Steps in the neoplastic transformation of hamster embryo cells by polyoma virus

Karyotype alteration generates the neoplastic phenotypes of SV40-infected human and rodent cells

BACKGROUND

Despite over 50 years of research, it remains unclear how the DNA tumor viruses SV40 and Polyoma cause cancers. Prevailing theories hold that virus-coded Tumor (T)-antigens cause cancer by inactivating cellular tumor suppressor genes. But these theories don't explain four characteristics of viral carcinogenesis: (1) less than one in 10,000 infected cells become cancer cells, (2) cancers have complex individual phenotypes and transcriptomes, (3) recurrent tumors without viral DNA and proteins, (4) preneoplastic aneuploidies and immortal neoplastic clones with individual karyotypes.

RESULTS

As an alternative theory we propose that viral carcinogenesis is a form of speciation, initiated by virus-induced aneuploidy. Since aneuploidy destabilizes the karyotype by unbalancing thousands of genes it catalyzes chain reactions of karyotypic and transcriptomic evolutions. Eventually rare karyotypes evolve that encode cancer-specific autonomy of growth. The low probability of forming new autonomous cancer-species by random karyotypic and transcriptomic variations predicts individual and clonal cancers. Although cancer karyotypes are congenitally aneuploid and thus variable, they are stabilized or immortalized by selections for variants with cancer-specific autonomy. Owing to these inherent variations cancer karyotypes are heterogeneous within clonal margins. To test this theory we analyzed karyotypes and phenotypes of SV40-infected human, rat and mouse cells developing into neoplastic clones. In all three systems we found (1) preneoplastic aneuploidies, (2) neoplastic clones with individual clonal but flexible karyotypes and phenotypes, which arose from less than one in 10,000 infected cells, survived over 200 generations, but were either T-antigen positive or negative, (3) spontaneous and drug-induced variations of neoplastic phenotypes correlating 1-to-1 with karyotypic variations.

CONCLUSIONS

Since all 14 virus-induced neoplastic clones tested contained individual clonal karyotypes and phenotypes, we conclude that these karyotypes have generated and since maintained these neoplastic clones. Thus SV40 causes cancer indirectly, like carcinogens, by inducing aneuploidy from which new cancer-specific karyotypes evolve automatically at low rates. This theory explains the (1) low probability of carcinogenesis per virus-infected cell, (2) the individuality and clonal flexibility of cancer karyotypes, (3) recurrence of neoplasias without viral T-antigens, and (4) the individual clonal karyotypes, transcriptomes and immortality of virus-induced neoplasias - all unexplained by current viral theories.

Immortality of cancers: a consequence of inherent karyotypic variations and selections for autonomy

Immortality is a common characteristic of cancers, but its origin and purpose are still unclear. Here we advance a karyotypic theory of immortality based on the theory that carcinogenesis is a form of speciation. Accordingly, cancers are generated from normal cells by random karyotypic rearrangements and selection for cancer-specific reproductive autonomy. Since such rearrangements unbalance long-established mitosis genes, cancer karyotypes vary spontaneously but are stabilized perpetually by clonal selections for autonomy. To test this theory we have analyzed neoplastic clones, presumably immortalized by transfection with overexpressed telomerase or with SV40 tumor virus, for the predicted clonal yet flexible karyotypes. The following results were obtained: (1) All immortal tumorigenic lines from cells transfected with overexpressed telomerase had clonal and flexible karyotypes; (2) Searching for the origin of such karyotypes, we found spontaneously increasing, random aneuploidy in human fibroblasts early after transfection with overexpressed telomerase; (3) Late after transfection, new immortal tumorigenic clones with new clonal and flexible karyotypes were found; (4) Testing immortality of one clone during 848 unselected generations showed the chromosome number was stable, but the copy numbers of 36% of chromosomes drifted ± 1; (5) Independent immortal tumorigenic clones with individual, flexible karyotypes arose after individual latencies; (6) Immortal tumorigenic clones with new flexible karyotypes also arose late from cells of a telomerase-deficient mouse rendered aneuploid by SV40 virus. Because immortality and tumorigenicity: (1) correlated exactly with individual clonal but flexible karyotypes; (2) originated simultaneously with such karyotypes; and (3) arose in the absence of telomerase, we conclude that clonal and flexible karyotypes generate the immortality of cancers.

