Revertants of an ASV-transformed rat cell line have lost the complete provius or sustained mutations in src

Inducing stable reversion to achieve cancer control

How can we stop cancer progression? Current strategies depend on modelling progression as the balanced outcome of mutations in, and expression of, tumour suppressor genes and oncogenes. New treatments emerge from successful attempts to tip that balance, but secondary mutational escape from those treatments has become a major impediment because it leads to resistance. In this Opinion article, we argue for a return to an earlier stratagem: tumour cell reversion. Treatments based on selection and analysis of stable revertants could create more durable remissions by reducing the selective pressure that leads to rapid drug resistance.

The RNA-binding protein Musashi1 affects medulloblastoma growth via a network of cancer-related genes and is an indicator of poor prognosis

Musashi1 (Msi1) is a highly conserved RNA-binding protein that is required during the development of the nervous system. Msi1 has been characterized as a stem cell marker, controlling the balance between self-renewal and differentiation, and has also been implicated in tumorigenesis, being highly expressed in multiple tumor types. We analyzed Msi1 expression in a large cohort of medulloblastoma samples and found that Msi1 is highly expressed in tumor tissue compared with normal cerebellum. Notably, high Msi1 expression levels proved to be a sign of poor prognosis. Msi1 expression was determined to be particularly high in molecular subgroups 3 and 4 of medulloblastoma. We determined that Msi1 is required for tumorigenesis because inhibition of Msi1 expression by small-interfering RNAs reduced the growth of Daoy medulloblastoma cells in xenografts. To characterize the participation of Msi1 in medulloblastoma, we conducted different high-throughput analyses. Ribonucleoprotein immunoprecipitation followed by microarray analysis (RIP-chip) was used to identify mRNA species preferentially associated with Msi1 protein in Daoy cells. We also used cluster analysis to identify genes with similar or opposite expression patterns to Msi1 in our medulloblastoma cohort. A network study identified RAC1, CTGF, SDCBP, SRC, PRL, and SHC1 as major nodes of an Msi1-associated network. Our results suggest that Msi1 functions as a regulator of multiple processes in medulloblastoma formation and could become an important therapeutic target.

Src kinase and pancreatic cancer

The c-Src non-receptor tyrosine kinase is overexpressed in a large number of human malignancies. It is linked to tumour development and progression to distant metastases by promoting cell proliferation, invasion, and motility. Recently, promising anticancer therapeutics targeting c-Src have been developed that are under clinical investigation.

Transgenic mice and host cell mutants resistant to transformation as model systems for identifying multiple components in oncogenesis

Tumorigenesis appears to be a multistep process involving mutations of conventional, dominantly acting proto-oncogenes, mutations of other genes that may act in a recessive manner, and interactions (or a lack of interactions) between the products of mutant and wild-type genes. Our laboratory is using a few well-established, dominant oncogenes to pose experimental questions that could lead to a better understanding of the more elusive genetic interactions which occur during tumour development. Two such situations are described: (1) We have created a line of transgenic mice that carry the int-1 proto-oncogene under the control of the enhancer element in the mouse mammary tumour virus long terminal repeat. Such mice express the transgene in mammary glands, salivary glands and male reproductive tract; mammary glands from both male and female animals are grossly hyperplastic, yet tumours arise rarely in the males and sporadically in the females (80% of female mice have one or a few tumours by six months of age). Thus expression of int-1 in these mice appears to place a large number of mammary cells at risk for secondary events that lead to carcinogenesis, providing a provocative experimental context for identifying such secondary events. (2) We have isolated a rat cell line that lacks most of the characteristics of transformed cells, despite the expression of two wild-type copies of the v-src gene of Rous sarcoma virus. This line harbours what appears to be a dominant mutation in an unidentified gene that renders the cell resistant to transformation by v-src and several other oncogenes. Isolation of the mutant gene responsible for suppressing transformation in this line should provide new insights into the interactions between oncoproteins and other cellular proteins.

Proviral position effects: possible probes for genes that suppress transcription

Rous sarcoma virus, an oncogenic avian retrovirus, readily causes morphological transformation of chick cells, but in infected rat cells transformation is rare because proviral transcription is inefficient. This constraint is not due to a lack of positive transcriptional factors, or an excess of negative ones, but reflects the site of proviral integration in rat cell DNA. In most sites the provirus is almost invariably silent, in others it is correspondingly active, whilst in a third category expression fluctuates in concert with transitions in chromatin structure. Transcriptional fluctuations are mediated both by flanking cell DNA in cis and by trans-acting cell genes, suggesting that proviral position effects are sensors for genes that down-regulate transcription, perhaps by determining chromatin configuration. We have tried to identify such genes by gene transfer, karyology and insertional mutagenesis. The variable success of these three approaches indicates that the transcriptional down-regulator(s) need act only transiently. This is consistent with a function that operates in ontogeny or differentiation to down-regulate genes whose silence is then perpetuated by other means. The loss of such functions may predispose to neoplasia.

Pyrazolo[3,4-d]pyrimidines c-Src inhibitors reduce epidermal growth factor-induced migration in prostate cancer cells

During its biological progression, prostate cancer frequently develops dependence on growth factor receptors and their downstream signalling messengers, including c-Src. Evidence for this supports the choice of c-Src as a therapeutic target in the prevention of tumour spreading. Two new pyrazolo[3,4-d]pyrimidines c-Src inhibitors, SI35 and SI40, were used to investigate the role of c-Src in the control of the aggressive phenotype of prostate carcinoma cell line, PC3. SI molecules reduced the proliferation of PC3 cells in a time- and dose-dependent manner, with an IC50 of approximately 50 microM. PC3 cells responded to the presence of epidermal growth factor (EGF) by increasing their migratory ability, and this effect was strongly reduced by the addition of SI at concentrations less than IC50. Further observations demonstrated that SI molecules modulated cell morphology and their adhesive capacity on different physiological substrates. The action of SI molecules appeared to involve, in parallel with c-Src inhibition, the down-modulation of the active forms of paxillin and extracellular signal-regulated kinase (ERK). Our data suggest a promising role for pyrazolo[3,4-d]pyrimidines c-Src inhibitors in the control of a highly invasive tumour phenotype.

