Glucocorticoids and aging

Female baboon adrenal zona fasciculata and zona reticularis regulatory and functional proteins decrease across the life course

Debate exists on life-course adrenocortical zonal function trajectories. Rapid, phasic blood steroid concentration changes, such as circadian rhythms and acute stress responses, complicate quantification. To avoid pitfalls and account for life-stage changes in adrenocortical activity indices, we quantified zonae fasciculata (ZF) and reticularis (ZR) across the life-course, by immunohistochemistry of key regulatory and functional proteins. In 28 female baboon adrenals (7.5-22.1 years), we quantified 12 key proteins involved in cell metabolism, division, proliferation, steroidogenesis (including steroid acute regulatory protein, StAR), oxidative stress, and glucocorticoid and mitochondrial function. Life-course abundance of ten ZF proteins decreased with age. Cell cycle inhibitor and oxidative stress markers increased. Seven of the 12 proteins changed in the same direction for ZR and ZF. Importantly, ZF StAR decreased, while ZR StAR was unchanged. Findings indicate ZF function decreased, and less markedly ZR function, with age. Causes and aging consequences of these changes remain to be determined.

The Aging Features of Thyrotoxicosis Mice: Malnutrition, Immunosenescence and Lipotoxicity

The problem of aging is mainly the increase of age-related diseases, and elderly patients have longer hospitalization and worse prognosis. Poorer nutritional status and immunosenescence may be predisposing and severe factors. The mechanism of the high incidence of diseases and poor prognosis behind aging is complex. Finding suitable aging models is of great significance to find strategies to prevent aging related events. In this study, the relationship between thyrotoxicosis and aging was investigated in mice. The results of routine blood tests and flow cytometry showed that immunosenescence occurred in thyrotoxicosis mice, which was characterized by a significant decrease in neutrophils, lymphocytes, CD4+/CD8+ and CD4+IFN-γ+ lymphocytes. Biochemical examination results showed that there were hypocholesterolemia, hypolipoproteinemia, and hyperlipidemia in thyrotoxicosis mice. Serum proteomics analysis showed that the downregulation of complement and coagulation proteins was another manifestation of declined immunity. Moreover, proteomics analysis showed that many downregulated proteins were related to homeostasis, mainly transport proteins. Their downregulation led to the disturbance of osmotic pressure, ion homeostasis, vitamin utilization, lipid transport, hyaluronic acid processing, and pH maintenance. Serum metabolomics analysis provided more detailed evidence of homeostasis disturbance, especially lipid metabolism disorder, including the downregulation of cholesterol, vitamin D, bile acids, docosanoids, and the upregulation of glucocorticoids, triglycerides, sphingolipids, and free fatty acids. The upregulated lipid metabolites were related to lipotoxicity, which might be one cause of immunosenescence and many aging related syndromes. This study provides evidence for the aging model of thyrotoxicosis mice, which can be used for exploring anti-aging drugs and strategies.

Stress hypothesis overload: 131 hypotheses exploring the role of stress in tradeoffs, transitions, and health

Stress is ubiquitous and thus, not surprisingly, many hypotheses and models have been created to better study the role stress plays in life. Stress spans fields and is found in the literature of biology, psychology, psychophysiology, sociology, economics, and medicine, just to name a few. Stress, and the hypothalamic-pituitaryadrenal/interrenal (HPA/I) axis and sympathetic nervous system (SNS), are involved in a multitude of behaviors and physiological processes, including life-history and ecological tradeoffs, developmental transitions, health, and survival. The goal of this review is to highlight and summarize the large number of available hypotheses and models, to aid in comparative and interdisciplinary thinking, and to increase reproducibility by a) discouraging hypothesizing after results are known (HARKing) and b) encouraging a priori hypothesis testing. For this review I collected 214 published hypotheses or models dealing broadly with stress. In the main paper, I summarized and categorized 131 of those hypotheses and models which made direct connections among stress and/or HPA/I and SNS, tradeoffs, transitions, and health. Of those 131, the majority made predictions about reproduction (n = 43), the transition from health to disease (n = 38), development (n = 23), and stress coping (n = 18). Additional hypotheses were classified as stage-spanning or models (n = 37). The additional 83 hypotheses found during searches were tangentially related, or pertained to immune function or oxidative stress, and these are listed separately. Many of the hypotheses share underlying rationale and suggest similar, if not identical, predictions, and are thus not mutually exclusive; some hypotheses spanned classification categories. Some of the hypotheses have been tested multiple times, whereas others have only been examined a few times. It is the hope that multi-disciplinary stress researchers will begin to harmonize their naming of hypotheses in the literature so as to build a clearer picture of how stress impacts various outcomes across fields. The paper concludes with some considerations and recommendations for robust testing of stress hypotheses.

