Collagen substrata for studies on cell behavior

A simple technique is described for the preparation of collagen substrata containing 0 1% of collagen by weight, in the form of native bundles with a 640 A period, the substrata are similar in these respects to soft-tissue matrices These substrate are hydrated collagen lattices (HCLs) in which the watery milieu is held within a fibrous collagen net mainly by capillary forces. HCLs have been characterized in terms of the course of collagen precipitation and aggregation, ultrastructure, and their stability under various conditions. The ways in which HCLs can be employed as both two- and three-dimensional substrata in cell behavioral studies are illustrated with some preliminary observations on the form, motility, adhesion, and growth of human diploid cells and two lines of malignant cells.

Establishment of a pancreatic beta cell line that retains glucose-inducible insulin secretion: special reference to expression of glucose transporter isoforms

Two cell lines have been established from insulinomas obtained by targeted expression of the simian virus 40 T antigen gene in transgenic mice. These cell lines, designated MIN6 and MIN7, produce insulin and T antigen and have morphological characteristics of pancreatic beta cells. MIN6 cells exhibit glucose-inducible insulin secretion comparable with cultured normal mouse islet cells, whereas MIN7 cells do not. Both cell lines produce liver-type glucose transporter (GT) mRNA at high level. Brain-type GT mRNA is also present at considerable level in MIN7 cells, but is barely detectable in MIN6 cells, suggesting that exclusive expression of the liver-type GT is related to glucose-inducible insulin secretion. MIN6 cells do not express either major histocompatibility (MHC) class I or class II antigens on the cell surface. However, treatment with interferon-gamma induces high levels of MHC class I antigens, and a combination of interferon-gamma and tumor necrosis factor-alpha induces a MHC class II antigen on the cell surface. These results emphasize that the MIN6 cell line retains physiological characteristics of normal beta cells. The MIN6 cell line will be especially useful to analyze the molecular mechanisms by which beta cells regulate insulin secretion in response to extracellular glucose concentrations. We discuss a possible role of GT isoforms in glucose sensing by beta cells.

In vivo sequence requirements of the SV40 early promotor region

To investigate the sequences necessary for proper initiation of transcription of SV40 early genes, we have constructed several deletion mutants in the promoter region. The TATA box region is apparently involved in fixing initiation precisely within a narrow area, but is dispensable for gene expression, while the sequences located more than 150 base pairs upstream are indispensable.

Telomere elongation in immortal human cells without detectable telomerase activity

Immortalization of human cells is often associated with reactivation of telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether telomerase activation is necessary for immortalization. All normal human fibroblasts tested were negative for telomerase activity. Thirteen out of 13 DNA tumor virus-transformed cell cultures were also negative in the pre-crisis (i.e. non-immortalized) stage. Of 35 immortalized cell lines, 20 had telomerase activity as expected, but 15 had no detectable telomerase. The 15 telomerase-negative immortalized cell lines all had very long and heterogeneous telomeres of up to 50 kb. Hybrids between telomerase-negative and telomerase-positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that activation of telomerase is not sufficient for immortalization. Some hybrid clones subsequently recommenced proliferation and became immortalized either with or without telomerase activity. Those without telomerase activity also had very long and heterogeneous telomeres. Taken together, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achieved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.

DNA gyrase: an enzyme that introduces superhelical turns into DNA

Relaxed closed-circular DNA is converted to negatively supercoiled DNA by DNA gyrase. This enzyme has been purified from Escherichia coli cells. The reaction requires ATP and Mg++ and is stimulated by spermidine. The enzyme acts equally well on relaxed closed-circular colicin E1, phage lambda, and simian virus 40 DNA. The final superhelix density of the DNA can be considerably greater than that found in intracellularly supercoiled DNA.

