Augmented expression of normal c-myc is sufficient for cotransformation of rat embryo cells with a mutant ras gene

Aurora-A overexpression enhances cell-aggregation of Ha-ras transformants through the MEK/ERK signaling pathway

BACKGROUND

Overexpression of Aurora-A and mutant Ras (RasV12) together has been detected in human bladder cancer tissue. However, it is not clear whether this phenomenon is a general event or not. Although crosstalk between Aurora-A and Ras signaling pathways has been reported, the role of these two genes acting together in tumorigenesis remains unclear.

METHODS

Real-time PCR and sequence analysis were utilized to identify Ha- and Ki-ras mutation (Gly -> Val). Immunohistochemistry staining was used to measure the level of Aurora-A expression in bladder and colon cancer specimens. To reveal the effect of overexpression of the above two genes on cellular responses, mouse NIH3T3 fibroblast derived cell lines over-expressing either RasV12 and wild-type Aurora-A (designated WT) or RasV12 and kinase-inactivated Aurora-A (KD) were established. MTT and focus formation assays were conducted to measure proliferation rate and focus formation capability of the cells. Small interfering RNA, pharmacological inhibitors and dominant negative genes were used to dissect the signaling pathways involved.

RESULTS

Overexpression of wild-type Aurora-A and mutation of RasV12 were detected in human bladder and colon cancer tissues. Wild-type Aurora-A induces focus formation and aggregation of the RasV12 transformants. Aurora-A activates Ral A and the phosphorylation of AKT as well as enhances the phosphorylation of MEK, ERK of WT cells. Finally, the Ras/MEK/ERK signaling pathway is responsible for Aurora-A induced aggregation of the RasV12 transformants.

CONCLUSION

Wild-type-Aurora-A enhances focus formation and aggregation of the RasV12 transformants and the latter occurs through modulating the Ras/MEK/ERK signaling pathway.

A Novel Role for H-Ras in the Regulation of Very Late Antigen-4 Integrin and VCAM-1 Via c-Myc-Dependent and -Independent Mechanisms

Despite extensive studies on the crucial functions of Ras and c-Myc in cellular proliferation and transformation, their roles in regulating cell adhesion are not yet fully understood. Involvement of Ras in modulating integrin activity by inside-out signaling has been recently reported. However, in contrast to R-Ras, H-Ras was found to exhibit a suppressive effect. Here we show that ectopic expression of a constitutively active H-Rasv12, but not c-Myc alone, in a hemopoietic cell line induces activation of very late Ag-4 (VLA-4, α4β1) integrin without changing its surface expression. Intriguingly, coexpression of H-Rasv12 and c-Myc in these cells results in not only the activation of VLA-4, but also the induction of expression of VCAM-1, the counterreceptor for VLA-4, thereby mediating a marked homotypic cell aggregation. In addition, H-Rasv12-induced VLA-4 activation appears to be partly down-regulated by coexpression with c-Myc. Our results represent an unprecedented example demonstrating a novel role for H-Rasv12 in the regulation of cell adhesion via c-Myc-independent and -dependent mechanisms.

Dominant transformation by mutated human ras genes in vitro requires more than 100 times higher expression than is observed in cancers

The gene-mutation-cancer hypothesis holds that mutated cellular protooncogenes, such as point-mutated proto-ras, "play a dominant part in cancer," because they are sufficient to transform transfected mouse cell lines in vitro [Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K. & Watson, J. D. (1994) Molecular Biology of the Cell (Garland, New York)]. However, in cells transformed in vitro mutated human ras genes are expressed more than 100-fold than in the cancers from which they are isolated. In view of the discrepancy between the very low levels of ras transcription in cancers and the very high levels in cells transformed in vitro, we have investigated the minimal level of human ras expression for transformation in vitro. Using point-mutated human ras genes recombined with different promoters from either human metallothionein-IIA or human fibronectin or from retroviruses we found dominant in vitro transformation of the mouse C3H cell line only with ras genes linked to viral promoters. These ras genes were expressed more than 120-fold higher than are native ras genes of C3H cells. The copy number of transfected ras genes ranged from 2-6 in our system. In addition, nondominant transformation was observed in a small percentage (2-7%) of C3H cells transfected with ras genes that are expressed less than 20 times higher than native C3H ras genes. Because over 90% of cells expressing ras at this moderately enhanced level were untransformed, transformation must follow either a nondominant ras mechanism or a non-ras mechanism. We conclude that the mutated, but normally expressed, ras genes found in human and animal cancers are not likely to "play a dominant part in cancer." The conclusion that mutated ras genes are not sufficient or dominant for cancer is directly supported by recent discoveries of mutated ras in normal animals, and in benign human tissue, "which has little potential to progress" [Jen, J., Powell, S. M., Papadopoulos, N., Smith, K. J., Hamilton, S. R., Vogelstein, B. & Kinzler, K. W. (1994) Cancer Res. 54, 5523-5526]. Even the view that mutated ras is necessary for cancer is hard to reconcile with (i) otherwise indistinguishable cancers with and without ras mutations, (ii) metastases of the same human cancers with and without ras mutations, (iii) retroviral ras genes that are oncogenic without point mutations, and (iv) human tumor cells having spontaneously lost ras mutation but not tumorigencity.

Host range restrictions of oncogenes: myc genes transform avian but not mammalian cells and mht/raf genes transform mammalian but not avian cells

The host range of retroviral oncogenes is naturally limited by the host range of the retroviral vector. The question of whether the transforming host range of retroviral oncogenes is also restricted by the host species has not been directly addressed. Here we have tested in avian and murine host species the transforming host range of two retroviral onc genes, myc of avian carcinoma viruses MH2 and MC29 and mht/raf of avian carcinoma virus MH2 and murine sarcoma virus MSV 3611. Virus vector-mediated host restriction was bypassed by recombining viral oncogenes with retroviral vectors that can readily infect the host to be tested. It was found that, despite high expression, transforming function of retroviral myc genes is restricted to avian cells, and that of retroviral mht/raf genes is restricted to murine cells. Since retroviral oncogenes encode the same proteins as certain cellular genes, termed protooncogenes, our data must also be relevant to the oncogene hypothesis of cancer. According to this hypothesis, cancer is caused by mutation of protooncogenes. Because protooncogenes are conserved in evolution and are presumed to have conserved functions, the oncogene hypothesis assumes no host range restriction of transforming function. For example, mutated human proto-myc is postulated to cause Burkitt lymphoma, because avian retroviruses with myc genes cause cancer in birds. But there is no evidence that known mutated protooncogenes can transform human cells. The findings reported here indicate that host range restriction appears to be one of the reasons (in addition to insufficient transcriptional activation) why known, mutated protooncogenes lack transforming function in human cells.

