Thioredoxin-mediated Negative Autoregulation of Peroxisome Proliferator-activated Receptor α Transcriptional Activity

Abstract

PPAR␣, a member of the nuclear receptor superfamily, and thioredoxin, a critical redox-regulator in cells, were found to form a negative feedback loop, which autoregulates transcriptional activity of PPAR␣. Thioredoxin was identified as a target gene of PPAR␣. Activation of PPAR␣ leads to increase of thioredoxin expression as well as its translocation from cytoplasm to nucleus, whereas ectopic overexpression of thioredoxin in the nucleus dramatically inhibited both constitutive and ligand-dependent PPAR␣ activation. As PPAR␣-target genes, the expression of muscle carnitine palmitoyltransferase I, medium chain acyl CoA dehydrogenase, and apolipoprotein A-I were significantly down-regulated by nucleus-targeted thioredoxin at transcriptional or protein level. The suppression of PPAR␣ transcriptional activity by Trx could be enhanced by overexpression of thioredoxin reductase or knockdown of thioredoxin-interacting protein, but abrogated by mutating the redox-active sites of thioredoxin. Mammalian one-hybrid assays showed that thioredoxin inhibited PPAR␣ activity by modulating its AF-1 transactivation domain. It was also demonstrated by electrophoretic mobility-shift assay that thioredoxin inhibited the binding of PPAR␣ to the PPAR-response element. Together, it is speculated that the reported negative-feedback loop may be essential for maintaining the homeostasis of PPAR␣ activity. INTRODUCTIONAs the first identified genetic sensor for fats, PPAR␣ is a ligandactivated transcription factor belonging to the nuclear receptor superfamily. The target genes of PPAR␣ are relatively homogenous group of genes that participate in various aspects of lipid catabolism such as fatty acid uptake through membranes, fatty acid binding in cells, fatty acid oxidation in microsomes, peroxisomes and mitochondria, and lipoprotein assembly and transportation (Lemberger et al., 1996). The significance of PPAR␣ in physiology and disease is evidenced by the fact that it and PPAR␥, another subtype of PPARs, are molecular targets for the lipid-lowering fibrate drugs and insulin-sensitizing thiazolidinedione (TZD), respectively (Evans et al., 2004). Besides activation of PPAR␣ by fatty acid and various exogenous ligands such as fibrates, the regulation of the PPAR␣ transcription activity by coactivators (Dowell et al., 1997), corepressors (Dowell et al., 1999), other transcription factors (TFs; Zhou et al., 1999), and posttranslational modifications including phosphorylation (Juge-Aubry et al., 1999) and ubiquination (Blanquart et al., 2002) have been extensively studied. However, redox regulation of PPAR␣ activity has not been reported so far.Multicellular organisms have evolved complex homeostatic mechanisms to sense and respond to a diverse range of exogenous and endogenous signals. It may be reasonable to expect some mechanisms that rapidly sense and tightly control the activity of PPAR␣ according to metabolic needs under normal physiological condition. Degradation of PPAR␣ by the ubiquitin-proteasome system and decrease of the ubiquitinat...

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“…The -galactosidase-normalized luciferase activities were assayed as previously described (Liu et al, 2006). Each transfection was performed with 300 ng of ARELuc construct and 50 ng of -galactosidase plasmid.…”

Visualization of aging-associated chromatin alterations with an engineered TALE system

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