Inhibitors of glycosylation reverse retroviral interference

Overexpression of Pax5 is not sufficient for neoplastic transformation of mouse neuroectoderm

The developmental control genes of the Pax family are essential for brain development. Several Pax genes are also involved in chromosomal translocations causing malignancies in humans, and Pax5 expression is deregulated in medulloblastomas. We have investigated whether Pax5 can induce tumors in the developing mouse brain. Primary mouse embryonic neuroectodermal cells were retrovirally transduced with mouse Pax5 and transplanted into the brain of syngeneic host mice. No tumors developed in 36 transplants after one year, and there were no alterations in the differentiation pattern of the neural transplants. We then generated transgenic mice expressing human Pax5 under control of the Engrailed-2 promoter, which is expressed in the cerebellar external granule cell layer and in medulloblastomas. Sustained expression was achieved in the cerebellum of transgenic animals throughout lifetime. Expression levels were similar to those observed in human medulloblastomas. Again, cerebellar morphogenesis was undisturbed, and no tumors arose. These results strongly argue against a dominant transforming activity of PAX5 in NEC and in cerebellar granule cell precursors of mice, and underline the restricted tissue-specificity of PAX5 related oncogenesis.

Cellular and species resistance to murine amphotropic, gibbon ape, and feline subgroup C leukemia viruses is strongly influenced by receptor expression levels and by receptor masking mechanisms

Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an inhibitor of N-linked glycosylation. These viruses use the related sodium-phosphate symporters Pit1 and Pit2, respectively, as receptors in nonhamster cells, and evidence has suggested that the corresponding transporters of CHO cells may be masked by tunicamycin-sensitive secreted inhibitors. Although the E36 line of Chinese hamster cells was reported to secrete the putative Pit2 inhibitor and to be sensitive to the inhibitory CHO factors, E36 cells are highly susceptible to both GALV and A-MLV in the absence of tunicamycin. Moreover, expression of E36 Pit2 in CHO cells conferred tunicamycin-independent susceptibilities to both viruses. Based on the latter results, it was suggested that E36 Pit2 must functionally differ from the endogenous Pit2 of CHO cells. To test these ideas, we analyzed the receptor properties of CHO Pit1 and Pit2 in CHO cells. Surprisingly, and counterintuitively, transfection of a CHO Pit2 expression vector into CHO cells conferred strong susceptibility to both GALV and A-MLV, and similar overexpression of CHO Pit1 conferred susceptibility to GALV. Thus, CHO Pit2 is a promiscuous functional receptor for both viruses, and CHO Pit1 is a functional receptor for GALV. Similarly, we found that the natural resistance of Mus dunni tail fibroblasts to subgroup C feline leukemia viruses (FeLV-C) was eliminated simply by overexpression of the endogenous FeLV-C receptor homologue. These results demonstrate a novel and simple method to unmask latent retroviral receptor activities that occur in some cells. Specifically, resistances to retroviruses that are caused by subthreshold levels of receptor expression or by stoichiometrically limited masking or interference mechanisms can be efficiently overcome simply by overexpressing the endogenous receptors in the same cells.

Differential glycosylation of the Cas-Br-E env protein is associated with retrovirus-induced spongiform neurodegeneration

The wild mouse ecotropic retrovirus, Cas-Br-E, induces progressive, noninflammatory spongiform neurodegenerative disease in susceptible mice. Functional genetic analysis of the Cas-Br-E genome indicates that neurovirulence maps to the env gene, which encodes the surface glycoprotein responsible for binding and fusion of virus to host cells. To understand how the envelope protein might be involved in the induction of disease, we examined the regional and temporal expression of Cas-Br-E Env protein in the central nervous systems (CNS) of mice infected with the highly neurovirulent chimeric virus FrCas(E). We observed that multiple isoforms of Cas-Br-E Env were expressed in the CNS, with different brain regions exhibiting unique patterns of processed Env glycoprotein. Specifically, the expression of gp70 correlated with regions showing microglial infection and spongiform neurodegeneration. In contrast, regions high in neuronal infection and without neurodegenerative changes (the cerebellum and olfactory bulb) were characterized by a gp65 Env protein isoform. Sedimentation analysis of brain region extracts indicated that gp65 rather than gp70 was incorporated into virions. Biochemical analysis of the Cas-Br-E Env isoforms indicated that they result from differential processing of N-linked sugars. Taken together, these results indicate that differential posttranslational modification of the Cas-Br-E Env is associated with a failure to incorporate certain Env isoforms into virions in vivo, suggesting that defective viral assembly may be associated with the induction of spongiform neurodegeneration.

Simultaneous infection with retroviruses pseudotyped with different envelope proteins bypasses viral receptor interference associated with colocalization of gp70 and target cells on fibronectin CH-296

Several factors are thought to limit the efficiency of retroviral transduction in clinical gene therapy protocols that target hematopoietic stem cells. For example, the level of expression of the amphotropic receptor Pit-2, a phosphate symporter, appears to be low in human and murine hematopoietic stem cells. We have previously demonstrated that transduction of hematopoietic cells in the presence of the fibronectin (FN) fragment CH-296 is extremely efficient (H. Hanenberg, X. L. Xiao, D. Dilloo, K. Hashino, I. Kato, and D. A. Williams, Nat. Med. 2:876-882, 1996). To examine functionally whether the retrovirus receptor is a limiting factor in transduction of hematopoietic cells, we performed competition experiments in the presence of FN CH-296 with retrovirus vectors pseudotyped with the same or a different envelope protein. We demonstrate in both human erythroleukemia (HEL) cells and primary human CD34(+) hematopoietic cells inhibition of efficient infection due to receptor interference when two vectors targeting the amphotropic receptor are used simultaneously. Receptor interference lasted up to 24 h. No interference was demonstrated when vectors targeting the amphotropic receptor and the gibbon ape leukemia virus (GALV) receptor Pit-1 were used concurrently. In contrast, simultaneous infection with vectors targeting both Pit-1 and Pit-2 yielded transduction efficiencies consistently higher than with either vector alone in both HEL cells and human CD34(+) hematopoietic cells. These data demonstrate that the use of FN CH-296 leads to amphotropic receptor saturation in these cells. Simultaneous infection with vectors targeting both amphotropic and GALV receptors may prove to be of additional benefit in the design of gene therapy protocols.

