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

Immortalized mouse articular cartilage cell lines retain chondrocyte phenotype and respond to both anabolic factor BMP-2 and pro-inflammatory factor IL-1

Articular cartilage chondrocytes help in the maintenance of tissue homeostasis and function of the articular joint. Study of primary chondrocytes in culture provides information closely related to in vivo functions of these cells. Limitations in the primary culture of chondrocytes have lead to the development of cells lines that serve as good surrogate models for the study of chondrocyte biology. In this study, we report the establishment and characterization of chondrocyte cell lines, MM-Sv/HP and MM-Sv/HP-2 from mouse articular cartilage. Cells were isolated from mouse femoral head articular cartilage, immortalized and maintained in culture through numerous passages. The morphology of the cells was from fibroblastic to polygonal in nature. Gene expression studies using quantitative PCR (Q-PCR) were performed on cells in monolayer culture and cells embedded in a three-dimensional alginate matrix. Stimulation of cells in monolayer culture with anabolic factor, BMP-2, resulted in increased gene expression of the extracellular matrix molecules, aggrecan and type II collagen and their regulator transcription factor, Sox9. Treatment by pro-inflammatory IL-1 resulted in increased gene expression of catabolic effectors including Aggrecanases (ADAMTS4, ADAMTS5), MMP-13 and nitric oxide synthase (Nos2). Cells in alginate treated with BMP-2 resulted in increased synthesis of proteoglycan which was released into the conditioned media on IL-1 stimulation. Western analysis of conditioned media showed the presence of Aggrecanase-cleaved aggrecan fragments. In summary, MM-Sv/HP and MM-Sv/HP-2 show preservation of important characteristics of articular chondrocytes as examined under multiple culture conditions and would provide a useful reagent in the study of chondrocyte biology.

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.

BCL2 is a downstream effector of MIZ-1 essential for blocking c-MYC-induced apoptosis

The c-MYC oncoprotein is among the most potent transforming agents in human cells. Ironically, c-MYC is also capable of inducing massive apoptosis under certain conditions. A clear understanding of the distinct pathways activated by c-MYC during apoptosis induction and transformation is crucial to the design of therapeutic strategies aimed at selectively reactivating the apoptotic potential of c-MYC in cancer cells. We recently demonstrated that apoptosis induction in primary human cells strictly requires that c-MYC bind and inactivate the transcription factor MIZ-1. This presumably blocked the ability of MIZ-1 to activate the transcription of an unidentified pro-survival gene. Here we report that MIZ-1 activates the transcription of BCL2. More importantly, inhibition of the MIZ-1/BCL2 signal is an essential event during the apoptotic response. Furthermore, targeting BCL2 with short hairpin RNA or small molecule inhibitors restores the apoptotic potential of a c-MYC mutant that is defective for MIZ-1 inhibition. These observations suggest that repression of BCL2 transcription is the single essential consequence of targeting the MIZ-1 pathway during apoptosis induction. These data define a genetic pathway that helps to explain historical observations documenting cooperation between c-MYC and BCL2 overexpression in human cancer.

Immortalized cell lines from mouse xiphisternum preserve chondrocyte phenotype

Chondrocytes are unique to cartilage and the study of these cells in vitro is important for advancing our understanding of the role of these cells in normal homeostasis and disease including osteoarthritis (OA). As there are limitations to the culture of primary chondrocytes, cell lines have been developed to overcome some of these obstacles. In this study, we developed a procedure to immortalize and characterize chondrocyte cell lines from mouse xiphisternum. The cells displayed a polygonal to fibroblastic morphology in monolayer culture. Gene expression studies using quantitative PCR showed that the cell lines responded to bone morphogenetic protein 2 (BMP-2) by increased expression of matrix molecules, aggrecan, and type II collagen together with transcriptional factor, Sox9. Stimulation by IL-1 results in the increased expression of catabolic effectors including MMP-13, nitric oxide synthase, ADAMTS4, and ADAMTS5. Cells cultured in alginate responded to BMP-2 by increased synthesis of proteoglycan (PG), a major matrix molecule of cartilage. IL-1 treatment of cells in alginate results in increased release of PG into the conditioned media. Further analysis of the media showed the presence of Aggrecanase-cleaved aggrecan fragments, a signature of matrix degradation. These results show that the xiphisternum chondrocyte cell lines preserve their chondrocyte phenotype cultured in either monolayer or 3-dimensional alginate bead culture systems. In summary, this study describes the establishment of chondrocyte cell lines from the mouse xiphisternum that may be useful as a surrogate model system to understand chondrocyte biology and to shed light on the underlying mechanism of pathogenesis in OA.

