Transcriptional networks and cellular senescence in human mammary fibroblasts

Senescence, the molecular program that limits the finite proliferative potential of a cell, acts as an important barrier to protect the body from cancer. Techniques for measuring transcriptome changes and for modulating their expression suggest that it may be possible to dissect the transcriptional networks underlying complex cellular processes. HMF3A cells are conditionally immortalized human mammary fibroblasts that can be induced to undergo coordinated senescence. Here, we used these cells in conjunction with microarrays, RNA interference, and in silico promoter analysis to promote the dissection of the transcriptional networks responsible for regulating cellular senescence. We first identified changes in the transcriptome when HMF3A cells undergo senescence and then compared them with those observed upon replicative senescence in primary human mammary fibroblasts. In addition to DUSP1 and known p53 and E2F targets, a number of genes such as PHLDA1, NR4A3, and a novel splice variant of STAC were implicated in senescence. Their role in senescence was then analyzed by RNA silencing followed by microarray analysis. In silico promoter analysis of all differential genes predicted that nuclear factor-kappaB and C/EBP transcription factors are activated upon senescence, and we confirmed this by electrophoretic mobility shift assay. The results suggest a putative signaling network for cellular senescence.

Establishment and characterization of equine autonomic ganglion cell lines to enable direct testing of candidate toxins involved in equine dysautonomia (grass sickness)

To enable direct testing of a range of potential toxins or pathogens that might be involved in grass sickness, equine thoracic sympathetic chain ganglion cell lines were established from primary cell cultures by retroviral-mediated transduction of the temperature-sensitive mutant of the establishment oncogene encoding SV40 large T antigen. Morphological and behavioral features, temperature dependence, and immunocytochemical characteristics of the cell lines were investigated. The majority of cells were noradrenergic neurons in which dopamine-beta-hydroxylase, the enzyme that catalyzes norepinephrine synthesis, and neuropeptide Y coexisted. Cells treated with plasma from grass sickness cases that had previously been shown to induce autonomic nervous system damage when injected into normal horses showed significantly decreased mitochondrial function after 1 day. After 3 days exposure most cells showed severe degeneration in contrast to those treated with normal plasma. Liver and lung cell lines were also susceptible to plasma, suggesting that the toxin is not specifically neurotoxic.

Conditional immortalization of freshly isolated human mammary fibroblasts and endothelial cells

Reports differ as to whether reconstitution of telomerase activity alone is sufficient for immortalization of different types of human somatic cells or whether additional activities encoded by other "immortalizing" genes are also required. Here we show that ectopic expression of either the catalytic subunit of human telomerase (hTERT) or a temperature-sensitive mutant (U19tsA58) of simian virus 40 large-tumor antigen alone was not sufficient for immortalization of freshly isolated normal adult human mammary fibroblasts and endothelial cells. However, a combination of both genes resulted in the efficient generation of immortal cell lines irrespective of the order in which they were introduced or whether they were introduced early or late in the normal proliferative lifespan of the cultures. The order and timing of transduction, however, did influence genomic stability. Karyotype analysis indicated that introduction of both transgenes at early passage, with hTERT first, yielded diploid cell lines. Temperature-shift experiments revealed that maintenance of the immortalized state depended on continued expression of functional U19tsA58 large-tumor antigen, with hTERT alone unable to maintain growth at nonpermissive temperatures for U19tsA58 large-tumor antigen. Such conditional diploid lines may provide a useful resource for both cell engineering and for studies on immortalization and in vitro transformation.

BAP37 and Prohibitin are specifically recognized by an SV40 T antigen antibody

We have identified two cellular proteins that are specifically immunoprecipitated by an anti-SV40 T antigen monoclonal antibody. This antibody, PAb419, recognizes an epitope contained within a region of T antigen which we have recently demonstrated is required for the initiation of immortalization by SV40 T antigen, but is not essential for maintenance of the immortal state. The two proteins were identified as BAP37 and Prohibitin. Recent results suggest Prohibitin may enhance the transcriptional inactivation of E2F by the retinoblastoma family of pocket proteins (pRb, p107, p130). BAP37 and Prohibitin are specifically recognized by PAb419 and PAb210, another anti-SV40 T antigen monoclonal antibody, which has an overlapping epitope, but not by other anti-SV40 T antigen monoclonal antibodies, demonstrating the specificity of the interaction.

