VDR and gemini ligands.

The active form of vitamin D, 1α,25-dihydroxyvitamin D3 [1,25(OH) 2 D 3 ; calcitriol], plays a key role in mineral and bone homeostasis, and exerts potent anti-inflammatory and anti-proliferative activities [1]. It is thus a potential pharmacological agent to treat various diseases, including autoimmune disorders, infections and cancer [1]. However, the 1,25(OH) 2 D 3 doses required to elicit such effects induce hypercalcemia, resulting in ectopic calcification of the vascular wall, kidney and other soft tissues, that can lead to organ failure and death [1]. 1,25(OH) 2 D 3 activities are mediated by the Vitamin D receptor (VDR; NR1I1), a member of the nuclear receptor superfamily [1]. More than 3000 VDR ligands were synthesized using medicinal chemistry approaches, but all those exhibiting potent anti-inflammatory and/or anti-proliferative properties still have hypercalcemic activities, which limit their clinical use [1]. VDR loss-of-function mutations in humans, termed Vitamin D-Dependent Rickets type-II (VDDR-II), and VDR-null mice develop skeletal deformities, osteomalacia, hypocalcemia and hypophosphatemia [2]. In addition, VDR DNA-binding deficient patients and VDR-null mice display alopecia [2]. Hair follicle defects in VDR-null mice are prevented by transgenic expression of ligand-binding deficient VDR in keratinocytes [3]. In contrast, severe deficiencies in 1,25(OH) 2 D 3 induced by dysfunctional 25(OH)-vitaminD 3-1α-hydroxylase (Cyp27b1), the enzyme that converts 25(OH)D 3 to 1,25(OH) 2 D 3 in VDDR-I patients, as well as in Cyp27b1-null mice, do not induce alopecia, even though they lead to rickets, which can be treated by 1,25(OH) 2 D 3 [2, 4]. Similarly, patients and mice expressing a mutated VDR with reduced affinity for 1,25(OH) 2 D 3 without impairing DNA binding have skeletal but no hair defects [2]. Thus, it appears that liganded VDR is essential for mineral ion homeostasis and skeletal metabolism, whereas ligand-independent VDR activities control hair cycling. Phenotypic analyses of mutant mice suggested that bone and mineral homeostasis alterations might be more pronounced in Cyp27b1-null mice than in VDR-null [4, 5]. Moreover, data presented in a recent report indicated that the size of mice expressing VDR L233S , a VDR bearing a mutation in the ligand binding domain that impairs calcitriol binding, was reduced compared to VDR-null [3]. Taken together, these results indicated that unliganded VDR might induce more severe skeletal defects than VDR deficiency. To characterise VDR ligand-independent activities, we generated, based on VDR structural analyses, mice expressing a ligand binding domain point-mutated VDR (VDRgem) that is unresponsive to 1,25(OH) 2 D …