The Steroid/Nuclear Receptors: From Three-Dimensional Structure to Complex Function

Ben F. Luisi*; John W.R. Schwabe†; Leonard P. Freedman‡    * Medical Research Council, Virology Unit, Glasgow G11 5JR, United Kingdom
† Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom
‡ Cell Biology and Genetics, Memorial Sloan–Kettering Cancer Center, New York, New York 10021

I INTRODUCTION

Since their discovery, lipophilic hormones such as the steroids and thyroids have provided a time-honored paradigm for the mechanism of long-distance intercellular communication. The diffusable hormone, representing the “signal,” originates in one tissue but subsequently affects the growth or activity of target cells in a second tissue at some distance. In contrast to these hormones, the chemically related vitamins have traditionally been thought to play more static roles, being metabolites or ingested factors required throughout the body for routine metabolism–or so it seemed. Despite these descriptive differences, it has emerged that both groups of chemical messengers exert their physiological effects by activating specialized receptor proteins. In contrast to the membrane-bound receptors, exemplified by the growth hormone receptor, the vitamin and hormone receptors are found within either the cell cytoplasm or nucleus. The thyroid and vitamin receptors appear to be primarily nuclear, whereas the steroid receptors are cytoplasmic, but on binding the hormone translocate to the nucleus. Once there, the steroid receptors, like the ligand-activated thyroid or vitamin receptors, bind avidly to specific DNA sequences and modulate the expression of target genes. In some cases, the ligand-induced activation of the receptor brings about an enhancement of transcription; in other cases, repression results. To date, some 30 distinct receptors have been identified from organisms as diverse as arthropods, such as the fruit fly Drosophila, to mammals. This group includes a number of “orphan receptors” whose ligands and functions are presently not known. All of these proteins are clearly related functionally and may be classified into a collective group, which we refer to as the steroid/nuclear receptor family.

The receptor proteins are evolutionarily related, as evidenced by their high sequence homology (Evans, 1989). They share a characteristic modular organization whereby separable functions are encoded by discrete functional domains (Fig. 1) that encompass ligand-binding, DNA-binding, nuclear localization, and transcriptional modulation. The most strongly conserved region is the DNA-binding domain, a segment of approximately 90 residues (Fig. 2). Molecular genetic analyses of a number of receptors show that, despite the small size, this domain is sufficient to direct the recognition of DNA targets. A compilation of sequences from the DNA-binding domains of representative receptors is shown in Fig. 2B. This region behaves as a true domain in that it folds stably in isolation and retains full DNA-binding activity. The folded structure is stabilized by two zinc ions, each of which is coordinated by four cysteines (Freedman et al., 1988a). The spacing of the zinc ligands is reminiscent of the zinc-finger motif (Rhodes and Klug, 1993), but three-dimensional structural analyses have shown that these metal-binding motifs are structurally distinct (Hard et al., 1990a; Schwabe et al., 1990, 1993a; Luisi et al., 1991; Knegtel et al., 1993; Lee et al., 1993).

In all steroid/nuclear receptors, the DNA-binding domain is followed by a region of extensive sequence variability, which in turn is followed by the comparatively well conserved ligand-binding domain. The transcriptional activation function maps to the ligand-binding domain. The amino termini of the receptors, preceding the DNA-binding domain, are hypervariable in length and sequence composition. In the steroid receptors, this region encodes a second transcriptional activation domain. Their sequence homology and phylogenetic diversity suggest that the receptors diverged some 500 million years ago from a primordial ancestor, and have subsequently evolved to serve many important regulatory roles in most multicellular eukaryotes (Amero et al., 1992; Laudet et al., 1992).

The steroid/nuclear receptors are involved in both the control of development and the maintenance of homeostasis. For example, specialized nuclear receptors regulate Drosophila development in response to ecdysone and related molting hormones (Seagraves, 1991). The Drosophila FTZ-F1 protein (which also has a mouse homologue) plays an active role in early embryogenesis (Tsukiyama et al., 1992). The glucocorticoid and mineralocorticoid receptors regulate the homeostatic processes of gluconeogenesis, whereas the sexual steroid receptors affect sex gland development and activity in all vertebrates. The regulatory roles of vitamins is a topic of expanding interest. The retinoids, a group of vitamin A derivatives, have been found to play central roles in development, growth, reproduction, vision, and general homeostasis of numerous tissues (Lohnes et al., 1992a,b; Lufkin et al., 1993). Vitamin D3 mediates control of intestinal calcium and phosphorus absorption, bone remodeling, and conservation of minerals in the kidney, but also appears to play an important role in regulating cells of immune system origin (Reichel et al., 1992).

In light of their key control of cellular functions, it is unsurprising that mutant forms of several receptors have been implicated in oncogenic processes. The oncogenic v-erbA gene product is related to the thyroid hormone receptor. A truncated version of the estrogen receptor (missing the ligand-binding domain and therefore constitutively active) has been implicated in certain breast cancers. The fusion of a putative transcription factor called PML and the retinoic acid receptor is associated with acute promyelocytic leukemias (de-Thé et al., 1991; Kastner et al., 1992, Kakizuka et al., 1991, and references therein). The administration of retinoic acid induces these leukemia cell lines to differentiate and causes remission of the disease in patients. Interestingly, the PML-RAR fusion protein appears to disrupt a novel nuclear organelle which contains the native PML protein, and retinoic acid reverses this disruption (Dyck et al., 1994; Weis et al., 1994; Koken et al., 1994). Evidence suggests that the vitamin D3 receptor can also suppress activation of T lymphocytes and induce the differentiation of promyelocytic leukemia cell lines (Reichel et al., 1992).

It is recently been established that certain receptors modulate each other’s function by forming heterodimeric complexes. The receptors for retinoic acid, vitamin D3, and thyroid form heterodimers with the receptor for 9-cis-retinoic acid, also known as RXR. In analogy with the regulatory heterodimerization found in other classes of transcription factors, such as max/myc, the resulting interplay between receptors results in an additional level of physiological control and hierarchical complexity (Segars et al., 1993).

Despite the distinct physiological effects mediated by the different hormones, the upstream regulatory elements to which their receptors bind share remarkably similar consensus sequences. In particular, the hormone response elements (HREs) of the steroid receptors are very closely related (Fig. 3). The near palindromic nature of their response...