Different functions are required for initiation and maintenance of immortalization of rat embryo fibroblasts by SV40 large T antigen

We have used two different, but complementary assays to characterize functions of SV40 T antigen that are necessary for its ability to immortalize rat embryo fibroblasts. In accordance with previous work, we found that several functions were required. These include activities that map to the p53 binding domain and the amino terminal 176 amino acids which contain the J domain as well as the CR1 and CR2 domain required for binding and sequestering the RB family of pocket proteins. Moreover, we found that even though activities dependent only upon the amino terminus were sufficient for immortalization they were unable to maintain it. This suggests that immortalization by these amino terminal functions requires either additional events or immortalization of a subset of cells within the heterogeneous rat embryo fibroblast population. We further found that an activity dependent upon amino acids 17 - 27 which remove a portion of the CR1 domain and the predicted alpha-1 helix of the J domain was not necessary to maintain growth but was required for direct immortalization suggesting that at least one of the functions required initially was not required to maintain the immortal state. This represents the first demonstration that some of the functions required for maintenance of the immortal state differ from those required for initiation of immortalization.

Immortalization of rat embryo fibroblasts by a 3'-untranslated region

We have exploited a cross-species expression screen to search for cellular immortalizing activities. A newt blastemal cDNA expression library was transfected into rat embryo fibroblasts and immortal cell lines were selected. This identified a 1-kb cDNA fragment which has a low representation in the cDNA library and is derived from the 3'-UTR of an alpha-glucosidase-related mRNA. Expression of this sequence in rat embryo fibroblasts has shown that it is active in promoting colony formation and immortalization. It is also able to cooperate with an immortalization-defective deletion mutant of SV40 T antigen, indicating that it can exert its growth-stimulatory activity in the pathway activated by a viral immortalizing oncogene. This is the first example of an immortalizing activity mediated by an RNA sequence, and further analysis of its mechanism should provide new insights into senescence and immortalization.

Cloning and identification of genes that associate with mammalian replicative senescence

Cellular senescence and limited proliferative capacity of normal diploid cells has a dominant phenotype over immortality of cancerous cells, suggesting its regulation by the expression of a set of genes. In order to isolate the genes that associate with senescence, we have employed a clonal system of conditional SV40 T antigen rat embryo fibroblast cell lines which undergo senescence upon T antigen inactivation. Construction of cDNA libraries from two conditional cell lines and application of differential screening and subtractive hybridization techniques have resulted in the cloning of eight senescence-induced genes (SGP-2/Apo J, alpha 1-procollagen, osteonectin, fibronectin, SM22, cytochrome C oxidase, GTP-alpha, and a novel gene) and a senescence-repressed gene (FRS-2). Three of these genes encode for extracellular matrix proteins, others are involved in the calcium-dependent signal transduction pathways, while the SGP-2/Apo J gene may have a cellular protective function. RNA analysis has shown that the senescence-associated genes are overexpressed in both normal rat embryonic fibroblasts and human osteoblasts cell cultures undergoing aging in vitro. In comparison, the expression of these genes in a rat fibroblast immortalized cell line (208F cells) was down-regulated after both its partial and its full transformation by ras oncogenes. Thus, cloning of senescence-associated genes opens up new ways to elucidate and/or to modulate aging and cancer.

Recovery of spatial learning by grafts of a conditionally immortalized hippocampal neuroepithelial cell line into the ischaemia-lesioned hippocampus

Transient global cerebral ischaemia in rats causes relatively circumscribed and specific damage to the CA1 pyramidal cells of the dorsal hippocampus, along with a cognitive deficit manifest as difficulties in the performance of a range of spatial learning and memory tasks. Our previous studies have shown that restoration of behavioural performance in ischaemic rats by neural grafts taken relatively late in fetal development occurs only after local replacement of cells homotypic to those lost through the ischaemic insult. This lesion-plus-behaviour model therefore offers a powerful means for establishing whether multipotent embryonic neuroepithelial cells will engraft the damaged CA1, develop into appropriate neuronal phenotypes and produce behavioural recovery. Here we report that, in rats subjected to 15 min of global cerebral ischaemia, intrahippocampal implants of a conditionally immortal, multipotent cell line, directly derived from the embryonic day 14 hippocampal neuroepithelium of the H-2Kb-tsA58 transgenic mouse, selectively repopulated the lesioned CA1 pyramidal layer and restored ischaemia-induced deficits in acquisition of a hidden platform location in the Morris water maze.

