TRANSFORMATION OF PROPERTIES OF AN ESTABLISHED CELL LINE BY SV40 AND POLYOMA VIRUS

Allele-specific CRISPR-Cas9 editing of dominant epidermolysis bullosa simplex in human epidermal stem cells

Epidermolysis bullosa simplex (EBS) is a rare skin disease inherited mostly in an autosomal dominant manner. Patients display a skin fragility that leads to blisters and erosions caused by minor mechanical trauma. EBS phenotypic and genotypic variants are caused by genetic defects in intracellular proteins whose function is to provide the attachment of basal keratinocytes to the basement membrane zone and most EBS cases display mutations in keratin 5 (KRT5) and keratin 14 (KRT14) genes. Besides palliative treatments, there is still no long-lasting effective cure to correct the mutant gene and abolish the dominant negative effect of the pathogenic protein over its wild-type counterpart. Here, we propose a molecular strategy for EBS01 patient's keratinocytes carrying a monoallelic c.475/495del21 mutation in KRT14 exon 1. Through the CRISPR-Cas9 system, we perform a specific cleavage only on the mutant allele and restore a normal cellular phenotype and a correct intermediate filament network, without affecting the epidermal stem cell, referred to as holoclones, which play a crucial role in epidermal regeneration.

Fibroblasts: Origins, definitions, and functions in health and disease

Fibroblasts are diverse mesenchymal cells that participate in tissue homeostasis and disease by producing complex extracellular matrix and creating signaling niches through biophysical and biochemical cues. Transcriptionally and functionally heterogeneous across and within organs, fibroblasts encode regional positional information and maintain distinct cellular progeny. We summarize their development, lineages, functions, and contributions to fibrosis in four fibroblast-rich organs: skin, lung, skeletal muscle, and heart. We propose that fibroblasts are uniquely poised for tissue repair by easily reentering the cell cycle and exhibiting a reversible plasticity in phenotype and cell fate. These properties, when activated aberrantly, drive fibrotic disorders in humans.

Evidence for immortality and autonomy in animal cancer models is often not provided, which causes confusion on key issues of cancer biology

Modern research into carcinogenesis has undergone three phases. Surgeons and pathologists started the first phase roughly 250 years ago, establishing morphological traits of tumors for pathologic diagnosis, and setting immortality and autonomy as indispensable criteria for neoplasms. A century ago, medical doctors, biologists and chemists started to enhance "experimental cancer research" by establishing many animal models of chemical-induced carcinogenesis for studies of cellular mechanisms. In this second phase, the two-hit theory and stepwise carcinogenesis of "initiation-promotion" or "initiation-promotion-progression" were established, with an illustrious finding that outgrowths induced in animals depend on the inducers, and thus are not authentically neoplastic, until late stages. The last 40 years are the third incarnation, molecular biologists have gradually dominated the carcinogenesis research fraternity and have established numerous genetically-modified animal models of carcinogenesis. However, evidence has not been provided for immortality and autonomy of the lesions from most of these models. Probably, many lesions had already been collected from animals for analyses of molecular mechanisms of "cancer" before the lesions became autonomous. We herein review the monumental work of many predecessors to reinforce that evidence for immortality and autonomy is essential for confirming a neoplastic nature. We extrapolate that immortality and autonomy are established early during sporadic human carcinogenesis, unlike the late establishment in most animal models. It is imperative to resume many forerunners' work by determining the genetic bases for initiation, promotion and progression, the genetic bases for immortality and autonomy, and which animal models are, in fact, good for identifying such genetic bases.

Regulation of reactive oxygen species homeostasis by peroxiredoxins and c-Myc

Peroxiredoxins (Prxs) are highly conserved proteins found in most organisms, where they function primarily to scavenge reactive oxygen species (ROS). Loss of the most ubiquitous member of the family, Prx1, is associated with the accumulation of oxidatively damaged DNA and a tumor-prone phenotype. Prx1 interacts with the transcriptional regulatory domain of the c-Myc oncoprotein and suppresses its transforming activity. The DNA damage in tissues of prx1-/- mice is associated in some cases with only modest increases in total ROS levels. However, these cells show dramatic increases in nuclear ROS and reduced levels of cytoplasmic ROS, which explains their mutational susceptibility. In the current work, we have investigated whether changes in other ROS scavengers might account for the observed ROS redistribution pattern in prx1-/- cells. We show approximately 5-fold increases in Prx5 levels in prx1-/- embryo fibroblasts relative to prx1+/+ cells. Moreover, Prx5 levels normalize when Prx1 expression is restored. Prx5 levels also appear to be highly dependent on c-Myc, and chromatin immunoprecipitation experiments showed differential occupancy of c-Myc and Prx1 complexes at E-box elements in the prx5 gene proximal promoter. This study represents a heretofore unreported mechanism for the c-Myc-dependent regulation of one Prx family member by another and identifies a novel means by which cells reestablish ROS homeostasis when one of these family members is compromised.

