Isolation and characterization of the human cellular myc gene product

Antibodies against the product of the human cellular myc gene (c-myc) were prepared against a bacterially expressed human c-myc protein by inserting the ClaI/BclI fragment of the human c-myc DNA clone in an expression vector derived from pPLc24. These antibodies cross-react with viral-coded myc (v-myc) proteins from MC29 and OK10 viruses. Furthermore, IgGs specific for synthetic peptides, corresponding to the 12 carboxy-terminal amino acids of the human c-myc gene and 16 internal amino acids, were isolated. By use of the various myc-specific antisera or IgGs, a protein of Mr 64 000 was detected in several human tumor cell lines including Colo320, small cell cancer of the lung (417d), HL60, Raji, and HeLa. This protein is larger than the corresponding v-myc or chicken c-myc proteins from avian virus transformed cells or avian bursa lymphoma cells (RP9), both of which are proteins of Mr 55 000. The human c-myc protein is located in the nucleus of Colo320 cells, exhibits a half-life of about 15 min, and is expressed at significantly lower levels than the viral protein. The human c-myc protein was enriched about 3000-fold from Colo320 cells using c-myc-specific IgG coupled to Sepharose beads. The protein binds to double-stranded DNA in vitro, a reaction that can be inhibited to more than 90% by c-myc specific IgG.     

V-myc- and c-myc-encoded proteins are associated with the nuclear matrix.

A series of extraction procedures were applied to avian nuclei which allowed us to define three types of association of v-myc- and c-myc-encoded proteins with nuclei: (i) a major fraction (60 to 90%) which is retained in DNA- and RNA-depleted nuclei after low- and high-salt extraction, (ii) a small fraction (1%) released during nuclease digestion of DNA in intact nuclei in the presence of low-salt buffer, and (iii) a fraction of myc protein (less than 10%) extractable with salt or detergents and found to have affinity for both single- and double-stranded DNA. Immunofluorescence analysis with anti-myc peptide sera on cells extracted sequentially with nucleases and salts confirmed the idea that myc proteins were associated with a complex residual nuclear structure (matrix-lamin fraction) which also contained avian nuclear lamin protein. Dispersal of myc proteins into the cytoplasm was found to occur during mitosis. Both c-myc and v-myc proteins were associated with the matrix-lamin, suggesting that the function of myc may relate to nuclear structural organization.     

Molecular Mechanism of Mutant p53 Stabilization: The Role of HSP70 and MDM2

Numerous p53 missense mutations possess gain-of-function activities. Studies in mouse models have demonstrated that the stabilization of p53 R172H (R175H in human) mutant protein, by currently unknown factors, is a prerequisite for its oncogenic gain-of-function phenotype such as tumour progression and metastasis. Here we show that MDM2-dependent ubiquitination and degradation of p53 R175H mutant protein in mouse embryonic fibroblasts is partially inhibited by increasing concentration of heat shock protein 70 (HSP70/HSPA1-A). These phenomena correlate well with the appearance of HSP70-dependent folding intermediates in the form of dynamic cytoplasmic spots containing aggregate-prone p53 R175H and several molecular chaperones. We propose that a transient but recurrent interaction with HSP70 may lead to an increase in mutant p53 protein half-life. In the presence of MDM2 these pseudoaggregates can form stable amyloid-like structures, which occasionally merge into an aggresome. Interestingly, formation of folding intermediates is not observed in the presence of HSC70/HSPA8, the dominant-negative K71S variant of HSP70 or HSP70 inhibitor. In cancer cells, where endogenous HSP70 levels are already elevated, mutant p53 protein forms nuclear aggregates without the addition of exogenous HSP70. Aggregates containing p53 are also visible under conditions where p53 is partially unfolded: 37°C for temperature-sensitive variant p53 V143A and 42°C for wild-type p53. Refolding kinetics of p53 indicate that HSP70 causes transient exposure of p53 aggregate-prone domain(s). We propose that formation of HSP70- and MDM2-dependent protein coaggregates in tumours with high levels of these two proteins could be one of the mechanisms by which mutant p53 is stabilized. Moreover, sequestration of p73 tumour suppressor protein by these nuclear aggregates may lead to gain-of-function phenotypes.     

