Epidermal growth factor, its receptor, and related proteins

An interesting aspect of the developments forthcoming from the study of cell growth control by growth factors is the structural and functional homologies that have been found to exist between growth factors or their receptors and other molecules. The epidermal growth factor (EGF) system has been particularly fruitful in this regard. The information in this article is meant to summarize the relationships that have been recently described between EGF and other EGF-like molecules and between the EGF receptor and related macromolecules.

Polyproteins containing a domain encoded by the V-SKI oncogene are located in the nuclei of SKV-transformed cells

SKV-transformed nonproducer clones were isolated from infected quail and chicken embryo cells. Analysis of intracellular viral RNAs by the Northern technique revealed that each clone contained a single SKV genome (either 5.7 or 8.9 kb) but no genome of the helper virus. Analysis of intracellular viral proteins containing gag determinants revealed that each clone contained a single species of either 55, 110, or 125 kDa. The intracellular location of these proteins was determined by indirect immunofluorescence employing either monoclonal antibodies (anti-p19gag) or conventional antiserum against gag proteins. All three of the SKV-specific proteins were localized to the nuclei of the transformed cells.

Protein phosphorylation at tyrosine residues in v-abl transformed mouse lymphocytes and fibroblasts

Phosphotyrosine antibodies were employed to immunodecorate and immunoprecipitate proteins phosphorylated at tyrosine residues in cells transformed by Abelson murine leukemia virus (A-MuLV). In pre-B and pre-T lymphoma cells transformed by A-MuLV, the major phosphotyrosine-containing protein has an MW of 160 kDa and shares immunologically detectable sequences with the v-abl oncogene product. Moreover, two different proteins of approximately 100 and 68 kDa, heavily phosphorylated at tyrosine, were identified. Lack of immunological cross-reactivity with viral products and phosphopeptide mapping showed that the 100 and 68 kDa proteins are coded by cellular genes. Phosphoproteins were undetectable in control resting lymphocytes. The 68 and the 100 kDa proteins were phosphorylated to different extents in proliferating lymphocytes, either stimulated by the growth factor IL-2, or transformed by M-MuLV (lacking the oncogene coded kinase). In fibroblasts transformed by A-MuLV, phosphotyrosine antibodies identified 2 proteins of 120 and 70 kDa. By immunological cross-reaction and by phosphopeptide mapping, the first was identified as a 120 kDa form of the v-abl coded kinase. The 70 kDa protein is coded by a cellular gene, is not structurally related to the 120 kDa v-abl kinase, and is different from any phosphotyrosine-containing protein detected in A-MuLV-transformed lymphocytes. These data show that, upon v-abl-induced transformation, phosphorylation at tyrosine takes place also on proteins other than the 160 or 120-kDa oncogene products. In lymphocytes and fibroblasts these proteins are different, suggesting that the cascade of events triggered by the v-abl gene in different cell types involves tyrosine phosphorylation of different specific proteins.

Induction and characterization of antisera against terminal and internal peptides of SV40 large T antigen

Thirteen synthetic peptides corresponding to different regions of the SV40 large T antigen were used as immunogens after coupling to a carrier protein. All peptide conjugates elicited sera that recognized the inducing peptide. In 10 cases the corresponding sites in the native large T antigen also were recognized, as determined by immunoprecipitation. The degree of recognition of the native protein varied between 0.5 and 80%, the most reactive sera being those induced by the terminal peptides. The ability of internal peptides to induce antibodies reactive with native large T antigen appeared to be correlated with peptide hydrophilicity and possibly atomic mobility. No such correlation was apparent with predicted features of secondary structure. The influence of peptide length on induction of protein-recognizing antisera will also be discussed.