Adenovirus E1b-58kd tumor antigen and SV40 large tumor antigen are physically associated with the same 54 kd cellular protein in transformed cells

The adenovirus E1b-58kd tumor antigen has been detected in a physical association with a 54 kilodalton cellular protein in adenovirus-transformed mouse cells. Antibody specific for the E1b-58kd protein coimmunoprecipitates a 54 kd protein from transformed, but not from productively infected, cells. Monoclonal antibody specific for the cellular 54 kd protein coimmunoprecipitates the adenovirus E1b-58kd protein from transformed cell extracts. The same or closely related cellular 54 kd protein, associated with the adenovirus E1b-58kd protein, was present in the SV40 large T antigen-54 kd complex previously detected in SV40-transformed mouse cells. The identity of the 54 kd protein is based on the immunological specificities of the anti-54 kd monoclonal antibodies and partial peptide maps of the 54 kd protein associated with the adenovirus and SV40 tumor antigens. The adenovirus E1b-58kd-54 kd complex, like the SV40 large T antigen-54 kd complex, is heterogeneous in size or mass. While all of the cellular 54 kd protein in the adenovirus-transformed cell extract is found in a complex with the E1b-58kd protein, some of the viral 58 kd antigen is detected in a form not associated with the 54 kd protein. The fact that the adenovirus and Sv40 tumor antigens, both required for transformation, can be found in physical association with the same cellular protein in a transformed cell is a good indication that these two diverse viral proteins share some common mechanisms or functions.

Interaction between CD4 and class II MHC molecules mediates cell adhesion

The CD4 glycoprotein is expressed on T-helper and cytotoxic lymphocytes which are restricted to class II major histocompatibility complex (MHC) antigens on target cells. Antibody inhibition studies imply that CD4 acts to increase the avidity of effector-target cell interactions. These observations have led to the speculation that CD4 binds to a monomorphic class II antigen determinant, thereby augmenting low affinity T-cell receptor-antigen interactions. However, no direct evidence has been presented indicating that CD4 and class II molecules interact. To address this issue, we have used a vector derived from simian virus 40 (SV40) to express a complementary DNA (cDNA) encoding the human CD4 glycoprotein. When CV1 cells expressing large amounts of the CD4 protein at the cell surface are incubated with human B cells bearing MHC-encoded class II molecules, they are bound tightly to the infected monolayer, whereas mutant B cells which lack class II molecules fail to bind. Furthermore, the binding reaction is specifically inhibited by anti-class II and anti-CD4 antibodies. Thus, the CD4 protein, even in the absence of T-cell receptor-antigen interactions, can interact directly with class II antigens to function as a cell surface adhesion molecule.

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.

Regulation of p16CDKN2 expression and its implications for cell immortalization and senescence

p16CDKN2 specifically binds to and inhibits the cyclin-dependent kinases CDK4 and CDK6, which function as regulators of cell cycle progression in G1 by contributing to the phosphorylation of the retinoblastoma protein (pRB). Human cell lines lacking functional pRB contain high levels of p16 RNA and protein, suggesting a negative feedback loop by which pRB might regulate p16 expression in late G1. By a combination of nuclear run-on assays and promoter analyses in human fibroblasts expressing a temperature-sensitive simian virus 40 T antigen, we show that p16 transcription is affected by the status of pRB and define a region in the p16 promoter that is required for this response. However, the effect is not sufficient to account for the differences in p16 RNA levels between pRB-positive and -negative cells. Moreover, p16 RNA is extremely stable, and the levels do not change appreciably during the cell cycle. Primary human fibroblasts express very low levels of p16, but the RNA and protein accumulate in late-passage, senescent cells. The apparent overexpression of p16 in pRB-negative cell lines is therefore caused by at least two factors: loss of repression by pRB and an increase in the number of population doublings.

Teratocarcinomas and mammalian embryogenesis

In the last decade there has emerged an appreciation of the remarkable similarity between the cells that give rise to teratocarcinomas in mice and the cells that give rise to the developing mouse embryo. The resemblance is so close that in certain instances the tumor stem cells can join with their embryonic counterparts and develop into a completely normal mouse. The availability of stem cell lines isolated from mouse teratocarcinomas has made possible a number of new biochemical, immunological, and genetic approahes to the study of early mammalian development.

Purification and characterization of CAF-I, a human cell factor required for chromatin assembly during DNA replication in vitro

The purification and characterization of a replication-dependent chromatin assembly factor (CAF-I) from the nuclei of human cells is described. CAF-I is a multisubunit protein that, when added to a crude cytosol replication extract, promotes chromatin assembly on replicating SV40 DNA. Chromatin assembly by CAF-I requires and is coupled with DNA replication. The minichromosomes assembled de novo by CAF-I consist of correctly spaced nucleosomes containing the four core histones H2A, H2B, H3, and H4, which are supplied in a soluble form by the cytosol replication extract. Thus, by several criteria, the CAF-I-dependent chromatin assembly reaction described herein reflects the process of chromatin formation during DNA replication in vivo.

