Temperature-sensitive expression of differentiation in transformed myoblasts

Memories of a young student: the early days of splicing regulation with François Gros

I joined the laboratory of François Gros as a young student in the mid-1980s and worked on the characterization of the β -tropomyosin gene in chicken and the regulation of alternative splicing of its transcript, under the supervision of Marc Fiszman. In particular, I was interested in how secondary structures of the RNA influence the recognition of exons specifically used in muscle cells. I will recall a few memories on how interacting with François on this project shaped my perception of the scientific process and of the relationships between models and data. Later I worked on many aspects of RNA biology, from transcription to mRNP biogenesis and non-coding RNAs.

Transformation of chicken embryo fibroblasts by v-src uncouples beta1 integrin-mediated outside-in but not inside-out signaling

Adhesion of cells to extracellular matrix is mediated by integrin family receptors. The process of receptor-ligand binding is dependent on metabolic energy and is regulated by intracellular signals, termed inside-out signals. The strength of the initial alpha5beta1-mediated adhesion of v-src-transformed chicken embryo fibroblasts (v-srcCEF) was similar to that of normal CEF. A chemically cross-linked fibronectin substrate was able to restore cell spreading and the ability of v-srcCEF to assemble a fibronectin matrix. Over time, v-srcCEF showed decreased adhesion due to the reduction of alpha5beta1-fibronectin bonds consequent on the reduction of substrate-bound fibronectin due to the secretion of proteases by v-srcCEF. Excess synthesis of hyaluronic acid by v-srcCEF also reduced the alpha5beta1-fibronectin bonds and contributed to cell detachment at later times in culture. Thus, the adhesion defects were not due to a failure of alpha5beta1 function and adhesion of the v-srcCEF was alpha5beta1 dependent. Integrin-mediated adhesion also produces signals that affect cell proliferation and cell differentiation. An early consequence of these "outside-in" signals was the phosphorylation of FAK Y397 in direct proportion to the number of alpha5beta1-fibronectin bonds formed. In contrast, v-srcCEF had an increased level of phosphorylation on five different tyrosines in FAK, and none of these phosphorylation levels were sensitive to the number of alpha5beta1-fibronectin bonds. In the absence of serum, CEF proliferation was sensitive to changes in alpha5beta1-mediated adhesion levels. Transformation by v-src increased the serum-free proliferation rate and made it insensitive to alpha5beta1-mediated adhesion. Thus, the v-srcCEF were insensitive to the normal outside-in signals from alpha5beta1 integrin.

Delta 1-activated notch inhibits muscle differentiation without affecting Myf5 and Pax3 expression in chick limb myogenesis

The myogenic basic helix-loop-helix (bHLH) transcription factors, Myf5, MyoD, myogenin and MRF4, are unique in their ability to direct a program of specific gene transcription leading to skeletal muscle phenotype. The observation that Myf5 and MyoD can force myogenic conversion in non-muscle cells in vitro does not imply that they are equivalent. In this paper, we show that Myf5 transcripts are detected before those of MyoD during chick limb development. The Myf5 expression domain resembles that of Pax3 and is larger than that of MyoD. Moreover, Myf5 and Pax3 expression is correlated with myoblast proliferation, while MyoD is detected in post-mitotic myoblasts. These data indicate that Myf5 and MyoD are involved in different steps during chick limb bud myogenesis, Myf5 acting upstream of MyoD. The progression of myoblasts through the differentiation steps must be carefully controlled to ensure myogenesis at the right place and time during wing development. Because Notch signalling is known to prevent differentiation in different systems and species, we sought to determine whether these molecules regulate the steps occurring during chick limb myogenesis. Notch1 transcripts are associated with immature myoblasts, while cells expressing the ligands Delta1 and Serrate2 are more advanced in myogenesis. Misexpression of Delta1 using a replication-competent retrovirus activates the Notch pathway. After activation of this pathway, myoblasts still express Myf5 and Pax3 but have downregulated MyoD, resulting in inhibition of terminal muscle differentiation. We conclude that activation of Notch signalling during chick limb myogenesis prevents Myf5-expressing myoblasts from progressing to the MyoD-expressing stage.

Promotion of skeletal muscle differentiation by K252a with tyrosine phosphorylation of focal adhesion: a possible involvement of small GTPase Rho

K252a, a protein kinase inhibitor, acts as a neurotrophic factor in several neuronal cells. In this study we show that K252a enhanced the differentiation of C2C12 myoblasts as well as tyrosine phosphorylation of several focal adhesion-associated proteins including p130(Cas), focal adhesion kinase, and paxillin. The tyrosine phosphorylation of these proteins, reaching a maximum at 30 min after K252a treatment, closely correlated with the colocalization of these proteins in focal adhesion complexes and the coimmunoprecipitation of these proteins with p130(Cas). In addition, K252a stimulated longitudinal development of stress fiber-like structures and cell-matrix interaction in postmitotic myoblasts and eventually formation of well-developed myofibrils in multinucleated myotubes. Herbimycin A, a potent inhibitor of Src family kinases, and cytochalasin D, a selective disrupting-agent of actin filament, completely inhibited K252a-induced tyrosine phosphorylation as well as myoblast differentiation. Similar inhibitory effect was observed in the cells scrape loaded with a Rho inhibitor, C3 transferase, and the treatment of K252a induced a rapid translocation of Rho. These results are consistent with the model that Rho-dependent tyrosine phosphorylation of focal adhesion-associated proteins plays an important role in skeletal muscle differentiation.

The transcriptional activity of a muscle-specific promoter depends critically on the structure of the TATA element and its binding protein

We have previously characterized the proximal promoter of the mouse IIB myosin heavy chain (MyHC) gene, which is expressed only in fast-contracting glycolytic skeletal muscle fibers. We show here that the substitution into this promoter of a non-canonical TATA sequence from the IgH gene results in inactivity in muscle cells, even though TATA-binding protein (TBP) can bind strongly to this mutated promoter. Chemical foot-printing data show, however, that TBP makes different DNA contacts on this heterologous TATA sequence. The inactivity of such a non-canonical TATA motif in the IIB promoter context appears to be caused by a non-functional conformation of the bound TBP-DNA complex that is incapable of sustaining transcription. The conclusions imply that the precise sequence of the promoter TATA motif needs to be matched with the specific functional class of upstream activator proteins present in a given cell type in order for the gene to be transcriptionally active.

