Cellular transforming genes and oncogenesis

The genetics of radiation-induced and sporadic osteosarcoma: a unifying theory?

Cancer is a disease of the genome, with the neoplastic phenotype being passed from one cell generation to the other. Radiation-induced cancer has often been considered to represent a unique entity amongst neoplasia, with the energy deposition being held responsible for both direct (gene mutations) and indirect (bystander effects, induced instability etc) alterations to the cellular genome. However, radiogenic tumours in man and experimental animals appear to be physiologically and genetically indistinguishable from their sporadic counterparts, suggesting that the aetiologies of these two tumour types are in fact closely related. We have conducted a general screen of the genetic alterations in radiation-induced mouse osteosarcoma, a tumour that is histopathologically indistinguishable from human sporadic osteosarcoma. Comparison of the two tumour types indicates the existence of a common set of genetic changes, providing additional evidence to support the concept that the molecular pathology of radiation-induced malignancy is no different to that of sporadic cancers.

C-myc oncogene expression in anal squamous neoplasia

AIMS

To determine the pattern of c-myc oncogene expression in anal squamous neoplasia and to determine if this could be used as a marker of disease progression.

METHODS

The presence and localisation of the c-myc gene product p62 in archival specimens of anal squamous epithelium, normal and neoplastic, was examined using immunohistochemical staining with the monoclonal antibody Myc1-6E10. Ten normal and epithelia, 10 anal intraepithelial neoplasia (AIN) III, and 31 anal squamous cancers were examined.

RESULTS

There was a noticeable difference between the staining characteristics of invasive tumours, normal anal epithelium, and AIN III. Intense, diffuse, mixed nuclear and cytoplasmic (n = 14) and exclusively nuclear (n = 8) staining in 22 of 31 (71%) of invasive anal tumours was observed. All positively staining tumours were well differentiated histologically, while the negatively staining nine of 31 (29%) were poorly differentiated (n = 7) and moderately well differentiated (n = 2). In six positively staining tumour sections adjacent areas of AIN III and non-dysplastic anal epithelium had staining characteristics similar to those of the invasive component. Staining in both normal anal epithelium (4/10) and AIN III specimens obtained from patients without a history of invasive disease (8/10) was less intense, focal in distribution, and exclusively nuclear. No difference in staining characteristics could be detected in these two groups.

CONCLUSIONS

The results of this study suggest that c-myc oncogene expression is implicated in the pathogenesis of anal squamous neoplasia, and that immunohistochemical staining for c-myc protein may be helpful in identifying those AIN III lesions most likely to progress to invasive tumours.

Metabolism of debrisoquine and susceptibility to breast cancer

There may exist an association between the genetically determined oxidation status of the antihypertensive agent debrisoquine (DEB) and the propensity to develop tumours. The metabolism of DEB is extensive in 90% of healthy subjects (metabolic ratio = MR = 0-12.6; MR = % DEB excreted divided by % 4-hydroxy-DEB excreted) and poor in 10% (MR greater than 12.6). In patients with cancer of the lung, urinary bladder, and gastrointestinum, the percentage of high metabolizers is increased to greater than 98%. The poor metabolizer mode is almost devoid of cancer patients. It was investigated whether breast cancer patients show a similar association with respect to the oxidative status of DEB. 108 breast cancer patients and 123 women with benign gynecologic disorders received 1 tablet of 10 mg DEB orally in the evening. Urine was collected for the subsequent 8 hrs and analysed for its content of DEB and its main urinary metabolite 4-OH-DEB by means of HPLC. No decreased amount of poor metabolizers was seen in the cancer group.

