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Keywords:

  • aging;
  • cellular senescence;
  • gene expression;
  • GLB1;
  • HeLa cells;
  • lysosomes;
  • molecular biology of aging;
  • senescence

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

Replicative senescence limits the proliferation of somatic cells passaged in culture and may reflect cellular aging in vivo. The most widely used biomarker for senescent and aging cells is senescence-associated β-galactosidase (SA-β-gal), which is defined as β-galactosidase activity detectable at pH 6.0 in senescent cells, but the origin of SA-β-gal and its cellular roles in senescence are not known. We demonstrate here that SA-β-gal activity is expressed from GLB1, the gene encoding lysosomal β-D-galactosidase, the activity of which is typically measured at acidic pH 4.5. Fibroblasts from patients with autosomal recessive GM1-gangliosidosis, which have defective lysosomal β-galactosidase, did not express SA-β-gal at late passages even though they underwent replicative senescence. In addition, late passage normal fibroblasts expressing small-hairpin interfering RNA that depleted GLB1 mRNA underwent senescence but failed to express SA-β-gal. GLB1 mRNA depletion also prevented expression of SA-β-gal activity in HeLa cervical carcinoma cells induced to enter a senescent state by repression of their endogenous human papillomavirus E7 oncogene. SA-β-gal induction during senescence was due at least in part to increased expression of the lysosomal β-galactosidase protein. These results also indicate that SA-β-gal is not required for senescence.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

Normal somatic cells proliferate for a limited number of doublings in culture and then enter an irreversible growth-arrested stage called replicative senescence (Campisi, 2005). Key phenotypes of senescence include enlarged and flat cell morphology, β-galactosidase activity detectable at pH 6.0 [defined as senescence-associated β-galactosidase (SA-β-gal) activity] (Dimri et al., 1995), and high-level autofluorescence due to lipofuscin accumulation (von Ziglinicki et al., 1995). Levels of p53, p21WAF1, and p16INK4a often gradually increase as fibroblasts approach senescence (Dimri et al., 1996), and senescent cells produce high levels of reactive oxygen species (ROS) and contain elevated levels of oxidative DNA damage (Chen et al., 1995; Song et al., 2005). Permanent growth arrest that displays many of the features of replicative senescence can also be acutely induced by a variety of manipulations including expression of activated oncogenes (Serrano et al., 1997; Zhu et al., 1998), exposure to sublethal levels of DNA damaging agents or oxidative stress (Chen et al., 1995; Shay & Roninson, 2004), and introduction or activation of tumor suppressor genes (Bringold & Serrano, 2000; Hwang, 2002). Cellular senescence is viewed as a model for certain aspects of cellular and organismal aging (Campisi, 2005), but the relationship between senescence and aging remains to be established. In addition, several recent studies strongly suggest that senescence plays a role in tumor suppression (Braig, 2005; Campisi, 2005; Collado, 2005; Michaloglou, 2005).

SA-β-gal is a β-galactosidase activity detectable at pH 6.0 in cultured cells undergoing replicative or induced senescence but absent from proliferating cells (Dimri et al., 1995). SA-β-gal activity is typically measured by in situ staining using a chromogenic substrate such as X-gal. Since it was first reported, SA-β-gal activity has been the most extensively utilized biomarker for senescence because of the simplicity of the assay method and its apparent specificity for senescent cells. It is not clear whether a process analogous to senescence occurs during aging in animals. SA-β-gal activity has been detected in organs of old individuals and animals, suggesting that cellular senescence is a feature of organismal aging and that senescent cells accumulate with age in tissue (e.g. Dimri et al., 1995; Mishima et al., 1999; Pendergrass et al., 1999; Sigal et al., 1999; Melk et al., 2003). In some cases, the identification of cells as senescent rests solely on the detection of SA-β-gal activity. However, despite the widespread reliance on SA-β-gal as a marker of senescence, the origin of SA-β-gal activity has not been conclusively determined, and its role in senescence, if any, is unknown. In fact, there are a number of reports that β-galactosidase activity at pH 6.0 can be detected in cells in various nonsenescent states, such as extended incubation at high density, and there are conflicting reports regarding the presence of SA-β-gal activity in aged tissues (e.g. Yegorov et al., 1998; Krishna et al., 1999; Kurz et al., 2000; Severino et al., 2000; Untergasser et al., 2003). Thus, the suitability of SA-β-gal as a marker of senescence has been challenged (Krishna et al., 1999; Severino et al., 2000; Coates, 2002; Cristofalo, 2005; Yang & Hu, 2005).

