CRM1 protein-mediated regulation of nuclear clusterin (nCLU), an ionizing radiation-stimulated, Bax-dependent pro-death factor

Expression of the clusterin (CLU) gene results in the synthesis of a conventional secretory isoform set (pre- and mature secretory clusterin proteins, psCLU/sCLU), as well as another set of intracellular isoforms, appearing in the cytoplasm (pre-nuclear CLU, pnCLU) and in the nucleus as an ∼55-kDa mature nuclear clusterin (nCLU) form. These two isoform sets have opposing cell functions: pro-survival and pro-death, respectively. Although much is known about the regulation and function of sCLU as a pro-survival factor, the regulation and function of endogenous nCLU in cell death are relatively unexplored. Here, we show that depletion of endogenous nCLU protein using siRNA specific to its truncated mRNA increased clonogenic survival of ionizing radiation (IR)-exposed cells. nCLU-mediated apoptosis was Bax-dependent, and lethality correlated with accumulation of mature nCLU protein. nCLU accumulation was regulated by CRM1 because binding between CRM1 and nCLU proteins was significantly diminished by leptomycin B (LMB), and nuclear levels of nCLU protein were significantly enhanced by LMB and IR co-treatment. Moreover, LMB treatment significantly enhanced IR-induced nCLU-mediated cell death responses. Importantly, bax(-/-) and bax(-/-)/bak(-/-) double knock-out cells were resistant to nCLU-mediated cell death, whereas bak(-/-) or wild-type bax(+/+)/bak(+/+) cells were hypersensitive. The regulation of nCLU by CRM1 nuclear export/import may explain recent clinical results showing that highly malignant tumors have lost the ability to accumulate nCLU levels, thereby avoiding growth inhibition and cell death.

CHIP E3 ligase regulates mammalian senescence by modulating the levels of oxidized proteins

Senescence can be induced by various stressors including oxidative stress. It has been reported that CHIP (C-terminus of Hsp70-interacting protein) ligase is induced during senescence while CHIP(-/-) mice exhibit accelerated aging. Here, we explore the effects of modulating CHIP expression on mammalian senescence. We demonstrate that CHIP silencing induces premature senescence that is accompanied by elevated levels of oxidized proteins. On the contrary, ectopic expression of CHIP leads to oxidized proteins levels reduction. Moreover, we reveal that CHIP(-/-) mouse fibroblasts have an impaired ubiquitin proteasome system. Taken together, we propose that CHIP influences cellular senescence by modulating the oxidative load.

CHIP-dependent p53 regulation occurs specifically during cellular senescence

p53 regulates several biological processes, including senescence. Its protein stability is regulated by ubiquitination and proteasomal degradation, mainly mediated by Mdm2. However, other E3 ligases have been identified, such as the chaperone-associated ligase CHIP, although their precise function regarding p53 degradation remains elusive. Interestingly, CHIP deficiency has been recently shown to result in accelerated aging in mice, although the molecular basis of this phenotype was not completely understood. In this study, we explore the role of CHIP in regulating p53 in senescence. We demonstrate that in senescent human fibroblasts, CHIP is up-regulated concomitant with a significant down-regulation of p53. Moreover, CHIP partially translocates to the nucleus and acquires higher ubiquitination levels in senescent cells. Notably, CHIP overexpression in young cells, to levels similar to those recorded during senescence, leads to p53 degradation to below its basal levels. In addition, whereas CHIP silencing has no effect on p53 stability in young cells, a considerable p53 accumulation occurs in their senescent counterparts. Finally, we have observed an attenuation of the CHIP-associated molecular folding-refolding machinery during senescence, and supportively, inhibition of Hsp90 activity leads to rapid p53 degradation only in senescent cells. Taking these results together, we conclude that CHIP-dependent p53 regulation occurs specifically during senescence.

