Down-regulation of steroidogenic acute regulatory (StAR) protein in rat Leydig cells: implications for regulation of testosterone production during aging

Prevention of Male Late-Onset Hypogonadism by Natural Polyphenolic Antioxidants

Androgen production primarily occurs in Leydig cells located in the interstitial compartment of the testis. In aging males, testosterone is crucial for maintaining muscle mass and strength, bone density, sexual function, metabolic health, energy levels, cognitive function, as well as overall well-being. As men age, testosterone production by Leydig cells of the testes begins to decline at a rate of approximately 1% per year starting from their 30s. This review highlights recent findings concerning the use of natural polyphenolics compounds, such as flavonoids, resveratrol, and phenolic acids, to enhance testosterone production, thereby preventing age-related degenerative conditions associated with testosterone insufficiency. Interestingly, most of the natural polyphenolic antioxidants having beneficial effects on testosterone production tend to enhance the expression of the steroidogenic acute regulatory protein (Star) gene in Leydig cells. The STAR protein facilitates the entry of the steroid precursor cholesterol inside mitochondria, a rate-limiting step for androgen biosynthesis. Natural polyphenolic compounds can also improve the activities of steroidogenic enzymes, hypothalamus-pituitary gland axis signaling, and testosterone bioavailability. Thus, many polyphenolic compounds such as luteolin, quercetin, resveratrol, ferulic acid phenethyl ester or gigantol may be promising in delaying the initiation of late-onset hypogonadism accompanying aging in males.

De novo assembly, characterization and comparative transcriptome analysis of gonads reveals sex-biased genes in Coreoperca whiteheadi

The wild Coreoperca whiteheadi is considered as the primordial species in sinipercine fish, which has valuable genetic information. Unfortunately, C. whiteheadi was listed as a near-threatened species because of the environmental pollution, over-exploitation and species invasion. Therefore, more genetic information is needed to have a better understanding of gonadal development in C. whiteheadi. Here, the first gonadal transcriptomes analysis of C. whiteheadi was conducted and 277.14 million clean reads were generated. A total of 96,753 unigenes were successfully annotated. By comparing ovary and testis transcriptomes, a total of 21,741 differentially expressed genes (DEGs) were identified, of which 12,057 were upregulated and 9684 were downregulated in testes. Among them, we also identified about 53 differentially expressed sex-biased genes. Subsequently, the expression of twenty-four DEGs were confirmed by real-time fluorescence quantitative PCR. Furthermore, the histological analysis was conducted on ovaries and testes of one-year-old C. whiteheadi. Our results provided basic support for further studies on the function of sex-biased genes and the molecular mechanism of sex determination and reproduction in C. whiteheadi.

Male sex hormones, aging, and inflammation

Adequate levels of androgens (eugonadism), and specifically testosterone, are vital compounds for male quality of life, longevity, and positive health outcomes. Testosterone exerts its effects by binding to the androgen receptor, which is expressed in numerous tissues throughout the body. Significant research has been conducted on the impact of this steroid hormone on skeletal, muscle and adipose tissues and on the cardiovascular, immune, and nervous systems. Testosterone levels have also been studied in relation to the impact of diseases, aging, nutrition and the environment on its circulating levels. Conversely, the impact of testosterone on health has also been evaluated with respect to its cardiac and vascular protective effects, body composition, autoimmunity and all-cause mortality. The male aging process results in decreasing testosterone levels over time. The exact mechanisms and impact of these changes in testosterone levels with age on health- and life-span are still not completely clear. Further research is needed to determine the optimal testosterone and androgen levels to protect from chronic age-related conditions such as frailty and osteoporosis.

Gigantol Improves Cholesterol Metabolism and Progesterone Biosynthesis in MA-10 Leydig Cells

In aging males, androgen production by testicular Leydig cells decreases at a rate of approximately 1% per year. Phenolic compounds may enhance testosterone biosynthesis and delay the onset of male hypogonadism. Gigantol is a bibenzyl compound isolated from several types of orchids of the genus Dendrobium. This compound has various biological activities, including antioxidant activity. However, its capacity to regulate gene expression and steroid production in testicular Leydig cells has never been evaluated. We investigated the effect of gigantol on MA-10 Leydig cells' gene expression using an RNA-Seq approach. To further investigate the structure-function relationship of the hydroxy-methoxyphenyl moiety of gigantol, experiments were also performed with ferulic acid and isoferulic acid. According to transcriptomic analysis, all genes coding for cholesterol biosynthesis-related enzymes are increased in response to gigantol treatment, resulting in increased lipid droplets accumulation. Moreover, treatments with 10 μM gigantol increased StAR protein levels and progesterone production from MA-10 Leydig cells. However, neither ferulic acid nor isoferulic acid influenced StAR protein synthesis and progesterone production in MA-10 Leydig cells. Thus, our findings indicate that gigantol improves cholesterol and steroid biosynthesis within testicular Leydig cells.

