HSP90 chaperone cycle for steroid hormone receptors (SHR) in the presence of ligand

The chaperoning function of HSP90 is coupled to its ATPase activity. Our current understanding of the ATPase mechanism of Hsp90 is based largely on structural and functional studied for the Saccharomyces cerevisiae Hsp90 complexes (Meyer P et al. 2003, 2004; Ali MM et al. 2006; Prodromou C et al. 2000; Prodromou C 2012). The ATPase cycle of human HSP90 is less well understood, however several studies suggest that the underlying enzymatic mechanisms and a set of conformational changes that accompany the ATPase cycle are highly similar in both species Once ATP is bound it helps to stabilize the closed ATP lid state, in which the gamma-phosphate of ATP provides a hydrogen bonding that promotes a stable association of the ATP lid with N-terminal domain (NTD) (Ali MM et al. 2006; Prodromou C et al. 2000; Chadli A et al. 2000). The association of ATP with NTD then stimulates structural changes in NTD and in the middle domain that are likely to involve movements of the ATP lid segment within each N-terminal domain that locates over the bound ATP. The movement of the lids exposes surface residues that are subsequently involved in transient dimerization of the N-terminal domains of HSP90 (Ali MM et al. 2006; Prodromou C et al. 2000; Chadli A et al. 2000). Furthermore, the in-trachain associations of NTD with the middle domain leads to the active conformation of the catalytic loop of HSP90, which commits the ATP for hydrolysis (Meyer P et al. 2003). The subsequent conformational changes upon ATP binding are regulated by co-chaperone activities. For example, arrangement of the STIP1 domains in the complex seems to prevent the NTDs dimerization of HSP90 monomers and total closure of the HSP90 dimer that

androgen receptors (AR) are mostly located in the cytoplasm and completely translocate to the nucleus only after binding hormone (Htun H et al. 1999;Stenoien D et al. 2000;Tyagi RK et al. 2000;Cadepond F et al. 1992;Jewell CM et al. 1995;Kumar S et al. 2006). The unliganded mineralocorticoid receptor (MR) is partially cytoplasmic but can be found in nucleus in the ligand-bound or ligand-free form (Nishi M & Kawata M 2007). The progesterone receptor (PR) exists in two forms (PRA and PRB) with different ratios of nuclear versus cytoplasmic localization of the unliganded receptor. In most cell contexts, the PRA isoform is a repressor of the shorter PRB isoform, and without hormone induction it is mostly located in the nucleus, whereas PRB distributes both in the nucleus and in the cytoplasm (Lim CS et al. 1999;Griekspoor A et al. 2007). In the absence of ligand, members of the steroid receptor family remain sequestered in the cytoplasm and/or nucleus in the complex with proteins of HSP70/HSP90 chaperone machinery (Pratt WB & Dittmar KD1998). The highly dynamic ATP-dependent interactions of SHRs with HSP90 complexes regulate SHR cellular location, protein stability, competency to bind steroid hormones and transcriptional activity (Echeverria PC & Picard D 2010). Understanding the mechanism of ATPase activity of HSP90 is mostly based on structural and functional studies of the Saccharomyces cerevisiae Hsp90 complexes (Meyer P et al. 2003(Meyer P et al. , 2004Ali MM et al. 2006;Prodromou C et al. 2000;Prodromou C 2012). The ATPase cycle of human HSP90 is less well understood, however several studies suggest that the underlyhttps://reactome.org ing enzymatic mechanisms and a set of conformational changes that accompany the ATPase cycle are highly similar in both species (Richter K et al. 2008;Vaughan CK et al. 2009). Nascent SHR proteins are chaperoned by HSP70 and HSP40 to HSP90 cycle via STIP1 (HOP) (and its TPR domains) (Hernández MP et al. 2002a,b;EcheverriaPC & Picard D 2010;Li J et al. 2011). The ATP-bound form of HSP90 leads to the displacement of STIP1 by immunophilins FKBP5 or FKBP4 resulting in conformational changes that allow efficient hormone binding (Li J et al. 2011). PTGES3 (p23) binds to HSP90 complex finally stabilizing it in the conformation with a high hormone binding affinity. After hydrolysis of ATP the hormone bound SHR is released from HSP90 complex. The cytosolic hormone-bound SHR can be transported to the nucleus by several import pathways such as the dynein-based nuclear transport along microtubules involving the transport of the entire HSP90 complex or nuclear localization signals (NLS)-mediated nuclear targeting