Genetic complementation of the immortal phenotype in group D cell lines by introduction of chromosome 7

GNG11 (G-protein subunit γ 11) suppresses cell growth with induction of reactive oxygen species and abnormal nuclear morphology in human SUSM-1 cells

Enforced expression of GNG11, G-protein subunit γ 11, induces cellular senescence in normal human diploid fibroblasts. We here examined the effect of the expression of GNG11 on the growth of immortalized human cell lines, and found that it suppressed the growth of SUSM-1 cells, but not of HeLa cells. We then compared these two cell lines to understand the molecular basis for the action of GNG11. We found that expression of GNG11 induced the generation of reactive oxygen species (ROS) and abnormal nuclear morphology in SUSM-1 cells but not in HeLa cells. Increased ROS generation by GNG11 would likely be caused by the down-regulation of the antioxidant enzymes in SUSM-1 cells. We also found that SUSM-1 cells, even under normal culture conditions, showed higher levels of ROS and higher incidence of abnormal nuclear morphology than HeLa cells, and that abnormal nuclear morphology was relevant to the increased ROS generation in SUSM-1 cells. Thus, SUSM-1 and HeLa cells showed differences in the regulation of ROS and nuclear morphology, which might account for their different responses to the expression of GNG11. Thus, SUSM-1 cells may provide a unique system to study the regulatory relationship between ROS generation, nuclear morphology, and G-protein signaling.

The role of the MORF/MRG family of genes in cell growth, differentiation, DNA repair, and thereby aging

The discovery that replicative cellular senescence is a dominant phenotype over immortality led to the discovery that there are at least four unique genetic subgroups of immortal cell lines that use distinct mechanistic pathways to evade cell cycle exit. Study of one of these genetic complementation groups demonstrated that one gene, MORF4, possessed the ability to induce senescence in group B cell lines. The MRG family of genes, of which MORF4 is a member, has since proven important for cellular aging, proliferation, positive and negative transcriptional regulation, and DNA damage repair. MRG15, the evolutionary ancestor of the family, is highly conserved in yeast, C. elegans, drosophila, plants, and mammals and has been implicated in chromatin remodeling in these species. Our proteomics studies have found that MRG15 is unique among mammalian genes in that it associates with both histone deacetylases and histone acetyl transferase complexes, and thus potentially plays a role in both transcriptional silencing and activation. Its knockout in mice is embryonic lethal, resulting in improper organogenesis, as well as cell proliferation and DNA damage repair defects. Future study of these genes will help clarify the role of chromatin remodeling in aging, cellular proliferation, and DNA damage repair.

Stat1 expression is not sufficient to regulate the interferon signaling pathway in cellular immortalization

DNA hypermethylation in gene promoters is an epigenetic mechanism regulating gene expression in cellular immortalization, an important step in carcinogenesis. Previously, we studied the genes dysregulated during immortalization using spontaneously immortalized fibroblasts from patients with Li-Fraumeni syndrome (LFS), who carry a germline mutation in the tumor suppressor gene p53. We found that multiple interferon (IFN) signaling pathway genes were regulated by epigenetic silencing. In this study we focused on a key regulator of that pathway, the signal transducer and transcription activator 1 (Stat1) gene. Although Stat1 is downregulated after cellular immortalization and upregulated in immortal MDAH041 cells after 5-aza-2'-deoxycytidine (5-aza-dC) treatment, we detected no methylation of the Stat1 promoter region in these cells before or after immortalization. To analyze the function of Stat1 in immortalization, we expressed Stat1 in immortal MDAH041 cells by stable infection, expecting to induce IFN-regulated genes or cellular senescence or both. However, the overexpression of Stat1 alone was not sufficient to repress the proliferation rate of immortal MDAH041 cells or induce senescence in immortal MDAH041 cells. We concluded that factor(s) additional to Stat1 (whether IFN dependent or not) are required for the immortalization of LFS fibroblasts.

