Disruption of the function of tumor-suppressor gene p53 by the hepatitis B virus X protein and hepatocarcinogenesis

The X gene of the hepatitis B virus codes for a small basic protein and is able to transactivate viral and cellular genes, although the X protein exhibits no DNA-binding activity. The mechanism of transactivation by X protein has been suggested to be via protein-protein interaction(s). We first demonstrated that X protein had amino acid sequences homologous to the functionally essential domain of Kunitz-type serine protease inhibitors and that those sequences were indispensable for the transactivation function. We demonstrated that X protein exhibited an inhibitor activity against hepatic serine proteases, and subsequently found that the protein activated X gene transcription in HepG2 cells and that the X responsive element was localized in the minimal promoter of the X gene. In contrast, the tumor-suppressor gene p53, but not mutant p53, remarkably reduced transcription from the minimal promoter. This p53 repression on the X gene promoter was cancelled by X gene co-expression, probably indicating that the X protein disrupts the p53 tumor suppressor function in the nucleus. All data suggest that X protein leads to transactivation of cellular oncogenes by preventing an interaction between p53 and cellular transcription factor(s) consisting of the basal transcriptional machinery.

Cloning and sequence analysis of the aminopeptidase My gene from Mycoplasma salivarium

The complete nucleotide sequence of a major component of aminopeptidase My purified from Mycoplasma salivarium was determined. The protein gene encoded a protein consisting of 520 amino acids with a molecular mass of 58079 Da. The protein contained two tryptophan residues, one of which was encoded by UGA. A computer-aided homology search suggested that aminopeptidase My had properties similar to those of leucine aminopeptidase (EC 3.4.11.1).

A human temperature-sensitive p53 mutant p53Val-138: modulation of the cell cycle, viability and expression of p53-responsive genes

A human p53 mutant, p53Val-138 (amino acid 138, Alanine-->Valine), generated by in vitro mutagenesis was introduced into Saos-2 human osteosarcoma and Jurkat acute T-lymphoblastic leukemia cell lines, both lacking p53 protein expression. p53Val-138 caused growth arrest in Saos-2 cell line and apoptosis in Jurkat cell line at 32.5 degrees C while it allowed both cell lines to grow continuously at 37.5 degrees C. p53Val-138 activated expression of p53-responsive genes including MDM2, GADD45 and WAF1/CIP1/SD11 in Saos-2 cell line upon the temperature shift-down from 37.5 degrees C to 32.5 degrees C. Thus, p53Val-138 acted as a temperature-sensitive p53 mutant. Taking advantage of these human cell systems, we demonstrated that p53-mediated cell cycle arrest occurred in G1 and G2/M phases of Saos-2 cell line but not in Jurkat cell line. The induced level of WAF1/CIP1/SDI1 mRNA by p53 was extremely lower in Jurkat cell line than that of Saos-2 cell line. However, MDM2 mRNA accumulated to the similar levels in these two cell lines. These results suggest that a factor(s) other than p53 may be involved in differential expression of WAF1/CIP1/SDI1 and MDM2 mRNA.

Apoptosis is induced in BHK cells by the tsBN462/13 mutation in the CCG1/TAFII250 subunit of the TFIID basal transcription factor

A temperature-sensitive (ts) mutant of the BHK21 cell line derived from golden hamsters, tsBN462 has a mutation in the gene encoding the largest subunit of the TFIID complex, TAFII250/p230/CCG1, and arrests in the G1 phase at the nonpermissive temperature, 39.5 degrees C. We found that tsBN462 cells underwent apoptosis following growth arrest at 39.5 degrees C, suggesting a role for CCG1 as a repressor of apoptosis. By electron microscopic observation, tsBN462 cells at 39.5 degrees C showed characteristic features of apoptosis. Apoptosis was not suppressed by expression of Bc1-2 or the adenovirus E1B 19 kDa protein. Cell death was suppressed completely by expression of wild-type CCG1 and partially by wild-type p53, a growth suppressor protein. Cell cycle arrest induced by p53 may help survival of tsBN462 cells at 39.5 degrees C. Apoptosis was accelerated in SV40 large T antigen-transformed tsBN462 cells at 39.5 degrees C where SV40 large T antigen formed a complex with p53, implying that the apoptosis of tsBN462 cells at 39.5 degrees C occurred in a p53-independent manner. Our results suggest that CCG1/TAFII250 is required for the expression of factors regulating apoptosis.

