Insulin-like growth factor binding protein (IGFBP)-3 binds IGFs with high affinity and affects their biological activity. IGFBP-3 that is not bound to IGF also affects cells via mechanisms involving binding to specific cell surface receptors and/or transport into the cell. IGFBP-3 is produced by porcine embryonic myogenic cell (PEMC) cultures. Additionally, IGFBP-3 facilitates the proliferation-suppressing actions of TGF-β1 and myostatin in PEMC cultures via mechanisms that do not involve IGF binding. Moreover, these mechanisms do not involve preventing myostatin or TGF-β1-induced increases in phosphosmad2 or phosphosmad3 level. Consequently, the mechanism(s) by which IGFBP-3 facilitates the proliferation-suppressing actions of TGF-β1 and myostatin in PEMC is unclear. Since IGFBP-3 reportedly interacts with nuclear proteins that regulate transcription, TGF-β1 or myostatin-induced translocation of IGFBP-3 into the nucleus may facilitate the proliferation-suppressing actions of these cytokines. Here, we show that IGFBP-3 is localized in cells containing the muscle specific protein desmin, thus establishing the presence of this IGFBP in myogenic cells. IGFBP-3 is present in the cytoplasm of all myogenic cells and approximately 50% of the nuclei of proliferating PEMC. IGFBP-3 is also detectable in fused myotubes. IGFBP-3 suppresses IGF-I-stimulated differentiation of PEMC but has no affect on Long-R3-IGF-I-stimulated differentiation of PEMC. Treatment of PEMC for 24 h with TGF-β1 (20 ng/ml) results in a 78% (p < 0.01) increase in the number of nuclei that contain detectable IGFBP-3. These results suggest that translocation of IGFBP-3 into the nucleus of PEMC could play a role in mediating the proliferation-suppressing action of TGF-β1.
Bibliographical noteFunding Information:
This research was supported by National Research Initiative Competitive Grants no. 2000-35206-9342 and 2006-35206-16632 from the USDA Cooperative State Research, Education, and Extension Service and by the Minnesota Agricultural Experiment Station.