Interaction with normal cells suppresses the transformed phenotype of v-myc-transformed quail muscle cells

SRC points the way to biomarkers and chemotherapeutic targets

The role of Src in tumorigenesis has been extensively studied since the work of Peyton Rous over a hundred years ago. Src is a non-receptor tyrosine kinase that plays key roles in signaling pathways controlling tumor cell growth and migration. Src regulates the activities of numerous molecules to induce cell transformation. However, transformed cells do not always migrate and realize their tumorigenic potential. They can be normalized by surrounding nontransformed cells by a process called contact normalization. Tumor cells need to override contact normalization to become malignant or metastatic. In this review, we discuss the role of Src in cell migration and contact normalization, with emphasis on Cas and Abl pathways. This paradigm illuminates several chemotherapeutic targets and may lead to the identification of new biomarkers and the development of effective anticancer treatments.

Differential control of muscle-specific gene expression specified by src and myc oncogenes in myogenic cells

Myogenic cells can be transformed in vitro by the introduction of several exogenous viral oncogenes. Transformed myoblasts are prevented from terminal differentiation into myotubes by the continuous expression of oncogenes such as myc and src, chosen as prototypes of nuclear and cytoplasmic oncogenes. A comparative analysis of the relationship between transformation and differentiation in myoblasts and cells belonging to other lineages has led to the proposal that terminal differentiation of myc-transformed quail myoblasts is indirectly prevented by the loss of growth control and that myc-bearing cells remain susceptible to growth regulation by interaction with adjacent normal cells. On the contrary, the src oncogene appears to affect expression of the myogenic programme via a direct mechanism, independent from abnormal growth control. There is increasing evidence for the existence of master regulatory genes that govern and influence muscle development in vivo and myogenic differentiation in vitro. Expression of cytoplasmic oncogenes such as src, ras and polyoma middle T in the mouse myogenic cell line, C2, results in inhibition of biochemical differentiation and a marked down-regulation of the MyoD1 and myogenin genes.

The mesenchyme in cancer therapy as a target tumor component, effector cell modality and cytokine expression vehicle

Tissues and organs harbor a component of supportive mesenchymal stroma. The organ stroma is vital for normal functioning since it expresses factors instructing growth and differentiation along with molecules that restrain these processes. Similarly, the growth of tumors is strictly dependent on the tumor stroma. This review first discusses the possibility of developing tools to block the propagation of the tumor-associated stroma, that may halt tumor progression. It further describes how the tropism of mesenchymal stroma to tumor sites may be utilized to cause regression of the cancerous tissue. Mesenchyme can be genetically modified to overexpress specific regulatory molecules with known effects on specific tumors, such as interferon beta, studied in the context of melanoma and glioma and activin A, a transforming growth factor beta cytokine, examined in multiple myeloma. These studies point to the possibility that genetically modified mesenchymal cells may be used as a therapeutic modality for incurable human diseases. It is proposed that further development of methods of tumor stroma targeting, or alternatively the use of stromal mesenchyme as a cell or cell/gene therapy modalities, may yield novel clinical tools for the treatment of human cancers.

Microdosimetric Model for the Induction of Cell Killing through Medium‐Borne Signals

Microbeam, medium-transfer and low-dose experiments have demonstrated that intercellular signals can initiate many of the same biological events and processes as direct exposure to ionizing radiation. These phenomena cast doubt on cell-autonomous modes of action and the linear, no-threshold carcinogenesis paradigm. To account for the effects of intercellular signals, new approaches are needed to relate dosimetric quantities to the emission and processing of signals by irradiated and unirradiated cells. In this paper, microdosimetric principles are used to develop a stochastic model to relate absorbed dose to the emission and processing of cell death signals by unirradiated cells. Our analyses of published results of medium transfer experiments performed using HPV-G human keratinocytes suggest that the emission of death signals is a bi-exponential function of dose with a distinct plateau in the 5- to 100-mGy range. However, the emission of death signals by HPV-G cells may not become fully saturated until the absorbed dose becomes larger than 0.6 Gy. Similar saturation effects have been observed in microbeam and medium-transfer experiments with other mammalian cell lines. The model predicts that the cell-killing effect of medium-borne death signals decreases exponentially as the absorbed dose becomes small compared to the frequency-mean specific energy per radiation event.

Reduced protein degradation rates and low expression of proteolytic systems support skeletal muscle hypertrophy in transgenic mice overexpressing the c-ski oncogene

We have investigated the protein turnover modulations involved in the hypertrophic muscle phenotype of c-ski overexpressing transgenic mice. In these animals, the body weight is increased and all the muscles examined show a definite hypertrophy. The protein degradation rate is significantly reduced in the fast twitch muscles of c-ski transgenic animals with respect to controls; in contrast, there are no detectable differences in the synthesis rates. The down-regulation of protein breakdown is paralleled by decreased expression of genes belonging to the lysosomal as well as to the ATP-ubiquitin-dependent proteolytic pathways.