A temperature sensitive mutant of the human p53, Val138, arrests rat cell growth without induced expression of cip1/waf1/sdi1 after temperature shift-down

To investigate functions of wild type p53 in human cells, we introduced a (Ala-->Val) mutation at the 138th codon of the human p53 (Val138), which corresponds to the Val135 mutation of the temperature sensitive mouse p53. The human Val138 mutant showed temperature-sensitive transformation of rat embryo fibroblasts (REFs) in collaboration assay with activated ras, and arrested cell proliferation of transformed clones in G1 at 32.5 degrees C. Transient CAT assay for transcriptional activation in human Saos2 cells revealed activity equivalent to that of wild type at 32.5 degrees C but undetectable at 37.5 degrees C. These results suggest that the human Val138 mutant also exhibited the wild type phenotype at the permissive temperature as is for the mouse Val135 mutant, although we observed differences between the two mutants such as in transactivational activities in CV-1 and HeLa cells. Further, the role of cip1/waf1/sdi1 in the cell growth arrest of the Val138/ras-transformed REFs and Val138-introduced Saos2 cells was studied by northern hybridization analysis. Although rapid induction of cip1/waf1/sdi1 mRNA was observed in the Saos2 cells, no detectable induction of mRNAs for cip1/waf1/sdi1 and gadd45 was observed in the transformed REFs upon temperature shift-down, while mdm2 mRNA was enhanced, suggesting that the p53 gene could arrest cell growth by a mechanism other than that with induced expression of the gene for p21 cdk-cycline inhibitor.

Roles of p53 mutation in cell line establishment and identification of the minimum transactivation and transform suppression domains

The mutation of the p53 tumour suppressor gene is the most frequently recognised genetic alteration in human cancer. We recently showed that the frequency of p53 gene mutations in oral squamous cell carcinomas (SCCs) from which cell lines were established (group A) did not significantly differ from that in SCCs from which cell lines could not be established (group B), suggesting that the presence of a p53 mutation by itself is not sufficient. To assess the relevance of p53 mutations to cell line establishment, we determined sequences of the mutated genes, constructed the expression plasmids, and compared biological and biochemical activities. Both groups contained typical mutant type mutations at a similar frequency. However, two mutations in group A had strong transforming activity. One of the mutants, codon 306 Stop mutant with C-terminal truncation, was found to have the transactivation and transform suppression activities similar to wild type. The minimum transactivation and transform suppression domains of p53 were thus determined based on analysis of various C-terminal deletions. Activity disappeared between codons 300 and 282, an interval which contains the C-terminal end of the sequence-specific DNA binding domain, which suggests that the DNA binding domain is essential for the above activities.

Interaction between hsp70 and hsp40, eukaryotic homologues of DnaK and DnaJ, in human cells expressing mutant-type p53

We have recently identified a novel 40-kDa heatshock protein hsp40 as a mammalian homologue of bacterial DnaJ protein. Here we demonstrate the physical interaction between hsp70 (DnaK homologue) and hsp40 in human cells as determined by immunoprecipitation methods. Co-immunoprecipitation of hsp70 with hsp40 was dependent on the presence of ATP or unfolded protein (reduced carboxymethylated alpha-lactalbumin). A mutant type of tumor suppressor gene product, mtp53, was co-immunoprecipitated not only with hsp70 but also with hsp40. These results suggest the existence of a hsp70(DnaK)/hsp40(DnaJ) chaperone system in mammalian cells.

