Characterization of rat cornea aldehyde dehydrogenase

Investigating the Functional Roles of Aldehyde Dehydrogenase 3A1 in Human Corneal Epithelial Cells

Aldehyde dehydrogenase 3A1 (ALDH3A1) oxidizes medium-chain aldehydes to their corresponding carboxylic acids. It is expressed at high rates in the human cornea, where it has been characterized as a multi-functional protein displaying various cytoprotective modes of action. Previous studies identified its association with the DNA damage response (DDR) pathway. Here, we utilized a stable transfected HCE-2 (human corneal epithelium) cell line expressing ALDH3A1, to investigate the molecular mechanisms underlying the cytoprotective role(s) of ALDH3A1. Our data revealed morphological differences among the ALDH3A1-expressing and the mock-transfected HCE-2 cells accompanied by differential expression of E-cadherin. Similarly, the ALDH3A1/HCE-2 cells demonstrated higher mobility, reduced proliferation, upregulation of ZEB1, and downregulation of CDK3, and p57. The expression of ALDH3A1 also affected cell cycle progression by inducing the sequestration of HCE-2 cells at the G2/M phase. Following 16 h cell treatments with either H₂O₂ or etoposide, a significantly lower percentage of ALDH3A1/HCE-2 cells were apoptotic compared to the respective treated mock/HCE-2 cells. Interestingly, the protective effect of ALDH3A1 expression under these oxidative and genotoxic conditions was accompanied by a reduced formation of γ-H2AX foci and higher levels of total and phospho (Ser15) p53. Finally, ALDH3A1 was found to be localized both in the cytoplasm and the nucleus of transfected HCE-2 cells. Its cellular compartmentalization was not affected by oxidant treatment, while the mechanism by which ALDH3A1 translocates to the nucleus remains unknown. In conclusion, ALDH3A1 protects cells from both apoptosis and DNA damage by interacting with key homeostatic mechanisms associated with cellular morphology, cell cycle, and DDR.

Aldehyde dehydrogenase 3A1 confers oxidative stress resistance accompanied by altered DNA damage response in human corneal epithelial cells

Aldehyde dehydrogenase 3A1 is constitutively expressed in a taxon-specific manner in the cornea, where, due to its high abundance, it has been characterized as a corneal crystallin. ALDH3A1 has been proposed to be a multifaceted protein that protects cellular homeostasis through several modes of action. The present study examines the mechanisms by which ALDH3A1 exerts its cytoprotective role under conditions of oxidative stress. To this end, we have utilized an isogenic HCE-2 (human corneal epithelium) cell line pair differing in the expression of ALDH3A1. Single cell gel electrophoresis assay and H2DCFDA analysis revealed that the expression of ALDH3A1 protected HCE-2 cells from H₂O₂-, tert-butyl peroxide- and etoposide-induced oxidative and genotoxic effects. Furthermore, comparative qPCR analysis revealed that a panel of cell cycle (Cyclins B1, B2, D, E), apoptosis (p53, BAX, BCL-2, BCL-XL) and DNA damage response (DNA-PK, NBS1) genes were up-regulated in the ALDH3A1 expressing HCE-2 cells. Moreover, the expression profile of a variety of DNA damage signaling (DDS)-related genes, was investigated (under normal and oxidative stress conditions) by utilizing the RT² profiler™ PCR array in both isogenic HCE-2 cell lines. Our results demonstrated that several genes associated with ATM/ATR signaling, cell cycle regulation, apoptosis and DNA damage repair were differentially expressed under all conditions tested. In conclusion, this study suggests that ALDH3A1 significantly contributes to the antioxidant defense of corneal homeostasis by maintaining DNA integrity possibly through altering the expression of specific DDS-related genes. Further studies will shed light on the precise role(s) of this multifunctional protein.

Structure and mechanism of benzaldehyde dehydrogenase from Pseudomonas putida ATCC 12633, a member of the Class 3 aldehyde dehydrogenase superfamily

Benzaldehyde dehydrogenase from Pseudomonas putida (PpBADH) belongs to the Class 3 aldehyde dehydrogenase (ALDH) family. The Class 3 ALDHs are unusual in that they are generally dimeric (rather than tetrameric), relatively non-specific and utilize both NAD+ and NADP+. To date, X-ray structures of three Class 3 ALDHs have been determined, of which only two have cofactor bound, both in the NAD+ form. Here we report the crystal structure of PpBADH in complex with NADP+ and a thioacyl intermediate adduct. The overall architecture of PpBADH resembles that of most other members of the ALDH superfamily, and the cofactor binding residues are well conserved. Conversely, the pattern of cofactor binding for the rat Class 3 ALDH differs from that of PpBADH and other ALDHs. This has been interpreted in terms of a different mechanism for the rat enzyme. Comparison with the PpBADH structure, as well as multiple sequence alignments, suggest that one of two conserved glutamates, at positions 215 (209 in rat) and 337 (333 in rat), would act as the general base necessary to hydrolyze the thioacyl intermediate. While the latter is the general base in the rat Class 3 ALDH, site-specific mutagenesis indicates that Glu215 is the likely candidate for PpBADH, a result more typical of the Class 1 and 2 ALDH families. Finally, this study shows that hydride transfer is not rate limiting, lending further credence to the suggestion that PpBADH is more similar to the Class 1 and 2 ALDHs than it is to other Class 3 ALDHs.