Selective repression of myoD transcription by v-Myc prevents terminal differentiation of quail embryo myoblasts transformed by the MC29 strain of avian myelocytomatosis virus

We have investigated the mechanism by which expression of the v-myc oncogene interferes with the competence of primary quail myoblasts to undergo terminal differentiation. Previous studies have established that quail myoblasts transformed by myc oncogenes are severely impaired in the accumulation of mRNAs encoding the myogenic transcription factors Myf-5, MyoD and Myogenin. However, the mechanism responsible for such a repression remains largely unknown. Here we present evidence that v-Myc selectively interferes with quail myoD expression at the transcriptional level. Cis-regulatory elements involved in the auto-activation of qmyoD are specifically targeted in this unique example of transrepression by v-Myc, without the apparent participation of Myc-specific E-boxes or InR sequences. Transiently expressed v-Myc efficiently interfered with MyoD-dependent transactivation of the qmyoD regulatory elements, while the myogenin promoter was unaffected. Finally, we show that forced expression of MyoD in v-myc-transformed quail myoblasts restored myogenin expression and promoted extensive terminal differentiation. These data suggest that transcriptional repression of qmyoD is a major and rate-limiting step in the molecular pathway by which v-Myc severely inhibits terminal differentiation in myogenic cells.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Distinct effects of Rac1 on differentiation of primary avian myoblasts

Rho family GTPases have been implicated in the regulation of the actin cytoskeleton in response to extracellular cues and in the transduction of signals from the membrane to the nucleus. Their role in development and cell differentiation, however, is little understood. Here we show that the transient expression of constitutively active Rac1 and Cdc42 in unestablished avian myoblasts is sufficient to cause inhibition of myogenin expression and block of the transition to the myocyte compartment, whereas activated RhoA affects myogenic differentiation only marginally. Activation of c-Jun N-terminal kinase (JNK) appears not to be essential for block of differentiation because, although Rac1 and Cdc42 GTPases modestly activate JNK in quail myoblasts, a Rac1 mutant defective for JNK activation can still inhibit myogenic differentiation. Stable expression of active Rac1, attained by infection with a recombinant retrovirus, is permissive for terminal differentiation, but the resulting myotubes accumulate severely reduced levels of muscle-specific proteins. This inhibition is the consequence of posttranscriptional events and suggests the presence of a novel level of regulation of myogenesis. We also show that myotubes expressing constitutively active Rac1 fail to assemble ordered sarcomeres. Conversely, a dominant-negative Rac1 variant accelerates sarcomere maturation and inhibits v-Src-induced selective disassembly of I-Z-I complexes. Collectively, our findings provide a role for Rac1 during skeletal muscle differentiation and strongly suggest that Rac1 is required downstream of v-Src in the signaling pathways responsible for the dismantling of tissue-specific supramolecular structures.

Myogenic conversion of NIH3T3 cells by exogenous MyoD family members: dissociation of terminal differentiation from myotube formation

Myogenic regulatory factors (MRF) of the MyoD family regulate the skeletal muscle differentiation program. Non-muscle cells transfected with exogenous MRF either are converted to the myogenic lineage or fail to express the muscle phenotype, depending on the cell type analysed. We report here that MRF-induced myogenic conversion of NIH3T3 cells results in an incomplete reprogramming of these cells. Transfected cells withdrew from the cell cycle and underwent biochemical differentiation but, surprisingly, terminally differentiated myocytes absolutely failed to fuse into multinucleated myotubes. Analysis of muscle regulatory and structural gene expression failed to provide an explanation for the fusion defectiveness. However, myogenic derivatives of NIH3T3 cells were shown to be unable to accumulate the transcripts encoding muscle-specific isoforms of the integrin subunit beta1D and the transcription factor MEF2D1b2, that depend on muscle-specific alternative splicing. Our results suggest that the fusion into myotubes is under a distinct genetic control that might depend, at least partially, on differential splicing.

Carcinoembryonic antigen, a human tumor marker, cooperates with Myc and Bcl-2 in cellular transformation

Carcinoembryonic antigen (CEA) is a tumor marker that is overexpressed in many human cancers and functions in vitro as a homotypic intercellular adhesion molecule. We have investigated the possibility of synergy between CEA, v-Myc, and Bcl-2 in the transformation of cells with differentiation capacity. We find that v-Myc increases the cell division rate and maximum density of rat L6 myoblasts but also markedly stimulates both apoptosis and surprisingly, differentiation, thus preventing transformation. The superposition of Bcl-2 blocks the apoptotic stimulation of v-Myc and independently promotes further cell division at confluence, but still allows differentiation. The further expression of CEA has a dominant effect in blocking differentiation, regardless of the presence of the other activated oncogenes, generating cells that enter a reversible quiescent G0-like state in medium promoting differentiation. Transfectants expressing CEA with or without v-myc and bcl-2 allow the emergence of cells with the property of heritable, efficient, anchorage-independent growth in soft agar and the ability to markedly reduce the latency for tumor formation in nude mice. We propose that by prolonging cell survival in the presence of differentiation signals, CEA represents a novel class of dominant differentiation-blocking oncogene.

