Increased cholesterol biosynthesis and hypercholesterolemia in mice overexpressing squalene synthase in the liver

Abstract

Squalene synthase (SS) is the first committed enzyme for cholesterol biosynthesis, located at a branch point in the mevalonate pathway. To examine the role of SS in the overall cholesterol metabolism, we transiently overexpressed mouse SS in the livers of mice using adenovirusmediated gene transfer. Overexpression of SS increased de novo cholesterol biosynthesis with increased 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase activity, in spite of the downregulation of its own mRNA expression. Furthermore, overexpression of SS increased plasma concentrations of LDL, irrespective of the presence of functional LDL receptor (LDLR). Thus, the hypercholesterolemia is primarily caused by increased hepatic production of cholesterol-rich VLDL, as demonstrated by the increases in plasma cholesterol levels after intravenous injection of Triton WR1339. mRNA expression of LDLR was decreased, suggesting that defective LDL clearance contributed to the development of hypercholesterolemia. Curiously, the liver was enlarged, with a larger number of Ki-67-positive cells. These results demonstrate that transient upregulation of SS stimulates cholesterol biosynthesis as well as lipoprotein production, providing the first in vivo evidence that SS plays a regulatory role in cholesterol metabolism through modulation of HMG-CoA reductase activity and cholesterol biosynthesis. Cholesterol biosynthesis is subject to tight regulation by a multivalent feedback mechanism at both the transcriptional and the posttranscriptional level (1). The transcriptional regulation is mediated through the action of sterol-regulatory element binding proteins (SREBPs), membrane-bound transcription factors that enhance transcription of genes encoding cholesterol biosynthetic enzymes and the LDL receptor (LDLR) (2). The translational regulation of 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase (EC1.1.1.34), the rate-limiting enzyme in cholesterol biosynthesis, is mediated by nonsterol mevalonate-derived isoprenoids, which act by an undefined mechanism (3). Its degradation is regulated by both sterols and nonsterol end products of mevalonate metabolism (4, 5). The sterolregulated degradation of HMG-CoA reductase is mediated Abbreviations: Ad-SS, recombinant adenovirus carrying SS cDNA under the control of cytomegalovirus promoter; apoB, apolipoprotein B; E, embryonic day; ER, endoplasmic reticulum; LDLR, LDL receptor; m.o.i., multiplicity of infection; SREBP, sterol-regulatory element binding protein; SS, squalene synthase; TC total cholesterol; TG, triglyceride; TUNEL, terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling.

K45R variant of squalene synthase increases total cholesterol levels in two study samples from a French Canadian population

“…This is the first report of a common variant in a gene involved in the cholesterol biosynthetic pathway influencing TC levels. Our results, combined with the fact that overexpression of squalene synthase confers hypercholesterolemic properties [Okazaki et al, 2006] …”

“…Studies in mice have shown that the homozygous knockout of squalene synthase results in embryonic lethality, while the heterozygous mice had phenotypically normal plasma levels of total cholesterol (TC), TG, very low-density lipoprotein cholesterol (VLDL-C), LDL-C, and HDL-C [Tozawa et al, 1999]. However, overexpression of squalene synthase in mouse liver increased plasma TC, LDL-C, and HDL-C levels [Okazaki et al, 2006].…”

Plasma cholesterol-lowering and transient liver dysfunction in mice lacking squalene synthase in the liver

“…Reduced VLDL production and subsequent plasma lipid lowering have also been reported in rats ( 29 ) or Watanabe heritable hyperlipidemic rabbits ( 30 ) treated with an SS inhibitor, TAK-475. Conversely, the mice overexpressing SS in the liver showed an opposite phenotype in terms of plasma lipoprotein metabolism: they were hyperlipidemic due to an increase in hepatic VLDL production ( 31 ).…”

Growth Performance, Organ Weights and Some Blood Parameters of Replacement Laying Pullets Fed with Increasing Levels of Wheat Bran

“…As previously explained, the unsaturated lipids present in soybean oil may reduce the rate of intestinal transit (Aguilar et al, 2011), and therefore, may have counteracted the effect of fiber in the ceca. Moreover, because the crude fat intake was not excessive in the present experiment, liver relative weight and function were not affected (Okazaki et al, 2006). According to Connor et al (1969), lipid digestion and transport to the liver in poultry greatly differs from mammals, especially cholesterol, which is stored in the liver of newly-hatched chicks, whereas it is stored in brain of newly-born mammals.…”