Molecular characterization, expression analysis, and role of ALDH3B1 in the cellular protection against oxidative stress

Aldehyde dehydrogenase (ALDH) enzymes are critical in the detoxification of aldehydes. The human genome contains 19 ALDH genes, mutations in which are the basis of several diseases. The expression, subcellular localization, enzyme kinetics, and role of ALDH3B1 in aldehyde- and oxidant-induced cytotoxicity were investigated. ALDH3B1 was purified from Sf9 cells using chromatographic methods, and enzyme kinetics were determined spectrophotometrically. ALDH3B1 demonstrated high affinity for hexanal (K(m)=62 μM), octanal (K(m)=8 μM), 4-hydroxy-2-nonenal (4HNE; K(m)=52 μM), and benzaldehyde (K(m)=46 μM). Low affinity was seen toward acetaldehyde (K(m)=23.3 mM), malondialdehyde (K(m)=152 mM), and the ester p-nitrophenyl acetate (K(m)=3.6 mM). ALDH3B1 mRNA was abundant in testis, lung, kidney, and ovary. ALDH3B1 protein was highly expressed in these tissues and the liver. Immunofluorescence microscopy of ALDH3B1-transfected human embryonic kidney (HEK293) cells and subcellular fractionation of mouse kidney and liver revealed a cytosolic protein localization. ALDH3B1-transfected HEK293 cells were significantly protected from the lipid peroxidation-derived aldehydes trans-2-octenal, 4HNE, and hexanal and the oxidants H(2)O(2) and menadione. In addition, ALDH3B1 protein expression was up-regulated by 4HNE in ARPE-19 cells. The results detailed in this study support a pathophysiological role for ALDH3B1 in protecting cells from the damaging effects of oxidative stress.

Extent of Corneal Injury as a Biomarker for Hazard Assessment and the Development of Alternative Models to the Draize Rabbit Eye Test

We have characterized 22 ocular irritants differing in type (surfactants, acid, alkali, bleaches, alcohol, aldehyde, acetone) and severity (slight to severe) by using the low-volume rabbit eye test. Ocular irritation was evaluated by 1) light microscopy to assess pathological changes, 2) in vivo confocal microscopy (CM) to quantify 4-dimensionally (x, y, z, and t) initial corneal injury and later responses in the same eye, and 3) laser scanning CM to quantify initial cell death. These studies revealed that regardless of the processes leading to injury, slight irritants injure the corneal epithelium, mild irritants injure the corneal epithelium and the superficial stroma, and moderate/severe irritants injure the epithelium, deep stroma, and at times the corneal endothelium. Furthermore, extent of initial corneal injury was shown to predict subsequent responses and final outcomes. These findings suggest that extent of corneal injury may be used as a basis for the development of alternative ocular irritation tests. To test the validity of this approach, we have used an ex vivo, rabbit cornea culture model to measure extent of corneal injury following exposure to ocular irritants. Data indicate that the extent of ex vivo corneal injury significantly correlate with the extent of initial injury measured previously in live animals. Overall, these findings indicate that extent of initial corneal injury can be used as a new "gold standard" for the continued refinement and ultimate replacement of the Draize rabbit eye Ocular Irritation Test.

Physical, kinetic and spectrophotometric studies of a NAD(P)-dependent benzaldehyde dehydrogenase from Pseudomonas putida ATCC 12633

The mandelate pathway of Pseudomonas putida ATCC 12633 comprises five enzymes and catalyzes the conversion of R- and S-mandelamide to benzoic acid which subsequently enters the beta-ketoadipate pathway. Although the first four enzymes have been extensively characterized the terminal enzyme, a NAD(P)+-dependent benzaldehyde dehydrogenase (BADH), remains largely undescribed. Here we report that BADH is a dimer in solution, and that DTT is necessary both to maintain the activity of BADH and to prevent oligimerization of the enzyme. Site-directed mutagenesis confirms that Cys249 is the catalytic cysteine and identifies Cys140 as the cysteine likely to be involved in inter-monomer disulfide formation. BADH can utilize a range of aromatic substrates and will also operate efficiently with cyclohexanal as well as medium-chain aliphatic aldehydes. The logV and logV/K pH-rate profiles for benzaldehyde with either NAD+ or NADP+ as the coenzyme are both bell-shaped. The pKa values on the ascending limb range from 6.2 to 7.1 while those on the descending limb range from 9.6 to 9.9. A spectrophotometric approach was used to show that the pKa of Cys249 was 8.4, i.e., Cys249 is not responsible for the pKas observed in the pH-rate profiles.

Corneal keratocytes: phenotypic and species differences in abundant protein expression and in vitro light-scattering

PURPOSE

Previous studies suggest that corneal haze after injury involves changes in the light-scattering properties of keratocytes that are possibly linked to the abundant expression of water-soluble proteins. The purpose of this study was to determine the protein expression pattern of keratocytes from different species and different cultured rabbit keratocyte phenotypes and to assess differences in light-scattering in vitro.

METHODS

Water-soluble proteins were isolated from corneal epithelial cells and keratocytes of several species, including human (Hu), mouse (Mo), rabbit (Ra), chicken (Ch), and pig (P) and different cultured rabbit keratocyte phenotypes. Proteins were then characterized by SDS-PAGE, tryptic peptide sequence analysis, and Western blot analysis. Light-scattering and actin organization from cultured cells were determined with confocal reflectance and fluorescence microscopy, respectively.

RESULTS

Protein expression patterns varied substantially between species and cell types, with five new abundantly expressed proteins identified including, LDH (Ra, Ch), G3PDH (Hu, Ch), pyruvate kinase (Ch), Annexin II (Ch), and protein disulfide isomerase (Ch). Different rabbit keratocyte phenotypes also showed different levels of expression of ALDH1A1 and TKT, with myofibroblasts showing the greatest reduction. Myofibroblasts showed significantly greater (P < 0.05) light-scattering but also showed the greatest organization of actin filaments.

CONCLUSIONS

Abundant protein expression is a characteristic feature of corneal keratocytes that is lost when cells are phenotypically modulated in culture. Greater light-scattering by myofibroblasts also provides support for a link between cellular transparency and haze after injury that is possibly related to loss of protein expression or development of prominent actin filament bundles.