Response of heat shock element within the human HSP70 promoter to mutated p53 genes

Recent studies have demonstrated that mutation of the p53 gene caused a gain of new functions such as transforming activation, binding to heat shock cognate protein 70 and/or transactivation of a variety of promoters. In the course of seeking the biochemical basis for the gain of these functions, we have noticed the correlation between transforming activity of different mutated p53 genes and their transactivational activity on the human heat shock protein 70 promoter. Analysis of 5' deletion constructs of the heat shock protein 70 promoter showed that some specific elements within the heat shock domain containing two heat shock elements (HSEs) could respond to mutant p53 species but not basic promoter elements such as the TATA box, CCAAT box, and GC box. Subsequently, we identified the HSE as a responsive element using reporter constructs of minimal promoter containing synthetic proximal HSE, distal HSE, or GC/CCAAT box. Further analysis using in vitro mutagenesis of HSE suggests that HSE with heat shock factor binding ability is required for transactivation of the heat shock protein 70 promoter by mutated p53 genes.

Hypertriglyceridemic serum, very low density lipoprotein, and iron enhance Mycobacterium avium replication in human macrophages

The growth of Mycobacterium avium 7497, serovar 4, in cultured human macrophages is enhanced by Fe3+ and serum lipids over 7 days. Iron (1-80 micrograms/mL) added to macrophages cultured in normal serum resulted in 10-fold increases in growth. If iron-supplemented macrophages were cultured in serum from hypertriglyceridemic donors after infection, M. avium growth increased 10(3)- to 10(4)-fold. Without macrophages, differences in bacterial growth between sera were not seen. Removal of very low density lipoprotein (VLDL) eliminated the differences between sera. Isolated VLDL from hyperlipidemic serum resulted in 10(5)-fold increases in growth over that seen with VLDL from normal sera. Accelerated M. avium growth in macrophages cultured with hyperlipidemic serum was partly inhibited by the addition of superoxide dismutase (1000 IU/mL). Results suggest that iron stimulates O2-induced oxidation of VLDL and its subsequent accumulation in macrophages. The resultant iron- and lipid-laden cells become excellent hosts for mycobacterial growth.

A novel human gene that is preferentially transcribed in heart muscle

We have cloned a human cDNA fragment for a gene that is expressed primarily in the heart. To explore its biological function, we have isolated full-length cDNA clones of the gene. DNA sequencing of the 2.7-kb cDNA revealed a 2274-bp ORF. Close to the N terminus of the deduced amino acid sequence is a possible ATP-binding domain that has been assembled by a fusion of B- and A-type adenine nucleotide-binding motifs. In the middle of the sequence is a long stretch of alpha-helical residues that could form a coiled-coil. These features imply that this is a sequence encoding a new human motor protein.

Aberrant expression of the p53 tumor suppressor gene in adult T-cell leukemia and HTLV-I-infected cells

By immunoprecipitation analysis, enhanced p53 expression was detected in 3 of 4 adult T-cell leukemia (ATL) cell lines, 1 of 3 HTLV-I-infected cell lines and 1 of 5 fresh ATL samples, compared with phytohemagglutinin-stimulated peripheral blood lymphocytes. Among these 5 high expressers, p53 missense mutations were indicated in 2 ATL cell lines and 1 fresh ATL sample by extensive p53 cDNA and genomic DNA polymerase chain reaction single-strand conformation polymorphism analysis. No mutation was found throughout the entire coding region of the remaining 2 high expressers (1 ATL and 1 HTLV-I-infected cell lines) and low expressers of p53 (2 HTLV-I-infected cell lines). Tax oncoprotein expression was found in these 2 high p53 expressers in which p53 mutation was not present, but not in low p53 expressers or cells carrying this mutation. The levels of p53 mRNA were similar among the samples regardless of p53 levels. Posttranscriptional mechanisms other than missense mutation would thus appear to increase p53 in the Tax-expressing cells but not in cells containing undetectable levels of Tax. No complex formation between p53 and Tax was observed.