Gibbon ape leukemia virus receptor functions of type III phosphate transporters from CHOK1 cells are disrupted by two distinct mechanisms

The Chinese hamster cell lines E36 and CHOK1 dramatically differ in susceptibility to amphotropic murine leukemia virus (A-MuLV) and gibbon ape leukemia virus (GALV); E36 cells are highly susceptible to both viruses, CHOK1 cells are not. We have previously shown that GALV can infect E36 cells by using both its own receptor, HaPit1, and the A-MuLV receptor, HaPit2. Given that the two cell lines are from the same species, the loss of function of both of these receptors in CHOK1 cells is surprising. Other studies have shown that CHOK1 cells secrete proteins that block A-MuLV entry into CHOK1 as well as E36, suggesting the two A-MuLV receptors are functionally identical. However, CHOK1 conditioned medium does not block GALV entry into E36, indicating the secreted inhibitors do not block HaPit1. HaPit1 and ChoPit1 therefore differ as receptors for GALV; ChoPit1 is either inactivated by secreted factors or intrinsically nonfunctional. To determine why GALV cannot infect CHOK1, we cloned and sequenced ChoPit1 and ChoPit2. ChoPit2 is almost identical to HaPit2, which explains why CHOK1 conditioned medium blocks A-MuLV entry via both receptors. Although ChoPit1 and HaPit1 are 91% identical, a notable difference is at position 550 in the fourth extracellular region, shown by several studies to be crucial for GALV infection. Pit1 and HaPit1 have aspartate at 550, whereas ChoPit1 has threonine at this position. We assessed the significance of this difference for GALV infection by replacing the aspartate 550 in Pit1 with threonine. This single substitution rendered Pit1 nonfunctional for GALV and suggests that threonine at 550 inactivates ChoPit1 as a GALV receptor. Whether native ChoPit1 functions for GALV was determined by interference assays using Lec8, a glycosylation-deficient derivative of CHOK1 that is susceptible to both viruses and that has the same receptors as CHOK1. Unlike with E36, GALV and A-MuLV exhibited reciprocal interference when infecting Lec8, suggesting that they use the same receptor. We conclude both viruses can use ChoPit2 in the absence of the inhibitors secreted by CHOK1 and ChoPit1 is nonfunctional.

Cell-surface receptors for retroviruses and implications for gene transfer

Retroviruses can utilize a variety of cell-surface proteins for binding and entry into cells, and the cloning of several of these viral receptors has allowed refinement of models to explain retrovirus tropism. A single receptor appears to be necessary and sufficient for entry of many retroviruses, but exceptions to this simple model are accumulating. For example, HIV requires two proteins for cell entry, neither of which alone is sufficient; 10A1 murine leukemia virus can enter cells by using either of two distinct receptors; two retroviruses can use different receptors in some cells but use the same receptor for entry into other cells; and posttranslational protein modifications and secreted factors can dramatically influence virus entry. These findings greatly complicate the rules governing retrovirus tropism. The mechanism underlying retrovirus evolution to use many receptors for cell entry is not clear, although some evidence supports a mutational model for the evolution of new receptor specificities. Further study of factors that govern retrovirus entry into cells are important for achieving high-efficiency gene transduction to specific cells and for the design of retroviral vectors to target additional receptors for cell entry.

Mutations affecting the cytoplasmic domain of the Moloney murine leukemia virus envelope protein: rapid reversion during replication

Five premature termination mutations and five missense mutations were introduced into the portion of cloned Moloney murine leukemia virus (M-MuLV) DNA encoding the Env cytoplasmic domain. All of the mutant DNAs gave rise to replication-competent virus after transfection of NIH/3T3 cells, but several of the mutant DNAs scored as replication-defective when introduced into Rat2 cells. Cell lines stably expressing the mutant DNAs all released virion particles, and in all but one case infectious virus were generated. These viable mutants were all found to have reverted to the wild-type sequence. To generate fully mutant virus stocks, the mutant DNAs were introduced transiently into COS cells, which are resistant to infection with MuLV, thus prohibiting reversion by error-prone mechanisms involving reverse transcription. Virions harvested from the COS cells were confirmed as mutant by analyzing both virion proteins and the viral DNA they generated, and were then tested for infectivity in NIH/3T3 cells. The mutant viruses were infectious, but still rapidly gave rise to revertants. We conclude that the mutations within the cytoplasmic domain do not provide an absolute block to virus replication, but that the mutants replicate more slowly than the wild-type and quickly give rise to revertants with selective advantage for replication.

