Insulin-like growth factor 1 receptor

The Insulin-like Growth Factor 1 (IGF-1) Receptor is a transmembrane receptor that is activated by IGF-1 and by the related growth factor IGF-2. It belongs to the large class of tyrosine kinase receptors. This receptor mediates the effects of IGF-1, which is a polypeptide protein hormone similar in molecular structure to insulin. IGF-1 plays an important role in growth and continues to have anabolic effects in adults - meaning that it can induce hypertrophy of skeletal muscle and other target tissues.

Structure of receptor
Two alpha subunits and two beta subunits make up the IGF-1 receptor. The beta subunits pass through the cellular membrane and are linked by disulfide bonds. The receptor is a member of a family which consists of the Insulin Receptor and the IGF-2R (and their respective ligands IGF-1 and IGF-2), along with several IGF-binding proteins.

IGF-1R and IR both have a binding site for ATP, which is used to provide the phoshates for autophosphorylation (see below). There is a 60% homology between IGF-1R and the insulin receptor.

Receptor family
Tyrosine kinase receptors, including, the IGF-1 receptor, mediate their activity by causing the addition of a phosphate groups  to particular tyrosines on certain proteins within a cell. This addition of phosphate induces what are called "cell signaling" cascades - and the usual result of activation of the IGF-1 receptor is survival and proliferation in mitosis-competent cells, and growth (hypertrophy) in tissues such as skeletal muscle and cardiac muscle.

During embryonic development, the IGF-1R pathway is involved with the developing limb buds.

The IGFR signalling pathway is of critical importance during normal development of mammary gland tissue during pregnancy and lactation. During pregnancy, there is intense proliferation of epithelial cells which form the duct and gland tissue. Following weaning, the cells undergo apoptosis and all the tissue is destroyed. Several growth factors and hormones are involved in this overall process, and IGF-1R is believed to have roles in the differentiation of the cells and a key role in inhibiting apoptosis until weaning is complete.

The IGF-1R is implicated in several cancers, most notably breast cancer. In some instances its anti-apoptotic properties allow cancerous cells to resist the cytotoxic properties of chemotheraputic drugs or radiotherapy. In others, where EGFR inhibitors such as erlotinib are being used to inhibit the EGFR signalling pathway, IGF-1R confers resistance by forming one half of a heterodimer (see the description of EGFR signal transduction in the erlotinib page), allowing EGFR signalling to resume in the presence of a suitable inhibitor. This process is referred to as crosstalk between EGFR and IGF-1R.

It is further implicated in breast cancer by increasing the metastatic potential of the original tumour by inferring the ability to promote vascularisation.

IGF-1 vs Insulin Receptor Signaling
IGF-1 binds to at least two cell surface receptors: the IGF1 Receptor (IGFR), and the insulin receptor. The IGF-1 receptor seems to be the "physiologic" receptor - it binds IGF-1 at significantly higher affinity than it binds the insulin receptor. Like the insulin receptor, the IGF-1 receptor is a receptor tyrosine kinase - meaning it signals by causing the addition of a phosphate molecule on particular tyrosines. IGF-1 activates the Insulin receptor at approximately 0.1x the potency of insulin. Part of this signaling may be via IGF1R-InsulinReceptor heterodimers (the reason for the confusion is that binding studies show that IGF1 binds the insulin receptor 100-fold less well than insulin, yet that does not correlate with the actual potency of IGF1 in vivo at inducing phosphorylation of the Insulin receptor, and hypoglycemia).

Inhibitors of IGF-1R
Due to the similarity of the structures of IGF-1R and the insulin receptor, especially in the regions of the ATP binding site and tyrosine kinase regions, synthesising selective inhibitors of IGF-1R is difficult. Prominent in current research are three main classes of inhibitor:


 * 1) Tyrphostins such as AG538 and AG1024. These are in early pre-clinical testing.  They are not thought to be ATP-competitive, although they are when used in EGFR as described in QSAR studies.  These show some selectivity towards IGF-1R over IR.
 * 2) Pyrrolo[2,3-d]-pyrimidine derivatives such as NVP-AEW541, which show far greater (100 fold) selectivity towards IFG-1R over IR.
 * 3) Monoclonal antibodies are probably the most specific and promising therapeutic compounds.

Effects of aging on IGF-1R
Studies in female mice have shown that both Supraoptic nucleus (SON) and Paraventricular nucleus (PVN) lose approximately one-third of IGF-1R immunoreactive cells with normal aging. Also, Old calorically restricted (CR) mice lost higher numbers of IGF-1R non-immunoreactive cells while maintaining similar counts of IGF-1R immunoreactive cells in comparison to Old-Al mice. Consequently, Old-CR mice show a higher percentage of IGF-1R immunoreactive cells reflecting increased hypothalamic sensitivity to IGF-1 in comparison to normally aging mice.