The role of the transcription factor Sp1 in regulating the expression of the WAF1/CIP1 gene in U937 leukemic cells.

The Waf1/Cip1 protein induces cell cycle arrest through inhibition of the activity of cyclin-dependent kinases and proliferating cell nuclear antigen. Expression of the WAF1/CIP1 gene is induced in a p53-dependent manner in response to DNA damage but can also be induced in the absence of p53 by agents such as growth factors, phorbol esters, and okadaic acid. WAF1/CIP1 expression in U937 human leukemic cells is induced by both phorbol ester, a protein kinase C activator, and by okaidaic acid, an inhibitor of phosphatases 1 and 2A. Both of these agents induce the differentiation of these leukemic cells toward macrophages. We demonstrate that phorbol esters and okadaic acid stimulate transcription from the WAF1/CIP1 promoter in U937 cells. This transcription is mediated by a region of the promoter between -154 and +16, which contains two binding sites for the transcription factor Sp1. Deletion or mutation of these Sp1 sites reduces WAF1/CIP1 promoter response to phorbol ester and okadaic acid, while a reporter gene under the control of a promoter containing only multiple Sp1 binding sites and a TATA box is induced by phorbol ester and okadaic acid. The WAF1/CIP1 promoter is also highly induced by exogenous Sp1 in the Sp1-deficient Drosophila Schnieder SL 2 cell line. These results suggest that phorbol ester and okadaic acid activate transcription of the WAF1/CIP1 promoter through a complex of proteins that includes Sp1 and basal transcription factors.