In vitro transformation of chicken bone marrow cells with avian erythroblastosis virus

Proteomic characterization of HIV-modulated membrane receptors, kinases and signaling proteins involved in novel angiogenic pathways

Background

Kaposi's sarcoma (KS), hemangioma, and other angioproliferative diseases are highly prevalent in HIV-infected individuals. While KS is etiologically linked to the human herpesvirus-8 (HHV8) infection, HIV-patients without HHV-8 and those infected with unrelated viruses also develop angiopathies. Further, HIV-Tat can activate protein-tyrosine-kinase (PTK-activity) of the vascular endothelial growth factor receptor involved in stimulating angiogenic processes. However, Tat by itself or HHV8-genes alone cannot induce angiogenesis in vivo unless specific proteins/enzymes are produced synchronously by different cell-types. We therefore tested a hypothesis that chronic HIV-replication in non-endothelial cells may produce novel factors that provoke angiogenic pathways.

Methods

Genome-wide proteins from HIV-infected and uninfected T-lymphocytes were tested by subtractive proteomics analyses at various stages of virus and cell growth in vitro over a period of two years. Several thousand differentially regulated proteins were identified by mass spectrometry (MS) and >200 proteins were confirmed in multiple gels. Each protein was scrutinized extensively by protein-interaction-pathways, bioinformatics, and statistical analyses.

Results

By functional categorization, 31 proteins were identified to be associated with various signaling events involved in angiogenesis. 88% proteins were located in the plasma membrane or extracellular matrix and >90% were found to be essential for regeneration, neovascularization and angiogenic processes during embryonic development.

Conclusion

Chronic HIV-infection of T-cells produces membrane receptor-PTKs, serine-threonine kinases, growth factors, adhesion molecules and many diffusible signaling proteins that have not been previously reported in HIV-infected cells. Each protein has been associated with endothelial cell-growth, morphogenesis, sprouting, microvessel-formation and other biological processes involved in angiogenesis (p = 10^-4 to 10^-12). Bioinformatics analyses suggest that overproduction of PTKs and other kinases in HIV-infected cells has suppressed VEGF/VEGFR-PTK expression and promoted VEGFR-independent pathways. This unique mechanism is similar to that observed in neovascularization and angiogenesis during embryogenesis. Validation of clinically relevant proteins by gene-silencing and translational studies in vivo would identify specific targets that can be used for early diagnosis of angiogenic disorders and future development of inhibitors of angiopathies. This is the first comprehensive study to demonstrate that HIV-infection alone, without any co-infection or treatment, can induce numerous "embryonic" proteins and kinases capable of generating novel VEGF-independent angiogenic pathways.

Transmembrane domain of the AEV erb B oncogene protein is not required for partial manifestation of the transformed phenotype

The transmembrane domain was deleted from within the v-erb B protein coding region of avian erythroblastosis virus. The mutant oncogene encoded a shortened, apparently soluble form of the normally membrane bound v-erb B protein. Despite this alteration in subcellular distribution, the mutant polypeptide retained the ability to induce fibroblast transformation by several parameters, including the ability to display anchorage-independent growth. It appears that the transmembrane domain, although important for full manifestation of the transformed phenotype, is not essential for v-erb B-mediated oncogenic transformation.

Creation of a chimeric oncogene: analysis of the biochemical and biological properties of v-erbB/src fusion polypeptide

A novel gene was created that linked complementary portions of two different tyrosine kinase oncogenes: v-erB and v-src. The v-erbB/src chimera encoded a glycoprotein exhibiting the subcellular distribution of the v-erbB protein but containing the kinase catalytic domain of the v-src parent. Fibroblasts expressing the v-erbB/src gene product became transformed to an oncogenic state and closely resembled cells expressing the v-erbB parent oncogene. Our results indicated that v-erbB sequences can be functionally replaced by sequences derived from a different oncogene, v-src, and that important determinants of the transformed phenotype appear to be encoded in oncogene sequences distinct from those defining the kinase catalytic domain itself.

Mutagenesis of the avian erythroblastosis virus erbB coding region: an intact extracellular domain is not required for oncogenic transformation

Avian erythroblastosis virus (AEV) is an oncogenic retrovirus of birds. The AEV-encoded erbB polypeptide, a transmembrane glycoprotein bearing an N-terminal domain exposed on the surface of virally transformed cells, plays a crucial role in AEV-mediated oncogenesis. We report here a characterization of a mutated form of the AEV erbB protein which lacks over two-thirds of the extracellular region of this oncogenic protein. This mutant v-erbB protein, although lacking the three possible extracellular sites of N-linked protein glycosylation, appears unimpaired in the ability to transform cells to an oncogenic phenotype.