Role of infectious diseases in human carcinogenesis

The burden of human infectious diseases remains a public health problem worldwide. At least 2 billion people are affected by viral infections, and a similar number by bacteria or helminths. The long-term effects of these maladies have raised particular concern since some infectious agents have been associated with chronic human diseases, especially cancer. It is estimated that 13-20% of the world cancer cases are associated with some virus, bacteria, or helminth, e.g., human papillomavirus, Helicobacter pylori, and Schistosoma haematobium that cause cervical, stomach, and urinary bladder cancer, respectively. Certain associations between infection and malignancy are strong and irrefutable; others are still speculative. This article reviews the infectious agents that have been associated with cancer and current knowledge about the mechanisms underlying these associations.

The road to Src

More than a quarter of a century has elapsed since the identification of the c-src proto-oncogene. During that period, we have learned that cancer arises as the result of mutations in proto-oncogenes and tumor suppressor genes, and we are now seeing the first fruits of these discoveries, in the form of targeted therapies directed against activated tyrosine kinases such as Bcr-Abl, c-Kit and the EGF receptor. But the discovery of the c-src proto-oncogene was in turn based on decades of study on an avian RNA tumor virus, Rous sarcoma virus (RSV). Here I review the work that led up to the identification of the RSV transforming gene and its protein product, and how this information in turn led to the discovery of cellular Src.

The hunting of the Src

The non-receptor tyrosine kinase Src is important for many aspects of cell physiology. The viral src gene was the first retroviral oncogene to be identified, and its cellular counterpart was the first proto-oncogene to be discovered in the vertebrate genome. Src has been important, not only as an object of study in itself, but also as an entry point into the molecular genetics of cancer.

Bcr: a negative regulator of the Bcr-Abl oncoprotein

Chronic myelogenous leukemia is typically characterized by the presence of the Philadelphia chromosome (Ph) in which 5' portions of the BCR gene are fused to a large portion of the ABL gene. Our studies and those of others indicate that Bcr sequences within the Bcr-Abl oncoprotein are critically involved in activating the Abl tyrosine kinase and actively participate in the oncogenic response, which is generated by the Bcr-Abl oncoprotein. We investigated the role of the Bcr protein in the oncogenic effects of Bcr-Abl. Reduction of the level of the Bcr protein by incubating cells with a 3' BCR anti-sense oligodeoxynucleotide increased the growth rate and survival of hematopoietic cell lines expressing Bcr-Abl. Also, enforced expression of Bcr in Bcr-Abl cell lines strongly reduced transformation efficiency. Induction of Bcr expression drastically reduced the phosphotyrosine content of Bcr-Abl in Rat-1 fibroblasts transformed by P185 BCR-ABL and in hematopoietic cells expressing P210 Bcr-Abl within days following induction of Bcr. Rat-1/P185 cells maintained for three weeks after Bcr induction had dramatically reduced amounts of phosphotyrosine proteins compared to cells in which Bcr expression was repressed by the addition of Tet. In contrast Bcr expression did not decrease the phosphotyrosine content of either v-Src or activated Neu tyrosine kinase. Importantly, the phosphotyrosine content of total P160 BCR (induced plus endogenous) was strongly reduced by inducing expression of Bcr, indicating that the induced Bcr protein was not a target of the tyrosine kinase activity of Bcr-Abl but instead functioned as an inhibitor of Bcr-Abl. These results show that the Bcr protein can function as a negative regulator of Bcr-Abl, but that the inhibitory effects of Bcr are dependent on achieving an elevated level of Bcr expression relative to Bcr-Abl.

Expression, purification, and regulation of two isoforms of the inositol 1,4,5-trisphosphate 3-kinase

The level of inositol 1,4,5-trisphosphate in the cytoplasm is tightly regulated by two enzymes, the inositol 1,4,5,5-phosphatase and the inositol 1,4,5-trisphosphate 3-kinase. Two isoforms of the inositol 1,4,5-trisphosphate 3-kinase have been identified, the A form and the B form. The regulatory properties of the two isoforms were compared following overexpression and purification of the proteins from a v-src transformed mammalian cell line. The highly purified, recombinant inositol 1,4,5-trisphosphate 3-kinases were differentially regulated by calcium/calmodulin and via phosphorylation by protein kinase C or the cyclic AMP-dependent protein kinase. Both enzymes had similar affinities for inositol 1,4, 5-trisphosphate (Km 2-5 mu M). Calcium/calmodulin stimulated the activity of isoform A about 2.5-fold, whereas the activity of isoform B was increased 20-fold. The cyclic AMP-dependent protein kinase phosphorylated the inositol 1,4,5-trisphosphate 3-kinase A to the extent of 0.9 mol/mol and isoform B to 1 mol/mol. Protein kinase C phosphorylated isoform A to the extent of 2 mol/mol and isoform B to 2.7 mol/mol. Phosphorylation of isoform A by the cyclic AMP-dependent protein kinase caused a 2.5-fold increase in its activity when assayed in the absence of calcium/calmodulin, whereas phosphorylation by protein kinase C decreased activity by 72%. The activity of isoform B in the absence of calcium/calmodulin was not affected by phosphorylation using either kinase. When assayed in the presence of calcium/calmodulin, phosphorylation of isoform A by the cyclic AMP-dependent protein kinase increased activity 1.5-fold, whereas phosphorylation of isoform B decreased activity by 45%. Phosphorylation of either isoform A or B by protein kinase C resulted in a 70% reduction of calcium/calmodulin-stimulated activity. Differential expression and regulation of the two inositol 1,4,5-trisphosphate 3-kinase isoforms provides multiple mechanisms for regulating the cytosolic level of inositol 1,4,5-trisphosphate in cells.

