Structure-function relationships in 3alpha-hydroxysteroid dehydrogenases: a comparison of the rat and human isoforms

Aldo-keto reductase (AKR) superfamily website and database: An update

The aldo-keto reductase (AKR) superfamily is a large family of proteins found across the kingdoms of life. Shared features of the family include 1) structural similarities such as an (α/β)8-barrel structure, disordered loop structure, cofactor binding site, and a catalytic tetrad, and 2) the ability to catalyze the nicotinamide adenine dinucleotide (phosphate) reduced (NAD(P)H)-dependent reduction of a carbonyl group. A criteria of family membership is that the protein must have a measured function, and thus, genomic sequences suggesting the transcription of potential AKR proteins are considered pseudo-members until evidence of a functionally expressed protein is available. Currently, over 200 confirmed AKR superfamily members are reported to exist. A systematic nomenclature for the AKR superfamily exists to facilitate family and subfamily designations of the member to be communicated easily. Specifically, protein names include the root "AKR", followed by the family represented by an Arabic number, the subfamily-if one exists-represented by a letter, and finally, the individual member represented by an Arabic number. The AKR superfamily database has been dedicated to tracking and reporting the current knowledge of the AKRs since 1997, and the website was last updated in 2003. Here, we present an updated version of the website and database that were released in 2023. The database contains genetic, functional, and structural data drawn from various sources, while the website provides alignment information and family tree structure derived from bioinformatics analyses.

Relationship between cholestasis and altered progesterone metabolism in the placenta-maternal liver tandem

BACKGROUND

In intrahepatic cholestasis of pregnancy (ICP), there are elevated maternal serum levels of total bile acids, progesterone, and some sulfated metabolites, such as allopregnanolone sulfate, which inhibits canalicular function.

AIM

To investigate the relationship between cholestasis and the expression of crucial enzymes involved in progesterone metabolism in the liver and placenta.

METHODS

Obstructive cholestasis was induced by bile duct ligation (BDL). RT-qPCR (mRNA) and western blot (protein) were used to determine expression levels. Srd5a1 and Akr1c2 enzymatic activities were assayed by substrate disappearance (progesterone and 5α-dihydroprogesterone, respectively), measured by HPLC-MS/MS.

RESULTS

BDL induced decreased Srd5a1 and Akr1c2 expression and activity in rat liver, whereas both enzymes were up-regulated in rat placenta. Regarding sulfotransferases, Sult2b1 was also moderately up-regulated in the liver. In placenta from ICP patients, SRD5A1 and AKR1C2 expression was elevated, whereas both genes were down-regulated in liver biopsies collected from patients with several liver diseases accompanied by cholestasis. SRD5A1 and AKR1C2 expression was not affected by incubating human hepatoma HepG2 cells with FXR agonists (chenodeoxycholic acid and GW4064). Knocking-out Fxr in mice did not reduce Srd5a1 and Akr1c14 expression, which was similarly down-regulated by BDL.

CONCLUSION

SRD5A1 and AKR1C2 expression was markedly altered by cholestasis. This was enhanced in the placenta but decreased in the liver, which is not mediated by FXR. These results suggest that the excess of progesterone metabolites in the serum of ICP patients can involve both enhanced placental production and decreased hepatic clearance. The latter may also occur in other cholestatic conditions.

Modulatory role of neurosteroidogenesis in the spinal cord during peripheral nerve injury-induced chronic pain

The brain and spinal cord (SC) are both targeted by various hormones, including steroid hormones. However, investigations of the modulatory role of hormones on neurobiological functions usually focus only on the brain. The SC received little attention although this structure pivotally controls motor and sensory functions. Here, we critically reviewed key data showing that the process of neurosteroid biosynthesis or neurosteroidogenesis occurring in the SC plays a pivotal role in the modulation of peripheral nerve injury-induced chronic pain (PNICP) or neuropathic pain. Indeed, several active steroidogenic enzymes expressed in the SC produce endogenous neurosteroids that interact with receptors of neurotransmitters controlling pain. The spinal neurosteroidogenesis is differentially regulated during PNICP condition and its blockade modifies painful sensations. The paper suggests that future investigations aiming to develop effective strategies against PNICP or neuropathic pain must integrate in a gender or sex dependent manner the regulatory effects exerted by spinal neurosteroidogenesis.

Allopregnanolone: An overview on its synthesis and effects

Allopregnanolone, a 3α,5α-progesterone metabolite, acts as a potent allosteric modulator of the γ-aminobutyric acid type A receptor. In the present review, the synthesis of this neuroactive steroid occurring in the nervous system is discussed with respect to physiological and pathological conditions. In addition, its physiological and neuroprotective effects are also reported. Interestingly, the levels of this neuroactive steroid, as well as its effects, are sex-dimorphic, suggesting a possible gender medicine based on this neuroactive steroid for neurological disorders. However, allopregnanolone presents low bioavailability and extensive hepatic metabolism, limiting its use as a drug. Therefore, synthetic analogues or a different therapeutic strategy able to increase allopregnanolone levels have been proposed to overcome any pharmacokinetic issues.

Overview of human 20 alpha-hydroxysteroid dehydrogenase (AKR1C1): Functions, regulation, and structural insights of inhibitors

Human aldo-keto reductase family 1C1 (AKR1C1) is an important enzyme involved in human hormone metabolism, which is mainly responsible for the metabolism of progesterone in the human body. AKR1C1 is highly expressed and has an important relationship with the occurrence and development of various diseases, especially some cancers related to hormone metabolism. Nowadays, many inhibitors against AKR1C1 have been discovered, including some synthetic compounds and natural products, which have certain inhibitory activity against AKR1C1 at the target level. Here we briefly reviewed the physiological and pathological functions of AKR1C1 and the relationship with the disease, and then summarized the development of AKR1C1 inhibitors, elucidated the interaction between inhibitors and AKR1C1 through molecular docking results and existing co-crystal structures. Finally, we discussed the design ideals of selective AKR1C1 inhibitors from the perspective of AKR1C1 structure, discussed the prospects of AKR1C1 in the treatment of human diseases in terms of biomarkers, pre-receptor regulation and single nucleotide polymorphisms, aiming to provide new ideas for drug research targeting AKR1C1.

Developmental Considerations for the Use of Naltrexone in Children and Adolescents

Naltrexone (NTX) is a well-tolerated drug with a wide safety margin and mechanism of action that affords use across a wide variety of indications in adults and children. By antagonizing the opioid reward system, NTX can modulate behaviors that involve compulsivity or impulsivity, such as substance use, obesity, and eating disorders. Evidence regarding the disposition and efficacy of NTX is mainly derived from adult studies of substance use disorders and considerable variability exists. Developmental changes, plausible disease-specific alterations and genetic polymorphisms in NTX disposition, and pharmacodynamic pathways should be taken into consideration when optimizing the use of NTX in the pediatric population. This review highlights the current state of the evidence and gaps in knowledge regarding NTX to facilitate evidence-based pharmacotherapy of mental health conditions, for which few pharmacologic options exist.

