Takashi Tokino

p53AIP1, a Potential Mediator of p53-Dependent Apoptosis, and Its Regulation by Ser-46-Phosphorylated p53

Through direct cloning of p53 binding sequences from human genomic DNA, we have isolated a novel gene, designated p53AIP1 (p53-regulated Apoptosis-Inducing Protein 1), whose expression is inducible by wild-type p53. Ectopically expressed p53AIP1, which is localized within mitochondria, leads to apoptotic cell death through dissipation of mitochondrial A(psi)m. We have found that upon severe DNA damage, Ser-46 on p53 is phosphorylated and apoptosis is induced. In addition, substitution of Ser-46 inhibits the ability of p53 to induce apoptosis and selectively blocks expression of p53AIP1. Our results suggest that p53AIP1 is likely to play an important role in mediating p53-dependent apoptosis, and phosphorylation of Ser-46 regulates the transcriptional activation of this apoptosis-inducing gene.

Epigenetic inactivation of SFRP genes allows constitutive WNT signaling in colorectal cancer

Aberrant WNT pathway signaling is an early progression event in 90% of colorectal cancers. It occurs through mutations mainly of APC and less often of CTNNB1 (encoding β-catenin) or AXIN2 (encoding axin-2, also known as conductin). These mutations allow ligand-independent WNT signaling that culminates in abnormal accumulation of free β-catenin in the nucleus. We previously identified frequent promoter hypermethylation and gene silencing of the genes encoding secreted frizzled-related proteins (SFRPs) in colorectal cancer. SFRPs possess a domain similar to one in the WNT-receptor frizzled proteins and can inhibit WNT receptor binding to downregulate pathway signaling during development. Here we show that restoration of SFRP function in colorectal cancer cells attenuates WNT signaling even in the presence of downstream mutations. We also show that the epigenetic loss of SFRP function occurs early in colorectal cancer progression and may thus provide constitutive WNT signaling that is required to complement downstream mutations in the evolution of colorectal cancer.

AXIN1 mutations in hepatocellular carcinomas, and growth suppression in cancer cells by virus-mediated transfer of AXIN1

The Wnt signalling pathway is essential for development and organogenesis. Wnt signalling stabilizes β-catenin, which accumulates in the cytoplasm, binds to T-cell factor (TCF; also known as lymphocyte enhancer-binding factor, LEF) and then upregulates downstream genes. Mutations in CTNNB1 (encoding β-catenin) or APC (adenomatous polyposis coli) have been reported in human neoplasms including colon cancers and hepatocellular carcinomas (HCCs). Because HCCs tend to show accumulation of β-catenin more often than mutations in CTNNB1 , we looked for mutations in AXIN1, encoding a key factor for Wnt signalling, in 6 HCC cell lines and 100 primary HCCs. Among the 4 cell lines and 87 HCCs in which we did not detect CTNNB1 mutations, we identified AXIN1 mutations in 3 cell lines and 6 mutations in 5 of the primary HCCs. In cell lines containing mutations in either gene, we observed increased DNA binding of TCF associated with β-catenin in nuclei. Adenovirus mediated gene transfer of wild-type AXIN1 induced apoptosis in hepatocellular and colorectal cancer cells that had accumulated β-catenin as a consequence of either APC, CTNNB1 or AXIN1 mutation, suggesting that axin may be an effective therapeutic molecule for suppressing growth of hepatocellular and colorectal cancers.

A novel brain-specific p53-target gene, BAI1, containing thrombospondin type 1 repeats inhibits experimental angiogenesis

The genetic alteration of p53 is associated with neovascularization during progression of glioma to its more malignant form, glioblastoma. Hence, one or more of the genes transactivated by p53 is likely to function as an angiogenesis inhibitors. We isolated a novel p53-inducible gene that encodes a 1584-amino-acid product containing five thrombospondin type 1 (TSP-type 1) repeats and is specifically expressed in the brain. A recombinant protein corresponding to the TSP-type 1 repeats of this gene product inhibited in vivo neovascularization induced by bFGF in the rat cornea. The expression of this gene, designated BAI1 (brain-specific angiogenesis inhibitor 1) was absent or significantly reduced in eight of nine glioblastoma cell lines, suggesting BAI1 plays a significant role in angiogenesis inhibition, as a mediator of p53.

Upregulation of miR-196a and HOTAIR Drive Malignant Character in Gastrointestinal Stromal Tumors

Large intergenic noncoding RNAs (lincRNA) have been less studied than miRNAs in cancer, although both offer considerable theranostic potential. In this study, we identified frequent upregulation of miR-196a and lincRNA HOTAIR in high-risk gastrointestinal stromal tumors (GIST). Overexpression of miR-196a was associated with high-risk grade, metastasis and poor survival among GIST specimens. miR-196a genes are located within the HOX gene clusters and microarray expression analysis revealed that the HOXC and HOTAIR gene were also coordinately upregulated in GISTs which overexpress miR-196a. In like manner, overexpression of HOTAIR was also strongly associated with high-risk grade and metastasis among GIST specimens. RNA interference-mediated knockdown of HOTAIR altered the expression of reported HOTAIR target genes and suppressed GIST cell invasiveness. These findings reveal concurrent overexpression of HOX genes with noncoding RNAs in human cancer in this setting, revealing miR-196a and HOTAIR as potentially useful biomarkers and therapeutic targets in malignant GISTs.