Spontaneously immortalized human T lymphocytes develop gain of chromosomal region 2p13-24 as an early and common genetic event

To gain further insight into the molecular events responsible for the extended life span and immortalization of human lymphoid cells, we analyzed a series of spontaneously immortalized, IL2-dependent human T-cell lines using molecular cytogenetic techniques. Two of the cell lines were derived from normal spleen and three from patients with Nijmegen breakage syndrome (NBS), a recessive disorder characterized by a high incidence of lymphoid malignancies. Here we show that spontaneous immortalization of the five T-cell lines was associated with the acquisition of copy number gains involving chromosomal region 2p13-24 as common early alterations. In addition, we found an amplification of 8q21-24 after prolonged propagations in all three NBS-derived cell lines as well as early development of near-tetraploidy in two of these lines. Gains involving the short arm of chromosome 2 recently were found in several lymphoid malignancies. Therefore, the cell lines described here can be used for identification and characterization of genes involved in the pathogenesis of lymphoid neoplasms and would also provide a useful tool for better understanding the mechanisms responsible for cell immortalization.

Genetics of cellular senescence

Cellular senescence or replicative senescence is a state of irreversible growth arrest that somatic cells enter as a result of replicative exhaustion. This can be mimicked by culture manipulations such as Ras oncogene overexpression or treatment with various agents such as sodium butyrate and 5-azacytidine. It is believed that cellular senescence is one of the protective mechanisms against tumor formation. Genetic analyses of cellular senescence have revealed that it is dominant over immortality because whole cell fusion of normal with immortal cells yields hybrids with limited division potential. Only four complementation groups for indefinite division have been identified from extensive studies fusing different immortal human cell lines with each other. The senescence-related genes for three of the complementation groups B-D have been identified on human chromosomes 4, 1, and 7, respectively, by microcell-mediated chromosome transfer, though the existence of senescence-related genes on other chromosomes has been suggested. MORF4 was cloned as the senescence-related gene on human chromosome 4 and is a member of a new gene family, which has multiple transcription factor-like motifs. This gene family may affect cell division by modulating gene expression. Study of this novel gene family should lead to new insights regarding the mechanisms and function of cellular senescence in aging and immortalization.

Alternative lengthening of telomeres in mammalian cells

Some immortalized mammalian cell lines and tumors maintain or increase the overall length of their telomeres in the absence of telomerase activity by one or more mechanisms referred to as alternative lengthening of telomeres (ALT). Characteristics of human ALT cells include great heterogeneity of telomere size (ranging from undetectable to abnormally long) within individual cells, and ALT-associated PML bodies (APBs) that contain extrachromosomal telomeric DNA, telomere-specific binding proteins, and proteins involved in DNA recombination and replication. Activation of ALT during immortalization involves recessive mutations in genes that are as yet unidentified. Repressors of ALT activity are present in normal cells and some telomerase-positive cells. Telomere length dynamics in ALT cells suggest a recombinational mechanism. Inter-telomeric copying occurs, consistent with a mechanism in which single-stranded DNA at one telomere terminus invades another telomere and uses it as a copy template resulting in net increase in telomeric sequence. It is possible that t-loops, linear and/or circular extrachromosomal telomeric DNA, and the proteins found in APBs, may be involved in the mechanism. ALT and telomerase activity can co-exist within cultured cells, and within tumors. The existence of ALT adds some complexity to proposed uses of telomere-related parameters in cancer diagnosis and prognosis, and poses challenges for the design of anticancer therapeutics designed to inhibit telomere maintenance.

Upregulation of the gene encoding a cytoplasmic dynein intermediate chain in senescent human cells

Normal human somatic cells, unlike cancer cells, stop dividing after a limited number of cell divisions through the process termed cellular senescence or replicative senescence, which functions as a tumor-suppressive mechanism and may be related to organismal aging. By means of the cDNA subtractive hybridization, we identified eight genes upregulated during normal chromosome 3-induced cellular senescence in a human renal cell carcinoma cell line. Among them is the DNCI1 gene encoding an intermediate chain 1 of the cytoplasmic dynein, a microtubule motor that plays a role in chromosome movement and organelle transport. The DNCI1 mRNA was also upregulated during in vitro aging of primary human fibroblasts. In contrast, other components of cytoplasmic dynein showed no significant change in mRNA expression during cellular aging. Cell growth arrest by serum starvation, contact inhibition, or gamma-irradiation did not induce the DNCI1 mRNA, suggesting its specific role in cellular senescence. The DNCI1 gene is on the long arm of chromosome 7 where tumor suppressor genes and a senescence-inducing gene for a group of immortal cell lines (complementation group D) are mapped. This is the first report that links a component of molecular motor complex to cellular senescence, providing a new insight into molecular mechanisms of cellular senescence.

