Noninvasive approach to mend the broken heart: Is "remote conditioning" a promising strategy for application in humans?
Peroxisome proliferator-activated receptors (PPARalpha, -gamma and -delta) are nuclear receptors involved in transcriptional regulations of lipid metabolism. The effect of PPARalpha in regulation of cardiac fatty acid oxidation has been well characterized. Whether PPARdelta also independently regulates fatty acid oxidation in the heart remains unclear. In this study, we tested the hypothesis that PPARdelta activates fatty acids oxidation in cardiomyocytes through transcriptional activation that are independent of PPARalpha. Our results first indicate that PPARdelta abundantly expresses in nucleus of cardiomyocytes. Palmitate oxidation rates were significantly increased in both neonatal and adult cardiomyocytes after treatment of a PPARdelta-selective ligand (GW0742). Further increases of fatty acid oxidation were evident when the treatment was applied to cardiomyocytes overexpressing a wild type PPARdelta, but not a mutant PPARdelta that lacks the intact carboxyl ligand-binding domain. Furthermore, genes of fatty acid oxidation enzymes were significantly upregulated in cultured rat neonatal cardiomyocytes when exposed to GW0742. GW0742 can restore partly the expression of certain key genes of fatty acid oxidation in mouse adult cardiomyocytes isolated from PPARalpha knockout mice. Therefore, while active crosstalk between PPARdelta and -alpha may exist, PPARdelta regulates cardiac fatty acid oxidation in the heart at least partly independent of PPARalpha. We conclude that PPARdelta may play a key role in cardiac energy balance and may serve as a "sensor" of fatty acid of other endogenous ligands in controlling fatty acids oxidation levels in the hearts under normal and pathological conditions.