Agonist-specific differential changes of cellular signal transduction pathways in senescent human diploid fibroblasts

AMP-activated protein kinase-dependent nuclear localization of glyceraldehyde 3-phosphate dehydrogenase in senescent human diploid fibroblasts

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme that participates in various cellular events, such as DNA repair and apoptosis. The functional diversity of GAPDH depends on its intracellular localization. Because AMP-activated protein kinase (AMPK) regulates the nuclear translocation of GAPDH in young cells and AMPK activity significantly increases during aging, we investigated whether altered AMPK activity is involved in the nuclear localization of GAPDH in senescent cells. Age-dependent nuclear translocation of GAPDH was confirmed by confocal laser scanning microscopy in human diploid fibroblasts (HDFs) and by immunohistochemical analysis in aged rat skin cells. Senescence-induced nuclear localization was reversed by lysophosphatidic acid but not by platelet-derived growth factor. The extracellular matrix from young cells also induced the nuclear export of GAPDH in senescent HDFs. An activator of AMPK, 5-Aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), increased the level of nuclear GAPDH, whereas an inhibitor of AMPK, Compound C, decreased the level of nuclear GAPDH in senescent HDFs. Transfection with AMPKα siRNA prevented nuclear translocation of GAPDH in senescent HDFs. The stimulatory effect of AICAR and serum depletion on GAPDH nuclear translocation was reduced in AMPKα1/α2-knockout mouse embryonic fibroblasts. Overall, increased AMPK activity may play a role in the senescence-associated nuclear translocation of GAPDH.

HMGB1 and Caveolin-1 related to RPE cell senescence in age-related macular degeneration

Accumulation of lipofuscin in the retinal pigment epithelium (RPE) is considered a major cause of RPE dysfunction and senescence in age-related macular degeneration (AMD), and N-retinylidene-N-retinylethanolamine (A2E) is the main fluorophore identified in lipofuscin from aged human eyes. Here, human-induced pluripotent stem cell (iPSC)-RPE was generated from healthy individuals to reveal proteomic changes associated with A2E-related RPE cell senescence. A novel RPE cell senescence-related protein, high-mobility group box 1 (HMGB1), was identified based on proteomic mass spectrometry measurements on iPSC-RPE with A2E treatment. Furthermore, HMGB1 upregulated Caveolin-1, which also was related RPE cell senescence. To investigate whether changes in HMGB1 and Caveolin-1 expression under A2E exposure contribute to RPE cell senescence, human ARPE-19 cells were stimulated with A2E; expression of HMGB1, Caveolin-1, tight junction proteins and senescent phenotypes were verified. HMGB1 inhibition alleviated A2E induced cell senescence. Migration of RPE cells was evaluated. Notably, A2E less than or equal to 10μM induced both HMGB1 and Caveolin-1 protein upregulation and HMGB1 translocation, while Caveolin-1 expression was downregulated when there was more than 10μM A2E. Our data indicate that A2E-induced upregulation of HMGB1、Caveolin-1 and HMGB1 release may relate to RPE cell senescence and play a role in the pathogenesis of AMD.

G Protein-Coupled Receptor Systems as Crucial Regulators of DNA Damage Response Processes

G protein-coupled receptors (GPCRs) and their associated proteins represent one of the most diverse cellular signaling systems involved in both physiological and pathophysiological processes. Aging represents perhaps the most complex biological process in humans and involves a progressive degradation of systemic integrity and physiological resilience. This is in part mediated by age-related aberrations in energy metabolism, mitochondrial function, protein folding and sorting, inflammatory activity and genomic stability. Indeed, an increased rate of unrepaired DNA damage is considered to be one of the 'hallmarks' of aging. Over the last two decades our appreciation of the complexity of GPCR signaling systems has expanded their functional signaling repertoire. One such example of this is the incipient role of GPCRs and GPCR-interacting proteins in DNA damage and repair mechanisms. Emerging data now suggest that GPCRs could function as stress sensors for intracellular damage, e.g., oxidative stress. Given this role of GPCRs in the DNA damage response process, coupled to the effective history of drug targeting of these receptors, this suggests that one important future activity of GPCR therapeutics is the rational control of DNA damage repair systems.

