p15^INK4b in HDAC inhibitor-induced growth arrest

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We investigate the effect of histone deacetylase (HDAC) inhibitors for the INK4 family genes. In this meeting of last year, we demonstrated that HDAC inhibitors, including Trichostatin A (TSA) and sodium butyrate, activate p19^INK4d through the Sp1 binding site on the p19^INK4d gene promoter in the human T cell leukemia cell line Jurkat. However, Jurkat cells delete p16^INK4a and p15^INK4b genes. Therefore, using human immortalized keratinocyte HaCaT cells normally conserve p15^INK4b gene, we examine the effect of HDAC inhibitors for p15^INK4b gene. HDAC inhibitors arrest the cell cycle of HaCaT cells at the G1 phase and inhibit cell growth. HDAC inhibitors stimulate the p15^INK4b promoter activity and up-regulate the p15^INK4b mRNA and protein levels, and then a hyperphosphorylated form of the RB protein is converted into a hypophosphorylated form in HaCaT cells. In addition, the overexpression of p15^INK4b completely suppressed the colony formation of HaCaT cells. Our results suggest that p15^INK4b is one of the important molecular targets of HDAC inhibitors. As a member of the INK4 family, the functional similarity to p16^INK4a suggests that p15^INK4b may function as a replacement for p16^INK4a in case p16^INK4a is inactivated. Therefore, transcriptionally regulated agents of the p15^INK4b gene may contribute to new strategies for the prevention or therapy of malignancies, which we have termed ‘gene-regulating chemoprevention or chemotherapy’.