Etoricoxib mg RDH belongs to C family of the short chain deh
RDH10 belongs to 16C family of the short-chain dehydrogenase/reductase (SDRs) superfamily of proteins , . Notably, in human genome adjacent to the gene encoding RDH10 on chromosome 8 are located two other genes encoding members of the SDR16C family: retinol dehydrogenase epidermal 2 (RDHE2, SDR16C5) and retinol dehydrogenase epidermal 2-similar (RDHE2S, SDR16C6) , . The deduced RDHE2 and RDHE2S proteins share the highest sequence similarity (∼56–59%) with RDH10. Previously, we reported that frog rdhe2, which represents the single genomic equivalent of human RDHE2 and RDHE2S in Xenopus laevis, functions as a retinol dehydrogenase in vivo and is essential for embryonic development in frogs . This finding suggests that the human orthologs of the frog rdhe2 may also have a role in RA biosynthesis in humans, except that this role has been split between two proteins, RDHE2 and RDHE2S. Two lines of evidence support the notion that both RDHE2 and RDHE2S may complement RDH10 activity in mammals. First, our previous report demonstrated that the partially purified recombinant human RDHE2 expressed in Sf9 Etoricoxib mg recognizes retinol as a substrate with NAD+ as the preferred cofactor . Second, while the human RDHE2S has never been characterized, we showed that its murine ortholog is highly active as a retinol dehydrogenase and contributes to RA biosynthesis when expressed in living human embryonic kidney (HEK) 293 cells . The goal of the present study was to evaluate the properties of human RDHE2 and RDHE2S in order to assess their potential for contributing to RA biosynthesis in humans.
Materials and methods
Discussion While RDH10 is the major enzyme responsible for the biosynthesis of RA during embryogenesis, it does not account for all of retinaldehyde synthesis for RA production in vivo. Among members of the SDR superfamily, RDH10 is most closely related to RDHE2 and RDHE2S , . Phylogenetic analysis places RDHE2, RDHE2S, and RDH10 into the same clade within SDR16C family, separately from other members, thus confirming their common origin . This observation suggests that RDHE2 and RDHE2S may represent the missing retinol dehydrogenases that support the residual RA biosynthesis during late embryogenesis and in adult tissues. The results of this study demonstrate that RDHE2 may, in fact, serve as a functional retinol dehydrogenase in certain human tissues. Based on the GenBank EST database, transcripts encoding human RDHE2 are found in brain, esophagus, heart, intestine, lung, lymph node, mammary gland, mouth, pancreas, pharynx, stomach, and trachea, suggesting a broad impact on human physiology. Remarkably, the expression of human RDHE2 is very sensitive to RA levels. The strong downregulation of RDHE2 expression in response to RA in skin epidermis observed using the organotypic skin raft culture may have several implications. First, because RDHE2 is suppressed by RA, the upregulation of RDHE2 in psoriasis might signal that the steady-state levels of RA in psoriatic skin are actually reduced compared to healthy skin; and hence, proliferation of keratinocytes in psoriasis is not caused by overproduction of RA, as initially suggested , . There is a possibility that the upregulation of RDHE2 occurs in response to RA insufficiency in order to normalize RA levels in psoriatic epidermis. We speculate that human RDHE2 functions as a highly inducible low-activity retinol dehydrogenase, which provides the means for adjusting the rate of RA production depending on physiological status of the tissue. A surprising finding of this study is that human RDHE2S gene does not encode a functional protein, despite the full conservation of the primary structure and catalytic residues required for activity. The two putative variants of human RDHE2S protein are correctly targeted to the microsomal and mitochondrial membranes when expressed in insect Sf9 cells and both protein variants are stable in these cells. Nevertheless, neither variant exhibits any detectable activity towards retinol. Taking into account that no ESTs encoding human RDHE2S have been reported thus far, and that neither version of the protein is stable in HEK293 cells, it appears that the human ortholog of murine Rdhe2s gene lost its functional significance and represents a pseudogene, and that in humans, as in frogs, a single RDHE2 enzyme fulfills the functions of both enzymes.