Amplification of MDM2 inhibits MyoD-mediated myogenesis

One obvious phenotype of tumor cells is the lack of terminal differentiation. We previously classified rhabdomyosarcoma cell lines as having either a recessive or a dominant nondifferentiating phenotype. To study the genetic basis of the dominant nondifferentiating phenotype, we utilized microcell fusion to transfer chromosomes from rhabdomyosarcoma cells into C2C12 myoblasts. Transfer of a derivative chromosome 14 inhibits differentiation. The derivative chromosome 14 contains a DNA amplification. MDM2 is amplified and overexpressed in these nondifferentiating hybrids and in the parental rhabdomyosarcoma. Forced expression of MDM2 inhibits MyoD-dependent transcription. Expression of antisense MDM2 restores MyoD-dependent transcriptional activity. We conclude that amplification and overexpression of MDM2 inhibit MyoD function, resulting in a dominant nondifferentiating phenotype.

Conditional differentiation of heart- and smooth muscle-derived cells transformed by a temperature-sensitive mutant of SV40 T antigen

To create muscle cell lines that conditionally differentiate in vitro we introduced a temperature-sensitive SV40 T antigen by retroviral infection into rat aortic smooth muscle cells (SMCs) and neonatal heart-derived cells. After G418 selection cell lines isolated were characterized at permissive (33 degrees C) and non-permissive (39 degrees C) temperatures. [3H]Thymidine uptake showed tht progression through the cell cycle is greatly reduced at 39 degrees C. Cytoskeletal proteins, such as actins and vimentin did not change significantly after temperature shift, while the number of desmin-positive SMCs significantly increased when cells were switched to 39 degrees C. Heart-derived muscle cells showed sarcomeric myosin heavy chain reactivity only when grown at 39 degrees C. After thrombin stimulation intracellular calcium in both cell types increased severalfold in 39 degrees C-cells but not in 33 degrees C-cells. Whole cell patch-clamp recordings of SMCs and heart-derived cells revealed a strong increase in nicardipine-sensitive Ca2+ current when cells were switched to 39 degrees C. Nicardipine-insensitive Ca2+ current also increased in both cell types at the non-permissive temperature. Na+ current in SMCs was large at 33 degrees C and small or not detectable at 39 degrees C and absent in heart-derived cells. Using a cDNA probe specific for the alpha 1 subunit of the dihydropyridine-sensitive Ca2+ channel we demonstrate a temperature-sensitive expression of the dihydropyridine receptor mRNA in smooth muscle-derived cells but not in heart-derived H10 cells. Our results suggest that upon downregulation of SV40 T antigen these cells become quiescent and exhibit a more differentiated phenotype. These cell lines may provide a useful tool to investigate ion channel- and receptor signal transduction, as well as cell cycle control in smooth and possibly cardiac muscle cell differentiation.

Transcriptional down regulation of the nov proto-oncogene in fibroblasts transformed by p60v-src

We have sought to identify genes whose expression is altered as a consequence of transformation by p60v-src. Using the mRNA differential display method, we have identified the nov proto-oncogene as one gene that is down regulated in chicken embryo fibroblasts (CEFs) transformed by p60v-src. nov transcripts were also found to be present at only very low levels in proliferating CEFs in comparison with quiescent CEFs. Serum stimulation of quiescent CEFs also resulted in a decline in the steady-state level of nov transcripts. Taken together, these findings suggest that the nov gene is expressed only in quiescent fibroblasts and that its down regulation may contribute to cellular transformation by the v-src oncogene. Down regulation of the nov gene appears to occur at both the transcriptional and posttranscriptional levels. Results obtained from experiments with a protein kinase inhibitor suggest that protein kinase C may be a key downstream effector in mediating the down regulation of nov transcripts in response to activation of p60src or serum stimulation. In addition, we found that transcription of an unknown gene is required for the decline in the steady-state level of nov transcripts in response to serum stimulation.

An analysis of astrocytic cell lines with different abilities to promote axon growth

The adult mammalian central nervous system (CNS) lacks the capacity to support axonal regeneration. There is increasing evidence to suggest that astrocytes, the major glial population in the CNS, may possess both axon-growth promoting and axon-growth inhibitory properties and the latter may contribute to the poor regenerative capacity of the CNS. In order to examine the molecular differences between axon-growth permissive and axon-growth inhibitory astrocytes, a panel of astrocyte cell lines exhibiting a range of axon-growth promoting properties was generated and analysed. No clear correlation was found between the axon-growth promoting properties of these astrocyte cell lines with: (i) the expression of known neurite-outgrowth promoting molecules such as laminin, fibronectin and N-cadherin; (ii) the expression of known inhibitory molecules such tenascin and chondroitin sulphate proteoglycan; (iii) plasminogen activator and plasminogen activator inhibitor activity; and (iv) growth cone collapsing activity. EM studies on aggregates formed from astrocyte cell lines, however, revealed the presence of an abundance of extracellular matrix material associated with the more inhibitory astrocyte cell lines. When matrix deposited by astrocyte cell lines was assessed for axon-growth promoting activity, matrix from permissive lines was found to be a good substrate, whereas matrix from the inhibitory astrocyte lines was a poor substrate for neuritic growth. Our findings, taken together, suggest that the functional differences between the permissive and the inhibitory astrocyte cell lines reside largely with the ECM.