Genome-wide transcriptome profile of the human osteosarcoma Sa OS and U-2 OS cell lines

With the use of genome-wide cDNA microarrays, we investigated the transcriptome profile of the human osteosarcoma Sa OS and U-2 OS cell lines. In all, 1,098 chip entries were differentially regulated in the two cell lines; of these, 796 entries corresponded to characterized mRNAs. The identified genes are mostly expressed in epithelial tissues and localize on chromosomes 1, 10, and 20. Furthermore, signaling cascades for cell cycle, glycolysis, and gluconeogenesis, the p53 pathway, cell communication, and focal adhesion were found to be differently regulated in the two cell lines. The transcriptome profiles reported here provide novel information about the considerable molecular differences between these two widely used human osteosarcoma cell lines.

Nuclear erythroid factor 2-mediated proteasome activation delays senescence in human fibroblasts

Replicative senescence in human fibroblasts is accompanied with alterations of various biological processes, including the impaired function of the proteasome. The proteasome is responsible for the removal of both normal and damaged proteins. Due to its latter function, proteasome is also considered a representative secondary antioxidant cellular mechanism. Nrf2 is a basic transcription factor responsible for the regulation of the cellular antioxidant response that has also been shown to regulate several proteasome subunits in mice. We have established in this study the proteasome-related function of Nrf2 in human fibroblasts undergoing replicative senescence. We demonstrate that Nrf2 has a declined function in senescence, whereas its silencing leads to premature senescence. However, upon its activation by a novel Nrf2 inducer, elevated levels of proteasome activity and content are recorded only in cell lines possessing a functional Nrf2. Moreover, treatment by the Nrf2 inducer results in the enhanced survival of cells following oxidative stress, whereas continuous treatment leads to lifespan extension of human fibroblasts. Importantly the Nrf2-proteasome axis is functional in terminally senescent cultures as these cells retain their responsiveness to the Nrf2 stimuli. In conclusion, these findings open up new directions for future manipulation of the senescence phenotype.

The proteasome is an integral part of solar ultraviolet a radiation-induced gene expression

Solar ultraviolet (UV) A radiation is a well known trigger of signaling responses in human skin fibroblasts. One important consequence of this stress response is the increased expression of matrix metalloproteinase-1 (MMP-1), which causes extracellular protein degradation and thereby contributes to photoaging of human skin. In the present study we identify the proteasome as an integral part of the UVA-induced, intracellular signaling cascade in human dermal fibroblasts. UVA-induced singlet oxygen formation was accompanied by protein oxidation, the cross-linking of oxidized proteins, and an inhibition of the proteasomal system. This proteasomal inhibition subsequently led to an accumulation of c-Jun and phosphorylated c-Jun and activation of activator protein-1, i.e. transcription factors known to control MMP-1 expression. Increased transcription factor activation was also observed if the proteasome was inhibited by cross-linked proteins or lactacystin, indicating a general mechanism. Most importantly, inhibition of the proteasome was of functional relevance for UVA-induced MMP-1 expression, because overexpression of the proteasome or the protein repair enzyme methionine sulfoxide reductase prevented the UVA-induced induction of MMP-1. These studies show that an environmentally relevant stimulus can trigger a signaling pathway, which links intracellular and extracellular protein degradation. They also identify the proteasome as an integral part of the UVA stress response.

Vanadium-induced apoptosis of HaCaT cells is mediated by c-fos and involves nuclear accumulation of clusterin

Vanadium exerts a variety of biological effects, including antiproliferative responses through activation of the respective signaling pathways and the generation of reactive oxygen species. As epidermal cells are exposed to environmental insults, human keratinocytes (HaCaT) were used to investigate the mechanism of the antiproliferative effects of vanadyl(IV) sulfate (VOSO(4)). Treatment of HaCaT cells with VOSO(4) inhibited proliferation and induced apoptosis in a dose-dependent manner. Inhibition of proliferation was associated with downregulation of cyclins D1 and E, E2F1, and the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). Induction of apoptosis correlated with upregulation of the c-fos oncoprotein, changes in the expression of clusterin (CLU), an altered ratio of antiapoptotic to proapoptotic Bcl-2 protein family members, and poly(ADP-ribose) polymerase-1 cleavage. Forced overexpression of c-fos induced apoptosis in HaCaT cells that correlated with secretory CLU downregulation and upregulation of nuclear CLU (nCLU), a pro-death protein. Overexpression of Bcl-2 protected HaCaT cells from vanadium-induced apoptosis, whereas secretory CLU overexpression offered no cytoprotection. In contrast, nCLU sensitized HaCaT cells to apoptosis. Our data suggest that vanadium-mediated apoptosis was promoted by c-fos, leading to alterations in CLU isoform processing and induction of the pro-death nCLU protein.