Survive or thrive: tradeoff strategy for cellular senescence

Aging-dependent cellular behaviors toward extrinsic stress are characterized by the confined localization of certain molecules to either nuclear or perinuclear regions. Although most growth factors can activate downstream signaling in aging cells, they do not in fact have any impact on the cells because the signals cannot reach their genetic targets in the nucleus. For the same reason, varying apoptotic stress factors cannot stimulate the apoptotic pathway in senescent cells. Thus, the operation of a functional nuclear barrier in an aging-dependent manner has been investigated. To elucidate the mechanism for this process, the housekeeping transcription factor Sp1 was identified as a general regulator of nucleocytoplasmic trafficking (NCT) genes, including various nucleoporins, importins, exportins and Ran GTPase cycle-related genes. Interestingly, the posttranslational modification of Sp1 is readily influenced by extrinsic stress, including oxidative and metabolic stress. The decrease in SP1 O-GlcNAcylation under oxidative stress or during replicative senescence makes it susceptible to proteosomal degradation, resulting in defective NCT functions and leading to nuclear barrier formation. The operation of the nuclear barrier in aging provides a fundamental mechanism for cellular protection against stress and promotes survival at the expense of growth via stress-sensitive transcriptional control.

Effects of low-level laser therapy and epidermal growth factor on the activities of gingival fibroblasts obtained from young or elderly individuals

This study evaluated the effects of low-level laser therapy (LLLT) and epidermal growth factor (EGF) on fibroblasts obtained from young and elderly individuals. Gingival fibroblasts from young (Y) and elderly (E) individuals were seeded in wells of 24-well plates with Dulbecco's modified Eagle's medium (DMEM) containing 10 % of fetal bovine serum (FBS). After 24 h, the cells were irradiated (LASERTable-InGaAsP-780 ± 3 nm, 25 mW, 3 J/cm²) or exposed to EGF (100 μM). After 72 h, cells were evaluated for viability, migration, collagen and vascular endothelial growth factor (VEGF) synthesis, and gene expression of growth factors. Data were analyzed by Kruskal-Wallis and Mann-Whitney tests (α = 5 %). Y and E fibroblasts irradiated with laser or exposed to EGF showed increased viability and collagen synthesis. Enhanced cell migration was observed for Y fibroblasts after both treatments, whereas only the LLLT stimulated migration of E cells. VEGF synthesis was higher for Y and E cells exposed to EGF, while this synthesis was reduced when E fibroblasts were irradiated. Increased gene expression of VEGF was observed only for Y and E fibroblasts treated with LLLT. Regardless of a patient's age, the LLLT and EGF applications can biostimulate gingival fibroblast functions involved in tissue repair.

Phenyl 2-pyridyl ketoxime induces cellular senescence-like alterations via nitric oxide production in human diploid fibroblasts

Phenyl-2-pyridyl ketoxime (PPKO) was found to be one of the small molecules enriched in the extracellular matrix of near-senescent human diploid fibroblasts (HDFs). Treatment of young HDFs with PPKO reduced the viability of young HDFs in a dose- and time-dependent manner and resulted in senescence-associated β-galactosidase (SA-β-gal) staining and G2/M cell cycle arrest. In addition, the levels of some senescence-associated proteins, such as phosphorylated ERK1/2, caveolin-1, p53, p16(ink4a), and p21(waf1), were elevated in PPKO-treated cells. To monitor the effect of PPKO on cell stress responses, reactive oxygen species (ROS) production was examined by flow cytometry. After PPKO treatment, ROS levels transiently increased at 30 min but then returned to baseline at 60 min. The levels of some antioxidant enzymes, such as catalase, peroxiredoxin II and glutathione peroxidase I, were transiently induced by PPKO treatment. SOD II levels increased gradually, whereas the SOD I and III levels were biphasic during the experimental periods after PPKO treatment. Cellular senescence induced by PPKO was suppressed by chemical antioxidants, such as N-acetylcysteine, 2,2,6,6-tetramethylpiperidinyloxy, and L-buthionine-(S,R)-sulfoximine. Furthermore, PPKO increased nitric oxide (NO) production via inducible NO synthase (iNOS) in HDFs. In the presence of NOS inhibitors, such as L-NG-nitroarginine methyl ester and L-NG-monomethylarginine, PPKO-induced transient NO production and SA-β-gal staining were abrogated. Taken together, these results suggest that PPKO induces cellular senescence in association with transient ROS and NO production and the subsequent induction of senescence-associated proteins.

