most notably the Fanconi anemia D2 protein (FANCD2) as well as BRCA2 and MUS81. In line with these results, we found that FANCD2-containing APBs form in an ATR-dependent manner in HPV-16 E7-expressing cells. To directly show a role of FANCD2 in ALT, we provide evidence that knockdown of FANCD2 rapidly causes telomere dysfunction in cells that rely on ALT to maintain telomeres. Taken together, our results suggest a novel link between replication stress and recombination-based telomere maintenance that may play a role in HPV-16 E7-mediated extension of host cell life span and immortalization.

Mesenchymal stem cells: molecular targets for tissue engineering

Mesenchymal stem cells (MSCs) represent an adherent, fibroblast-like population present not only in the bone marrow, but in a number of tissues, including blood, adipose tissue, muscle, and dermis. Their extensive proliferation and transdifferentiation potential makes them best suited for tissue engineering applications. Identification of growth factors and signaling pathways involved in self-renewal and differentiation is important for designing strategies to overcome replicative senescence and attain directed differentiation. Wnt, bone morphogenetic protein (BMP), and Notch pathways have been implicated to play key roles in self-renewal and differentiation of hematopoietic, intestinal, and epidermal stem cells. They are also involved in regulating MSC differentiation. However, MSC self-renewal has not received much attention, with Nucleostemin being the only recently identified proliferation molecule. Although immortalization using viral oncogenes and telomerase has been achieved, transformation in long-term cultures is a potential risk. Understanding of the mechanisms governing osteogenic differentiation of MSCs is expanding with the recent identification of two major transcription factors, Osterix and Runx2. Enhanced expansion as well as osteogenic differentiation of MSCs can be attained using a combinatorial approach involving co-expression of proliferation and differentiation genes. However, a thorough understanding of the molecular mechanism is necessary for enhancing the self-renewal ability and osteogenic potential in vitro.

p16INK4A-silencing augments DNA damage-induced apoptosis in cervical cancer cells

p16(INK4A) (p16) has been suggested to be an early biomarker for the detection of cervical cancer. However, its functional role in cervical cancer is not well characterized. In this study, we reported the consistent and significant upregulation of p16 in cervical cancer tissues when compared to both matched non-tumourous tissues of the same patient and normal cervical tissues from non-cancer patients. We have employed p16 small interfering RNA (siRNA) to dissect the role of p16 in cervical carcinogenesis. Although the silencing of p16 was accompanied by the upregulation of p53, p21 and RB in the p16 siRNA-transfected cells, no significant effect on cell cycle progression was observed. When the p16 siRNA-silenced cells were subjected to DNA damage stress including ultraviolet-irradiation and cisplatin treatments, a significantly higher percentage of apoptotic cells could be observed in the p16-siRNA silenced cells compared to control siRNA-treated cells. Moreover, induction of apoptosis was associated with the activation of p53 through phosphorylation, and this process, when studied by gene profiling experiments, involved both the intrinsic and extrinsic apoptotic pathways. The observation that silencing of p16 expression augments DNA damage-induced apoptosis in cervical cancer cells offers alternative strategies for anti-cancer therapies for human cervical cancer.

The human papillomavirus E6 oncogene dysregulates the cell cycle and contributes to cervical carcinogenesis through two independent activities

Cervical cancer is a leading cause of death due to cancer among women worldwide. Using transgenic mice to dissect the contributions of the human papillomavirus (HPV) 16 E6 and E7 oncogenes in cervical cancer, E7 was identified previously to be the dominant oncogene. Specifically, when treated with exogenous estrogen for 6 months, E7 transgenic mice developed cancer throughout the reproductive tract, but E6 transgenic mice did not. E6 contributed to carcinogenesis of the reproductive tract, as E6/E7 double transgenic mice treated for 6 months with estrogen developed larger cancers than E7 transgenic mice. In the current study, we investigated whether the E6 oncogene alone could cooperate with estrogen to induce cervical cancer after an extended estrogen treatment period of 9 months. We found that the E6 oncogene synergizes with estrogen to induce cervical cancer after 9 months, indicating that E6 has a weaker but detectable oncogenic potential in the reproductive tract compared with the E7 oncogene. Using transgenic mice that express mutant forms of HPV16 E6, we determined that the interactions of E6 with cellular alpha-helix and PDZ partners correlate with its ability to induce cervical carcinogenesis. In analyzing the tumors arising in E6 transgenic mice, we learned that E6 induces expression of the E2F-responsive genes, Mcm7 and cyclin E, in the absence of the E7 oncogene. E6 also prevented the expression of p16 in tumors of the reproductive tract through a mechanism mediated by the interaction of E6 with alpha-helix partners.

