Thyroid hormone receptor isoforms localize to cardiac mitochondrial matrix with potential for binding to receptor elements on mtDNA Fionnuala Morrish c , Norman E. Buroker a , Ming Ge a,b , Xue-Han Ning a,b , Jesus Lopez-Guisa a , David Hockenbery c , Michael A. Portman a,b, * a Children’s Hospital and Regional Medical Center, Seattle, WA, USA b Division of Cardiology, Department of Pediatrics, University of Washington School of Medicine, Seattle, WA, USA c Fred Hutchinson Cancer Research Center, Seattle, WA, USA Received 3 February 2006; received in revised form 13 April 2006; accepted 21 April 2006 Available online 3 May 2006 Abstract Thyroid hormone (T 3 ) rapidly promotes both nuclear and mitochondrial DNA transcription in cardiomyocytes, suggesting that T3 directly binds and activates mitochondrial genes. We showed for the first time mitochondrial localization for multiple TRa isoforms in heart, including truncated versions. Additionally, we demonstrated novel mitochondrial localization for versions of TRa 2 , the dominant negative isoform lacking a functional ligand-binding domain. We also confirmed by electromobility shift assays, that TRa 2 in mitochondrial extracts binds to thyroid receptor response elements present in the 12S rRNA (DRO) and D-loop region (DR2) of mitochondrial DNA. Thus, TRa isoforms may directly regulate T 3 responses at mtDNA in the heart. q 2006 Elsevier B.V. and Mitochondria Research Society. All rights reserved. Keywords: Energy metabolism; Transactivation; Mitochondrial import; Alternate translation 1. Introduction Mitochondrial biogenesis requires cooperative regulation of protein synthesis by the nuclear and mitochondrial genomes. The vast majority of the genes contributing to the respiratory chain and oxidative phosphorylation system are encoded in the nucleus (Scarpulla, 2005). The coding capacity of the mammalian mitochondrial genome is restricted to synthesis of 13 proteins, comprising vital respiratory complex subunits. Bigenomic coordination occurs in part through nuclear respiratory factor-1 (NRF-1), which can promote mito- chondrial transcription factor A and other nuclear encoded proteins involved in mitochondrial transcription, while simultaneously enhancing synthesis of nuclear encoded respiratory chain components. However, prevailing theories for bigenomic regulation do not encompass the direct binding of similar or even identical transcriptional factors to both nuclear and mitochondrial DNA promoter sites (Scarpulla, 2005). The heart, which maintains a high oxidative capacity relative to other organs, must respond rapidly to changing energy requirements encountered physiologically or during stress conditions. Recent data show that thyroid hormone (T 3 ) coordinates cardiac mitochondrial and nuclear transcription (Goldenthal et al., 2004, 2005). However, time-course analysis seems to refute the theory that T 3 promotes mitochondrial transcription machinery prior to increasing respiratory complex proteins and/or activity, and suggests that an alternate more direct mechanism promotes mtDNA transcription. Casas et al. showed in liver cells that truncated versions of nuclear receptors, TRa 1 and RXR, directly bind to receptor elements on mtDNA and promote ligand dependent transcrip- tion (Casas et al., 1999, 2003). Their data provide a compelling explanation for rapid T 3 coordination of bigenomic Mitochondrion 6 (2006) 143–148 www.elsevier.com/locate/mito 1567-7249/$ - see front matter q 2006 Elsevier B.V. and Mitochondria Research Society. All rights reserved. doi:10.1016/j.mito.2006.04.002 Abbreviations T 3 , thyroid hormone; TREs, thyroid hormone response element; COXIV, cytochrome c oxidase subunit IV; VDAC, voltage dependent anion channel; PCNA, proliferative cell nuclear antigen; GAPDH, glycer- aldehyde-3-phosphate dehydrogenase; mtHSP70, mitochondrial heat shock protein 70; mtDNA, mitochondrial DNA; aa, amino acid; DBD, DNA binding domain. * Corresponding author. Address: Children’s Hospital and Regional Medical Center, Cardiology MSW4841, 4800 Sand Point Way NE, Seattle, WA 98105, USA. Tel.: C1 206 987 2015; fax: C1 206 987 4036. E-mail address: michael.portman@seattlechildrens.org (M.A. Portman).