Cell density and paradoxical transcriptional properties of c-Myc and Max in cultured mouse fibroblasts

Deregulated expression of the c-Myc oncoprotein occurs in several human malignancies. The c-Myc protein behaves as a transcription factor, and undoubtedly its role in carcinogenesis involves its ability to affect the expression of genes involved in cell growth. c-Myc has been reported to both activate and repress transcription in transient transfection experiments using reporter constructs bearing multiple copies of the c-Myc binding site, CAC (G/A) TG. We investigated these apparently paradoxical effects of c-Myc by determining if they arose from differences in the cell proliferation states of transfected cells. We found that endogenous c-Myc protein levels vary inversely with the degree of cell confluency, such that at low cell confluency, where endogenous levels of c-Myc are high and presumably endogenous levels of Max are limiting, exogenous c-Myc fails to affect basal transcription. In cells at high cell confluency, in which endogenous c-Myc levels are low, exogenous c-Myc augments transactivation by titrating the relative excess endogenous Max. These observations suggest that the apparently paradoxical behavior of c-Myc in transfection experiments is partially dependent on ambient cellular levels of c-Myc.

Ornithine decarboxylase is a mediator of c-Myc-induced apoptosis

c-Myc plays a central role in the regulation of cell cycle progression, differentiation, and apoptosis. However, the proteins which mediate c-Myc function(s) remain to be determined. Enforced c-myc expression rapidly induces apoptosis in interleukin-3 (IL-3)-dependent 32D.3 murine myeloid cells following IL-3 withdrawal, and this is associated with the constitutive, growth factor-independent expression of ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis. Here we have examined the role of ODC in c-Myc-induced apoptosis. Enforced expression of ODC, like c-myc, is sufficient to induce accelerated death following IL-3 withdrawal. ODC induced cell death in a dose-dependent fashion, and alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, effectively blocked ODC-induced cell death. ODC-induced cell death was due to the induction of apoptosis. We also demonstrate that ODC is a mediator of c-Myc-induced apoptosis. 32D.3-derived c-myc clones have augmented levels of ODC enzyme activity, and their rates of death were also a function of their ODC enzyme levels. Importantly, the rates of death of c-myc clones were inhibited by treatment with DFMO. These findings demonstrate that ODC is an important mediator of c-Myc-induced apoptosis and suggest that ODC mediates other c-Myc functions.

Ornithine decarboxylase is a mediator of c-Myc-induced apoptosis

c-Myc plays a central role in the regulation of cell cycle progression, differentiation, and apoptosis. However, the proteins which mediate c-Myc function(s) remain to be determined. Enforced c-myc expression rapidly induces apoptosis in interleukin-3 (IL-3)-dependent 32D.3 murine myeloid cells following IL-3 withdrawal, and this is associated with the constitutive, growth factor-independent expression of ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis. Here we have examined the role of ODC in c-Myc-induced apoptosis. Enforced expression of ODC, like c-myc, is sufficient to induce accelerated death following IL-3 withdrawal. ODC induced cell death in a dose-dependent fashion, and alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, effectively blocked ODC-induced cell death. ODC-induced cell death was due to the induction of apoptosis. We also demonstrate that ODC is a mediator of c-Myc-induced apoptosis. 32D.3-derived c-myc clones have augmented levels of ODC enzyme activity, and their rates of death were also a function of their ODC enzyme levels. Importantly, the rates of death of c-myc clones were inhibited by treatment with DFMO. These findings demonstrate that ODC is an important mediator of c-Myc-induced apoptosis and suggest that ODC mediates other c-Myc functions.

Activation domains of L-Myc and c-Myc determine their transforming potencies in rat embryo cells

Members of the Myc family of proteins share a number of protein motifs that are found in regulators of gene transcription. Conserved stretches of amino acids found in the N-terminal transcriptional activation domain of c-Myc are required for cotransforming activity. Most of the Myc proteins contain the basic helix-loop-helix zipper (bHLH-Zip) DNA-binding motif which is also required for the cotransforming activity of c-Myc. L-Myc, the product of a myc family gene that is highly amplified in many human lung carcinomas, was found to cotransform primary rat embryo cells with an activated ras gene. However, L-Myc cotransforming activity was only 1 to 10% of that of c-Myc (M. J. Birrer, S. Segal, J. S. DeGreve, F. Kaye, E. A. Sausville, and J. D. Minna, Mol. Cell. Biol. 8:2668-2673, 1988). We sought to determine whether functional differences between c-Myc and L-Myc in either the N-terminal or the C-terminal domain could account for the relatively diminished L-Myc cotransforming activity. Although the N-terminal domain of L-Myc could activate transcription when fused to the yeast GAL4 DNA-binding domain, the activity was only 5% of that of a comparable c-Myc domain. We next determined that the interaction of the C-terminal bHLH-Zip region of L-Myc or c-Myc with that of a Myc partner protein, Max, was equivalent in transfected cells. A Max expression vector was found to augment the cotransforming activity of L-Myc as well as that of c-Myc. In addition, a bacterially synthesized DNA-binding domain of L-Myc, like that o c-Myc, heterodimerizes with purified Max protein to bind the core DNA sequence CACGTG. To determine the region of L-Myc responsible for its relatively diminished cotransforming activity, we constructed chimeras containing exons 2 (constituting activation domains) and 3 (constituting DNA-binding domains) of c-Myc fused to those of L-Myc. The cotransforming potencies of these chimeras were compared with those of full-length L-Myc of c-Myc in rat embryo cells. The relative cotransforming activities suggest that the potencies of the activation domains determine the cotransforming efficiencies for c-Myc and L-Myc. This correlation supports the hypothesis that the Myc proteins function in neoplastic cotransformation as transcription factors.

Activation domains of L-Myc and c-Myc determine their transforming potencies in rat embryo cells

