Limited lifespan in somatic cell hybrids and cybrids

Pancreatic ductal cells acquire mesenchymal characteristics through cell fusion with bone marrow-derived mesenchymal stem cells and SIRT1 attenuates the apoptosis of hybrid cells

Bone marrow-derived mesenchymal stem cells (bMSCs) contribute to tissue repair and regeneration. Cell fusion between somatic cells and bMSCs to form hybrid cells may have an important role in tissue repair through the subsequent reprogramming of the somatic cell nucleus. Few studies have assessed the mesenchymal characteristics of fusion-induced hybrid cells and their survival mechanisms. In this study, we investigated the effect of cell fusion on the biological characteristics of pancreatic ductal cells (PDCs) and on the survival mechanism of hybrid cells. To this end, we generated mouse-mouse hybrid cells in vitro by polyethylene glycol-mediated fusion of primary mouse bMSCs with primary mouse PDCs. Hybrid cells showed an enhanced capacity for proliferation and self-renewal compared with PDCs. No PDC had the capacity for anchorage-independent growth or invasion into Matrigel, but some hybrid cells were able to form colonies in soft agar and invade Matrigel. Expression of the tumor suppressor protein p53, which initiates apoptosis, was detected in hybrid cells but not in PDCs or bMSCs. However, the p53 deacetylase, sirtuin 1 (SIRT1), was also detected in hybrid cells, and the level of acetylated p53, the active form, was low. The addition of nicotinamide (Nam) inhibited the deacetylation activity of SIRT1 on p53 and induced cell apoptosis in hybrid cells. This study demonstrated that PDCs could obtain high proliferation rates, self-renewal capabilities, and mesenchymal characteristics by fusion with bMSCs. SIRT1 expression in the hybrid cells attenuated their apoptosis.

Therapy-induced senescence in cancer

Cellular senescence is a response to nonlethal stress that results in persistent cytostasis with a distinct morphological and biochemical phenotype. The senescence phenotype, detected in tumors through the expression of mRNA and protein markers, can be generated in cancer cells lacking functional p53 and retinoblastoma protein. Current research suggests that therapy-induced senescence (TIS) represents a novel functional target that may improve cancer therapy. TIS can be induced in immortal and transformed cancer cells by selected anticancer compounds or radiation, and accumulating data indicate that TIS may produce reduced toxicity-related side effects and increased tumor-specific immune activity. This review examines the current status of TIS-regulated mechanisms, agents, and senescence biomarkers with the goal of encouraging further development of this approach to cancer therapy. Remaining hurdles include the lack of efficient senescence-inducing agents and incomplete biological data on tumor response. The identification of additional compounds and other targeted approaches to senescence induction will further the development of TIS in the clinical treatment of cancer.

Polyethylene Glycol-Mediated Fusion between Primary Mouse Mesenchymal Stem Cells and Mouse Fibroblasts Generates Hybrid Cells with Increased Proliferation and Altered Differentiation

Bone marrow-derived mesenchymal stem cells (MSCs) can differentiate into different cell lineages with the appropriate stimulation in vitro. Transplantation of MSCs in human and other animal models was found to repair tissues through the fusion of transplanted MSCs with indigenous cells. We have generated mouseâmouse hybrid cell lines in vitro by polyethylene glycol-mediated fusion of primary mouse MSCs with mouse fibroblasts to investigate the characteristics of hybrid cells, including their potentials for proliferation and differentiation. Similar to the parental MSCs, hybrid cells are positive for the cell-surface markers CD29, CD44, CD49e, and Sca-1, and negative for Gr-1, CD11b, CD13, CD18, CD31, CD43, CD45, CD49d, CD90.2, CD445R/B220, and CD117 markers. The hybrid cells also produce a high level of tissue nonspecific alkaline phosphatase compared to the parental cells. Conditioned medium of hybrid cells contain biologically active factors that are capable of stimulating proliferation of other cells. Although the parental MSCs can differentiate into adipogenic and osteogenic lineages, hybrid cells held disparate differentiation capacity. Hybrid cell lines in general have increased proliferative capacity than the primary MSCs. Our study demonstrates that proliferative hybrid cell lines can be generated in vitro by induced fusion of both immortal and primary somatic cells with primary MSCs.

Identification of a </= 600-kb region on human chromosome 1q42.3 inducing cellular senescence

The introduction of a human chromosome 1 via microcell-mediated chromosome transfer (MMCT) induces the cellular senescence in mouse melanoma B16-F10 cells. The senescent cells maintained still the telomerase activity, which is frequently associated with immortal growth of human cells, suggesting that a telomerase-independent mechanism is involved in the senescence observed in this mouse cell line. To map the senescence-inducing gene to a specific chromosomal region, we took two experimental approaches: identification of a minimal region with the senescence-inducing activity via MMCT of a series of subchromosomal transferrable fragments (STFs), each consisting of a different profile of human chromosome 1-derived regions, and identification of a region commonly deleted from the transferred chromosome 1 in the revertant clones that escaped cellular senescence. These approaches identified a 2.7-3.0 Mb of senescence-inducing region shared among the active STFs and a 2.4-3.0 Mb of commonly deleted region in the revertant clones. These two regions overlapped each other to map the responsible gene at the 450 to 600-kb interval between UniSTS93710 and D1S3542 on chromosome 1q42.3. This study provides essential information and materials for cloning and characterization of a novel senescence-inducing gene that functions in a telomerase-independent pathway, which is likely to be conserved between mice and humans.

