Developmentally Regulated N-terminal Variants of the Nuclear Receptor Hepatocyte Nuclear Factor 4α Mediate Multiple Interactions through Coactivator and Corepressor-Histone Deacetylase Complexes

Abstract

To understand the mechanisms governing the regulation of nuclear receptor (NR) function, we compared the parameters of activation and repression of two isoforms of the orphan receptor hepatocyte nuclear factor (HNF) 4␣. HNF4␣7 and HNF4␣1 differ only in their N-terminal domains, and their expression in the liver is regulated developmentally. We show that the N-terminal activation function (AF)-1 of HNF4␣1 possesses significant activity that can be enhanced through interaction with glucocorticoid receptor-interacting protein 1 (GRIP-1) and cAMP response element-binding protein-binding protein (CBP). In striking contrast, HNF4␣7 possesses no measurable AF-1, implying major functional differences between the isoforms. Indeed, although HNF4␣1 and HNF4␣7 are able to interact via AF-2 with GRIP-1, p300, and silencing mediator for retinoid and thyroid receptors (SMRT), only HNF4␣1 interacts in a synergistic fashion with GRIP-1 and p300. Although both isoforms interact physically and functionally with SMRT, the repression of HNF4␣7 is less robust than that of HNF4␣1, which may be caused by an increased ability of the latter to recruit histone deacetylase (HDAC) activity to target promoters. Moreover, association of SMRT with HDACs enhanced recruitment of HNF4␣1 but not of HNF4␣7. These observations suggest that NR isoformspecific association with SMRT could affect activity of the SMRT complex, implying that selection of HDAC partners is a novel point of regulation for NR activity. Possible physiological consequences of the multiple interactions with these coregulators are discussed. Members of the nuclear receptor (NR)1 superfamily regulate a broad array of physiological processes (1). Their effects are elicited by either activation or repression of a set of genes that harbor DNA motifs recognized by NRs. Although mechanisms of activation are well understood today, repression is beginning to be recognized as a key player in homeostasis (2-4).Gene activation is generally associated with relaxed chromatin structure, which is facilitated by acetylation and methylation of histone N-terminal tails through the recruitment of acetyltransferases or methyltransferases (5, 6). Recruitment of these coregulators to target promoters occurs mainly through direct physical interaction with one of the two activation function modules of NRs, the ligand-dependent AF-2 (7). The role of the other AF, AF-1, is less well studied. Nevertheless, increasing evidence suggests that AF-1 is involved in recruitment of coactivators such as Src-1, GRIP-1, and CBP (8 -11). The mechanisms of cooperative action of the two AFs within a single NR are by and large poorly understood, but it has been shown for estrogen receptor ␣ that the transcription intermediary factor TIF2 mediates such cooperation (12).Gene repression is associated with recruitment of histone deacetylases (HDACs) that remove acetyl groups from histones, provoking the recondensation of chromatin (5, 6). Repression is achieved by recruiting corepressors to regulatory regions through direct ...

DAX-1 Acts as a Novel Corepressor of Orphan Nuclear Receptor HNF4α and Negatively Regulates Gluconeogenic Enzyme Gene Expression

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Differential regulation of constitutive androstane receptor expression by hepatocyte nuclear factor4α isoforms

“…28,[45][46][47] Indeed, we observed that while PGC1␣ equally potentiates HNF4␣1 and HNF4␣7 transcriptional activity, p160/ SRC1 potentiates only HNF4␣1. This could explain why, although HNF4␣1 and HNF4␣7 bind CAR/ HNF4RE with equal efficiency, hCAR promoter transactivation by HNF4␣1 is much greater.…”

Differential Effects of Hepatocyte Nuclear Factor 4α Isoforms on Tumor Growth and T-Cell Factor 4/AP-1 Interactions in Human Colorectal Cancer Cells

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