Abstract Body (Do not enter title and authors here): Background: Almost all cardiac diseases are difficult to cure entirely because cardiomyocytes rarely proliferate to recover cardiac damage. In fact, most mammals have a capacity to regenerate cardiomyocytes immediately after birth. Newborn infant drastically changes cardiac metabolism from glycolysis to oxidative phosphorylation using fatty acids. Previous research revealed that lipid metabolism had a negative influence on cardiomyocyte cell cycle but the mechanism has not been clarified completely.
Objective: This study was performed to elucidate the way fatty acid metabolism affects cardiomyocyte proliferation.
Methods and results: Isolated and cultured neonatal rat cardiomyocytes (NRCM) were treated with the mixture of palmitic acid, oleic acid, and L-carnitine (FA mix) for 24 hours. Realtime RT-PCR revealed that FA mix increased the mRNA expressions of pyruvate dehydrogenase kinase 4 (PDK4), a fatty acid metabolism regulator, and HMG-CoA synthase 2 (HMGCS2), a ketogenic factor, as well as β-oxidation-related enzymes in dose dependent manner. Simultaneously, the results of immunofluorescent staining showed that FA mix decreased cardiomyocytes expressing Ki67, a cell cycle marker. GW501516, a peroxisome proliferator-activated receptor δ (PPARδ) agonist, also increased fatty acid metabolism factors with the reduction of Ki67 positive cardiomyocytes, whereas fenofibrate, a PPARα agonist, or pioglitazone, a PPARγ agonist, caused little or no change in these phenotypes. Moreover, NRCM pretreated with GSK3787, a PPARδ antagonist, before FA mix stimulation suppressed the upregulations of fatty acid metabolism factors and the reduction of Ki67 expressing cardiomyocytes. Finally, the lentiviral vectors which expressed PDK4 or HMGCS2 were produced to investigate these proteins suppressed NRCM cell cycle directly. Either overexpressing PDK4 or HMGCS2 decreased Ki67 positive NRCM without the upregulation of each other or β-oxidation-related enzymes.
Conclusion: Fatty acids disturbed NRCM proliferation by upregulating PDK4 and HMGCS2 through the activation of PPARδ independently from β-oxidation.