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.

Inhibition of testosterone synthesis induced by oral TiO2 NPs is associated with ROS-MAPK(ERK1/2)-StAR signaling pathway in SD rat

Titanium dioxide nanoparticles (TiO₂ NPs) have been widely used in food, medical, and other fields; their reproductive toxicity has been reported in numerous studies. However, the relevant toxicity mechanism still requires further exploration. In this paper, the effect of oral exposure to 500 mg/kg TiO₂ NPs (anatase and rutile) in adult male SD rats was studied over 3 and 7 days. Results showed that the total sperm count and testosterone level of 7 days of exposure in serum decreased in the experimental group. Testicular tissue lesions, such as disappearance of Leydig cells, disorder of arrangement of spermatogenic cells in the lumen of convoluted seminiferous tubules, and disorder of arrangement of germ cells, were observed. Meanwhile, the expression of steroidogenic acute regulatory (StAR; the key factors of testosterone synthesis), MAPK (ERK1/2), and phosphorylated ERK1/2 in testes of SD rats after exposure to TiO₂ NPs for 7 days decreased, while the malondialdehyde content increased and superoxide dismutase activity decreased in serum. The present study showed that TiO₂ NPs could cause reproductive toxicity. Notably, anatase is more toxic than rutile. In addition, exposure to 500 mg/kg TiO₂ NPs for 7 days inhibited testosterone synthesis in male rat, which may be related to the reactive oxygen species (ROS)-MAPK (ERK1/2)-StAR signal pathway. Warning that the use of TiO₂ NPs should be regulated.

Histological analysis of the effects of thymoquinone on testicular damage in pentylenetetrazole-induced temporal lobe epilepsy model

In this study, it was aimed to investigate possible ameliorating effects of thymoquinone on testicular damage in an epilepsy model. Adult male Wistar rats were divided into 4 groups. The animals in sham-operated groups were given saline or thymoquinone (s.c.); and the animals in pentylenetetrazole (PTZ) group were applied PTZ. The animals in PTZ+thymoquinone group were given thymoquinone (i.p) for 6 days after applying PTZ. Hematoxylin-eosin, periodic acid-Schiff and TUNEL staining and PCNA, StAR, inhibin β-B immunohistochemistry and ZO-1 immunofluorescence methods were applied. Staining intensity and cell numbers were determined. Degeneration of seminiferous tubules was observed in PTZ group. Most of the tubules showed normal morphology in the PTZ+thymoquinone group. Apoptotic cell index was found to be increased and proliferative index decreased in PTZ group. Thymoquinone administration decreased apoptotic index and increased proliferation index. In PTZ group, ZO-1, StAR and inhibin β-B immunohistochemical staining intensity was observed to be decreased and after thymoquinone application, ZO-1 was increased. StAR and inhibin β-B-positive cell numbers were decreased in PTZ group and increased in the PTZ +thymoquinone group. In this study, it was observed that PTZ-induced epileptic seizures caused testicular damage in the rat and thymoquinone ameliorated these effects.

Improvement of Testicular Steroidogenesis Using Flavonoids and Isoflavonoids for Prevention of Late-Onset Male Hypogonadism

Androgen production, being important for male fertility, is mainly accomplished by the Leydig cells from the interstitial compartment of the testis. Testosterone plays a critical role in testis development, normal masculinization, and the maintenance of spermatogenesis. Within seminiferous tubules, appropriate Sertoli cell function is highly dependent on testicular androgen levels and is essential to initiate and maintain spermatogenesis. During aging, testosterone production by the testicular Leydig cells declines from the 30s in humans at a rate of 1% per year. This review outlines the recent findings regarding the use of flavonoids and isoflavonoids to improve testosterone production, contributing to normal spermatogenesis and preventing age-related degenerative diseases associated with testosterone deficiency. With the cumulation of information on the actions of different flavonoids and isoflavonoids on steroidogenesis in Leydig cells, we can now draw conclusions regarding the structure-activity relationship on androgen production. Indeed, flavonoids having a 5,7-dihydroxychromen-4-one backbone tend to increase the expression of the steroidogenic acute regulatory protein (StAR), being critical for the entry of cholesterol into the mitochondria, leading to increased testosterone production from testis Leydig cells. Therefore, flavonoids and isoflavonoids such as chrysin, apigenin, luteolin, quercetin, and daidzein may be effective in delaying the initiation of late-onset hypogonadism associated with aging in males.