Combined MD and QM/MM Investigations of Hydride Reduction of 5α-Dihydrotestosterone Catalyzed by Human 3α-Hydroxysteroid Dehydrogenase Type 3: Importance of Noncovalent Interactions

3α-Hydroxysteroid dehydrogenase (3α-HSD) is an enzyme that is essential in the regulation of the concentration of 5α-dihydrotestosterone (5α-DHT) in the prostate. It catalyzes the hydride reduction of 5α-DHT to 3α-androstanediol, which activates androgen receptors. Elucidating details about the hydride reduction of 5α-DHT by 3α-HSD and the environment around the active site of the enzyme could lead to the development of effective drugs for the treatment of prostate cancer. In this study, the X-ray crystal structure of human 3α-HSD type 3 was comprehensively evaluated. Moreover, molecular dynamics (MD) simulations and hybrid ONIOM-type quantum mechanics/molecular mechanics (QM/MM) calculations were performed using a large QM region (maximum 232 atoms). It was determined that the reaction proceeded in a single step without the formation of an alkoxide ion owing to the direct hydride reduction of the substrate by nicotinamide adenine dinucleotide phosphate (NADPH) and concerted proton transfer by Tyr55 and Lys84. Noncovalent interaction (NCI) analysis highlighted the roles of Tyr216 and Trp227 in 3α-HSD. Specifically, Tyr216 assisted the reaction by π/π interactions with the neighboring nicotinamide ring of NADP(H), whereas Trp227 played an important role in recognition of the size of the substrate by CH/π interactions.

Characterization of aldo-keto reductase 1C subfamily members encoded in two rat genes (akr1c19 and RGD1564865). Relationship to 9-hydroxyprostaglandin dehydrogenase

Rat genes, akr1c19 and RGD1564865, encode members (R1C19 and 20HSDL, respectively) of the aldo-keto reductase (AKR) 1C subfamily, whose functions, however, remain unknown. Here, we show that recombinant R1C19 and 20HSDL exhibit NAD⁺-dependent dehydrogenase activity for prostaglandins (PGs) with 9α-hydroxy group (PGF_2α, its 13,14-dihydro- and 15-keto derivatives, 9α,11β-PGF₂ and PGD₂). 20HSDL oxidized the PGs with much lower K_m (0.3-14 μM) and higher k_cat/K_m values (0.064-2.6 min^-1μM^-1) than those of R1C19. They also differed in other properties: R1C19, but not 20HSDL, oxidized some 17β-hydroxysteroids (5β-androstane-3α,17β-diol and 5β-androstan-17β-ol-3-one). 20HSDL was specifically inhibited by zomepirac, but not by R1C19-selective inhibitors (hexestrol, flavonoids, ibuprofen and flufenamic acid), although the two enzymes were sensitive to indomethacin and cis-unsaturated fatty acids. The mRNA for 20HSDL was expressed abundantly in rat kidney and at low levels in the liver, testis, brain, heart and colon, in contrast to ubiquitous expression of R1C19 mRNA. The comparison of enzymic features of R1C19 and 20HSDL with rat PG dehydrogenases and other AKRs suggests not only a close relationship of 20HSDL with 9-hydroxy-PG dehydrogenase in rat kidney, but also roles of R1C19 and rat AKRs (1C16 and 1C24) in the metabolism of PGF_2α, PGD₂ and 9α,11β-PGF₂ in other tissues.

Physiopathological role of the enzymatic complex 5α-reductase and 3α/β-hydroxysteroid oxidoreductase in the generation of progesterone and testosterone neuroactive metabolites

The enzymatic complex 5α-reductase (5α-R) and 3α/3β-hydroxysteroid oxidoreductase (HSOR) is expressed in the nervous system, where it transforms progesterone (PROG) and testosterone (T) into neuroactive metabolites. These metabolites regulate myelination, brain maturation, neurotransmission, reproductive behavior and the stress response. The expression of 5α-R and 3α-HSOR and the levels of PROG and T reduced metabolites show regional and sex differences in the nervous system and are affected by changing physiological conditions as well as by neurodegenerative and psychiatric disorders. A decrease in their nervous tissue levels may negatively impact the course and outcome of some pathological events. However, in other pathological conditions their increased levels may have a negative impact. Thus, the use of synthetic analogues of these steroids or 5α-R modulation have been proposed as therapeutic approaches for several nervous system pathologies. However, further research is needed to fully understand the consequences of these manipulations, in particular with 5α-R inhibitors.

Neurosteroid Actions in Memory and Neurologic/Neuropsychiatric Disorders

Memory dysfunction is a symptomatic feature of many neurologic and neuropsychiatric disorders; however, the basic underlying mechanisms of memory and altered states of circuitry function associated with disorders of memory remain a vast unexplored territory. The initial discovery of endogenous neurosteroids triggered a quest to elucidate their role as neuromodulators in normal and diseased brain function. In this review, based on the perspective of our own research, the advances leading to the discovery of positive and negative neurosteroid allosteric modulators of GABA type-A (GABA_A), NMDA, and non-NMDA type glutamate receptors are brought together in a historical and conceptual framework. We extend the analysis toward a state-of-the art view of how neurosteroid modulation of neural circuitry function may affect memory and memory deficits. By aggregating the results from multiple laboratories using both animal models for disease and human clinical research on neuropsychiatric and age-related neurodegenerative disorders, elements of a circuitry level view begins to emerge. Lastly, the effects of both endogenously active and exogenously administered neurosteroids on neural networks across the life span of women and men point to a possible underlying pharmacological connectome by which these neuromodulators might act to modulate memory across diverse altered states of mind.

Overview of the Molecular Steps in Steroidogenesis of the GABAergic Neurosteroids Allopregnanolone and Pregnanolone

Allopregnanolone and pregnanolone-neurosteroids synthesized from progesterone in the brain, adrenal gland, ovary and testis-have been implicated in a range of neuropsychiatric conditions including seizure disorders, post-traumatic stress disorder, major depression, post-partum depression, pre-menstrual dysphoric disorder, chronic pain, Parkinson's disease, Alzheimer's disease, neurotrauma, and stroke. Allopregnanolone and pregnanolone equipotently facilitate the effects of gamma-amino-butyric acid (GABA) at GABAA receptors, and when sulfated, antagonize N-methyl-D-aspartate receptors. They play myriad roles in neurophysiological homeostasis and adaptation to stress while exerting anxiolytic, antidepressant, anti-nociceptive, anticonvulsant, anti-inflammatory, sleep promoting, memory stabilizing, neuroprotective, pro-myelinating, and neurogenic effects. Given that these neurosteroids are synthesized de novo on demand, this review details the molecular steps involved in the biochemical conversion of cholesterol to allopregnanolone and pregnanolone within steroidogenic cells. Although much is known about the early steps in neurosteroidogenesis, less is known about transcriptional, translational, and post-translational processes in allopregnanolone- and pregnanolone-specific synthesis. Further research to elucidate these mechanisms as well as to optimize the timing and dose of interventions aimed at altering the synthesis or levels of these neurosteroids is much needed. This should include the development of novel therapeutics for the many neuropsychiatric conditions to which dysregulation of these neurosteroids contributes.