Transformation of Rat-1 fibroblasts with the v-src oncogene induces inositol 1,4,5-trisphosphate 3-kinase expression

Transformation of Rat-1 fibroblasts with the v-src oncogene leads to a 6- to 8-fold enhancement of the activity of the Ins(1,4,5)P3 3-kinase in cytosolic extracts [Johnson, Wasilenko, Mattingly, Weber and Garrison (1989) Science 246, 121-124]. This study confirms these results using another v-src-transformed Rat-1 cell line (B31 cells) and investigates the molecular mechanism by which pp60v-src activates Ins(1,4,5)P3 3-kinase. The mRNA and protein levels for two rat isoforms of Ins(1,4,5)P3 3-kinase were determined in the v-src-transformed cell line. Both the mRNA and protein levels for isoform A were elevated in v-src-transformed Rat-1 cells while those for isoform B were not significantly affected. Moreover, stable expression of either form of Ins(1,4,5)P3 3-kinase in the B31 v-src-transformed Rat-1 cell line did not result in tyrosine phosphorylation of Ins(1,4,5)P3 3-kinase A or B. These results suggest that at least one mechanism by which the v-src oncogene increases the activity of the Ins(1,4,5)P3 3-kinase in the Rat-1 transformed fibroblast is by increasing the level of expression of Ins(1,4,5)P3 3-kinase A.

Transformation and pp60v-src autophosphorylation correlate with SHC-GRB2 complex formation in rat and chicken cells expressing host-range and kinase-active, transformation-defective alleles of v-src

The biochemical properties of several pp60v-src substrates believed to participate in src-mediated transformation were examined in cells expressing a kinase-active, transformation-defective v-src allele (v-src-F172 delta/Y416F) and its parental allele, v-src-F172 delta, a host-range--dependent allele that transforms chicken cells to a fusiform morphology, but does not transform rat cells. Because pp60v-src-F172 delta is dependent on autophosphorylation for transforming ability, these alleles provide a unique opportunity to examine the role of pp60v-src autophosphorylation in regulating substrate interactions. Increased pp125FAK tyrosine phosphorylation and high levels of pp60v-src-associated phosphotidylinositol-3' kinase activity were detected specifically in chicken cells exhibiting round, refractile transformation but not in cells transformed to a fusiform morphology. Increased pp125FAK kinase activity, but not increased pp125FAK tyrosine-phosphorylation correlated with pp60v-src autophosphorylation and increased anchorage-independent growth. Thus, pp125FAK and PI3'K may participate in morphological transformation by v-src. Furthermore, association of phosphorylated SHC with the adapter GRB2 correlated with increased anchorage-independent growth (and autophosphorylation) in both rat and chicken cells independent of the morphological phenotype induced. Therefore, host-range dependence for transformation may be regulated through association of SHC with GRB2, thus implicating SHC as a crucial substrate for src-dependent transformation.

Autophosphorylation is required for high kinase activity and efficient transformation ability of proteins encoded by host range alleles of v-src

pp60v-src is a nonreceptor protein tyrosine kinase that can transform both chicken and rodent fibroblasts. The src homology 2 (SH2) domain of this protein serves a critical role in the regulation of protein tyrosine kinase activity. The host range proteins pp60v-src-L, which contains a deletion of a highly conserved residue (Phe-172) in the SH2 domain, and pp60v-src-PPP, which contains a change from a Leu to a Phe at amino acid 186 in the SH2 domain, transform chicken but not rat cells and have slightly reduced kinase activity measured in vitro. The data presented here show that these altered proteins require autophosphorylation on Tyr-416 for high kinase activity and transforming ability. In the absence of autophosphorylation, there is a further decrease of at least threefold in in vitro kinase activity relative to the phosphorylated host range parental protein, no morphological transformation, a reduction in anchorage independent growth, and no disruption of the actin cytoskeleton. In addition, these SH2 mutations abolish the ability of the SH2 domain to bind a phosphorylated peptide that corresponds to the autophosphorylation site of pp60src. Thus, like mutant alleles of c-src encoding transformation competent proteins, and unlike v-src, transformation by pp60v-src-F172 delta and pp60v-src-L186F is dependent on phosphorylation of Y-416 for high kinase activity and transformation ability. The dependence of transformation on phosphotyrosine is not a reflection of an intramolecular interaction between the autophosphorylation site and the SH2 domains since purified SH2 domains are incapable of binding phosphorylated autophosphorylation site peptides in vitro.

Possible activity of acidic fibroblast growth factor as a progression factor rather than a transforming factor

Acidic and basic fibroblast growth factors (aFGF and bFGF) are mitogens for mesoderm- and neuroectoderm-derived cells. The facts that FGF-related proteins are oncogenic and that FGFs are expressed in a variety of tumor cell lines or tumor tissues suggest the transforming activities of FGFs. To examine such an activity of aFGF, we introduced a human aFGF expression vector into two populations of Rat-1 cells; one was non-transformed (nRat-1), the other was partially-transformed (tRat-1). tRat-1 cells transfected with aFGF cDNA formed larger colonies in soft agar and produced larger and more malignant tumors in nude mice than those of parental cells. In contrast, nRat-1 cells transfected with aFGF cDNA neither formed colonies in soft agar nor produced tumors in nude mice. Our results suggest that high expression of aFGF may enhance a tumorigenic potential of Rat-1 cells rather than confer such a potential de novo.

A mutation in the catalytic domain of pp60v-src is responsible for the host- and temperature-dependent phenotype of the Rous sarcoma virus mutant tsLA33-1

We have analyzed a host- and temperature-dependent mutant of Rous sarcoma virus in order to learn more about the nature of mutations which lead to a host range phenotype. We have cloned and sequenced the v-src genes from this mutant, tsLA33-1, and from its presumed parent, tsLA33. Both the tsLA33 and the tsLA33-1 pp60v-src proteins contain multiple mutations. The tsLA33 v-src gene product has amino acid alterations at four positions. In the tsLA33-1 v-src gene product, two of these four mutations have reverted to wild type. We have constructed chimeras between the two mutant v-src gene products and between each mutant and the Prague A v-src gene product. To assess the contribution of each amino acid change to the transformation phenotypes of tsLA33 and tsLA33-1, we expressed the hybrid proteins in both chicken embryo fibroblasts and Rat-3 fibroblasts. Additionally, we have measured the protein tyrosine kinase activity of chimeras constructed between the tsLA33 and tsLA33-1 pp60v-src proteins. Our results indicate that mutations in the catalytic domain of each protein are the principal determinants of the transforming ability and protein tyrosine kinase activity of the tsLA33 and tsLA33-1 pp60v-src proteins.