A brief history of the search for the protein(s) involved in the acute regulation of steroidogenesis

The synthesis of steroid hormones occurs in specific cells and tissues in the body in response to trophic hormones and other signals. In order to synthesize steroids de novo, cholesterol, the precursor of all steroid hormones, must be mobilized from cellular stores to the inner mitochondrial membrane (IMM) to be converted into the first steroid formed, pregnenolone. This delivery of cholesterol to the IMM is the rate-limiting step in this process, and has long been known to require the rapid synthesis of a new protein(s) in response to stimulation. Although several possibilities for this protein have arisen over the past few decades, most of the recent attention to fill this role has centered on the candidacies of the proteins the Translocator Protein (TSPO) and the Steroidogenic Acute Regulatory Protein (StAR). In this review, the process of regulating steroidogenesis is briefly described, the characteristics of the candidate proteins and the data supporting their candidacies summarized, and some recent findings that propose a serious challenge for the role of TSPO in this process are discussed.

Effects of fluoride and aluminum on expressions of StAR and P450scc of related steroidogenesis in guinea pigs' testis

A lot of studies have shown that fluoride and aluminum have toxic effect on male reproductive system, but the mechanism of which and the interaction between fluoride and aluminum is still unknown. This study investigated the effects of fluoride (NaF) or/and aluminum (AlCl3) on serum testosterone level, gene and protein expression levels of Steroidogenic Acute Regulatory Protein (StAR) and Cytochrome P450 cholesterol side chain cleavage enzyme (P450scc) in the testes of guinea pigs. Fifty-two guinea pigs were divided randomly into four groups (Control, HiF, HiAl and HiF + HiAl). Fluoride (150 mg NaF/L) or/and aluminum (300 mg AlCl3/L) were orally administrated to male guinea pigs for 13 weeks. The results showed that F and Al reduced number and elevated abnormal ratio of sperm. Meanwhile, the concentrations of serum testosterone in all experimental groups were decreased. P450scc protein expression was significantly reduced in all treatment groups, and StAR expression was decreased remarkably in HiF group and HiF + HiAl group. The levels of StAR mRNA in three groups were reduced by 53.9%, 21.4% and 33.4%, respectively, while the expressions of P450scc mRNA were reduced by 67.8%, 17.0% and 47.8%. Therefore, we concluded that F induced the reduction in testosterone and sperm amount, and thus in lower fertility, which might occur as a consequence of depressed StAR and P450scc mRNA expression. There were no synergistic effects between F and Al, instead, Al weakened the toxicity of F to some extents. The results indicated that Al had antagonism effects on F.

Knockout of the transcription factor Nrf2: Effects on testosterone production by aging mouse Leydig cells

Aging in rodents and men is associated with reduced serum levels of testosterone and Leydig cell testosterone productions. To further investigate the mechanism by which Leydig cell testosterone production declines, the effect of knocking out Nrf2, a master regulator of phase 2 antioxidant genes, was examined. In wild-type mice, testosterone production and serum testosterone levels remained unchanged through middle age (8 months), but then were reduced significantly by old age (21-24 months). In contrast, serum testosterone levels and Leydig cell testosterone production were reduced significantly in the Nrf2-/- mice as early as middle age, and were reduced further in the aged mice. Reduced steroidogenesis in the knockout mice was associated with reduced antioxidant capacity, and increased expression of protein nitrotyrosine residues, a marker of ROS. These results support the hypothesis that, over time, increases in oxidative stress contribute to or cause the reduced testosterone production that characterizes Leydig cell aging.

p38 MAPK regulates steroidogenesis through transcriptional repression of STAR gene

STAR/StarD1, part of a protein complex, mediates the transport of cholesterol from the outer to inner mitochondrial membrane, which is the rate-limiting step for steroidogenesis, and where steroid hormone synthesis begins. Herein, we examined the role of oxidant-sensitive p38 MAPKs in the regulation of STAR gene transcription, using model steroidogenic cell lines. Our data indicate that oxidant activation of p38 MAPK exhibits a negative regulatory role in the induction of functional expression of STAR, as evidenced by enhanced induction of STAR (mRNA/protein) expression and increased steroidogenesis during pharmacological inhibition of p38 MAPK or in cells with increased transient overexpression of a dominant-negative (dn) form of p38 MAPKα or p38 MAPKβ. Studies with rat Star-promoter demonstrated that overexpression of p38 MAPKα-wt, -β, or -γ significantly reduced both basal and cAMP-sensitive promoter activity. In contrast, overexpression of p38 MAPKα-dn, -β, or -γ enhanced the Star promoter activity under basal conditions and in response to cAMP stimulation. Use of various constitutively active and dn constructs and designer knock-out cell lines demonstrated that MKK3 and MKK6, the upstream activators of p38 MAPKs, play a role in p38 MAPKα-mediated inhibition of Star promoter activity. In addition, our studies raised the possibility of CREB being a potential target of the p38 MAPK inhibitory effect on Star promoter activity. Collectively, these data provide novel mechanistic information about how oxidant-sensitive p38 MAPKs, particularly p38 MAPKα, contribute to the negative regulation of Star gene expression and inhibit steroidogenesis.

