AR, apoE, and cognitive function

Reduced androgen levels in aged men and women might be risk factors for age-related cognitive decline and Alzheimer's disease (AD). Ongoing clinical trials are designed to evaluate the potential benefit of estrogen in women and of testosterone in men. In this review, we discuss the potential beneficial effects of androgens and androgen receptors (ARs) in males and females. In addition, we discuss the hypothesis that AR interacts with apolipoprotein (apoE)4, encoded by epsilon4 and a risk factor for age-related cognitive decline and AD, and the potential consequences of this interaction.     

Androgens modulate beta-amyloid levels in male rat brain

As a normal consequence of aging, men experience a significant decline in androgen levels. Although the neural consequences of age-related androgen depletion remain unclear, recent evidence suggests a link between low androgen levels and the development of Alzheimer's disease (AD). Here, we test the hypothesis that androgens act as endogenous modulators of beta-amyloid protein (Abeta) levels. To investigate this possibility, brain and plasma levels of Abeta were measured in male rats with varying hormonal conditions. Depletion of endogenous sex steroid hormones via gonadectomy (GDX) resulted in increased brain levels of Abeta in comparison to gonadally intact male rats. This GDX-induced increase in Abeta levels was reversed by DHT supplementation, demonstrating a functional role for androgens in modulating brain levels of Abeta. These findings suggest that age-related androgen depletion may result in accumulation of Abeta in the male brain and thereby act as a risk factor for the development of AD.     

Androgen deficiency in the aging male: benefits and risks of androgen supplementation

There is now convincing evidence that in a subset of aging men, increasing with age, plasma testosterone levels fall below a critical level resulting in hypogonadism. This state of testosterone deficiency has an impact on bone, muscle and brain function and is maybe a factor in the accumulation of visceral fat which again has a significant impact on the cardiovascular risk profile. From the above it follows that androgen replacement to selected men with proven androgen deficiency will have beneficial effects. There is, however a concern that androgen administration to aging men may be harmful in view of effects on prostate disease. Benign prostate hyperplasia (BPH) and prostate cancer are typically diseases of the aging male, steeply increasing with age. But epidemiological studies provide no clues that the levels of circulating androgen are correlated with or predict prostate disease. Similarly, androgen replacement studies in men do not suggest that these men suffer in a higher degree from prostate disease than control subjects. It seems a defensible practice to treat aging men with androgens if and when they are testosterone-deficient, but long-term studies including sufficient numbers of men are needed.     

Androgens activate mitogen-activated protein kinase signaling: role in neuroprotection

Recent evidence indicates that testosterone is neuroprotective, however, the underlying mechanism(s) remains to be elucidated. In this study, we investigated the hypothesis that androgens induce mitogen-activated protein kinase (MAPK) signaling in neurons, which subsequently drives neuroprotection. We observed that testosterone and its non-aromatizable metabolite dihydrotestosterone (DHT) rapidly and transiently activate MAPK in cultured hippocampal neurons, as evidenced by phosphorylation of extracellular signal-regulated kinase (ERK)-1 and ERK-2. Importantly, pharmacological suppression of MAPK/ERK signaling blocked androgen-mediated neuroprotection against beta-amyloid toxicity. Androgen activation of MAPK/ERK and neuroprotection also was observed in PC12 cells stably transfected with androgen receptor (AR), but in neither wild-type nor empty vector-transfected PC12 cells. Downstream of ERK phosphorylation, we observed that DHT sequentially increases p90 kDa ribosomal S6 kinase (Rsk) phosphorylation and phosphorylation-dependent inactivation of Bcl-2-associated death protein (Bad). Prevention of androgen-induced phosphorylation of Rsk and Bad blocked androgen neuroprotection. These findings demonstrate AR-dependent androgen activation of MAPK/ERK signaling in neurons, and specifically identify a neuroprotective pathway involving downstream activation of Rsk and inactivation of Bad. Elucidation of androgen-mediated neural signaling cascades will provide important insights into the mechanisms of androgen action in brain, and may present a framework for therapeutic intervention of age-related neurodegenerative disorders.     

