Phorbol esters enhance 1α,25-dihydroxyvitamin D3-regulated 25-hydroxyvitamin d-24-hydroxylase (CYP24A1) gene expression through ERK-mediated phosphorylation of specific protein 3 (Sp3) in Caco-2 cells

► Phorbol 12-myristate 13-acetate (PMA) enhances 1,25(OH)2D3 action. ► PMA enhances vitamin D receptor binding to the human hCYP24A1 gene promoter. ► Specific protein (Sp) family members 1 and 3 are involved. ► PMA increased Sp3-VDR interactions and Sp3 binding to the hCYP24A1 promoter. ► The effect of PMA on Sp3 is dependent on mitogen activated protein kinases.