A comparison of the steroidogenic acute regulatory protein-related lipid transfer (START) domain-containing 6 on the brain and testes between young and aged rats

The START domain-containing 6 (StarD6) was originally reported to play a role during male germ cell maturation. We have since reported on StarD6 in the developing hypothyroid rat brain. Therefore, we investigated qualitative and quantitative changes of StarD6 in the aging rat brain and testes of male Sprague-Dawley rats. Serum testosterone levels decreased with aging and total protein levels of StarD6 in the testes decreased. While the immunolocalization of StarD6 in the spermatocytes decreased, cytoplasmic localization appeared in the aged testes. Compared with young rats, aged rats showed decreased StarD6 in the cerebrum and cerebellum without changes in immunolocalization in the cortical neurons of the cerebral cortex and Purkinje cells of the cerebellar cortex. Aged rats also showed increases in StarD6 in the hippocampus with changes in its immunolocalization from the Stratum pyramidale to the Stratum radiatum and Stratum lacunosum-moleculare. Taken together, StarD6 decreased with aging in the testes, which implies that StarD6 might play a role in impaired spermatogenesis in the aged rat. StarD6 decreased in the cerebrum and the cerebellum, but slightly increased in the hippocampus, which suggests that StarD6 might also play a role for neurosteroidogenesis in the hippocampus of aged rats.

Autophagic deficiency is related to steroidogenic decline in aged rat Leydig cells

Late-onset hypogonadism (LOH) is closely related to secondary androgen deficiency in aged males, but the mechanism remains unclear. In this study, we found that reduced testosterone production in aged rat Leydig cells is associated with decreased autophagic activity. Primary rat Leydig cells and the TM3 mouse Leydig cell line were used to study the effect of autophagic deficiency on Leydig cell testosterone production. In Leydig cells from young and aged rats, treatment with wortmannin, an autophagy inhibitor, inhibited luteinising hormone (LH)-stimulated steroidogenic acute regulatory (StAR) protein expression and decreased testosterone production. In contrast, treatment with rapamycin, an autophagy activator, enhanced LH-stimulated steroidogenesis in Leydig cells from aged, but not young, rats. Intracellular reactive oxygen species (ROS) levels were increased in both young and aged Leydig cells treated with wortmannin but decreased only in aged Leydig cells treated with rapamycin. Furthermore, an increased level of ROS, induced by H(2)O(2), resulted in LH-stimulated steroidogenic inhibition. Finally, knockdown of Beclin 1 decreased LH-stimulated StAR expression and testosterone production in TM3 mouse Leydig cells, which were associated with increased intracellular ROS level. These results suggested that autophagic deficiency is related to steroidogenic decline in aged rat Leydig cells, which might be influenced by intracellular ROS levels.

Effects of apigenin on steroidogenesis and steroidogenic acute regulatory gene expression in mouse Leydig cells

Previous studies reported that the age-related decline in testosterone biosynthesis is associated with a decrease in the steroidogenic acute regulatory (StAR) protein which regulates the rate-limiting step of testosterone biosynthesis. To explore the possibility of delaying this decline using a dietary approach, we have examined the effect of a natural flavonoid, apigenin, on StAR gene expression in mouse Leydig cells. Incubation of these cells with the flavonoid enhanced cyclic adenosine monophosphate (cAMP)-induced steroidogenesis and StAR protein expression. The results from the analyses of StAR mRNA by reverse transcription-polymerase chain reaction and the luciferase assays of StAR promoter activity indicated that this flavonoid enhanced StAR gene expression at the level of transcription. Further studies showed that apigenin blocked the thromboxane A2 receptor and interrupted the signaling through the cyclooxygenase-2-thromboxane A synthase-thromboxane A2-receptor pathway, resulting in a reduction of DAX-1 (dosage sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene-1) protein, a transcriptional repressor of StAR gene expression. When DAX-1 protein was reduced, the sensitivity of the Leydig cells was dramatically enhanced, with sub-threshold level of cAMP being able to induce maximal levels of StAR protein expression and steroid hormone production. The present study suggests a potential application of apigenin to improve StAR protein expression and steroidogenic sensitivity of aging Leydig cells.