Transgenic oncogenes induce oncogene-independent cancers with individual karyotypes and phenotypes

Cancers are clones of autonomous cells defined by individual karyotypes, much like species. Despite such karyotypic evidence for causality, three to six synergistic mutations, termed oncogenes, are generally thought to cause cancer. To test single oncogenes, they are artificially activated with heterologous promoters and spliced into the germ line of mice to initiate cancers with collaborating spontaneous oncogenes. Because such cancers are studied as models for the treatment of natural cancers with related oncogenes, the following must be answered: 1) which oncogenes collaborate with the transgenes in cancers; 2) how do single transgenic oncogenes induce diverse cancers and hyperplasias; 3) what maintains cancers that lose initiating transgenes; 4) why are cancers aneuploid, over- and underexpressing thousands of normal genes? Here we try to answer these questions with the theory that carcinogenesis is a form of speciation. We postulate that transgenic oncogenes initiate carcinogenesis by inducing aneuploidy. Aneuploidy destabilizes the karyotype by unbalancing teams of mitosis genes. This instability thus catalyzes the evolution of new cancer species with individual karyotypes. Depending on their degree of aneuploidy, these cancers then evolve new subspecies. To test this theory, we have analyzed the karyotypes and phenotypes of mammary carcinomas of mice with transgenic SV40 tumor virus- and hepatitis B virus-derived oncogenes. We found that (1) a given transgene induced diverse carcinomas with individual karyotypes and phenotypes; (2) these karyotypes coevolved with newly acquired phenotypes such as drug resistance; (3) 8 of 12 carcinomas were transgene negative. Having found one-to-one correlations between individual karyotypes and phenotypes and consistent coevolutions of karyotypes and phenotypes, we conclude that carcinogenesis is a form of speciation and that individual karyotypes maintain cancers as they maintain species. Because activated oncogenes destabilize karyotypes and are dispensable in cancers, we conclude that they function indirectly, like carcinogens. Such oncogenes would thus not be valid models for the treatment of cancers.

Cerium and yttrium oxide nanoparticles are neuroprotective

The responses of cells exposed to nanoparticles have been studied with regard to toxicity, but very little attention has been paid to the possibility that some types of particles can protect cells from various forms of lethal stress. It is shown here that nanoparticles composed of cerium oxide or yttrium oxide protect nerve cells from oxidative stress and that the neuroprotection is independent of particle size. The ceria and yttria nanoparticles act as direct antioxidants to limit the amount of reactive oxygen species required to kill the cells. It follows that this group of nanoparticles could be used to modulate oxidative stress in biological systems.

Specificity of resistance to oxidative stress

Two clonal nerve-like cell lines derived from HT22 and PC12 have been selected for resistance to glutamate toxicity and amyloid toxicity, respectively. In the following experiments it was asked if these cell lines show cross-resistance toward amyloid beta peptide (Abeta) and glutamate as well as toward a variety of additional neurotoxins. Conversely, it was determined if inhibitors of oxytosis, a well-defined oxidative stress pathway, also protect cells from the neurotoxins. It is shown that both glutamate and amyloid resistant cells are cross resistant to most of the other toxins or toxic conditions, while inhibitors of oxytosis protect from glutathione and cystine depletion and H2O2 toxicity, but not from the toxic effects of nitric oxide, rotenone, arsenite or cisplatin. It is concluded that while there is a great deal of cross-resistance to neurotoxins, the components of the cell death pathway which has been defined for oxytosis are not used by many of the neurotoxins.

