Recent neuroendocrine facets of male reproductive aging

Sirt1 and Nrf2: regulation of Leydig cell oxidant/antioxidant intracellular environment and steroid formation†

Previous studies reported that, with aging, Leydig cell intracellular antioxidants are reduced in concentration and intracellular ROS levels increase, suggesting that oxidant/antioxidant imbalance may contribute to the reduced testosterone production that characterizes the aging cells. As yet, little is known about how the Leydig cell oxidant/antioxidant environment is regulated. Sirt1, an enzyme that deacetylates transcription factors, and the transcription factor Nrf2, have been shown to be associated with cellular response to oxidative stress. We hypothesized that Sirt1 and/or Nrf2 might be involved in regulating the oxidant/antioxidant environment of Leydig cells, and therefore, the testosterone production. We found that Sirt1 and Nrf2 are present in the Leydig cells of Brown Norway rats, though reduced in aged cells. In MA-10 cells in which Sirt1 or Nrf2 were suppressed by nicotinamide (NAM) or ML385, respectively, or in which siRNAs were used for knockdown of Sirt1 or Nrf2, increased ROS levels and decreased progesterone production occurred. In rat Leydig cells, inhibition of Sirt1 by culturing the cells with NAM resulted in increased ROS and reduced testosterone production, and subsequent removal of NAM from the culture medium resulted in increased testosterone production. Activation of rat Leydig cells Sirt1 with honokiol or of Nrf2 with sulforaphane resulted in the maintenance of testosterone production despite the exposure of the cells to oxidizing agent. These results, taken together, suggest that Sirt1 and Nrf2 are involved in maintaining the Leydig cell oxidant/antioxidant environment, and thus in maintaining steroid production.

Pig Hsd17b3: Alternative splice variants expression, insertion/deletion (indel) in promoter region and their associations with male reproductive traits

Hydroxysteroid 17-Beta Dehydrogenase 3 (Hsd17b3), primarily expressed in Leydig cells (LCs) of the mammalian testes, is essential for testosterone biosynthesis and male fertility. The aim of our study was to profile the expression, splice variants (SV) and novel insertion/deletion (indel) of Hsd17b3 in boars. Quantitative analysis showed that the expression level of Hsd17b3 in the testis was significantly highest. Among different testicular cell types, the Hsd17b3 mRNA expression level of LCs was significantly higher than that of SSCs (spermatogonial stem cells) and SCs (Sertoli cells). Furthermore, the SV was firstly identified in pigs and it was highly expressed in LCs comparing with SSCs and SCs. In addition, two mutations were identified in pig Hsd17b3 gene promotor and intron, respectively, which were associated with male reproductive traits (P <  0.05). In conclusion, both transcripts of Hsd17b3 gene were highly expressed in pig testes and LCs; the two novel indel variants of Hsd17b3 gene can be used as potential DNA makers for the marker-assisted selection in pigs. All these findings would enrich the study of Hsd17b3 gene in pig genetic breeding.

Role of hypothalamus in aging and its underlying cellular mechanisms

Aging is characterized by a progressive loss of several physiological functions that can cause various age-related disorders. Several factors have been identified as causes of aging to elucidate the decline in functions. Various aspects of physiological deterioration are controlled by the hypothalamus, a critical brain region that connects the neuroendocrine system to physiological functions. In addition, functional alterations in a set of agouti-related peptide/neuropeptide Y (AgRP/NPY) and pro-opiomelanocortin (POMC) neurons, a set of growth hormone-releasing hormone (GHRH) and somatostatin (SST) neurons, a set of arginine vasopressin (AVP) and vasoactive intestinal peptide (VIP) neurons, and a set of gonadotropin-releasing hormone (GnRH) and kisspeptin/neurokinin B/dynorphin (KNDy) neurons contribute to age-related physiological decline in energy metabolism, hormone regulation, circadian rhythm, and reproduction, respectively. The underlying cellular mechanism for the hypothalamus-mediated aging progression comprises dysregulation of nutrient sensing, altered intercellular communication, stem cell exhaustion, loss of proteostasis, and epigenetic alterations. Furthermore, mammalian target of rapamycin (mTOR), NF-kB, hypothalamic stem cell, autophagy, and SIRT1 have been recognized as critical factors or pathways mediating the mechanism. Perhaps, further dissection of these pathways or components could provide the potential for developing a therapeutic intervention for age-related diseases or the extension of healthy lifespan.

