myc and src oncogenes have complementary effects on cell proliferation and expression of specific extracellular matrix components in definitive chondroblasts

Prognostic markers in chondrosarcoma: evaluation of cell proliferation and of regulators of the cell cycle

Purpose. The prognosis, treatment principles and prediction of clinical outcome of patients with chondrosarcoma currently rest on histologic grading which is somewhat ambiguous due to difficulty in pathologic interpretation of this neoplasm. Immunohistochemistry, flow cytometry and oncogene/tumor suppressor gene expression have been examined as alternative indices to predict the biologic behavior of these tumors. Because of partial successes obtained with flow cytometry and because of the improvement in predicting recurrence offered by examining the S-phase fraction, we undertook the current study to determine if expression of specific regulators of the cell cycle would act as prognostic indicators for these patients.Subjects/methods. We examined archival pathologic specimens from 39 patients with at least 2 years' clinical follow-up for the presence of p53, Rb, src and MIB-1 by immunohistochemistry and correlated this with clinical histories and incidence of recurrence.Results. While Rb, p53 and src gene products were identified to a variable extent in these specimens, there was no prognostic significance to their expression. In contrast, MIB-1, an epitope expressed only during semiconservative replication and an accepted marker of cell proliferation, served as a significant prognostic indicator. MIB-1 staining was present in 14.5% of tumor cells in all specimens (range 0-59%). When MIB-1 staining was examined with respect to disease recurrence, there was a statistically significant association between staining and histologic grade (p < 0.05) as well as event-free survival (p < 0.02). Comparing survival curves stratified by MIB-1 expression, there was a significant decrease in event-free survival associated with increasing MIB-1 indices (p < 0.003). Covariates that were associated with event-free survival include histologic grade (p = 0.025) and stage (Musculoskeletal Tumor Society) (p = 0.014). There was no statistical association with patient age (p = 0.15), tumor size (p = 0.47), tumor histology (p = 0.62) or anatomic location (p = 0.316).Discussion. These results indicate that determination of the proliferation index by MIB-1 immunostaining may serve as a useful adjunct to current histopathologic classification. Patients with a high proliferation index may benefit from established adjuvant therapies or experimental approaches including immunotherapy or biologic modulation.

Transcriptional stimulation of the retina-specific QR1 gene upon growth arrest involves a Maf-related protein

The avian neural retina (NR) is derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of postmitotic NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-Src protein. QR1 is a retina-specific gene expressed exclusively at the stage of growth arrest and differentiation during retinal development. In NR cells infected with tsPA101, an RSV mutant conditionally defective in pp60v-src mitogenic capacity, QR1 expression is downregulated in proliferating cells at 37 degrees C and is fully restored when the cells become quiescent as a result of pp60v-src inactivation at 41 degrees C. We were able to arrest proliferation of tsPA101-infected quail NR cells expressing an active v-Src protein by serum starvation at 37 degrees C. This allowed us to investigate the role of cell growth in regulating QR1 transcription. We report that QR1 transcription is stimulated in growth-arrested cells at 37 degrees C compared with that in proliferating cells maintained at the same temperature. Growth arrest-dependent stimulation of QR1 transcription requires the integrity of the A box, a previously characterized cis-acting element responsible for QR1 transcriptional stimulation upon v-Src inactivation and during retinal differentiation. We also show that formation of the C1 complex on the A box is increased upon growth arrest by serum starvation in the presence of an active v-Src oncoprotein. Thus, the C1 complex represents an important link between cell cycle and developmental control of QR1 gene transcription during NR differentiation and RSV infection. By using antibodies directed against different Maf proteins of the leucine zipper family and competition with Maf consensus site-containing oligonucleotides in a gel shift assay, we show that the C1 complex is likely to contain a Maf-related protein. We also show that a purified bacterially expressed v-Maf protein is able to bind the A box and that the level of a 43-kDa Maf-related protein is increased upon growth arrest in infected retinal cells. Moreover, ectopic expression of c-mafI, c-mafII, and mafB cDNAs in quiescent tsPA101-infected quail NR cells is able to stimulate transcription of a QR1 reporter gene through the A box. Therefore, QR1 appears to be the first target gene for a Maf-related protein(s) in the NR.