Proteoglycan production by immortalized human chondrocyte cell lines cultured under conditions that promote expression of the differentiated phenotype

Large and small proteoglycans are essential components of articular cartilage. How to induce chondrocytes to repair damaged cartilage with normal ratios of matrix components after their loss due to degenerative joint disease has been a major research focus. We have developed immortalized human chondrocyte cell lines for examining the regulation of cartilage-specific matrix gene expression. However, the decreased synthesis and deposition of cartilage matrix associated with a rapid rate of proliferation has presented difficulties for further examination at the protein level. In these studies, proteoglycan synthesis was characterized in two chondrocyte cell lines, T/C-28a2 and tsT/AC62, derived, respectively, from juvenile costal and adult articular cartilage, under culture conditions that either promoted or decreased cell proliferation. Analysis of proteo[36S]glycans by Sepharose CL-4B chromatography and SDS-PAGE showed that the large proteoglycan aggrecan and the small, leucine-rich proteoglycans, decorin and biglycan, were produced under every culture condition studied. In monolayer cultures, a high initial cell density and conditions that promoted proliferation (presence of serum for T/C-28a2 cells or permissive temperature for the temperature-sensitive tsT/AC62 cells) favored cell survival and ratios of proteoglycans expected for differentiated chondrocytes. However, the tsT/AC62 cells produced more proteoglycans at the nonpermissive temperature. Culture of cells suspended in alginate resulted in a significant decrease in proteoglycan production in all culture conditions. While the tsT/AC62 cells continued to produce a larger amount of aggrecan than small proteoglycans, the T/C-28a2 cells lost the ability to produce significant amounts of aggrecan in alginate culture. In addition, our data indicate that immortalized chondrocytes may alter their ability to retain pericellular matrix under changing culture conditions, although the production of the individual matrix components does not change. These findings provide critical information that will assist in the development of a reproducible chondrocyte culture model for the study of regulation of proteoglycan biosynthesis in cartilage.

Immortalized human adult articular chondrocytes maintain cartilage-specific phenotype and responses to interleukin-1beta

OBJECTIVE

To develop a reproducible immortalized human chondrocyte culture model for studying the regulation of chondrocyte functions relevant to arthritic diseases in adult humans.

METHODS

Primary adult articular chondrocytes were immortalized with a retrovirus expressing a temperature-sensitive mutant of SV40-large T antigen (tsTAg). The established tsT/AC62 chondrocyte cell line was examined in monolayer and alginate culture systems. The levels of messenger RNA (mRNA) encoding cartilage matrix proteins and interleukin-1beta (IL-1beta)-inducible mRNA were analyzed by reverse transcriptase-polymerase chain reaction. Matrix protein synthesis was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 35S-sulfate-labeled proteoglycans and Western blotting of type II collagen and aggrecan. Type II collagen (COL2A1)-luciferase reporter gene expression was analyzed by transient transfection. Phosphorylated stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38 mitogen-activated protein kinase (p38 MAPK), and activating transcription factor 2 (ATF-2) were detected by Western blotting.

RESULTS

The tsT/AC62 cells expressed TAg at the permissive temperature (32degrees C), and the loss of TAg at 37 degrees C and 39 degrees C correlated with decreased cell proliferation. Cells in alginate culture deposited abundant alcian blue-stainable matrix and continued to proliferate at 32 degrees C. Preferential retention of aggrecan was observed in the cell-associated matrix, while biglycan and decorin were secreted into the medium of monolayer and alginate cultures. The levels of COL2A1 and aggrecan mRNA were increased after transfer from monolayer to alginate culture at 32 degrees C. Treatment with IL-1beta decreased COL2A1 and aggrecan mRNA levels and increased the levels of matrix metalloproteinases 1, 3, and 13 mRNA, as well as those of cyclooxygenase 2, type I collagen, and secretory phospholipase A2 type IIA mRNA, but not those of inducible nitric oxide synthase mRNA. IL-1beta also stimulated phosphorylation of p38 MAPK, SAPK/JNK, and ATF-2. The p38 MAPK-selective inhibitor, SB203580, partially reversed IL-1beta-induced inhibition of COL2A1 mRNA levels and COL2A1-luciferase reporter gene expression.