Stabilization of the p53 tumor suppressor is induced by adenovirus 5 E1A and accompanies apoptosis

Oncogenic transformation by human adenoviruses requires early regions 1A and 1B (E1A and E1B) and provides a model of multistep carcinogenesis. This study shows that the metabolic stabilization of p53 observed in adenovirus 5 (Ad5)-transformed cells can occur in untransformed cells expressing E1A alone. Stabilized p53 was localized to the nucleus and was indistinguishable from wild-type p53 with respect to its interactions with hsc70, PAb420, Ad5 p55E1B, and SV40 large T antigen. Moreover, binding of Ad5 p55E1B or SV40 large T antigen had no additional effect on p53 levels or turnover. Higher levels of p53 were also induced in a variety of cell types within 40 hr after transferring E1A genes. E1A also caused cells to lose viability by a process resembling apoptosis. The apoptosis appeared to involve p53, because p53 levels reverted to normal in surviving cells that had lost E1A, and E1B protected cells from the toxic effects of E1A. These results suggest that (1) the involvement of p53 in tumor suppression and/or apoptosis can be regulated at the level of protein turnover, and (2) a major oncogenic role for E1B is to counter cellular responses to E1A (i.e., stabilization of p53 and associated apoptosis) that preclude transformation by E1A alone. This represents the first physiological setting in which high levels of endogenous p53 are induced in response to an oncogenic challenge, with the apparent consequence of suppressing transformation.

The arrangement of simian virus 40 sequences in the DNA of transformed cells

High molecular weight DNA, isolated from eleven cloned lines of rat cells independently transformed by SV40, was cleaved with various restriction endonucleases. The DNA was fractionated by electrophoresis through agarose gels, denatured in situ, transferred directly to sheets of nitrocellulose as described by Southern (1975), and hybridized to SV40 DNA labeled in vitro to high specific activity. The location of viral sequences among the fragments of transformed cell DNA was determined by autoradiography. The DNAs of seven of the cell lines contained viral sequences in fragments of many different sizes. The remaining four cell lines each contain a single insertion of viral DNA at a different chromosomal location. The junctions between viral and cellular sequences map at different places on the viral genome.

FVB/N: an inbred mouse strain preferable for transgenic analyses

FVB/N mice offer a system suitable for most transgenic experiments and subsequent genetic analyses. The inbred FVB/N strain is characterized by vigorous reproductive performance and consistently large litters. Moreover, fertilized FVB/N eggs contain large and prominent pronuclei, which facilitate microinjection of DNA. The phenotype of large pronuclei in the zygote is a dominant trait associated with the FVB/N oocyte but not the FVB/N sperm. In experiments to generate transgenic mice, the same DNA constructs were injected into three different types of zygotes: FVB/N, C57BL/6J, and (C57BL/6J x SJL/J)F1. FVB/N zygotes survived well after injection, and transgenic animals were obtained with efficiencies similar to the F1 zygotes and much better than the C57BL/6J zygotes. Genetic markers of the FVB/N strain have been analyzed for 44 loci that cover 15 chromosomes and were compared with those of commonly used inbred strains. In addition to the albino FVB/N strain, pigmented congenic strains of FVB/N are being constructed. These features make the FVB/N strain advantageous to use for research with transgenic mice.

A difference in the architecture of the surface membrane of normal and virally transformed cells

Several tissue culture cell lines that were transformed by a tumor virus have been found to react with an agglutinin, while under identical conditions their untransformed parent cell lines did not agglutinate. Since a short treatment of the parent cell line with low concentrations of proteases exposed the same agglutinin receptor sites in a fashion indistinguishable from the transformed cells, it is proposed that both viral and chemical transformation produce changes in the architecture of the membrane, identical to those of the proteases.

Conformational fluctuations of DNA helix

Treatment of a homogeneous sample of circular DNA molecules that contain at least one single-strand scission per molecule with ligase yields a population of covalently closed circles heterogeneous in the topological winding number alpha. Under proper conditions, the product molecules with different values of alpha can all be resolved by gel electrophoresis. The distributions in alpha for several DNAs, covalently closed at different temperatures, have been determined and the results have been interpreted in terms of thermal fluctuations of the DNA helix.