Bmi-1 cooperates with human papillomavirus type 16 E6 to immortalize normal human oral keratinocytes

Bmi-1 is a member of the polycomb group (PcG) transcription repressors and is implicated in human carcinogenesis. In normal human oral keratinocytes (NHOK), we found that exogenous Bmi-1 expression significantly extended the replicative life span without causing cellular immortalization. Immortalization of NHOK occurs only in combination with human papillomavirus type 16 E6 (HPV-16 E6) but not with E7. During immortalization of NHOK by sequential expression of exogenous Bmi-1 and E6, telomerase activation was observed only after the cells had overcome crisis. Genetic analysis with E6 deletion mutants revealed that the intact second zinc finger domain (amino acids 118-122) was necessary for its cooperative effects with Bmi-1 in the immortalization process. Using these mutants, we found that the increased telomerase activity was closely associated with cell immortalization by Bmi-1 and E6, whereas p53 degradation was not. Using microarray analysis, we identified genes that are immortalization-specific and may participate in the process of NHOK immortalization by Bmi-1 and HPV-16 E6. Our results provide new information on the roles of Bmi-1 and HPV-16 E6 in the multi-step process of oral epithelial carcinogenesis.

The Rb-related p130 protein controls telomere lengthening through an interaction with a Rad50-interacting protein, RINT-1

The oncogenic process often leads to a loss of normal telomere length control, usually as a result of activation of telomerase. Nevertheless, there are also telomerase-independent events that involve a Rad50-dependent recombination mechanism to maintain telomere length. Previous work has implicated the Rb family of proteins in the control of telomere length, and we now demonstrate that the p130 member of the Rb family is critical for telomere length control. p130 interacts specifically with the RINT-1 protein, previously identified as a Rad50-interacting protein. We further show that RINT-1 is essential for telomere length control. We propose that p130, forming a complex with Rad50 through RINT-1, blocks telomerase-independent telomere lengthening in normal cells. Given previous work implicating E2F in the control of telomerase gene expression, these results thus point to complementary roles for the Rb/E2F pathway in the control of telomere length.

Immortalized feeders for the scale-up of human embryonic stem cells in feeder and feeder-free conditions

Human embryonic stem cells (hESC) are pluripotent cells that proliferate indefinitely in culture, whilst retaining their capacity for differentiation into different cell types. However, hESC cultures require culture in direct contact with feeder cells or conditioned medium (CM) from feeder cells. The most common source of feeders has been primary mouse embryonic fibroblast (MEF). In this study, we immortalized a primary MEF line with the E6 and E7 genes from HPV16. The immortal line, DeltaE-MEF, was able to proliferate beyond 7-9 passages and has an extended lifespan beyond 70 passages. When tested for its ability to support hESC growth, it was found that hESC continue to maintain the undifferentiated morphology for >40 passages both in co-culture with DeltaE-MEF and in feeder-free cultures supplemented with CM from DeltaE-MEF. The cultures also continue to express the pluripotent markers, Oct-4, SSEA-4, Tra-1-60, Tra-1-81, alkaline phosphatase and maintain a normal karyotype. In addition, these hESC formed teratomas when injected into SCID mice. Lastly, we demonstrated the feasibility of scaling-up significant quantities of undifferentiated hESC (>10(8) cells) using DeltaE-MEF in cell factories. The results from this study suggest that immortalized feeders can provide a consistent and reproducible source of feeders for hESC expansion and research.

TGF-beta2 allows pluripotent human embryonic stem cell proliferation on E6/E7 immortalized mouse embryonic fibroblasts

In this study we report observations that mouse embryonic fibroblasts (MEF) capable of supporting expansion of pluripotent, human embryonic stem cells (hESC) fail to support after immortalization using E6/E7 oncogenes in serum conditions; however this can be reversed following addition of exogenous TGF-beta2. Microarray analysis of immortalized and non-immortalized MEF revealed differential gene expression of several TGF-beta related genes. By supplementing TGF-beta2 into E6/E7 immortalized MEF cultures, this enabled proliferation of undifferentiated, pluripotent hESC as demonstrated by marker expression (Oct-4, SSEA-4, alkaline phosphatase) and teratoma formation representing three germ layers following hESC injection into immuno-deficient mice. Subsequent investigation using quantitative real-time PCR highlighted differential gene expression of several extracellular matrix related transcripts in primary and immortal (+/-TGF-beta2) feeder cells including the induction of osteopontin following addition of TGF-beta2. Our results demonstrate that TGF-beta2 and its related genes in MEF play a role in the support of pluripotent hESC expansion.