Abundant corneal gelsolin in Zebrafish and the 'four-eyed' fish, Anableps anableps: possible analogy with multifunctional lens crystallins

The cornea accumulates high proportions (can be up to 50%) of taxon-specific, water-soluble, cytoplasmic proteins (often enzymes) that have been considered analogous to the multifunctional lens crystallins. We have shown that gelsolin (an actin-severing protein) is the major water-soluble corneal protein of the zebrafish (Danio rerio) and the 'four-eyed' fish (Anableps anableps). Each Anableps eye contains one lens, an aquatic ventral cornea with an epithelium comprising 5-7 cell layers, and an air-exposed flatter dorsal cornea with an epithelium comprising >20 cell layers and appreciably enriched with glycogen. Gelsolin accounts for 38 and 21% of the dorsal and ventral cornea, respectively, suggesting that the abundance of gelsolin in the cornea is not incompatible with its function in air. The thicker, glycogen-enriched, air-exposed dorsal cornea may protect against UV irradiation and desiccation. Gelsolin comprises approximately 50% of the 5 cell-layer thick aquatic corneal epithelium of zebrafish. Reported zebrafish ESTs have indicated the presence of a second gelsolin gene in this species. We show by RT-PCR that the abundant corneal gelsolin (also expressed weakly in lens) (C/L-gelsolin) is also expressed in early development and differs from a ubiquitously expressed gelsolin (U-gelsolin) that is not specialized for cornea. Microinjection tests showed that overexpression of C/L-gelsolin dorsalizes the embryo and can lead to axis duplication, while interruption of C/L-gelsolin expression with a specific morpholino oligonucleotide ventralizes the embryo and interferes with brain and eye development. The evidence that C/L-gelsolin participates in the bone morphogenetic protein (BMP)/Smad dorsal-ventral signaling pathway is reviewed. Finally, we speculate that soluble C/L-gelsolin:actin complexes in the cornea may be analogous to soluble alphaA:alphaB-crystallin complexes in the lens. Together, our data are consistent with an analogy between the abundance of gelsolin in fish corneas and taxon-specific multifunctional crystallins in lenses.

Molecular cloning and baculovirus expression of the rabbit corneal aldehyde dehydrogenase (ALDH1A1) cDNA

Most mammalian species express high concentrations of ALDH3A1 in corneal epithelium with the exception of the rabbit, which expresses high amounts of ALDH1A1 rather than ALDH3A1. Several hypotheses that involve catalytic and/or structural functions have been postulated regarding the role of these corneal ALDHs. The aim of the present study was to characterize the biochemical properties of the rabbit ALDH1A1. We have cloned and sequenced the rabbit ALDH1A1 cDNA, which is 2,073 bp in length (excluding the poly(A+) tail), and has 5' and 3' nontranslated regions of 46 and 536 bp, respectively. This ALDH1A1 cDNA encodes a protein of 496 amino acids (Mr = 54,340) that is: 86-91% identical to mammalian ALDH1A1 proteins, 83-85% identical to phenobarbital-inducible mouse and rat ALDH1A7 proteins, 84% identical to elephant shrew ALDH1A8 proteins (eta-crystallins), 69-73% identical to vertebrate ALDH1A2 and ALDH1A3 proteins, 65% identical to scallop ALDH1A9 protein (omega-crystallin), and 55-57% to cephalopod ALDH1C1 and ALDH1C2 (omega-crystallins). Recombinant rabbit ALDH1A1 protein was expressed using the baculovirus system and purified to homogeneity with affinity chromatography. We found that rabbit ALDH1A1 is catalytically active and efficiently oxidizes hexanal (Km = 3.5 microM), 4-hydroxynonenal (Km = 2.1 microM) and malondialdehyde (Km = 14.0 microM), which are among the major products of lipid peroxidation. Similar kinetic constants were observed with the human recombinant ALDH1A1 protein, which was expressed and purified using similar experimental conditions. These data suggest that ALDH1A1 may contribute to corneal cellular defense against oxidative damage by metabolizing toxic aldehydes produced during UV-induced lipid peroxidation.

Structurally normal corneas in aldehyde dehydrogenase 3a1-deficient mice

We have constructed an ALDH3a1 null mouse to investigate the role of this enzyme that comprises nearly one-half of the total water-soluble protein in the mouse corneal epithelium. ALDH3a1-deficient mice are viable and fertile, have a corneal epithelium with a water-soluble protein content approximately half that of wild-type mice, and contain no ALDH3a1 as determined by zymograms and immunoblots. Despite the loss of protein content and ALDH3a1 activity, the ALDH3a1(-/-) mouse corneas appear indistinguishable from wild-type corneas when examined by histological analysis and electron microscopy and are transparent as determined by light and slit lamp microscopy. There is no evidence for a compensating protein or enzyme. Even though the function of ALDH3a1 in the mouse cornea remains unknown, our data indicate that its enzymatic activity is unnecessary for corneal clarity and maintenance, at least under laboratory conditions.

Enigma of the abundant water-soluble cytoplasmic proteins of the cornea: the "refracton" hypothesis

It is accepted that the taxon-specific, multifunctional crystallins (small heat-shock proteins and enzymes) serve structural roles contributing to the transparent and refractive properties of the lens. The transparent cornea also accumulates unexpectedly high proportions of taxon-specific, multifunctional proteins particularly, but not only, in the epithelium. For example, aldehyde dehydrogenase 3 (ALDH3) is the main water-soluble protein in corneal epithelial cells of most mammals (but ALDH1 predominates in the rabbit), whereas gelsolin predominates in the zebrafish corneal epithelium. Moreover, some invertebrates (e.g., squid and scallop) accumulate proteins in their corneas that are similar to their lens crystallins. Pax-6, among other transcription factors, is implicated in development and tissue-specific gene expression of the lens and cornea. Environmental factors appear to influence gene expression in the cornea, but not the lens. Although no direct proof exists, the diverse, abundant corneal proteins may have evolved a crystallinlike role, in addition to their enzymatic or cytoskeletal functions, by a gene sharing mechanism similar to the lens crystallins. Consequently, it is proposed that the cornea and lens be considered as a single refractive unit, called here the "refracton," to emphasize their similarities and common function.

