Abstract
Induction of terminal differentiation represents a potentially less toxic cancer therapy. Treatment of HO‐1 human metastatic melanoma cells with IFN‐β plus mezerein (MEZ) promotes terminal differentiation with an irreversible loss of growth potential. During this process, the transcription factor FOXM1 is down‐regulated potentially inhibiting transactivation of target genes including those involved in G~2~/M progression and cell proliferation. We investigated the mechanism of FOXM1 down‐regulation and its physiological role in terminal differentiation. Genetic and pharmacological studies revealed that FOXM1 down‐regulation was primarily caused by MEZ activation of PKCα and co‐treatment with IFN‐β plus MEZ augmented the effect of PKCα. Promoter analysis with a mutated E‐box on the FOXM1 promoter, and in vitro and in vivo binding assays confirm a direct role of c‐Myc on FOXM1 expression. Reduction of c‐Myc and overexpression of Mad1 by IFN‐β plus MEZ treatment should cause potent and persistent reduction of FOXM1 expression during terminal differentiation. Overexpression of FOXM1 restored expression of cell cycle‐associated genes and increased the proportion of cells in the S phase. Our experiments support a model for terminal differentiation in which FOXM1 down‐regulation via activation of PKCα followed by suppression of c‐Myc expression, are causal events in promoting growth inhibition during terminal differentiation. J. Cell. Physiol. 226: 194–204, 2010. © 2010 Wiley‐Liss, Inc.