The t-unique coding domain is important to the transformation maintenance function of the simian virus 40 small t antigen

The small t antigen (t) of simian virus 40, a 174-amino-acid-containing protein, when present together with the other early viral protein, large T antigen (T), plays an important role in the maintenance of simian virus 40-induced neoplastic phenotype in certain cells. Indeed, each protein functions in a complementary manner in this process. The t coding unit is composed of two segments, a 5' region of 246 nucleotides which is identical to that of the corresponding 5' region of the T coding unit and a 3' segment of 276 nucleotides which is unique. Two mutant, t-encoding genomes, one bearing a missense and the other a nonsense mutation at the same point in the t-unique coding region were constructed in vitro and found to be defective in their ability to dissolve the actin cytoskeleton of rat fibroblasts and to complement T in the growth of mouse fibroblasts in soft agar. Therefore, the unique segment of the t gene encodes a portion of the t molecule which is essential to its transformation maintenance function.

Creutzfeldt-Jakob infection increases adenylate cyclase activity in specific regions of guinea pig brain

Creutzfeldt-Jakob disease is a slow, infectious, progressive neurological disorder which results in human dementia. Synaptic membranes from various brain regions of guinea pigs infected with Creutzfeldt-Jakob disease show increased guanyl nucleotide- or 5-hydroxytryptamine-mediated activation of adenylate cyclase. This increased enzyme activity appears due, primarily, to facilitated 'coupling' between the GTP-binding protein which stimulates adenylate cyclase (GNs) and the catalytic moiety of that enzyme rather than increased sensitivity to 5-hydroxytryptamine. It is possible that this phenomenon is due to direct effects of the Creutzfeldt-Jakob infectious agent, or a pathological product resulting from that agent, upon synaptic membrane adenylate cyclase.

Enhanced spontaneous metastasis of mouse carcinoma cells transfected with an activated c-Ha-ras-1 gene

To investigate whether the presence of an activated ras oncogene influences the ability of tumour cells to metastasize, the c-Ha-ras-1 oncogene cloned from EJ/T24 cells was introduced into MT1 Cl.5/7 mouse mammary carcinoma cells. Since the MT1 Cl.5/7 cells are already tumorigenic but have a low metastatic capacity, this experimental design allows a distinction to be made between the effects of the ras gene on metastasis and tumorigenicity. MT1 Cl.5/7 containing the EJ c-Ha-ras-1 metastasized more readily and to more tissue sites than control cells (2.8 sites/mouse vs 0.9 sites/mouse). The metastases expressed the EJ c-Ha-ras-1 p21 ras proteins; however, one metastasis was discovered that had lost the expression of the c-Ha-ras-1 gene. When these cells were re-tested for metastasis, the rate of metastasis was indistinguishable from that of controls. This observation, coupled with a demonstration that lung colonization potential following intravenous inoculation is unaffected by the presence of the activated ras gene, argues that the effect of mutant ras genes is exerted on the ability of cells to escape from the primary tumour, rather than on a survival in the circulatory systems and ability to seed a second site.

Transforming Sloan-Kettering viruses generated from the cloned v-ski oncogene by in vitro and in vivo recombinations