SV40: Cell transformation and tumorigenesis

The story of SV40-induced tumorigenesis and cellular transformation is intimately entwined with the development of modern molecular biology. Because SV40 and other viruses have small genomes and are relatively easy to manipulate in the laboratory, they offered tractable systems for molecular analysis. Thus, many of the early efforts to understand how eukaryotes replicate their DNA, regulate expression of their genes, and translate mRNA were focused on viral systems. The discovery that SV40 induces tumors in certain laboratory animals and transforms many types of cultured cells offered the first opportunity to explore the molecular basis for cancer. The goal of this article is to highlight some of the experiments that have led to our current view of SV40-induced transformation and to provide some context as to how they contributed to basic research in molecular biology and to our understanding of cancer.

Assessing the tumorigenic phenotype of VERO cells in adult and newborn nude mice

VERO cell lines are important substrates for viral vaccine manufacture. The mechanism by which these cells became neoplastically transformed is unknown. During tissue-culture passage, VERO cells can develop the capacity to form tumors. Although at the passage levels (around p140) currently used for vaccine manufacture, VERO cells are non-tumorigenic, questions have been raised about safety issues that might be associated with this capacity to acquire a tumorigenic phenotype. To begin to address these issues, the tumorigenicity of VERO cell lines, derived at different passage levels under different growth conditions, were evaluated in 365-day assays in adult and newborn nude mice. High passage (p>200) VERO cell lines established by random passaging in tissue culture produced tumors in adult (10 out of 27) mice and newborn (21 out of 30) mice, respectively. In contrast, a high passage (p>250) cell line established by passage at sub-confluence produced tumors only in newborn mice (16 out of 30). Progressively growing tumors began forming at 36 days in newborns and at 69 days in adults. Higher tumor incidences and shorter tumor latencies suggest that newborn nude mice may be more sensitive than adults in detecting the expression of a tumorigenic phenotype by some VERO cell lines.

Action of a 50 Hz magnetic field on proliferation of cells in culture

Proliferation of SV40-3T3 mouse fibroblasts and human HL-60 promyelocytes was studied after treatment with a sinusoidal 2 mT 50 Hz magnetic field. A single exposure of 60 minutes caused quasicyclic changes in the number of SV40-3T3 cultures as function of time after treatment, which was interpreted to be due to the induction of chronobiological mechanisms by the field. Moreover, small variations in cell cycle distribution were measured during postexposure incubation for both cell lines. To discriminate between the effect of the magnetic vector and the induced electric field, HL-60 cell exposure was also performed on organ culture dishes. These dishes consist of two coaxially centered, isolated compartments in which different electric field levels are induced in the medium during treatment. Cell growth was affected in the outer compartment only where the induced electric field ranged from 8 to 12 mVpeak/meter at 2 mT, but it was not affected in the inner compartment (field range 0-4 mVpeak/meter). This suggests that the effects on cell growth are due to the induced electric field and are expressed only above a threshold of between 4 and 8 mVpeak/meter.

p21 is a critical CDK2 regulator essential for proliferation control in Rb-deficient cells

Proliferation in mammalian cells is controlled primarily in the G1-phase of the cell cycle through the action of the G1 cyclin-dependent kinases, CDK4 and CDK2. To explore the mechanism of cellular response to extrinsic factors, specific loss of function mutations were generated in two negative regulators of G1 progression, p21 and pRB. Individually, these mutations were shown to have significant effects in G1 regulation, and when combined, Rb and p21 mutations caused more profound defects in G1. Moreover, cells deficient for pRB and p21 were uniquely capable of anchorage-independent growth. In contrast, combined absence of pRB and p21 function was not sufficient to overcome contact inhibition of growth nor for tumor formation in nude mice. Finally, animals with the genotype Rb+/-;p21(-/-) succumbed to tumors more rapidly than Rb+/- mice, suggesting that in certain contexts mutations in these two cell cycle regulators can cooperate in tumor development.