Overexpression of normal and mutated forms of HRAS induces orthotopic bladder invasion in a human transitional cell carcinoma

Recent studies have shown that orthotopic (transurethral) transplantation of human bladder cancer cell lines into nude mice permits tumor growth that accurately reflects their clinical malignant status in the original host. Thus, such a system allows a unique opportunity to analyze the genetic events involved in the conversion of low-grade bladder cancer, the vast majority of which are curable, to the high-grade life-threatening form of the disease. Since 5-10% of transitional cell carcinomas (TCCs) have been shown to contain a mutated HRAS gene, and protein expression levels of all forms of HRAS have been correlated with TCC progression, we chose to study the contribution of the HRAS oncogene in bladder tumor progression. We evaluated the effects of transfection of normal or mutated HRAS genes into a human TCC, called RT-4, that behaves as a superficial noninvasive papillary tumor after transurethral orthotopic inoculation into athymic nude mice. We found that overexpression of either transfected normal or mutated HRAS genes converted RT-4 cells to express an invasive phenotype remarkably similar in nature to the clinical behavior of high-grade bladder carcinomas. These results suggest a role for overexpressed normal or mutated RAS genes in human bladder carcinoma progression, and highlight the importance of using orthotopic inoculation systems for evaluation of the contribution of oncogenes to malignant tumor progression.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. II. Two signaling pathways distinguished by pertussis toxin and a potential role for the ras oncogene

In the preceding report (Kelvin, D.J., G. Simard, H.H. Tai, T.P. Yamaguchi, and J.A. Connolly. 1989. J. Cell Biol. 108:159-167) we demonstrated that pertussis toxin (PT) blocked proliferation and induced differentiation in BC3H1 muscle cells. In the present study, we have used PT to examine specific growth factor signaling pathways that may regulate these processes. Inhibition of [3H]thymidine by PT in 20% FBS was reversed in a dose-dependent fashion by purified fibroblast growth factor (FGF). In 0.5% FBS, the normally induced increase in creatine kinase (CK) activity was blocked by FGF in both the presence and absence of PT. Similar results were obtained with purified epidermal growth factor (EGF). We subsequently examined the effect of a family of growth factors linked to inositol lipid hydrolysis and found that thrombin, like FGF, would increase [3H]thymidine incorporation and block CK synthesis. However, PT blocked thymidine incorporation induced by thrombin, and blocked the inhibition of CK turn-on in 0.5% FBS by thrombin. The ras oncogene, a G protein homologue, has previously been shown to block muscle cell differentiation in C2 muscle cells (Olson, E.N., G. Spizz, and M.A. Tainsky. 1987. Mol. Cell. Biol. 7:2104-2111); we have characterized a BC3H1 cell line, BCT31, which we transfected with the val12 oncogenic Harvey ras gene. This cell line did not express CK in response to serum deprivation. Whereas [3H]thymidine incorporation was inhibited by 70-80% by increasing doses of PT in control cells, BCT31 cells were only inhibited by 15-20%. ADP ribosylation studies indicate this PT-insensitivity is not because of the lack of a PT substrate in this cell line. Furthermore, PT could not induce CK expression in BCT31 cells as it did in parental cells. We conclude that there are at least two distinct growth factor pathways that play a key role in regulating proliferation and differentiation in BC3H1 muscle cells, one of which is PT sensitive, and postulate that a G protein is involved in transducing signals from the thrombin receptor. We believe that ras functions in the transduction of growth factor signals in the nonPT-sensitive pathway or downstream from the PT substrate in the second pathway.

Alteration of glycolipids in ras-transfected NIH 3T3 cells

Glycosphingolipid alterations upon viral transformation are well documented. Transformation of mouse 3T3 cells with murine sarcoma viruses results in marked decreases in the levels of gangliosides GM1 and GD1a and an increase in gangliotriaosylceramide. The transforming oncogenes of these viruses have been identified as members of the ras gene family. We analyzed NIH 3T3 cells transfected with human H-, K- and N-ras oncogenes for their glycolipid composition and expression of cell surface gangliosides. Using conventional thin-layer chromatographic analysis, we found that the level of GM3 was increased and that of GD1a was slightly decreased or unchanged, and GM1 was present but not in quantifiable levels. Cell surface levels of GM1 were determined by 125I-labeled cholera toxin binding to intact cells. GD1a was determined by cholera toxin binding to cells treated with sialidase prior to toxin binding. All ras-transfected cells had decreased levels of surface GM1 and GD1a as compared to logarithmically growing normal NIH 3T3 cells. Levels of GM1 and, to a lesser extent, GD1a increased as the latter cells became confluent. Using a monoclonal antibody assay, we found that gangliotriaosylceramide was present in all ras-transfected cells studied but not in logarithmically growing untransfected cells. Interestingly, gangliotriaosylceramide appeared when the latter cells became confluent. These results indicated that ras oncogenes derived from human tumors are capable of inducing alterations in glycolipid composition.

