Membrane binding proteins are the major determinants for the hepatocellular transmembrane flux of long-chain fatty acids bound to albumin

Liver-derived metabolites as signaling molecules in fatty liver disease

Excessive fat accumulation in the liver has become a major health threat worldwide. Unresolved fat deposition in the liver can go undetected until it develops into fatty liver disease, followed by steatohepatitis, fibrosis, cirrhosis, and eventually hepatocellular carcinoma. Lipid deposition in the liver is governed by complex communication, primarily between metabolic organs. This can be mediated by hormones, organokines, and also, as has been more recently discovered, metabolites. Although how metabolites from peripheral organs affect the liver is well documented, the effect of metabolic players released from the liver during the development of fatty liver disease or associated comorbidities needs further attention. Here we focus on interorgan crosstalk based on metabolites released from the liver and how these molecules act as signaling molecules in peripheral tissues. Due to the liver's specific role, we are covering lipid and bile mechanism-derived metabolites. We also discuss the high sucrose intake associated with uric acid release from the liver. Excessive fat deposition in the liver during fatty liver disease development reflects disrupted metabolic processes. As a response, the liver secretes a variety of signaling molecules as well as metabolites which act as a footprint of the metabolic disruption. In the coming years, the reciprocal exchange of metabolites between the liver and other metabolic organs will gain further importance and will help to better understand the development of fatty liver disease and associated diseases.

Hepatic fatty acid trafficking: multiple forks in the road

The liver plays a unique, central role in regulating lipid metabolism. In addition to influencing hepatic function and disease, changes in specific pathways of fatty acid (FA) metabolism have wide-ranging effects on the metabolism of other nutrients, extra-hepatic physiology, and the development of metabolic diseases. The high prevalence of nonalcoholic fatty liver disease (NAFLD) has led to increased efforts to characterize the underlying biology of hepatic energy metabolism and FA trafficking that leads to disease development. Recent advances have uncovered novel roles of metabolic pathways and specific enzymes in generating lipids important for cellular processes such as signal transduction and transcriptional activation. These studies have also advanced our understanding of key branch points involving FA partitioning between metabolic pathways and have identified new roles for lipid droplets in these events. This review covers recent advances in our understanding of FA trafficking and its regulation. An emphasis will be placed on branch points in these pathways and how alterations in FA trafficking contribute to NAFLD and related comorbidities.

Strategies to address low drug solubility in discovery and development

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

Regulation of liver fatty acid binding protein expression by clofibrate in hepatoma cells

Peroxisome proliferator-activated receptor (PPAR) agonists such as clofibrate are known to affect liver fatty acid binding protein (L-FABP) levels, which in turn influence hepatocellular oxidant status. The mechanism of clofibrate’s modulation of L-FABP levels is not clear. In this study we used clofibrate (PPARα agonist), MK886 (PPARα antagonist), and GW9662 (PPARγ antagonist) in determining the regulating mechanism of L-FABP expression and its antioxidant activity in CRL-1548 hepatoma cells. Antioxidant activity was assessed by determining intracellular reactive oxygen species (ROS) using dichlorofluorescein (DCF) fluorescence. The effect of clofibrate on cytosolic activity of the intracellular antioxidant enzymes was also assessed. RT-PCR and mRNA stability assay showed that clofibrate treatment enhanced L-FABP mRNA stability, which resulted in increased L-FABP levels. A nuclear run-off assay and RT-PCR measurements of L-FABP mRNA revealed that clofibrate increased the L-FABP gene transcription rate. The increased L-FABP was associated with reduced cytosolic ROS. Levels of superoxide dismutase, glutathione peroxidase, and catalase were not affected by clofibrate treatment. L-FABP siRNA knockdown studies showed that a reduction in L-FABP expression was associated with increased DCF fluorescence. We conclude that clofibrate enhanced L-FABP gene transcription and mRNA stability, thus affecting L-FABP expression and ultimately cellular antioxidant activity.

Transmembrane diffusion of gemcitabine by a nanoparticulate squalenoyl prodrug: an original drug delivery pathway

We have designed an amphiphilic prodrug of gemcitabine (dFdC) by its covalent coupling to a derivative of squalene, a natural lipid. The resulting bioconjugate self-assembled spontaneously in water as nanoparticles that displayed a promising in vivo anticancer activity. The aim of the present study was to provide further insight into the in vitro subcellular localization and on the metabolization pathway of the prodrug. Cells treated with radiolabelled squalenoyl gemcitabine (SQdFdC) were studied by differential detergent permeation, and microautography coupled to fluorescent immunolabeling and confocal microscopy. This revealed that the bioconjugate accumulated within cellular membranes, especially in those of the endoplasmic reticulum. Radio-chromatography analysis proved that SQdFdC delivered dFdC directly in the cell cytoplasm. Mass spectrometry studies confirmed that gemcitabine was then either converted into its biologically active triphosphate metabolite or exported from the cells through membrane transporters. To our knowledge, this is the first description of such an intracellular drug delivery pathway. In vitro cytotoxicity assays revealed that SQdFdC was more active than dFdC on a transporter-deficient human resistant leukemia model, which was explained by the subcellular distribution of the drugs and their metabolites. The squalenoylation drug delivery strategy might, therefore, dramatically improve the efficacy of gemcitabine on transporter-deficient resistant cancer in the clinical context.

