Antisense knockout of HOXB4 blocks 1,25-dihydroxyvitamin D3 inhibition of c-myc expression

Abstract

The expression of c-myc is decreased by 1,25-(OH)2D3 during HL-60 cell differentiation. Concomitantly, 1,25-(OH)2D3 increases the expression and DNA binding activity of HOXB4, a homeobox gene. HOXB4 binds to the c-myc gene at sites involved in blocking c-myc transcription elongation. In this study, a phosphorothioate antisense oligonucleotide targeted against HOXB4 was examined for its effect on 1,25-(OH)2D3 inhibition of c-myc expression. Alone, 1,25-(OH)2D3 (20 nM) increased HOXB4 levels by 103 ± 7% (mean ± s.e., n=3) and decreased c-myc levels by 89 ± 5% (mean ± s.e.m., n=3) at 48 h of treatment. HOXB4 antisense treatment completely blocked the induction of HOXB4 by 1,25-(OH)2D3. In addition, HOXB4 antisense partially blocked 1,25-(OH)2D3-mediated decrease in c-myc levels (46 ± 6% inhibition) and promotion of HL-60 cell differentiation (20 ± 2% and 25 ± 3% inhibition as assessed by nitroblue tetrazolium and non-specific esterase assays respectively). The data further establish that HOXB4 levels are regulated by 1,25-(OH)2D3 and reveal that HOXB4 participates in the down-regulation of c-myc expression.