Inhibitors of retrovirus infection are secreted by several hamster cell lines and are also present in hamster sera

We have previously shown that Chinese hamster ovary (CHO) cells are resistant to infection by gibbon ape leukemia virus and amphotropic pseudotype retroviral vectors because of the secretion of factors that inhibit retrovirus infection. Such factors were not secreted by any mouse or human cell lines tested. Secretion of the inhibitors and resistance to infection are abrogated by treatment of CHO cells with the glycosylation inhibitor tunicamycin. Here we show that the inhibitory activities against gibbon ape leukemia virus and amphotropic viruses are partially separable and that glycosylation mutations in CHO cells mimic the effects of tunicamycin treatment. We find that several hamster cell lines derived from both Chinese and Syrian hamsters secrete inhibitors of retrovirus infection, showing that these inhibitors are not unique to the CHO cell line. Inhibitory factors are also present in the sera of Chinese and Syrian hamsters but were not detected in bovine serum. These results suggest the presence of specific factors that function to inhibit retrovirus infection in hamsters.

A phenotypic host range alteration determines RD114 virus restriction in feline embryonic cells

We have characterized the restriction mechanism for RD114 virus replication in embryonic feline cells (FeF). By comparing growth properties of the virus in FeF cells with its behavior in a fetal feline glial cell line (G355) permissive for RD114, we showed that both cell lines were readily infectible by virus grown in permissive cells and that no significant differences in viral integration or viral RNA expression could be detected. However, analysis of viral protein expression revealed differences in viral env gene processing in the two cell types. Envelope precursor pR85 was produced, but the expected processed gp70 product was detectable only in permissive (G355) cells. An envelope product of 85 kDa was packaged into virions produced by FeF cells, while virions produced by G355 cells contained the expected RD114 gp70. While the gp85 env-containing virions were infectious for permissive G355 cells, they were unable to infect FeF cells. The block to infection by the gp85-containing particles in FeF cells could be abrogated by treatment with the glycosylation inhibitor tunicamycin. Our results indicate that restriction of RD114 virus involves a novel mechanism dependent on two factors: altered glycosylation of the envelope to a gp85 form and an altered RD114 receptor in FeF cells.

Mutational analysis of the N-linked glycosylation sites of the SU envelope protein of Moloney murine leukemia virus

The role of the N-linked glycosylation sites in the major envelope glycoprotein, SU (gp70), of Moloney murine leukemia virus has been examined. By using site-specific oligonucleotide-directed mutagenesis, each of the seven glycan addition sites has been individually eliminated. Mutations resulting in the loss of a single glycosylation site produced, intracellularly, stable precursor SU-TM proteins which were 4 to 5 kDa smaller than the wild-type virus SU-TM protein. Mutant delta 1,4,7, a trimutant lacking three N-linked glycan addition sites, resulted in a viable, infectious virus with a stable SU-TM protein approximately 12 to 15 kDa smaller than the wild-type SU-TM protein. Five of the seven single-site mutations resulted in viable virus as judged by the release of reverse transcriptase in transient-expression assays and XC syncytium assays. Mutations at two of the sites resulted in a detectable phenotype. Virus mutated at position 2 was temperature sensitive in Rat2 cells; viable virus was produced at 32 degrees C but not at 37 degrees C. Virus mutated at position 3 was noninfectious and yielded virions lacking detectable mature SU protein. The mutation results in the block of transport of the protein to the cell surface and assembly into virion particles.

Tunicamycin treatment of CHO cells abrogates multiple blocks to retrovirus infection, one of which is due to a secreted inhibitor

Chinese hamster ovary (CHO) cells are resistant to infection by all of the major classes of murine retroviruses and are partially resistant to infection by gibbon ape leukemia virus. Treatment of CHO cells with the glycosylation inhibitor tunicamycin rendered these cells susceptible to infection by retroviral vectors with ecotropic, xenotropic, and amphotropic host ranges and increased the titer of gibbon ape leukemia virus pseudotyped vectors 10-fold. Vectors having a polytropic host range did not infect CHO cells in the presence or absence of tunicamycin, showing that the effect of tunicamycin was specific and related to the pseudotype of the vector. We present evidence for three mechanisms of resistance to infection: lack of viral receptors on CHO cells, the presence of nonfunctional receptors which can be made functional by treatment with tunicamycin, and the secretion of a protein factor that blocks retroviral infection of CHO cells. Several criteria indicate that the secreted inhibitor is not an interferon, and secretion of this factor was not detected in several other cell lines that were examined.

Evaluation of lymphoid-specific enhancer addition or substitution in a basic retrovirus vector

Two novel retroviral vectors bearing lymphoid-specific enhancers were tested for improved expression of human adenosine deaminase (hADA) in tissue culture cells and in mouse bone marrow transplant recipients. These vectors carried either an added human T-cell receptor alpha-chain enhancer (delta N2TADA) or a substitution of the Moloney long terminal repeat (LTR) enhancer with the murine immunoglobulin mu heavy-chain first intron enhancer (delta N2 mu ADA). Each vector was produced at a titer of approximately 10(6) infectious units/ml and efficiently transduced hADA into murine fibroblast and myeloma cells in culture. No quantitative difference in expression was observed between the enhancer modified vectors and the basic retrovirus vector (delta N2ADA). In addition, each vector efficiently conferred hADA expression in lymphoid, myeloid, and erythroid cells of long-term transplanted mice. The majority of the transduced-marrow recipients demonstrated expression of the human enzyme for 4-8 months with each of the three vectors.

Viral and cellular factors governing hamster cell infection by murine and gibbon ape leukemia viruses

Hamster cells are resistant to infection by most retroviruses, including Moloney murine leukemia virus (MoMLV) and gibbon ape leukemia viruses (GaLVs). We have constructed MoMLV-GaLV hybrid virions to identify viral and cellular determinants responsible for the inability of GaLV and MoMLV to infect hamster cells. The substitution of MoMLV core components for GaLV core components circumvents the resistance of hamster cells to infection by GaLV, demonstrating that hamster cells have receptors for GaLV but are not efficiently infected by this primate retrovirus because of a postpenetration block. In contrast, hamster cells are apparently resistant to MoMLV infection because although they bear a receptor for MoMLV, the receptor is nonfunctional. Treatment of CHO K1 or BHK 21 hamster cells with the glycosylation inhibitor tunicamycin allows the cells to be infected by MoMLV. The construction of MoMLV-GaLV hybrid virions that can efficiently infect resistant cells has allowed the identification of viral and cellular factors responsible for restricting infection of hamster cells by MoMLV and GaLV.

