Physiological functions and hormonal regulation of mouse vas deferens protein (AKR1B7) in steroidogenic tissues

Non-Canonical Effects of ACTH: Insights Into Adrenal Insufficiency

Introduction

Adrenocorticotropic hormone (ACTH) is produced from proopiomelanocortin, which is predominantly synthetized in the corticotroph and melanotroph cells of the anterior and intermediate lobes of the pituitary gland and the arcuate nucleus of the hypothalamus. Although ACTH clearly has an effect on adrenal homeostasis and maintenance of steroid hormone production, it also has extra-adrenal effects that require further elucidation.

Methods

We comprehensively reviewed English language articles, regardless of whether they reported the presence or absence of adrenal and extra-adrenal ACTH effects.

Results

In the present review, we provide an overview on the current knowledge on adrenal and extra-adrenal effects of ACTH. In the section on adrenal ACTH effects, we focused on corticosteroid rhythmicity and effects on steroidogenesis, mineralocorticoids and adrenal growth. In the section on extra-adrenal effects, we have analyzed the effects of ACTH on the osteoarticular and reproductive systems, adipocytes, immune system, brain and skin. Finally, we focused on adrenal insufficiency.

Conclusions

The role of ACTH in maintaining the function of the hypothalamic-pituitary-adrenal axis is well known. Conversely, if we broaden our vision and analyze its role as a potential treatment strategy in other conditions, it will be evident in the literature that researchers seem to have abandoned this aspect in studies conducted several years ago. We believe it is worth re-evaluating the role of ACTH considering its noncanonical effects on the adrenal gland itself and on extra-adrenal organs and tissues; however, this would not have been possible without the recent advances in the pertinent technologies.

Ablation of glucocorticoid receptor in the hindbrain of the mouse provides a novel model to investigate stress disorders

The hypothalamic-pituitary-adrenal (HPA) axis regulates responses to internal and external stressors. Many patients diagnosed with conditions such as depression or anxiety also have hyperactivity of the HPA axis. Hyper-stimulation of the HPA axis results in sustained elevated levels of glucocorticoids which impair neuronal function and can ultimately result in a psychiatric disorder. Studies investigating Glucocorticoid Receptor (GR/NR3C1) in the brain have primarily focused on the forebrain, however in recent years, the hindbrain has become a region of interest for research into the development of anxiety and depression, though the role of GR signalling in the hindbrain remains poorly characterised. To determine the role of glucocorticoid signalling in the hindbrain we have developed a novel mouse model that specifically ablates hindbrain GR to ascertain its role in behaviour, HPA-axis regulation and adrenal structure. Our study highlights that ablation of GR in the hindbrain results in excessive barbering, obsessive compulsive digging and lack of cage exploration. These mice also develop kyphosis, elevated circulating corticosterone and severe adrenal cortex disruption. Together, this data demonstrates a role for hindbrain GR signalling in regulating stress-related behaviour and identifies a novel mouse model to allow further investigation into the pathways impacting stress and anxiety.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

60 YEARS OF POMC: Adrenal and extra-adrenal functions of ACTH

The pituitary adrenocorticotropic hormone (ACTH) plays a pivotal role in homeostasis and stress response and is thus the major component of the hypothalamo-pituitary-adrenal axis. After a brief summary of ACTH production from proopiomelanocortin (POMC) and on ACTH receptor properties, the first part of the review covers the role of ACTH in steroidogenesis and steroid secretion. We highlight the mechanisms explaining the differential acute vs chronic effects of ACTH on aldosterone and glucocorticoid secretion. The second part summarizes the effects of ACTH on adrenal growth, addressing its role as either a mitogenic or a differentiating factor. We then review the mechanisms involved in steroid secretion, from the classical Cyclic adenosine monophosphate second messenger system to various signaling cascades. We also consider how the interaction between the extracellular matrix and the cytoskeleton may trigger activation of signaling platforms potentially stimulating or repressing the steroidogenic potency of ACTH. Finally, we consider the extra-adrenal actions of ACTH, in particular its role in differentiation in a variety of cell types, in addition to its known lipolytic effects on adipocytes. In each section, we endeavor to correlate basic mechanisms of ACTH function with the pathological consequences of ACTH signaling deficiency and of overproduction of ACTH.

Aldo-Keto Reductases 1B in Adrenal Cortex Physiology

Aldose reductase (AKR1B) proteins are monomeric enzymes, belonging to the aldo-keto reductase (AKR) superfamily. They perform oxidoreduction of carbonyl groups from a wide variety of substrates, such as aliphatic and aromatic aldehydes or ketones. Due to the involvement of human aldose reductases in pathologies, such as diabetic complications and cancer, AKR1B subgroup enzymatic properties have been extensively characterized. However, the issue of AKR1B function in non-pathologic conditions remains poorly resolved. Adrenal activities generated large amount of harmful aldehydes from lipid peroxidation and steroidogenesis, including 4-hydroxynonenal (4-HNE) and isocaproaldehyde (4-methylpentanal), which can both be reduced by AKR1B proteins. More recently, some AKR1B isoforms have been shown to be endowed with prostaglandin F synthase (PGFS) activity, suggesting that, in addition to possible scavenger function, they could instigate paracrine signals. Interestingly, the adrenal gland is one of the major sites for human and murine AKR1B expression, suggesting that their detoxifying/signaling activity could be specifically required for the correct handling of adrenal function. Moreover, chronic effects of ACTH result in a coordinated regulation of genes encoding the steroidogenic enzymes and some AKR1B isoforms. This review presents the molecular mechanisms accounting for the adrenal-specific expression of some AKR1B genes. Using data from recent mouse genetic models, we will try to connect their enzymatic properties and regulation with adrenal functions.

