Whereas the role of liver fatty acid-binding protein (L-FABP) in the uptake, transport, mitochondrial oxidation, and esterification of normal straight-chain fatty acids has been studied extensively, almost nothing is known regarding the function of L-FABP in peroxisomal oxidation and metabolism of branched-chain fatty acids. Therefore, phytanic acid (most common dietary branched-chain fatty acid) was chosen to address these issues in cultured primary hepatocytes isolated from livers of L-FABP gene-ablated (؊/؊) and wild type (؉/؉) mice. These studies provided three new insights: First, L-FABP gene ablation reduced maximal, but not initial, uptake of phytanic acid 3.2-fold. Initial uptake of phytanic acid uptake was unaltered apparently due to concomitant 5. , 7), specific effects of L-FABP gene ablation on liver uptake of straight-chain fatty acid and liver lipid distribution are complex, apparently depending on feeding status, gender, and age of the mice (6 -9).Although relatively little is known about the role of L-FABP in the oxidation of straight-chain fatty acids, which occurs primarily in mitochondria (reviewed in Refs. 1-3), recent studies with L-FABP gene-ablated mice suggest that L-FABP may affect liver oxidation of straight-chain fatty acids under high fatty acid load. Under fed conditions, serum free fatty acid levels were found to be low, and ␤-hydroxybutyrate levels were unaltered in L-FABP (Ϫ/Ϫ) male or female mice, suggesting that L-FABP may not play a role in fatty acid oxidation (8, 9). However, under starvation conditions, serum fatty acid levels were highly elevated, and serum ␤-hydroxybutyrate levels were reduced. These findings led to the conclusion that under fasting conditions L-FABP gene ablation reduces fatty acid oxidation (8, 9). However, other in vitro studies measuring fatty acid oxidation and ␤-hydroxybutyrate production in liver homogenates showed that L-FABP gene ablation had no effect on oxidation of straight-chain, radiolabeled palmitic acid at high levels (1 mM) (9). In contrast, when fatty acid oxidation and ␤-hydroxybutyrate production were measured with hepatocyte suspensions freshly isolated from female mice, L-FABP gene ablation reduced oxidation of high levels (1 mM) of straight-chain palmitic acid by about 30% (9). Whereas the above results appear contradictory, the intact hepatocyte and in vivo data suggest that L-FABP gene ablation does not affect oxidization of straight-chain fatty acids under normal fed conditions when serum fatty acid levels are low but may do so when serum fatty acids are high as in starvation.In contrast to the above studies with straight-chain fatty acids, almost nothing is known regarding potential roles of L-FABP in the uptake, oxidation, and esterification of branched-chain fatty acids. The most common dietary branched-chain fatty acid, phytanic acid, is produced by ruminants in the gut by bacterial cleavage of the side chain of chlorophyll to yield phytol, followed by conversion to phytanic acid (reviewed in Ref. 10). Consequently, levels of...

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The effect of liver type fatty acid binding protein (L-FABP) gene ablation on the uptake and distribution of long chain fatty acids (LCFA) to the nucleus by real-time laser scanning confocal imaging and peroxisome proliferator activated receptor-α (PPARα) activity was examined in cultured primary hepatocytes from livers wild-type L-FABP+/+ and gene ablated L-FABP−/− mice. Cultured primary hepatocytes from livers of L-FABP−/− mice exhibited: (i) reduced oxidation of palmitic acid, a common dietary long chain fatty acid (LCFA); (ii) reduced expression of fatty acid oxidative enzymes-proteins transcriptionally regulated by PPARα; (iii) reduced palmitic acid-induced PPARα coimmunoprecipitation with coactivator SRC1 concomitant with increased PPARα coimmunoprecipitation with coinhibitor N-CoR; (iv) reduced palmitic acid-induced PPARα. Diminished PPARα activation in L-FABP null hepatocytes was associated with lower uptake of common dietary LCFA (palmitic acid as well as its fluorescent derivative BODIPY FL C 16 ), reduced level of total unesterified LCFA, and real-time redistribution of BODIPY FL C 16 from the central nucleoplasm to the nuclear envelope. Taken together, these studies support the hypothesis that L-FABP may facilitate ligand (LCFA)-activated PPARα transcriptional activity at least in part by increasing total LCFA ligand available to PPARα for inducing PPARα-mediated transcription of proteins involved in LCFA metabolism.