Transformation of erythroid cells by Rous sarcoma virus (RSV)

RSV transforms several nonhematopoietic cell types and as reported here also has the capacity to transform hematopoietic cells of the erythroid lineage. In vitro, the three RSV isolates tested induced erythroblast-like colonies in infected bone marrow cells that were distinguishable by size and cell arrangement from those induced by avian erythroblastosis virus (AEV). Also in contrast to AEV-transformed erythroblast cultures, isolated cell colonies induced by RSV required complex growth conditions in liquid medium similar to the in vitro conditions necessary for erythroblasts transformed by the acute leukemia virus E26. Temperature-shift experiments using temperature-sensitive (ts) NY68 RSV revealed that when grown at the nonpermissive temperature (42 degrees), mutant-infected cells became benzidine positive and partially differentiated into erythrocytes. Wild-type (wt) RSV-transformed cells did not undergo similar changes. However, both wt RSV-, and to a greater extent, ts RSV-transformed cultures at the permissive temperature (37 degrees) did contain populations of spontaneously differentiating erythroid cells signifying that the transforming activity of the virus did not fully arrest erythroid maturation. In addition, the RSV-transformed cells did express tyrosine kinase activity. When injected intravenously into birds, RSV induced an erythroblastosis-like disease similar to AEV but also caused fibrosarcomas and leg paralysis. These results show that RSV can alter the pattern of erythroid differentiation in a manner similar to, but distinct from, AEV and indicate that the tyrosine-specific pp60src kinase is involved in erythroid cell transformation. Since the src and erb B proteins share a significant amino acid homology, these data suggest that both may also share a common functional homology.

src- and fps-containing avian sarcoma viruses transform chicken erythroid cells

We report here that several oncogene-transducing avian sarcoma virus strains, namely Rous sarcoma virus (src), Fujinami sarcoma virus (fps), and PRCII (fps), transform avian erythroid cells in vitro and in vivo. The src- and fps-transformed erythroblasts grow in vitro for 20-30 generations, require special growth conditions, and tend to differentiate spontaneously. In these properties, they resemble erythroid cells transformed with the erbB-containing H strain of avian erythroblastosis virus (AEV-H) but differ from those transformed with AEV-ES4 (erbA, erbB), which grow under standard culture conditions and rarely differentiate spontaneously. Erythroblasts transformed with viruses carrying temperature-sensitive mutations in the src or fps oncogene and then shifted to the nonpermissive temperature in the presence of anemic serum (as a source of an erythropoietin-like factor) differentiate terminally into erythrocytes. These results demonstrate that several members of the src gene family other than erbB have the capacity to transform erythroid cells.

Identification and characterization of the avian erythroblastosis virus erbB gene product as a membrane glycoprotein

Avian erythroblastosis virus causes erythroid leukemia and sarcomas in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions known as erbA and erbB, and recent evidence suggests that it is the erbB gene that is responsible for the transforming activity. From rats bearing avian erythroblastosis virus-induced sarcomas, we have obtained antisera which are specific for the erb gene products. Using such antisera, we have been able to characterize the erbB gene product as a 68,000 molecular weight protein. Pulse-chase and cell-free in vitro translation experiments show that the initial product is a 62,500 dalton protein which is initially modified to a 66,000 dalton protein, and then further modified to a 68,000 dalton form. These modifications could be shown to be associated with glycosylation and phosphorylation. Cell fractionation experiments revealed that the 66,000 and 68,000 dalton proteins were located in cell membrane fractions, and immunofluorescence results showed the erbB gene product to be expressed on the cell surface.

