Background: Diabetes is at epidemic proportions, with the major form of fatality due to congestive heart failure triggered by myocardial infarction (MI). The impaired insulin signalling in the diabetic heart leads to myocardial energy dysregulation that compromises the cardioprotective mechanism against ischemic injury. Therefore understanding how mitochondrial energetics is altered in the diabetic ischemic heart would greatly advance the knowledge base for improving outcomes from heart failure in diabetic patients.
Methods/Findings: We observed that db/db mice (leptin deficient, type 2 diabetic mice) have increased infarction size (>30%) compared to wild type mice after ischemia/reperfusion (IR) injury by TTC stain. We also found that activity of Hypoxia inducible factor-1 (HIF1) is involved in the cardioprotective response to ischemia, is impaired in db/db hearts. HIF1 is known to transcriptionally regulate genes involved in myocardial energetics. We recently found that HIF1 transcriptionally regulates the mitochondrial protein frataxin (Fxn) in cardiomyocytes as determined by luciferase assays (>3 fold). In vitro studies indicate that hypoxic conditions increase Fxn protein expression in cardiomyocytes as determined by western analysis (2 fold). Fxn plays an important role in the Fe-S cluster biogenesis required for aconitase, succinate dehydrogenase and complexes in the mitochondria. Interestingly, we observed decreased expression of Fxn in the ischemic diabetic heart.
Conclusion: we postulate that attenuated HIF1-Fxn signalling in ischemic diabetic heart leads to abnormally enlarged infarction size in response to IR. The decline in HIF-1 activity in response to hypoxia was further validated in cardiomyocytes cultured in high glucose media. The significance for Fxn against hypoxic injury was confirmed by utilizing overexpressed Fxn cardiomyocytes via MTT, ATP and aconitase activity assays. Current and future work: currently we are attempting to identify the HIF response element (HRE) in Fxn promoter to further validate the transcriptional activity of HIF1. In addition, we are completing the IR surgeries on HIF1 KO mice to address the cardioprotective nature of HIF1-Fxn signalling against MI.