Members of the Myc family of proteins share a number of protein motifs that are found in regulators of gene transcription. Conserved stretches of amino acids found in the N-terminal transcriptional activation domain of c-Myc are required for cotransforming activity. Most of the Myc proteins contain the basic helix-loop-helix zipper (bHLH-Zip) DNA-binding motif which is also required for the cotransforming activity of c-Myc. L-Myc, the product of a myc family gene that is highly amplified in many human lung carcinomas, was found to cotransform primary rat embryo cells with an activated ras gene. However, L-Myc cotransforming activity was only 1 to 10% of that of c-Myc (M. J. Birrer, S. Segal, J. S. DeGreve, F. Kaye, E. A. Sausville, and J. D. Minna, Mol. Cell. Biol. 8:2668-2673, 1988). We sought to determine whether functional differences between c-Myc and L-Myc in either the N-terminal or the C-terminal domain could account for the relatively diminished L-Myc cotransforming activity. Although the N-terminal domain of L-Myc could activate transcription when fused to the yeast GAL4 DNA-binding domain, the activity was only 5% of that of a comparable c-Myc domain. We next determined that the interaction of the C-terminal bHLH-Zip region of L-Myc or c-Myc with that of a Myc partner protein, Max, was equivalent in transfected cells. A Max expression vector was found to augment the cotransforming activity of L-Myc as well as that of c-Myc. In addition, a bacterially synthesized DNA-binding domain of L-Myc, like that o c-Myc, heterodimerizes with purified Max protein to bind the core DNA sequence CACGTG. To determine the region of L-Myc responsible for its relatively diminished cotransforming activity, we constructed chimeras containing exons 2 (constituting activation domains) and 3 (constituting DNA-binding domains) of c-Myc fused to those of L-Myc. The cotransforming potencies of these chimeras were compared with those of full-length L-Myc of c-Myc in rat embryo cells. The relative cotransforming activities suggest that the potencies of the activation domains determine the cotransforming efficiencies for c-Myc and L-Myc. This correlation supports the hypothesis that the Myc proteins function in neoplastic cotransformation as transcription factors.

The mRNA 5' cap-binding protein, eIF-4E, cooperates with v-myc or E1A in the transformation of primary rodent fibroblasts

We present evidence that eIF-4E, the mRNA 5' cap-binding protein, cooperates with two immortalizing oncogenes, v-myc and E1A, to cause transformation of rat embryo fibroblasts. eIF-4E alone can transform rat embryo fibroblasts when selection is applied. The pattern of transformation by eIF-4E is similar to that of p21 Ras, raising the possibility that eIF-4E shares a common signal transduction pathway with p21 Ras.

The mRNA 5' cap-binding protein, eIF-4E, cooperates with v-myc or E1A in the transformation of primary rodent fibroblasts

We present evidence that eIF-4E, the mRNA 5' cap-binding protein, cooperates with two immortalizing oncogenes, v-myc and E1A, to cause transformation of rat embryo fibroblasts. eIF-4E alone can transform rat embryo fibroblasts when selection is applied. The pattern of transformation by eIF-4E is similar to that of p21 Ras, raising the possibility that eIF-4E shares a common signal transduction pathway with p21 Ras.

Progestins both stimulate and inhibit breast cancer cell cycle progression while increasing expression of transforming growth factor alpha, epidermal growth factor receptor, c-fos, and c-myc genes

This study documents a biphasic change in the rate of cell cycle progression and proliferation of T-47D human breast cancer cells treated with synthetic progestins, consisting of an initial transient acceleration in transit through G1, followed by cell cycle arrest and growth inhibition. Both components of the response were mediated via the progesterone receptor. The data are consistent with a model in which the action of progestins is to accelerate cells already progressing through G1, which are then arrested early in G1 after completing a round of replication, as are cells initially in other phases of the cell cycle. Such acceleration implies that progestins act on genes or gene products which are rate limiting for cell cycle progression. Increased production of epidermal growth factor and transforming growth factor alpha, putative autocrine growth factors in breast cancer cells, does not appear to account for the initial response to progestins, since although the mRNA abundance for these growth factors is rapidly induced by progestins, cells treated with epidermal growth factor or transforming growth factor alpha did not enter S phase until 5 to 6 h later than those stimulated by progestin. The proto-oncogenes c-fos and c-myc were rapidly but transiently induced by progestin treatment, paralleling the well-known response of these genes to mitogenic signals in other cell types. The progestin antagonist RU 486 inhibited progestin regulation of both cell cycle progression and c-myc expression, suggesting that this proto-oncogene may participate in growth modulation by progestins.

Progestins both stimulate and inhibit breast cancer cell cycle progression while increasing expression of transforming growth factor alpha, epidermal growth factor receptor, c-fos, and c-myc genes

This study documents a biphasic change in the rate of cell cycle progression and proliferation of T-47D human breast cancer cells treated with synthetic progestins, consisting of an initial transient acceleration in transit through G1, followed by cell cycle arrest and growth inhibition. Both components of the response were mediated via the progesterone receptor. The data are consistent with a model in which the action of progestins is to accelerate cells already progressing through G1, which are then arrested early in G1 after completing a round of replication, as are cells initially in other phases of the cell cycle. Such acceleration implies that progestins act on genes or gene products which are rate limiting for cell cycle progression. Increased production of epidermal growth factor and transforming growth factor alpha, putative autocrine growth factors in breast cancer cells, does not appear to account for the initial response to progestins, since although the mRNA abundance for these growth factors is rapidly induced by progestins, cells treated with epidermal growth factor or transforming growth factor alpha did not enter S phase until 5 to 6 h later than those stimulated by progestin. The proto-oncogenes c-fos and c-myc were rapidly but transiently induced by progestin treatment, paralleling the well-known response of these genes to mitogenic signals in other cell types. The progestin antagonist RU 486 inhibited progestin regulation of both cell cycle progression and c-myc expression, suggesting that this proto-oncogene may participate in growth modulation by progestins.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Transforming function of proto-ras genes depends on heterologous promoters and is enhanced by specific point mutations

Based on transfection into cells in culture or natural transduction into retroviruses, proto-ras genes seem to derive transforming function either from heterologous promoters or from point mutations. Here we ask how such different events could achieve the same results. To identify homologous regulatory elements, about 3 kilobases of rat DNA upstream of the first untranslated proto-Ha-ras exon was sequenced. Surprisingly, the sequence shares at -1858 a homology of 148 nucleotides with Harvey (Ha) sarcoma virus, 5' of viral ras, signaling possibly a second untranslated proto-Ha-ras exon. In addition the sequence contains a perfect repeat of 25 CA dinucleotides at -2655. A retroviral promoter, even from upstream of the poly(CA), conferred transforming function on proto-Ha-ras and increased transcription greater than 100-fold compared with that of unrearranged proto-ras. Point mutations were not necessary for transforming function of rat and human proto-Ha-ras genes with retroviral promoters but did enhance it greater than 10-fold. A unifying hypothesis proposes that proto-ras genes depend on high expression from heterologous promoters or enhancers for transforming function, which is modulated by ras point mutations. The hypothesis makes two testable predictions. (i) Unrearranged proto-ras genes with point mutations, which occur in some cancers, have no transforming function. Indeed, tumors with mutated proto-ras genes, even those that also lack hypothetical tumor-suppressor genes, are indistinguishable from counterparts with normal proto-ras genes. (ii) Proto-ras genes in transfected cells derive transforming function from heterologous promoters or enhancers acquired via illegitimate recombination from vector DNAs and particularly from viral helper genes that must be cotransfected for transformation of primary cells. Indeed, expression of exogenous proto-ras genes in cells transformed by transfection is as high as for viral ras genes and is much higher than in the cells of origin.