Apoptotic cell death of a temperature-sensitive central neuronal cell line

A neuron-like cell line HS-2, derived from a primary fetal rat (E17) hippocampal cell culture using the temperature-sensitive SV 40 large T antigen, exhibits flat shape and grows well in culture medium with 5% fetal calf serum (FCS) at the permissive temperature (PT, 33.5 degrees C). At the non-permissive temperature (NPT, 38.5 degree C), many, but not all cells, have a neuronal shape with processes. The addition of dibutyryl-cAMP promotes the morphological changes in the cells to a neuron-like shape with long neurite-like processes and the cells exhibit neuron-specific enolase- and glutamic acid decarboxylase-immunoreactivity. Apoptotic cell death also occurs in these cultures at the NPT. DNA fragmentation and chromatin condensation that are characteristic of apoptosis occur within 8 h of being placed at the NPT. By 48 h after being placed at the NPT, the number of surviving cells decreases by 40% in the presence of 5% FCS. This cell line should be useful for investigating the mechanisms of 'programmed cell death' of neurons, which appears to occur during brain development and possibly in CNS degenerative diseases.

Promoter elements and transcriptional control of the chicken tropomyosin gene [corrected]

The chicken beta tropomyosin (beta TM) gene has two alternative transcription start sites (sk and nmCAP sites) which are used in muscle or non muscle tissues respectively. In order to understand the mechanisms involved in the tissue-specific and developmentally-regulated expression of the beta TM gene, we have analyzed the 5' regions associated with each CAP site. Truncated regions 5' to the nmCAP site were inserted upstream to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene and these constructs were transfected into avian myogenic and non myogenic cells. The maximum transcription is driven by the CAT construct (-168/ + 216 nt) in all cell types. Previous deletion analysis of the region 5' to the beta TMskCAP site has indicated that 805 nt confer myotube-specific transcription. In this work, we characterized an enhancer element (-201/-68 nt) which contains an E box (-177), a variant CArG box (-104) and a stretch of 7Cs (-147). Mutation of any of these motifs results in a decrease of the myotube-specific transcriptional activity. Electrophoretic mobility shift assays indicate that these cis-acting sequences specifically bind nuclear proteins. This enhancer functions in an orientation-dependent manner.

Isolation and characterization of conditionally immortalized astrocyte cell lines derived from adult human spinal cord

As an approach to develop both oligodendrocytic and astrocytic cell lines from adult human spinal cord, a cellular preparation of highly enriched oligodendrocytes and their precursors was infected with a replication-deficient retrovirus containing DNA sequences encoding the temperature-sensitive mutant of SV40 large T antigen. Six immortal cell lines were obtained. At both permissive (33 degrees C) and non-permissive (38.5 degrees C) temperatures, all cell lines were positive for vimentin, two demonstrated glial fibrillary acidic protein (GFAP) immunoreactivity, and none expressed oligodendrocyte or microglial markers. The 2 GFAP-positive cell lines [human spinal cord (HSC)2 and HSC6] were further characterized. Karyotype analysis revealed that both HSC2 and HSC6 cells showed gain of chromosomal material and structural chromosomal abnormalities. However, at non-permissive temperature both cell lines were indistinguishable from primary human astrocytes by a number of criteria. These properties included glutamine synthetase activity, Na(+)-dependent glutamate uptake, K+ flux, purine-evoked Ca2+ mobilization and entry, and the ability to support neurite outgrowth from embryonic rat retinal explants. The HSC2 and HSC6 cell lines may prove to be valuable models for studying the physiological properties of adult human astrocytes.

Infection with the avian polyomavirus, BFDV, selectively affects myofibril structure in embryonic chick ventricle cardiomyocytes

Embryonic cardiomyocytes can both beat and divide. They assemble cardiac muscle-specific proteins into sarcomeric myofibrils and contract. In addition, they periodically synthesize DNA, complete mitosis, disassemble sarcomeric myofibrils in the area of the mitotic spindle, assemble cytoplasmic isoform-specific proteins into a cleavage furrow contractile ring, undergo cytokinesis, and then reform sarcomeric myofibrils in daughter cells. Little is known about how embryonic cardiomyocytes disassemble their myofibrils as they traverse the cell cycle and divide. In the present study, beating embryonic avian ventricular cardiomyocytes in primary culture were stimulated to initiate DNA synthesis without subsequent mitosis or cytokinesis by infection with the lytic avian polyomavirus, Budgerigar Fledgling Disease Virus (BFDV). Within 48 hours, infected, adherent cardiomyocytes disassemble most of their sarcomeric myofibrils, retaining cardiac myosin only in thin myofibrils with disrupted sarcomeric periodicity and in amorphous nonfibrillar pools. By 72 hours, infected cardiomyocytes contain no myofibrils and no longer react with antibodies to cardiac myosin. In contrast, infected cardiomyocytes continue to display cytoplasmic myosin localized in stress-fiber-like-structures in adherent cells, or in disrupted fibers and dispersed pools in detaching cells. Infected cardiomyocytes also continue to display interphase-like arrays of polymerized microtubules, even when rounded-up just prior to lysis. These results suggest that polyomavirus infection may provide a useful model system for further study of the regulation of myofibrils disassembly in embryonic cardiomyocytes.

A novel cDNA corresponding to transcripts expressed in retina post-mitotic neurons

The long term objective of this study is to isolate genes specifically expressed at the onset of neuronal cell cycle withdrawal. As an experimental paradigm we have used a quail neuroretinal cell clone (clone K2) immortalized by a thermosensitive mutant of Rous Sarcoma Virus. K2 cells proliferate at 36 degrees C but stop synthesizing DNA after a shift to 41.5 degrees C. We have constructed a cDNA library from K2 cells transferred to 41.5 degrees C and autosubtracted with RNAs from K2 cells maintained at 36 degrees C. This strategy has led to the isolation of cDNAs which recognize mRNAs expressed in quail neuroretina (NR) during development. We report here one of these cDNAs, cDNA QN1, that hybridizes with transcripts expressed in retina neurons, in parallel with their withdrawal from the cell cycle. QN1 ORF codes for a 138 kDa polypeptide corresponding to the protein observed in Western blot analysis. A role of QN1 product(s) on neuronal quiescence is suggested by the positive effect of an antisense oligonucleotide on DNA synthesis of K2 cells.