Frequent epigenetic inactivation of Wnt antagonist genes in breast cancer.

Although mutation of APC or CTNNB1 (β-catenin) is rare in breast cancer, activation of Wnt signalling is nonetheless thought to play an important role in breast tumorigenesis, and epigenetic silencing of Wnt antagonist genes, including the secreted frizzled-related protein (SFRP) and Dickkopf (DKK) families, has been observed in various tumours. In breast cancer, frequent methylation and silencing of SFRP1 was recently documented; however, altered expression of other Wnt antagonist genes is largely unknown. In the present study, we found frequent methylation of SFRP family genes in breast cancer cell lines (SFRP1, 7 out of 11, 64%; SFRP2, 11 out of 11, 100%; SFRP5, 10 out of 11, 91%) and primary breast tumours (SFRP1, 31 out of 78, 40%; SFRP2, 60 out of 78, 77%; SFRP5, 55 out of 78, 71%). We also observed methylation of DKK1, although less frequently, in cell lines (3 out of 11, 27%) and primary tumours (15 out of 78, 19%). Breast cancer cell lines express various Wnt ligands, and overexpression of SFRPs inhibited cancer cell growth. In addition, overexpression of a β-catenin mutant and depletion of SFRP1 using small interfering RNA synergistically upregulated transcriptional activity of T-cell factor/lymphocyte enhancer factor. Our results confirm the frequent methylation and silencing of Wnt antagonist genes in breast cancer, and suggest that their loss of function contributes to activation of Wnt signalling in breast carcinogenesis.

Frequent epigenetic inactivation of SFRP genes and constitutive activation of Wnt signaling in gastric cancer

Activation of Wnt signaling has been implicated in gastric tumorigenesis, although mutations in APC (adenomatous polyposis coli), CTNNB1 (β-catenin) and AXIN are seen much less frequently in gastric cancer (GC) than in colorectal cancer. In the present study, we investigated the relationship between activation of Wnt signaling and changes in the expression of secreted frizzled-related protein (SFRP) family genes in GC. We frequently observed nuclear β-catenin accumulation (13/15; 87%) and detected the active form of β-catenin in most (12/16; 75%) GC cell lines. CpG methylation-dependent silencing of SFRP1, SFRP2 and SFRP5 was frequently seen among GC cell lines (SFRP1, 16/16, 100%; SFRP2, 16/16, 100%; SFRP5, 13/16, 81%) and primary GC specimens (SFRP1, 42/46, 91%; SFRP2, 44/46, 96%; SFRP5, 30/46, 65%), and treatment with the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine rapidly restored SFRP expression. Ectopic expression of SFRPs downregulated T-cell factor/lymphocyte enhancer factor transcriptional activity, suppressed cell growth and induced apoptosis in GC cells. Analysis of global expression revealed that overexpression of SFRP2 repressed Wnt target genes and induced changes in the expression of numerous genes related to proliferation, growth and apoptosis in GC cells. It thus appears that aberrant SFRP methylation is one of the major mechanisms by which Wnt signaling is activated in GC.

Epigenetic inactivation of CHFR in human tumors

Cell-cycle checkpoints controlling the orderly progression through mitosis are frequently disrupted in human cancers. One such checkpoint, entry into metaphase, is regulated by the CHFR gene encoding a protein possessing forkhead-associated and RING finger domains as well as ubiquitin–ligase activity. Although defects in this checkpoint have been described, the molecular basis and prevalence of CHFR inactivation in human tumors are still not fully understood. To address this question, we analyzed the pattern of CHFR expression in a number of human cancer cell lines and primary tumors. We found CpG methylation-dependent silencing of CHFR expression in 45% of cancer cell lines, 40% of primary colorectal cancers, 53% of colorectal adenomas, and 30% of primary head and neck cancers. Expression of CHFR was precisely correlated with both CpG methylation and deacetylation of histones H3 and H4 in the CpG-rich regulatory region. Moreover, CpG methylation and thus silencing of CHFR depended on the activities of two DNA methyltransferases, DNMT1 and DNMT3b, as their genetic inactivation restored CHFR expression. Finally, cells with CHFR methylation had an intrinsically high mitotic index when treated with microtubule inhibitor. This means that cells in which CHFR was epigenetically inactivated constitute loss-of-function alleles for mitotic checkpoint control. Taken together, these findings shed light on a pathway by which mitotic checkpoint is bypassed in cancer cells and suggest that inactivation of checkpoint genes is much more widespread than previously suspected.

The role of p53-target genes in human cancer

The tumor suppressor gene p53 is mutated in a large proportion of human cancers. In some cellular conditions like DNA damage, the p53 gene is induced and its gene product is posttranscriptionally activated. p53 works as a transcriptional activator and induces the expression of its downstream target genes. This review will explain why expression of the normal p53 gene leads to tumor growth suppression. The p53 has several biological effects involving cell-cycle arrest, DNA replication and repair, proliferation, apoptosis, angiogenesis inhibition, and cellular stress response. These effects of the p53 result mainly from the activation of expression of a large number of p53-target genes. Here we have focused on the biological functions of the transcriptional targets of p53.

Genetic, epigenetic, and clinicopathologic features of gastric carcinomas with the CpG island methylator phenotype and an association with Epstein–Barr virus

The CpG island methylator phenotype (CIMP), which is characterized by simultaneous methylation of the CpG islands of multiple genes, has been recognized as one of the important mechanisms in gastrointestinal carcinogenesis.