by importins (Tyagi RK et al. 2000;Cadepond F et al. 1992;Jewell CM et al. 1995;Kumar S et al. 2006). It is worth noting that GR-importin interactions can be ligand-dependent or independent (Freedman & Yamamoto 2004;Picard & Yamamoto 1987). In the nucleus ligand-activated SHR dimerizes, binds specific sequences in the DNA, called Hormone Responsive Elements (HRE), and recruits a number of coregulators that facilitate gene transcription. Nuclear localization is essential for SHRs to transactivate their target genes, but the same receptors also possess non-genomic functions in the cytoplasm. The human HSP70 family comprises at least eight unique gene products that differ from each other by amino acid sequence, expression level and sub-cellular localization (Daugaard M et al. 2007). HSP70 family members display highly conserved amino acid sequences and domain structures consisting of: the ATPase N-terminal domain that binds and hydrolyzes ATP (NBD), the substrate domain (SBD) that binds to exposed hydrophobic segments of client polypeptides and promote their solubility and/or folding in a dynamic ATP-dependent manner, and the C-domain that provides a "lid" for the substrate domain (Zhang P et al. 2014;Brocchieri L et al. 2008;Wisniewska M et al. 2010). The conserved domain structure consolidates the chaperone function of the Hsp70 proteins and enables them to bind and release extended stretches of hydrophobic amino acids, exposed by incorrectly folded globular proteins in an ATP-dependent manner (Takayama S et al. 1999;Mayer MP 2013;Daugaard M et al. 2007). The initial binding of an unfolded client protein by a heat shock protein 40 (HSP40) prevents its aggregation and 'delivers' it to HSP70. The substrate binding ability of HSP70 is dependent on its bound state to either ATP or ADP (Kityk R et al. 2012;Qi R et al. 2013). Client substrates enter the HSP70 functional cycle by binding the ATP form of the chaperone, which has lower substrate affinity but faster binding and release rates compared with the ADP state. Interaction of the client in the cleft results in conformational changes in NBD that modestly increase ATP hydrolysis. Second, a transient interaction of HSP70 with J-protein co-chaperone HSP40, which has a higher affinity to ATP-bound HSP70 than ADP-bound HSP70, also stimulates the ATPase activity of HSP70 (Wittung-Stafshede P et al. 2003). Mayer, MP. (2013). Hsp70 chaperone dynamics and molecular mechanism. Trends Biochem. Sci., 38, 507-14. ↗ Brocchieri, L., Conway de Macario, E., Macario, AJ. (2008) 1996;Russell R et al. 1999;Laufen T et al. 1999;Landry SJ 2003;Wittung-Stafshede P et al. 2003).

Literature references
The HSP70 family of chaperone proteins is one of the most conserved protein families in evolution (Takayama S et al. 1999;Boorstein WR et al. 1994;Brocchieri L et al. 2008  Biochemical and crystallographic analysis revealed that TPR domains of STIP1 interact specifically with  Meyer P et al. 2003Meyer P et al. , 2004Colombo G et al. 2008;Li J et al. 2012). The structural studies of NTD of human HSP90 with antitumor agent geldanamycin (that acts as an ADP/ATP mimetic) support the polar interactions in the binding pocket described for yeast Hsp90 and ADP or ATP (Stebbins CE et al. 1997;Prodromou C et al.1997;Grenert JP et al. 1997). Once ATP is bound it helps to stabilize the closed ATP lid state, in which the gamma-phosphate of ATP provides a hydrogen bonding that promotes a stable association of the ATP lid with NTD. The association of ATP or AMP-PNP with NTD then stimulates structural changes in NTD. NMR analysis of human full-length HSP90 protein with and without ATP confirmed that ATP binding led to conformational changes in NTD (Karagöz GE et al. 2010

Literature references
Patel, NA., Morgner, N., Beilsten-Edmands, V., Robinson, CV., Ebong, IO. (2016). The interchange of immunophilins leads to parallel pathways and different intermediates in the assembly of Hsp90 glucocorticoid receptor complexes.  (Ray DW et AL. 1999;Williams SP & Sigler PB 1998;. LBD (E-region) resides in the C-terminal half of the receptor and in addition to ligand binding function contains a transcriptional activation function (AF2), sequences for dimerization, heat shock protein association, intermolecular silencing and intramolecular repression . The binding of hormone acts as an allosteric switch to regulate SHR-DNA and SHR-protein interactions, including interdomain interactions and/or dimerization .
