High-affinity nuclear receptor binding of 20-epi analogues of 1,25-dihydroxyvitamin D3 correlates well with gene activation

Functional conformations of the nuclear 1alpha,25-dihydroxyvitamin D3 receptor

The nuclear hormone 1alpha,25-dihydroxyvitamin D3 (VD) has important cell regulatory functions. Various synthetic VD analogues are under investigation to identify candidates with an improved therapeutic profile against hyperproliferative diseases. VD directly activates the transcription factor VD receptor (VDR), which in turn stimulates the expression of a cascade of primary and secondary VD-responsive genes. The activation of the VDR through binding of its natural and synthetic ligands is linked to a conformational change presenting the interface with co-activator proteins, referred to as the (trans)activation function 2 (AF-2) domain. Multiple conformations of the VDR might be the key to understanding a selective action of VD analogues. The method of limited protease digestion was used here to characterize up to three different functional VDR conformations stabilized individually by VD and its analogues. The relative potency of VDR ligands can be quantified in the interaction with these VDR conformations by determination of a functional dissociation constant, where a two-concentration-point comparison has already provided important information. In this way seven amino acid residues in the AF-2 domain have been analysed as potential ligand contact points. Interestingly, residues Phe-422 and Val-418 seem to interact with all tested VDR ligands, whereas VD analogues such as the anti-psoriatic drug MC903 displayed additional contact points within the AF-2 domain. Taken together, limited protease digestion is a powerful method for studying functional VDR conformations and seems to be very appropriate for screening VD analogues.

The high affinity ligand binding conformation of the nuclear 1,25-dihydroxyvitamin D3 receptor is functionally linked to the transactivation domain 2 (AF-2)

The nuclear receptor for 1,25-dihydroxyvitamin D3 (VD), VDR, is a transcription factor that mediates all genomic actions of the hormone. The activation of VDR by ligand induces a conformational change within its ligand binding domain (LBD). Due to the lack of a crystal structure analysis, biochemical methods have to be applied in order to investigate the details of this receptor-ligand interaction. The limited protease digestion assay can be used as a tool for the determination of a functional dissociation constant (K(df)) of VDR with any potential ligand. This method provided with the natural hormone VD two protease-resistant fragments of the VDR LBD and with the 20-epi conformation of VD, known as MC1288, even an additional fragment of intermediate size. These fragments were interpreted as different receptor conformations and their decreasing size was found to be associated with decreasing ligand binding affinity. A critical amino acid for VDR's high ligand binding conformation has been identified by C-terminal receptor truncations and point mutations as phenylalanine 422. This amino acid appears to directly contact the ligand and belongs to the ligand-inducible activation function-2 (AF-2) domain. Moreover, functional assays supported the observation that high affinity ligand binding is directly linked to transactivation function.