Perturbation in connexin 43 and connexin 26 gap-junction expression in mouse skin hyperplasia and neoplasia

To examine the possible role of gap junctions in mouse skin tumor progression, we generated a panel of mouse skin tissue samples exhibiting normal, hyperplastic, or neoplastic changes and characterized the expression of the gap-junction genes connexin 43 (Cx43) and connexin 26 (Cx26) by in situ hybridization and immunohistochemical analyses. In normal skin, these two gap junction genes were differentially expressed; Cx43 was found predominantly in the less differentiated lower spinous layers, whereas Cx26 was found in terminally differentiating upper spinous and granular layers. In hyperplastic epidermis exhibiting an expansion of the differentiated upper layer, i.e., epidermis with a thickened granular layer or in which the granular layer was replaced with keratinocytes exhibiting tricholemmal differentiation, expression of Cx43 and Cx26 remained segregated in the lower and upper spinous layers, respectively. However, in papillomas, Cx26 was localized in the lower but not upper spinous layer, an expression pattern identical to that of Cx43. In addition, the overall expression levels of both Cx43 and Cx26 appeared to be greatly elevated in the papillomas. It is interesting that such marked alteration in the pattern of Cx26 expression occurred within the context of hyperplastic changes histologically identical to those seen in the nonpapillomous hyperplasias. Interestingly, in neoplastic skin lesions containing a squamous cell carcinoma, Cx43 and Cx26 expression was extinguished. Moreover, expression of Cx43 was also significantly reduced in adjacent apparently nonneoplastic tissues. Overall, these observations show that perturbations in gap-junction gene expression are associated with skin hyperplasia and neoplasia. Such findings suggest a possible role for gap junctions in the malignant conversion of mouse epidermal cells.

A mathematical model of periodically pulsed chemotherapy: tumor recurrence and metastasis in a competitive environment

A competition model describing tumor-normal cell interaction with the added effects of periodically pulsed chemotherapy is discussed. The model describes parameter conditions needed to prevent relapse following attempts to remove the tumor or tumor metastasis. The effects of resistant tumor subpopulations are also investigated and recurrence prevention strategies are explored.

Inhibition of gap junctional intercellular communication and enhancement of growth in BALB/c 3T3 cells treated with connexin43 antisense oligonucleotides

Many studies have correlated reductions in gap junctional intercellular communication (GJIC) with altered cellular growth, tumor promotion, and neoplastic transformation. To test directly whether reduced GJIC affects cellular growth, GJIC was inhibited in murine BALB/c 3T3 fibroblasts by treatment with a phosphorothioate-modified antisense oligonucleotide targeted against the connexin43 translation start codon, and in vitro cell growth was monitored. The cells were incubated with the oligonucleotide (0.1-0.5 microM) in liposomes in serumless culture medium for 16 h; washed and refed with serum-containing medium; and analyzed for dye-coupling, connexin43 protein and mRNA levels, and cell growth over the next 5 d. The antisense oligonucleotide inhibited dye-coupling and reduced connexin43 protein levels in a concentration-dependent manner but had no effect on connexin43 mRNA levels. Cell growth rate was not affected, but saturation density was increased approximately threefold by the oligonucleotide. These data support a role for GJIC in the establishment of contact inhibition of in vitro cell growth.

Ciliary neurotrophic factor promotes the terminal differentiation of v-myc immortalized sympathoadrenal progenitor cells in vivo

Survival and differentiation of a sympathoadrenal progenitor cell line (termed MAH), transduced with a v-myc oncogene, was studied subsequent to transplantation in the peripheral and central nervous system of adult rats. In the brain, MAH cell survival depended on the secretion of ciliary neurotrophic factor (CNTF) by co-grafts of genetically modified glioma cells. No trophic factor supplement was required for development of the MAH cells in the peripheral nerve environment. Transplanted progenitor cells withdrew from the cell cycle within 48 h and differentiated into a prominent population of large sympathetic-like neurons. The neurons expressed the alpha subunit of the CNTF receptor and appropriate spatial distributions of cytoskeletal proteins and catecholamine related enzymes. The results identify a role for CNTF in the development of the sympathoadrenal cell lineage and support the concept of immortalized progenitor cells as alternatives to primary cells for cell replacement strategies in the nervous system.

