Peroxisomal and mitochondrial enzymes involved in lipid metabolism : studies on function and regulation

Abstract

Fatty acids constitute a major part of the energy that we obtain from the diet and are also the principal source for mammals to store energy. To use the incoming or stored fatty acids as energy, the fatty acids needs to be metabolized of which the majority of fatty acids will be degraded by the mitochondrial β-oxidation system that in the end generates energy to the cell in the form of ATP. However, this organelle is not able to handle all kinds of fatty acids of which very long chain fatty acids, long chain methyl-branched fatty acids and dicarboxylic acids are such cumbersome fatty acids. Therefore a second organelle, the peroxisome, is required for metabolism of these particular fatty acids. Also peroxisomes contain a β-oxidation system and similar to the mitochondrial system is the initial substrate a CoA-esterified fatty acid, so-called acyl-CoA.

This thesis will focus on some enzymes that are active on these acyl-CoA esters, but that are not directly involved in the β-oxidation per se. Instead they contribute to the regulation of both acyl-CoA and free coenzyme A levels in different cellular compartments. This thesis will also include how these fatty acid degrading systems can be regulated at gene level by affecting different transcription factors by dietary ligands and by fasting.

The peroxisomal Nudix hydrolase 7α (NUD7α), previously believed to be a CoASH degrading enzyme, was demonstrated to be a medium chain diphosphatase, most active on medium chain acyl-CoA esters, to produce 3’,5’-ADP and the corresponding 4’-acylphosphopantetheine thereof. NUDT7α expression and activity was down regulated by PPARα activation, which would prevent CoASH degradation and support a high rate of the β-oxidation in peroxisomes during these conditions.

Peroxisomes are not only needed for the degradation of complex lipids, but are also essential for many other metabolic pathways such as bile acid and etherphospholipid synthesis and the degradation of D-amino acids and glyoxylate. The expression of gene transcripts that code for the proteins involved in these peroxisomal pathways was investigated almost throughout the whole mouse body with the aim to map the tissue expression of these pathways. The peroxisomal β-oxidation system is present in all examined tissues, however with differences in magnitude. More specifically expressed pathways are e.g. glyoxylate and D-amino acid degradation pathways. Auxiliary enzymes to the peroxisomal β-oxidation showed tissue specific expression, suggesting a high degree of tissue specific metabolite patterns, also being dependent on the metabolic state. The study also shows that PPARα is of major importance for the regulation in liver of the peroxisomal “transcriptome” during fasting.

Mitochondria degrade both fatty acids and amino acids and the mitochondrial acyl-CoA thioesterase 9 (ACOT9) was shown to hydrolyze both long chain acyl-CoAs as well as short chain acyl-CoA intermediates and products of branched-chain amino acid metabolism. Kinetic characterization of the enzyme suggests a thigh regulation of the activity during different metabolic conditions in the mitochondria.

Dietary ω-3 PUFAs from fish oil (FO) and krill oil (KO) cause different changes in lipid profiles and gene regulation when supplemented to mice. FO lowered most plasma lipids whereas KO only significantly lowered non-esterified fatty acids in plasma. FO showed a classical PPARα activation response by up regulating genes for fatty acid utilization and oxidation whereas KO down regulates genes for cholesterol and fatty acid synthesis.