Androgens as therapy for androgen receptor-positive castration-resistant prostate cancer

Prostate cancer is the most frequently diagnosed non-cutaneous tumor of men in Western countries. While surgery is often successful for organ-confined prostate cancer, androgen ablation therapy is the primary treatment for metastatic prostate cancer. However, this therapy is associated with several undesired side-effects, including increased risk of cardiovascular diseases. Shortening the period of androgen ablation therapy may benefit prostate cancer patients. Intermittent Androgen Deprivation therapy improves quality of life, reduces toxicity and medical costs, and delays disease progression in some patients. Cell culture and xenograft studies using androgen receptor (AR)-positive castration-resistant human prostate cancers cells (LNCaP, ARCaP, and PC-3 cells over-expressing AR) suggest that androgens may suppress the growth of AR-rich prostate cancer cells. Androgens cause growth inhibition and G1 cell cycle arrest in these cells by regulating c-Myc, Skp2, and p27^Kip via AR. Higher dosages of testosterone cause greater growth inhibition of relapsed tumors. Manipulating androgen/AR signaling may therefore be a potential therapy for AR-positive advanced prostate cancer.     

Prostate cancer risk in testosterone-treated men

Men with classical androgen deficiency have reduced prostate volume and blood prostate-specific antigen (PSA) levels compared with their age peers. As it is plausible that androgen deficiency partially protects against prostate disease, and that restoring androgen exposure increases risk to that of eugonadal men of the same age, men using ART should have age-appropriate surveillance for prostate disease. This should comprise rectal examination and blood PSA measurement at regular intervals (determined by age and family history) according to the recommendations, permanently revisited, published by ISSAM, EAU, Endocrine Society….

Testosterone replacement therapy is now being prescribed more often for aging men, the same population in which prostate cancer incidence increases; it has been suggested that administration in men with unrecognised prostate cancer might promote the development of clinically significant disease. In hypogonadal men who were candidates for testosterone therapy, a 14% incidence of occult cancer was found. A percentage (15.2%) of prostate cancer has been found in the placebo group (with normal DRE and PSA) in the prostate cancer prevention study investigating the chemoprevention potential of finasteride.

The hypothesis that high levels of circulating androgens is a risk factor for prostate cancer is supported by the dramatic regression, after castration, of tumour symptoms in men with advanced prostate cancer. However these effects, seen at a very late stage of cancer development, may not be relevant to reflect the effects of variations within a physiological range at an earlier stage.

Data from all published prospective studies on circulating level of total and free testosterone do not support the hypothesis that high levels of circulating androgens are associated with an increased risk of prostate cancer. A study on a large prospective cohort of 10,049 men, contributes to the gathering evidence that the long standing “androgen hypothesis” of increasing risk with increasing androgen levels can be rejected, suggesting instead that high levels within the reference range of androgens, estrogens and adrenal androgens decrease aggressive prostate cancer risk. Indeed, high-grade prostate cancer has been associated with low plasma level of testosterone. Furthermore, pre-treatment total testosterone was an independent predictor of extraprostatic disease in patients with localized prostate cancer; as testosterone decreases, patients have an increased likelihood of non-organ confined disease and low serum testosterone levels are associated with positive surgical margins in radical retropubic prostatectomy.

A clinical implication of these results concerns androgen supplementation which has become easier to administer with the advent of transdermal preparations (patch or gel) that achieve physiological testosterone serum levels without supra physiological escape levels. During the clinical development of a new testosterone patch in more than 200 primary or secondary hypogonadal patients, no prostate cancer was diagnosed.     

