Structural Determinants for High-Affinity Binding of Insulin-Like Growth Factor II to Insulin Receptor (IR)-A, the Exon 11 Minus Isoform of the IR

Abstract

The insulin receptor (IR) lacking the alternatively spliced exon 11 (IR-A) is preferentially expressed in fetal and cancer cells. The IR-A has been identified as a high-affinity receptor for insulin and IGF-II but not IGF-I, which it binds with substantially lower affinity. Several cancer cell types that express the IR-A also overexpress IGF-II, suggesting a possible autocrine proliferative loop. To determine the regions of IGF-I and IGF-II responsible for this differential affinity, chimeras were made where the C and D domains were exchanged between IGF-I and IGF-II either singly or together. The abilities of these chimeras to bind to, and activate, the IR-A were investigated. We also investigated the ability of these chimeras to bind and activate the IR exon 11+ isoform (IR-B) and as a positive control, the IGF-I receptor (IGF-1R). We show that the C domain and, to a lesser extent, the D domains represent the principal determinants of the binding differences between IGF-I and IGF-II to IR-A. The C and D domains of IGF-II promote higher affinity binding to the IR-A than the equivalent domains of IGF-I, resulting in an affinity close to that of insulin for the IR-A. The C and D domains also regulate the IR-B binding specificity of the IGFs in a similar manner, although the level of binding for all IGF ligands to IR-B is lower than to IR-A. In contrast, the C and D domains of IGF-I allow higher affinity binding to the IGF-1R than the analogous domains of IGF-II. Activation of IGF-1R by the chimeras reflected their binding affinities whereas the phosphorylation of the two IR isoforms was more complex.

The insulin receptor is essential for virus-induced tumorigenesis of Kaposi's sarcoma

“…While increased levels of both isoforms were seen following KSHV infection, their ratio did not change. Thus, IR-A is the predominantly expressed isoform in KSHV-infected E-DMVEC as has been observed in other cancers (Denley et al, 2004).…”

“…While insulin does stimulate Shc phosphorylation, other growth factor stimuli are stronger activators of this adaptor protein. This may suggest that IGF-II signaling is the major activator of IR phosphorylation, which would be in line with the predominance of the IR-A isoform, which has a higher affinity for IGF-II (Denley et al, 2003(Denley et al, , 2004.…”

“…The IR is necessary for KSHV-induced tumorigenesis The IR is encoded by two splice variants that differ by 12 amino acids in the cytoplasmic C-terminus: IR-A, which lacks exon 11, and a full-length IR-B (Denley et al, 2004). To determine if these isoforms are differentially induced by KSHV, we performed RT-PCR on two independently derived lines of E-DMVEC, either uninfected or latently infected with KSHV.…”

Differential effects of IGF-I, IGF-II and insulin in human preadipocytes and adipocytes – Role of insulin and IGF-I receptors

“…This is well in agreement with our results (2x10 -11 M for insulin, 3x10 -9 M for IGF-II and 0.6x10 -8 M for IGF-I). IGF-II is known to have a higher affinity for IR than IGF-I (Denley et al, 2004) and in our study IR was phosphorylated at a 10-fold lower concentration of IGF-II than of IGF-I. Our study shows that when preadipocytes are developed into mature adipocytes there is a huge increase in their response to insulin.…”

“…An augmented mitogenic effect of IGF-I compared to insulin has also been observed in vascular smooth muscle cells (Bornfeldt et al, 1991) and in cultured human skeletal muscle IGF-I has an increased mitogenic activity compared with insulin (Ciaraldi et al, 2001). It is believed that the C-domain of IGF-I somehow is important for interaction with the IGF-I receptor as suggested by Denley (Denley et al, 2004) and that this may in part explain the differences in maximal effects between IGF-I and insulin. Another very potent activator of mitogenic signalling via the IGF-IR is IGF-II.…”