Two Isomeric C16 Oxo-Fatty Acids from the Diatom Chaetoceros karianus Show Dual Agonist Activity towards Human Peroxisome Proliferator-Activated Receptors (PPARs) α/γ

The peroxisome proliferator-activated receptors (PPARs) function as ligand-activated transcription factors that convert signals in the form of lipids to physiological responses through the activation of metabolic target genes. Due to their key roles in lipid and carbohydrate metabolism, the PPARs are important drug targets. However, for several of the PPAR drugs currently in use, adverse side effects have been reported. In an effort to identify compounds from marine organisms that may serve as molecular scaffolds for the development of novel and safer PPAR-targeting drugs, we performed a bioassay-guided screening of organic extracts made from organisms supplied by the Norwegian Biobank of Arctic Marine Organisms (Marbank). Among several interesting hits, we identified two poorly described isomeric oxo-fatty acids from the microalgae Chaetoceros karianus for which we provide the first evidence that they might display dual specificity towards human PPARα and PPARγ. Principal component analysis showed that C. karianus stood out from other Chaetoceros species, both with respect to the metabolic profile and the PPAR activity. The isolation of these compounds holds the potential of uncovering a PPAR pharmacophore with tunable activity and specificity

Synthesis and biological evaluations of marine oxohexadecenoic acids: PPARα/γ dual agonism and anti-diabetic target gene effects

Obesity and associated disorders such as metabolic syndrome and type 2 diabetes (T2D) have reached epidemic proportions. Several natural products have been reported as Peroxisome Proliferator-Activated Receptor (PPAR) agonists, functioning as lead compounds towards developing new anti-diabetic drugs due to adverse side effects of existing PPAR drugs. We recently isolated and identified (7E)-9-oxohexadec-7-enoic acid (1) and (10E)-9-oxohexadec-10-enoic acid (2) from the marine algae Chaetoceros karianus. Herein we report the total synthesis, pharmacological characterization, and biological evaluations of these naturally occurring oxo-fatty acids (oFAs). The syntheses of 1 and 2 afforded sufficient material for extensive biological evaluations. Both oFAs show an appreciable dose-dependent activation of PPARα and -γ, with EC50 values in the micromolar range, and an ability to regulate important PPAR target genes in hepatocytes and adipocytes. Moreover, both 1 and 2 are able to drive adipogenesis when evaluated in the Simpson-Golabi-Behmel syndrome (SGBS) pre-adipocyte cell model, but with lowered expression of adipocyte markers and reduced lipid accumulation compared to the drug rosiglitazone. This seems to be caused by a transient upregulation of PPARγ and C/EBPα expression. Importantly, whole transcriptome analysis shows that both compounds induce anti-diabetic gene programs in adipocytes by upregulating insulin-sensitizing adipokines and repressing pro-inflammatory cytokines

Molecular modelling, synthesis, and biological evaluations of a 3,5-disubstituted isoxazole fatty acid analogue as a PPARα-selective agonist

The peroxisome proliferator activated receptors (PPARs) are important drug targets in treatment of metabolic and inflammatory disorders. Fibrates, acting as PPARα agonists, have been widely used lipid-lowering agents for decades. However, the currently available PPARα targeting agents show low subtype-specificity and consequently a search for more potent agonists have emerged. In this study, previously isolated oxohexadecenoic acids from the marine algae Chaetoceros karianus were used to design a PPARα-specific analogue. Herein we report the design, synthesis, molecular modelling studies and biological evaluations of the novel 3,5-disubstituted isoxazole analogue 6-(5-heptyl-1,2-oxazol-3-yl)hexanoic acid (1), named ADAM. ADAM shows a clear receptor preference and significant dose-dependent activation of PPARα (EC50 = 47 µM) through its ligand-binding domain (LBD). Moreover, ADAM induces expression of important PPARα target genes, such as CPT1A, in the Huh7 cell line and primary mouse hepatocytes. In addition, ADAM exhibits a moderate ability to regulate PPARγ target genes and drive adipogenesis. Molecular modelling studies indicated that ADAM docks its carboxyl group into opposite ends of the PPARα and -γ LBD. ADAM interacts with the receptor-activating polar network of amino acids (Tyr501, His447 and Ser317) in PPARα, but not in PPARγ LBD. This may explain the lack of PPARγ agonism, and argues for a PPARα-dependent adipogenic function. Such compounds are of interest towards developing new lipid-lowering remedies

