Effect of ursodeoxycholic acid on hepatic LDL binding and uptake in dietary hypercholesterolemic hamsters
Introduction
The low density lipoprotein (LDL) receptor plays a central role in the regulation of cholesterol homeostasis, while the liver is a key organ in the maintenance of whole body cholesterol balance [1], [2]. In a number of species, including the hamster, most of receptor-mediated clearance of LDL occurs in the liver [3], [4], [5] by rapid internalization of the complex formed by the binding of the apolipoprotein-B100-containing particle to the LDL receptor [6]. Once in the cell, the LDL particle dissociates, and the receptor generally is cycled back to the cell surface membrane [7].
The activity of the LDL receptor is tightly regulated by changes in cholesterol supply and demand. Hepatic LDL receptor synthesis and cycling can be markedly suppressed by dietary cholesterol [8], [9]. Conversely, administration of both bile acid binding resins, which augment hepatocellular bile acid synthesis, and cholesterol synthesis inhibitors, increase the number of LDL receptors, by increasing the cellular cholesterol demand [10], [11]. The consequent changes in cellular cholesterol pools modulate the production of LDL receptor mRNA and the synthesis of the LDL receptor.
It has previously been shown in vivo, in the golden Syrian hamster, that chronic feeding of ursodeoxycholic acid (UDCA), in contrast to that of its 7α-hydroxyl epimer, chenodeoxycholic acid (CDCA), evoked a significant increment in hepatic LDL uptake. However, this occurred in spite of both a 55–71% enrichment of the bile acid pool with CDCA and a marked suppression of bile acid synthesis [12]. This increased LDL receptor-dependent LDL uptake and degradation in vitro have also been documented, upon acute exposure to UDCA, in hepatocytes isolated from standard rodent chow-fed hamsters [13], [14]. Furthermore, recent studies from our laboratory have demonstrated that UDCA interacts directly with the LDL receptor, in the absence of any effects on either the LDL particle or on the membrane lipid composition, and independently of any effects on LDL receptor synthesis or cycling [13], [15]. Thus, the ability of UDCA to directly augment LDL receptor-dependent LDL uptake is independent of changes in cellular cholesterol pools.
Administration of UDCA has been shown to decrease serum total and LDL cholesterol in hypercholesterolemic patients with primary biliary cirrhosis [16]. In light of results of previous studies, it was postulated that the cholesterol-lowering effect of UDCA may be due, in part, to a direct increment in LDL receptor binding [13], [15]. The present investigation was undertaken in order to determine the ability of UDCA to enhance hepatocellular LDL receptor recruitment, as determined by its effect both in vivo on LDL uptake, and in vitro on LDL binding, under conditions of moderately elevated serum cholesterol concentrations [17]. This was achieved by feeding male golden Syrian hamsters an excess of cholesterol in order to induce mild hypercholesterolemia. Under these conditions, the effects of UDCA on parameters of hepatic LDL metabolism were studied both in vivo and in vitro.
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Materials
Sodium 125I (specific activity 16–20 mCi/μg) and [U-14C]sucrose (specific activity 380 μCi/μmol) were purchased from Amersham Corporation (Arlington Heights, IL). Gelatin and bovine serum albumin (fraction V) were purchased from Sigma (St. Louis, MO). UDCA was supplied by Tokyo Tanabe (Tokyo, Japan), and CDCA by Dr Falk GmbH (Freiburg, Germany). Both UDCA and CDCA were 98–99% pure, as judged by gas–liquid chromatography. All other chemicals used were of analytical grade available from
Serum lipid concentrations
After 3 weeks of feeding a 0.15% cholesterol-supplemented diet, total serum cholesterol concentrations increased by 44%, (Table 1). The percentage of serum cholesterol associated to HDL particles was significantly decreased from 48 to 44% of the total serum cholesterol in the hypercholesterolemic animals.
In those hamsters fed a 0.15% cholesterol-supplemented diet, CDCA supplementation induced a significant decrease in the percentage of serum HDL cholesterol, and a consequent increase in the
Discussion
Results of the present study in dietary hypercholesterolemic hamsters indicate that both chronic, in-vivo and acute, in-vitro treatments with UDCA were able to restore the respective hepatic LDL binding and uptake to the levels observed in normocholesterolemic hamsters. Furthermore, the administration of UDCA in conjunction with a high cholesterol diet suppressed the biotransformation of UDCA to CDCA. This possibly removed some of the repressive effects of CDCA on hepatic LDL uptake, namely, an
Acknowledgments
The authors dedicate this work to Mauro Malavolti. His friendship and collaboration will be greatly missed. The authors are grateful to Joan Fasulo for assistance in the studies of hepatocellular lipid determinations. The studies were supported, in part, by grants from the American Heart Association Nation's Capital Affiliate, Inc. (B.B.), and the General Medical Research Service of the Veterans Administration (S.J.R.). B.B. is supported, in part, by NIH/NIDDK DK-46954. These studies were
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