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Endocrine
Vertebrates
Vitamin D Signaling
Vitamin D3 activation and nuclear receptor signaling for calcium homeostasis.
Overview
Vitamin D3 (cholecalciferol) is produced in skin by UV-B radiation or obtained from diet. It is hydroxylated to 25(OH)D in the liver (by CYP2R1) and to active 1,25(OH)₂D (calcitriol) in the kidney (by CYP27B1). Calcitriol binds the vitamin D receptor (VDR), which heterodimerizes with RXR to regulate genes controlling calcium absorption (intestine), bone mineralization, and immune function. PTH stimulates CYP27B1 when calcium is low.
Cellular Location
Skin → Liver → Kidney → Target tissues
Clinical Significance
Deficiency causes rickets (children) and osteomalacia (adults); linked to immune function, cancer risk, and cardiovascular health; pandemic of vitamin D insufficiency worldwide.