ReviewWhat's new in the IGF-binding proteins?
Section snippets
Historical introduction and overview
Over the last few years we have witnessed a renewed interest in the chemistry and biology of the IGFBPs. The new awareness of these proteins stems from numerous reports demonstrating that they exhibit a multitude of direct cellular actions that appear to be separate from their roles as IGF carrier proteins. A number of observations, which could not be explained on the basis of the paradigm of IGFBP action modulating IGF availability and function, led to the hypothesis that some of the IGFBPs
IGFBP structure: will the real IGF-binding domain please stand up?
The IGFBPs comprise a family of six soluble proteins ranging in length from 216 to 289 amino acids (Fig. 1). Based on their primary structures, each IGFBP may be divided into three distinct domains of approximately equal size. The NH2- and COOH-termini are highly conserved and contain 16–18 spatially conserved cysteine residues that form the various intra-domain disulfide bonds [1], [5]. The middle or linker domain is the least conserved region of the protein family, exhibiting <30% primary
Analysis of IGF-I:IGFBP interactions
Ligand blotting is a rapid, qualitative, and at least semi-quantitative technique for studying the IGFBPs [27]. This method additionally provides a means of examining “biologically relevant” IGFBPs, as misfolded, degraded or denatured proteins will not be detected. Few changes have been introduced into this method since its inception in 1986. A number of laboratories have applied the use of substituted biotinylated IGF-I or IGF-II in place of 125I-labeled ligand, coupled with detection using an
IGFBP function: IGF-dependent vs. independent actions
Based on the exquisitely high affinities of IGF-I and IGF-II for IGFBPs, which exceeds the affinity toward the IGF-IR, the IGFBPs have been viewed as proteins involved in the sequestration of ligand away from the IGF-IR. Over the last few years, this concept has changed with the emerging idea that IGFBPs have additional activities that are independent of their IGF binding [33]. A variety of sometimes paradoxical and conflicting biological effects have been reported, including both stimulation
IGFBP cell biology: cell surface receptors, internalization and nuclear targets
In addition to cell surface interactions, considerable interest has been generated by observations of nuclear targeting of IGFBPs and in identification of potential nuclear proteins as binding partners in two-hybrid screening protocols. Fluorescent derivatives of IGFBP-3 and IGFBP-5 were observed to translocate to the nucleus [57] via an importin β-dependent pathway [58]. Once in the nucleus, these IGFBPs have been proposed to regulate transcriptional processes involved in the cell death
IGFBPs and cancer
Over the last few years the role of the IGF system in cancer has become an area of considerable interest. IGFBP-3 has been defined in epidemiological studies as a negative risk factor for breast, prostate and colorectal cancers, with low serum IGFBP-3 and high IGF-I levels posing the greatest statistical risk [67]. Consistent with this anti-cancer role are findings in cell culture models that IGFBP-3 inhibits neoplastic cell growth [14], [68]. It has further been postulated that other IGFBPs
The future of IGFBP-based therapeutics
As natural antagonists of the IGFs, IGFBPs logically could be significant therapeutic agents for inhibiting IGF actions in a variety of human diseases, including cancer [19], diabetic renal, vascular, and eye disease [76], [77], and the “wet” form of age-related macular degeneration [78], [79]. To this end, a number of investigators have designed potential small molecule IGFBP-mimetics capable of complexing with IGFs and blocking their access to the IGF-IR. Deshayes et al. recently reported the
Conclusions
Over the last several years the interest in IGFBPs has evolved from a focus on their roles as carrier proteins that modulate the levels of free IGF, to interest in a variety of other biological effects. Clearly, many new and old questions remain to be answered in the field, including the three-dimensional structure of each IGFBP and the precise biochemistry of their interactions with IGFs, with extra-cellular matrix proteins, with putative receptors, and with intracellular targets. The
Acknowledgements
This work was supported by NIH Grant CA78887 and DoD Grant GC-3532-03-42153CM awarded to Hollings Cancer Center.
References (90)
- et al.
Localization of an insulin-like growth factor (IGF) binding site of bovine IGF binding protein-2 using disulfide mapping and deletion mutation analysis of the C-terminal domain
J. Biol. Chem.
(1998) - et al.
Structure–function analysis of the human insulin-like growth factor binding protein-4
J. Biol. Chem.
(1998) - et al.
Site-directed mutagenesis of the NH2-terminal region of IGF binding protein 1; analysis of IGF binding capability
FEBS Lett.
(1991) - et al.
Substitutions for hydrophobic amino acids in the NH2-terminal domains of IGFBP-3 and -5 markedly reduce IGF-I binding and alter their biologic actions
J. Biol. Chem.
(2000) - et al.
Insulin-like growth factor (IGF)-binding protein-3 mutants that do not bind IGF-I or IGF-II stimulate apoptosis in human prostate cancer cells
J. Biol. Chem.
(2002) - et al.
BIAcore analysis of bovine insulin-like growth factor (IGF)-binding protein-2 identifies major IGF binding site determinants in both the amino- and carboxyl-terminal domains
J. Biol. Chem.
(2001) - et al.
Synthesis and characterization of insulin-like growth factor (IGF)-1 photoprobes selective for the IGF-binding proteins (IGFBPS). Photoaffinity labeling of the IGF-binding domain on IGFBP-2
J. Biol. Chem.
(2001) - et al.
Isolation of a biologically active fragment from the carboxy terminus of the fetal rat binding protein for insulin-like growth factors
Biochem. Biophys. Res. Commun.
(1988) - et al.
Analysis of serum insulin-like growth factor binding proteins using Western blotting: use of the method for titration of the binding proteins and competitive binding studies
Anal. Biochem.
(1986) The insulin-like growth factor system as a treatment target in breast cancer
Semin. Oncol.
(2002)