The role of specific mitogen-activated protein kinase signaling cascades in the regulation of steroidogenesis

Mitogen-activated protein kinases (MAPKs) comprise a family of serine/threonine kinases that are activated by a large variety of extracellular stimuli and play integral roles in controlling many cellular processes, from the cell surface to the nucleus. The MAPK family includes four distinct MAPK cascades, that is, extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK, c-Jun N-terminal kinase or stress-activated protein kinase, and ERK5. These MAPKs are essentially operated through three-tiered consecutive phosphorylation events catalyzed by a MAPK kinase kinase, a MAPK kinase, and a MAPK. MAPKs lie in protein kinase cascades. The MAPK signaling pathways have been demonstrated to be associated with events regulating the expression of the steroidogenic acute regulatory protein (StAR) and steroidogenesis in steroidogenic tissues. However, it has become clear that the regulation of MAPK-dependent StAR expression and steroid synthesis is a complex process and is context dependent. This paper summarizes the current level of understanding concerning the roles of the MAPK signaling cascades in the regulation of StAR expression and steroidogenesis in different steroidogenic cell models.

Effects of the β2 -agonist clenbuterol on testicular steroidogenic acute regulatory protein mRNA expression in adult rats

This study was carried out to investigate the effects of clenbuterol (CLB) on the testicular (steroidogenic acute regulatory, StAR) protein mRNA expression in rats. Thirty adult male rats were administered CLB by gavage daily at the doses of 0.4, 2.0 and 18.5 mg/kg bw for 14 days in the subacute experiment, whereas 20 rats received a single treatment with CLB at the doses of 20 and 40 mg/kg bw in the acute experiment and 20 rats were treated with 0.9% NaCl solution as vehicle groups. Testicular tissues were collected and snap-frozen in liquid nitrogen and stored at -70 °C until use. The levels of StAR mRNA were detected by RT-PCR. The levels of StAR mRNA were markedly increased (P < 0.05) at both dosages of 20 and 40 mg/kg bw but the effects were not dose-dependent and the mRNA levels of StAR were returned to near normal level after 7 days of CLB withdrawal, compared with the control animals. In the subacute experiment, CLB induced a dose-dependent but no statistical significant reduction (P > 0.05) in the expression levels of StAR mRNA, and the mRNA levels were recovered to near normal level in the groups treated with CLB at dosages of 0.4 and 2.0 mg/kg bw/day following a 7-day withdrawal period, compared with the control animals. The mRNA levels of StAR showed a significant decrease in the groups treated with CLB at the dosage of 18.5 mg/kg bw/day (P < 0.05) after a 1- or 7-day withdrawal period with respect to the control animals. These results demonstrated transient stimulative effects of CLB on testicular StAR mRNA levels and inhibitory effects after treatment with CLB for 14 consecutive days.

ROS inhibit the expression of testicular steroidogenic enzyme genes via the suppression of Nur77 transactivation

Steroidogenesis decreases with aging in the testis, whereas the levels of reactive oxygen species (ROS) increase. In addition, ROS have been reported to inhibit testicular steroidogenesis. Here, we investigated the effects of ROS on the transcriptional activity of Nur77, one of the major transcription factors that regulate the expression of steroidogenic enzyme genes. ROS signaling inhibited Nur77 transactivation, which was diminished by either treatment with c-Jun N-terminal kinase (JNK) inhibitor or the expression of a dominant negative form of JNK. This suggests the involvement of JNK signaling, which elevates the expression of c-Jun as well as its phosphorylation in Leydig cells. In transient transfection assays, c-Jun suppressed Nur77 transactivation in a dose-dependent manner. Further studies using c-Jun mutants revealed that the protein level of c-Jun, but not phosphorylation itself, was important for the suppression of Nur77 transactivation. Nur77 directly interacted with c-Jun in vivo, which blocked the DNA binding activity of Nur77. Together, these results suggest that ROS signaling-mediated c-Jun upregulation suppresses the expression of steroidogenic enzyme genes by inhibiting Nur77 transactivation, resulting in the reduction of testicular steroidogenesis. These findings may provide a mechanistic explanation for the age-related decline in testicular steroid hormone production.