Expression of p24, a novel p21Waf1/Cip1/Sdi1-related protein, correlates with measurement of the finite proliferative potential of rodent embryo fibroblasts

Normal mammalian fibroblasts undergo a limited number of divisions when cultured in vitro before entering a state of replicative senescence. The molecular basis for the determination of the finite mitotic potential is not known. Nevertheless, simian virus 40 T antigen, among other oncogenes, is able to prevent senescence in rodent embryo fibroblasts. T antigen immortalized cells are dependent upon this protein for maintaining growth once their normal mitotic life span has elapsed. Even though the mechanism that measures the finite mitotic potential of rodent fibroblasts is not known, it has been shown that it continues to function normally in the presence of this immortalizing gene. Accumulation of cyclin-dependent kinase inhibitors such as p21Waf1/Cip1/Sdi1 could potentially be a component of the mechanism that determines the finite life span. Here we show that accumulation of p21Waf1/Cip1/Sdi1 does not correlate with this biological counting mechanism, but we have identified p24, a p21Waf1/Cip1/Sdi1-related protein, whose accumulation does correlate with the measurement of the finite proliferative potential of rodent embryo fibroblasts and suggest that sequestration might be a mechanism by which its activity is regulated.

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.

Immortalization of epithelial cells

The methodologies for isolating cell lines have become very powerful, particularly in terms of retaining differentiated features of the parent cells. Cell lines can be developed from primary or early passage cells as well as from transgenic animals that carry an immortalizing gene. Cell lines from epithelia have been selected for their polar orientation, tight junction formation, and expression of differentiated markers or functions. These cell lines provide useful models for studying cell biology of specific tissues, tumorigenicity, genetic abnormalities, or to help screen for effective methods of gene therapy.

Vascular smooth muscle cells of H-2Kb-tsA58 transgenic mice. Characterization of cell lines with distinct properties

BACKGROUND

The vascular wall is composed of at least two different populations of smooth muscle cells that are distinct in their structure and protein composition. According to the developmental stage of tissue taken for culture, the ratio between cells of epithelioid phenotype and spindle-shaped cells is variable. In particular, the epithelioid cells display characteristic features associated with immaturity. Because their increased appearance can be observed in endothelial denudation, the represent a dedifferentiated, proliferative smooth muscle cell type with a repair function in vascular injury.

METHODS AND RESULTS

To investigate this cellular heterogeneity, we established vascular smooth muscle cell lines from H-2Kb-tsA58 transgenic mice. Due to temperature-sensitive expression of the SV 40 large T-antigen in cells derived from this mouse strain, our smooth muscle lines were conditionally immortalized from the onset of their life in culture. Thus, we were able to clone cell lines representing the two different phenotypes described so far. Epithelioid cells derived from newborn animals are characterized by their expression of cytokeratins and the development of tight junctional complexes. Spindle-shaped cells, which could be isolated from newborn or adult animals, corresponded in phenotype and protein expression to smooth muscle cell lines established previously.

CONCLUSIONS

The special properties of vascular smooth muscle cells of the epithelioid phenotype suggest an endothelial replacement function in the course of injury to the vascular wall.

The H-2KbtsA58 transgenic mouse: a new tool for the rapid generation of novel cell lines

The ability to generate expanded populations of individual cell types able to undergo normal differentiation in vitro and in vivo is of critical importance in the investigation of the mechanisms that underly differentiation and in studies on the use of cell transplantation to repair damaged tissues. This review discusses the development of a strain of transgenic mice that allows the direct derivation of conditionally immortal cell lines from a variety of tissues, simply by dissociation of the tissue of interest and growth of cells in appropriate conditions. In these mice the tsA58 mutant of SV40 large T antigen is controlled by the interferon-inducible Class I antigen promoter. Cells can be grown for extended periods in vitro simply by growing them at 33 degrees C in the presence of interferon, while still retaining the capacity to undergo normal differentiation in vivo and in vitro. In addition, it appears that cell lines expressing mutant phenotypes can readily be generated by preparing cultures from appropriate offspring of matings between H-2KbtsA58 transgenic mice and mutant mice of interest.

Roles of hepatocyte growth factor/scatter factor and the met receptor in the early development of the metanephros

Several lines of evidence suggest that hepatocyte growth factor/scatter factor (HGF/SF), a soluble protein secreted by embryo fibroblasts and several fibroblast lines, may elicit morphogenesis in adjacent epithelial cells. We investigated the role of HGF/SF and its membrane receptor, the product of the c-met protooncogene, in the early development of the metanephric kidney. At the inception of the mouse metanephros at embryonic day 11, HGF/SF was expressed in the mesenchyme, while met was expressed in both the ureteric bud and the mesenchyme, as assessed by reverse transcription PCR, in situ hybridization, and immunohistochemistry. To further investigate the expression of met in renal mesenchyme, we isolated 13 conditionally immortal clonal cell lines from transgenic mice expressing a temperature-sensitive mutant of the SV-40 large T antigen. Five had the HGF/SF+/met+ phenotype and eight had the HGF/SF-/met+ phenotype. None had the HGF/SF+/met- nor the HGF/SF-/met- phenotypes. Thus the renal mesenchyme contains cells that express HGF/SF and met or met alone. When metanephric rudiments were grown in serum-free organ culture, anti-HGF/SF antibodies (a) inhibited the differentiation of metanephric mesenchymal cells into the epithelial precursors of the nephron; (b) increased cell death within the renal mesenchyme; and (c) perturbed branching morphogenesis of the ureteric bud. These data provide the first demonstration for coexpression of the HGF/SF and met genes in mesenchymal cells during embryonic development and also imply an autocrine and/or paracrine role for HGF/SF and met in the survival of the renal mesenchyme and in the mesenchymal-epithelial transition that occurs during nephrogenesis. They also confirm the postulated paracrine role of HGF/SF in the branching of the ureteric bud.