Genetic analysis of the Rous sarcoma virus subgroup D env gene: mammal tropism correlates with temperature sensitivity of gp85

Subgroup D avian sarcoma and leukosis viruses can penetrate a variety of mammalian cells in addition to cells from their natural host, chickens. Sequences derived from the gp85-coding domain within the env gene of a mammal-tropic subgroup D virus (Schmidt-Ruppin D strain of Rous sarcoma virus [SR-D RSV]) and a non-mammal-tropic subgroup B virus (Rous-associated virus type 2) were recombined to map genetic determinants that allow penetration of mammalian cells. The following conclusions were based on host range analysis of the recombinant viruses. (i) The determinants of gp85 that result in the mammal tropism phenotype of SR-D RSV are encoded within the 160 codons that lie 3' of codon 121 from the corresponding amino terminus of the gp85 protein. (ii) Small linear domains of the SR-D RSV gp85-coding domain placed in the subgroup B background did not yield viruses with titers equal to that of the subgroup D virus in a human cell line. (iii) Recombinant viruses that contained subgroup D sequences within the hr1 variable domain of gp85 showed modest-to-significant increases in infectivity on human cells relative to chicken cells. A recombinant virus that contained three fortuitous amino acid substitutions in the gp85-coding domain was found to penetrate the human cell line and give a titer similar to that of the subgroup D virus. In addition, we found that the subgroup D virus, the mutant virus, and recombinant viruses with an increased mammal tropism phenotype were unstable at 42 degrees C. These results suggest that the mammal tropism of the SR-D strain is not related to altered receptor specificity but rather to an unstable and fusogenic viral glycoprotein. A temperature sensitivity phenotype for infectivity of mammalian cells was also observed for another mammal-tropic avian retrovirus, the Bratislava 77 strain of RSV, a subgroup C virus, but was not seen for any other avian retrovirus tested, strengthening the correlation between mammal tropism and temperature sensitivity.

Fusion of Rous-sarcoma-virus-transformed rat cells to morphologically normal human or rat cells results in transcriptional suppression of the provirus that depends on its chromosomal integration site

The fusion of a Rous sarcoma virus (RSV)-transformed rat fibroblast clone to at least 2 different human cell types reproducibly produces phenotypically normal hybrids. Analysis of such hybrids reveals that proviral silence is the result of transcriptional down-regulation, presumably by a trans-acting human molecule. Furthermore, this phenomenon seems to be strongly influenced by the proviral chromosomal integration site and its imposition may entail a mechanism that is required only transiently.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

Site-directed mutagenesis of the SH2- and SH3-coding domains of c-src produces varied phenotypes, including oncogenic activation of p60c-src

The products of the viral and cellular src genes, p60v-src and p60c-src, appear to be composed of multiple functional domains. Highly conserved regions called src homology 2 and 3 (SH2 and SH3), comprising amino acid residues 88 to 250, are believed to modulate the protein-tyrosine kinase activity present in the carboxy-terminal halves of the src proteins. To explore the functions of these regions more fully, we have made 34 site-directed mutations in a transformation-competent c-src gene encoding phenylalanine in place of tyrosine 527 (Y527F c-src). Twenty of the new mutations change only one or two amino acids, and the remainder delete small or large portions of the SH2-SH3 region. These mutant alleles have been incorporated into a replication-competent Rous sarcoma virus vector to examine the biochemical and biological properties of the mutant proteins after infection of chicken embryo fibroblasts. Four classes of mutant proteins were observed: class 1, mutants with only slight differences from the parental gene products; class 2, mutant proteins with diminished transforming and specific kinase activities; class 3, mutant proteins with normal or enhanced specific kinase activity but impaired biological activity, often as a consequence of instability; and class 4, mutant proteins with augmented biological and catalytic activities. In general, there was a strong correlation between total kinase activity (or amounts of intracellular phosphotyrosine-containing proteins) and transforming activity. Deletion mutations and some point mutations affecting residues 109 to 156 inhibited kinase and transforming functions, whereas deletions affecting residues 187 to 226 generally had positive effects on one or both of those functions, confirming that SH2-SH3 has complex regulatory properties. Five mutations that augmented the transforming and kinase activities of Y527F c-src [F172P, R175L, delta(198-205), delta(206-226), and delta(176-226)] conferred transformation competence on an otherwise normal c-src gene, indicating that mutations in SH2 (like previously described lesions in SH3, the kinase domain, and a carboxy-terminal inhibitory domain) can activate c-src.

Factors affecting long-term stability of Moloney murine leukemia virus-based vectors

We have examined the long-term functional and structural stability of retroviral vectors in infected murine cells. We have used Moloney murine leukemia virus-based vectors expressing human HPRT, firefly luciferase (luc), and Escherichia coli beta-galactosidase (lacZ) as reporter genes, and the human HPRT and the transposon Tn5 neomycin resistance (neo) gene as selectable markers. All vectors, whether single or double gene, yielded both stable and unstable clones. Stability of the proviruses was dependent on a number of factors, including the nature of the infected cell, the reporter gene, the integration site of the provirus, the relative positions of the component genes in multigene vectors, and the presence or absence of selection pressure. Selection pressure was helpful, but not universally effective, in maintaining provirus structural and functional integrity. Reporter gene expression from an internal promoter was likely to be unstable with or without selection for an upstream, LTR-driven neo gene. In some clones, loss of proviral gene expression was accompanied by deletions, while other inactive clones retained an apparently intact provirus. In the latter clones, treatment with 5-azacytidine failed to reactivate the reporter genes, but superinfection with helper virus resulted in the reappearance of transmissible vector, indicating a reversible epigenetic mechanism for proviral shutdown. The design of effective retroviral vectors and their possible use in vivo will require further characterization of these determinants of provirus stability.