In vitro mRNA degradation system to study the virion host shutoff function of herpes simplex virus

The virion host shutoff (vhs) gene of herpes simplex virus encodes a virion polypeptide that induces degradation of host mRNAs at early times and rapid turnover of viral mRNAs throughout infection. To better investigate the vhs function, an in vitro mRNA degradation system was developed, consisting of cytoplasmic extracts from HeLa cells infected with wild-type herpes simplex virus type 1 or a mutant encoding a defective vhs polypeptide. Host and viral mRNAs were degraded rapidly in extracts from cells productively infected with wild-type herpes simplex virus type 1 but not in extracts from mock-infected cells or cells infected with the mutant vhs1. In contrast, 28S rRNA was stable in all three kinds of extract. Accelerated turnover of host mRNAs was also observed in extracts from cells infected with wild-type virus in the presence of dactinomycin, indicating that the activity was induced by a structural component of the infecting virions. The in vitro vhs activity was inactivated by heat or proteinase K digestion but was insensitive to brief treatment of the extracts with micrococcal nuclease. It was not inhibited by placental RNase inhibitor, it exhibited a strong dependence upon added Mg2+, it was active at concentrations of K+ up to 200 mM, and it did not require the components of an energy-generating system. In summary, the in vitro mRNA degradation system appears to accurately reproduce the vhs-mediated decay of host and viral mRNAs and should be useful for studies of the mechanism of vhs action.

Neoplastic transformation of mouse mammary epithelial cells by deregulated myc expression

A spontaneously immortalized, nontumorigenic mouse mammary epithelial cell line (MMEC) was transfected with an activated myc construct by electroporation. Constitutive expression of myc in MMEC resulted in anchorage independence in soft agar and tumorigenicity in nude mice. The myc-expressing MMEC showed higher saturation density, faster growth rate, and partial abrogation of serum-derived growth factor(s) requirement compared with parent MMEC. Epidermal growth factor or transforming growth factor alpha stimulated the anchorage-independent growth, but not the anchorage-dependent growth, of MMEC-myc cells. Type 1 transforming growth factor beta, on the other hand, inhibited both the anchorage-independent and anchorage-dependent growth of MMEC-myc cells. These results demonstrate that deregulated expression of myc results in neoplastic transformation iin mammary epithelial cells. Accompanying the transformation is altered sensitivity to polypeptide growth factors.

Induction of differentiation in v-Ha-ras-transformed MDCK cells by prostaglandin E2 and 8-bromo-cyclic AMP is associated with a decrease in steady-state level of inositol 1,4,5-trisphosphate

We used Ha-ras-transformed Madin-Darby canine kidney (MDCK) cells as a model to study possible signal transduction mechanisms underlying the induction of glucagon responsiveness by the differentiation inducers prostaglandin E2 (PGE2) and 8-bromo-cyclic (8-Br-cAMP) AMP and the inhibition of induction by phorbol ester or a serum factor. The steady-state level of inositol 1,4,5-trisphosphate (IP3) was higher in Ha-ras-transformed MDCK cells than in parental MDCK cells. In contrast, the steady-state level of intracellular cAMP of transformed cells was similar to that of normal cells. PGE2 and 8-Br-cAMP increased cAMP content but decreased IP3 levels in a concentration-dependent fashion after 5 days of treatment. We examined the time course for effects of PGE2 and 8-Br-cAMP and found that there was a lag period of 8 to 16 h between elevation of cAMP after the addition of 8-Br-cAMP or PGE2 and the decrease of IP3 levels. Another lag period of 2 days existed before the induction of differentiation. Both the reduction of IP3 levels and the induction of glucagon responsiveness were blocked by phorbol-12-myristate-13-acetate or serum, suggesting that a decrease in the IP3 level might be causally involved in induction of differentiation in transformed MDCK cells. However, induction of differentiation was not due to changes in the expression or guanine nucleotide-binding properties of p21 protein. It is likely that cAMP has a direct regulatory effect on the phospholipid signaling pathway. We conclude that perturbation of the inositol phosphate signaling pathway may be responsible for the induction of differentiation by PGE2 and 8-Br-cAMP in transformed MDCK cells.

Induction of differentiation in v-Ha-ras-transformed MDCK cells by prostaglandin E2 and 8-bromo-cyclic AMP is associated with a decrease in steady-state level of inositol 1,4,5-trisphosphate

We used Ha-ras-transformed Madin-Darby canine kidney (MDCK) cells as a model to study possible signal transduction mechanisms underlying the induction of glucagon responsiveness by the differentiation inducers prostaglandin E2 (PGE2) and 8-bromo-cyclic (8-Br-cAMP) AMP and the inhibition of induction by phorbol ester or a serum factor. The steady-state level of inositol 1,4,5-trisphosphate (IP3) was higher in Ha-ras-transformed MDCK cells than in parental MDCK cells. In contrast, the steady-state level of intracellular cAMP of transformed cells was similar to that of normal cells. PGE2 and 8-Br-cAMP increased cAMP content but decreased IP3 levels in a concentration-dependent fashion after 5 days of treatment. We examined the time course for effects of PGE2 and 8-Br-cAMP and found that there was a lag period of 8 to 16 h between elevation of cAMP after the addition of 8-Br-cAMP or PGE2 and the decrease of IP3 levels. Another lag period of 2 days existed before the induction of differentiation. Both the reduction of IP3 levels and the induction of glucagon responsiveness were blocked by phorbol-12-myristate-13-acetate or serum, suggesting that a decrease in the IP3 level might be causally involved in induction of differentiation in transformed MDCK cells. However, induction of differentiation was not due to changes in the expression or guanine nucleotide-binding properties of p21 protein. It is likely that cAMP has a direct regulatory effect on the phospholipid signaling pathway. We conclude that perturbation of the inositol phosphate signaling pathway may be responsible for the induction of differentiation by PGE2 and 8-Br-cAMP in transformed MDCK cells.

DNA-binding domain of human c-Myc produced in Escherichia coli

We have identified the domain of the human c-myc protein (c-Myc) produced in Escherichia coli that is responsible for the ability of the protein to bind sequence-nonspecific DNA. Using analysis of binding of DNA by proteins transferred to nitrocellulose, DNA-cellulose chromatography, and a nitrocellulose filter binding assay, we examined the binding properties of c-Myc peptides generated by cyanogen bromide cleavage, of mutant c-Myc, and of proteins that fuse portions of c-Myc to staphylococcal protein A. The results of these analyses indicated that c-Myc amino acids 265 to 318 were responsible for DNA binding and that other regions of the protein (including a highly conserved basic region and a region containing the leucine zipper motif) were not required. Some mutant c-Mycs that did not bind DNA maintained rat embryo cell-cotransforming activity, which indicated that the c-Myc property of in vitro DNA binding was not essential for this activity. These mutants, however, were unable to transform established rat fibroblasts (Rat-1a cells) that were susceptible to transformation by wild-type c-Myc, although this lack of activity may not have been due to their inability to bind DNA.

