Studies on the transcriptional regulation of cholesterol 24-hydroxylase (CYP46A1): marked insensitivity toward different regulatory axes.
Mammalian CNS contains a disproportionally large and remarkably stable pool of cholesterol. Despite an efficient recycling there is some requirement for elimination of brain cholesterol. Conversion of cholesterol into 24S-hydroxycholesterol by the cholesterol 24-hydroxylase (CYP46A1) is the quantitatively most important mechanism. Based on the protein expression and plasma levels of 24S-hydroxycholesterol, CYP46A1 activity appears to be highly stable in adults. Here we have made a structural and functional characterization of the promoter of the human CYP46A1 gene. No canonical TATA or CAAT boxes were found in the promoter region. Moreover this region had a high GC content, a feature often found in genes considered to have a largely housekeeping function. A broad spectrum of regulatory axes using a variety of promoter constructs did not result in a significant transcriptional regulation. Oxidative stress caused a significant increase in transcriptional activity. The possibility of a substrate-dependent transcriptional regulation was explored in vivo in a sterol-deficient mouse model (Dhcr24 null) in which almost all cholesterol had been replaced with desmosterol, which is not a substrate for CYP46A1. Compared with heterozygous littermates there was no statistically significant difference in the mRNA levels of Cyp46a1. During the first 2 weeks of life in the wild-type mouse, however, a significant increase of Cyp46a1 mRNA levels was found, in parallel with an increase in 24S-hydroxycholesterol level and a reduction of cholesterol synthesis. The failure to demonstrate a significant transcriptional regulation under most conditions is discussed in relation to the turnover of brain and neuronal cholesterol.
Ohyama
Yoshihiko
Y
Divisions of Clinical Chemistry and Clinical Pharmacology, Department of Laboratory Medicine, Karolinska University Hospital, Huddinge, Stockholm, Sweden.
Meaney
Steve
S
Heverin
Maura
M
Ekström
Lena
L
Brafman
Anat
A
Shafir
Millicent
M
Andersson
Ulla
U
Olin
Maria
M
Eggertsen
Gösta
G
Diczfalusy
Ulf
U
Feinstein
Elena
E
Björkhem
Ingemar
I
D000544:Alzheimer Disease / Q000201:enzymology; D000818:Animals; D001483:Base Sequence; D001921:Brain / Q000378:metabolism; D045744:Cell Line, Tumor; D002784:Cholesterol / Q000378:metabolism; D000072556:Cholesterol 24-Hydroxylase; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000032:analysis; D013250:Steroid Hydroxylases / Q000235:genetics*; D024363:Transcription Initiation Site; D014158:Transcription, Genetic
D012333:RNA, Messenger; D002784:Cholesterol; D013250:Steroid Hydroxylases; D000072556:Cholesterol 24-Hydroxylase