Leydig cells: From stem cells to aging

Leydig cells are the testosterone-producing cells of the testis. The adult Leydig cell population ultimately develops from undifferentiated mesenchymal-like stem cells present in the interstitial compartment of the neonatal testis. Four distinct stages of adult Leydig cell development have been identified and characterized: stem Leydig cells, progenitor Leydig cells, immature Leydig cells and adult Leydig cells. The stem Leydig cells are undifferentiated cells that are capable of indefinite self-renewal, differentiation, and replenishment of the Leydig cell niche. Progenitor Leydig cells are derived from the stem Leydig cells. These spindle-shaped cells are luteinizing hormone (LH) receptor positive, have high mitotic activity, and produce little testosterone but rather testosterone metabolites. The progenitor Leydig cells give rise to immature Leydig cells which are round, contain large amounts of smooth endoplasmic reticulum, and produce some testosterone but also very high levels of testosterone metabolites. A single division of these cells produces adult Leydig cells, which are terminally differentiated cells that produce high levels of testosterone. As men age, serum testosterone levels decline, and this is associated with alterations in body composition, energy level, muscle strength, physical, sexual and cognitive functions, and mood. In the Brown Norway rat, used extensively as a model for male reproductive aging, age-related reductions in serum testosterone result from significant decline in the ability of aged Leydig cells to produce testosterone in response to LH stimulation. This review describes Leydig cell development and aging. Additionally, the molecular mechanisms by which testosterone synthesis declines with aging are discussed.

Evidence that age-related changes in p38 MAP kinase contribute to the decreased steroid production by the adrenocortical cells from old rats

The current studies were initiated to investigate whether excessive oxidative stress exerts its antisteroidogenic action through modulation of oxidant-sensitive mitogen-activated protein kinase (MAPK) signaling pathways. Western blot analysis indicated that aging caused increased phosphorylation and activation of rat adrenal p38 MAPK, but not the ERK1/2 or JNK1/2. Lipid peroxidation measurements (an index of cellular oxidative stress) indicated that adrenal membranes from young animals contained only minimal levels of endogenous thiobarbituric acid-reactive substances (TBARS), and exposure of membranes to enzymatic and non-enzymatic pro-oxidants enhanced TBARS formation approximately 12- and 20-fold, respectively. The adrenal membranes from old animals showed much more susceptibility to lipid peroxidation and exhibited roughly 4- to 6-fold higher TBARS formation than young controls both under basal conditions and in response to pro-oxidants. Qualitatively similar results were obtained when lipid peroxide formation was measured using a sensitive FOXRS (ferrous oxidation-xylenol orange-reactive substances) technique. We next tested whether aging-induced excessive oxidative insult alters steroidogenesis through modulation of MAPK signaling pathway. Treatment of adrenocortical cells from old rats with specific p38 MAPK inhibitors restored Bt(2)cAMP-stimulated steroidogenesis approximately 60-70% of the value seen in cells of young animals. Likewise, pretreatment of cells with reactive oxygen species (ROS) scavengers MnTMPyP and N-acetyl cysteine also partially rescued age-induced loss of steroid production. In contrast, simultaneous treatment of cells with ROS scavengers and p38 MAPK inhibitor did not produce any additional effect suggesting that both types of inhibitors exert their stimulatory action through inhibition of p38 MAPK activation. Collectively, these results indicate that p38 MAPK functions as a signaling effector in oxidative stress-induced inhibition of steroidogenesis during aging.

Cyclooxygenase-2 regulation of the age-related decline in testosterone biosynthesis

The age-related decline in testosterone biosynthesis in testicular Leydig cells has been well documented, but the mechanisms involved in the decline are not clear. Recent studies have described a cyclooxygenase-2 (COX2)-dependent tonic inhibition of Leydig cell steroidogenesis and expression of the steroidogenic acute regulatory protein (StAR). The present study was conducted to determine whether COX2 protein increases with age in rat Leydig cells and whether COX2 plays a role in the age-related decline in testosterone biosynthesis. Our results indicate that from 3 months of age to 30 months, COX2 protein in aged rat Leydig cells increased by 346% over that of young Leydig cells, StAR protein decreased to 33%, and blood testosterone concentration and testosterone biosynthesis in Leydig cells decreased to 41 and 33%, respectively. Further experiments demonstrated that overexpressing COX2 in MA-10 mouse Leydig cells inhibited StAR gene expression and steroidogenesis and that the inhibitory effects of COX2 could be reversed by blocking COX2 activity. Notably, incubation of aged Leydig cells with the COX2 inhibitor NS398 enhanced their testosterone biosynthesis. Blood testosterone concentrations in aged rats fed the COX2 inhibitor DFU, at doses of 5, 10, 15, and 20 mg/kg body weight per day were increased by 15, 23, 56, and 120%, respectively, over the levels in the rats receiving no DFU. The present study suggests a novel mechanism in male aging involving COX2 and a potential application of the mechanism to delay the age-related decline in testosterone biosynthesis.