Shuttle vector system for the analysis of mutational events in mammalian chromosomal DNA

cDNA of the human hprt gene was introduced into the BamHI cloning site of the retroviral shuttle vector pZipNeoSV(X)1. The mouse cell line 2TGOR, a hypoxanthine phosphoribosyltransferase-deficient derivative of Balb/c 3T3, was transformed with the vector and some stably transformed HATrNEOr clones were established. One of the clones, VH-12, contained a single copy of the vector integrated stably into a chromosome in a proviral form. From this clone, we were able to recover efficiently the vector sequence preserving its intact structure by use of COS cell fusion. The relatively small size of the hprt cDNA (657 base pairs for the coding region) allowed quick determination of the entire DNA sequence. It was also notable that use of 6TG NEO double selection for mutant isolation could eliminate the 6TGr derivatives of VH-12 cells which arose from loss of the total vector sequence or from some epigenetic event, because such alterations would lead to inactivation of the neo gene as well as the hprt cDNA. The properties of our shuttle vector system were particularly useful for analysis of the molecular mechanisms of mutational events in chromosomal DNA of mammalian cells.

A mutation in v-src that removes a single conserved residue in the SH-2 domain of pp60v-src restricts transformation in a host-dependent manner

The v-src oncogene of Rous sarcoma virus (RSV) is able to transform both avian and mammalian cells, but the mutant allele v-src-L displays a host range dependence for transformation, transforming chicken but not rat cells with wild-type efficiency. This host range restriction can be detected by measuring growth in low serum, saturation density, and anchorage independent growth. In addition, rat cells expressing v-src-L do not form tumors in syngeneic rats or nude mice, but RSV carrying the mutant allele causes tumors in chicks, although at a reduced efficiency and with increased latency. To determine the lesion responsible for this phenotype, we sequenced the entire v-src gene from the parental B77 strain of RSV, as well as the mutant allele. v-src-L is missing 3 nucleotides present in the wild-type parent, RSV B31, eliminating Phe-172, an invariant residue in a conserved region of src-related proteins known as SH-2. The kinase activity of pp60v-src-L was indistinguishable from that of the wild type in chicken cells but was significantly reduced in rat cells as assayed by an in vitro immune complex assay; in vivo phosphorylation of one specific substrate, p36 (calpactin I heavy chain); and total phosphotyrosine-containing proteins. In addition, the pattern of phosphotyrosine-containing proteins in rat cells was qualitatively different when cells containing pp60v-src-L were compared with cells with wild-type pp60v-src, even though both pp60v-src proteins were membrane associated. The data are consistent with a role for the SH-2 region in substrate specificity.

A rat mutant cell clone showing temperature-dependent transformed phenotypes with functional expression of the src gene product

The cellular mutant B814 isolated from a Fischer rat cell line shows temperature-sensitivity of focus formation on infection with Moloney murine sarcoma virus (Mo-MSV) and Rous sarcoma virus (RSV). An RSV-transformed clone (S814-2) isolated from B814 cells shows temperature-sensitive transformed phenotypes for morphology, growth in soft agar, and glucose uptake. The expression, phosphorylation, and tyrosine kinase activity of pp60v-src in S814-2 were not affected at the nonpermissive temperature, and virus rescued from this clone had wild-type transforming ability, suggesting that a cellular factor altered in S814-2 is responsible for the cellular steps of transformation after the function of pp60v-src. In addition, the cellular 36K protein, a possible candidate as a target of pp60v-src, was phosphorylated at the nonpermissive temperature in S814-2, indicating that phosphorylation of the 36K protein is not correlated with transformed phenotypes.

Detection of genes with a potential for suppressing the transformed phenotype associated with activated ras genes

Seven morphologically nontransformed (flat) revertants with reduced tumorigenicity in vivo have been isolated from populations of Kirsten sarcoma virus-transformed NIH 3T3 cells transfected with a cDNA expression library of normal human fibroblasts. Each revertant harbors 1-10 recombinant plasmids per cell and retains a rescuable transforming virus as well as high level expression of v-Ki-ras-specific RNA and the viral oncogene product, p21v-Ki-ras. Transformed phenotypes are suppressed in cell hybrids generated by fusing each revertant to v-Ki-ras-transformed NIH 3T3 cells. From two of the revertant lines, plasmids capable of giving rise to flat secondary transfectants have been recovered. Thus, in some, if not all, of the revertants, transfected cDNAs seem to be responsible for the suppression of specific transformed phenotypes.

A host-dependent temperature-sensitive mutant of Rous sarcoma virus: evidence for host factors affecting transformation

We have characterized a host range mutant of Rous sarcoma virus in order to identify host cell factors involved in transformation. This mutant, tsLA33-1, which was isolated from a stock of the temperature-sensitive mutant tsLA33, is not temperature-sensitive for transformation of chicken embryo fibroblasts, as judged by its ability to induce morphological changes and agar colony formation at both 36 and 41.5 degrees. In Rat-3 cells, however, this mutant induced a temperature-dependent transformation: infected Rat-3 cells were transformed at 34 degrees but not at 39.5 degrees. Retransformants were isolated from tsLA33-1-infected Rat-3 cells by growth in agar suspension at 39.5 degrees. Virus rescued from these retransformants induced a temperature-dependent transformation when reintroduced into rat cells. The level of expression of pp60v-src at 39.5 degrees was unchanged in the retransformants. When the retransformants were treated with herbimycin, an antibiotic which induces turnover of certain protein-tyrosine kinases, they reverted to a normal phenotype, indicating that the transformed phenotype of the retransformants was dependent on continued expression of pp60v-src. The retransformants are therefore pseudorevertants in which a cellular alteration has occurred that allows transformation at 39.5 degrees by the mutant pp60v-src. Thus the temperature-dependence of transformation by tsLA33-1 is affected by the cellular environment, and is suppressed or complemented both in chicken cells and in the rat cell pseudorevertants. No clear correlation between levels of phosphorylation at tyrosine and transformation was observed. In Rat-3 cells the pp60v-src encoded by tsLA33-1 may be defective in its interaction with low abundance substrates that are critical for transformation; alternatively the nonpermissive cells may require a higher threshold dose of pp60v-src for transformation.