Human chromosome 5 carries a putative telomerase repressor gene

Telomerase, the ribonucleoprotein enzyme that maintains the telomere, is active in human germ and stem cells and in a majority of tumor tissues and immortalized cell lines. In contrast, telomerase activity is not detected in most somatic cells, suggesting that normal human cells contain a regulatory factor(s) to repress this activity. To identify which human chromosomes carry a gene or genes that function as telomerase repressors, we investigated telomerase activity in hybrids of the B16-F10 cell line, which contain individual human chromosomes transferred previously by microcell fusion and therefore represent a hybrid panel for the entire genome except for the Y chromosome. Microcell hybrids with an introduced normal human chromosome 5 showed inhibition of telomerase activity, but clones at a late passage exhibited reactivation of telomerase activity. Reactivation of telomerase activity was accompanied by deletion and/or rearrangement of the transferred human chromosome 5. The introduction of other human chromosomes did not significantly affect the telomerase activity of B16-F10 cells. The effect of suppression of telomerase activity in microcell hybrids containing chromosome 5 was accompanied by a reduction in the level of mTERT mRNA, which encodes a component of the telomerase complex. The putative telomerase repressor gene was mapped to human chromosome bands 5p11-p13 by a combination of functional analysis using transfer of subchromosomal transferable fragments of chromosome 5 into B16-F10 cells and deletion mapping of revertant clones with reactivated telomerase activity. Thus, these results suggest that loss of a gene(s) on this chromosome was responsible for telomerase reactivation, indicating that human chromosome 5 contains a gene or genes that can regulate the expression of mTERT in B16-F10 cells.

Loss of heterozygosity on chromosome 7q in in vitro-immortalized human oral keratinocyte cell lines

Loss of heterozygosity in two in vitro-immortalized human oral keratinocyte cell lines was analysed by polymerase chain reaction using 42 polymorphic microsatellite markers on chromosomes 4, 6, 7 and 15. These chromosomes are regarded as candidates for harbouring genes involved in the immortalization of human cells or tumour-suppressor genes in several tumours, including oral cancers, and karyotypic analysis has revealed that both cell lines have non-random alterations in these chromosomes. No allele losses were detected at any informative loci on chromosomes 4 and 6 in the cell lines, including genomic regions adjacent to putative human tumour-suppressor genes and putative senescence genes. When analysed for loss of heterozygosity on chromosomes 7 and 15, allele losses common to both cell lines were detected in the regions at 7q11.2, 7q21.1-21.3 and 7q31.1. High frequencies of loss of heterozygosity on chromosome 7q in at least two distinct regions, particularly centred around 7q31, are observed in a variety of tumours, including oral squamous-cell carcinoma, suggesting that multiple genes involved in immortalization of these cell lines might be present on chromosome 7q.

Twenty years of ageing research at the Mill Hill laboratories

Research on ageing was carried out in the Genetics Division laboratories, Mill Hill, London, from 1970 to 1990, resulting in more than 100 publications. The work centred around the in vitro ageing of human diploid fibroblasts, but there was also research on transformed cells, rat and mouse tissues, human lymphocytes, chick cells, mice and a microbial model system. The major conclusion from all this research, together with a broad overview of the whole field of gerontology, is that ageing has multiple causes, and that adult animals become senescent through the eventual failure of several important maintenance mechanisms.

Replicative senescence revisited

Forty years after its discovery, replicative senescence remains a rich source of information about cell-cycle regulation and the progression from a normal to a transformed phenotype. Effectors of this growth-arrested state are being discovered at a great pace. This review discusses the latest findings on the players responsible for establishing replicative senescence, as well as the associated telomere shortening.

Telomeres and telomerase: implications for cancer and aging

Maintenance of telomere stability is required for cells to escape from replicative senescence and proliferate indefinitely. Telomere length is maintained by a balance between processes that lengthen telomeres (telomerase) and processes that shorten telomeres (the end-replication problem). Telomerase is a cellular ribonucleoprotein reverse transcriptase which stabilizes telomere length by adding hexameric (TTAGGG) repeats to the telomeric ends of the chromosomes, thus compensating for the continued erosion of telomeres. Introduction of the telomerase catalytic protein component into normal telomerase-negative human cells results in restoration of telomerase activity and extension of cellular life span. Human cells with introduced telomerase maintain a normal chromosome complement and continue to grow in a normal manner. Telomerase-induced manipulations of telomere length may thus be important not only for cell and tissue engineering but also for dissecting the molecular mechanisms underlying inherited genetic diseases, as well as defining the genetic pathways leading to cancer. Because almost all human tumors express telomerase activity, inhibition of telomerase may result in gradual erosion of telomeres and eventual cessation of cell proliferation or induction of apoptosis. Thus telomerase may also be a promising target for cancer therapy.