Flavonoids protect neuronal cells from oxidative stress by three distinct mechanisms

Flavonoids are a family of antioxidants found in fruits and vegetables as well as in popular beverages such as red wine and tea. Although the physiological benefits of flavonoids have been largely attributed to their antioxidant properties in plasma, flavonoids may also protect cells from various insults. Nerve cell death from oxidative stress has been implicated in a variety of pathologies, including stroke, trauma, and diseases such as Alzheimer's and Parkinson's. To determine the potential protective mechanisms of flavonoids in cell death, the mouse hippocampal cell line HT-22, a model system for oxidative stress, was used. In this system, exogenous glutamate inhibits cystine uptake and depletes intracellular glutathione (GSH), leading to the accumulation of reactive oxygen species (ROS) and an increase in Ca(2+) influx, which ultimately causes neuronal death. Many, but not all, flavonoids protect HT-22 cells and rat primary neurons from glutamate toxicity as well as from five other oxidative injuries. Three structural requirements of flavonoids for protection from glutamate are the hydroxylated C3, an unsaturated C ring, and hydrophobicity. We also found three distinct mechanisms of protection. These include increasing intracellular GSH, directly lowering levels of ROS, and preventing the influx of Ca(2+) despite high levels of ROS. These data show that the mechanism of protection from oxidative insults by flavonoids is highly specific for each compound.

Propofol hemisuccinate protects neuronal cells from oxidative injury

Oxidative stress contributes to the neuronal death observed in neurodegenerative disorders and neurotrauma. Some antioxidants for CNS injuries, however, have yet to show mitigating effects in clinical trials, possibly due to the impermeability of antioxidants across the blood-brain barrier (BBB). Propofol (2,6-diisopropylphenol), the active ingredient of a commonly used anesthetic, acts as an antioxidant, but it is insoluble in water. Therefore, we synthesized its water-soluble prodrug, propofol hemisuccinate sodium salt (PHS), and tested for its protective efficacy in neuronal death caused by non-receptor-mediated, oxidative glutamate toxicity. Glutamate induces apoptotic death in rat cortical neurons and the mouse hippocampal cell line HT-22 by blocking cystine uptake and causing the depletion of intracellular glutathione, resulting in the accumulation of reactive oxygen species (ROS). PHS has minimal toxicity and protects both cortical neurons and HT-22 cells from glutamate. The mechanism of protection is attributable to the antioxidative property of PHS because PHS decreases the ROS accumulation caused by glutamate. Furthermore, PHS protects HT-22 cells from oxidative injury induced by homocysteic acid, buthionine sulfoximine, and hydrogen peroxide. For comparison, we also tested alpha-tocopherol succinate (TS) and methylprednisolone succinate (MPS) in the glutamate assay. Although TS is protective against glutamate at lower concentrations than PHS, TS is toxic to HT-22 cells. In contrast, MPS is nontoxic but also nonprotective against glutamate. Taken together, PHS, a water-soluble prodrug of propofol, is a candidate drug to treat CNS injuries owing to its antioxidative properties, low toxicity, and permeability across the BBB.

The activation of metabotropic glutamate receptors protects nerve cells from oxidative stress

Metabotropic glutamate receptors (mGluRs) have been implicated in a variety of cellular responses to glutamic acid. The work described in this manuscript extends the role of mGluRs to include protection from oxidative stress-induced programmed cell death. Glutamate analogs regulate inositol-1,4,5 triphosphate mass accumulation in accordance with their ability to protect cells from oxidative glutamate toxicity, and protection appears to take place at the level of glutathione metabolism. Short-term exposure of cells to low concentrations of glutamate desensitizes cells to a subsequent challenge from glutamate. Glutamate exposure upregulates the expression of mGluR5 in hippocampal HT-22 cells and mGluR1 in cortical primary cultures. Finally, group I mGluR agonists also protect cells from death programs initiated by glucose starvation and cystine deprivation.