It is essential to establish the origin of SA-β-gal activity to determine the basis for its induction during senescence and its suitability as a marker that unambiguously identifies senescent cells. One candidate protein that may underlie SA-β-gal activity is a well-characterized β-D-galactosidase (EC 3.3.1.23) that is localized to lysosomes of mammalian cells. Consistent with localization in this acidic organelle, lysosomal β-galactosidase displays maximal activity between pH 4.0 and 4.5 but markedly lower activity at pH 6.0 (Zhang et al., 1994). Indeed, β-galactosidase activity is not detectable in proliferating cells by in situ staining with X-gal at pH 6.0, the conditions used to detect SA-β-gal activity, even though lysosomal β-galactosidase activity is readily detectible in these cells at acidic pH. Nevertheless, based on indirect physiological experiments, it has been proposed that increased lysosomal-β-galactosidase activity in senescent cells accounts for SA-β-gal activity (Kurz et al., 2000). Specifically, the levels of total cellular β-galactosidase activity is higher in late-passage compared to early-passage cells at pH 4.5 as well as at pH 6.0, and maximal β-galactosidase activity is measurable at low pH in both early- and late-passage cells (Ferland et al., 1990; Kurz et al., 2000; Gerland et al., 2003; Gary & Kindell, 2005; Yang & Hu, 2005). Furthermore, the number and size of lysosomes increase in cells at late passage (Robbins et al., 1970; Brunk et al., 1973). These results suggest that lysosomal β-galactosidase activity increases in senescent cells due to increased lysosome content, surpassing a threshold level so that it is detectable at the suboptimal pH 6.0 (Kurz et al., 2000; Gary & Kindell, 2005). However, no experiments have been reported that directly test the hypothesis that SA-β-gal activity is due to increased amounts or activity of the lysosomal-β-galactosidase protein in senescent cells, and not due to increased activity of another enzyme capable of catalyzing the hydrolysis of terminal β-D-galactose residues from β-galactosides. In this paper, we provide compelling genetic evidence that SA-β-gal activity is in fact encoded by the lysosomal-β-galactosidase gene (designated GLB1) and that levels of lysosomal-β-galactosidase protein increase during senescence. In addition, we demonstrate that SA-β-gal activity is not required for senescence.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

Low lysosomal β-galactosidase activity in GM1-gangliosidosis fibroblasts with inactivating mutations in the GLB1 gene

SKa and JNa primary fibroblasts were established from skin biopsies of GM1-gangliosidosis patients. Patients with this autosomal recessive disease lack lysosomal β-galactosidase (EC 3.2.1.23) activity (Dacremont & Kint, 1968; Norden & O’Brien, 1975). DNA sequencing of the lysosomal β-galactosidase gene (designated GLB1) revealed the existence of a homozygous arg201cys mutation in SKa cells and a homozygous arg59his mutation in JNa cells. Both mutations are known to result in the loss of lysosomal β-galactosidase activity and GM1-gangliosidosis (Silva et al., 1993; Oshima et al., 1994; Caciotti et al., 2005). As expected, extracts of cultured SKa and JNa fibroblasts contained low levels of β-galactosidase activity measured at pH 4.5 as compared to the normal fibroblasts (Fig. 1A). Furthermore, when SKa and JNa fibroblasts were incubated in the presence of X-gal at pH 4.5, no cells stained positively, whereas virtually all of the normal fibroblasts stained intensely (data not shown).

image

Figure 1. Characterization of GM1-gangliosidosis fibroblasts. (A) 105 early-passage normal, JNa, and SKa fibroblasts were lysed in 100 µL of 0.1 m citrate buffer (pH 4.5). Equal volumes of extract were assayed in solution for acidic β-galactosidase activity (pH 4.5) using ONPG as substrate. Each bar shows the average of three assays with the cells prepared at either PD23 (black bars) or PD26 (gray bars). (B) p21WAF1 levels in normal, SKa, and JNa cells. Extracts (20 µg protein) from cells collected either at early passage (E) (PD18 for the normal and SKa cells, and PD24 for JNa) or late passage (L) (PD74, PD58, and PD56 for the normal, SKa, and JNa cells, respectively) were subjected to Western blotting for p21WAF1 and Erk-1/2. (C) Early- and late-passage fibroblasts were mounted and examined by fluorescence (bottom panels) and phase contrast microscopy (top panels). NHF, normal human fibroblasts.