Neural cell adhesion molecule (N-CAM) domains and intracellular signaling pathways involved in the inhibition of astrocyte proliferation

The neural cell adhesion molecule (N-CAM) inhibits astrocyte proliferation in vitro and in vivo, and this effect is partially reversed by the glucocorticoid antagonist RU-486. The present studies have tested the hypothesis that N-CAM-mediated inhibition of astrocyte proliferation is caused by homophilic binding and involves the activation of glucocorticoid receptors. It was observed that all N-CAM Ig domains inhibited astrocyte proliferation in parallel with their ability to influence N-CAM binding. The proliferation of other N-CAM-expressing cells also was inhibited by the addition of N-CAM. In contrast, the proliferation of astrocytes from knockout mice lacking N-CAM was not inhibited by added N-CAM. These findings support the hypothesis that it is binding of soluble N-CAM to N-CAM on the astrocyte surface that leads to decreased proliferation. Signaling pathways stimulated by growth factors include activation of mitogen-activated protein (MAP) kinase. Addition of N-CAM inhibited MAP kinase activity induced by basic fibroblast growth factor in astrocytes. In accord with previous findings that RU-486 could partially prevent the proliferative effects of N-CAM, inhibition of MAP kinase activity by N-CAM was reversed by RU-486. The ability of N-CAM to inhibit astrocyte proliferation was unaffected, however, by agents that block the ability of N-CAM to promote neurite outgrowth. Together, these findings indicate that homophilic N-CAM binding leads to inhibition of astrocyte proliferation via a pathway involving the glucocorticoid receptor and that the ability of N-CAM to influence astrocyte proliferation and neurite outgrowth involves different signal pathways.

The neural cell adhesion molecule (N-CAM) inhibits proliferation in primary cultures of rat astrocytes

Cell proliferation is a key primary process during neural development and also plays an important role in the regenerative response of neural tissue to injury. It has been reported that glial cell proliferation is, at least in part, controlled by a neuronal signal, possibly involving cell surface molecules. We report here that the addition of purified rat neural cell adhesion molecule (N-CAM) to primary cultures of rat forebrain astrocytes inhibits their proliferation. This inhibitory effect can be elicited in cultures grown in chemically defined serum-free medium or in medium that had been supplemented with growth factors. Polyclonal antibodies to N-CAM or their Fab' fragments elicited a similar inhibitory effect. The magnitude of the inhibitory effect of N-CAM was dependent on cell density: it was maximal at low cell densities and weakened progressively as cells approached confluency. Synthetic peptides with sequences identical to a putative homophilic binding region of N-CAM mimicked the effect of purified N-CAM, while peptides of the same length and amino acid composition but with a randomized sequence did not. The addition of N-CAM antisense oligonucleotides to primary astrocyte cultures for 48 h resulted in reduced levels of N-CAM expression. After N-CAM levels on astrocytes were diminished by this treatment, the antiproliferative effect of N-CAM added to the medium was significantly reduced. The combined results suggest that N-CAM homophilic binding may be involved in the control of glial cell proliferation.

Cell density, negative proliferation control, and phosphorylation of retinoblastoma protein

Cell density negative control (CDNC) of normal human fibroblast proliferation occurs after stimulation by mitogens with different signal transduction mechanism. Delayed exposure to agents that interfere with CDNC, such as double-stranded RNA and vanadate, reveals the existence of a biochemical event, involved in CDNC, that occurs 5-8 hr after the beginning of mitogenic stimulation. This is earlier than the point of "mitogenic commitment," defined by the duration of mitogen exposure required for cell cycle entry (8-18 hr). Phosphorylation of the retinoblastoma gene product (pRB) begins 8-10 hr after mitogen stimulation and is nearly complete at 18 hr, just as the first cells enter S-phase. CDNC prevents pRB phosphorylation. Interferon beta delays pRB phosphorylation by up to 20 hr but has little effect on the timing of mitogenic commitment. Thus mitogenic commitment is located in time between CDNC and pRB phosphorylation. When agents that cause a release from CDNC are applied to dense, negatively controlled cultures after 18 hr of EGF stimulation, pRB phosphorylation occurs 6-8 hr after release. This suggests that the negatively controlled cells process the mitogenic signal but accumulate at a restriction point. The relatively early timing of CDNC-related events in the prereplicative phase raises the possibility that pRB phosphorylation is a consequence rather than a prerequisite for release from cell density negative control.