The Sloan-Kettering viruses (SKVs) are replication-defective retroviruses that transform avian cells in vitro. Each of the three SKV isolates is a mixture of viruses with genomes ranging in size from 4.1 to 8.9 kilobases (kb) with a predominant genome of 5.7 kb. Using a cDNA representing a sequence, v-ski, that is SKV specific and held in common by the multiple SKV genomes, we generated a restriction map of the 5.7-kb SKV genome and molecularly cloned a ski-containing fragment from SKV proviral DNA. Southern hybridization and sequence analysis showed that the cloned DNA fragment consisted of the 1.3-kb ski sequence embedded in the p19gag sequence and followed by the remaining 5' half of the gag gene and small portions of both the pol and env genes. A large deletion encompassing the 3' half of gag and the 5' 80% of pol was mapped to a position about 1 kb downstream from the 3' ski-gag junction. To determine whether the cloned ski sequence had transforming activity, the ski-containing fragment and a cloned Rous-associated virus 1 (RAV-1) genome were used to construct an analog of the 5.7-kb SKV genome, RAV-SKV. Cotransfection of chicken embryo cells with RAV-SKV and RAV-1 yielded foci of transformed cells whose morphology was identical to that induced by the natural SKVs. The transformed transfected cells produced transforming virus with a 5.7-kb ski-containing genome and synthesized a gag-containing polyprotein of 110 kilodaltons (kDa). Several nonproducer clones of RAV-SKV-transformed cells were analyzed, and most were found to synthesize a 5.7-kb SKV RNA and a 110-kDa polyprotein. One clone was found to contain an 8.9-kb SKV RNA, and this clone synthesized a 125-kDa polyprotein. Since both the 5.7- and 8.9-kb genomes and the 110- and 125-kDa polyproteins had been identified in studies on the natural SKVs, the present results not only demonstrate the transforming activity of these individual SKVs but also suggest mechanisms for their generation.

DNA-binding region of the simian virus 40 tumor antigen

The simian virus 40 (SV40) tumor (T) antigen was purified by immunoaffinity chromatography and cleaved with small amounts of trypsin, and the resulting fragments were subjected to SV40 DNA cellulose chromatography. A 44,000-molecular-weight fragment (44K fragment) from the left end of the molecule and a 30K fragment mapping from approximately Lys 131 to Lys 371 bound to the column and were eluted with 1 M NaCl. In a second series of experiments, T antigen was immunoprecipitated with hamster anti-T serum or various monoclonal antibodies and partially digested with trypsin. Fragments that were solubilized by this treatment were tested for DNA-binding activity by using an SV40 DNA fragment-binding assay. A 17K fragment which originated from the amino-terminal region of the polypeptide had no apparent binding activity in this assay. On the other hand, larger fragments (76K, 46K, and 30K) whose amino termini were mapped around Lys 131 did display DNA-binding activity. Finally, complexes consisting of SV40 DNA and T-antigen fragments were precipitated in the DNA-binding assay with monoclonal antibodies that recognize the central region of the protein; however, antibodies with specificities to the amino- or carboxy-terminal regions were inactive. These results strongly suggest that the DNA-binding region of T antigen lies approximately between Lys 131 and Lys 371, corresponding to 0.51 and 0.37 map units on the DNA.

Subcellular localisation of the middle and large T-antigens of polyoma virus

The distribution of two of the polyoma virus early proteins (the large and middle T-antigens) in lytically infected mouse cells and transformed rat cells has been investigated by indirect immunofluorescence and immuno-electron microscopy using well-characterised monoclonal antibodies. By these techniques, the viral large T-antigen was found almost exclusively in the nucleus, sometimes in association with nuclear pores, but never in the nucleolus. In lytically infected, but not transformed cells, fluorescence was detected in discrete areas ('hot spots') within the nucleus and, in a minor population of lytically infected cells, cytoplasmic immunoreactive material was observed. The viral middle T-antigen was found in association with most cytoplasmic membranes and in the majority of cells mainly in the endoplasmic reticulum. Only a fraction of the staining was observed in the plasma membrane and no staining in the nucleoplasm was observed. The data suggest that the site of action of the major transforming activity of polyoma virus need not be at the plasma membrane. Functions associated with the viral antigens are discussed in terms of their subcellular distributions within cells.