Dynamic expression of L-FABP and PPAR-α mRNAs in nonalcoholic fatty liver disease in rats

AIM: To establish a rat model of nonalcoholic fatty liver disease (NAFLD) to detect the changes in the expression of liver fatty acid-binding protein (L-FABP) and peroxisome proliferator-activated receptor-α (PPAR-α) mRNAs in NAFLD and to explore the mechanism underlying the pathogenesis of the disease.

METHODS: Eighty-four male Wistar rats were randomly and equally divided into two groups: control group (fed a normal diet for 18 weeks) and model group (fed a high-fat diet for 12 wk and a normal diet for another 6 wk). Each group was further divided into 7 subgroups for testing at weeks 0, 2, 4, 8, 12, 16 and 18. Rats in the two groups were sacrificed at each time point. Blood samples were taken to measure serum ALT, TG, CHOL, HDL-C and LDL-C. Liver samples were taken for HE staining and for detection of the expression of L-FABP and PPAR-α mRNAs by real-time fluorescence quantitative RT-PCR.

RESULTS: The expression of L-FABP and PPAR-α mRNAs in control rats showed no obvious changes. No steatosis was observed at week 2 in rats fed a high-fat diet. The expression of L-FABP mRNA increased obviously in rats fed a high-fat diet at week 4 (0.59 ± 0.06 vs 0.52 ± 0.03, P < 0.05), reaching the peak at weeks 8 and 12 (0.91 ± 0.07 and 0.92 ± 0.08 vs 0.52 ± 0.03, respectively; both P < 0.01). At week 18, the expression level of L-FABP mRNA declined significantly (0.59 ± 0.04 vs 0.92 ± 0.08, P < 0.01) but was still higher than that in the control group (P < 0.05). The expression of PPAR-α mRNA decreased obviously in rats fed a high-fat diet at week 4 (1.05 ± 0.09 vs 1.13 ± 0.07, P < 0.05), reaching the lowest level at weeks 8 and 12 (0.89 ± 0.04 and 0.85 ± 0.07 vs 1.13 ± 0.07, respectively; both P < 0.01). At week 18, the expression level of PPAR-α mRNA was elevated obviously (1.04 ± 0.07 vs 0.85 ± 0.07, P < 0.01) but was still lower than that in the control group. Steatosis become worst at week 12 but was improved greatly at week 18.

CONCLUSION: In the progression of rat NAFLD, PPAR-α mRNA expression decreases and L-FABP mRNA expression increases as steatosis becomes worse. Steatosis can be improved through diet.

Liver-type fatty acid-binding protein and lipid transport

Liver-type fatty acid-binding protein (L-FABP), a member of the genetically related cytosolic fatty acid binding protein (FABP) family, has both similar and different function and conformation compared with other family members. L-FABP, which is mainly found in liver and small intestine, participates in transporting fatty acids and combining a variety of hydrophobic group and is associated with many diseases. In recent years, the transfer mechanism of L-FABP was attracting great attention, and the research methods were developed from in vitro to in vivo, and from cell molecular level to gene ablation animals. Although a part of the mechanism has been revealed, the research should be continued to demonstrate it in deepth and resolve some new questions. This article aims to review the characteristics, conformation and in vivo study status of L-FABP.

Lipid-based delivery systems and intestinal lymphatic drug transport: a mechanistic update

After oral administration, the majority of drug molecules are absorbed across the small intestine and enter the systemic circulation via the portal vein and the liver. For some highly lipophilic drugs (typically log P > 5, lipid solubility > 50 mg/g), however, association with lymph lipoproteins in the enterocyte leads to transport to the systemic circulation via the intestinal lymph. The attendant delivery benefits associated with lymphatic drug transport include a reduction in first-pass metabolism and lymphatic exposure to drug concentrations orders of magnitude higher than that attained in systemic blood. In the current review we briefly describe the mechanisms by which drug molecules access the lymph and the formulation strategies that may be utilised to enhance lymphatic drug transport. Specific focus is directed toward recent advances in understanding regarding the impact of lipid source (both endogenous and exogenous) and intracellular lipid trafficking pathways on lymphatic drug transport and enterocyte-based first-pass metabolism.

Diet-induced obesity and hepatic steatosis in L-Fabp / mice is abrogated with SF, but not PUFA, feeding and attenuated after cholesterol supplementation

Liver fatty acid (FA)-binding protein (L-Fabp), a cytoplasmic protein expressed in liver and small intestine, regulates FA trafficking in vitro and plays an important role in diet-induced obesity. We observed that L-Fabp(-/-) mice are protected against Western diet-induced obesity and hepatic steatosis. These findings are in conflict, however, with another report of exaggerated obesity and increased hepatic steatosis in female L-Fabp(-/-) mice fed a cholesterol-supplemented diet. To resolve this apparent paradox, we fed female L-Fabp(-/-) mice two different cholesterol-supplemented low-fat diets and discovered (on both diets) lower body weight in L-Fabp(-/-) mice than in congenic wild-type C57BL/6J controls and similar or reduced hepatic triglyceride content. We extended these comparisons to mice fed low-cholesterol, high-fat diets. Female L-Fabp(-/-) mice fed a high-saturated fat (SF) diet were dramatically protected against obesity and hepatic steatosis, whereas weight gain and hepatic lipid content were indistinguishable between mice fed a high-polyunsaturated FA (PUFA) diet and control mice. These findings demonstrate that L-Fabp functions as a metabolic sensor with a distinct hierarchy of FA sensitivity. We further conclude that cholesterol supplementation does not induce an obesity phenotype in L-Fabp(-/-) mice, nor does it play a significant role in the protection against Western diet-induced obesity in this background.