A specific regulatory role for SGTα on the maturation and activation of steroid hormone receptors

Steroid hormone receptors (SHRs) are ligand-dependent transcription factors belonging to the nuclear receptor superfamily. These receptors regulate various physiological functions in higher ordered eukaryotes. In the absence of hormone these receptors form a complex with molecular chaperones such as Hsp90 and Hsp70 and other cochaperones in the cytoplasm. Association with the Hsp90-Hsp70 chaperone machinery leads to the maturation of the Ligand Binding Domain (LBD) of the receptors and enables the receptors to bind hormone with high affinity. Upon binding with a specific hormone the receptors are translocated into the nucleus where they initiate transcription of specific genes. Although the involvement of the Hsp90-Hsp70 chaperone machinery in steroid hormone receptor function is well-characterized, other proteins have also been found in the receptor-chaperone complex which are known as Hsp90 and Hsp70 cochaperones. One such cochaperone is human Small Glutamine Rich Tetratricopeptide repeat (TPR) containing protein α (SGTα). SGTα is a Tetratricopeptide (TPR) containing protein which belongs to the Hsp90 chaperone family. SGTα consists of three TPR motifs which mediate interaction with the C-terminal EEVD motif of Hsp90, Hsp70 and an N-terminal domain which facilitates self-dimerization of the protein. In addition, recent studies indicated SGTα is a key participant in Androgen receptor (AR) signaling pathway and negatively affects its activity. However, the role of SGTα on other steroid hormone receptors still remains elusive. In this study, we investigated the role of SGTα on other steroid hormone receptors such as Glucocorticoid (GR), Progesterone (PR), Estrogen (ER) and Mineralocorticoid (MR). In order to determine the possible role played by SGTα in other steroid hormone receptor maturation pathways, we used two model systems, Saccharomyces cerevisiae and mammalian cell culture. Our results showed that SGTα is also a regulator of glucocorticoid (GR) and progesterone (PR) receptors’ maturation and activation in a ligand-dependent manner. For both GR and PR, SGTα was found to downregulate receptors’ activity in yeast in the presence of hormone. However, this cochaperone was not found to be involved with the mineralocorticoid and estrogen receptor maturation pathways. In Hela cells, stable knockdown of endogenous SGTα resulted in an upregulation of the GR and PR-mediated reporter activity in comparison to the stably transfected scrambled shRNA as well as wild type cells. Transfection of SGTα in the stable SGTα knockdown cells resulted in restoration of receptor activity to the wild type level. Furthermore, our studies in the yeast model system demonstrate that SGTα is a competitive inhibitor of FK506 binding protein 52 (FKBP52), a well-characterized immunophilin, which is involved in AR, GR and PR maturation pathways and potentiates their activity. Assays done in yeast showed that SGTα inhibits FKBP52-mediated potentiation of AR, GR and PR. Moreover, in vitro studies showed that SGTα and FKBP52 compete for binding on Hsp90. Collectively, our results indicate that SGTα partakes in GR and PR complex maturation and down-regulates these receptors’ activity in the presence of hormone. These findings cast new insights on how different cochaperones specifically interact with different steroid hormone receptors and regulate their functions