Corneal and stomach expression of aldehyde dehydrogenases: from fish to mammals

We have studied the distribution of the ALDH3A1, ALDH1A1 and ALDH2 proteins in the cornea and stomach of several animal species, including mammals (C57BL/6J and SWR/J mice, rat and pig), birds (chicken and turkey), amphibians (frog) and fish (trout and zebrafish). High ALDH3A1 protein levels and catalytic activities were detected in C57BL/6J mouse, rat and pig. We found complete absence of the ALDH3A1 protein in SWR/J mice, which carry the Aldh3a1(c) allele characterized by four amino acid substitutions (G88R, I154N, H305R and I352V) and lack of enzymatic activity. This indicates that the SWR/J mouse strain is a natural gene knockout model for ALDH3A1. Traces of ALDH3A1 were detected in rabbit, whereas expression was absent from chicken, turkey, frog, trout, and zebrafish. Interestingly, significant levels of the cytosolic ALDH1A1 and mitochondrial ALDH2 proteins were detected by immunoblot analysis in all examined species that are deficient in ALDH3A1 expression. In contrast, no ALDH1A1 or ALDH2 protein was detected in the species expressing ALDH3A1. It can, therefore, be concluded that corneal expression of ALDH3A1 or ALDH1A1/ALDH2 occurs in a taxon-specific manner, supporting the protective role of these ALDHs in cornea against the UV-induced oxidative damage.

Omega -crystallin of the scallop lens. A dimeric aldehyde dehydrogenase class 1/2 enzyme-crystallin

While many of the diverse crystallins of the transparent lens of vertebrates are related or identical to metabolic enzymes, much less is known about the lens crystallins of invertebrates. Here we investigate the complex eye of scallops. Electron microscopic inspection revealed that the anterior, single layered corneal epithelium overlying the cellular lens contains a regular array of microvilli that we propose might contribute to its optical properties. The sole crystallin of the scallop eye lens was found to be homologous to Omega-crystallin, a minor crystallin in cephalopods related to aldehyde dehydrogenase (ALDH) class 1/2. Scallop Omega-crystallin (officially designated ALDH1A9) is 55-56% identical to its cephalopod homologues, while it is 67 and 64% identical to human ALDH 2 and 1, respectively, and 61% identical to retinaldehyde dehydrogenase/eta-crystallin of elephant shrews. Like other enzyme-crystallins, scallop Omega-crystallin appears to be present in low amounts in non-ocular tissues. Within the scallop eye, immunofluorescence tests indicated that Omega-crystallin expression is confined to the lens and cornea. Although it has conserved the critical residues required for activity in other ALDHs and appears by homology modeling to have a structure very similar to human ALDH2, scallop Omega-crystallin was enzymatically inactive with diverse substrates and did not bind NAD or NADP. In contrast to mammalian ALDH1 and -2 and other cephalopod Omega-crystallins, which are tetrameric proteins, scallop Omega-crystallin is a dimeric protein. Thus, ALDH is the most diverse lens enzyme-crystallin identified so far, having been used as a lens crystallin in at least two classes of molluscs as well as elephant shrews.

CLED: a calcium-linked protein associated with early epithelial differentiation

Although it has been well established that Ca(2+) plays a key role in triggering keratinocyte differentiation, relatively little is known about the molecules that mediate this signaling process. By analyzing a bovine corneal epithelial subtraction cDNA library, we have identified a novel gene that we named CLED (calcium-linked epithelial differentiation), which encodes a messenger RNA present in all stratified squamous epithelia, hair follicle, the bladder transitional epithelium, and small intestinal epithelium. The deduced amino acid sequence of CLED, based on a bovine partial cDNA and its full-length, human and mouse homologues that have been described only as ESTs, contains 2 EF-hand Ca(2+)-binding domains, a myristoylation motif, and several potential protein kinase phosphorylation sites; the CLED protein is therefore related to the S100 protein family. In all stratified squamous epithelia, the CLED message is associated with the intermediate cell layers. Similar CLED association with cells that are above the proliferative compartment but below the terminally differentiated compartment is seen in hair follicle, bladder, and small intestinal epithelia. The only exception is corneal epithelium, where CLED is expressed in both basal and intermediate cells. The presence of CLED in corneal epithelial basal cells, but not in the adjacent limbal basal (stem) cells, provides additional, strong evidence for the unique lateral heterogeneity of the limbal/corneal epithelium. These results suggest that CLED, via Ca(2+)-related mechanisms, may play a role in the epithelial cell's commitment to undergo early differentiation, and that its down-regulation is required before the cells can undergo the final stages of terminal differentiation.

Identification of a cytosolic NADP+-dependent isocitrate dehydrogenase that is preferentially expressed in bovine corneal epithelium. A corneal epithelial crystallin

Recently, metabolic enzymes have been observed in both the lens and corneal epithelium at levels greatly exceeding what is necessary for normal metabolic functions. These proteins have been termed taxon-specific crystallins and are thought to play a role in maintaining tissue transparency. We report here that cytosolic NADP+-dependent isocitrate dehydrogenase (ICDH) represents a new corneal crystallin. Using suppression subtractive hybridization, we identified a gene (with a deduced amino acid sequence that showed 94% identity to rat cytosolic NADP+-dependent ICDH) that is preferentially expressed in bovine corneal epithelium. Northern blots established that its mRNA level in the corneal epithelium was 31-, 39-, 133-, 230-, and 929-fold more than in the liver, bladder epithelium, stomach epithelium, brain, and heart, respectively. This mRNA was detected primarily in corneal epithelial basal cells by in situ hybridization. SDS-polyacrylamide gel electrophoresis, two-dimensional gel analysis, and Western blotting showed that this protein was overexpressed in the corneal epithelium, constituting approximately 13% of the total soluble bovine corneal epithelial proteins. Enzyme assays showed a corresponding overabundance of this protein in bovine corneal epithelium. Taken together, these data indicate that bovine cytosolic ICDH fulfills the criteria for a corneal epithelial crystallin and may be involved in maintaining corneal epithelial transparency.