Molecular modelling, synthesis, and biological evaluations of a 3,5-disubstituted isoxazole fatty acid analogue as a PPARα-selective agonist

The peroxisome proliferator activated receptors (PPARs) are important drug targets in treatment of metabolic and inflammatory disorders. Fibrates, acting as PPARα agonists, have been widely used lipid-lowering agents for decades. However, the currently available PPARα targeting agents show low subtype-specificity and consequently a search for more potent agonists have emerged. In this study, previously isolated oxohexadecenoic acids from the marine algae Chaetoceros karianus were used to design a PPARα-specific analogue. Herein we report the design, synthesis, molecular modelling studies and biological evaluations of the novel 3,5-disubstituted isoxazole analogue 6-(5-heptyl-1,2-oxazol-3-yl)hexanoic acid (1), named ADAM. ADAM shows a clear receptor preference and significant dose-dependent activation of PPARα (EC50 = 47 µM) through its ligand-binding domain (LBD). Moreover, ADAM induces expression of important PPARα target genes, such as CPT1A, in the Huh7 cell line and primary mouse hepatocytes. In addition, ADAM exhibits a moderate ability to regulate PPARγ target genes and drive adipogenesis. Molecular modelling studies indicated that ADAM docks its carboxyl group into opposite ends of the PPARα and -γ LBD. ADAM interacts with the receptor-activating polar network of amino acids (Tyr501, His447 and Ser317) in PPARα, but not in PPARγ LBD. This may explain the lack of PPARγ agonism, and argues for a PPARα-dependent adipogenic function. Such compounds are of interest towards developing new lipid-lowering remedies

LXRα Regulates ChREBPα Transactivity in a Target Gene-Specific Manner through an Agonist-Modulated LBD-LID Interaction

The cholesterol-sensing nuclear receptor liver X receptor (LXR) and the glucose-sensing transcription factor carbohydrate responsive element-binding protein (ChREBP) are central players in regulating glucose and lipid metabolism in the liver. More knowledge of their mechanistic interplay is needed to understand their role in pathological conditions like fatty liver disease and insulin resistance. In the current study, LXR and ChREBP co-occupancy was examined by analyzing ChIP-seq datasets from mice livers. LXR and ChREBP interaction was determined by Co-immunoprecipitation (CoIP) and their transactivity was assessed by real-time quantitative polymerase chain reaction (qPCR) of target genes and gene reporter assays. Chromatin binding capacity was determined by ChIP-qPCR assays. Our data show that LXRα and ChREBPα interact physically and show a high co-occupancy at regulatory regions in the mouse genome. LXRα co-activates ChREBPα and regulates ChREBP-specific target genes in vitro and in vivo. This co-activation is dependent on functional recognition elements for ChREBP but not for LXR, indicating that ChREBPα recruits LXRα to chromatin in trans. The two factors interact via their key activation domains; the low glucose inhibitory domain (LID) of ChREBPα and the ligand-binding domain (LBD) of LXRα. While unliganded LXRα co-activates ChREBPα, ligand-bound LXRα surprisingly represses ChREBPα activity on ChREBP-specific target genes. Mechanistically, this is due to a destabilized LXRα:ChREBPα interaction, leading to reduced ChREBP-binding to chromatin and restricted activation of glycolytic and lipogenic target genes. This ligand-driven molecular switch highlights an unappreciated role of LXRα in responding to nutritional cues that was overlooked due to LXR lipogenesis-promoting function

Activation of LXR increases acyl-CoA synthetase activity through direct regulation of ACSL3 in human placental trophoblast cells

Placental fatty acid transport and metabolism are important for proper growth and development of the feto-placental unit. The nuclear receptors, liver X receptors α and β (LXRα and LXRβ), are key regulators of lipid metabolism in many tissues, but little is known about their role in fatty acid transport and metabolism in placenta. The current study investigates the LXR-mediated regulation of long-chain acyl-CoA synthetase 3 (ACSL3) and its functions in human placental trophoblast cells. We demonstrate that activation of LXR increases ACSL3 expression, acyl-CoA synthetase activity, and fatty acid uptake in human tropholast cells. Silencing of ACSL3 in these cells attenuates the LXR-mediated increase in acyl-CoA synthetase activity. Furthermore, we show that ACSL3 is directly regulated by LXR through a conserved LXR responsive element in the ACSL3 promoter. Our results suggest that LXR plays a regulatory role in fatty acid metabolism by direct regulation of ACSL3 in human placental trophoblast cells