Age-related decline in Ras/ERK mitogen-activated protein kinase cascade is linked to a reduced association between Shc and EGF receptor

Augmentation of the heat shock axis during exceptional longevity in Ames dwarf mice

How the heat shock axis, repair pathways, and proteostasis impact the rate of aging is not fully understood. Recent reports indicate that normal aging leads to a 50% change in several regulatory elements of the heat shock axis. Most notably is the age-dependent enhancement of inhibitory signals associated with accumulated heat shock proteins and hyper-acetylation associated with marked attenuation of heat shock factor 1 (HSF1)–DNA binding activity. Because exceptional longevity is associated with increased resistance to stress, this study evaluated regulatory check points of the heat shock axis in liver extracts from 12 months and 24 months long-lived Ames dwarf mice and compared these findings with aging wild-type mice. This analysis showed that 12M dwarf and wild-type mice have comparable stress responses, whereas old dwarf mice, unlike old wild-type mice, preserve and enhance activating elements of the heat shock axis. Old dwarf mice thwart negative regulation of the heat shock axis typically observed in usual aging such as noted in HSF1 phosphorylation at Ser307 residue, acetylation within its DNA binding domain, and reduction in proteins that attenuate HSF1–DNA binding. Unlike usual aging, dwarf HSF1 protein and mRNA levels increase with age and further enhance by stress. Together these observations suggest that exceptional longevity is associated with compensatory and enhanced HSF1 regulation as an adaptation to age-dependent forces that otherwise downregulate the heat shock axis.

Identification of Host Factors Associated with the Development of Equine Herpesvirus Myeloencephalopathy by Transcriptomic Analysis of Peripheral Blood Mononuclear Cells from Horses

Equine herpesvirus-1 is the cause of respiratory disease, abortion, and equine herpesvirus myeloencephalopathy (EHM) in horses worldwide. EHM affects as many as 14% of infected horses and a cell-associated viremia is thought to be central for EHM pathogenesis. While EHM is infrequent in younger horses, up to 70% of aged horses develop EHM. The aging immune system likely contributes to EHM pathogenesis; however, little is known about the host factors associated with clinical EHM. Here, we used the “old mare model” to induce EHM following EHV-1 infection. Peripheral blood mononuclear cells (PBMCs) of horses prior to infection and during viremia were collected and RNA sequencing with differential gene expression was used to compare the transcriptome of horses that did (EHM group) and did not (non-EHM group) develop clinical EHM. Interestingly, horses exhibiting EHM did not show respiratory disease, while non-EHM horses showed significant respiratory disease starting on day 2 post infection. Multiple immune pathways differed in EHM horses in response to EHV-1. These included an upregulation of IL-6 gene expression, a dysregulation of T-cell activation through AP-1 and responses skewed towards a T-helper 2 phenotype. Further, a dysregulation of coagulation and an upregulation of elements in the progesterone response were observed in EHM horses.

Multifactorial Attenuation of the Murine Heat Shock Response With Age

Age-dependent perturbation of the cellular stress response affects proteostasis and other key functions relevant to cellular action and survival. Central to age-related changes in the stress response is loss of heat shock factor 1 (HSF1)-DNA binding and transactivation properties. This report elucidates how age alters different checkpoints of HSF1 activation related to posttranslational modification and protein interactions. When comparing liver extracts from middle aged (12 M) and old (24 M) mice, significant differences are found in HSF1 phosphorylation and acetylation. HSF1 protein levels and messenger RNA decline with age, but its protein levels are stress-inducible and exempt from age-dependent changes. This surprising adaptive change in the stress response has additional implications for aging and chronic physiological stress that might explain an age-dependent dichotomy of HSF1 protein levels that are low in neurodegeneration and elevated in cancer.

Fucoidan⁻Fucoxanthin Ameliorated Cardiac Function via IRS1/GRB2/ SOS1, GSK3β/CREB Pathways and Metabolic Pathways in Senescent Mice

The effects of low molecular weight fucoidan (LMWF) in combination with high-stability fucoxanthin (HSFUCO) on cardiac function and the metabolic pathways of aging mice (Mus musculus) were investigated. We demonstrated that LMWF and HSFUCO could improve cardiac function in aging mice. Aging mice were treated with LMWF and HSFUCO, either on their own or in combination, on 28 consecutive days. Electrocardiography and whole-cell patch-clamp were used to measure QT interval and action potential duration (APD) of the subjects. Cardiac tissue morphology, reactive oxygen species, and Western blot were also applied. Ultra-high-performance liquid chromatography⁻quadrupole time-of-flight (UPLC-QTOF) mass spectrometry was used for investigating metabolic alterations. The use of LMWF and HSFUCO resulted in improvements in both ventricular rhythms (QT and APD). Treatment with fucoidan and fucoxanthin reduced the expression levels of SOS1 and GRB2 while increasing GSK3β, CREB and IRS1 proteins expression in the aging process. Three main metabolic pathways, namely the TCA cycle, glycolysis, and steroid hormone biosynthesis, were highly enriched in the pathway enrichment analysis. When taken together, the LMWF and HSFUCO treatment improved both the ventricular rhythm and the muscular function of aging subjects by interfering with the metabolism and gene function.