The p53 tumor-suppressor gene and ras oncogene mutations in oral squamous-cell carcinoma

The frequencies of mutations in the p53 tumor-suppressor gene and ras proto-oncogenes were investigated systematically in surgically resected oral squamous-cell carcinomas (SCCs) using single-strand conformation polymorphism (SSCP) and/or dot-blot hybridization analysis of DNA fragments which had been amplified by the polymerase chain reaction (PCR). p53 gene mutations, within the region of exons 5 to 8, were detected in 17 out of 27 (63%) tumor specimens. The role of p53 mutations in cell-line establishment was investigated. p53 gene mutations were detected in 5 out of 6 tissue samples from which cell lines were established and in 4 out of 5 specimens from which cell lines could not be established, suggesting that the presence of p53 gene mutations is not by itself sufficient for cell-line establishment. Tumor samples were also analyzed for point mutational activation of the ras proto-oncogenes. One out of 30 (3%) tumors showed an activating point mutation in codon 12 of H-ras, this being consistent with reports from Europe and USA but not with any from India. Compared to frequencies of the other genetic changes so far reported for oral SCC, the p53 mutations have been observed most often to undergo genetic change. p53 gene mutation is thus intimately involved in the genesis of oral SCC and consequently should be useful as a marker for the diagnosis of this neoplasm.

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.

Detection of p53 gene mutations in human ovarian and endometrial cancers by polymerase chain reaction-single strand conformation polymorphism analysis

The presence of mutations in the p53 gene was examined in ovarian cancers by a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. The primers were designed to amplify exons 5 through 9 that contain phylogenetically conserved domains of the p53 gene. Mutations were detected in 5 out of 10 cases, one of which contained a deletion in the second allele. A single base substitution was detected in 4 cases at codons 162, 175, 205 and 273 and a single base insertion in one case within codon 315. A high frequency of p53 mutations in ovarian cancers and lack of mutation in 6 benign ovarian tumors and 2 normal ovaries suggested that the mutation of the p53 gene was associated with the genesis and/or progression of ovarian cancer. In 1 of 7 endometrial cancers, two mutations at codons 239 and 254 were detected.

Most human squamous cell carcinomas in the oral cavity contain mutated p53 tumor-suppressor genes

Oral squamous cell carcinoma cell lines and tumor tissues used for cell line establishment were examined for p53 tumor-suppressor gene mutation using cellular DNAs and RNAs. For sensitive and rapid detection, a newly designed two-stage filtration strategy was used. Full advantage was taken of the single-strand conformation polymorphism (SSCP) of cDNA and cellular DNA fragments amplified by the polymerase chain reaction (PCR), followed by DNA sequencing of mutated fragments. Fourteen out of 15 cells lines and the corresponding five tumor tissues had p53 mutations within the region of exons 5-8. Loss of normal alleles was noted in 14 lines, but not in one in which only mutated transcripts were detected. DNA sequencing indicated six out of 14 mutations to be in positions that have so far not been reported. In two special cases, novel mutations were found in the splicing donor sequence of exon 6, and consequently the cryptic splice site had to be used. Extremely frequent p53 gene mutations indicated that the mutations are likely be intimately involved in the carcinogenesis of oral squamous cell carcinoma.

Mechanism of induction of cellular DNA synthesis by the adenovirus E1A 12S cDNA product

The mechanism of induction of DNA synthesis in quiescent rat 3Y1 cells by the adenovirus E1A gene was investigated using the 3Y1 derivative cell lines g12-21, gn12RB1, and gn12RB2. The g12-21 cells express the E1A 12S cDNA and the latter two cells express both the E1A 12S cDNA and the human retinoblastoma susceptibility (Rb) gene at different levels in response to dexamethasone (dex). The cDNA sequences of E1A-inducible cell cycle-dependent genes, clone 3 and clone 16, were isolated by differential screening of a cDNA library constructed from dex-treated g12-21 cells. The quiescent 3Y1 cells induced c-fos and c-myc expression within 2 h after serum stimulation and expressed clone 16 and clone 3 transiently at around 8 h before the onset of DNA synthesis (10 h). In contrast, the quiescent g12-21 cells treated with dex expressed a high level of E1A at 6 to 8 h after treatment and expressed clone 16 and clone 3 at around 8 h without stimulation of c-fos and c-myc expression, suggesting that E1A bypasses the cell cycle early in G1. The half-maximal rate of DNA synthesis was reached in a much shorter time in dex-treated g12-21 cells (12 h) than in serum-treated 3Y1 cells (18 h), suggesting that E1A also bypasses the cell cycle at the G1/S boundary. The gn12RB1 and gn12RB2 cells were unable to induce DNA synthesis in response to dex presumably due to lower levels of E1A expression, although gn12RB2 but not gn12RB1 cells could express clone 16 and clone 3. These results suggest that the level of E1A required for bypass at the G1/S boundary is higher than that required early in G1.

