Lower mitochondrial ketone body oxidation capacity in the human diabetic heart

Introduction/Purpose Type 2 Diabetes (T2D) is associated with impairment of cardiac mitochondrial metabolism contributing to diabetic cardiomyopathy. Recent studies suggest that ketone body (KB) uptake and ketolysis are impaired in the diabetic heart. However, whether this translates to reduced utilization of KBs for mitochondrial oxidative phosphorylation in diabetic hearts remains unclear. Thus, the objective of this study was to quantify KB-dependent myocardial mitochondrial respiration in T2D compared to non-diabetic hearts. Methods Between March 2023 and October 2024, as part of a single-center prospective cohort study, we included heart transplanted individuals undergoing routine endomyocardial biopsies aged between 45 and 70 years. Participants were divided into T2D and non-DM using the following criteria: current hemoglobin A1c (HbA1c) ≥ 6.5% or presence of specific antihyperglycemic treatment. Two distinct high-resolution respirometry (HRR) protocols were subsequently applied. In the first protocol the maximum mitochondrial oxidative phosphorylation (OXPHOS) capacity was quantified using conventional respirometry substrates, such as fatty acids, complex I- and complex II-linked substrates. In the second protocol, the KBs β-Hydroxybutyrate (HBA) and acetoacetate (ACA) were utilized as respirometry substrates to quantify KB-dependent respiration and different aspects of ketolysis. The relative contribution of the KBs to maximum OXPHOS capacity was calculated by comparing HBA- and ACA-derived respiration to OXPHOS. Results Non-DM (n=35) and T2D (n=22) did not differ in demographic characteristics, with similar age distribution and sex, and showed significant differences in HbA1c (mean±SD, 5.5±0.5 vs. 7.1±1.2 %, p<0.0001). Consistent with prior research, we observed a tendency for reduced OXPHOS capacity for the conventional respirometry substrates tested in T2D, albeit not statistically significant. Myocardial mitochondrial KB-derived respiration was significantly decreased in T2D, both for using HBA only (38.94±11.07 vs. 29.65±9.48 pmol O2 mg-1 s-1, p=0.0014), as well as after addition of ACA (33.99±9.74 vs. 23.17±9.61 pmol O2 mg-1 s-1, p=0.0002). Also, the T2D group exhibited lower relative ACA-dependent respiration (0.38±0.18 vs. 0.28±0.11 [AU], p=0.0136), but showed no statistically significant difference in relative HBA-dependent respiration. Conclusion Our results suggest that the ability of the diabetic heart to utilize KBs as mitochondrial substrates is reduced. ACA-linked respiration appears to be affected to a higher extent than HBA-linked respiration, indicating an involvement of enzymes downstream of the HBA metabolizing enzyme β-hydroxybutyrate dehydrogenase. Impaired myocardial metabolic flexibility and substrate utilization in T2D may contribute to the development of diabetic cardiomyopathy and thus could be a potential therapeutic target.