The biological clock that measures the mitotic life-span of mouse embryo fibroblasts continues to function in the presence of simian virus 40 large tumor antigen

Normal mammalian fibroblasts cultured in vitro undergo a limited number of divisions before entering a senescent phase in which they can be maintained for long periods but cannot be induced to divide. In rodent fibroblasts senescence can be prevented by expression of simian virus 40 large tumor antigen (T antigen). Cells expressing T antigen can proliferate indefinitely; however, such cells are absolutely dependent upon continued expression of T antigen for maintenance of growth; inactivation of T antigen results in a rapid and irreversible entry into a postmitotic state. To determine when, after the initial expression of T antigen, fibroblasts become dependent upon it for continued growth, we serially cultivated embryonic fibroblasts prepared from H-2Kb-tsA58 transgenic mice. We show that these fibroblasts become dependent upon T antigen for maintenance of proliferation only when their normal mitotic life-span has elapsed and that the biological clock that limits the mitotic potential continues to function normally, even in cells expressing this immortalizing gene. Our results suggest that random accumulation of cellular damage is unlikely to be the factor that limits fibroblast division but support the hypothesis that senescence is regulated via a genetic program.

Myogenic cell lines derived from transgenic mice carrying a thermolabile T antigen: a model system for the derivation of tissue-specific and mutation-specific cell lines

Skeletal myoblasts cloned from limb muscles of H-2Kb-tsA58 transgenic mice remained proliferative through at least 80 generations under conditions permissive for expression and function of the tsA58 gene product. When switched to nonpermissive conditions or implanted into muscles of nude mdx mice they underwent differentiation but, in one clonal cell line, a small proportion appeared to become quiescent muscle precursors in vivo. H-2Kb-tsA58 X mdx/mdx F1 male mice yielded dystrophin-deficient myoblasts. By such simple genetic crosses, H-2Kb-tsA58 transgenic mice provide a valuable tool for the rapid isolation of cell lines, myogenic or otherwise, bearing mutations of interest.

A model for primitive neuroectodermal tumors in transgenic neural transplants harboring the SV40 large T antigen

Using retrovirus-mediated transfer of the SV40 virus large T antigen into neural transplants, we have observed a high incidence of primitive neuroectodermal tumors (PNET). These neoplasms developed in 8 of 14 (57%) neural grafts after latency periods of 176 to 311 days. Histopathologically, the tumors exhibited features of human PNET such as formation of neuroblastic rosettes and immunocytochemical evidence for neuronal differentiation, synaptogenesis, and focal astrocytic differentiation. All neoplasms showed a striking migratory potential. The presence of the large T gene in the tumors was demonstrated by polymerase chain reaction-mediated amplification of a specific 242 bp segment of large T and DNA sequence analysis. Large T antigen was identified in tissue sections using an immunocytochemical reaction with the monoclonal antibody Pab 108. Cell lines were established from several tumors and subjected to G418 selection. Secondary tumors induced by intracerebral transplantation of these cells retained the characteristic morphological and immunocytochemical properties of PNETs. These experiments demonstrate a considerable transforming potential of SV40 large T antigen for neural precursor cells. The long latency period suggests that neoplastic transformation initiated by the large T gene requires additional spontaneous mutations of cooperating cellular genes. Because the mechanism of transformation by large T antigen appears to involve complex formation with and inactivation of cellular tumor suppressor gene products, these cell lines may serve as an interesting tool to search for novel neural tumor suppressor genes.

