Baicalein 5,6,7-trimethyl ether activates peroxisomal but not mitochondrial fatty acid beta-oxidation

Role of fatty acid transport protein 4 in metabolic tissues: insights into obesity and fatty liver disease

Fatty acid (FA) metabolism is a series of processes that provide structural substances, signalling molecules and energy. Ample evidence has shown that FA uptake is mediated by plasma membrane transporters including FA transport proteins (FATPs), caveolin-1, fatty-acid translocase (FAT)/CD36, and fatty-acid binding proteins. Unlike other FA transporters, the functions of FATPs have been controversial because they contain both motifs of FA transport and fatty acyl-CoA synthetase (ACS). The widely distributed FATP4 is not a direct FA transporter but plays a predominant function as an ACS. FATP4 deficiency causes ichthyosis premature syndrome in mice and humans associated with suppression of polar lipids but an increase in neutral lipids including triglycerides (TGs). Such a shift has been extensively characterized in enterocyte-, hepatocyte-, and adipocyte-specific Fatp4-deficient mice. The mutants under obese and non-obese fatty livers induced by different diets persistently show an increase in blood non-esterified free fatty acids and glycerol indicating the lipolysis of TGs. This review also focuses on FATP4 role on regulatory networks and factors that modulate FATP4 expression in metabolic tissues including intestine, liver, muscle, and adipose tissues. Metabolic disorders especially regarding blood lipids by FATP4 deficiency in different cell types are herein discussed. Our results may be applicable to not only patients with FATP4 mutations but also represent a model of dysregulated lipid homeostasis, thus providing mechanistic insights into obesity and development of fatty liver disease.

Biogenesis and Function of Peroxisomes in Human Disease with a Focus on the ABC Transporter

Peroxisomes are indispensable organelles in mammals including humans. They are involved in the β-oxidation of very long chain fatty acids, and the synthesis of ether phospholipids and bile acids. Pre-peroxisomes bud from endoplasmic reticulum and peroxisomal membrane and matrix proteins are imported to the pre-peroxisomes. Then, matured peroxisomes grow by division. Impairment of the biogenesis and function of peroxisomes results in severe diseases. Since I first undertook peroxisome research in Prof. de Duve's laboratory at Rockefeller University in 1985, I have continuously studied peroxisomes for more than 30 years, with a particular focus on the ATP-binding cassette (ABC) transporters. Here, I review the history of peroxisome research, the biogenesis and function of peroxisomes, and peroxisome disease including X-linked adrenoleukodystrophy. The review includes the targeting and function of the ABC transporter subfamily D.

Isoform-specific domain organization determines conformation and function of the peroxisomal biogenesis factor PEX26

Peroxisomal biogenesis factor PEX26 is a membrane anchor for the multi-subunit PEX1-PEX6 protein complex that controls ubiquitination and dislocation of PEX5 cargo receptors for peroxisomal matrix protein import. PEX26 associates with the peroxisomal translocation pore via PEX14 and a splice variant (PEX26Δex5) of unknown function has been reported. Here, we demonstrate PEX26 homooligomerization mediated by two heptad repeat domains adjacent to the transmembrane domain. We show that isoform-specific domain organization determines PEX26 oligomerization and impacts peroxisomal β-oxidation and proliferation. PEX26 and PEX26Δex5 displayed different patterns of interaction with PEX2-PEX10 or PEX13-PEX14 complexes, which relate to distinct pre-peroxisomes in the de novo synthesis pathway. Our data support an alternative PEX14-dependent mechanism of peroxisomal membrane association for the splice variant, which lacks a transmembrane domain. Structure-function relationships of PEX26 isoforms explain an extended function in peroxisomal homeostasis and these findings may improve our understanding of the broad phenotype of PEX26-associated human disorders.

ABC Transporter Subfamily D: Distinct Differences in Behavior between ABCD1-3 and ABCD4 in Subcellular Localization, Function, and Human Disease

ATP-binding cassette (ABC) transporters are one of the largest families of membrane-bound proteins and transport a wide variety of substrates across both extra- and intracellular membranes. They play a critical role in maintaining cellular homeostasis. To date, four ABC transporters belonging to subfamily D have been identified. ABCD1-3 and ABCD4 are localized to peroxisomes and lysosomes, respectively. ABCD1 and ABCD2 are involved in the transport of long and very long chain fatty acids (VLCFA) or their CoA-derivatives into peroxisomes with different substrate specificities, while ABCD3 is involved in the transport of branched chain acyl-CoA into peroxisomes. On the other hand, ABCD4 is deduced to take part in the transport of vitamin B₁₂ from lysosomes into the cytosol. It is well known that the dysfunction of ABCD1 results in X-linked adrenoleukodystrophy, a severe neurodegenerative disease. Recently, it is reported that ABCD3 and ABCD4 are responsible for hepatosplenomegaly and vitamin B₁₂ deficiency, respectively. In this review, the targeting mechanism and physiological functions of the ABCD transporters are summarized along with the related disease.

Modulation of the Nrf2 signalling pathway in Hct116 colon carcinoma cells by baicalein and its methylated derivative negletein

CONTEXT

Baicalein is a major compound in extracts derived from Scutellaria baicalensis Georgi (Lamiaceae) which are used in the Traditional Chinese Medicine for the treatment of inflammatory and gastrointestinal diseases. This flavonoid is an activator of the Nrf2 signalling pathway but the molecular mechanism is not clearly established.

OBJECTIVE

We investigated the molecular mode of baicalein-mediated Nrf2-activation in Hct116 cells by the analysis of proteasomal activity, radical-scavenging activity and the comparison with baicalein derivatives.

