Abstract 16841: Cardiac Deletion of Aryl Hydrocarbon Nuclear Translocator (Hypoxia Inducible Factor 1-beta) in the Adult Heart Leads to Cardiomyopathy Due to Lipid Accumulation via Peroxisome Proliferator Activated Receptor Activation

Abstract

Background: Aryl hydrocarbon receptor nuclear translocator (ARNT), also known as hypoxia-inducible factor 1-beta (HIF1-beta), serves as a binding partner for numerous basic helix-loop-helix Per/ARNT/Sim proteins, including HIF1alpha. Although HIF1alpha is known to be required for normal cardiac development, the role of ARNT in basal cardiac function in the adult is not known. We hypothesized that ARNT is required for normal cardiac physiology, and that ARNT deletion in the adult heart results in cardiomyopathy. Methods and Results: Cardiac-specific ARNT deletion was achieved by crossing ARNT flox/flox with αMHC-MCM (tamoxifen-inducible heart specific Cre) transgenic mice, followed by oral administration of tamoxifen. MCM/ARNT +/+ (WT) littermates were used as a control. Echocardiography of the ARNT knockout (KO) mice showed enlarged left ventricle with a reduction in ejection fraction (KO vs. WT: 35.8 ± 3.6% vs. 61.2 ± 2.8%, n=12, p<0.01). Closed-chest catheterization indicated that +dP/dt (KO vs. WT: 4355 ± 538 vs. 9426 ± 180 mmHg, p<0.01) was reduced in KO mice but not in WT mice. Electron microscopy revealed lipid droplets in ARNT KO heart tissues that were confirmed by oil-red staining. Furthermore, genes involved in fatty acid oxidation were up-regulated in KO hearts, as shown by microarray analysis and qRT-PCR. We further confirmed these results in vitro with siRNA-mediated knock down of ARNT in H9c2 cardiac myoblasts. Compared to control siRNA transfection, ARNT knockdown led to increases in PPAR-alpha (1.7-fold, p<0.001), fatty acid translocase (1.4-fold, P<0.01), and acyl-coenzyme A dehydrogenase (1.6-fold, p<0.0001) mRNA levels and to enhanced triglyceride accumulation (p<0.05). Moreover, insulin resistance was noted in ARNT knockdown cells, as demonstrated by significantly reduced glucose uptake in response to insulin stimulation. These results suggest that impaired fatty acid oxidation leads to lipid accumulation in response to a reduction in ARNT levels. Conclusion: ARNT is essential for normal cardiac function, and a reduction in ARNT levels leads to cardiomyopathy by increasing triglyceride accumulation through a PPAR-alpha pathway.