Induction of Myc-intron-binding polypeptides MIBP1 and RFX1 during retinoic acid-mediated differentiation of haemopoietic cells

Retinoic acid-mediated differentiation of HL60 cells is associated with an alteration of chromatin structure that maps to protein-binding sequences wi thin intron I of the c-myc gene and with down-regulation of c-myc expressio n. By using HeLa cell extracts, we previously identified two polypeptides, designated MIBP1 (for Myc-intron-binding peptide) and RFX1, that interact i n vivo and bind to the intron I element; we showed that tandem repeats of a n MIBP1/RFX1-binding site can exhibit silencer activity on a heterologous p romoter. Here we demonstrate that p160 MIBP1 and p130 RFX1 are absent from undifferentiated HL60 cells. In addition, we show that treatment with retin oic acid induces both MIBP1 and RFX1 protein, as well as their DNA-binding activity, upon granulocytic differentiation of HL60 cells, with a gel mobil ity pattern identical to that of HeLa cells. In the absence of p160 MIBP1 a nd p130 RFX1, we observed that the altered gel mobility-shift pattern detec ted in undifferentiated HL60 cells reflects the binding of two novel polype ptides, p30 and p97, that can be cross-linked to the same recognition intro n sequence. We also show that the time course of MIBP1 and RFX1 induction i s inversely correlated with the down-regulation of c-myc levels during the treatment of HL60 cells with retinoic acid.