Target regulation of neuronal differentiation in a temperature-sensitive cell line derived from medullary raphe

Following infection of dissociated embryonic day 13 rat medullary raphe cells with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen, a clonal cell line, RN33B, was isolated by serial dilution. At 33 degrees C, RN33B cells divide with a doubling time of 48 h and show T antigen, vimentin, nestin, diffuse neuron-specific enolase, and low and medium molecular weight neurofilament immunoreactivities. RN33B cells are immortal, but not transformed, as they will not grow in soft agar. At non-permissive temperature (38.5 degrees C), T antigen expression is markedly decreased and RN33B cells cease mitotic activity and differentiate with phase bright cell bodies and 'neuritic-like' processes. Differentiated RN33B cells express enhanced neuronal-specific protein expression but do not synthesize astrocytic or oligodendrocytic-specific proteins. Moreover, differentiated RN33B cells returned to 33 degrees C re-express T antigen, but do not de-differentiate or begin dividing. Co-culture with embryonic hippocampus and cerebral cortex, but not medullary raphe or spinal cord, resulted in significantly greater survival, more complex neuronal morphology, and enhanced expression of neuronal-specific antigens. Immunohistochemical and Northern blot analysis revealed high levels of low affinity NGF receptor protein and mRNA in differentiated RN33B cells. PCR analysis demonstrated the presence of trkB, but not trkA or trkC, mRNA in both undifferentiated and differentiated RN33B cells. These data suggest that the observed target regulation of RN33B cell neuronal differentiation in co-culture may be mediated by neurotrophin(s).

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.

Biochemical and cytological changes associated with expression of deregulated pp60src in Xenopus oocytes

We have examined the effects of Xenopus pp60c-src with constitutive kinase activity on the morphology and maturation of Xenopus laevis oocytes. When RNA encoding this deregulated variant was injected into stage VI oocytes, we observed a gross alteration in the cortex of the oocyte. This alteration involved aggregation of pigment and invagination of the cortex in a large area proximal to the site of injection. This phenomenon was not seen in oocytes injected with RNA encoding wild-type pp60c-src. We have correlated this phenomenon with the tyrosine phosphorylation of 84- and 100-kDa proteins. These phosphorylated proteins colocalized with the alteration in the oocyte cortex when assayed by both biochemical and immunocytochemical methods. Neither the pigment aggregation nor phosphorylation of the 84- and 100-kDa proteins was observed in oocytes expressing a nonmyristoylated version of the deregulated pp60c-src. Expression of deregulated Xenopus fyn, a src-family member, resulted in a phenotype similar to that seen with deregulated src. However, in the fyn-injected oocytes, many more proteins were phosphorylated on tyrosine than in the src-injected oocytes. Progesterone stimulation of oocytes expressing deregulated pp60c-src resulted in an increase in the number of tyrosine-phosphorylated proteins. This change may represent the response of pp60src to the resumption of the cell cycle in maturing oocytes. These data suggest that the oocyte may be a particularly useful system for investigating the role of pp60c-src in the regulation of cytoskeletal structure and in the regulation of events associated with the cell cycle.

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.

Biochemical and cytological changes associated with expression of deregulated pp60src in Xenopus oocytes

We have examined the effects of Xenopus pp60c-src with constitutive kinase activity on the morphology and maturation of Xenopus laevis oocytes. When RNA encoding this deregulated variant was injected into stage VI oocytes, we observed a gross alteration in the cortex of the oocyte. This alteration involved aggregation of pigment and invagination of the cortex in a large area proximal to the site of injection. This phenomenon was not seen in oocytes injected with RNA encoding wild-type pp60c-src. We have correlated this phenomenon with the tyrosine phosphorylation of 84- and 100-kDa proteins. These phosphorylated proteins colocalized with the alteration in the oocyte cortex when assayed by both biochemical and immunocytochemical methods. Neither the pigment aggregation nor phosphorylation of the 84- and 100-kDa proteins was observed in oocytes expressing a nonmyristoylated version of the deregulated pp60c-src. Expression of deregulated Xenopus fyn, a src-family member, resulted in a phenotype similar to that seen with deregulated src. However, in the fyn-injected oocytes, many more proteins were phosphorylated on tyrosine than in the src-injected oocytes. Progesterone stimulation of oocytes expressing deregulated pp60c-src resulted in an increase in the number of tyrosine-phosphorylated proteins. This change may represent the response of pp60src to the resumption of the cell cycle in maturing oocytes. These data suggest that the oocyte may be a particularly useful system for investigating the role of pp60c-src in the regulation of cytoskeletal structure and in the regulation of events associated with the cell cycle.

Overexpression of Mos, Ras, Src, and Fos inhibits mouse mammary epithelial cell differentiation

Mammary epithelial cells terminally differentiate in response to lactogenic hormones. We present evidence that oncoprotein overexpression is incompatible with this hormone-inducible differentiation and results in striking cellular morphological changes. In mammary epithelial cells in culture, lactogenic hormones (glucocorticoid and prolactin) activated a transfected beta-casein promoter and endogenous beta-casein gene expression. This response to lactogenic hormone treatment was paralleled by a decrease in cellular AP-1 DNA-binding activity. Expression of the mos, ras, or src (but not myc) oncogene blocked the activation of the beta-casein promoter induced by the lactogenic hormones and was associated with the maintenance of high levels of AP-1. Mos expression also increased c-fos and c-jun mRNA levels. Overexpression of Fos and Jun from transiently transfected constructs resulted in a functional inhibition of the glucocorticoid receptor in these mouse mammary epithelial cells. This finding clearly suggests that glucocorticoid receptor inhibition arising from oncogene expression will contribute to the block in hormonally induced mammary epithelial cell differentiation. Expression of Src resulted in the loss of the normal organization and morphological phenotype of mammary epithelial cells in the epithelial/fibroblastic line IM-2. Activation of a conditional c-fos/estrogen receptor gene encoding an estrogen-dependent Fos/estrogen receptor fusion protein also morphologically transformed mammary epithelial cells and inhibited initiation of mammary epithelial differentiation-associated expression of the beta-casein and WDNM 1 genes. In response to estrogen treatment, the cells displayed a high level of AP-1 DNA-binding activity. Our results demonstrate that high cellular AP-1 levels contribute to blocking the ability of mammary epithelial cells in culture to respond to lactogenic hormones. This and other studies indicate that the oncogene products Mos, Ras, and Src exert their effects, at least in part, by stimulating cellular Fos and probably cellular Jun activity.