Signal Transduction Pathways in Ageing

It is now widely recognised that ageing and its associated functional decline are regulated by a wide range of molecules that fit into specific cellular pathways. Here, we describe several of the evolutionary conserved cellular signalling pathways that govern organismal ageing and discuss how their identification, and work on the individual molecules that contribute to them, has aided in the design of therapeutic strategies to alleviate the adverse effects of ageing and age-related disease.

Membrane ion Channels and Receptors in Animal lifespan Modulation

Acting in the interfaces between environment and membrane compartments, membrane ion channels, and receptors transduce various physical and chemical cues into downstream signaling events. Not surprisingly, these membrane proteins play essential roles in a wide range of cellular processes such as sensory perception, synaptic transmission, cellular growth and development, fate determination, and apoptosis. However, except insulin and insulin-like growth factor receptors, the functions of membrane receptors in animal lifespan modulation have not been well appreciated. On the other hand, although ion channels are popular therapeutic targets for many age-related diseases, their potential roles in aging itself are largely neglected. In this review, we will discuss our current understanding of the conserved functions and mechanisms of membrane ion channels and receptors in the modulation of lifespan across multiple species including Caenorhabditis elegans, Drosophila, mouse, and human.

Inhibition of adenylyl cyclase type 5 increases longevity and healthful aging through oxidative stress protection

Mice with disruption of adenylyl cyclase type 5 (AC5 knockout, KO) live a third longer than littermates. The mechanism, in part, involves the MEK/ERK pathway, which in turn is related to protection against oxidative stress. The AC5 KO model also protects against diabetes, obesity, and the cardiomyopathy induced by aging, diabetes, and cardiac stress and also demonstrates improved exercise capacity. All of these salutary features are also mediated, in part, by oxidative stress protection. For example, chronic beta adrenergic receptor stimulation induced cardiomyopathy was rescued by AC5 KO. Conversely, in AC5 transgenic (Tg) mice, where AC5 is overexpressed in the heart, the cardiomyopathy was exacerbated and was rescued by enhancing oxidative stress resistance. Thus, the AC5 KO model, which resists oxidative stress, is uniquely designed for clinical translation, since it not only increases longevity and exercise, but also protects against diabetes, obesity, and cardiomyopathy. Importantly, inhibition of AC5's action to prolong longevity and enhance healthful aging, as well as its mechanism through resistance to oxidative stress, is unique among all of the nine AC isoforms.

Targeting SHCBP1 inhibits cell proliferation in human hepatocellular carcinoma cells

Src homology 2 domain containing (SHC) is a proto-oncogene which mediates cell proliferation and carcinogenesis in human carcinomas. Here, the SHC SH2-domain binding protein 1 (SHCBP1) was first established to be up-regulated in human hepatocellular carcinoma (HCC) tissues by array-base comparative genome hybridization (aCGH). Meanwhile, we examine and verify it by quantitative real-time PCR and western blot. Our current data show that SHCBP1 was up-regulated in HCC tissues. Overexpression of SHCBP1 could significantly promote HCC cell proliferation, survival and colony formation in HCC cell lines. Furthermore, knockdown of SHCBP1 induced cell cycle delay and suppressed cell proliferation. Furthermore, SHCBP1 could regulate the expression of activate extracellular signal-regulated kinase 1/2 (ERK1/2) and cyclin D1. Together, our findings indicate that SHCBP1 may contribute to human hepatocellular carcinoma by promoting cell proliferation and may serve as a molecular target of cancer therapy.

Aging and regeneration in vertebrates

Aging is marked by changes that affect organs and resident stem cell function. Shorting of telomeres, DNA damage, oxidative stress, deregulation of genes and proteins, impaired cell-cell communication, and an altered systemic environment cause the eventual demise of cells. At the same time, reparative activities also decline. It is intriguing to correlate aging with the decline of regenerative abilities. Animal models with strong regenerative capabilities imply that aging processes might not be affecting regeneration. In this review, we selectively present age-dependent changes in stem/progenitor cells that are vital for tissue homeostasis and repair. In addition, the aging effect on regeneration following injury in organs such as lung, skeletal muscle, heart, nervous system, cochlear hair, lens, and liver are discussed. These tissues are also known for diseases such as heart attack, stroke, cognitive impairment, cataract, and hearing loss that occur mostly during aging in humans. Conclusively, vertebrate regeneration declines with age with the loss of stem/progenitor cell function. Future studies on improving the function of stem cells, along with studies in fish and amphibians where regeneration does not decline with age, will undoubtedly provide insights into both processes.

