Disruption of the Oxysterol 7α-Hydroxylase Gene in Mice

Li-Hawkins, Jia; Lund, Erik; Turley, Stephen D.; Russell, David W.

doi:10.1074/jbc.m001811200

Cited by 190 publications

(162 citation statements)

References 32 publications

(25 reference statements)

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confidence: 62%

“…In agreement with the latter preference, Cyp7b1 knockout mice accumulate these two oxysterols in their plasma and tissues (6). The levels of the other major oxysterol, 24-hydroxycholesterol (cholest-5-ene-3␤, 24-diol), are near normal in these animals (6). In contrast, a human with a complete absence of oxysterol 7␣-hydroxylase activity accumulated 24-hydroxycholesterol as well as 25-and 27-hydroxycholesterol in his plasma (7).…”

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confidence: 85%

“…The cell medium was extracted with chloroform/ methanol (2:1, v/v); the aqueous phase was aspirated; and the remaining organic phase was taken to dryness under a stream of nitrogen. Solids were redissolved in 1 ml of toluene and purified on Isolute Silica columns (14), with the modifications described in the accompanying paper (6). After derivatization to trimethylsilyl ethers, samples were analyzed by gas chromatography-mass spectrometry with the quadrupole scanned from m/z 50 to 650 at a rate of one scan/1.5 s.…”

Section: Methodsmentioning

confidence: 99%

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Expression Cloning of an Oxysterol 7α-Hydroxylase Selective for 24-Hydroxycholesterol

Li-Hawkins

Lund

Bronson

et al. 2000

Journal of Biological Chemistry

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168

131

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The synthesis of 7␣-hydroxylated bile acids from oxysterols requires an oxysterol 7␣-hydroxylase encoded by the Cyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25-and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7␣-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7␣-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1 locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7␣-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7␣-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes.Two metabolic pathways that differ in their initial steps produce 7␣-hydroxylated bile acids (1). In one pathway, cholesterol (5-cholesten-3␤-ol) is first converted into 7␣-hydroxycholesterol (cholest-5-ene-3␤,7␣-diol) by the enzyme cholesterol 7␣-hydroxylase, which is encoded by the Cyp7a1 gene in mice. In the other pathway, cholesterol is first converted into one of several oxysterols prior to being 7␣-hydroxylated by oxysterol 7␣-hydroxylase, which is encoded by the Cyp7b1 gene. The 7␣-hydroxylated intermediates produced by these different initiating steps are subsequently converted into primary bile acids by a series of shared enzymes in the liver (2).Cholesterol 7␣-hydroxylase shows a marked preference for cholesterol as a substrate and is only weakly active against other sterols (3), whereas oxysterol 7␣-hydroxylase prefers 25-hydroxycholesterol (cholest-5-ene-3␤,25-diol) and 27-hydroxycholesterol (cholest-5-ene-3␤,27-diol) (4, 5). In agreement with the latter preference, Cyp7b1 knockout mice accumulate these two oxysterols in their plasma and tissues (6). The levels of the other major oxysterol, 24-hydroxycholesterol (cholest-5-ene-3␤,24-diol), are near normal in these animals (6). In contrast, a human with a complete absence of oxysterol 7␣-hydroxylase activity accumulated 24-hydroxycholesterol as well as 25-and 27-hydroxycholesterol in his plasma (7). These observations suggest that the human enzyme, unlike the mouse enzyme, may act on all three oxysterol substrates.The observed differences in the oxysterol accumulation patterns in mice and humans that express no oxysterol 7␣-hydroxylase are not due to differences in the biosynthesis of oxysterols, as cholester...

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confidence: 62%

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confidence: 85%

Section: Methodsmentioning

confidence: 99%

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Expression Cloning of an Oxysterol 7α-Hydroxylase Selective for 24-Hydroxycholesterol

Li-Hawkins

Lund

Bronson

et al. 2000

Journal of Biological Chemistry

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168

131

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“…Although CYP7B1 has been studied by several groups, the focus has been on its possible role in oxysterol metabolism (23)(24)(25)(26). Our data show that this enzyme may have a far more important role in the body, i.e., regulating the cellular specificity of estrogen signaling and influencing the balance between ER and AR signaling.…”

