Steroid Receptor Interactions with Heat Shock Protein and Immunophilin Chaperones

Abstract

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a ...

Vanadate increases glucocorticoid receptor-mediated gene expression: a novel mechanism for potentiation of a steroid receptor

“…In the absence of hormone, the GR is known to reside in the cytoplasm of most cells as a heteromeric complex containing the Hsp90-based chaperone complex (for a comprehensive review of this and related topics, see [1]). Based on this structure and localization, it is generally accepted that the major stages of the GR signaling pathway include the following sequence of events: hormone-binding, dissociation of the GR/Hsp complex, translocation to the nucleus of hormone-bound GR, binding by GR to high-affinity response elements within chromatin, and, finally, enhancement of Pol II-based transcription through mediation by co-activators.…”

Tamoxifen-regulated adenoviral E1A chimeras for the control of tumor selective oncolytic adenovirus replication in vitro and in vivo

“…After binding of the ligand to the HBD, the fusion protein is released from this complex. 28 This mechanism seems to be very efficient. It was shown that E1A fused to the HBD of the glucocorticoid receptor is fully hormonedependent for adenoviral E3 promoter stimulation, although the fusion protein is located exclusively in the nucleus even in the absence of the inducer.…”

“…34,39 This domain was fused to an adenoviral E1A protein containing a deletion in the pRB-binding site (amino acids 122-129 within CR-2 41 ) at either the N-terminus (ME) or C-terminus (EM) or at both the Nand C-terminus (MEM) (Figure 2a). Based on a model describing the functional activation/inactivation of chimeric proteins containing the HBD of steroid receptors, 28 the E1A-Mer fusion proteins were assumed to promote viral gene expression and replication only in the presence of tamoxifen or 4-OH-Tam ( Figure 1). …”

“…27 In the absence of ligand, HBD fusion proteins seems to be inactivated by a large complex of heat-shock proteins including hsp90, hsp70 and hsp56, most probably through direct interaction of hsp90 with the HBD. 27,28 Binding of a functional ligand leads to an alteration in the structure of the HBD, which allows release of the chimera from hsp90, refolding to an active form and stabilization by dimerization of the HBD. 28,29 Several intracellular proteins have been rendered functionally hormone-dependent by fusing them with steroid receptors, among them are several viral and cellular transcription factors like E1A, 26,30 HIV Rev, 31 c-Myc 32 and v-Myb.…”

“…27,28 Binding of a functional ligand leads to an alteration in the structure of the HBD, which allows release of the chimera from hsp90, refolding to an active form and stabilization by dimerization of the HBD. 28,29 Several intracellular proteins have been rendered functionally hormone-dependent by fusing them with steroid receptors, among them are several viral and cellular transcription factors like E1A, 26,30 HIV Rev, 31 c-Myc 32 and v-Myb. 33 Previously it has been shown that fusion proteins of a mutated HBD of murine estrogen receptor (Mer) 34 or its human analogue (ER) 35 enable very tight regulation of the corresponding effector proteins in vitro and in vivo.…”