Nuclear effects: unexpected intracellular actions of insulin-like growth factor binding protein-3

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has been shown to be a growth inhibitory, apoptosis-inducing molecule by virtue of its ability to bind IGFs, in addition to previously demonstrated IGF-independent effects. The recent discovery of the interaction between nuclear IGFBP-3 and 9-cis retinoic acid receptor-alpha (retinoid X receptor alpha RXRalpha), a nuclear receptor, and its involvement in the regulation of transcriptional signaling and apoptosis represents an important paradigm shift in the understanding of IGFBP function. RXRalpha is required for the apoptosis-inducing effects of IGFBP-3. IGFBP-3 and RXR ligands are additive in inducing apoptosis in cancer cells. IGFBP-3 has direct effects on gene transcription, as RXR response element reporter signaling was enhanced and the all-trans retinoic acid receptor response element reporter signaling was inhibited. Accumulating evidence further confirms IGF-independent functions of this multifunction binding protein. Other binding proteins, in addition to other members of the IGF axis, have now been described in the nucleus and are postulated to have effects on transcriptional events. Investigation into these new interactions will expose new protein partners in the interface between the nuclear receptor and growth factor pathways and reveal new targets to be exploited in the treatment of cancer and other diseases.