Polyunsaturated fatty acyl-coenzyme As are inhibitors of cholesterol biosynthesis in zebrafish and mice

Abstract

SUMMARYLipid disorders pose therapeutic challenges. Previously we discovered that mutation of the hepatocyte β-hydroxybutyrate transporter Slc16a6a in zebrafish causes hepatic steatosis during fasting, marked by increased hepatic triacylglycerol, but not cholesterol. This selective diversion of trapped ketogenic carbon atoms is surprising because acetate and acetoacetate can exit mitochondria and can be incorporated into both fatty acids and cholesterol in normal hepatocytes. To elucidate the mechanism of this selective diversion of carbon atoms to fatty acids, we fed wild-type and slc16a6a mutant animals high-protein ketogenic diets. We find that slc16a6a mutants have decreased activity of the rate-limiting enzyme of cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmgcr), despite increased Hmgcr protein abundance and relative incorporation of mevalonate into cholesterol. These observations suggest the presence of an endogenous Hmgcr inhibitor. We took a candidate approach to identify such inhibitors. First, we found that mutant livers accumulate multiple polyunsaturated fatty acids (PUFAs) and PUFA-CoAs, and we showed that human HMGCR is inhibited by PUFA-CoAs in vitro. Second, we injected mice with an ethyl ester of the PUFA eicosapentaenoic acid and observed an acute decrease in hepatic Hmgcr activity, without alteration in Hmgcr protein abundance. These results elucidate a mechanism for PUFA-mediated cholesterol lowering through direct inhibition of Hmgcr.

Zebrafish: An Important Tool for Liver Disease Research

“…65 Although these mutants have no overt phenotype and are viable as adults, they have increased levels of hepatic fatty acid and triglycerides when placed on a ketogenic diet. Surprisingly, however, they do not develop high levels of cholesterol, 78 which is characteristic of most other models of FLD. This observation led to the finding that polyunsaturated fatty acids accumulate when these mutants are fed a ketogenic diet and function as endogenous inhibitors of HMG-CoA reductase (Hmgcra), 78 the rate-limiting step in cholesterol biogenesis.…”

“…Surprisingly, however, they do not develop high levels of cholesterol, 78 which is characteristic of most other models of FLD. This observation led to the finding that polyunsaturated fatty acids accumulate when these mutants are fed a ketogenic diet and function as endogenous inhibitors of HMG-CoA reductase (Hmgcra), 78 the rate-limiting step in cholesterol biogenesis. This effect of polyunsaturated fatty acids on cholesterol metabolism was also observed in mice.…”

“…This effect of polyunsaturated fatty acids on cholesterol metabolism was also observed in mice. 78 Findings from zebrafish have therefore provided important insights into cholesterol metabolism, a topic with significant clinical implications.…”

Lxr-driven enterocyte lipid droplet formation delays transport of ingested lipids

“…Many dyslipidemic animal models lack Cetp, frustrating drug development ( 20 ). Similarly, we have optimized a series of serum and tissue metabolite composition assays to study lipid metabolism (21)(22)(23). Vital to this study, zebrafi sh have a single Lxr ortholog ( nr1h3 ) (24)(25)(26), which is similar to mammalian Lxra ( 25 ).…”

A genetic screen for zebrafish mutants with hepatic steatosis identifies a locus required for larval growth

“…Larvae were fixed in 2.5% paraformaldehyde and 1% glutaraldehyde in 100 mM sodium cacodylate (pH 7.4), and counterstained with OsO 4 . Electron microscopy was performed using a JEOL JEM-1400 Plus transmission electron microscope at the University of Utah Electron Microscopy Core facility (Hugo et al 2012;Karanth et al 2013;Cruz-Garcia & Schlegel, 2014).…”