Maintenance of the differentiated state in skeletal muscle: activation of v-Src disrupts sarcomeres in quail myotubes

We have used quail skeletal myotubes expressing a temperature-sensitive allele of the v-src oncogene to address the issue of the homeostasis of sarcomeric myofibrils in differentiated muscle cells. Reactivation of the v-Src tyrosine kinase by shifting the cultures to the permissive temperature leads within minutes to the formation of F-actin-containing bodies (ABs), that originate in the ventral region of the myotubes and increase in number concomitantly with the dismantling of the I-Z-I complex of the sarcomeres. This process is detailed by confocal and electron microscopy. Indirect immunofluorescence reveals that ABs contain muscle-specific protein isoforms associated with the I-Z-I complexes and vinculin, a component of the cytoskeletal network. Anti-phosphotyrosine antibodies label proteins in ABs and Z-discs. Evidence is presented indicating that this phenomenon specifically depends on the persistent activation of v-Src, rather than on a general increase in phosphotyrosine content such as that induced by vanadate. AB formation is prevented by activation of protein kinase C by phorbol ester or by treatment with the kinase inhibitor 2-aminopurine, without any detectable effect on tyrosine phosphorylation. Taken together these findings indicate that phosphorylation of specific target proteins by v-Src, although necessary, is not sufficient per se to induce AB formation. In addition, the signal transduction cascade that culminates in MAP kinase activation and its nuclear translocation is activated both by v-Src and phorbol ester, and is relatively unaffected by 2-aminopurine. These findings imply that both phorbol esters and 2-aminopurine operate, at least in part, at the level of alternative pathways that may diverge upstream of the MAP kinase and are presumably mediating the early effects of v-Src on the differentiated phenotype.

Transformation by myc prevents fusion but not biochemical differentiation of C2C12 myoblasts: mechanisms of phenotypic correction in mixed culture with normal cells

To study the effects of myc oncogene on muscle differentiation, we infected the murine skeletal muscle cell line C2C12 with retroviral vectors encoding various forms of avian c- or v-myc oncogene. myc expression induced cell transformation but, unlike many other oncogenes, prevented neither biochemical differentiation, nor commitment (irreversible withdrawal from the cell cycle). Yet, myotube formation by fusion of differentiated cells was strongly inhibited. Comparison of uninfected C2C12 myotubes with differentiated myc-expressing C2C12 did not reveal consistent differences in the expression of several muscle regulatory or structural genes. The present results lead us to conclude that transformation by myc is compatible with differentiation in C2C12 cells. myc expression induced cell death under growth restricting conditions. Differentiated cells escaped cell death despite continuing expression of myc, suggesting that the muscle differentiation programme interferes with the mechanism of myc-induced cell death. Cocultivation of v-myc-transformed C2C12 cells with normal fibroblasts or myoblasts restored fusion competence and revealed two distinguishable mechanisms that lead to correction of the fusion defect.

Patterns of integration and expression of retroviral, non-replicative vectors in avian embryos: embryo developmental stage and virus subgroup envelope modulate tissue-tropism

We previously demonstrated that Avian Leukemia Viruses (ALV) carrying the v-myc gene specifically induce two types of tumors, cardiomyocytic tumors when the virus is injected before embryonic day 3 (E3), skin tumors when the virus is injected at E3 or E5. Aiming to elucidate the mechanisms which determine this time-dependent change in target, we infected chick and quail embryos at E3 and E5 with replication-deficient, lacZ gene-carrying, ALV-based viruses produced by a packaging cell line. Three constructs driven by 3 different Long Terminal Repeats (LTRs) were tested and yielded similar results. When the constructs were inoculated at E3 and the lacZ gene product revealed 5 days later, around 70% of the embryos carried lacZ+ clones in the heart, around 50% had positive clones in the skin anywhere on the body, while a few embryos displayed clones in internal organs (liver, stomach, lungs). Immunocytological identification of the heart cell type(s) expressing the virus revealed that the only cells infected were cardiomyocytes. When the constructs were inoculated at E5, no lacZ+ clones appeared in the heart but all were located in the cephalic skin. In order to examine the relationship between viral integration and expression, DNA of different organs or tissues from lacZ stained embryos was analyzed by PCR. A tight correlation between integration and expression in the heart and in the skin was revealed in most cases. In contrast, a significant PCR signal was often detected in the liver or the stomach despite weak or absent expression as revealed by lacZ+ clones. We then investigated the influence of envelope glycoprotein subgroups on the tropism of these constructs. The lacZ vector driven by RAV-2 LTRs was packaged as subgroups A, B or E viral particles. The A subgroup, used in the part of the study described above, infects both chick and quail while the B and E subgroups are specific for chick or quail respectively. These B and E subgroups induced lacZ+ clones in the heart (after E3 injection) while no clones or only a few were detected in the skin either after E3 or E5 injection. The following conclusions can be drawn: 1) cardiomyocytes are at E3 the major target for integration and expression of ALV-derived viruses in vivo; 2) targets change rapidly with embryonic age; and 3) tissue-specific infections depend on the envelope subgroup, thus presumably on the presence of the cognate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of myogenic factors and the retinoblastoma protein mediates muscle cell commitment and differentiation

