6. Biophysics Division

Apoptosis is an active form of cell death which is genetically encoded and functions as a safeguard system in mammalian cells, preventing cell transformation through elimination of aberrant cells with deregulated oncogene expression and/or DNA damage exceeding the limit of repair. In tumor cells, however, the apoptotic pathway is potentially disrupted. The Biophysics Division has focused on elucidating the mechanism by which tumor cells acquire resistance to apoptosis. Identification of the determinants conferring apoptotic resistance to tumor cells could lead to innovative approaches to killing tumor cells, which might facilitate the development a new therapeutic methods for human cancer.

Signaling Pathway in Apoptosis

Signal trunsduction in apoptosis is regulated by a variety of extra- and intracellular factors such as oncoproteins, tumor suppressors, radiation, viruses and chemotherapeutic agents. The Biophysics Division has been interested in the intracellular signaling pathways involved in apoptosis. Recent studies have shown that activation of the ICE/CED-3 family protease cascade is required for induction of Fas- and TNFR-mediated apoptosis. However, it remains unknown whether activation of this protease cascade is generally involved in apoptosis mediated by other stimuli such as oncoproteins and tumor suppressors. Gene transfection experiments showed that expression of the s-myc gene, a unique myc family gene isolated from a rat genomic library, can effectively induce apoptosis in human and rat glioma cells.^(78-80) In contrast to c-Myc, s-Myc expression can induce apoptosis in glioma cells cultured in the presence of 10% fetal calf serum in a wt-p53-independent manner. Recently, similar intronless myc genes having the ability to induce apoptosis but lacking tranforming activity were isolated from mouse and human genomic libraries.⁽⁸⁰⁾ Preliminary data suggest that specific inhibitors of ICE/CED-3 family proteases such as Z-Asp-CH2-DCB, Ac-DEVD-CHO, and CrmA can inhibit s-Myc- and c-Myc-mediated apoptosis. These findings suggest that death signals triggered by s-Myc and c-Myc expression are transmitted through ICE-family proteases as well as in Fas- and TNFR-mediated apoptosis. Deprivation of growth factors such as IGFs and PDGF can also induce apoptosis in mammalian cells. Reduction of schwannoma-derived growth factor (SDGF) synthesis by the introduction of antisense SDGF mRNAs inhibited the growth of tumor cells but did not induce apoptosis in these cells.⁽⁸¹⁾ Mitochondria are important organelles in regulating apoptosis induction in mammalian cells. Using the H₂O₂-resistant cell line established from Chinese hamster V79 cells, it was found that mitochondria are one of the lethal targets of H₂O₂-induced cell death.⁽⁸²⁾ Mitochondria are also required for regulation of glucose-stimulated insulin secretion in pancreatic beta cells.⁽⁸³⁾ Diseases of the immune system such as AIDS caused by HIV infection as an apoptosis inducer may be alleviated by long term administration of megadose vitamin C infusions.^(84,85)

Transcriptional Regulation of tRNA Genes

Some of the eukaryotic and retroviral mRNAs that produce a protein required for the expression of specific cellular and viral functions contain translatable nonsense codons. For effective translation of these nonsense codons, specific components of the translational machinery such as suppressor tRNA and a unique secondary or ternary structure of mRNAs surrounding the nonsense codons are required. A typical translatable nonsense codon, UAG, has been detected at the gag-pol junction of Moloney murine leukemia virus. Translation of this UAG codon preceded by splicing of viral mRNA at the unique site located in the gag-pol region is necessary for maturation of the viral proteins and for vegetative viral replication.⁽⁸⁶⁾ Transcriptional expression of the mammalian suppressor tRNA gene encoding glutamine tRNA which can translate the UAG codon is under negative control, mediated by binding of an E-box binding nuclear protein. The E-box binding protein has been regarded as a regulator of the genes transcribed by RNA polymerase II. The initiator methionine tRNA gene was isolated from Pyrodictium occultum, a hyperthermophilic archaeum growing at 105ĄC, and characterized.⁽⁸⁷⁾

Three-dimensional Structures of Biological Molecules

Analysis of the three-dimensional (3D) structures of biologically active molecules is crucially important for elucidating and for understanding their biological functions at the molecular level. In addition to NMR and X-ray diffraction methods, the molecular dynamics calculation method is a useful means of analyzing the 3D structures of these molecules. The 3D structures of the thymine dimer produced in DNA by far-UV irradiation and a tripeptide, N-acetyl-Pro-Gly-Phe, that contributes to formation of the b-turn type I form in proteins, were analyzed by the ab initio molecular orbital method.^(88,89)

List of papers from this division

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