A follistatin-like gene, mac25, may act as a growth suppressor of osteosarcoma cells.

mac25, a retinoic acid-inducible gene that is expressed at high levels in senescent epithelial cells, was initially cloned as a gene that is differentially expressed in meningioma. Although the homology of its product with members of family of insulin-like growth factor-binding proteins was suggested, the product also exhibits strong homology to follistatin, an activin-binding protein. However, a domain corresponding to the carboxyl terminus of follistatin is not found in mac25. The carboxyl-terminally truncated form of follistatin, generated by alternative splicing, has stronger activin-binding activity than the complete form. This result suggests that mac25 might act as an activated follistatin. Clonal growth of a p53-deficient osteosarcoma cell line was strongly inhibited when the murine mac25 gene, as well as the p53 gene, was introduced. Resembling activins that belong to the transforming growth factor-beta (TGF-beta) superfamily, mac25 and p53 might associate with similar but distinct targets, namely cyclin-dependent kinase inhibitors. However, there is no evidence for compensation of p53 function by mac25 in the development of p53-deficient mice, as judged from the pattern of expression of mac25 in mice. mac25 might act as a tumor suppressor, modulating signaling of the TGF-beta family, as does alpha-inhibin.

Up-regulation of cell cycle-associated genes by p53 in apoptosis of an erythroleukemic cell line.

A murine erythroleukemic cell line (1-2-3) which expresses only the temperature-sensitive mutant p53 gene (Ala-to-Val substitution at codon 135) was established. When these cells were cultured at 32 degrees C, the growth rate was reduced significantly and DNA fragmentation, a typical character of apoptosis, was observed. In this process, p53 migrated from cytoplasm to nucleus and protein complexes binding to the p53-responsive element were detected in nuclear extracts of the cells cultured at 32 degrees C by gel-shift assay and transactivation from the p53-responsive element was detected. The expression of the p21 (waf1/cip1/sdi1), cyclin G and gadd45 genes was increased (about 3 to 4 fold that at 37 degrees C), when the cells were cultured at 32 degrees C. However, the expression of the bax gene was increased slightly (about 1.5 fold that at 37 degrees C) and no significant change was detected in expression of the mdm2 gene. No change in the amount of Fas antigen was observed by flow cytometric analysis. Transcripts of the bcl-2 and fasl gene were not detected in the cells both at 37 degrees C and 32 degrees C. These results suggest that up-regulation of the genes associated with the cell cycle and/or DNA replication, such as p21, cyclinG and gadd45 rather than bax, fas, fasl and bcl-2 may be important for induction of apoptosis of this erythroleukemic cell line by p53.

Inactivation of oncoprotein binding by a single Cys706-to-Tyr substitution in the retinoblastoma protein.

We previously found a new single amino acid substitution at codon 706 (Cys-to-Tyr) of the retinoblastoma (RB) gene in a sporadic retinoblastoma patient. The glutathione S-transferase-RB fused protein containing this mutation was here tested for binding to SV40 large T antigen and adenovirus E1A protein, and was shown to have lost its binding affinity. Thus, Tyr, as well as Phe, residues substituted for Cys706 were found to abolish the RB protein activity.

Loss of heterozygosity on chromosome 17 and mutation of the p53 gene in retinoblastoma.

Loss of heterozygosity (LOH) on chromosome 17 and mutations of the p53 gene were examined in 25 retinoblastomas (RB), consisting of three familial tumors, nine hereditary tumors without family history, 11 non-hereditary tumors, one recurrent tumor and one lung-metastatic tumor. LOH on chromosome 17 was detected in only one of the 23 primary RB. No mutations of the p53 gene were detected in the primary tumors. A recurrent tumor showed LOH on the short arm region of chromosome 17. LOH on chromosome 17 and a point mutation of the p53 gene were also detected in a metastatic tumor. These results suggest that LOH on chromosome 17 and mutation of the p53 gene may not be associated with the development of primary RB, but may play a role in the progression of RB.

SMRT as a T3SF-binding protein.

p53-binding consensus-like sequence (T3SF) is located in the murine promoter region of tissue inhibitor of metalloproteinase 3 gene. To identify the genes that encode proteins that bind to T3SF DNA sequence, we screened a cDNA library using the Southwestern technique. The SMRT gene was cloned as one of the candidates. Addition of antibody against SMRT reduced the intensity of a band that is supposed to contain SMRT in electrophoresis mobility shift assay, although antibody against p53 had no effect. Ultraviolet (UV)-irradiation reduced the intensity of the SMRT complex whereas p53 complex was stabilized by UV-irradiation. These results suggest that SMRT may bind to T3SF sequence in p53-independent manner and dissociate from the sequence by UV irradiation.