The cellular basis of corneal transparency: evidence for ‘corneal crystallins’

In vivo corneal light scattering measurements using a novel confocal microscope demonstrated greatly increased backscatter from corneal stromal fibrocytes (keratocytes) in opaque compared to transparent corneal tissue in both humans and rabbits. Additionally, two water-soluble proteins, transketolase (TKT) and aldehyde dehydrogenase class 1 (ALDH1), isolated from rabbit keratocytes showed unexpectedly abundant expression (approximately 30% of the soluble protein) in transparent corneas and markedly reduced levels in opaque scleral fibroblasts or keratocytes from hazy, freeze injured regions of the cornea. Together these data suggest that the relatively high expressions of TKT and ALDH1 contribute to corneal transparency in the rabbit at the cellular level, reminiscent of enzyme-crystallins in the lens. We also note that ALDH1 accumulates in the rabbit corneal epithelial cells, rather than ALDH3 as seen in other mammals, consistent with the taxon-specificity observed among lens enzyme-crystallins. Our results suggest that corneal cells, like lens cells, may preferentially express water-soluble proteins, often enzymes, for controlling their optical properties.

Aldehyde dehydrogenase class 3 expression: identification of a cornea-preferred gene promoter in transgenic mice

Aldehyde dehydrogenase class 3 (ALDH3) constitutes 20-40% of the total water-soluble proteins in the mammalian cornea. Here, we show by Northern blot analysis that ALDH3 expression in the mouse is at least 500-fold higher in the cornea than in any other tissue examined, with very low levels of expression detected in the stomach, urinary bladder, ocular lens, and lung. Histochemical localization reveals that this exceptional level of expression in the mouse cornea occurs in the anterior epithelial cells and that little ALDH3 is present in the keratocytes or corneal endothelial cells. A 13-kbp mouse ALDH3 promoter fragment containing >12 kbp of the 5' flanking sequence, the 40-bp untranslated first exon, and 29 bp of intron 1 directed cat reporter gene expression to tissues that express the endogenous ALDH3 gene, except that transgene promoter activity was higher in the stomach and bladder than in the cornea. By contrast, when driven by a 4.4-kbp mouse ALDH3 promoter fragment [1,050-bp 5' flanking region, exon 1, intron 1 (3.4 kbp), and 7 bp of exon 2] expression of the cat reporter gene was confined to the corneal epithelial cells, except for very low levels in the liver, effectively reproducing the corneal expression pattern of the endogenous ALDH3 gene. These results indicate that tissue-specific expression of ALDH3 is determined by positive and negative elements in the 5' flanking region of the gene and suggests putative silencers located in intron 1. We demonstrate regulatory sequences capable of directing cornea-specific gene expression, affording the opportunity for genetic engineering in this transparent tissue.

Characterization of the rat Class 3 aldehyde dehydrogenase gene promoter

The Class 3 aldehyde dehydrogenase gene (ALDH-3) is differentially expressed. Expression is either constitutive or xenobiotic inducible via an aromatic hydrocarbon (Ah) receptor-mediated pathway, depending upon the tissue. A series of studies were performed to examine the regulation of rat ALDH-3 basal expression. DNase I footprint analysis identified four DNA regions within the proximal 1 kb of the 5' flanking region of rat ALDH-3 which interact with regulatory proteins. Reporter gene and gel mobility shift assays indicate that Sp1-like proteins interact with two proximal DNase I footprinted sites to confer strong promoter activity. Two distal DNase I footprinted sites are found within a region that inhibits rat ALDH-3 promoter activity. This negative region is bound by NF1-like proteins and/or unique proteins. This 1 kb 5' flanking region of rat ALDH-3 may act constitutively in many cell types. In contrast with other Ah receptor regulated genes, no DNA elements or transcription factors acting within this region appear to be involved in regulating xenobiotic-inducible expression of rat ALDH-3.

Constitutive expression of class 3 aldehyde dehydrogenase in cultured rat corneal epithelium

Mammalian Class 3 aldehyde dehydrogenase (ALDH) is normally associated with neoplastic transformation or xenobiotic induction by aromatic hydrocarbons in liver. However, Class 3 ALDH is constitutively expressed at it's highest specific activity in corneal epithelium. Tissue-specific, differential gene expression is often controlled by alternative, independent molecular pathways. We report here the development of an in vitro corneal epithelium culture system that retains constitutive high expression of the ALDH3 gene. This model system was used to establish, by enzymatic assays, Western and Northern analyses, histochemical and immunocytochemical staining, and 5'3' RACE methodologies that constitutive and xenobiotic induction of Class 3 ALDHs occurs from a single gene. Our results also provide a plausible explanation for the very high Class 3 ALDH activity in mammalian cornea, as the primary mechanism of oxidation of lipid peroxidation-derived aldehydes. Further studies with corneal epithelium suggest the presence of additional mechanisms, other than Ah-receptor-mediated, by which the ALDH3 gene can be differentially regulated in a tissue-specific manner.