The characterization of astrocyte cell lines that display properties of glial scar tissue

The glial scar has been proposed to be a major impediment to regeneration in the adult CNS. Analysis of glial scars in vivo is complicated, however, by the large number of cell types present in such lesions. We have attempted to simplify analysis of the glial scar environment by deriving a series of conditionally immortal astrocyte cell lines that display several properties expressed by glial scar tissue in vitro. The astrocyte lines, which were derived from H-2KbtsA58 transgenic mice, expressed macromolecules associated with glial scars in vivo and were significantly less effective than neonatal astrocytes at promoting neurite outgrowth from postnatal central and peripheral neurons. The astrocyte lines also inhibited migration of oligodendrocyte type-2 astrocyte progenitor cells in vitro. We propose that certain properties shown previously to be expressed by glial scars may be reconstituted in vitro by astrocytes alone.

Generation of osteoclast-inductive and osteoclastogenic cell lines from the H-2KbtsA58 transgenic mouse

The development of osteoclastic cell lines would greatly facilitate analysis of the cellular and molecular biology of bone resorption. Several cell lines have previously been reported to be capable of osteoclastic differentiation. However, such cell lines form at best only occasional excavations, suggesting that osteoclastic differentiation is either incomplete or that osteoclasts represent a very small proportion of the cells present. We have used the recently developed H-2KbtsA58 transgenic mouse, in which the interferon-inducible major mouse histocompatibility complex H-2Kb promoter drives the temperature-sensitive (ts) immortalizing gene of simian virus 40 (tsA58), to develop cell lines from bone marrow with high efficiency. Bone marrow cells were incubated with gamma interferon at 33 degrees C, then cloned, and expanded. The cell lines were characterized at 39.5 degrees C in the absence of gamma interferon. First, stromal cell lines were established that induced osteclast formation (resorption of bone slices) when cocultured with hemopoietic spleen cells. Some of the stromal cell lines so generated were able to resorb approximately 30 mm2/cm2 of bone surface. We then established cell lines of hemopoietic origin, several of which possess osteoclastic potential. When these osteoclast-precursor cell lines were cocultured with stromal cell lines, extensive bone resorption was observed. Osteoclast formation did not occur if the precursor cell lines were incubated on bone slices without stromal cells; osteoclast formation was also dependent upon the presence of 1 alpha,25-dihydroxyvitamin D3. These cell lines represent a model for osteoclast formation and a valuable resource for identification of the mechanisms and factors that regulate osteoclast differentiation and function.

Establishment of conditionally immortalized epithelial cell lines from both colon and small intestine of adult H-2Kb-tsA58 transgenic mice

Intestinal mucosal cells have proved difficult to culture in vitro. Many attempts have been made to develop long-term cultures of these cells either by direct culturing or by attempting to immortalize these cells by using a range of transforming viral genes, but with little success. The recent development of a transgenic mouse bearing a temperature-sensitive mutation of the simian virus 40 large tumor antigen gene (tsA58) has enabled us to initiate conditionally immortalized cultures of epithelial cells from both small intestinal and colonic mucosa of adult mice. Crypts were isolated from either the small intestines or colons of young adult mice and cultured at the permissive temperature (33 degrees C) in medium containing conditioned medium from a human colon carcinoma cell line, LIM1863. Crypts from both tissues yielded cultures of epithelial cells that have now been in culture for more than 12 months with regular passaging. The epithelial nature of the cells has been confirmed by staining with anti-keratin antibodies. The intestinal origin of the cells was demonstrated by the ability of the cells to synthesize low levels of both brush border peptidases and a disaccharidase. The levels of expression of these enzymes were modulated by the addition of sodium butyrate or phorbol myristate acetate to the medium, which resulted in an increase in the synthesis of the peptidases and a decrease in the synthesis of the disaccharidase. The cells proliferate continuously at the permissive temperature (33 degrees C), but proliferation ceases at the nonpermissive temperature (39.5 degrees C). To our knowledge, this is the first description of the establishment of epithelial cell lines from both small intestine and colon of the same mouse strain. The success reported here indicates that this transgenic mouse will be a useful source of tissue for the study of the mechanisms that control the proliferation and eventual differentiation and senescence of the cells of the intestinal mucosa. These mice will also be a useful source of cells for attempts to culture cells from other tissues that have proved difficult to culture in vitro.