Hypermethylation in the retinoblastoma gene is associated with unilateral, sporadic retinoblastoma.

We previously reported 9 unilateral, sporadic retinoblastomas with hypermethylation in the 5' region of the RB gene, and we found that CpG methylation in the RB promoter inhibits the binding of the retinoblastoma binding factor 1 (RBF-1) and the activating transcription factor (ATF)-like factors, thereby resulting in a considerable reduction in RB promoter activity. In this study, we screened for hypermethylation in 121 additional cases of retinoblastoma, and found 5 tumors with hypermethylation, including 4 unilateral, sporadic tumors, and one hereditary tumor. The hereditary tumor had a germline deletion of one allele, and the hypermethylation was an acquired, epigenetic change in the other allele. Another tumor had hypermethylation restricted to approximately 800 base pairs in the RB promoter region including the essential RBF-1 and ATF sites. The frequency of hypermethylation in unilateral, sporadic tumors was 9.3% combining our previous and present examinations (13 among 140), whereas the frequency was 1.0% in bilateral hereditary tumors (one among 101). The statistical analyses using the chi-square test indicated significant correlation between hypermethylation and unilateral, sporadic tumors (p < 0.05). These results suggest that hypermethylation in the RB gene is always an acquired, epigenetic change and causes about 9% of unilateral, sporadic tumors.

The mechanisms of death of an erythroleukemic cell line by p53: involvement of the microtubule and mitochondria.

A murine erythroleukemic cell line (1-2-3) which expresses only the temperature-sensitive mutant p53 gene (Ala-to-Val substitution at codon 135) was established. These cells showed typical characteristics of apoptosis, when they were cultured at 32 degrees C. In this process, p53 recovered the wild-type p53 function and the expression of the p21 (waf1/cip1/sdi1), cyclin G1 and gadd45 genes was increased. However, no significant changes were detected in the expression of the mdm2, bcl-2, bax, fas and fasl genes, suggesting the existence of other genes associated with apoptosis. Genes up-regulated by p53 were screened by the mRNA differential display method. One of the up-regulated genes was identified as the elongation factor 1 alpha (EF-1 alpha) gene. EF-1 alpha is also a microtubule-severing protein. Upon the temperature-shift, the cells developed the morphology and the localization of alpha-tubulin similar to those of the cells treated with vincristine, a drug that affects microtubules. The microtubule-severing associated with up-regulation of EF-1 alpha by p53 may be a cause of the cell death. On the other hand, the function of cyclin G1 is not so clear despite the fact that 1-2-3 cells showed a significant increase of the cyclin G1 gene during the early stage of apoptosis. The yeast two-hybrid system was used to identify cyclin G1-associated proteins. One is a cytochrome c (Cyt c) oxidase subunit II (COXII). Cyclin G1 and COXII were co-immunoprecipitated from an extract of human osteosarcoma cell line that expressed high levels of cyclin G1. COX activity was also increased by temperature-shift in this cell line. The pattern of changes in COX activity was closely reflected by the expression of the cyclin G1 gene. Cyclin G1 and COXII associate physically with each other in vivo and that activation of COXII by binding to cyclin G1 upregulated by p53 may be associated with apoptosis. These two new pathways, p53-EF-1 alpha-microtubule-severing (-distortion of cytoskeleton) and p53-cyclin G1-COXII (-CytC, ATP-caspase-3 activation), may cooperate to induce apoptosis in this cell line.

Correlation between induction of the mac25 gene and anti-proliferative effects of 1alpha,25(OH)2-D3 on breast cancer and leukemic cells.

In the differentiation of a myelomonocytic cell line U937 treated with 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2-D3], transient proliferation was observed prior to cell growth arrest. The expression of the p21 and p27 genes increased transiently and decreased quickly in the proliferation, suggesting that other genes may contribute to the growth arrest of the cell line after reduction of the p21 and p27 genes. The mac25 gene was isolated as a gene associated with cellular senescence and growth suppression. Despite a previous report that retinoic acid (RA) induced the mac25 gene, the mac25 gene did not increase in U937 cells treated with RA but did increase in the cells treated with 1alpha,25(OH)2-D3. The high level of the expression of the mac25 gene was detected for four days after the 1alpha,25(OH)2-D3 treatment. Therefore, mac25 may contribute to the growth arrest of U937 cells treated with 1,25-D3. The growth responses to 1alpha,25(OH)2-D3 and the expression of the mac25 gene of three other cancer cell lines (Saos-2, U2OS and MCF7) were studied. Although the growth suppression was observed in MCF7 cells treated with 1alpha,25(OH)2-D3 dose-dependently (1-100 nM of 1alpha,25(OH)2-D3), the treatment of 100 nM of 1alpha,25(OH)2-D3 had no effect on the growth of Saos-2 and U2OS cells. The expression of the mac25 gene was up-regulated in MCF7 cells treated with 100 nM of 1alpha,25(OH)2-D3, whereas no transcript of the mac25 gene was detected in Saos-2 and U2OS cells even when they were treated with 100 nM of 1alpha,25(OH)2-D3. These results suggest that the cellular response to 1alpha,25(OH)2-D3 may depend on the induction of the mac25 gene.