Telomerase and human tumorigenesis

Human cancer cells, unlike their normal counterparts, have shed the molecular restraints to limited cell growth and are immortal. Exactly how cancer cells manage this at the molecular level is beginning to be understood. Human cells must overcome two barriers to cellular proliferation. The first barrier, referred to as senescence, minimally involves the p53 and Rb tumor-suppressor pathways. Inactivation of these pathways results in some extension of lifespan. However, inactivation of these pathways is insufficient for immortalization. As normal cells undergo repeated rounds of DNA replication, their telomeres shorten due to the inability of traditional DNA polymerases to completely replicate the end of the chromosomal DNA. This shortening continues until the cells reach a second proliferative block referred to as crisis, which is characterized by chromosomal instability, end-to-end fusions, and cell death. Stabilization of the telomeric DNA through either telomerase activation or the activation of the alternative mechanism of telomere maintenance (ALT) is essential if the cells are to survive and proliferate indefinitely. Conversely, loss of telomere stabilization by an already-immortalized cell results in loss of immortality and cell death. Together this indicates that telomere maintenance is a critical component of immortality. In this review we attempt to describe our current understanding of the role of telomere maintenance in senescence, crisis, and tumorigenesis.

Identification of a novel human mitochondrial D-loop RNA species which exhibits upregulated expression following cellular immortalization

We report the identification and characterization of a novel human mitochondrial RNA species approximately 0.47 kb long that is transcribed from the mtDNA L-strand and is derived from the D-loop. Its expression increases when human cells become immortal, a key event in tumorigenesis. The RNA is therefore designated IDL (Immortalization-associated D-Loop). Sequence and hybrid cell analyses suggest that the increased level of IDL RNA in immortal cells is due to a recessive change, possibly in the activity of a trans-acting factor that controls IDL RNA expression.

Extramitochondrial localization of mortalin/mthsp70/PBP74/GRP75

Subcellular fractionation and immunofluorescence microscopy were used to identify the specific sites of intracellular residence of mortalin, also called a mitochondrial homologue of the hsp70 family, in immortal human cell lines previously assigned to four distinct complementation groups (A-D) for indefinite cell division. In addition to the mitochondria it was seen in the endoplasmic reticulum (ER) fractions of all the cell lines analyzed. Interestingly, three of the group A cells lines (EJ, GM639, and HT1080), in addition to the mitochondria and ER, exhibited cytosolically (extra-organelle) localized pool of mortalin. These findings demonstrate that mortalin is not present exclusively in mitochondria. Its residence in different organelles may be the basis of differential distribution observed previously in different human cell lines.

Cellular senescence of a human bladder carcinoma cell line (JTC-32) induced by a normal chromosome 11

Human chromosome 11 is expected to carry tumor suppressor genes for a variety of human cancers, including bladder carcinoma. To examine the functional role of a putative tumor suppressor gene(s) on this chromosome in the development of bladder carcinoma, we performed microcell-mediated transfer of chromosome 11 into the bladder carcinoma cell line, JTC-32. Fifteen of 20 colonies formed by the transfer experiment showed a remarkable change in cell morphology. They flattened and ceased growing, or senesced, prior to 10 population doublings. The presence of transferred chromosome 11-derived fragments in the growth-arrested cells was confirmed by PCR-based polymorphism analyses. The remaining 5 microcell hybrid clones exhibited a parental cell-like morphology, and presumably escaped from senescence, which was accompanied by deletions and/or rearrangements of the transferred chromosome 11. On the other hand, a transferred normal chromosome 7 neither changed the cell morphology nor arrested the cell growth. These results support the hypothesis that chromosome 11 contains a gene or genes which restore the senescence program lost during the immortalization process of JTC-32 cells.

Identification of a gene at 16q24.3 that restores cellular senescence in immortal mammary tumor cells

We have mapped a cellular senescence gene, SEN16, within a genetic distance of 3 - 7 cM, at 16q24.3. Microcell mediated transfer of a normal human chromosome 16, 16q22-qter or 16q23-qter restored cellular senescence in four immortal cell lines, derived from human and rat mammary tumors. The resumption of indefinite cell proliferation, concordant with the segregation of the donor chromosome, confirmed the presence of a senescence gene at 16q23-qter. While microcell hybrids were maintained in selection medium to retain the donor chromosome, sporadic immortal revertant clones arose among senescent cells. Reversion to immortal growth could occur due to inactivation of the senescence gene either by a mutation or a deletion. The analysis for chromosome 16 specific DNA markers, in revertant clones of senescent microcell hybrids, revealed a consensus deletion, spanning a genetic interval of approximately 3 - 7 cM at 16q24.3.