Novel strategies for immortalization of human hepatocytes

Normal somatic cells have a finite life span due in part to their inability to maintain telomere length and chromosome stability. Immortalization strategies based on recent advances in telomere biology and aging research have led to the creation of genetically stable, nontumorigenic immortalized cell lines. Reversible immortalization, using the Cre-lox recombination and excision system, has been developed for the expansion of primary cells for cell based clinical therapies. Immortalized human hepatocyte cell lines with differentiated liver functions would find broad applications in biomedical research, especially for pharmacology and toxicology, artificial liver support, and hepatocyte transplantation. The biological basis of these new immortalization methods and their application to human hepatocytes is reviewed.

Mutational and functional analyses reveal that ST7 is a highly conserved tumor-suppressor gene on human chromosome 7q31

Loss of heterozygosity (LOH) of markers on human chromosome 7q31 is frequently encountered in a variety of human neoplasias, indicating the presence of a tumor-suppressor gene (TSG). By a combination of microcell-fusion and deletion-mapping studies, we previously established that this TSG resides within a critical region flanked by the genetic markers D7S522 and D7S677. Using a positional cloning strategy and aided by the availability of near-complete sequence of this genomic interval, we have identified a TSG within 7q31, named ST7 (for suppression of tumorigenicity 7; this same gene was recently reported in another context and called RAY1). ST7 is ubiquitously expressed in human tissues. Analysis of a series of cell lines derived from breast tumors and primary colon carcinomas revealed the presence of mutations in ST7. Introduction of the ST7 cDNA into the prostate-cancer-derived cell line PC3 had no effect on the in vitro proliferation of the cells, but abrogated their in vivo tumorigenicity. Our data indicate that ST7 is a TSG within chromosome 7q31 and may have an important role in the development of some types of human cancer.

Identification of Mad as a repressor of the human telomerase (hTERT) gene

Activation of telomerase, which has been frequently associated with cellular immortality, may constitute a key step in the development of human cancer. De-repression in the expression of its catalytic subunit hTERT gene has been proposed to directly link to the telomerase activation in tumor cells. Little is known about the mechanism how the hTERT gene is repressed in telomerase-negative mortal cells. This study was conducted, using an expression cloning approach, with the aim of identifying the gene(s) responsible for repressing the hTERT gene expression. Using this genetic screen, we isolated the transcription factor Mad as a repressor. Mutation of its DNA binding sites caused significant de-repression of hTERT promoter activity in mortal cells. This Mad-mediated repression of the hTERT promoter in mortal cells was counteracted by ectopic expression of Myc. The antagonism between Mad and Myc was also observed with an endogenous hTERT promoter. Their potential roles in differential hTERT promoter activities were further supported by the relative amounts of Mad and Myc proteins detected in immortal and mortal cells. Thus, Mad may be a direct negative regulator of hTERT in mortal cells and this repression mechanism can be inhibited by induction of Myc in immortal cells.

The Wilms' tumor 1 tumor suppressor gene represses transcription of the human telomerase reverse transcriptase gene

Regulation of the human telomerase reverse transcriptase (hTERT) gene is the primary determinant for telomerase enzyme activity, which is found in tumor cells but is largely absent from normal somatic cells. Recent studies have shown that Myc protein can transcriptionally activate the hTERT gene. However, little is known about the repression mechanism of the hTERT gene and telomerase enzyme. Here, we developed an expression cloning strategy to identify cDNAs whose products can repress hTERT promoter activity in telomerase-positive immortal cells. Using this screen, we isolated the Wilms' tumor 1 suppressor gene (WT1). WT1 can repress hTERT promoter activity in 293 kidney cells. The WT1 binding site on the hTERT promoter was identified by deletional analysis. Alteration of the WT1 binding site markedly derepresses transcription from an isolated hTERT promoter by inhibiting interaction of WT1 with DNA. These specific repression effects of WT1 were not observed in HeLa cells, which express no endogenous WT1. Furthermore, we show that WT1 can repress the endogenous hTERT promoter and telomerase enzyme activities. These results suggest that WT1 may be a transcriptional repressor of the hTERT gene, at least in some specific cells.