Aldo-keto reductase 1b7, a novel marker for renin cells, is regulated by cyclic AMP signaling

We previously identified aldo-keto reductase 1b7 (AKR1B7) as a marker for juxtaglomerular renin cells in the adult mouse kidney. However, the distribution of renin cells varies dynamically, and it was unknown whether AKR1B7 maintains coexpression with renin in response to different developmental, physiological, and pathological situations, and furthermore, whether similar factor(s) simultaneously regulate both proteins. We show here that throughout kidney development, AKR1B7 expression-together with renin-is progressively restricted in the kidney arteries toward the glomerulus. Subsequently, when formerly renin-expressing cells reacquire renin expression, AKR1B7 is reexpressed as well. This pattern of coexpression persists in extreme pathological situations, such as deletion of the genes for aldosterone synthase or Dicer. However, the two proteins do not colocalize within the same organelles: renin is found in the secretory granules, whereas AKR1B7 localizes to the endoplasmic reticulum. Interestingly, upon deletion of the renin gene, AKR1B7 expression is maintained in a pattern mimicking the embryonic expression of renin, while ablation of renin cells resulted in complete abolition of AKR1B7 expression. Finally, we demonstrate that AKR1B7 transcription is controlled by cAMP. Cultured cells of the renin lineage reacquire the ability to express both renin and AKR1B7 upon elevation of intracellular cAMP. In vivo, deleting elements of the cAMP-response pathway (CBP/P300) results in a stark decrease in AKR1B7- and renin-positive cells. In summary, AKR1B7 is expressed within the renin cell throughout development and perturbations to homeostasis, and AKR1B7 is regulated by cAMP levels within the renin cell.

Double NF1 inactivation affects adrenocortical function in NF1Prx1 mice and a human patient

Background

Neurofibromatosis type I (NF1, MIM#162200) is a relatively frequent genetic condition, which predisposes to tumor formation. Apart from tumors, individuals with NF1 often exhibit endocrine abnormalities such as precocious puberty (2,5–5% of NF1 patients) and some cases of hypertension (16% of NF1 patients). Several cases of adrenal cortex adenomas have been described in NF1 individuals supporting the notion that neurofibromin might play a role in adrenal cortex homeostasis. However, no experimental data were available to prove this hypothesis.

Materials and Methods

We analysed Nf1Prx1 mice and one case of adrenal cortical hyperplasia in a NF1patient.

Results

In Nf1Prx1 mice Nf1 is inactivated in the developing limbs, head mesenchyme as well as in the adrenal gland cortex, but not the adrenal medulla or brain. We show that adrenal gland size is increased in NF1Prx1 mice. Nf1Prx1 female mice showed corticosterone and aldosterone overproduction. Molecular analysis of Nf1 deficient adrenals revealed deregulation of multiple proteins, including steroidogenic acute regulatory protein (StAR), a vital mitochondrial factor promoting transfer of cholesterol into steroid making mitochondria. This was associated with a marked upregulation of MAPK pathway and a female specific increase of cAMP concentration in murine adrenal lysates. Complementarily, we characterized a patient with neurofibromatosis type I with macronodular adrenal hyperplasia with ACTH-independent cortisol overproduction. Comparison of normal control tissue- and adrenal hyperplasia- derived genomic DNA revealed loss of heterozygosity (LOH) of the wild type NF1 allele, showing that biallelic NF1 gene inactivation occurred in the hyperplastic adrenal gland.

Conclusions

Our data suggest that biallelic loss of Nf1 induces autonomous adrenal hyper-activity. We conclude that Nf1 is involved in the regulation of adrenal cortex function in mice and humans.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Oxidative and reductive metabolism of lipid-peroxidation derived carbonyls

Extensive research has shown that increased production of reactive oxygen species (ROS) results in tissue injury under a variety of pathological conditions and chronic degenerative diseases. While ROS are highly reactive and can incite significant injury, polyunsaturated lipids in membranes and lipoproteins are their main targets. ROS-triggered lipid-peroxidation reactions generate a range of reactive carbonyl species (RCS), and these RCS spread and amplify ROS-related injury. Several RCS generated in oxidizing lipids, such as 4-hydroxy trans-2-nonenal (HNE), 4-oxo-2-(E)-nonenal (ONE), acrolein, malondialdehyde (MDA) and phospholipid aldehydes have been shown to be produced under conditions of oxidative stress and contribute to tissue injury and dysfunction by depleting glutathione and other reductants leading to the modification of proteins, lipids, and DNA. To prevent tissue injury, these RCS are metabolized by several oxidoreductases, including members of the aldo-keto reductase (AKR) superfamily, aldehyde dehydrogenases (ALDHs), and alcohol dehydrogenases (ADHs). Metabolism via these enzymes results in RCS inactivation and detoxification, although under some conditions, it can also lead to the generation of signaling molecules that trigger adaptive responses. Metabolic transformation and detoxification of RCS by oxidoreductases prevent indiscriminate ROS toxicity, while at the same time, preserving ROS signaling. A better understanding of RCS metabolism by oxidoreductases could lead to the development of novel therapeutic interventions to decrease oxidative injury in several disease states and to enhance resistance to ROS-induced toxicity.