Initiation of human parturition: signaling from senescent fetal tissues via extracellular vesicle mediated paracrine mechanism

A better understanding of the underlying mechanisms by which signals from the fetus initiate human parturition is required. Our recent findings support the core hypothesis that oxidative stress (OS) and cellular senescence of the fetal membranes (amnion and chorion) trigger human parturition. Fetal membrane cell senescence at term is a natural physiological response to OS that occurs as a result of increased metabolic demands by the maturing fetus. Fetal membrane senescence is affected by the activation of the p38 mitogen activated kinase-mediated pathway. Similarly, various risk factors of preterm labor and premature rupture of the membranes also cause OS-induced senescence. Data suggest that fetal cell senescence causes inflammatory senescence-associated secretory phenotype (SASP) release. Besides SASP, high mobility group box 1 and cell-free fetal telomere fragments translocate from the nucleus to the cytosol in senescent cells, where they represent damage-associated molecular pattern markers (DAMPs). In fetal membranes, both SASPs and DAMPs augment fetal cell senescence and an associated ‘sterile’ inflammatory reaction. In senescent cells, DAMPs are encapsulated in extracellular vesicles, specifically exosomes, which are 30–150 nm particles, and propagated to distant sites. Exosomes traffic from the fetus to the maternal side and cause labor-associated inflammatory changes in maternal uterine tissues. Thus, fetal membrane senescence and the inflammation generated from this process functions as a paracrine signaling system during parturition. A better understanding of the premature activation of these signals can provide insights into the mechanisms by which fetal signals initiate preterm parturition.

Novel concepts on pregnancy clocks and alarms: redundancy and synergy in human parturition

The signals and mechanisms that synchronize the timing of human parturition remain a mystery and a better understanding of these processes is essential to avert adverse pregnancy outcomes. Although our insights into human labor initiation have been informed by studies in animal models, the timing of parturition relative to fetal maturation varies among viviparous species, indicative of phylogenetically different clocks and alarms; but what is clear is that important common pathways must converge to control the birth process. For example, in all species, parturition involves the transition of the myometrium from a relaxed to a highly excitable state, where the muscle rhythmically and forcefully contracts, softening the cervical extracellular matrix to allow distensibility and dilatation and thus a shearing of the fetal membranes to facilitate their rupture. We review a number of theories promulgated to explain how a variety of different timing mechanisms, including fetal membrane cell senescence, circadian endocrine clocks, and inflammatory and mechanical factors, are coordinated as initiators and effectors of parturition. Many of these factors have been independently described with a focus on specific tissue compartments.In this review, we put forth the core hypothesis that fetal membrane (amnion and chorion) senescence is the initiator of a coordinated, redundant signal cascade leading to parturition. Whether modified by oxidative stress or other factors, this process constitutes a counting device, i.e. a clock, that measures maturation of the fetal organ systems and the production of hormones and other soluble mediators (including alarmins) and that promotes inflammation and orchestrates an immune cascade to propagate signals across different uterine compartments. This mechanism in turn sensitizes decidual responsiveness and eventually promotes functional progesterone withdrawal in the myometrium, leading to increased myometrial cell contraction and the triggering of parturition. Linkage of these processes allows convergence and integration of the gestational clocks and alarms, prompting a timely and safe birth. In summary, we provide a comprehensive synthesis of the mediators that contribute to the timing of human labor. Integrating these concepts will provide a better understanding of human parturition and ultimately improve pregnancy outcomes.

Human fetal membranes at term: Dead tissue or signalers of parturition?

Various endocrine, immune, and mechanical factors produced by feto-maternal compartments at term increase intrauterine inflammatory loads to induce labor. The role of fetal (placental) membranes (amniochorion) as providers of parturition signals has not been well investigated. Fetal membranes line the intrauterine cavity and grow with and protect the fetus. Fetal membranes exist as an entity between the mother and fetus and perform unique functions during pregnancy. Membranes undergo a telomere-dependent p38 MAPK-induced senescence and demonstrate a decline in functional and mechanical abilities at term, showing signs of aging. Fetal membrane senescence is also allied with completion of fetal maturation at term as the fetus readies for delivery, which may also indicate the end of independent life and longevity of fetal membranes as their functional role concludes. Fetal membrane senescence is accelerated at term because of oxidative stress and increased stretching. Senescent fetal membranes cells produce senescence-associated secretory phenotype (SASP-inflammation) and also release proinflammatory damage-associated molecular patterns (DAMPs), namely HMGB1 and cell-free fetal telomere fragments. In a feedback loop, SASP and DAMPs increase senescence and enhance the inflammatory load to promote labor. Membranes increase the inflammatory load to disrupt homeostatic balance to transition quiescent uterine tissues toward a labor phenotype. Therefore, along with other well-described labor-promoting signals, senescent fetal membranes may also contribute to human term parturition.