Teaching an old dogma new tricks: twenty years of Shc adaptor signalling

Shc (Src homology and collagen homology) proteins are considered prototypical signalling adaptors in mammalian cells. Consisting of four unique members, ShcA, B, C and D, and multiple splice isoforms, the family is represented in nearly every cell type in the body, where it engages in an array of fundamental processes to transduce environmental stimuli. Two decades of investigation have begun to illuminate the mechanisms of the flagship ShcA protein, whereas much remains to be learned about the newest discovery, ShcD. It is clear, however, that the distinctive modular architecture of Shc proteins, their promiscuous phosphotyrosine-based interactions with a multitude of membrane receptors, involvement in central cascades including MAPK (mitogen-activated protein kinase) and Akt, and unconventional contributions to oxidative stress and apoptosis all require intricate regulation, and underlie diverse physiological function. From early cardiovascular development and neuronal differentiation to lifespan determination and tumorigenesis, Shc adaptors have proven to be more ubiquitous, versatile and dynamic than their structures alone suggest.

Constitutively active MEK1 rescues cardiac dysfunction caused by overexpressed GSK-3α during aging and hemodynamic pressure overload

Expression of GSK-3α is increased in aging hearts and those subjected to hemodynamic overload. Overexpressed GSK-3α inhibits ERK and enhances pressure overload (PO)-induced cardiac dysfunction. We studied whether suppression of the MEK1/ERK pathway contributes to cardiac responses induced by overexpressed GSK-3α using constitutively active MEK1 (CA-MEK1)/GSK-3α bigenic mice (bigenic mice), which were obtained by crossing cardiac-specific GSK-3α transgenic mice (Tg-GSK) and cardiac-specific CA-MEK1 transgenic mice (Tg-MEK1). The suppression of ERK phosphorylation observed in Tg-GSK was eliminated in bigenic mice. At 12 mo, left ventricular (LV) weight/tibia length, LV weight/body weight, and cardiac myocyte size were significantly smaller in Tg-GSK than in nontransgenic mice (NTg), but were not significantly different between Tg-MEK1 and bigenic mice. The LV ejection fraction (LVEF), fractional shortening (FS), and change in pressure over time were significantly lower in Tg-GSK than in NTg, but were not significantly different between bigenic mice and Tg-MEK1. The increase in apoptosis in Tg-GSK was abolished in bigenic mice, although the increase in fibrosis was not. After PO, the decrease in cardiac hypertrophy and the enhancement of apoptosis seen in Tg-GSK were abrogated in bigenic mice. After PO, the LVEF and FS were significantly reduced in Tg-GSK compared with its sham, but not in NTg, Tg-MEK1, or bigenic mice compared with their respective shams. There was no significant difference in LVEF and FS between bigenic mice and Tg-MEK1 after PO. In conclusion, inhibition of the MEK1/ERK pathway mediates the hypertrophy suppression and cardiac dysfunction caused by GSK-3α overexpression in cardiac myocytes.

Liver regeneration and aging: a current perspective

Many organ systems exhibit significant age-related deficits, but, based on studies in old rodents and elderly humans, the liver appears to be relatively protected from such changes. A remarkable feature of the liver is its capacity to regenerate its mass following partial hepatectomy. Reports suggests that aging compromises the liver's regenerative capacity, both in the rate and to the extent the organ's original volume is restored. There has been modest definitive information as to which cellular and molecular mechanisms regulating hepatic regeneration are affected by aging. Changes in hepatic sensitivity to growth factors, for example, epidermal growth factor (EGF), appear to influence regeneration in old animals. Studies have demonstrated (a) a 60% decline in EGF binding to hepatocyte plasma membranes, (b) reduced expression of the hepatic high affinity EGF receptor and (c) a block between G1 and S-phases of the cell cycle in old rats following EGF stimulation. Recent studies suggest that reduced phosphorylation and dimerization of the EGF receptor, critical steps in the activation of the extracellular signal-regulated kinase pathway and subsequent cell proliferation are responsible. Other studies have demonstrated that aging affects the upregulation of a Forkhead Box transcription factor, FoxM1B, which is essential for growth hormone-stimulated liver regeneration in hepatectomized mice. Aging appears to compromise liver regeneration by influencing several pathways, the result of which is a reduction in the rate of regeneration, but not in the capacity to restore the organ to its original volume.

Reduced expression of epidermal growth factor receptors in rat liver during aging

Proliferative responsiveness of hepatocytes to epidermal growth factor (EGF) declines during aging. The role of EGF receptors in mediating age-dependent changes of EGF-induced mitogenic signaling in liver remains incompletely understood. We assessed EGF receptor expression levels in whole liver specimens as well as in freshly isolated and cultured hepatocytes from young adult and senescent Fischer 344 male rats. Hepatic EGF receptor messenger RNA and protein levels, and the number of high- and low-affinity receptor binding sites, decreased with aging. Ligand-induced EGF receptor activation, determined by receptor dimerization and tyrosine phosphorylation, was reduced in old animals in parallel with the age-related decline in receptor expression. Stimulation of the extracellular signal-regulated kinase pathway by EGF was also attenuated in hepatocytes from old animals. Our results implicate decreased expression of EGF receptors as a key determinant of reduced mitogenic signaling responsive to EGF stimulation of liver during aging.