Regulation of c-myc mRNA stability in vitro by a labile destabilizer with an essential nucleic acid component

The turnover rates of some mRNAs vary by an order of magnitude or more when cells change their growth pattern or differentiate. To identify regulatory factors that might be responsible for this variability, we investigated how cytosolic fractions affect mRNA decay in an in vitro system. A 130,000 X g supernatant (S130) from the cytosol of exponentially growing erythroleukemia cells contains a destabilizer that accelerates the decay of polysome-bound c-myc mRNA by eightfold or more compared with reactions lacking S130. The destabilizer is deficient in or absent from the S130 of cycloheximide-treated cells, indicating that it is labile or is repressed when translation is blocked. It is not a generic RNase, because it does not affect the turnover of delta-globin, gamma-globin, or histone mRNA and does not destabilize a major portion of polysomal polyadenylated mRNA. The destabilizer accelerates the turnover of the c-myc mRNA 3' region, as well as subsequent 3'-to-5' degradation of the mRNA body. It is inactivated in vitro by mild heating and by micrococcal nuclease, suggesting that it contains a nucleic acid component. c-myb mRNA is also destabilized in S130-supplemented in vitro reactions. These results imply that the stability of some mRNAs is regulated by cytosolic factors that are not associated with polysomes.

The BCR-ABL oncogene transforms Rat-1 cells and cooperates with v-myc

The tyrosine kinase P210 is the gene product of the rearranged BCR-ABL locus on the Philadelphia chromosome (Ph1), which is found in leukemic cells of patients with chronic myelogenous leukemia. It has a weakly oncogenic effect in immature murine hematopoietic cells and does not transform NIH 3T3 cells. We have found that P210 has a strikingly different effect in Rat-1 cells, another line of established rodent fibroblasts. Stable expression of P210 in Rat-1 cells caused a distinct morphological change and conferred both tumorigenicity and capacity for anchorage-independent growth. The introduction of v-myc into Rat-1 cells expressing P210 led to complete morphological transformation and enhanced tumorigenicity. No such interaction took place in NIH 3T3 cells. Thus, Rat-1 cells can be used to detect cooperation between BCR-ABL and other oncogenes and may prove useful for the identification of secondary oncogenic events in chronic myelogenous leukemia.

DNA-binding domain of human c-Myc produced in Escherichia coli

We have identified the domain of the human c-myc protein (c-Myc) produced in Escherichia coli that is responsible for the ability of the protein to bind sequence-nonspecific DNA. Using analysis of binding of DNA by proteins transferred to nitrocellulose, DNA-cellulose chromatography, and a nitrocellulose filter binding assay, we examined the binding properties of c-Myc peptides generated by cyanogen bromide cleavage, of mutant c-Myc, and of proteins that fuse portions of c-Myc to staphylococcal protein A. The results of these analyses indicated that c-Myc amino acids 265 to 318 were responsible for DNA binding and that other regions of the protein (including a highly conserved basic region and a region containing the leucine zipper motif) were not required. Some mutant c-Mycs that did not bind DNA maintained rat embryo cell-cotransforming activity, which indicated that the c-Myc property of in vitro DNA binding was not essential for this activity. These mutants, however, were unable to transform established rat fibroblasts (Rat-1a cells) that were susceptible to transformation by wild-type c-Myc, although this lack of activity may not have been due to their inability to bind DNA.

Regulation of c-myc mRNA stability in vitro by a labile destabilizer with an essential nucleic acid component

The turnover rates of some mRNAs vary by an order of magnitude or more when cells change their growth pattern or differentiate. To identify regulatory factors that might be responsible for this variability, we investigated how cytosolic fractions affect mRNA decay in an in vitro system. A 130,000 X g supernatant (S130) from the cytosol of exponentially growing erythroleukemia cells contains a destabilizer that accelerates the decay of polysome-bound c-myc mRNA by eightfold or more compared with reactions lacking S130. The destabilizer is deficient in or absent from the S130 of cycloheximide-treated cells, indicating that it is labile or is repressed when translation is blocked. It is not a generic RNase, because it does not affect the turnover of delta-globin, gamma-globin, or histone mRNA and does not destabilize a major portion of polysomal polyadenylated mRNA. The destabilizer accelerates the turnover of the c-myc mRNA 3' region, as well as subsequent 3'-to-5' degradation of the mRNA body. It is inactivated in vitro by mild heating and by micrococcal nuclease, suggesting that it contains a nucleic acid component. c-myb mRNA is also destabilized in S130-supplemented in vitro reactions. These results imply that the stability of some mRNAs is regulated by cytosolic factors that are not associated with polysomes.

The BCR-ABL oncogene transforms Rat-1 cells and cooperates with v-myc

The tyrosine kinase P210 is the gene product of the rearranged BCR-ABL locus on the Philadelphia chromosome (Ph1), which is found in leukemic cells of patients with chronic myelogenous leukemia. It has a weakly oncogenic effect in immature murine hematopoietic cells and does not transform NIH 3T3 cells. We have found that P210 has a strikingly different effect in Rat-1 cells, another line of established rodent fibroblasts. Stable expression of P210 in Rat-1 cells caused a distinct morphological change and conferred both tumorigenicity and capacity for anchorage-independent growth. The introduction of v-myc into Rat-1 cells expressing P210 led to complete morphological transformation and enhanced tumorigenicity. No such interaction took place in NIH 3T3 cells. Thus, Rat-1 cells can be used to detect cooperation between BCR-ABL and other oncogenes and may prove useful for the identification of secondary oncogenic events in chronic myelogenous leukemia.

Sequential expression and cooperative interaction of c-Ha-ras and c-erbB genes in in vivo chemical carcinogenesis

The level of expression of several cellular protooncogenes is examined at different stages of 7,12-dimethylbenzanthracene (DMBA)-induced tumor development in hamster buccal pouch epithelium (HBPE). Results presented demonstrate overexpression of c-Ha-ras gene at a very early stage of tumor development, and this elevated level of expression of the gene persists throughout the tumorigenesis process. The expression of the cellular protooncogene c-erbB, on the other hand, can be detected only after 8-10 weeks of DMBA treatment of the tissue and increases with the progression of the disease. The overexpression of c-erbB gene can be correlated with the stage of extensive proliferation and subsequent invasion of the HBPE cells into the underlying connective tissue. This sequential pattern of stage-specific expression of the two cellular protooncogenes can be observed in (i) treated tissues, (ii) stage-representative cultured cells, and (iii) NIH 3T3 transformants derived with DNA from HBPE cells. The low-level expression of c-myc and c-sis genes detected in control tissues remains unaffected, while c-fos gene activity cannot be detected at any stage of tumor development. The overexpression of c-Ha-ras gene alone in HBPE cells derived from tissues treated for 5 weeks (DM5) is not sufficient to induce tumors in athymic mice, whereas expression of c-Ha-ras and c-erbB genes at later stages of tumor development (DM10 and HCPC cells) induce histopathologically defined epithelial cell carcinoma in athymic mice within 2-3 weeks. The sequential overexpression of c-Ha-ras and c-erbB genes in a stage-specific manner and their cooperative interaction in the DMBA-induced in vivo oral carcinogenesis have been demonstrated.

