microorganisms Review Microbial Hydroxysteroid Dehydrogenases: From Alpha to Omega Heidi L. Doden 1,2 and Jason M. Ridlon 1,2,3,4,5,* 1 Microbiome Metabolic Engineering Theme, Carl R. Woese Institute for Genomic Biology, Urbana, IL 61801, USA; [email protected] 2 Department of Animal Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 3 Division of Nutritional Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 4 Cancer Center of Illinois, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 5 Department of Microbiology and Immunology, Virginia Commonwealth University School of Medicine, Richmond, VA 23298, USA * Correspondence: [email protected] Abstract: Bile acids (BAs) and glucocorticoids are steroid hormones derived from cholesterol that are important signaling molecules in humans and other vertebrates. Hydroxysteroid dehydrogenases (HSDHs) are encoded both by the host and by their resident gut microbiota, and they reversibly convert steroid hydroxyl groups to keto groups. Pairs of HSDHs can reversibly epimerize steroids from α-hydroxy conformations to β-hydroxy, or β-hydroxy to !-hydroxy in the case of !-muricholic acid. These reactions often result in products with drastically different physicochemical properties than their precursors, which can result in steroids being activators or inhibitors of host receptors, can affect solubility in fecal water, and can modulate toxicity. Microbial HSDHs modulate sterols associated with diseases such as colorectal cancer, liver cancer, prostate cancer, and polycystic ovary syndrome. Although the role of microbial HSDHs is not yet fully elucidated, they may have therapeutic potential as steroid pool modulators or druggable targets in the future. In this Citation: Doden, H.L.; Ridlon, J.M. review, we explore metabolism of BAs and glucocorticoids with a focus on biotransformation by Microbial Hydroxysteroid Dehydrogenases: From Alpha to microbial HSDHs. Omega. Microorganisms 2021, 9, 469. https://doi.org/10.3390/ Keywords: hydroxysteroid dehydrogenase; sterolbiome; cholesterol; bile acid; cortisol; androgen; microorganisms9030469 deoxycholic acid Academic Editor: Harsharn Gill Received: 17 January 2021 1. Introduction Accepted: 18 February 2021 Steroid hormones are signaling molecules derived from cholesterol that include gluco- Published: 24 February 2021 corticoids, mineralocorticoids, androgens, estrogens, progestogens, and bile acids (BAs) [1]. Steroid hormones are essential for the regulation of various physiological processes, such as Publisher’s Note: MDPI stays neutral metabolism, salt and water balance, reproduction, inflammation, and stress response [2]. with regard to jurisdictional claims in These cholesterol-derived molecules are synthesized in the human adrenal glands, gonads, published maps and institutional affil- placenta, and liver [3,4]. All steroids have a cyclopentanoperhydrophenanthrene ring iations. structure, composed of three six-carbon rings denoted A, B, and C along with a five-carbon D ring (Figure1), with differing hydroxyl groups and side-chains [ 1]. Hydroxysteroid dehydrogenases (HSDH) are an important class of enzyme expressed by both host tissues and host-associated microbiota that modify the hydroxyl groups on steroids. These small Copyright: © 2021 by the authors. modifications to steroids greatly impact their physicochemical properties and can change Licensee MDPI, Basel, Switzerland. the steroid solubility, toxicity, host receptor affinity, and ability to activate or inhibit host This article is an open access article receptors [5–8]. The current review focuses on the importance of gut microbial HSDHs in distributed under the terms and cholesterol, BA, and glucocorticoid metabolism. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Microorganisms 2021, 9, 469. https://doi.org/10.3390/microorganisms9030469 https://www.mdpi.com/journal/microorganisms Microorganisms 2021, 9, 469 2 of 24 Microorganisms 2021, 9, 469 2 of 23 Figure 1. Steroid structure. Steroids have a cyclopentanoperhydrophenanthrene ring structure. Cholesterol, the precursor Figure 1. Steroid structure. Steroids have a cyclopentanoperhydrophenanthrene ring structure. Cholesterol, the precursor to human steroid hormones, contains 27 carbons, while the major classes of steroid hormones contain the following: C24 to human steroid hormones, contains 27 carbons, while the major classes of steroid hormones contain the following: C24 bile acids, C19 androgens, C18 estrogens, and C21 glucocorticoids, mineralocorticoids, and progestogens. bile acids, C19 androgens, C18 estrogens, and C21 glucocorticoids, mineralocorticoids, and progestogens. 