IL-3 Signaling

IL-3 (Interleukin-3) is a cytokine that regulates haemopoiesis, the formation of blood cells in the body. IL-3, also called multi-CSF (multi-lineage colony stimulating factor), is produced by T cells and mast cells, after activation with mitogens or antigens. The molecular weight of IL-3, a protein of 140 amino acids, ranges from 14 to 36 kDa. It stimulates eosinophils and B cell differentiation while it inhibits LAK (Lymphokine-Activated Killer) cell activity. IL-3 shares several biological activities with GM-CSF. IL-3 is capable of inducing the growth and differentiation of multi-potential haematopoetic stem cells, neutrophils, eosinophils, megakaryocytes, macrophages, lymphoid and erythroid cells (Ref.1). IL-3R (IL-3 receptor) is composed of two polypeptide chains, an Alpha subunit of 60-70kDa and a Beta subunit of 130-140kDa. Both subunits contain the extracellular conserved motifs found in the cytokine receptor superfamily. The common Beta -chain receptor subunit (c), which is shared among the receptors for IL-3, IL-5 and granulocyte/macrophage colony-stimulating factor, is essential for signal transduction in cell proliferation and differentiation induced by these cytokines, and plays a major role in recruiting intracellular signaling molecules such as JAK2 (Janus Kinase2) tyrosine kinase and STAT5 (Signal Transducers And Activators Of Transcription) (Ref.2).

The Alpha and Betac chains of the IL-3 receptor are not associated in the absence of ligand. The presence of ligand induces alpha and Betac chain association to form a heterodimeric complex. Receptor activation is followed by activation of receptor associated JAK2 kinase and tyrosine and serine phosphorylation of the Betac chain cytoplasmic tail. Activation of JAK2 leads to phosphorylation of the IL-3R Betac chain on multiple tyrosine residues which in turn serve as docking sites for other signal transducing proteins, the most important of which are STATs. IL-3 activation of hematopoetic cells appears to lead to the activation of multiple STATs, which includes STAT1, STAT3, STAT5 and STAT6. In addition, stimulation with IL-3 leads to morphological changes of cells through tyrosine phosphorylation of Betac and its associated protein pp90. In unstimulated cells, ectopically expressed RON tyrosine kinase localizes with the IL-3 receptor Betac (Ref.3).

Src family kinases mediate the phosphorylation of STAT3 mediated by IL-3/receptor interactions and play a critical role in signal transduction pathways associated with myeloid cell proliferation. One or both isoforms of STAT5 interact directly with JAK2, which in turn mediates their phosphorylation, STAT3 activation require its interaction with c-Src, which in turn mediates its phosphorylation. In addition to the activation of STATs, IL-3 activates multiple signal transduction pathways, which includes the Ras and PI3K (Phosphatidylinositol-3 Kinase) pathways. Upon IL-3 stimulation, the adapter molecule SHC (Src Homology 2 Domain Containing) transforming protein is rapidly phosphorylated and associates with the phosphorylated Betac subunit of IL-3. IL-3 stimulation also results in tyrosine phosphorylation of the inositol phosphatase SHIP (SH2-Containing Inositol Phosphatase), which forms a complex with SHC, GRB2 (Growth Factor Receptor-Bound Protein 2) and SOS (Son Of Sevenless). This is followed by the activation of Ras and c-Raf, which results in downstream activation of ERK1 and ERK2 (Extracellular Signal-Regulated Kinases). Activation of the cascades culminates in the increased expression of transcription factors c-Jun and c-Fos. In addition to activation of ERKs, IL-3 also activates p38 and JNK. IL-3 induces a rapid activation of the lipid kinase PI3K. Downstream proteins recruited by the PI3K pathways upon IL-3 stimulation includes the Akt protein. Another downstream protein activated in response to IL-3 stimulation is p70S6k (p70 Ribosomal S6 Kinase), which also mediates its effect via interaction with Betac chain. Another protein that feeds into the PI3K-PKB/AKT pathway is the Cbl protein, which also docks onto the adaptor protein GRB2 and SHC. The BCL2 family mediates the cell survival function of IL-3. BCL2 and BCLXL are rapidly induced by IL-3, which depends upon JAK2 activation. IL-3 also regulates the glycolytic pathway. In Baf-3 cells IL-3 starvation leads to a decrease in glucose uptake and in lactate production. It is found that the eosinophils activated by IL-3 may contribute to T cell activation in allergic and parasitic diseases by presenting superantigens and peptides to T-Cells (Ref.4).