A C3H strain-specific allele (alpha va126) of the murine alpha-globin gene newly detected by UT-PAGE and RT-PCR-SSCP analysis.

An extra band. distinct from the well-characterized globin chains (alpha, beta-maj, beta-min, beta-s), was detected in an adult erythrocyte sample of the C3H strain by urea triton polyacrylamide gel electrophoresis (UT-PAGE) analysis. The extra band was recognized by an antibody against the alpha-globin chain by Western blot analysis. Reverse transcription, polymerase chain reaction and single strand conformation polymorphism (RT-PCR-SSCP) analysis and direct sequencing analysis of cDNA of the alpha-globin gene revealed a nucleotide substitution (GGA to GTA) corresponding to an amino acid substitution (Gly to Val) at codon 26 in the alpha-globin gene only in the erythrocyte sample of the C3H strain. Polypeptides generated by in vitro translation from the alpha-globin gene with the nucleotide substitution at codon 26 (alpha Val26) had the same mobility as that of the extra band of the C3H strain in UT-PAGE. These results suggest that the substitution GGA (Gly) to GTA (Val) at codon 26 of the murine alpha-globin gene may directly affect the mobility of alpha-globin in UT-PAGE and the base substitution may be a C3H strain-specific polymorphism.

A secreted tumor-suppressor, mac25, with activin-binding activity.

BACKGROUND: mac25 is a follistatin (FS)-like protein that has a growth-suppressing effect on a p53-deficient osteosarcoma cell line (Saos-2). The protein exhibits a strong homology to FS, an activin-binding protein, and part of its sequence includes the consensus sequence of the member of the Kazal serine protease inhibitor family. MATERIALS AND METHODS: Localization of mac25 protein was analyzed using mac25 protein fused with green fluorescent protein (GFP). Recombinant mac25 protein was expressed in E. coli and purified. The recombinant mac25 protein was added in culture medium for analysis of growth suppression and cell cycle analysis. Binding of mac25 protein to activin A was studied by immunoprecipitation and Western blots analysis. RESULTS: mac25 protein was localized in the cytoplasm and secreted into culture medium. Addition of recombinant mac25 protein (10-7 M) into the culture medium induced significant suppression of the growth of human cervical carcinoma cells (HeLa) and murine embryonic carcinoma cells (P19), as well as osteosarcoma cells (Saos-2). mac25 protein was co-immunoprecipitated with activin A, a result that suggests that mac25 may be a secreted tumor-suppressor that binds activin A. CONCLUSION: mac25 exhibits homology to insulin-like growth factor-binding proteins (IGF-BPs) and to fibroblast growth factor receptor. The multi-functional nature of mac25 protein may be important for growth-suppression and/or cellular senescence.

A specific chromosome change and distinctive transforming genes are necessary for malignant progression of spontaneous transformation in cultured Chinese hamster embryo cells.

Chinese hamster embryo (CHE) cell strains, each initiated from a separate cell stock obtained from different mothers, were transferred successively at intervals of 3 days and the changes in growth properties and karyotypes at various passages were examined. All nine cell strains proliferated at varying growth rates for 60 passages but only 2 (designated CHE A1 and CHE A2) of them expressed malignant phenotypes. The acquisition of tumorigenicity in nude mice was observed in CHE A1 and CHE A2 cells at passages 40 and 10, respectively. After 5 passages, 8 of 9 cell strains contained one or two common additional chromosomes, chromosome 3q and/or chromosome 5, although one cell strain (designated CHE A3) maintained a normal diploid karyotype for 60 passages. Trisomy of chromosome 3q was observed in all tumorigenic CHE A1 and A2 cells. One or two 3q chromosomes were detected in all tumor-derived cell lines established from tumors produced by these tumorigenic cells. DNA from tumorigenic cells and tumor-derived cell lines exhibited a high ability to transform mouse NIH3T3 cells, but we could not detect any activation of Ha-ras, Ki-ras, hst, erbB-2, mos, met or raf in any of the transformed NIH3T3 cells. These results suggest that even though cultured CHE cells can transform spontaneously, without any specific chromosome change, to immortal cells, activation of unknown oncogene(s) in addition to a specific chromosome change may be required for their malignant progression. Our results suggest that trisomy of chromosome 3q is this specific chromosome change.