Reversible Conversion of Immortal Human Cells from Telomerase-Positive to Telomerase-Negative Cells

Immortal cell lines and tumors maintain their telomeres via the telomerase pathway or via a telomerase-independent pathway, referred to as alternative lengthening of telomeres (ALT). Here, we show the reversible conversion of the human papillomavirus type 16 E6-induced immortal human fibroblasts E6 Cl 6 from telomerase-positive (Tel(+)) to telomerase-negative (Tel(-)) cells. Tel(+) cells converted spontaneously to Tel(-) cells that reverted to Tel(+) cells following treatment with trichostatin A (TSA) and/or 5-aza-2'-deoxycytidine (5-AZC), which induced the reversion from complete to partial methylation of the CpG islands of the human telomerase reverse transcriptase (hTERT) promoter in Tel(-) E6 Cl 6 cells. Tel(-) E6 Cl 6 cells lacked the phenotypes characteristic of ALT cell lines such as very long and heterogenous telomeres and ALT-associated promyelocytic leukemia nuclear bodies (APB) but grew for >240 population doublings (PD) after they became telomerase negative. The ratios of histone H3 (H3) lysine (K) 9 methylation to each of H3-K4 methylation, H3-K9 acetylation, and H3-K14 acetylation of the chromatin containing the hTERT promoter in Tel(-) E6 Cl 6 cells and ALT cell lines were greater than those in Tel(+) cells and decreased following treatment with TSA and/or 5-AZC, inversely corresponding to telomerase activity. Our findings suggest the possibility that human tumors may be able to reversibly interconvert their telomere maintenance phenotypes by chromatin structure-mediated regulation of hTERT expression.

The role of HPV oncoproteins and cellular factors in maintenance of hTERT expression in cervical carcinoma cells

OBJECTIVE

E6 and E7 oncoproteins of high risk type HPV modulate activities of host components in cell cycle regulation. Many of these factors are also involved in the regulation of telomerase activity or the expression of hTERT, the catalytic subunit. Transcription of E6 and E7 is inhibited by the papillomavirus E2 protein, and ectopic expression of E2 in HeLa cells has been shown to cause activation of the p53-growth inhibitory pathway and downregulation of the hTERT gene. In this study, using E2 transduction in HeLa cells, the relative importance of host and viral factors in the maintenance of hTERT and telomerase activity in cervical carcinoma cells was investigated.

METHODS

Depletion of E6/E7 proteins, concomitant upregulation of p53, p21WAF1, and hypophosphorylated Rb, and downregulation of E2F1, c-Myc, and hTERT were achieved in HeLa cells through SV40-mediated E2 transduction. And, through gene transduction, E6 and E7 proteins were separately re-expressed in HeLa cells that were devoid of these proteins. As well, E2F1, c-Myc, and p53 were ectopically expressed in HeLa cells to ascertain their effect on the level of hTERT expression through RT-PCR, Western blotting, and TRAP assays.

RESULTS

Continued expression of E2F1 and c-Myc could not prevent hTERT downregulation caused by E2 transduction, but re-expression of either E6 or E7 individually reactivated hTERT expression. Finally, p53 overexpression caused repression of the hTERT gene in the presence of E6 and E7.

CONCLUSION

HPV E6 plays an important role in the induction and maintenance of high levels of hTERT in cervical carcinoma cells through direct stimulation of hTERT promoter and prevention of the inhibitory effects of p53. E7, but not E2F1, may contribute to high telomerase activity in cancer cells.

Immortalization without neoplastic transformation of human mesenchymal stem cells by transduction with HPV16 E6/E7 genes

hMSCs derived from bone marrow are useful as a species-specific cell culture system for studying cell lineage differentiation and tissue remodeling. However, hMSCs usually have a short in vitro life span due to replicative senescence. We therefore used a high dose of retroviral vector LXSN-16E6E7 to transduce hMSCs of an aging donor and obtained an actively proliferating cell line, designated KP-hMSCs, which expressed HPV16 E6/E7 mRNA. Whereas parental hMSCs ceased to grow after 30 PDs, KP-hMSCs could be propagated beyond 100 PDs. With culture procedures to avoid selection pressure and crowded cell growth, KP-hMSCs showed no signs of neoplastic transformation as examined by soft-agar anchorage-independent growth and NOD-SCID mouse tumorigenicity assays. KP-hMSCs gave similar cytofluorimetric profiles of 31 CD markers to those of the parental primary hMSCs, except with some morphologic changes and expansion of an originally very minor CD34(dim)CD38(+)CD50+ cell population. Upon exposure to specific stimulating conditions in vitro, KP-hMSCs could respond and differentiate along the mesenchymal (bone, fat and cartilage) and nonmesenchymal (neuron) cell lineages. Our results indicated that hMSCs could be immortalized by transduction with HPV16 E6/E7, maintained without neoplastic transformation by careful culture procedures and thus useful for stem cell research and clinical application.