Rachel Hertz

Fatty acyl-CoA thioesters are ligands of hepatic nuclear factor-4α

Dietary fatty acids specifically modulate the onset and progression of various diseases, including cancer,, atherogenesis, hyperlipidaemia, insulin resistance and hypertension, as well as blood coagulability and fibrinolytic defects; their effects depend on their chain length and degree of saturation. Hepatocyte nuclear factor-4α (ref. 8) (HNF-4α) is an orphan transcription factor of the superfamily of nuclear receptors and controls the expression of genes (reviewed in ref. 9) that govern the pathogenesis and course of some of these diseases. Here we show that long-chain fatty acids directly modulate the transcriptional activity of HNF-4α by binding as their acyl-CoA thioesters to the ligand-binding domain of HNF-4α. This binding may shift the oligomeric–dimeric equilibrium of HNF-4α or may modulate the affinity of HNF-4α for its cognate promoter element, resulting in either activation or inhibition of HNF-4α transcriptional activity as a function of chain length and the degree of saturation of the fatty acyl-CoA ligands. In addition to their roles as substrates to yield energy, as an energy store, or as constituents of membrane phospholipids, dietary fatty acids may affect the course of a disease by modulating the expression of HNF-4α-controlled genes.

Kinase-independent transcriptional co-activation of peroxisome proliferator-activated receptor α by AMP-activated protein kinase

AMPK (AMP-activated protein kinase) responds to intracellular ATP depletion, while PPARalpha (peroxisome proliferator-activated receptor alpha) induces the expression of genes coding for enzymes and proteins involved in increasing cellular ATP yields. PPARalpha-mediated transcription is shown here to be co-activated by the alpha subunit of AMPK, as well as by kinase-deficient (Thr172Ala) and kinase-less (Asp157Ala, Asp139Ala) mutants of AMPKalpha. The Ser452Ala mutant of mPPARalpha mutated in its putative consensus AMPKalpha phosphorylation site is similarly co-activated by AMPKalpha. AMPKalpha or its kinase-less mutants bind to PPARalpha; binding is increased by MgATP, to a lesser extent by MgADP, but not at all by AMP or ZMP [AICAR (5-aminoimidazole-4-carboxamide ribonucleoside) monophosphate]. ATP-activated binding of AMPKalpha to PPARalpha is mediated primarily by the C-terminal regulatory domain of AMPKalpha. PPARalpha co-activation by AMPKalpha may, however, require its secondary interaction with the N-terminal catalytic domain of AMPKalpha, independently of its kinase activity. While AMPK catalytic activity is activated by AICAR, PPARalpha co-activation and PPARalpha-controlled transcription are robustly inhibited by AICAR, with concomitant translocation of nuclear AMPKalpha or its kinase-less mutants to the cytosol. In conclusion, AMPKalpha, independently of its kinase activity, co-activates PPARalpha both in primary rat hepatocytes and in PPARalpha-transfected cells. The kinase and transcriptional co-activation modes of AMPKalpha are both regulated by the cellular ATP/AMP ratio. Co-activation of PPARalpha by AMPKalpha may transcriptionally complement AMPK in maintaining cellular ATP status.

The effect of bezafibrate and long-chain fatty acids on peroxisomal activities in cultured rat hepatocytes

Peroxisomal activities have been evaluated in cultured rat hepatocytes in the presence of bezafibrate or long-chain fatty acids added to the culture medium. All activities decreased continuously over a time period of 100 h in culture but selected activities were relatively increased as a function of the added effectors. This relative increase in peroxisomal activities was dose-dependent, discernible within the first 24 h in culture and consisted of activities related specifically to peroxisomal fatty acyl beta-oxidation, e.g., cyanide-insensitive palmitoyl-CoA oxidation, H2O2-forming palmitoyl-CoA oxidase and heat-labile enoyl-CoA hydratase. Peroxisomal catalase or mitochondrial fatty acyl beta-oxidation (cyanide-sensitive) remained relatively unchanged. The relative increase in peroxisomal activities was accompanied by a respective increase in the number of peroxisomes as well as in thymidine incorporation rate.