Upregulation of the elongation factor-1alpha gene by p53 in association with death of an erythroleukemic cell line.

Genes upregulated by p53 were screened using an erythroleukemic cell line (1-2-3) that expresses only the temperature-sensitive p53 by the mRNA differential display method. One of the upregulated genes was identified as the elongation factor-1alpha (EF-1alpha) gene, an essential component of the eukaryotic translation apparatus. Three p53-responsive elements were found in the mouse EF-1alpha gene and in the corresponding human, rat, and frog genes. These elements conferred the capacity for induction by p53. EF-1alpha is also a microtubule-severing protein. Upon the temperature-shift, the cells developed the morphology and the localization of alpha-tubulin similar to those of the cells treated with vincristine, a drug that affects microtubules. The microtubule-severing associated with upregulation of EF-1alpha by p53 may be a cause of the cell death.

Loss of heterozygosity on chromosome 13 and its association with delayed growth of retinoblastoma.

Loss of heterozygosity (LOH) on chromosome 13 and the age of patients at operation were studied in 46 cases of retinoblastoma (RB) tumors, of which 25 were hereditary and 21 were non-hereditary. The frequency of LOH was 70% for all informative tumors, but significantly higher in non-hereditary tumors (90%) than in hereditary ones (52%). Our results suggest that LOH might be involved in the initial somatic events in non-hereditary tumors. Age at operation of patients with hereditary tumors was significantly lower than that of patients with non-hereditary tumors. Even when tumors associated with a family history were omitted from among the hereditary cases, the difference was still significant. In the case of hereditary tumors, age at operation of LOH-negative patients was significantly lower than that of LOH-positive patients. When tumors associated with a family history were omitted, the difference was still significant. The delay in development of LOH-positive tumors suggests that LOH for one chromosome 13 may be disadvantageous with respect to growth of RB tumors.

Delayed development of retinoblastoma associated with loss of a maternal allele on chromosome 13.

Loss of heterozygosity (LOH) on chromosome 13, which is associated with the functional inactivation of the retinoblastoma (RB) gene, is critical for the development of RB. To date, we have found that LOH-negative tumors develop earlier than LOH-positive tumors in hereditary cases of RB, an observation which suggests that loss of one allele on chromosome 13 may be disadvantageous with respect to growth of RB tumors. In this study, the parental origin of the lost allele on chromosome 13 and the age at operation of 13 patients with non-hereditary RB tumors that had been enucleated at the same stage were studied, in an attempt to determine whether there are any differences between tumors with loss of a maternal allele on chromosome 13 and tumors with loss of a paternal allele. Six tumors had lost the maternal allele and 7 tumors had lost the paternal allele on chromosome 13. The age (average 694 days) of patients at operation in the case of tumors with loss of the paternal allele was significantly lower than the age (average 1,079 days) of patients at operation for removal of tumors with loss of the maternal allele. RB tumors that had lost the maternal allele on chromosome 13 developed later than tumors that had lost the paternal allele. The possibility is discussed that loss of the maternal allele on chromosome 13 might be disadvantageous for growth of RB tumors.

Detection of mutations of the RB1 gene in retinoblastoma patients by using exon-by-exon PCR-SSCP analysis.

Most sporadic cases of retinoblastoma, malignant eye tumor of children, may require the identification of a mutation of the retinoblastoma gene (RB1 gene) for precise genetic counseling. We established a mutation detection system of and screened for the RB1 gene mutation in 24 patients with retinoblastoma--12 bilateral patients and 12 unilateral patients. Mutation analysis was performed by PCR-mediated SSCP analysis in the entire coding region and promoter region, as an initial screening method, followed by direct genomic sequencing. Possible oncogenic mutations were identified in 14 (58%) of 24 tumors, of which 6 were single base substitutions, 4 were small deletions, 3 were small insertions, and 1 was a complex alteration due to deletion-insertion. A constitutional somatic mosaicism was suggested in one bilateral patient. A majority (57%) of mutations were found in E1A binding domains, and all were presumed to truncate the normal gene products. The mutation analysis presented here may provide a basis for the screening system of RB1 gene mutations in retinoblastoma patients.

Repression of p53-dependent sequence-specific transactivation by MEF2c.