Transcriptional and posttranslational regulation of clusterin by the two main cellular proteolytic pathways

Clusterin/apolipoprotein J (CLU) is a secreted glycoprotein associated with many severe physiological disturbances that represent states of increased oxidative stress, such as aging, cancer, atherosclerosis, diabetes, and renal and neurodegenerative diseases. The aim of our work was to examine the effect of proteasome and lysosome inhibition on CLU expression and to determine whether those proteolytic pathways are implicated in CLU gene regulation and protein degradation. To this end we used two different model systems, namely the U-2 OS osteosarcoma cell line and the WI38 primary human embryonic lung fibroblasts. We report that proteasome inhibition promotes both heat-shock factor 1 (HSF-1)-dependent CLU gene expression induction and protein accumulation due to reduced degradation. In contrast, lysosome inhibition results in elevated levels of CLU protein but does not affect the CLU mRNA levels. We also provide direct evidence that both the intracellular precursor, psCLU, and the mature secreted, sCLU, isoforms constitute proteolytic substrates of the proteasome and the lysosome. Overall our findings indicate that CLU overexpression after proteasome inhibition relates to both positive gene transcriptional regulation by HSF-1 and posttranslational protein accumulation due to reduced proteasomal and lysosomal degradation.

Intracellular clusterin inhibits mitochondrial apoptosis by suppressing p53-activating stress signals and stabilizing the cytosolic Ku70-Bax protein complex

PURPOSE

Secretory clusterin (sCLU)/apolipoprotein J is an extracellular chaperone that has been functionally implicated in DNA repair, cell cycle regulation, apoptotic cell death, and tumorigenesis. It exerts a prosurvival function against most therapeutic treatments for cancer and is currently an antisense target in clinical trials for tumor therapy. However, the molecular mechanisms underlying its function remained largely unknown.

EXPERIMENTAL DESIGN

The molecular effects of small interfering RNA-mediated sCLU depletion in nonstressed human cancer cells were examined by focusing entirely on the endogenously expressed sCLU protein molecules and combining molecular, biochemical, and microscopic approaches.

RESULTS

We report here that sCLU depletion in nonstressed human cancer cells signals stress that induces p53-dependent growth retardation and high rates of endogenous apoptosis. We discovered that increased apoptosis in sCLU-depleted cells correlates to altered ratios of proapoptotic to antiapoptotic Bcl-2 protein family members, is amplified by p53, and is executed by mitochondrial dysfunction. sCLU depletion-related stress signals originate from several sites, because sCLU is an integral component of not only the secretory pathway but also the nucleocytosolic continuum and mitochondria. In the cytoplasm, sCLU depletion disrupts the Ku70-Bax complex and triggers Bax activation and relocation to mitochondria. We show that sCLU binds and thereby stabilizes the Ku70-Bax protein complex serving as a cytosol retention factor for Bax.

CONCLUSIONS

We suggest that elevated sCLU levels may enhance tumorigenesis by interfering with Bax proapoptotic activities and contribute to one of the major characteristics of cancer cells, that is, resistance to apoptosis.

Proteasome activation as a novel antiaging strategy

Homeostasis is a key feature of cellular lifespan. Maintenance of cellular homeostasis influences the rate of aging and is determined by several factors, including efficient proteolysis of damaged proteins. Protein degradation is predominantly catalyzed by the proteasome. Specifically, the proteasome is responsible for cell clearance of abnormal, denatured or in general damaged proteins as well as for the regulated degradation of short-lived proteins. As proteasome has an impaired function during aging, emphasis has been given recently in identifying ways of its activation. A number of studies have shown that the proteasome can be activated by genetic manipulations as well as by factors that affect its conformation and stability. Importantly the developed proteasome activated cell lines exhibit an extended lifespan. This review article discusses in details the various factors that are involved in proteasome biosynthesis and assembly and how they contribute to its activation. Finally as few natural compounds have been identified having proteasome activation properties, we discuss the advantages of this novel antiaging strategy.