The relations between the composition of liposomes and their interaction with triton X-100

Abstract The interaction of Triton X-100 with phospholipid bilayers of multilamellar liposomes formed from mixtures of spinal cord sphingomyelin and egg yolk lecithin in various mole ratios (all containing 1 mole of dicetylphosphate per 10 moles of phospholipid) was studied. The results indicate that the process is time dependent and is much slower than the formation of simple micelles. The time to reach the final equilibrium state is dependent on the SPM to PC mole ratio, on the Triton to phospholipid mole ratio, and on the Tirton concentration. Titration with increasing Triton concentration shows that the behavior of Triton is biphasic for all the various lipid compositions tested. For low Triton to phospholipid mole ratio there is no mass formation of mixed micelles; in addition, the Triton seems to radically increase the leakage of glucose without reducing the turbidity. This range is limited by a turning point where most of the phospholipids and about half of the Triton coprecipitate. Above this Triton to phospholipid mole ratio formation of mixed Triton-phospholipid micelles occurred followed by a drastic decline in turbidity. This turning point as well as the exact profile of the Triton effect are strongly related to the SPM:PC mole ratio. The higher the mole fraction of SPM in the membrane, the less Triton is required to reach the turning point and to cause a complete solubilization. These effects can be explained by tighter packing and stronger phospholipid-phospholipid interactions imposed by SPM and expressed as apparent microviscosity which increases upon increasing the mole fraction of SPM in the bilayer.

AMPK activation by long chain fatty acyl analogs

The antidiabetic efficacy of first-line insulin sensitizers (e.g., metformin, glitazones) is accounted for by activation of AMP-activated protein kinase (AMPK). Long chain fatty acids (LCFA) activate AMPK, but their putative antidiabetic efficacy is masked by their beta-oxidized or esterified lipid products. Substituted alpha,omega-dicarboxylic acids of 14-18 carbon atoms in length (MEDICA analogs) are not metabolized beyond their acyl-CoA thioesters, and may therefore simulate AMPK activation by LCFA while avoiding LCFA turnover into beta-oxidized or esterified lipid products. MEDICA analogs are shown here to activate AMPK and some of its downstream targets in vivo, in cultured cells and in a cell-free system consisting of the (alpha(1)beta(1)gamma(1))AMPK recombinant and LKB1-MO25-STRAD (AMPK-kinase) recombinant proteins. AMPK activation by MEDICA is accompanied by normalizing the hyperglycemia-hyperinsulinemia of diabetic db/db mice in vivo with suppression of hepatic glucose production in cultured liver cells. Activation of AMPK by MEDICA or LCFA is accounted for by (a) decreased intracellular ATP/AMP ratio and energy charge by the free acid, (b) activation of LKB1 phosphorylation of AMPK(Thr172) by the acyl-CoA thioester. The two activation modes are complementary since LKB1/AMPK activation by the CoA-thioester is fully evident under conditions of excess AMP. MEDICA analogs may expand the arsenal of AMPK activators used for treating diabetes type 2.

Solubilization of sphingomyelin by triton X-100 effect of the method of mixing and the concentrations of detergent and lipid☆

Abstract Mixed dispersions of sphingomyelin and Triton X-100 were prepared by two procedures. In method A, aqueous dispersions of sphingomyelin were mixed with aqueous solutions of Triton X-100. In method B, solutions of sphingomyelin and Triton X-100 in organic solvent were mixed, the solvent was evaporated and the dry residue was dispersed in buffer. Measurement of turbidities, electron microscopy and sedimentation of the mixed dispersions suggested the following: Below the critical micellar concentration of Triton X-100, the sphingomyelin is present as liposomes which sediment in the ultracentrifuge. Above the CMC, mixed micelles of sphingomyelin and Triton form. Method B resulted in aggregates of sphingomyelin which contain Triton X-100 even below its critical micellar concentration and which are smaller than those obtained by method A.

Stability of fatty acyl-coenzyme a thioester ligands of hepatocyte nuclear factor-4α and peroxisome proliferator-activated receptor-α

