Activation and alleviation of cellular stress responses in fatty acid oxidation disorders

Authors:

Martin Lund*, Niels Gregersen*, Rikke Olsen*

* Research unit for molecular medicine, Aarhus University, Denmark

Corresponding author: Rikke Olsen rikke.olsen@clin.au.dk +4578455409

Background:

The primary cellular pathogenic factor in long-chain fatty acid oxidation disorders is reduced enzymatic capacity of specific fatty acid β-oxidation enzyme(s), which results in failure to maintain ATP/ADP ratio, coupled with lipotoxicity and misfolded protein stress. Enzyme deficiency may explain the majority of symptoms in these disorders. However, certain complications are less easily explained by enzyme deficiency, such as the associated myopathies, and may therefore be caused by secondary pathogenic disease drivers.

Methods: A mixed molecular biology approach primarily using image cytometry was employed. Sample material was anonymized very long-chain acyl-CoA dehydrogenase (VLCAD) deficient dermal fibroblasts and control dermal fibroblasts without known disorders.

Results: We found the levels of the essential peptide antioxidant glutathione to be elevated compared to controls, which was supported by increased levels of mitochondrial reactive oxygen species (ROS), specifically matrix superoxide levels. Bezafibrate has been of interest in recent years as a potential treatment. Bezafibrate treatment increases not only mitochondrial fatty acid β-oxidation, but rather boost several component systems of the mitochondria. We were interested in the effects of boosting not only VLCAD protein levels, but also overall mitochondrial activity in VLCAD deficiency and found that the bezafibrate treatment further increased ROS production and decreased glutathione levels. Since glutathione in some patient fibroblast cultures was reduced to below that of controls levels, we decided to apply stress to the model-system to investigate what effect bezafibrate treatment would have on cellular resilience. This was done by subjecting VLCAD deficient fibroblasts to mild heat stress and fasting, i.e. cessation of culture media refreshment. This caused a potent increase in VLCAD ROS levels and failure to maintain glutathione levels. Pretreatment with bezafibrate further exacerbated this issue. Even or odd medium-chained fatty acids partially alleviated the depletion of glutathione, most likely by helping to maintain NADPH levels in mitochondrial matrix.

Discussion: These results implicate deregulation of redox homeostasis as a key pathogenic mechanism in VLCAD deficiency that may be managed by the use of glutathione or NAD(P)H boosting drugs and/or prevention of catabolic stress. The results also questions the therapeutic benefit of using bezafibrate or other PPAR agonists as solo therapy. It seems that there is a negative selective pressure on mitochondrial function/quality, which is exacerbated by PPAR (over)activation. PPAR agonist therapy can potentially be improved by co-application of appropriate additional treatment.