The activation of metabotropic glutamate receptors protects nerve cells from oxidative stress

Metabotropic glutamate receptors (mGluRs) have been implicated in a variety of cellular responses to glutamic acid. The work described in this manuscript extends the role of mGluRs to include protection from oxidative stress-induced programmed cell death. Glutamate analogs regulate inositol-1,4,5 triphosphate mass accumulation in accordance with their ability to protect cells from oxidative glutamate toxicity, and protection appears to take place at the level of glutathione metabolism. Short-term exposure of cells to low concentrations of glutamate desensitizes cells to a subsequent challenge from glutamate. Glutamate exposure upregulates the expression of mGluR5 in hippocampal HT-22 cells and mGluR1 in cortical primary cultures. Finally, group I mGluR agonists also protect cells from death programs initiated by glucose starvation and cystine deprivation.

Cellular mechanisms of resistance to chronic oxidative stress

Oxidative stress is implicated in several pathologies such as AIDS, Alzheimer's disease, and Parkinson's disease, as well as in normal aging. As a model system to study the response of cells to oxidative insults, glutamate toxicity on a mouse nerve cell line, HT-22, was examined. Glutamate exposure kills HT-22 via a nonreceptor-mediated oxidative pathway by blocking cystine uptake and causing depletion of intracellular glutathione (GSH), leading to the accumulation of reactive oxygen species and, ultimately, apoptotic cell death. Several HT-22 subclones that are 10-fold resistant to exogenous glutamate were isolated and the mechanisms involved in resistance characterized. The expression levels of neither heat shock proteins nor apoptosis-related proteins are changed in the resistant cells. In contrast, the antioxidant enzyme catalase, but not glutathione peroxidase nor superoxide dismutase, is more highly expressed in the resistant than in the parental cells. In addition, the resistant cells have enhanced rates of GSH regeneration due to higher activities of the GSH metabolic enzymes gamma-glutamylcysteine synthetase and GSH reductase, and GSH S-transferases activities are also elevated. As a consequence of these alterations, the glutamate resistant cells are also more resistant to organic hydroperoxides and anticancer drugs that affect these GSH enzymes. These results indicate that resistance to apoptotic oxidative stress may be acquired by coordinated changes in multiple antioxidant pathways.

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.

Requirement for cGMP in nerve cell death caused by glutathione depletion

Glutathione depletion occurs in several forms of apoptosis and is associated with Parkinson's disease and HIV toxicity. The neurotransmitter glutamate kills immature cortical neurons and a hippocampal nerve cell line via an oxidative pathway associated with glutathione depletion. It is shown here that soluble guanylyl cyclase (sGC) activity is required for nerve cell death caused by glutathione depletion. Inhibitors of sGC block glutamate toxicity and a cGMP analogue potentiates cell death. Glutamate also induces an elevation of cGMP that occurs late in the cell death pathway. The resultant cGMP modulates the increase in intracellular calcium that precedes cell death because sGC inhibitors prevent calcium elevation and the cGMP analogue potentiates the increase in intracellular calcium. These results suggest that the final pathway of glutamate induced nerve cell death is through a cGMP-modulated calcium channel.

A role for 12-lipoxygenase in nerve cell death caused by glutathione depletion

An early and highly specific decrease in glutathione (GSH) in the substantia nigra is associated with Parkinson's disease, and low levels of GSH lead to the degeneration of cultured dopaminergic neurons. Using immature cortical neurons and a clonal nerve cell line, it is shown that a decrease in GSH triggers the activation of neuronal 12-lipoxygenase (12-LOX), which leads to the production of peroxides, the influx of Ca2+, and ultimately to cell death. The supporting evidence includes: 1) inhibitors of arachidonate metabolism and 12-LOX block cell death induced by GSH depletion; 2) there is an increase in 12-LOX activity and a membrane translocation in HT22 cells, and an induction of the enzyme in primary cortical neurons following the reduction of GSH; 3) 12-LOX is directly inhibited by GSH; and 4) exogenous arachidonic acid potentiates cell death. These data show that the LOX pathway is a critical intermediate in at least some forms of neuronal degeneration.

Review Lecture - Configurational and biological properties of polyoma virus DNA

The experimental study of viral carcinogenesis has made considerable advances in the last few years. On the basis of recently discovered facts, many aspects of this interesting process can now be understood. I intend to discuss some of the newly acquired facts, in part discovered in our laboratory, in relation to the mechanisms of viral carcinogenesis. There are two types of cancer-producing viruses: those containing DNA and those containing RNA . We shall consider here only those that contain DNA , of which there are three main groups: (1) polyoma virus and simian virus 40 (SV 40); (2) papilloma viruses of rabbit, man, bovine and dog; (3) adenoviruses, of the types 7, 12, and 18.

