Human wig-1, a p53 target gene that encodes a growth inhibitory zinc finger protein

We previously identified a novel p53-induced mouse gene, wig-1, that encodes a 290 amino acid zinc finger protein (Varmeh-Ziaie et al., 1997). Here we have identified and characterized the human homolog of mouse wig-1. The human wig-1 protein is 87% identical to the mouse protein and contains three zinc finger domains and a putative nuclear localization signal. Human wig-1 mRNA and protein is induced following activation of wild type p53 expression in our BL41-ts p53 Burkitt lymphoma cells. Wig-1 is also induced in MCF7 cells following treatment with the DNA-damaging agent mitomycin C. Northern blotting detected low levels of wig-1 mRNA in normal human tissues. Fluorescence in situ hybridization mapped wig-1 to human chromosome 3q26.3-27. FLAG-tagged human wig-1 localizes to the nucleus. Ectopic overexpression of human wig-1 inhibits tumor cell growth in a colony formation assay. These results suggest that human wig-1 has a role in the p53-dependent growth regulatory pathway.

Inhibition of cell growth and apoptosis by inducible expression of the transcriptional repressor Mad1

Mad1 is a Myc antagonist that heterodimerizes with Max and functions as a transcriptional repressor. We have studied the effects of Mad1 on cell growth, cell cycle distribution, and apoptosis using Mad1-inducible cell lines. Expression of Mad1 inhibited cell proliferation, S-phase entry, and colony formation, changes that were accompanied by a reduction in CDK2 activity. The inhibition of Mad1 on cell proliferation was potentiated by serum starvation and was paralleled by accumulation of cells in the G0/G1 and the G2 phases of the cell cycle. Mad1 also reduced apoptosis induced by serum withdrawal and by the cytostatic drug cisplatinum. The effects on both cell growth and apoptosis were dependent on the mSin3 interaction domain of Mad1, which is necessary for recruitment of histone deacetylases and corepressors, suggesting that transcriptional repression is mediating these functions. Taken together with the expression pattern of Mad1, these results suggest that Mad1 plays an important role during initiation of differentiation by inhibiting cell proliferation and blocking apoptosis.

Myc is an essential negative regulator of platelet-derived growth factor beta receptor expression

Platelet-derived growth factor BB (PDGF BB) is a potent mitogen for fibroblasts as well as many other cell types. Interaction of PDGF BB with the PDGF beta receptor (PDGF-betaR) activates numerous signaling pathways and leads to a decrease in receptor expression on the cell surface. PDGF-betaR downregulation is effected at two levels, the immediate internalization of ligand-receptor complexes and the reduction in pdgf-betar mRNA expression. Our studies show that pdgf-betar mRNA suppression is regulated by the c-myc proto-oncogene. Both constitutive and inducible ectopic Myc protein can suppress pdgf-betar mRNA and protein. Suppression of pdgf-betar mRNA in response to Myc is specific, since expression of the related receptor pdgf-alphar is not affected. We further show that Myc suppresses pdgf-betar mRNA expression by a mechanism which is distinguishable from Myc autosuppression. Analysis of c-Myc-null fibroblasts demonstrates that Myc is required for the repression of pdgf-betar mRNA expression in quiescent fibroblasts following mitogen stimulation. In addition, it is evident that the Myc-mediated repression of pdgf-betar mRNA levels plays an important role in the regulation of basal pdgf-betar expression in proliferating cells. Thus, our studies suggest an essential role for Myc in a negative-feedback loop regulating the expression of the PDGF-betaR.

p53-induced apoptosis as a safeguard against cancer

p53 acts as a potent tumor suppressor largely through its ability to induce cell death by apoptosis. Diverse cellular stress conditions, e.g., DNA damage, hypoxia, and oncogene activation, trigger p53-dependent apoptosis. ARF is a 14-kDa protein encoded by an alternative reading frame within the human INK4a locus that also encodes the p16 protein. ARF induces p53 in response to oncogene activation by preventing its degradation. This ensures the elimination of emerging tumor cells by p53-dependent apoptosis. p53 promotes apoptosis through multiple mechanisms, including transactivation of specific target genes, down-regulation of a distinct set of genes, and transcription-independent mechanisms. This may explain the frequent inactivation of ARF/p53 rather than downstream effectors during tumor development.

Induction of the human ARF protein by serum starvation

The ARF protein encoded by the alternative transcript of the INK4a gene inhibits cell growth by stabilization of p53. ARF is induced by activated oncogenes sucll as c-myc, E1A and E2F-1. We show here that ARF protein expression is also induced by serum deprivation in the human tumor cell line MDA-MB-157 and in the SV40 large T-immortalized keratinocyte line Rhek. This increase of expression was reversed by the addition of serum. ARF mRNA levels also increased after serum starvation, suggesting that ARF upregulation is mediated, at least in part, by increased transcription and/or mRNA stability. These results indicate that ARF responds not only to oncogenic hyper-proliferative signals but also to suboptimal growth conditions.