CONCLUSION

The tsT/AC62 cells provide a reproducible model that mimics the adult articular chondrocyte phenotype, particularly in alginate culture, and demonstrates characteristic responses to IL-1beta. These studies also show, for the first time, that p38 MAPK is one of the signals required for IL-1beta-induced inhibition of COL2A1 gene expression. Availability of this model will permit identification of signals that regulate cytokine responses, and will also provide rational strategies for targeting these pathways.

SV40 large T antigen expression driven by col2a1 regulatory sequences immortalizes articular chondrocytes but does not allow stabilization of type II collagen expression

Immortalization of chondrocytes by SV40 T Ag has often been reported to trigger the loss of expression of type II collagen, one of the main differentiation markers, although some immortalized chondrocyte lines maintaining a differentiated phenotype have also been described. Here, we show using transient cotransfections in differentiated chondrocytes that, in contrast to c-src, neither SV40 T Ag, nor c-myc, decreases col2a1 transcriptional activity. Then, we report the possibility of immortalizing rabbit articular chondrocytes by expression of SV40 T Ag controlled by the col2a1 promoter and enhancer (pCol2SV). This strategy allows one to select within a population of differentiated chondrocytes those which are able to maintain functional regulation of the col2a1 gene through long-term culture. In precrisis pCol2SV-transfected chondrocytes, all-trans-retinoic acid, a down-regulator of col2a1 expression, induced apoptosis, strongly suggesting the strict control of T Ag expression by col2a1 regulatory sequences. Some pCol2SV-transfected chondrocytes were definitively immortalized, after a short crisis period. However, type II collagen synthesis was restricted to a small proportion of cells, which went on to decrease with subculture, while the proportion of cells expressing T Ag was not affected. In these postcrisis cells, T Ag remained at least partially under the control of functional col2a1 regulatory elements as assessed by all-trans-retinoic acid down-regulation.

Characterization of a new melanocyte-specific gene (QNR-71) expressed in v-myc-transformed quail neuroretina

Quail neuroretina cells (QNR) infected with the v-myc-expressing retrovirus MC29 become pigmented after several passages in vitro. After differential screening of a cDNA library constructed from these cells, we have isolated a cDNA clone (QNR-71) which identifies an RNA expressed only in the pigmented layer of the retina and in the epidermis. This gene can also be induced in other cell types transformed by MC29, suggesting that QNR-71 may be regulated by the v-myc protein. Sequence analysis showed that the QNR-71 cDNA exhibits stretches of homologies with melanosomal proteins encoding genes. From bacterially expressed QNR-71 peptides we obtained rabbit antisera able to specifically recognize two proteins of 95 and 100 kDa in pigmented retinal cells, but not in the neuroretina. To study the regulation of QNR-71, we used promoter fragments linked to the CAT reporter gene, in transient co-expression assay. We observed an increase in CAT expression with a c-MYC and microphtalmia (mi) expression vectors. Both MYC and mi activate the QNR-71 promoter through direct binding to a CATGTG site present in the promoter fragment.