Androgens and estrogens in benign prostatic hyperplasia: past, present and future

Benign prostatic hyperplasia (BPH) and associated lower urinary tract symptoms (LUTS) are common clinical problems in urology. While the precise molecular etiology remains unclear, sex steroids have been implicated in the development and maintenance of BPH. Sufficient data exists linking androgens and androgen receptor pathways to BPH and use of androgen reducing compounds, such as 5α-reductase inhibitors which block the conversion of testosterone into dihydrotestosterone, are a component of the standard of care for men with LUTS attributed to an enlarged prostate. However, BPH is a multifactorial disease and not all men respond well to currently available treatments, suggesting factors other than androgens are involved. Testosterone, the primary circulating androgen in men, can also be metabolized via CYP19/aromatase into the potent estrogen, estradiol-17β. The prostate is an estrogen target tissue and estrogens directly and indirectly affect growth and differentiation of prostate. The precise role of endogenous and exogenous estrogens in directly affecting prostate growth and differentiation in the context of BPH is an understudied area. Estrogens and selective estrogen receptor modulators (SERMs) have been shown to promote or inhibit prostate proliferation signifying potential roles in BPH. Recent research has demonstrated that estrogen receptor signaling pathways may be important in the development and maintenance of BPH and LUTS; however, new models are needed to genetically dissect estrogen regulated molecular mechanisms involved in BPH. More work is needed to identify estrogens and associated signaling pathways in BPH in order to target BPH with dietary and therapeutic SERMs.     

Protective effects of vitamin E against hypercholesterolemia-induced age-related diseases

Hypercholesterolemia is a major risk factor for age-related diseases such as atherosclerosis and Alzheimer’s disease (AD). Changes in human plasma cholesterol levels results from the interaction between multiple genetic and environmental factors. The accumulation of excess cholesterol in blood vessels leads to atherosclerosis. Many studies on this field show that differential expression of oxidative stress-related proteins, lipid metabolism-related enzymes, and receptors response to atherogenic diet. Additionally, excess brain cholesterol has been associated with increased formation and deposition of amyloid-β peptide from amyloid precursor protein which may contribute to the risk and pathogenesis of AD. To consider genetically, more than 50 genes have been reported to influence the risk of late-onset AD. Some of these genes might be also important in cholesterol metabolism and transport. Epidemiological studies have shown an association between high intake and high serum concentrations of antioxidant vitamins like vitamin E and lower rates of ischemic heart diseases. It has been known that vitamin E also inhibits smooth muscle cell proliferation by non-antioxidant mechanism. On the basis of the previous results, vitamin E has been accepted as an important protective factor against hypercholesterolemia-induced age-related diseases.     

Estradiol protects against ischemic brain injury in middle-aged rats

Several clinical studies suggest that estradiol acts as a potent growth and protective factor in the adult brain. Postmenopausal women experience permanent hypoestrogenicity and suffer from increased risk of brain injury associated with neurodegenerative diseases such as stroke and Alzheimer's disease. Estrogen replacement therapy appears to decrease the risk and severity of these neurodegenerative conditions. Studies using animal models have shown that estradiol exerts similar effects in rodents and can enhance cell survival and induce synaptic plasticity. Therefore, we undertook studies to assess whether estradiol treatment can decrease brain injury and cell death induced by an experimental model of ischemia and whether aging animals remain responsive to the protective effects of estradiol. We will review results from recent studies that demonstrate that 1) in young animals, estrogens exert profound protective effects against ischemic brain injury induced by cerebral artery occlusion and 2) the response of aging animals has been tested with varying results. We will discuss and compare our experimental findings that utilize a permanent cerebral artery occlusion model and physiological levels of estradiol replacement therapy in young and middle-aged rats with those of previous studies. These observations provide important insights into the potential protective actions of estrogen replacement therapy on age- and disease-related processes in the brain.     

Roles of humanin and derivatives on the pathology of neurodegenerative diseases and cognition