Molecular mechanisms in the evolution of chronic myelocytic leukemia

Chronic myelocytic or Ph1-positive acute lymphoblastic leukemias have been analyzed for alterations in a variety of proto-oncogenes and anti-oncogenes implicated in the progression of chronic myeloid leukemia (CML) from its chronic phase to blast crisis. The most frequent genetic change found in disease evolution is an alteration of the p53 gene involving a point mutation, a rearrangement or a deletion. These gene changes are common in myeloid and undifferentiated variants of blast crisis but are usually undetectable in lymphoid leukemic transformants. Other molecular changes also occur in the clonal evolution of CML. The retinoblastoma-susceptibility (Rb) gene is an anti-oncogene. Structural abnormalities of Rb are frequent in all types of human acute leukemia, but are particularly common in Ph1-positive leukemia of lymphoid phenotype including both Ph1-positive ALL and lymphoid blast crisis of CML. Changes in Rb occur early in the transition to blast crisis with loss of Rb protein being the common factor. Mutations in the N-RAS gene also occur, but are rare in typical blast crisis. They are sometimes seen in Ph1-negative myeloid blast crisis. Since changes in the p53 gene are generally associated with progression of disease of a myeloid phenotype and changes in the Rb gene occur more often with a lymphoid phenotype, a particular molecular alteration may influence the character of disease evolution in CML.

From chance to choice in the generation of neural cell lines

Despite the central importance of cell lines in contemporary studies in cellular and molecular biology, many areas of potential investigation remain impeded by the limited number of lines available and by the difficulty in generating new lines of interest. Thus, there has been a constant pressure to develop improved methods for obtaining cell lines of particular interest. This review examines some of the problems associated with in vitro approaches to cell line generation. In addition, two different ways in which transgenic animals can be used to overcome the limitations of in vitro production of cell lines are discussed. In the first approach, specific promoters are utilized to target expression of immortalizing genes to cells of interest. The second approach is concerned with development of a strain of transgenic animals (the H-2KbtsA58 transgenic mouse) designed to obviate the need for identification of cell-type specific promoters, and in which it is theoretically possible to directly generate conditionally immortal cell lines from any tissue of the body by simple dissection and growth of cells in appropriate tissue culture conditions. Finally, approaches are also discussed in which investigations on the control of precursor differentiation have been applied so as to bypass the need for expression of activated immortalizing oncogenes in the generation of large quantities of conditionally immortalized cells with the capacity to undergo normal differentiation in vitro and in vivo.

Abnormalities of the retinoblastoma gene in the pathogenesis of acute leukemia

The retinoblastoma-susceptibility (Rb) gene is an antioncogene that is frequently altered in retinoblastomas, sarcomas, and some epithelial tumors. We examined the structure of the Rb gene by Southern blotting in 215 cases of leukemias and lymphomas of diverse phenotype and in 15 leukemic cell lines. In selected cases Rb protein expression was examined with specific monoclonal antibodies. Structural abnormalities of the Rb gene with absent protein expression were frequent in all types of human acute leukemia, but were particularly common (27% incidence) in M4 and M5 myeloid leukemia with monocytic differentiation and in Philadelphia chromosome (Ph1)-positive leukemia of lymphoid phenotype (11% to 29% incidence). Changes in Rb were observed early in the transition to acute leukemia in cases of myelodysplastic syndrome and in the accelerated phase of chronic myelocytic leukemia in transition to blast crisis. In one case, molecular changes in Rb could be correlated with leukemia remission and relapse. We conclude that the Rb antioncogene is commonly involved in the evolution of human acute leukemias, particularly in those of a monocytic phenotype and in lymphoid leukemia in which there is an antecedent alteration of the Ph1 chromosome.

Direct derivation of conditionally immortal cell lines from an H-2Kb-tsA58 transgenic mouse

Studies on cell lines have greatly improved our understanding of many important biological questions. Generation of cell lines is facilitated by the introduction of immortalizing oncogenes into cell types of interest. One gene known to immortalize many different cell types in vitro encodes the simian virus 40 (SV40) large tumor (T) antigen (TAg). To circumvent the need for gene insertion in vitro to generate cell lines, we created transgenic mice harboring the SV40 TAg gene. Since previous studies have shown that TAg expression in transgenic mice is associated with tumorigenesis and aberrant development, we utilized a thermolabile TAg [from a SV40 strain, tsA58, temperature sensitive (ts) for transformation] to reduce the levels of functional TAg present in vivo. To direct expression to a broad range of tissues, we used the mouse major histocompatibility complex H-2Kb promoter, which is both widely active and can be further induced by interferons. tsA58 TAg mRNA was expressed in tissues of all animals harboring the hybrid construct. Development of all tissues was macroscopically normal except for thymus, which consistently showed hyperplasia. Fibroblast and cytokeratin+ thymic epithelial cultures from these mice were readily established without undergoing crisis and were conditionally immortal in their growth; the degree of conditionality was correlated with the levels of tsA58 TAg detected. One strain of H-2Kb-tsA58 mice has been bred through several generations to homozygosity and transmits a functional copy of the transgene.

The cellular measurement of time

This review describes three biological processes in which there is evidence for single cells being able to measure elapsed time. We describe the work that has led to this view, and review more recent work that has provided new insights into possible mechanisms for the measurement of time.

