The IκBα gene is a peroxisome proliferator-activated receptor cardiac target gene


N. E. Buroker, Department of Cardiology, Seattle Children’s Hospital, Seattle, WA 98101 and Department of Pediatrics, University of Washington, School of Medicine, Seattle, WA 98195, USA
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The purpose of this study was to provide a better understanding of the regulatory role of the nuclear steroid receptor on the nuclear factor of kappa light polypeptide gene enhancer in B cells (NFκB) in mouse heart. NFκB regulates many nuclear genes and has been associated with many human cardiac diseases. NFκB’s protein regulator gene, nuclear factor of kappa light polypeptide gene enhancer in B cells inhibitor alpha gene (IκBα), was found in this study to be regulated by peroxisome proliferator-activated receptors (PPARs). PPARs, retinoid X receptors (RXRs) and thyroid hormone receptors (THRs) are members of the nuclear receptor superfamily, which consists of a large number of transcription factors whose activities are regulated by their cognate ligands. These steroid hormone receptors are important regulators of gene expression and differentiation in the heart. These receptors form homo-(RXR, THR) and hetero-(PPAR–RXR, RXR–THR) dimers that bind DNA at various response elements (PPAR, RXR and THR) in the promoter regions of target genes. The PPAR/RXR response elements in the promoter of IκBα are described in this article. A known PPAR activator (Wy14643) and dimethylsulfoxide (vehicle) were introduced into control (FVB) and δ337T thyroid hormone receptor (TRβ) transgenic mice. The δ337T TRβ transgenic mouse has a resistance to the thyroid hormone (RTH) phenotype. Affymetrix 430_2 chip gene expression was examined for four study groups (control, control with Wy14643, δ337T TRβ and δ337T TRβ with Wy14643), consisting of seven mice each. IκBα mRNA expression in the Wy14643 control and in transgenic mice was upregulated significantly in microarray (P < 0.05) and quantitative RT-PCR (P < 0.01) analyses. The increase in mRNA level was also accompanied by an increase in IκBα protein in cells, as measured by Western blot analysis. Duplex oligo-DNAs containing the putative PPAR/RXR motif (AGGTCA/TCCAGT) from the IκBα promoter were used in gel shift assays to verify the binding of PPAR and RXR to their response elements. pGL4.0 [Luc] constructs of the IκBα promoter, with and without the PPAR/RXR motifs, were co-transfected with mouse PPAR α, β and γ1 into HepG2 cells and used in luciferase assays to verify gene activation. In conclusion, our study revealed that PPAR regulates the mouse cardiac IκBα gene in both control and transgenic mouse heart. The implications of this finding are discussed in relation to possible changes in cardiac function.