Regulating genes with electromagnetic response elements

A 900 base pair segment of the c-myc promoter, containing eight nCTCTn sequences, is required for the induction of c-myc expression by electromagnetic (EM) fields. Similarly, a 70 bp region of the HSP70 promoter, containing three nCTCTn sequences, is required for the induction of HSP70 expression by EM fields. Removal of the 900 base pair segment of the c-myc promoter eliminates the ability of EM fields to induce c-myc expression. Similarly, removal of the 70 bp region of the HSP70 promoter, with its three nCTCTn sequences, eliminates the response to EM fields. The nCTCTn sequences apparently act as electromagnetic field response elements (EMRE). To test if introducing EMREs imparts the ability to respond to applied EM fields, the 900 bp segment of the c-myc promoter (containing eight EMREs) was placed upstream of CAT or luciferase reporter constructs that were otherwise unresponsive to EM fields. EMREs-reporter constructs were transfected into HeLa cells and exposed to 8 microT 60 Hz fields. Protein extracts from EM field-exposed transfectants had significant increases in activity of both CAT and luciferase, compared with identical transfectants that were sham-exposed. Transfectants with CAT or luciferase constructs lacking EMREs remained unresponsive to EM fields, i.e., there was no increase in either CAT or luciferase activity. These data support the idea that EMREs can be used as switches to regulate exogenously introduced genes in gene therapy.     

In situ study of c-myc protein expression during avian development

The distribution of the c-myc protein was studied in the developing embryo from the two-somite stage to embryonic day 17 (E17). A triple labelling method was used, with a polyclonal serum recognizing the human and avian c-myc proteins as the first marker followed by Hoechst 33258 for nuclear staining and the monoclonal antibody 13F4 which reveals the avian myogenic lineage. In situ hybridization was carried out at three selected stages (E3, E6 and E8), in order to compare the distribution of myc mRNA and myc protein. The c-myc protein signal was barely detectable in blastodisc nuclei during the period of somite formation, after which it became ubiquitous in the embryonic body until E4. Myotomal cell nuclei displayed a strong signal until their organization into premuscular masses. On day 4, the level of c-myc protein decreased in all embryonic tissues. By doubling the antibody titre and amplifying the signal by means of the streptavidin-biotin method, c-myc could still be detected in nuclei of defined groups of cells. Such was the case in some mesenchyme-derived tissues at critical periods of organogenesis, for instance in prechondrogenic condensations or hemopoietic cell foci at E6, the latter becoming negative at E9. The heart ventricle displayed a patch-work of positive and negative nuclei from E6 to E10. A myc signal restricted to the quail species was found in the wall of the carotid arteries. Cell nuclei in the nervous system displayed a detectable signal which became restricted to postmitotic neurones. In the ectoderm, the c-myc protein was generally not present after E4, except in presumptive feather buds at the time of epitheliomesenchymal interactions. Endodermal cells (such as hepatocytes, oesophageal and tracheal epithelia) did not express detectable levels of c-myc at any time. Our results reveal a time- and tissue-specific expression of c-myc during avian development. It is noteworthy that the expression of the c-myc protein often appears dissociated from cell proliferation as shown by the absence of the signal in endodermal cells at E3-E13 as well as its presence in postmitotic neurones. Finally, although RNA and protein are simultaneously detected in some structures such as presumptive feather buds, their expression is dissociated in endodermal tissues, notably hepatocytes, where in situ hybridization detects a large number of RNA copies with no detectable protein signal.     

Transfer of ‘immortalizing’ oncogenes into rat fibroblasts induces both high rates of sister chromatid exchange and appearance of abnormal karyotypes

Cell lines established after transfer into FR3T3 rat fibroblast cells of 'immortalizing' oncogenes (plt gene (large T protein) of polyoma virus, v-myc gene of MC29 virus, rearranged forms of c-myc) exhibited increased rates of sister chromatid exchange (SCE). This was observed neither in cells which expressed one of the oncogenes responsible for the terminal stages of tumorigenic transformation (polyoma virus pmt (middle T protein), mutated ras genes), nor in cell lines carrying oncogenes of both types. Abnormal chromosome numbers were observed in cell lines expressing plt or myc, but not after transformation by pmt or ras oncogenes.     

