YOUNG INVESTIGATOR PERSPECTIVES Nuclear Receptor Coactivators in Neuroendocrine Function

Steroid hormones in ̄uence a variety of neuroendocrine events, including brain development, sexual differentiation and reproduction. Hormones elicit many of these effects by binding to neuronal steroid receptors, which are members of a nuclear receptor superfamily of transcriptional activators. However, the mechanisms by which activated steroid receptors regulate gene expression in brain are not well understood. Recently, a new class of proteins, known as nuclear receptor coactivators, have been found to dramatically enhance steroid receptor mediated transactivation of genes in vitro. Here, the proposed molecular mechanisms of how these coactivators enhance the transcriptional activity of steroid receptors are summarized. While much is known about the mechanisms of these coactivators in vitro, it is unclear how these cofactors function in hormone action in vivo or in the brain. This paper discusses some of the initial and enticing investigations into the role of these important coregulatory proteins in neuroendocrine events. Finally, some of the critical issues and future directions in nuclear receptor coactivator function in neuroendocrinology are highlighted. Function of steroid hormones in brain Steroid hormones act throughout the body to regulate development, differentiation, metabolism and reproduction. During the last half of the 20th Century, there was an explosion in the knowledge of how steroid hormones in ̄uence the central nervous system. A variety of studies reveals that the gonadal steroid hormones, oestrogens, progestins and androgens, act in the brain to in ̄uence many neuroendocrine events, including those associated with reproduction. These hormones have also been shown in animal and human studies to in ̄uence memory and cognition (1). In addition, oestrogens have been linked to depression and delaying the onset of Alzheimer's disease, further implicating a role for steroid hormones in higher order mental processes in humans. However, the mechanisms by which steroid hormones in ̄uence gene expression in the brain remain poorly understood. To address these basic cellular and molecular neuroendocrine mechanisms, a variety of model systems have been used to investigate steroid hormone action in brain. Two widely exploited models for studying steroid hormone action in brain are the hormonal regulation of brain development and hormone-dependent reproduction in rodents. Exposure of the developing brain to gonadal hormones during critical periods results in profound changes in morphology that are sexually differentiated. In adulthood, the ovarian steroid hormones, oestradiol and progesterone, are secreted sequentially during the rat oestrous cycle and act in the brain to regulate female reproductive behaviour (2). Thus, sexual differentiation of the brain and female reproductive behaviour in rats offer excellent models for elucidating how hormones act in the brain to modulate gene expression and behaviour. Steroid hormones elicit many of their biological effects by binding to their respective intracellular receptors, which belong to a large class of nuclear receptors. Recently, a novel class of proteins, known as nuclear receptor coactivators, has been shown to enhance the activity of nuclear receptors (3, 4). These nuclear receptor coactivators represent an important and critical level of regulation of receptor transcriptional activity. While much is known about how these coactivators in ̄uence hormone action in vitro, research on the in vivo function of nuclear receptor coactivators in neuroendocrine events is in its infancy. Here, the molecular mechanisms Correspondence to: Marc J. Tetel, Center for Neuroendocrine Studies and Neuroscience and Behaviour Program, University of Massachusetts, Amherst, MA 01003, USA (e-mail: [email protected]). Journal of Neuroendocrinology, 2000, Vol. 12, 927±932 # 2000 Blackwell Science Ltd underlying nuclear receptor coactivator modulation of steroid receptor action are brie ̄y described and recent ®ndings on the role of these coactivators in neuroendocrine function are discussed. Mechanisms of action of steroid receptors Nuclear receptors represent a superfamily of transcriptional activators that can be divided into subfamilies based on phylogenetic analysis. Receptors for oestrogens (ER), progestins (PR), androgens (AR), glucocorticoids (GR) and mineralocorticoids (MR) represent the type I subfamily known as classic steroid receptors. Receptors for thyroid hormone (TR), vitamin D3 (VDR), all-trans retinoic acid and 9-cis retinoic acid comprise the type II receptors. The third subclass includes the orphan nuclear receptors, which have no known ligands (5). These nuclear receptors share a modular structure consisting of a carboxyl-terminal ligand binding domain (LBD), a hinge region, a