Analysis of mutations in the envelope gene of Moloney murine leukemia virus: separation of infectivity from superinfection resistance

Six deletion mutations and an insertion were generated in the env gene of cloned copies of Moloney murine leukemia virus DNA. All seven mutants were replication-defective as tested by transformation of NIH/3T3 cells. The mutant DNAs were introduced into NIH/3T3 cells to generate stable producer lines; all released virion particles into the medium, suggesting that none of the mutations affected overall viral gene expression, gag and pol gene expression, gag and pol gene functions, or virion budding. Several of the mutations reduced the lifetime of the env protein or blocked its export to the cell surface. One mutation altering the membrane-spanning region and the cytoplasmic tail of the TM protein had no effect on export of the protein, proteolytic processing, or incorporation into virion particles, but still blocked the infectivity of the resulting virus. The results suggest that alterations in the transmembrane region can affect early steps of infection, such as the fusion of virion and host membranes. Cells expressing this mutant env protein were fully resistant to superinfection by wild-type virus. Thus, induction of virus resistance, presumably reflecting blocking the virus receptor, can be separated from virus infectivity.

N-linked glycosylation and reticuloendotheliosis retrovirus envelope glycoprotein function

Different properties of the spleen necrosis virus (SNV) envelope glycoprotein were analyzed following biosynthesis in the presence of glycosylation inhibitors. Tunicamycin, which inhibits all asparagine N-linked glycosylation, prevented intracellular processing and translocation to the cell surface of the envelope protein. In contrast, castanospermine or deoxymannojirimycin, which block glycosidase trimming of the early high-mannose chains and subsequent complex type N-glycosylation, did not inhibit proteolytic cleavage or cellular translocation. The ability of unglycosylated and partially glycosylated envelope protein to bind the viral receptor was assayed using an infection interference assay. Tunicamycin abrogated SNV envelope glycoprotein-induced receptor interference, whereas the trimming glycosidase inhibitors had no effect on interference. Similarly, tunicamycin but not the glycosidase inhibitors reduced the titers of released virus 100-fold. We conclude that carbohydrate trimming and complex N-glycosylation are not essential for envelope glycoprotein translocation, proteolytic cleavage, receptor binding, or infectivity, whereas cotranslational high-mannose N-glycosylation is essential for all of the SNV envelope glycoprotein properties tested. Syncytia formation can be induced following transfection into D17 cells of an envelope glycoprotein expression plasmid. Unlike virus particle infectivity, cell fusion is strongly inhibited by the glycosidase inhibitors.

v-rasH induces non-small cell phenotype, with associated growth factors and receptors, in a small cell lung cancer cell line

Small cell lung cancer (SCLC) tumor progression can involve partial or complete conversion to a more treatment-resistant non-small cell (NSCLC) phenotype. In a cell culture model of this phenomenon, we have previously demonstrated that insertion of the viral Harvey ras gene (v-Ha-ras) into SCLC cell lines with amplification and overexpression of the c-myc gene induced many NSCLC phenotypic features. We now report that the v-Ha-ras gene can also induce morphologic, biochemical, and growth characteristics consistent with the NSCLC phenotype in an N-myc amplified SCLC cell line, NCI-H249. We show that v-Ha-ras has novel effects on these cells, abrogating an SCLC-specific growth requirement for gastrin-releasing peptide, and inducing mRNA expression of three NSCLC-associated growth factors and receptors, platelet-derived growth factor B chain, transforming growth factor-alpha (TGF-alpha), and epidermal growth factor receptor (EGF-R). TGF-alpha secretion and EGF-R also appear, consistent with the induction of an autocrine loop previously shown to be growth stimulatory for NSCLC in culture. These data suggest that N-myc and v-Ha-ras represent functional classes of genes that may complement each other in bringing about the phenotypic alterations seen during SCLC tumor progression, and suggest that such alterations might include the appearance of growth factors and receptors of potential importance for the growth of the tumor and its surrounding stroma.

Assembly of gag-beta-galactosidase proteins into retrovirus particles

We studied the expression of beta-galactosidase (beta-gal) and 15 gag-beta-gal fusion proteins in the presence of Moloney murine leukemia virus wild-type core (gag) proteins. Analysis indicated that proteins retaining the amino-terminal portion of gag through the capsid protein-coding region were incorporated into retrovirus particles. Proteins which deleted portions of the capsid protein were assembled into virions at low efficiency, indicating the importance of capsid protein interactions in retrovirus assembly. Fusion proteins which retained the amino-terminal matrix protein of the gag polyprotein but which lacked the capsid protein were released efficiently from cells in a nonviral form. The nonviral form was characterized by a high sedimentation coefficient and a low density, suggestive of membrane vesicles. While beta-gal was present in the cytoplasm of expressing cells, all fusion constructs were associated with cellular membranes. gag-beta-gal proteins which were capable of release from cells demonstrated a two-component immunofluorescence staining pattern consisting of a circle of fluorescence around the nucleus and a punctate pattern of staining throughout the remainder of the cell. Interestingly, fusions within the matrix protein were trapped intracellularly and yielded distinct perinuclear staining patterns, possibly localizing to the rough endoplasmic reticulum and/or Golgi. This observation suggests that Moloney murine leukemia virus gag proteins travel to the plasma membrane by vesicular transport associated with the cytoplasmic face of intracellular vesicles.