Developmental control of transcription of a retina-specific gene, QR1, during differentiation: involvement of factors from the POU family

Developmental control of gene expression often results from the coupling of growth arrest with the establishment of differentiation programs. QR1 is a gene specifically expressed in retinas during the late phase of embryogenesis. At this stage neuroectodermal precursors have reached terminal mitosis and are undergoing differentiation into distinct cell types. Transcription of the QR1 gene is tightly regulated during retinal development: this gene is expressed between embryonic day 9 (ED9) and ED17 and is completely repressed at hatching in quail. Moreover, QR1 transcription is downregulated when postmitotic neural retina cells are induced to proliferate by pp60v-src. We studied the stage-dependent transcriptional control of this gene during quail neural retina (QNR) cell development. Transient transfection experiments with QR1/CAT constructs at various stages of development showed that a region located between -935 and -1265 bp upstream of the transcription start site is necessary to promote transcription in retina cells during the late phase of embryonal development (QNR9, corresponding to ED9). By in vivo footprinting assays we identified at least two elements that are occupied by DNA-protein complexes in QNR cells: the A and B boxes. The A box allows formation of several biochemically distinct complexes: C1, C2, C3, and C4. Formation of the C2 complex mainly during early stages (ED7) and of C2, C3, and C4 complexes during postnatal life correlates with repression of QR1 transcription, whereas the C1 complex is strongly induced at ED11 when the QR1 gene is expressed. We previously showed that C1 was involved in downregulation of QR1 transcription by pp60v-src. Several complexes are also formed on the B box. We show that these complexes are exclusively present in neural tissues and that they involve members of the POU family of transcription factors. Mutations of each one of the two regions which abolish the binding of the C1 factor(s) on the A box and of the POU factor(s) on the B box also prevent stimulation of QR1 transcription in QNR9. Therefore, both elements appear to be required for the stage-specific transcription of the QR1 gene. We also show that the regulatory region from position -1265 to position -935 is able to confer stage-specific transcription upon a heterologous promoter (thymidine kinase). Indeed, this region stimulates transcription in differentiating retinas (QNR9) and represses transcription in terminally differentiated retinas (QNR17, corresponding to postnatal life). Our results suggest that cell growth regulation and developmental control are coordinated through the A and B boxes in regulating QR1 transcription during retinal differentiation.

Interleukin-1 beta-modulated gene expression in immortalized human chondrocytes

Immortalized human chondrocytes were established by transfection of primary cultures of juvenile costal chondrocytes with vectors encoding simian virus 40 large T antigen and selection in suspension culture over agarose. Stable cell lines were generated that exhibited chondrocyte morphology, continuous proliferative capacity (> 80 passages) in monolayer culture in serum-containing medium, and expression of mRNAs encoding chondrocyte-specific collagens II, IX, and XI and proteoglycans in an insulin-containing serum substitute. They did not express type X collagen or versican mRNA. These cells synthesized and secreted extracellular matrix molecules that were reactive with monoclonal antibodies against type II collagen, large proteoglycan (PG-H, aggrecan), and chondroitin-4- and chondroitin-6-sulfate. Interleukin-1 beta (IL-1 beta) decreased the levels of type II collagen mRNA and increased the levels of mRNAs for collagenase, stromelysin, and immediate early genes (egr-1, c-fos, c-jun, and jun-B). These cell lines also expressed reporter gene constructs containing regulatory sequences (-577/+3,428 bp) of the type II collagen gene (COL2A1) in transient transfection experiments, and IL-1 beta suppressed this expression by 50-80%. These results show that immortalized human chondrocytes displaying cartilage-specific modulation by IL-1 beta can be used as a model for studying normal and pathological repair mechanisms.

v-src transformation of rat embryo fibroblasts. Inefficient conversion to anchorage-independent growth involves heterogeneity of primary cultures