Nuclear barrier hypothesis of aging as mechanism for trade-off growth to survival

When the aging-dependent cellular behaviors toward growth factors and toxic stress have been analyzed, the perinuclear accumulation of the activated signals, either mitogenic or apoptotic, has been observed, suggesting the aging-dependent inefficiency of the nucleocytoplasmic trafficking of the signals. Thereby, it would be natural to assume the operation of the functional nuclear barrier in aging-dependent manner, which would be designated as "Park and Lim's Barrier." And for the ultimate transcriptional factor for these aging-dependent changes of the functional nuclear barrier, Sp1 transcriptional factor has been suggested to be the most probable candidate. This novel mechanism of aging-dependent operation of the functional nuclear barrier is proposed as the ultimate checking mechanism for cellular protection against toxic environment and the general mechanism for the trade-off growth to survival in aging.

Caveolin-1 as a novel indicator of wound-healing capacity in aged human corneal epithelium

Excess caveolin-1 has been reported to play a role in age-dependent hyporesponsiveness to growth factors in vitro. Therefore, we hypothesized that caveolin-1-dependent hyporesponsiveness to growth factors in aged corneal epithelial cells might be responsible for delayed wound healing in vivo. To test this hypothesis, we evaluated corneal wound-healing time by vital staining using fluorescein after laser epithelial keratomileusis (LASEK). We compared wound-healing times in young, middle-aged and elderly patients. We also examined caveolin-1 levels and other aging markers, such as p53 and p21, in the corneal epithelium. Elderly patients generally had higher caveolin-1 levels in the corneal epithelia than young patients. There were, however, variations among individuals with increased caveolin-1 in some young patients and decreased levels in some elderly patients. Wound-healing time after LASEK correlated well with the corneal caveolin-1 status. Therefore, we suggest that caveolin-1 status might be responsible for delayed wound healing in elderly patients after LASEK. Caveolin-1 status might be a regulator for wound-healing capacity and a novel target for in vivo adjustment.

Time-dependent differential gene expression in lysophosphatidic acid-treated young and senescent human diploid fibroblasts

The gene expression profiles of lysophosphatidic acid (LPA)-treated young and senescent human diploid fibroblasts (HDFs) were examined using cDNA microarray analysis. The expression of some genes, including EGR 1/3 and MRRF, was controlled by LPA similarly in young and senescent cells, showing a typical time-dependent up-and-down expression profile. In contrast, some other genes, including DUSP6, CYR61, and F3, showed sustained upregulation in senescent HDFs later after LPA treatment. These genes might be involved in altered LPA responsiveness during the aging process.

Influence of aging on biological properties of periodontal ligament cells

The majority of patients eligible for periodontal regenerative therapies are aged subjects. Since periodontal ligament cells (PDLC) are essential for periodontal regeneration, the aim of the present study was to determine the effect of cellular aging on PDLC, including genes associated with extracellular matrix metabolism and growth-associated factors. PDLC cultures were obtained from subjects aged 15 to 20 years and subjects aged more than 60 years. Proliferation, cell viability, mineralization assays, and mRNA levels were assessed for type I and III collagen, platelet-derived growth factor (PDGF)-1, basic fibroblast growth factor (bFGF), metalloproteinase (MMP)-2 and-8, and tissue inhibitor of metalloproteinases (TIMP)-1 and-2. Data analysis demonstrated that aging negatively influenced cell proliferation and mineral nodule formation (p < 0.05). Gene expression analysis further showed that mRNA levels for bFGF, PDGF-1, and TIMP-2 were not affected by aging (p > 0.05). In addition, mRNA levels for type I and III collagen were significantly lower in aged cells (p < 0.05), whereas MMP-2 and-8 and TIMP-1 mRNA levels were higher (p < 0.05). Within the limits of the present study, data analysis suggests that aging modulates important biological properties of periodontal ligament cells, diminishes the potential for mineral nodule formation, and favors extracellular matrix degradation.