Friend spleen focus-forming virus induces factor independence in an erythropoietin-dependent erythroleukemia cell line

Erythroid cells from mice infected with the polycythemia-inducing strain of Friend spleen focus-forming virus (SFFVP), unlike normal erythroid cells, can proliferate and differentiate in apparent absence of the erythroid hormone erythropoietin (Epo). The unique envelope glycoprotein encoded by SFFV has been shown to be responsible for this biological effect. The recent isolation of an Epo-dependent erythroleukemia cell line, HCD-57, derived from a mouse infected at birth with Friend murine leukemia virus, afforded us the opportunity to study the direct effect of SFFVP on a homogeneous population of factor-dependent cells. The introduction of SFFVP in complex with various helper viruses into these Epo-dependent cells efficiently and reproducibly gave rise to lines which expressed high levels of SFFV and were factor independent. SFFV appears to be unique in its ability to abrogate the factor dependence of Epo-dependent HCD-57 cells, since infection of these cells with retroviruses carrying a variety of different oncogenes had no effect. The induction of Epo independence by SFFV does not appear to involve a classical autocrine mechanism, since there is no evidence that the factor-independent cells synthesize or secrete Epo or depend on it for their growth. However, the SFFV-infected, factor-independent cells had significantly fewer receptors available for binding Epo than their factor-dependent counterparts had, raising the possibility that the induction of factor independence by the virus may be due to the interaction of an SFFV-encoded protein with the Epo receptor.

Insertion of the v-Ha-ras oncogene induces differentiation of calcitonin-producing human small cell lung cancer

Human small cell lung cancers (SCLC) and cell lines derived therefrom are phenotypically heterogeneous concerning neuroendocrine differentiation. Unlike most SCLC tumors and cell lines that express poorly differentiated neuroendocrine phenotypes, the SCLC cell line DMS 53 exhibits mature endocrine differentiation features, including unusually high expression of the gene for the peptide hormone, calcitonin (CT). We now report that introduction of the viral Harvey ras (v-rasH) oncogene into DMS 53 cells via retroviral infection, with resultant constitutive expression, results in increased features of neuroendocrine differentiation. 7-10 d after infection the cells demonstrated altered morphology, increased CT secretion, increased CT gene expression, markedly diminished cellular proliferation, and nearly abolished methylcellulose cloning efficiency. This response of DMS 53 cells to v-rasH is unlike the tumor progression effects we have previously observed in other SCLC lines. Significantly, the differentiation response that follows expression of the virally introduced v-rasH oncogene in DMS 53 cells is similar to that of neoplastic neuroendocrine cell lines derived from adrenal pheochromocytes and thyroid C cells. The effects of constitutive v-rasH expression in DMS 53 SCLC cells and other neuroendocrine cell lines suggest an important role for rasH or related genes in neuroendocrine differentiation.

Fibroblasts transfected with Torpedo acetylcholine receptor beta-, gamma-, and delta-subunit cDNAs express functional receptors when infected with a retroviral alpha recombinant

Torpedo californica acetylcholine receptor (AChR) alpha-, beta-, gamma-, and delta-subunit cDNAs were each stably introduced into muscle and/or fibroblast cell lines using recombinant retroviral vectors and viral infection, or using SV-40 vectors and DNA-mediated cotransfection. The expressed proteins were characterized in terms of their molecular mass, antigenicity, posttranslational processing, cell surface expression, stability in fibroblasts, stability in differentiated and undifferentiated muscle cells, and ability (of alpha) to bind alpha-bungarotoxin (BuTx). We demonstrated that the alpha, beta, gamma, and delta polypeptides acquired one, one, two, and three units of oligosaccharide, respectively. If all four subunits were expressed in the same cell, fully functional cell surface AChRs were produced which had a Kd for BuTx of 7.8 X 10(-11) M. In contrast, subunits expressed individually were not detected on the surface of fibroblasts and the Kd for BuTx binding to individual alpha polypeptides was only approximately 4 X 10(-7) M. The half-lives of the alpha, gamma, and delta subunits at 37 degrees C were all found to be quite short (approximately 43 min), while the half-life of the beta subunit was found to be even shorter (approximately 12 min). The unique half-life of the beta subunit suggests that it might perform a key regulatory role in the process of AChR subunit assembly. One stable fibroblast cell line was established by transfection that expressed beta, gamma, and delta subunits simultaneously. When this cell line was infected with a retroviral alpha recombinant, fully functional cell surface AChRs were produced. The successful expression of this pentameric protein complex combining transfection and infection techniques demonstrates one strategy for stably introducing the genes of a heterologous multisubunit protein complex into cells.

Role of reticuloendotheliosis virus envelope glycoprotein in superinfection interference

Cells expressing specific proviruses are resistant to superinfection by viruses of the same subgroup. To investigate the role of the reticuloendotheliosis virus (REV) envelope glycoprotein (env-gp) in the establishment of resistance to superinfection, we constructed plasmids that express either the wild-type env-gp or an env-gp derivative that lacks part of the transmembrane (TM) protein. After transfection, transient expression of the wild-type env gene resulted in syncytium formation in a mammalian cell line permissive for virus replication, whereas synthesis of the TM-defective env-gp did not result in syncytium formation. Several stable cell lines expressing either the normal or TM-defective env-gp were isolated. Expression of the normal env-gp in the absence of expression of other viral genes induced resistance to infection by REV. Immunofluorescence analysis of cells expressing the TM-defective env derivative and an examination of the glycosylation pattern of this peptide indicated that it is not translocated to the cell surface but resides primarily in the rough endoplasmic reticulum. However, these cells were also resistant to REV infection. Thus, interaction between the env derivative and the cellular component that functions as a receptor for the virus can occur in the endoplasmic reticulum and renders the cell immune to superinfection.