Reconciled rat and human metabolic networks for comparative toxicogenomics and biomarker predictions

The laboratory rat has been used as a surrogate to study human biology for more than a century. Here we present the first genome-scale network reconstruction of Rattus norvegicus metabolism, iRno, and a significantly improved reconstruction of human metabolism, iHsa. These curated models comprehensively capture metabolic features known to distinguish rats from humans including vitamin C and bile acid synthesis pathways. After reconciling network differences between iRno and iHsa, we integrate toxicogenomics data from rat and human hepatocytes, to generate biomarker predictions in response to 76 drugs. We validate comparative predictions for xanthine derivatives with new experimental data and literature-based evidence delineating metabolite biomarkers unique to humans. Our results provide mechanistic insights into species-specific metabolism and facilitate the selection of biomarkers consistent with rat and human biology. These models can serve as powerful computational platforms for contextualizing experimental data and making functional predictions for clinical and basic science applications.

The rat is a widely-used model for human biology, but we must be aware of metabolic differences. Here, the authors reconstruct the genome-scale metabolic network of the rat, and after reconciling it with an improved human metabolic model, demonstrate the power of the models to integrate toxicogenomics data, providing species-specific biomarker predictions in response to a panel of drugs.

Revisiting the roles of progesterone and allopregnanolone in the nervous system: resurgence of the progesterone receptors

Progesterone is commonly considered as a female reproductive hormone and is well-known for its role in pregnancy. It is less well appreciated that progesterone and its metabolite allopregnanolone are also male hormones, as they are produced in both sexes by the adrenal glands. In addition, they are synthesized within the nervous system. Progesterone and allopregnanolone are associated with adaptation to stress, and increased production of progesterone within the brain may be part of the response of neural cells to injury. Progesterone receptors (PR) are widely distributed throughout the brain, but their study has been mainly limited to the hypothalamus and reproductive functions, and the extra-hypothalamic receptors have been neglected. This lack of information about brain functions of PR is unexpected, as the protective and trophic effects of progesterone are much investigated, and as the therapeutic potential of progesterone as a neuroprotective and promyelinating agent is currently being assessed in clinical trials. The little attention devoted to the brain functions of PR may relate to the widely accepted assumption that non-reproductive actions of progesterone may be mainly mediated by allopregnanolone, which does not bind to PR, but acts as a potent positive modulator of γ-aminobutyric acid type A (GABA(A) receptors. The aim of this review is to critically discuss effects of progesterone on the nervous system via PR, and of allopregnanolone via its modulation of GABA(A) receptors, with main focus on the brain.

Progesterone synthesis in the nervous system: implications for myelination and myelin repair

Progesterone is well known as a female reproductive hormone and in particular for its role in uterine receptivity, implantation, and the maintenance of pregnancy. However, neuroendocrine research over the past decades has established that progesterone has multiple functions beyond reproduction. Within the nervous system, its neuromodulatory and neuroprotective effects are much studied. Although progesterone has been shown to also promote myelin repair, its influence and that of other steroids on myelination and remyelination is relatively neglected. Reasons for this are that hormonal influences are still not considered as a central problem by most myelin biologists, and that neuroendocrinologists are not sufficiently concerned with the importance of myelin in neuron functions and viability. The effects of progesterone in the nervous system involve a variety of signaling mechanisms. The identification of the classical intracellular progesterone receptors as therapeutic targets for myelin repair suggests new health benefits for synthetic progestins, specifically designed for contraceptive use and hormone replacement therapies. There are also major advantages to use natural progesterone in neuroprotective and myelin repair strategies, because progesterone is converted to biologically active metabolites in nervous tissues and interacts with multiple target proteins. The delivery of progesterone however represents a challenge because of its first-pass metabolism in digestive tract and liver. Recently, the intranasal route of progesterone administration has received attention for easy and efficient targeting of the brain. Progesterone in the brain is derived from the steroidogenic endocrine glands or from local synthesis by neural cells. Stimulating the formation of endogenous progesterone is currently explored as an alternative strategy for neuroprotection, axonal regeneration, and myelin repair.

Estrogen receptor dependent gene expression by osteoblasts - direct, indirect, circumspect, and speculative effects

Hormone activated estrogen receptors (ERs) have long been appreciated as potent mediators of gene expression in female reproductive tissues. These highly targeted responses likely evolved from more elemental roles in lower organisms, in agreement with their widespread effects in the cardiovascular, immunological, central nervous, and skeletal tissue systems. Still, despite intense investigation, the multiple and often perplexing roles of ERs retain significant attention. In the skeleton, this in part derives from apparently opposing effects by ER agonists on bone growth versus bone remodeling, and in younger versus older individuals. The complexity associated with ER activation can also derive from their interactions with other hormone and growth factor systems, and their direct and indirect effects on gene expression. We propose that part of this complexity results from essential interactions between ERs and other transcription factors, each with their own biochemical and molecular intricacies. Solving some of the many questions that persist may help to achieve better, or better directed, use of agents that can drive ER activation in focused and possibly tissue restricted ways.

Aldo-keto reductase 1C3 is expressed in differentiated human epidermis, affects keratinocyte differentiation, and is upregulated in atopic dermatitis

Aldo-keto reductase 1C3 (AKR1C3) has been shown to mediate the metabolism of sex hormones and prostaglandin D(2) (PGD(2)), a lipid mediator that promotes skin inflammation in atopic dermatitis (AD). As both have a role in skin function and pathology, we first sought to investigate the expression pattern of AKR1C3 in normal human epidermis. Immunofluorescence revealed a strong expression of AKR1C3 in the differentiated suprabasal layers compared with the basal layer. Western blot analysis and quantitative PCR confirmed that AKR1C3 expression was also upregulated in differentiation-induced primary human keratinocytes (PHKs). To investigate the functional role of AKR1C3 during PHK differentiation, its expression and activity (measured as PGD(2) reduction to 9α,11β-PGF(2) by ELISA) were impaired by small interfering RNA or 2'-hydroxyflavanone, respectively. Cytokeratin 10 (K10) and loricrin expression were then examined by western blot analysis, thus revealing altered expression of these differentiation markers. Finally, following an observation that the AD-associated mediator, PGD(2), upregulated AKR1C3 expression in PHKs, we used immunofluorescence to examine AKR1C3 expression in AD and psoriasis lesions. AKR1C3 was found to be upregulated in AD but not in psoriasis lesions compared with non-lesional skin. Our work demonstrates a function for AKR1C3 in differentiation-associated gene regulation and also suggests a role in supporting inflammation in AD.