Delayed testicular aging in pituitary adenylate cyclase-activating peptide (PACAP) null mice

Age-related decline in male sex hormones is a direct consequence of testicular aging. These changes in the hormonal complement cause physiological disturbances affecting the quality of life for millions of aging men. To assess the influence on testicular aging of pituitary adenylate cyclase-activating peptide (PACAP), a polypeptide that regulates testicular steroidogenesis in vitro, we compared the testicular structure and function between C57BL/6 wild-type and PACAP-/- male mice, at 4 and 15 months of age. We show that, in 4-month-old PACAP-/- mice, steroidogenesis (evaluated by levels of testosterone, steroidogenic acute regulatory protein, 3beta-hydroxysteroid dehydrogenase, and P450c17) was impaired. However, the testicular structure of these animals was not affected. At 15 months of age, wild-type testis displayed typical signs of aging (patchy seminiferous tubules, germ cell depletion, and vacuolization), whereas testicular structure was remarkably well conserved in PACAP-/- animals. The depletion of germ cells found in wild-type animals was associated with a higher content of peroxynitrites, a marker of reactive oxygen species, and a higher number of apoptotic cells compared with PACAP-/- mice. Our results show that testicular aging is delayed in PACAP-/- animals. Because the expression levels of steroidogenic factors are low and constant over time in knockout animals, a proposed mechanism for the protection against testicular degeneration is that production of reactive oxygen species, a byproduct of steroidogenesis that induces apoptosis, is down-regulated in PACAP-/- animals.

Therapie des „Aging male“

Serum testosterone levels decline in men with increasing age. Late-onset hypogonadism with its characteristic symptoms can occur in men as they age. Typical symptoms of late-onset hypogonadism are decreased libido and sexual function, osteoporosis, altered distribution of body fat, overall reduction in physical strength, and alterations in general mood. Late-onset hypogonadism can be treated with testosterone, and different forms of testosterone have become available for this indication. The aim of testosterone replacement therapy is to produce serum testosterone levels within the physiological range avoiding levels above and below this range. Although hormone replacement therapy has become accepted in aging males, careful consideration of the indications and therapy monitoring are still required since there are major concerns about the long-term outcome of this therapy and particularly its effects on the prostate gland.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Androgen receptor gene expression in leucocytes is hormonally regulated: implications for gender differences in disease pathogenesis

OBJECTIVE

There is evidence that male sex hormones influence the rate of progression of inflammatory and cardiovascular diseases. We have previously shown that human leucocytes and arterial cells isolated from male donors express more androgen receptor (AR) than those from female cells, with potentially pro-atherogenic effects. We now investigate whether the gender difference in AR expression is due to genetic or hormonal regulation.

DESIGN AND PATIENTS

The influence of hormones on AR expression were studied in hpg mice (a mouse model of androgen deficiency) treated with testosterone, oestradiol or dihydrotestosterone (DHT). Blood samples were obtained for leucocyte AR expression and hormone levels from 53 subjects, grouped into: 12 male [six young adult (27-45 years), six elderly (71-79 years)] and six female (young adult 25-45 years) healthy controls; six male-to-female transsexuals (M2F; 20-50 years) receiving stable pharmacological oral oestrogen treatment; six female-to-male transsexuals (F2M; 31-51 years) receiving stable androgen replacement therapy; five younger men (18-56 years) who had been receiving long-term androgen replacement therapy for hypogonadal disease; six elderly men (72-88 years) who had undergone medical castration for prostate cancer treatment; and 12 male bone marrow transplant recipients (BMT; 23-65 years) from either male or female donors.

MEASUREMENTS

Serum testosterone and oestradiol concentrations were measured by established immunoflurometric assays from unextracted human serum. AR mRNA levels were measured by RT-PCR and AR protein levels by western blot (cell culture) or immunohistochemistry (mouse arteries).

RESULTS

We found that AR mRNA levels were significantly down-regulated in the leucocytes of hpg mice that were treated with exogenous testosterone, oestradiol or DHT. AR protein levels were also lower in aortic tissue from the same mice. In humans, we found AR expression was significantly down-regulated by exogenous treatment with testosterone in F2M (31 +/- 13%, compared with control) or oestradiol in M2F (22 +/- 5%) but was significantly up-regulated by endogenous testosterone in BMT (128 +/- 17%). Low androgen levels measured in castrated older men were associated with markedly increased AR expression (207 +/- 26%, P < 0.05) compared with age-matched older male controls (100 +/- 2%).

CONCLUSIONS

Our results indicate a regulated ability of vascular cells to respond to sex hormones, with the effects of exogenous therapies differing markedly from those due to endogenous sex hormones. We conclude that the gender difference in AR expression in vascular cells is hormonally, rather than genetically, controlled.