Depressed levels of prostaglandin F2α in mice lacking Akr1b7 increase basal adiposity and predispose to diet-induced obesity

Negative regulators of white adipose tissue (WAT) expansion are poorly documented in vivo. Prostaglandin F(2α) (PGF(2α)) is a potent antiadipogenic factor in cultured preadipocytes, but evidence for its involvement in physiological context is lacking. We previously reported that Akr1b7, an aldo-keto reductase enriched in adipose stromal vascular fraction but absent from mature adipocytes, has antiadipogenic properties possibly supported by PGF(2α) synthase activity. To test whether lack of Akr1b7 could influence WAT homeostasis in vivo, we generated Akr1b7(-/-) mice in 129/Sv background. Akr1b7(-/-) mice displayed excessive basal adiposity resulting from adipocyte hyperplasia/hypertrophy and exhibited greater sensitivity to diet-induced obesity. Following adipose enlargement and irrespective of the diet, they developed liver steatosis and progressive insulin resistance. Akr1b7 loss was associated with decreased PGF(2α) WAT contents. Cloprostenol (PGF(2α) agonist) administration to Akr1b7(-/-) mice normalized WAT expansion by affecting both de novo adipocyte differentiation and size. Treatment of 3T3-L1 adipocytes and Akr1b7(-/-) mice with cloprostenol suggested that decreased adipocyte size resulted from inhibition of lipogenic gene expression. Hence, Akr1b7 is a major regulator of WAT development through at least two PGF(2α)-dependent mechanisms: inhibition of adipogenesis and lipogenesis. These findings provide molecular rationale to explore the status of aldo-keto reductases in dysregulations of adipose tissue homeostasis.

Aldo-Keto Reductases 1B in Endocrinology and Metabolism

The aldose reductase (AR; human AKR1B1/mouse Akr1b3) has been the focus of many research because of its role in diabetic complications. The starting point of these alterations is the massive entry of glucose in polyol pathway where it is converted into sorbitol by this enzyme. However, the issue of AR function in non-diabetic condition remains unresolved. AR-like enzymes (AKR1B10, Akr1b7, and Akr1b8) are highly related isoforms often co-expressed with bona fide AR, making functional analysis of one or the other isoform a challenging task. AKR1B/Akr1b members share at least 65% protein identity and the general ability to reduce many redundant substrates such as aldehydes provided from lipid peroxidation, steroids and their by-products, and xenobiotics in vitro. Based on these properties, AKR1B/Akr1b are generally considered as detoxifying enzymes. Considering that divergences should be more informative than similarities to help understanding their physiological functions, we chose to review specific hallmarks of each human/mouse isoforms by focusing on tissue distribution and specific mechanisms of gene regulation. Indeed, although the AR shows ubiquitous expression, AR-like proteins exhibit tissue-specific patterns of expression. We focused on three organs where certain isoforms are enriched, the adrenal gland, enterohepatic, and adipose tissues and tried to connect recent enzymatic and regulation data with endocrine and metabolic functions of these organs. We presented recent mouse models showing unsuspected physiological functions in the regulation of glucido-lipidic metabolism and adipose tissue homeostasis. Beyond the widely accepted idea that AKR1B/Akr1b are detoxification enzymes, these recent reports provide growing evidences that they are able to modify or generate signal molecules. This conceptually shifts this class of enzymes from unenviable status of scavenger to upper class of messengers.

Structure of the His269Arg mutant of the rat aldose reductase-like protein AKR1B14 complexed with NADPH

Rat aldose reductase-like protein (AKR1B14) is an orthologue of mouse vas deferens protein (AKR1B7) and plays roles in the detoxification of reactive aldehydes and synthesis of prostaglandin F(2α). Here, the 1.87 Å resolution crystal structure of the His269Arg mutant of AKR1B14 complexed with NADPH is described and shows that the negatively charged 2'-phosphate group of the coenzyme forms an ionic interaction with the positively charged guanidinium group of Arg269 that is also observed in the human aldose reductase (AKR1B1) structure. Previous experiments on the site-directed mutagenesis of His269 to Arg, Phe and Met revealed fourfold, sevenfold and 127-fold increases in the K(m) for NADPH, respectively, which are in agreement with the present molecular-modelling and X-ray crystallographic studies. This is the first tertiary structure of a mutant form of this AKR1B7 orthologue to be reported in order to investigate the structure-function relationship of the nonconserved His269 and its role in coenzyme binding.

Endogenous acetaldehyde toxicity during antral follicular development in the mouse ovary

The biosynthesis of androgens requires multiple steps and during the conversion of pregnenolone to 17α-hydroxypregnenolone and dehydroepiandrosterone (DHEA) by CYP17a1. Acetaldehyde is potentially formed as a by-product in theca cells during antral follicular development. In this study, acetaldehyde level was significantly increased after eCG stimulation and reached a maximum level at 36-h post-eCG. By 48 h, the level of acetaldehyde decreased in association with the induction of aldehyde dehydrogenase (ALDH) type 1 family members. When immature mice were co-injected with the ALDH inhibitor, cyanamide, and eCG, the expression of genes involved in the differentiations of granulosa cells was suppressed and the number of ovulated oocytes was reduced. The in vitro studies showed that ALDH inhibitors prevented FSH-induced granulosa cell differentiation. These results indicate that acetaldehyde is generated as a by-product during steroidogenesis and can exert toxic effects to impair the differentiation of granulosa cells, reduce ovulation and decrease oocyte quality.

