Calcitriol receptor

Calcitriol receptor, also known as the vitamin D receptor (VDR), is a member of the nuclear receptor family of transcription factors. Calcitriol, the hormonally active form of vitamin D, binds to the VDR, which then forms a heterodimer with the retinoid-X receptor (RXR). This complex then binds to Vitamin D Response Elements (VDREs) in the DNA, regulating the expression of genes involved in calcium and phosphorus homeostasis, immune function, and cell proliferation.

Function

The primary function of the calcitriol receptor is to maintain normal blood levels of calcium and phosphorus, which is essential for bone formation, mineralization, and growth. The VDR is expressed in various tissues, including the intestine, kidney, and bone, where it mediates the effects of vitamin D. In the intestine, VDR increases the absorption of calcium and phosphorus. In the kidneys, it promotes the reabsorption of calcium and the excretion of phosphorus. In bone, VDR influences the activity of osteoblasts and osteoclasts, cells responsible for bone formation and resorption, respectively.

Beyond its role in mineral homeostasis, the VDR also plays a significant role in the immune system. It modulates the activity of immune cells, such as macrophages and dendritic cells, and regulates the production of antimicrobial peptides, which are part of the innate immune response. The VDR is also involved in cell proliferation and differentiation, and its activation has been shown to inhibit the growth of certain cancer cells.

Genetic Aspects

The gene encoding the calcitriol receptor is located on chromosome 12q13.11. Variations in this gene have been associated with susceptibility to several diseases, including osteoporosis, rickets, and certain autoimmune diseases. Polymorphisms in the VDR gene can affect the expression and activity of the receptor, influencing the individual's response to vitamin D.

Clinical Significance

The calcitriol receptor is a target for drugs used to treat conditions related to calcium and phosphorus imbalance, such as osteoporosis and secondary hyperparathyroidism. Vitamin D analogs, which activate the VDR, are used to increase calcium absorption and decrease parathyroid hormone levels. Understanding the function and regulation of the VDR is crucial for developing new therapies for bone diseases, immune disorders, and possibly cancer.

See Also

• Vitamin D
• Osteoporosis
• Rickets
• Hyperparathyroidism
• Immune system

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