p21WAF1/CIP1 plays a major role in the induction of G1 arrest following DNA damage. Although p21WAF1/CIP1 expression is regulated by the tumor suppressor p53, induction of p21WAF1/CIP1 expression through p53-independent pathways has been described in numerous cell types. In this report, we describe the mechanism by which the retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) induces p21WAF1/CIP1 in breast carcinoma cells possessing either a wild-type (MCF-7 cells) or mutated (MDA-MB-468 cells) p53. Exposure of MDA-MB-468 cells to this retinoid results in an approximately 10-fold increase in p21WAF1/CIP1 mRNA levels, whereas less than a 2-fold increase in p21WAF1/CIP1 gene transcription was observed as indicated by transient transfection experiments utilizing a p21WAF1/CIP1 promoter firefly luciferase reporter gene construct and nuclear run-off studies. We found similar results in the MCF-7 cells (Z-M. Shao et al., Oncogene, 11: 493–504, 1995). We have now found that while enhancing p21WAF1/CIP1 gene transcription minimally, this retinoid increases p21WAF1/CIP1 mRNA stability by 3-fold in both cell types. We also demonstrate that ∼1.5 kb of the 3′ untranslated region causes enhanced instability of p21WAF1/CIP1 mRNA. The retinoid-dependent increase in p21WAF1/CIP1 mRNA stability is accompanied by an increase in p21WAF1/CIP1 protein expression, as indicated by Western blot experiments utilizing anti-p21WAF1/CIP1 monoclonal antibody. This increase in p21WAF1/CIP1 is subsequently followed by the onset of programmed cell death in both cell types. Thus, CD437 is a novel retinoid which enhances p21WAF1/CIP1 mRNA levels through stabilization of the message regardless of the p53 status of the cell.
Supported in part by the Medical Research Services of the Department of Veterans Affairs and NIH Grants CA 63335 (J. A. F.) and CA 51993 (M. I. D.).