Growth factors and vitamin E modify neuronal glutamate toxicity

The sympathetic nerve cell line PC-12 is killed by glutamate in a concentration-dependent manner. Although glycine and the deletion of magnesium weakly potentiate glutamate toxicity and PC-12 cells express N-methyl-D-aspartate-receptor mRNA, most toxicity is mediated by means of a mechanism independent of typical N-methyl-D-aspartate receptors. Glutamate toxicity is, however, greatly enhanced by prior exposure to nerve growth factor or basic fibroblast growth factor. Glutamate killing is blocked by epidermal growth factor and, to a lesser extent, by vitamin E. These observations show that synergistic interactions between growth factors and excitotoxic amino acids may play critical roles in the developing nervous system and that antioxidants attenuate this toxicity.

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.

Induction of neurite outgrowth by v-src mimics critical aspects of nerve growth factor-induced differentiation

PC12 cells treated with nerve growth factor (NGF) or infected with Rous sarcoma virus differentiate into sympathetic, neuronlike cells. To compare the differentiation programs induced by NGF and v-src, we have established a PC12 cell line expressing a temperature-sensitive v-src protein. The v-src-expressing PC12 cell line was shown to elaborate neuritic processes in a temperature-inducible manner, indicating that the differentiation process was dependent on the activity of the v-src protein. Further characterization of this cell line, in comparison with NGF-treated PC12 cells, indicated that the events associated with neurite outgrowth induced by these two agents shared features but could be distinguished by others. Both NGF- and v-src-induced neurite outgrowths were reversible. In addition, NGF and v-src could prime PC12 cells for NGF-induced neurite outgrowth, and representative early and late NGF-responsive genes were also induced by v-src. However, unlike NGF-induced neurite growth, v-src-induced neurite outgrowth was not blocked at high cell density. A comparison of phosphotyrosine containing-protein profiles showed that v-src and NGF each increase tyrosine phosphorylation of multiple cellular proteins. There was overlap in substrates; however, both NGF-specific and v-src-specific tyrosine phosphorylations were observed. One protein which was found to be phosphorylated in both the NGF- and v-src-induced PC12 cells was phospholipase C-gamma 1. Taken together, these results suggest that v-src's ability to function as an inducing agent may be a consequence of its ability to mimic critical aspects of the NGF differentiation program and raise the possibility that Src-like tyrosine kinases are involved in mediating some of the events triggered by NGF.

Induction of neurite outgrowth by v-src mimics critical aspects of nerve growth factor-induced differentiation

PC12 cells treated with nerve growth factor (NGF) or infected with Rous sarcoma virus differentiate into sympathetic, neuronlike cells. To compare the differentiation programs induced by NGF and v-src, we have established a PC12 cell line expressing a temperature-sensitive v-src protein. The v-src-expressing PC12 cell line was shown to elaborate neuritic processes in a temperature-inducible manner, indicating that the differentiation process was dependent on the activity of the v-src protein. Further characterization of this cell line, in comparison with NGF-treated PC12 cells, indicated that the events associated with neurite outgrowth induced by these two agents shared features but could be distinguished by others. Both NGF- and v-src-induced neurite outgrowths were reversible. In addition, NGF and v-src could prime PC12 cells for NGF-induced neurite outgrowth, and representative early and late NGF-responsive genes were also induced by v-src. However, unlike NGF-induced neurite growth, v-src-induced neurite outgrowth was not blocked at high cell density. A comparison of phosphotyrosine containing-protein profiles showed that v-src and NGF each increase tyrosine phosphorylation of multiple cellular proteins. There was overlap in substrates; however, both NGF-specific and v-src-specific tyrosine phosphorylations were observed. One protein which was found to be phosphorylated in both the NGF- and v-src-induced PC12 cells was phospholipase C-gamma 1. Taken together, these results suggest that v-src's ability to function as an inducing agent may be a consequence of its ability to mimic critical aspects of the NGF differentiation program and raise the possibility that Src-like tyrosine kinases are involved in mediating some of the events triggered by NGF.