Effect of aging on EGF-induced proliferative response in primary cultured periportal and perivenous hepatocytes

BACKGROUND/AIMS

Aging relates to declined proliferative capacity of the liver, but the molecular mechanism is not well understood. We examined whether functional changes of epidermal growth factor (EGF) receptor (EGFR) are involved in age-related decline in EGF-induced DNA synthesis using hepatocytes isolated in periportal and perivenous regions of the liver, which differ in the proliferative capacity.

METHODS

Periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) in 7-, 30-, and 90-week-old rats were isolated using the digitonin/collagenase perfusion technique. DNA synthesis was assessed by [methyl-(3)H]thymidine incorporation. EGFR binding affinity to EGF was analyzed by Scatchard analysis using [(125)I]EGF. EGFR dimerization and phosphorylation were determined by Western blot analysis.

RESULTS

EGF-induced DNA synthesis was greater in PPH than in PVH from rats of 7 weeks, but the zonal difference disappeared with aging. [(125)I]EGF binding studies indicated that high-affinity EGFR in both subpopulations also disappeared with aging. Furthermore, EGF-induced dimerization in both subpopulations was down-regulated with aging, and the pattern of EGFR phosphorylation was parallel to that of dimerization.

CONCLUSIONS

These data suggest that age-related decline in EGF-induced DNA synthesis of PPH and PVH is caused by down-regulation of EGFR dimerization through the decrease of high-affinity EGFR.

Type 5 adenylyl cyclase disruption increases longevity and protects against stress

Mammalian models of longevity are related primarily to caloric restriction and alterations in metabolism. We examined mice in which type 5 adenylyl cyclase (AC5) is knocked out (AC5 KO) and which are resistant to cardiac stress and have increased median lifespan of approximately 30%. AC5 KO mice are protected from reduced bone density and susceptibility to fractures of aging. Old AC5 KO mice are also protected from aging-induced cardiomyopathy, e.g., hypertrophy, apoptosis, fibrosis, and reduced cardiac function. Using a proteomic-based approach, we demonstrate a significant activation of the Raf/MEK/ERK signaling pathway and upregulation of cell protective molecules, including superoxide dismutase. Fibroblasts isolated from AC5 KO mice exhibited ERK-dependent resistance to oxidative stress. These results suggest that AC is a fundamentally important mechanism regulating lifespan and stress resistance.

Calorie restriction protects against age-related rat aorta sclerosis

Many theories have been advanced to account for the ageing process, among which the free radical theory deserves much attention. Numerous studies have also shown an association between tissue fibrosis and oxidative stress. Of note, fibrosis may be considered a significant index of tissue ageing. Calorie restriction (CR) has been demonstrated to maintain many physiological processes in a youthful state until a very advanced age. However the anti-ageing mechanisms of CR are still not fully understood. We thus evaluated the effect of CR on oxidative damage and its relationship with fibrosis during ageing. We found a significant increase of both oxidative stress and fibrosis parameters in the aortae from aged vs. young rats. CR reversed both phenomena. CR also protected against the age-associated increase of Jun-N-terminal kinase and p-38 activities, involved in TGFbeta1 expression and signaling. On the contrary, extracellular regulated kinases did not show any age-related change. CR similarly reversed the age-related increase of AP-1 DNA binding activity and the AP-1-dependent increase of vimentin gene expression. In parallel, CR reversed the age-related morphological alterations of the aorta wall cell composition. These data further support the relationship between oxidative stress and fibrosis in different diseases and during ageing. The protection exerted by CR against fibrosclerosis might be due to a decrease of oxidative stress, with consequent decreased MAPK activity and down-regulation of AP-1 activation and of TGFbeta1 expression and signaling.

Epidermal growth factor (EGF)-mediated DNA-binding activity of AP-1 is attenuated in senescent human epidermal keratinocytes

The proliferative responses of cells to mitogens decrease during aging, and this may result from age-related defects in signal transduction in response to mitogens. In this study, we have investigated the age-related alteration of responses to epidermal growth factor (EGF) in cultured human keratinocytes that were senesced in vitro by repeated passage. The stimulation with EGF increased the DNA-binding activity of activator protein 1 (AP-1), an important transcription factor for cell proliferation, in young keratinocytes, whereas the binding activity showed little or slight change in the senescent cells. The induced DNA-binding activity of AP-1 in young cells was inhibited by PD 98059, an inhibitor of MEK, and partially inhibited by GF 109203X, an inhibitor of protein kinase C. Western blot analysis demonstrated that EGF induced dramatic increase in the phosphorylation of EGF receptor (EGFR) and extracellular signal-regulated kinases (ERK) in young cells, while this phosphorylation was much less profound in senescent cells. Finally, the application of EGF to young cells resulted in increased phosphorylation of Fra-2, a Fos protein component of the Jun/Fos heterodimer AP-1 complex. This EGF-induced Fra-2 phosphorylation was attenuated in senescent cells. Taken together, our study suggests that the signal transduction mediated by EGF/ERK pathway is altered in senescent human keratinocytes, and this change may be attributed, in part, to the decreased AP-1 transcription activity observed in senescent keratinocytes.

