Mitochondrial biogenesis in human fibroblasts

Abstract

It is not known whether limitation of lifespan represents a programmed genetic event or is a result of environmental factors imposed by the conditions of culture. An investigation of the factors surrounding the limitedin vitro lifespan of human diploid fibroblasts has been undertaken. We have investigated the role of mitochondria in the finite lifespan of WI-38 human lung fibroblasts. Mitochondrial function was depressed in a controlled manner by treating cells with ethidium bromide and chloramphenicol both of which inhibit normal biogenesis. These antibiotics decrease cytochrome oxidase activity, change cell ultrastructure, and inhibit growth at high concentration. At lower concentrations the antibiotics do not affect cell proliferation for several generations. However, their effect is cumulative and after several generations the cells enlarge, stop dividing and die. Removal of antibiotics from the culture media before death restores proliferative capacity. At still lower concentrations cytochrome oxidase activity was decreased but continuous growth in the presence of the antibiotics caused no decrease inin vitro lifespan. Thus, the potential for oxidative metabolism appears to be in excess of that needed for cell proliferation at all stages of thein vitro lifespan of a culture. The importance of cytoplasmic protein synthesis was evaluated using cycloheximide, a specific inhibitor of this process. Cycloheximide was used to try to distinguish between the effects due to general inhibition and that due to specific inhibition of mitochondrial biogenesis. Exposure of cultures to concentrations of cycloheximide which inhibited growth drastically caused no decrease in cytochrome oxidase activity.