Design of lead antagonists for transcriptional regulation of Glucocorticoid Responsive Elements

 

Wely B. Floriano1, Nagarajan Vaidehi1, Eugene Roberts2 and Willian A. Goddard III.1

1Materials and Process Simulation Center,
California Institute of Technology

2City of Hope

 

Abstract

The glucocorticoid receptor (GR) belongs to the nuclear receptors (NRs) family. Most members of this family display an identical structural organization with an amino-terminal region A/B, followed by a DNA-binding domain (DBD) consisting of two zinc fingers (region C), a linker region D, and the ligand-binding domain (LBD). GRs are involved in selective regulation of eukaryotic hormone responsive genes and affect both stimulation and repression of target genes. This selective regulation elicit cytostatic and cytotoxic effects in a variety of cells (Moras et Gronemeyer, 1998; Rogatsky et al, 1999; Nazareth et al, 1991; Wyllie, 1980).

Activation of target genes relies in most cases on the ability of the GR to bind to specific DNA sequences named glucocorticoid responsive elements (GREs) (Mangelsdorf et al., 1995; Resche-Rigon et al. 1998). Transcriptional repression of GREs has been linked to cell death (apoptosis) (Rogatsky et al, 1999; Wyllie, 1980). The minimal GR fragment necessary for causing apoptosis in lymphoblastic leukemic cells (Nazareth et al, 1991) has being identified as the DNA-binding domain. GR can also stimulate or repress gene expression independent of its binding to the DNA, through a mechanism mediated by protein-protein interaction with a second transcription factor (McEwan et al., 1997; Drouin, 1993; Akerblom and Mellon, 1991).

It has been suggested (Rogatsky et al., 1999) that multiple GR transcriptional regulatory mechanisms employing distinct receptor surfaces are used to induce the cytostatic or cytotoxic response to glucocorticoids. However, the exact molecular mechanisms regulating all these events and the target genes affected by the activated receptor remain unclear. The GR activity affects a vast repertoire of functions and glucocorticoid receptor-dependent initiation of transcription and translation has been implicated even in events such learning and memory alterations due to depression and stress (Xu et al., 1998; Roozendaal et al., 1996; Montkowski et al., 1995).

In this work, we investigate the GR transcriptional regulatory mechanisms and design lead compounds for transcriptional control by performing a series of docking studies. We used the known tridimensional (3D) structure for the rat Glucocorticoid Receptor (pdb code 1gdc; Baumann et al, 1993) as target. A series of 16 compounds known for their transcriptional regulation activities is used to explore different docking sites of the GR. The steroid Dexamethasone (dxmz) and other six compounds for which data on competitive displacement of dxmz is available are used as control. Two sites are being used for docking: the DNA-binding site and the second Zinc finger site of the GR. We present the predicted Binding Energies for all 23 compounds in the second Zinc finger site. We compare our Binding Energies to experimental results for the phenylbutyrate (Hudgins et al, 1996) and acetamide (Chun et al, 1986) series. Based on our docking studies we suggest a set of molecules that are predicted to have regulatory effect on GR activity.

Our ultimate goal is the identification of novel cancer therapies that explore the cytotoxic action of transcriptional GREs repression.

 

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