An Isoform of Branched-chain Aminotransferase Is a Novel Co-repressor for Thyroid Hormone Nuclear Receptors

Abstract

The functions of thyroid hormone receptors (TRs) are regulated by a host of co-regulatory proteins. Tissue-specific expression of these co-regulators leads to distinct expression patterns and regulation of thyroid hormone (T3) target genes in tissues. Previously we have found that human colon carcinoma RKO cells exhibit strong T3-independent transcriptional activity. We therefore searched for co-regulatory proteins in RKO cells using a yeast two-hybrid system with the intact TR␤1 as bait. One of the three positive clones, designated as P3, was identified to be an isoform of human mitochondria branched-chain aminotransferase (BCATm). P3 was a spliced variant of BCATm with an internal 12-amino acid deletion near the carboxylterminal region and was abundantly expressed in RKO cells. The expressed protein localized both to the mitochondria and the nucleus of transfected CV1 cells. P3 physically interacted with TR␤1 in a T3-independent manner that led to the inhibition in binding of TR␤1 to thyroid hormoneresponsive element. P3 not only enhanced the repressor activity of the unliganded TR but also repressed the liganddependent activation of TR. This repression was reversed by treatment of cells with trichostatin A, suggesting that in addition to the inhibition of DNA binding, the repression activity of P3 on TR may also be mediated by histone deacetylase activity. Thus, unlike the currently known corepressors, P3 is a novel ligand-independent co-repressor for TR. Thyroid hormone receptor (TR)1 is a member of the nuclear hormone receptor superfamily that acts as ligand-dependent transcription factor to control cell proliferation, differentiation, and homeostasis (1). Two distinct genes, TR␣ and TR␤, give rise to four hormone-binding TR isoforms (␣1, ␤1, ␤2, and ␤3) by alternative splicing (1, 2). The tissue specificity and differential expression of TR isoforms indicate that they play distinct functional roles in vivo (3,4). This is demonstrated by the finding that TR␤-knockout mice show very different phenotypes from TR␣1-deficient mice. TR regulates transcription by binding to thyroid hormone response element (TRE) as a homodimer or heterodimer in the promoter region of target genes. The transcriptional activity of TR relies on the types of TRE as well as on T3. In the presence of T3, TR functions as a transcriptional activator that binds to positive TREs and as a repressor that binds to negative TREs.Modulation of gene expression by TR involves the coordination of a network of co-regulatory proteins, including co-activators and co-repressors. Binding of ligand to TR induces a conformational change that promotes dissociation of the corepressors and recruitment of the co-activators, leading to gene activation. Unlike other nuclear hormone receptors, TR and retinoic acid receptors are able to bind to their target genes in the absence of ligands and actively repress transcription (5-8). The repression is mediated by a silencing domain in the carboxyl terminus (5, 9 -11) that also harbors several other functions, includ...

Distribution of the branched chain aminotransferase proteins in the human brain and their role in glutamate regulation

“…Two other variants have also been described that are homologous to hBCATm: a novel alternatively spliced variant found in placental tissue and a splice variant that is reported to act as a co‐repressor of thyroid hormone nuclear receptors (Lin et al . ; Than et al . ).…”

Branched-chain amino acid catabolism: unique segregation of pathway enzymes in organ systems and peripheral nerves

“…Recent work has shown that a splice variant of BCATm with an internal deletion of 12 amino acids is expressed in colon carcinoma cells (41). This isoform of BCATm binds to the thyroid hormone receptor and enhances its effects on nuclear transcription activity (41). Furthermore, BCATm has a redox-active CXXC center (9) and has been found to associate with BCKD and other proteins (27).…”

“…Finally, it should be noted that, for both BCATm and BCATc, nonenzymatic roles in intracellular signaling may be possible. Recent work has shown that a splice variant of BCATm with an internal deletion of 12 amino acids is expressed in colon carcinoma cells (41). This isoform of BCATm binds to the thyroid hormone receptor and enhances its effects on nuclear transcription activity (41).…”

The CymR Regulator in Complex with the Enzyme CysK Controls Cysteine Metabolism in Bacillus subtilis

“…However, the involvement of MalY in the transduction of an unknown signal to MalT remains to be established. Finally, a short isoform of the human branched chain aminotransferase is also a co-repressor for thyroid hormone receptor-mediated transcription by a physical interaction between these two proteins (42).…”