Contrasting expression patterns of three members of the myc family of protooncogenes in the developing and adult mouse kidney

The myc family of protooncogenes encode similar but distinct nuclear proteins. Since N-myc, c-myc, and L-myc have been found to be expressed in the newborn kidney, we studied their expression during murine kidney development. By organ culture studies and in situ hybridization of tissue sections, we found that each of the three members of the myc gene family shows a remarkably distinct expression pattern during kidney development. It is known that mesenchymal stem cells of the embryonic kidney convert into epithelium if properly induced. We demonstrate the N-myc expression increases during the first 24 h of in vitro culture as an early response to induction. Moreover, the upregulation was transient and expression levels were already low during the first stages of overt epithelial cell polarization. In contrast, neither c-myc nor L-myc were upregulated by induction of epithelial differentiation. c-myc was expressed in the uninduced mesenchyme but subsequently became restricted to the newly formed epithelium and was not expressed in the surrounding loose mesenchyme. At onset of terminal differentiation c-myc expression was turned off also from the epithelial tubules. We conclude that N-myc is a marker for induction and early epithelial differentiation states. That the undifferentiated mesenchyme, unlike stromal cells of later developmental stages, express c-myc demonstrates that the undifferentiated mesenchymal stem cells are distinct from the stromal cells. The most astonishing finding, however, was the high level of L-myc mRNA in the ureter, ureter-derived renal pelvis, papilla, and collecting ducts. In the ureter, expression increased, rather than decreased, with advancing maturation and was highest in adult tissue. Our results suggest that each of the three members of the myc gene family are involved in quite disparate differentiation processes, even within one tissue.     

Studies on the interaction of the human c-myc protein with cell nuclei: p62c-myc as a member of a discrete subset of nuclear proteins

We have analyzed the localization of the human c-myc product (p62c-myc) at steady state in cells by immunoprecipitation and immunoblotting. We show that p62c-myc is extracted from nuclei by mild salt concentrations (below 200 mM), without affecting gross nuclear structure or causing extraction of major chromatin components. We observe no association between p62c-myc and the nuclear matrix. We also demonstrate that p62c-myc is a member of a discrete subset of nuclear proteins that are all rendered irreversibly insoluble in situ by exposure of isolated nuclei to physiological temperatures (37 degrees C). p62c-myc is sequestered into a similar insoluble complex in cells that have been subjected to heat shock. Finally, we show that avian v-myc and v-myb proteins in isolated nuclei also become insoluble after exposure to temperatures above 37 degrees C. We discuss the possible implications of these results.     

The effect of alterations in myc gene expression on B cell development in the bursa of Fabricius

Infection of 18-day embryonic bursal lymphocytes with a v-myc-containing retrovirus leads directly to a polyclonal proliferation of surface immunoglobulin-positive (slg+) cells in the bursa of Fabricius detected four weeks after hatching. These v-myc-expressing bursal cells repopulate the follicles of chemically ablated bursae more efficiently than total normal 18-day embryonic bursal cells. In contrast, comparable normal bursal cells lose the ability to repopulate follicles by four weeks. Bursal lymphocytes expressing either a retroviral v-myc or a c-myc gene deregulated by adjacent retroviral integration retain the ability of embryonic bursal lymphocytes to diversify their immunoglobulin light chain genes. These results suggest that retroviral deregulation of myc expression during avian B cell development induces outgrowth of a population of cells with the cardinal phenotypic characteristics of bursal stem cells.     

The myc oncogene and transdifferentiation of the retinal pigment epithelium]

The retinal pigment epithelium (RPE) develops from the same sheet of neuroepithelium as the neuroretina. When infected with MC29, a v-myc expressing virus, the RPE cells can be induced to transdifferentiate and to take a neuroretinal epithelium fate. After a PCR-based differential screening from these cells we have identified three genes of interest. Qath5, a quail basic helix-loop-helix (bHLH) gene that is closely related to the Drosophila atonal, and whose expression is found in the developing neuroretina. A Chx10-related homeobox gene also expressed in the developing neuroretina and HuD, a RNA-binding protein not expressed in the RPE but expressed during neurogenesis. Beside these genes whose function is involved in regulating neuronal differentiation myc also induced a transient Mitf expression. Mitf is expressed in the entire optic cup, later restricted to the pigmented retina. Mitf is involved in the regulation of the pigmented differentiation. We conclude that v-myc can reverse the RPE to the bipotential retinal primordia.