Identification, cloning and regulation of cDNA encoding aldo-keto reductase 1B7 in the adrenal gland of two Saharan rodents Meriones libycus (Libyan jird) and Gerbillus gerbillus (gerbil)

Aldo-Keto Reductase 1B7 (AKR1B7) is a mouse aldose reductase-like protein with two major sites of expression, the vas deferens and the adrenal cortex. In the adrenal cortex, Akr1b7 is an adrenocorticotropin (ACTH)-responsive-gene whose product scavenges harmful byproducts of steroidogenesis and limits stress response through the biosynthesis of prostaglandin F2α. The purpose of the present study was to explore the possible expression of AKR1B7 in the adrenal glands of two saharan rodents, Libyan jird and Lesser Egyptian gerbil. Western blot analyses demonstrated that a protein related to murine/rat AKR1B7 was highly expressed in adrenals and absent from vas deferens of both saharan species. Based on conserved sequences between mouse and rat, full length cDNA were cloned and sequenced in both species while hormonal regulation and tissue localization were explored in Libyan jird. Both cDNA encoded the expected 316 amino acids protein typical of AKR1B subfamily and contained the highly conserved catalytic tetrad consisting in Asp-44, Tyr-49, Lys-78 and His-111 residues. The deduced proteins shared higher identities with aldose reductase-like, i.e. AKR1B7 (86-94%), AKR1B8 and AKR1B10 (83-86%) than with aldose reductase group, i.e. AKR1B1 and AKR1B3 (70%). Phylogenetic analysis showed that the Libyan jird and gerbil enzymes were more closely related to murine and rat AKR1B7 than to the other AKR1B members. Northern blot analyses of total RNA from Libyan jird adrenals showed a single mRNA transcript of 1.4 kb whose expression was dependent on circulating ACTH levels. In conclusion, we demonstrate here that adrenal glands of Libyan jird and gerbil express both an ortholog of the murine/rat Akr1b7 gene and that ACTH-responsiveness is at least conserved in Libyan jird.

Rat aldose reductase-like protein (AKR1B14) efficiently reduces the lipid peroxidation product 4-oxo-2-nonenal

In this study, we examined the substrate specificity, inhibitor sensitivity and kinetic mechanism of a rat aldose reductase-like protein, which is named AKR1B14 in the aldo-keto reductase (AKR) superfamily. AKR1B14 catalyzed the nicotinamide adenine dinucleotide phosphate reduced form (NADPH)-dependent reduction of carbonyl compounds (derived from lipid peroxidation and glycation), xenobiotic aromatic aldehydes and some aromatic ketones. 4-Oxo-2-nonenal, the best substrate showing a K(m) value of 0.16 µM, was reduced into less reactive 4-oxo-2-nonenol, and its cytotoxicity was attenuated by the overexpression of the enzyme in cultured cells. The enzyme also showed low K(m) values (0.9-10 µM) for medium-chain aliphatic aldehydes (such as 4-hydroxynonenal, 1-hexenal and farnesal) and 3-deoxyglucosone, although the K(m) values for short-chain substrates (such as isocaproaldehyde, acrolein and methylglyoxal) were high (16-600 µM). In the reverse reaction, aliphatic and aromatic alcohols were oxidized by AKR1B14 at low rates. AKR1B14 was inhibited by aldose reductase inhibitors such as tolrestat and epalrestat, and their inhibition patterns were noncompetitive versus the aldehyde substrate and competitive with respect to the alcohol substrate. Kinetic analyses of the oxidoreduction and dead-end inhibition suggest that the reaction follows an ordered sequential mechanism.

Activation of aldo-keto reductase family member 1B14 (AKR1B14) by bile acids: Activation mechanism and bile acid-binding site

Aldo-keto reductase (AKR) 1B14, a rat ortholog of mouse androgen-dependent vas deferens protein (AKR1B7), is involved in the synthesis of prostaglandin F(2α) and detoxification of 4-oxononenal formed by lipid peroxidation. The NADPH-linked reductase activity of AKR1B14 was activated by various bile acids. Although the activation was increased by decreasing pH from 9.0 to 6.0, the concentrations giving maximum stimulation (2- to 18-fold) were 0.2-6.0 μM for bile acids at pH 7.4. Kinetic analyses of the activation by glycochenodeoxycholic acid in the forward and reverse reactions, together with fluorescence changes and protection against 4-oxononenal-induced inactivation by bile acid, indicate that the bile acid binds to the enzyme and its coenzyme binary complex as a non-essential activator. The bile acid binding to AKR1B14 mainly accelerates the NADP(+) dissociation, the rate-limited step of the enzyme reaction. AKR1B7 was also activated by bile acids, but the activation was low and independent of pH. The mutagenesis of His269 and Leu267 of AKR1B14 into the corresponding residues (Arg and Pro, respectively) of AKR1B7 resulted in low and pH-independent activation by bile acids. The results, together with the docking of the bile acid in the recently determined crystal structure of AKR1B14, identify the bile acid-binding site of which His269 plays a key role in significant activation through its electrostatic interaction with the carboxyl group of bile acid, facilitating the release of NADP(+).

Structure of rat aldose reductase-like protein AKR1B14 holoenzyme: Probing the role of His269 in coenzyme binding by site-directed mutagenesis

Rat aldose reductase-like protein (AKR1B14) is the ortholog of mouse vas deferens protein (AKR1B7) playing roles in detoxification of reactive aldehydes and synthesis of prostaglandin F(2α). The crystal structure of the binary complex (AKR1B14-NADPH) was determined at 1.86Å resolution, and showed that the adenine ring and the 2'-phosphate group of the coenzyme formed π-stacking and electrostatic interactions with the imidazole ring and ND1 atom, respectively, of His269, which is not conserved in other aldose reductase-like proteins. The interactions were supported by site-directed mutagenesis of His269 to Arg, Phe and Met, which increased the K(m) for NADPH by 4, 7 and 127-fold, respectively. This is the first report of the tertiary structure of a rodent AKR1B7 ortholog, which describes the role of a novel dual interaction for the non-conserved His269 in coenzyme binding.