Glucocorticoid excess induces a prolonged leucine resistance on muscle protein synthesis in old rats

This experiment was undertaken to examine leucine responsiveness of muscle protein synthesis during dexamethasone treatment and the subsequent recovery in young (4-5 weeks), adult (10-11 months) and old rats (21-22 months). Rats received dexamethasone in their drinking water. The dose and length of the treatment was adapted in order to generate the same muscle atrophy. Protein synthesis was assessed in vitro by incorporation of radiolabelled phenylalanine into proteins at the end of the treatment and after 3 or 7-day recovery. Results showed that dexamethasone did not alter muscle protein synthesis stimulation by leucine in young rats. In contrast, muscles from adult and old rats became totally resistant to leucine. Furthermore, the recovery of leucine responsiveness after dexamethasone withdrawal was slowed down in old rats when compared to younger rats. We concluded that glucocorticoids exert their catabolic action in adult and old rats partly through antagonising the stimulatory effect of leucine and may contribute to sarcopenia in old rats.

Effect of cAMP-induced transcription on the repair of the phosphoenolpyruvate carboxykinase gene by hepatocytes isolated from young and old rats

The repair of UV-induced DNA damage in the phosphoenolpyruvate carboxykinase (PEPCK) gene was studied in primary cultures of hepatocytes isolated from young (6-month-old) and old (24-month-old) rats fed ad libitum and old rats fed a calorie-restricted diet. Incubation of the hepatocytes with cAMP rapidly induced PEPCK transcription and mRNA levels 4- to 5-fold. In absence of cAMP, the repair of the PEPCK fragment was similar in cultured hepatocytes isolated from young and old rats fed ad libitum. However, cAMP significantly increased the percentage of cyclobutane pyrimidine dimers (CPDs) removed from the PEPCK fragment 12 h after UV-irradiation in cultured hepatocytes isolated from young rats fed ad libitum. This increase was due to an increase in the repair of the transcribed strand of the PEPCK fragment. In contrast, cAMP did not increase the repair of the PEPCK fragment in cultured hepatocytes isolated from old rats fed ad libitum in spite of an increase in PEPCK transcription. Thus, it appears that the coupling of transcription and DNA repair is compromised in cultured hepatocytes isolated from old rats fed ad libitum. However, cultured hepatocytes isolated from old rats fed a calorie-restricted diet showed an induction in the rate of repair of the transcribed strand of the PEPCK fragment by cAMP that was similar to hepatocytes isolated from young rats fed ad libitum.

Glucocorticoid-targeting of the adenylyl cyclase signaling pathway in the cerebellum of young vs. aged rats

Glucocorticoids exacerbate aging-induced cell death, but relatively little is known about other CNS effects in senescence. We examined noradrenergic/adenylyl cyclase signaling in the cerebellum, which is a brain region that is susceptible to deterioration of synaptic function in aging. Aged control rats had increased total cyclase catalytic activity, but showed deficits in basal adenylyl cyclase. Deficits resolved when G-proteins were stimulated with GTP, GTP and fluoride, or GTP and isoproterenol, despite reductions in beta-receptors. In young rats, long-term dexamethasone infusions evoked the same types of changes that had been seen in aging, including induction of cyclase catalytic activity and enhanced G-protein responsiveness. The same dexamethasone regimens given to aged rats failed to cause stimulation of these processes in the cerebellum, but did so in a peripheral tissue (kidney). These data indicate homology between the cellular events involved in noradrenergic signaling during aging and after glucocorticoid administration to young animals; the absence of glucocorticoid effects in the elderly cohort supports a convergent mechanism with aging. Given the high incidence of HPA axis dysregulation in the elderly, and particularly in elderly depression, effects of glucocorticoids on cell signaling may contribute to disrupted function and to altered drug reactivity.