To clarify whether a single oncogene can transform primary cells in culture, we compared the transforming effect of a recombinant retrovirus (ZSV) containing the v-src gene in rat embryo fibroblasts (REFs) to that in the rat cell line 3Y1. In the focus assay, REFs exhibited resistance to transformation as only six foci were observed in the primary cultures as opposed to 98 in 3Y1 cells. After G418 selection, efficiency of transformation was again somewhat lower with REFs compared to that with 3Y1 cells, but the number of G418-resistant REF colonies was much greater than the number of foci in REF cultures. Furthermore, while 98% of G418-resistant colonies of ZSV-infected REFs were morphologically transformed, only 25% were converted to anchorage-independent growth, as opposed to 100% conversion seen in ZSV-infected 3Y1 cells. The poor susceptibility of REFs to anchorage-independent transformation did not involve differences in expression and subcellular distribution of p60v-src, or its kinase activity in vitro and in vivo. It rather reflected a property of the primary cultures, as cloning of REFs before ZSV infection demonstrated that only 2 out of 6 REF clones tested were permissive for anchorage-independent growth. The nonpermissive phenotype was dominant over the permissive one in somatic hybrid cells, and associated with organized actin filament bundles and a lower growth rate, both before and after ZSV infection. These results indicate that the poor susceptibility of REFs to anchorage-independent transformation by p60v-src reflects the heterogeneity of the primary cultures. REFs can be morphologically transformed by p60v-src with high efficiency but only a small fraction is convertible to anchorage-independent growth. REF resistance seems to involve the presence of a suppressor factor which may emerge from REF differentiation during embryonic development.

Proto-oncogene expression in a human chondrosarcoma cell line: HCS-2/8

HCS-2/8 is a stable human chondrosarcoma cell line with many chondrocytic characteristics and has the capacity to form chondrosarcomas in nude mice. The cells display both biochemically and morphologically definable changes in sparse, subconfluent, confluent and over-confluent phases of in vitro culture. Such features of HCS-2/8 cells may reflect the processes of both proliferation and differentiation of chondrocytes in vivo. We examined the correlations of these changes of HCS-2/8 cells with their transcript levels of 21 proto-oncogenes by Northern analysis. We found no detectable transcripts of 9 proto-oncogenes (c-sis, c-met, c-src, c-lyn, c-fgr, c-ros, c-pim, Blym and N-myc), but detected transcripts of 12 other proto-oncogenes (int-2, erbB, c-abl, c-raf-1, c-fyn, K-ras, H-ras, c-mos, c-myc, c-myb, c-fos, and c-jun). In the over-confluent phase, the levels of c-fos and c-raf-1 were increased several dozen times and about 5 times, respectively, while the level of c-abl was about 1/5th of that in the sparse, subconfluent and confluent phases of culture. The level of int-2 increased about 10-fold in the confluent and over-confluent phases of in vitro culture. The transcript levels of c-mos and K-ras were high in the sparse phase, low in the subconfluent and confluent phases and high in the over-confluent phase. The levels of the other 6 proto-oncogenes in HCS-2/8 cells were constant in all phases of in vitro culture.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Expression of simian virus 40 large T (tumor) oncogene in mouse chondrocytes induces cell proliferation without loss of the differentiated phenotype

We have infected primary embryonic mouse limb chondrocytes with a retrovirus carrying simian virus 40 early regions and have obtained a monoclonal mouse chondrocyte line, MC615, that was able to grow on culture dishes for at least 7 months and 20 passages. MC615 cells show expression of simian virus 40 large T (tumor) antigen and express markers characteristic of cartilage in vivo, such as types II, IX, and XI collagen, as well as cartilage aggrecan and link protein. These data show that cell growth induced by large T oncogene expression does not prevent the maintenance of the chondrocytic phenotype.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

myc products induce the expression of catecholaminergic traits in quail neural crest-derived cells

The avian myelocytomatosis virus strain MC29 v-myc oncogene transforms a wide panel of avian cells in vitro and either blocks or maintains differentiation, depending on the cell type. In the present work, we have investigated the effect of this oncogene on the differentiation of early embryonic cells, neural crest cells, grown in vitro. We report that the MC29 v-myc gene product induces a strong cellular proliferation of 2-day quail neural crest with the appearance of catecholaminergic traits. Other v-myc as well as the c-myc gene products also trigger this phenotype. Retroviruses carrying some other oncogenes do not elicit this phenotypic expression, although they activate cell multiplication. Thus, our results indicate that myc gene products induce (directly or indirectly) a differentiated phenotype in a subpopulation of neural crest cells.

