Insulin-like growth factor binding protein-3 antagonizes the effects of retinoids in myeloid leukemia cells

Insulin-like growth factor binding protein-3 (IGFBP-3) can cause growth suppressive and proapoptotic effects on retinoids in many types of cancer cells. However, the expression and effects of IGFBP-3 in myeloid leukemia cells have not been elucidated. In this study, we found no IGFBP-3 expression in the human myeloid leukemia cell lines either at baseline or after stimulation with all-trans retinoic acid (ATRA). Human recombinant IGFBP-3 induced growth arrest and apoptosis of HL-60 and NB4 cells. We have previously identified RXRalpha as a nuclear receptor for IGFBP-3 and have proceeded to examine further the role of this interaction in leukemia cell lines. In signaling assays, IGFBP-3 potently suppressed RAR- and VDR-mediated signaling while enhancing RXR signaling. Interestingly, when IGFBP-3 was administered to these cells in combination with an RAR-selective ligand, the ability of these retinoids to induce differentiation was blunted. On the other hand, IGFBP-3 enhanced the effect of an RXR-selective ligand to induce differentiation of HL-60 and NB4 cells. Further studies showed that IGFBP-3 down-regulated (at the transcriptional level) the retinoid-induced expression of C/EBPepsilon in NB4 cells. Taken together, these results indicate that IGFBP-3 has antiproliferative activity against myeloid leukemia cells; while it enhances signaling through RXR/RXR, it blunts signaling by activated RAR/RXR.