Identification of the human c-myc protein nuclear translocation signal

We identified and characterized two regions of the human c-myc protein that target proteins into the nucleus. Using mutant c-myc proteins and proteins that fuse portions of c-myc to chicken muscle pyruvate kinase, we found that residues 320 to 328 (PAAKRVKLD; peptide M1) induced complete nuclear localization, and their removal from c-myc resulted in mutant proteins that distributed in both the nucleus and cytoplasm but retained rat embryo cell cotransforming activity. Residues 364 to 374 (RQRRNELKRSP; peptide M2) induced only partial nuclear targeting, and their removal from c-myc resulted in mutant proteins that remained nuclear but were cotransformationally inactive. We conjugated synthetic peptides containing M1 or M2 to human serum albumin and microinjected the conjugate into the cytoplasm of Vero cells. The peptide containing M1 caused rapid and complete nuclear accumulation, whereas that containing M2 caused slower and only partial nuclear localization. Thus, M1 functions as the nuclear localization signal of c-myc, and M2 serves some other and essential function.

Sustained expression of the human protooncogene MYCN rescues rat embryo cells from senescence

Amplification of the human gene MYCN may play a role in the malignant progression of human neuroblastomas. In pursuit of this possibility, previous studies have shown that the abundant expression of MYCN in cultured cells can elicit several aspects of the transformed phenotype. We now extend those findings by demonstrating that rat embryo cells transfected with MYCN can proliferate for at least 200 generations. Isolation of established cells was dependent on high expression of MYCN and on biological selection to eliminate untransfected cells. The established cells were not tumorigenic in syngeneic rats or athymic mice, failed to grow in soft agar, and required relatively high concentrations of serum for proliferation in culture. Our results show that enhanced expression of MYCN can rescue normal cells from senescence, add to the credentials of MYCN as an authentic protooncogene, and identify an additional biological activity that can be used in the characterization of MYCN.

Identification of the human c-myc protein nuclear translocation signal

We identified and characterized two regions of the human c-myc protein that target proteins into the nucleus. Using mutant c-myc proteins and proteins that fuse portions of c-myc to chicken muscle pyruvate kinase, we found that residues 320 to 328 (PAAKRVKLD; peptide M1) induced complete nuclear localization, and their removal from c-myc resulted in mutant proteins that distributed in both the nucleus and cytoplasm but retained rat embryo cell cotransforming activity. Residues 364 to 374 (RQRRNELKRSP; peptide M2) induced only partial nuclear targeting, and their removal from c-myc resulted in mutant proteins that remained nuclear but were cotransformationally inactive. We conjugated synthetic peptides containing M1 or M2 to human serum albumin and microinjected the conjugate into the cytoplasm of Vero cells. The peptide containing M1 caused rapid and complete nuclear accumulation, whereas that containing M2 caused slower and only partial nuclear localization. Thus, M1 functions as the nuclear localization signal of c-myc, and M2 serves some other and essential function.

Absence of missense mutations in activated c-myc genes in avian leukosis virus-induced B-cell lymphomas

We have determined the nucleotide sequences of two independent DNA clones which contained the activated c-myc genes from avian leukosis virus-induced B-cell lymphomas. Neither of these c-myc genes contained missense mutations. This strongly supports the notion that the c-myc proto-oncogene in avian leukosis virus-induced B-cell lymphomas can be oncogenically activated by altered expression of the gene without a change in the primary structure of the gene product.

L-myc cooperates with ras to transform primary rat embryo fibroblasts

Recent molecular analysis has revealed that L-myc has several domains of extremely conserved amino acid sequence homology with c-myc and N-myc, suggesting similarity of function. We tested the biologic activity of L-myc by using an expression vector containing a cDNA clone coding for the major open reading frame in the 3.9-kilobase mRNA of L-myc under the control of a strong promoter (Moloney long terminal repeat) and found that L-myc complemented an activated ras gene in transforming primary rat embryo fibroblasts. However, the efficiency of transformation was 1 to 10% of that seen with the c-myc and simian virus 40 (SV40) controls. The L-myc/ras transformants initially grew more slowly than c-myc or SV40 transformants, but once established as continuous cell lines, they were indistinguishable from cell lines derived from c-myc/ras or SV40/ras transfectants as determined by morphology, soft-agar cloning, and tumorigenicity in nude mice.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

L-myc cooperates with ras to transform primary rat embryo fibroblasts

Recent molecular analysis has revealed that L-myc has several domains of extremely conserved amino acid sequence homology with c-myc and N-myc, suggesting similarity of function. We tested the biologic activity of L-myc by using an expression vector containing a cDNA clone coding for the major open reading frame in the 3.9-kilobase mRNA of L-myc under the control of a strong promoter (Moloney long terminal repeat) and found that L-myc complemented an activated ras gene in transforming primary rat embryo fibroblasts. However, the efficiency of transformation was 1 to 10% of that seen with the c-myc and simian virus 40 (SV40) controls. The L-myc/ras transformants initially grew more slowly than c-myc or SV40 transformants, but once established as continuous cell lines, they were indistinguishable from cell lines derived from c-myc/ras or SV40/ras transfectants as determined by morphology, soft-agar cloning, and tumorigenicity in nude mice.