Establishment and characterization of two clonal cell lines derived from murine mandibular condyles

We have established two clonal cell lines, designated SM1/9 and SM25/3 from the mandibular condyles of newborn BALB/c mice by immortalization with the SV40 large T antigen. These cells have a high proliferative activity and have been maintained in culture for over 50 passages. They are polygonal in shape. Electron microscopic studies indicate an immature phenotype for both clones and a lack of prominent intracellular filaments typical of fibroblasts. SM25/3 demonstrates different biological properties as compared to SM1/9, it is tumourigenic in nude mice, has a faster growth rate and exhibits less differentiated features. Both cell lines have low constitutive levels of alkaline phosphatase, and the activity of this enzyme is increased significantly in a dose and confluency dependent manner by retinoic acid and 1,25 (OH)2 vitamin D3. The cells express transcripts for retinoic acid receptors mRAR-alpha and mRAR-gamma but not for mRAR-beta. They also express mRNA for the 1,25 (OH)2 vitamin D3 receptor. They co-express transcripts for collagen types I, II, III. Expression of mRNA for extracellular matrix proteins such as biglycan, osteopontin, PAI-1 is detected. Cultured cells do not express mRNA for osteocalcin and this transcript is not inducible with 1,25 (OH)2 vitamin D3 or retinoic acid. Chondrocyte markers such as link protein and aggrecan are not detected. In vitro assays indicate that the cell lines have a limited capacity for osteogenic or chondrogenic differentiation. Similarly agarose culture experiments and extended treatment with retinoic acid indicate that they do not resemble dedifferentiated chondrocytes. Both the cell lines appear to express a phenotype intermediate to osteoblasts and chondroblasts and possibly represent transitional differentiation stages of the progenitor cells of the mandibular condyle. These cells could serve as useful models in elucidating the pathways of early mesenchymal cell differentiation.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Responsiveness of clonal limb bud cell lines to bone morphogenetic protein 2 reveals a sequential relationship between cartilage and bone cell phenotypes

There is growing evidence to suggest that BMPs are among the signals necessary to create the embryonic skeleton, but how these regulatory molecules enter the pathways of embryonic bone formation remains to be defined. The earliest steps of endochondral bone formation, consisting of mesenchymal condensation and chondrogenesis, have been shown to result directly from BMP-2 action. To determine whether the transition from chondrogenesis to osteogenesis occurring later in endochondral bone formation is also the result of BMP activity, we tested the effects of BMP-2 on immortalized endochondral skeletal progenitor cells derived from mouse limb bud. The cell lines established by this process were found to fall into three general categories: undifferentiated skeletal progenitor cells, which in the presence of BMP-2 first express cartilage matrix proteins and then switch to production of bone matrix proteins; prechondroblast-like cells that constitutively express a subset of markers associated with chondrogenesis and, in the presence of BMP-2, shut off synthesis of these molecules and are induced to produce bone matrix molecules; and osteoblast-like cells that are not significantly affected by BMP-2 treatment. These data suggest that BMP-2 initiates the differentiation of limb bud cells into cells of both the cartilage and bone lineages in a sequential manner, making BMP-2 a potent regulator of skeletal cell differentiation.

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

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

In vitro differentiation of the human osteosarcoma cell lines, HOS and KHOS

The osteogenic potential of the two human osteosarcoma cell lines HOS and KHOS; a cell line produced by the transformation of the HOS cells by the Kirsten murine sarcoma virus, was studied in vitro. HOS cells cultured more than 2 weeks formed nodules composed of two morphologically distinct layers, an epithelial-like surface cell layer and a collagen-rich inner cell layer. Alkaline phosphatase (ALPase) activity occurred in the plasma membrane of the surface cell layer, and calcified substances developing along collagen fibers were detected in the collagen-rich inner cell layer. The calcified substances were further examined by analytical electron microscopy and were shown to be hydroxyapatite crystals. In contrast, there was neither ALPase nor the deposition of a calcified substance in the KHOS cells.

SV40-immortalization of rabbit articular chondrocytes: alteration of differentiated functions