Differential response to retinoic acid of Syrian hamster embryo fibroblasts expressing v-src or v-Ha-ras oncogenes

It has been shown that treatment of many but not all tumor cell lines with retinoids affects cell proliferation and expression of the transformed phenotype. To determine whether the response of the tumor cell to retinoids is influenced by specific oncogenes activated in the cell, we studied the action of these agents in the immortal, nontumorigenic Syrian hamster embryo cell lines DES-4 and 10W transfected with either v-Ha-ras or v-src oncogenes. In this paper we show that in transformed DES-4 cells expressing v-src, retinoic acid inhibited anchorage-independent growth, reduced saturation density, and inhibited the induction of ornithine decarboxylase by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. In contrast, retinoic acid enhances the expression of the transformed phenotype in DES-4-derived cells that express v-Ha-ras. In these cells retinoic acid increases the number and the average size of colonies formed in soft agar. Moreover, retinoic acid enhances ornithine decarboxylase activity and acts in a synergistic fashion with 12-O-tetradecanoylphorbol-13-acetate. These results indicate that oncogenes activated in cells can indeed influence the response of cells to retinoids. Retinoic acid does not appear to alter the levels of pp60src or p21ras proteins in these cells, suggesting that retinoic acid does not affect the synthesis of these oncogene products. Furthermore, retinoic acid does not affect the protein kinase activity of pp60src. Transformed cell lines derived from 10W cells responded differently, indicating that the presence of a specific oncogene is not the only factor determining the response to retinoids. Possible mechanisms by which retinoic acid may interfere with the expression of the oncogene products are discussed.

Characterization of two members of the rho gene family from the yeast Saccharomyces cerevisiae

The rho genes comprise an evolutionarily conserved family with significant homology to the ras oncogene family. Two members of the rho family were isolated from the yeast Saccharomyces cerevisiae and characterized by DNA sequence analysis. The yeast genes RHO1 and RHO2 are 70% and 57% identical, respectively, to the rho gene of the marine snail Aplysia, and they are 53% identical to each other. Inactivation of these genes showed that RHO1 is required for cell viability, while RHO2 is not an essential gene. A mutant allele of RHO1 (RHO1-His68) was constructed with a mutation analogous to one that activates the transforming potential of the human HRAS gene. Diploid strains containing RHO1-His68 in either low or high copy number are unable to sporulate, and the mutant allele is dominant over wild-type RHO1. The requirement for RHO1 cannot be circumvented by introduction of high copy number plasmids containing either the gene encoding the catalytic subunit of cAMP-dependent protein kinase or the mutant allele RAS2-Val19. Despite the conservation between the rho and ras gene families, the finding that RHO1 functions independently of the adenylate cyclase cAMP-dependent protein kinase cascade suggests that rho and ras are involved in distinct biochemical pathways.

Lineage-specific transformation after differentiation of multipotential murine stem cells containing a human oncogene

We transfected the human EJ bladder carcinoma oncogene (Ha-rasEJ-1) into multipotential embryonal carcinoma cell line P19. The transgenic P19(ras+) cells expressed high levels of both the mRNA and the p21EJ protein derived from the oncogene. When cultured in the presence of retinoic acid, P19(ras+) cells differentiated and developed into the same spectrum of differentiated cell types as the parental P19 cells (namely, neurons, astrocytes, and fibroblast-like cells). Thus, it seems unlikely that the Ha-ras-1 proto-oncogene product plays a role in initiation of differentiation or in the choice of differentiated cell lineage. Most of the P19(ras+)-derived differentiated cells contained relatively low levels of p21EJ and were nontransformed, whereas certain cells with fibroblast-like morphology continued to express the Ha-rasEJ-1 gene at high levels and were transformed (i.e., immortal and anchorage independent). Fibroblasts derived from P19 cells did not become transformed following transfection of the Ha-rasEJ-1 oncogene, suggesting that transformation of the fibroblast cells only occurred if the oncogene was present and expressed during the early stages of the developmental lineage.