The first seven amino acids encoded by the v-src oncogene act as a myristylation signal: lysine 7 is a critical determinant

The transforming protein of Rous sarcoma virus, pp60v-src, is covalently coupled to myristic acid by an amide linkage to glycine 2. Myristylation promotes the association of pp60v-src with cellular membranes, and this subcellular location is essential for transforming activity. The findings presented here, in conjunction with the previous reports of others, imply that the seventh amino acid encoded by v-src might be important in the myristylation reaction. Replacement of lysine 7 by asparagine greatly reduced the myristylation, membrane association, and transforming activity of pp60v-src. In contrast, substitution of arginine at residue 7 had no effect on any of these properties of pp60v-src. Addition of amino acids 1 to 7 encoded by v-src was sufficient to cause myristylation of a src-pyruvate kinase fusion protein. We conclude that the recognition sequence for myristylation of pp60v-src comprises amino acids 1 to 7 and that lysine 7 is a critical component of this sequence.

The first seven amino acids encoded by the v-src oncogene act as a myristylation signal: lysine 7 is a critical determinant

The transforming protein of Rous sarcoma virus, pp60v-src, is covalently coupled to myristic acid by an amide linkage to glycine 2. Myristylation promotes the association of pp60v-src with cellular membranes, and this subcellular location is essential for transforming activity. The findings presented here, in conjunction with the previous reports of others, imply that the seventh amino acid encoded by v-src might be important in the myristylation reaction. Replacement of lysine 7 by asparagine greatly reduced the myristylation, membrane association, and transforming activity of pp60v-src. In contrast, substitution of arginine at residue 7 had no effect on any of these properties of pp60v-src. Addition of amino acids 1 to 7 encoded by v-src was sufficient to cause myristylation of a src-pyruvate kinase fusion protein. We conclude that the recognition sequence for myristylation of pp60v-src comprises amino acids 1 to 7 and that lysine 7 is a critical component of this sequence.

Temperature-sensitive cellular mutant for expression of mRNA from murine retrovirus

The cellular mutant B812 isolated from a Fisher rat cell line shows temperature sensitivity of focus formation induced by various retroviruses such as recombinant murine retrovirus containing the middle T gene of polyomavirus (PyMLV), Kirsten murine sarcoma virus, Moloney murine sarcoma virus, and recombinant murine retrovirus containing the src gene of Rous sarcoma virus. B812 cells, however, show normal ability to proliferate and synthesize protein at the nonpermissive temperature, suggesting that their mutation is in a gene specifically concerned with the process of transformation by retroviruses. In this work, experiments with hybrids of mutant and wild-type cells showed that the temperature-dependent defect of this mutant was complemented by wild-type cells. To determine the step of transformation that is restricted at the nonpermissive temperature in B812, we examined the expressions of the oncogene (middle T antigen) in no. 7 (wild-type cells) and B812 cultures infected with PyMLV (the chimeric retrovirus containing the middle T gene of polyomavirus) at the permissive and nonpermissive temperatures. Middle T-associated protein kinase activity, the expression of middle T antigen, and PyMLV-specific mRNA were reduced at the nonpermissive temperature in B812 cultures infected with PyMLV. However, integration of PyMLV into the chromosomal DNA of the mutant was not affected at the nonpermissive temperature. These results suggest that B812 cells have a mutation affecting the expression of viral mRNAs from integrated proviral DNA at the nonpermissive temperature.

A conserved domain regulates interactions of the v-fps protein-tyrosine kinase with the host cell

All cytoplasmic protein-tyrosine kinases (PTKs) share a noncatalytic domain, termed SH2, which comprises approximately 100 residues located immediately N-terminal to the kinase domain. A linker in the AX9m mutant of Fujinami avian sarcoma virus (FSV) introduces a dipeptide insertion into the SH2 domain of the P130gag-fps PTK, which abolishes its ability to transform Rat-2 cells. However, at 36 degrees C AX9m FSV elicits focus formation and agar colony formation in infected chicken embryo fibroblasts (CEF) with single hit kinetics. At 41.5 degrees C AX9m FSV is nontransforming for CEF, and the mutant is therefore both host and temperature dependent for transforming activity. Both in vitro and in vivo, the specific kinase activity of AX9m FSV P130gag-fps, measured by autophosphorylation and phosphorylation of exogenous substrates, correlated with transforming activity. The consequences of the AX9m mutation for enzymatic function and transforming activity therefore depend on the cellular environment in which the altered v-fps protein is expressed. We conclude that the SH2 domain directs the interaction of the P130gag-fps catalytic domain with cellular proteins such as substrates for phosphorylation or regulators of kinase activity important for its transforming ability.

Revertants of v-fos-transformed fibroblasts have mutations in cellular genes essential for transformation by other oncogenes

Morphologic revertants of FBJ murine sarcoma virus (v-fos)-transformed rat-1 fibroblasts were isolated using a novel selection procedure based on prolonged retention of rhodamine 123 within mitochondria of v-fos-transformed versus normal fibroblasts. Two classes of revertants were isolated: class I revertants have sustained mutations in cellular genes, and a class II revertant has a nonfunctional v-fos provirus. Somatic-cell hybridization studies suggested that the revertant phenotype was recessive to the transformed phenotype. Class I revertants were also resistant to retransformation by v-gag-fos-fox, v-Ha-ras, v-abl, and v-mos, but could be retransformed by the trk oncogene and polyoma virus middle T antigen. These results suggest that the class I revertants sustained mutations in one or more cellular genes essential for transformation by some, but not all, oncogenes. Our data suggest the existence of common biochemical pathways for transformation.

Instability of v-src sequences in nonhuman primate tumors cultured in vitro

We have analyzed the DNA of marmoset tumors induced and marmoset cells transformed by Rous sarcoma virus (RSV) and derivative viruses of various types. Southern blot hybridization was used to determine the presence of v-src gene sequences. We failed to detect v-src DNA in high-passage cells derived from marmoset tumors induced in vivo or from marmoset cell lines transformed in vitro. The inability to detect src sequences was not related to selection of revertants in culture, since all cell lines retained transformed morphology and cells transformed in vitro retained the ability to induce sarcomas after transplantation into adult allogeneic marmosets. By contrast, we detected integrated proviruses in cells analyzed 32 to 60 days after in vitro transformation. The proviral sequences appeared to be identical to the transforming virus but were apparently unstable and continued to transpose.