Recombinant plasmid expressing the entire coding region of the Bmyc putative protein

An expression vector with the entire coding region of Bmyc oncogene was constructed. The longest predicted open reading frame of Bmyc (178 amino acid residues) was expressed as a fusion protein with the trpE protein (308 amino acid residues) in transformed bacteria. The newly synthesized 58 Kd protein reacted with an anti pan-myc serum. The fusion protein isolated from a preparative Western blot was used as immunogen to generate rabbit anti myc specific immune sera.

Nucleotide sequence of the rat Bmyc gene

We have cloned and sequenced the rat Bmyc gene. The rat Bmyc gene contains sequences related to the central part of c-myc, namely the first intron, the second exon, and the noncoding part of the third exon. The homology drops in the 3' part of the c-myc second exon, but continues in the noncoding part of the third exon. We have sequenced the total predicted coding region of the Bmyc. The longest open reading frame in Bmyc suggests a protein of 178 amino acids, which is only 41% of the c-myc protein size. To confirm the putative open reading frame, we have produced a trpE-Bmyc protein that is detected with a pan-myc antibody. We discuss these findings in the context of potential functional domains and the possibility of overlapping and distinct activities of myc-family proteins.

Chromosome localization and expression pattern of Lmyc and Bmyc in murine embryonal carcinoma cells

Using Southern blot analysis of DNA from mouse-hamster somatic cell hybrids, we have mapped Lmyc and Bmyc, two members of the myc family of genes, to mouse chromosomes 4 and 2, respectively. Furthermore, we have compared the regulation of Lmyc and Bmyc expression under different growth conditions and during in vitro differentiation of the murine EC line F9 and considered the findings in relation to our previous studies on Nmyc and c-myc expression in the same line (Sejersen et al., 1987). Lmyc was down-regulated at an early stage of visceral endoderm differentiation, similarly to c-myc and Nmyc, while Bmyc was expressed at a constant low level at all stages. Lmyc, but not c-myc and Nmyc, was upregulated in terminally differentiated visceral endoderm cells. Inhibition of protein synthesis by cycloheximide for 4 h induced a 70% increase in Lmyc and 30% increase in Bmyc transcript levels, indicating that the expression of these genes is negatively regulated by a short-lived protein. Mitogenic stimulation with insulin and transferrin did not affect Lmyc and Bmyc mRNA levels. Lmyc transcripts have a half life of 30 min, whereas the Bmyc transcript is highly stable, with a half life of 6 h. The half-lives of the c-myc and Nmyc transcripts have been estimated previously as 40 and 130 min, respectively.

Structure and expression of B-myc, a new member of the myc gene family

The myc family of genes contains five functional members. We describe the cloning of a new member of the myc family from rat genomic and cDNA libraries, designated B-myc. A fragment of cloned B-myc was used to map the corresponding rat locus by Southern blotting of DNA prepared from rat X mouse somatic cell hybrids. B-myc mapped to rat chromosome 3. We have previously mapped the c-myc to rat chromosome 7 (J. Sümegi, J. Spira, H. Bazin, J. Szpirer, G. Levan, and G. Klein, Nature [London] 306:497-498, 1983) and N-myc and L-myc to rat chromosomes 6 and 5, respectively (S. Ingvarsson, C. Asker, Z. Wirschubsky, J. Szpirer, G. Levan, G. Klein, and J. Sümegi, Somat. Cell Mol. Genet. 13:335-339, 1987). A partial sequence of B-myc had extensive sequence homology to the c-myc protein-coding region, and the detection of intron homology further indicated that these two genes are closely related. The DNA regions conserved among the myc family members, designated myc boxes, were highly conserved between c-myc and B-myc. A lower degree of homology was detected in other parts of the coding region in c-myc and B-myc not present in N-myc and L-myc. A 1.3-kilobase B-myc-specific mRNA was detected in most rat tissues, with the highest expression in the brain. This resembled the expression pattern of c-myc, although at different relative levels, and was in contrast to the more tissue-specific expression of N-myc and L-myc. B-myc was expressed at uniformly high levels in all fetal tissues and during subsequent postnatal development, in contrast to the stage-specific expression of c-myc.

Rat c-raf oncogene is located on chromosome 4 and may be activated by sequences from chromosome 13

Activated forms of the protooncogene c-raf have been found to transform established lines of rodent fibroblasts after transfection with DNA from several human and rat tumors. Using Southern blot analysis of DNAs from rat x mouse somatic cell hybrids, we have mapped c-raf to rat chromosome 4. An exogenous sequence that was found juxtaposed to c-raf within transforming DNA originally derived from a rat hepatocellular carcinoma was localized to chromosome 13.

Amplification of c-myc and pvt-1 homologous sequences in acute nonlymphatic leukemia

Leukemic cells with double minute (DM) chromosomes from an ANLL(M1) patient were found to carry 10-15 fold amplified c-myc sequences. The linked pvt-1-like locus was amplified at the same level, suggesting that the c-myc amplicon is at least 300 kb in size.

Mapping of Lmyc and Nmyc to rat chromosomes 5 and 6

Using Southern blot analysis of DNAs from rat X mouse somatic cell hybrids, we have mapped Nmyc and Lmyc, two members of the myc family of proto-oncogenes, to rat chromosomes 6 and 5, respectively.