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.

The limited reproductive life span of normal human cells in culture

It has been suggested that the limited reproductive life span of normal (diploid) cells in culture may be explained by an inevitable shortening of one or more telomeres. The hypothesis is that one of the shortened telomeres will either generate a specific signal or will invoke a DNA damage checkpoint, in either case causing that cell to leave the cell cycle irreversibly. To assess this hypothesis, I review what constitutes the limited life span of cells in culture. Careful inspection of the kinetics of the life span of diploid cells in culture has shown that the limited life span arises because a fraction of newborn cells irreversibly leave the cell cycle at each division; and this fraction of reproductively sterile cells increases steadily throughout the life span of the culture. Cell fusion experiments suggest that only a small number of genes are involved in preventing continued cell growth, but that at least two independent mutation events are required to immortalize human cells, although only one event is sufficient in some rodent species. Human genetic diseases such as Werner's syndrome indicate that the duration of the life span is also genetically regulated, and is independent of the cessation of cell proliferation.

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.

Telomeres and telomerase in normal and malignant haematopoietic cells

The normal haematopoietic system harbours telomerase-competent cells with a capacity to upregulate the activity to notable levels in a telomere length-independent manner. Strong telomerase activity is found in progenitor stem cells and activated lymphocytes in vitro as well as in vivo, indicating that cells with high growth requirements can readily upregulate telomerase. Despite detection of telomerase activity, a gradual telomere erosion occurs in stem cells and lymphocytes, with significantly shortened telomeres at higher ages, a phenomenon that might be of importance for developing immunosenescence and exhausted haematopoiesis. In malignant haematopoietic disorders telomerase activity is a general finding with large differences in activity levels. The strongest telomerase expression has been shown in acute leukaemias and non-Hodgkin's lymphomas, especially high grade cases. There are indications that the level of activity might parallel tumour progression and be of prognostic relevance, but studies of larger patient materials are needed. An association between the cell cycle and telomerase activity exists, especially for normal haematopoietic cells, and induction of a differentiation programme in immortalised cell lines downregulates telomerase activity. The expression of telomerase activity seems to be regulated at different levels, since for immature bone marrow cells the level of activity seemed to parallel better the phenotype than the proliferation state. The frequent expression of telomerase in leukaemias and lymphomas makes these disorders interesting targets for future anti-telomerase therapy.

Chromosomal changes and progressive tumorigenesis of human bronchial epithelial cell lines

A simian virus 40 (SV40)-transformed human bronchial epithelial cell line, BEAS-2B, underwent progressive changes, including the development of tumorigenicity, during extended in vitro passaging. Karyotypic changes occurred in parallel with the phenotypic changes. For the first 12 passages following viral transformation, there were random karyotypic changes. Immortalization occurred between passages 12 and 21, corresponding with the accumulation of four characteristic abnormal chromosomes-m-1: add(15)(p11.1); m-2: der(8;9)(q10;q10); m-3: add(16)(p13); and m-4: mar4- and the loss of one homolog of chromosomes 8, 15, 16, 21, and 22. With further passaging (from 21 to 63), the acquisition of weak tumorigenicity was observed, accompanied by an increased frequency of cells containing all four common abnormal chromosomes, m-1 through m-4, and missing one normal homolog of chromosomes 8, 15, 16, and 22. Four tumor cell lines (B39-TL, B39-TR, B61-T4 and B61-T7) were established from tumors induced by the injection of these weakly tumorigenic BEAS-2B 39th- and 61st- passage cells into athymic nude mice. One of the cell lines, B39-TL, is significantly more tumorigenic than the others. It is notable that B39-TL showed two specific abnormal chromosomes, del(3p);der(3;15) (q10;q10) and m-6; der(21)t(3;21)(p14.2;p12) inducing deletion of a short arm of chromosome 3. Fluorescence in situ hybridization analysis with a probe for protein tyrosine phosphatase-gamma demonstrated loss of heterozygosity in the 3p14 region. The development of step-wise karyotypic changes in this in vitro carcinogenesis model parallels changes documented in several common human cancers.