Biosynthesis and metabolism of steroids in molluscs

Molluscs are the second most diverse animal group, they are ecologically important and they are considered excellent indicators of ecosystem health. Some species have been widely used in pollution biomonitoring programs; however, their endocrinology is still poorly known. Despite some studies reporting the presence of (vertebrate-type) steroids in molluscs, information regarding enzymatic pathways involved in steroid synthesis and further catabolism of those steroids is still fragmentary. Regarding steroidogenesis, a number of excellent studies were performed in the 70s using different radio-labelled steroid precursors and detecting the formation of different metabolites. But, since then a long gap of research exist until the late 90s when the 'endocrine disruption' issue raised the need of a better knowledge of mollusc (and invertebrate) endocrinology in order to assess alterations caused by pollutants. Here we summarize past and recent studies dealing with steroid biosynthesis and metabolism in different mollusc species. Most of these studies suggest the involvement of steroids in mollusc reproduction. However, the knowledge is still fragmentary and many questions remain to be answered.

Human aldo-keto reductases: structure, substrate specificity and roles in tumorigenesis

The aldo-keto reductase (AKR) superfamily consists of over 150 protein members sharing similar structure and enzymatic activities. To date, 13 human AKRs have been identified, and they participate in xenobiotic detoxification, biosynthesis and metabolism. Increasing evidence suggests the involvement of human AKR proteins in cancer development, progression and treatment. Some proteins demonstrate multiple functional features in addition to being a reductase for carbonyl groups. This review article discusses the most recent progress made in the study of humans AKRs.

Neuroactive steroids in affective disorders: target for novel antidepressant or anxiolytic drugs?

In the past decades considerable evidence has emerged that so-called neuroactive steroids do not only act as transcriptional factors in the regulation of gene expression but may also alter neuronal excitability through interactions with specific neurotransmitter receptors such as the GABA(A) receptor. In particular, 3α-reduced neuroactive steroids such as allopregnanolone or allotetrahydrodeoxycorticosterone have been shown to act as positive allosteric modulators of the GABA(A) receptor and to play an important role in the pathophysiology of depression and anxiety. During depression, the concentrations of 3α,5α-tetrahydroprogesterone and 3α,5β-tetrahydroprogesterone are decreased, while the levels of 3β,5α-tetrahydroprogesterone, a stereoisomer of 3α,5α-tetrahydroprogesterone, which may act as an antagonist for GABAergic steroids, are increased. Antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs) or mirtazapine apparently have an impact on key enzymes of neurosteroidogenesis and have been shown to normalize the disequilibrium of neuroactive steroids in depression by increasing 3α-reduced pregnane steroids and decreasing 3β,5α-tetrahydroprogesterone. Moreover, 3α-reduced neuroactive steroids have been demonstrated to possess antidepressant- and anxiolytic-like effects both in animal and human studies for themselves. In addition, the translacator protein (18 kDa) (TSPO), previously called peripheral benzodiazepine receptor, is the key element of the mitochondrial import machinery supplying the substrate cholesterol to the first steroidogenic enzyme (P450scc), which transforms cholesterol into pregnenolone, the precursor of all neurosteroids. TSPO ligands increase neurosteroidogenesis and are a target of novel anxiolytic drugs producing anxiolytic effects without causing the side effects normally associated with conventional benzodiazepines such as sedation or tolerance. This article is part of a Special Issue entitled: Neuroactive Steroids: Focus on Human Brain.

Steroids, spinal cord and pain sensation

During the whole life, the nervous system is continuously submitted to the actions of different categories of hormones, including steroids. Therefore, the interactions between hormonal compounds and neural tissues are subjected to intense investigations. While a majority of studies focus on the brain, the spinal cord (SC) has received little attention, although this structure is also an important part of the central nervous system, controlling motor and sensory functions. To point out the importance of interactions between hormones and the SC in the regulation of neurobiological activities, we recapitulated and discussed herein various key data, revealing that the pivotal role played by the SC in nociception and pain modulation, directly depends on the SC ability to metabolize and synthesize steroidal molecules. The paper suggests that future investigations aiming to develop effective strategies against chronic pain, must integrate regulatory effects exerted by hormonal steroids on the SC activity, as well as the actions of endogenous neurosteroids locally synthesized in spinal neural networks.

Structure-based optimization and biological evaluation of human 20α-hydroxysteroid dehydrogenase (AKR1C1) salicylic acid-based inhibitors

The tertiary structure of the Leu308Val mutant of human 20α-hydroxysteroid dehydrogenase (AKR1C1) in complex with the inhibitor 3,5-dichlorosalicylic acid (DCL) has been determined. Structures and kinetic properties of the wild-type and mutant enzymes indicate that Leu308 is a selectivity determinant for inhibitor binding. The Leu308Val mutation resulted in 13-fold and 3-fold reductions in the inhibitory potencies of DCL and 3-bromo-5-phenylsalicylic acid (BPSA), respectively. The replacement of Leu308 with an alanine resulted in 473-fold and 27-fold reductions in the potencies for DCL and BPSA, respectively. In our attempts to optimize inhibitor potency and selectivity we synthesized 5-substituted 3-chlorosalicylic acid derivatives, of which the most potent compound, 3-chloro-5-phenylsalicylic acid (K(i) = 0.86 nM), was 24-fold more selective for AKR1C1 relative to the structurally similar 3α-hydroxysteroid dehydrogenase (AKR1C2). Furthermore, the compound inhibited the metabolism of progesterone in AKR1C1-overexpressed cells with an IC(50) value equal to 100 nM.

Factorizing the role of a critical leucine residue in the binding of substrate to human 20alpha-hydroxysteroid dehydrogenase (AKR1C1): molecular modeling and kinetic studies of the Leu308Val mutant enzyme

A comparison of the structures and kinetic properties of human 20alpha-hydroxysteroid dehydrogenase (AKR1C1) and its mutant enzymes (Leu308Val and Leu308Ala) indicates that Leu308 is a selectivity determinant for substrate binding. While the Leu308Val mutation improved the catalytic efficiency (k(cat)/K(m)) of AKR1C1 towards the two substrates 5alpha-pregnane-3alpha,20alpha-diol (PregA) and 5beta-pregnan-3alpha-ol-20-one (PregB), the Leu308Ala mutation rendered the enzyme inactive. In the docked model of PregA the conformation of the steroid molecule was similar to that of 20alpha-hydroxyprogesterone in the crystal structure of the AKR1C1 complex where the steroid did not interact with the catalytic residues Tyr55 and His117. In the case of PregB the steroid interacted with the catalytic residue His117 and formed close contacts with Leu308, suggesting that the binding mechanism of 3alpha-hydroxysteroids in the active site of AKR1C1 is different from that of 20alpha-hydroxysteroids.