MC29 deletion mutants which fail to transform chicken macrophages are competent for transformation of quail macrophages

A number of MC29 mutants with deleted myc genes have been previously characterized. Many of these mutants have been found to be defective for transformation of chicken macrophages in vitro and for tumor induction in chickens. Such mutants are capable of transforming Japanese quail macrophages in vitro and inducing a high incidence of tumors in Japanese quail. Thus, Japanese quail may contain a factor(s) capable of complementing the defective transforming proteins encoded by some deleted v-myc genes.

Fusion-mediated microinjection of active amine and diamine oxidases into cultured cells: effect on protein and DNA synthesis in chick embryo fibroblasts and in glioma cells

Serum amine oxidase and/or porcine kidney diamine oxidase were trapped within reconstituted Sendai virus envelopes, and retained their activity. The trapped enzymes that were detected by radioimmunoblots were microinjected into cultured cells by fusion. When diamine oxidase was microinjected into cultured fibroblasts of chick or rat embryos, a temporary arrest in protein and DNA synthesis was observed. The inhibitory effect was more significant when both serum amine oxidase and kidney diamine oxidase were microinjected into those cultured cells. Fibroblasts of either chick or rat embryos transformed by Rous sarcoma virus were more susceptible to the injected enzymes than the normal cultures, showing a complete arrest in protein and DNA synthesis within 4 hours. Similar results were obtained by microinjecting diamine oxidase into cultured glioma cells. The injected enzyme catalyzed the oxidation of intracellular polyamines. The resulting oxidation product (hydrogen peroxide and aminoaldehydes) apparently caused the arrest in the synthesis of macromolecules.

Variable stability of a selectable provirus after retroviral vector gene transfer into human cells

Human lymphoblasts deficient in the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) were infected with an amphotropic helper-free retroviral vector expressing human HPRT cDNA. The stability and expression of the HPRT provirus in five cell lines with different proviral integration sites were examined by determining HPRT mutation and reversion frequencies and by blot hybridization studies. Mutation to the HPRT-negative phenotype occurred at frequencies of approximately 4 X 10(-5) to 3 X 10(-6) per generation. Most mutations in each of the five cell lines were associated with partial or complete deletions or rearrangements of the provirus. Several mutants retained a grossly intact HPRT provirus, and in one such mutant HPRT shutdown resulted from a revertible epigenetic mechanism that was not associated with global changes in proviral methylation. Therefore, mutation and shutdown of the HPRT provirus in human lymphoblasts result from mechanisms similar to those reported for several other avian and mammalian replication-competent retroviruses.

Perturbed hemopoiesis and the generation of multipotential stem cell clones in src-infected bone marrow cultures is an indirect or transient effect of the oncogene

Multipotential stem cell lines, derived specifically from long-term bone marrow cultures infected with a recombinant retrovirus carrying v-src, lack v-src. Stable consequences thus result from transient actions or indirect effects of v-src on other cells, with the latter possibility being favored by its mosaic expression in marrow cultures.

Stability of retrovirally transduced markers in a rat cell line

A MoMLV-based retroviral vector capable of transmitting and expressing both the human hypoxanthine phosphoribosyltransferase (hprt) coding sequence and the Herpes simplex type 1 thymidine kinase (tk) gene has been constructed. After infection of a rat cell line, cell clones were selected on the basis of expressing both markers. They were subsequently found to contain a single provirus of the expected topology. The ease with which loss of expression of the markers can be monitored has allowed us to make observations on the stability of proviral genes. In particular, we have found indirect evidence of strong position effects on proviral gene expression by comparing the characteristic frequency of marker loss in different clonal proviral lines. Effects of the selection protocol on the apparent frequency of variants have also been noted. Finally, a combination of molecular and genetic observations lead us to invoke chromosome loss as the major factor influencing marker stability in this system.

Regulation of cellular morphology by the Rous sarcoma virus src gene: analysis of fusiform mutants

We have been interested in how Rous sarcoma virus (RSV) influences transformed cell morphology and compared the molecular properties of chicken embryo cells (CEC) infected with mutants of RSV that induce the fusiform transformed cell morphology with those of CEC infected by wild-type RSV, which induces the more normal round transformed cell morphology. We looked for properties shared by all fusiform mutant-infected cells, because these may be responsible for maintaining the fusiform morphology. Five different fusiform mutants, two wild-type RSVs, and one wild-type back revertant of a fusiform mutant were studied. In the fusiform mutant-infected cells, the localization and myristylation of pp60src were determined and the extent of expression of the extracellular matrix protein fibronectin was examined at both the mRNA and protein levels. The phosphorylation of vinculin on tyrosine also was examined in the same CEC. Within all fusiform mutant-transformed CEC, pp60src was dramatically absent from the adhesion plaque sites normally seen in cells transformed with wild-type RSV, and these transformed CEC all expressed more fibronectin mRNA and protein in the extracellular matrix than did the wild-type RSV-transformed CEC. The absence of pp60src from the adhesion plaques was not due to lack of myristylation of the src protein, and tyrosine phosphorylation of vinculin was not related to fibronectin expression. These results suggest that the inverse relationship between pp60src in the adhesion plaques and fibronectin expression in the extracellular matrix may be interconnected phenomena and could be related to the maintenance of the fusiform transformed morphology.

Variable stability of a selectable provirus after retroviral vector gene transfer into human cells

Human lymphoblasts deficient in the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) were infected with an amphotropic helper-free retroviral vector expressing human HPRT cDNA. The stability and expression of the HPRT provirus in five cell lines with different proviral integration sites were examined by determining HPRT mutation and reversion frequencies and by blot hybridization studies. Mutation to the HPRT-negative phenotype occurred at frequencies of approximately 4 X 10(-5) to 3 X 10(-6) per generation. Most mutations in each of the five cell lines were associated with partial or complete deletions or rearrangements of the provirus. Several mutants retained a grossly intact HPRT provirus, and in one such mutant HPRT shutdown resulted from a revertible epigenetic mechanism that was not associated with global changes in proviral methylation. Therefore, mutation and shutdown of the HPRT provirus in human lymphoblasts result from mechanisms similar to those reported for several other avian and mammalian replication-competent retroviruses.

