Protein-tyrosine Phosphatase PTPεC Inhibits Jak-STAT Signaling and Differentiation Induced by Interleukin-6 and Leukemia Inhibitory Factor in M1 Leukemia Cells

Abstract

We engineered and expressed both a wild-type and mutant cytosolic isoform of PTPε (PTPεC) in murine M1 leukemic cells, which can be induced to growth arrest and monocytic differentiation by interleukin-6 (IL-6) and leukemia inhibitory factor (LIF). Forced expression of PTPεC inhibited IL-6-and LIF-induced monocytic differentiation and apoptosis in M1 cells, whereas expression of PTPεM, a transmembrane isoform of PTPε, did not. PTPεC expression resulted in lower levels of IL-6-induced tyrosine phosphorylation of Jak1, Tyk2, gp130 and Stat3 compared to parent cells. In M1 transfectants expressing an inactive mutant of PTPεC, both tyrosine phosphorylation and apoptosis induced by IL-6 and LIF were potentiated rather than inhibited. These results suggest an important role for PTPεC in negative regulation of IL-6and LIF-induced Jak-STAT signaling. cytokines. The Jak-STAT pathway mediates many of these functions and constitutes an important signaling pathway. Binding of cytokines to their receptors activates a family of transcription factors, designated STAT(s), through phosphorylation of a specific tyrosine residue by receptor-associated Jak tyrosine kinases (1-3). Interleukin (IL)-6, a typical multifunctional cytokine, induces both differentiation of normal B lymphocytes and production of acute phase protein in hepatocytes. IL-6 stimulates growth in cells such as multiple myelomas and B cell plasmacytomas/hybridomas, while it inhibits growth in several myeloid leukemia cells. Murine M1 myeloid leukemia cells, for example, undergo cell cycle arrest and monocytic differentiation following IL-6 stimulation and have been used as a model system for studying IL-6-signaling (4-9). The receptor for IL-6 is composed of an IL-6-specific receptor α-chain subunit, gp80, and a signal transducing chain, gp130. gp130 is a shared signaling moiety for cytokine receptors of the IL-6 family, including IL-6, leukemia inhibitory factor (LIF), oncostatin M, IL-11, ciliary neurotrophic factor and cardiotrophin 1 (10-15). In the Jak family, Jak1, Jak2 and Tyk2 associate constitutively with gp130 and are activated by IL-6 family cytokines, while Jak1 is thought to play an essential role in the signaling (16-18). Activated Jak(s), in turn, induces tyrosine phosphorylation of the heart, lung and testis (39, 42). We and others have shown that expression of PTPεC, but not PTPεM, is up-regulated during monocytic differentiation of several cell lines (39, 42). To elucidate roles of PTPεC in IL-6 signaling, we engineered and over-expressed wild-type and a dominant-negative form of PTPεC in M1 cells and analysed the biological consequences of ectopic expression. Our findings identified an isoform-specific role for PTPεC in inhibition of Jak-STAT signaling elicited by cytokines belonging to the IL-6 family. EXPERIMENTAL PROCEDURES Materials. Human recombinant IL-6 was kindly provided by Ajinomoto Co. (Yokohama, Japan). Murine LIF was obtained from Gibco-BRL. FCS was purchased from Intergen. Cell culture. All cells were cultured in RPMI1640 medium con...

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