The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3, acting through its cognate nuclear receptor (vitamin D3 receptor, VDR) will induce myeloid leukemic cell lines to terminally differentiate into monocytes/macrophages. Because VDR acts by transcriptionally regulating responsive genes in a ligand-dependent manner, we sought target genes of the receptor that initiate, the differentiation process in response to ligand. We screened a cDNA library prepared from the myelomonocytic U937 cell line with probes generated from either 1,25-dihydroxyvitamin D3-treated or untreated cells. We report here that a candidate clone that hybridized differentially is the Cdk inhibitor p21WAF1, CIP1. Furthermore, we show that p21 is transcriptionally induced by 1,25-dihydroxyvitamin D3 in a VDR-dependent, but not p53-dependent, manner, and we 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-dihydroxyvitamin D3 results in the cell-surface expression of monocyte/macrophage-specific markers, suggesting that ligand-modulated transcriptional induction of the p21 gene facilitates the induced differentiation of this monoblastic cell line. We believe that this is the first report demonstrating that the ectopic overexpression of a Cdk inhibitor such as p21 or p27 directly leads to a terminal differentiation program.

Progression through the cell cycle is catalyzed by cyclin-dependent kinases (CDKs) and is negatively controlled by CDK inhibitors (CDIs). We have isolated a new member of the p21CIP1/p27KIP1 CDI family and named it p57KIP2 to denote its apparent molecular mass and higher similarity to p27KIP1. Three distinct p57 cDNAs were cloned that differ at the start of their open reading frames and correspond to messages generated by the use of distinct splice acceptor sites. p57 is distinguished from p21 and p27 by its unique domain structure. Four distinct domains follow the heterogeneous amino-terminal region and include, in order, a p21/p27-related CDK inhibitory domain, a proline-rich (28% proline) domain, an acidic (36% glutamic or aspartic acid) domain, and a carboxy-terminal nuclear targeting domain that contains a putative CDK phosphorylation site and has sequence similarity to p27 but not to p21. Most of the acidic domain consists of a novel, tandemly repeated 4-amino acid motif. p57 is a potent inhibitor of G1- and S-phase CDKs (cyclin E-cdk2, cyclin D2-cdk4, and cyclin A-cdk2) and, to lesser extent, of the mitotic cyclin B-Cdc2. In mammalian cells, p57 localizes to the nucleus, associates with G1 CDK components, and its overexpression causes a complete cell cycle arrest in G1 phase. In contrast to the widespread expression of p21 and p27 in human tissues, p57 is expressed in a tissue-specific manner, as a 1.5-kb species in placenta and at lower levels in various other tissues and a 7-kb mRNA species observed in skeletal muscle and heart. The expression pattern and unique domain structure of p57 suggest that this CDI may play a specialized role in cell cycle control.