Senescence and immortalization of human cells.

Following a limited number of population doublings (PD), human diploid somatic cells enter the terminal proliferation arrest state of senescence. This is an intrinsic mechanism which involves p53- and pRB/p16INK4-mediated pathways. The most popular candidate for the counting mechanism which measures the age of a cell in PD is telomere shortening. Recent studies have shown that senescence can also be induced independently of a PD level by various factors; this premature senescence also appears to involve the activity of p53 and/or p16INK4. Immortalization of cells requires abrogation of p53 and pRB-mediated terminal proliferation arrest and/or activation of a telomere maintenance mechanism. The central role of telomeres in human cell senescence and immortalization has received much attention; however there is evidence that senescence can occur independently of telomere length and that genes that are not necessarily involved in telomere maintenance are involved in immortalization.