AKR1B7 is induced by the farnesoid X receptor and metabolizes bile acids

Although bile acids are crucial for the absorption of lipophilic nutrients in the intestine, they are cytotoxic at high concentrations and can cause liver damage and promote colorectal carcinogenesis. The farnesoid X receptor (FXR), which is activated by bile acids and abundantly expressed in enterohepatic tissues, plays a crucial role in maintaining bile acids at safe concentrations. Here, we show that FXR induces expression of Akr1b7 (aldo-keto reductase 1b7) in murine small intestine, colon, and liver by binding directly to a response element in the Akr1b7 promoter. We further show that AKR1B7 metabolizes 3-keto bile acids to 3β-hydroxy bile acids that are less toxic to cultured cells than their 3α-hydroxy precursors. These findings reveal a feed-forward, protective pathway operative in murine enterohepatic tissues wherein FXR induces AKR1B7 to detoxify bile acids.

Properties and tissue distribution of a novel aldo-keto reductase encoding in a rat gene (Akr1b10)

A recent rat genomic sequencing predicts a gene Akr1b10 that encodes a protein with 83% sequence similarity to human aldo-keto reductase (AKR) 1B10. In this study, we isolated the cDNA for the rat AKR1B10 (R1B10) from rat brain, and examined the enzymatic properties of the recombinant protein. R1B10 utilized NADPH as the preferable coenzyme, and reduced various aldehydes (including cytotoxic 4-hydroxy-2-hexenal and 4-hydroxy- and 4-oxo-2-nonenals) and α-dicarbonyl compounds (such as methylglyoxal and 3-deoxyglucosone), showing low K(m) values of 0.8-6.1μM and 3.7-67μM, respectively. The enzyme also reduced glyceraldehyde and tetroses (K(m)=96-390μM), although hexoses and pentoses were inactive and poor substrates, respectively. Among the substrates, 4-oxo-2-nonenal was most efficiently reduced into 4-oxo-2-nonenol, and its cytotoxicity against bovine endothelial cells was decreased by the overexpression of R1B10. R1B10 showed low sensitivity to aldose reductase inhibitors, and was activated to approximately two folds by valproic acid, and alicyclic and aromatic carboxylic acids. The mRNA for R1B10 was expressed highly in rat brain and heart, and at low levels in other rat tissues and skin fibroblasts. The results suggest that R1B10 functions as a defense system against oxidative stress and glycation in rat tissues.

Aldo keto reductase 1B7 and prostaglandin F2alpha are regulators of adrenal endocrine functions

Prostaglandin F_2α (PGF_2α), represses ovarian steroidogenesis and initiates parturition in mammals but its impact on adrenal gland is unknown. Prostaglandins biosynthesis depends on the sequential action of upstream cyclooxygenases (COX) and terminal synthases but no PGF_2α synthases (PGFS) were functionally identified in mammalian cells. In vitro, the most efficient mammalian PGFS belong to aldo-keto reductase 1B (AKR1B) family. The adrenal gland is a major site of AKR1B expression in both human (AKR1B1) and mouse (AKR1B3, AKR1B7). Thus, we examined the PGF_2α biosynthetic pathway and its functional impact on both cortical and medullary zones. Both compartments produced PGF_2α but expressed different biosynthetic isozymes. In chromaffin cells, PGF_2α secretion appeared constitutive and correlated to continuous expression of COX1 and AKR1B3. In steroidogenic cells, PGF_2α secretion was stimulated by adrenocorticotropic hormone (ACTH) and correlated to ACTH-responsiveness of both COX2 and AKR1B7/B1. The pivotal role of AKR1B7 in ACTH-induced PGF_2α release and functional coupling with COX2 was demonstrated using over- and down-expression in cell lines. PGF_2α receptor was only detected in chromaffin cells, making medulla the primary target of PGF_2α action. By comparing PGF_2α-responsiveness of isolated cells and whole adrenal cultures, we demonstrated that PGF_2α repressed glucocorticoid secretion by an indirect mechanism involving a decrease in catecholamine release which in turn decreased adrenal steroidogenesis. PGF_2α may be regarded as a negative autocrine/paracrine regulator within a novel intra-adrenal feedback loop. The coordinated cell-specific regulation of COX2 and AKR1B7 ensures the generation of this stress-induced corticostatic signal.

24-dehydrocholesterol reductase/seladin-1: a key protein differentially involved in adrenocorticotropin effects observed in human and rat adrenal cortex

DHCR24 (24-dehydrocholesterol reductase), or seladin-1, is one of the most expressed genes in the adrenal gland. Because the rat and human adult adrenal cortex differ in their respective functional properties, the aim of the present study was to verify whether seladin-1 may be differentially involved in basal and ACTH-stimulated steroidogenesis and oxidative stress management. Seladin-1 expression was predominantly observed in both human and rat zona fasciculata, with a predominant cytoplasmic localization in human cells and a nucleo-cytoplasmic distribution in rat cells. In human fasciculata cells, localization of the protein was primarily associated with the endoplasmic reticulum. Although its expression was increased by ACTH, its intracellular localization was not altered by ACTH treatment (10 nm) or by the seladin-1 inhibitor U18666A (75 nm). Preincubation with U18666A did not modify the ACTH-induced increase in cortisol secretion but abolished the ACTH-induced increase in dehydroepiandrosterone secretion. In rat fasciculata cells, ACTH induced a massive redistribution of seladin-1 from the cytoplasm (cis-Golgi apparatus) to the nucleus, which was inhibited by preincubation with U18666A. Preincubation with U18666A also decreased ACTH-induced seladin-1 and 11beta-hydroxylase protein expression as well as corticosterone production, increased ACTH-induced ROS production but decreased ACTH-induced expression of the detoxifying protein aldo-ketoreductase 1b7. Thus, protection against acutely elevated ACTH-induced oxidative stress in rat fasciculata cells is correlated with nuclear relocalization of seladin-1 and its effects on cellular detoxifying machinery. Altogether, these results indicate that seladin-1 expression and intracellular localization are correlated with both the intensity and nature of ACTH-induced steroidogenesis and resultant oxidative stress.