Gene amplification and overexpression of epidermal growth factor receptor in squamous cell carcinoma of the head and neck

The degree of gene amplification for epidermal growth factor receptor (EGFR) and its expression levels were examined in 4 cases of tumor lesions and their cell lines of human squamous cell carcinoma (SCC) of the oral cavity. The amplification was detected in 1 case (ZA), but not significantly in 3 other cases (HOC605, HOC815, and HOC927) in which the amplification did not occur during the cell line establishment. In those 3 cases, levels of EGFR synthesis and human EGF (hEGF) binding capacity were varied: HOC605 and HOC815 had slightly increased levels of hEGF binding capacity and EGFR synthesis, respectively. While HOC927 had the lowest levels of both, the hEGF binding capacity was elevated in the tumor lesion when compared with the normal counterpart of the same patient. These results suggest that the increased capacity for EGF binding plays a more important role than does gene amplification on the tumorigenesis of SCC of the head and neck.

cDNA and deduced amino acid sequence of mouse matrix gla protein: one of five glutamic acid residues potentially modified to gla is not conserved in the mouse sequence

A cDNA library was constructed using the mouse osteoblastic cell line MC3T3-E1 treated with 1 alpha,25-dihydroxyvitamin D3, based on the finding that the treatment increased ninefold the expression of 0.7 kb matrix gla protein (MGP) mRNA. cDNA clones encoding mouse MGP were isolated from the library. The nucleotide sequence showed an open reading frame of 312 nucleotides encoding 104 amino acids. Murine MGP shared 84-89% amino acid sequence homology with bovine, rat, and human MGP. However, there are five glutamic acid residues potentially modified to gamma-carboxyglutamic acid (gla) in those species; in murine MGP, lysine replaced glutamic acid 37. Also, an extra tyrosine was added at the carboxyl terminus. The significance of the substitution is discussed in relation to the gamma-carboxylation sites in MGP protein.

Nonlethal G0-ts mutant tsJT60 becomes lethal at the nonpermissive temperature after transformation: a hint for new cancer chemotherapeutics

tsJT60 is a nonlethal temperature-sensitive (ts) mutant of a Fischer rat cell line (3Y1) classified as a G0 mutant; i.e., the ts defect is not expressed within the cell growth cycle but is expressed only between the G0 and S phase. tsJT60 clones transformed with oncogenes such as adenovirus E1A, polyoma large T, polyoma middle T, v-Ki-ras, and LTR activated c-myc, or with a chemical carcinogen N-methyl-N'-nitro-N-nitrosoguanidine, grew well at 34 degrees C. However, most of these clones grew slowly at 40 degrees C, producing many floating dead cells, and some clones were killed at 40 degrees C. When they were cultured under conditions inadequate for growth of untransformed cells, such as high cell density or serum restriction, they were killed at 40 degrees C. These and previous results from SV40- and adenovirus-transformed tsJT60 clones favour the idea that transformed tsJT60 cells occasionally enter the G0 phase and are metabolically imbalanced at 40 degrees C during self-stimulation from the G0 to S phase. We propose that a drug which exclusively block, G0-G1 transition would be cytocidal to transformed cells but cytostatic to normal cells.