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Replicative senescence in GM1-gangliosidosis fibroblasts

Proliferation of the SKa and JNa fibroblasts reached a Hayflick limit (Hayflick & Moorhead, 1961) after comparable numbers of population doublings (PD) in cell culture. As judged by the lack of a significant increase in cell number during a 20-day culture period, the control normal fibroblasts proliferated to PD75, whereas the SKa and JNa fibroblasts proliferated to approximately PD59 and PD58, respectively (data not shown). (The PD number for the GM1-gangliosidosis cells is an underestimate because PD numbers at some early passages were not recorded). The acquisition of a senescent state in these late-passage cells was confirmed by several criteria in addition to growth retardation. In senescent normal fibroblasts, the levels of p53 and p21WAF1 or p16INK4a are high, the p105Rb retinoblastoma protein is primarily hypophosphorylated, and cellular levels of autofluorescence and ROS increase. At late passages, the normal and GM1-gangliosidosis cell strains displayed elevated levels of p53 and p21WAF1 as well as hypophosphorylated Rb (Fig. 1B shows the status of p21WAF1 as a representative example). All three cell strains also displayed increased cell size, flattened morphology, autofluorescence, and ROS at the late passages but not at the early passages (Fig. 1C and data not shown). Similar changes in all three cell strains at early passage were also elicited by adriamycin treatment, which induces a state of senescence in normal fibroblasts (B.Y.L. and E.S.H., unpublished result). Taken together, these results indicate that senescence could be imposed on the GM1-gangliosidosis fibroblasts either by prolonged replication in culture or by chemical stress.

Absence of SA-β-gal activity in GM1-gangliosidosis fibroblasts

The senescent SKa and JNa fibroblasts were analyzed for SA-β-gal activity in situ by incubation with X-gal at pH 6.0. Although the great majority of normal cells stained positively for SA-β-gal activity at late passage, none of the late-passage SKa or JNa cells stained (Fig. 2A). Enzymatic activity assays in cell extracts also demonstrated very low β-galactosidase activity at pH 6.0 in senescent GM1-gangliosidosis fibroblasts, whereas the late-passage normal cells displayed high level SA-β-gal activity (approximately sixfold higher than the level in early-passage cells) (Fig. 2B). Similarly, adriamycin-treated normal fibroblasts, but not GM1-gangiosidosis fibroblasts, displayed in situ SA-β-gal activity (data not shown). Therefore, SA-β-gal activity is absent or markedly reduced in GM1-gangliosidosis fibroblasts, which are devoid of active lysosomal β-galactosidase due to inactivating mutations in the GLB1 gene, strongly suggesting that lysosomal β-galactosidase is the origin of SA β-gal activity.

image

Figure 2. Low SA-β-gal activity in GM1-gangliosidosis fibroblasts. (A) Fibroblasts at either early or late passage (at PD indicated in legend of Fig. 1B) were subjected to in situ SA-β-gal staining at pH 6, and examined by bright field microscopy. (B) 105 normal (black bar), SKa (grey bar), and JNa (white bar) cells were collected at either early or late passage as in panel A and lysed in 100 µL of 0.1 m phosphate buffer (pH 6.0), and equal volumes were assayed in solution for β-galactosidase activity at pH 6.0.

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Decreased SA-β-gal activity in late-passage fibroblasts expressing GLB1 interfering RNA

