Hypoxia Increases IGFBP-1 Phosphorylation Mediated by mTOR Inhibition

Area of Science:

• Endocrinology
• Developmental Biology
• Cellular Signaling

Background:

• Fetal growth restriction (FGR) involves reduced nutrient/oxygen supply, limiting fetal growth.
• Insulin-like growth factor I (IGF-I) regulates fetal growth; IGF binding protein -1 (IGFBP-1) controls its bioavailability.
• Hypoxia induces IGFBP-1 hyperphosphorylation, decreasing IGF-I availability and potentially contributing to FGR.

Purpose of the Study:

• To investigate if mechanistic target of rapamycin (mTOR) inhibition mediates IGFBP-1 hyperphosphorylation under hypoxic conditions.
• To elucidate the role of mTORC1 and mTORC2 pathways in regulating IGFBP-1 phosphorylation in response to hypoxia.

Main Methods:

• Utilized HepG2 cells cultured under hypoxia (1% O2) or normoxia (20% O2).
• Inhibited or activated mTORC1/mTORC2 using rapamycin, siRNA, or by silencing endogenous mTOR inhibitors.
• Analyzed IGFBP-1 phosphorylation via immunoblotting and mass spectrometry; assessed IGF-I receptor activity.

Main Results:

• Hypoxia and mTOR inhibition (mTORC1/mTORC2) induced significant IGFBP-1 phosphorylation at Ser101/119/169.
• Activation of mTORC1/mTORC2 pathways prevented hypoxia-induced IGFBP-1 hyperphosphorylation.
• Mass spectrometry identified novel phosphorylation sites (Ser98, Ser174) on IGFBP-1, with Ser174 near the IGF-I binding site.

Conclusions:

• Signaling through mTORC1 or mTORC2 is sufficient to induce IGFBP-1 hyperphosphorylation under hypoxic conditions.
• mTOR inhibition represents a key mechanism linking hypoxia, reduced IGF-I bioavailability, and FGR.
• This study offers new insights into the cellular control of IGFBP-1 phosphorylation in FGR.