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),vith 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 tumorigeni c in nude mice, although they became tumorigenic after extended passag ing in culture (p. > 30), Differential mRNA display analyses of normal cells (84-3) and radiation-initiated cell lines at early passage show ed that the tatter contained increased steady-state levels of the prec ursor (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 irradi ated (HDR) cell lines, and were stably maintained during continuous pa ssaging (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 ampl ification of the mt DNA, Radiation-initiated cells contained elevated levels of the CO I protein, showed a 75% reduction in cytochrome c oxi dase (GO) activity, and a 5-fold increase in the concentration of hydr ogen peroxide secreted into their culture medium compared with cells w ith no alterations in CO I mRNA processing, Our findings suggest that alterations in mt CO I processing may play a role in the neoplastic co nversion of mammalian cells by ionizing radiation.