Granulocyte macrophage colony‐stimulating factor enhances retinoic acid‐induced gene expression

We reported previously that treatment of human myeloblastic leukemia ML‐1 cells with all‐ trans retinoic acid (ATRA) in combination with GM‐CSF enhances the granulocytic differentiation, which is induced only slightly by ATRA alone. To investigate the mechanism underlying this differentiation and the synergistic effect of ATRA and GM‐CSF, we used cDNA microarray to examine gene expression profiles of ML‐1 cells treated with ATRA and/or GM‐CSF. We identified 22 up‐regulated genes in ML‐1 cells treated with both reagents and examined the expression of these genes in cells treated with ATRA and/or GM‐CSF by Northen blot analysis. Comparison of cells treated with both reagents and cells treated with ATRA or GM‐CSF alone revealed that expression of nine of the 19 genes was induced synergistically by combined treatment with ATRA and GM‐CSF. Expression of most of these genes was increased only slightly by ATRA alone, and this induction was enhanced by the addition of GM‐CSF. These results indicate that GM‐CSF enhances ATRA‐induced gene expression. Moreover, studies with inhibitors of signaling molecules suggested that activation of JAK2 is associated with the synergistic induction of several genes by ATRA and GM‐CSF. JAK2 inhibitor suppressed induction of NBT‐reducing activity in ML‐1 cells treated with both reagents. It is likely that the enhancer effect of GM‐CSF on ATRA‐induced gene expression leads to the differentiation induced synergistically by ATRA combined with GM‐CSF. Further studies of the mechanism underlying this effect may identify better approaches for the treatment of RA‐insensitive leukemia.