Date Published: June 28, 2012
Publisher: Impact Journals LLC
Author(s): Zhiyong Mao, Zhonghe Ke, Vera Gorbunova, Andrei Seluanov.
Most human somatic cells do not divide indefinitely but enter a terminal growth arrest termed replicative senescence. Replicatively senescent cells are generally believed to arrest in G1 or G0 stage of the cell cycle. While doing cell cycle analysis on three different lines of normal human fibroblasts we observed that 36-60% of the replicatively senescent cells had 4N DNA content. Only up to 5% of senescent cells had more than one nucleus ruling out the possibility that the 4N cell population were G1-arrested bi-nucleated cells. Furthermore, it is unlikely that the 4N cells are tetraploids, because actively dividing pre-senescent cultures lacked the 8N tetraploid G2 population. Collectively these results suggest that the 4N population consists of G2 arrested cells. The notion that a large fraction of senescent cell population is arrested in G2 is important for understanding the biology of replicative senescence.
The phenomenon of replicative senescence has been described in 1961 by Hayflick and Moorhead , and has since been actively investigated by biogerontologists. Senescent cells were later found to affect surrounding cells by secreting inflammatory cytokines  and were recently shown to promote age-related pathology in vivo . Replicatively senescent cells are believed to arrest in G1 or G0 stage of the cell cycle. Interestingly, early studies performed in 1970th noted that populations of replicatively senescent cells contained a sizable fraction of cells with 4N DNA content [4-6]. It was not possible at that time to conclusively distinguish G2 cells from tetraploid G1 cells. In some reports it was speculated that these cells were tertaploids arrested in G1 [4, 6]. Later these studies forgotten and G1/G0 arrest in senescent cells became a dogma.
We observed that 60% of the population of replicatively senescent normal human foreskin fibroblasts HCA2 consisted of cells with 4N DNA content (Figure 1A), while the remaining cells were in G1 stage. Intrigued by the result we repeated propidium idodide (PI) staining on replicatively senescent WI-38 and IMR-90 cells, which are the two commonly used normal human fibroblast strains. The senescent WI-38 and IMR-90 cells contained 37% and 39% of cells with 4N DNA content, respectively (Figure 1B).