rasH mutants deficient in GTP binding

Single amino acid substitutions were introduced into a region of the rasH protein (residues 116, 117, and 119) homologous to a variety of diverse GTP-binding proteins. Each of the mutant p21 proteins displayed a significant reduction (10- to 5,000-fold) in GTP binding affinity. Activated rasH proteins deficient in GTP binding were unaltered in their ability to morphologically transform NIH 3T3 cells.

Identification of the protein encoded by the human diffuse B-cell lymphoma (dbl) oncogene

The dbl oncogene was initially isolated from a human diffuse B-cell lymphoma. Antisera from mice bearing tumors induced by this oncogene specifically detected a protein of about 66 kDa (p66) in dbl transformants. dbl cDNA-selected poly(A)+ RNA isolated from a transfectant clone expressing p66 directed the in vitro synthesis of this protein, establishing that it is encoded by dbl. Subcellular localization studies revealed that p66 is a cytoplasmic protein distributed between cytosol and crude membrane fractions. Moreover, p66 was shown to be a phosphoprotein, with phosphorylation specific to serine residues. Our characterization of the dbl-encoded protein appears to distinguish this transforming gene product from those of other known oncogenes.

Localisation of lung cancer by a radiolabelled monoclonal antibody against the c-myc oncogene product

A set of mouse nonoclonal antibodies against the c-myc oncogene product, a 62,000 dalton nuclear binding protein involved in cell cycle control, has been constructed by immunisation with synthetic peptide fragments. One such antibody, CT14, was radiolabelled with 131I and administered to 20 patients with different malignant diseases. Good tumour localisation was observed in 12 out of 14 patients with primary bronchial carcinoma but not in patients with pulmonary metastases from primary tumours elsewhere. Successfully localised tumours were all 3 cm or more in diameter. Monoclonal antibodies against oncogene products may provide novel selective tools for the diagnosis and therapy of cancer.

Transforming activity of DNA fragments from normal human lymphocytes results from spontaneous activation of a c-Ha-ras1 gene

An activated human Ha-ras gene was present in a secondary NIH 3T3 transformant isolated after serial transfection of originally low-molecular-weight DNA fragments from normal human cells. This gene appeared to have acquired its transforming properties by a spontaneous mutation in codon 12 by substitution of a deoxythymidine residue for a deoxyguanosine residue. DNA rearrangements in the flanking sequences of the transferred Ha-ras gene were not involved in the activation of the protooncogene.

Monoclonal antibody specific for an activated RAS protein

Activated RAS transforming genes that encode proteins (p21s) with amino acid substitutions at positions 12, 13, or 61 have been detected in 10-20% of human neoplasms. This report describes a monoclonal antibody (DWP) raised against a synthetic peptide corresponding to amino acids 5-16 of a mutated RAS gene encoding Val instead of Gly at position 12. DWP reacted in competition assays with peptides containing Val or Cys at position 12, but did not react with peptides containing Gly, Arg, Ser, Ala, Asp, or Glu at position 12. Immunoblot analysis of transformed NIH cells and human carcinoma cell lines showed that DWP reacts specifically with activated RAS proteins containing Val at position 12 and not with normal p21s or p21s activated by other amino acid substitutions at positions 12 and 61. Immunohistochemical studies showed that DWP-labeled transformed NIH cells and human carcinoma cells contained p21s with either Val or Cys at position 12 but not normal or other activated p21s. In contrast to the specificity seen with human carcinoma cell lines, analysis of formalin-fixed, primary carcinoma specimens indicated that positive immunoperoxidase staining with DWP did not necessarily correlate with immunoblot and transfection assays for the presence of activated RAS proteins. Immunohistochemical studies did show, however, that DWP preferentially binds human carcinoma cells.

