is an important estrogen steroid hormone in both women and men. It is a typical steroid with core four-ring system (ABCD), composed of 17 carbon atoms. Cutting Ring B produces secosteroids one of which is vitamin D3. (5ien). Calcitriol is the active form of vitamin D pro-hormone.

• Steroid Hormones and their receptors

Types by function

Corticosteroids

Glucocorticoids

Cortisol, a glucocorticoid whose functions include immunosuppression

Mineralocorticoids

Aldosterone, a mineralocorticoid which helps regulate blood pressure through water and electrolyte balance

Sex steroids

Progestogens

Progesterone, which regulates cyclical changes in the endometrium of the uterus and maintains a pregnancy

Androgens

Testosterone, which contributes to the development and maintenance of male secondary sex characteristics

Estrogens

Estradiol, which contributes to the development and maintenance of female secondary sex characteristics

Additional classes of steroids include:

• Neurosteroids

Neurosteroids such as DHEA and allopregnanolone Aminosteroid neuromuscular blocking agents such as pancuronium bromide

Secosteroids (open-ring steroids)

Vitamin D forms such as ergocalciferol, cholecalciferol, and calcitriol.

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(5ien)

Calcitriol is the active form of vitamin D pro-hormone.

• Vitamin D receptor (also called calcitriol receptor)

is a transcription factor. Upon binding to vitamin D, VDR forms a heterodimer with retinoid-X receptor and binds to hormone response receptors on DNA causing gene expression. The (green) binds to receptors in its target cells, controlling the synthesis of many different proteins involved in Ca transport and utilization.

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. VDR contains 2 domains: a , that binds to the hormone (grey) and that binds to DNA (green and blue are 2 same VDR structures). It pairs up with a similar protein, 9-cis retinoic acid receptor (RXR), and together they bind to the DNA, activating synthesis in some cases and repressing it in others. When is mutated it is replaced with a which results in an inhibition of transcriptional activation. When transcription is inhibited it results in p53 accumulation, which activates and promotes p53 translocation into mitochondria leading to apoptosis. is replaced with when mutated creating a negative charge. The negative charge at the residue inhibits DNA binding which cause a downregulation of VDR activity. VDR needs DNA binding in order for it to be activated which is only possible with a serine residue.

The vitamin D nuclear receptor is a ligand-dependent transcription factor that controls multiple biological responses such as cell proliferation, immune responses, and bone mineralization. Numerous 1 α,25(OH)(2)D(3) analogues, which exhibit low calcemic side effects and/or antitumoral properties, have been synthesized. It was shown that acts as a 1α,25(OH)(2)D(3) superagonist and exhibits both antiproliferative and prodifferentiating properties in vitro. Using this information and on the basis of the crystal structures of human VDR ligand binding domain (hVDR LBD) bound to 1α,25(OH)(2)D(3), 2α-methyl-1α,25(OH)(2)D(3), or 2a, a novel analogue, 2α-methyl-(20S,23S)-epoxymethano-1α,25-dihydroxyvitamin D(3) (4a) was designed, in order to increase its transactivation potency.