Altered processing of precursor transcripts and increased levels of the subunit I of mitochondrial cytochrome c oxidase in Syrian hamster fetal cells initiated with ionizing radiation.
Treatment of Syrian hamster fetal cells (SHFC) with ionizing radiation resulted in the establishment of 21 transformed cell lines. Relative to unirradiated controls, cells from early post-irradiation passages (p.3) showed marked morphologic alterations, increased growth rate and extended life span, and they were contact-inhibited and not tumorigenic in nude mice, although they became tumorigenic after extended passaging in culture (p. > 30). Differential mRNA display analyses of normal cells (84-3) and radiation-initiated cell lines at early passage showed that the latter contained increased steady-state levels of the precursor (4-fold) and mature (1.7-fold) transcripts of the mitochondrial (mt) gene encoding the subunit I of cytochrome c oxidase (CO I). These molecular alterations were consistently observed in 57% of the irradiated (HDR) cell lines, and were stably maintained during continuous passaging (p. > 50). Further analyses of one of these cell lines (HDR-3) demonstrated that the accumulation of CO I precursor transcripts was the result of mRNA stabilization and increased replication and/or amplification of the mt DNA. Radiation-initiated cells contained elevated levels of the CO I protein, showed a 75% reduction in cytochrome c oxidase (CO) activity, and a 5-fold increase in the concentration of hydrogen peroxide secreted into their culture medium compared with cells with no alterations in CO I mRNA processing. Our findings suggest that alterations in mt CO I processing may play a role in the neoplastic conversion of mammalian cells by ionizing radiation.