Insulin-like growth factor 1 (IGF1) control many biological processes such as cellular metabolism, proliferation, differentiation, and apoptosis. These effects are mediated through ligand activation of the tyrosine kinase activity intrinsic to their receptors IGF1 Receptor (IGF1R) (Ref.1). Activation of the IGF1R is a particularly important survival-promoting signal (Ref.2). In addition to signaling through the classical tyrosine kinase pathway, recent studies indicate that IGF1Rs can emit signals in the unoccupied state through some yet-to-be-defined noncanonical pathways (Ref.1).
IGF1R is a transmembrane, ligand-activated tyrosine protein kinase that consists of Alpha2Beta2 heterotetramers held together by disulfide bridges. Both IGF1 and IGF2 exhibit high-affinity binding to IGF1R and most of the biological effects of IGF1 and IGF2 are mediated by IGF1R. Once both alpha and beta residue are phosphorylated, the intrinsic tyrosine kinase activity of the IGF1R increases, leading to phosphorylation of other sites in the receptor and associated substrate proteins. Recruitment of these molecules activates phosphatidylinositol-3-kinase (PI3K)/Akt and Ras/extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase (MAPK) signaling pathways. All these pathways, however, result in maintenance of cell survival by antagonizing the processes and proteins involved in apoptosis (Ref.2 and 3).
The binding of IGF1 or IGF2 to IGF1R activates the receptor's intrinsic tyrosine kinase activities, which results in the phosphorylation of the IRSs (Insulin Receptor Substrates) (Ref.5). Tyrosine-phosphorylated IRSs interact with the cytoplasmic protein PI3K through its SH2 (Src Homology -2) domains. Activated PI3K catalyzes the conversion of PIP2 (Phosphatidylinositol 4, 5-Bisphosphate) to PIP3 (Phosphatidylinositol 3, 4, 5-Trisphosphate), and the reverse reaction is catalyzed by PTEN (Phosphatase and Tensin Homolog). PDK-1 then phosphorylates residue Thr308 on Akt. PDK-1 also phosphorylates PKC (Protein Kinase-C) proteins: PKC-Lambda (Protein Kinase-C-Lambda) and PKC-Zeta (Protein Kinase-C-Zeta) (Ref.4).
These PKCs, along with Akt enhance the rate of Glucose uptake by the cell by facilitating the GLUT4 (Glucose Transporter-4) translocation from the GLUT4
vesicle to the membrane (Ref.6). Activated Akt phosphorylates and inactivates several proteins that are involved in apoptosis. A primary target is the BAD. In its non-phosphorylated state, BAD locates at the mitochondrial membrane where it interacts with BCL2 (B-Cell CLL/Lymphoma-2) and prevents it from performing its anti-apoptotic functions. Once phosphorylated by Akt, BAD associates with the cytosolic protein 14-3-3
and is unable to interfere with BCL2. Akt also phosphorylates several pro-apoptotic members of the forkhead transcription factor family, FKHRL1, FKHR
and prevents their activity (Ref.7). Action of Akt diminishes expression of FasL (Fas Ligand), thus decreasing Fas-mediated apoptosis. In addition to the inhibition of pro-apoptotic transcription factors, the activity of Akt also increases the levels of anti-apoptotic proteins including BCL2 and BCL-X and several extracellular matrix adhesion molecules. Induced activity of Akt also leads to expression of the anti-apoptotic transcription factor NF-KappaB through regulation of IKKs (I-KappaB Kinases). In response to IGFs, the inhibition of GSK3 promotes the dephosphorylation and activation of Glycogen Synthase, contributing to the stimulation of glycogen synthesis. GSK3 also catalyses the phosphorylation and inhibition of eIF2B (Eukaryotic Protein Synthesis Initiation Factor-2B), thereby inhibiting protein synthesis. Hence IGF1R, by inhibiting GSK3, stimulates the dephosphorylation and activation of eIF2B, contributing to an increased rate of protein synthesis. IGF1 promotes protein synthesis by activating eIF4E (Eukaryotic Initiation Factor-4E). Phosphorylation of 4EBP results in dissolution of the eIF4E-4EBP complex, freeing eIF4E to promote the initiation phase of protein translation. Phosphorylated mTOR promotes phosphorylation and inhibition of 4EBP1, and promotes protein synthesis by relieving 4EBP-mediated inhibition of eIF4E. When phosphorylated by mTOR, the ribosomal p70S6K and S6 (Ribosomal Protein-S6) becomes activated and increases protein synthesis (Ref.8).
Upon IGF1R autophosphorylation the protein SHC (SH2 Containing Protein) is recruited to the receptor and becomes phosphorylated on tyrosine residues. Activated SHC then binds the adaptor GRB2 (Growth Factor Receptor Bound Protein-2), recruiting the SOS in an IRS-independent manner. This complex then activates Ras and initiates sequential phosphorylation cascades involving serine/threonine kinase Raf, MEK1/2 (MAP Kinase Kinases), and ERK1/2 (Extracellular Signal Regulated Kinases). An endpoint of the MAPK pathway is modification of transcription factor activity, such as activation of Elk transcription factors. Similar to the Akt pathway, the downstream target of ERKs that prevents apoptosis is BAD. However, JNK promotes apoptosis and the role of IGF1R signaling is to prevent the activation of JNK. The Ras-->Raf-->ERK1/2 pathway can also be activated by the tyrosine phosphorylation of IRSs, with the resultant formation of the IRS-GRB2-SOS complex that activates Ras, which in turn binds to and activates Raf, subsequently phosphorylating and activating MEKs, followed by ERKs. Phosphorylated ERKs, in turn, transmit signals to the nucleus, with resultant mitogenic response: progression of the cell cycle and cell proliferation (Ref.9).
IGF1R acts as modulator of ion channels. Ligand binding to IGF1R leads to activation of voltage-dependent Calcium channels; thus, causing large transient increases in the Ca2+ levels. The downstream effects include regulation of Calcium-dependent transcription factors such as MEF2 (Mads Box Transcription Enhancer Factor-2), NFAT (Nuclear Factors of Activated T-cells) and CREB. These transcription factors promote the expression of several anti-apoptotic proteins including BCL2. The increased Ca2+ levels in the cytoplasm activate the protein phosphatase Calcineurin by disrupting the inhibitory effects of Calm (Calmodulin). Calcineurin activation leads to the dephosphorylation of NFAT, allowing it to enter the nucleus, where it cooperates with other transcription factors to bind promoters (Ref.10).
IGF1R signalling plays numerous roles in both physiological and pathophysiological conditions. IGFs protect cells from apoptosis induced by a wide variety of conditions, including growth factor withdrawal, chemotherapy and oncogene expression. Activation of IGF1R by its ligand also initiates metabolic cascades that result in the stimulation of protein synthesis, glucose intake, glycogen synthesis, and lipid storage. Since the ability to regulate apoptosis may impact upon several types of serious diseases in humans, including different human Cancers, drugs to disrupt IGF1R function have been developed and are now entering clinical trial. Understanding of the cellular pathways by IGF1R that signal to a cell to live or die is crucially important for the development of appropriate clinical treatments (Ref.11 and 12).