A New Regulator of Glucocorticoid Receptor Transcriptional Activity: The Chicken Ovalbumin Upstream Promoter Transcription Factor II (COUP-TFII)

M. U. De Martino

Pediatric and Reproductive Endocrinology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda MD 20 892 - 1583, U.S.A.

The Glucocorticoid Receptor (GR) is a transcription factor that belongs to the nuclear receptor superfamily. The GR gene, located in the chromosome 5, encodes for two isoforms, GRα and β, depending on the alternative use of terminal exon 9α or 9β, respectively. GRα is the classic GR that binds glucocorticoids and transduces their biologic activities; in the GRβ, the last 50 amino acids of GRα are replaced with a unique 15 amino acid sequence that renders the molecule incapable of binding glucocorticoids and therefore transcriptionally inactive. GRβ has dominant negative activity upon GRα and unclear physiologic roles. The ligand dependent activation of GRα modulates the transcription of a large number of target genes whose coordinate expression plays important roles in the control of metabolic, cardiovascular, immune and behavioral homeostasis. Besides the classical mode of action, via direct binding to GRE-DNA, the ligand activated GRα affects the transcriptional events of other signal transduction cascades via mutual protein-protein interaction with several transcription factors, such as nuclear factor-κB (NF-κB), activator protein-1 (AP-1) and Smad3 that transduces the signal of transforming growth factor β (TGFβ). We identified the chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) as a novel partner of GR isoforms in a yeast two hybrid screening assay using GR components as baits. COUP-TFII is an orphan receptor, already known as a suppressor of the transcriptional activity of several nuclear receptors such as retinoic acid, thyroid hormone, vitamin D and others. This transcription factor homodimerizes or heterodimerizes with nuclear hormone receptors and binds a wide variety of response elements that contain imperfect AGGTCA direct repeats separated by a variable number of nucleotides. Moreover COUP-TFII, along with its closely related protein COUP-TFI in humans, plays important roles in glucose, fatty acids, cholesterol and xenobiotic metabolism as well as embryonic development. In a GST pull-down assay, COUP-TFII interacted with the hinge regions of both GRα and GRβ via its DNA binding domain, while COUP-TFII formed a complex with GRα, but not with GRβ on the COUP-TFII-responsive cholesterol 7α-hydroxylase (CYP7A) promoter in a chromatin immunoprecipitation assay. In functional reporter assays, COUP-TFII repressed the transcriptional activity of ligand-activated GRα on the glucocorticoid-responsive mouse mammary tumor virus promoter, while GRα, but not GRβ, enhanced the transcriptional activity of COUP-TFII on the CYP7A promoter. It is known that phospoenolpyruvate carboxykinase (PEPCK), a key enzyme of the gluconeogenetic pathway and its promoter activity, is regulated by both GR and COUP-TFII. We found that COUP-TFII is necessary for the enhancement of this promoter activity by glucocorticoids, using morpholino oligo-anitisense. We conclude that COUP-TFII may mediate some part of glucocorticoid actions in the regulation of intermediary metabolism through protein-protein interaction with GRα.