Overexpression of Mos, Ras, Src, and Fos inhibits mouse mammary epithelial cell differentiation

Mammary epithelial cells terminally differentiate in response to lactogenic hormones. We present evidence that oncoprotein overexpression is incompatible with this hormone-inducible differentiation and results in striking cellular morphological changes. In mammary epithelial cells in culture, lactogenic hormones (glucocorticoid and prolactin) activated a transfected beta-casein promoter and endogenous beta-casein gene expression. This response to lactogenic hormone treatment was paralleled by a decrease in cellular AP-1 DNA-binding activity. Expression of the mos, ras, or src (but not myc) oncogene blocked the activation of the beta-casein promoter induced by the lactogenic hormones and was associated with the maintenance of high levels of AP-1. Mos expression also increased c-fos and c-jun mRNA levels. Overexpression of Fos and Jun from transiently transfected constructs resulted in a functional inhibition of the glucocorticoid receptor in these mouse mammary epithelial cells. This finding clearly suggests that glucocorticoid receptor inhibition arising from oncogene expression will contribute to the block in hormonally induced mammary epithelial cell differentiation. Expression of Src resulted in the loss of the normal organization and morphological phenotype of mammary epithelial cells in the epithelial/fibroblastic line IM-2. Activation of a conditional c-fos/estrogen receptor gene encoding an estrogen-dependent Fos/estrogen receptor fusion protein also morphologically transformed mammary epithelial cells and inhibited initiation of mammary epithelial differentiation-associated expression of the beta-casein and WDNM 1 genes. In response to estrogen treatment, the cells displayed a high level of AP-1 DNA-binding activity. Our results demonstrate that high cellular AP-1 levels contribute to blocking the ability of mammary epithelial cells in culture to respond to lactogenic hormones. This and other studies indicate that the oncogene products Mos, Ras, and Src exert their effects, at least in part, by stimulating cellular Fos and probably cellular Jun activity.

Retinoic acid induces myogenin synthesis and myogenic differentiation in the rat rhabdomyosarcoma cell line BA-Han-1C

Two clonal rat rhabdomyosarcoma cell lines BA-Han-1B and BA-Han-1C with different capacities for myogenic differentiation have been examined for the expression of muscle regulatory basic helix-loop-helix (bHLH) proteins of the MyoD family. Whereas cells of the BA-Han-1C subpopulation constitutively expressed MyoD1 and could be induced to differentiate with retinoic acid (RA), BA-Han-1B cells did not express any of the myogenic control factors and appeared to be largely differentiation-defective. Upon induction with RA, BA-Han-1C cells expressed also myogenin, in contrast to BA-Han-1B cells which never activated any of the genes encoding muscle bHLH factors. The onset of myogenin transcription in BA-Han-1C cells required de novo protein synthesis and DNA replication suggesting that RA probably did not act directly on the myogenin gene. Although MyoD1 was expressed in proliferating BA-Han-1C myoblasts, muscle-specific reporter genes were not activated indicating that MyoD was biologically inactive. However, transfections with plasmid expressing additional MyoD1 protein resulted in the transactivation of muscle genes even in the absence of RA. mRNA encoding the negative regulatory HLH protein Id was expressed in proliferating BA-Han-1C cells and disappeared later after RA induction which suggested that it may be involved in the regulation of MyoD1 activity. The myogenic differentiation of malignant rhabdomyosarcoma cells strictly correlated with the activation of the myogenin gene. In fact, stable transfections of BA-Han-1C cells with myogenin expressing plasmids resulted in spontaneous differentiation. Together, our results suggest that the transformed and undifferentiated phenotype of BA-Han-1C rhabdomyosarcoma cells is associated with the inactivation of the myogenic factor MyoD1 as well as lack of myogenin expression. RA alleviates the inhibition of myogenic differentiation, probably by activating MyoD protein and myogenin gene transcription. BA-Han-1B cells did not respond to RA and the differentiated phenotype could not be restored by overexpression of MyoD1 or myogenin.

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.

Density dependent polarized secretion of a prostatic epithelial cell line

The polarized secretions (apical/basal) of newly synthesized total protein and proteases from prostatic epithelial sheets of PA-III cells grown in dual compartment chambers were investigated at various cell densities and culture conditions. PA-III cells grown in a serum free defined medium (SFDM) form morphologically polarized monolayers of epithelial cells. These cells secreted their 35S-methionine labeled total protein in a predominantly apical direction (apical/basal ratio, 4-8 fold), with a lesser proportion of protein secreted apically at lower cell densities of the PA-III cell monolayer. PA-III cells grown in 5% fetal calf serum (FCS) are morphologically squamous, comparable to the anaplastic phenotype, and exhibited an inversion of polarized total protein secretion (apical/basal ratio, 0.4-0.9 fold), with an increased proportion of total protein secreted in a basal direction at lower cell densities. Since the culture of PA-III cells in FCS may approximate the anaplastic phenotype we investigated the polarized secretion of proteases from these cells at various cell densities, and compared them with the secretory pattern of protease secretion from polarized PA-III cells cultured in SFDM. At lower cell densities of the PA-III cells grown in FCS the polarity of protease secretion was inverted such that metalloproteinases, tissue type plasminogen activator, and a 72 kD gelatinase were secreted in a predominantly basal direction, as well as urokinase and a gelatinase of 26 kD that were secreted more or less equally into the apical and basal compartments of the chambers. On the other hand, for cultures of PA-III cells grown in SFDM the aforementioned proteases exhibited predominantly an apically directed polarity of secretion. These results suggest that the anaplastic phenotype characterized by a loss of polarized structure may also be characterized by a functional loss or inversion of polarized secretion. The consequences of such a loss or inversion of polarized secretion would be to increase the localized concentrations of proteases along the basal domain of cells thereby facilitating degradation of the basement membrane and interstitial tissue in vivo.

