Hypothyroid Phenotype Is Contributed by Mitochondrial Complex I Inactivation Due to Translocated Neuronal Nitric-oxide Synthase

Abstract

Although transcriptional effects of thyroid hormones have substantial influence on oxidative metabolism, how thyroid sets basal metabolic rate remains obscure. Compartmental localization of nitric-oxide synthases is important for nitric oxide signaling. We therefore examined liver neuronal nitric-oxide synthase-␣ (nNOS) subcellular distribution as a putative mechanism for thyroid effects on rat metabolic rate. At low 3,3 ,5-triiodo-L-thyronine levels, nNOS mRNA increased by 3-fold, protein expression by one-fold, and nNOS was selectively translocated to mitochondria without changes in other isoforms. In contrast, under thyroid hormone administration, mRNA level did not change and nNOS remained predominantly localized in cytosol. In hypothyroidism, nNOS translocation resulted in enhanced mitochondrial nitric-oxide synthase activity with low O 2 uptake. In this context, NO utilization increased active O 2 species and peroxynitrite yields and tyrosine nitration of complex I proteins that reduced complex activity. Hypothyroidism was also associated to high phospho-p38 mitogenactivated protein kinase and decreased phospho-extracellular signal-regulated kinase 1/2 and cyclin D1 levels. Similarly to thyroid hormones, but without changing thyroid status, nitric-oxide synthase inhibitor N -nitro-L-arginine methyl ester increased basal metabolic rate, prevented mitochondrial nitration and complex I derangement, and turned mitogen-activated protein kinase signaling and cyclin D1 expression back to control pattern. We surmise that nNOS spatial confinement in mitochondria is a significant downstream effector of thyroid hormone and hypothyroid phenotype.Hypothyroidism is a prevalent disorder associated to low oxygen utilization and low tissue proliferation rate (1). In addition to non-genomic effects (2), thyroid hormones influence transcription of a number of nuclear and mitochondrial-encoded respiratory genes (3). Although direct or transcriptional effects have considerable impact on oxidative metabolism and hemodynamic function, much is still unknown about how thyroid hormones set the metabolic rate of the body (4); consonant with slowness of transcriptional mechanisms, treatment of hypothyroidism may require weeks of hormone administration to normalize the altered functions (5). In the last decade, the effects of nitric oxide (NO) 2 expanded from the vascular system to the intracellular milieu. In this context, subcellular localization of nitric oxide-synthases (NOS) with effector molecules is an important regulatory mechanism for NO signaling (6, 7). Accordingly, we are interested in the traffic of a posttranslationally modified variant of neuronal nitric-oxide synthase-␣ (nNOS) to mitochondria (formerly named mitochondrial nitric-oxide synthase or mtNOS), which vectorially directs NO to the matrix compartment (8, 9). mtNOS could be low in adult rodents, but a modulated increase has been associated to thyroid status (10), release of cytochrome c (11), mitochondrial protein nitration (12), liver and brain development (13,...

Nitric oxide: promoter or suppressor of programmed cell death?

“…In addition, new isoforms or mitochondrial variants of NOS (mtNOS) have since been described in rat liver (Ghafourifar and Richter, 1997;Giulivi et al, 1998), thymus (Bustamante et al, 2000), and brain (Riobo et al, 2002). mtNOS is bound to mitochondrial cytochrome c oxidase (COX) (Persichini et al, 2005) and to complex I (Franco et al, 2006), thus favoring a steric relationship between the released NO and the control of respiration. NO is critical for numerous biological processes, including PCD, and it functions through both cGMPdependent and-independent pathways.…”

Mitochondrial nitric oxide in the signaling of cell integrated responses

“…There exist three canonical isoforms [neuronal (NOS I or nNOS), inducible (NOS II), and endothelial (NOS III)] and a significant number of spliced and posttranslationally modified variants. In addition, new isoforms or mitochondrial variants of NOS (mtNOS) were recently described in rat liver (60,63), thymus (29), and brain (119); mtNOS is bound to mitochondrial PDZ domains of COX (57,103) and to complex I (57), thus favoring a steric relationship between the released NO, vectorially directed to the matrix and inner membrane, and the control of respiration. In pathological conditions such as extreme hypoxia, mitochondrial NO could come either from stimulated NOS (116,142) or from the reduction of nitrite by COX (39).…”

A novel CARD containing splice-isoform of CIITA regulates nitric oxide synthesis in dendritic cells

“…In addition, new isoforms or mitochondrial variants of NOS (mtNOS) were recently described in rat liver (Ghafourifar and Richter, 1997;Giulivi et al, 1998), thymus (Bustamante et al, 2000) and brain (Riobo et al, 2002). It was previously reported that mtNOS forms complexes with mitochondrial COX (Persichini et al, 2005;Franco et al, 2006) and complex I (Franco et al, 2006), thus favoring a steric relationship of newly synthesized NO to the control of mitochondrial-dependent respiration reaction. Interestingly, the mtNOS had been reported to be a NOS1 isoform that is translocated to mitochondria.…”