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

The expression of c-myc is decreased by 1,25-(OH)2D(3) during HL-60 cell differentiation. Concomitantly, 1,25-(OH)2D(3) increases the expression and DNA binding activity of HOXB4, a homeobox gene. HOXB4 binds to the c-my c 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)2D(3) inhibition of c-myc expression. Alone, 1,25-(OH)2D(3) (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)2D(3). In addition, HOXB4 antisense partially blocked 1,25-(OH)2D(3)-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)2D(3) and reveal that HOXB4 participates in the down-regulation of c-myc expression.