Molecular mechanism of null expression of aldehyde dehydrogenase-1 in rat liver

Alcohol and Aldehyde Dehydrogenases Contribute to Sex-Related Differences in Clearance of Zolpidem in Rats

OBJECTIVES

The recommended zolpidem starting dose was lowered in females (5 mg vs. 10 mg) since side effects were more frequent and severe than those of males; the mechanism underlying sex differences in pharmacokinetics (PK) is unknown. We hypothesized that such differences were caused by known sex-related variability in alcohol dehydrogenase (ADH) expression.

METHODS

Male, female, and castrated male rats were administered 2.6 mg/kg zolpidem, ± disulfiram (ADH/ALDH pathway inhibitor) to compare PK changes induced by sex and gonadal hormones. PK analyses were conducted in rat plasma and rat brain.

KEY FINDINGS

Sex differences in PK were evident: females had a higher C MAX (112.4 vs. 68.1 ug/L) and AUC (537.8 vs. 231.8 h(∗)ug/L) than uncastrated males. Castration induced an earlier T MAX (0.25 vs. 1 h), greater C MAX (109.1 vs. 68.1 ug/L), and a corresponding AUC increase (339.7 vs. 231.8 h(∗)ug/L). Administration of disulfiram caused more drastic C MAX and T MAX changes in male vs. female rats that mirrored the effects of castration on first-pass metabolism, suggesting that the observed PK differences may be caused by ADH/ALDH expression. Brain concentrations paralleled plasma concentrations.

CONCLUSION

These findings indicate that sex differences in zolpidem PK are influenced by variation in the expression of ADH/ALDH due to gonadal androgens.

Hairpin Ribozyme Genes Curtail Alcohol Drinking: from Rational Design to in vivo Effects in the Rat

Ribozyme genes were designed to reduce voluntary alcohol drinking in a rat model of alcohol dependence. Acetaldehyde generated from alcohol in the liver is metabolized by the mitochondrial aldehyde dehydrogenase (ALDH2) such that diminishing ALDH2 activity leads to the aversive effects of blood acetaldehyde upon alcohol intake. A stepwise approach was followed to design genes encoding ribozymes targeted to the rat ALDH2 mRNA. In vitro studies of accessibility to oligonucleotides identified suitable target sites in the mRNA, one of which fulfilled hammerhead and hairpin ribozyme requirements (CGGUC). Ribozyme genes delivered in plasmid constructs were tested in rat cells in culture. While the hairpin ribozyme reduced ALDH2 activity 56% by cleavage and blockade (P < 0.0001), the hammerhead ribozyme elicited minor effects by blockade. The hairpin ribozyme was tested in vivo by adenoviral gene delivery to UChB alcohol drinker rats. Ethanol intake was curtailed 47% for 34 days (P < 0.0001), while blood acetaldehyde more than doubled upon ethanol administration and ALDH2 activity dropped 25% in liver homogenates, not affecting other ALDH isoforms. Thus, hairpin ribozymes targeted to 16 nt in the ALDH2 mRNA provide durable and specific effects in vivo, representing an improvement on previous work and encouraging development of gene therapy for alcoholism.

Aldehyde dehydrogenase (ALDH2) activity in hepatoma cells is reduced by an adenoviral vector coding for an ALDH2 antisense mRNA

BACKGROUND

Individuals carrying the Glu487Lys coding mutation in the gene for mitochondrial aldehyde dehydrogenase (ALDH2) have a diminished capacity to metabolize acetaldehyde. This deficiency leads to increases in blood acetaldehyde levels when they consume ethanol, which results in an aversion to alcohol and in marked protection against alcoholism. In the present studies, we aimed to mimic the high-acetaldehyde low-ALDH2 activity phenotype in a rat hepatoma cell line by inhibiting Aldh2 gene expression by an Aldh2 antisense-coding gene carried by an adenoviral vector.

