Growth and Differentiation through the Cytokine Receptors

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Interaction between erythropoietin (EPO) and its membrane receptor induces the proliferation and differentiation of erythroid progenitors. EPO has been shown to activate the JAK2-STAT5 pathway in various hematopoietic cell lines, although the physiological role of this pathway is unclear. We introduced various deletion and tyrosine (Y) to phenylalanine (F) substitution in the cytoplasmic domain of the chimeric receptor and expressed these mutant chimeras in an EPO-responsive erythroleukemia cell line, ELM-I-1. We found that Y343 or Y401 of the EPO receptor are independently necessary for erythroid differentiation as well as STAT5 activation. We searched for immediate early cytokine responsive genes and isolated a novel gene, CIS (Cytokine Inducible SH2 containing protein) that is induced through STAT5. We cloned the 5'-flanking region of the CIS gene, and found that about 200 bases upstream of the transcription-initiation site contain four potential START5 binding sites (MGF boxes). CIS contains an SH2 domain and binds to tyrosine-phosphorylated EPO and IL3 receptors. CIS is a feedback modulator of STAT5 ; its expression is induced by STAT5 and it negatively modulates STAT5 activation by binding to the receoptor. We are currently investigating the function of CIS by creating transgenic and knockout mice. We also found a novel CIS related molecule, JAB,JAB was cloned by yeast two-hybrid using JAK2 tyrosine kinase domain as bait. JAB is a negative regulator of JAK kinases ans probably inhibits various action of cytokines. We are studying the function of JAB.