Genomic structure of rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD, AKR1C9)

Koumine enhances spinal cord 3α-hydroxysteroid oxidoreductase expression and activity in a rat model of neuropathic pain

BACKGROUND

Koumine is an alkaloid monomer found abundantly in Gelsemium plants. It has been shown to reverse thermal hyperalgesia and mechanical allodynia induced by sciatic nerve chronic constriction injury (CCI) in rats in a dose-dependent manner. Interestingly, this effect is mediated by elevated allopregnanolone levels in the spinal cord (SC). Since 3α-hydroxysteroid oxidoreductase (3α-HSOR), the key synthetase of allopregnanolone, is responsible for allopregnanolone upregulation in the SC, the objective of the present study was to investigate the role of its expression in the SC in koumine-induced analgesia using a rat model of neuropathic pain following peripheral nerve injury.

RESULTS

Time-course investigations of immunohistochemistry and real-time polymerase chain reaction revealed that the immunoreactivity and mRNA expression of 3α-HSOR markedly increased in a time-dependent manner in the SC of koumine-treated CCI rats. Furthermore, 3α-HSOR activity in the SC of koumine-treated CCI rats increased by 15.8% compared to the activity in untreated CCI rats. Intrathecal injection of medroxyprogesterone acetate, a selective 3α-HSOR inhibitor, reversed the analgesic effect of koumine on CCI-induced mechanical pain perception. Our results confirm that koumine alleviates neuropathic pain in rats with CCI by enhancing 3α-HSOR mRNA expression and bioactivity in the SC.

CONCLUSION

This study demonstrates that 3α-HSOR is an important molecular target of koumine for alleviating neuropathic pain. Koumine may prove a promising compound for the development of novel analgesic agents effective against intractable neuropathic pain.

Fluoxetine elevates allopregnanolone in female rat brain but inhibits a steroid microsomal dehydrogenase rather than activating an aldo-keto reductase

Background and Purpose

Fluoxetine, a selective serotonin reuptake inhibitor, elevates brain concentrations of the neuroactive progesterone metabolite allopregnanolone, an effect suggested to underlie its use in the treatment of premenstrual dysphoria. One report showed fluoxetine to activate the aldo-keto reductase (AKR) component of 3α-hydroxysteroid dehydrogenase (3α-HSD), which catalyses production of allopregnanolone from 5α-dihydroprogesterone. However, this action was not observed by others. The present study sought to clarify the site of action for fluoxetine in elevating brain allopregnanolone.

Experimental Approach

Adult male rats and female rats in dioestrus were treated with fluoxetine and their brains assayed for allopregnanolone and its precursors, progesterone and 5α-dihydroprogesterone. Subcellular fractions of rat brain were also used to investigate the actions of fluoxetine on 3α-HSD activity in both the reductive direction, producing allopregnanolone from 5α-dihydroprogesterone, and the reverse oxidative direction. Fluoxetine was also tested on these recombinant enzyme activities expressed in HEK cells.

Key Results

Short-term treatment with fluoxetine increased brain allopregnanolone concentrations in female, but not male, rats. Enzyme assays on native rat brain fractions and on activities expressed in HEK cells showed fluoxetine did not affect the AKR producing allopregnanolone from 5α-dihydroprogesterone but did inhibit the microsomal dehydrogenase oxidizing allopregnanolone to 5α-dihydroprogesterone.

Conclusions and Implications

Fluoxetine elevated allopregnanolone in female rat brain by inhibiting its oxidation to 5α-dihydroprogesterone by a microsomal dehydrogenase. This is a novel site of action for fluoxetine, with implications for the development of new agents and/or dosing regimens to raise brain allopregnanolone.

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.

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Local synthesis and dual actions of progesterone in the nervous system: neuroprotection and myelination

Progesterone (PROG) is synthesized in the brain, spinal cord and peripheral nerves. Its direct precursor pregnenolone is either derived from the circulation or from local de novo synthesis as cytochrome P450scc, which converts cholesterol to pregnenolone, is expressed in the nervous system. Pregnenolone is converted to PROG by 3beta-hydroxysteroid dehydrogenase (3beta-HSD). In situ hybridization studies have shown that this enzyme is expressed throughout the rat brain, spinal cord and dorsal root ganglia (DRG) mainly by neurons. Macroglial cells, including astrocytes, oligodendroglial cells and Schwann cells, also have the capacity to synthesize PROG, but expression and activity of 3beta-HSD in these cells are regulated by cellular interactions. Thus, Schwann cells convert pregnenolone to PROG in response to a neuronal signal. There is now strong evidence that P450scc and 3beta-HSD are expressed in the human nervous system, where PROG synthesis also takes place. Although there are only a few studies addressing the biological significance of PROG synthesis in the brain, the autocrine/paracrine actions of locally synthesized PROG are likely to play an important role in the viability of neurons and in the formation of myelin sheaths. The neuroprotective effects of PROG have recently been documented in a murine model of spinal cord motoneuron degeneration, the Wobbler mouse. The treatment of symptomatic Wobbler mice with PROG for 15 days attenuated the neuropathological changes in spinal motoneurons and had beneficial effects on muscle strength and the survival rate of the animals. PROG may exert its neuroprotective effects by regulating expression of specific genes in neurons and glial cells, which may become hormone-sensitive after injury. The promyelinating effects of PROG were first documented in the mouse sciatic nerve and in co-cultures of sensory neurons and Schwann cells. PROG also promotes myelination in the brain, as shown in vitro in explant cultures of cerebellar slices and in vivo in the cerebellar peduncle of aged rats after toxin-induced demyelination. Local synthesis of PROG in the brain and the neuroprotective and promyelinating effects of this neurosteroid offer interesting therapeutic possibilities for the prevention and treatment of neurodegenerative diseases, for accelerating regenerative processes and for preserving cognitive functions during aging.