G protein-coupled receptor systems and their lipid environment in health disorders during aging

Cells, tissues and organs undergo phenotypic changes and deteriorate as they age. Cell growth arrest and hyporesponsiveness to extrinsic stimuli are all hallmarks of senescent cells. Most such external stimuli received by a cell are processed by two different cell membrane systems: receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). GPCRs form the largest gene family in the human genome and they are involved in most relevant physiological functions. Given the changes observed in the expression and activity of GPCRs during aging, it is possible that these receptors are directly involved in aging and certain age-related pathologies. On the other hand, both GPCRs and G proteins are associated with the plasma membrane and since lipid-protein interactions regulate their activity, they can both be considered to be sensitive to the lipid environment. Changes in membrane lipid composition and structure have been described in aged cells and furthermore, these membrane changes have been associated with alterations in GPCR mediated signaling in some of the main health disorders in elderly subjects. Although senescence could be considered a physiologic process, not all aging humans develop the same health disorders. Here, we review the involvement of GPCRs and their lipid environment in the development of the major human pathologies associated with aging such as cancer, neurodegenerative disorders and cardiovascular pathologies.

Hippocalcin increases phospholipase D2 expression through extracellular signal-regulated kinase activation and lysophosphatidic acid potentiates the hippocalcin-induced phospholipase D2 expression

We have previously isolated a 22 kDa protein from a rat brain which was found to be involved in activating phospholipsae D (PLD), and identified the protein as hippocalcin through sequence analysis. Nevertheless, the function of hippocalcin for PLD activation still remains to be resolved. Here, we proposed that hippocalcin was involved in extracellular signal-regulated kinase (ERK)-mediated PLD2 expression. To elucidate a role of hippocalcin, we made hippocalcin transfected NIH3T3 cells and showed that the expression of PLD2 and basal PLD activity were increased in hippocalcin transfected cells. We performed PLD assay with dominant negative PLD2 (DN-PLD2) and hippocalcin co-transfected cells. DN-PLD2 suppressed increase of basal PLD activity in hippocalcin transfected cells, suggesting that increased basal PLD activity is due to PLD2 over-expression. Hippocalcin is a Ca2+-binding protein, which is expressed mainly in the hippocampus. Since it is known that lysophosphatidic acid (LPA) increases intracellular Ca2+, we investigated the possible role of hippocalcin in the LPA-induced elevation of intracellular Ca2+. When the intracellular Ca2+ level was increased by LPA, hippocalcin was translocated to the membrane after LPA treatment in hippocalcin transfected cells. In addition, treatment with LPA in hippocalcin transfected cells markedly potentiated PLD2 expression and showed morphological changes of cell shape suggesting that increased PLD2 expression acts as one of the major factors to cause change of cell shape by making altered membrane lipid composition. Hippocalcin-induced PLD2 expression potentiated by LPA in hippocalcin transfected cells was inhibited by a PI-PLC inhibitor, U73122 and a chelator of intracellular Ca2+, BAPTA-AM suggesting that activation of hippocalcin caused by increased intracellular Ca2+ is important to induce over-expression of PLD2. However, downregulation of PKC and treatment of a chelator of extracellular Ca2+, EGTA had little or no effect on the inhibition of hippocalcin-induced PLD2 expression potentiated by LPA in the hippocalcin transfected cells. Interestingly, when we over-express hippocalcin, ERK was activated, and treatment with LPA in hippocalcin transfected cells significantly potentiated ERK activation. Specific inhibition of ERK dramatically abolished hippocalcin-induced PLD2 expression. Taken together, these results suggest for the first time that hippocalcin can induce PLD2 expression and LPA potentiates hippocalcin-induced PLD2 expression, which is mediated by ERK activation.