Influence of the carbohydrate moiety on the growth inhibitory activity and adhesiveness of 3T3 cell plasma membranes

Treatment of 3T3 cell plasma membranes with glycosidase enzymes decreased their ability to inhibit cell growth and also decreased their binding to 3T3 cells. This suggests that carbohydrate is required for complete function of inhibitory activity and that inhibition is associated with membrane adhesion.

Increased membrane permeability for an antitumoral alkyl lysophospholipid in sensitive tumor cells

We have investigated cellular sensitivity to the antitumoral alkyl lysophospholipid (ALP) 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) in vitro. The permeation of this lipid into the cell was not influenced by metabolic inhibitors of ATP biosynthesis. ET-18-OCH3 uptake was not saturable within sublytic concentrations, but could be inhibited in part by cytochalasin B (CB) and dipyridamole. The activation energy of the CB-sensitive uptake process was increased up to threefold compared to CB-insensitive uptake. ET-18-OCH3 influx and equilibrium binding of ET-18-OCH3 were decreased in a fibrosarcoma cell variant (MethA) selected for ET-18-OCH3 resistance. The resistant MethA cells were also less sensitive to cytolysis by lysophosphatidylcholine and other ALP. After 72 hr, the resistant MethA cells had metabolized only 11.8% more of the absorbed ET-18-OCH3 than sensitive MethA cells. However, they tolerated at least a 30-fold concentration of this ALP. The uptake mechanism, which could be inhibited by CB, was less active in resistant MethA cells and several other ALP-resistant cell lines. The concentration of CB, required for maximal uptake inhibition, was increased more than four times in the ALP-sensitive tumor cell lines. CB-specific ET-18-OCH3 uptake was also enhanced after virus transformation of 3T3 fibroblasts by SV 40. Dipyridamole retarded the ET-18-OCH3-mediated cell destruction.

Variations in polyoma virus genotype in relation to tumor induction in mice. Characterization of wild type strains with widely differing tumor profiles

The authors have explored the effects of variations in mouse polyoma virus genotype on patterns of tumor formation in the mouse. Four "wild type" virus strains were surveyed. Two were highly oncogenic, inducing multiple tumors of epithelial and mesenchymal origin, at high frequency and with short latency. The other two strains were weakly oncogenic, inducing fewer tumors, solely of mesenchymal origin, and after a long latency. These sharply contrasting tumor profiles were reproduced with virus stocks derived from molecularly cloned viral genomes. Though vastly different in their oncogenic properties, these cloned viruses proved equally effective in transforming established rat fibroblasts in culture and showed the same patterns of tumor antigen expression in cultured mouse cells. Complexes of polyoma middle T antigen and pp60c-src were demonstrated in extracts of epithelial tumors induced by a highly oncogenic virus strain. It is concluded that polyoma viral genetic determinants for tumor induction in the mouse are more complex than those previously defined by the use of cell transformation systems.

Loss of reciprocal modulations of growth and liver function of hepatoma cells in culture by contact with cells or cell membranes

In mature rat hepatocytes in primary culture, many metabolic functions and cell growth are controlled reciprocally by cell density, and this reciprocal regulation is mediated by a cell-surface modulator through cell-cell contact. Cultured RY-121B cells from Reuber hepatoma and MH1C1 cells from Morris hepatoma, which retain some liver-specific functions, did not show any cell-density dependency of either cell growth or hepatocyte-specific functions, such as induction of tyrosine aminotransferase by dexamethasone. However, when RY-121B cells were cocultured with a low density of rat hepatocytes as monolayers in direct contact, they exerted contact-dependent control of DNA synthesis and of differentiated function of the hepatocytes. Furthermore, plasma membranes from various tumor cells including these hepatoma cells had strong modulator activity on primary cultures of normal rat hepatocytes, and their activity mimicked the reciprocal effects of cell density on DNA synthesis and induction of tyrosine aminotransferase. On the contrary, addition of plasma membranes from normal adult rat liver to sparse cultures of RY-121B or MH1C1 cells did not cause any inhibition of active DNA synthesis or enhancement of induction of tyrosine aminotransferase in these cells. These results suggest that hepatoma cells have lost cell density-dependent regulations of many cellular activities and cell growth because they have lost the ability to respond to the cell surface modulator, although they retain modulator activity on their plasma membranes.