Structure-function of human 3 alpha-hydroxysteroid dehydrogenases: genes and proteins

Four soluble human 3 alpha-hydroxysteroid dehydrogenase (HSD) isoforms exist which are aldo-keto reductase (AKR) superfamily members. They share 86% sequence identity and correspond to: AKR1C1 (20 alpha(3 alpha)-HSD); AKR1C2 (type 3 3 alpha-HSD and bile-acid binding protein); AKR1C3 (type 2 3 alpha-HSD and type 5 17 beta-HSD); and AKR1C4 (type 1 3 alpha-HSD). Each of the homogeneous recombinant enzymes are plastic and display 3-, 17- and 20-ketosteroid reductase and 3 alpha- 17 beta- and 20 alpha-hydroxysteroid oxidase activities with different k(cat)/K(m) ratios in vitro. The crystal structure of the AKR1C2.NADP(+).ursodeoxycholate complex provides an explanation for this functional plasticity. Ursodeoxycholate is bound backwards (D-ring in the A-ring position) and upside down (beta-face of steroid inverted) relative to the position of 3-ketosteroids in the related rat liver 3 alpha-HSD (AKR1C9) structure. Transient transfection indicates that in COS-1 cells, AKR1C enzymes function as ketosteroid reductases due to potent inhibition of their oxidase activity by NADPH. By acting as ketosteroid reductases they may regulate the occupancy of the androgen, estrogen and progesterone receptors. RT-PCR showed that AKRs are discretely localized. AKR1C4 is virtually liver specific, while AKR1C2 and AKR1C3 are dominantly expressed in prostate and mammary gland. AKR1C genes are highly conserved in structure and may be transcriptionally regulated by steroid hormones and stress.

A cluster of eight hydroxysteroid dehydrogenase genes belonging to the aldo-keto reductase supergene family on mouse chromosome 13

A subclass of hydroxysteroid dehydrogenases (HSD) are NADP(H)-dependent oxidoreductases that belong to the aldo-keto reductase (AKR) superfamily. They are involved in prereceptor or intracrine steroid modulation, and also act as bile acid-binding proteins. The HSD family members characterized thus far in human and rat have a high degree of protein sequence similarity but exhibit distinct substrate specificity. Here we report the identification of nine murine AKR genes in a cluster on chromosome 13 by a combination of molecular cloning and in silico analysis of this region. These include four previously isolated mouse HSD genes (Akr1c18, Akr1c6, Akr1c12, Akr1c13), the more distantly related Akr1e1, and four novel HSD genes. These genes exhibit highly conserved exon/intron organization and protein sequence predictions indicate 75% amino acid similarity. The previously identified AKR protein active site residues are invariant among all nine proteins, but differences are observed in regions that have been implicated in determining substrate specificity. Differences also occur in tissue expression patterns, with expression of some genes restricted to specific tissues and others expressed at high levels in multiple tissues. Our findings dramatically expand the repertoire of AKR genes and identify unrecognized family members with potential roles in the regulation of steroid metabolism.

Biosynthesis of the neurosteroid 3 alpha-hydroxy-4-pregnen-20-one (3 alpha hp), a specific inhibitor of FSH release

The gonadal steroid 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP) is a neuroactive steroid with anxiolytic and analgesic actions. In addition, 3 alpha HP has been shown to inhibit GnRH activity on gonadotropes and selectively suppress FSH release from pituitary cells, without an effect on LH. The enzyme 3 alpha-hydroxysteroid dehydrogenase (3 alpha HSD) has been presumed to be the enzyme responsible for the conversion of progesterone to 3 alpha HP, but this has never been confirmed in vitro or in vivo. We have now determined the mechanism of 3 alpha HP synthesis in vivo using specific enzyme inhibitors and in vitro using recombinant proteins. Incubation of [(3)H]progesterone with purified recombinant rat and human 3 alpha HSD isoforms showed that both the rat 3 alpha HSD and the human type 2(brain) 3 alpha HSD converted progesterone to 3 alpha HP. Age-dependent 3 alpha HP production was demonstrated in pituitary and cortex. Incubation of both tissues with indomethacin, a known 3 alpha HSD inhibitor, decreased the conversion of progesterone to 3 alpha HP by at least 70%, indicating that 3 alpha HSD was responsible for this conversion. As human type 2 3 alpha HSD is expressed in a region-specific fashion in the brain, 3 alpha HP may only be made in specific regions of the brain. Furthermore, the data suggest that the pituitary has the capacity for 3 alpha HP production, which may provide an additional mechanism for regulation of GnRH action.

