Expression of rat aldehyde reductase AKR7A1: influence of age and sex and tissue-specific inducibility☆, 1

Abstract

The regulation of the aldo-keto reductase AKR7A1 was examined in the livers of male and female rats during development by using Western blots, and its contribution to carbonyl metabolism was assessed by using enzyme assays. Hepatic levels of AKR7A1 are low in fetal rats and rise to a peak at around 6 weeks of age in animals of both sexes. Higher levels of the enzyme are found in adult male rat liver than in adult female rat liver. The reductase, therefore, appears to be subject to sex-specific regulation. The effect of growth hormone in mediating this difference in expression was examined by using hypophysectomized animals whose serum growth hormone levels had been feminized by continuous administration. Results demonstrate that such treatment leads to a reduction in AKR7A1 expression. AKR7A1 was found to be constitutively expressed in rat tissues such as liver, kidney, small intestine, and testis, but it was not detected in nasal mucosa, skeletal muscle, heart, adrenal gland, brain, or spleen. However, AKR7A1 was inducible by the synthetic antioxidant ethoxyquin in liver, kidney, and small intestine, but not in the other tissues examined. These results show that levels of this important detoxication enzyme vary considerably according to age and sex and that dietary antioxidants can also influence its level in several tissues.

Introduction

Aldehydes and ketones are ubiquitous in the environment, and some of these compounds have been shown to cause damage to protein, DNA, and membranes, leading to toxicity and mutagenicity [1]. Most organisms have evolved protective mechanisms to detoxify carbonyl-containing compounds. Enzymatic defences include the glutathione S-transferases, which can conjugate glutathione (GSH) to reactive carbonyls [2]; aldehyde dehydrogenases, which can oxidize aldehydes to acids [3]; and enzymes, which can reduce aldehydes and ketones to alcohols [4]. This latter biotransformation is catalysed by enzymes that fall into three separate families: the aldo-keto reductases [4]; the alcohol dehydrogenases, and NAD(P)H:quinone oxidoreductase (NQO1; also called DT-diaphorase). The aldo-keto reductases comprise a large superfamily of NAD(P)H-dependent reductases, members of which have been isolated from a variety of species and tissues [5], [6]. Several members of this family have been identified in the rat, including aldehyde reductase (AKR1A3) [7], aldose reductase (AKR1B4) [8], and 3α-hydroxysteroid dehydrogenase (AKR1C9) [9], [10]. In addition to their potential roles in detoxification, aldo-keto reductases can reduce sugars, steroids, and prostaglandins, but because of their overlapping substrate specificity, definitive functions have been difficult to ascribe.

We have previously identified and characterized an aldehyde reductase from rat liver (AKR7A1) that is capable of detoxifying a dialdehyde metabolite of aflatoxin B₁[11], [12]. This enzyme is also capable of reducing a range of cytotoxic and mutagenic aldehydes and ketones, including products of lipid peroxidation such as 4-hydroxynonenal and hexanal [13]. We have shown that the hepatic level of this enzyme, unlike most other members of the aldo-keto reductase family, is increased in rats after dietary exposure to certain xenobiotics. Such inducing agents include the phenolic antioxidants butylated hydroxyanisole and ethoxyquin, as well as naturally occurring compounds including coumarin [12], [14]. Levels of AKR7A1 are also elevated in the liver of rats raised on a selenium-deficient diet [15]. Modulation of the level of AKR7A1 correlates with profound differences in susceptibility to chemical carcinogenesis: animals with high levels of AKR7A1 in the liver appear to be more resistant to the hepatocarcinogen aflatoxin B₁ than are animals with low levels of AKR7A1 [11].

This study was designed to investigate whether factors such as age and sex also influence the expression of AKR7A1, as well as to examine the tissue-specific inducibility of this enzyme. Such information can provide insights into factors that may affect susceptibility to the effects of toxic aldehydes and ketones, as well suggesting additional biological roles for this important enzyme in normal cellular processes.