Growth-inhibitory activity of lymphoid cell plasma membranes. II. Partial characterization of the inhibitor

We have shown that plasma membranes from lymphoid cells have inhibitory activity for the growth of normal lymphocytes and lymphoid tumor cells (Stallcup, K. C., A. Dawson, and M. F. Mescher, J. Cell Biol. 99:1221-1226). This growth-inhibitory activity has been found to co-purify with major histocompatibility complex class I antigens (H-2K and D) when these cell surface glycoproteins are isolated from detergent lysates of cells by affinity chromatography on monoclonal antibody columns. When incorporated into liposomes, the affinity-purified H-2 antigens inhibited the growth of both normal lymphocytes and tumor cells at concentrations of 1-3 micrograms/ml. Inhibition was readily reversed upon removal of the liposomes from the cell cultures, even after several days of exposure of cells to the inhibitor. Inhibitory activity was insensitive to protease digestion or heat treatment, indicating that it was not due to the H-2 glycoproteins. This was confirmed by the demonstration that inhibitory activity could be separated from the H-2 protein by gel filtration in the presence of deoxycholate and could be extracted from membranes or H-2 antigen preparations with organic solvents. The results demonstrate that the growth-inhibitory component(s) of the plasma membrane is a minor lipid or lipid-like molecule which retains activity in the absence of other membrane components. The findings reported here and in the preceding article suggest that this novel membrane component may have a role in control of lymphoid cell growth, possibly mediated by cell contacts.

Reduced insulin endocytosis in serum-transformed fibroblasts demonstrated by flow cytometry

Neoplastic transformation often results in the loss of growth control and concomitant changes in cell surface properties. The changes in endocytosis of a variety of probes after serum or anchorage transformation were measured for mouse fibroblasts by flow cytofluorometry. No major differences in dextran (fluid phase) or histone (nonspecific-adsorptive) endocytosis were observed among four cell lines with different growth properties. However, decreased receptor-mediated internalization of alpha 2-macroglobulin was observed for cell lines transformed to either serum or anchorage independence. Furthermore, increased wheat germ agglutinin and decreased insulin endocytosis were observed, but only in serum transformants. The changes specific to serum transformants were not accounted for by changes in binding of wheat germ agglutinin or insulin. The possible implications of these observations regarding serum transformation and the insulin requirement for growth in serum-free medium are discussed.

Density-dependent inhibition of growth: inhibitory diffusible factors from 3T3- and Rous sarcoma virus (RSV)-transformed 3T3 cells

We recently fractionated, from the culture medium of 3T3 cells, a thermolabile inhibitory diffusible factor (IDFN) with a molecular weight of about 40,000 daltons, which decreased nucleic acids synthesis of stimulated target 3T3 cells. In the present publication the inhibitory activities of IDFN (produced by 3T3 cells) and IDFT (produced by RSV-transformed 3T3 [3T3 SRA/H] cells) on 3T3 and 3T3 SRA/H cells have been compared. The inhibitory activity of IDFN decreased (by a mean of 57%) when it was tested on transformed instead of 3T3 cells. On the other hand, IDFT was able to decrease 14C-inosine incorporation in target 3T3 cells. However, the inhibitory activity of IDFT decreased (by mean 50%) when tested on 3T3 SRA/H instead of 3T3 cells. Therefore, transformed cells produced an inhibitory factor but were less sensitive than 3T3 cells to its inhibitory activity. The inhibitory activity of IDFT on 3T3 SRA/H cells was only 20% of the inhibitory activity of IDFN on 3T3 cells. This appreciable difference is of particular interest, since it could explain the release of density-dependent inhibition of growth (DDI) in transformed 3T3 SRA/H cells. Furthermore, it provides more evidence for the hypothesis that, in 3T3 cells, DDI of growth is due to the release of an inhibitory molecule into the medium, and that IDFN is in fact, the inhibitory molecule involved in this phenomenon.

Diverse mitogenic agents induce the phosphorylation of two related 42,000-dalton proteins on tyrosine in quiescent chick cells

Incubation of quiescent chicken embryo cells with platelet-derived growth factor, epidermal growth factor, or serum was found to stimulate phosphorylation of two proteins of ca. 42,000 daltons on tyrosine. These proteins are structurally related to each other and to two proteins phosphorylated on tyrosine under similar conditions in mitogen-treated mouse fibroblasts. Three other very different mitogenic agents, the protease trypsin and the chemically unrelated tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate and teleocidin, stimulated phosphorylation of the same proteins. In all cases, phosphotyrosine was detected in these phosphoproteins. Although additional changes in protein phosphorylation were evident, no other proteins were observed by two-dimensional gel electrophoresis which contained increased amounts of phosphotyrosine in mitogen-treated chicken embryo cells. One of these 42,000-dalton proteins was shown previously to be phosphorylated on tyrosine in chicken embryo cells transformed with various retroviruses whose transforming proteins possess tyrosine protein kinase activity. Phosphorylation of the 42,000-dalton proteins could be important in the regulation of cell division.