The experiments reported here document that the tumor suppressor retinoblastoma protein (pRB) plays an important role in the production and maintenance of the terminally differentiated phenotype of muscle cells. We show that pRB inactivation, through either phosphorylation, binding to T antigen, or genetic alteration, inhibits myogenesis. Moreover, inactivation of pRB in terminally differentiated cells allows them to reenter the cell cycle. In addition to its involvement in the myogenic activities of MyoD, pRB is also required for the cell growth-inhibitory activity of this myogenic factor. We also show that pRB and MyoD directly bind to each other, both in vivo and in vitro, through a region that involves the pocket and the basic-helix-loop-helix domains, respectively. All the results obtained are consistent with the proposal that the effects of MyoD on the cell cycle and of pRB on the myogenic pathway result from the direct binding of the two molecules.

Interplay between proliferation and differentiation within the myogenic lineage

In muscle cells, as in a variety of cell types, proliferation and differentiation are mutually exclusive events controlled by a balance of opposing cellular signals. Members of the MyoD family of muscle-specific helix-loop-helix proteins which, in collaboration with ubiquitous factors, activate muscle differentiation and inhibit cell proliferation function at the nexus of the cellular circuits that control proliferation and differentiation of muscle cells. The activities of these myogenic regulators are negatively regulated by peptide growth factors and activated oncogenes whose products transmit growth signals from the membrane to the nucleus. Recent studies have revealed multiple mechanisms through which intracellular growth factor signals may interfere with the functions of the myogenic regulators. When expressed at high levels, members of the MyoD family can override mitogenic signals and can cause growth arrest independent of their effects on differentiation. The ability of these myogenic regulators to inhibit proliferation of normal as well as transformed cells from multiple lineages suggests that they interact with conserved components of the cellular machinery involved in cell cycle progression and that similar types of regulatory factors participate in differentiation and cell cycle control in diverse cell types.

Growth retardation in glioma cells cocultured with cells overexpressing a gap junction protein

To examine the role of gap-junctional intercellular communication in controlling cell proliferation, we have transfected C6 glioma cells with connexin 43 cDNA. The growth of transfected clones was dramatically reduced compared with nontransfected glioma cells. To further characterize the role of gap junctions in controlling proliferation, we have examined the growth of C6 cells cocultured with transfected cells overexpressing connexin 43. Although C6 cells grew at their normal rate when cocultured with nontransfected C6 cells, when cocultured with connexin 43-overexpressing cells they displayed a dramatic reduction in growth rate. Furthermore, a significant, dose-dependent reduction in cell proliferation was noted when C6 cells were cultured in medium conditioned by transfected cells. This effect correlated with the level of connexin 43 expression. These results suggest that the decreased cell proliferation rate of transfected cells and C6 cells cultured with them is due to the secretion of a growth inhibitory factor(s) and that the secretion of this factor may be linked to the level of gap junctional intercellular communication.

A point mutation in the MyoD basic domain imparts c-Myc-like properties

MyoD and c-Myc, members of the large "basic-helix-loop-helix" family of proteins, regulate diverse aspects of both normal and neoplastic growth and specific gene regulation. These two proteins differ at 9 of the 14 amino acids that comprise the basic domains necessary for DNA binding and transcriptional control. Individual amino acids in the MyoD basic domain were mutated to those found at the analogous positions in c-Myc. Four classes of mutants were obtained: (i) those with no effects on MyoD-site binding or activation of MyoD-responsive genes, (ii) those with no effect on MyoD-site binding but with a loss of activation potential, (iii) those with a loss of both DNA binding and activation potential, and (iv) one mutant (mut 9, Leu122----Arg) that left MyoD-site binding unaffected but imparted a new c-Myc-site binding capability. mut 9 competed with wild-type protein for the activation of MyoD-responsive reporter genes but could, like c-Myc, also suppress the adenovirus major-late promoter, which contains a c-Myc binding site. Our studies thus identify specific amino acid residues in the MyoD basic domain that are important for its activity as a DNA-binding transcriptional activator. Most significantly, our results with mut 9 indicate that Leu122 of MyoD is a critical determinant of specific DNA binding and that mutation at this residue can alter this specificity.

Proto-oncogenes in the regulatory circuit for myogenesis

Skeletal muscle cells have provided an auspicious system for dissecting the mechanisms through which growth factor signals disrupt programs for cellular differentiation. Insight into the molecular mechanisms that control muscle differentiation has recently been obtained through the cloning of a family of muscle-specific transcription factors, often referred to as the MyoD family, that can activate myogenesis. The expression and activity of these factors are negatively regulated by growth factor signals and by activated oncogenes whose products transduce growth signals from the cell membrane to the nucleus. This review will focus on the role of proto-oncogenes in the transduction of growth factor signals that regulate myogenesis and on the cross-talk between the regulatory circuits that control myoblast proliferation and differentiation.

