Background: The late gestational surge in glucocorticoids is vital for the maturation of fetal organs in preparation for birth and survival during the neonatal period. Metabolic maturation of cardiomyocytes involves a switch in fuel substrate from glucose utilization to fatty acid (FA) oxidation. In fetal cardiomyocytes, glucocorticoids induce expression of Ppargc1a (encoding PGC1a, a master regulator of mitochondrial capacity). We hypothesized that glucocorticoids promote the metabolic switch to FA oxidation during cardiomyocyte maturation.
Methods: Isolated embryonic day 14.515.5 mouse C57Bl/6 fetal cardiomyocytes were pre-treated with the glucocorticoid receptor antagonist RU486, or vehicle for 30mins prior to treatment with dexamethasone (dex) or vehicle for 24 h. A Seahorse XF Analyzer was used to measure glycolysis and mitochondrial respiration. Palmitate was used to measure FA oxidation; with etomoxir to block mitochondrial FA uptake. RNA was extracted from cardiomyocytes following 24 h dex or vehicle treatment and from C57Bl/6 E14.5 fetal hearts collected from pregnant dams injected (i.p.) at E13.5 with dex or vehicle. Genes involved in FA oxidation were analysed by qRT-PCR.
Results: Fetal cardiomyocytes exhibited little dependence on glycolysis and this was unaltered by dex treatment. With palmitate, dex treatment increased the basal respiration rate (518±48 vs. 367±71 pmol/min/protein, mean±S.D., n=5) and oxygen consumption (a measure of ATP production, 160±63 vs. 298±36 pmol/min/protein, mean±S.D., n=5) compared to vehicle. Etomoxir and RU486 inhibited these dex-dependent increases. In fetal cardiomyocytes, dex increased the expression of genes involved in FA uptake (Cd36, Cpt1a, Cpt1b) and utilisation (Lcad, Mcad, Lipin1, Ppargca1) but not Sirt1 (involved in autophagy) and Scad (short chain FA utilisation). Analysis of genes regulated in vivo is underway.
Conclusions: These data support a glucocorticoid-induced switch in substrate preference towards FA oxidation in fetal cardiomyocytes. The mechanism involves upregulation of genes involved in mitochondrial capacity and FA oxidation.