Lens crystallins of invertebrates--diversity and recruitment from detoxification enzymes and novel proteins

The major proteins (crystallins) of the transparent, refractive eye lens of vertebrates are a surprisingly diverse group of multifunctional proteins. A number of lens crystallins display taxon-specificity. In general, vertebrate crystallins have been recruited from stress-protective proteins (i.e. the small heat-shock proteins) and a number of metabolic enzymes by a gene-sharing mechanism. Despite the existence of refractive lenses in the complex and compound eyes of many invertebrates, relatively little is known about their crystallins. Here we review for the first time the state of knowledge of invertebrate crystallins. The major cephalopod (squid, octopus, and cuttlefish) crystallins (S-crystallins) have, like vertebrate crystallins, been recruited from a stress protective metabolic enzyme, glutathione S-transferase. The presence of overlapping AP-1 and antioxidant responsive-like sequences that appear functional in transfected vertebrate cells suggest that the recruitment of glutathione S-transferase to S-crystallins involved response to oxidative stress. Cephalopods also have at least two taxon-specific crystallins: omega-crystallin, related to aldehyde dehydrogenase, and omega-crystallin, related to a superfamily of lipid-binding proteins. L-crystallin (probably identical to O-crystallin) is the major protein of the lens of the squid photophore, a specialized structure for emitting light. The use of L/omega-crystallin in the ectodermal lens of the eye and the mesodermal lens of the photophore of the squid contrasts with the recruitment of different crystallins in the ectodermal lenses of the eye and photophore of fish. S-and omega-crystallins appear to be lens-specific (some S-crystallins are also expressed in cornea) and, except for one S-crystallin polypeptide (SL11/Lops4; possibly a molecular fossil), lack enzymatic activity. The S-crystallins (except SL11/Lops4) contain a variable peptide that has been inserted by exon shuffling. The only other invertebrate crystallins that have been examined are in one marine gastropod (Aplysia, a sea hare), in jellyfish and in the compound eyes of some arthropods; all are different and novel proteins. Drosocrystallin is one of three calcium binding taxon-specific crystallins found selectively in the acellular corneal lens of Drosophila, while antigen 3G6 is a highly conserved protein present in the ommatidial crystallin cone and central nervous system of numerous arthropods. Cubomedusan jellyfish have three novel crystallin families (the J-crystallins); the J1-crystallins are encoded in three very similar intronless genes with markedly different 5' flanking sequences despite their almost identical encoded proteins and high lens expression. The numerous refractive structures that have evolved in the eyes of invertebrates contrast markedly with the limited information on their protein composition, making this field as exciting as it is underdeveloped. The similar requirement of Pax-6 (and possibly other common transcription factors) for eye development as well as the diversity, taxon-specificity and recruitment of stress-protective enzymes as crystallins suggest that borrowing multifunctional proteins for refraction by a gene sharing strategy may have occurred in invertebrates as did in vertebrates.

Aldehyde oxidoreductase activity in Desulfovibrio gigas: in vitro reconstitution of an electron-transfer chain from aldehydes to the production of molecular hydrogen

The molybdenum [iron-sulfur] protein, first isolated from Desulfovibrio gigas by Moura et al. [Moura, J. J. G., Xavier, A. V., Bruschi, M., Le Gall, J., Hall, D. O., & Cammack, R. (1976) Biochem. Biophys. Res. Commun. 72, 782-789], was later shown to mediate the electronic flow from salicylaldehyde to a suitable electron acceptor, 2,6-dichlorophenolindophenol (DCPIP) [Turner, N., Barata, B., Bray, R. C., Deistung, J., LeGall, J., & Moura, J. J. G. (1987) Biochem. J. 243, 755-761]. The DCPIP-dependent aldehyde oxidoreductase activity was studied in detail using a wide range of aldehydes and analogues. Steady-state kinetic analysis (KM and Vmax) was performed for acetaldehyde, propionaldehyde, benzaldehyde, and salicylaldehyde in excess DCPIP concentration, and a simple Michaelis-Menten model was shown to be applicable as a first kinetic approach. Xanthine, purine, allopurinol, and N1-methylnicotinamide (NMN) could not be utilized as enzyme substrates. DCPIP and ferricyanide were shown to be capable of cycling the electronic flow, whereas other cation and anion dyes [O2 and NAD(P)+] were not active in this process. The enzyme showed an optimal pH activity profile around 7.8. This molybdenum hydroxylase was shown to be part of an electron-transfer chain comprising four different soluble proteins from D. gigas, with a total of 11 discrete redox centers, which is capable of linking the oxidation of aldehydes to the reduction of protons.

Kinetic properties of the bovine corneal aldehyde dehydrogenase (BCP 54)

The major soluble protein of bovine cornea (BCP 54: bovine corneal protein 54 kDa) was isolated successively by gel filtration, anion-exchange chromatography and chromatofocusing. The amino acid sequence of a fragment of the purified BCP 54 obtained by lysyl-endopeptidase digestion showed marked homology with tumor-associated and 2,3,7,8-tetrachloro-dibenzo-p-dioxin-inducible aldehyde dehydrogenase (AIDH). From the high similarity of BCP 54 with tumor-associated AIDH in structural form, it is suggested that BCP 54 has AIDH activity. We confirmed a high AIDH activity of BCP 54 by immunoprecipitation using a mouse anti-BCP 54 monoclonal antibody followed by a spectrophotometric assay for AIDH activity. Next we demonstrated the unique properties of the purified BCP 54 as AIDH. The major isoelectric point is 6.41. BCP 54 preferentially oxidizes aromatic aldehyde such as benzaldehyde with NAD as coenzyme, but cannot oxidize phenylacetaldehyde. After heat treatment the AIDH activity is more stable with propionaldehyde-NAD than with benzaldehyde-NADP. With propionaldehyde-NAD the pH profile shows a broad plateau from pH 6-9 followed by a sharp rise up to pH 10. In contrast, with benzaldehyde-NADP there is a sharp optimum at pH 9.0. The activity with only benzaldehyde-NADP is inhibited by p-hydroxymercuribenzoate, but is not affected by disulfiram and diethylstilbestrol. So we suggested that BCP 54 is an AIDH with kinetic properties different from the rat tumor-associated AIDH.