Rous-associated virus 1-induced erythroleukemic cells exhibit a weakly transformed phenotype in vitro and release c-erbB-containing retroviruses unable to transform fibroblasts

Avian leukosis viruses induce erythroblastosis in chicks by integrating into the c-erbB gene and thus activating c-erbB transcription. We characterized Rous-associated virus 1-induced leukemic erythroblasts in vitro and showed that they mostly resemble erythropoietin-independent but otherwise normal erythroid progenitors. Some leukemic cells, however, were able to both differentiate and proliferate extensively in vitro. All 14 leukemias studied expressed high levels of erbB-related proteins that were 5 to 10 kilodaltons larger but otherwise very similar to the gp74erbB protein of avian erythroblastosis virus ES4 with respect to biosynthesis, glycosylation, and cell surface expression. Two leukemias contained and released retroviruses that transduced erbB. Chicken embryo fibroblasts fully infected with these viruses expressed high levels of erbB RNA and protein but retained a normal phenotype. Our results suggest that certain forms of c-erbB, activated by long terminal repeat insertion or viral transduction, are capable of inducing erythroleukemia but unable to transform fibroblasts.

Detection and characterization of the protein encoded by the v-rel oncogene

To identify the protein encoded by v-rel, the oncogene of reticuloendotheliosis virus (REV-T), antisera have been raised to three synthetic peptides derived from the translation of our previously published v-rel DNA sequence [R.M. Stephens, N.R. Rice, R.R. Hiebsch, H.R. Bose, Jr., and R.V. Gilden, Proc. Natl. Acad. Sci. USA 80, 6229-6233 (1983)]. Sera to all three peptides precipitate a 59,000 Da protein from REV-T-transformed chicken lymphoid cells. This protein is not detectable in uninfected chick embryo fibroblasts, and its observed size is in good agreement with the 56,000 Da predicted by the DNA sequence. We conclude that this protein is the v-rel product and designate it p59rel. To search for evidence of post-translational processing of this protein, cells were grown in the presence of glycosylation inhibitors. These resulted in no detectable difference in the size of p59rel. Nor was its size detectably altered during the course of a pulse-chase experiment. Growth of cells in the presence of [32P] orthophosphate, however, revealed that p59rel is a phosphoprotein. It is also closely associated with a protein kinase activity, for precipitation with one of the peptide antisera (but not the other two) resulted in strong kinase activity in the immune complex pellet. During this reaction, p59rel itself becomes phosphorylated. Kinase activity was retained in the immune complex following detergent and high salt washes, leaving open the possibility that p59rel is itself a kinase.

A new sensitive target-bound DNA binding assay for SV40 large T antigen

We have developed a new sensitive target-bound DNA binding assay (TB assay) for SV40 large T antigen (large T). The major advantage of this assay is that in contrast to commonly used DNA binding assays, DNA binding is not performed in large T extracts, but instead is performed with immunopurified target-bound large T. Thereby interference of cellular components present in large T extracts is avoided. Thus the TB assay allows DNA binding analysis of large T from different sources (extracts, cell lines) under standardized conditions. Large T is first immunopurified with an anti-T monoclonal antibody not interfering with DNA binding and protein A-Sepharose. Then SV40 DNA is added to the large T immune complex. For analysis of bound DNA and large T, we developed a two-step elution procedure by which bound DNA and large T in the immune complex can be analyzed separately and which allows the determination of the actual amounts of bound DNA and large T. Binding data obtained with the TB assay allowed us to determine an equilibrium dissociation constant (Kd). As a further application of this assay, we analyzed the ORI binding of SVR9D mutant large T which has been reported to exhibit no ORI binding activity. We found that a small percentage of SVR9D large T binds specifically to the SV40 ORI.

Immunocytochemical search for JC papovavirus large T-antigen in multiple sclerosis brain tissue

The large T-antigens of papovaviruses JC (JCV) and BK share a C-terminal subsequence with myelin basic protein (MBP). Since this sequence functions as a phosphate acceptor site in MBP, expression of a competing T-antigen sequence in oligodendroglia might adversely affect their ability to post-translationally process MBP and thus to maintain myelin. We have used techniques which demonstrate JCV T-antigen in small oligodendroglial cells from progressive multifocal leukoencephalopathy tissue to search for a possible latent JCV infection expressing T-antigen in nine cases of multiple sclerosis (MS) and three normal brains. No cells expressing T-antigen were detected in plaque or periplaque regions of the MS brains or in control CNS tissue.