BackgroundAlzheimer's disease (AD), Parkinson's disease (PD), and age-related macular degeneration (AMD) are common among neurodegenerative diseases, but investigations into novel therapeutic approaches are currently limited. Humanin (HN) is a mitochondrial-derived peptide found in brain tissues of patients with familial AD and has been increasingly investigated in AD and other neurodegenerative diseases.Scope of reviewIn this review, we summarize and discuss the effects of HN on the pathology of neurodegenerative diseases and cognition based on several studies from preclinical to clinical models. The association between cardiac ischemia-reperfusion (I/R) injury and brain are also included. Findings from in vitro studies and those involving mice provide the most fundamental information on the impact of HN and its potential association with clinical studies.Major conclusionsHN plays a considerable role in countering the progression and neuropathology of AD. Inhibition and reduction of oxidative stress and neuroinflammation of the original amyloid hypothesis is the mainstay mechanism. Multiple intracellular mechanisms will be elucidated, including those involved in the anti-apoptotic signaling cascades, the insulin signaling pathway, and mitochondrial function, and especially autophagic activity. These beneficial roles are also found following cardiac I/R injury. Cognitive improvement was found to be related to maintenance of synaptic integrity and neurotransmitter modulation. Small humanin-like peptide 2 demonstrates the neuroprotective effects in PD and AMD via prevention of mitochondrial loss.General significanceComprehensive knowledge of HN effects on cognition and neurodegenerative diseases emphasizes its potential to treat a viable disease, as it ameliorates the pathogenesis of the disease.     

Metals, oxidative stress and neurodegenerative disorders

The neurodegenerative diseases, Alzheimer’s disease (AD) and Parkinson’s disease (PD), are age-related disorders characterized by the deposition of abnormal forms of specific proteins in the brain. AD is characterized by the presence of extracellular amyloid plaques and intraneuronal neurofibrillary tangles in the brain. Biochemical analysis of amyloid plaques revealed that the main constituent is fibrillar aggregates of a 39–42 residue peptide referred to as the amyloid-β protein (Aβ). PD is associated with the degeneration of dopaminergic neurons in the substantia nigra pars compacta. One of the pathological hallmarks of PD is the presence of intracellular inclusions called Lewy bodies that consist of aggregates of the presynaptic soluble protein called α-synuclein. There are various factors influencing the pathological depositions, and in general, the cause of neuronal death in neurological disorders appears to be multifactorial. However, it is clear, that the underlying factor in the neurological disorders is increased oxidative stress substantiated by the findings that the protein side-chains are modified either directly by reactive oxygen species (ROS) or reactive nitrogen species (RNS), or indirectly, by the products of lipid peroxidation. The increased level of oxidative stress in AD brain is reflected by the increased brain content of iron (Fe) and copper (Cu) both capable of stimulating free radical formation (e.g. hydroxyl radicals via Fenton reaction), increased protein and DNA oxidation in the AD brain, enhanced lipid peroxidation, decreased level of cytochrome c oxidase and advanced glycation end products (AGEs), carbonyls, malondialdehyde (MDA), peroxynitrite, and heme oxygenase-1 (HO-1). AGEs, mainly through their interaction with receptors for advanced glycation end products (RAGEs), further activate signaling pathways, inducing formation of proinflammatory cytokines such as interleukin-6 (IL-6). The conjugated aromatic ring of tyrosine residues is a target for free-radical attack, and accumulation of dityrosine and 3-nitrotyrosine has also been reported in AD brain. The oxidative stress linked with PD is supported by both postmortem studies and by studies showing the increased level of oxidative stress in the substantia nigra pars compacta, demonstrating thus the capacity of oxidative stress to induce nigral cell degeneration. Markers of lipid peroxidation include 4-hydroxy-trans-2-nonenal (HNE), 4-oxo-trans-2-nonenal (4-ONE), acrolein, and 4-oxo-trans-2-hexenal, all of which are well recognized neurotoxic agents. In addition, other important factors, involving inflammation, toxic action of nitric oxide (NO·), defects in protein clearance, and mitochondrial dysfunction all contribute to the etiology of PD. It has been suggested that several individual antioxidants or their combinations can be neuroprotective and decrease the risk of AD or slow its progression. The aim of this review is to discuss the role of redox metals Fe and Cu and non-redox metal zinc (Zn) in oxidative stress-related etiology of AD and PD. Attention is focused on the metal-induced formation of free radicals and the protective role of antioxidants [glutathione (GSH), vitamin C (ascorbic acid)], vitamin E (α-Tocopherol), lipoic acid, flavonoids [catechins, epigallocatechin gallate (EGCG)], and curcumin. An alternate hypothesis topic in AD is also discussed.     

