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Distinct arrhythmia patterns occur after myocardial infarction (with or without reperfusion) and after direct necrosis. They are not produced by differences in sympathetic activation and are likely related to the evolution of myocardial injury. The necrosis rat model may be useful in studies of arrhythmogenesis.

Krüppel-like factors (KLF) have important roles in metabolism. We previously found that KLF5 is a positive transcriptional regulator of peroxisome proliferator-activated receptor α ( Ppara) , a central regulator of cardiac fatty acid oxidation (FAO). Mice with cardiomyocyte-specific Klf5 ablation ( α MHC-Klf5 -/- ) had reduced cardiac Ppara expression and FAO. At age 6-12 months these mice develop distinct cardiac dysfunction. The role of PPARα activation in I/R injury is unclear as both beneficial and detrimental effects have been reported. We aimed to study if Ppara expression changes during I/R are driven by KLF5 and explore its protective or detrimental role. Wild type mice were subjected to in vivo I/R or sham surgery. I/R resulted in an initial increase in Ppara , and its target gene pyruvate dehydrogenase kinase 4 ( Pdk4) mRNA after 2h reperfusion, followed by decreased expression after 24h reperfusion. The Ppara expression is associated with parallel changes in cardiac Klf5 mRNA expression. Concurrent, there was a decrease of cardiac FAO-related genes carnitine palmitoyl-transferase 1β ( Cpt1b), very long chain acyl-CoA dehydrogenase (Vlcad), and acyl-CoA oxidase ( Aox) in mice with I/R. To define the cell type causing the temporal changes in Klf5 and Ppara after I/R we isolated primary cardiomyocytes and fibroblasts. Our data suggest a similar effect in primary isolated cardiomyocytes only. Klf5 mRNA expression is increased after 2 hour hypoxia and normalized after 4 hour re-oxygenation in cardiomyocytes, whereas there are no changes after hypoxia/normoxia in fibroblasts. To assess the importance of cardiomyocyte KLF5 in I/R we used α MHC-Klf5 -/- mice. Interestingly, despite reduced FAO, 7 month old αMHC-Klf5 -/- mice subjected to I/R had a marked increase in mortality; 4 of 7 αMHC-Klf5 -/- mice died within the first 24h of reperfusion while no mortality was observed in age-matched wild type mice that underwent I/R. In conclusion, I/R is associated with an increase in Klf5 and Ppara in the first hours of reperfusion followed by a decrease in Klf5 and Ppara , likely accounted for by cardiomyocytes. Increased mortality for α MHC-Klf5 -/- mice with I/R injury suggests that the initial increase may be an adaptive response that is critical for survival.

Inhibition of cardiac fatty acid oxidation (FAO) is considered beneficial after ischemia reperfusion (I/R). Krüppel-like factors (KLF) have an important role in metabolism. In a model of cardiac energetic deficiency (LPS-treated mice), in silico promoter analysis and whole genome array analysis indicated cardiac KLF5 as important regulator of Ppara . Gene expression analysis in human ventricular myocytes (AC16) treated with Ad-Klf5 followed by ChIP analysis confirmed that KLF5 is a positive transcriptional regulator of Ppara . Mice with cardiomyocyte-specific Klf5 ablation (αMHC-Klf5 -/- ) that we made had reduced cardiac Ppara expression and other FAO-related genes. The role of PPARα activation in I/R injury is unclear as both beneficial and detrimental effects have been reported. We aimed to gain insight in the effects of KLF5 and PPARα on acute I/R injury. To mimic I/R in vitro , AC16 cells were subjected to hypoxia (9.5 h) followed by short (1h) or prolonged (14h) normoxia. Ppara expression was initially (1h) increased (p<0.05) and then (14 h) decreased (p<0.05). Klf5 expression pattern showed a trend of similar changes . Infarction was performed by 30 min of left coronary artery occlusion followed by 2 or 24 hours of reperfusion in wild type mice. I/R resulted in a trend for an initial increase in Klf5 (2-fold, p=0.05), Ppara (3-fold, p=0.08), and Pdk4 (8-fold, p<0.01) mRNA at 2h post-surgery. Klf5 (2-fold; p<0.01) and Ppara (9-fold; p<0.05) expression were decreased 24h post-surgery. Consistent with Ppara changes, there was a 2-fold decrease of Cpt1 (p<0.01) and Vlcad (p<0.01) and a trend for decreased Aox (2-fold), Lcad (2-fold), and Pdk4 (6-fold) compared to sham. Echocardiography indicated normal cardiac function after 24h post-surgery. Despite reduced FAO, αMHC-Klf5 -/- mice subjected to I/R had a marked increase in mortality; 40% (4 of 10) of αMHC-Klf5 -/- mice died within the first 24h of reperfusion while no mortality was observed in wild type mice that underwent I/R. In conclusion, I/R is associated with an increase in Klf5 and Ppara in the first hours of reperfusion followed by a decrease in Klf5 and Ppara . Increased mortality for αMHC-Klf5 -/- mice with I/R injury suggests that the initial increase may be an adaptive response that is critical for survival.