VDR and colorectal cancer: 25(OH)D: 25-hydroxycholecalciferol; APC: adenomatous polyposis coli; CRC: colorectal cancer; COX2: cyclooxygenase-2; 5-FU: 5-fluorouracil; IL: interleukin; NF-κB: nuclear factor-κB; TNF-α: tumor necrosis factor-α; TGF-β1: transforming growth factor-β1; VDR: vitamin D receptor