We used an independent approach to confirm the origin of SA-β-gal activity. Pre-senescent (PD62) normal fibroblasts were infected in parallel with retroviruses that expressed two different small hairpin interfering RNAs (shRNA) targeting GLB1 mRNA, along with the gene for puromycin-resistance, or with a control retrovirus. Following puromycin selection, pooled cell lines were established and analyzed. Real-time RT-PCR demonstrated that the level of GLB1 mRNA decreased by 7.3- and 5.6-fold, respectively, in two independent lines that express shRNA targeting either of two different sites in the GLB1 mRNA (data not shown). Repression of lysosomal β-galactosidase expression was confirmed by Western blotting (see Fig. 4, lane 5). The level of GLB1 mRNA was not affected in control cells that express Photinus luciferase shRNA (shLUC). In both GLB1-knockdown fibroblast lines, β-galactosidase activity measured at pH 4.5 in solution was reduced to less than 20% of that in the cells expressing luciferase shRNA or the parent cells at similar passage (Fig. 3A), and fewer than 10% of GLB1-knockdown fibroblasts positively stained in situ for acidic β-galactosidase at pH 4.5 (data not shown). Cells expressing GLB1 shRNA and control cells were passaged until they ceased proliferation and assumed a senescent morphology (at estimated PD73), at which time they were analyzed for in situ SA-β-gal activity at pH 6.0 (Fig. 3B and Table 1). As expected, many cells in the population of both the parental and the control shLUC lines positively stained. Strikingly, only rare GLB1-knockdown cells stained for SA-β-gal activity at pH 6.0, and the intensity of staining was in general lower in the rare positive cells (e.g. shGLB#2, Fig. 3B). In addition, β-galactosidase activity measured in solution at pH 6.0 was approximately 16.7% of the activity of the parent normal human fibroblasts (data not shown). Thus, GLB1 mRNA suppression markedly inhibited SA-β-gal activity in senescent fibroblasts.

image

Figure 4. Biochemical analysis of lysosomal β-galactosidase expression. 4.5 x 105 normal human fibroblasts at two early passages (PD18 and 20) and two late passages (PD72 and 73) as well as NHF-shGLB#2 at PD73 (lane 5) were lysed in 80 µL of RIPA buffer. Either equal mass of extracted protein (10 µg) (lanes 1–5) or equal volumes of extract (4 × 104 cell equivalent) (lanes 6–9) were subjected to Western blotting for human lysosomal β-galactosidase (top panel) or Erk-1/2 protein (bottom panel). The arrow indicates the lysosomal β-galactosidase band at approximately 64 kDa.

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image

Figure 3. Analysis of normal fibroblasts and HeLa cells expressing GLB1 shRNA. (A) and (B) Normal fibroblasts were infected with retroviruses expressing shRNA targeting the GLB1 mRNA (shGLB#1 and shGLB#2) or with a control retrovirus (shLUC), as indicated. Pooled puromycin-resistant cells were subjected to an assay for β-galactosidase in solution at pH 4.5 (A) or to in situ staining for β-galactosidase at pH 6.0 (B). Normal late-passage fibroblasts were stained as controls. (C) HeLa or HeLa-shGLB#2 cells were either mock infected or infected with the E2 virus, incubated for 10 days, and subjected to in situ staining for β-galactosidase at pH 6.0.

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Table 1.  Percentage of cells positive for SA-β-gal*
VectorNHF (PD73)HeLa
(–E2)(+E2)
  • *

    Two or more different fields with 50 or more cells were photographed, and number of cells stained blue with X-gal at pH 6.0 was divided by the total cell number.

  • n.d., not determined.

Empty54.0 ± 8.42.5 ± 1.080.7 ± 5.9
ShGLB#2 5.1 ± 1.46.5 ± 1.6 4.6 ± 1.2
ShLUC49.6 ± 11.9n.d.n.d.

Decreased SA-β-gal activity in senescent HeLa cells expressing GLB1 interfering RNA

HeLa cervical carcinoma cells can be induced to enter a senescent state by transduction of the bovine papillomavirus E2 gene, which suppresses HPV18 oncogene expression and activates tumor suppressor pathways in these cells (Hwang et al., 1993; Goodwin et al., 2000; Wells et al., 2000). After expression of the E2 protein, cells rapidly and irreversibly arrest in G1 phase, and nearly 100% of them express a senescent phenotype, including elevated SA-β-gal activity. HeLa cells were modified to express decreased amounts of GLB1 mRNA by infecting with the shGLB-retroviruses described above. In cells expressing shGLB#2, GLB1 mRNA level was 8.1-fold lower than that in the mock-treated cells as determined by real-time RT-PCR (data not shown). The GLB1-knockdown and control HeLa cells were infected with a virus that expressed the E2 protein, incubated for 10 days, and assayed for the SA-β-gal activity in situ. The parental HeLa cells infected with the E2 virus expressed the expected senescent phenotype including growth arrest, increased cell size and flattening, elevated autofluorescence and high level SA-β-gal activity (Fig. 3C, left panels, and data not shown). The GLB1-knockdown HeLa cells treated in parallel with the E2 gene also underwent growth arrest and displayed increased cell size and flattening and elevated autofluorescence (Fig. 3C, right panels, and data not shown). Strikingly, on average < 5% of infected HeLa cells expressing the shGLB#2 RNA stained positively for SA-β-gal activity in situ, whereas 80% of the parental cells were positive (Fig. 3C and Table 1). Similar results were obtained in an independent population of HeLa cells generated by using the shGLB#1 virus (data not shown). β-Galactosidase activity measured in solution at pH 6.0 showed a progressive increase with increasing time after E2-infection of the parental cells, with little activity present in the GLB1-knockdown cells (data not shown). Thus, SA-β-gal activity in cancer cells induced to enter senescence also requires expression of the GLB1 gene. Taken together, these results indicate that SA-β-gal activity originates from the GLB1 gene in both replicative and induced senescence and that this activity is not required for senescence.