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.

Age affects ERK1/2 and NRF2 signaling in the regulation of GCLC expression

We previously reported that activator protein-1 (AP-1) DNA binding activity was increased in vascular smooth muscle cells (VSMC) from old rats when exposed to high glucose or tumor necrosis factor (TNF-alpha) (Li et al., 2003. J Cell Physiol 197:418-425). We have now examined the relationship between the age-dependent activation of the ERK1/2-AP-1 pathway and modulation of constitutive gene expression of the catalytic subunit of glutamate-cysteine ligase (GCLC) in response to high glucose and TNF-alpha. GCLC mRNA levels were higher in VSMC from old rats compared to young, a pattern consistent with its protein levels. To determine whether age-related activation of ERK1/2-AP-1 signaling is responsible for the up-regulation of GCLC, the MEK inhibitors, PD98059 and U0126, were used to block ERK1/2 in VSMC from old rats. An increase in GCLC with inhibitors was observed, diminishing the likelihood of ERK1/2-AP-1 activation as the up-regulating signal for GCLC. However, the transcription factor Nrf2 was higher in nuclei and accompanied by increased Nrf2-ARE binding in VSMC from old rats. Furthermore, MEK inhibitors increased nuclear Nrf2 and Nrf2/ARE binding. These data suggest opposing effects of Nrf2 and ERK1/2 signaling in the modulation of GCLC expression in old animals.

The function of mitogen-activated protein kinase phosphatase-1 in peptidoglycan-stimulated macrophages

Mitogen-activated protein (MAP) kinases play a pivotal role in the macrophages in the production of proinflammatory cytokines triggered by lipopolysaccharides. However, their function in the responses of macrophages to Gram-positive bacteria is poorly understood. Even less is known about the attenuation of MAP kinase signaling in macrophages exposed to Gram-positive bacteria. In the present study, we have investigated the regulation of MAP kinases and the role of MAP kinase phosphatase (MKP)-1 in the production of pro-inflammatory cytokines using murine RAW264.7 and primary peritoneal macrophages after peptidoglycan stimulation. Treatment of macrophages with peptidoglycan resulted in a transient activation of JNK, p38, and extracellular signal-regulated kinase. Most interestingly, MKP-1 expression was potently induced by peptidoglycan, and this induction was concurrent with MAP kinase dephosphorylation. Triptolide, a diterpenoid triepoxide, potently blocked the induction of MKP-1 by peptidoglycan and prolonged the activation of JNK and p38. Overexpression of MKP-1 substantially attenuated the production of tumor necrosis factor (TNF)-alpha induced by peptidoglycan, whereas knockdown of MKP-1 by small interfering RNA substantially increased the production of both TNF-alpha and interleukin-1 beta. Finally, we found that in primary murine peritoneal macrophages, MKP-1 induction following peptidoglycan stimulation also coincided with inactivation of JNK and p38. Blockade of MKP-1 induction resulted in a sustained activation of both JNK and p38 in primary macrophages. Our results reveal that MKP-1 critically regulates the expression of TNF-alpha and interleukin-1 beta in RAW264.7 cells and further suggest a central role for this phosphatase in controlling the inflammatory responses of primary macrophages to Gram-positive bacterial infection.

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.

Striatal GDNF administration increases tyrosine hydroxylase phosphorylation in the rat striatum and substantia nigra

Glial cell line-derived neurotrophic factor (GDNF) improves motor dysfunction associated with aging in rats and non-human primates, in animal models of Parkinson's disease, and may improve motoric function in patients with advanced Parkinson's disease. These improvements are associated with increased dopamine function in the nigrostriatal system, but the molecular events associated with this increase are unknown. In these studies, 100 micro g of GDNF was injected into the striatum of normal aged (24-month-old) male Fischer 344 rats. The protein levels and phosphorylation of TH, ERK1/2, and related proteins were determined by blot-immunolabeling of striatum and substantia nigra harvested 30 days after injection. In GDNF-treated rats, TH phosphorylation at Ser31 increased approximately 40% in striatum and approximately 250% in the substantia nigra. In the substantia nigra, there was a significant increase in ERK1 phosphorylation. In striatum, there was a significant increase in ERK2 phosphorylation. Microdialysis studies in striatum showed that both amphetamine- and potassium-evoked dopamine release in GDNF recipients were significantly increased. These data show that GDNF-induced increases in dopamine function are associated with a sustained increase in TH phosphorylation at Ser31, which is greatest in the substantia nigra and maintained for at least one month following a single striatal administration of GDNF. These findings, taken from the nigrostriatal system of normal aged rats, may help explain the long lasting effects of GDNF on dopamine function and prior studies supporting that a major effect of GDNF involves its effects on dopamine storage and somatodendritic release of dopamine in the substantia nigra.

