Socs-1 Inhibits TEL-JAK2-Mediated Transformation of Hematopoietic Cells through Inhibition of JAK2 Kinase Activity and Induction of Proteasome-Mediated Degradation

Abstract

TEL-JAK2 fusion proteins, which are a result of t(9;12)(p24;p13) translocations associated with human leukemia, activate Stat5 in vitro and in vivo and cause a myelo- and lymphoproliferative disease in a murine bone marrow transplant model. We report that Socs-1, a member of the SOCS family of endogenous inhibitors of JAKs and STATs, inhibits transformation of Ba/F3 cells by TEL-JAK2 but has no effect on Ba/F3 cells transformed by BCR-ABL, TEL-ABL, or TEL–platelet-derived growth factor receptor beta. TEL-JAK2, in addition to activating Stat5, associates with Shc and Grb2 and induces activation of Erk2, and expression of Socs-1 inhibits engagement of each of these signaling molecules. TEL-JAK2 kinase activity is inhibited by Socs-1, as assessed by in vitro kinase assays. In addition, Socs-1 induces proteasomal degradation of TEL-JAK2. Mutational analysis indicates that the SOCS box of Socs-1 is required for proteasomal degradation and for abrogation of growth of TEL-JAK2-transformed cells. Furthermore, murine bone marrow transplant assays demonstrate that expression of Socs-1 prolongs latency of TEL-JAK2-mediated disease in vivo. Collectively, these data indicate that Socs-1 inhibits TEL-JAK2 in vitro and in vivo through inhibition of kinase activity and induction of TEL-JAK2 protein degradation.

ACKNOWLEDGMENTS

We acknowledge the invaluable assistance of D. Cain for murine bone marrow transplant experiments and K. Shannon (UCSF) for MAPK assays. We thank R. Van Etten and T. Golub for critical review of the manuscript, and we thank B. Neel and T. Roberts for valuable discussions and critical review of this work.

This work was supported in part by NIH grants POI DK50654 and POI CA66996 (D.G.G.), NIH grant CA82261 (D.W.S.), and the Leukemia Society of America (J.S.). D.G.G. is an Associate Investigator for Howard Hughes Medical Institute.