Vitamin D deficiency or insufficiency is overwhelmingly find more associated with viral hepatitis, cirrhosis, and fatty liver diseases. Recent clinical trials have shown that vitamin D supplements
significantly enhance the efficacy of interferon plus ribavirin therapy through sustained virological response. A recent study showed that 25-dihydroxyvitamin D rather than 1,25-dihydroxyvitamin D could directly suppress hepatitis C virus assembly. Moreover, clinical evidence has shown that vitamin D deficiency is associated with alcoholic and non-alcoholic fatty liver diseases. In this review, we highlight some recent advances in vitamin D researches and clinical trails. Different from the classical definition of a vitamin, VD is neither
a co-enzyme factor nor an essential nutrient component. In addition to dietary sources from animals (VD3) or plants (VD2), VD can be synthesized in the skin from cholesterol under sunlight. Driven by sunlight, 7-dehydrocholesterol in the skin cells is converted to pre-vitamin D3, which consequently undergoes an isomerization process to vitamin D3, also known as cholecalciferol. The first step in the synthesis of biologically active VD from vitamin D3 occurs in hepatocytes through 25-hydroxylation, catalyzed by Cyp2R1 or Cyp27A1. Secreted from hepatocytes, 25(OH)D3 is conveyed by VD-binding protein (VDBP) to the kidneys, where it is additionally hydroxylated by 1-alpha-hydroxylase (Cyp27B1) to generate fully http://www.selleckchem.com/products/Nolvadex.html activated form, 1,25-dihydroxyvitamin D, namely calcitriol. VD levels are also regulated by its degradation processes. Through a negative feedback loop, calcitriol can induce the catabolic enzyme 24-hydroxylase (Cyp24A1) in the kidneys as well as in other VD-targeting tissues, which inactivates VD and promotes it breakdown. Furthermore, to ensure bioavailability, Cyp24A1 transcription is negatively regulated by parathyroid hormone (PTH) driven by low calcium levels. Serum 25(OH)D levels are usually a thousand times higher than 1,25(OH)2D levels, indicating that the limiting step for
synthesis of active VD is conducted mostly by 1-hydroxylation via the relative activities of between its synthesis by Cyp27B1 and degradation by Cyp24A1 in the Nintedanib (BIBF 1120) same cells. In healthy individuals, serum levels are normally 25–40 ng/mL (62–99 nM) for 25(OH)VD3, and 20–45 pg/mL (48–108 pM) for 1,25(OH)2D3. In addition to the liver–kidney loop for the synthesis of 1,25(OH)2D3, the immune system can independently generate bioactive VD through a distinct regulatory mechanism. It was initially recognized that activated macrophages in sarcoidosis, a form of calcified lung fibrosis, could generate abundant calcitriol. Later it was found that normal macrophages under lipopolysaccharide (LPS) and interferon-gamma stimulation could also produce calcitriol.