We determined the molecular mechanisms by which trichostatin A (TSA) induced insulin-like growth factor-binding protein 3 (IGFBP-3) gene expression in Hep3B cells, a p53-mutant human hepatocellular carcinoma (HCC) cell line. TSA induced the expressions of the IGFBP-3 mRNA and protein and the activation of its promoter. Using IGFBP-3 promoter deletion constructs, the TSA-responsive element was mapped to a region between -115 and -30, relative to the transcription start site. Promoter mutation analysis confirmed that the TSA-responsive element coincides with the Sp1/GC-rich region on the IGFBP-3 promoter. This transcriptional activation appears to be mediated by both the Sp1 and Sp3 transcription factors and, in particular, by the phosphorylation of Sp1, because treatment of Hep3B cells and Schneider (SL2) cells with TSA significantly activated phosphorylation of Sp1 in a dose-dependent manner. Consistent with the transcriptional activation of the IGFBP-3 promoter by TSA, TSA treatment led to the release of HDAC1 and Sp3 from the Sp1 transcriptional factor complex, indicating the involvement of multiprotein complexes containing Sp1, Sp3, p300, and HDAC-1 in IGFBP-3 activation by TSA. Taken together, these results show that Sp1 phosphorylation and the modulation of the Sp1/Sp3/HDAC1 multiprotein complex play a pivotal role in the transcriptional activation of the IGFBP-3 promoter through the Sp1/GC-rich site by TSA.