Insulin-Like Growth Factor Binding Protein-5 in Physiology and Disease

Figures

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  Figure 1. The structure of IGFBP-5. IGFBP-5 consists of 3 structurally domains: a highly conserved globular N-terminal domain, a central non-conserved linker domain, and a conserved C-terminal globular domain. The N-domain contains 12 conserved cysteine residues and a hydrophobic patch important for IGF binding. The L-domain contains several proteolytic cleavage sites. The C-domain contains six conserved cysteine residues, a RK-rich sequence (red) important for IGF binding, ALS binding, nuclear localization, and a thyroglobulin-like fold and other sites of interaction with ECM components.

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  Figure 2. Proposed modes of IGFBP-5 actions. (A) IGFBP-5 modulates circulating IGFs by forming a binary complex with IGF or a ternary complex with IGF and acid labile subunit (ALS) in the blood. (B) IGFBP-5 inhibits IGF signaling in target cells by sequestrating IGF away from the IGF1R. (C) IGFBP-5 potentiates IGF signaling in target cells by #1) releasing of bound IGF to the IGF-1R upon interaction with ECM and cell surface molecules or #2) protease-mediated IGFBP-5 proteolysis. (D) IGF-independent action of IGFBP-5 via #1) its putative membrane receptor(s) or #2) interacting with co-factors in the nucleus.

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  Figure 3. IGFBP-5 regulates osteosarcoma cell survival by binding to IGFs. (A) Schematic diagram showing the structure of IGFBP-5 and three siRNA resistant IGFBP-5 expression constructs. Three point mutations were introduced into the target region of the siRNA to make the IGFBP-5 resistant to the RNA interference (BP5::GFP). Since the mutations are on the third position of the codons, the amino acid sequence is unchanged. Nuclear localization mutations (NLS) and IGF ligand binding deficient mutations (LBD) were further introduced into the siRNA resistant BP5::GFP plasmid, resulting in the NLS::GFP and LBD::GFP construct. (B) Left panel: Western immunoblot showing the expression levels of the three siRNA resistant IGFBP-5::GFP proteins in transfected U2 osteosarcoma cells. Right panel: Western ligand blot using DIG labeled IGF-I showing the ligand binding capability of BP5::GFP, NLS::GFP and LBD::GFP. Note the lack of IGF binding of LBD::GFP. pRC is the empty GFP vector. (C) Percentage of transfected cells with nuclear GFP signal. (D) The three siRNA resistant constructs were co-transfected into human U2 osteosarcoma cells with pSuper (the empty siRNA vector) or pSuper-BP5 (IGFBP-5 siRNA…

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  Figure 4. IGFBP-5 is part of a molecular switch that turns IGF signaling on or off in target cells. Zebrafish IGFBP-5a and the conserved zinc metalloproteinase Papp-aa are expressed in ionocytes. Left panel: under normal [Ca2+] conditions, Papp-a proteolysis activity is low. Igfbp5a is intact and it inhibits IGF signaling in these cells by binding to IGFs and prevents their binding to the IGF1R in ionocytes. Right panel: under low [Ca2+] conditions, Papp-a activity is increased. This increases IGFBP-5a proteolytic cleavage and releases IGFs from the IGFBP-5a/IGF complex to activate IGF-1 receptor-mediated PI3 kinase-Akt-Tor signaling and promotes ionocytes to proliferate.