Aldo-keto reductase (AKR) superfamily: genomics and annotation

Aldo-keto reductases (AKRs) are phase I metabolising enzymes that catalyse the reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)-dependent reduction of carbonyl groups to yield primary and secondary alcohols on a wide range of substrates, including aliphatic and aromatic aldehydes and ketones, ketoprostaglan-dins, ketosteroids and xenobiotics. In so doing they functionalise the carbonyl group for conjugation (phase II enzyme reactions). Although functionally diverse, AKRs form a protein superfamily based on their high sequence identity and common protein fold, the (α/(β)₈-barrel structure. Well over 150 AKR enzymes, from diverse organisms, have been annotated so far and given systematic names according to a nomenclature that is based on multiple protein sequence alignment and degree of identity. Annotation of non-vertebrate AKRs at the National Center for Biotechnology Information or Vertebrate Genome Annotation (vega) database does not often include the systematic nomenclature name, so the most comprehensive overview of all annotated AKRs is found on the AKR website (http://www.med.upenn.edu/akr/). This site also hosts links to more detailed and specialised information (eg on crystal structures, gene expression and single nucleotide polymorphisms [SNPs]). The protein-based AKR nomenclature allows unambiguous identification of a given enzyme but does not reflect the wealth of genomic and transcriptomic variation that exists in the various databases. In this context, identification of putative new AKRs and their distinction from pseudogenes are challenging. This review provides a short summary of the characteristic features of AKR biochemistry and structure that have been reviewed in great detail elsewhere, and focuses mainly on nomenclature and database entries of human AKRs that so far have not been subject to systematic annotation. Recent developments in the annotation of SNP and transcript variance in AKRs are also summarised.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Transcriptome response of enterocytes to dietary lipids: impact on cell architecture, signaling, and metabolism genes

Intestine contributes to lipid homeostasis through the absorption of dietary lipids, which reach the apical pole of enterocytes as micelles. The present study aimed to identify the specific impact of these dietary lipid-containing micelles on gene expression in enterocytes. We analyzed, by microarray, the modulation of gene expression in Caco-2/TC7 cells in response to different lipid supply conditions that reproduced either the permanent presence of albumin-bound lipids at the basal pole of enterocytes or the physiological delivery, at the apical pole, of lipid micelles, which differ in their composition during the interprandial (IPM) or the postprandial (PPM) state. These different conditions led to distinct gene expression profiles. We observed that, contrary to lipids supplied at the basal pole, apical lipid micelles modulated a large number of genes. Moreover, compared with the apical supply of IPM, PPM specifically impacted 46 genes from three major cell function categories: signal transduction, lipid metabolism, and cell adhesion/architecture. Results from this first large-scale analysis underline the importance of the mode and polarity of lipid delivery on enterocyte gene expression. They demonstrate specific and coordinated transcriptional effects of dietary lipid-containing micelles that could impact the structure and polarization of enterocytes and their functions in nutrient transfer.

Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

Carbonyl reduction of aldehydes, ketones, and quinones to their corresponding hydroxy derivatives plays an important role in the phase I metabolism of many endogenous (biogenic aldehydes, steroids, prostaglandins, reactive lipid peroxidation products) and xenobiotic (pharmacologic drugs, carcinogens, toxicants) compounds. Carbonyl-reducing enzymes are grouped into two large protein superfamilies: the aldo-keto reductases (AKR) and the short-chain dehydrogenases/reductases (SDR). Whereas aldehyde reductase and aldose reductase are AKRs, several forms of carbonyl reductase belong to the SDRs. In addition, there exist a variety of pluripotent hydroxysteroid dehydrogenases (HSDs) of both superfamilies that specifically catalyze the oxidoreduction at different positions of the steroid nucleus and also catalyze, rather nonspecifically, the reductive metabolism of a great number of nonsteroidal carbonyl compounds. The present review summarizes recent findings on carbonyl reductases and pluripotent HSDs of the SDR protein superfamily.

Endogenous steroid production in the spinal cord and potential involvement in neuropathic pain modulation

It has recently been demonstrated that the spinal cord (SC) is an active production center of neuroactive steroids including pregnenolone, dehydroepiandrosterone, progesterone and allopregnanolone. Indeed, anatomical, cellular and biochemical investigations have shown that the SC dorsal horn (DH), a pivotal structure in nociception, contains various active steroidogenic enzymes such as cytochrome P450side-chain-cleavage, cytochrome P450c17, 3beta-hydroxysteroid dehydrogenase, 5alpha-reductase and 3alpha-hydroxysteroid oxido-reductase. Reviewed here are several data obtained with in vitro and vivo experiments showing that endogenous steroids synthesized in the SC are involved in the modulation of nociceptive mechanisms. Various approaches were used as the real-time polymerase chain reaction after reverse transcription to determine the effects of neuropathic pain on the expression of genes encoding steroidogenic enzymes in the DH. Combination of the pulse-chase technique with high performance liquid chromatography and continuous flow scintillation detection allowed investigations of the impact of noxious signals on the activity of steroid-producing enzymes in the SC in vitro. Radioimmunological analyses of spinal tissue extracts contributed to determine the link between the painful state and endogenous steroid secretion in the SC in vivo. Finally, the physiological relevance of the modification of endogenous steroid formation in the SC during painful situation was discussed.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Regulation of human 3 alpha-hydroxysteroid dehydrogenase (AKR1C4) expression by the liver X receptor alpha

Type I human hepatic 3alpha-hydroxysteroid dehydrogenase (AKR1C4) plays a significant role in bile acid biosynthesis, steroid hormone metabolism, and xenobiotic metabolism. Utilization of a hidden Markov model for predictive modeling of nuclear hormone receptor response elements coupled with chromatin immunoprecipitation/microarray technology revealed a putative binding site in the AKR1C4 promoter for the nuclear hormone receptor known as liver X receptor alpha, (LXRalpha [NR1H3]), which is the physiological receptor for oxidized cholesterol metabolites. The putative LXRalpha response element (LXRE), identified by chromatin immunoprecipitation, was approximately 1.5 kilobase pairs upstream of the transcription start site. LXRalpha was shown to bind specifically to this LXRE and mediate transcriptional activation of the AKR1C4 gene, leading to increased AKR1C4 protein expression. These data suggest that LXRalpha may modulate the bile acid biosynthetic pathway at a unique site downstream of CYP7A1 and may also modulate the metabolism of steroid hormones and certain xenobiotics.

