Proliferative and metabolic capacity of rat embryo fibroblasts immortalized by c-myc depends on cellular age at oncogenic transfection.

It is well known that secondary rat embryo fibroblasts are immortalized and transformed with respect to requirements of growth factors by transfection with an overexpressed c-myc protooncogene. On the other hand, c-myc expression of nontransformed cells was shown to be independent of cellular age in vitro. In order to elucidate further the role of the c-myc protooncogene in the process of aging of rat embryo fibroblasts, we have transfected these cells at low (< or = 2) and at high (> or = 16) cumulative population doublings with SV40-promoter/enhancer-driven murine c-myc. These cells transformed young or aged, as well as their nontransformed, young and aged controls, were characterized with respect to their expression of c-myc at the mRNA and the protein level. Furthermore, we have investigated in detail their cell density-dependent growth, dependence of cell proliferation on stimulation by combinations of growth factors, and dependence of entry into cell cycle on cell size. In addition, we have measured rates of synthesis and degradation of cellular RNA and protein. The main result was that cells transformed at old age cannot be distinguished from nontransformed old cells by any of the characteristics investigated except by their immortalization. Thus, the cell lines overexpressing c-myc are immortalized and fixed in the proliferative/metabolic state achieved at the time of transfection. It is concluded that the intracellular effects of c-myc depend on the epigenetic status of the cells.