High levels of saturated, branched-chain fatty acids are deleterious to cells and animals, resulting in lipid accumulation and cytotoxicity. Although fatty acid binding proteins (FABPs) are thought to be protective, this hypothesis has not previously been examined. Phytanic acid (branched chain, 16-carbon backbone) induced lipid accumulation in L cell fibroblasts similar to that observed with palmitic acid (unbranched, C16): triacylglycerol ≫ free fatty acid > cholesterol > cholesteryl ester ≫ phospholipid. Although expression of sterol carrier protein (SCP)-2, SCP-x, or liver FABP (L-FABP) in transfected L cells reduced [3H]phytanic acid uptake (57–87%) and lipid accumulation (21–27%), nevertheless [3H]phytanic acid oxidation was inhibited (74–100%) and phytanic acid toxicity was enhanced in the order L-FABP ≫ SCP-x > SCP-2. These effects differed markedly from those of [3H]palmitic acid, whose uptake, oxidation, and induction of lipid accumulation were not reduced by L-FABP, SCP-2, or SCP-x expression. Furthermore, these proteins did not enhance the cytotoxicity of palmitic acid. In summary, intracellular FABPs reduce lipid accumulation induced by high levels of branched-chain but not straight-chain saturated fatty acids. These beneficial effects were offset by inhibition of branched-chain fatty acid oxidation that correlated with the enhanced toxicity of high levels of branched-chain fatty acid.

Although in vitro studies suggest a role for sterol carrier protein-2 (SCP-2) in cholesterol trafficking and metabolism, the physiological significance of these observations remains unclear. This issue was addressed by examining the response of mice overexpressing physiologically relevant levels of SCP-2 to a cholesterol-rich diet. While neither SCP-2 overexpression nor cholesterol-rich diet altered food consumption, increased weight gain, hepatic lipid, and bile acid accumulation were observed in wild-type mice fed the cholesterol-rich diet. SCP-2 overexpression further exacerbated hepatic lipid accumulation in cholesterol-fed females (cholesterol/cholesteryl esters) and males (cholesterol/ cholesteryl esters and triacyglycerol). Primarily in female mice, hepatic cholesterol accumulation induced by SCP-2 overexpression was associated with increased levels of LDLreceptor, HDL-receptor scavenger receptor-B1 (SR-B1) (as well as PDZK1 and/or membrane-associated protein 17 kDa), SCP-2, liver fatty acid binding protein (L-FABP), and 3a-hydroxysteroid dehydrogenase, without alteration of other proteins involved in cholesterol uptake (caveolin), esterification (ACAT2), efflux (ATP binding cassette A-1 receptor, ABCG5/8, and apolipoprotein A1), or oxidation/ transport of bile salts (cholesterol 7a-hydroxylase, sterol 27a-hydroxylase, Na 1 /taurocholate cotransporter, Oatp1a1, and Oatp1a4). The effects of SCP-2 overexpression and cholesterol-rich diet was downregulation of proteins involved in cholesterol transport (L-FABP and SR-B1), cholesterol synthesis (related to sterol regulatory element binding protein 2 and HMG-CoA reductase), and bile acid oxidation/ transport (via Oapt1a1, Oatp1a4, and SCP-x). Levels of serum and hepatic bile acids were decreased in cholesterol-fed SCP-2 overexpression mice, especially in females, while the total bile acid pool was minimally affected. Taken together, these findings support an important role for SCP-2 in hepatic cholesterol homeostasis. Due to the deleterious effects associated with cholesterol accumulation leading to atherosclerosis, levels of cholesterol in cells and tissues are carefully regulated (1, 2). Cholesterol is derived from both diet and endogenous synthesis, and in order to maintain cholesterol homeostasis, human liver excretes nearly 2 g of cholesterol per day into bile (3). Decreased cholesterol disposal results in hepatic cholesterol accumulation and elevated blood cholesterol, while excessive cholesterol secretion or disproportionate biliary constituents may lead to cholelithiasis and inflammatory gallbladder disease (3, 4). Cholesterol is removed from peripheral tissues to the liver for elimination by oxidation and/or biliary excretion via a process termed reverse cholesterol transport (RCT). Recent novel experiments have elucidated many molecular details of the RCT pathway, including those involving scavenger receptor-B1 (SR-B1)-mediated uptake of HDL-cholesteryl esters and ABC transporter- Abbreviations: 3a-HSD, 3a-hydroxysteroid dehydrogenase; ABCA-...