Dibutyryl cyclic adenosine monophosphate restores the ability of aged Leydig cells to produce testosterone at the high levels characteristic of young cells

The wealth of knowledge about the function and regulation of adult Leydig cells, the cells within the mammalian testis that produce testosterone, make these cells ideal for studying principles and mechanisms of aging. A hallmark of mammalian aging is decreased serum testosterone concentration. In the Brown Norway rat, this has been shown to be associated with the reduced ability of aged Leydig cells to produce testosterone in response to LH. Herein, we demonstrate that culturing the aged cells with dibutyryl cAMP, a membrane-permeable cAMP agonist that bypasses the LH receptor-adenlyly cyclase cascade, restores testosterone production to levels comparable to those of young cells and also restores steroidogenic acute regulatory protein and P450scc, the proteins involved in the rate-limiting steps of steroidogenesis. These results strongly suggest that signal transduction deficits are responsible for reduced steroidogenesis by aged Leydig cells and that bypassing signal transduction reverses the steroidogenic decline by the aged cells.

Gene expression by the anterior pituitary gland: effects of age and caloric restriction

Biological aging is associated with functional deficits at the cellular, organ, and system levels. The pituitary gland, the central organ of the neuroendocrine system, has been shown to play an important role in the aging process. To gain a better understanding of its functional changes with aging, we compared the gene expression profiles of the anterior pituitary of young and old Brown Norway rats, focusing on the major pituitary hormone genes. We also explored the effects of caloric restriction, an intervention shown to delay or inhibit age-associated pathologic and biologic changes in a number of systems and organisms, on the expression of these genes. Of the total of 1176 genes arrayed on each of the six membranes per group that we used, 542 (46%) were detectable in the anterior pituitary of young and old rats. Significance analysis of microarrays (SAM) of these 542 detectable genes revealed 28 genes that changed significantly with age, among which 24 decreased and 4 increased. Among the five major hormone genes on the membrane, growth hormone (GH) and prolactin decreased with age, the glycoprotein hormone common alpha subunit gene increased, and follicle-stimulating hormone-beta subunit (FSH-beta) and thyrotropin-beta (TSH-beta) subunit did not change. Among these genes, the three found to change by array analysis were confirmed to do so by Northern blot analysis. For the two genes among the five that were not selected (i.e. did not change) by array analysis, TSH-beta also showed no significant change by Northern blot; but the other, FSH-beta, showed significant increase. Thus, of the five genes checked by Northern blot analysis, the results were consistent with the array data in four cases. Short-term caloric restriction (5 weeks) of young adult animals resulted in 19 genes being significantly down-regulated, while no significantly up-regulated genes were identified. Among the genes that were down-regulated were GH, gonadotropin releasing hormone receptor (GnRH-R), three cytochrome c oxidase subunits and two heat shock proteins. With long-term (21 month) caloric restriction, about 30% of the genes that changed with aging (8/28) were prevented from doing so, and none of the age-related changes was enhanced with long-term caloric restriction. The genes that showed most significant rescue were neuropeptide Y, GnRH-R, DNA-binding protein inhibitor Id-3, and nerve growth factor-induced protein I-B. These results indicate that long-term caloric restriction can partially prevent some of the age-related changes in gene expression in the anterior pituitary of Brown Norway rats, suggesting a benefit of this regimen to be the slowing of the aging process. The fact that fewer than 30% genes derived benefit also suggests that the effect of caloric restriction is rather limit, which is consistent with the thesis that caloric restriction may slow, but not prevent, the aging process.

Age-related increase in mitochondrial superoxide generation in the testosterone-producing cells of Brown Norway rat testes: relationship to reduced steroidogenic function?

Aging in Brown Norway rats is accompanied by the reduced production of testosterone by the Leydig cells, the testicular cells responsible for synthesizing and secreting this essential steroid. As yet, the mechanism by which Leydig cell steroidogenesis is reduced is unknown. Herein we assess the production of mitochondrial reactive oxygen species by intact Leydig cells isolated from the testes of young and old rats. To this end, Leydig cells were incubated with lucigenin (bis-N-methylacridinium nitrate), a probe that enters cells, localizes to mitochondria, and yields a significant chemiluminescent response following its reaction with intramitochondrial superoxide. Leydig cells from old rats elicited significantly greater lucigenin-derived chemiluminescence (LDCL) than those from young rats. Electron microscopic stereological analysis revealed that the absolute volume of mitochondria in the old cells was reduced from that in the young. These results, taken together, suggest that there are age-related changes in the production of reactive oxygen species by the mitochondria of Leydig cells, with those of old Leydig cells producing significantly greater levels than those of young Leydig cells. The results are consistent with the proposal that mitochondrial-derived reactive oxygen may play a role in the irreversible decline in the ability of old Leydig cells to produce testosterone.