Suppression of steroidogenesis and activator protein-1 transcription factor activity in rat adrenals by vitamin E deficiency-induced chronic oxidative stress

Excessive oxidative stress and associated macromolecular damage are considered to be key features of aging, and appear to contribute to the age-related decline in steroid hormone production in adrenal and testicular Leydig cells. The current studies were initiated to examine the potential mechanism by which excessive oxidative stress during aging attenuates the functional expression of the oxidant-responsive transcription factor Activator protein-1. Chronic oxidative stress was induced in vivo by maintaining groups of rats on a diet deficient in vitamin E for 6 months. Plasma, liver, and adrenal tissues from vitamin E-deficient animals had negligible levels of this vitamin and showed high susceptibility to in vitro lipid peroxidation. Synthesis and secretion of corticosterone in response to corticotropin (ACTH), dibutyryl-cAMP, or 20alpha-hydroxycholesterol in vitro was significantly reduced in adrenocortical cells prepared cells from rats deficient in vitamin E. AP-1 DNA-binding activity was diminished approximately 55 % in adrenal extracts from vitamin E-deficient rats with no corresponding change in the binding activity of SP-1. The vitamin E deficiency-mediated loss of AP-1 activity was not due to an alteration in the dimeric composition of constituent proteins, but rather to a general down-regulation of steady-state levels of members of the Fos and Jun families of proteins. Interestingly, vitamin E deficiency also reduced the expression of the redox-regulated Ref-1 protein. Collectively these data demonstrate that chronic oxidative stress specifically down-regulates essential components of the AP-1 transcription factor complex, and suggest that aberrancies in AP-1 expression may adversely affect processes crucial for intracellular cholesterol transport and steroid hormone production.

Aging alters the functional expression of enzymatic and non-enzymatic anti-oxidant defense systems in testicular rat Leydig cells

In aged rats, trophic hormone-stimulated testosterone secretion by isolated Leydig cells is greatly reduced. The current studies were initiated to establish a functional link between excess oxidative stress and the age-related decline in steroidogenesis. Highly purified Leydig cell preparations obtained from 5-month (young mature) and 24-month (old) Sprague-Dawley rats were employed to measure and compare levels of lipid peroxidation, non-enzymatic (alpha-tocopherol, ascorbic acid, and reduced/oxidized glutathione) and enzymatic (Cu, Zn-superoxide dismutase, Cu, Zn-SOD; Mn-superoxide dismutase, Mn-SOD; glutathione peroxidase-1, GPX-1, and catalase, CAT) anti-oxidants. The extent of lipid peroxidation (oxidative damage) in isolated membrane fractions was quantified by measuring the content of thiobarbituric acid-reactive substances (TBARS) under basal conditions, or in the presence of non-enzymatic or enzymatic pro-oxidants. Membrane preparations isolated from Leydig cells from old rats exhibited two- to three-fold enhancement of basal TBARS formation. However, aging had no significant effect on TBARS formation in response to either non-enzymatic or enzymatic pro-oxidants. Among the non-enzymatic anti-oxidants, the levels of reduced glutathione were drastically reduced during aging, while levels of alpha-tocopherol and ascorbic acid remained unchanged. Both steady-state mRNA levels and catalytic activities of Cu, Zn-SOD, Mn-SOD, and GPX-1 were also significantly lower in Leydig cells from 24-month-old rats as compared with 5-month-old control rats. In contrast, neither mRNA levels nor enzyme activity of catalase was sensitive to aging. From these data we conclude that aging is accompanied by reduced expression of key enzymatic and non-enzymatic anti-oxidants in Leydig cells leading to excessive oxidative stress and enhanced oxidative damage (lipid peroxidation). It is postulated that such excessive oxidative insult may contribute to the observed age-related decline in testosterone secretion by testicular Leydig cells.

