The role of p21 in interferon gamma-mediated growth inhibition of human breast cancer cells

IFN-gamma-mediated growth inhibition requires signal transducers and activa tors of transcription (STAT)-1 activation and may require induction of the cyclin-dependent kinase inhibitor p21. Using an electrophoretic mobility sh ift assay, we identified STAT1 activation after IFN-gamma treatment in brea st cancer cell lines. Accordingly, IFN-gamma inhibited proliferation of mon olayer cultured MCF-7 and MDA-MB-231 breast cancer cells. Interestingly, IF N-gamma inhibited anchorage-independent growth of MCF-7 cells but had no ef fect on MDA-MB-231 colony formation. Because p21 has been shown to play a r ole in anchorage-independent growth and is a transcriptional target of STAT 1, we examined the effect of IFN-gamma on p21 mRNA. We found that IFN-gamma induced p21 mRNA in MCF-7 cells but not in MDA-MB-231 cells. Furthermore, IFN-gamma induced activation of a p21 promoter-luciferase reporter construc t that contained the STAT1-inducible element in MCF-7 cells, but not in MDA -MB-231 cells. IFN-gamma treatment resulted in increased p21 protein in MCF -7 cells, whereas MDA-MB-231 cells did not appear to express detectable p21 , even after IFN-gamma treatment. However, in MDA-MB-231 cells, p21 protein was detected only after proteosome inhibition, suggesting that degradation may be responsible for the undetectable level of p21 in these cells, despi te the abundant mRNA levels. Finally, focus formation of MDA-MB-231 cells w as inhibited by overexpression of p21. In conclusion, STAT1 activation does not appear to be sufficient for IFN-gamma-mediated growth inhibition. Furt hermore, the role of p21 appears to be complex because monolayer growth inh ibition occurs in the absence of p21, but anchorage-independent growth inhi bition may require p21. Breast cancer cells may provide a unique model for further study of IFN-gamma signaling.