Oncogene v-src transforms and establishes embryonic rodent fibroblasts but not diploid human fibroblasts

The conversion of cells from a normal phenotype to full malignancy apparently requires multiple genetic events. Efforts to reconstruct multistep tumorigenesis in cell culture have shown that two types of oncogenes (typified by HRAS and MYC) can cooperate to elicit complete transformation. Transformation of embryonic rodent cells by single oncogenes is reputed either not to occur or to require specialized circumstances. It has not been known how the large group of oncogenes that encode protein-tyrosine kinases might fit into this scheme. We now report that v-src, a prototype for the kinase oncogenes, can convert rat embryo fibroblasts to a fully transformed and tumorigenic phenotype when the gene is expressed vigorously. By contrast, v-src had no demonstrable effect on diploid human fibroblasts. Our results sustain the view that it is possible for at least some oncogenes to achieve a potency sufficient for unilateral tumorigenesis.

myc protooncogene linked to retroviral promoter, but not to enhancer, transforms embryo cells

To define conditions under which the chicken protooncogene p-myc is converted to a viral and possibly to a cellular transforming gene, we assayed transforming function of hybrid genes put together from cloned retroviral and p-myc elements and of p-myc genes isolated from spontaneous viral lymphomas. Transforming function was measured in quail embryo cells transfected with cloned myc genes. We found that only myc genes with a promoter of a retroviral long terminal repeat (LTR) located between the native p-myc promoter and the second p-myc exon have transforming function. Transforming efficiencies decreased with increasing lengths of unspliced sequences between the LTR and p-myc exon 2. p-myc DNAs with LTRs downstream of the coding region or upstream but in the opposite transcriptional orientation failed to transform embryo cells. Likewise, only those retroviral-p-myc combinations from chicken B-cell lymphomas with a LTR positioned as promoter upstream of p-myc exon 2 had transforming function. We conclude that substitution of a retroviral LTR for the promoter and for as yet poorly defined, untranscribed regulatory elements of p-myc is sufficient to convert chicken p-myc to a transforming gene. However, retroviral LTRs can only convert p-myc genes to embryo-cell-transforming genes from a limited number of positions, and not as position-independent enhancers. Further, we deduce that there are two classes of viral chicken B-cell lymphomas, those with and those without embryo-cell-transforming p-myc genes.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Neoplastic transformation by the human gene N-myc

Amplification and abundant expression of a gene known as N-myc are found frequently in advanced stages of human neuroblastoma and may play a role in the genesis of several malignant human tumors. Previous studies have shown that N-myc can cooperate with a mutant allele of the proto-oncogene c-Ha-ras to transform embryonic rat cells in culture. Here we show that N-myc can also act alone to elicit neoplastic growth of an established line of rat fibroblasts (Rat-1). We used recombinant DNA vectors to express either N-myc or its kindred gene c-myc in transfected cells. Both genes caused morphological transformation, anchorage-independent growth, and tumorigenicity. We noticed two variables that appeared to influence the ability to isolate cells transformed by N-myc and c-myc: the abundance in which the genes were expressed and biological selection to eliminate untransformed cells from the cultures. Our findings sustain the belief that N-myc is an authentic proto-oncogene, lend further credibility to the role of N-myc in the genesis of human tumors, and establish a convenient assay that can be used to explore further the properties of both N-myc and c-myc.

Definition of regions in human c-myc that are involved in transformation and nuclear localization

To study the relationship between the primary structure of the c-myc protein and some of its functional properties, we made in-frame insertion and deletion mutants of the normal human c-myc coding domain that was expressed from a retroviral promoter-enhancer. We assessed the effects of these mutations on the ability of c-myc protein to cotransform normal rat embryo cells with a mutant ras gene, induce foci in a Rat-1-derived cell line (Rat-1a), and localize in nuclei. Using the cotransformation assay, we found two regions of the protein (amino acids 105 to 143 and 321 to 439) where integrity was critical: one region (amino acids 1 to 104) that tolerated insertion and small deletion mutations, but not large deletions, and another region (amino acids 144) to 320) that was largely dispensable. Comparison with regions that were important for transformation of Rat-1a cells revealed that some are essential for both activities, but others are important for only one or the other, suggesting that the two assays require different properties of the c-myc protein. Deletion of each of three regions of the c-myc protein (amino acids 106 to 143, 320 to 368, and 370 to 412) resulted in partial cytoplasmic localization, as determined by immunofluorescence or immunoprecipitation following subcellular fractionation. Some abnormally located proteins retained transforming activity; most proteins lacking transforming activity appeared to be normally located.

Neoplastic transformation by the human gene N-myc

Amplification and abundant expression of a gene known as N-myc are found frequently in advanced stages of human neuroblastoma and may play a role in the genesis of several malignant human tumors. Previous studies have shown that N-myc can cooperate with a mutant allele of the proto-oncogene c-Ha-ras to transform embryonic rat cells in culture. Here we show that N-myc can also act alone to elicit neoplastic growth of an established line of rat fibroblasts (Rat-1). We used recombinant DNA vectors to express either N-myc or its kindred gene c-myc in transfected cells. Both genes caused morphological transformation, anchorage-independent growth, and tumorigenicity. We noticed two variables that appeared to influence the ability to isolate cells transformed by N-myc and c-myc: the abundance in which the genes were expressed and biological selection to eliminate untransformed cells from the cultures. Our findings sustain the belief that N-myc is an authentic proto-oncogene, lend further credibility to the role of N-myc in the genesis of human tumors, and establish a convenient assay that can be used to explore further the properties of both N-myc and c-myc.

Definition of regions in human c-myc that are involved in transformation and nuclear localization

To study the relationship between the primary structure of the c-myc protein and some of its functional properties, we made in-frame insertion and deletion mutants of the normal human c-myc coding domain that was expressed from a retroviral promoter-enhancer. We assessed the effects of these mutations on the ability of c-myc protein to cotransform normal rat embryo cells with a mutant ras gene, induce foci in a Rat-1-derived cell line (Rat-1a), and localize in nuclei. Using the cotransformation assay, we found two regions of the protein (amino acids 105 to 143 and 321 to 439) where integrity was critical: one region (amino acids 1 to 104) that tolerated insertion and small deletion mutations, but not large deletions, and another region (amino acids 144) to 320) that was largely dispensable. Comparison with regions that were important for transformation of Rat-1a cells revealed that some are essential for both activities, but others are important for only one or the other, suggesting that the two assays require different properties of the c-myc protein. Deletion of each of three regions of the c-myc protein (amino acids 106 to 143, 320 to 368, and 370 to 412) resulted in partial cytoplasmic localization, as determined by immunofluorescence or immunoprecipitation following subcellular fractionation. Some abnormally located proteins retained transforming activity; most proteins lacking transforming activity appeared to be normally located.

Coinfection with viruses carrying the v-Ha-ras and v-myc oncogenes leads to growth factor independence by an indirect mechanism

The concomitant expression of certain oncogenes can transform normal diploid rodent cells into transplantable tumorigenic cells. The mechanism by which these oncogenes collaborate is unclear. Recent findings (M. Oshimura, T. M. Gilmer, and J. C. Barrett, Nature [London] 316:636-639, 1985) raise the possibility that karyotypic changes, including monosomy for chromosome 15, are required to induce tumorigenicity in Syrian hamster embryo cells transfected in vitro with v-Ha-ras and v-myc DNAs. We studied the effect of the oncogenes v-Ha-ras and v-myc, introduced by viral infection, on murine hematopoietic cells. The induction of growth factor independence by the two oncogenes was used as an in vitro correlate of tumorigenicity. After a period of reduced growth rate reminiscent of the growth rate of cells in crisis, the doubly infected cells became growth factor independent. These cells showed a great variability in their karyotypes.