Cell lines were established from rabbit articular chondrocytes following transfection with a plasmid encoding SV40 early function genes. This resulted in cell immortalization (130 passages have been completed for the oldest cell line) with acquisition of characteristics of partial transformation such as reduced serum requirements for normal and clonal growth. The immortalized chondrocytes, called SVRAC, did not form multilayer foci when maintained in postconfluent culture. Their ability to form colonies in soft agar was not increased in comparison with normal chondrocytes, but they were weakly tumorigenic in nude mice. SVRAC lost the ability to synthesize type II collagen and Alcian blue-stainable matrix, which are markers of the differentiated chondrocyte phenotype, and synthesized predominantly type I collagen. Studies of collagen gene expression showed that pro alpha 1 (II) mRNA was undetectable, whereas pro alpha 1 (I) collagen mRNA was expressed even in late passage cultures. Unlike normal dedifferentiated chondrocytes, SVRAC were unable to re-express the differentiated phenotype in response to tridimensional culture or microfilament depolymerization. Cell lines obtained from chondrocytes transfected either in primary culture or just after release of cells from cartilage displayed the same behaviour. Thus SV40 early genes were able to immortalize rabbit articular chondrocytes, but the resulting cell lines displayed an apparently irreversibly dedifferentiated phenotype. These cell lines can be used as models to identify regulatory pathways that are required for the maintenance or reexpression of differentiated function in chondrocytes.

Constitutive myc expression impairs hypertrophy and calcification in cartilage

The myc oncogene is expressed by proliferating quail embryo chondrocytes (QEC) grown as adherent cells and is repressed in QEC maintained in suspension culture. To investigate the interference of myc expression during chondrocyte differentiation, QEC were infected with a retrovirus carrying the v-myc oncogene (QEC-v-myc). Uninfected or helper virus-infected QEC were used as control. In adherent culture, QEC-v-myc displayed a chondrocytic phenotype and synthesized type II collagen and Ch21 protein, while control chondrocytes synthesized type I and type II collagen with no Ch21 protein detected as long as the attachment to the plastic was kept. In suspension culture, QEC-v-myc readily aggregated and within 1 week the cell aggregates released small single cells; still they secreted only type II collagen and Ch21 protein. In the same conditions control cell aggregates released hypertrophic chondrocytes producing type II and type X collagens and Ch21 protein. In the appropriate culture conditions, QEC-v-myc reconstituted a tissue defined as nonhypertrophic, noncalcifying cartilage by the high cellularity, the low levels of alkaline phosphatase enzymatic activity, and the absence of type X collagen synthesis and of calcium deposition. We conclude that the constitutive expression of the v-myc oncogene keeps chondrocytes in stage I (active proliferation and synthesis of type II collagen) and prevents these cells from reconstituting hypertrophic calcifying cartilage.

Karyotype instability and altered differentiation of rat sarcoma cells after retroviral infection

The karyotypic and phenotypic stability of cultured rat fibrosarcoma cells was challenged by infection with Moloney murine sarcoma virus (MoMuSV). After transformation, the spindle-like morphology of the parental HH-16 cl.2/1 cells had altered to a rounded phenotype, which was maintained in tumors produced by inoculating transformed cells into congenic animals. In contrast to the parental cells, transformed cells lacked cables of cytokeratins 14-16 and 19 and showed reduction of the mesenchymal marker protein vimentin. Additionally, the morphologically altered cell clones tf-1 to tf-3 had lost growth arrest in the presence of dexamethasone. The DNA of the transformed cells contained between four and six randomly integrated proviral copies. Karyotypic alterations were manifested by reduction of morphologically intact chromosomes in the MoMuSV-transformed cells together with increase of structural aberrations. Three additional markers were identified in the virus-transformed cell clones. Karyotypic instability induced by MoMuSV infection appeared closely related to reduction of the cellular differentiation status, although only cells of clone tf-1 had increased metastatic potential.

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.

Loss of a tumor suppressor gene function is correlated with downregulation of chondrocyte-specific collagen expression in Syrian hamster embryo cells

