IGFBP-3 binds to the retinoid X receptor (RXR) and has been shown to induce apoptosis in murine embryonal carcinoma cells only if the cells expressed RXR. In the present study, we examine whether direct binding of IGFBP-3 to RXR is necessary for IGFBP-3 to induce apoptosis in human prostate cancer cells. First, we mutated different combinations of amino acids in an 18-residue COOH-terminal basic domain of IGFBP-3 to the corresponding sequence of IGFBP-1, a closely related protein that does not bind RXR. Mutation of all 11 residues that differed between the two IGFBPs to the IGFBP-1 sequence was required to abolish RXR binding. Expression of the RXR-non-binding mutant in PC-3 human prostate cancer cells still induced apoptosis in an IGF-independent manner as determined by Annexin V staining and flow cytometry. These studies indicate that IGFBP-3 can induce apoptosis in human prostate cancer cells without binding RXR.