A transgenic mouse line with specific Cre recombinase expression in the adrenal cortex

The Cre-loxP system combined with gene targeting strategies has proven to be very useful for gene inactivation in specific tissues and/or cell types. To achieve adrenal cortex specific recombination in vivo, we used a 0.5-kb fragment of the 5'-flanking region of the akr1b7 gene to drive Cre expression in adrenocortical cells. The resulting 0.5 akr1b7-Cre mice express Cre in all steroidogenic zones of the adrenal cortex but not in the gonads. Although recombination of the ROSA26R reporter locus was not observed in all cortical cells, we provide evidence that Cre is expressed in all the cells of the cortex in adult mice. In addition, Cre activity was found in collecting ducts and maturing glomeruli of the kidney. This line is the first to show specific Cre expression in the adrenal cortex in the absence of Cre expression in the gonads. This transgene thus provides a valuable tool for specific gene recombination in the adrenal cortex and kidney.

Atlas of dihydrotestosterone actions on the transcriptome of prostate in vivo

BACKGROUND

Using serial analysis of gene expression (SAGE), we studied the transcriptomic changes in vivo by dihydrotestosterone (DHT) treatment in mice to better understand androgen effects in the prostate.

METHODS

Approximately 872,000 SAGE tags were isolated from intact and castrated (GDX) mice with and without DHT injection.

RESULTS

GDX significantly altered 431 transcripts, including 110 transcripts restored by DHT, and 146 potentially new transcripts. Totally, 187 transcripts were significantly affected by DHT treatment, of which 124 were induced and 63 were repressed. Interestingly and consistent with the prostate's secretory role, DHT up-regulated the expression of many genes involved in various steps of protein metabolism such as synthesis, folding, and secretion. GDX modulated the expression of genes which induce cell apoptosis and inhibit cell proliferation through polyamine biosynthesis, retinoid X receptor actions as well as several signaling pathways and some related factors. These results clarify DHT effects on prostate transcriptome in the areas of protein metabolism, cell proliferation and apoptosis. In addition, we detected gene expression changes in metabolic pathways, cytoskeleton, immunity and endoplasmic reticulum stress. Furthermore, knockdown of S-adenosylmethionine decarboxylase 1 in LNCaP cells confirmed the importance of androgen-regulated genes (ARGs) in prostate cancer cell growth.

CONCLUSION

Our data support the idea that ARGs are essential for the normal development of the prostate and can also be responsible for the pathogenesis of the prostate cancer.

Characterization of a rat NADPH-dependent aldo-keto reductase (AKR1B13) induced by oxidative stress

A rat aldo-keto reductase (AKR1B13) was identified as a hepatoma-derived protein, exhibiting high sequence identity with mouse fibroblast growth factor (FGF)-induced reductase, AKR1B8. In this study, AKR1B13 was characterized in terms of its enzymatic properties, tissue distribution and regulation. Recombinant AKR1B13 exhibited NADPH-linked reductase activity towards various aldehydes and alpha-dicarbonyl compounds, which include reactive compounds such as methylglyoxal, glyoxal, acrolein, 4-hydroxynonenal and 3-deoxyglucosone. The enzyme exhibited low NADP(+)-linked dehydrogenase activity towards aliphatic and aromatic alcohols, and was inhibited by aldose reductase inhibitors, flavonoids, benzbromarone and hexestrol. Immunochemical and reverse transcription-PCR analyses revealed that the enzyme is expressed in many rat tissues, endothelial cells and fibroblasts. Gene expression in YPEN-1 and NRK cells was up-regulated by treatments with submicromolar concentrations of hydrogen peroxide and 1,4-naphthoquinone, but not with FGF-1, FGF-2, 5alpha-dihydrotestosterone and 17beta-estradiol. These results indicate that AKR1B13 differs from AKR1B8 in tissue distribution and gene regulation, and suggest that it functions as a defense system against oxidative stress in rat tissues.

Substrate specificity and catalytic efficiency of aldo-keto reductases with phospholipid aldehydes

Phospholipid oxidation generates several bioactive aldehydes that remain esterified to the glycerol backbone ('core' aldehydes). These aldehydes induce endothelial cells to produce monocyte chemotactic factors and enhance monocyte-endothelium adhesion. They also serve as ligands of scavenger receptors for the uptake of oxidized lipoproteins or apoptotic cells. The biochemical pathways involved in phospholipid aldehyde metabolism, however, remain largely unknown. In the present study, we have examined the efficacy of the three mammalian AKR (aldo-keto reductase) families in catalysing the reduction of phospholipid aldehydes. The model phospholipid aldehyde POVPC [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine] was efficiently reduced by members of the AKR1, but not by the AKR6 or the ARK7 family. In the AKR1 family, POVPC reductase activity was limited to AKR1A and B. No significant activity was observed with AKR1C enzymes. Among the active proteins, human AR (aldose reductase) (AKR1B1) showed the highest catalytic activity. The catalytic efficiency of human small intestinal AR (AKR1B10) was comparable with the murine AKR1B proteins 1B3 and 1B8. Among the murine proteins AKR1A4 and AKR1B7 showed appreciably lower catalytic activity as compared with 1B3 and 1B8. The human AKRs, 1B1 and 1B10, and the murine proteins, 1B3 and 1B8, also reduced C-7 and C-9 sn-2 aldehydes as well as POVPE [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphoethanolamine]. AKR1A4, B1, B7 and B8 catalysed the reduction of aldehydes generated in oxidized C(16:0-20:4) phosphatidylcholine with acyl, plasmenyl or alkyl linkage at the sn-1 position or C(16:0-20:4) phosphatidylglycerol or phosphatidic acid. AKR1B1 displayed the highest activity with phosphatidic acids; AKR1A4 was more efficient with long-chain aldehydes such as 5-hydroxy-8-oxo-6-octenoyl derivatives, whereas AKR1B8 preferred phosphatidylglycerol. These results suggest that proteins of the AKR1A and B families are efficient phospholipid aldehyde reductases, with non-overlapping substrate specificity, and may be involved in tissue-specific metabolism of endogenous or dietary phospholipid aldehydes.