Large T antigen-rich viral DNA replication loci in SV40-infected monkey kidney cells

The nuclear distribution of the large T antigen (T-Ag) during lytic infection of CV1 monkey kidney cells with SV40 virus was studied by immunoelectron microscopy. The viral protein was associated with the cellular chromatin and also accumulated within a small number of clearly delimited areas of the nucleoplasm. These T-Ag-rich areas were devoid of viral particles but contain 3-10 nm DNA filaments in an amorphous matrix. We have named these areas 'viral DNA/T-Ag loci.' The combination of the immunostaining for T-Ag with ultrastructural autoradiography revealed that these viral DNA/T-Ag loci were the sites of active SV40 DNA synthesis. We suggest that the viral DNA/T-Ag loci may represent definite structural domains specifically involved in viral DNA replication regulated by SV40-T antigen.

Leukemogenicity of avian oncovirus S13

Avian oncovirus S13 induces erythroblastic and granulocytic leukemias in line 6 and Spafas chickens. It also causes anemia, sarcomas, and endothelial proliferation. The leukemic cells contain the transformation-specific protein of S13, gp155.

Analysis of a tyrosine-specific protein kinase activity associated with the retroviral erbB oncogene product

The transforming protein erbB of avian erythroblastosis virus (AEV) has considerable sequence homology with the epidermal growth factor (EGF) and appears to represent a truncated form of this receptor. The sequence of the erbB gene is furthermore related to that of other viral transforming genes such as src, fps, yes or abl. The transforming proteins of these src-related oncogenes as well as receptors for EGF, platelet-derived growth factor (PDGF), and insulin are associated with tyrosine-specific protein kinases. It has been difficult to demonstrate this activity for the erbB protein. To analyze the erbB gene product, we prepared polyclonal antibodies against a bacterially expressed erbB DNA restriction fragment (BamHI/BamHI). The antiserum is shown to immunoprecipitate the erbB protein from AEV-transformed chicken fibroblasts and also recognizes the EGF receptor protein. Both proteins become phosphorylated in vitro on tyrosine residues upon the addition of [gamma-32P]ATP. The protein kinase activity is low compared to other oncogene-specific kinases. This is not due to kinase blocking by the serum, because erbB carboxyterminal synthetic peptide antibodies give rise to low levels of protein kinase activity as well indicating that this may be a characteristic property of erbB in vitro.

Studies on the interaction of the human c-myc protein with cell nuclei: p62c-myc as a member of a discrete subset of nuclear proteins

We have analyzed the localization of the human c-myc product (p62c-myc) at steady state in cells by immunoprecipitation and immunoblotting. We show that p62c-myc is extracted from nuclei by mild salt concentrations (below 200 mM), without affecting gross nuclear structure or causing extraction of major chromatin components. We observe no association between p62c-myc and the nuclear matrix. We also demonstrate that p62c-myc is a member of a discrete subset of nuclear proteins that are all rendered irreversibly insoluble in situ by exposure of isolated nuclei to physiological temperatures (37 degrees C). p62c-myc is sequestered into a similar insoluble complex in cells that have been subjected to heat shock. Finally, we show that avian v-myc and v-myb proteins in isolated nuclei also become insoluble after exposure to temperatures above 37 degrees C. We discuss the possible implications of these results.