METHODS

We designed and produced elevated titers of adenoviral vectors (10 virions/ml) carrying Aldh2 cDNA cloned in the reverse orientation preceded by a CMV promoter and followed by a poly-A termination signal. Rat hepatoma cells were infected with these vectors.

RESULTS

Studies showed that 1) the antisense gene is actively transcribed in the cells and high levels of antisense mRNA are attained, 2) the antisense gene reduced ALDH2 activity by 65%, and 3) when incubated with 10 mM ethanol, acetaldehyde accumulation by cells increased 8-fold to levels (80-90 microM) known to be aversive to animals and humans.

CONCLUSIONS

Data presented show that antialcohol drugs that inhibit Aldh2 gene expression can be generated endogenously in liver cells infected by an adenoviral vector carrying an antisense-coding gene, thus mimicking the high-acetaldehyde phenotype that exists in humans carrying the Glu487Lys mutation who are protected against alcoholism.

Use of an "acetaldehyde clamp" in the determination of low-KM aldehyde dehydrogenase activity in H4-II-E-C3 rat hepatoma cells

The high-affinity (K(M)<1 microM) mitochondrial class 2 aldehyde dehydrogenase (ALDH2) metabolizes most of the acetaldehyde generated in the hepatic oxidation of ethanol. H4-II-E-C3 rat hepatoma cells have been found to express ALDH2. We report a method to assess ALDH2 activity in intact hepatoma cells that does not require mitochondrial isolation. To determine only the high-affinity ALDH2 activity it is necessary to keep constant low concentrations of acetaldehyde in the cells to minimize its metabolism by high-K(M) aldehyde dehydrogenases. To maintain both low and constant concentrations of acetaldehyde we used an "acetaldehyde clamp," which keeps acetaldehyde at a concentration of 4.2+/-0.4 microM. The clamp is attained by addition of excess yeast alcohol dehydrogenase, 14C-ethanol, and oxidized form of nicotinamide adenine dinucleotide (NAD(+)) to the hepatoma cell culture medium. The concentration of 14C-acetaldehyde attained follows the equilibrium constant of the alcohol dehydrogenase reaction. Thus, 14C-acetate is generated virtually by the low-K(M) aldehyde dehydrogenase activity. 14C-acetate is separated from the culture medium by an anionic resin and its radioactivity is determined. We showed that (1) acetate production is linear for 120 min, (2) addition of 160 microM cyanamide to the culture medium leads to a 75%-80% reduction of acetate generated, and (3) ALDH2 activity is dependent on cell-to-cell contact and increases after cells reach confluence. The clamp system allows the determination of ALDH2 activity in less than one million H4-II-E-C3 rat hepatoma cells. The specificity and sensitivity of the "acetaldehyde clamp" assay should be of value in evaluation of the effects of new agents that modify Aldh2 gene expression, as well as in the study of ALDH2 regulation in intact cells.

Differential regulation of the aldehyde dehydrogenase 1 gene in embryonic chick retina and liver

Aldehyde dehydrogenase (ALDH1) is highly expressed in the dorsal cells of the undifferentiated retina, where it has been proposed to play a role in the formation of a retinoic acid gradient along the ventrodorsal axis. In contrast to the retina, ALDH1 levels increase with differentiation in the liver and remain elevated in the adult tissue. To understand the molecular basis for differential expression of ALDH1 during development, we characterized the ALDH1 transcripts expressed in chick retina and liver. By sequencing, primer extension, and S1 nuclease analysis, we show that retina ALDH1 mRNA has an additional 300 nucleotides of 5'-untranslated sequence resulting from the transcription of two 5' noncoding exons. There is a 24-29-kilobase pair (kb) gap between exons 1 and 2 and a 290-base pair gap between exons 2 and 3. Exon 3, which contains the ALDH1 start codon, represents the first exon of the liver transcript. Using a reporter gene assay, we have identified tissue-specific regulatory elements that govern ALDH1 expression in primary retina and liver cultures. Constructs with >1.6 kb of DNA flanking the 5'-end of exon 1 showed elevated activity in retinal cultures but only basal activity in liver cultures. In contrast, constructs with <1 kb of 5'-flanking DNA were active in both retina and liver cultures. Our results suggest that an important mechanism for the control of ALDH1 transcriptional activity is through the presence of inhibitory elements located 0.7-1.6 kb upstream of the ALDH1 gene. DNase I footprint analysis reveal four sites of protein-DNA interaction within this region, one of which is specific to the liver and corresponds to a NF-kappaB/Rel binding site.