Partial proteasome inhibition in human fibroblasts triggers accelerated M1 senescence or M2 crisis depending on p53 and Rb status

Proteasome-dependent degradation has been extensively investigated and has been shown to play a vital role in the maintenance of cellular homeostasis. Proteasome activity and expression are reduced during aging and replicative senescence. Its activation has been shown to confer lifespan extension in human diploid fibroblasts (HDFs), whereas partial proteasome inhibition triggers an irreversible premature senescent state in young HDFs. As p53 and Rb tumor suppressors regulate both replicative and premature senescence (RS and PS, respectively), in this study we investigated their implication in proteasome inhibition-mediated PS. By taking advantage of a variety of HDFs with defective p53 or/and Rb pathways, we reveal that proteasome activity inhibition to levels normally found in senescent human cells results in immediate growth arrest and/or moderate increase of apoptotic death. These effects are independent of the cellular genetic context. However, in the long term, proteasome inhibition-mediated PS can only be initiated and maintained in the presence of functional p53. More specifically, we demonstrate that following partial proteasome inhibition, senescence is dominant in HDFs with functional p53 and Rb molecules, crisis/death is induced in cells with high p53 levels and defective Rb pathway, whereas stress recovery and restoration of normal cycling occurs in cells that lack functional p53. These data reveal the continuous interplay between the integrity of proteasome function, senescence and cell survival.

The olive constituent oleuropein exhibits proteasome stimulatory properties in vitro and confers life span extension of human embryonic fibroblasts

Normal human fibroblasts undergo replicative senescence due to both genetic and environmental factors. Senescence and aging can be further accelerated by exposure of cells to a variety of oxidative agents that contribute among other effects to the accumulation of damaged proteins. The proteasome, a multicatalytic nonlysosomal protease, has impaired function during aging, while its increased expression delays senescence in human fibroblasts. The aim of this study was to identify natural compounds that enhance proteasome activity and exhibit antiaging properties. We demonstrate that oleuropein, the major constituent of Olea europea leaf extract, olive oil and olives, enhances the proteasome activities in vitro stronger than other known chemical activators, possibly through conformational changes of the proteasome. Moreover, continuous treatment of early passage human embryonic fibroblasts with oleuropein decreases the intracellular levels of reactive oxygen species (ROS), reduces the amount of oxidized proteins through increased proteasome-mediated degradation rates and retains proteasome function during replicative senescence. Importantly, oleuropein-treated cultures exhibit a delay in the appearance of senescence morphology and their life span is extended by approximately 15%. In summary, these data demonstrate the beneficial effect of oleuropein on human fibroblasts undergoing replicative senescence and provide new insights towards enhancement of cellular antioxidant mechanisms by natural compounds that can be easily up-taken through normal diet.

Overexpression of hUMP1/POMP proteasome accessory protein enhances proteasome-mediated antioxidant defence

The proteasome is the major cellular proteolytic machinery. It is involved in the regulation of various pathways via the selective degradation of either short-lived normal proteins or damaged proteins permitting the cellular detoxification. Proteasome has impaired function during several biological processes, including aging and diseases; however, it can be activated through overexpression of beta(5)- or beta(1)-subunits, resulting to enhanced survival and extended lifespan. In the current study, we have investigated proteasomal up-regulation via overexpression of hUMP1/POMP protein, the known accessory factor for proteasome assembly in humans. hUMP1/POMP overexpressing fibroblasts have increased levels of functional proteasome and enhanced capacity to cope better and faster with various oxidative stressors. These data highlight hUMP1/POMP role in proteasome assembly and further strengthen the prospect of genetic manipulation of the proteasomal system.

