Arginine and Glutamate-rich 1 (ARGLU1) Interacts with Mediator Subunit 1 (MED1) and Is Required for Estrogen Receptor-mediated Gene Transcription and Breast Cancer Cell Growth

Abstract

Estrogen receptor is a nuclear receptor superfamily member of transcriptional activators that regulate gene expression by recruiting diverese transcriptional coregulators. The Mediator complex is a central transcriptional coactivator complex that acts as a bridge between transcriptional activators and RNA polymerase II. MED1 (Mediator subunit 1) is the key Mediator subunit that directly interacts with estrogen receptor to mediate its functions both in vitro and in vivo. Interestingly, our previous biochemical analyses indicated that MED1 exists only in a subpopulation of the Mediator complex that is enriched with a number of distinct Mediator subunits and RNA polymerase II. Here, we report ARGLU1 as a MED1/Mediator-associated protein. We found that ARGLU1 (arginine and glutamate rich 1) not only colocalizes with MED1 in the nucleus, but also directly interacts with a far C-terminal region of MED1. Reporter assays indicate that ARGLU1 is able to cooperate with MED1 to regulate estrogen receptor-mediated gene transcription. Importantly, ARGLU1 is recruited, in a ligand-dependent manner, to endogenous estrogen receptor target gene promoters and is required for their expression. Furthermore, by ChIP-reChIP assay, we confirm that ARGLU1 and MED1 colocalize on the same estrogen receptor target gene promoter upon estrogen induction. Moreover, we found that depletion of ARGLU1 significantly impairs the growth, as well as anchorage-dependent and -independent colony formation of breast cancer cells. Taken together, these results establish ARGLU1 as a new MED1-interacting protein required for estrogen-dependent gene transcription and breast cancer cell growth.Estrogen receptor belongs to a nuclear receptor superfamily of transcriptional activators that, in a ligand-dependent manner, regulates the expression of specific genes controlling the development, reproduction, homeostasis, and metabolism of an organism (1, 2). Like other nuclear receptors, estrogen receptor shares a common organization of functional domains: a highly variable N-terminal activation function-1 domain that mediates ligand-independent transcription, a highly conserved central DNA-binding domain that specifically binds to hormone-response elements in cognate promoters of target genes, and a moderately conserved C-terminal ligand-binding domain containing activation domain 2, which mediates ligand-dependent transcription (1, 2). Upon ligand binding, the ultimate action of these receptors on regulating target gene expression is to enhance the recruitment and/or function of the general transcriptional machinery, including RNA polymerase II and general transcription factors (3-6). In the past decades, increasingly diverse groups of transcriptional coregulators have been found to play key roles in this process (3, 4). Among them, Mediator has emerged as a key transcriptional coregulator complex that is responsible for communicating the signals from transcription activators to RNA polymerase II and the general transcription machinery (7-9).Mediator is a large ...

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