cDNA cloning of cholesterol 24-hydroxylase, a mediator of cholesterol homeostasis in the brain

Abstract

The turnover of cholesterol in the brain is thought to occur via conversion of excess cholesterol into 24S-hydroxycholesterol, an oxysterol that is readily secreted from the central nervous system into the plasma. To gain molecular insight into this pathway of cholesterol metabolism, we used expression cloning to isolate cDNAs that encode murine and human cholesterol 24-hydroxylases. DNA sequence analysis indicates that both proteins are localized to the endoplasmic reticulum, share 95% identity, and represent a new cytochrome P450 subfamily (CYP46). When transfected into cultured cells, the cDNAs produce an enzymatic activity that converts cholesterol into 24S-hydroxycholesterol, and to a lesser extent, 25-hydroxycholesterol. The cholesterol 24-hydroxylase gene contains 15 exons and is located on human chromosome 14q32.1. Cholesterol 24-hydroxylase is expressed predominantly in the brain as judged by RNA and protein blotting. In situ mRNA hybridization and immunohistochemistry localize the expression of this P450 to neurons in multiple subregions of the brain. The concentrations of 24S-hydroxycholesterol in serum are low in newborn mice, reach a peak between postnatal days 12 and 15, and thereafter decline to baseline levels. In contrast, cholesterol 24-hydroxylase protein is first detected in the brain of mice at birth and continues to accumulate with age. We conclude that the cloned cDNAs encode cholesterol 24-hydroxylases that synthesize oxysterols in neurons of the brain and that secretion of 24S-hydroxycholesterol from this tissue in the mouse is developmentally regulated.

Bi-lateral changes to hippocampal cholesterol levels during epileptogenesis and in chronic epilepsy following focal-onset status epilepticus in mice

“…In contrast to cholesterol, 24S--OHC can pass the blood-brain barrier and directly enter the general circulation. As 24S--OHC is ultimately cleared by the liver, this system effectively acts as a brain--initiated pathway of reverse cholesterol transport ( Lund et al, 1999). The cytochrome P450 enzyme, cholesterol 24--hydroxylase (CYP46A1) mediates this transformation and is therefore a key determinant of the rate of cholesterol removal from the brain ( Lund et al, 2003).…”

“…The cytochrome P450 enzyme, cholesterol 24--hydroxylase (CYP46A1) mediates this transformation and is therefore a key determinant of the rate of cholesterol removal from the brain ( Lund et al, 2003). In humans, CYP46A1 is exclusively expressed in the brain and under normal conditions the enzyme protein is detected only in neurons, in particular in the pyramidal neurons and interneurons of the hippocampus ( Lund et al, 1999 andRamirez et al, 2008). Seizures are a co--morbidity in a variety of diseases associated with pathologic cholesterol metabolism.…”

Analysis of oxysterols by electrospray tandem mass spectrometry

“…The 3␤-hydroxycholest-5-en-27-oic acid in human plasma (formed from 27-hydroxycholesterol [23]) originates mostly from the lung [7]. 24S-Hydroxycholesterol in the human circulation originates mainly from the brain, where it is formed by the action of CYP46A1 [24]. While 7␣-hydroxycholesterol, 27-hydroxycholesterol and 3␤-hydroxycholest-5-en-27-oic acid are efficiently converted into bile acids in the liver, about half of the 24S-hydroxycholesterol formed in humans is excreted in bile in a sulphated or glucuronidated form.…”

Do oxysterols control cholesterol homeostasis?

“…In vitro, the sterol 27-hydroxylase pathway appears to be most important when cells are not provided with an extracellular acceptor such as HDL (10). Indeed, we (unpublished observations) have found some signs that the sterol 27-hydroxylase mechanism is modestly upregulated in Tangier disease, where HDL is deficient (see Tall et CNS neurons contain the CYP46 cytochrome P-450 species, which hydroxylates cholesterol to yield 24S-hydroxycholesterol (10,14). Unlike cholesterol, this oxysterol can pass the blood-brain barrier.…”