Epidermal Growth Factor Potentiates Cholecystokinin/Gastrin Receptor-mediated Ca2+ Release by Activation of Mitogen-activated Protein Kinases

Small differences in amplitude, duration, and temporal patterns of change in the concentration of free intracellular Ca2+ ([Ca2+](i)) can profoundly affect cell physiology, altering programs of gene expression, cell proliferation, secretory activity, and cell survival. We report a novel mechanism for amplitude modulation of [Ca2+](i) that involves mitogen-activated protein kinase (MAPK). We show that epidermal growth factor (EGF) potentiates gastrin-(1-17) (G17)-stimulated Ca2+ release from intracellular Ca2+ stores through a MAPK-dependent pathway. G17 activation of the cholecystokinin/gastrin receptor (CCK(2)R), a G protein-coupled receptor, stimulates release of Ca2+ from inositol 1,4,5-triphosphate-sensitive Ca2+ stores. Pretreating rat intestinal epithelial cells expressing CCK(2)R with EGF increased the level of G17-stimulated Ca2+ release from intracellular stores. The stimulatory effect of EGF on CCK(2)R-mediated Ca2+ release requires activation of the MAPK kinase (MEK)1,2/extracellular signal-regulated kinase (ERK)1,2 pathway. Inhibition of the MEK1,2/ERK1,2 pathway by either serum starvation or treatment with selective MEK1,2 inhibitors PD98059 and U0126 or expression of a dominant-negative mutant form of MEK1 decreased the amplitude of the G17-stimulated Ca2+ release response. Activation of the MEK1,2/ERK1,2 pathway either by pretreating cells with EGF or by expression of constitutively active K-ras (K-rasV12G) or MEK1 (MEK1*) increased the amplitude of G17-stimulated Ca2+ release. Although EGF, MEK1*, and K-rasV12G activated the MEK1,2/ERK1,2 pathway, they did not increase [Ca2+](i) in the absence of G17. These data demonstrate that the activation state of the MEK1,2/ERK1,2 pathway can modulate the amplitude of the CCK(2)R-mediated Ca2+ release response and identify a novel mechanism for cross-talk between EGF receptor- and CCK(2)R-regulated signaling pathways.

Ageing, autoimmunity and arthritis: Perturbations of TCR signal transduction pathways with ageing - a biochemical paradigm for the ageing immune system

It is widely accepted that cell-mediated immune functions decline with age, rendering an individual more susceptible to infection and possibly cancer, as well as to age-associated autoimmune diseases. The exact causes of T-cell functional decline are not known. One possible cause could be the development of defects in the transduction of mitogenic signals following TCR stimulation. This T-cell hyporesponsiveness due to defects of signalling through the TCR either from healthy elderly subjects or from individuals with autoimmune diseases such as rheumatoid arthritis or systemic lupus erythematosus results in an impaired ability to mount efficient immune responses and to maintain responsiveness to foreign antigens. This implies that a high proportion of autoreactive T cells might accumulate either intrathymically or in the periphery. T-cell anergy and differential TCR signalling could thus also be key players in the disruption of tolerance and the onset of autoimmune diseases. The increasing number of the elderly may lead to an increase of clinically important autoimmune diseases. We will review the signal transduction changes through the TCR-CD3 complex in T lymphocytes from healthy elderly subjects, which result in a modification of the activation of transcription factors involved in IL-2 gene expression leading to decreased IL-2 production. The putative contribution of altered T-cell signalling with ageing in the development of autoimmune diseases will be also discussed.

Common mechanisms for declines in oxidative stress tolerance and proliferation with aging

Aging is often characterized by reduced stress tolerance and decreased proliferative capacity, but little is known about the effects of aging on signaling pathways important in regulating these responses. Recent studies from our laboratory have implicated impairments in epidermal growth factor receptor (EGFR) signaling and extracellular signal-regulated kinase (ERK) activation to both effects in rat hepatocytes. Here we investigated the responsiveness of hepatocytes derived from young (4-5 months) and aged (24-29 months) mice to proliferative signals (low concentrations of H2O2 and epidermal growth factor [EGF] stimulation), and oxidant injury (high H2O2 concentrations). Old hepatocytes displayed lower levels of DNA synthesis in response to low H(2)O(2) concentrations (5-10 microM) and EGF stimulation, and reduced survival following treatment with high H2O2 concentrations (20-50 microM). Both effects were associated with reduced activation of ERK and diminished phosphorylation of EGFR tyrosine residue 1173. p38 was also activated by H2O2, but to a greater extent in old cells. Pharmacologic inhibition of ERK increased the sensitivity of young cells to H2O2-induced cell death, while inhibition of p38 decreased the sensitivity of old cells. Our findings suggest that impairments in common signaling events underlie age-related declines in proliferative capacity and oxidative stress tolerance in mouse hepatocytes, and that an imbalance in ERK and p38 activities contributes to the greater sensitivity of aged cells to H2O2.