MATERIALS AND METHODS

The radical-scavenging activity (TEAC, DCF) up to 25 μM, cytotoxicity (MTT assay, 48 h) up to 100 μM, proteasomal activity and the Nrf2-activation (luciferase assay, ubiquitinylation, western blot, Ser40-phosphorylation; incubation for 1 or 4 h) by concentrations up to 40 or 50 μM of the compounds were analysed in Hct116 human colon carcinoma cells.

RESULTS

No change in the ubiquitinylation of Nrf2, proteasomal activity and transcription of the NRF2 gene were detectable. Baicalein decreased the phosphorylation of Nrf2 (IC50-value approximately 20 μM) suggesting an inhibitory effect of the flavonoid on protein kinases. Since the activation of the Nrf2 pathway by baicalein might be also due to redox-activity of the compound, we investigated the effects of methylated baicalein derivatives oroxylin A, negeletein and baicaleintrimethylether. Oroxylin A and negletein showed a comparable redox-active potential, but only negletein (50 μM, 4 h) was able to activate Nrf2.

CONCLUSION

This result confirms the hypothesis that baicalein, a component of extracts derived from Baical Skullcap, causes an activation of Nrf2 independent of a modulation of the cellular redox potential.

A novel method for determining peroxisomal fatty acid β-oxidation

The purpose of this study is to establish an assay method to screen for chemical compounds that stimulate peroxisomal fatty acid β-oxidation activity in X-linked adrenoleukodystropy (X-ALD) fibroblasts. In this investigation, we used 12-(1-pyrene)dodecanoic acid (pyrene-C12:0), a fluorescent fatty acid analog, as a substrate for fatty acid β-oxidation. When human skin fibroblasts were incubated with pyrene-C12:0, β-oxidation products such as pyrene-C10:0 and pyrene-C8:0 were generated time-dependently. These β-oxidation products were scarcely detected in the fibroblasts from patients with Zellweger syndrome, a peroxisomal biogenesis disorder. In contrast, in fibroblasts with mitochondrial carnitine-acylcarnitine translocase deficiency, the β-oxidation products were detected at a level similar to control fibroblasts. These results indicate that the β-oxidation of pyrene-C12:0 takes place in peroxisomes, but not mitochondria, so pyrene-C12:0 is useful for measuring peroxisomal fatty acid β-oxidation activity. In X-ALD fibroblasts, the β-oxidation activity for pyrene-C12:0 was approximately 40 % of control fibroblasts, which is consistent with previous results using [1-(14)C]lignoceric acid as the substrate. The present study provides a convenient procedure for screening chemical compounds that stimulate the peroxisomal fatty acid β-oxidation in X-ALD fibroblasts.

Very long chain fatty acid β-oxidation in astrocytes: contribution of the ABCD1-dependent and -independent pathways

Very long chain fatty acid (VLCFA) metabolism in astrocytes is important for the maintenance of myelin structure in central nervous system. To analyze the contribution of the ABCD1-dependent and -independent pathways to VLCFA metabolism in astrocytes, we prepared human glioblastoma U87 cells with a silencing of ABCD1 and primary astrocytes from abcd1-deficient mice, and measured fatty acid β-oxidation in the presence or absence of a potent inhibitor of carnitine palmitoyltransferase I, 2-[5-(4-chlorophenyl)pentyl]oxirane-2-carboxylate (POCA). In U87 cells, C24:0 β-oxidation was decreased to ca. 70% of the control in the presence of POCA, and the activity was further decreased to ca. 20% by the silencing of ABCD1. In mouse primary astrocytes, C24:0 β-oxidation was also decreased to ca. 70% of the control in the presence of POCA. The C24:0 β-oxidation in Abcd1-deficient primary astrocytes was ca. 60% of the wild-type cells and the activity was further decreased to ca. 25% in the presence of POCA. Compared to human skin fibroblasts, in which VLCFA β-oxidation is not significantly inhibited by POCA, approximately one-third of the overall VLCFA β-oxidation was inhibited in both types of astrocytic cells. These results suggest that VLCFA is indeed β-oxidized in ABCD1-dependent pathway, but the ABCD1-independent peroxisomal and mitochondrial β-oxidation pathways significantly contribute to VLCFA β-oxidation in astrocytic cells.

Recovery of PEX1-Gly843Asp peroxisome dysfunction by small-molecule compounds

Zellweger spectrum disorder (ZSD) is a heterogeneous group of diseases with high morbidity and mortality caused by failure to assemble normal peroxisomes. There is no therapy for ZSD, but management is supportive. Nevertheless, one-half of the patients have a phenotype milder than classic Zellweger syndrome and exhibit a progressive disease course. Thus, patients would benefit if therapies became available and were instituted early. Recent reports indicate several interventions that result in partial peroxisome recovery in ZSD fibroblasts. To identify drugs that recover peroxisome functions, we expressed a GFP-peroxisome targeting signal 1 reporter in fibroblasts containing the common disease allele, PEX1-p.Gly843Asp. The GFP reporter remained cytosolic at baseline, and improvement in peroxisome functions was detected by the redistribution of the GFP reporter from the cytosol to the peroxisome. We established a high-content screening assay based on this phenotype assay and evaluated 2,080 small molecules. The cells were cultured in chemical for 2 days and then, were fixed and imaged by epifluorescent microscopy on a high-content imaging platform. We identified four compounds that partially recover matrix protein import, and we confirmed three using independent assays. Our results suggest that PEX1-p.G843D is a misfolded protein amenable to chaperone therapy.