Rat embryo fibroblasts immortalized with simian virus 40 large T antigen undergo senescence upon its inactivation

Introduction of simian virus 40 T antigen into rodent fibroblasts gives rise to cells that can proliferate indefinitely but are dependent upon it for maintenance of their growth once the normal mitotic life span has elapsed. Inactivation of T antigen in these immortalized cells causes rapid and irreversible cessation of growth. To determine whether this growth arrest is associated with entry into senescence, we have undertaken a genetic and biological analysis of conditionally immortal (tsa) cell lines derived by immortalizing rat embryo fibroblasts with the thermolabile tsA58 T antigen. This analysis has identified the following parallels between the tsa cells after inactivation of T antigen and senescent rat embryo fibroblasts: (i) growth arrest is irreversible; (ii) it occurs in G1 as well as G2; (iii) the G1 block can be partially overcome by stimulation with 20% fetal calf serum, but the G2 block cannot be overcome; (iv) 20% fetal calf serum induces c-fos, but c-myc is unaltered; and (v) fibronectin and p21(Waf1/Cip1/Sdi1) are upregulated upon growth arrest. These results suggest that T-antigen-immortalized fibroblasts are committed to undergo senescence but are prevented from undergoing this process by T antigen. Inactivation of T antigen removes this block and results in senescence of the cells. Thus, these cell lines may represent a powerful system for study of the molecular basis of entry into senescence.

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.

Differential CDK-inhibitor gene expression in aging human diploid fibroblasts

Cellular aging is accompanied by a reduction in proliferative activity and changes in gene expression. To further elucidate the mRNA phenotype of aging fibroblasts we have monitored the expression of an array of genes implicated in regulating cell-cycle progression. Fourteen genes, including 3 cyclin-dependent kinase (CDK) inhibitors (p16INK4, p21SDI/CIP/WAF and p27KIP), 5 cyclins, 4 CDKs, Cdi-1, and PCNA were tested in four primary fibroblast strains. Relative mRNA expression levels were assessed using a rapid and sensitive Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) assay called the "Primer-dropping" method. p16INK4, a specific inhibitor of the cyclin D-associated kinases CDK4 and CDK6, was, in addition to p21 and cyclin D1, overexpressed in higher passage cells, while the abundance of the D-type kinase mRNAs remained relatively constant. Levels of cyclin H, a component of the CDK-activating kinase (CAK) were markedly reduced in all strains examined, suggesting that the activity of target cyclin/CDK complexes may not be activated in aging cells. These results corroborate and extend previous observations demonstrating elevated expression of specific cell cycle genes in higher passage cells and suggest that overexpression of the CDK-inhibitors p16INK4 and p21SDI/CIP/WAF, but not p27KIP, may contribute to lower proliferative activity of senescing primary fibroblasts.

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

Human immortal cell lines have been classified into at least four (A-D) genetic complementation groups by cell-cell hybrid analysis, i.e., a hybrid derived from different groups becomes mortal. Recently we have demonstrated that introduction of human chromosome 7 suppresses indefinite division potential in the non-tumorigenic human immortalized fibroblast lines KMST-6 and SUSM-1, both assigned to complementation group D. By extending our microcell-mediated chromosome transfer, we found that chromosome 7 also suppresses division potential in the human hepatoma line HepG2 (again, assigned to group D). Chromosome 7 was thus shown to suppress indefinite growth in the above group D cell lines irrespective of their cell types, or whether they are tumorigenic or not. Since chromosome 7 had no such effect on representative cell lines derived from complementation group A, B or C, these results indicate that the senescence gene(s) commonly mutated in the group D cell lines is located on chromosome 7.

A gene involved in control of human cellular senescence on human chromosome 1q

Normal cells in culture exhibit limited division potential and have been used as a model for cellular senescence. In contrast, tumor-derived or carcinogen- or virus-transformed cells are capable of indefinite division. Fusion of normal human diploid fibroblasts with immortal human cells yielded hybrids having limited life spans, indicating that cellular senescence was dominant. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. The purpose of this study was to determine whether human chromosome 1 could complement the recessive immortal defect of human cell lines assigned to one of the four complementation groups. Using microcell fusion, we introduced a single normal human chromosome 1 into immortal human cell lines representing the complementation groups and determined that it caused loss of proliferative potential of an osteosarcoma-derived cell line (TE85), a cytomegalovirus-transformed lung fibroblast cell line (CMV-Mj-HEL-1), and a Ki-ras(+)-transformed derivative of TE85 (143B TK-), all of which were assigned to complementation group C. This chromosome 1 caused no change in proliferative potential of cell lines representing the other complementation groups. A derivative of human chromosome 1 that had lost most of the q arm by spontaneous deletion was unable to induce senescence in any of the immortal cell lines. This finding indicates that the q arm of human chromosome 1 carries a gene or set of genes which is altered in the cell lines assigned to complementation group C and is involved in the control of cellular senescence.

