Steroidogenesis: Unanswered Questions

Until the mid-1980s studies of steroidogenesis largely depended on identifying steroid structures and measuring steroid concentrations in body fluids. The molecular biology revolution radically revolutionized studies of steroidogenesis with the cloning of known steroidogenic enzymes, by identifying novel factors, and delineating the genetic basis of known and newly discovered diseases. Unfortunately, this dramatic success has led many young research-oriented endocrinologists to regard steroidogenesis as a 'solved area'. However, many important and exciting questions remain, especially concerning the mechanisms of cholesterol delivery to the steroidogenic machinery, the biochemistry of androgen synthesis, the regulation and biological role of adrenarche, fetal adrenal development and involution, the roles of steroids made in 'extraglandular' cells, and the search for genetic disorders. This review outlines some of these questions, but this list is necessarily incomplete.

Dose-dependent effects of cisplatin on the severity of testicular injury in Sprague Dawley rats: reactive oxygen species and endoplasmic reticulum stress

Cisplatin (CIS) is used in the treatment of cancer, but its nonspecific systemic actions lead to toxic effects on other parts of the body. This study investigated the severity of CIS toxicity by increasing its dose over a constant time period. Sprague Dawley rats were divided into five treatment groups and control group with CIS (2, 4, 6, 8, and 10 mg/kg) administered intraperitoneally for 5 days. The body and organs were weighed, epididymal sperm was counted, and sperm motility and sperm apoptosis were evaluated. Blood samples were evaluated for complete blood count, reactive oxygen and nitrogen species, malondialdehyde levels, and total testosterone. The testicular tissue was examined for steroidogenic acute regulatory protein and endoplasmic reticulum stress protein. Epididymal sperm was collected for CatSper Western blot. The toxic effects of different doses of CIS on the testis and kidney were compared histologically. The weights of body, testis, epididymis, prostate, seminal vesicle, and kidney; sperm count; sperm motility; steroidogenic acute regulatory protein level; and epididymal sperm count were significantly lower in the CIS-treated groups than in the control group. In contrast, sperm apoptosis, plasma reactive oxygen and nitrogen species, and malondialdehyde, testosterone, red blood cell, hematocrit, hemoglobin, and endoplasmic reticulum stress protein levels all increased. Though CIS effectively treats cancer, at an increased dose it is toxic and life-threatening to the genitourinary system and other parts of the body.

Impairment of Macrophage Cholesterol Efflux by Cholesterol Hydroperoxide Trafficking: Implications for Atherogenesis Under Oxidative Stress

OBJECTIVE

Oxidative stress associated with cardiovascular disease can produce various oxidized lipids, including cholesterol oxides, such as 7-hydroperoxide (7-OOH), 7-hydroxide (7-OH), and 7-ketone (7=O). Unlike 7=O and 7-OH, 7-OOH is redox active, giving rise to the others via potentially toxic-free radical reactions. We tested the novel hypothesis that under oxidative stress conditions, steroidogenic acute regulatory (StAR) family proteins not only deliver cholesterol to/into mitochondria of vascular macrophages, but also 7-OOH, which induces peroxidative damage that impairs early stage reverse cholesterol transport.

APPROACH AND RESULTS

Stimulation of human monocyte-derived THP-1 macrophages with dibutyryl-cAMP resulted in substantial upregulation of StarD1 and ATP-binding cassette (ABC) transporter, ABCA1. Small interfering RNA-induced StarD1 knockdown before stimulation had no effect on StarD4, but reduced ABCA1 upregulation, linking the latter to StarD1 functionality. Mitochondria in stimulated StarD1-knockdown cells internalized 7-OOH slower than nonstimulated controls and underwent less 7-OOH-induced lipid peroxidation and membrane depolarization, as probed with C11-BODIPY (4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-inda-cene-3-undecanoic acid) and JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolylcarbocyanine iodide), respectively. Major functional consequences of 7-OOH exposure were (1) loss of mitochondrial CYP27A1 activity, (2) reduced 27-hydroxycholesterol (27-OH) output, and (3) downregulation of cholesterol-exporting ABCA1 and ABCG1. Consistently, 7-OOH-challenged macrophages exported less cholesterol to apoA-I or high-density lipoprotein than did nonchallenged controls. StarD1-mediated 7-OOH transport was also found to be highly cytotoxic, whereas 7=O and 7-OH were minimally toxic.

CONCLUSIONS

This study describes a previously unrecognized mechanism by which macrophage cholesterol efflux can be incapacitated under oxidative stress-linked disorders, such as chronic obesity and hypertension. Our findings provide new insights into the role of macrophage redox damage/dysfunction in atherogenesis.

SOD2 deficiency-induced oxidative stress attenuates steroidogenesis in mouse ovarian granulosa cells

This study investigated the effects of SOD2 (MnSOD)-deficiency-induced excessive oxidative stress on ovarian steroidogenesis in vivo and isolated and cultured granulosa cells using WT and Sod2+/- mice. Basal and 48 h eCG-stimulated plasma progesterone levels were decreased ~50% in female Sod2+/- mice, whereas plasma progesterone levels were decreased ~70% in Sod2+/- mice after sequential stimulation with eCG followed by hCG. Sod2+/- deficiency caused about 50% reduction in SOD2 activity in granulosa cells. SOD2-deficiency also caused a marked reduction in progestins and estradiol in isolated granulosa cells. qRT-PCR measurements indicated that the mRNA expression levels of StAR protein and steroidogenic enzymes are decreased in the ovaries of Sod2+/- mice. Further studies showed a defect in the movement of mobilized cytosolic cholesterol to mitochondria. The ovarian membrane from Sod2+/- mice showed higher susceptibility to lipid peroxidation. These data indicates that SOD2-deficiency induced oxidative stress inhibits ovarian granulosa cell steroidogenesis primarily by interfering with cholesterol transport to mitochondria and attenuating the expression of Star protein gene and key steroidogenic enzyme genes.

De novo Neurosteroidogenesis in Human Microglia: Involvement of the 18 kDa Translocator Protein

Neuroactive steroids are potent modulators of microglial functions and are capable of counteracting their excessive reactivity. This action has mainly been ascribed to neuroactive steroids released from other sources, as microglia have been defined unable to produce neurosteroids de novo. Unexpectedly, immortalized murine microglia recently exhibited this de novo biosynthesis; herein, de novo neurosteroidogenesis was characterized in immortalized human microglia. The results demonstrated that C20 and HMC3 microglial cells constitutively express members of the neurosteroidogenesis multiprotein machinery-in particular, the transduceosome members StAR and TSPO, and the enzyme CYP11A1. Moreover, both cell lines produce pregnenolone and transcriptionally express the enzymes involved in neurosteroidogenesis. The high TSPO expression levels observed in microglia prompted us to assess its role in de novo neurosteroidogenesis. TSPO siRNA and TSPO synthetic ligand treatments were used to reduce and prompt TSPO function, respectively. The TSPO expression downregulation compromised the de novo neurosteroidogenesis and led to an increase in StAR expression, probably as a compensatory mechanism. The pharmacological TSPO stimulation the de novo neurosteroidogenesis improved in turn the neurosteroid-mediated release of Brain-Derived Neurotrophic Factor. In conclusion, these results demonstrated that de novo neurosteroidogenesis occurs in human microglia, unravelling a new mechanism potentially useful for future therapeutic purposes.

Exposure to benzo(a)pyrene from juvenile period to peripubertal impairs male reproductive parameters in adult rats

Benzo(a)pyrene (BaP) is a persistent organic pollutant and endocrine disruptor that can compromise the steroidogenesis process by interacting with the StAR protein, causing adverse effects on male reproduction. However, consequences of prepubertal BaP exposure and its impacts on adult life are yet unknown. This study investigated the effects of BaP exposure from the juvenile period to peripubertal on reproductive parameters in adult male rats. Males were exposed to 0; 0.1; 1 or 10 μg/kg/day of BaP from post-natal (PND) 23 to PND 53 (by gavage). The lowest dose of BaP was able to compromise the male copulatory behavior, as evidenced by the delay in the first mount, intromission and ejaculation. Furthermore, BaP-treated groups showed lower sperm quality (disrupted motility and morphology) and quantity, reduced relative weights of the thyroid and seminal gland. Serum testosterone levels and the Leydig cells nuclei volume were decreased by BaP exposure whereas the StAR expression was increased. Histopathological changes in the testis also were detected in the males exposed to BaP. These results showed that prepubertal BaP-exposure adversely influenced the male reproductive system in the adult life, indicating that a comprehensive risk assessment of BaP-exposure on prepubertal period is necessary.