Telomerase activity in hybrids between telomerase-negative and telomerase-positive immortal human cells is repressed in the different complementation groups but not in the same complementation group of immortality

The expression of telomerase is essential for cells to be immortalized, and most immortal cell lines possessed telomerase activity. Using the cell fusion technique, it has been shown that mortal and telomerase-negative phenotypes of normal cells are dominant over immortal and telomerase-positive phenotypes, suggesting that the normal cells possessed dominant repressor-type activity for telomerase expression. Several telomerase-negative immortal human cell lines were reported, in which telomerase-independent mechanisms was supposed to maintain telomere length. We aimed at seeing whether the telomerase-negative phenotype of these immortal cells is dominant over telomerase-positive phenotype of other immortal cells in correlation with cellular mortality. Results showed that, when telomerase-positive and -negative immortal parental cell lines belonging to the different complementation groups were fused, telomerase-negative mortal hybrid clones arose, i.e. telomerase-negative phenotype was dominant as well as mortal phenotype. However, when immortal hybrid cells arose from telomerase-positive and -negative immortal parents belonging to either the same or different complementation groups, they were all telomerase-positive, i.e. telomerase-negative phenotype appeared to be recessive. Telomerase-negative immortal hybrid was never established from any combinations between telomerase-negative and -positive immortal parental cells.

Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Some immortalized cell lines maintain their telomeres in the absence of detectable telomerase activity by an alternative (ALT) mechanism. To study how telomere maintenance is controlled in ALT cells, we have fused an ALT cell line GM847 (SV40 immortalized human skin fibroblasts) with normal fibroblasts or with telomerase positive immortal human cell lines and have examined their proliferative potential and telomere dynamics. The telomeres in ALT cells are characteristically very heterogeneous in length, ranging from very short to very long. The ALT x normal hybrids underwent a rapid reduction in telomeric DNA and entered a senescence-like state. Immortal segregants rapidly reverted to the ALT telomere phenotype. Fusion of ALT cells to telomerase-positive immortal cells in the same immortalization complementation group resulted in hybrids that appeared immortal and also exhibited repression of the ALT telomere phenotype. In these hybrids, which were all telomerase-positive, we observed an initial rapid loss of most long telomeres, followed either by gradual loss of the remaining long telomeres at a rate similar to the rate of telomere shortening in normal telomerase-negative cells, or by maintenance of shortened telomeres. These data indicate the existence of a mechanism of rapid telomere deletion in human cells. They also demonstrate that normal cells and at least some telomerase-positive immortal cells contain repressors of the ALT telomere phenotype.

Identification of a gene that reverses the immortal phenotype of a subset of cells and is a member of a novel family of transcription factor-like genes

Based on the dominance of cellular senescence over immortality, immortal human cell lines have been assigned to four complementation groups for indefinite division. Human chromosomes carrying senescence genes have been identified, including chromosome 4. We report the cloning and identification of a gene, mortality factor 4 (MORF 4), which induces a senescent-like phenotype in immortal cell lines assigned to complementation group B with concomitant changes in two markers for senescence. MORF 4 is a member of a novel family of genes with transcription factor-like motifs. We present here the sequences of the seven family members, their chromosomal locations, and a partial characterization of the three members that are expressed. Elucidation of the mechanism of action of these genes should enhance our understanding of growth regulation and cellular aging.

