Dual Subcellular Localization in the Endoplasmic Reticulum and Peroxisomes and a Vital Role in Protecting against Oxidative Stress of Fatty Aldehyde Dehydrogenase Are Achieved by Alternative Splicing

Abstract

Fatty aldehyde dehydrogenase (FALDH, ALDH3A2) is thought to be involved in the degradation of phytanic acid, a saturated branched chain fatty acid derived from chlorophyll. However, the identity, subcellular distribution, and physiological roles of FALDH are unclear because several variants produced by alternative splicing are present in varying amounts at different subcellular locations. Subcellular fractionation experiments do not provide a clear-cut conclusion because of the incomplete separation of organelles. We established human cell lines heterologously expressing mouse FALDH from each cDNA without tagging under the control of an inducible promoter and detected the variant FALDH proteins using a mouse FALDHspecific antibody. One variant, FALDH-V, was exclusively detected in peroxisomal membranes. Human FALDH-V with an amino-terminal Myc sequence also localized to peroxisomes. The most dominant form, FALDH-N, and other variants examined, however, were distributed in the endoplasmic reticulum. A gas chromatography-mass spectrometry-based analysis of metabolites in FALDH-expressing cells incubated with phytol or phytanic acid showed that FALDH-V, not FALDH-N, is the key aldehyde dehydrogenase in the degradation pathway and that it protects peroxisomes from oxidative stress. In contrast, both FALDHs had a protective effect against oxidative stress induced by a model aldehyde for lipid peroxidation, dodecanal. These results suggest that FALDH variants are produced by alternative splicing and share an important role in protecting against oxidative stress in an organelle-specific manner.Plants produce a variety of secondary metabolites, and some of these are potentially toxic to animals (1). Herbivora have developed behavioral and physiological strategies to avoid specific plants and to detoxify any toxins ingested. Detoxification can occur in the mouth and the gut rumen with or without the help of microbes (2). The absorbed toxins must be detoxified in the intestine and liver, but studies on these mechanisms are limited because to date most animal experiments have been carried out using laboratory diets. Recently we found that a nuclear receptor, peroxisome proliferator-activated receptor ␣ (PPAR␣), 2 is involved in the detoxification by using plant seeds as a diet for mice (3).PPAR␣ is activated by fatty acid ligands and is an important regulator of lipid metabolism in animals (4). Despite the claimed essential role of this receptor in the liver, the PPAR␣-null mouse shows little phenotypic change when fed a normal laboratory diet (5, 6). We have examined its extrahepatic roles and found that PPAR␣ induces the expression of 17␤-hydroxysteroid dehydrogenase type 11 in the intestine (7). Recent studies on the substrates of 17␤-hydroxysteroid dehydrogenase type 11 showed that they include not only glucocorticoids and sex steroids but also bile acids, fatty acids, and branched amino acids (8, 9). So we examined the possibility that PPAR␣ plays a vital role in inducing enzymes for metabolizing secondary metabolites...

Mouse aldehyde dehydrogenase ALDH3B2 is localized to lipid droplets via two C-terminal tryptophan residues and lipid modification

“…Thus, the differences in their functions cannot be attributed to their substrate specificity. However, their intracellular localizations varied greatly; ALDH3A2 is found in the ER and peroxisomes, ALDH3B1 and ALDH3B3 are localized to the plasma membrane, and ALDH3B2 locates to lipid droplets [11,17] (Figure 2B). It therefore follows that they function to remove lipid-derived aldehydes generated at their specific sites of localization.…”

“…(ER and peroxisomes) [10,11], and human ALDH3B1 (plasma membrane) [17]. However, the localization of ALDH3B2 or ALDH3B3 had yet to be identified.…”

“…ALDH3A2 is expressed ubiquitously and exhibits activity toward saturated and unsaturated aliphatic aldehydes of C6 to C24 in length [9]. The major splicing isoform of ALDH3A2 is a type II endoplasmic reticulum (ER) membrane protein, whereas its minor isoform is localized to peroxisomes [10,11]. A variety of ALDH3A2 substrates are generated through the metabolism of lipids such as fatty alcohols, leukotriene B 4 , phytol, and sphingosine-1-phosphate (S1P) and through lipid peroxidation [12][13][14].…”

Phytosphingosine degradation pathway includes fatty acid α-oxidation reactions in the endoplasmic reticulum

“…We speculate that the differences in the localization of HACL1 (peroxisome) and HACL2 (ER) determines the degree of their contribution to the PHS degradation pathway. All of the enzymes acting downstream of PHS 1-phosphate, i.e., SGPL1, ALDH3A2, and ACSLs, are all localized in the ER (22,35,40): In other words, PHS degradation occurs in the ER. In Hacl2-deficient cells, unmetabolized 2-OH C16:0-CoA in the ER may be unusually delivered to peroxisomes and is subjected to the C1 removal reaction by HACL1.…”

Novel functions of acyl-CoA thioesterases and acyltransferases as auxiliary enzymes in peroxisomal lipid metabolism

“…This α-oxidation process occurs in peroxisomes by a dedicated set of enzymes, although it has been disputed if the final enzymatic step by a fatty aldehyde dehydrogenase also occurs in peroxisomes [110]. Recently however, alternative splicing of the fatty aldehyde dehydrogenase was shown to result in a dual localization in the endoplasmic reticulum and peroxisomes, indicating that all enzymes required for α-oxidation are indeed localized in peroxisomes [111]. The conversion of a 3-methyl branched fatty acid (phytanic acid) to a 2-methyl branched fatty acid (pristanic acid (2,6,10,14-tetramethylpentadecanoic acid)) results in a carboxylic acid that can then undergo β-oxidation.…”