Differentiation and heterogeneity in T-antigen immortalized precursor cell lines from mouse cerebellum

Recently, various techniques have been developed to transfer oncogenes into brain cells in order to generate immortalized neural cell lines. It is of interest to establish how well such cell lines reflect their cellular origin. Here we report the characterization of sixteen cell lines from mouse cerebellum and, as a control, six cell lines from skin. Lines were established by immortalizing postnatal primary cell cultures with a retrovirus carrying a modified temperature-sensitive variant of SV40 large T antigen. The cell lines reflect many properties of the cell type from which they were derived. All of the sixteen cerebellar lines expressed one or more markers of the neural precursor cells, namely, nestin and epitopes for NG2 and A2B5. In contrast, none of the six skin lines expressed neural precursor markers. Both types of cell lines expressed vimentin and fibronectin. Differentiation occurred in some of the cerebellar lines and was enhanced in defined medium. A small percentage of cerebellar cells, usually less than 5%, was positive for a marker of differentiation, e.g., glial fibrillary acidic protein (GFAP), galactocerebroside (GalC), or L1. Expression of GFAP colocalized with that of nestin at varying levels of intensity, indicating a gradual replacement of nestin by GFAP in the cytoskeleton. Both the cells positive for precursor markers and those positive for differentiation markers tended to be located in clusters, suggesting that stochastic processes or cell-cell interactions are important for the determination of the fate of cells within a clonal cell line in vitro. The degree of differentiation seemed to correlate with a shift from serum-containing to defined medium, but not with a shift from the permissive to the nonpermissive temperature for T antigen expression. The immortalization approach described here thus allows the establishment of cell lines which are "captured" in the precursor state of the developing mouse neuroepithelium.

The chicken gene encoding the alpha isoform of tropomyosin of fast-twitch muscle fibers: organization, expression and identification of the major proteins synthesized

The chicken gene alpha fTM encoding the alpha-tropomyosin of fast-twitch muscle fibers (alpha fTM) covers 20 kb and consists of 15 exons. From this gene, three types of mature transcripts (1.3 kb, 2 kb and 2.8 kb) are expressed through the use of alternative promoters, alternatively spliced exons and multiple 3' end processing. Northern analysis and S1 mapping have shown that the 1.3-kb transcript (exons 1a, 2b, 3, 4, 5, 6b, 7, 8, 9a-9b) is expressed in fast-twitch skeletal muscles and that 2-kb transcripts are expressed in smooth muscle (exons 1a, 2a, 3, 4, 5, 6b, 7, 8, 9d) and in fibroblasts (exons 1a, 2b, 3, 4, 5, 6a or 6b, 7, 8, 9d). These 2-kb transcripts encode distinct proteins which we have identified by two-dimensional (2D) gel electrophoresis. The 2.8-kb transcript which has not been so far characterized in birds is expressed in brain (exons 1b, 3, 4, 5, 6b, 7, 8, 9c-9d). This transcript has been characterized by a cDNA polymerase chain reaction assay and by S1 nuclease mapping. It produces a major TM isoform of chick brain which we have identified by 2D gels.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation 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.

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.

Reversible inhibition of a thyroid-specific trans-acting factor by Ras

Exposure of rat thyroid cells for 1 week to a temperature-sensitive variant of Kirsten murine sarcoma virus (KiMSV) Ras inactivated the thyroglobulin promoter (pTg). Cellular dedifferentiation was paralleled by the loss of the thyroid-specific trans-acting factor, TgTF1, which binds to pTg. When Ras was denatured by shifting cells to 39 degrees C, TgTF1 binding and pTg function recovered rapidly without the synthesis of new protein. TgTF1 could be reactivated in vitro by treating nuclear extracts with protein kinase A. After 4 weeks of exposure to the oncogene, denaturation of Ras no longer restored TgTF1 binding or reactivated pTg. Incubation of nuclear extracts with protein kinase A likewise did not reactivate TgTF1. Cells chronically exposed to Ras did, however, yield differentiated clones after treatment with 5-azacytidine. We suggest that Ras induces dedifferentiation in two sequential steps: (1) Ras reduces PKA activity; TgTF1 (or an auxiliary protein) becomes dephosphorylated, and binding to pTg is abolished. (2) The effects of Ras become imprinted by methylation, possibly of the TgTF1 gene.

The current state of oncogenes and cancer: experimental approaches for analyzing oncogenetic events in human cancer

The development of cancer is a multistage process. The activation of proto-oncogenes and the inactivation of tumor suppressor genes play a critical role in the induction of tumors. Using human cell model systems of carcinogenesis, we have studied how oncogenes, tumor suppressor genes, and recessive cancer susceptibility genes participate in this multistep process. Normal human cells are resistant to the transforming potential of oncogenes, such as ras oncogenes, which are activated by specific point mutations. Since as many as 40% of some tumor types contain activated ras oncogenes, a preneoplastic transition in multistage carcinogenesis must involve changing from an oncogene-resistant stage to an oncogene-susceptible stage. The analysis of such critical steps in carcinogenesis using rodent systems has usually not represented the human disease with fidelity. In order to study this carcinogenic process, we have developed human cell, in vitro systems that represent some of the genetic changes that occur in cellular genes during human carcinogenesis. Using these systems, we have learned some of the functions of dominant activated-transforming oncogenes, tumor suppressor genes, and cellular immortalization genes and how they influence the carcinogenic process in human cells. Using our understanding of these processes, we are attempting to clone critical genes involved in the etiology of familial cancers. These investigations may help us to develop procedures that allow us to predict, in these cancer families, which individuals are at high risk for developing cancer.

