Adams Mark K, Lee Seung-Ah, Belyaeva Olga V, Wu Lizhi, Kedishvili Natalia Y
Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
Chem Biol Interact. 2017 Oct 1;276:88-94. doi: 10.1016/j.cbi.2016.10.019. Epub 2016 Oct 25.
All-trans-retinoic acid (RA) is a bioactive derivative of vitamin A that serves as an activating ligand for nuclear transcription factors, retinoic acid receptors. RA biosynthesis is initiated by the enzymes that oxidize retinol to retinaldehyde. It is well established that retinol dehydrogenase 10 (RDH10, SDR16C4), which belongs to the 16C family of the short chain dehydrogenase/reductase (SDR) superfamily of proteins, is the major enzyme responsible for the oxidation of retinol to retinaldehyde for RA biosynthesis during embryogenesis. However, several lines of evidence point towards the existence of additional retinol dehydrogenases that contribute to RA biosynthesis in vivo. In close proximity to RDH10 gene on human chromosome 8 are located two genes that are phylogenetically related to RDH10. The predicted protein products of these genes, retinol dehydrogenase epidermal 2 (RDHE2, SDR16C5) and retinol dehydrogenase epidermal 2-similar (RDHE2S, SDR16C6), share 59% and 56% sequence similarity with RDH10, respectively. Previously, we showed that the single ortholog of the human RDHE2 and RDHE2S in frogs, Xenopus laevis rdhe2, oxidizes retinol to retinaldehyde and is essential for frog embryonic development. In this study, we explored the potential of each of the two human proteins to contribute to RA biosynthesis. The results of this study demonstrate that human RDHE2 exhibits a relatively low but reproducible activity when expressed in either HepG2 or HEK293 cells. Expression of the native RDHE2 is downregulated in the presence of elevated levels of RA. On the other hand, the protein encoded by the human RDHE2S gene is unstable when expressed in HEK293 cells. RDHE2S protein produced in Sf9 cells is stable but has no detectable catalytic activity towards retinol. We conclude that the human RDHE2S does not contribute to RA biosynthesis, whereas the low-activity RA-sensitive human RDHE2 may have a role in adjusting the cellular levels of RA in accord with specific physiological conditions.
全反式维甲酸（RA）是维生素A的一种生物活性衍生物，作为核转录因子维甲酸受体的激活配体。RA生物合成由将视黄醇氧化为视黄醛的酶启动。众所周知，属于短链脱氢酶/还原酶（SDR）超家族16C家族的视黄醇脱氢酶10（RDH10，SDR16C4）是胚胎发育过程中RA生物合成中负责将视黄醇氧化为视黄醛的主要酶。然而，有几条证据表明存在其他视黄醇脱氢酶，它们在体内对RA生物合成有贡献。在人类8号染色体上靠近RDH10基因的位置有两个与RDH10有系统发育关系的基因。这些基因预测的蛋白质产物，视黄醇脱氢酶表皮2（RDHE2，SDR16C5）和视黄醇脱氢酶表皮2-相似物（RDHE2S，SDR16C6），分别与RDH10有59%和56%的序列相似性。此前，我们表明青蛙（非洲爪蟾）中人类RDHE2和RDHE2S的单一直系同源物，非洲爪蟾rdhe2，将视黄醇氧化为视黄醛，对青蛙胚胎发育至关重要。在本研究中，我们探究了这两种人类蛋白质各自对RA生物合成的潜在作用。这项研究的结果表明，人类RDHE2在HepG2或HEK293细胞中表达时表现出相对较低但可重复的活性。在RA水平升高时，天然RDHE2的表达下调。另一方面，人类RDHE2S基因编码的蛋白质在HEK293细胞中表达时不稳定。在Sf9细胞中产生的RDHE2S蛋白是稳定的，但对视黄醇没有可检测到的催化活性。我们得出结论，人类RDHE2S对RA生物合成没有贡献，而低活性的RA敏感型人类RDHE2可能在根据特定生理条件调节细胞内RA水平方面发挥作用。