3-ketosteroid reductase activity and expression by fetal rat osteoblasts

In addition to reproductive tissue, sex hormones induce transcriptional events in many connective tissue cells, including osteoblasts. Some sex hormone receptor modulators with bone sparing effects selectively target estrogen or androgen receptors, whereas others appear more promiscuous, in part through enzymatic metabolism. Rat osteoblasts express significant oxidative 3alpha-hydroxysteroid dehydrogenase activity, which can convert precursor substrates to potent androgen receptor agonists. Here we show that they also express 3-ketosteroid reductase activity, exemplified by 7-methyl-17-ethynyl-19-norandrostan-5 (10)en-3-one (tibolone) conversion to potent estrogen receptor alpha agonists. Conversion was rapid and quantitative, with 3alpha-hydroxytibolone as the primary metabolite. Consistently, tibolone induced estrogen receptor alpha-dependent gene promoter activity through cis-acting estrogen response elements, increased the stimulatory effect of TGF-beta on Smad-dependent gene promoter activity, and enhanced prostaglandin E2-induced activity of transcription factor Runx2. Rat osteoblasts express the 3-ketosteroid reductase AKR1C9, an aldo-keto reductase gene family member. Exposure to prostaglandin E2 increased AKR1C9 gene promoter activity and mRNA expression. AKR1C9 promoter activity was also enhanced by overexpression of protein kinase A catalytic subunit or transcription factor C/EBPdelta, and the effect of PGE2 was reduced by dominant negative C/EBPdelta competition or C/EBPdelta antisense expression. Moreover, prostaglandin E2 increased the amount of functional endogenous nuclear C/EBPdelta that could bind specifically to a distinct domain approximately 1.8-kb upstream from the start site of AKR1C9 transcription. In summary, in addition to 3alpha-hydroxysteroid dehydrogenase, rat osteoblasts express significant and regulatable 3-ketosteroid reductase activity. Through these enzymes, they may selectively metabolize precursor compounds into potent steroid receptor agonists locally within bone.

Sex steroids and potential mechanisms of non-genomic endocrine disruption in invertebrates

The review reports on the presence and metabolism of sex steroids in several invertebrate species and provides detailed information on possible mechanisms of endocrine disruption other than the interaction with nuclear receptors. The presence of most vertebrate sex steroids in invertebrate tissues has been demonstrated by liquid or gas chromatography coupled to mass spectrometry. In addition, enzymatic pathways involved in the steroidogenic pathway have been described in at least some invertebrate phyla. Some endocrine disruptors induce alterations in these metabolic pathways and might lead to changes in steroid levels. Growing evidence suggests that estradiol can act through non-genomic pathways in molluscs, and that xenobiotics can as well interfere in these signalling cascades. In spite of these recent advances, most question marks on the action and function of sex steroids in invertebrates remain to be answered.

Sex, drugs and sports: Prostaglandins, epitestosterone and sexual development

Amateau and McCarthy's findings published in Nature Neuroscience (June 2004) are noteworthy for suggesting a role for prostaglandins in sexual development. However, evidence suggests that in manipulating PGE2, they unknowingly implicated 3alpha-hydroxysteroid dehydrogenase [E.C. 1.1.1.50], 3(or 17)alpha-hydroxysteroid dehydrogenase [E.C. 1.1.1.209] and their respective products, androsterone (ADT) and epitestosterone (EpiT), in the developmental masculinization of sex behavior. EpiT is generally regarded as a hormonally inactive 17alpha-epimer of testosterone (T). In rats, the kidney is the primary site of EpiT formation, whereas in humans it originates from the gonads, with only a small contribution secreted by the adrenals. Because the ratio of T to EpiT is nearly constant, it is presently used for assessing steroid abuse in competitive sports, where the World Anti-Doping Agency (WADA) considers a T/EpiT ratio >4 evidence of T doping. Despite its central role in the detection of illict anabolic steroid use, our knowledge of factors effecting EpiT production is poor. Clues in the literature, however, reveal that prostaglandin-mediated processes, such as LHRH release, may influence its production. Antimycotics, NSAIDs, and opioid analgesics used in sports medicine are all known to effect prostaglandin E2 synthesis. Primary PGs are potent inhibitors of ADT oxidation, while indomethacin, a prostaglandin blocker, powerfully inhibits 3alpha-HSD reduction and ADT oxidation. This is significant because ADT inhibits the oxidation of EpiT, and may modulate its antiandrogenic and neuroprotective effects. It is hypothesized that the T/EpiT ratio is increased by COX-2 inhibitors and opiod analgesics, and decreased by antimycotics that do not impair testosterone biosynthesis. Given the devastating personal and career consequences that may result from false positive drug tests, substantive research on the effects of PGE2 manipulations on EpiT is warranted.

Multiple steps determine the overall rate of the reduction of 5alpha-dihydrotestosterone catalyzed by human type 3 3alpha-hydroxysteroid dehydrogenase: implications for the elimination of androgens

Human type 3 3alpha-hydroxysteroid dehydrogenase, or aldo-keto reductase (AKR) 1C2, eliminates the androgen signal in human prostate by reducing 5alpha-dihydrotestosterone (DHT, potent androgen) to form 3alpha-androstanediol (inactive androgen), thereby depriving the androgen receptor of its ligand. The k(cat) for the NADPH-dependent reduction of DHT catalyzed by AKR1C2 is 0.033 s(-1). We employed transient kinetics and kinetic isotope effects to dissect the contribution of discrete steps to this low k(cat) value. Stopped-flow experiments to measure the formation of the AKR1C2.NADP(H) binary complex indicated that two slow isomerization events occur to yield a tight complex. A small primary deuterium isotope effect on k(cat) (1.5) and a slightly larger effect on k(cat)/K(m) (2.1) were observed in the steady state. In the transient state, the maximum rate constant for the single turnover of DHT (k(trans)) was determined to be 0.11 s(-1) for the NADPH-dependent reaction, which was approximately 4-fold greater than the corresponding k(cat) x k(trans) was significantly reduced when NADPD was substituted for NADPH, resulting in an apparent (D)k(trans) of 3.5. Thus, the effects of isotopic substitution on the hydride transfer step were masked by slow events that follow or precede the chemical transformation. Transient multiple-turnover reactions generated curvilinear reaction traces, consistent with the product formation and release occurring at comparable rates. Global fitting analysis of the transient kinetic data enabled the estimate of the rate constants for the three-step cofactor binding/release model and for the minimal ordered bi-bi turnover mechanism. Results were consistent with a kinetic mechanism in which a series of slow events, including the chemical step (0.12 s(-1)), the release of the steroid product (0.081 s(-1)), and the release of the cofactor product (0.21 s(-1)), combine to yield the overall observed low turnover number.

