Rodent embryo cells immortalized with temperature-sensitive mutants of simian virus 40 large tumor (T) antigen have a proliferative potential that depends on temperature. At the restrictive temperature, heat-inactivation oflarge T antigen causes p53 release, growth arrest, and cell death.Morphological and molecular analysis indicate that the induced cell death corresponds to apoptosis. Flow cytometric analysis using a combination of forward light scatter and side scatter allows a discrimination of cells committed to apoptosis within the whole population. These cells display a reduction in cell size and a higher cellular density, confirming the apoptotic nature of the cell death. When cells exhibiting the morphological features of apoptosis were stained with a fluorescent probe of the mitochondrial membrane potential, a decreased accumulation of the dye was recorded. Measures of cellular respiration, performed with whole-cell populations, showed that the lower mitochondrial membrane potential (At,) correlates, as expected, with an uncoupling of electron transport from ATP production and is linked to the induction of apoptosis. We also show that this decrease in A'I is associated with a decrease in the rate ofmitochondrial translation. These events are detected at early stages of the apoptotic process, when most of the cells are not irreversibly committed to death, suggesting that mitochondria could be a primary target during apoptosis.Mammalian cells grown in culture exhibit a finite life-span for proliferation. After a variable number of divisions they stop dividing, undergo a variety of changes, and finally die. Some oncogenes have the ability to confer an unlimited proliferative potential (immortality) to primary cells in culture. In the case of polyomavirus-induced immortalization of rodent embryo fibroblasts, it has been shown that the unlimited proliferative capacity is maintained by the large tumor (T) antigen (1)(2)(3)(4)
MATERIALS AND METHODSCell Lines and Cell Culture. The REtsAF and RELPB cell lines were isolated at low cell density from a rat embryo fibroblast culture infected with SV40 (2). REtsAF was obtained by using a temperature-sensitive tsA58 mutant and is temperature sensitive for immortalization, whereas RELPB was obtained with wild-type SV40 and is immortal at both 330 and 39.50C. REtsAF-Revl was derived from REtsAF by selection for proliferation at 39.50C (8