Changes in in vitro growth behaviour of the mammary epithelial cell line NMuMG caused by the v-fos oncogene

A defective retrovirus was constructed to investigate the effect of the expression of the v-fos oncogene from FBJ-MSV on the in vitro growth properties of the mammary epithelial cell line NMuMG. Clearly visible areas of overgrowth in monolayer cultures of NMuMG were seen in cells infected with the v-fos-containing retrovirus but not in cells infected with control virus which did not contain an oncogene. Two cell lines, representing two morphological types of infected cell, were isolated from a morphologically altered region and further characterized. Fos.3.1.NMuMG grew as very spindly cells, achieving a higher density than control cells in 5% foetal calf serum (FCS) but growing very poorly in 1% FCS or in soft agar. Fos.3.3.NMuMG grew to a high density in 5% FCS and to a limited extent in low serum. This cell line also grew in soft agar. Fos.3.3.NMuMG seemed to be more transformed than fos.3.1.NMuMG using the criteria of growth in soft agar and low serum. All the cells used in this study were shown to retain epithelial characteristics by staining for cytokeratins and to contain at least one viral genome by Southern blotting. fos mRNA expression was raised over control levels in the two transformed cell lines.

v-Ha-ras oncogene insertion: a model for tumor progression of human small cell lung cancer

Small cell lung cancer (SCLC) manifests a range of phenotypes in culture that may be important in understanding its relationship to non-SCLCs and to tumor progression events in patients. Most SCLC-derived cell lines, termed "classic" SCLC lines, have properties similar to SCLC tumors in patients, including high expression of neuroendocrine markers and low c-myc oncogene expression. A significant number of SCLC lines characterized as "biochemical or morphologic variant" SCLC lines have decreased levels of endocrine differentiation markers associated with increased proliferative indices, amplification of the c-myc oncogene, and growth patterns and biochemical markers more typical of non-SCLCs. To delineate further the relationships between these phenotypes and the molecular events involved, we have inserted the v-Ha-ras gene in SCLC cell lines with (biochemical variant) and without (classic) an amplified c-myc gene. These two SCLC subtypes had markedly different phenotypic responses to similar levels of expression of v-Ha-ras RNA. No biochemical or morphologic changes were observed in classic SCLC cells. In contrast, in biochemical variant SCLC cells, v-Ha-ras expression induced features typical of large cell undifferentiated lung carcinoma, including adherent monolayer growth patterns, increased cloning efficiency, increased levels of non-SCLC cell markers, ultrastructural characteristics and an acquired resistance to polyamine depletion typical of large cell carcinoma, but not SCLC, in vitro. Expression of v-Ha-ras in biochemical variant SCLC cells directly demonstrates that important transitions can occur between phenotypes of human lung cancer cells and that these may play a critical role in tumor progression events in patients. The findings provide a model system to study molecular events involved in tumor progression steps within a series of related tumor types.

Comparison of expression in hemopoietic cells by retroviral vectors carrying two genes

In order to identify factors that influence expression by retroviral vectors in hemopoietic cells, we have compared viral RNA levels in cells infected with several different recombinant viruses. All of the vectors tested carry the neomycin resistance gene and provide for the insertion of a second gene which, in these studies, comprised sequences from the myc or myb oncogenes or the gene encoding granulocyte-macrophage colony-stimulating factor. The vectors utilize two different strategies for the coexpression of the two genes: alternate splicing and the use of a separate internal promoter. We found that expression in hemopoietic cells could be increased by substituting sequences from the myeloproliferative sarcoma virus long terminal repeat for those of the Moloney murine leukemia virus long terminal repeat. However, none of the vectors examined was able to express a second gene at levels equivalent to those achieved by the parental vectors carrying only the neomycin resistance gene. The reasons for this varied with the different vectors and included inefficient splicing and/or a reduction in the level of unspliced transcripts upon insertion of a second gene. Although the basis of the latter phenomenon is not clear, it is probably related to the position--near the 5' long terminal repeat--at which the second gene was inserted, since insertion of the same genes near the 3' end of another vector had no effect on viral RNA levels. In an attempt to circumvent some of these problems, we constructed a vector that employs an internal beta-actin promoter. Although this vector could express granulocyte-macrophage colony-stimulating factor sequences in a responsive hemopoietic cell line, the level of granulocyte-macrophage colony-stimulating factor produced was disappointingly low. The results from these studies suggest approaches to the design of improved vectors for effective expression of genes in hemopoietic cells.

A retrovirus expressing the 12S adenoviral E1A gene product can immortalize epithelial cells from a broad range of rat tissues

An epithelial cell-transforming virus could be of great use, both in the culture of epithelial cell lines and in the study of carcinogenesis. Since the adenoviral E1A gene has been shown to partially transform some epithelial cells from primary rat cell cultures, we constructed retrovirus vectors containing either the 12S or 13S E1A cDNA sequences to facilitate the transfer of these genes into a variety of primary cell types. The 12S E1A virus induced proliferation and immortalization of epithelial cells in rat kidney, liver, heart, pancreas, and thyroid primary cultures. In the two cases tested, heart and liver cultures, E1A-immortalized cells were nontumorigenic, but could be completely transformed by subsequent introduction of the ras oncogene. To our surprise, the 13S virus had a greatly reduced immortalization potential. We discuss these data in light of the model of Spindler et al. (K. R. Spindler, C. Y. Eng, and A.-J. Berk, J. Virol. 53:742-750, 1985), in which the 12S E1A protein is required for the complete induction of the cellular DNA replication machinery in the quiescent human epithelial cells in which adenoviruses normally replicate.