Differential response to retinoic acid of Syrian hamster embryo fibroblasts expressing v-src or v-Ha-ras oncogenes

It has been shown that treatment of many but not all tumor cell lines with retinoids affects cell proliferation and expression of the transformed phenotype. To determine whether the response of the tumor cell to retinoids is influenced by specific oncogenes activated in the cell, we studied the action of these agents in the immortal, nontumorigenic Syrian hamster embryo cell lines DES-4 and 10W transfected with either v-Ha-ras or v-src oncogenes. In this paper we show that in transformed DES-4 cells expressing v-src, retinoic acid inhibited anchorage-independent growth, reduced saturation density, and inhibited the induction of ornithine decarboxylase by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. In contrast, retinoic acid enhances the expression of the transformed phenotype in DES-4-derived cells that express v-Ha-ras. In these cells retinoic acid increases the number and the average size of colonies formed in soft agar. Moreover, retinoic acid enhances ornithine decarboxylase activity and acts in a synergistic fashion with 12-O-tetradecanoylphorbol-13-acetate. These results indicate that oncogenes activated in cells can indeed influence the response of cells to retinoids. Retinoic acid does not appear to alter the levels of pp60src or p21ras proteins in these cells, suggesting that retinoic acid does not affect the synthesis of these oncogene products. Furthermore, retinoic acid does not affect the protein kinase activity of pp60src. Transformed cell lines derived from 10W cells responded differently, indicating that the presence of a specific oncogene is not the only factor determining the response to retinoids. Possible mechanisms by which retinoic acid may interfere with the expression of the oncogene products are discussed.

rasH mutants deficient in GTP binding

Single amino acid substitutions were introduced into a region of the rasH protein (residues 116, 117, and 119) homologous to a variety of diverse GTP-binding proteins. Each of the mutant p21 proteins displayed a significant reduction (10- to 5,000-fold) in GTP binding affinity. Activated rasH proteins deficient in GTP binding were unaltered in their ability to morphologically transform NIH 3T3 cells.

Transforming gene from human stomach cancers and a noncancerous portion of stomach mucosa

DNAs from 21 human stomach cancers, 16 metastatic stomach cancers to lymph nodes, and 21 apparently noncancerous specimens of stomach mucosae from a total of 26 patients with stomach cancer were tested for their ability to induce neoplastic transformation of NIH 3T3 cells on transfection by the calcium phosphate precipitation technique. Three samples of DNA were shown to have transforming activity; one was from a primary stomach cancer of one patient, the second was from a noncancerous portion of stomach mucosa of the same patient, and the third was from a lymph node metastasis of stomach cancer from another patient. These transformants were tumorigenic in nude mice, and DNAs from the cells could induce secondary transformants. A portion of the transforming gene from the stomach cancer of one patient, which contained the sequences expressed in the NIH 3T3 transformants, was cloned. The transforming gene did not have any homology with the transforming sequences reported previously. We have applied the term hst to this novel human transforming gene. The transforming gene, hst, was found to be present in all the primary and secondary transformants induced by the other two samples of DNA.

The clinical significance of flow cytometric c-myc oncoprotein quantitation in testicular cancer

A sensitive flow cytometric assay has been developed using a monoclonal antibody, Myc 1-6E10, to quantitate c-myc oncoprotein levels in nuclei isolated from wax embedded testicular tumours. The oncoprotein (p62c-myc) level increased significantly with increasing teratoma differentiation. Patients with intermediate and undifferentiated tumours who developed recurrence had lower p62c-myc levels than those who were disease free since their initial treatment. Such quantitative biochemical methods may provide new prognostic indices for cancer patients.