Enzymatic characteristics of an aldo-keto reductase family protein (AKR1C15) and its localization in rat tissues

A member of the aldo-keto reductase superfamily, AKR1C15, was isolated via cDNA cloning, but its physiological function remains unknown. Here, we show that recombinant AKR1C15 is an NADPH-dependent reductase with broad substrate specificity for aromatic, alicyclic and aliphatic carbonyl compounds, including acetoin, 2,5-hexanedione, methylglyoxal, farnesal, retinals, 17-ketosteroids and monosaccharides. Especially, all-trans-retinal, alpha-diketones and lipid-derived aldehydes including 4-hydroxynonenal were excellent substrates showing low K(m) values (0.3-5.5 microM). Immunohistochemical and reverse transcription-PCR analyses revealed that AKR1C15 is highly expressed in rat bronchiolar Clara cells, type II alveolar cells, gastric parietal cells, the epithelial cells of the stomach and colon, and the brown adipocytes. The enzyme was not detected in cells of other rat tissues, but is consistently expressed in the vascular endothelial cells. These results suggest that AKR1C15 plays a role in retinoid, steroid, isoprenoid and carbohydrate metabolism, as well as a defense system, protecting against reactive carbonyl compounds.

Role of exosomes in sperm maturation during the transit along the male reproductive tract

Even tough differentiated spermatozoa are unable of transcriptional or translational activity; the sperm surface undergoes major modifications in macromolecules composition during the transit along the male reproductive tract. This is the result of sequential, well orchestrated interactions between the male reproductive tract secretions and the transiting male gamete. This is particularly true when spermatozoa transit along the epididymis. The epididymis is a long convoluted tubules in which the spermatozoa leaving the testis have to transit. The unraveled epididymal tubule can be as long as 80 m in stallion, and the transit time of spermatozoa is of 3-12 days depending on the species. The epididymis is usually divided in three segments: the caput (proximal part), the corpus, and cauda. While the cauda epididymides acts as a sperm reservoir, the caput and corpus are responsible for sperm maturation. This means that, under androgen control, the epididymal epithelium secretes proteins that will interact sequentially with sperm surface. Some of the sperm proteins acquired during maturation along the excurrent duct behave as integral membrane proteins. In fact, some epididymal originating proteins are glycosylphosphatidylinositol (GPI)-anchored to the sperm plasma membrane. Our laboratory has shown that some of these proteins are secreted in an apocrine manner by the epididymal epithelium and are associated to exosomes, called epididymosomes. Epididymosomes are rich in sphingomyelin and are characterized by a high cholesterol/phospholipids ratio. Many proteins are associated to epididymosomes, some of which are selectively transferred to spermatozoa during the epididymal transit. We have identified some of these exosomes associated proteins transferred to the maturing spermatozoa. These include two enzymes involved in the polyol pathway: an aldose reductase and a sorbitol dehydrogenase. A cytokine named MIF (macrophage migration inhibitory factor) is another protein associated to exosomes who is transferred to spermatozoa during the epididymal transit. We hypothesized that both the polyol pathway and MIF secreted in an apocrine fashion by the epididymal epithelium modulate sperm motility during the transit along the male reproductive tract. Finally, P25b, belonging to a family of sperm surface proteins (P26h/P34H) necessary for the binding to the surface of the egg, is also acquired through the interaction between epididymosomes and the male gamete. In vitro studies have defined the conditions of protein transfer when epididymal spermatozoa are co-incubated with epididymosomes. The transfer of selected proteins to specific membrane domains of spermatozoa is saturable, temperature and pH-dependent, being optimal at pH 6.5. The presence of zinc in the incubation medium, but not of calcium neither magnesium, significantly increases the efficiency of protein transfer. These results show that exosomes play a role in sperm epididymal maturation which is an essential event to produce male gametes with optimal fertilizing ability.

Glomerulosa function and aldosterone synthesis in the rat

Since its discovery, it has been generally assumed that the primary function of the zona glomerulosa of the adrenal cortex is the secretion of aldosterone. Taking evidence from the rat, and recognising that there is probably considerable species variation, I argue here that the glomerulosa in fact has many functions, including aldosterone synthesis, but is probably only a relatively poor de novo source of steroid. In vitro, the CYP11B2 (aldosterone synthase) of the glomerulosa can and does utilise as substrates products arising from CYP11B1 (11beta-hydroxylase) activity in fasciculata cells. Whether it does in vivo is open to question, but corticosterone and 18-hydroxydeoxycorticosterone are both present in circulating rat plasma at suitable concentrations. Such a mechanism would explain several inconsistencies in the literature, including the anomalous distribution of steroidogenic enzymes in the glomerulosa, the stimulation of CYP11B1 products by aldosterone secretagogues such as potassium ions or angiotensin II, the partial dependence of aldosterone secretion in vivo on an intact pituitary, the sensitivity of aldosterone secretion to tissue disruption in vitro, and the "late pathway" regulation of aldosterone synthesis.