A retrovirus expressing the 12S adenoviral E1A gene product can immortalize epithelial cells from a broad range of rat tissues

An epithelial cell-transforming virus could be of great use, both in the culture of epithelial cell lines and in the study of carcinogenesis. Since the adenoviral E1A gene has been shown to partially transform some epithelial cells from primary rat cell cultures, we constructed retrovirus vectors containing either the 12S or 13S E1A cDNA sequences to facilitate the transfer of these genes into a variety of primary cell types. The 12S E1A virus induced proliferation and immortalization of epithelial cells in rat kidney, liver, heart, pancreas, and thyroid primary cultures. In the two cases tested, heart and liver cultures, E1A-immortalized cells were nontumorigenic, but could be completely transformed by subsequent introduction of the ras oncogene. To our surprise, the 13S virus had a greatly reduced immortalization potential. We discuss these data in light of the model of Spindler et al. (K. R. Spindler, C. Y. Eng, and A.-J. Berk, J. Virol. 53:742-750, 1985), in which the 12S E1A protein is required for the complete induction of the cellular DNA replication machinery in the quiescent human epithelial cells in which adenoviruses normally replicate.

Introduction of v-Ha-ras oncogene induces differentiation of cultured human medullary thyroid carcinoma cells

Medullary thyroid carcinoma (MTC) is an endocrine tumor of the thyroid C cells that expresses high levels of the neuroendocrine peptide hormone calcitonin. During tumor progression in the host, there is an apparent loss of differentiation in MTC cells that involves a consistent decrease in calcitonin content of the tumor cells associated with decreased expression of the calcitonin gene and/or changes in a mRNA alternative-processing pattern away from that characteristic of the parent thyroid C cell. We now report that introduction of the viral Harvey ras (v-Ha-ras) oncogene into cultured human MTC cells can reverse such changes in gene expression and can induce endocrine differentiation of the tumor cells. The expression of v-Ha-ras is associated with decreased cellular proliferation and DNA synthesis. There is a marked increase in the number of cytoplasmic secretory granules that are a classic feature of differentiated thyroid C cells. v-Ha-ras expression induces increased expression of the calcitonin gene and the processing of the primary gene transcript is shifted to favor calcitonin mRNA rather than calcitonin-gene-related peptide (CGRP) mRNA production. These studies with cultured human MTC cells provide a model system to study the role of Ha-ras and related genes in neuroendocrine differentiation. The findings suggest an important approach for identifying genes in solid tumors whose altered expression may play a role in the impaired maturational capacity characteristic of cancer cells during tumor progression.

Expression of the Saccharomyces cerevisiae glycoprotein invertase in mouse fibroblasts: glycosylation, secretion, and enzymatic activity

Oligosaccharide processing is controlled by host- and protein-dependent factors. To increase our understanding of the relative contribution of those factors we studied the glycosylation of yeast invertase expressed in a heterologous system. Invertase synthesized in psi-2 cells (an NIH 3T3-derived packaging line) is secreted efficiently, enzymatically active, and heavily glycosylated. It was estimated that the protein contains 8 or 9 carbohydrate chains. Two classes can be observed, of an approximate size of 100-110 kDa and 115-130 kDa, respectively. The size differences are due to differences in glycosylation. The smaller class contains two high-mannose carbohydrate chains; the remainder is of the complex type, sialylated and most likely tri- or tetraantennary. This profile parallels the situation observed with invertase glycosylation in yeast, where 2 of 9 or 10 chains remain unprocessed. The larger size class of invertase expressed in mouse fibroblasts has a different profile, since it contains probably only complex-type glycans. There are no apparent differences, however, in the size of the protein backbone between the two size classes. When invertase is synthesized in the presence of the mannosidase inhibitor 1-deoxymannojirimycin, processing is blocked completely, since all glycans are susceptible to endo-beta-N-acetylglucosaminidase H. The glucosidase inhibitor 1-deoxynojirimycin does not inhibit processing completely. In both cases secretion of the protein is not affected. The glycosylation inhibitor tunicamycin prevents secretion of invertase completely when cells are cultured at 37 degrees C. At 26 degrees C, however, nonglycosylated invertase can be detected in the medium. These data suggest that glycosylation of invertase seems to be essential for the early steps of the secretory pathway but is less critical for later events.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Infection of immune mast cells by Harvey sarcoma virus: immortalization without loss of requirement for interleukin-3

Cells from adult mouse spleens were cultured in WEHI-3 cell-conditioned medium, which contains the lymphokine interleukin-3 (IL-3). Under these conditions, cells grow well for 4 to 8 weeks; the cultures contain a variety of cell types for the first 1 to 2 weeks but are subsequently composed largely of immune mast cells. We found that infection of these cultures with Harvey sarcoma virus (HaSV) profoundly enhanced the growth potential of the cells, resulting in the reproducible isolation of long-term cell lines. These HaSV-infected cells appeared to be phenotypically identical to the immune mast cells found in uninfected cultures as determined by biochemical, immunological, and cytological tests. Although the cells expressed protein p21Ha-ras at levels similar to those in HaSV-transformed fibroblasts, they continued to require IL-3 for growth in vitro. Similar IL-3-dependent, long-term mast cell lines were also cultured from the enlarged spleens present in HaSV-infected mice. These results suggest that high-level expression of an activated Ha-ras oncogene enhances growth in these cells, perhaps by stimulating the progression of the cells into S, without affecting differentiation or altering the requirements for normal growth factor.