Species- and cell type-specific requirements for cellular transformation

Recent evidence suggests that human cells require more genetic changes for neoplastic transformation than do their murine counterparts. However, a precise enumeration of these differences has never been undertaken. We have determined that perturbation of two signaling pathways-involving p53 and Raf-suffices for the tumorigenic conversion of normal murine fibroblasts, while perturbation of six pathways-involving p53, pRb, PP2A, telomerase, Raf, and Ral-GEFs-is needed for human fibroblasts. Cell type-specific differences also exist in the requirements for tumorigenic transformation: immortalized human fibroblasts require the activation of Raf and Ral-GEFs, human embryonic kidney cells require the activation of PI3K and Ral-GEFs, and human mammary epithelial cells require the activation of Raf, PI3K, and Ral-GEFs.

Impairment of mycobacterial but not viral immunity by a germline human STAT1 mutation

Interferons (IFN) alpha/beta and gamma induce the formation of two transcriptional activators: gamma-activating factor (GAF) and interferon-stimulated gamma factor 3 (ISGF3). We report a natural heterozygous germline STAT1 mutation associated with susceptibility to mycobacterial but not viral disease. This mutation causes a loss of GAF and ISGF3 activation but is dominant for one cellular phenotype and recessive for the other. It impairs the nuclear accumulation of GAF but not of ISGF3 in heterozygous cells stimulated by IFNs. Thus, the antimycobacterial, but not the antiviral, effects of human IFNs are principally mediated by GAF.

The fibronexus: a transmembrane association of fibronectin-containing fibers and bundles of 5 nm microfilaments in hamster and human fibroblasts

A possible connection between external fibronectin-containing fibers and cytoplasmic 5 nm actin microfilaments within dense submembranous plaques has been observed by transmission electron microscopy. We refer to this transmembranous association as the fibronexus. Hamster embryo fibroblasts, transformed by wild-type or temperature-sensitive mutant (A28) SV40 virus, and human lung fibroblasts (WI-38, MRC-5) were studied using the tannic acid method of Simionescu and Simionescu (1976), which preferentially stained external carbohydrates. Fibronectin antigens were also localized on the extracellular fibers of the fibronexus with fibronectin antibody and immunoferritin staining. Goniometric studies of sections cut parallel to the plasmalemma demonstrated that the actin- and fibronectin-containing fibers of the fibronexus remained colinear when the specimen was tilted through a 40 degree angle about the fibrillar long axis. Sections cut perpendicular to the cell surface also showed that these fibers were apparently colinear. Our results suggest that the fibronectin and actin fibers of the fibronexus are closely associated (maximum separation distances of 8--22 nm), if not co-axial. Fibronexuses remained after expression of SV40-induced transformation, despite alteration of microfilament bundles and reduction in the amount of fibronectin (observed by immunofluorescence microscopy). The possible roles of fibronectin and the fibronexus in regulating actin polymerization are discussed.

Subunit rearrangement of the cyclin-dependent kinases is associated with cellular transformation

In normal human diploid fibroblasts, cyclins of the A, B, and D classes each associate with cyclin-dependent kinases (CDKs), proliferating cell nuclear antigen (PCNA), and p21, thereby forming multiple independent quaternary complexes. Upon transformation of diploid fibroblasts with the DNA tumor virus SV40, or its transforming tumor antigen (T), the cyclin D/p21/CDK/PCNA complexes are disrupted. In transformed cells, CDK4 totally dissociates from cyclin D, PCNA, and p21 and, instead, associates exclusively with a polypeptide of 16 kD (p16). Quaternary complexes containing cyclins A or B1 and p21/CDK/PCNA also undergo subunit rearrangement in transformed cells. Both PCNA and p21 are no longer associated with CDC2-cyclin B1 binary complexes. Cyclin A complexes no longer contain p21, and a new 19-kD polypeptide (p19) is found in association with cyclin A. The pattern of subunit rearrangement of cyclin-CDK complexes in SV40-transformed cells is also shared in those containing adeno- or papilloma viral oncoproteins. Rearrangement also occurs in p53-deficient cells derived from Li-Fraumeni patients that carry no known DNA tumor virus. These findings suggest a mechanism by which oncogenic proteins alter the cell cycle of transformed cells.