Knockout of the Cholesterol 24-Hydroxylase Gene in Mice Reveals a Brain-specific Mechanism of Cholesterol Turnover

Abstract

Most cholesterol turnover takes place in the liver and involves the conversion of cholesterol into soluble and readily excreted bile acids. The synthesis of bile acids is limited to the liver, but several enzymes in the bile acid biosynthetic pathway are expressed in extra-hepatic tissues and there also may contribute to cholesterol turnover. An example of the latter type of enzyme is cholesterol 24-hydroxylase, a cytochrome P450 (CYP46A1) that is expressed at 100-fold higher levels in the brain than in the liver. Cholesterol 24-hydroxylase catalyzes the synthesis of the oxysterol 24(S)-hydroxycholesterol. To assess the relative contribution of the 24-hydroxylation pathway to cholesterol turnover, we performed balance studies in mice lacking the cholesterol 24-hydroxylase gene (Cyp46a1 ؊/؊ mice). Parameters of hepatic cholesterol and bile acid metabolism in the mutant mice remained unchanged relative to wild type controls. In contrast to the liver, the synthesis of new cholesterol was reduced by ϳ40% in the brain, despite steady-state levels of cholesterol being similar in the knockout mice. These data suggest that the synthesis of new cholesterol and the secretion of 24(S)-hydroxycholesterol are closely coupled and that at least 40% of cholesterol turnover in the brain is dependent on the action of cholesterol 24-hydroxylase. We conclude that cholesterol 24-hydroxylase constitutes a major tissue-specific pathway for cholesterol turnover in the brain.The major site for the turnover of cholesterol 1 in the body is the liver. This process involves the secretion of cholesterol and of bile acids derived from cholesterol. A pathway of 16 enzymes is responsible for the conversion of cholesterol to primary bile acids like cholic acid and chenodeoxycholic acid, which are then secreted into the bile and eventually excreted from the body (1). Cholesterol for bile acid synthesis is derived from three sources, including de novo synthesis in the liver, absorption from the diet, and delivery from peripheral tissues via lipoprotein-mediated transport. The transfer of cholesterol from the peripheral tissues to circulating lipoprotein particles occurs at the surfaces of cells and represents the major pathway by which extra-hepatic tissues turn over cholesterol (2); an exception is the brain, where the blood-brain barrier prevents transfer of cholesterol to circulating lipoproteins. The brain is thought to utilize an alternate mechanism of turnover in which cholesterol is converted into 24(S)-hydroxycholesterol, an oxysterol that may be able to diffuse across the blood-brain barrier (3-6). Once in the circulation, 24(S)-hydroxycholesterol is cleared by the liver and therein converted into 7␣-hydroxylated intermediates in the bile acid synthetic pathway by a dedicated enzyme (7-9). In humans, the mass of cholesterol converted into 24(S)-hydroxycholesterol by the brain is estimated to be 0.09 mg/day/kg of body weight (10). The role of 24-hydroxylation has not been defined in other species, but the total amount of sterol excreted fr...

Cholesterol 24S-Hydroxylase Overexpression Inhibits the Liver X Receptor (LXR) Pathway by Activating Small Guanosine Triphosphate-Binding Proteins (sGTPases) in Neuronal Cells

“…As previously mentioned, inactivation of Cyp46a1 has been associated with a selective reduction of cholesterol synthesis [13], while a significant increase in the levels of several cholesterol precursors was observed in the brain of CYP46A1 transgenic mice [16,20]. Moreover, herein we have also observed an increase in HMGR activity in CYP46A1 transfected cells.…”

“…In accordance to this, selective overexpression of CYP46A1, in the cortex and hippocampus of transgenic mice model for AD, ameliorated the amyloid β pathology, without activation of the LXR pathway [17]. Knockout mice lacking Cyp46a1 have a≈40 % reduction in brain cholesterol excretion, but this decrease is compensated for by a reduction in the de novo synthesis that maintains normal steady-state levels [13]. Cyp46a1 null mice exhibit severe deficiencies in spatial, associative and motor learning, and hippocampal long-term potentiation (LTP) [18,19].…”

“…Thus to meet cholesterol needs, de novo and in situ synthesis occur in the CNS. The conversion of cholesterol into 24(S)-hydroxycholesterol (24OHC), by the neuronal-specific cytochrome P450 cholesterol 24S-hydroxylase (CYP46A1), has been described as the major cholesterol elimination mechanism [11][12][13]. In vitro, 24OHC has been shown to be a ligand for the liver X receptor (LXR) [14,15].…”

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

“…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). 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).…”

Neuronal cholesterol esterification by ACAT1 in Alzheimer's disease

“…In the brain, cholesterol can be enzymatically converted to 24SOH by Cyp46A1 (56); the concentration of 24SOH far exceeds those of other oxysterols in the brain (57)(58)(59). Cyp46A1 is a brain-specific enzyme; in mice lacking Cyp46A1, the 24SOH content in the brains is reduced by more than 95% (60), implying that almost all of the 24SOH present in the brain is produced through Cyp46A1. Various oxysterols, including 24SOH, can downregulate sterol synthesis in intact cells and in vitro (61,62), and can bind and activate the LXRs (52,63).…”