A gene involved in control of human cellular senescence on human chromosome 1q

Normal cells in culture exhibit limited division potential and have been used as a model for cellular senescence. In contrast, tumor-derived or carcinogen- or virus-transformed cells are capable of indefinite division. Fusion of normal human diploid fibroblasts with immortal human cells yielded hybrids having limited life spans, indicating that cellular senescence was dominant. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. The purpose of this study was to determine whether human chromosome 1 could complement the recessive immortal defect of human cell lines assigned to one of the four complementation groups. Using microcell fusion, we introduced a single normal human chromosome 1 into immortal human cell lines representing the complementation groups and determined that it caused loss of proliferative potential of an osteosarcoma-derived cell line (TE85), a cytomegalovirus-transformed lung fibroblast cell line (CMV-Mj-HEL-1), and a Ki-ras(+)-transformed derivative of TE85 (143B TK-), all of which were assigned to complementation group C. This chromosome 1 caused no change in proliferative potential of cell lines representing the other complementation groups. A derivative of human chromosome 1 that had lost most of the q arm by spontaneous deletion was unable to induce senescence in any of the immortal cell lines. This finding indicates that the q arm of human chromosome 1 carries a gene or set of genes which is altered in the cell lines assigned to complementation group C and is involved in the control of cellular senescence.

Failure of infinite life span human cells from different immortality complementation groups to yield finite life span hybrids

The observation that fusion of infinite life span cells with finite life span cells produces hybrid cells with finite life spans led to the conclusion that an infinite life span in culture is a recessive trait resulting from loss of the function of a gene or genes that contribute to an active program for cellular senescence. Furthermore, finding that certain pairs of infinite life span cells, when fused to one another, can complement each other to yield finite life span hybrids allowed 30 infinite life span cell lines to be assigned to four immortality complementation groups (Pereira-Smith and Smith, 1988, Proc. Natl. Acad. Sci. U.S.A., 85:6042). In the present study, we fused a chromosomally stable, near diploid, morphologically normal, infinite life span cell strain, designated MSU-1.1, with its normal, finite life span, precursor cell strain and obtained finite life span hybrids, as expected if infinite life span in culture is a recessive trait. However, 14 of the 14 hybrids from our fusions of MSU-1.1 cells with representative cell lines from each of the four immortality complementation groups, and 38 of the 39 hybrids from our fusions of infinite life span cells that have been reported to complement each other, failed to exhibit finite life spans. This result suggests that infinite life span cells cannot complement each other to yield finite life span hybrids. In examining this unexpected result, we obtained evidence that long-term dual drug selection can be deleterious to hybrid cells even though they carry resistance markers for both drugs, indicating that the cell death of such hybrids observed in other studies may have resulted from the cytotoxic effect of long-term drug selection, rather than from senescence.

Assignment of two human epidermal squamous cell carcinomas cell lines to more than one complementation group for the immortal phenotype

Two human cell lines derived from squamous cell carcinomas (SCCs) of the epidermis, SCC-12 clone F and SCC-13 clone Y, were made to be independent of the Swiss 3T3 feeder layer to perform somatic-cell genetic experiments. We fused these SCC lines with normal human fibroblasts, and all resulting hybrids senesced after completing 12-17 population doublings, suggesting that in part, immortalization of the keratinocyte during SCC development results from the loss of gene function. We also tested whether these two SCC lines mapped to known complementation groups for immortality by fusing them with representatives of groups A (GM847), B (HeLa), and C (143B), but most of these hybrids were indistinguishable from those derived from homotypic crosses set up as immortal hybrid controls. As reported by others, fusions of cell lines from different complementation groups-143B (group C) x HeLa (group B) or GM847 (group A) x Hela (group B)--resulted in predominantly senescent hybrids. Our results confirmed and extended previous observations by others that the phenomenon of senescence is dominant to that of immortality, but they did not allow us to assign either of the SCC lines we studied to a complementation group for immortality.

Chromosome 7 suppresses indefinite division of nontumorigenic immortalized human fibroblast cell lines KMST-6 and SUSM-1

Using nontumorigenic immortalized human cell lines KMST-6 (KMST) and SUSM-1 (SUSM), we attempted to identify the chromosome that carries a putative senescence-related gene(s). These cell lines are the only ones that have been established independently from normal human diploid fibroblasts following in vitro mutagenesis. We first examined restriction fragment length polymorphisms on each chromosome of these immortalized cell lines and their parental cell lines and found specific chromosomal alterations common to these cell lines (a loss of heterozygosity in KMST and a deletion in SUSM) on the long arm of chromosome 7. In addition to these, we also found that introduction of chromosome 7 into these cell lines by means of microcell fusion resulted in the cessation of cell division, giving rise to cells resembling cells in senescence. Introduction of other chromosomes, such as chromosomes 1 and 11, on which losses of heterozygosity were also detected in one of the cell lines (KMST), to either KMST or SUSM cells or of chromosome 7 to several tumor-derived cell lines had no effect on their division potential. These results strongly suggest that a gene(s) affecting limited-division potential or senescence of normal human fibroblasts is located on chromosome 7, probably at the long arm of the chromosome, representing the first case in which a specific chromosome reverses the immortal phenotype of otherwise normal human cell lines.

Is 'senescence' of diploid cells an ad hoc mechanism suppressing 'replicon' or not?