Lambda ZAP cDNA library constructed from spleen of a p53-deficient mouse was screened by South-Western technique using Fragment A, a DNA sequence that p53 specifically binds to, as a probe. One (clone 2) of six clones isolated was identical to MEF2c, a MADS-family transcription factor. Transcripts of the mef2c gene was also detected in spleen where the expression has not been reported so far. Isolated clones except clone 2 had a growth-suppression activity on p53-deficient osteosarcoma cell line (Saos II). Clone 2 repressed the transactivation from Fragment A by p53, suggesting that MEF2c may act as a negative regulator of p53-responsive element.

Genomic imprinting of the human serotonin-receptor (HTR2) gene involved in development of retinoblastoma.

Epidemiological and genetic studies of retinoblastoma (RB) suggested that imprinted genes might be genetically linked to the RB gene. In this study, we found that the human serotonin-receptor, HTR2, gene, which had been mapped nearby the RB gene on chromosome 13, was expressed only in human fibroblasts with a maternal allele and not in cells without a maternal allele. The 5' genomic region of the human HTR2 gene was cloned by PCR-mediated method. Only the 5' region of the gene was methylated in cells with the maternal gene, and it was not methylated in cells without the maternal gene. A polymorphism of PvuII site of the gene was also found and useful for the segregation analysis in a family of a RB patient and for analysis of loss of heterozygosity on chromosome 13 in tumor and its parental origin. These results suggest that the human HTR2 gene might be affected by genomic imprinting and that exclusive expression of the maternal HTR2 gene may be associated with the delayed occurrence of RB, which had lost the maternal chromosome 13.

Function and conformation of wild-type p53 protein are influenced by mutations in bovine leukemia virus-induced B-cell lymphosarcoma.

The mutations of the p53 gene previously represented one of several genetic changes involved in the development of bovine leukemia virus (BLV)-induced lymphosarcoma, while the effects of these mutations on the function of p53 are unknown. We identified four mutations of p53 gene in BLV-infected cattle with lymphosarcoma and demonstrated clearly the existence of two functionally distinct groups of mutants: (i) the mutant forms with substitutions at codons 241 and 242, which were mapped within an evolutionally conserved region and corresponded to the human "hot-spot" mutations, had completely lost the capacities for transactivation and growth suppression and gained transdominant repression activity in p53-null SAOS-2 cells; and (ii) the mutations at codons 206 and 207 were located outside the evolutionally conserved regions. These mutants partially retained the capacity for transactivation and growth suppression and failed to inhibit the transactivation activity of coexpressed wild-type p53, instead showing an enhancement of this activity. In addition, protein analysis using an antibody specific for the mutant form revealed that the mutations at codons 206 and 242 induced a "mutant" conformation of the bovine p53 proteins. Collectively, these results show that mutations of p53 gene in BLV-infected cattle with lymphosarcoma can potentially alter its physiological function and may play an important role in BLV-induced leukemogenesis.

Characterization of the promoter of the murine mac25 gene.

It is important to know the regulation of the expression of the mac25 gene because of its reduced expression in several cancer cells and of its induction by some hormonal factors. We cloned the promoter region of the murine mac25 gene and found five repeats of CCAAT sequences, four Sp1 sites, a TATA-like sequence, and an initiator (INR) sequence. Analysis using luciferase reporter plasmids indicated that CCAAT repeats have a strong enhancer activity and the second to fourth Sp1 sites are essential for basal activity of the expression of the mac25 gene. The 1 kb region that contains the promoter and exon 1 of the mac25 gene was in a typical CpG island. As hypermethylation and reduced expression of the mac25 gene were reported in murine liver tumors, methylation of this CpG island may be directly associated with the expression of the mac25 gene and tumorigenesis.

Parental origin of germ-line and somatic mutations in the retinoblastoma gene.

Segregation analysis of polymorphic sites within the retinoblastoma (RB) gene and on chromosome 13, as well as the parental origin of the lost allele in the tumor, were analyzed in 24 families with RB patients. Four mutant alleles transmitted through the germ-line and seven de novo germ-line mutant alleles were identified in 11 patients with hereditary RB. Segregation analysis within the RB gene and on chromosome 13 was useful for DNA diagnosis of susceptibility to RB in relatives of hereditary patients, even if mutations were not identified. All seven de novo germ-line mutant alleles were paternally derived. The bias toward the paternal allele for de novo germ-line mutations of the RB gene was statistically significant. Seven paternal alleles and six maternal alleles were lost in 13 non-hereditary RB tumors with no bias in the parental origin of the somatic allele loss. These results suggest that the physical environment or a deficiency in DNA repair during spermatogenesis may be associated with significant risk factors for de novo germ-line mutations.