Purified plasma membranes inhibit polypeptide growth factor-induced DNA synthesis in subconfluent 3T3 cells

Plasma membranes derived from NR-6 cells, a variant line of Swiss mouse 3T3 cells that does not have cell surface receptors for epidermal growth factor (EGF), inhibited EGF-induced stimulation of DNA synthesis by 50% in serum-starved, subconfluent 3T3 cells. Membranes derived from SV3T3 cells were much less effective in inhibiting EGF-induced DNA synthesis. This inhibition on DNA synthesis by NR-6 membranes was not a direct effect of membranes on EGF, nor could it be overcome by high concentrations of EGF. NR-6 membranes were most effective when added 3 h before EGF addition and had little effect when added 2 h or more after EGF. NR-6 membranes also reduced the stimulation of DNA synthesis induced by platelet-derived growth factor or fibroblast growth factor in serum-starved 3T3 cells. These findings indicate that membrane-membrane interactions between nontransformed cells may diminish their ability to proliferate in response to serum polypeptide growth factors.

Diverse mitogenic agents induce the phosphorylation of two related 42,000-dalton proteins on tyrosine in quiescent chick cells

Incubation of quiescent chicken embryo cells with platelet-derived growth factor, epidermal growth factor, or serum was found to stimulate phosphorylation of two proteins of ca. 42,000 daltons on tyrosine. These proteins are structurally related to each other and to two proteins phosphorylated on tyrosine under similar conditions in mitogen-treated mouse fibroblasts. Three other very different mitogenic agents, the protease trypsin and the chemically unrelated tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate and teleocidin, stimulated phosphorylation of the same proteins. In all cases, phosphotyrosine was detected in these phosphoproteins. Although additional changes in protein phosphorylation were evident, no other proteins were observed by two-dimensional gel electrophoresis which contained increased amounts of phosphotyrosine in mitogen-treated chicken embryo cells. One of these 42,000-dalton proteins was shown previously to be phosphorylated on tyrosine in chicken embryo cells transformed with various retroviruses whose transforming proteins possess tyrosine protein kinase activity. Phosphorylation of the 42,000-dalton proteins could be important in the regulation of cell division.

Paravertebral malignant rhabdoid tumor in infancy. In vitro studies of a familial tumor

Two female siblings died within three months after presenting with paravertebral tumors in the first year of life. The pathology of the two tumors was identical and characteristic of a malignant rhabdoid tumor. There were no identifiable tumor patterns within the kindred which have been associated with any hereditary cancer or precancer syndromes. Fibroblasts were cultured from skin biopsies obtained from the second patient and both parents. Assays of growth kinetics associated with cellular transformation revealed that fibroblasts from the affected sibling can be distinguished from those of the parents and age-matched controls by increased in vitro occurrence of tetraploidy. Such evidence suggests that increased in vitro tetraploidy occurring spontaneously in cultured fibroblasts is an expression of a cancer-prone gene. Increased in vitro tetraploidy has previously been demonstrated in some kindreds with heritable colon cancer syndromes, and may extend our understanding of the genetic etiology of some childhood cancers.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Nuclease S1 sensitive sites in parental deoxyribonucleic acid of cold-and temperature-sensitive mammalian cells

Temperature-sensitive mutants of 3T3 cells (H6-15) express the transformed phenotype at 33 degrees C and the normal phenotype at 39 degrees C. Cold-sensitive mutants of Chinese hamster ovary cells (cs4-D3) express the transformed phenotype at 39 degrees C and the normal phenotype, along with a G1 block, at 33 degrees C. When either cell type is under conditions such that it is normal and in a G0 state, the number of S1-sensitive sites in purified DNA, labeled in parental chains only, is zero. When the normal cells are stimulated by 10% serum, the number of S1 sites per 10(5) base pairs increases slightly, to 0.7 in cs4-D3 and 1.1 in H6-15. Under conditions permitting the expression of the transformed phenotype, but not proliferation, the maximum number of S1 sites per 10(5) base pairs is 5 in cs4-D3 and 44 in H6-15. When the stationary transformed cells are stimulated by 10% serum, the number of S1 sites per 10(5) base pairs increases to 6 in cs4-D3 and 43 in H6-15. Furthermore, the DNA from the stimulated transformed H6-15 cells contains at least twice as many S1 sites as the total number of breaks (nicks plus gaps), raising the possibility of the acquisition of stable looped or cruciform structures as the cells are stimulated.