Lineage-specific transformation after differentiation of multipotential murine stem cells containing a human oncogene

We transfected the human EJ bladder carcinoma oncogene (Ha-rasEJ-1) into multipotential embryonal carcinoma cell line P19. The transgenic P19(ras+) cells expressed high levels of both the mRNA and the p21EJ protein derived from the oncogene. When cultured in the presence of retinoic acid, P19(ras+) cells differentiated and developed into the same spectrum of differentiated cell types as the parental P19 cells (namely, neurons, astrocytes, and fibroblast-like cells). Thus, it seems unlikely that the Ha-ras-1 proto-oncogene product plays a role in initiation of differentiation or in the choice of differentiated cell lineage. Most of the P19(ras+)-derived differentiated cells contained relatively low levels of p21EJ and were nontransformed, whereas certain cells with fibroblast-like morphology continued to express the Ha-rasEJ-1 gene at high levels and were transformed (i.e., immortal and anchorage independent). Fibroblasts derived from P19 cells did not become transformed following transfection of the Ha-rasEJ-1 oncogene, suggesting that transformation of the fibroblast cells only occurred if the oncogene was present and expressed during the early stages of the developmental lineage.

Biological and biochemical properties of human rasH genes mutated at codon 61

Using site-directed mutagenesis, we have introduced mutations encoding 17 different amino acids at codon 61 of the human rasH gene. Fifteen of these substitutions increased rasH transforming activity. The remaining two mutants, encoding proline and glutamic acid, displayed transforming activities similar to the normal gene. Overall, these mutants vary over 1000-fold in transforming potency. Increased levels of p21 expression were required for transformation by weakly transforming mutants. The mutant proteins were unaltered in guanine nucleotide binding properties. However, all 17 different mutant proteins displayed equivalently reduced rates of GTP hydrolysis, 8- to 10-fold lower than the normal protein. There was no quantitative correlation between reduction in GTPase activity and transformation, indicating that reduced GTP hydrolysis is not sufficient to activate ras transforming potential.

Detection of the c-myc oncogene product in colonic polyps and carcinomas

The c-myc oncogene has been implicated in the processes of normal cell proliferation and differentiation. Elevated levels of c-myc mRNA and its gene product (p62c-myc), have been detected in a variety of solid tumours and cultured cel lines. Its precise role in normal cell function and in neoplastic transformation and progression has yet to be elucidated. We have used a monoclonal antibody, raised by peptide immunisation, to determine the distribution by immunoperoxidase staining of the c-myc oncogene product in archival specimens of colonic polyps and carcinomas. Samples from 42 patients with colon carcinoma, 24 with benign polyps and 15 normal colon biopsies were examined. Normal colon revealed maximum staining in the mid-zone of the crypts, corresponding to the zone of differentiation and maturation. The staining was predominantly cytoplasmic. Adenomatous polyps revealed the most intense pattern of staining in areas of dysplastic change. Colonic tumours showed a wide range of staining. Well differentiated tumours contained more cytoplasmic p62c-myc than poorly differentiated tumours. These findings suggest that the c-myc oncogene product may play an important role in the evolution of colonic neoplasia.

Concerted DNA rearrangements in Moloney murine leukemia virus-induced thymomas: a potential synergistic relationship in oncogenesis

Rat thymic lymphomas induced by Moloney murine leukemia virus carry DNA rearrangements due to provirus integration in at least five independent cellular DNA domains (Mlvi-1, Mlvi-2, Mlvi-3, RMoInt-1, and c-myc). We had previously shown that rearrangements in more than one of these domains could occur in the same tumor. In this report we extend these findings by showing that, with one exception, tumors containing provirus insertions in Mlvi-1 always contained provirus insertions in a second locus, Mlvi-2. To determine whether both events occurred in the same population of tumor cells, we examined the clonal nature of these tumors by taking advantage of allelic polymorphisms that occur naturally in both Mlvi-1 and Mlvi-2. Tumors with provirus insertions in both Mlvi-1 and Mlvi-2 arising in rats heterozygous at one of these loci were identified. DNA from these tumors was analyzed by restriction endonuclease digestion and hybridization to DNA probes derived from both Mlvi-1 and Mlvi-2. Thus, we determined the clonal nature of three thymomas and showed that in these tumors both insertion events occurred in the same population of tumor cells. The concomitant appearance of provirus insertions in Mlvi-1 and Mlvi-2 suggests a synergism of these two events that may be important in tumor induction and progression.

