TRANSCRIPTIONAL ACTIVATION OF THE CDK INHIBITOR P21 BY VITAMIN-D-3 LEADS TO THE INDUCED-DIFFERENTIATION OF THE MYELOMONOCYTIC CELL-LINE U937

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D-3, acting thro ugh its cognate nuclear receptor (vitamin D-3 receptor, VDR) will indu ce myeloid leukemic cell lines to terminally differentiate into monocy tes/macrophages. Because VDR acts by transcriptionally regulating resp onsive genes in a ligand-dependent manner, we sought target genes of t he receptor that initiate the differentiation process in response to l igand. We screened a cDNA library prepared from the myelomonocytic U93 7 cell line with probes generated from either 1,25-dihydroxyvitamin D- 3-treated or untreated cells. We report here that a candidate clone th at hybridized differentially is the Cdk inhibitor p21(WAF1, CIP1). Fur thermore, we show that p21 is transcriptionally induced by 1,25-dihydr oxvitamin D-3 in a VDR-dependent, but not p53-dependent, manner, and w e identify a functional vitamin D response element in the p21 promoter . Transient overexpression of p21 and/or the related Cdk inhibitor p27 in U937 cells in the absence of 1,25-dihydroxvitamin D-3 results in t he cell-surface expression of monocyte/macrophage-specific markers, su ggesting that ligand-modulated transcriptional induction of the p21 ge ne facilitates the induced differentiation of this monoblastic cell li ne. We believe that this is the first report demonstrating that the ec topic overexpression of a Cdk inhibitor such as p21 or p27 directly le ads to a terminal differentiation program.