2. Hydroxysteroid Dehydrogenases 2. Hydroxysteroid Dehydrogenases 2.1. Hydroxysteroid Dehydrogenase Function 2.1. Hydroxysteroid Dehydrogenase Function Hydroxysteroid dehydrogenases are nicotinamide adenine dinucleotide (phosphate) (NAD(P)(H))-dependentHydroxysteroid dehydrogenases oxidoreductases are nicotinamide that catalyze adenine the reversible dinucleotide conversion (phosphate) of hy- (NAD(P)(H)droxyl groups)-dependent to keto groups oxidoreductases on steroids [ 9that]. HSDHs catalyze are the regio- reversible and stereospecific, conversion meaning of hy- droxylthey are groups specific to forketo the groups hydroxyl on steroids position [9] on. the HSDHs steroid are (C-3 regio vs.- C-7)and andstereospecific, for the orientation mean- ing(α theyvs. β are) of specific the hydroxyl for the group,hydroxyl respectively position on [5 the]. Pairs steroid of HSDHs(C-3 vs. C can-7) convertand for steroidsthe ori- entationfrom the (αα -orientation,vs. β) of the through hydroxyl an group oxo-intermediate,, respectively to [5] the. Pairs epimerized of HSDHsβ-orientation can convert and steroidsvice versa. from the α-orientation, through an oxo-intermediate, to the epimerized β-orien- tationHydroxysteroid and vice versa. dehydrogenases are found in both host and microbial genomes, althoughHydroxysteroid more is known dehydrogenase about the physiologicals are found in function both host of hostand hydroxysteroidmicrobial genomes, dehydro- alt- houghgenases, more which is known are typically about the abbreviated physiological HSDs function in literature. of host In thishydroxysteroid review, host dehydro- hydroxys- genaseteroids dehydrogenases, which are typically are denotedabbreviated “HSD” HSDs while in literature. bacterial enzymesIn this review, are denoted host hydroxys- “HSDH”. teroidHost dehydrogenases HSDs are key enzymes are denoted in the “HSD biosynthesis” while bacterial of steroids enzymes in steroidogenic are denoted tissues “HSDH [10”.]. HostThey HSDs also functionare key enzymes to activate in or the inactivate biosynthesis steroids of steroids in peripheral in steroidogenic tissues, thus tissues regulating [10]. Theylocal also concentrations function to ofactivate steroid or hormones inactivate [ 5steroids]. Even thoughin peripheral host HSDs tissues, catalyze thus regulating reversible localreactions concentrationsin vitro, they of steroid typically hormones function [5] primarily. Even though in one directionhost HSDsin catalyze vivo on thereversible basis of reactionscofactor balance:in vitro, they either typically as dehydrogenases function primarily or as reductases in one direction [11]. in vivo on the basis of cofactorHost balance: HSDs either are druggable as dehydrogenases targets important or as reductases in the treatment [11]. of endocrine-dependent disorders,Host HSDs including are druggable cancers [targets12]. Host-associated important in the microbial treatment HSDHs of endocrine may also-dependent serve as disorders,pharmacological including targets cancers or, alternatively, [12]. Host-associated may be enriched microbial in theHSDHs host through may also engineering serve as pharmacologicaland delivering probiotic targets or bacteria, alternatively, with rational may sterolbiome be enriched phenotypes. in the host Onethrough recent engineer- example inginvolves and delivering identification probiotic of a cholesterol bacteria with 3β-HSDH rational involved sterolbiome in conversion phenotypes. of cholesterol One recent to examplecoprostanol, involves the enrichment identification of which of a cholesterol may be important 3β-HSDH as ainvolved probiotic in approach conversion to reducing of cho- lesterolserum cholesterolto coprostanol, [13]. the enrichment of which may be important as a probiotic approach to reducing serum cholesterol [13]. 2.2. Structural Biology of Hydroxysteroid Dehydrogenases 2.2. StructuralHydroxysteroid Biology of dehydrogenases Hydroxysteroid belongDehydrogenases to one of the following three large and diverse protein superfamilies: short-chain dehydrogenase/reductase (SDR), medium-chain dehy- Hydroxysteroid dehydrogenases belong to one of the following three large and di- drogenase/reductase (MDR), or aldo-keto reductase (AKR) [5,14]. Many SDR and MDR verse protein superfamilies: short-chain dehydrogenase/reductase (SDR), medium-chain family hydroxysteroid dehydrogenases have been identified in the gut microbiome [14–17]. dehydrogenase/reductase (MDR), or aldo-keto reductase (AKR) [5,14]. Many SDR and Microorganisms 2021, 9, 469 3 of 23 HSDHs in the AKR superfamily are generally found within mammals [12], although microbial AKR family HSDHs have been reported [18]. The SDR superfamily is one of the largest, containing proteins spanning all three domains of life [19].