Relationship between the cellular resistance to Friend murine leukemia virus infection and the expression of murine leukemia virus-gp70-related glycoprotein on cell surface of BALB/c-Fv-4wr mice

Fv-4r is a dominant resistance gene which controls resistance of mice to exogenous infection with ecotropic murine leukemia viruses. Cell lines with various degrees of resistance to ecotropic Friend murine leukemia virus infection were established from BALB/c-Fv-4wr mice which are partially congenic with BALB/c. The degree of resistance of these cell lines correlated well to the amount of glycoprotein with mol wt 80 K on cell surface. The resistance of cells was reduced by the treatment of glycosylation inhibitors, tunicamycin and 2-deoxy-D-glucose. The results indicate that the Fv-4 resistance is ascribed to the unique glycoprotein on cell surface.

Factors involved in production of helper virus-free retrovirus vectors

Retrovirus vectors allow efficient transfer of genetic material into cells. We describe an improved method for making cell lines which secrete broad host range retrovirus vectors in the absence of helper virus. This method was used to make virus-producing cell lines from several retrovirus vector constructions that encode dominant selectable markers. Virus titers from such lines exceeded 10(6) colony-forming units per milliliter of medium exposed to the cells. Cell lines that secreted certain vectors remained free of helper virus, while cell lines made using other vectors always secreted helper virus. Secretion of helper virus apparently depended on recombination between vector and the retrovirus packaging system, and factors involved in this event were investigated.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Susceptibility of wild mouse cells to exogenous infection with xenotropic leukemia viruses: control by a single dominant locus on chromosome 1

Although xenotropic murine leukemia viruses cannot productively infect cells of laboratory mice, cells from various wild-derived mice can support replication of these viruses. Although the virus-sensitive wild mice generally lack all or most of the xenotropic proviral genes characteristic of inbred strains, susceptibility to exogenous infection is unrelated to inheritance of these sequences. Instead, susceptibility is controlled by a single dominant gene, designated Sxv, which maps to chromosome 1. Sxv is closely linked to, but distinct from Bxv-1, the major locus for induction of xenotropic murine leukemia viruses in laboratory mice. Genetic experiments designed to characterize Sxv show that this gene also controls sensitivity to a wild mouse virus with the interference properties of mink cell focus-forming murine leukemia viruses, and that Sxv-mediated susceptibility to xenotropic murine leukemia viruses is restricted by the mink cell focus-forming virus resistance gene Rmcf. These data, together with genetic mapping of the mink cell focus-forming virus cell surface receptor locus to this same region of chromosome 1, suggest that Sxv may encode a wild mouse variant of the mink cell focus-forming virus receptor that allows penetration by xenotropic murine leukemia viruses.

Infection of immune mast cells by Harvey sarcoma virus: immortalization without loss of requirement for interleukin-3

Cells from adult mouse spleens were cultured in WEHI-3 cell-conditioned medium, which contains the lymphokine interleukin-3 (IL-3). Under these conditions, cells grow well for 4 to 8 weeks; the cultures contain a variety of cell types for the first 1 to 2 weeks but are subsequently composed largely of immune mast cells. We found that infection of these cultures with Harvey sarcoma virus (HaSV) profoundly enhanced the growth potential of the cells, resulting in the reproducible isolation of long-term cell lines. These HaSV-infected cells appeared to be phenotypically identical to the immune mast cells found in uninfected cultures as determined by biochemical, immunological, and cytological tests. Although the cells expressed protein p21Ha-ras at levels similar to those in HaSV-transformed fibroblasts, they continued to require IL-3 for growth in vitro. Similar IL-3-dependent, long-term mast cell lines were also cultured from the enlarged spleens present in HaSV-infected mice. These results suggest that high-level expression of an activated Ha-ras oncogene enhances growth in these cells, perhaps by stimulating the progression of the cells into S, without affecting differentiation or altering the requirements for normal growth factor.

Different recombinant murine leukemia viruses use different cell surface receptors

Retroviruses can be grouped by viral interference measurements into classes which use common cell surface receptors. We previously tested a large number of isolates of mink cell focus-inducing (MCF) murine leukemia viruses (MuLVs), and reported that all of them share a distinct receptor on NIH/3T3 cells (A. Rein, Virology 120, 251, 1982). We now extend this generalization to several additional recombinant isolates, including two (SL3-2 and GPA-V2) which would not be considered MCFs on the basis of host-range data. We note the superiority of interference tests, based on positive, unambiguous data, over host-range tests for virus classification. We also show that in contrast to the MCFs, which are all derived from ecotropic MuLVs, a recombinant derived from wild mouse amphotropic MuLV (S. Rasheed et al., Int. J. Cancer 29, 345, 1982) uses a unique receptor on NIH/3T3 cells. This suggests that (a) mouse cells contain more than one type of endogenous env sequence; and (b) there is some specificity in the generation of recombinants, since ecotropic MuLVs appear to give rise only to MCFs, while amphotropic MuLV has generated a distinct type of recombinant. It also represents a second case (in addition to the MCFs) in which an env gene recombinant is more pathogenic than its exogenous parent. We also show that xenotropic MuLV does not interfere with MCFs in NZB mouse cells; thus, despite the close homology between MCF and xenotropic env sequences, the gp70 of xenotropic MuLV appears to have no detectable affinity for the MCF receptor.

Liposome encapsulation of retrovirus allows efficient superinfection of resistant cell lines

Cell lines which are infected with retrovirus are resistant to superinfection by a related retrovirus. Packaging of whole virions within synthetic lipid vesicles allows efficient infection of such resistant cell lines. This system is more efficient in introducing encapsulated virus into infected cells than into uninfected cells.