Functional antagonism between c-Jun and MyoD proteins: a direct physical association

The product of the proto-oncogene Jun inhibits myogenesis. Constitutive expression of Jun in myoblasts interferes with the expression and the function of MyoD protein. In transient transfection assays Jun inhibits transactivation of the MyoD promoter, the muscle creatine kinase enhancer, and a reporter gene linked to MyoD DNA-binding sites. Conversely, MyoD suppresses the transactivation by Jun of genes linked to an AP-1 site. We demonstrate that both in vivo and in vitro MyoD and Jun proteins physically interact. Mutational analysis suggests that this interaction occurs via the leucine zipper domain of Jun and the helix-loop-helix region of MyoD.

The cell-cell channel in the control of growth

Several lines of evidence indicate that the cell-cell channels in gap junction are conduits for growth-regulating signals. Experimental upregulation of the channels by retinoids causes inhibition of cellular growth and, conversely, their downregulation by oncogenes, e.g. activated src, stimulates growth. In either direction, the extent of growth correlates tightly with the degree of communication. Cogent evidence of the channel's function in growth regulation is now on hand: incorporation of a channel-protein gene into the genome of a transformed communication-deficient cell line normalizes communication and growth. The current data conform to a model of growth control with discrete regulatory centers.

Multipotent neural cell lines can engraft and participate in development of mouse cerebellum

Multipotent neural cell lines were generated via retrovirus-mediated v-myc transfer into murine cerebellar progenitor cells. When transplanted back into the cerebellum of newborn mice, these cells integrated into the cerebellum in a nontumorigenic, cytoarchitecturally appropriate manner. Cells from the same clonal line differentiated into neurons or glia in a manner appropriate to their site of engraftment. Engrafted cells, identified by lacZ expression and PCR-mediated detection of a unique sequence arrangement, could be identified in animals up to 22 months postengraftment. Electron microscopic and immunohistochemical analysis demonstrated that some engrafted cells were similar to host neurons and glia. Some transplant-derived neurons received appropriate synapses and formed normal intercellular contacts. These data indicate that generating immortalized cell lines for repair of, or transport of genes into, the CNS may be feasible. Such lines may also provide a model for commitment and differentiation of cerebellar progenitor cells.

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.

An avian retrovirus expressing chicken pp59c-myc possesses weak transforming activity distinct from v-myc that may be modulated by adjacent normal cell neighbors

We demonstrate that EF168, an avian retrovirus that expresses the chicken pp59c-myc proto-oncogene, transforms quail embryo fibroblasts in vitro. An EF168-transformed quail clone, EF168-28, containing a single provirus, synthesizes several hundred copies of c-myc RNA and expresses elevated levels of the pp59c-myc gene product. The EF168 provirus in EF168-28 was isolated as a molecular clone, and the nucleotide sequence of its c-myc allele was confirmed as identical to that of exons 2 and 3 of the chicken c-myc proto-oncogene. Extended infection of quail embryo fibroblast cultures with EF168 induced a number of in vitro transformation-associated parameters similar to those elicited by the oncogenic v-myc-encoding retrovirus MC29, including alteration of cellular morphology, anchorage-independent growth, and induction of immortalized cell lines. Despite the fact that EF168 and MC29 shared these biological activities, further analysis revealed that EF168 initiated transformation in quail embryo fibroblasts, bone marrow, or adherent peripheral blood cultures 100- to 1,000-fold less efficiently than did MC29. Further, in contrast to MC29-induced foci, EF168 foci were smaller, morphologically diffuse, and less prominent. Analysis of newly infected cells demonstrated efficient expression of EF168 viral RNA in the absence of transformation. These differences suggest that while the pp59v-myc gene product can exert dominant transforming activity on quail embryo fibroblasts, its ability to initiate transformation is distinct from that of the pp110gag-v-myc gene product encoded by MC29 and may be suppressed by adjacent nontransformed cell neighbors.

Escape from transforming growth factor beta control and oncogene cooperation in skin tumor development

Control of tumor development by surrounding normal cells has been suggested by a number of in vitro studies. In vivo, tumorigenicity of ras-transformed primary keratinocytes can be suppressed by addition of normal dermal fibroblasts. Here, we report that dermal fibroblasts produce a diffusible inhibitory factor belonging to the transforming growth factor beta (TGF-beta) family and possibly corresponding to TGF-beta 3. This factor can suppress growth of ras-transformed primary keratinocytes in culture and after injection into mice. As with primary cells, tumorigenicity of a ras-transformed, TGF-beta-sensitive keratinocyte line is substantially inhibited by adding dermal fibroblasts, leading to the formation of much smaller and differentiated tumors. Introduction of an intact E1a oncogene into these cells induces concomitant resistance to TGF-beta, to the effect of dermal-fibroblast inhibitory factor, and to dermal-fibroblast tumor suppression. Similar results are obtained with a transformation-deficient truncated E1a mutant, which binds to a reduced subset of cellular proteins (including the retinoblastoma gene product). Thus, genetic events such as those elicited by E1a transformation enable keratinocytes to escape from the inhibitory influences of a normal cellular environment and lead, together with ras transformation, to skin tumor development.