Androgen actions and the ovary

Although androgens and the androgen receptor (AR) have defining roles in male reproductive development and function, previously no role in female reproductive physiology beyond testosterone (T) as the precursor in estradiol (E(2)) biosynthesis was firmly established. Understanding the role and specific mechanisms of androgen action via the AR in the ovary has been limited by confusion on how to interpret results from pharmacological studies, because many androgens can be metabolized in vivo and in vitro to steroids that can also exert actions via the estrogen receptor (ESR). Recent genetic studies using mouse models with specific disruption of the Ar gene have highlighted the role that AR-mediated actions play in maintaining female fertility through key roles in the regulation of follicle health, development, and ovulation. Furthermore, these genetic studies have revealed that AR-mediated effects influence age-related female fertility, possibly via mechanisms acting predominantly at the hypothalamic-pituitary axis in a dose-dependent manner. This review focuses on combining the findings from pharmacological studies and novel genetic mouse models to unravel the roles of ovarian androgen actions in relation to female fertility and ovarian aging, as well as creating new insights into the role of androgens in androgen-associated reproductive disorders such as polycystic ovarian syndrome.     

Inflammatory,Apoptotic, and Survival Gene Signaling in Alzheimer’s Disease

Aging is associated with an enhanced susceptibility to brain dysfunction, loss of memory, and cognitive decline and significantly influences the quality of life for the affected individual. Recent molecular–genetic approaches have provided powerful insights into common age-related diseases that are both progressive and multifactorial, such as Alzheimer’s disease (AD), and in vitro in AD models. These investigations have uncovered consistent deficits in brain gene signaling mechanisms and neurotrophic substances known to contribute to normal brain function. Inflammatory signaling pathways involving up-regulation of cytosolic phospholipase A₂ and the arachidonic acid cycle, the depletion of the brain-essential fatty acid docosahexaenoic acid (DHA) and DHA-derived neuroprotectin D1, and changes in the expression of key proapoptotic and antiapoptotic members of the Bcl-2 gene family are thought to be major contributors to pathogenic processes in degenerating brain tissue. This review will focus on the roles of stress genes, apoptosis-related genes, and inflammation in the molecular genetics of AD with emphasis on the interactive nature of inflammatory, neurotrophic, and apoptotic signaling and will highlight areas of rapid progress in the characterization of action of DHA and neuroprotectin D1 and address important research challenges. We also attempt to integrate these molecular, genetic, and neurochemical changes with cellular pathways involved in brain aging to formulate an integrated understanding of multifactorial age-related neurologic disease and pharmacotherapeutic strategies that may be useful in the restoration of homeostatic brain function.     

Proteomic identification of oxidatively modified proteins in Alzheimer's disease brain. Part I: creatine kinase BB,glutamine synthase,and ubiquitin carboxy-terminal hydrolase L-1

Oxidative alterations of proteins by reactive oxygen species (ROS) have been implicated in the progression of aging and age-related neurodegenerative disorders such as Alzheimer's disease (AD). Protein carbonyls, a marker of protein oxidation, are increased in AD brain, indicating that oxidative modification of proteins is relevant in AD. Oxidative damage can lead to several events such as loss in specific protein function, abnormal protein clearance, depletion of the cellular redox-balance and interference with the cell cycle, and, ultimately, to neuronal death. Identification of specific targets of protein oxidation represents a crucial step in establishing a relationship between oxidative modification and neuronal death in AD, and was partially achieved previously in our laboratory through immunochemical detection of creatine kinase BB and beta-actin as specifically oxidized proteins in AD brain versus control brain. However, this process is laborious, requires the availability of specific antibodies, and, most importantly, requires a reasonable guess as to the identity of the protein in the first place. In this study, we present the first proteomics approach to identify specifically oxidized proteins in AD, by coupling 2D fingerprinting with immunological detection of carbonyls and identification of proteins by mass spectrometry. The powerful techniques, emerging from application of proteomics to neurodegenerative disease, reveal the presence of specific targets of protein oxidation in Alzheimer's disease (AD) brain: creatine kinase BB, glutamine synthase, and ubiquitin carboxy-terminal hydrolase L-1. These results are discussed with reference to potential involvement of these oxidatively modified proteins in neurodegeneration in AD brain. Proteomics offers a rapid means of identifying oxidatively modified proteins in aging and age-related neurodegenerative disorders without the limitations of the immunochemical detection method.     