central highly conserved DNA binding domain and a variable amino terminal domain. In general, nuclear receptors have two transcriptional activation function domains: one in the amino-terminus (AF-1) and one in the carboxyl-terminal LBD (AF-2). A genomic mechanism of action for type I steroid receptors is summarized in Fig. 1. In vitro studies reveal that upon binding hormone, steroid receptors undergo a conformational change that causes dissociation from heat shock proteins (hsp), allowing receptor dimerization. These receptor dimers bind to palindromic steroid response elements of steroid-responsive target genes and alter the rate of gene transcription. While hormone action in brain is poorly understood, it is thought that steroid hormones act via their respective receptors to alter neuronal gene transcription, resulting in changes in steroidregulated neuroendocrine events. In contrast to type I receptors, type II receptors bind to DNA in the absence of ligand and often exert a repressive effect on the promoters of target DNA. This repressive effect, known as silencing, is relieved upon receptor binding to ligand. While it is beyond the scope of this review, it should be noted that a variety of studies provide evidence for nongenomic mechanisms of steroid hormone action in brain (6). Nuclear receptor coregulators Recent and exciting discoveries have revealed a class of proteins known as nuclear receptor coregulators. These coregulators consist of coactivators and corepressors that are required for ef®cient transcriptional regulation of nuclear receptors (3, 4). Nuclear receptor coactivators are proteins that interact with nuclear receptors and enhance their transcriptional activity. In vitro experiments reveal that these coactivators are often rate-limiting for receptor activation and appear to act as bridging proteins between the receptor and the basal transcriptional machinery (Fig. 1) (3, 4). Corepressors decrease transcriptional activity of nuclear receptors and appear to function more in action of type II receptors (3, 4). Here, I will focus on the behaviourally and physiologically relevant roles of nuclear receptor coactivators in steroid hormone action in brain. Function of nuclear receptor coactivators in steroid hormone action Steroid receptor coactivator family Steroid receptor coactivator-1 (SRC-1) was the ®rst coactivator to be cloned and characterized that dramatically enhances the transcriptional activity of nuclear receptors (7). The following list provides some of the general properties (in italics) that de®ne nuclear receptor coactivators as proposed by McKenna et al. (8) and the experimental evidence that identi®es SRC-1 as a coactivator of steroid receptors. (i) Receptors physically interact with coactivators in a ligand-dependent manner. SRC-1, which was identi®ed using the LBD of PR in a yeast two-hybrid screen, interacts with steroid receptors in the presence of an agonist, but not when unbound or in the presence of an antagonist (7). (ii) Coactivators increase the transcriptional activity of steroid receptors. In reporter assays, SRC-1 enhances the transactivation of steroid receptors, including PR, ER, GR, and the nuclear receptors thyroid hormone (TR) and 9-cis retinoic acid (RXR) (3, 7). (iii) Coactivators are rate-limiting and required for ef®cient transcriptional activity of steroid receptors. Depletion of SRC-1 in cultured cells by microinjection of SRC-1 antibodies prevents steroid receptor-dependent transcription (9). (iv) Transcriptional interference, or squelching, occurs when one receptor type represses the transcriptional activity of another receptor type by sequestering cofactors required by both receptors. Expression of coactivators can reverse this transcriptional interference. In cell culture systems, hormone-induced transactivation of PR is reduced by coexpression of ERa. This squelching of PR activity by ERa was relieved by over-expression of SRC-1 (7). (v) Coactivators contain activation domains that are able to enhance gene transcription when fused to a DNA binding domain. SRC-1 contains two autonomous activation domains that increase gene transcription when fused to a DNA binding domain in a reporter assay. A recent two-step model by Lui et al. (10) proposes that SRC-1 enhances steroid receptor activity by (a) stabilizing interactions between receptor and general transcription factors and (b) remodelling chromatin through its intrinsic histone acetyltransferase activity (11), which unwraps DNA from its condensed histone complex making it more accessible to the transcriptional machinery. SRC-1 belongs to a novel family of proteins about 160 kDa in size, called the p160s, that also includes SRC-2 and SRC-3 (3, 7). SRC-2 (also known as GRIP1 and related to TIF2) is Steroid Inactive steroid receptor Active receptor dimers Steroid responsive gene Protein SRE mRNA + hsp90 SRCs CBP/p300