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. 2020 May 5;8:286. doi: 10.3389/fcell.2020.00286

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Copyright © 2020 Varma Shrivastav, Bhardwaj, Pathak and Shrivastav.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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IGF-dependent and IGF-independent roles of IGFBP-3. (A) IGF-dependent roles of IGFBP-3 include the transport of IGFs in a binary complex, the state in which it cannot associate with IGF-1R. (B) Regulated proteolysis of the binary complex releases IGF, and free IGF can associate with the receptors (IGF-1R) to activate Akt/mTOR or MAP kinase signaling pathways. (C) IGFBP-3 can be internalized independent of IGFs into the cytosol through various endocytic mechanisms, including fluid-phase uptake, caveolae-mediated and clathrin-mediated endocytosis. (D) IGFBP-3 can associate with membrane proteins, including TGFβVR or IGFBP-3R and induce apoptosis. (E) IGFBP-3 can directly associate with TGF-β receptor to regulate the downstream Smad signaling. (F) IGFBP-3 can interact with importin-β through its nuclear localization signal (NLS) sequence and is transported into the nucleus, where IGFBP-3 can bind with nuclear receptors (NR) to inhibit gene transcription that can induce apoptosis.