Age related mitochondrial degenerative disorders in humans

Disorders caused by mitochondrial respiratory chain deficiency due to mutations in mitochondrial DNA have varied phenotypes but many involve neurological features often associated with cell loss within specific brain regions. These disorders, along with the increasing evidence of decline in mitochondrial function with ageing, have raised speculation that primary changes in mitochondria could have an important role in age-related neurodegenerative diseases such as Parkinson's disease (PD) and Alzheimer's disease (AD). Evidence supporting a role for mitochondria in common neurodegenerative diseases comes from studies with the toxin MPP+ and familial PD, which has been shown to involve proteins such as DJ-1 and Pink1 (both of which are predicted to have a role in mitochondrial function and oxidative stress). Mutations within the mitochondrial genome have been shown to accumulate with age and in common neurodegenerative diseases. Mitochondrial DNA haplogroups have also been shown to be associated with certain neurodegenerative conditions. This review covers the primary mitochondrial diseases but also discuss the potential role of mitochondria and mitochondrial DNA mutations in mitochondrial and neurodegenerative diseases, in particular in PD and in AD.     

Increased adiposity in DNA binding-dependent androgen receptor knockout male mice associated with decreased voluntary activity and not insulin resistance

In men, as testosterone levels decrease, fat mass increases and muscle mass decreases. Increased fat mass in men, in particular central obesity, is a major risk factor for type 2 diabetes, cardiovascular disease, and all-cause mortality. Testosterone treatment has been shown to decrease fat mass and increase fat-free mass. We hypothesize that androgens act directly via the DNA binding-dependent actions of the androgen receptor (AR) to regulate genes controlling fat mass and metabolism. The aim of this study was to determine the effect of a global DNA binding-dependent (DBD) AR knockout (DBD-ARKO) on the metabolic phenotype in male mice by measuring body mass, fat mass, food intake, voluntary physical activity, resting energy expenditure, substrate oxidation rates, serum glucose, insulin, lipid, and hormone levels, and metabolic gene expression levels and second messenger protein levels. DBD-ARKO males have increased adiposity despite a decreased total body mass compared with wild-type (WT) males. DBD-ARKO males showed reduced voluntary activity, decreased food intake, increased serum leptin and adiponectin levels, an altered lipid metabolism gene profile, and increased phosphorylated CREB levels compared with WT males. This study demonstrates that androgens acting via the DNA binding-dependent actions of the AR regulate fat mass and metabolism in males and that the increased adiposity in DBD-ARKO male mice is associated with decreased voluntary activity, hyperleptinemia and hyperadiponectinemia and not with insulin resistance, increased food intake, or decreased resting energy expenditure.     

Do DHEA/DHEAS play a protective role in coronary heart disease?

Dehydroepiandrosterone (DHEA) and dehydroepiandrosterone-sulfate (DHEAS), the major androgens secreted by human adrenal glands, were suggested to play a protective role in the pathogenesis of atherosclerosis and coronary heart disease. On the basis of a critical review of all existing studies we concluded that 1) there is no evidence of a protective role of DHEA and DHEAS in women, and 2) men with low plasma DHEA and DHEAS levels can be considered as beings at risk of developing a fatal cardiovascular event. These androgens can interfere with atherogenic process by several mechanisms. They influence enzymes such as glucoso-6-phosphate dehydrogenase, which can modify the lipid spectrum. Furthermore, they can inhibit human platelet aggregation, enhance fibrinolysis, slow down cell proliferation and reduce plasma levels of plasminogen activator inhibitor type 1 and tissue plasminogen activator antigen. We suggest that all these DHEA(S) actions are dependent on sex hormone metabolic pathways. There are still insufficient data to advise DHEA supplementation in elderly men, but this type of hormone replacement therapy merits further studies.