Evaluation of the mutagenic effects of SV40 in mouse, hamster, and mouse-human hybrid cells

We have examined the ability of SV40 to induce changes in drug or temperature resistance in mouse, hamster, and mouse-human hybrid cells. SV40 induced a substantial increase of cells resistant to 5-bromodeoxyuridine + trifluorothymidine in Balb/c 3T3 cells and induced an increase of hybrid cells resistant to 6-thioguanine. SV40 was found to be nonmutagenic or weakly mutagenic in other test systems. The 3T3 cells were T-antigen positive, exhibited a marked reduction in TK activity, were heterogeneous for [3H]BrdU incorporation by autoradiography, and exhibited instability of the drug-resistance phenotype, suggesting that SV40 may be inducing resistance by an epigenetic process. SV40-induced 6-thioguanine resistance in the hybrids appears to occur predominantly by chromosome loss.

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.

Nerve growth factor action is mediated by cyclic AMP- and Ca+2/phospholipid-dependent protein kinases

Nerve growth factor (NGF) mediates the phosphorylation of tyrosine hydroxylase in PC12 cells on two distinct peptide fragments, separable by two-dimensional tryptic phosphopeptide mapping (phosphopeptides T1 and T3). Phorbol diester derivatives capable of activating Ca+2/phospholipid-dependent protein kinase (C-kinase) cause a specific phosphorylation of peptide T3 in a dose-dependent, saturable manner. Derivatives of the endogenous C-kinase activator diacylglycerol, also cause the phosphorylation of tyrosine hydroxylase on peptide T3. The C-kinase inhibitors chlorpromazine and trifluoperazine inhibit the phorbol diester stimulated phosphorylation of site T3 in a dose-dependent manner. These agents inhibit the phosphorylation of T3 in response to NGF, but have no effect on NGF's ability to cause T1 phosphorylation. In a PC12 mutant deficient in cAMP-dependent protein kinase activity, NGF mediates the phosphorylation of tyrosine hydroxylase on peptide T3 but not on T1. We conclude that NGF mediates the activation of both the cAMP-dependent protein kinase and the C-kinase to phosphorylate substrate proteins. These kinases can act independently to phosphorylate tyrosine hydroxylase, each at a different site, and each of which results in the enzyme activation. A molecular framework is thus provided for events underlying NGF action.

Actin filaments and tumorigenicity in a Fischer rat embryo fibroblast cell line (3Y1) transformed by ultraviolet-irradiated HSV

The characteristics of the cytoskeleton of a Fischer rat embryo fibroblast cell line (3Y1) transformed by ultraviolet (UV)-irradiated HSV were studied by indirect immunofluorescence using anti-actin IgG. Parental 3Y1 cells possessed well-developed actin filaments, while 3Y1 cells transformed by UV-irradiated HSV also retained well-developed actin filaments. Transformed cells were divided into 2 groups according to tumorigenicity in newborn Fischer rats; one had a strongly tumorigenic potential and the other a weakly tumorigenic potential. Tumor-derived cell lines exhibited a highly tumorigenic potential, and were also divided into 2 groups, one with well-developed actin filaments and the other without well-developed actin filaments. Our results suggested that transformation or tumor formation by HSV is a multi-step process and that morphological loss of actin filaments in the cells is not essential to the tumorigenic potential of the cells transformed by HSV.