Retinoblastoma gene deletions in human glioblastomas

The retinoblastoma susceptibility gene, RB, is the best characterised of the tumour suppressor genes, or 'anti-oncogenes'. Abnormal function of the RB protein is thought to result in loss of an inhibitory effect on cell growth, and thus contribute towards the development of certain human cancers. One group of human cancers of particular interest in relationship to retinoblastoma gene function are the gliomas, which are central nervous system tumours thought to originate from the neuroectoderm, the embryological tissue which also gives rise to retinoblastomas. We have therefore examined a group of benign and malignant gliomas for evidence of structural alterations of the RB gene. Four out of nine (44%) glioblastomas, the most malignant gliomas, showed loss of heterozygosity of a locus within this gene. In addition, one of these hemizygous tumours showed deletion of part of the RB protein-coding region, and this abnormality was also present in cells cultured from the tumour. These findings suggest that RB gene abnormalities may contribute to the development of glioblastomas.

Characterization of murine thymic stromal-cell lines immortalized by temperature-sensitive simian virus 40 large T or adenovirus 5 E1a

The heterogeneity of thymic stromal cells is probably related to their role in providing different microenvironments where T cells can develop. We have immortalized thymic stromal elements using recombinant retroviral constructs containing a temperature-sensitive simian virus 40 (SV40tsA58) large-T antigen gene or the adenovirus 5 E1a region linked to the gene coding for resistance to G418. Cell lines containing the thermolabile large T antigen encoded by SV40 proliferate at the permissive temperature of 33 degrees C and arrest growth when transferred to the nonpermissive temperature of 39 degrees C. At the nonpermissive temperature, ts-derived cell lines are shown to alter their phenotype but remain metabolically active, as indicated by the inducible expression of class I and class II MHC antigens. Here we describe the generation of a total of 84 thymic stromal-cell lines, many of which show distinct morphologic, phenotypic, and functional properties consistent with fibroblastoid, epithelial, or monocytoid origins. Several E1a and SV40tsA58-derived cell lines generated exhibit the epithelial characteristic of desmosome formation and, in addition, two of these lines (15.5 and 15.18) form multicellular complexes (rosettes) when incubated with unfractionated thymocytes from syngeneic mice. A single line (14.5) displays very strong nonspecific esterase activity, suggesting it may represent a macrophagelike cell type. We describe the generation of stromal cell lines with different properties, which is consistent with the heterogeneity found in the thymic microenvironment. In addition to documenting this diversity, these cell lines may be useful tools for studying T-cell development in vitro and give access to model systems in which stromal-thymocyte interactions can be examined.

Maintenance of cellular proliferation by adenovirus early region 1A in fibroblasts conditionally immortalized by using simian virus 40 large T antigen requires conserved region 1

Various mutants of adenovirus E1A were assayed for their ability to complement the growth defect at the nonpermissive temperature for the cell line tsa14 which was isolated by immortalizing rat embryo fibroblasts with the thermolabile large T antigen of tsA58. This cell line grows indefinitely at the permissive temperature but undergoes rapid growth arrest upon shift up to the nonpermissive temperature. Since this growth arrest can be overcome by introduction of wild-type simian virus 40 large T antigen, human papillomavirus 16 E7, and adenovirus E1A, the tsa14 cells provided an excellent system for defining regions of E1A necessary for complementation of the growth defect. We demonstrate that conserved region 1 (CR1) is the region of E1A required for complementation. While CR2 of E1A has been shown to be required for the immortalization of primary cells and is also necessary for the binding of the 105-kDa retinoblastoma protein, mutations within this region did not abrogate complementation of the growth defect. However, since both CR1 and CR2 have previously been shown to be absolutely required for immortalization of primary cells by adenovirus E1A, this evidence suggests that the tsa14 system assays for the maintenance of proliferation and that this requires CR1.

Cell lines established by a temperature-sensitive simian virus 40 large-T-antigen gene are growth restricted at the nonpermissive temperature

The thermolabile large T antigen, encoded by the simian virus 40 early-region mutant tsA58, was used to establish clonal cell lines derived from rat embryo fibroblasts. These cell lines grew continuously at the permissive temperature but upon shift-up to the nonpermissive temperature showed rapidly arrested growth. The growth arrest occurred in either the G1 or G2 phase of the cell cycle. After growth arrest, the cells remained metabolically active as assayed by general protein synthesis and the ability to exclude trypan blue. The inability of these cell lines to divide at the nonpermissive temperature was not readily complemented by the exogenous introduction of other nuclear oncogenes. This finding suggests that either these genes establish cells via different pathways or that immortalization by one oncogene results in a finely balanced cellular state which cannot be adequately complemented by another establishment gene.

