Decreased Fatty Acid β-Oxidation in Riboflavin-Responsive, Multiple Acylcoenzyme A Dehydrogenase-Deficient Patients Is Associated with an Increase in Uncoupling Protein-3

Abstract

Riboflavin-responsive, multiple acylcoenzyme A dehydrogenase deficiency (RR-MAD), a lipid storage myopathy, is characterized by, among others, a decrease in fatty acid (FA) beta-oxidation capacity. Muscle uncoupling protein 3 (UCP3) is up-regulated under conditions that either increase the levels of circulating free FA and/or decrease FA beta-oxidation. Using a relatively large cohort of seven RR-MAD patients, we aimed to better characterize the metabolic disturbances of this disease and to explore the possibility that it might increase UCP3 expression. A battery of biochemical and molecular tests were performed, which demonstrated decreases in FA beta-oxidation and in the activities of respiratory chain complexes I and II. These metabolic alterations were associated with increases of 3.1- and 1.7-fold in UCP3 mRNA and protein expression, respectively. All parameters were restored to control values after riboflavin treatment. We postulate that the up-regulation of UCP3 in RR-MAD is due to the accumulation of muscle FA/acylCoA. RR-MAD is an optimal model to support the hypothesis that UCP3 is involved in the outward translocation of an excess of FA from the mitochondria and to show that, in humans, the effects of FA on UCP3 expression are direct and independent of fatty acid beta-oxidation.

Coordinated and reversible reduction of enzymes involved in terminal oxidative metabolism in skeletal muscle mitochondria from a riboflavin‐responsive, multiple acyl‐CoA dehydrogenase deficiency patient

“…Intramyocellular lipid was detected histochemically by staining with Oil Red O as described in [20], and NADH dehydrogenase was determined by zymography as described in [17].…”

“…The activities of respiratory chain complexes and citrate synthase were assayed spectrophotometrically in the 6006g supernatant of tissue homogenates, as described in [20].…”

“…All have a urinary organic acid profile compatible with either glutaric aciduria (a severe form) or ethylmalonic-adipic aciduria (a mild form) and high levels of blood acylcarnitine (for a review, see [11][12][13][14]). RR-MAD can be cured by high doses of riboflavin [15][16][17][18][19][20]: The biochemical mechanism underlying this therapeutic effect is unclear.…”

“…The deficit in fatty acid b-oxidation results in increased intramyocellular lipid content. Probably in response to the need for export of accumulated fatty acids from mitochondria, uncoupling protein-3 (UCP3) is overexpressed in RR-MAD patients [20]. Riboflavin therapy restores to their normal values both the activities of flavin-dependent mitochondrial enzymes and the amount of UCP3 [15][16][17][18][19][20].…”

Adaptive thermogenesis and uncoupling proteins: a reappraisal of their roles in fat metabolism and energy balance

“…Within these models of fatty acid cycling, therefore, the UCP1-homologues Table 4 A link between skeletal muscle UCP2/UCP3 gene expression and regulation of lipids as fuel substrate in response to fasting would exert an uncoupling effect by being involved in translocating an excessive amount of fatty acid anions out of the mitochondrion. Support for this model is drawn from the observations of an elevated skeletal muscle UCP3 expression under conditions when one could expect cytosolic accumulation of free FFA due to diminished fatty acid oxidation, such as in patients who show multiple acyl-CoA dehydrogenase-deficiency [82] or after pharmacological inhibition using etomoxir [83,84]. These studies, however, do not provide evidence for a cause-effect relation between the up-regulation of UCP3 and mitochondrial export of FFA, since up-regulation of this UCP1 homologue (for some other undefined function) may simply be consequential to the cytosolic accumulation of fatty acids inherent in this hypothesis.…”