Distinct and different effects of the oncogenes v-myc and v-src on avian sympathetic neurons: retroviral transfer of v-myc stimulates neuronal proliferation whereas v-src transfer enhances neuronal differentiation

Immature avian sympathetic neurons are able to proliferate in culture for a limited number of divisions albeit expressing several neuron-specific properties. The effect of avian retroviral transfer of oncogenes on proliferation and differentiation of sympathetic neurons was investigated. Primary cultures of 6-d-old quail sympathetic ganglia, consisting of 90% neuronal cells, were infected by Myelocytomatosis virus (MC29), which contains the oncogene v-myc, and by the v-src-containing Rous sarcoma virus (RSV). RSV infection, in contrast to findings in other cellular systems, resulted in a reduction of neuronal proliferation as determined by 3H-thymidine incorporation (50% of control 4 d after infection) and in increased morphological differentiation. This is reflected by increased neurite production, cell size, and expression of neurofilament protein. In addition, RSV-infected neurons, unlike uninfected cells, are able to survive in culture for time periods up to 14 d in the absence of added neurotrophic factors. In contrast, retroviral transfer of v-myc stimulated the proliferation of immature sympathetic neurons preserving many properties of uninfected cells. The neuron-specific cell surface antigen Q211 and the adrenergic marker enzyme tyrosine hydroxylase were maintained in MC29-infected cells and in the presence of chick embryo extract the cells could be propagated over several weeks and five passages. Within 7 d after infection, the number of Q211-positive neurons increased approximately 100-fold. These data demonstrate distinct and different effects of v-src and v-myc-containing retroviruses on proliferation and differentiation of sympathetic neurons: v-src transfer results in increased differentiation, whereas v-myc transfer maintains an immature status reflected by proliferation, immature morphology, and complex growth requirements. The possibility of expanding immature neuronal populations by transfer of v-myc will be of considerable importance for the molecular analysis of neuronal proliferation and differentiation.

SV40 immortalizes myogenic cells: DNA synthesis and mitosis in differentiating myotubes

Primary skeletal muscle myoblasts have a limited proliferative capacity in cell culture and cease to proliferate after several passages. We examined the effects of several oncogenes on the immortalization and differentiation of primary cultures of rat skeletal muscle myoblasts. Retroviruses containing a SV40 large T antigen (LT) gene very efficiently immortalize myogenic cells. The immortalized cell lines retain a very high differentiation capacity and form, in the appropriate culture conditions, a very dense network of muscle fibers. As in primary culture, cell fusion is associated with the synthesis of large amounts of muscle-specific proteins. However, unlike normal myoblasts (and previously established myogenic cell lines), nuclei in the multinucleated fibers of SV40-immortalized cells synthesize DNA and enter mitosis. Thus, withdrawal from DNA synthesis is not obligatory for cell fusion and biochemical differentiation. Using a retrovirus coding for a temperature-sensitive SV40 LT, myogenic cell lines were produced in which the SV40 LT could be inactivated by a shift from 33 degrees C to 39 degrees C. The inactivation of LT induced massive cell fusion and synthesis of muscle proteins. The nuclei in those fibers did not synthesize DNA, nor did they undergo mitosis. This approach enabled the reproducible establishment of myogenic cell lines from very small populations of myoblasts or single primary myogenic clones. Activated p53 also readily immortalized cells in primary muscle cultures, however the cells of eight out of the nine cell lines isolated had a fibroblastic morphology and could not be induced to form multinucleated fibers.

Carcinogenic implications of the neighborhood coherence principle (NCP)

A new hypothesis on carcinogenesis is set forth on the basis of the neighborhood coherence principle (NCP). NCP constitutes a general rule of pattern formation and maintenance. According to this principle, a system of interacting cells can produce and maintain a spatial organization by virtue of cell-cell communication. This hypothesis suggests that this homeostasis primarily results from a NCP-like process implying cell-cell communication. Each cell is constrained by its neighbors to maintain the mature phenotype despite its inherent individual variability. If the cell-cell mature communication happens to be impaired, tissue homeostasis is disrupted and a proliferative state can be initiated. A further potential effect may result from the establishment of NCP-like communication specific for proliferative cells allied to paracrine and outocrine factors which can lock the cells into the proliferative mode. Most mechanisms implied in this hypothesis have already been investigated. There is a large body of experimental results supporting the role of junctional communication in cooperative metabolism, growth, differentiation and tumour-related events. This new hypothesis provides a framework within which these known facts may be put in a theoretical perspective; it might well constitute the unifying theory--as yet missing--in carcinogenesis.

The use of cell lines in neurobiology

The mature nervous system is made up of a large number of terminally differentiated neuronal and glial cell types, which develop from precursor cells in the embryonic nervous system. Many aspects of the differentiation pathways leading to the formation of neurons and glia remain elusive because of the cellular and molecular complexity of the brain, with cells of different types intermingled and differentiating at different times. One way to reduce the complexity is to study particular developmental stages and steps in neuronal differentiation in cell lines, i.e. clonal, homogeneous populations of cells that can be grown indefinitely in vitro. Urban Lendahl and Ronald McKay discuss how cell lines are used to dissect the cellular differentiation of the nervous system. Recent technical progress may allow the construction of 'custom-made' cell lines from different regions and developmental stages in the nervous system. Such cell lines retain features of the cells from which they originated and make possible detailed molecular studies of features only transiently present in the developing brain. New strategies are being developed which can be used to assess the effect of genetic changes in cell lines both in tissue culture and in the whole animal. This review attempts to show that cell lines are not a 'reductio ad absurdum' but an additional and critical tool in understanding the genetic contribution to the organization and function of the brain.

Mesodermal cell determination and differentiation

Many in vitro systems have been designed to study the processes governing cell determination and differentiation during development. Mammalian culture systems have been particularly helpful in elucidating the mechanisms regulating gene expression during differentiation in cells of mesodermal origin, namely, myoblasts, preadipocytes, and chondroblasts. Studies have shown that particular cis-acting sequences and trans-acting factors are important in determining tissue-specific and developmental gene expression in these systems. The role of growth factors, oncogenes, and other agents during differentiation has also been examined. Recently four putative muscle determination genes have been isolated and are being characterized. These studies have been useful in postulating models of how development proceeds in vivo and how differentiation and transformation to a neoplastic phenotype may be related.

