Posttranscriptional regulation of p21(WAF1/CIP1) expression in human breast carcinoma cells

p21(WAF1/CIP1) plays a major role in the induction of G(1) arrest following DNA damage. Although p21(WAF1/CIP1) expression is regulated by the tumor suppressor p53, induction of p21(WAF1/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 p21(WAF1/CIP1) in breast carcinoma cells possessing either a wild-type (MCP-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 p21(WAF1/CIP1) mRNA levels, whereas less than a 2-fold increase in p21(WAF1/CIP1) gene transcription was observed as indicated by transient transfection experiments utilizing a p21(WAF1/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 p21(WAF1/CIP1) gene transcription minimally, this retinoid increases p21(WAF1/CIP1) mRNA stability by 3-fold in both cell types. We also demonstrate that similar to 1.5 kb of the 3' untranslated region causes enhanced instability of p21(WAF1/CIP1) mRNA. The retinoid-dependent increase in p21(WAF1/CIP1) mRNA stability is accompanied by an increase in p21(WAF1/CIP1) protein expression, as indicated by Western blot experiments utilizing anti-p21(WAF1/CIP1) monoclonal antibody. This increase in p21(WAF1/CIP1) is subsequently followed by the onset of programmed cell death in both cell types. Thus, CD437 is a novel retinoid which enhances p21(WAF1/CIP1) mRNA levels through stabilization of the message regardless of the p53 status of the cell.