Proteome analysis of rat hepatomas: carcinogen-dependent tumor-associated protein variants

Proteome analysis led to the identification and characterization of tumor-associated protein variants by two-dimensional electrophoresis and mass spectrometry. We focused on comparing the influence of genotoxic nitroso compounds N-methyl-N-nitrosourea, diethylnitrosamine and N-nitrosomorpholine and the nongenotoxic peroxisome proliferator Nafenopin as tumor-inducing agents on the protein pattern of rat hepatomas. We found several tumor-associated variants that represent members of the aldo-keto reductase superfamily. Their induction and/or inhibition was specifically related to the carcinogen used for tumor induction. The most prominent tumor-associated protein, rat aldose reductase-like protein-1 (rARLP-1) (69% sequence identity to lens aldose reductase) and three additional types of rARLP-1 were detected in nitroso compound-induced rat hepatomas, while rat aldo-keto reductase protein-c (Rak-c), a novel tumor-associated variant (65% sequence identity with 3alpha-hydroxysteroid dehydrogenase) was discovered in N-methyl-N-nitrosourea-induced hepatomas only. 3Alpha-hydroxysteroid dehydrogenase and delta4-3-ketosteroid-5beta-reductase, both liver-specific enzymes, were reduced in amount in all hepatomas investigated, independent of their mode of induction. We conclude, that detoxification enzymes like 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) and delta4-3-ketosteroid-5beta-reductase (5beta-Red) might be replaced in hepatomas by tumor-associated proteins that are often present in the embryonal state, like the rARLPs or the Rak-c protein. Their induction appears to reflect an altered constitutive pattern of detoxification enzymes, detoxifying toxic aldehydes being induced by nitroso compounds. In contrast, members of the aldo-keto reductase superfamily have not been found in Nafenopin-induced hepatomas. The pattern of tumor-associated protein variants is apparently characteristic for a given group of initiating carcinogens. The hypothesis is proposed that carcinogens leave specific fingerprints at the proteome level of manifest liver tumors.

Members of the nuclear factor 1 transcription factor family regulate rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD AKR1C9) gene expression: a member of the aldo-keto reductase superfamily

Rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD; AKR1C9), a member of the aldo-keto reductase (AKR) superfamily, inactivates nearly all steroid hormones by converting 5alpha- and 5beta-dihydrosteroids to their respective 3alpha,5alpha- and 3alpha,5beta-tetrahydrosteroids and protects against circulating steroid hormone excess. It is highly expressed in rat liver comprising 0.5-1.0% of the soluble protein. Previously, we identified a powerful distal enhancer resident at about -4.0 kb to -2.0 kb in the 5'-flanking region of the 3alpha-HSD/DD gene. We now report the functional dissection of this enhancer. Transfection of nested deletions of the 5'-end of the gene promoter linked to chloramphenicol acetyltransferase (CAT) into HepG2 cells located the enhancer activity between (-4673 to -4179 bp). Further internal and 5'-end deletion mutants revealed that a 73-bp fragment (from -4351 to -4279 bp) contained a major enhancer element. This fragment spanned two imperfect direct repeats GTGGAAAAACCCAGGAA and GTGGAAAAAACCCAGGAA and contained three direct repeats of GGAAAAA. This fragment also contained three potential half-nuclear factor 1 (NF1) sites (TGGA-NNNNNGCCA) and a putative CCAAT-enhancer binding protein (C/EBP) binding site. The 73-bp fragment enhanced CAT activity from the basal 3alpha-HSD/DD gene promoter. Recombinant C/EBPalpha and C/EBPbeta did not bind to this fragment. Electrophoretic mobility shift assays showed that HepG2 and rat liver nuclear extracts bound to this 73-bp fragment. The 73-bp protein complex was competed out by a NF1 oligonucleotide and was supershifted by an NF1 antibody. When the 73-bp fragment was fused to an alpha1-globin promoter-CAT construct and cotransfected with CCAAT transcription factor 1 (CTF1)/NF1 into Drosophila Schneider SL2 insect cells (which lack NF1-like proteins) trans-activation of CAT activity was observed. These results indicate that members of the NF1 transcription factor family regulate high constitutive expression of the rat 3alpha-HSD/DD gene that is responsible for steroid hormone inactivation. The potential role of NF1 in regulating other AKR genes that have protective roles is discussed.