26,27-Dihomo-1 alpha-hydroxy- and 26,27-dihomo-24-epi-1 alpha,25-dihydroxyvitamin D2 analogs that differ markedly in biological activity in vivo

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Investigating the Role of VDR and Megalin in Semi-Selectivity of Side-Chain Modified 19-nor Analogs of Vitamin D

1,25-dihydroxyvitamin D₃ (1,25D3) is implicated in many cellular functions, including cell proliferation and differentiation, thus exerting potential antitumor effects. A major limitation for therapeutic use of 1,25D3 are potent calcemic activities. Therefore, synthetic analogs of 1,25D3 for use in anticancer therapy should retain cell differentiating potential, with calcemic activity being reduced. To obtain this goal, the analogs should effectively activate transcription of genes responsible for cell differentiation, leaving the genes responsible for calcium homeostasis less active. In order to better understand this phenomenon, we selected a series of structurally related 19-nor analogs of 1,25D (PRI-5100, PRI-5101, PRI-5105, and PRI-5106) and tested their activities in blood cells and in cells connected to calcium homeostasis. Affinities of analogs to recombinant vitamin D receptor (VDR) protein were not correlated to their pro-differentiating activities. Moreover, the pattern of transcriptional activities of the analogs was different in cell lines originating from various vitamin D-responsive tissues. We thus hypothesized that receptors which participate in transport of the analogs to the cells might contribute to the observed differences. In order to study this hypothesis, we produced renal cells with knock-out of the megalin gene. Our results indicate that megalin has a minor effect on semi-selective activities of vitamin D analogs.

Mechanisms and functions of vitamin D

The vitamin D hormone, 1,25-dihydroxyvitamin D3, functions by way of a nuclear receptor (vitamin D receptor [VDR]) in a manner analogous to the other members of the steroid-thyroid hormone superfamily. Although the vitamin D receptor has been cloned, its three-dimensional structure remains unknown. The VDR binds to the direct repeat response elements called DR-3 in the promoter region of target genes to stimulate or suppress transcription of those genes encoding for proteins that carry out a wide variety of functions. The binding of the VDR to a DR-3 requires the presence of its ligand and a companion protein, namely the RXR group of retinoid receptors. The RXR binds to the 5' arm of the response element while the VDR binds to the 3' arm. In addition, the transcription factor TFIIB has been shown to bind VDR but there is currently no evidence that a co-repressor or co-activator of VDR is also involved. Phosphorylation of VDR in the transcription complex occurs as does bending of the DNA prior to the initiation or suppression of transcription. As VDR has been detected in cells not previously thought to be target organs, scientists continue to discover new functions of vitamin D. Among these new functions are those noted in the immune system. Experiments in mice have illustrated that the autoimmune diseases of multiple sclerosis and rheumatoid arthritis can be successfully treated with the vitamin D hormone and its analogs. New experiments illustrating the use of the vitamin D hormone and its analogs in suppressing transplant rejection indicate that these compounds may be superior to cyclosporin and may not have the side effects attributed to the cyclosporin immunosuppression therapies.