Functional similarity between HPV16E7, SV40 large T and adenovirus E1a proteins

We have analysed the immortalizing function of Human Papillomavirus type 16 (HPV16) in a rat cell line which has been derived by immortalizing rat embryo fibroblasts with a thermolabile SV40 large T antigen and is temperature sensitive for growth. Introduction of wild type SV40 large T antigen or the adenovirus E1a 12s gene product has previously been shown to readily overcome the inability of these cells to divide at the non-permissive temperature. In contrast, the introduction of myc, another known immortalizing gene, cannot complement the growth defect of these cells. This cell line therefore provides a novel assay system which can distinguish two groups of immortalizing oncogenes. We have shown here that expression of HPV16 can readily complement the growth defect in this rat cell line and that this function can be genetically localised to E7. Cells expressing HPV16 E6 sequences are also weakly rescued from growth arrest at the non-permissive temperature. These results demonstrate a functional similarity between HPV16 E7, SV40 large T and adenovirus E1a and suggest that these genes may immortalize cells by a common mechanism. Interestingly, the limited sequence homology between these three gene products is restricted to the domain which has recently been implicated in binding the retinoblastoma gene product.

Immortalization of precursor cells from the mammalian CNS

Recent studies show that the nervous system contains many molecularly distinct cell types. Clonal cell marking experiments demonstrate that different cell types in some areas of the CNS are products of a multipotential stem cell. The factors controlling the differentiation of vertebrate CNS precursor cells would be more accessible to molecular analysis if cell lines with precursor properties could be established. Here we show that cell lines expressing an antigenic marker specific for a major brain precursor cell population can be established from rat cerebellum. We demonstrate that cell lines express the precursor, neuronal or glial properties depending on the growth conditions. This work supports the view that brain precursor cells expressing the marker Rat 401 are multipotential and can differentiate into cells with either neuronal or glial properties. Cell lines capable of differentiation should be useful in defining the signaling systems generating the cell types of the brain.

Localization of a repressive sequence contributing to B-cell specificity in the immunoglobulin heavy-chain enhancer

The immunoglobulin heavy-chain enhancer is a cis-acting element which activates transcription of nearby genes only in cells of the lymphoid lineage. To identify the minimal sequences necessary to impart cell type transcriptional specificity, we tested the activity of several deletions and internal mutations in the mu enhancer. Experiments involving measurement of both chloramphenicol acetyltransferase activity and RNA levels indicated the presence of a dominant repressor element within the mu enhancer. This repressive activity was detected in fibroblasts but not in myeloma cells. Removal or disruption of this repressor element revealed the presence of elements within the mu enhancer that activate transcription in fibroblasts. Thus, enhancer tissue specificity is in part due to the composite of both constitutive activation and cell-type-specific repressive activity. The possible biological roles of this phenomenon are discussed.

Large T antigens of simian virus 40 and polyomavirus efficiently establish primary fibroblasts

Recombinant retroviruses that transduce the simian virus 40 (SV40) large T antigen or the polyomavirus large T antigen as well as encoding resistance to antibiotic G418 were used to investigate whether these genes alone were sufficient for immortalization of primary cells. The results provided definitive evidence that either viral gene can efficiently establish primary fibroblasts. The capability of the SV40 large T antigen to establish primary fibroblasts was undiminished by a mutation that alters its binding to sequences within the origin of replication. Surprisingly, most of the primary cells established by the expression of the SV40 large T antigen did not have a transformed phenotype. This suggests that transformation by SV40 is not simply due to a high level of expression of the SV40 large T antigen and stabilization of cellular p53.

Recombinant retroviruses encoding simian virus 40 large T antigen and polyomavirus large and middle T antigens

We used a murine retrovirus shuttle vector system to construct recombinants capable of constitutively expressing the simian virus 40 (SV40) large T antigen and the polyomavirus large and middle T antigens as well as resistance to G418. Subsequently, these recombinants were used to generate cell lines that produced defective helper-free retroviruses carrying each of the viral oncogenes. These recombinant retroviruses were used to analyze the role of the viral genes in transformation of rat F111 cells. Expression of the polyomavirus middle T antigen alone resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were unaltered by the criteria of morphology, anchorage-independent growth, and tumorigenicity. More surprisingly, SV40 large T-expressing cell lines were not tumorigenic despite the fact that they contained elevated levels of cellular p53 and had a high plating efficiency in soft agar. These results suggest that the SV40 large T antigen is not an acute transforming gene like the polyomavirus middle T antigen but is similar to the establishment genes such as myc and adenovirus EIa.