IL-4 (Interleukin-4) is a T-Cell derived multifunctional cytokine with a molecular weight of approximately 15 to 19 kD that plays a critical role in the regulation of immune responses. It plays an important role in modulating the balance of TH (T Helper) cell subsets, favouring expansion of the TH2 lineage relative to TH1. IL-4 may mediate its biological effects, at least in part, by activating a tyrosine-phosphorylated DNA binding protein. Its effects depend upon binding to and signaling through a receptor complex consisting of the IL-4R (IL-4 Receptor) chain and the common gamma chain, resulting in a series of phosphorylation events mediated by receptor-associated kinases (Ref.1). In turn, these cause the recruitment of mediators of cell growth, of resistance to apoptosis, and of gene activation and differentiation.
The receptor consists of a 140-kDa IL-4RAlpha chain that binds IL-4 with high affinity. Physiologic signaling depends upon IL-4-mediated heterodimerization of the IL-4RAlpha chain with a second chain i.e. the gamma chain (Ref.2). Three members of the JAK (Janus Kinases) family, JAK1, JAK2, and JAK3, have been demonstrated to be activated in response to IL-4R engagement and to associate with components of the IL-4R complex. JAK1 has been proposed to associate with the IL-4RAlpha chain while JAK3 associates with the IL-4Rgamma chain. IL-4 engagement of the IL-4RAlpha chain results in tyrosine phosphorylation of JAK1 and JAK3. In addition to these JAK-family kinases, the Src-family kinase Fes has also been reported to associate with the IL-4RAlpha and to be activated in response to IL-4 stimulation. STATs (Signal Transducer and Activator of Transcription) act as second messenger after binding of a signaling molecule to its receptor (Ref.3). STAT6 and IRS2 (Insulin Receptor Substrate-2) are two important signaling proteins that associate with the cytoplasmic tail of the IL-4R. The IRS2 signaling pathway provides mitogenic and antiapoptotic signals. Phosphorylated STAT6 dimerizes and is translocated to the nucleus, where it mediates many of the gene activation functions of IL-4, including determination of immunoglobulin class switching to IgE and differentiation of naive cells into TH2 cells. Interaction of SOCS (Suppressor of Cytokine Signaling) protein with the IL-4R results in the inhibition of IL-4-mediated STAT6 activation (Ref.4).
IRS1/2 becomes phosphorylated as a result of interaction with phosphorylated IL-4Ralpha, presumably through the action of receptor-associated kinases (Ref.5). IRS1/2 molecules each have approximately 20 potential sites for tyrosine phosphorylation. Tyrosine phosphorylation of sites within both IRS1 and IRS2 allows their high affinity association with cellular proteins that contain SH2 domains including the p85 regulatory subunit of PI3K (Phosphoinositide-3 Kinase), GRB2 (Growth-Factor Receptor-Bound Protein-2), and the SHP2 (Src-Homology Protein Tyrosine Phosphatase-2), as well as other signaling molecules. GRB2 is constitutively complexed to the guanine nucleotide exchange protein SOS (Son of Sevenless). The primary function of SOS is to catalyze the exchange of GDP in inactive Ras for GTP, producing the active GTP-bound form of Ras. The MAPK (Mitogen-Activated Protein Kinases) pathway is initiated by the Ser/Thr kinase Raf following its activation by Ras-GTP (Ref.6). Active Raf initiates a cascade of kinase activation events that ultimately result in the phosphorylation and activation of the MAPKs ERK1/2 (Extracellular Signal Regulated Kinase). Active ERK1/2 translocates to the nucleus and activates the expression of genes such as c-Fos by phosphorylating specific transactivating factors. Distinct kinases with functions similar to the ERKs, such as the JNK (Jun N-terminal Kinase), can be activated through kinase cascades initiated by Ras as well as by other small GTPases related to Ras. Activation of these kinases results in the nuclear phosphorylation and activation of c-Jun as well as of other transcription factors.
The PI3K complex also interacts with Fes kinase after IL-4R engagement. Once activated, the p110 catalytic subunit is capable of phosphorylating membrane lipids as well as Ser/Thr residues of proteins. The lipid kinase activity mediates the transfer of phosphate from ATP to the D3 position of inositol in phosphotidylinositol in the cellular membrane. Forms of phosphorylated phosphotidylinositol are PIP3 (phosphotidylinositol- 3,4,5-Triphosphate) and PIP2 (phosphotidylinositol-3,4-Bisphosphate). Phosphoinositides have been implicated in the activation of a number of downstream kinases including different forms of PKC (Protein Kinase-C) and the Akt (also known as Protein Kinase-B) that play a key role in cell survival (Ref.7). IL-4 has been demonstrated to enhance the survival of hematopoietic cells. IL-4 is an important B-Cell survival and growth factor. It plays an important role in the activation of resting B-Cells and the subsequent proliferation and differentiation of B-Cells. IL-4 inhibits the secretion of IL-1B, TNF-Alpha (Tumor Necrosis Factor-Alpha) and IL-6 of human monocytes, to down-regulate the surface expression of CD5 on B-Cells and to promote the growth of human T-Cells. Also anti-inflammatory effects of IL-4 have been reported.