Phorbol ester-induced terminal differentiation is inhibited in human U-937 monoblastic cells expressing a v-myc oncogene

Induction of differentiation of the human monoblastic cell line U-937 in vitro by several physiologic and nonphysiologic inducers is accompanied by a rapid decrease in expression of MYC, the endogenous human myc protooncogene. To investigate whether this reduction is a prerequisite for terminal differentiation, we introduced a constitutively expressed v-myc gene into U-937 cells. The results show that constitutive expression of an avian v-myc oncogene does not interfere with phorbol 12-myristate 13-acetate-induced differentiation of U-937 cells early after stimulation. However, after 24 hr the differentiation process is reversed, as judged by a full recovery of the proliferative capacity and reexpression of the immature phenotype, within the next 2-4 days. We conclude that the terminal stage of macrophage differentiation is inhibited in U-937 cells constitutively expressing a v-myc oncogene.

Expression of c-src in cultured human neuroblastoma and small-cell lung carcinoma cell lines correlates with neurocrine differentiation

Human cell lines with neuronal and neuroendocrine features were examined for their expression of pp60c-src, the cellular homolog of the transforming gene product pp60v-src of Rous sarcoma virus. Four neuroblastoma (LA-N-5, SH-SY5Y, Paju, and SK-N-MC) and three small-cell lung carcinoma (U-2020, U-1690, and U-1285) cell lines were selected on the basis of their stage of neurocrine differentiation, as determined by the expression of neuron-specific enolase. In an immune complex protein kinase assay, all seven cell lines displayed c-src kinase activity which was considerably higher than that found in nonneurocrine cells (human diploid fibroblasts, glioma, and non-small cell lung carcinoma cell lines). Furthermore, the c-src kinase activity, as determined by autophosphorylation or phosphorylation of an exogenous substrate, enolase, correlated with the stage of neurocrine differentiation. There was an approximately 30-fold difference in c-src kinase autophosphorylation activity between the cell lines representing the highest and lowest stages of neurocrine differentiation. A similar variation was found in the steady-state levels of the c-src protein of these cell lines. Highly differentiated neuroblastoma cells expressed two forms of the src protein. Digestion by Staphylococcus aureus V8 protease did reveal structural diversity in the amino-terminal ends of these c-src molecules. In summary, we found a clear correlation between c-src kinase activity and the stage of neuronal and neuroendocrine differentiation. Thus, the phenotypic similarity between neurons and neuroendocrine cells includes high c-src expression.

Independent regulation of transcription of the two strands of the c-myc gene

Previously we demonstrated the existence of transcripts from the noncoding strand of a rearranged, truncated c-myc gene in murine plasmacytomas in which this oncogene is translocated to an immunoglobulin constant-region gene element (M. Dean, R. B. Kent, and G. E. Sonenshein, Nature [London] 305:443-446, 1983). Here we report on the transcription of the two strands of a normal, unrearranged c-myc gene. We examined the effects of gene rearrangements, growth state transitions, and differentiation on the relative levels of usage of the two strands. Transcription from intron 1 to exon 3 of the murine c-myc gene was studied in in vitro nuclear runoff assays. The level of transcription of the noncoding strand across this region of a germ line c-myc gene in a murine B-cell lymphoma line was comparable to the level observed in plasmacytomas with translocated c-myc genes. Rapid changes in transcription of the coding strand of the c-myc gene could be seen during growth arrest of WEHI 231 cells and during activation of splenic T lymphocytes. Transcription of the noncoding strand was constitutive during these growth state transitions and during activation of primary cultures of quiescent calf aortic smooth muscle cells as well. In contrast, differentiation of murine erythroleukemia cells was accompanied by an early drop in transcription of the two strands of this gene. The ramifications of these findings with respect to measurements of c-myc gene transcription and to the regulation of this gene are discussed.

