Hypercholesterolemia in rats with hepatomas: increased oxysterols accelerate efflux but do not inhibit biosynthesis of cholesterol.
Hypercholesterolemia is an important paraneoplastic syndrome in patients with hepatoma, but the nature of this defect has not yet been identified. We investigated the molecular mechanisms of hypercholesterolemia in a hepatoma-bearing rat model. Buffalo rats were implanted in both flanks with Morris hepatoma 7777 (McA-RH7777) cells. After 4 weeks, tumor weight was 5.5+/-1.7 g, and serum cholesterol level increased from 60+/-2 to 90+/-2 mg/dL. Protein and mRNA expression of the ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1) was markedly higher in tumors than in livers. These increases were associated with activation of liver X receptor alpha (LXRalpha) as a result of the increased tissue oxysterol concentrations. The accumulation of oxysterols in the hepatomas appeared to be caused mainly by the upregulation of cholesterol biosynthesis, despite the increased tissue sterol concentrations. Overexpression of the sterol regulatory element-binding protein (SREBP) processing system relative to sterol concentration contributed to the resistance to sterols in this tumor. In addition, bile acid biosynthesis was inhibited despite the reduced expression of the small heterodimer partner (SHP) and activated LXRalpha, which also appeared to contribute to the accumulation of oxysterols followed by the acceleration of cholesterol efflux. In conclusion, hypercholesterolemia in McA-RH7777 hepatoma-bearing rats was caused by increased cholesterol efflux from tumors as a result of activation of LXRalpha. Overexpression of the SREBP processing system contributed to the activation of LXRalpha by maintaining high oxysterol levels in tissue.
Hirayama
Takeshi
T
Division of Gastroenterology and Hepatology, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, and Kasumigaura Hospital, Ibaraki, Japan.
Honda
Akira
A
Matsuzaki
Yasushi
Y
Miyazaki
Teruo
T
Ikegami
Tadashi
T
Doy
Mikio
M
Xu
Guorong
G
Lea
Michael
M
Salen
Gerald
G
D064286:ATP Binding Cassette Transporter 1; D000070998:ATP Binding Cassette Transporter, Subfamily G, Member 1; D018528:ATP-Binding Cassette Transporters / Q000235:genetics; D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000150:complications* / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D002784:Cholesterol / Q000096:biosynthesis*; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D015972:Gene Expression Regulation, Neoplastic; D006937:Hypercholesterolemia / Q000150:complications / Q000378:metabolism*; D015151:Immunoblotting; D008114:Liver Neoplasms, Experimental / Q000150:complications* / Q000378:metabolism / Q000473:pathology; D000071518:Liver X Receptors; D008297:Male; D057093:Orphan Nuclear Receptors; D012334:RNA, Neoplasm / Q000235:genetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051779:Sterol Regulatory Element Binding Proteins / Q000378:metabolism*
C577836:ABCA1 protein, human; C419353:ABCG1 protein, human; D064286:ATP Binding Cassette Transporter 1; D000070998:ATP Binding Cassette Transporter, Subfamily G, Member 1; D018528:ATP-Binding Cassette Transporters; D004268:DNA-Binding Proteins; D000071518:Liver X Receptors; C000605213:NR1H3 protein, human; C000605215:Nr1h3 protein, rat; D057093:Orphan Nuclear Receptors; D012334:RNA, Neoplasm; D018160:Receptors, Cytoplasmic and Nuclear; D051779:Sterol Regulatory Element Binding Proteins; D002784:Cholesterol