Induction of class 3 aldehyde dehydrogenase in the mouse hepatoma cell line Hepa-1 by various chemicals

The mouse hepatoma cell line Hepa-1 was shown to express an aldehyde dehydrogenase (ALDH) isozyme which was inducible by TCDD and carcinogenic polycyclic aromatic hydrocarbons. The induced activity could be detected with benzaldehyde as substrate and NADP as cofactor (B/NADP ALDH). As compared with rat liver and hepatoma cell lines, the response was moderate (maximally 5-fold). There was an apparent correlation between this specific form of ALDH and aryl hydrocarbon hydroxylase (AHH) in the Hepa-1 wild-type cell line--in terms of inducibility by several chemicals. However, the magnitude of the response was clearly smaller for ALDH than for AHH. Southern blot analysis showed that a homologous gene (class 3 ALDH) was present in the rat and mouse genome. The gene was also expressed in Hepa-1 and there was a good correlation between the increase of class 3 ALDH-specific mRNA and B/NADP ALDH enzyme activity after exposure of the Hepa-1 cells to TCDD. It is concluded that class 3 ALDH is inducible by certain chemicals in the mouse hepatoma cell line, although the respective enzyme is not inducible in mouse liver in vivo.

Taxon-specific recruitment of enzymes as major soluble proteins in the corneal epithelium of three mammals, chicken, and squid

Studies of others have shown that class 3 aldehyde dehydrogenase is a major component of the epithelial cells of the mammalian cornea. Here we demonstrate by peptide sequencing that other major proteins of the corneal epithelium are also identical or related to enzymes in the human, mouse, kangaroo, chicken, and squid. Aldehyde dehydrogenase class 3 was found to be the major protein of human, mouse, and kangaroo corneal epithelial cells. Peptidyl prolyl cis-trans isomerase (cyclophilin) or a homologue thereof is strikingly abundant in the corneal epithelial cells of chicken, but not mammals, and appears to be absent from the cornea of squid. By contrast, enolase or its homologue is relatively abundant in both the mammalian and chicken corneal epithelial cells. In some instances, abundant enzymes are common to cornea and lens in the same species--for example, arginino-succinate lyase/delta 1-crystallin in the chicken and glutathione S-transferase-like protein in the squid; in other cases, the abundant proteins in the cornea have not been found as lens crystallins in any species--for example, aldehyde dehydrogenase class 3 and cyclophilin. These data suggest that enzymes and certain enzyme-crystallins have been recruited as major corneal proteins in a taxon-specific manner and may serve structural rather than, or as well as, enzymatic roles in corneal epithelial cells.

Molecular weight forms of corneal aldehyde dehydrogenase

Aldehyde dehydrogenase has recently been shown to be one of the major soluble proteins in the mammalian cornea. The enzyme has a subunit molecular weight of 54 kd and gel filtration experiments indicate that a dimer molecule is the enzymatically active species. The purpose of the studies described here was to investigate whether oligomeric forms of this enzyme could also be detected using the much faster SDS-PAGE mini-gel electrophoresis technique combined with immunoblotting and "in gel" enzyme detection. Low temperature treatment of samples prior to electrophoresis revealed that both human and bovine corneal ALDH are mainly present as a 54 kd and as a dimer molecule with an apparent molecular weight of 88 kd. Bovine corneal ALDH also contained larger oligomers with a molecular weight of 110, 154 and 210 kd respectively. The classical 3 minutes boiling procedure prior to SDS-PAGE dissociated the oligomers into the 54 kd subunit. Zymography experiments showed that enzyme activity was only present in the 88 kd form of corneal ALDH. Pretreatment of corneal ALDH at various temperatures showed that the temperature induced shift of the 88 kd species to the 54 kd subunit paralleled the decrease in enzymatic activity. The fact that reduction of samples with DTT did not dissociate the 88 kd form suggests that disulfide bridge formation is not involved in the oligomerisation of corneal ALDH.

Aldehyde dehydrogenases and their role in carcinogenesis

Aldehydes are highly reactive molecules that may have a variety of effects on biological systems. They can be generated from a virtually limitless number of endogenous and exogenous sources. Although some aldehyde-mediated effects such as vision are beneficial, many effects are deleterious, including cytotoxicity, mutagenicity, and carcinogenicity. A variety of enzymes have evolved to metabolize aldehydes to less reactive forms. Among the most effective pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). ALDHs are a family of NADP-dependent enzymes with common structural and functional features that catalyze the oxidation of a broad spectrum of aliphatic and aromatic aldehydes. Based on primary sequence analysis, three major classes of mammalian ALDHs--1, 2, and 3--have been identified. Classes 1 and 3 contain both constitutively expressed and inducible cytosolic forms. Class 2 consists of constitutive mitochondrial enzymes. Each class appears to oxidize a variety of substrates that may be derived either from endogenous sources such as amino acid, biogenic amine, or lipid metabolism or from exogenous sources, including aldehydes derived from xenobiotic metabolism. Changes in ALDH activity have been observed during experimental liver and urinary bladder carcinogenesis and in a number of human tumors, including some liver, colon, and mammary cancers. Changes in ALDH define at least one population of preneoplastic cells having a high probability of progressing to overt neoplasms. The most common change is the appearance of class 3 ALDH dehydrogenase activity in tumors arising in tissues that normally do not express this form. The changes in enzyme activity occur early in tumorigenesis and are the result of permanent changes in ALDH gene expression. This review discusses several aspects of ALDH expression during carcinogenesis. A brief introduction examines the variety of sources of aldehydes. This is followed by a discussion of the mammalian ALDHs. Because the ALDHs are a relatively understudied family of enzymes, this section presents what is currently known about the general structural and functional properties of the enzymes and the interrelationships of the various forms. The remainder of the review discusses various aspects of the ALDHs in relation to tumorigenesis. The expression of ALDH during experimental carcinogenesis and what is known about the molecular mechanisms underlying those changes are discussed. This is followed by an extended discussion of the potential roles for ALDH in tumorigenesis. The role of ALDH in the metabolism of cyclophosphamidelike chemotherapeutic agents is described. This work suggests that modulation of ALDH activity may an important determinant of the effectiveness of certain chemotherapeutic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of aldehyde dehydrogenase activity in human corneal extracts

