AKR1B15 – a novel human mitochondrial oxidoreductase participating in steroid metabolism

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SAT 358-380-Steroid Hormone Biosynthesis & Metabolism
Saturday, June 15, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SAT-375
Susanne Weber*1, Joshua K Salabei2, Gabriele Möller1, Aruni Bhatnagar2, Oleg A Barski2 and Jerzy Adamski3
1Helmholtz Zentrum Muenchen, 2University of Louisville, Louisville, KY, 3Helmholtz Zentrum München, Neuherberg, Germany
Aldo-Keto Reductases (AKRs) are a superfamily of proteins, which includes members throughout various kingdoms of life. Most of the AKR enzymes are involved in the reduction of biogenic and xenobiotic carbonyls. In humans, 15 AKR superfamily members have been identified so far. One of them is a newly identified gene locus, AKR1B15, which clusters with the other human AKR1B subfamily members, AKR1B1 and AKR1B10, on chromosome 7 [1]. We show that alternative splicing of the AKR1B15 gene transcript gives rise to two protein isoforms with different N-termini: AKR1B15.1 is a 316 amino acid protein with 91 % amino acid identity to AKR1B10; AKR1B15.2 has a prolonged N-terminus and consists of 344 amino acid residues. So far, no ortholog of AKR1B15 has been identified in mouse and rat, despite the fact that genomes of these species include 4 genes belonging to the AKR1B family.

To elucidate the function of AKR1B15, we performed tissue distribution and subcellular localization studies as well as activity assays with various physiological substrates. We found that AKR1B15.1 and AKR1B15.2 differ strongly in expression level, subcellular localization and activity. Although sharing high amino acid homology with AKR1B10, we ascertained AKR1B15.1 to co-localize with mitochondria in contrast to other AKR enzymes which are mostly cytosolic. AKR1B15.1 is a predominately reductive enzyme converting 3-keto-acyl CoAs as well as androgens and estrogens with high position selectivity. It exhibits  strong cofactor selectivity towards NADPH. In contrast, AKR1B15.2 localizes to the cytosol and displays no enzymatic activity with tested substrates. In accordance to the substrate spectrum, we found the highest expression levels of AKR1B15.1 in steroid-related tissues, namely placenta and testis. Placental expression could be verified on RNA and protein level in BeWo cell line.

As public record links the S8R mutation in AKR1B15.1 to a strong mitochondrial disease phenotype [2], we aimed to reach a deeper insight into the function of AKR1B15.1 by analyzing the subcellular localization and activity properties of the mutant.

In conclusion, our studies demonstrate the existence of a novel functional and mitochondria-associated AKR enzyme, which is mainly expressed in steroid related tissues. In addition, we analyzed the characteristics of the AKR1B15.1 S8R mutant, claimed to be responsible for a severe mitochondrial defect.

1. Salabei, J.K., Li, X.P, et al. Functional expression of novel human and murine AKR1B genes. Chem Biol Interact. 2011; 191(1-3): 177–184.

2. Calvo, S.E., Compton, A.G., et al. Molecular diagnosis of infantile mitochondrial disease with targeted next-generation sequencing. Sci Transl Med. 2012; 4(118):118ra10.

Nothing to Disclose: SW, JKS, GM, AB, OAB, JA

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm