The need for an effective biomarker of senescent cells

An effective biomarker of cellular senescence is required so senescent cells can be visualised both in vitro and in vivo, allowing their frequency and distribution to be monitored in ageing and diseased tissues.

At present, the most commonly used method to detect senescent cells is a modified beta-galactosidase assay (Dimri et al, 1995). Detectable β-galactosidase at pH 6 was found to increase during replicative senescence of fibroblast cultures in vitro and in vivo and was absent in immortal cell cultures. This was termed senescent-associated β-galactosidase or SA-β-Gal. However, since this first report, there have been numerous studies that have demonstrated SA-β-Gal staining in non-senescent cells.

For example, it has been reported that SA-β-Gal activity is detectable in quiescent cultures of Swiss 3T3 as well as some types of human cancer cells that were chemically stimulated to differentiate (Yegorov et al, 1998). After 21 days in culture, Swiss 3T3 cells in low serum displayed 40-50% SA-β-Gal positive cells and cells treated to differentiate after 13 days displayed as high as 75% staining. Another study looked at the expression of SA-β-Gal in human ovarian surface epithelial cells (HOSE 6-3) undergoing immortalisation by the human papilloma viral oncogene E6 and E7 (Litaker et al, 1998). They found that HOSE 6-3 cells expressing SA-β-Gal was highest (39%) when cells were at crisis. After this stage when cells achieved immortalisation status SA-β-Gal activity sharply decreased (1.3%).

Severino et al (2000) specifically focused on determining the robustness of SA-β-Gal activity as a marker of replicative senescence . This study characterised changes in SA-β-Gal staining in a variety of different conditions. SA-β-Gal activity was found to be elevated in confluent non-transformed fibroblast cultures, in immortal fibroblast cultures that had reached a high cell density and in low-density young, normal cultures oxidatively challenged by treatment with H2O2. They concluded that although SA-β-Gal staining is increased under a variety of different conditions, the interpretation of increased staining remains unclear.

SA-β-Gal staining has also been shown to be a marker for differentiation of human prostate epithelial cells (HPEC) (Untergasser et al, 2003). HPEC cells stimulated with transforming growth factor beta (TGF-β), resulted in an increase in SA-β-Gal activity but showed no terminal growth arrest nor induction of important senescent-associated genes such as p16. It was therefore suggested that TGF-β could contribute to the increased number of SA-β-Gal positive epithelial cells observed in benign prostatic hyperplasia (BPH).

A recent report demonstrated that fibroblasts from patients with autosomal recessive G(M1)-gangliosidosis, which have defective lysosomal beta-galactosidase did not express SA-β-Gal at late passage even though they underwent replicative senescence (Lee et al, 2006). It was also demonstrated that cells depleted of GLB1 (the gene encoding lysosomal beta-D-galactosidase) mRNA underwent senescence but failed to express SA-β-Gal. SA-β-Gal activity is therefore dependent upon lysosomal mass rather than growth state. If this is indeed the case, SA-β-GAL staining would most likely underestimate the percentage of senescent cells in a sample.

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The main focus of ageing research is to prevent/combat age-related disease and disability, allowing everyone to live healthier lives for longer.