Fibroblast cell interactions with human melanoma cells affect tumor cell growth as a function of tumor progression

It is known from a variety of experimental systems that the ability of tumor cells to grow locally and metastasize can be affected by the presence of adjacent normal tissues and cells, particularly mesenchymally derived stromal cells such as fibroblasts. However, the comparative influence of such normal cell-tumor cell interactions on tumor behavior has not been thoroughly investigated from the perspective of different stages of tumor progression. To address this question we assessed the influence of normal dermal fibroblasts on the growth of human melanoma cells obtained from different stages of tumor progression. We found that the in vitro growth of most (4 out of 5) melanoma cell lines derived from early-stage radial growth phase or vertical growth phase metastatically incompetent primary lesions is repressed by coculture with normal dermal fibroblasts, suggesting that negative homeostatic growth controls are still operative on melanoma cells from early stages of disease. On the other hand, 9 out of 11 melanoma cell lines derived from advanced metastatically competent vertical growth phase primary lesions, or from distant metastases, were found to be consistently stimulated to grow in the presence of dermal fibroblasts. Evidence was obtained to show that this discriminatory fibroblastic influence is mediated by soluble inhibitory and stimulatory growth factor(s). Taken together, these results indicate that fibroblast-derived signals can have antithetical growth effects on metastatic versus metastatically incompetent tumor subpopulations. This resultant conversion in responsiveness to host tissue environmental factors may confer upon small numbers of metastatically competent cells a growth advantage, allowing them to escape local growth constraints both in the primary tumor site and at distant ectopic tissue sites.

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.

The myoD gene family: nodal point during specification of the muscle cell lineage

The myoD gene converts many differentiated cell types into muscle. MyoD is a member of the basic-helix-loop-helix family of proteins; this 68-amino acid domain in MyoD is necessary and sufficient for myogenesis. MyoD binds cooperatively to muscle-specific enhancers and activates transcription. The helix-loop-helix motif is responsible for dimerization, and, depending on its dimerization partner, MyoD activity can be controlled. MyoD senses and integrates many facets of cell state. MyoD is expressed only in skeletal muscle and its precursors; in nonmuscle cells myoD is repressed by specific genes. MyoD activates its own transcription; this may stabilize commitment to myogenesis.

MC29-immortalized clonal avian heart cell lines can partially differentiate in vitro

We established quail clonal heart muscle cell lines from cardiac rhabdomyosarcomas developed in embryos injected in ovo with the MC29 virus containing the v-myc oncogene. These clones were characterized by means of antibodies detecting markers of striated muscle cells. Two clones were selected for further characterization on the basis of a distribution of myogenic markers similar to that in normal early embryonic cardiac muscle cells. However, these muscle markers progressively disappeared with time in culture. Cardiomyocytic differentiation could be reinduced in culture, by associating the avain cardiac cells with 3T3 cells in a defined synthetic medium. Muscle markers were then reexpressed in all cardiac cells as soon as Day 1 after coculture. Multiplication of cardiac cells continued at the same time. This is characteristic of cardiac clones since MC29-infected quail myoblasts and MC29-infected quail fibroblasts exhibited a split response to 3T3 association, i.e., decreased growth and enhanced differentiation. The cardiac clones were maintained in vitro for more than 60 generations (6 months) without morphological changes. To our knowledge, this is the first description of clonal embryonic avian heart cell lines.

The c-myc proto-oncogene regulates cardiac development in transgenic mice

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.

The embryonic environment strongly attenuates v-src oncogenesis in mesenchymal and epithelial tissues, but not in endothelia

We demonstrate that the behavior of cells expressing v-src, a tyrosine kinase oncogene, differs profoundly between the embryonic and culture environments. V-src was introduced into avian embryo cells both in culture and in stage-24 embryo limbs, using replication-defective retroviral vectors. These vectors were used as single-hit, cellular markers to determine the environmental influences imposed by normal cells and tissues on clonal cell growth. The marker gene lacZ was coexpressed with v-src in order to locate the descendent cells. In culture, v-src induced rapid morphological transformation and anchorage-independent growth of embryo fibroblasts; the vectors were also tumorigenic in hatchling chickens. In contrast, most of the cell clones expressing v-src in the embryo grew normally without neoplasia. Expression of v-src vectors could be found in a wide range of cell types, demonstrating not only that neoplastic transformation is attenuated in ovo, but also that differentiation commitment in many lineages can be maintained concurrently with oncogene expression. Significantly, the embryonic control of cell growth could be perturbed by v-src under certain conditions. Rare, marked clones showed hyperplasia or dysplasia, and the primitive endothelium could succumb to rapid neoplasia; thus, these embryonic tissues are not inherently deficient in transformation factors. We propose that the environmental conditions imposed on cells in ovo are critical for the attenuation of neoplasia, while cultured cells lose this requisite environment.