Characterization of brain neurons that express enzymes mediating neurosteroid biosynthesis

Allopregnanolone (ALLO) and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of GABA action at GABA(A) receptors. ALLO and THDOC are synthesized in the brain from progesterone or deoxycorticosterone, respectively, by the sequential action of two enzymes: 5alpha-reductase (5alpha-R) type I and 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD). This study evaluates 5alpha-R type I and 3alpha-HSD mRNA expression level in mouse brain by using in situ hybridization combined with glutamic acid decarboxylase 67/65, vesicular glutamate transporter 2, glial fibrillary acidic protein, and S100beta immunohistochemistry. We demonstrate that 5alpha-R type I and 3alpha-HSD colocalize in cortical, hippocampal, and olfactory bulb glutamatergic principal neurons and in some output neurons of the amygdala and thalamus. Neither 5alpha-R type I nor 3alpha-HSD mRNAs are expressed in S100beta- or glial fibrillary acidic protein-positive glial cells. Using glutamic acid decarboxylase 67/65 antibodies to mark GABAergic neurons, we failed to detect 5alpha-R type I and 3alpha-HSD in cortical and hippocampal GABAergic interneurons. However, 5alpha-R type I and 3alpha-HSD are significantly expressed in principal GABAergic output neurons, such as striatal medium spiny, reticular thalamic nucleus, and cerebellar Purkinje neurons. A similar distribution and cellular location of neurosteroidogenic enzymes was observed in rat brain. Taken together, these data suggest that ALLO and THDOC, which can be synthesized in principal output neurons, modulate GABA action at GABA(A) receptors, either with an autocrine or a paracrine mechanism or by reaching GABA(A) receptor intracellular sites through lateral membrane diffusion.

Influence of mirtazapine on plasma concentrations of neuroactive steroids in major depression and on 3alpha-hydroxysteroid dehydrogenase activity

Concentrations of 3alpha-reduced neuroactive steroids are altered in depression and normalize after antidepressant pharmacotherapy with selective serotonin re-uptake inhibitors (SSRIs). We investigated the impact of mirtazapine on the activity of a key neurosteroidogenic enzyme, the 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), and on the levels of neuroactive steroids in relation to clinical response. A total of 23 drug-free in-patients suffering from a major depressive episode (DSM-IV criteria) underwent 5-week treatment with mirtazapine (45 mg/day). Plasma samples were taken weekly at 0800 and quantified for neuroactive steroids by means of combined gas chromatography/mass spectrometry analysis. Enzyme activity was determined by assessment of steroid conversion rates. Irrespective of clinical outcome, there were significant increases in 3alpha,5alpha-tetrahydroprogesterone, 3alpha,5beta-tetrahydroprogesterone, 5alpha-dihydroprogesterone, and 5beta-dihydroprogesterone after mirtazapine treatment, whereas 3beta,5alpha-tetrahydroprogesterone levels were significantly decreased. In vitro investigations demonstrated a dose-dependent inhibitory effect of mirtazapine on the activity of the microsomal 3alpha-HSD in the oxidative direction (conversion of 3alpha,5alpha-tetrahydroprogesterone to 5alpha-dihydroprogesterone). Mirtazapine affects neuroactive steroid composition similarly as do SSRIs. The inhibition of the oxidative pathway catalyzed by the microsomal 3alpha-HSD is compatible with an enhanced formation of 3alpha-reduced neuroactive steroids. However, the changes in neuroactive steroid concentrations more likely reflect direct pharmacological effects of this antidepressant rather than clinical improvement in general.

Green tea polyphenol epigallocatechin-3 gallate (EGCG) affects gene expression of breast cancer cells transformed by the carcinogen 7,12-dimethylbenz[a]anthracene

Since the 1980s, the incidence of late-onset breast cancer has been increasing in the United States. Known risk factors, such as genetic modifications, have been estimated to account for approximately 5 to 10% of breast cancer cases, and these tend to be early onset. Thus, exposure to and bioaccumulation of ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAHs), have been proposed to play a role in this increased incidence. Treatment of female Sprague-Dawley rats with a single dose of the PAH 7,12-dimethylbenz[a]anthracene (DMBA) induces mammary tumors in approximately 90 to 95% of test animals. We showed previously that female rats treated with DMBA and given green tea as drinking fluid displayed significantly decreased mammary tumor burden and invasiveness and a significantly increased latency to first tumor. Here we used cDNA microarray analysis to elucidate the effects of the green tea polyphenol epigallocatechin-3 gallate (EGCG) on the gene expression profile in a DMBA-transformed breast cancer cell line. RNA was isolated, in quadruplicate, from D3-1 cells treated with 60 mug/mL EGCG for 2, 7, or 24 h and subjected to analysis. Semiquantitative RT-PCR and Northern blot analyses confirmed the changes in the expression of 12 representative genes seen in the microarray experiments. Overall, our results documented EGCG-altered expression of genes involved in nuclear and cytoplasmic transport, transformation, redox signaling, response to hypoxia, and PAHs.

Developmental expression and function of aldehyde reductase in proximal tubules of the kidney

Aldehyde reductase reduces a wide variety of toxic and physiological aldehydes with a marked preference for negatively charged substrates such as glucuronate. Reduction of glucuronate to gulonate is a step in inositol catabolism, a process specific to the kidney cortex. Administration of the aldehyde reductase inhibitor AL-1576 to mice increases urinary output of glucuronate and decreases output of vitamin C. Aldehyde reductase mRNA with a 319-bp 5′-untranslated region is expressed ubiquitously in murine tissues. A new isoform with a short 64-bp 5′-untranslated region is found predominantly in the kidney, resulting in 10-fold higher enzymatic activity observed in this organ compared with other tissues. A moderate level of the new transcript is found in liver, intestine, and stomach, whereas brain, heart, lung, spleen, ovary, and testis have low to insignificant levels. The short transcript is absent during embryonic development and is first observed in the murine kidney on postnatal day 6. The abundance of the short transcript and enzyme activity increase sigmoidally with age; the sharpest increase occurs during the third week of life. As shown by immunohistochemistry, aldehyde reductase expression is limited to the proximal tubules and parietal epithelium of Bowman’s capsule. In the mouse, the intensity of staining in tubules increases with age, suggesting that induction of aldehyde reductase expression is part of renal tubular maturation. The human kidney also exhibits proximal tubular localization and the two mRNA transcripts of aldehyde reductase. Immunoreactive protein is present in the 9-wk-old fetal kidney, indicating that the induction of aldehyde reductase in humans occurs early in development.

Intracellular oxidation of allopregnanolone by human brain type 10 17beta-hydroxysteroid dehydrogenase

Allopregnanolone is a positive allosteric modulator of GABAA receptors, generated by the reduction of 5alpha-dihydroprogesterone (5alpha-DHP) in astrocytes. This neuroactive steroid can be inactivated by its 3alpha-oxidation to yield 5alpha-DHP. It was found that 5alpha-DHP levels in HEK293 cells expressing type 10 17beta-hydroxysteroid dehydrogenase (17beta-HSD10), but not its catalytic inactive mutant, increased significantly as allopregnanolone was added to culture media. The results demonstrate that mitochondrial 17beta-HSD10 effectively catalyzes the intracellular oxidation of allopregnanolone. Moreover, brain astrocytes contain a moderate level of 17beta-HSD10, which is elevated in activated astrocytes of brains with Alzheimer type pathology, including sporadic Alzheimer's disease (AD) and Down's syndrome with AD. The distribution of 17beta-HSD10 was found not to parallel that of 3alpha-HSD3. Cerebral cortex has the lowest level of 17beta-HSD10; whereas the hippocampus, hypothalamus, and amygdala possess relatively higher levels of this enzyme. The catalysis of 17beta-HSD10 appears to be essential for maintaining normal functions of GABAergic neurons. The elevated level of 17beta-HSD10 in activated astrocytes is a new feature found in brains of people with AD, and it may have important impact on AD pathogenesis.