Clusterin/Apolipoprotein J up-regulation after zinc exposure, replicative senescence or differentiation of human haematopoietic cells

Clusterin/Apolipoprotein J (CLU) is a cellular senescence biomarker implicated in several physiological processes. In this work we have investigated CLU expression and function in human haematopoietic cells. We found that early passage human T cell clones (TCC) express minimal endogenous amounts of CLU, which are significantly elevated in late passage cells. Moreover, exposure of TCC to increased levels of the essential micronutrient zinc in culture resulted in intense induction of CLU. Because haematopoietic cells cease proliferation following induction of terminal differentiation, we also studied the expression profile of CLU in the leukemic progenitor cell lines K562 and HL-60. We found that, like TCC, both cell lines express minimal endogenous levels of CLU in their actively proliferating state. However, when induced to differentiate into their distinct cell types, CLU was found to be up-regulated specifically in those cells expressing the main differentiation markers. Enforced stable over-expression of CLU in K562 cells inhibited the expression of the CD14 differentiation marker and blocked differentiation to either monocytes/megacaryoblasts or to erythrocytes. Overall, our results suggest that CLU is actively involved in both replicative senescence and terminal differentiation in different types of human haematopoietic cells.

Development of resistance to chemotherapeutic drugs in human osteosarcoma cell lines largely depends on up-regulation of Clusterin/Apolipoprotein J

Clusterin/Apolipoprotein J (CLU) is differentially regulated during in vivo cancer progression. We have addressed the role of CLU during the acquisition and maintenance of human cancer cells resistance to chemotherapeutic drugs. We used two osteosarcoma (OS) cell lines, namely U-2 OS and KH OS, and selected three generations of doxorubicin (DXR)-resistant cells (R1, R2 and R3; resistant to 0.0035, 0.035 and 0.35 microM DXR, respectively) by continuous exposure to increasing, clinically relevant, DXR concentrations. Our studies showed that the DXR-resistant OS cell lines were cross-resistant to a variety of unrelated cytotoxic agents. Analysis of the CLU mRNA and protein expression levels revealed a minimal CLU up-regulation in the U-2 OS R2 cells and a significant, more than 4-fold, induction in the KH OS R2 and R3 cells. Antibody-mediated neutralization of the extracellular CLU, or silencing of CLU gene expression via small interfering RNA (siRNA) partially sensitized KH OS R2 cells to the drugs assayed. Moreover, siRNA-mediated CLU knock down in the absence of DXR induced high levels of endogenous spontaneous apoptosis in both the parental and R2 OS cell lines. This effect was enhanced by more than 60% in the KH OS R2 cells as compared to their parental counterparts, indicating that the high CLU levels in the KH OS R2 cells are essential for survival. Overall, we suggest that CLU up-regulation in the multi-drug resistant OS cells relates to enhanced drug resistance. Therefore, CLU may represent a predictive marker, which correlates to response of cancer cells to chemotherapy.

Regulation of clusterin/apolipoprotein J, a functional homologue to the small heat shock proteins, by oxidative stress in ageing and age-related diseases

Clusterin/apolipoprotein J (CLU) gene has a nearly ubiquitous expression pattern in human tissues. The two main CLU protein isoforms in human cells include the conventional glycosylated secreted heterodimer (sCLU) and a truncated nuclear form (nCLU). CLU has been implicated in various physiological processes and in many severe physiological disturbance states including ageing, cancer progression, vascular damage, diabetes, kidney and neuron degeneration. Although unrelated in their etiology and clinical manifestation, these diseases represent states of increased oxidative stress, which in turn, promotes amorphous aggregation of target proteins, increased genomic instability and high rates of cellular death. Among the various properties attributed to CLU so far, those mostly investigated and invariably appreciated are its small heat shock proteins-like chaperone activity and its involvement in cell death regulation, which are both directly correlated to the main features of oxidant injury. Moreover, the presence of both a heat shock transcription factor-1 and an activator protein-1 element in the CLU gene promoter indicate that CLU gene can be an extremely sensitive biosensor to reactive oxygen species. This review emphasizes on CLU gene regulation by oxidative stress that is the common link between all pathological conditions where CLU has been implicated.

