Hepatic Glycosphingolipid Deficiency and Liver Function in Mice

Abstract

Recent studies have reported that glycosphingolipids (GSLs) might be involved in obesityinduced insulin resistance. Those reports suggested that inhibition of GSL biosynthesis in animals ameliorated insulin resistance accompanied by improved glycemic control and decreased liver steatosis in obese mice. In addition, pharmacologic GSL depletion altered hepatic secretory function. In those studies, ubiquitously acting inhibitors for GSL biosynthesis have been used to inhibit the enzyme Ugcg (UDP-glucose:ceramide glucosyltransferase), catalyzing the first step of the glucosylceramide-based GSL-synthesis pathway. In the present study a genetic approach for selective GSL deletion in hepatocytes was chosen to achieve complete inhibition of GSL synthesis and to avoid possible adverse effects caused by Ugcg inhibitors. Using the Cre/loxP system under control of the albumin promoter, GSL biosynthesis in hepatocytes and their release into the plasma could be effectively blocked. Deletion of GSL in hepatocytes did not change the quantity of bile excretion through the biliary duct. Total bile salt content in bile, feces, and plasma from mutant mice showed no difference as compared to control animals. Cholesterol concentration in liver, bile, feces, and plasma samples remained unaffected. Lipoprotein concentrations in plasma samples in mutant animals reached similar levels as in their control littermates. No alteration in glucose tolerance after intraperitoneal application of glucose and insulin appeared in mutant animals. A preventive effect of GSL deficiency on development of liver steatosis after a high-fat diet was not observed. Conclusion: The data suggest that GSL in hepatocytes are not essential for sterol, glucose, or lipoprotein metabolism and do not prevent high-fat diet-induced liver steatosis, indicating that Ugcg inhibitors exert their effect on hepatocytes either independently of GSL or mediated by other (liver) cell types. (HEPATOLOGY 2010;51:1799-1809 T he liver exerts a central role in metabolic events. Its location interposed between the intestinal tract and the systemic circulation enables an exocrine secretion of bile acids and cholesterol to the intestine and release of serum proteins, coagulation factors, and lipoproteins into the blood system. The excretion and transport of bile acids and cholesterol is regulated by lipid transporters located in the canalicular membrane Abbreviations: AlbCre, albumin promoted hepatocyte specific Cre-recombinase expression; ALT, alanine transferase; AMP-DNM, N-(5-adamantane-1-yl-methoxypentyl)-deoxynojirimycin; GlcCer, glucosylceramide; GM2 and GM3, glycosphingolipids are abbreviated according to the recommendations of the International

Compounds of the sphingomyelin-ceramide-glycosphingolipid pathways as secondary messenger molecules: new targets for novel therapies for fatty liver disease and insulin resistance

“…Enzyme deficiency leads to the accumulation of GC in lysosomes, which is central to disease pathogenesis involving visceral organs, bone, or brain. GSLs are endocytosed to the Golgi apparatus in normal cells, although they are mistargeted to lysosomes in GD (84,158,162).…”

“…GC modulates endolysosomal pH in lymphocytes, suggesting that this mechanism is disrupted in GD (84,158,162). Substrate reduction therapy utilizes inhibitors of GC synthase increases cholesterol, triacylglycerol, and the endolysosomal pH in macrophages.…”

“…Meanwhile partial GSL reduction is a therapeutic venue for obesity-induced insulin resistance and Type 2 diabetes. In vivo, inhibition of GSL biosynthesis in animals ameliorated insulin resistance, and this effect was accompanied by improved glycemic control and decreased liver steatosis in obese mice (84,88,154). Pharmacological GSL depletion altered hepatic secretory function, whereas SM inhibits intestinal cholesterol absorption.…”

“…In contrast, inhibitors of GSL biosynthesis via inhibition of the enzyme Ugcg (UDP-glucose: Cer glucosyltransferase), that catalyze the first step of the glucosylceramide-based GSLsynthesis pathway, did not exert a similar effect (84,88,154). Ugcg inhibitors may exert their effect on hepatocytes independently of GSLs (157,161,173).…”

Glycosphingolipids Are Essential for Intestinal Endocytic Function

“…Isolated stools were freeze-dried, and the dry weight was determined and extracted essentially as described previously (16). Lipids were quantified using a Shimadzu CS-9310PC TLC scanner (Shimadzu Europe, Duisburg, Germany) at a wavelength of 580 nm.…”