Programmed cell death (apoptosis) is an active process by which cells initiate their own self-destruction. Growing evidence shows that this event is controlled by the activation of unique gene expressions; some function as survival genes, such as bcl2, and others as killer genes, such as ced3 or interleukin converting enzyme. Likewise, external factors, such as the presence or absence of stimuli in the microenvironment of a cell, play a key role in ushering it towards survival or suicidal fate. Previously, I and others have reported that withdrawal of serum from culture medium can induce contact-inhibited quiescent mouse 3T3 fibroblasts to undergo rapid programmed cell death, as evidenced by the presence of massive DNA fragmentation within 24 h. I now report that, although the same process of serum withdrawal is capable of inducing apoptotic death in quiescent young human fibroblasts, the process takes as long as 2 weeks. Repeated attempts at the same serum withdrawal with cultures of senescent human fibroblasts show that phenotypic signs of apoptosis, such as DNA fragmentation and loss of cell viability, are not observed for up to 4 weeks; I suggest that in vitro aged human fibroblasts are resistant to undergoing programmed cell death. I have investigated the level of bcl2 presence as a possible protector of senescent human fibroblasts from apoptotic death; biochemical characterization shows that in mouse as well as human fibroblasts, bcl2 is present as an easily extractable (0.1% Triton) cytoplasmic protein. bcl2 level is in inverse relationship with the ease of induction of apoptotic death between young and senescent human fibroblasts. Immunofluorescence staining shows that, in senescent human fibroblasts, bcl2 is present not only in the cytoplasmic punctate spots seen in both mouse and young human fibroblasts but also in the nuclei as well as large granules surrounding the nuclei. Upon serum deprivation, the bcl2 level is reduced to undetectable in mouse 3T3 fibroblasts within 24 h and in young and intermediate aged human fibroblasts within 2 weeks; however, it remains unchanged in senescent human fibroblasts after the deprivation of serum for 2 weeks. These findings lead me to conclude that senescent fibroblasts are resistant to the induction of apoptotic death by serum deprivation. Furthermore, I suggest that repeated serial passaging during the in vitro aging process has inadvertently instituted a molecular mechanism whereby the bcl2 level cannot be repressed upon serum deprivation, which may subsequently allow senescent fibroblasts to be long-lived and protected from self-destruction.
This work was supported by Grant R01 AG09278 (to E. W.) from the National Institute on Aging of the NIH.