Rearrangement at the 5' end of amplified c-myc in human COLO 320 cells is associated with abnormal transcription

The proto-oncogene c-myc is amplified in sublines of human COLO 320 cells carrying either homogeneously staining chromosomal regions or double minutes. COLO 320 cells carrying homogeneously staining chromosomal regions have 15 to 20 copies of an apparently normal c-myc allele and 1 to 2 copies of an abnormal c-myc allele lacking exon 1 and express high levels of a normal c-myc mRNA 2.5 kilobases in size. COLO 320 cells carrying double minutes have about 25 copies each of the normal allele and the abnormal allele but express preferentially an abnormal c-myc mRNA 2.2 kilobases in size. Nucleotide sequence analyses revealed that the break point of rearrangement resulting in the loss of exon 1 in the abnormal allele lies within a region frequently rearranged in human and murine B-cell tumors.

Behavior of myc and ras oncogenes in transformation of rat embryo fibroblasts

The requirements for transformation of rat embryo fibroblasts (REFs) by transfected ras and myc oncogenes were explored. Under conditions of dense monolayer culture, neither oncogene was able to transform REFs on its own. However, the introduction of a ras oncogene together with a selectable neomycin resistance marker into REFs allowed killing of the normal nontransfected cells and the outgrowth of colonies of ras transformants, 10% of which survived crisis and became tumorigenic. These cells expressed greater than 10-fold-higher levels of ras p21 than tumorigenic cells cotransfected with ras and myc oncogenes. The myc oncogene similarly was unable to induce tumorigenic conversion of REFs unless especially refractile colonies of oncogene-bearing cells, produced by use of a cotransfected selectable marker, were picked and subcultured. Tumorigenic conversion of REFs by single transfected oncogenes appears to require special culture conditions and high levels of gene expression.

Synergism of v-myc and v-Ha-ras in the in vitro neoplastic progression of murine lymphoid cells

Murine bone marrow was either singly or doubly infected with retroviral vectors expressing v-myc (OK10) or v-Ha-ras. The infected bone marrow was cultured in a system that supports the long-term growth of B-lineage lymphoid cells. While the v-myc vector by itself had no apparent effect on lymphoid culture establishment and growth, infection with the v-Ha-ras vector or coinfection with both v-myc and v-Ha-ras vectors led to the appearance of growth-stimulated cell populations. Clonal pre-B-cell lines stably expressing v-Ha-ras alone or both v-myc and v-Ha-ras grew out of these cultures. In comparison with cell lines expressing v-Ha-ras alone, cell lines expressing both v-myc and v-Ha-ras grew to higher densities, had reduced dependence on a feeder layer for growth, and had a marked increase in ability to grow in soft-agar medium. The cell lines expressing both oncogenes were highly tumorigenic in syngeneic animals. These experiments show that the v-myc oncogene in synergy with v-Ha-ras can play a direct role in the in vitro transformation of murine B lymphoid cells.

Coinfection with viruses carrying the v-Ha-ras and v-myc oncogenes leads to growth factor independence by an indirect mechanism

The concomitant expression of certain oncogenes can transform normal diploid rodent cells into transplantable tumorigenic cells. The mechanism by which these oncogenes collaborate is unclear. Recent findings (M. Oshimura, T. M. Gilmer, and J. C. Barrett, Nature [London] 316:636-639, 1985) raise the possibility that karyotypic changes, including monosomy for chromosome 15, are required to induce tumorigenicity in Syrian hamster embryo cells transfected in vitro with v-Ha-ras and v-myc DNAs. We studied the effect of the oncogenes v-Ha-ras and v-myc, introduced by viral infection, on murine hematopoietic cells. The induction of growth factor independence by the two oncogenes was used as an in vitro correlate of tumorigenicity. After a period of reduced growth rate reminiscent of the growth rate of cells in crisis, the doubly infected cells became growth factor independent. These cells showed a great variability in their karyotypes.

Synergism of v-myc and v-Ha-ras in the in vitro neoplastic progression of murine lymphoid cells

Murine bone marrow was either singly or doubly infected with retroviral vectors expressing v-myc (OK10) or v-Ha-ras. The infected bone marrow was cultured in a system that supports the long-term growth of B-lineage lymphoid cells. While the v-myc vector by itself had no apparent effect on lymphoid culture establishment and growth, infection with the v-Ha-ras vector or coinfection with both v-myc and v-Ha-ras vectors led to the appearance of growth-stimulated cell populations. Clonal pre-B-cell lines stably expressing v-Ha-ras alone or both v-myc and v-Ha-ras grew out of these cultures. In comparison with cell lines expressing v-Ha-ras alone, cell lines expressing both v-myc and v-Ha-ras grew to higher densities, had reduced dependence on a feeder layer for growth, and had a marked increase in ability to grow in soft-agar medium. The cell lines expressing both oncogenes were highly tumorigenic in syngeneic animals. These experiments show that the v-myc oncogene in synergy with v-Ha-ras can play a direct role in the in vitro transformation of murine B lymphoid cells.

Rearrangement at the 5' end of amplified c-myc in human COLO 320 cells is associated with abnormal transcription

The proto-oncogene c-myc is amplified in sublines of human COLO 320 cells carrying either homogeneously staining chromosomal regions or double minutes. COLO 320 cells carrying homogeneously staining chromosomal regions have 15 to 20 copies of an apparently normal c-myc allele and 1 to 2 copies of an abnormal c-myc allele lacking exon 1 and express high levels of a normal c-myc mRNA 2.5 kilobases in size. COLO 320 cells carrying double minutes have about 25 copies each of the normal allele and the abnormal allele but express preferentially an abnormal c-myc mRNA 2.2 kilobases in size. Nucleotide sequence analyses revealed that the break point of rearrangement resulting in the loss of exon 1 in the abnormal allele lies within a region frequently rearranged in human and murine B-cell tumors.

Behavior of myc and ras oncogenes in transformation of rat embryo fibroblasts

The requirements for transformation of rat embryo fibroblasts (REFs) by transfected ras and myc oncogenes were explored. Under conditions of dense monolayer culture, neither oncogene was able to transform REFs on its own. However, the introduction of a ras oncogene together with a selectable neomycin resistance marker into REFs allowed killing of the normal nontransfected cells and the outgrowth of colonies of ras transformants, 10% of which survived crisis and became tumorigenic. These cells expressed greater than 10-fold-higher levels of ras p21 than tumorigenic cells cotransfected with ras and myc oncogenes. The myc oncogene similarly was unable to induce tumorigenic conversion of REFs unless especially refractile colonies of oncogene-bearing cells, produced by use of a cotransfected selectable marker, were picked and subcultured. Tumorigenic conversion of REFs by single transfected oncogenes appears to require special culture conditions and high levels of gene expression.