Adenovirus serotype determines association and localization of the large E1B tumor antigen with cellular tumor antigen p53 in transformed cells

The distribution and stability of the cellular tumor antigen p53 were studied in baby rat kidney cells transformed by region E1 sequences of nononcogenic adenovirus (Ad) type 5 (Ad5) or oncogenic type 12 (Ad12). In transformed cells expressing the large E1B T antigen of Ad5, p53 was associated with this T antigen. The complexed proteins were concentrated in a cytoplasmic body, which has been shown to consist of a cluster of 8-nm filaments (A. Zantema et al., Virology 142:44-58, 1985). In transformed cells expressing the E1B region of Ad12, however, no association between the viral large T antigen and p53 was detectable. In the latter case, both proteins were found almost exclusively in the nucleus. The stability of p53 in both Ad5- and Ad12-transformed cells was increased relative to that in primary cells or cells immortalized by the E1A region only. Thus, the increased stability of p53 in Ad-transformed cells is not caused by association with a viral T antigen, but it correlates with expression of E1B and with morphological transformation.

Detection of cellular proteins associated with human adenovirus type 5 early region 1A polypeptides

Antisera prepared against synthetic peptides corresponding to the amino and carboxy termini of human adenovirus type 5 (Ad5) early region 1A (E1A) proteins were used to identify polypeptides that are associated with these viral species in lytically infected KB cells. Proteins were sought which coprecipitated with E1A polypeptides using both sera and which were not recognized in extracts from mock-infected cells by either serum. Four such species were identified with apparent molecular weights of 68K, 65K, and a doublet at about 105K. A fifth species migrating with a molecular weight in excess of 250K was also identified consistently with E1A-C1 but not E1A-N1 serum. Addition of an excess of the appropriate synthetic peptide to the immunoprecipitation mixtures prevented the precipitation of all of these species. Mixing experiments demonstrated that all species were cellular proteins expressed in normal uninfected KB cells and in addition showed that an association with E1A proteins could take place in vitro. Studies carried out with the mutants pm975 and hr1 indicated that while the 105K doublet and the greater than 250K species were found with the products of both the 1.1- and 0.9-kb E1A mRNAs, 65K and 68K appeared to be primarily associated with those of the 1.1-kb mRNA. Finally, the 105K doublet and greater than 250K were shown to be phosphoproteins. These data indicated that Ad5 E1A proteins may function in a complex with cellular polypeptides which includes species of 105K, 68K, 65K, and possibly a large protein of greater than 250K.

Reinitiation of cellular DNA synthesis in BrdU-selected nondividing senescent WI-38 cells by simian virus 40 infection

Bromodeoxyuridine-selected nondividing senescent WI-38 cells were stimulated to synthesize DNA, as evidenced by incorporation of [3H]thymidine into nuclei of senescent cells, after infection with simian virus 40 (SV40). Cellular DNA synthesis was confirmed by DNA-DNA hybridization experiments and the use of temperature-sensitive A gene mutants. The DNA synthesis was, at least in part, semiconservative, as microdensitometry of Feulgen-stained nuclei revealed increased DNA content in a large fraction of the cells in the infected population. Thus, senescent cells retain the capacity to replicate their DNA, despite their intrinsic inability to initiate DNA synthesis.

Epithelial HBL-100 cell line derived from milk of an apparently healthy woman harbours SV40 genetic information

The epithelial HBL-100 cell line was established in vitro from milk of an apparently healthy woman. It exhibits characteristics of transformation from the very beginning and evolves during in vitro maintenance, until becoming tumorigenic in nude mice. This immortal cell line represents a useful model for studying the progression of human epithelial cells toward malignancy. In the course of our investigations we detected a 94K protein in HBL-100 cells obtained from four different sources. This protein is shown to be indistinguishable from the SV40 large T-antigen on the basis of: Recognition by polyclonal and different monoclonal antibodies. Partial peptide map analysis. Specific binding capacity to the SV40 DNA origin of replication. The presence of a tandemly integrated SV40 genome is demonstrated by Southern blotting. Successful rescue of SV40 DNA by fusion with permissive COS-7, but not CV-1 cells, indicates that the SV40 T-antigen from HBL-100 cells is defective in a function(s) essential to the replication of the viral DNA. The possible origin of the SV40 genetic information that we have detected in HBL-100 cells and the implications of this finding on studies involving this cell line are discussed.