Growth control and cell spreading: differential response in preneoplastic and in metastatic cell variants

Growth control and sensitivity to changes in cell shape were studied in anchorage-dependent mouse fibroblasts (diploid fibroblasts, 3T3 and 3T6), in DNa tumor-virus-transformed mouse fibroblasts (SVPy 3T3), in four B16 melanoma and five uv-2237 fibrosarcoma cell variants that exhibit distinct metastatic properties. Differential adhesive conditions were established by precoating the plastic plates with poly (2-hydroxyethylmethacrylate) that allowed an accurate and reproducible control of cell shape, from flat to spherical. Mouse fibroblasts that form a continuum between rigorously controlled cells to fully anchorage-independent cells, display a direct correlation between degree of growth control and sensitivity to changes in cell spreading. In contrast, there is no apparent direct correlation between sensitivity of growth control to changes in cellular configuration and the metastatic potential of tumor cells.

Growth control in cultured 3T3 fibroblasts II. Molecular properties of a fraction enriched in growth inhibitory activity

Treatment of sparse, proliferating cultures of 3T3 cells with medium conditioned by exposure to density-inhibited 3T3 cultures resulted in an inhibition of growth and division in the target cells when compared to similar treatment with unconditioned medium. This growth inhibitory activity was fractionated by ammonium sulfate precipitation and gel filtration, yielding one fraction that was 35-fold enriched in specific activity. Analysis of the chemical and biological properties of this highly active fraction indicated that: (a) it is an endogenous cell product, synthesized by the 3T3 cells and shed into the medium; (b) it is a protein and its activity is sensitive to treatment with pronase; (c) the constituent polypeptide chains have molecular weights of 10,000 and 13,000; and (d) it is not cytotoxic and its effect on target cells are reversible. These results suggest that we have partially purified from conditioned medium an endogenous growth regulatory factor that may play a role in density-dependent inhibition of growth in cultured fibroblasts. We propose the term Fibroblast Growth Regulator to describe this class of molecules.

An inhibitor of animal cell growth increases cell-to-cell adhesion

The interactions of both normal and transformed cells with their environment is mediated to a large extent by the cell surface. Succinylated concanavalin A (succinyl-Con A) is a nontoxic and nonagglutinating derivative of the jack-bean lectin concanavalin A. Succinyl-Con A, presumably through an interaction with the cell surface, reversibly inhibits the growth of normal cells and restores a normal growth phenotype to transformed cells. Whereas at high cell densities migration was inhibited, it turned out that at low cell densities where cells are not in contact with each other, cell movement was not affected by succinyl-Con A. Together with some additional observations, this suggests that this lectin derivative increases cell-to-cell adhesion in culture and thereby may influence cell migration. An increase in cell-to-cell adhesion by this lectin derivative may not be brought about simply by physically linking cells together. It occurs after a lag time, possibly by inducing surface changes. The relationship between cell adhesion in culture, cell movement, and cell growth is discussed.

Elimination of Mycoplasma hyorhinis infections from four cell lines

Four monolayer mammalian cell lines were cured of Mycoplasma hyorhinis infections by cloning in microtiter dishes in the presence of tetracycline and kanamycin. During cloning, cultures were refed with fresh antibiotic containing medium every 2 or 3 d for 14 day and were then cultured without effective antibiotics for at least 21 d. From the four lines we recovered 29 clones, none of which were infected after treatment as judged by the lack of extranuclear fluorescence after staining with the fluorochrome Hoechst 33258, and by normal autoradiographic labeling of the cells by tritiated nucleosides. One clone from each line was tested further by attempted culture of mycoplasmas and was also judged to be uninfected. Infection has not reappeared in any of the clones after extensive culture in the absence of the effective antibiotics.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Phosphorylation patterns of tumour antigens in cells lytically infected or transformed by simian virus 40