Multiple functions of type 10 17beta-hydroxysteroid dehydrogenase

Human 17beta-hydroxysteroid dehydrogenase type 10 (17beta-HSD10) is a mitochondrial enzyme encoded by the SCHAD gene, which escapes chromosome X inactivation. 17Beta-HSD10/SCHAD mutations cause a spectrum of clinical conditions, from mild mental retardation to progressive infantile neurodegeneration. 17Beta-HSD10/SCHAD is essential for the metabolism of isoleucine and branched-chain fatty acids. It can inactivate 17beta-estradiol and steroid modulators of GABA(A) receptors, and convert 5alpha-androstanediol into 5alpha-dihydrotestosterone (DHT). Certain malignant prostatic epithelial cells contain high levels of 17beta-HSD10, generating 5alpha-DHT in the absence of testosterone. 17Beta-HSD10 has an affinity for amyloid-beta peptide, and might be linked to the mitochondrial dysfunction seen in Alzheimer's disease. This versatile enzyme might provide a new drug target for neuronal excitability control and for intervention in Alzheimer's disease and certain cancers.

Topography and function of androgen-metabolizing enzymes in the central nervous system

The present review describes concisely the topography and function of the three androgen-metabolizing enzymes, namely aromatase, 5alpha-reductase and 3alpha-hydroxysteroid dehydrogenase, in the central nervous system (CNS). Aromatase, estrogen synthetase, is the key enzyme for converting androgens to estrogens. Aromatase is indispensable for the sexual differentiation of the brain and the enzyme activity and expression of aromatase are high during the critical period of neural development, which extends from the late embryonal to the early neonatal period in rodents. Aromatase is expressed in neurons within specific hypothalamic and limbic regions. The locations of aromatase-immunoreactive neurons are divided into three groups according to the period of enzyme expression. Steroid 5alpha-reductase converts a number of steroids with a C3 ketone group and a C4-C5 double bond (delta4; androgens, progestins and glucocorticoids) to their 5alpha-reduced metabolites. Two isoforms of 5alpha-reductase are found and type 1 is predominant in neural tissues. The enzyme activity of 5alpha-reductase is found widely in the CNS and is high in white matter regions. The enzyme expression of 5alpha-reductase peaks during the late embryonic period. 3alpha-Hydroxysteroid dehydrogenase is the oxidoreductase that interconverts 3-ketosteroids to 3alpha-hydroxysteroids. Four isozymes have been found in humans and only one type has been found in rats. The enzyme converts 5alpha-reduced steroids (e.g. 5alpha-dihydroprogesterone) to tetrahydrosteroids (e.g. 3alpha,5alpha-tetrahydroprogesterone). The latter steroid is a potent stimulator of the GABA(A) receptor. The activity of 3alpha-hydroxysteroid dehydrogenase is high during the first 1-2 postnatal weeks, decreases with development and this enzyme is highly expressed in astrocytes.

Type 10 17beta-hydroxysteroid dehydrogenase catalyzing the oxidation of steroid modulators of gamma-aminobutyric acid type A receptors

The steroids allopregnanolone and allotetrahydrodeoxycorticosterone (3alpha,5alpha-THDOC) are positive allosteric modulators of GABA(A) receptors, generated by the reduction of 5alpha-dihydroprogesterone (5alpha-DHP) and 5alpha-DHDOC, respectively, under the catalysis of human type 3 3alpha-hydroxysteroid dehydrogenase (HSD). However, brain enzymes catalyzing the conversion of such tetrahydrosteroids back to the corresponding 5alpha-dihydrosteroids remain to be identified. Characterization of human type 10 17beta-HSD provides a new insight into its importance for the oxidation of steroid modulators of GABA(A) receptors. The apparent catalytic efficiency (k(cat)/K(m)) of this enzyme for the oxidation of allopregnanolone and 3alpha,5alpha-THDOC are 432 and 1381 min(-1) mM(-1), respectively. This enzyme has negligible 3-ketosteroid reductase activity for 5alpha-DHP and 5alpha-DHDOC even in an acidic environment. Immunoreactivity against 17beta-HSD10 was found in a number of neuronal populations. Taken together, evidence suggests that 17beta-HSD10 is the brain enzyme capable of catalyzing the oxidation of steroid modulators of GABA(A) receptors.

Anatomical and cellular localization of neuroactive 5?/3?-reduced steroid-synthesizing enzymes in the spinal cord

The complementary activities of 5 alpha-reductase (5 alpha-R) and 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) are crucial for the synthesis of neuroactive 5 alpha/3 alpha-reduced steroids, such as 3 alpha-androstanediol, allopregnanolone, and tetrahydrodeoxycorticosterone, which control several important neurophysiological mechanisms through allosteric modulation of gamma-aminobutyric acid type A receptors. Immunocytochemical localization of 3 alpha-HSD in the central nervous system (CNS) has never been determined. The presence and activity of 5 alpha-R have been investigated in the CNS, but only the brain was considered; the spinal cord (SC) received little attention, although this structure is crucial for many sensorimotor activities. We have determined the first cellular distribution of 5 alpha-reductase type 1 (5 alpha-R1) and type 2 (5 alpha-R2) and 3 alpha-HSD immunoreactivities in adult rat SC. 5 alpha-R1 immunostaining was detected mainly in the white matter (Wm). In contrast, intense 5 alpha-R2 labeling was observed in dorsal (DH) and ventral horns of gray matter (Gm). 3 alpha-HSD immunoreactivity was largely distributed in the Wm and Gm, but the highest density was found in sensory areas of the DH. Double-labeling experiments combined with confocal analysis revealed that, in the Wm, 5 alpha-R1 was localized in glial cells, whereas 35% of 5 alpha-R2 and 3 alpha-HSD immunoreactivities were found in neurons. In the DH, 60% of 5 alpha-R2 immunostaining colocalized with oligodendrocyte, 25% with neuron, and 15% with astrocyte markers. Similarly, 45% of 3 alpha-HSD immunoreactivity was found in oligodendrocytes, 35% in neurons, and 20% in astrocytes. These results are the first demonstrating that oligodendrocytes and neurons of the SC possess the key enzymatic complex for synthesizing potent neuroactive steroids that may control spinal sensorimotor processes.