Nuclear location of the putative transforming protein of avian myelocytomatosis virus

The putative transforming protein of avian myelocytomatosis virus MC29 is a 110,000 dalton (P110gag-myc) polyprotein comprised of sequences derived from both the gag region and the MC29-specific myc region. Two approaches have been taken to determine the location of the MC29 gag-related proteins in transformed cells: subcellular fractionation and immunofluorescence. Analysis of subcellular fractions of MC29-transformed cells by immunoprecipitation indicates that the majority of the gag-myc polyprotein is found in the nuclear fractions of Q8 cells (a nonproducer line of MC29-transformed quail embryo fibroblasts) and nonproducer cells derived from a liver tumor of MC20-infected quail. This is in contrast to the distribution of gag-related helper virus proteins lacking myc, which are found only in nonnuclear fractions of superinfected Q8 cells. The purity of unlabeled nuclei was assessed by electron microscopy and enzyme assays, revealing little contaminating material from other subcellular fractions. Immunofluorescence experiments using monospecific anti-gag serum showed specific, intense immunofluorescence in the nuclei of fixed Q8 cells. In contrast, the majority of P75gag-erb, a candidate transforming protein produced by avian erythroblastosis virus (AEV), is absent from the nuclei of nonproducer AEV-transformed chick embryo fibroblasts. The nuclear association of the MC29 transforming protein may be related to some of the unique properties of MC29-transformed cells.

In vitro translation of avian myeloblastosis virus RNA

Avian myeloblastosis virus (AMV)-infected cells contain two viral mRNA's, a genome-sized 34S (7.5-kilobase) mRNA and a 21S (2.5-kilobase) subgenomic mRNA, which contains the AMV-specific sequences (myb sequences). We found that AMV virions packaged both the 7.5-kilobase full-length genomic RNA and the 2.5-kilobase subgenomic RNA. In vitro translation of AMV virion RNA sized by sucrose density gradient centrifugation yielded 76,000-, 56,000-, 48,500-, 47,000-, and 32,000-dalton products. The 76,000-dalton protein was coded for by RNA throughout the gradient, but the peak of activity was at 34S to 35S. The 56,000-, and 48,500-, and 32,000-dalton proteins were encoded in a 21S RNA, and 47,000-dalton protein was encoded in an RNA of approximately 24S. The 76,000-dalton protein was identified as Pr76gag, based upon immunoprecipitation with specific antiserum and the presence of the 19* dipeptide. 7-Methylguanosine triphosphate inhibited the syntheses of Pr76gag and the 56,000-, 48,500-, and 32,000-dalton proteins, but not the synthesis of the 47,000-dalton protein. The 56,000-, 48,500-, 47,000-, and 32,000-dalton proteins were not immunoprecipitated by anti-gag, anti-reverse transcriptase, or anti-gp85 antiserum. Two-dimensional peptide maps of the 56,000- and 48,500-dalton proteins indicated that they were unique. In vitro translational products of myeloblastosis-associated virus 1 were also analyzed to aid in the identification of the AMV myb gene product(s); the translational products analyzed included Pr76gag, p60env, and a 56,000-dalton polypeptide which apparently was not identical to the 56,000-dalton AMV translational product, as determined by two-dimensional peptide mapping. Our data indicated that one of these proteins (56,000, 48,500, or 32,000 daltons) may represent the product of the AMV myb gene and, therefore, the putative transforming protein(s) of AMV.

PRCII, a representative of a new class of avian sarcoma viruses

The Poultry Research Center Virus II (PRC II) is a replication-defective avian sarcoma virus with envelope determinants of the A and B subgroups. In nonproducing cells transformed by PRCII the products of the replicative genes gag, pol, and env are not demonstrable, but a single polyprotein of Mr 105,000 (p105) can be detected. P105 contains peptides of the gag proteins p19 and p27 plus transformation-specific sequences. It does not contain peptides of gPr95env of Pr180gag-pol (with the possible exception of one pol peptide). The transformation-specific sequences of p105 are distinct form those of p100 of avian carcinoma virus MH2, of p110 coded for by avian myelocytoma virus MC29, and of p75 or p40 of avian erythroblastosis virus AEV. They also show no resemblance to p60src of Rous sarcoma virus. P105 is phosphorylated on a tyrosine residue and has an associated phosphokinase activity. P105 appears to be capable of autophosphorylation and of phosphorylating homologous immunoglobulin.