Lysophosphatidic acid-induced changes in cAMP profiles in young and senescent human fibroblasts as a clue to the ageing process

This study attempts to elucidate the molecular mechanisms underlying the ageing-dependent cAMP profiles in human diploid fibroblasts stimulated by lysophosphatidic acid (LPA). In senescent cells, LPA-dependent Gialpha activation was reduced, with a consequent reduction in Gi-suppressed cAMP levels, without alterations in the levels of Gialpha proteins. In young cells, when Gialpha activity was inhibited by pertussis toxin pretreatment, or when its expression was blocked by siRNA, the pattern of changes in cAMP levels in response to LPA was similar to that seen in senescent cells. An increase in protein kinase C (PKC)-dependent isoforms of adenylyl cyclase (AC) types II, IV, and VI was also observed in these senescent fibroblasts. In senescent cells treated with PKC-specific inhibitors, bis-indolylmaleimide, Gö6976, rottlerin, and PKCvarepsilonV1, LPA-induced cAMP accumulation was inhibited, indicating that increased ACs in response to LPA occur via the activation of protein kinase Cs. When the expression of AC II, IV, and VI was blocked by siRNA in senescent fibroblasts, LPA-induced cAMP accumulation was also blocked. These results suggest that the senescence-associated increase of cAMP levels after LPA treatment is associated with reduced Gialpha, increased AC II, IV, and VI proteins, and PKC-dependent stimulation of their activities and provide an explanation for the age-dependent differences in cAMP-related physiological responses.

New molecular target for modulation of aging process

Despite many endeavors, no satisfactory strategy has emerged for modulating the aging process, most probably because they were based on faulty rationales. In an extension of the "gate theory of aging" that we proposed recently, we propose here that caveolin, an essential component of caveolae structure, may offer a potential target for modulating the aging process. According to the gate theory, certain biomolecules such as caveolins, amphiphysins, G proteins, and integrins play decisive roles in determining the senescent phenotype and thus provide targets for modulating the aging process. Among these molecules, we chose caveolin, because it can associate with a variety of regulatory and structural molecules via their scaffolding domains and thereby influence a broad spectrum of biological phenomena including both the physiology and morphology of the senescent cells. This is an attempt to review the vast body of evidence available in the literature, both direct and indirect, supporting the accord of this pivotal role to the caveolin in the background of the gate theory for the aging process.

Analysis of expression of growth factor receptors in replicatively and oxidatively senescent human fibroblasts

Replicatively and oxidatively senescent human fibroblasts demonstrate an impaired response to mitogens. To investigate whether this is due to downregulation of growth factor receptors we examined their expression in these two types of senescence. mRNA and protein levels of the insulin receptor and platelet-derived growth factor (PDGF) alpha-receptor decreased in replicatively senescent cells. The PDGF beta-receptor and insulin-like growth factor 1 receptor at the protein level also decreased but remained readily detectable. However, these major growth factor receptors remained unchanged in oxidatively premature senescent cells. This suggests that mechanisms underlying diminished responsiveness to mitogens might be different in replicative senescence and oxidatively premature senescence.

Caveolin-1 as a prime modulator of aging: a new modality for phenotypic restoration?

Aging can be characterized by structural changes and functional deterioration during the lifetime, for which hundreds of explanations have been put forward. Recently, we have proposed the gate theory of aging, in which gatekeeper molecules at the membrane level would play the prime role in determining the senescent phenotype. Caveolin-1 would be a prime candidate for such a role as a major determinant of the aging process. Caveolin-1 can associate with a variety of molecules, involved in signal transduction, endocytosis and transcytosis, cytoskeletal arrangement, etc. The level of caveolin-1 is strictly regulated to maintain cellular integrity, leading to cellular transformation if depleted, and to the senescent phenotype if overexpressed. In case of senescent cells, the functional and physiological responses to the mitogenic stimuli can be restored and the morphological shape can be resumed by simple adjustment of caveolin-1 status. Therefore, it is suggested that prime modulator molecules, represented by caveolin-1, play a key role in determining the senescent phenotype, either as a physiological response or altered morphology.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Functional efficiency of the senescent cells: replace or restore?

