Background: The product of the obesity gene (ob), leptin, has a well-recognized role in regulating energy homeostasis. During the period of weight maintenance, circulating leptin concentration reflects total body fat mass. On the other hand, overnutrition is accompanied by progressive hyperleptinemia. In overnourished animals, the elevation in circulating fatty acids results in increased uptake and excessive deposition of lipids within muscle cells. Consequently, triglicerydes overload seems to strongly correlate to the impairment of insulin signaling in skeletal muscle, the primary target for insulin stimulated glucose disposal. High levels of leptin in the course of fat storage may protect non-adipose tissues from lipid accumulation.
Aim of the study: Here, we aim to evaluate in vitro the relationship between leptin treatment and expression of acyl-CoA oxidase (ACOX), a peroxisomal key enzyme involved in fatty acid catabolism. We also evaluate the adaptive response of cells to a putative oxidative insult, resulting from H(2)O(2) production.
Methods: The effects of increasing levels of leptin, at different times, were assessed on mouse C2C12 myotubes by semiquantitative PCR. Activation pathway was investigated by using extracellular signal-regulated kinase (ERK) and cytosolic phospholipase A(2) (cPLA(2)) inhibitors. Cellular adaptive response to oxidative stress was evaluated by measuring glutathione concentration, oxidized/reduced glutathione ratio and the main antioxidant enzymatic activities.
Results: A 1.8-fold increase in ACOX mRNA expression was evident at 20 ng/ml leptin, a dose comparable to that found in hyperleptinemic subjects. The induction was dose-dependent, with an increase of 3-fold at 100 ng/ml; the ability of leptin to stimulate ACOX mRNA reached a maximum at 20 min and was lost in myotubes continuously exposed for more than 1 h. ACOX enzymatic activity followed mRNA changes: it was doubled after 1 h treatment and remained elevated for 24 h. ERK and cPLA(2) pathway is involved, since their inhibitors abrogated the ACOX mRNA induction. Myotubes counteract the resulting oxidative insult by catalase and glutathione peroxidase activation, thus removing H(2)O(2) at the expenses of the reduced glutahione pool.
Conclusions: The present study shows that acute, but not chronic, leptin treatment of C2C12 myotubes induces ACOX expression. Peroxisomal fatty acid oxidation may work together with mitochondrial beta-oxidation to remove excessive lipids from non-adipose tissues, during early stages of overnutrition and before development of leptin resistance.