A repressor function for telomerase activity in telomerase-negative immortal cells

Human telomerase, a ribonucleoprotein that adds TTAGGG repeats onto telomeres and compensates for their shortening, is repressed in most normal human somatic cells. Human somatic cells are considered to have a limited proliferation capacity because of the telomere shortening. Although immortalization of somatic cells is often associated with telomerase reactivation, there are some immortal cells in which telomerase activity is undetectable. In these cells, telomeres may be maintained by an unknown mechanism other than telomerase reactivation. To examine the genetic regulation of telomerase activity, we constructed hybrids between immortal cells with (HepG2) and without (KMST6) telomerase activity. These two cell lines had relatively short and long telomeres, respectively. The hybrid cells continued to proliferate without detectable telomerase activity even after 100 population doublings. Telomerase-positive subpopulations occasionally appeared after serial passages. Southern blot analysis revealed that the hybrids had long terminal restriction fragments similar to that of KMST6, regardless of telomerase activity, and fluorescence in situ hybridization with a telomeric probe showed high-intensity hybridization signals on telomeres, indicating relatively long telomeric repeats. These results suggest that the telomerase-negative immortal cells contain a gene or genes functioning as a telomerase repressor and maintain telomere length by a dominant mechanism other than telomerase reactivation.

Reassessment of immortalization complementation group D

Previous somatic cell hybridization studies have assigned many human cell lines to one of four complementation groups (A-D) for immortalization. We report here that the A1698DM cell line, which contains selectable markers and has previously been defined as the immortalization group D representative, was derived from T24 cells rather than A1698. A1698DM did not undergo senescence when fused with cell lines assigned to groups A, B, or C. This raises the possibility that this cell line has undergone further evolution and lost multiple putative senescence genes so that it is now unable to complement any, or most, other cell lines for senescence. Cell lines previously assigned to group D may, therefore, be heterogeneous with respect to the genetic changes that resulted in their immortalization. This has important implications for strategies to clone senescence genes based on complementation groups.

Replicative senescence: implications for in vivo aging and tumor suppression

Normal cells have limited proliferative potential in culture, a fact that has been the basis of their use as a model for replicative senescence for many years. Recent molecular analyses have identified numerous changes in gene expression that occur as cells become senescent, and the results indicate that multiple levels of control contribute to the irreversible growth arrest. These include repression of growth stimulatory genes, overexpression of growth inhibitory genes, and interference with downstream pathways. Studies with cell types other than fibroblasts will better define the role of cell senescence in the aging process and in tumorigenesis.

Evidence for two senescence loci on human chromosome 1

Microcell-mediated introduction of a neo-tagged human chromosome 1 (HC-1-neo) into several immortal cell lines has previously been shown to induce growth arrest and phenotypic changes indicative of replicative senescence. Somatic cell hybridization studies have localized senescence activity for immortal hamster 10W-2 cells to a cytogenetically defined region between 1q23 and the q terminus. Previous microcell-mediated chromosome transfer experiments showed that a chromosome 1 with an interstitial q-arm deletion (del-1q) lacks senescence inducing activity for several immortal human cell lines that are sensitive to an intact HC-1-neo. In contrast, our studies reveal that the del-1q chromosome retains activity for 10W-2 cells, indicating that there are at least two senescence genes on human chromosome 1. Sequence-tagged site (STS) content analysis revealed that the q arm of the del-1q chromosome has an interstitial deletion of approximately 63 centimorgans (cM), between the proximal STS marker DIS534 and distal marker DIS412, approximately 1q12 to 1q31. This deletion analysis provides a candidate region for one of the senescence genes on 1q. In addition, because this deletion region extends distally beyond 1q23, it localizes the region containing a second senescence gene to approximately 1q31-qter, between DIS422 and the q terminus. STS content analysis of a panel of 11 10W-2 microcell hybrid clones that escaped senescence identified 2 common regions of loss of 1q material below the distal breakpoint of del-1q. One region is flanked by markers DIS459 and ACTN2, and the second lies between markers WI-4683 and DIS1609, indicating that the distal 1q senescence gene(s) localizes within 1q42-43.

Expression of the human cGMP-dependent protein kinase II gene is lost upon introduction of SV40 T antigen or immortalization in human cells

We have cloned a human cGMP-dependent protein kinase type II cDNA to examine its gene expression in terms of cellular senescence and/or immortalization. The genetic locus was mapped to band 4q21 by FISH. Northern blot analysis revealed that expression of the type II gene was markedly decreased or lost in mortal or immortal human fibroblasts producing SV40 T antigen. Also in various immortalized cell lines tested, the gene was not expressed. In normal diploid fibroblasts, the gene was constitutively expressed during cell-cycle and population doubling levels (PDLs).