Insulin-like growth factor-binding protein-5 (IGFBP-5): a critical member of the IGF axis

Abstract

The six members of the insulin-like growth factor-binding protein family (IGFBP-1-6) are important components of the IGF (insulin-like growth factor) axis. In this capacity, they serve to regulate the activity of both IGF-I and -II polypeptide growth factors. The IGFBPs are able to enhance or inhibit the activity of IGFs in a cell- and tissue-specific manner. One of these proteins, IGFBP-5, also has an important role in controlling cell survival, differentiation and apoptosis. In this review, we report on the structural and functional features of the protein which are important for these effects. We also examine the regulation of IGFBP-5 expression and comment on its potential role in tumour biology, with special reference to work with breast cancer cells.

Figure 1. The IGF axis consists of IGF-I and IGF-II polypeptides (orange), six IGFBPs (various colours), a family of IGFBP proteases (yellow) and cell-surface IGFBP receptor, IGF-IR and IGF-IIR (light grey)

Cell membrane receptors for IGFBP-1 and IGFBP-3 have been partially defined [211,212]. In the Figure, IGFBP-5 is shown interacting with components of the ECM at a site removed from the IGF-binding site. See text for further details.

Figure 2. Sequence alignment of rat IGFBP-1–6

Asterisks indicate the location of 18 conserved cysteine residues clustered in the N- and C-terminal domains for each IGFBP. The two extra cysteine residues in IGFBP-4 are underlined. When a residue is identical in three out of five of the IGFBPs, it is shown in bold. Dashes have been introduced in the N- and C-terminal domains to allow for better alignment of sequences. The central domain of the IGFBPs show less conserved sequence and these residues are bracketed. Figure reproduced from [14] with permission. © 1991 The American Society for Biochemistry and Molecular Biology.

Figure 3. Disulphide linkages in IGFBPs

Cysteine residues are numbered sequentially from the N-terminus. Numbers in parentheses represent the actual residue number (the first residue in the mature peptide is designated residue 1). r, rat; b, bovine; h, human. Reproduced from D. Byun, S. Mohan, D. J. Baylink and X. Qin (2001), Localization of the IGF binding domain and evaluation of the role of cysteine residues in IGF binding in IGF binding protein-4, J. Endocrinol., 169, 135–143. © Society for Endocrinology (2001). Reproduced by permission.

Figure 4. Location of putative transcription-factor-binding sites within the proximal promoter region of IGFBP-5, identified relative to the mRNA CAP site (+1)

Agents shown to regulate IGFBP-5 expression are indicated in parentheses below transcription factors. With the exception of OP-1, these act to increase IGFBP-5 expression. PGE₂-RE, PGE₂-responsive element.

Figure 5. Biosensor analysis of WT, AA–EA and IGF⁻ mutant IGFBP-5 binding to immobilized IGF-I, heparin, PAI-1 or tPA

Details of mutant proteins are described in [123] and biosensor methodology is described in detail in [141a]. The IGF⁻ mutant protein was originally described by Imai et al. [46] and shows no detectable binding to IGF-I, but has unimpaired binding to heparin, PAI-1 and tPA.

Figure 6. Analysis of the binding of wild-type (WT) and mutant (AA–FA) IGFBP-5 species to heparin and IGF-I was analysed by biosensor (upper panel) and blotting methodologies (lower panel)

See [123] for details of the IGFBP-5 mutants. Note the parallelism in the loss of affinity for both IGF-I and heparin with cumulative mutagenesis of positively charged residues within the region 201–218 of IGFBP-5. RU=resonance units; e⁻²=10⁻²; molecular-mass sizes are indicated in kDa. See the text for further details. Reproduced from Endocrinology, ‘Cumulative mutagenesis of the basic residues in the 201–218 region of insulin-like growth factor (IGF)-binding protein-5 results in progressive loss of both IGF-I binding and inhibition of IGF-I biological action' by G. J. Allan, E. Tonner, M. Szymanowska, J. H. Shand, S. M. Kelly, K. Phillips, R. A. Clegg, I. F. Gow, J. Beattie and D. J. Flint 147(1), pp. 338–349, with permission. © 2006, The Endocrine Society.

Figure 7. Stromal cell derived IGF-I and epithelial cell-derived IGFBP-5 interact to regulate mammary epithelial cell function

During lactation, IGF-I maintains secretory function in the mammary epithelial cell. At this stage, the activity of the pituitary hormones GH (stimulating IGF-I secretion) and prolactin (inhibiting IGFBP-5 secretion) act to maintain a high IGF-I/IGFBP-5 ratio. During mammary gland regression and apoptosis of epithelial cells, IGFBP-5 concentrations increase and neutralize the survival factor activity of IGF-I. See the text for further details.