The effect of aging on immune responses

Although vaccine manufacturers make no specific recommendations regarding the vaccination of older horses and ponies, the similarities in age-induced immunologic changes between human beings and equids suggests that similar vaccination recommendations should be followed. The need for vaccination of the older horse depends, of course, on the relative risk of exposure for the individual horse. Particular care should be taken when using attenuated vaccine products because these live agents may pose a unique risk to the older individual. Immunization with inactivated agent vaccines is likely to be safer. In general, annual vaccination against equine influenza virus infection, tetanus, rabies, and encephalomyelitis viruses is warranted.

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.

Age-related decline in cellular response to oxidative stress: links to growth factor signaling pathways with common defects

Accumulation of oxidative damage is believed to be a major contributor to the decline in physiologic function that characterizes mammalian aging, and recent studies suggest that how well you respond to acute oxidative stress is an important factor in determining longevity. Oxidant injury elicits a wide spectrum of responses ranging from proliferation to cell death. The particular outcome observed largely reflects the severity of the stress encountered and the relative degree of activation of various signal transduction pathways aimed at enhancing survival or inducing cell death. Herein we examine the relationship between pathways important in supporting cell survival in response to oxidant injury and those involved in regulating proliferation. We review evidence indicating that [Curr. Opin. Cell Biol. 10 (1998) 248] common pathways are indeed involved in regulating these responses, and [Physiol. Rev. 82 (2002) 47] alterations in shared signaling events likely account for the age-related decline in the ability of cells to respond to both proliferative signals and oxidant stimuli.

Functional recovery of senescent cells through restoration of receptor-mediated endocytosis

The functional deterioration of an organism with age causes the major problem of maintaining the quality of life at old age. Degenerative changes in the organism may to some extent reflect alterations that can be observed in cells during in vitro replicative senescence. At the cellular level, the receptor-mediated endocytosis in the membrane might be emphasized as a responsible mechanism for functional decay, since the endocytosis is in charge of many important biological phenomena: nutrient uptake, growth factor sensitivity, immune response, protection from environment and pathogen uptake, etc. We found that two major endocytotic pathways, i.e. clathrin-mediated and caveolae-dependent endocytosis, are down regulated in senescent cells. For the down regulation of the clathrin dependent receptor-mediated endocytosis, the reduction of amphiphysin-1 was found responsible, which was confirmed by Western blot analysis, dominant negative mutant transfection and restoration of gene activity by microinjection. With respect to the hypo-responsiveness of senescent cells to growth factors, the upregulation of caveolins has been suggested to be a causal factor. The overexpression of caveolins caused senescent-like changes in epidermal growth factor (EGF) response of the young cells, while down regulation of caveolins by use of antisense-oligonucleotides restored the EGF response in old cells, suggesting that caveolin system would be one of the major mechanisms responsible for decreased responses to growth factors in the senescent cells. Based on these results, it can be suggested that the functional deterioration of the senescent cells may be explained in terms of the down regulation of receptor mediated endocytosis, at least in part, and that the restoration of endocytosis apparatus either with amphiphysin supplementation or with reduction of caveolins might lead to functional recovery of the senescent cells.

Effects of aging on hepatic IGF-I signaling

Little is known regarding hepatic insulin-like growth factor-1 IGF-I signaling with aging despite the observation that other tissues demonstrate resistance to IGF-I with aging and declines in liver mass accompany aging. Our aim was to determine if the IGF-I-induced signaling process changes with aging. Young (5 months) and old (24 months) C57BL/6 mice hepatic tissues and blood samples were taken 20 min after an intraperitoneal injection of desIGF-I. Age had no significant effect on plasma glucose, insulin and total IGF-I levels. IRS-1 protein was significantly decreased (33%) with aging. Basal phosphorylation of IRS-1, PKB and ERK were unaffected whereas basal phosphorylation of CREB and FKHR were significantly increased (37 and 33%, respectively) with aging. desIGF-I caused a significant decrease in plasma glucose concentrations in both young (53%) and old (44%) mice. desIGF-I administration significantly increased the phosphorylation of IRS-1 in both young (104%) and old (89%) hepatic tissues. Similarly, the phosphorylation of PKB was dramatically enhanced in both young (527%) and old (350%) hepatic tissues after desIGF-I stimulation. By contrast, desIGF-I administration had no significant effects on the phosphorylation of ERK and phosphorylation of transcription factors CREB and FKHR in both young and old hepatic tissues. These data suggest that aging dose not impair IGF-I signaling in hepatic tissues.