Although long-chain fatty acyl-coenzyme A (LCFA-CoA) thioesters are specific high-affinity ligands for hepatocyte nuclear factor-4α (HNF-4α) and peroxisome proliferator-activated receptor-α (PPARα), X-ray crystals of the respective purified recombinant ligand-binding domains (LBD) do not contain LCFA-CoA, but instead exhibit bound LCFA or have lost all ligands during the purification process, respectively. As shown herein: (i) The acyl chain composition of LCFA bound to recombinant HNF-4α reflected that of the bacterial LCFA-CoA pool, rather than the bacterial LCFA pool. (ii) Bacteria used to produce the respective HNF-4α and PPARα contained nearly 100-fold less LCFA-CoA than LCFA. (iii) Under conditions used to crystallize LBD (at least 3 wk at room temperature in aqueous buffer), 16∶1-CoA was very unstable in buffer alone. (iv) In the presence of the respective nuclear receptor (i.e., HNF-4α and PPARα), LBD 70–75% of 16∶1-CoA was degraded after 1 d at room temperature in the crystallization buffer, whereas as much as 94–97% of 16∶1-CoA was degraded by 3 wk. (v) Cytoplasmic LCFA-CoA binding proteins such as acyl-CoA binding protein, sterol carrier protein-2, and liver-FA binding protein slowed the process of 16∶1-CoA degradation proportional to their respective affinities for this ligand. Taken together, these data for the first time indicated that the absence of LCFA-CoA in the crystallized HNF-4α and PPARα was due to the paucity of LCFA-CoA in bacteria as well as to the instability of LCFA-CoA in aqueous buffers and the conditions used for LBD crystallization. Furthermore, instead of protecting bound LCFA-CoA from autohydrolysis like several cytoplasmic LCFA-CoA binding proteins, these nuclear receptors facilitated LCFA-CoA degradation.

Clofibrate does not induce peroxisomal proliferation in human hepatoma cell lines PLC/PRF/5 and SK-HEP-1

The potential of fibrate drugs to induce peroxisomal proliferation in human liver cells was evaluated in athymic nude mice transplanted with human hepatoma cells and treated by clofibrate in vivo as well as in cultured human hepatoma cells in the presence of fibrate drugs added to the culture medium. Clofibrate did not induce peroxisomal activities and neither acted as a peroxisomal proliferator in human PLC/PRF/5 or SK-HEP-1 heterotransplants under conditions of induction of peroxisomal activities in the host rodent liver. Similarly, clofibric acid or bezafibrate did not induce peroxisomal activities in cultured human PLC/PRF/5 or SK-HEP-1 cells under conditions of induction of peroxisomal activities in cultured primary rat liver cells. The lack of response of the human cells to peroxisomal proliferators of the fibrate type may indicate a species specificity with respect to induction of peroxisomal activities by xenobiotic peroxisomal proliferators.

Prevention of peroxisomal proliferation by carnitine palmitoyltransferase inhibitors in cultured rat hepatocytes and in vivo

1. The induction of peroxisomal beta-oxidation activities by bezafibrate in cultured rat hepatocytes and in the rat in vivo was prevented by inhibitors of carnitine acyltransferase, e.g. 2-bromopalmitate, 2-[5-(4-chlorophenyl)pentyl]oxirane-2-carboxylate or 2-tetradecylglycidic acid. 2. The prevention of peroxisomal proliferation by carnitine palmitoyltransferase inhibitors could not be accounted for by inhibition of mitochondrial beta-oxidation, since 2-bromo-octanoate, acting as an inhibitor of beta-oxidation, did not prevent the induction of peroxisomal activities in cultured rat hepatocytes. 3. The putative role of the acylcarnitine derivative of bezafibrate was analysed by studying the formation of bezafibroylcarnitine with bezafibroyl-CoA as substrate. However, no bezafibroylcarnitine formation was demonstrated in the presence of rat liver preparations capable of catalysing transfer to carnitine of medium- or long-chain fatty acids. 4. The prevention of peroxisomal proliferation by carnitine acyltransferase inhibitors may help in dissecting the causal relationship between the multiple effects mediated by peroxisomal proliferators.

Negative autoregulation of HNF-4α gene expression by HNF-4α1

HNF-4α (hepatocyte nuclear factor-4α) is required for tissue-specific expression of many of the hepatic, pancreatic, enteric and renal traits. Heterozygous HNF-4α mutants are inflicted by MODY-1 (maturity onset diabetes of the young type-1). HNF-4α expression is reported here to be negatively autoregulated by HNF-4α1 and to be activated by dominant-negative HNF-4α1. Deletion and chromatin immunoprecipitation analysis indicated that negative autoregulation by HNF-4α1 was mediated by its association with the TATA-less HNF-4α core promoter enriched in Sp1, but lacking DR-1 response elements. Also, negative autoregulation by HNF-4α1 was independent of its transactivation function, being similarly exerted by transcriptional-defective MODY-1 missense mutants of HNF-4α1, or under conditions of suppressing or enhancing HNF-4α activity by small heterodimer partner or by inhibiting histone deacetylase respectively. Negative autoregulation by HNF-4α1 was abrogated by overexpressed Sp1. Transcriptional suppression by HNF-4α1 independently of its transactivation function may extend the scope of its transcriptional activity to interference with docking of the pre-transcriptional initiation complex to TATA-less promoters.