We previously described the isolation of closely related, preneoplastic Syrian hamster cell lines that have retained (supB+) or lost (supB-) the ability to suppress the anchorage-independent growth and tumorigenicity of a sarcoma cell line (BP6T) in cell hybrids. In this report, we have used differential cDNA screening to clone several genes that are expressed in supB+ cells and downregulated in supB- cells. The nontumorigenic supB+ and supB- variants are advantageous for differential cDNA cloning because multiple independent cell lines differing in their tumor suppressor activity have been isolated. Differentially expressed cDNAs were isolated and placed into one of four groups based on DNA cross-hybridization. Representative cDNAs from Groups I and II, which were expressed at relatively high levels in two independently derived supB+ cell lines (DES4 and 10W) and downregulated in the supB- and tumor cell lines, were sequenced. The DNA and predicted amino acid sequences of these genes were found to be highly homologous to the chondrocyte-specific collagens type II and type IX. In contrast to the chondrocyte-specific collagens, another collagen isoform, collagen type I, was expressed at similar levels in both supB+ and supB- cells. These results suggest that carcinogen-induced immortalization selected for chondrocyte-like cell lines from the mixed embryo cell population. As these cells progressed toward tumorigenicity, the ability to express the chondrocyte differentiation markers was lost concomitantly with the ability to suppress the tumorigenicity of the BP6T sarcoma cell line. These results are consistent with the hypothesis that the supB+ tumor suppressor gene is involved in the regulation of differentiation. The identification of genes regulated by this suppressor gene may aid in its isolation.

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

We have analyzed mixed cultures of normal mammalian fibroblastic cells and transformed quail myoblasts to investigate whether the presence of an excess of normal cells could suppress the phenotype of transformed quail cells. In such mixed cultures, only v-myc-transformed cells were growth-arrested, whereas v-src-transformed myoblasts were essentially unaffected. Growth arrest appeared to reflect reversion from the transformed state, including re-expression of the myogenic differentiation program. The v-myc-transformed myoblasts were phenotypically corrected also by differentiating normal quail myoblasts, giving rise to hybrid myotubes containing nuclei from both cell types. The differential behavior of transformed cells closely paralleled the efficiency with which they established metabolic cooperation with adjacent normal cells. Our results indicate that unrestrained proliferation associated with transformation is responsible for v-myc-induced block of myogenic differentiation.

An established rat cell line expressing chondrocyte properties

Chondrocytes express a well-characterized set of marker proteins making these cells useful for studies on differentiation and regulation of gene expression. Because of the inherent instability of primary rat chondrocytes in culture, and because several rat chondrocyte genes have been cloned and characterized (including the collagen II promoter and enhancer), a rat chondrocyte cell line would be especially useful. To obtain this line we infected primary fetal rat costal chondrocytes with a recombinant retrovirus (NIH/J-2) carrying the myc and raf oncogenes, which have been shown to have an "immortalizing" function. Following infection, a rapidly proliferating clonal line was isolated that maintained a stable phenotype through 45 passages (11/2 year in culture). This line, termed IRC, grows in suspension culture as multicellular aggregates and in monolayer culture as polygonal cells which accumulate an alcian blue-stainable matrix. IRC cells synthesize high levels of cartilage proteoglycan core protein, and link protein, but show reduced collagen II expression. In addition, the cells express virally derived myc mRNA and protein, but do not express v-raf. Retinoic acid, which is a known modulator of chondrocyte phenotype, down-regulates expression of chondrocyte marker proteins, while stimulating v-myc expression by IRC cells. These data suggest that v-myc expression by chondrocytes results in rapid cell division and maintenance of many aspects of the differentiated phenotype. These "immortalized" cells, however, remain responsive to agents such as retinoic acid which modulate cell phenotype. The potential exists for development of chondrocyte cell lines from diseased cartilage, as well as from human cartilage.

Inhibition of transforming activity of tyrosine kinase oncogenes by herbimycin A

We studied the effectiveness of herbimycin A, an inhibitor of the function of the temperature-sensitive src oncogene, to reverse the morphologies of chicken and mammalian cells transformed by various oncogenes. It was found that the antibiotic was effective against the cells transformed by tyrosine kinase oncogenes src, yes, fps, ros, abl, erbB, but did not reverse the transformed morphologies induced by oncogenes raf, ras, and myc. Moreover, decreases in phosphotyrosine content of the total cellular proteins and in 36K protein phosphorylation by herbimycin treatment supported the selective inhibition of the antibiotic on the transforming activity of tyrosine kinase oncogenes.

