SAITO Yoshinori Asahikawa Medical College, Pathology, Assistant, 医学部, 助手 (70241429)
LEE Gang-Hong Asahikawa Medical College, Pathology, Assistant Professor, 医学部, 助教授 (10261405)
Cell cycle checkpoint was investigated after DNA damage in normal, immortal and malignantly-transformed hepatocytes, and following points were clarified.
1. Many immortal hepatocyte clones were isolated from hepatocyte colonies developing during primary cultures of mouse hepatocytes. By introducing the mutationally-activated H-ras gene to these cells, many malignantly-transformed cell lines were also produced. Using these cells as well as primary hepatocyte cultures, it became possible to compare cell cycle checkpoint in normal, immortal and malignant cells carrying the same background.
2. When normal hepatocytes were exposed to DNA damaging agents, cell growth was completely inhibited, and cell cycle was arrested at G1 phase. During this period, half life of p53 was markedly extended, and p53 was overexpressed in the nuclei. Suppression of p53 protein using p53 antisense oligonucleotide resulted in abrogation of G1 arrest.
3. Although normal and immortal cells showed G1 arrest after UV damage, it was much less evident in H-rastransformed cells.
4. There was no p53 mutations in any of malignant cells, and overexpression of p53 occurred after UV damage. These observations indicate that downstream of the p53 pathway may be knockout by H-ras.
5. Malignant cells show much greater variation of chromosomal numbers than immortal cells, suggesting that loss of cell cycle checkpoint can result in chromosomal instability.