Selective inhibition of tyrosine protein kinase by a synthetic multisubstrate analog

Synthetic compounds were designed in an attempt to mimic the possible transition state of tyrosine protein kinases. One representative compound (RP 53801) inhibited the enzyme purified from RSV-transformed cells. A serine/threonine kinase (kinase C) was 45 fold less sensitive. The inhibition was competitive with respect to ATP and noncompetitive with respect to the phosphate acceptor poly glu4-tyr1. The degree of inhibition (IC50 = 22 microM) was however lower than that expected from a transition state analog. The compound was capable of reducing tyrosine protein kinase activity in intact cells with some selectivity at 100 microM.

Transformation by activated ras or fos prevents myogenesis by inhibiting expression of MyoD1

Transformation of myoblasts by activated ras inhibits myogenic differentiation. We demonstrate that this oncogene inhibits expression of the muscle regulatory factors MyoD1 and myogenin. Expression of retroviral-encoded MyoD1 in ras-transformed myoblasts leads to the re-expression of both terminal differentiation markers and lineage markers expressed in proliferating myoblasts (including endogenous MyoD1 and myogenin), suggesting that ras inhibits myogenic differentiation in a manner dependent on the loss of MyoD1 expression. In addition, we show that fos transformation of myoblasts inhibits muscle differentiation by a similar mechanism.

Elevated c-fos expression inhibits differentiation of L6 rat myoblasts

Expression of c-fos is induced by a number of signals in several cell systems. Although the exact function of the c-fos product is unknown, it has been implicated to be of importance for both cell growth and differentiation (Verma and Sassone-Corsi, 1987). To analyze how c-fos expression relates to in vitro myogenic differentiation, the kinetics of c-fos mRNA expression during spontaneous in vitro differentiation of L6J1 myoblasts was examined; c-fos transcripts were most abundant at day 4 of the differentiation process. Multinucleated myotubes and expression of alpha-actin and myosin heavy chain (MHC) mRNA appeared later, at day 6 or 7, and increased to maximal levels after 10 days in culture. To analyze further the relation between c-fos expression and L6J1 myogenic differentiation, L6J1 myoblasts were transfected with expression vectors containing the murine c-fos gene driven by a metallothionein promoter. The growth rate of c-fos-transfected L6J1 cells did not differ from that of control cells. However, formation of myotubes was significantly reduced in c-fos-transfected L6J1 cultures compared with neo-transfected controls. Myotube formation and expression of the myogenic markers alpha-actin and MHC were reduced in subclones expressing high levels of c-fos, but not in subclones with lower levels of c-fos expression. These results indicate that a marked elevation of c-fos expression at least partially inhibits L6J1 myogenic differentiation.

Changes in c-onc expression during embryonal carcinoma cell differentiation

Protooncogenes expressed in murine embryonal carcinoma (EC) cells or their differentiated daughter cells include more or less ubiquitously expressed protooncogenes such as c-myc, c-K-ras, and c-abl, as well as c-onc genes with a very restricted expression pattern. Examples of the latter are N-myc, c-mos, and int-2. These c-onc genes are transcriptionally active in EC cells, as well as in germ cells and/or early embryonic cells. When EC cells are induced to differentiate some protooncogenes or oncogene-related products undergo changes in expression. Thus, EC cell differentiation has been associated with increased expression of c-src, c-fos, int-1, int-2, and the epidermal growth factor (EGF) receptor, whereas decreased expression has been observed for c-mos, c-K-ras, c-myc, N-myc, and platelet-derived growth factor. The relationships between these changes in expression and EC cell differentiation are not understood. They may be important for the differentiation process or for expression of a differentiated phenotype. They may, however, also be secondary events with no functional significance to EC cell differentiation.

c-src and other proto-oncogenes implicated in neuronal differentiation

The proto-oncogene c-src has been implicated in the development and mature function of the nervous system. pp60c-src, the protein product of the c-src gene, is a tyrosine protein kinase that is highly enriched in fetal neural tissue. pp60c-src appears in two phases of neuronal development. Neuroectodermal cells of gastrulating embryos first express pp60c-src around the time of commitment to neuronal or glial pathways. Later, committed neuroepithelial cells express pp60c-src near the onset of terminal neuronal differentiation. Immunocytochemical analyses of pp60c-src in developing chick retina, telencephalon, and cerebellum show immunoreactivity concentrated in regions rich in growth cones and neurites. Moreover, pp60c-src is concentrated approximately 10-fold in a biochemical fraction from fetal rat brain that is enriched in nerve growth cone membranes. These results point toward a function for pp60c-src in neurite outgrowth. A functional role for other proto-oncogenes in the development of the nervous system was indicated from a study of the expression of a battery of proto-oncogenes during the retinoic acid-induced differentiation of the mouse embryonal carcinoma cell line P19 to a neuronal phenotype. Nuclear runoff transcription of the proto-oncogenes c-src, c-fms, c-sis, N-ras, c-myc, and c-fos was observed in proliferating and retinoic acid-treated cells.

Replicating myoblasts express a muscle-specific phenotype

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The developmentally regulated, muscle-specific, integral membrane protein H36 and the intermediate-filament protein desmin were scored as markers of the myogenic phenotype. The percentage of BrdUrd+ (i.e., proliferative) cells among H36+ and desmin+ myoblasts was equal to the percentage of BrdUrd+ cells in the entire population, indicating that the expression of H36 and desmin is uniformly characteristic of replicating myoblasts. Inhibition of protein synthesis before and during growth in BrdUrd did not alter the frequency of desmin and H36 immunofluorescence in BrdUrd+ cells. Thus, desmin and H36 were present in the replicating myoblasts prior to the onset of growth in BrdUrd. These results were confirmed using H36+ cells selected by flow cytometry: these purified H36+ myoblasts replicate, express desmin, and differentiate. Similar results were obtained with mouse myoblasts. Desmin expression in these mammalian cells differs from that in chicken embryo myoblasts: only a small proportion of replicating chicken embryo myoblasts express desmin. That replicating mammalian myoblasts have a muscle-specific phenotype serves to define a distinct stage in myogenic development and a specific cell in the myogenic lineage. Further, it implies that there is a regulatory event activated during myogenesis that precedes terminal differentiation and that is required for expression of those genes whose products distinguish the replicating myoblast.