So-called 'limited life span' of diploid cells does not depend on their unresponsiveness to growth factors from the data obtained by complete protein-free culture of tumor cells. Limited proliferation (called limited life span) of diploid cells is one of the 'ad-hoc' negative regulatory mechanisms in animals, and can not be differentiated from other inhibitory mechanisms called 'tumor suppression' and 'terminal differentiation'. Metabolic imbalance induced by proliferation-dependent time-bomb mechanisms including infidelity of DNA repair is suggested to explain limited proliferation of cells. After maturation, autonomic progression of negative regulators in cells is induced by no prohibition of terminal differentiation, since organisms prepare no programs to stop development and differentiation. It is an attractive hypothesis that a proliferation-dependent time-bomb has been developed to control organogenesis for maturation and to determine body size. It is true that limited proliferation of cells can not explain longevity of individuals, although the so-called longevity genes play roles in ageing. Teleologically, longevity of individuals has been developed to produce genetic heterogeneity according to the selfish gene theory. This inter- and intra-species genetic heterogeneity increases the probability of selfish gene replication in germ cell line. After maturation and reproduction of DNA in germ cell line, individuals as vehicles for the DNA can be discarded by the 'selfish' genes.

Chromosome 7 suppresses indefinite division of nontumorigenic immortalized human fibroblast cell lines KMST-6 and SUSM-1

Using nontumorigenic immortalized human cell lines KMST-6 (KMST) and SUSM-1 (SUSM), we attempted to identify the chromosome that carries a putative senescence-related gene(s). These cell lines are the only ones that have been established independently from normal human diploid fibroblasts following in vitro mutagenesis. We first examined restriction fragment length polymorphisms on each chromosome of these immortalized cell lines and their parental cell lines and found specific chromosomal alterations common to these cell lines (a loss of heterozygosity in KMST and a deletion in SUSM) on the long arm of chromosome 7. In addition to these, we also found that introduction of chromosome 7 into these cell lines by means of microcell fusion resulted in the cessation of cell division, giving rise to cells resembling cells in senescence. Introduction of other chromosomes, such as chromosomes 1 and 11, on which losses of heterozygosity were also detected in one of the cell lines (KMST), to either KMST or SUSM cells or of chromosome 7 to several tumor-derived cell lines had no effect on their division potential. These results strongly suggest that a gene(s) affecting limited-division potential or senescence of normal human fibroblasts is located on chromosome 7, probably at the long arm of the chromosome, representing the first case in which a specific chromosome reverses the immortal phenotype of otherwise normal human cell lines.

Cell immortalization as a key, rate-limiting event in malignant transformation: approaches toward a molecular genetic analysis

Recent advances using somatic cell genetic approaches have provided a convincing body of evidence that the senescence of mammalian cells in culture is controlled by a small group of genes, one or more of which are functionally deleted in the process of immortalization. Microcell-mediated mono-chromosomal transfer methods should permit precise mapping of these genes to specific chromosomal regions. Cloning of senescence genes, using either conventional 'positional cloning' techniques or retroviral insertion mutagenesis, is now a realistic possibility. The leap in our understanding of the molecular genetic events driving the alternative cellular states of limited proliferative capacity and immortality, which such advances should precipitate, will finally permit the question of the role of cell immortalization in cancer to be addressed, and may open the door to the design of new modes of cancer therapy. In addition, the precise mechanism underlying the wide difference in transformability between human and rodent cells, which should also emerge from these investigations, is likely to make a significant contribution towards resolving the key issue of the relevance of rodent tumour induction assays in assessing the potential carcinogenicity of environmental chemicals.

Cellular senescence

The ageing of cells, cellular senescence, is an event that is encountered in all normal cells. Cells grown in vitro have a limited life span and do not grow well after a certain number of divisions. They cease to divide and eventually die. In accordance with this, the life expectancy of an established cell culture depends on the age of the donor. Cells that have undergone immortalization via a crisis period of transformation by chemicals or viruses, as well as malignant cell lines in general, have an ability to divide indefinitely. A distinct form of cell death, apoptosis or programmed cell death, is encountered in many physiological situations like in keratinocyte differentiation.

Stimulation of DNA synthesis in senescent human cells following incubation with plasma membranes

DNA synthesis and mitosis were increased in mitogen-stimulated senescent WI-38 cells following incubation with plasma membranes prepared from young or senescent WI-38 cells, A431 cells, 3T3 cells, or NR6 cells. The percentage of [3H]thymidine-labeled nuclei in senescent cultures was two- to fivefold greater than that seen in controls in which cells were incubated in the absence of membranes or in the presence of boiled membranes. The effect was trypsin sensitive, suggesting that a protein moiety is necessary for stimulation of DNA synthesis. As the culture age increased, basal levels of DNA synthesis, as well as maximal stimulation of DNA synthesis following incubation with plasma membranes, decreased. These observations are consistent with the hypothesis that different subpopulations exist in senescing cultures and suggest a complex pattern of inhibitory and stimulatory regulation of cell proliferation.