SHs are synthesized from cholesterol in the adrenal cortex (glucocorticoids, mineralocorticoids, and adrenal androgens), the testes (testicular androgens, estrogen), and the ovary and placenta (estrogen and progestogen or progestins) (Payne AH & Hales DB 2004;). SHs reach their target cells via the blood, where they are bound to specific carrier proteins Hammond GL 2016). SHs detach from the carrier proteins and because of their lipophilic nature readily diffuse through the plasma membrane of cells . Within the target cells SHs bind to steroid hormone receptors (SHRs) which are present in a heterocomplex with heat shock protein HSP90 and co-chaperones (e.g., immunophilins p23) (Echeverria PC & Picard D 2010). The ATP-bound form of HSP90 and chaperone-mediated conformational changes are required to keep SHRs in a ligand binding-competent state . Here, the androgens testosterone (TEST), dihydrotestosterone (DHTEST), androst-4-en-3,17-dione (ANDST) and 6-dehydrotestosterone bind the androgen receptor (AR), within the HSP90 chaperone complex. ing specific ligands (i.e., steroid hormones (SH)) to the ligand-binding domain (LBD) (Ray DW et AL. 1999;Williams SP & Sigler PB 1998;. LBD (E-region) resides in the C-terminal half of the receptor and in addition to ligand binding function contains a transcriptional activation function (AF2), sequences for dimerization, heat shock protein association, intermolecular silencing and intramolecular repression . The binding of hormone acts as an allosteric switch to regulate SHR-DNA and SHR-protein interactions, including interdomain interactions and/or dimerization .
SHs are synthesized from cholesterol in the adrenal cortex (glucocorticoids, mineralocorticoids, and adrenal androgens), the testes (testicular androgens, estrogen), and the ovary and placenta (estrogen and progestogen or progestins) (Payne AH & Hales DB 2004;). SHs reach their target cells via the blood, where they are bound to specific carrier proteins Hammond GL 2016). SHs detach from the carrier proteins and because of their lipophilic nature readily diffuse through the plasma membrane of cells   The mineralocorticoid receptor (Nuclear receptor subfamily 3 group C member 2, NR3C2) is a receptor with equal affinity for both mineralocorticoids (MC) such as aldosterone and glucocorticoids (GC) such as corticosterone or cortisol. NR3C2 is expressed in many tissues such as kidney, heart, the CNS and sweat glands. Its activation leads to the expression of proteins regulating ionic and water transport resulting in the reabsorption of sodium. Consequently, there is an increase in extracellular volume, an increase in blood pressure, and increased excretion of potassium to maintain normal salt concentrations.
Synthetic NR3C2 antagonists competitively inhibit NR3C2 (Kagawa et al. 1957, Pollow et al. 1992, Rupprecht et al. 1993 in the kidney distal convoluted tubule to promote sodium and water excretion and potassium retention. These diuretic drugs are typically indicated for congestive heart failure, hypertension and chronic kidney disease. Synthetic antagonists of NR3C2 include the steroidal compounds spironolactone, eplerenone, and drospirenone. Nimodipine, a calcium channel blocker, can also act as an NR3C2 antagonist (Dietz et al. 2008, Luther 2014.
The broad clinical use of steroidal mineralocorticoid receptor antagonists is limited by the potential risk of inducing hyperkalemia. Novel, non-steroidal NR3C2 antagonists demonstrate an improved therapeutic index for hyperkalemic risk compared to their steroidal counterparts in preclinical models (reviews