The Heat Shock Protein 70 Cochaperone Hip Enhances Functional Maturation of Glucocorticoid Receptor

Abstract

Multiple molecular chaperones interact with steroid receptors to promote functional maturation and stability of receptor complexes. The heat shock protein (Hsp)70 cochaperone Hip has been identified in conjunction with Hsp70, Hsp90, and the Hsp70/Hsp90 cochaperone Hop/Sti1p in receptor complexes during an intermediate stage of receptor assembly, but a functional requirement for Hip in the receptor assembly process has not been established. Because the budding yeast Saccharomyces cerevisiae contains orthologs for most of the receptor-associated chaperones yet lacks an orthologous Hip gene, we exploited the well-established yeast model for steroid receptor function to ask whether Hip can alter steroid receptor function in vivo. Introducing human Hip into yeast enhances hormone-dependent activation of a reporter gene by glucocorticoid receptor (GR). Because Hip does not similarly enhance signaling by mineralocorticoid, progesterone, or estrogen receptors, a general effect on transcription can be excluded. Instead, Hip promotes functional maturation of GR without increasing steady-state levels of GR protein. Unexpectedly, Hip binding to Hsp70 is not critical for boosting GR responsiveness to hormone. In conclusion, Hip functions by a previously unrecognized mechanism to promote the efficiency of GR maturation in cells.

Glucocorticoid treatment and endocrine pancreas function: implications for glucose homeostasis, insulin resistance and diabetes

“…The GR is expressed in virtually all tissues; however, GR is able to regulate genes in a cell-specific manner, indicating that the response to GCs is regulated by factors beyond receptor expression. The GR is guided from the moment of synthesis to decay through signal transduction and by a variety of molecular chaperones such as HSP70 (Nelson et al 2004) and HSP90 (Pratt et al 2006), which facilitate folding, maturation and ligand binding. In addition, GR-mediated transcriptional activation is modulated both positively and negatively by phosphorylation (Ismaili & Garabedian 2004) of kinases and phosphatases.…”

Structure and function of Hip, an attenuator of the Hsp70 chaperone cycle

“…Deletion of 14 residues from the N-terminus of Hip resulted in the apparent loss of Hip homo-oligomerization. This indicates that dimerization was required for Hipassisted nuclear receptor maturation in vivo (Irmer and Höhfeld, 1997;Nelson et al, 2004;Nollen et al, 2001). …”

Functional Protein Interactions in Steroid Receptor-Chaperone Complexes

“…Hip acts to stabilize the ADP-bound state of Hsp70 that is necessary for high affinity interaction with unfolded substrates (Frydman &Höhfeld, 1997;Höhfeld et al, 1995). Structurally, Hip consists of an N-terminal oligomerization domain that is important for the functional maturation of GR in yeast (Nelson et al, 2004), a central TPR domain and an adjacent highly charged region which are both required for Hsp70 binding ) and a C-terminal DP-rich domain that helps direct the intermediate stage recruitment of Hop-Hsp90 during assembly of steroid receptor complexes (Prapapanich et al, 1998).…”