Gene cloning of an Actinobacillus actinomycetemcomitans Y4 antigen which reacts with peripheral blood sera in patients with advanced destructive periodontitis

Actinobacillus actinomycetemcomitans has been implicated in the aetiology of juvenile periodontitis and advanced destructive periodontitis. Levels of IgG antibody against A. actinomycetemcomitans in peripheral blood sera of patients with advanced destructive periodontitis are high, as are those against Bacteroides gingivalis. To clone the genes of antigens reactive with sera of such patients, a library of the A. actinomycetemcomitans strain Y4 DNA in lambda L47 was constructed and then screened, using an immunochemical detection method, with serum from a patient with the advanced disease. Six clones from among nearly 1000 reacted with the serum and also with that of another patient. They were designated 3, 4, 6, 7, 8 and 9. Restriction enzyme and Southern blot analyses indicated that clones 8 and 9 were identical and that all the clones were overlapping because they shared in common the 4 and 5 kbp HincII DNA fragments of A. actinomycetemcomitans. The cloned DNA fragment hybridized to the DNA of two other strains of A. actinomycetemcomitans but not to those of six periodontopathic bacteria examined. These findings suggest that a DNA sequence encoding an A. actinomycetemcomitans strain Y4 antigen strongly reactive with sera of patients with advanced destructive periodontitis was cloned. This sequence is present specifically in A. actinomycetemcomitans but not in other bacteria isolated from patients with periodontal diseases. Thus, the cloned DNA could serve as a probe for the diagnosis of periodontitis.

Inactivation of the N-myc gene product by single amino acid substitution of leucine residues located in the leucine-zipper region

To verify the importance of the hypothetical leucine-zipper structure in the N-myc protein, a series of mutants of the mouse N-myc gene were constructed, in which codons for the first and second leucine residues within this structure were systematically replaced by other amino acids. The expression plasmids which contained the mutated and wild type N-myc genes were cotransfected into rat embryo cells with activated c-Ha-ras gene and their transforming abilities were compared. It was shown that single amino acid substitutions in the leucine-zipper region inactivate the transforming ability of the N-myc gene product. In particular, proline, which is known to disrupt an alpha-helical structure, completely inactivated the transforming activity even when it was substituted for another amino acid located between these two leucine residues. Among several amino acid species used for substitution of the leucine residues, only methionine was able to retain the transforming activities in both the first and second leucine positions, although the activity was reduced as compared with wild-type N-myc gene product. It also appeared that the integrity of the first leucine is more important than the second leucine. Our results provide experimental evidence for the physiological importance of the hypothetical leucine-zipper structure.

Altered ganglioside expression in ras-oncogene-transformed cells

Alteration of ganglioside composition in mouse BALB/3T3 cells transformed either by DNA transfection with viral K-, H-, or cellular H-ras oncogene, or by infection with the K-ras oncogene-carrying murine sarcoma virus (Ki-KSV) was studied using a highly sensitive thin-layer chromatography/enzyme immunostaining method. Marked common decreases in the content of GD3 ganglioside and the increase of its metabolic precursor GM3 were bound in BALB/3T3 cell lines transformed by either K- or H-ras oncogenes. Moreover, a common decrease or loss in the contents of "A" series ganglio-tetraose gangliosides such as GM1a and GD1a was also found in all transformed cell lines, indicating that the alteration of cellular glycosphingolipids by ras oncogenes apparently does not depend on the type of ras-concogenes (K- and H-ras).

Activation of oncogenes in human oral cancer cells: a novel codon 13 mutation of c-H-ras-1 and concurrent amplifications of c-erbB-1 and c-myc

By NIH3T3 transfection assay in conjunction with in vitro transient neomycin selection, activated c-H-ras-1 oncogenes were detected in two squamous cell carcinoma cell lines, ZA and HOC-313, newly established from human oral cancer patients. ZA had a point mutational activation at the 13th codon, this activation of c-H-ras-1 being novel in human cancer cells, while HOC-313 appeared to have an activation at the 12th codon. In ZA, 16- to 32-fold amplification of the EGF receptor gene, c-erbB-1 and a few-fold amplification of c-myc were detected. The significance of these findings is discussed in relation to multistep carcinogenesis in human cells.