Altered chromosome 6 in immortal human fibroblasts

Human diploid fibroblasts have a limited life span in vitro, and spontaneous immortalization is an extremely rare event. We have used transformation of human diploid fibroblasts by an origin-defective simian virus 40 genome to develop series of genetically matched immortal cell lines to analyze immortalization. Comparison of a preimmortal transformant (SVtsA/HF-A) with its uncloned and cloned immortalized derivatives (AR5 and HAL) has failed to reveal any major alteration involving the simian virus 40 genome. Karyotypic analysis, however, demonstrated that all of the immortal cell lines in this series have alterations of chromosome 6 involving loss of the portion distal to 6q21. The karyotypic analysis was corroborated by DNA analyses. Southern analysis demonstrated that only one copy of three proto-oncogene loci (ros1, c-myb, and mas1) on 6q was retained in immortal cells. Polymerase chain reaction analysis of the microsatellite polymorphism at 6q22 (D6S87) showed loss of heterozygosity. In addition, elevated expression of c-myb (6q22-23) was observed. We hypothesize that the region at and/or distal to 6q21 plays a role in immortalization, consistent with the presence of a growth suppressor gene.

Altered chromosome 6 in immortal human fibroblasts

Human diploid fibroblasts have a limited life span in vitro, and spontaneous immortalization is an extremely rare event. We have used transformation of human diploid fibroblasts by an origin-defective simian virus 40 genome to develop series of genetically matched immortal cell lines to analyze immortalization. Comparison of a preimmortal transformant (SVtsA/HF-A) with its uncloned and cloned immortalized derivatives (AR5 and HAL) has failed to reveal any major alteration involving the simian virus 40 genome. Karyotypic analysis, however, demonstrated that all of the immortal cell lines in this series have alterations of chromosome 6 involving loss of the portion distal to 6q21. The karyotypic analysis was corroborated by DNA analyses. Southern analysis demonstrated that only one copy of three proto-oncogene loci (ros1, c-myb, and mas1) on 6q was retained in immortal cells. Polymerase chain reaction analysis of the microsatellite polymorphism at 6q22 (D6S87) showed loss of heterozygosity. In addition, elevated expression of c-myb (6q22-23) was observed. We hypothesize that the region at and/or distal to 6q21 plays a role in immortalization, consistent with the presence of a growth suppressor gene.

Finite life span of hybrids formed by fusion of different simian virus 40-immortalized human cell lines

Simian virus 40 (SV40) genes are able to induce immortalization of normal human cells after a culture crisis during which unknown cellular genetic changes presumably occur. To determine whether these genetic changes are always identical, we performed somatic cell hybridization analysis of an SV40-immortalized human bronchial epithelial cell line, BET-1A. Fusion of BET-1A with an SV40-immortalized fibroblast cell line resulted in hybrids that senesced, indicating that these cell lines are in different complementation groups for immortalization.

Molecular genetic approaches to the study of cellular senescence

Normal cells in culture exhibit limited division potential, which is used as a model for cellular aging. In contrast, tumor-derived, carcinogen- or virus-transformed cells are capable of dividing indefinitely (immortal). Fusion of normal with immortal human cells yielded hybrids having limited life span, indicating that cellular senescence is a dominant phenotype and that immortality is recessive. Fusions of various immortal human cell lines with each other led to the identification of four complementation groups for indefinite division. In order to identify the chromosomes and genes involved in growth regulation, that had been modified in immortal cells, we used the technique of microcell fusion to introduce either a normal human chromosome 11 or 4 into cell lines representative of the different complementation groups. Chromosome 11 had no effect on the in vitro life span of the different immortal human tumor lines. However, when a normal human chromosome 4 was introduced into cell lines assigned to complementation group B, the cells lost the immortal phenotype. No effect on the proliferation potential of cell lines representative of the other complementation groups was observed. These results suggest that a gene(s) on human chromosome 4 has been modified in immortal cell lines assigned to complementation group B, to allow escape from senescence. They also provide evidence for a genetic basis for cellular aging.

Protein markers for cellular mortality and immortality

Fixed mortality of normal somatic cells is a well-established fact though the mechanism underlying this universal phenomenon remains unknown. Use of immortal cells in conjunction with their normal mortal counterparts has delineated the dominant genetic nature of the senescent phenotype over immortalization. Although the involvement of proteins in determining the entry/exit/arrest of cells in the cell cycle is evident from the literature, none of them has been confirmed for its role in senescence-associated irreversible cell cycle exit/arrest. The identification of true mortality markers might be possible by selecting a system of natural and conditional aging achieved by the fusion of mortal and spontaneously immortalized cells of the same origin. We report here a few such protein markers which might serve as useful handles to tease out the molecular events determining mortality/immortality of cultured cells.