Effect of increasing the expression of cholesterol transporters (StAR, MLN64, and SCP-2) on bile acid synthesis.
There are two major pathways of bile acid synthesis: the "neutral" pathway, initiated by highly regulated microsomal cholesterol 7alpha-hydroxylase (CYP7A1), and an "alternative" pathway, initiated by mitochondrial sterol 27-hydroxylase (CYP27A1). In hepatocyte cultures, overexpression of CYP7A1 increases bile acid synthesis by >8-fold. However, overexpression of CYP27A1 in hepatocytes only increases it by 1.5-fold, suggesting that additional rate-limiting steps must be involved in the regulation of this pathway. The effects of intracellular cholesterol transport proteins on bile acid synthesis have been investigated in the current study. Under culture conditions in which the neutral pathway was inactive, selective overexpression of the gene encoding steroidogenic acute regulatory protein (StAR), MLN64 (StAR homolog protein), and sterol carrier protein-2 (SCP-2) led to 5.7-, 1.2-, and 1.7-fold increases, respectively, in the rates of bile acid synthesis in primary rat hepatocytes. Surprisingly, co-overexpression of MLN64 with StAR, SCP-2, or CYP7A1 blunted the upregulated bile acid synthesis by 48, 47, and 45%, respectively. These results suggest that MLN64, in its full-length form, is not responsible for the transport of cholesterol to the mitochondria or the endoplasmic reticulum, where CYP27A1 or CYP7A1 is located, respectively.
Ren
Shunlin
S
Department of Medicine, Veterans Affairs Medical Center and Virginia Commonwealth University, Richmond, VA, USA.
Hylemon
Phillip
P
Marques
Dalila
D
Hall
Elizabeth
E
Redford
Kaye
K
Gil
Gregorio
G
Pandak
William M
WM
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport; D015153:Blotting, Western; D002352:Carrier Proteins / Q000096:biosynthesis*; D002478:Cells, Cultured; D053493:Cholestanetriol 26-Monooxygenase; D002784:Cholesterol / Q000378:metabolism; D002790:Cholesterol 7-alpha-Hydroxylase / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D008297:Male; D026901:Membrane Transport Proteins / Q000096:biosynthesis*; D008954:Models, Biological; D010750:Phosphoproteins / Q000096:biosynthesis*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013250:Steroid Hydroxylases / Q000378:metabolism; D013997:Time Factors; D015854:Up-Regulation; D000098682:Steroidogenic Acute Regulatory Protein
D001647:Bile Acids and Salts; D002352:Carrier Proteins; C486529:MLN64 protein, mouse; D026901:Membrane Transport Proteins; D010750:Phosphoproteins; C580807:Stard3nl protein, rat; D000098682:Steroidogenic Acute Regulatory Protein; C072087:sterol carrier proteins; D002784:Cholesterol; D013250:Steroid Hydroxylases; D002790:Cholesterol 7-alpha-Hydroxylase; C506831:CYP27A1 protein, human; D053493:Cholestanetriol 26-Monooxygenase; C506832:Cyp27a1 protein, mouse