Expression of c-src in cultured human neuroblastoma and small-cell lung carcinoma cell lines correlates with neurocrine differentiation

Human cell lines with neuronal and neuroendocrine features were examined for their expression of pp60c-src, the cellular homolog of the transforming gene product pp60v-src of Rous sarcoma virus. Four neuroblastoma (LA-N-5, SH-SY5Y, Paju, and SK-N-MC) and three small-cell lung carcinoma (U-2020, U-1690, and U-1285) cell lines were selected on the basis of their stage of neurocrine differentiation, as determined by the expression of neuron-specific enolase. In an immune complex protein kinase assay, all seven cell lines displayed c-src kinase activity which was considerably higher than that found in nonneurocrine cells (human diploid fibroblasts, glioma, and non-small cell lung carcinoma cell lines). Furthermore, the c-src kinase activity, as determined by autophosphorylation or phosphorylation of an exogenous substrate, enolase, correlated with the stage of neurocrine differentiation. There was an approximately 30-fold difference in c-src kinase autophosphorylation activity between the cell lines representing the highest and lowest stages of neurocrine differentiation. A similar variation was found in the steady-state levels of the c-src protein of these cell lines. Highly differentiated neuroblastoma cells expressed two forms of the src protein. Digestion by Staphylococcus aureus V8 protease did reveal structural diversity in the amino-terminal ends of these c-src molecules. In summary, we found a clear correlation between c-src kinase activity and the stage of neuronal and neuroendocrine differentiation. Thus, the phenotypic similarity between neurons and neuroendocrine cells includes high c-src expression.

Independent regulation of transcription of the two strands of the c-myc gene

Previously we demonstrated the existence of transcripts from the noncoding strand of a rearranged, truncated c-myc gene in murine plasmacytomas in which this oncogene is translocated to an immunoglobulin constant-region gene element (M. Dean, R. B. Kent, and G. E. Sonenshein, Nature [London] 305:443-446, 1983). Here we report on the transcription of the two strands of a normal, unrearranged c-myc gene. We examined the effects of gene rearrangements, growth state transitions, and differentiation on the relative levels of usage of the two strands. Transcription from intron 1 to exon 3 of the murine c-myc gene was studied in in vitro nuclear runoff assays. The level of transcription of the noncoding strand across this region of a germ line c-myc gene in a murine B-cell lymphoma line was comparable to the level observed in plasmacytomas with translocated c-myc genes. Rapid changes in transcription of the coding strand of the c-myc gene could be seen during growth arrest of WEHI 231 cells and during activation of splenic T lymphocytes. Transcription of the noncoding strand was constitutive during these growth state transitions and during activation of primary cultures of quiescent calf aortic smooth muscle cells as well. In contrast, differentiation of murine erythroleukemia cells was accompanied by an early drop in transcription of the two strands of this gene. The ramifications of these findings with respect to measurements of c-myc gene transcription and to the regulation of this gene are discussed.

Expression of viral p21ras during acquisition of a transformed phenotype by rat adrenal cortex cells infected with Kirsten murine sarcoma virus

Rat adrenal cortex cells infected with Kirsten murine sarcoma virus acquire a transformed phenotype in a progressive fashion. The expression of the viral p21ras does not appear to correlate with the degree of transformation of the adrenocortical cells but rather is produced at similar levels as the culture becomes transformed. This indicates that the expression of an oncogenic form of p21ras is not of itself sufficient to completely transform rat adrenal cortex cells.

Characterization of a myc-containing retrovirus generated by propagation of an MH2 viral subgenomic RNA

We have previously isolated, from wild-type MH2 virus that contains the two oncogenes mil and myc, mutants defective in one or the other oncogene product. We report here the molecular cloning and extensive characterization of MH2 CL25 provirus lacking the v-mil oncogene. Our results indicate that this virus corresponds to the propagation of the 2.8-kilobase subgenomic RNA of MH21.