CELLULAR AGING, DESTABILIZATION AND CANCER

Three major characteristics of aging in animals are a slowdown of cell proliferation, an increase in residual bodies associated with age pig ments, and a marked increase in the likelihood of neoplastic transform ation. The 28 L subline of the NIH 3T3 line of mouse embryo fibroblast s exhibits all these characteristics when held at confluence for exten ded periods. The impairment of proliferation is the first behavioral c haracteristic detected in low density subcultures from the confluent c ultures, and it persists through many cell generations of exponential multiplication. There is an equal degree of growth impairment among re plicate cultures (lineages) recovered after each of 2 successive round s of confluence, although heterogeneity appears after the third round. The growth impairment pervades the entire cell population of each lin eage. The degree and duration of impairment increase with repeated rou nds of confluence. A marked increase of residual bodies characteristic of age pigments occurs in the cytoplasm of all the cells kept under p rolonged confluence. Neoplastic transformation first appears as foci o f multilayered cells on a monolayered background of nontransformed cel ls. The transformed cells arise at different times in the lineages and originate from a very small fraction of the population. The transform ed cells selectively overgrow the entire population in successive roun ds of confluence leading to an increase in saturation density of each lineage at different times. Under cloning conditions, isolated colonie s of transformed cells develop more slowly than colonies of nontransfo rmed cells but eventually reach a higher population density. The regul arity of persistent growth impairment among the lineages and the appea rance of large numbers of residual bodies in all the cells of each pop ulation are more characteristic of an epigenetic process than of speci fic local mutations, although random chromosomal lesions cannot be rul ed out. By contrast, the low frequency and stochastic character of neo plastic transformation are consistent with a conventional genetic orig in. The advent in long-term confluent NIH 3T3 cultures of three cardin al characteristics of cellular aging in vivo recommends it as a model for aging cells.