Figure 2.

Principles of vitamin D signaling. 7-dehydrolesterol is a direct precursor of cholesterol but under exposure with UV-B can also convert into vitamin D₃ (left). CYP2R1 (cytochrome P450 family 2 subfamily R member 1) hydroxylates vitamin D₃ into 25(OH)D₃ (25-hydroxyvitamin D₃) and CYP27B1 adds a further hydroxy group to C1. 1,25(OH)₂D₃ is a nuclear hormone that activates VDR (red). The pioneer transcription factors PU.1 (purine-rich box 1, dark blue) and CEBPα (CCAAT enhancer binding protein α, green) support the DNA binding of VDR-RXR (retinoid X receptor, light green) heterodimers. VDR-bound enhancers activate via DNA looping and the Mediator complex Pol II (RNA polymerase II) waiting on the TSS (transcription start site) region of a vitamin D target gene. This finally changes the expression of hundreds of vitamin D target genes (top right), a number of which are involved in the regulation of the immune system and have an impact on MS (Section 4).