The phosphorylation sites of simian virus 40 (SV40) large tumor (T) antigens have been analyzed by partial proteolysis peptide mapping and phosphoamino acid analysis of the resulting products. At least four sites were found to be phosphorylated. An amino-terminal part of the molecule contained both phosphoserine and phosphothreonine. One phosphothreonine residue was located in the proline-rich carboxy-terminal end of the molecule, either at position 701 or at position 708. The mutant dl 1265, which is defective in adenovirus helper function, lacked this phosphorylation site. In addition, the carboxy-terminal part of the molecule contained phosphoserine at a more central position. T-antigen-associated proteins of SV40-transformed cell (nonviral T; 51,000 to 55,000 daltons) also contained multiple phosphorylation sites involving at least two serine residues in mouse antigens and an additional threonine residue in rat, human, and monkey antigens. The latter residue and at least one phosphoserine residue were located near one terminus of the human NVT molecule. We did not find any evidence for phosphorylation of tyrosine residues in any of the multiple species of either large T or nonviral T molecules. Several forms of large T antigens were extracted from both SV40-transformed and SV40-infected permissive and nonpermissive cells, and their phosphorylation patterns were compared. No evidence was found for a different phosphorylation pattern of T antigen in transformed cells.

Density-dependent regulation of cell growth: an example of a cell-cell recognition phenomenon

Cell-to-cell contact can result in a variety of changes in the cell's physiology. For different cell types, this may include both the initiation as well as the cessation of cell growth and changes in the state of differentiation. This review examines in detail one such phenomenon, density-dependent inhibition of growth, which is observed with many fibroblasts in culture. Data are summarized which demonstrate that the cessation of growth at high cell density is in part a consequence of cell-to-cell contact. An approach to the study of the molecular basis of this phenomenon is presented based on the demonstration that plasma membranes, when bound to sparse growing cells, mimic contact inhibition of growth. The present status of attempts to purify plasma membrane proteins responsible for this effect are summarized, and the properties of these membrane proteins are compared to those of previously described "soluble" proteins that inhibit cellular growth.

Sequence of events in the transformation process in cells infected with a temperature-sensitive transformation mutant of Moloney murine sarcoma virus

Normal rat kidney cells infected with the temperature-sensitive transformation mutant of Moloney murine sarcoma virus were used to study the biochemical and morphological changes that occur during transformation. The infected cells exhibited a normal morphology at the nonpermissive temperature (39 degrees C) and a transformed morphology at the permissive temperature (33 degrees C). A new viral protein was detected 2 hr after shift to the permissive temperature as a polyprotein with an estimated Mr of 85,000 (p85). Scanning electron microscopy of the cells within 5 hr after shifting them to the permissive temperature showed that they became smaller and rounded with numerous elongated microvilli. In an earlier study, changes in hexose uptake were found to occur 8-12 hr after the shift [Horn, J. P., Wood, T. G., Blair, D. G. & Arlinghaus, R. B. (1980) Virology 105, 516-525]. By 48 hr, the cells had the morphology of a fully transformed cell. Concomitant with the changes in the morphology were alterations in the cytoplasmic microtubule complex. At the nonpermissive temperature, the complex consisted of a lacy network of microtubules. Within 5 hr at the permissive temperature, the lacy network was still present but the microtubules were more diffusely stained and less discernible. By 48 hr, the microtubules were so diffuse that the lacy network could not be recognized. Alterations in the F-actin cables did not occur until 24 hr after shifting the cells to the permissive temperature. Enucleation of the cells at the nonpermissive temperature and shifting the cytoplasts to the permissive temperature did not result in the synthesis of detectable p85 or in any alteration of the cytoplast morphology or microtubule complex, suggesting that the temperature-sensitive lesion affects some event occurring in the nucleus.

In vitro evidence of genetic heterogeneity within the heritable colon cancer syndromes with polyposis coli

Assays of in vitro biological properties associated with cellular transformation revealed differences in cultured skin cells within the heritable colon cancer syndromes with polyposis coli. Such evidence suggested that genetic heterogeneity, long assumed from in vivo differences in extracolonic lesions in these syndromes, could be detected in vitro. None of the properties studied nor any grouping of them made identification of a specific genotype possible.

Some aspects of the modulation and regulation of collagen synthesis in vitro

We reviewed here a number of publications containing data on the quantitative aspects of collagen synthesis in vitro. In one section we discussed the factors which modulate the amount of collagen synthesized in various culture systems and in another section we presented experimental evidence for regulatory mechanisms operating in collagen synthesis on the transcriptional and/or translational levels. We believe that growing knowledge of the mechanisms controlling collagen synthesis will help us to understand and deal with fibrotic processes better.