Use of [3H]triphenylmethylphosphonium cation for estimating membrane potential in neuroblastoma cells

The lipophilic permeant cation [3H]triphenylmethylphosphonium (TPMP) was used to estimate membrane potential in neuroblastoma N1E 115 cells under carefully controlled conditions. The cation distributes into the cells only in the presence of a lipophilic anion, and tetraphenylboron and picrate have been used for this purpose. The potassium salt of tetraphenylboron is poorly soluble, so that studies in high [K+] media are difficult with this anion whereas picrate, at the concentrations required, hyperpolarises the cells. The effect of muscarinic receptor activation was investigated by treating cells with carbachol but no effect was seen either on [3H] TPMP distribution or electrophysiological parameters. The use of [3H]TPMP for qualitative and quantitative evaluation of membrane potential in these cells is discussed.

A 61,000-dalton truncated large T-antigen is uniformly expressed in hamster cells transformed by polyomavirus

Various polyomavirus-transformed hamster cell lines derived from tumors or from infected hamster cell cultures synthesized polyoma middle and small tumor (T)-antigens but no full-size large T-antigen. Instead, all cell lines produced the same or similar polyoma T-antigen-related proteins of ca. 61 kilodaltons (kDal). Like large T-antigen synthesized in lytically infected mouse cells, the 61-kDal proteins were phosphoproteins showing electrophoretic and charge heterogeneities. Chromatographic analysis of the methionine-containing tryptic peptides indicated that the 61-kDal proteins were truncated forms of large T-antigen comprising amino acid residues 1 to 485 (+/- 25). Analysis of viral DNA present in hamster chromosomal DNA of three independently isolated cell lines confirmed that synthesis of the 61-kDal proteins was due to a discontinuity in the large T-antigen coding sequence, most likely located between 7 and 8.9 map units on the polyoma DNA map. The three cell lines yielded essentially the same patterns of viral DNA-containing restriction enzyme fragments, suggesting that insertion of viral DNA into the hamster chromosomes took place at closely similar sites.

Membrane antigens associated with infection, transformation, and tumorigenesis by polyoma virus

Infection, transformation, or tumorigenesis by Py virus leads inter alia to modifications in the membrane antigens of the affected cells. The modifications include antigenic gains or losses or quantitative changes in both directions. Although there is a pronounced common denominator in the antigenic alterations in the three distinct Py-induced biological processes, it is nontheless possible that each of them is characterized by specific antigenic modifications. This possibility has yet to be analyzed. Specific antigenic modifications, if they occur, are probably the result of different selective processes and adaptions to these pressures.

In this brief review, we have attempted to survey the literature pertinent to this aspect. While doing so, we discovered that most researchers have not considered the possibility that differences could exist between antigens of cells infected by Py, cells transformed by this virus, and Py-induced tumor cells. We feel that a comprehensive antigenic comparison between these cells utilizing well-defined reagents is an essential prerequisite to understanding of the successful immunological surveillance against Py-induced malignancy.

Tumorigenic conversion of primary embryo fibroblasts requires at least two cooperating oncogenes

Transfection of embryo fibroblasts by a human ras oncogene does not convert them into tumour cells unless the fibroblasts are established and immortalized before transfection. The embryo fibroblasts become tumorigenic if a second oncogene such as a viral or cellular myc gene or the gene for the polyoma large-T antigen is introduced together with the ras gene.

Expression of the large T protein of polyoma virus promotes the establishment in culture of "normal" rodent fibroblast cell lines

Transfer into mouse and rat embryo fibroblasts in primary culture of cloned polyoma virus genes encoding only the large T protein led to the establishment of flat colonies in sparse subcultures at a frequency equal to that of transformation by wild-type virus. Cell lines could be derived from such colonies and maintained in culture for large numbers of generations without entering crisis. They exhibited a normal phenotype, by the criteria of growth on plastic to a low saturation density and of anchorage dependency. However, they required a lower serum concentration for growth than spontaneously established 3T3 cells. Similar results were obtained after transfer of recombinant DNA molecules encoding only the amino-terminal 40% of the large T protein, suggesting that this "immortalization" function corresponds to the activity of an amino-terminal domain of the protein. Immunoprecipitation analysis of T antigens in cell lines established after transfer of the full-size and of the truncated large T genes demonstrated the expression of the full-size large T protein and of a Mr 40,000 antigen expressed from the amino-terminal part of the gene, respectively. After transfer of a "large T only" plasmid that carries a tsa mutation, cell lines were established at 33 degrees C with the same efficiency as with the wild-type large T gene, but their growth was arrested after a shift to 40 degrees C, with a progressive loss in cell viability. This result indicates a continuous requirement for a large T function in the maintenance of "immortality."

