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

Abstract

Mammals have developed sophisticated and complex systems to maintain cellular content of cholesterol, an essential component of cellular membranes and a precursor of bile acids and steroid hormones ( 1 ). In addition to dietary intake, cholesterol is supplied by de novo synthesis from acetate. Squalene synthase (SS; farnesyl-diphosphate farnesyltransferase, EC2.5.1.21) catalyzes the reductive head-to-head condensation of two molecules of farnesyl diphosphate (FPP) to form squalene, the fi rst committed intermediate in the cholesterol biosynthetic pathway ( 2, 3 ). SS contains ‫ف‬ 416 amino acids and is anchored to endoplasmic reticulum by a short C-terminal membrane-spanning domain, with its large N-terminal catalytic domain facing the cytosol, where water-soluble FPP and NADPH are present. Hepatic SS is highly regulated at the transcriptional level, not only by cellular cholesterol content ( 4 ) but also by proinfl ammatory cytokines: TNF-␣ and interleukin 1 ␤ ( 5 ). This enzyme has been an attractive target for cholesterol-lowering therapy because the inhibition of this step theoretically may not perturb the nonsterol pathway, which is a potential problem in the use of statins, inhibitors of HMG-CoA reductase Abstract Squalene synthase (SS) catalyzes the biosynthesis of squalene, the fi rst specifi c intermediate in the cholesterol biosynthetic pathway. To test the feasibility of lowering plasma cholesterol by inhibiting hepatic SS, we generated mice in which SS is specifi cally knocked out in the liver (L-SSKO) using Cre-loxP technology. Hepatic SS activity of L-SSKO mice was reduced by >90%. In addition, cholesterol biosynthesis in the liver slices was almost eliminated. Although the hepatic squalene contents were markedly reduced in L-SSKO mice, the hepatic contents of cholesterol and its precursors distal to squalene were indistinguishable from those of control mice, indicating the presence of suffi cient centripetal fl ow of cholesterol and/or its precursors from the extrahepatic tissues. L-SSKO mice showed a transient liver dysfunction with moderate hepatomegaly presumably secondary to increased farnesol production. In a fed state, the plasma total cholesterol and triglyceride were signifi cantly reduced in L-SSKO mice, primarily owing to reduced hepatic VLDL secretion. In a fasted state, the hypolipidemic effect was lost. mRNA expression of liver X receptor ␣ target genes was reduced, while that of sterol-regulatory element binding protein 2 target genes was increased. In conclusion, liver-specifi c ablation of SS inhibits hepatic cholesterol biosynthesis and induces hypolipidemia without increasing signifi cant mortality. -Nagashima, S

Transcriptional Regulation of Cytosolic Sulfotransferase 1C2 by Intermediates of the Cholesterol Biosynthetic Pathway in Primary Cultured Rat Hepatocytes

“…Mice deficient in hepatic squalene synthase were also found to have reduced LXR signaling in the liver (Nagashima et al, 2015), potentially due to decreased oxysterol synthesis as well as the antagonistic actions of the isoprenoid geranylgeranyl pyrophosphate on LXR activity (Forman et al, 1997). However, inhibition of basal LXR activity is unlikely to account for the SQ1-mediated SULT1C2 induction that was observed in this study because both Prav and NB-598 treatments would also reduce endogenous oxysterol levels but had differing effects on SULT1C2 expression.…”

Differential Regulation of Gene Expression by Cholesterol Biosynthesis Inhibitors That Reduce (Pravastatin) or Enhance (Squalestatin 1) Nonsterol Isoprenoid Levels in Primary Cultured Mouse and Rat Hepatocytes

“…Farnesol has been reported to induce apoptosis and cell cycle arrest in a number of different cell lines, with cancer cells more sensitive to the effects of farnesol (Wiseman et al, 2007;Joo and Jetten, 2010). Although SSIs had good hepatic safety in preclinical trials , Nagashima et al (2015) recently described a transient liver dysfunction and mild hepatomegaly in hepatic-specific squalene synthase knockout mice (Nagashima et al, 2015). This was associated with elevated serum alanine aminotransferase levels and an increase in apoptotic and proliferative markers at a time point that coincided with higher farnesol production.…”

“…Additional studies are needed to address the functional role(s) of identified gene expression changes on hepatocellular physiology. Notwithstanding these limitations, as there is continued interest in SSIs and the isoprenoid pathway (Goto et al, 2011a;Ichikawa et al, 2013;Nagashima et al, 2015;Saito et al, 2015), our findings provide a contextual framework for more mechanistic studies. Fig.…”

“…Although the SSI lapaquistat (TAK-475) showed promising effects on serum lipid profiles in primates , its development was terminated during phase III clinical trials because of safety concerns and the lack of commercial viability at lower doses (Stein et al, 2011). Nonetheless, there is still significant interest in SSIs and the isoprenoid pathway with respect to hepatic lipid metabolism (Goto et al, 2011a;Nagashima et al, 2015) and as a potential therapeutic target for a variety of other conditions (Shang et al, 2014;Yang et al, 2014;Saito et al, 2015;Healey et al, 2016).…”

Estudio del mecanismo de acción de distintos quimiotipos de <i>Lippia alba</i> sobre el metabolismo lipídico y la proliferación celular

“…Con respecto a la síntesis de ácidos grasos y de lípidos saponificables, la presencia de Además, cabe destacar que aunque el inhibidor de la escualeno sintasa (SSI) lapaquistat (TAK-475) mostró efectos prometedores en los perfiles de lípidos en suero en los primates (500), su desarrollo se dio por terminado durante la fase III de ensayos clínicos fundamentalmente debido a hepatotoxicidad asociada a su administración y la falta de viabilidad comercial en dosis más bajas (165). Sin embargo, recientemente se ha propuesto que, si una acumulación de farnesol es el factor causal de la hepatotoxicidad de los SSIs, la coadministración con las estatinas podría aliviar la toxicidad en el hígado mediante la reducción de la síntesis de FPP y resultar una estrategia novedosa para el tratamiento de dislipidemias y la prevención de aterosclerosis (501).…”