Section: Discussionmentioning

confidence: 83%

Early onset of puberty and early ovarian failure in CYP7B1 knockout mice

Omoto¹,

Lathe²,

Warner³

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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CYP7B1 is the enzyme responsible for hydroxylation and termination of the estrogenic actions of the androgen metabolite, 5α-androstane-3β, 17β-diol (3βAdiol). 3βAdiol is estrogenic in ERα or ERβ positive cells only if they do not express CYP7B1. In this study we show that female CYP7B1–/– mice experience early onset of growth of the uterus and mammary glands and commence estrus cycles 2 days earlier than their wild-type littermates. Adult mammary glands and uteri appear to be under continuous estrogenic stimulation. We conclude that, by cell-specific regulation of the estrogenicity of 3βAdiol, CYP7B1 performs two major tasks: (i) it allows 3βAdiol to have growth inhibitory effects through ERβ and (ii) it permits estradiol-specific activation of estrogen receptors by protection of certain cells from the estrogenic effects of 3βAdiol. When CYP7B1 is inactivated, 3βAdiol activates estrogen receptors indiscriminately, and the overall effect is prolonged and inappropriate exposure to estrogen

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“…CYP7B1 demonstrated broad oxysterol substrate 7a-Hydroxylase activity in the human brain 723 specificity and these compounds accumulate in the plasma and tissues of CYP7B1 knockout mice (Li-Hawkins et al 2000a,b). Interestingly, the loss of oxysterol 7a-hydroxylase activity is obviously compensated by other pathways in CYP7B1 knockout mice (Li-Hawkins et al 2000b). The significance of CYP7B1 expression in humans was emphasized by the fatal effects of a homozygous nonsense mutation of the gene, which results in severe neonatal liver disease and is apparently incompatible with post-natal survival (Setchell et al 1998).…”

Section: Discussionmentioning

confidence: 99%

Characterization of the dehydroepiandrosterone (DHEA) metabolism via oxysterol 7α‐hydroxylase and 17‐ketosteroid reductase activity in the human brain

Steckelbroeck¹,

Watzka²,

Lütjohann³

et al. 2002

Journal of Neurochemistry

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Dehydroepiandrosterone and its sulphate are important factors for vitality, development and functions of the CNS. They were found to be subjects to a series of enzyme-mediated conversions within the rodent CNS. In the present study, we were able to demonstrate for the first time that membraneassociated dehydroepiandrosterone 7a-hydroxylase activity occurs within the human brain. The cytochrome P450 enzyme demonstrated a sharp pH optimum between 7.5 and 8.0 and a mean K M value of 5.4 lM, corresponding with the presence of the oxysterol 7a-hydroxylase CYP7B1. Real-time RT-PCR analysis verified high levels of CYP7B1 mRNA expression in the human CNS. The additionally observed conversion of dehydroepiandrosterone via cytosolic 17b-hydroxysteroid dehydrogenase activity could be ascribed to the activity of an enzyme with a broad pH optimum and an undetectably high K M value. Subsequent experiments with cerebral neocortex and subcortical white matter specimens revealed that 7a-hydroxylase activity is significantly higher in the cerebral neocortex than in the subcortical white matter (p < 0.0005), whereas in the subcortical white matter, 17b-hydroxysteroid dehydrogenase activity is significantly higher than in the cerebral neocortex (p < 0.0005). No sex differences were observed. In conclusion, the high levels of CYP7B1 mRNA in brain tissue as well as in a variety of other tissues in combination with the ubiquitous presence of 7a-hydroxylase activity in the human temporal lobe led us to assume a neuroprotective function of the enzyme such as regulation of the immune response or counteracting the deleterious effects of neurotoxic glucocorticoids, rather than a distinct brain specific function such as neurostimulation or neuromodulation.

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Disruption of the Oxysterol 7α-Hydroxylase Gene in Mice

Cited by 190 publications

References 32 publications

Expression Cloning of an Oxysterol 7α-Hydroxylase Selective for 24-Hydroxycholesterol

Expression Cloning of an Oxysterol 7α-Hydroxylase Selective for 24-Hydroxycholesterol

Early onset of puberty and early ovarian failure in CYP7B1 knockout mice

Characterization of the dehydroepiandrosterone (DHEA) metabolism via oxysterol 7α‐hydroxylase and 17‐ketosteroid reductase activity in the human brain

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