FOXM1 expression mediates growth suppression during terminal differentiation of HO-1 human metastatic melanoma cells.

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.