The scope of the research at the Cancer Medicine and Biophysics Division is broad, covering numerous areas including the cloning of genes involved in carcinogenesis, biological and structural analyses of proteins, analyses of animal models, and the development of new strategies for cancer therapy. In particular, the genes that are directly regulated by the tumor suppressor gene, p53, have been isolated using a microarray technology and their functions have been studied to uncover the mechanism of p53-mediated tumor suppression, based on which new cancer gene therapies could be developed.

Using a combination of microarray analysis and a chromatin immunoprecipitation assay, identification of p53-target genes in the human genome has been conducted at the Division. In 2007, SEMA3F ⁽⁵⁶⁾, NRP2 ⁽⁵⁶⁾ and NCCRI3 were identified as new p53-target genes. The results of there studies enabled us to further understand the physiological functions of p53.

There are a number of p53-target genes, and the function of the p53-target varies from gene to gene. The important question is how p53 regulates a number of target genes and/or functions. Recently, modifications of p53, such as phosphorylation, acetylation and sumoylation were suggested to play an important role in this process. To clarify the mechanisms, classification of the p53-target genes is being conducted according to their individual functions.

A number of axon-guidance molecules have been shown to be genetically and epigenetically inactivated in human cancers, implying their roles as tumor suppressor genes. Furthermore, these molecules are directly regulated by the tumor suppressor gene, p53. This finding suggests that there might be an important common mechanism between axon guidance and tumorigenesis. Several axon-guidance molecules, including DCC, neogenin, UNC5B and netrin-1, are suggested to regulate apoptosis. Currently, the mechanisms of tumor suppression by axon-guidance molecules are being investigated at this Division.

To kill cancer cells by means of chemotherapy, radiotherapy and molecular- targeted therapy, it is critical to determine whether cancer cell death would be induced by these treatments. From that point of view, alterations of pro- and anti-cell death signaling in human cancer cells are crucial for determining the response to anti-cancer therapy. Therefore, these mechanisms have been investigated at this Division. Until now, the effort has enabled us to identify a new pathway of p53-dependent cell death, which is unlikely to be related to caspase- and transcription activation. Activation of this new pathway by chemical compounds may become a new strategy for cancer therapy.

Netrin-1 is an axon-guidance molecule involved in neural development and differentiation. Recently, we discovered that netrin-1 strongly inhibits p53-dependent cell death. To explore the underlying mechanism, we examined the expression levels of p53 and its target genes. The expression levels of p53 and a number of p53-target genes were increased in the cells infected with Ad-p53 in the presence of netrin-1, suggesting that netrin-1 may not inhibit p53 itself and the transcriptional activation of the p53-target genes.

In order to characterize the netrin-1 anti-cell death pathway, we have been making efforts to identify the receptors and the mediators involved by screening for netrin-1-binding proteins with FLAG-tagged-netrin-1, and have identified the receptor-X responsible for this pathway. Receptor-X strongly interacts with netrin-1, leading to activation of several prosurvival molecules and inhibition of p53-induced apoptosis. In addition, the signaling of the netrin-1 and receptor-X enhanced migration of cancer cells and endothelial cells, implying the role of the pathway in cancer metastasis and angiogenesis. To further clarify the role of netrin-1 and this receptor in tumorigenesis, alterations of the pathway in human cancers have been investigated at this Division.

Mitochondria plays a critical role in a number of cellular functions, being involved in aging and cancer. However, the mechanisms involved in maintenance of the stability of healthy mitochondria still remain unclear. This year, we reported that p53R2 plays an essential role in DNA synthesis of mitochondria, and that its mutations cause congenital mitochondrial diseases ⁽⁵⁷⁾. This finding suggests that p53 might regulate the biosynthesis of mitochondria in order to prevent tumor initiation and progression. Therefore, the mechanisms of maintenance of healthy mitochondria and their alterations in human cancers are currently being investigated at this Division.

Several p53-mutants are known to show enhanced apoptosis inducing activity, and are believed to be some kind of activated forms of p53. On the other hand, several p53-target genes have also been reported to induce marked apoptosis in cancer cells. Therefore, to improve p53 gene therapy, adenovirus-mediated gene transfer of the active forms of p53 or apoptotic p53-target genes may well become a new therapeutic strategy for the treatment of p53-resistant cancers. Adenovirus-mediated gene transfer of p53-46F was previously demonstrated to be useful for cancer therapy. Toward the development of new strategies for cancer therapy, the in vitro and in vivo antitumor effects of these genes are being examined at this Division.

Attempts are also being made to identify and analyze other cancer-related genes, including tumor suppressor genes and oncogenes at this Division ⁽⁵⁸⁾.