Age-dependent modulation of NF-kappaB expression in rat adrenal gland

The current studies were initiated to examine the expression and regulation of an oxidative stress-responsive transcription factor, NF-kappa B, in rat adrenals during aging. NF- kappa B DNA-binding activity and expression of constituent proteins (Rel family of proteins and I kappa Bs) was measured in adrenal nuclear and cytoplasmic extracts from young mature (5 month) and old (24 month) Sprague-Dawley rats before and after treatment with LPS; the latter was used to further invoke oxidative stress. Administration of LPS to either young or old rats induced a dramatic activation of NF- kappa B DNA binding activity as assayed by EMSA. NF- kappa B hetero-dimers, RelA/NF- kappa B1 (p65/p50) accounted for the majority of proteins that bound to consensus NF- kappa B sequences in LPS-treated young and old animals. The intensity of DNA binding complexes was significantly reduced in old animals. The age-related decline in the activation of NF- kappa B could not be attributed to an alteration in the composition of constituent subunits or degradation of NF- kappa B inhibitory proteins (I kappa B alpha and I kappa B beta) but rather was due to selective down-regulation of RelA/p65 and NF- kappa B2/p52 proteins. No age-related or LPS-induced changes in the constitutively active transcription factors SP-1 and OCT-1 were detected. These data suggest that aberrancies in the activation of NF- kappa B DNA-binding activity may contribute to the excessive oxidative damage observed in adrenal tissue with aging and may adversely affect cellular processes crucial for intracellular cholesterol transport and steroid hormone production.

Impaired activation of AP-1 and altered expression of constituent proteins in rat adrenal during ageing

Oxidative stress appears to be one of the primary factors contributing to an age related decline in steroidogenic response in rat adrenocortical and testicular Leydig cells. In this report we concentrate on age-related changes in the DNA binding activity of the transcription factor AP-1 which is particularly responsive to changes in cellular oxidative conditions: adrenal nuclear extracts from young mature (5 months) and old (24 months) rats treated with, and without, lipopolysaccharide (LPS) were studied. AP-1 binding activity, as measured by electrophoretic mobility shift assays (EMSA), was diminished approximately 70% with age in unstimulated adrenals. Following LPS treatment, AP-1 binding activity increased significantly in the adrenals of both young and old animals; however, the level of AP-1 binding achieved in LPS-stimulated old rats was less than that observed for LPS-stimulated young rats. There was no corresponding change in the binding activity of housekeeping transcription factors SP-1 and OCT-1. To further understand these observations, compositional changes in the members of the AP-1 DNA-binding complex were examined by a super-shift assay and Western blot analysis. In adrenals from old rats, a significant decrease in the amount of Fra2 was noted under basal conditions, whereas, substantial decreases in c-Fos, Jun D and c-Jun were observed in response to LPS treatment. In contrast, basal levels of JunB, an inhibitor of the trans-activating function of c-Jun and repressor of AP-1-dependent transcription, were significantly elevated in adrenals from old rats compared to young rats. Together, these findings suggest that ageing-induced oxidative stress may contribute to impaired functional expression of AP-1 by differentially regulating the steady state levels of AP-1 components. The observed decrease in AP-1 binding activity in ageing adrenals is most likely due to decreased expression of the AP-1 activating components (c-Fos, c-Jun, JunD, etc.) and increased expression of JunB, resulting in a switch from transcriptionally active AP-1 complexes observed in young rats to less efficient JunB containing complexes in old rats.

StAR protein and the regulation of steroid hormone biosynthesis

Steroid hormone biosynthesis is acutely regulated by pituitary trophic hormones and other steroidogenic stimuli. This regulation requires the synthesis of a protein whose function is to translocate cholesterol from the outer to the inner mitochondrial membrane in steroidogenic cells, the rate-limiting step in steroid hormone formation. The steroidogenic acute regulatory (StAR) protein is an indispensable component in this process and is the best candidate to fill the role of the putative regulator. StAR is expressed in steroidogenic tissues in response to agents that stimulate steroid production, and mutations in the StAR gene result in the disease congenital lipoid adrenal hyperplasia, in which steroid hormone biosynthesis is severely compromised. The StAR null mouse has a phenotype that is essentially identical to the human disease. The positive and negative expression of StAR is sensitive to agents that increase and inhibit steroid biosynthesis respectively. The mechanism by which StAR mediates cholesterol transfer in the mitochondria has not been fully characterized. However, the tertiary structure of the START domain of a StAR homolog has been solved, and identification of a cholesterol-binding hydrophobic tunnel within this domain raises the possibility that StAR acts as a cholesterol-shuttling protein.

Androgens and aging

This review supports the conclusion that the use of testosterone replacement therapy for the treatment of the symptoms of androgen deficiency in aging males (ADAM) may be appropriate if undertaken with due caution. The best safety data exists for using oral therapy with testosterone undeconoate. Less data exists for the treatment of postmenopausal women with testosterone. Tibolone (an estrogen--prostagen--testosterone) compound, may make the need for testosterone being added to estrogen in women unnecessary. Less data is available to support the use of DHEA or its sulfate or pregnenolone. Further studies are clearly necessary in all these areas. However, it may be tentatively concluded that we are at the dawning of the age of androgens.