The maximum life span of animals is proportional to a maximum population doubling (PD) number of animal fibroblasts. Human fibroblasts have about 50–70 PDs, while mouse fibroblasts have only 8–10 PDs. Although telomere shortening is one of the best candidates to explain the replicative senescence, this cannot explain why mouse fibroblasts have shorter replicative potential than human fibroblasts even though mouse cells have longer telomere and telomerase activity. We prepared four mammalian embryonic fibroblasts – human embryonic fibroblasts (HEFs), dog embryonic fibroblasts (DEFs), rat embryonic fibroblasts (REFs), and mouse embryonic fibroblasts (MEFs) –and then used to compare the immortalization frequency, p53/p21/p16 expression levels, p53 binding activity, telomere length, and telomerase activity. Immortalization frequency of HEFs by SV40 large T antigen was the highest followed by REF and DEFs. HEFs showed almost no immortalization frequency. Protein levels of p21 and p16 were increased, and DNA-binding activity of p53 was increased during replicative senescence. On the other hand, p21 protein level was decreased and p16 was significantly increased in all immortalized cells. Embryonic fibroblasts from mammals of shorter life span have longer telomere and stronger telomerase activity. These results indicate that telomere length and telomerase activity are related with immortalization frequency but not replicative senescence in mammalian fibroblasts, except human fibroblasts. Only human fibroblasts showed replicative senescence-related telomere shortening.
- mammalian embryonic fibroblasts
- replicative senescence