Detection of the c-myc oncogene product in testicular cancer

A set of monoclonal antibodies was constructed by immunising mice with peptide fragments of the c-myc oncogene product. One such antibody, Myc 1-6E10 was shown to bind to a 62,000 dalton protein identifiable with the c-myc product (p62c-myc). The antigen recognised was not destroyed by paraffin wax embedding. Myc 1-6E10 was used to characterise the distribution of p62c-myc in archival testicular tumour material. Normal testes expressed only small amounts of p62c-myc. Seminomas showed increased nuclear and cytoplasmic staining. Undifferentiated teratoma showed little activity, whereas p62c-myc was abundant in the nuclei of differentiating epithelial structures, yolk sacs and embryoid bodies. Only small amounts of p62c-myc were seen in the tumours of 5 patients who subsequently died from their disease.

Insulin inhibits the cholera-toxin-catalysed ribosylation of a Mr-25000 protein in rat liver plasma membranes

A method is described for preparing a plasma-membrane fraction from hepatocytes by a rapid, gentle, Percoll fractionation procedure. Cholera toxin elicited the ribosylation of a number of proteins in these membranes, including the components of the stimulatory guanine nucleotide regulatory protein, Ns. Insulin, however, inhibited the ability of cholera toxin to ribosylate a protein of Mr 25 000. The action was decreased in membranes from cells that had been pre-treated with glucagon. Ribosylation of both the components of Ns and the Mr-25 000 species occurred in whole cells treated with cholera toxin, because membranes from such treated cells exhibited decreased labelling when incubated with [32P]NAD+ and activated cholera toxin. The labelling of proteins, including the Mr-25 000 species, with [32P]NAD+ and cholera toxin in the plasma membranes was decreased by an inhibitor of ribosylation. Azido-GTP photoaffinity labelling identified several high-affinity GTP-binding proteins, including one of Mr 25 000. Cholera toxin failed to ribosylate the Mr-25 000 protein in membranes from cells that had been pre-treated with the tumour-promoting agent 12-O-tetradecanoylphorbol 13-acetate (TPA). In membranes from such treated cells, insulin actually allowed cholera toxin to label this species. As TPA activates protein kinase C, it is possible that the Mr-25 000 protein, or a species that interacts with it, is a substrate for phosphorylation. These observations may offer an explanation for some of the perturbing effects that TPA exerts on insulin's action. It is suggested that the insulin receptor interacts with the guanine nucleotide regulatory protein system in the liver, and that the Mr-25 000 species may be a component of Nin, a specific guanine nucleotide regulatory protein that has been proposed to mediate certain of the actions of insulin on target cells [Houslay & Heyworth (1983) Trends Biochem. Sci. 8, 449-452].

Frequency of molecular alterations affecting ras protooncogenes in human urinary tract tumors

Members of the ras gene family are activated as oncogenes in many different human cancers. To systematically determine the frequency at which such genes might be involved in the neoplastic process affecting a specific target tissue, urothelial cells, we surveyed a large series of urinary tract tumors for ras oncogenes by DNA transfection and by molecular genetic analysis. Harvey (Ha)-ras oncogenes were detected in 2 of 38 tumors by transfection, molecularly cloned in biologically active form, and shown to contain single base changes at codon 61 leading to substitutions of arginine and leucine, respectively, for glutamine at this position. One additional Ha-ras oncogene was identified in a bladder carcinoma by restriction polymorphisms at codon 12. In one of 21 tumors, we observed a 40-fold amplification of the Kirsten (Ki)-ras gene. No amplification of other ras genes was detected in any of the tumors analyzed. Our findings strengthen the conclusion that codons 12 and 61 are the major "hot spots" of ras oncogene activation and suggest that quantitative alterations in expression due to gene amplification may provide an alternative mechanism for ras gene activation in primary human tumors.