Physiological role of mGSTA4-4, a glutathione S-transferase metabolizing 4-hydroxynonenal: generation and analysis of mGsta4 null mouse

The lipid peroxidation product 4-hydroxynon-2-enal (4-HNE) is a strong electrophile that forms covalent adducts with proteins and, to a lesser extent, nucleic acids and phospholipids. The generation of 4-HNE appears to be an inevitable consequence of aerobic metabolism. The metabolism of 4-HNE is mainly, although not entirely, conjugative, and proceeds via Michael addition of glutathione to the double bond of 4-HNE. This reaction is catalyzed by specialized glutathione S-transferases (GSTs) exemplified by the murine mGSTA4-4. To study the (patho)physiological effects of 4-HNE in an intact organism, we disrupted the mGsta4 gene in the mouse. The resulting mGsta4 null mouse expressed no mGsta4 mRNA and no corresponding protein, had a reduced ability to conjugate 4-HNE, and had an increased steady-state level of this aldehyde in tissues. The residual conjugating activity for 4-HNE (23-64% depending on the tissue) is probably attributable to isoforms of glutathione S-transferases which have low catalytic efficiency for 4-HNE but are more abundant than mGSTA4-4, or are upregulated upon mGsta4 gene disruption. Mice homozygous for the disrupted mGsta4 allele were viable and appeared normal except for lower litter size, higher fat content in bones, and greater susceptibility to bacterial infection. The null mice had a significantly lower survival time than wild-type controls when chronically treated with relatively low doses of paraquat, a finding consistent with a role of mGSTA4-4 in the defense against oxidative stress. The mouse model should be useful for the study of degenerative conditions in which 4-HNE is postulated to be a contributing factor.

Preparation and characterization of polyclonal antibodies against ARL-1 protein

AIM: To prepare and characterize polyclonal antibodies against aldose reductase-like (ARL-1) protein.

METHODS: ARL-1 gene was inserted into the E. coli expression vector pGEX-4T-1(His)6C and vector pQE-30. Recombinant ARL-1 proteins named ARL-(His)₆ and ARL-GST were expressed. They were purified by affinity chromatography. Sera from domestic rabbits immunized with ARL-(His)₆ were purified by CNBr-activated sepharose 4B coupled ARL-GST. Polyclonal antibodies were detected by Western blotting.

RESULTS: Recombinant proteins of ARL-(His)₆ with molecular weight of 35.7 KD and ARL-GST with molecular weight of 60.8 KD were highly expressed. The expression levels of ARL-GST and ARL-(His)₆ were 15.1% and 27.7% among total bacteria proteins, respectively. They were soluble, predominantly in supernatant. After purification by non-denatured way, SDS-PAGE showed one band. In the course of polyclonal antibodies purification, only one elution peak could be seen. Western blotting showed positive signals in the two purified proteins and the bacteria transformed with pGEX-4T-1(His)₆ C-ARL and pQE-30-ARL individually.

CONCLUSION: Polyclonal antibodies are purified and highly specific against ARL-1 protein. ARL-GST and ARL-(His)₆ are highly expressed and purified.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.

Steroidogenic factor-1 controls the aldose reductase akr1b7 gene promoter in transgenic mice through an atypical binding site

Aldo-keto-reductase 1B7/mouse vas deferens protein (AKR1B7/MVDP) is expressed in rodent steroidogenic glands and in the mouse vas deferens. In steroidogenic organs, AKR1B7/MVDP scavenges isocaproaldehyde produced from the cholesterol side-chain cleavage reaction. Akr1b7/mvdp is responsive to ACTH in adrenals and to androgens in vas deferens. Using transgenic mice, we previously delimited the regulatory DNA sequences necessary for expression in both organs and identified by cell transfections, a cryptic steroidogenic factor-1 (SF-1) response element (SFRE) at -102 that overlaps a proximal androgen-responsive element. To address its in vivo functions in adrenals, we devised a transgenic mouse study using wild-type and mutant akr1b7 promoters driving the chloramphenol acetyltransferase reporter gene. Adrenal expression in adults was impaired in all lines mutant for -102 SFRE. This effect is linked to impaired SF-1 binding and not to impaired androgen receptor binding, because akr1b7 expression is not affected in adrenals of androgen receptor-defective Tfm mice. Triphasic developmental patterns of both AKR1B7 and wild-type transgene expression paralleled changes in SF-1 levels/binding activity; expression was maximal in late embryos, minimal in 6- to 15-d-old neonates, and thereafter progressively restored. Differences in developmental expression between wild-type and mutant transgenes revealed that requirement for the -102 SFRE appears stage specific, as its integrity is an absolute prerequisite for reinduction of gene expression after postnatal d 15. Further, mutation of this site did not affect transgene responsiveness to ACTH. These findings demonstrate a new function for SFRE in vivo, via influencing promoter sensibility to postnatal changes of SF-1 contents, in controlling promoter strength in adults without affecting adrenal targeting, hormonal control, or early gene expression.

Growth hormone regulation of rat liver gene expression assessed by SSH and microarray

The sexually dimorphic secretion of growth hormone (GH) that prevails in the rat leads to a sex-differentiated expression of GH target genes, particularly in the liver. We have used subtractive suppressive hybridization (SSH) to search for new target genes induced by the female-characteristic, near continuous, pattern of GH secretion. Microarrays and dot-blot hybridizations were used in an attempt to confirm differential ratios of expression of obtained SSH clones. Out of 173 unique SSH clones, 41 could be verified as differentially expressed. Among these, we identified 17 known genes not previously recognized as differentially regulated by the sex-specific GH pattern. Additional SSH clones may also represent genes subjected to sex-specific GH regulation since only transcripts abundantly expressed could be verified. Optimized analyses, specific for each gene, are required to fully characterize the degree of differential expression.