StAR protein and steroidogenic enzyme expressions in the rat Harderian gland

The Harderian gland (HG) of the rat (Rattus norvegicus) secretes copious amounts of lipids, such as cholesterol. Here we report a study of the expressions of the StAR protein and key steroidogenic enzymes in the HG of male and female rats. The objective of the present investigation was to ascertain (a) whether the rat HG is involved in steroid production starting with cholesterol, and (b) whether the pattern of gene and protein expressions together with the enzymatic activities display sexual dimorphism. The results demonstrate, for the first time, the expression of StAR gene and protein, and Cyp11a1, Hsd3b1, Hsd17b3, Srd5a1, Srd5a2 and Cyp19a1 genes in the rat HG. StAR mRNA and protein expressions were much greater in males than in females. Immunohistochemical analysis demonstrated a non-homogeneous StAR distribution among glandular cells. Hsd17b3 and Cyp19a1 mRNA levels were higher in males than in females, whereas Srd5a1 mRNA levels were higher in females than in males. No significant differences were observed in mRNA levels of Cyp11a1, Hsd3b1 and Srd5a2 between sexes. Furthermore, the in vitro experiments demonstrated a higher 5α-reductase activity in the female as compared to the male HG vice versa a higher P450 aro activity in males as compared to females. These results suggest that the Harderian gland can be classified as a steroidogenic tissue because it synthesizes cholesterol, expresses StAR and steroidogenic enzymes involved in both androgen and estrogen synthesis. The dimorphic expression and activity of the steroidogenic enzymes may suggest sex-specific hormonal effects into the HG physiology.

Macrophage mitochondrial damage from StAR transport of 7-hydroperoxycholesterol: implications for oxidative stress-impaired reverse cholesterol transport

StAR family proteins in vascular macrophages participate in reverse cholesterol transport (RCT). We hypothesize that under pathophysiological oxidative stress, StARs will transport not only cholesterol to macrophage mitochondria, but also pro-oxidant cholesterol hydroperoxides (7-OOHs), thereby impairing early-stage RCT. Upon stimulation with dibutyryl-cAMP, RAW264.7 macrophages exhibited a strong time-dependent induction of mitochondrial StarD1 and plasma membrane ABCA1, which exports cholesterol. 7α-OOH uptake by stimulated RAW cell mitochondria (like cholesterol uptake) was strongly reduced by StarD1 knockdown, consistent with StarD1 involvement. Upon uptake by mitochondria, 7α-OOH (but not redox-inactive 7α-OH) triggered lipid peroxidation and membrane depolarization while reducing ABCA1 upregulation. These findings provide strong initial support for our hypothesis.

Integration of in silico approaches to determination of endocrine-disrupting perfluorinated chemicals binding potency with steroidogenic acute regulatory protein

A myriad of perfluorinated compounds (PFCs) have the ability to interfere with steroidogenic acute regulatory (StAR) protein. Consequently, PFCs breaches cholesterol biotransformation in mitochondria and cause fatal consequences in steroidogenesis, however, these were poorly characterized. In the present study, we have evaluated toxic potencies, nuclear mediated probabilities and interaction profiles with StAR of PFCs using computational system biology tools. Toxicity endpoints revealed that PFCs contain high carcinogenicity, developmental toxicity, skin sensitization effects with low mutagenic activity. Consensus qualitative nuclear receptor agonist models show higher probability rates towards ER and PPAR-γ receptor than AR and AhR models were observed. To poise the subtle fluctuations of actual predictions, balanced accuracy and MCC were computed, and they signify perfect correlation ranges in all models. Screening studies resulting protein-ligand interaction profiles showed that residues Asn148, Asn150, Glu169, Ala171, Arg182, Phe184, Arg188, Trp241, Thr263 and Phe267 were identified as novel hotspots, participated in halogen bonds, H-bonds, atomic π-stacking, π-cation interactions and salt-bridges formation. Thus, the additional bonds contribute conformer stability that holds the protein structure at flexible state, so that PFCs acts as a barrier to cholesterol binding. From docking outcomes, representation space was created, that specifies high and medium StAR binders were occupied in toxic endpoints space with active concern. PFCs restrain molecular features and mitochondrial membrane disruption functions were revealed by efficient toxicogenomics studies. These data indicate toxicity and StAR protein binding levels of PFCs, sorted pinpoints could be useful in a promising way to know the other environmental pollutants and health risks.

