CDKN1A and cholangiocarcinoma: VDR expression in human cholangiocarcinoma tissue specimens (41-43) Treatment with vitamin D or analogs impairs proliferation and induces apoptosis in cultured cells. Proposed mechanisms include induction of cell cycle arrest through regulation of cyclin D1, cyclin D3, CDK4, CDK6, p21, and/or p27 (44-47) VDR dependent downregulation of LCN2 expression (46,47,49)