Reduced expression levels of the senescence biomarker clusterin/apolipoprotein j in lymphocytes from healthy centenarians

Clusterin/apolipoprotein J (CLU) is a conserved, ubiquitously expressed secreted glycoprotein that has been implicated in several physiological processes and was found to accumulate in many severe physiological disturbances. We have previously shown that the CLU gene and protein are upregulated during replicative senescence, stress-induced premature senescence, in vivo aging, and in several age-related diseases. In this study we have examined the CLU gene relationship to human longevity. We recruited and further analyzed 96 blood samples from Italian and Greek healthy donors of different ages, including 49 centenarians. We found that although the CLU gene expression levels increase during aging, in the centenarians' samples CLU levels were lower than those found in old donors. We then investigated the possible existence of a genetic polymorphism related to longevity at the CLU structural locus. A neutral noncoding sequence variant was detected 35 nucleotides upstream from exon 6, which does not correlate, however, with the age of the donor. We conclude that CLU gene accumulation during in vivo aging does not directly relate to chronological age, but rather indicates increased levels of organismal stress due to a progressive failure of homeostasis and/or to prolonged exposure to a stressful environment.

Proteasome response to interferon-gamma is altered in senescent human fibroblasts

We have investigated immunoproteasomes in human fibroblasts during replicative senescence. Unlike levels of constitutive proteasome catalytic subunits and 26S proteasome regulatory subunits, levels of immunosubunits did not decrease dramatically in senescent cells. However, the induction of immunosubunits by interferon-gamma (IFN-gamma) was lost in senescent cells. In contrast, levels of the 11S proteasome regulator, PA28, were increased by IFN-gamma even in senescent cells, and both immunosubunits and PA28 increased with the reversible growth arrest in confluent cell cultures. The results highlight differences in the mechanisms of regulation of immunoproteasomes compared to constitutive proteasomes and in the irreversible growth arrest of senescent cells compared to reversible contact-induced growth arrest.

The Genetics of Human Longevity

Aging is due to a complex interaction of genetic, epigenetic, and environmental factors, but a strong genetic component appears to have an impact on survival to extreme ages. In order to identify "longevity genes" in humans, different strategies are now available. In our laboratory, we performed association studies on a variety of "candidate" polymorphisms in Italian centenarians. Many genes/polymorphisms gave negative results, while others showed a positive association with human longevity and a sometimes-positive association with unsuccessful aging (myocardial infarction, Alzheimer's disease, and type 2 diabetes). Results regarding genes involved in inflammation (IL-1 cluster, IL-6, IL-10, TNF-alpha, TGF-beta, TLR-4, PPARgamma), insulin/IGF-1 signaling pathway and lipid metabolism (apolipoproteins, CETP, PON1), and oxidative stress (p53, p66(shc)) will be described. In addition, a strong role of the interaction between nuclear and mitochondrial genomes (mtDNA haplogroups and the C150T mutation) emerged from our findings. Thus, the genetics of human longevity appears to be quite peculiar in a context where antagonistic pleiotropy can play a major role and genes can have a different biological role at different ages.

Comparative effects of hypoxia on normal and immortalized human diploid fibroblasts