The major soluble protein in bovine corneal epithelial extracts is a 54 kD protein (BCP 54) which has recently been identified as the corneal aldehyde dehydrogenase. Although ALDH activity has been reported in human corneal extracts it was not yet clear whether this was identical with the 54 kD protein described in bovine corneas. To investigate this question, we studied human corneal extracts for the presence of ALDH using enzyme analysis, SDS-PAGE, native electrophoresis, isoelectric focusing and immunoblotting techniques. The corneal epithelium was the most active layer (8.46 +/- 1.9 IU/mg protein) followed by the stroma (2.83 +/- 0.56 IU/mg protein) and endothelium (0.06-3.6 IU/mg protein). When comparing substrate specificity between human and bovine corneal ALDH, using NADP as coenzyme, it was shown that the human enzyme preferred benzaldehyde whereas the bovine enzyme revealed the strongest enzymatic activity with hexanal. Human corneal ALDH was partly inhibited by disulfiram. Bovine and human cornea ALDH lost their enzymatic activity after heating at temperatures above 56 degrees C. Both human and bovine corneal extracts contained a prominent 54 kD protein which reacted with a rabbit anti BCP 54 antibody. Isoelectric focusing followed by enzyme staining in the gel revealed 5 human corneal isozyme species and 4 in bovine corneal extracts, migrating at a pH between 6.5 and 7.0. All isozymes could also be detected after immunoblotting with a rabbit anti BCP 54 antibody.

Lipid aldehyde oxidation as a physiological role for class 3 aldehyde dehydrogenases

A large number of different unsaturated, saturated and hydroxylated aliphatic aldehydes can be generated during the peroxidation of cellular lipids. This study examined the kinetic properties of purified Class 3 rat aldehyde dehydrogenase (ALDH) with respect to the oxidation of various lipid aldehyde substrates. It also compared the substrate preference of the prototypic Class 3 ALDH with that of the constitutive rat microsomal aldehyde dehydrogenase. The results suggest that (1) microsomal ALDH is a member of the Class 3 aldehyde dehydrogenase family, and (2) the physiological role of the Class 3 ALDHs, including the microsomal form, is the oxidation of medium (6 to 9 carbon) chain length saturated and unsaturated aldehydes generated by the peroxidation of cellular lipids. Short chain aliphatic aldehydes, such as a malondialdehyde and 4-hydroxyalkenals, are not substrates for the Class 3 aldehyde dehydrogenases.

Bovine corneal aldehyde dehydrogenase: the major soluble corneal protein with a possible dual protective role for the eye

Bovine corneal aldehyde dehydrogenase was purified to homogeneity and characterized with aldehyde substrates at pH 7.4. The enzyme was a dimer with a subunit size of 65 kDa. Using kcat/Km values as an indication of substrate efficacy, aldehyde products of lipid peroxidation were recognized as the likely 'natural' substrates. Protein yields from enzyme purification, as well as electrophoretic analyses of crude and purified enzyme preparations, demonstrated that this enzyme is the major soluble protein in bovine cornea, and constitutes around 0.5% wet weight of tissue. A dual role in protecting the eye against UV-B light is proposed--oxidation of aldehydes generated by light induced lipid peroxidation, and the direct absorption of UV-B light by bovine corneal ALDH.

Genetics of alcohol dehydrogenase and aldehyde dehydrogenase from Monodelphis domestica cornea: further evidence for identity of corneal aldehyde dehydrogenase with a major soluble protein

A didelphid marsupial, the gray short-tailed opossum (Monodelphis domestica), was used as a model species to study the biochemical genetics of alcohol dehydrogenases (ADHs) and aldehyde dehydrogenase (ALDH) in corneal tissue. Isoelectric point variants of corneal ALDH (designated ALDH3) and a major soluble protein in corneal extracts were observed among eight families of animals used in studying the genetics of these proteins. Both phenotypes exhibited identical patterns following PAGE-IEF and were inherited in a normal Mendelian fashion, with two alleles at a single locus (ALDH3) showing codominant expression. The data provided evidence for genetic identity of corneal ALDH with this major soluble protein, and supported biochemical evidence, recently reported for purified bovine corneal ALDH, that this enzyme constitutes a major portion of soluble corneal protein (Abedinia et al. 1990). Isoelectric point variants for corneal ADH were also observed, with patterns for the two major forms (ADH3 and ADH4) and one minor form (ADH5) being consistent with the presence of two ADH subunits (designated gamma and delta), and variant phenotypes existing for the gamma subunit. The genetics of this enzyme was studied in the eight families, and the results were consistent with codominant expression of two alleles at a single locus (designated ADH3). It is relevant that a major detoxification function has been proposed for corneal ADH and ALDH, in the oxidoreduction of peroxidic aldehydes induced by available oxygen and UV-B light (Holmes & VandeBerg, 1986a). In addition, a direct role for corneal ALDH as a UV-B photoreceptor in this anterior eye tissue has also been proposed (Abedinia et al. 1990).