Replicative senescence at the molecular level in vitro

Cell cycle independent senescence (CCIS)

In contrast to the TDS and TIS mechanisms, cell cycle independent senescence (CCIS) does not seem to require cell division. A number of signals have been shown to trigger CCIS, including oxidative stress (Von Zglinicki et al, 2000), histone deacetylase inhibitors (Munro et al, 2004), and expression of some activated oncogenes such as ras, raf and MEK (Di Micco et al, 2006). This section focuses on oxidative stress and activated oncogenes.

The proposal that oxidative stress may cause CCIS comes from studies that have treated cells with concentrations of H2O2 and found that those cells enter a long-term growth arrest resembling replicative senescence; often called stress induced premature senescence or SIPS. One such study treated human fibroblasts with sub-lethal concentrations of hydrogen peroxide which induced cell cycle arrest, with an initial 2-3 fold increase in the level of p53 protein and subsequently an increase in the level of p21 protein (Chen et al, 1998). One study has shown that the up-regulation of Caveolin-1 by oxidative stress is required to induce premature senescence (Volonte et al, 2002). Caveolin-1 is thought to function as a “transformation suppressor” protein. For example Caveolin-1 mRNA and protein expression are lost or reduced during cell transformations by activated oncogenes.

Oxidative stress is thought to result in damage to DNA and DNA mutations. The most severe type of DNA damage is double-strand breaks (DSBs). Studies have shown that cells treated with agents that cause DSBs lead to an increase in p16 expression and premature senescence (Robles and Adami, 1998). Induction of DNA damage resulted in the induction of p53 and p21. The concentration of p21 protein increases 100-fold and then begins to drop, followed by an increase in p16. This is similar to what occurs during replicative senescence.

Oxidative stress is also thought to result in DNA becoming mutated and this may impact oncogenes such as RAS. Studies have shown that mutated oncogenic Ras can result in a permanent G1 arrest (Serrano et al, 1997). Oncogenic Ras is commonly associated with the transformation of primary cells to an immortal state. It seems, however, that this transformation can only occur with either the co-operation of another oncogene or the inactivation of tumour suppressers such as p53 or p16. Serrano and co-workers also found that Ras induced cell senescence is accompanied by the accumulation of p53 and p16 and that the inactivation of either one of these prevents Ras induced arrest in rodent cells. More recent studies have also shown that Ras can induce cell senescence in vascular smooth muscle cells (Minamino et al, 2003) and endothelial cells (Spyridopoulos et al, 2002). In the first study, an activated ras allele was introduced into human vascular smooth muscle cells using retroviral infection. This resulted in growth arrest with phenotypic characteristics of cell senescence. In the second study, bovine aortic endothelial cells were infected using the adenovirus containing the activated ras gene. Over-expression of ras in this case led to G1 and G2/M-cell cycle arrest after 72 hours due to induction of p21. p21 induction again appears to be the initiating factor of Ras induced senescence and induction of p16 is required to maintain the senescent state. One group for example found that Ras was capable of causing growth arrest in both p21 and p53 negative human fibroblasts, suggesting Ras is activating the p16 pathway (Wei et al, 2001).

An overview of telomere-dependent, telomere-dependent and cell-cycle independent senescence is shown below.