Fibroblast growth factor stimulates colony formation of differentiated chondrocytes in soft agar

The effect of fibroblast growth factor (FGF) on the growth of chondrocytes in soft agar was examined. FGF induced colony formation by chick embryo and rabbit chondrocytes. The colony-forming efficiency of FGF-exposed chondrocytes was similar to that of Rous sarcoma virus-transformed chondrocytes (15-20%). Other mitogenic agents tested, such as epidermal growth factor, insulin, insulin-like growth factor-l, and platelet-derived growth factor, induced very low levels of colony formation. The induction of growth in soft agar of chondrocytes by FGF was not due to cells' phenotypic transformation, because chondrocytes grown in soft agar with FGF retained the ability to synthesize cartilage-characteristic proteoglycan. FGF did not induce growth in soft agar of chondrocytes whose phenotypic expression was suppressed by retinoic acid or 5-bromodeoxyuridine. In addition, FGF did not induce growth in soft agar of primary fibroblasts and normal rat kidney (NRK) cells. These results suggest that FGF selectively stimulates growth of differentiated chondrocytes in soft agar.

v-myb dominance over v-myc in doubly transformed chick myelomonocytic cells

Chick myelomonocytic cells transformed by the v-myc oncogene resemble mature macrophages; those transformed by v-myb or v-myb,ets exhibit an immature phenotype. We have analyzed whether these oncogenes are capable of altering the differentiation phenotype of transformed cells by introducing both v-myc plus either v-myb or v-myb,ets into the same cells. Surprisingly, the doubly transformed cells were found to be essentially indistinguishable from cells transformed by v-myb or v-myb,ets alone even when they expressed a high level of v-myc protein. These results demonstrate that v-myb is dominant over v-myc and that, while v-myc induces cell proliferation without affecting differentiation, v-myb induces in the same target cells both proliferation and a block or reversal of differentiation.

Retention or loss of v-mil sequences after propagation of MH2 virus in vivo or in vitro

During propagation of the defective avian retrovirus MH2 in the presence of replication-competent helper virus, deletion of portions of the viral genome occurred frequently. After transformation of quail cells in vitro, v-mil sequences were lost, leading to populations of MH2 viruses which were highly deficient for mil gene expression but which could transform macrophage and fibroblast cells in vitro with high efficiency. In contrast, after induction of tumors in quail with mil-deficient MH2 viral stocks, a majority of the tumor DNAs contained mil+ proviruses, suggesting that there is selection for retention of the v-mil gene in vivo and that the mil protein may play a role in the oncogenicity of MH2 virus. We also isolated MH2-transformed cell lines which contained deleted proviruses arising from packaging and subsequent integration of the subgenomic v-myc-encoding mRNA. Some of these cell lines produced viruses which encoded abnormal v-myc proteins and had altered in vitro transforming properties. These altered phenotypes may be caused by mutations within the v-myc gene.

The v-myc oncogene is sufficient to induce growth transformation of chick neuroretina cells

A number of studies have shown that full transformation of non-established rodent fibroblasts can be efficiently achieved in vitro by the concerted action of two oncogenes belonging to different complementation groups. Extension of the two-genes carcinogenesis model to other differentiated cell types, presumably endowed with different controls of growth, is desirable for a better understanding of questions such as the host cell selectivity of oncogene action. A recent report claimed that cooperation between two oncogenes, v-myc and v-mil, is required to achieve transformation of chicken embryo neuroretina cells, which are characterized by a limited growth capacity in monolayer culture. Here we present evidence that the v-myc oncogene alone is sufficient to induce growth transformation of glial and neuronal precursor cell types from chick neuroretina. We also report that induction of transformation by v-myc is accompanied by faithful preservation of some of the differentiated functions of the chick cells.

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

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

Differentiation of PC12 phaeochromocytoma cells induced by v-src oncogene

PC12 rat phaeochromocytoma cells are a model system that can be used to study both neuronal differentiation and the mechanism of action of nerve growth factor (NGF). PC12 cells respond to NGF protein by shifting from a chromaffin-cell-like phenotype to a neurite-bearing sympathetic neurone-like phenotype. Here we present data on the effect of infection of PC12 cells with retroviruses carrying the src oncogene of Rous sarcoma virus. Previous studies have demonstrated that the expression of src severely affects the synthesis and accumulation of differentiated cell products in a variety of cell types. We show that in the PC12 cell system, expression of v-src appears to have an inductive effect on differentiation that resembles the action of a 'physiological' growth factor.