Avian erythroblastosis virus produces two mRNA's

We analyzed the viral mRNA's present in fibroblast nonproducer clones transformed by avian erythroblastosis virus. Two size classes of mRNA (28 to 30S and 22 to 24S) were identified by solution hybridization with both complementary DNA strong stop and complementary DNA made against the unique sequences of avian erythroblastosis virus. Based upon the kinetics of hybridization with complementary DNA made against the unique sequences of avian erythroblastosis virus, we estimated that there were 400 to 500 copies of the 28 to 30S RNA per cell and 200 to 250 copies of the 22 to 24S RNA per cell. Both RNA species were packaged in the virion. In vitro translation of the 28 to 30S virion RNA yielded a 75,000-dalton protein which was the 75,000-dalton gag-related polyprotein found in avian erythroblastosis virus-transformed cells. In vitro translation of the 22 to 24S virion RNA yielded two proteins (46,000 and 48,000 daltons). This indicates that there may be two genes in avian erythroblastosis virus, one coding for the 75,000-dalton gag-related polyprotein and the second coding for the 46,000- or 48,000-dalton protein or both.

Mutant of avian erythroblastosis virus defective for erythroblast transformation: deletion in the erb portion of p75 suggests function of the protein in leukemogenesis

Previous studies have shown that td359 AEV, a mutant of avian erythroblastosis virus (AEV), is unable to transform erythroblasts in vitro or in vivo but is capable of transforming fibroblasts in vitro and of causing sarcomas in chicks. In this paper we show that the mutant synthesizes a gag-gene related protein (delta p75) which is about 1000 daltons smaller than the protein, p75, induced by wild-type AEV. The mutant protein lacks 3 of the approximately 53 lysine-arginine tryptic peptides resolved in p75 and also contains an additional peptide. By cleavage of delta p75 with p15 protease and analysis of the fragments for size and peptide composition, the deletion in delta p75 could be located in the non-gag region of the molecule. In contrast, with p40 AEV, a second AEV-specific protein synthesized in in vitro translation experiments, there is no change in size of translation products obtained from td359 AEV RNA. Our data provide direct evidence that p75 is required for erythroblast transformation.

Comparative tryptic peptide mapping studies suggest a role in cell transformation for the gag-related protein of avian erythroblastosis virus and avian myelocytomatosis virus strains CMII and MC29

The gag-related proteins found in cells transformed by avian erythroblastosis virus (AEV) and the avian myelocytomatosis viruses MC29 and CMII have been compared by tryptic peptide fingerprinting. A comparison of the methionine-containing tryptic peptides of the AEV 75-kilodalton protein, the CMII 90-kilodalton protein, and the MC29 110-kilodalton protein with the gag gene product Pr76 of their naturally occurring helper leukemia viruses enabled us to distinguish those peptides related to the gag gene from the non-gag-related peptides. The 12 non-gag peptides found in the AEV 75-kilodalton protein were unique to this protein and not found in the MC29 110-kilodalton or CMII 90-kilodalton proteins. In contrast, the MC29 110-kilodalton protein shared two methionine-containing non-gag tryptic peptides with the CMII 90-kilodalton protein. When these experiments were repeated with [14C]lysine and [14C]arginine as the labeled amino acids, the MC29 110-kilodalton protein and the CMII 90-kilodalton protein were found to share 30 out of approximately 40 non-gag-related peptides. These results demonstrate that viruses with a similar transformation spectrum synthesize related proteins and suggest that the gag-related proteins represent the transforming proteins of the replication-defective avian leukemia viruses.

Temperature-sensitive mutant of avian erythroblastosis virus suggests a block of differentiation as mechanism of leukaemogenesis

A temperature sensitive mutant has been isolated for the first time from a replication defective acute leukaemia virus, AEV. In vivo, at 41 degrees C, the mutant shows a reduced leukaemogenic potential. In vitro, in erythroblasts transformed at 35 degrees C, haemoglobin synthesis can be induced by a shift to 41 degrees C. This indicates that the continuous expression of a viral gene product is necessary to maintain the undifferentiated state of the virus-transformed leukaemia cells.

Avian myelocytomatosis and erythroblastosis viruses lack the transforming gene src of avian sarcoma viruses

Using labeled cDNA specific for the detection of the src gene of avian sarcoma viruses, we find that avian myelocytomatosis virus strain MC29 and avian erythroblastosis virus strain ES4 lack nucleotide sequences related to the src gene. Furthermore, chicken fibroblasts as well as hematopoietic cells, infected and transformed with these viruses, show no enhanced level of transcription of the cellular nucleotide sequences related to the src gene of avian sarcoma viruses. These two viruses may thus contain their own transforming gene(s) or induce cellular genes unrelated to the src-like cellular sequences.