Peroxisome proliferators-activated receptor (PPAR) regulation in cardiac metabolism and disease

Abstract

Peroxisome proliferators-activated receptors (PPARs) are members of the nuclear receptor family of ligand activated transcription factors and consist of the three isoforms, PPAR, PPAR/ and PPAR. Considerable evidence has established the importance of PPARs in myocardial lipid homeostasis and cardiovascular function. Yet, the PPAR isoform specific gene expression profile in the heart has not been fully delineated. By combining gene profiling and computational PPRE analysis following PPAR isoform activation in normal versus PPAR deficient heart muscle cells we have, for the first time, determined the PPAR isoform-specific endogenous targets in the heart. Unexpectedly, electromobility shift and chromatin immunoprecipitation assays demonstrated the existence of an evolutionary conserved PPRE consensus-binding site in an igf1 enhancer. In line, Wy-14643 mediated PPAR activation in the wild-type mouse heart resulted in upregulation of igf1 transcript abundance and provided protection against cardiomyocyte apoptosis following ischemia/reperfusion or biomechanical stress. In response to acute pressure overload stimulation induced by aortic banding, the mitogen-activated protein kinase kinase (MAPKK) MEK1 has been shown to be activated. In chapter 3, we show that activation of the MEK1-ERK1/2 pathway leads to specific inhibition of PPARα transcriptional activity. Furthermore we show, for the first time, that this inhibitory effect is mediated by MEK1, and not by its downstream effector kinase ERK1/2, through a mechanism involving direct binding to PPARα and subsequent stimulation of PPARα export out of the nucleus. Although PPAR is an abundant member of the peroxisome proliferator-activated receptors (PPARs) family, limited information is available on the function of PPAR in the adult myocardium. To circumvent the embryonic lethality associated with postnatal deletion of PPAR/, we triggered conditional PPAR/ loss through the use of a tamoxifen-inducible Cre recombinase in the postnatal murine myocardium. Targeted PPAR/ deletion in adult mice provoked premature death within one week accompanied by rapid and dramatic biventricular enlargement and dramatic atrial enlargement. Overall, these results indicate that PPAR loss suffices to impact both adult myocardial morphology and function. The transcription factor myocyte enhancer factor 2 (MEF2) an important co-factor of PPARs and a downstream target for several hypertrophic signaling pathways in the heart. In chapter 5, we subjected wildtype and transgenic mice expressing a dominant negative form of MEF2 (DN-MEF2 Tg) in the heart to transverse aortic constriction (TAC). Echocardiographic analysis revealed that DN-MEF2 Tg mice displayed a decrease in cardiac function. Analysis of the mitochondrial respiratory chain showed that DN-MEF2 Tg mice displayed lower expression of NADH dehydrogenase subunit 6 (ND6), part of the large Complex I enzyme, only following pressure overload. The reduced expression of ND6 in DN-MEF2 Tg mice after pressure overload resulted in an increase in cell death secondary to overproduction of reactive oxygen species (ROS). Although PPAR share many aspects such as protein homology, we here provide an insight in isoform specific target genes and consequently new mechanisms of PPAR function in the heart. In addition, we show that transcriptional co-regulators of PPAR have an important role in the indirect regulation of PPAR function.