The Homeodomain Proteins PBX and MEIS1 Are Accessory Factors That Enhance Thyroid Hormone Regulation of the Malic Enzyme Gene in Hepatocytes

Yutong Wang,Liya Yin,F Bradley Hillgartner

doi:10.1074/jbc.m102166200

Yutong Wang, Liya Yin + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m102166200

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Abstract

Triiodothyronine (T3) stimulates a robust increase (>40-fold) in transcription of the malic enzyme gene in chick embryo hepatocytes. Previous work has shown that optimal T3 regulation of malic enzyme transcription is dependent on the presence of an accessory element (designated as region E) that immediately flanks a cluster of five T3 response elements in the malic enzyme gene. Here, we have analyzed the binding of nuclear proteins to region E and investigated the mechanism by which region E enhances T3 responsiveness. In nuclear extracts from hepatocytes, region E binds heterodimeric complexes consisting of the homeodomain proteins PBX and MEIS1. Region E contains four consecutive PBX/MEIS1 half-sites. PBX-MEIS1 heterodimers bind the first and second half-sites, the third and fourth half-sites, and the first and fourth half-sites. The configuration conferring the greatest increase in T3 responsiveness consists of the first and fourth half-sites that are separated by 7 nucleotides. Stimulation of T3 response element functions by region E does not require the presence of additional malic enzyme sequences. In pull-down experiments, PBX1a and PBX1b specifically bind the nuclear T3 receptor-alpha, and this interaction is enhanced by the presence of T3. A T3 receptor-alpha region containing the DNA binding domain plus flanking sequences (amino acids 21-157) is necessary and sufficient for binding to PBX1a and PBX1b. These results indicate that PBX-MEIS1 complexes interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in hepatocytes.

Highlights

Malic enzyme catalyzes the oxidative decarboxylation of malate to pyruvate and CO2, simultaneously generating NADPH from NADPϩ
Less is known about the functions of PBX-MEIS1/PREP1 in adult tissues, data from recent studies suggest that these complexes are involved in controlling endocrine function
We show that PBX-MEIS1 complexes interact with T3 receptors (TR)-retinoid X receptor (RXR) complexes to enhance T3 regulation of malic enzyme transcription in avian hepatocytes

Summary

The abbreviations used are

T3, 3,5,3Ј-triiodo-L-thyronine; TR, nutors that are elevated during consumption of a high carbohydrate, low fat diet, increase malic enzyme activity in chick embryo hepatocytes [3, 4]. PBX binds cooperatively to DNA with other 3-amino acid loop extension class homeodomain proteins including MEIS1 and the closely related factor, PREP1 ( referred to as pKnox) [25,26,27,28]. In the case of the somatostatin gene, PBX-PREP1 stimulates transcription in pancreatic cells by potentiating the transcriptional activity of the pancreaticspecific homeodomain factor, PDX1 [32]. Still other studies have shown that PBX-PREP1 heterodimers interact with a cis-acting element that confers cell type-specific transcription of the human ␣2(V) collagen gene [35]. We have analyzed the binding of nuclear proteins to malic enzyme region E in hepatocytes We show that this TR accessory element binds PBX-MEIS1 heterodimers in multiple configurations. We have developed data suggesting that the stimulation of T3-induced malic enzyme transcription by region E is mediated by direct interactions between PBX and TR

EXPERIMENTAL PROCEDURES

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DISCUSSION