OxeR1 regulates angiotensin II and cAMP-stimulated steroid production in human H295R adrenocortical cells

Hormone-regulated steroidogenesis and StAR protein induction involve the action of lipoxygenated products. The products of 5-lipoxygenase act on inflammation and immunity by stimulation of a membrane receptor called OxeR1. The presence of OxeR1 in other systems has not been described up to date and little is known about its mechanism of action and other functions. In this context, the aim of this study was the identification and characterization of OxeR1 as a mediator of cAMP-dependent and independent pathways. Overexpression of OxeR1 in MA-10 Leydig cells increased cAMP-dependent progesterone production. Angiotensin II and cAMP stimulation of adrenocortical human H295R cells produced an increase in StAR protein induction and steroidogenesis in cells overexpressing OxeR1 as compared to mock-transfected cells. Additionally, activation of OxeR1 caused a time-dependent increase in ERK1/2 phosphorylation. In summary, membrane receptor OxeR1 is involved in StAR protein induction and activation of steroidogenesis triggered by cAMP or angiotensin II, acting, at least in part, through ERK1/2 activation.

Effects of acrolein on the production of corticosterone in male rats

Acrolein, an α, β-unsaturated aldehyde, exists in a wide range of sources. Acrolein can be not only generated from all types of smoke but also produced endogenously from the metabolism by lipid peroxidation. The cellular influence of acrolein is due to its electrophilic character via binding to and depleting cellular nucleophiles. Although the toxicity of acrolein has been extensively studied, there is relatively little information about its impact on hormone release. This study aimed at the effect of acrolein on hypothalamic-pituitary-adrenal (H-P-A) axis. In an in vivo study, male rats were administrated with acrolein for 1 or 3days. The plasma corticosterone in response to a single injection of adrenocorticotropic hormone (ACTH) increased slowly in acrolein-pretreated rats than in control rats. Further investigating the steroidogenic pathway, the protein expressions of steroidogenic acute regulatory protein (StAR) and the upper receptor-melanocortin 2 receptor (MC2R) were attenuated in acrolein-treated groups. Another experiment using trilostane showed less activity of P450scc in zona fasciculata-reticularis (ZFR) cells in acrolein-treated groups. In addition to the suppressed ability of corticosterone production in ZFR cells, acrolein even had extended influence at higher concentrations. The lower ACTH was observed in the plasma from acrolein-pretreated rats. In an in vitro study, ZFR cells were incubated with acrolein and the results showed that corticosterone concentrations in media were decreased in a dose-dependent manner. Acrolein also desensitized the response of the ZFR cells to ACTH. These results suggested that acrolein decreased the releasing ability of corticosterone via an inhibition on the response of ZFR cells to ACTH and the reduction of protein expressions of StAR and MC2R as well as the activity of P450scc in rat ZFR cells. The present evidences showed that the H-P-A axis was affected by the administration of acrolein.

Stimulatory Effect of Food Restriction on the Steroidogenesis of Aldosterone in Ovariectomized Rats

Food or calorie restriction (FR or CR) induces several physiological changes including weight loss, metabolic adaptations, mineral and hormonal changes. However, the effects of FR on aldosterone steroidogenesis in zona glomerulosa (ZG) cells have not been elucidated. Therefore, the present study was designed to investigate the effects of FR on aldosterone secretion and the involved mechanisms in ovariectomized (Ovx) rats. Ovx rats were divided into ad libitum fed (control) and FR groups. The FR rats exhibited decreased body weight, water intake, urine flow, sodium excretion and increased plasma aldosterone in comparison with control rats. FR elevated the basal and angiotensin II-stimulated aldosterone secretion from ZG cells. The conversions of 25-hydroxy-cholesterol to pregnenolone or corticosterone to aldosterone in ZG cells of FR group were greater than that in control group. FR group had a higher protein expression of steroidogenic acute regulatory (StAR) protein in ZG cells. However, there was no different protein expression of cytochrome P450 sidechain cleavage enzyme (P450scc) in ZG cells between control and FR groups. In summary, the increased activities of P450scc and aldosterone synthase as well as the protein expression of StAR protein in ZG cells are involved in the effects of FR on aldosterone steroidogenesis in Ovx rats. We also suggest that the increase of aldosterone might be associated with anti-diuresis and antinatriuresis in FR group. These results are helpful for understanding the role of aldosterone in physiological adaptation and renal sodium conservation during FR.