Function and Signal Transduction Mediated by the Interleukin 3 Receptor System in Hematopoiesis

Interleukin 3 (IL‐3) promotes development of hematopoietic cells through activation of the IL‐3 receptor (IL‐3R) complex consisting of α and β subunits. The α subunit binds IL‐3 with low affinity and forms a high‐affinity receptor with the common β subunit (β C ). The β C subunit does not bind any cytokine by itself but is involved in the formation of high‐affinity functional receptors for IL‐5 and GM‐CSF. As the α subunits provide the specificity to cytokines and β C plays a major role in signal transduction, IL‐3, GM‐CSF and IL‐5 exhibit similar functions when they act on the same cells. Surprisingly, no apparent hematological defect other than a reduced number of eosinophils was found in knock‐out mice lacking an entire function of IL‐3, GM‐CSF and IL‐5; this indicates a remarkable functional overlap with other cytokine systems for hematopoiesis. Binding of the cytokines to the receptor induces activation of the JAK2 tyrosine kinase that associates with β C and triggers the signaling events. The membrane proximal region of β C is responsible for activation of JAK2 and STAT5, as well as for induction of c ‐myc . The signals induced by this region are required for cell‐cycle progression and DNA synthesis. Activation of the Ras pathway requires the distal region of β C and is involved in the suppression of apoptosis. Proliferation of hematopoietic cells requires signals for both DNA synthesis and anti‐apoptosis. In this review, we describe the recent findings of the function and signal transduction mediated by the IL‐3R system.