Modulation of human lymphocyte proliferative response with aging

Previously, we have demonstrated age-associated alterations in transmembrane signaling. One of the most reproducible alterations found in the immune response with aging is the decrease of lymphocyte proliferation on stimulation with various different mitogens. Here, we confirm that proliferative responses to stimulation with phytohaemagglutin (PHA), recombinant human IL-2, or anti-CD3 monoclonal antibody are all greater in the young (20-25 years) than old (60-87 years) population. We attempted to modulate the proliferative response using various agents acting at different levels of transmembrane signaling (pertussis toxin, cholera toxin, isoproterenol, PMA, Ca ionophore A23187), as well as at the level of the lymphocyte plasma membrane (methyl-beta-cyclodextrin, MBCD), or by using antioxidant vitamins (Vitamin E or C). None of these agents was able to restore effectively the proliferative response of lymphocytes from the aged to the level of young subjects. Even the combination of A23187 and PMA acting directly on calcium metabolism and protein kinase C activity was insufficient to restore the decreased mitogenic capacity of T cells from elderly subjects. Cyclodextrin, which decreases the cholesterol content of the membrane, increased the proliferative response of lymphocytes of elderly subjects, but not to the level of the young. Vitamin E had a very strong inhibitory effect on lymphocyte stimulation in both the age groups, except in combination with MBCD in T cells of the elderly, while Vitamin C had no significant modulatory effect. MAPK ERK and p38 activation was found to be decreased with aging in T cells after anti-CD3 mAb stimulation. Vitamin E but not Vitamin C strongly inhibited MAPK ERK or p38 activation. The direct activation of certain molecules or the modulation of the cholesterol content of the membrane seems to be effective immunomodulatory interventions with aging.

Loss in oxidative stress tolerance with aging linked to reduced extracellular signal-regulated kinase and Akt kinase activities

Oxidative stress is believed to be an important factor in the development of age-related diseases, and studies in lower organisms have established links between oxidative stress tolerance and longevity. We have hypothesized that aging is associated with a reduced ability to mount acute host defenses to oxidant injury, which increases the vulnerability of aged cells to stress. We tested this hypothesis by using primary hepatocytes from young (4-6 months) and aged (24-26 months) rats. Old hepatocytes were more sensitive to H2O2-induced apoptosis than were young cells. Lower survival is associated with reduced activations of extracellular signal-regulated kinase (ERK) and Akt kinase, both of which protect against oxidant injury. That reduced ERK and Akt activities contribute to lower survival of aged cells was supported by additional findings. First, pharmacologic inhibition of ERK and Akt activation in young cells markedly increased their sensitivity to H2O2. Second, caloric restriction, which increases rodent life span and delays the onset of many age-related declines in physiologic function, prevented loss in ERK and Akt activation by H2O2 and enhanced survival of old hepatocytes to levels similar to those of young cells. Strategies aimed at boosting these host responses to acute oxidant injury could have significant anti-aging benefits.

Loss of functional caveolae during senescence of human fibroblasts

Primary human fibroblasts have a finite replicative lifespan in culture that culminates in a unique state of growth arrest, termed senescence that is accompanied by distinct morphological and biochemical alterations. Senescent cell responses to extracellular stimuli are believed to be altered at a point after receptors are bound by ligand, leading to improper integration of the signals which initiate DNA replication. In this study we demonstrate that one of the key organizing membrane microdomains for receptor signaling, caveolae, are absent in senescent cells. A comparison of young and senescent cells indicated that senescent cells contained a higher total amount of caveolins 1 and 2 but had significantly less of both proteins in the caveolar fraction. Additionally, caveolar fractions from senescent cells completely lacked the tyrosine-kinase activity associated with functional caveolae. Furthermore, old cells had little caveolar protein exposed to the outer plasma membrane as estimated by using an in vivo biotinylation assay and no detectable caveolin 1 on the cell surface when processed for immunofluoresence and confocal microscopy. Together, these data suggest that a fundamental loss of signal integration at the plasma membrane of senescent cells is due to the loss of signaling competent caveolae.

Attenuation of EGF signaling in senescent cells by caveolin

One of the characteristics of senescent cells is unresponsiveness to external stimuli like EGF. Although they have a normal level of receptors and downstream signaling molecules, EGF cannot induce the activation of Erk kinases and DNA synthesis in senescent cells as much as in young cells. Caveolin proteins directly interact with signaling molecules including EGF receptor and suppress the activation of EGFR upon EGF stimulation. We found that Erk activation after EGF stimulation in senescent human diploid fibroblasts was down-regulated. Those senescent cells showed an increased level of three isoforms of caveolin proteins. This change seems to lie in transcriptional control in senescent cells. We also demonstrated up-regulated caveolin proteins were co-localized with EGFR proteins in detergent-insoluble fractions. From these results, we suggest that the up-regulated expression of caveolin might explain the unresponsiveness of senescent fibroblasts to EGF stimulation.