Transformation of NIH 3T3 cells by DNA of the MCF-7 human mammary carcinoma cell line induces expression of an endogenous murine leukemia provirus

Previous studies identified two glycoproteins of 86 (gp86) and 72 (gp72) kilodaltons and two nonglycosylated proteins of 70 (p70) and 19 (p19) kilodaltons which were specifically expressed in NIH cells transformed by DNA of the MCF-7 human mammary carcinoma cell line. Pulse-chase experiments and the use of tunicamycin to inhibit glycosylation suggested that gp86, gp72, and p19 were related as precursor products. Characteristics of the four transformation-associated proteins resembled those of murine leukemia virus (MuLV) proteins. Sera raised against disrupted MuLV immunoprecipitated the same four proteins in extracts of NIH(MCF-7) cells and MuLV-infected NIH 3T3 cells. In addition, a monoclonal antibody against MuLV gp70 immunoprecipitated proteins gp86 and gp72, whereas a monoclonal antibody against MuLV p15(E) immunoprecipitated gp86 and p19. These results indicate that proteins gp86, gp72, and p19 expressed in NIH(MCF-7) transformants correspond to MuLV envelope proteins gp80env, gp70, and p15(E), respectively. The transformation-associated protein p70 appears to be a non-envelope MuLV protein, most likely p65gag. Northern blot analysis confirmed that transformation of NIH cells by MCF-7 mammary carcinoma DNA led to the induction of an endogenous MuLV provirus.

Transformation of NIH 3T3 cells by DNA of the MCF-7 human mammary carcinoma cell line induces expression of an endogenous murine leukemia provirus

Previous studies identified two glycoproteins of 86 (gp86) and 72 (gp72) kilodaltons and two nonglycosylated proteins of 70 (p70) and 19 (p19) kilodaltons which were specifically expressed in NIH cells transformed by DNA of the MCF-7 human mammary carcinoma cell line. Pulse-chase experiments and the use of tunicamycin to inhibit glycosylation suggested that gp86, gp72, and p19 were related as precursor products. Characteristics of the four transformation-associated proteins resembled those of murine leukemia virus (MuLV) proteins. Sera raised against disrupted MuLV immunoprecipitated the same four proteins in extracts of NIH(MCF-7) cells and MuLV-infected NIH 3T3 cells. In addition, a monoclonal antibody against MuLV gp70 immunoprecipitated proteins gp86 and gp72, whereas a monoclonal antibody against MuLV p15(E) immunoprecipitated gp86 and p19. These results indicate that proteins gp86, gp72, and p19 expressed in NIH(MCF-7) transformants correspond to MuLV envelope proteins gp80env, gp70, and p15(E), respectively. The transformation-associated protein p70 appears to be a non-envelope MuLV protein, most likely p65gag. Northern blot analysis confirmed that transformation of NIH cells by MCF-7 mammary carcinoma DNA led to the induction of an endogenous MuLV provirus.

Detection of a molecular complex between ras proteins and transferrin receptor

Immunoprecipitation of extracts of human carcinoma cell lines with three different monoclonal antibodies generated against ras proteins revealed the coprecipitation of a 90,000 dalton protein. The coprecipitated protein was identified as the transferrin receptor by comigration in both reducing and nonreducing SDS-polyacrylamide gels, by absorption with a monoclonal antibody directed against transferrin receptor, and by analysis of partial proteolysis products. Coprecipitation of the transferrin receptor with three monoclonal antibodies with differing specificities to ras proteins, as well as the inability to coprecipitate the transferrin receptor from cell extracts from which ras proteins were depleted by preabsorption, indicates that ras proteins and the transferrin receptor form a molecular complex. This complex is disrupted by addition of transferrin to cell extracts. These findings suggest that ras proteins function in regulation of cell growth via interaction with the cell surface receptor for transferrin.