Molecular analysis of two closely related mouse aldehyde dehydrogenase genes: identification of a role for Aldh1, but not Aldh-pb, in the biosynthesis of retinoic acid

Mammalian class I aldehyde dehydrogenase (ALDH1) has been implicated as a retinal dehydrogenase in the biosynthesis of retinoic acid, a modulator of gene expression and cell differentiation. As the first step towards studying the regulation of ALDH1 and its physiological role in the biosynthesis of retinoic acid, mouse ALDH1 cDNA and genomic clones have been characterized. During the cloning process, an additional closely related gene was also isolated and named Aldh-pb, owing to its high amino acid sequence identity (92%) with the rat phenobarbitol-inducible ALDH protein (ALDH-PB). Aldh1 spans about 45 kb in length, whereas Aldh-pb spans about 35 kb. Both genes are composed of 13 exons, and the positions of all the exon/intron boundaries are conserved with those of human ALDH1. The promoter regions of Aldh1 and Aldh-pb demonstrate high sequence similarity with those of human ALDH1 and rat ALDH-PB. Expression of Aldh1 and Aldh-pb is tissue-specific, with mRNAs for both genes being found in the liver, lung and testis, but not in the heart, spleen or muscle. Expression of Aldh-pb, but not Aldh1, was also detected at high levels in the kidney. Aldh1 and Aldh-pb encode proteins of 501 amino acids with 90% positional identity. To examine the relative roles of these two enzymes in retinoic acid synthesis in vivo, Xenopus embryos were injected with mRNAs encoding these enzymes to assay the effect on conversion of endogenous retinal into retinoic acid. Injection of ALDH1, but not ALDH-PB, mRNA stimulated retinoic acid synthesis in Xenopus embryos at the blastula stage. Thus our results indicate that Aldh1 can function in retinoic acid synthesis under physiological conditions, but that the closely related Aldh-pb does not share this property.

Purification and partial characterization of bovine kidney aldehyde dehydrogenase able to oxidize retinal to retinoic acid

A NAD-dependent enzyme that catalyzes the oxidation of retinal to retinoic acid has been purified to homogeneity from bovine kidney. The procedures used in the purification included ion-exchange chromatography on DEAE-Sepharose, affinity chromatography on Affi-gel blue and chromatography on a Mono-Q anion-exchange column. On the Mono-Q column, the enzyme aldehyde dehydrogenase (ALDH) resolved into two activity peaks designated as ALDH1 and ALDH2. The enzymes ALDH1 and ALDH2 were purified about 114- and 65-fold, respectively. Gel filtration chromatography of the partially purified native enzyme on Sephacryl S-200 HR exhibited a molecular mass of about 108 kDa. Electrophoresis of the purified enzymes under nondenaturing conditions showed a single protein band. However, sodium dodecyl sulfate--polyacrylamide gel electrophorsis indicated three protein bands in the 55, 30, and 22 kDa molecular mass regions. Both enzymes exhibited a broad substrate specificity oxidizing a wide variety of aliphatic and aromatic aldehydes. The ALDH1 enzyme had a pI of 7.45 and exhibited a low Km (6.37 microM) for retinal, while the ALDH2 enzyme was found to have very low Km for acetaldehyde (0.98 microM). Based on its kinetic properties, it is suggested that the ALDH1 enzyme may be the primary enzyme for oxidizing retinal to retinoic acid in bovine kidney.