Increased lysosomal β-galactosidase protein in fibroblasts undergoing senescence

Although a protein that cross-reacts with an antibody raised to Escherichia coliβ-galactosidase undergoes a progressive increase in senescing cells (Kurz et al., 2000), the levels of lysosomal β-galactosidase protein have not been directly measured during senescence. To determine whether increased levels of lysosomal β-galactosidase protein accompanied the appearance of SA-β-gal activity in senescent cells, we conducted Western blotting with an antibody specific for human lysosomal β-galactosidase (Zhang et al., 1994). Two samples each of early-passage and late-passage normal fibroblasts were prepared and normalized for either total protein content (Fig. 4, lanes 1–5) or cell number (Fig. 4, lanes 6–9). The 85 kDa precursor and the ∼64 kDa mature form of the lysosomal β-galactosidase are readily detectable in early- and late-passage fibroblasts, except in late-passage cells expressing the GLB1 shRNA (lane 5), as expected. Strikingly, there was an approximately sixfold increase in the amount of the 65 kDa protein band on a per cell basis in late-passage compared to early-passage cells (compare lanes 6 and 7 to 8 and 9). Correspondingly, β-galactosidase activity in solution increased approximately six- to sevenfold per cell in late-passage fibroblasts undergoing senescence compared to early-passage cells when the assay was conducted at either pH 4.5 or 6.0, although the absolute level of activity was higher at pH 4.5 in both early- and late-passage cells (data not shown). Thus, increased SA-β-gal activity in senescent fibroblasts is due at least in part to increased levels of lysosomal β-galactosidase. Senescent cells are larger than early-passage cells, and approximately 2.5-fold more protein or RNA is isolated from equal numbers of late-passage compared to early-passage cells. Thus, if lysosomal β-galactosidase protein (Fig. 4, lanes 1–4) or SA-β-gal activity (data not shown) is expressed after normalization for total protein content (rather than on a per-cell basis), the increase in the senescent cells is approximately two- to threefold.

Levels of GLB1 RNA were measured by quantitative real-time RT-PCR. This analysis revealed a 1.6-fold increase in the GLB1 mRNA in late-passage fibroblasts normalized to the levels of γ-actin mRNA (average of independent measurements of three different sets of RNA samples, data not shown). On a per-cell basis, this corresponds to an approximately fourfold increase in GLB1 mRNA in senescent cells.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

Since described in 1995 (Dimri et al., 1995), in situ SA-β-gal activity has been widely used as a biomarker of senescence. Although several lines of indirect evidence suggested that SA-β-gal may be due to increased lysosomal β-galactosidase in senescent cells, the origin of SA-β-gal activity and the basis for induction has not been unequivocally established. In the present study, we provide two compelling pieces of genetic evidence that lysosomal β-galactosidase is the origin of SA-β-gal activity. First, senescent human GM1-gangliosidosis fibroblasts lacking lysosomal β-galactosidase due to homozygous inactivating mutations in the GLB1 gene fail to express SA-β-gal activity; second, SA-β-gal activity is greatly reduced in senescent cells in which the concentration of GLB1 mRNA is reduced by RNA interference. Our results, together with published results from other laboratories (Kurz et al., 2000; Gerland et al., 2003; Gary & Kindell, 2005; Yang & Hu, 2005), demonstrate that lysosomal β-galactosidase activity increases enough in senescent cells to be detected at suboptimal pH 6.0, constituting SA-β-Gal activity. This increase appears to be due at least in part to the accumulation of increased levels of GLB1 mRNA and protein. However, the extent of the senescence-induced increase in lysosomal β-galactosidase measured by Western blotting or soluble enzymatic activity appears much less than the induction of SA-β-gal observed by in situ staining with X-gal. Thus, in addition to increased amounts of lysosomal β-galactosidase in senescent cells, functional differences in senescent lysosomes or other factors may contribute to the very high levels of β-galactosidase activity as assessed by in situ staining at pH 6.