Effect of transformation by Rous sarcoma virus on the character and distribution of actin in Rat-1 fibroblasts: a biochemical and microscopical study

Actin has been measured in subcellular fractions from Rat-1 fibroblasts and in Rous sarcoma virus-transformed Rat-1 cells (VIT), using the DNase 1 inhibition assay. The transformed cells showed a significant shift in the actin monomer (G)in equilibrium with polymer (F) equilibrium within the cell cytosol, and a significant increase in actin in the Triton-insoluble cytoskeletal core in comparison with untransformed cells. This incorporation of actin into the cytoskeletal core fraction is associated with a change in filamentous actin assemblies from 'stress fibre' patterns to punctate filament aggregates. These differences have been correlated with changes in morphology, in actin, vinculin and alpha-actinin distribution, in adhesion plaque formation and with the production of pp60v-src-associated protein kinase activity in the transformed cells. Changes in actin distribution and its polymerization in response to src-gene expression may play an important role in the determination of the transformed cell characteristics.

Control of expression of an integrated Rous sarcoma provirus in rat cells: role of 5' genomic duplications reveals unexpected patterns of gene transcription and its regulation

Rat cells transformed by Rous sarcoma virus frequently contain duplications of viral (and sometimes cellular) DNA 5' to the integrated provirus, suggesting that such rearrangements favor provirus expression. In one cell line, A11, the duplication includes the viral src gene and proviral sequences that flank it. We examined three possible roles for this structure. Since the proviral v-src gene transformed recipient cells upon DNA transfer and was the major template for v-src transcription in A11 cells, the presence of v-src in the duplication is presumably not necessary for transformation. Since the size and structure of transcripts from the proviral v-src gene in A11 cells were conventional, the duplication does not facilitate transformation by providing a novel transcriptional strategy. Thus, we favor the concept that the duplication either attenuates a negative effect of flanking elements at the host chromosome integration site or augments the positive regulation of conventional provirus expression or both. Gene transfer and transcription analyses with both genomic and cloned DNA showed that the mechanisms of such regulatory phenomena are complex. Identical sequences in the provirus and the 5' duplication displayed different patterns of expression in A11 cells that could be disrupted in segments of cloned DNA. Among the elements that influenced such expression were sequences from the gag-pol region of the provirus.

Perturbed hemopoiesis and the generation of multipotential stem cell clones in src-infected bone marrow cultures is an indirect or transient effect of the oncogene

Multipotential stem cell lines, derived specifically from long-term bone marrow cultures infected with a recombinant retrovirus carrying v-src, lack v-src. Stable consequences thus result from transient actions or indirect effects of v-src on other cells, with the latter possibility being favored by its mosaic expression in marrow cultures.

Regulation of cellular morphology by the Rous sarcoma virus src gene: analysis of fusiform mutants

We have been interested in how Rous sarcoma virus (RSV) influences transformed cell morphology and compared the molecular properties of chicken embryo cells (CEC) infected with mutants of RSV that induce the fusiform transformed cell morphology with those of CEC infected by wild-type RSV, which induces the more normal round transformed cell morphology. We looked for properties shared by all fusiform mutant-infected cells, because these may be responsible for maintaining the fusiform morphology. Five different fusiform mutants, two wild-type RSVs, and one wild-type back revertant of a fusiform mutant were studied. In the fusiform mutant-infected cells, the localization and myristylation of pp60src were determined and the extent of expression of the extracellular matrix protein fibronectin was examined at both the mRNA and protein levels. The phosphorylation of vinculin on tyrosine also was examined in the same CEC. Within all fusiform mutant-transformed CEC, pp60src was dramatically absent from the adhesion plaque sites normally seen in cells transformed with wild-type RSV, and these transformed CEC all expressed more fibronectin mRNA and protein in the extracellular matrix than did the wild-type RSV-transformed CEC. The absence of pp60src from the adhesion plaques was not due to lack of myristylation of the src protein, and tyrosine phosphorylation of vinculin was not related to fibronectin expression. These results suggest that the inverse relationship between pp60src in the adhesion plaques and fibronectin expression in the extracellular matrix may be interconnected phenomena and could be related to the maintenance of the fusiform transformed morphology.

Rearrangements of viral and cellular DNA are often associated with expression of Rous sarcoma virus in rat cells

Rous sarcoma virus (RSV) proviruses integrated within the DNA of transformed rat cells frequently display duplications of variable segments of proviral DNA upstream of an intact provirus. The rearrangement in the A11 clone of transformed rat cells consists of a partial duplication of both viral and cellular DNA segments whose origin is a region of approximately 4 kb encompassing the 3' virus-cell junction. Transposition of this DNA appears to have occurred at or after virus integration by a mechanism involving at least two recombination events. In every case examined, including A11, the transcriptional organization of the original provirus has been conserved and viral RNA expression appears to occur normally. The frequency of such rearranged proviruses in the DNA of transformed rat cells suggests that upstream rearrangements may influence provirus expression.

Chromatin structure around the c-myc gene in Burkitt lymphomas with upstream and downstream translocation points

Burkitt lymphoma cells seem to have abnormal c-myc gene activity resulting from chromosomal translocation. We have examined the consequences of translocation on putative control sequences near to the c-myc gene by DNase I hypersensitivity mapping of chromatin. There is no detectable difference in the pattern of hypersensitivity (compared with the actively transcribed c-myc gene of lymphoblastoid cells) in Burkitt lymphoma cells where the translocation point occurs at a considerable distance upstream or downstream of c-myc. When the translocation occurs near the 5' end of the c-myc gene, resulting in loss of hypersensitive sites, those that remain show the same sensitivity as in lymphoblastoid cell lines. We conclude that translocation has little general effect on the usual pattern of hypersensitive sites near to the c-myc gene but new sites can be observed in some cases in the immunoglobulin region near to the breakpoint. These may be sites normally involved in immunoglobulin gene transcription and may exert a subtle effect on the translocated c-myc gene.