The c-myc proto-oncogene regulates cardiac development in transgenic mice

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.

Cell proliferation inhibited by MyoD1 independently of myogenic differentiation

Cell growth and differentiation are usually mutually exclusive. Transformation of myoblasts by retroviruses containing the myc oncogene inhibits differentiation, preventing cells from withdrawing from the cell cycle. If cell-cycle withdrawal is a prerequisite for myoblast differentiation, it is probably an early event in terminal cell differentiation, but this has not yet been established. MyoD1 regulates myogenesis. It is expressed only in skeletal muscle, but can convert other cells to muscle cells. The MyoD1 protein, a nuclear phosphoprotein in part similar to the myc family of proteins, is a DNA-binding protein binding to the enhancer sequences of the muscle-specific creatine phosphokinase gene. Thus, introduction of MyoD1 into cells provides a simple approach to study the effect of induction of differentiation on cell growth. In cultured NIH 3T3 cells, inhibition of cell proliferation occurs within 18 hours, and expression of myosin starts after 72 hours. Furthermore, injection of MyoD1 into quiescent NIH 3T3 cells inhibit cell proliferation independently of induction of differentiation. Deletion of the myc-like domain in the MyoD1 gene eliminates the inhibition of DNA synthesis, but substitution of the basic domain with the analogous domain from the E12 transcription factor inhibits growth yet fails to induce differentiation. Inhibition of DNA synthesis, therefore, seems to be controlled separately from myogenic differentiation.

Transformation of chicken embryo fibroblasts by direct DNA transfection of single oncogenes: comparative analyses of src, erbB, myc, and ras

Chicken embryo fibroblasts (CEF) have been used extensively to study the transformation parameters of a number of avian sarcoma-leukemia viruses. Previously, oncogene transformation of CEF has been conducted almost exclusively with replicating viruses, because of perceived difficulties with direct DNA transfection. Here, we show that CEF can be efficiently and stably transfected by selection for the neomycin resistance gene (neo). Cotransfection of neo with various oncogenes resulted in CEF transformation in vitro and, in several instances, sarcoma formation in vivo. Transfection of src, myc, erbB, and ras, either singly or in combination, resulted in soft-agar colonies with unique morphologies. Transfection of a family of v-src, c-src, and v/c-src chimeric constructs demonstrated the ability of the assay to discriminate between transforming and nontransforming genes. Transfection of a number of erbB variants showed that internal mutations, primarily in the kinase domain, contribute significantly to the ability to transform fibroblasts. The tumorigenic potential detected by transfection of oncogenes faithfully reproduced those previously reported by using viral infections. Our studies establish the utility of CEF transformation by direct DNA transfection. This method should prove useful in analyzing oncogenes, (e.g., myc) that do not readily transform rodent cell lines and in studying host-range mutants of oncogenes, such as those recently identified for src and erbB.

Dihydropyridine receptor gene expression is regulated by inhibitors of myogenesis and is relatively insensitive to denervation

To evaluate developmental and physiological signals that may influence expression of the dihydropyridine-sensitive "slow" Ca2+ channel, we analyzed dihydropyridine receptor (DHPR) mRNA abundance in mouse skeletal muscle. Using synthetic oligonucleotide probes corresponding to the rabbit skeletal muscle DHPR, a 6.5 kb DHPR transcript was identified in postnatal skeletal muscle and differentiated C2 or BC3H1 myocytes, but not cardiac muscle or brain. DHPR gene expression was reversibly suppressed by 0.4 nM transforming growth factor beta-1 or by transfection with a mutant c-H-ras allele, nominal inhibitors of myogenesis that block the appearance of slow channels and DHPR. In contrast, both BC3H1 and C2 myocytes containing the activated ras vector expressed the gene encoding the nicotinic acetylcholine receptor delta subunit, demonstrating that not all muscle-specific genes are extinguished by ras. Denervation stimulated DHPR gene expression less than 0.6-fold, despite 8-fold upregulation of delta-subunit mRNA and reciprocal effects on the skeletal and cardiac alpha-actin genes. Thus, DHPR gene induction is prevented by inhibitors of other muscle-specific genes, whereas, at most, relatively small changes in DHPR mRNA abundance occur during adaptation to denervation.