Our finding that lysosomal β-galactosidase is the source of SA-β-gal activity in senescent cells indicates that SA-β-gal is not a specific marker of senescence per se, but rather a surrogate marker for increased lysosome number or activity, which has long been associated with replicative senescence and organismal aging (Robbins et al., 1970; Brunk et al., 1973; Turk & Milo, 1974; Cristofalo & Kabakjian, 1975; Knook et al., 1975; Bosmann et al., 1976; Sanchez-Martin & Cabezas, 1997; Gerland et al., 2003). This is consistent with increased SA-β-gal activity in a number of nonsenescent situations and suggests that other conditions characterized by increased lysosomal content will also display elevated β-galactosidase activity at pH 6.0, and, conversely, that other lysosomal proteins are likely to increase during senescence and may also serve as senescence markers. Hence, SA-β-gal activity cannot stand alone to define a senescent state. Rather, senescence is best defined as the appearance of a constellation of features, only one of which is β-galactosidase activity at pH 6. The identification of GLB1 as the gene encoding SA-β-gal will allow a systematic evaluation of the factors that control its expression in a variety of growth states, and thereby improve its use as a marker of senescence.

Our results highlight two additional points regarding senescence. First, GLB1 is essential for expression of SA-β-gal in both replicative senescence and induced senescence, indicating at least partial overlap in the programs that lead to these senescence states. Second, the GM1-gangliosidosis cells undergo senescence in response to serial passage or adriamycin treatment, and GLB1 mRNA knockdown did not interfere with senescence, even though it caused a substantial reduction in SA-β-gal activity. Thus, SA-β-gal activity is not required for senescence, and the increase in β-galactosidase activity in senescent cells is an outcome rather than a cause of senescence.

Experimental procedures

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

Cell culture

Normal primary fibroblasts were isolated from healthy newborn foreskins and provided by Dr Sang Chul Park (Seoul National University, Korea) or the Yale Skin Diseases Research Center. GM1-gangliosidosis fibroblasts (SKa and JNa cells) were isolated from skin biopsies of two patients clinically diagnosed as affected by GM1-gangliosidosis (patients 89RD0051 and 90RD0450, respectively). The DNA sequence of the GLB1 gene from these patients was determined as described (Caciotti et al., 2005). Fibroblast cultures were passaged in low glucose (5.5 mm) DMEM containing 10% FBS at 1 : 4 ratio until the Hayflick limit (Hayflick & Moorhead, 1961) was reached. The number of population doublings (PD) was calculated using the equation, PD = log2F/I, where F and I are the numbers of cells at the end and those seeded at the beginning of one passage, respectively. To induce a state of senescence in early-passage fibroblasts (PD18 for normal and SKa cells, and PD24 for JNa cells), cells were treated with 0.5 µm adriamycin for 4 h and then incubated in its absence for 5 days.

Senescence induction in HeLa cells

Senescence was induced in HeLa/E6.5K cells (DeFilippis et al., 2003; referred to here as HeLa cells) as previously described by infection at multiplicity of infection of 20 with an SV40-recombinant virus (Pava1, designated here the E2 virus) expressing the bovine papillomavirus E2 protein (Hwang et al., 1993; Goodwin et al., 2000). This treatment represses the human papillomavirus (HPV) type 18 E7 gene, resulting in the acquisition of a number of markers of senescence, including growth arrest and an acute increase in SA-β-gal activity (Goodwin et al., 2000; Kang et al., 2004).

Detection of autofluorescence

Cells grown on glass cover slips were washed with phosphate-buffered saline (PBS), mounted in Gel/Mount (Biomeda, Foster City, CA, USA), and observed by fluorescence microscopy with an excitation filter (450–480 nm) and emission filter (520 nm) at ×200 magnification. Autofluorescence in HeLa cells was measured by fluorescence activated cell sorting as described (Goodwin et al., 2000).

