Date Published: November 28, 2011
Publisher: Public Library of Science
Author(s): Clair L. Alvino, Shee Chee Ong, Kerrie A. McNeil, Carlie Delaine, Grant W. Booker, John C. Wallace, Briony E. Forbes, Elizabeth Wilson. http://doi.org/10.1371/journal.pone.0027488
Insulin-like growth factor-II (IGF-II) promotes cell proliferation and survival and plays an important role in normal fetal development and placental function. IGF-II binds both the insulin-like growth factor receptor (IGF-1R) and insulin receptor isoform A (IR-A) with high affinity. Interestingly both IGF-II and the IR-A are often upregulated in cancer and IGF-II acts via both receptors to promote cancer proliferation. There is relatively little known about the mechanism of ligand induced activation of the insulin (IR) and IGF-1R. The recently solved IR structure reveals a folded over dimer with two potential ligand binding pockets arising from residues on each receptor half. Site-directed mutagenesis has mapped receptor residues important for ligand binding to two separate sites within the ligand binding pocket and we have recently shown that the IGFs have two separate binding surfaces which interact with the receptor sites 1 and 2.
In this study we describe a series of partial IGF-1R and IR agonists generated by mutating Glu12 of IGF-II. By comparing receptor binding affinities, abilities to induce negative cooperativity and potencies in receptor activation, we provide evidence that residue Glu12 bridges the two receptor halves leading to receptor activation.
This study provides novel insight into the mechanism of receptor binding and activation by IGF-II, which may be important for the future development of inhibitors of its action for the treatment of cancer.
The insulin-like growth factors (IGF-I and IGF-II) share sequence and structural similarities with insulin. IGFs have four domains in the order B, C, A, and D from the N terminus (Figure 1) and three alpha helices making up the core structure. Insulin is produced as a propeptide which, when processed to the mature form, only has the B and A domains on separate chains linked as a dimer by disulphide bonds and having a similar helical structure to the IGFs. In contrast to the critical role of insulin in metabolic control the IGFs act via the type 1 IGF receptor (IGF-1R) to promote cell proliferation, survival and differentiation. IGFs are essential for normal growth and development, and perturbation of IGF-I expression is associated with acromegaly (increased)  or short stature (decreased) . Disruption of IGF-II imprinting during development is associated with overgrowth in Beckwith-Wiedemann syndrome, whereas reduced paternal allele expression results in growth retardation in Silver-Russell syndrome . Furthermore, IGFs acting via the IGF-1R play a major role in promoting cancer cell growth and survival . Therefore understanding the mechanism of receptor activation will aid in the understanding of the role of these ligands in disease.
IGF-II binds to the IGF-1R and the IR-A and activates mitogenic signalling leading to cell proliferation and survival. Receptor binding involves a two site binding mechanism . With a series of six novel IGF-II mutants we have demonstrated the importance of IGF-II residue Glu12 in IGF-1R and IR-A binding and activation and have shown that both the size and charge of the Glu sidechain at this position are important for achieving high affinity binding of IGF-II to both receptors.