It is generally accepted that aging is a phenomenon of irreversibility, inevitability, and universality with parenchymal loss and functional decline. Consequently, the major goals of aging research are focused on the development of a replace strategy of the aged organs or cells, based on immortalizing tools, stem cells, or artificial substitutes. Recently, however, a new concept of functional recovery has been introduced on the basis of the functional restoration of the responsiveness of the senescent cells toward a variety of agonists, including growth factors. The aging phenotypes of hyporesponsiveness and morphological alteration are shown to be readily adjusted by modulation of the several membrane-associated molecules, named gatekeeper molecules, among which caveolin is one of the major determinants. Caveolin is the essential component of the caveolae, responsible for regulation of signal transduction, endocytosis and trancytosis, and cytoskeletal arrangement via its scaffolding domain. The caveolin status is associated strictly with cellular transformation, if depleted, and with senescent phenotype, if overexpressed. Therefore, simple reduction of caveolin status in senescent cells leads to restoration of the functional responsiveness to mitogenic stimuli and even of the cellular shape. These data strongly suggest that the gatekeeper molecules, represented by caveolin, may play the prime role in determination of the senescent phenotypes. From these results, it can be summarized that the replace principle would not necessarily be the essential one, but the restore principle can be somehow substituted for the betterment of the aged cells and organisms.

Altered cAMP signaling induced by lysophosphatidic acid in senescent human diploid fibroblasts

Lysophosphatidic acid (LPA) is a lipid mitogen that acts through G-protein-coupled receptors. LPA responsiveness has been reported to be dependent on the senescent state of the cells. To solve the mechanism underlying, we observed LPA-dependent cAMP status and found its age-dependent contrasting profile such as high level of cAMP in the senescent cells vs its low level in the young cells. In order to clarify the molecular mechanism of the ageing effect, we examined various molecular species involved in the cAMP signaling pathway by semi-quantitative RT-PCR. EDG-1 and EDG-4 were unchanged, but EDG-2 and EDG-7 were reduced with age. Senescent cells showed a partial reduction of Gi1, Gi2, and Gi3, but no change in the level of Gq. Decreased Gis and Gi-coupled LPA receptors may reduce the inhibitory effect of Gi alpha on adenylyl cyclases (ACs), resulting in cAMP accumulation via activation of adenylyl cyclase in senescent fibroblasts. We also observed an age-dependent increase in some of AC isoforms: II, IV, and VI. In conclusion, multiple changes in the cAMP signaling pathway of the senescent cells might explain the altered responsiveness to the mitogenic stimuli.

Age-dependent agonist-specific dysregulation of membrane-mediated signal transduction: emergence of the gate theory of aging

Although a general mechanism for the limited responsiveness of senescent cells has yet to be established, reduced responsiveness may in part be ascribed to deficits in the apparatus required for cell surface receptor-mediated signal transduction. Age-related changes of receptor-mediated signal transduction occur at many levels, and are known to include quantitative and qualitative changes in growth factor receptors, G-protein coupled receptors, and many other downstream signaling molecules. Here, we emphasize the prime role of the cellular surface in the perception and transmission of external stimuli in response to the aging process. As major means of cellular signal transduction, the receptor tyrosine kinase (RTK) system and the G protein-coupled receptor (GPCR) system of senescent cells were investigated. We observed that the RTK system was severely damaged, while the GPCR system was only partially inactivated by aging. These results suggest that the agonist-dependent dysregulation of and imbalance of signal transduction pathways might be responsible for the functional deterioration of senescent cells, and indicate a possibility of the functional recovery of senescent cells through agonist-specific signal system activation. Moreover, those data evoke the emerging concept that the senescent phenotype may be modulated by the membrance-associated signal system, implying the gate theory of aging.