Identification of a mechanism promoting mitochondrial sterol accumulation during myocardial ischemia-reperfusion: role of TSPO and STAR.

Hypercholesterolemia is a major risk factor for coronary artery diseases and cardiac ischemic events. Cholesterol per se could also have negative effects on the myocardium, independently from hypercholesterolemia. Previously, we reported that myocardial ischemia-reperfusion induces a deleterious build-up of mitochondrial cholesterol and oxysterols, which is potentiated by hypercholesterolemia and prevented by translocator protein (TSPO) ligands. Here, we studied the mechanism by which sterols accumulate in cardiac mitochondria and promote mitochondrial dysfunction. We performed myocardial ischemia-reperfusion in rats to evaluate mitochondrial function, TSPO, and steroidogenic acute regulatory protein (STAR) levels and the related mitochondrial concentrations of sterols. Rats were treated with the cholesterol synthesis inhibitor pravastatin or the TSPO ligand 4'-chlorodiazepam. We used Tspo deleted rats, which were phenotypically characterized. Inhibition of cholesterol synthesis reduced mitochondrial sterol accumulation and protected mitochondria during myocardial ischemia-reperfusion. We found that cardiac mitochondrial sterol accumulation is the consequence of enhanced influx of cholesterol and not of the inhibition of its mitochondrial metabolism during ischemia-reperfusion. Mitochondrial cholesterol accumulation at reperfusion was related to an increase in mitochondrial STAR but not to changes in TSPO levels. 4'-Chlorodiazepam inhibited this mechanism and prevented mitochondrial sterol accumulation and mitochondrial ischemia-reperfusion injury, underlying the close cooperation between STAR and TSPO. Conversely, Tspo deletion, which did not alter cardiac phenotype, abolished the effects of 4'-chlorodiazepam. This study reveals a novel mitochondrial interaction between TSPO and STAR to promote cholesterol and deleterious sterol mitochondrial accumulation during myocardial ischemia-reperfusion. This interaction regulates mitochondrial homeostasis and plays a key role during mitochondrial injury.

Cardioprotective Signaling Pathways in Obese Mice Submitted to Regular Exercise: Effect on Oxysterols.

Exercise induces cardioprotection against myocardial infarction, despite obesity, by restoring pro-survival pathways and increasing resistance of mitochondrial permeability transition pore (mPTP) opening at reperfusion. Among the mechanisms involved in the inactivation of these pathways, oxysterols appear interesting. Thus, we investigated the influence of regular exercise on the reperfusion injury salvage kinase (RISK) pathway, oxysterols, and mitochondria, in the absence of ischemia-reperfusion. We also studied 7ß-hydroxycholesterol (7ßOH) concentration (mass spectrometry) in human lean and obese subjects. Wild-type (WT) and obese (ob/ob) mice were assigned to sedentary conditions or regular treadmill exercise. Exercise significantly increased Akt phosphorylation, whereas 7ßOH concentration was reduced. Moreover, exercise induced the translocation of PKCε from the cytosol to mitochondria. However, exercise did not affect the calcium concentration required to open mPTP in the mitochondria, neither in WT nor in ob/ob animals. Finally, human plasma 7ßOH concentration was consistent with observations made in mice. In conclusion, regular exercise enhanced the RISK pathway by increasing kinase phosphorylation and PKCε translocation and decreasing 7ßOH concentration. This activation needs the combination with stress conditions, i.e., ischemia-reperfusion, in order to inhibit mPTP opening at the onset of reperfusion. The human findings suggest 7ßOH as a candidate marker for evaluating cardiovascular risk factors in obesity.