Age-related levels of urinary free cortisol in the tree shrew

There are still controversies concerning the effect of aging on the basal glucocorticoid concentration in mammals, including humans. Some studies reported an elevated glucocorticoid concentration in older subjects, while other reports showed no increases with age. These discrepancies may be caused by different experimental designs, gender differences, or varying sampling time points. The bulk of animal studies reporting increases of glucocorticoids with age were performed in rats. The present study was designed to investigate the impact of age on adrenocortical activity in a non-rodent mammalian species, tree shrews (Tupaia belangeri). We analyzed the basal urinary free cortisol concentration in the morning urine of male tree shrews in different age groups. Immediately after birth, a large variation in basal urinary free cortisol concentration (10-818 pg/micromol crea) has been observed. Between 21-40 day of age, the urinary cortisol concentration was low (32.7 +/- 5.6 pg/micromol crea) and increased steadily during puberty until adulthood (201-500 days; 161.8 +/- 15.1 pg/micromol crea). Thereafter, no further rise in basal urinary free cortisol concentration was found with increasing age and after reaching senescence (7-8 years).

Effect of glucocorticoid excess on skeletal muscle and heart protein synthesis in adult and old rats

This study was carried out to analyse glucocorticoid-induced muscle wasting and subsequent recovery in adult (6-8 months) and old (18-24 months) rats because the increased incidence of various disease states results in hypersecretion of glucocorticoids in ageing. Adult and old rats received dexamethasone in their drinking water for 5 or 6 d and were then allowed to recover for 3 or 7 d. As dexamethasone decreased food intake, all groups were pair-fed to dexamethasone-treated old rats (i.e. the group that had the lowest food intake). At the end of the treatment, adult and old rats showed significant increases in blood glucose and plasma insulin concentrations. This increase disappeared during the recovery period. Protein synthesis of different muscles was assessed in vivo by a flooding dose of [13C]valine injected subcutaneously 50 min before slaughter. Dexamethasone induced a significant decrease in protein synthesis in fast-twitch glycolytic and oxidative glycolytic muscles (gastrocnemius, tibialis anterior, extensor digitorum longus). The treatment affected mostly ribosomal efficiency. Adult dexamethasone-treated rats showed an increase in protein synthesis compared with their pair-fed controls during the recovery period whereas old rats did not. Dexamethasone also significantly decreased protein synthesis in the predominantly oxidative soleus muscle but only in old rats, and increased protein synthesis in the heart of adult but not of old rats. Thus, in skeletal muscle, the catabolic effect of dexamethasone is maintained or amplified during ageing whereas the anabolic effect in heart is depressed. These results are consistent with muscle atrophy occurring with ageing.

Increased longevity and resistance to heat shock in Drosophila melanogaster flies exposed to hypergravity

In recent years, attempts have been made to increase longevity in animal models (caloric restriction in rodents or overexpression of catalase and superoxide dismutase in transgenic flies, for instance). We report here that flies submitted to hypergravity (3 or 5 g), for 1 or 4 weeks starting from the second day of imaginal life and transferred after that time to 1 g, have a higher resistance to heat shock than flies living continuously at 1 g. Furthermore, male flies that had lived for 2 weeks from the second day of life at 3 or 5 g, lived longer than those living all the time at 1 g; no longevity increase was observed in females. As far as we know, this is the first example in flies showing that a mild stress at a young age not only increases resistance to an acute stress but also increases longevity. A hypothesis to explain these results could be that heat-shock proteins, which are induced by various stress factors, are synthesized in conditions of hypergravity.

Serotonin transporter expression in rat brain regions and blood platelets: aging and glucocorticoid effects

Hyperactivity of the hypothalamus-pituitary-adrenal axis is more common in elderly depression than in younger cohorts and glucocorticoids are known to influence serotonergic systems. The current study explores the interaction of glucocorticoids with aging on serotonin transporter expression and function. Continuous infusions of dexamethasone (26 days) reduced transporter expression in the aged brain but the ability of imipramine to inhibit synaptosomal [3H]serotonin uptake was unimpaired. These effects were unique to aged animals, as prior work with young adults found no effects of dexamethasone on transporter expression. In contrast to the effects in the brain, there were no differences in platelet transporter expression between young and old rats nor did dexamethasone treatment affect the values in the aged group: thus, the platelet may not reliably model these aspects of CNS function. The results suggest that there are basic biologic differences in the effects of glucocorticoids in aged vs. young brain that could contribute to lowered effectiveness to antidepressants in elderly depression; if transport capacity is already reduced by the effects of increased glucocorticoids, further inhibition of transport by antidepressants would have proportionally less impact on synaptic serotonin concentrations.