Denervation increases the degradation rate of acetylcholine receptors at end-plates in vivo and in vitro

We have studied the effect of denervation on the degradation of the existing junctional acetylcholine (ACh) receptors at end-plates in rat muscles. ACh receptors were labelled by injecting animals with iodinated alpha-bungarotoxin (I-alpha BT); 1 day later the left hemidiaphragm was denervated. The degradation of bound I-alpha BT in normal and denervated muscles was examined in organ culture, beginning at various times after denervation in vivo. The original, pre-labelled end-plate ACh receptors are degraded more rapidly after denervation. The rate of degradation begins to increase shortly after the nerve is cut and reaches a maximum value at about 9 days of denervation. Muscles denervated only on transfer to organ culture also show an increase in the degradation rate of bound I-alpha BT with increasing time of denervation (time in culture). In normal diaphragm muscles, the initial rate of degradation of functional ACh receptors, after correcting for non-degradative loss of I-alpha BT, is 0.0018 h-1 (t1/2 = 383 h). The maximal rate at denervated end-plates is 0.0073 h-1 (t1/2 = 94 h). For soleus, sternomastoid, plantaris and intercostal innervated muscles the apparent rate of ACh receptor degradation either in vitro or in vivo ranged from 0.0005 h-1 to 0.002 h-1. The rate of loss of bound I-alpha BT in vivo is more rapid at denervated end-plates than at innervated end-plates. For diaphragm muscles, the rates of I-alpha BT degradation measured in organ culture are able to describe the relative rates of loss of I-alpha BT from innervated and denervated muscles in vivo. At short times after labelling, a fraction (10-20%) of the I-alpha BT bound to innervated muscles is degraded more rapidly than the remaining toxin. The possibility that these I-alpha BT binding sites are degraded at the rate characteristic of extrajunctional receptors on denervated muscle fibres is discussed.

The roles of individual polyoma virus early proteins in oncogenic transformation

The expression in normal rat cells of modified polyoma virus genomes, separately encoding large T, middle T or small T antigens, has allowed the investigation of the roles of these proteins in oncogenic transformation. Middle T is sufficient to transform cells of established lines but the transformants are serum dependent. Large T lacks intrinsic oncogenic potential but can relieve the serum dependence of normal and transformed cells. Middle T alone cannot transform primary rat embryo fibroblasts.

Factors which disorganize microtubules or microfilaments increase the frequency of cell transformation by polyoma virus

Griseofulvin, 12-O-tetradecanoyl phorbol-13-acetate, melittin, epidermal growth factor, vinblastine, cytochalasin B, podophyllotoxin, colcemid, and colchicine were unable to transform cells but could increase from 8- to 40-fold the frequency of cell transformation by polyoma virus. The 3T3-like cells were resting at confluence and were exposed to the drug only during the 1st week after viral infection. Griseofulvin, a tumor promoter, reduced or increased the frequency of transformation depending on the dose with which the infected cells were treated. The antitumor activity of tumor promoters is discussed.

Transformation phenotype of polyoma virus-transformed rat fibroblasts: plasminogen activator production is modulated by the growth state of the cells and regulated by the expression of an early viral gene function

The expression of two transformation parameters, namely, ability to grow in agar and plasminogen activator production, was studied in several rat fibroblasts transformed by either wild-type or thermo-sensitive (tsa and ts25) polyoma viruses. The production of plasminogen activator was found to be dependent upon the growth state of the infected cells during a period of several days after infection. The analysis of the transformed phenotype of 25 tsa transformants and of 19 ts25 transformants independently isolated under various growth conditions led to the conclusion that there is no correlation between the regulation processes involved in plasminogen activator production and ability to grow without anchorage. The results obtained also suggested that the production of plasminogen activator is under the control of a functional large T antigen.