Western blotting analysis

Cells were lysed in RIPA buffer [50 mm Tris-HCl (pH 7.5), 150 mm NaCl, 1% Nonidet P-40, 0.5% sodium deoxycholate, 0.1% SDS] supplemented with protease inhibitor mixture (Sigma-Aldrich, St. Louis, MO, USA), NaF, and NaVO4. Twenty µg protein were subjected to gel electrophophoresis as described (DeFilippis et al., 2003) and analyzed by Western blotting using antip53 (DO-1; Calbiochem, San Diego, CA, USA), antip21WAF1 (C-19; Santa Cruz Biotechnology, Santa Cruz, CA, USA), anti-Rb (BD Biosciences, Franklin Lakes, NJ, USA), or anti-ERK 1/2 antibody. To detect lysosomal β-galactosidase protein, lysate from an equal number of cells or 20 µg of extracted protein were electrophoresed and probed with a human lysosomal β-galactosidase specific polyclonal antiplacental β-galactosidase IgG, kindly provided by John Callahan (Hospital for Sick Children, Toronto, ON, Canada) (Zhang et al., 1994). Protein bands were quantitated by using the ImageQuant 5.2 program (Amersham Biosciences, Piscataway, NJ, USA).

In situ staining for β-galactosidase activity

Cultured cells were washed in PBS (pH 7.4), fixed with 3.7% formaldehyde, and incubated overnight at 37 °C in freshly prepared staining buffer [1 mg mL−1 X-gal (5-bromo-4-chloro-3-indolyl β-D-galactoside), 5 mm K3Fe[CN]6, 5 mm K4Fe [CN]6, and 2 mm MgCl2 in PBS, pH 6.0, or in citrate-buffered saline, pH 4.5]. At the end of the incubation, cells were washed with H2O and examined at ×200 magnification.

Soluble β-galactosidase assay

Equal numbers of cells were collected, washed and resuspended in either 0.1 m citrate (pH 4.5) or phosphate buffer (pH 6.0). Cells were lysed by freeze/thaw. The lysates were centrifuged at 12 000 g for 7 min. The supernatants were mixed with 2-nitrophenyl-β-D-galactopyranoside (ONPG) (Sigma) (2.2 µg µL−1), 1 mm MgCl2 in either the citrate or phosphate buffer. After incubation at 37 °C for 12 h, two volumes of 1 m sodium carbonate were added and absorbance at 420 nm was measured.

Knockdown of lysosomal β-galactosidase RNA

To generate retroviral vectors that express small hairpin RNA (shRNA) uniquely targeting the human GLB1 mRNA (M34423), oligodeoxyribonucleotides containing 21-base target sequences (sh#1: 5′-ATGTAGCGAAATGGCTGGCCA-3′; sh#2: 5′-AAGTGTTGTCCGGTACAGCAC-3′) were annealed and ligated into pSIREN-RetroQ (BD Biosciences, San Jose, CA, USA) 3′ to the human U6 promoter. The presence of the correct inserts was confirmed by sequencing. A control vector that expresses shRNA targeting Photinus luciferase (RV-shLUC) (BD Biosciences) was constructed in parallel. Retroviruses were packaged in 293T cells and concentrated. Pre-senescent normal fibroblasts (PD62) and HeLa cells were infected with these viruses individually and selected for 3–14 days for resistance to 0.4 µg mL−1 puromycin and then pooled. The resistant pools were maintained by continuous passage in 0.2 µg mL−1 puromycin. RNA was isolated using RNeasy kit (QIAGEN Sciences, Germantown, MD, USA) and converted to cDNA using iScript cDNA Synthesis kit (Bio-Rad, Hercules, CA, USA) by following manufacturer's protocols. The protocol used to quantitate GLB1 and γ-actin mRNA by single-color quantitative Real-Time PCR (Bio-Rad) is available from the authors upon request.

Acknowledgments

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References

We thank John Callahan for the gift of antilysosomal β-galactosidase antibody, Kristin Yates for valuable discussions, and Jan Zulkeski for assistance with manuscript preparation. K.J. was supported in part by an MSTP grant to Yale University. This work was supported by a grant to E.S.H. from Korea Science and Engineering Foundation (KOSEF) through the Center for Aging and Apoptosis Research (R11-2002-097-07002-0) and a grant to D.D. from the National Cancer Institute (CA16038).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgments
  8. References
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