Hypoxia, a condition of reduced oxygen concentration, is observed in many physiological and pathophysiological states. However, there is rather limited information regarding hypoxic effects in the processes of immortalization and senescence of human cells. Here, the effects of hypoxia induced by either 1.5% O2, or by a hypoxia-mimetic agent, CoCl2, on the protein expression of normal human diploid fibroblasts (HDFs) undergoing replicative senescence, and in their Simian Virus 40 (SV40) T antigen immortalized counterparts are described. The data demonstrated that, in all cell types assayed, either hypoxia or CoCl2 induced the main regulator of transcriptional responses to reduced oxygen tension, namely the hypoxia inducible factor-1alpha (HIF-1alpha), in a dose- and time-dependent manner. In the immortalized HDFs, the transcriptional activity of HIF-1alpha was also evident by the accumulation of its main downstream gene targets, namely erythropoietin (EPO) and the vascular endothelial growth factor (VEGF). Interestingly, the immortalized HDFs were found to exhibit higher HIF-1alpha endogenous levels and induction, following cell exposure to hypoxic conditions, as compared to either young or senescent cells. Subsequent analysis of the expression levels of two pro-survival proteins, bcl-2 and clusterin/apolipoprotein J (CLU), in cells exposed to hypoxic conditions, revealed that although bcl-2 was up-regulated independently of the cell type, CLU was induced only in the CoCl2-treated immortalized HDFs. These findings indicate that the distinct cellular contexts of normal and immortalized HDFs may induce differential responses to hypoxia.

Proteasome dysfunction in mammalian aging: Steps and factors involved

Mammalian aging is a natural biological process, determined by both genetic and environmental/stochastic factors, that results in the gradual decline of physiological function and the eventual failure of organism homeostasis. The proteasome is one of the major proteolytic systems of mammalian cells. It is responsible for the degradation of normal proteins as well as of abnormal proteins (like misfolded and oxidized proteins) that tend to accumulate during aging. Impaired proteasome function has been tightly correlated with aging both in vivo and in vitro. Given the fundamental function of proteasome for retaining cellular homeostasis, this review article examines the steps and the factors involved in proteasome dysfunction during mammalian aging. We discuss the proteasome structural organization, its activities and biosynthesis during aging and senescence as well as the genetic and environmental causes of its age-dependent alterations. Finally, we provide insights on the possibilities of proteasome activation that may retard the appearance of the senescent phenotype.

Bcl-2 but not clusterin/apolipoprotein J protected human diploid fibroblasts and immortalized keratinocytes from ceramide-induced apoptosis: role of p53 in the ceramide response

The role of clusterin/apolipoprotein J (Clu/ApoJ) and Bcl-2 on C(2)-ceramide-induced apoptosis of embryonic human diploid fibroblasts, MRC-5 and immortalized adult skin keratinocytes, HaCaT was investigated. C(2)-ceramide-induced apoptosis of HaCaT in a time- and dose-dependent manner, while in MRC-5 only at higher concentrations. There was a dose-dependent accumulation of Clu/ApoJ and downregulation of Bcl-2 which correlated with C(2)-ceramide-induced apoptosis of MRC-5. While overexpression of Bcl-2 suppressed C(2)-ceramide-mediated apoptosis in both cell types, Clu/ApoJ failed to do so, accessed by morphological changes, DNA fragmentation and PARP cleavage. There was no change in the expression of endogenous p53 or p21(Waf1/Cip1) upon C(2)-ceramide treatment of MRC-5. However, mutant p53(143ala) increased the sensitivity of MRC-5 to C(2)-ceramide-induced apoptosis by markedly downregulating Bcl-2, pointing to a role for p53. These results suggested that whereas downregulation of Bcl-2 may be a crucial factor involved in C(2)-ceramide-induced apoptosis, accumulation of Clu/ApoJ may be a signal of stress response. Moreover, the ceramide-activated apoptotic pathway may be regulated by p53.

Identification of differentially expressed proteins in senescent human embryonic fibroblasts

Normal human fibroblasts undergo a limited number of divisions in culture, a process known as replicative senescence (RS). Although several senescence-specific genes have been identified, analysis at the level of protein expression can provide additional insights into the mechanisms that regulate RS. We have performed a proteomic comparison between young and replicative senescent human embryonic WI-38 fibroblasts and we have identified 13 proteins, which are differentially expressed in senescent cells. Some of the identified proteins are components of the cellular cytoskeleton, while others are implicated in key cellular functions including metabolism and energy production, Ca(2+) signalling, nucleo-cytoplasmic trafficking and telomerase activity regulation. In summary, our analysis contributes to the list of senescence-associated proteins by identifying new biomarkers and provides novel information on functional protein networks that are perturbed during replicative senescence of human fibroblast cultures.