Involvement of Jak2 tyrosine phosphorylation in Bcr–Abl transformation

Abstract

We have previously reported that the Jak2 tyrosine kinase but not Jak1 is tyrosine phosphorylated in the absence of IL-3 in Bcr ± Abl positive M3.16 cells, which are rendered IL-3 independent by BCR ± ABL gene expression. We have explored the involvement of Jak2 tyrosine phosphorylation in Bcr-Abl oncogenic eects. Our results indicate that Jak2 became tyrosine-phosphorylated in a number of cell lines expressing Bcr ± Abl, when maintained in medium lacking IL-3, whereas Bcr ± Abl negative cells lacked Jak2 tyrosine phosphorylation. Jak2 was poorly tyrosine-phosphorylated in cells expressing the SH2 deletion mutant of Bcr ± Abl compared to either wild-type Bcr ± Abl or its SH3 deletion mutant. Moreover, tyrosine phosphorylation of Jak2 by Bcr ± Abl was inhibited by the Abl tyrosine kinase inhibitor, STI 571, in a dose-dependent manner. This inhibition of Bcr ± Abl kinase by the drug did not interfere with the ability of Jak2 and Bcr ± Abl to form a complex. Studies with deletion mutants of Bcr ± Abl indicated that the Cterminal domain of Abl within Bcr ± Abl was involved in complex formation with Jak2. Similarly, GST ± Abl pulldown assays con®rmed the strong binding to Jak2 by the C-terminus of Abl. Jak2 peptide substrate studies indicated that the Bcr ± Abl and Abl tyrosine kinases speci®cally phosphorylated Y1007 of Jak2 but only poorly phosphorylated Y1008. Phosphorylation of Y1007 of Jak2 is known to be critical for its tyrosine kinase activation. Tyrosine residue 1007 of Jak2 was phosphorylated in 32Dp210 cells as measured by Western blotting with a phosphotyrosine 1007 sequence-speci®c antibody. A kinase-inactive Jak2 mutant blocked the colony forming ability of K562 cells. Tumor formation of K562 cells in nude mice was similarly inhibited by this kinase-inactive Jak2 mutant. This inhibition was independent of Stat5 tyrosine phosphorylation. Furthermore, tyrosine-phosphorylated Jak2 was detected in blood cells from CML patients in blast crisis but not in a normal marrow sample. In summary, these ®ndings provide strong evidence that the Jak2 tyrosine kinase is a critical factor in Bcr ± Abl malignant transformation. Oncogene (2001) 20, 6188 ± 6195.

BCR-ABL oncogenic transformation of NIH 3T3 fibroblasts requires the IL-3 receptor

“…Previous findings by two groups have shown that Stat5 activation is directed by Bcr-Abl oncoprotein (Ilaria and Van Etten, 1996;Klejman et al, 2002). Our previous studies indicate that Stat5 activation is independent of Jak2 activation in BCR-ABL þ 32D cells (Xie et al, 2001), which is in agreement with the earlier findings of Ilaria and Van Etten (1996).…”

“…Since the expression of the IL-3 receptor chains was critical for the oncogenic transformation of NIH 3T3 cells by BCR-ABL, we asked whether Jak2 is involved in the oncogenic behavior of these cells. We have previously shown that mouse and human BCR-ABL þ hematopoietic cells require Jak2 for the expression of the oncogenic phenotype (Xie et al, 2001(Xie et al, , 2002Samanta et al, 2006). Importantly, Jak2 plays a critical role in activating the PI-3 kinase, Akt, and NF-kB and inactivating GSK3-b.…”

“…It is known that Jak2 forms a complex with Bcr-Abl and the oncogenic effects of BCR-ABL require Jak2 (Miyamoto et al, 2001;Xie et al,. 2001Xie et al,.…”

“…The Bcr-Abl oncoprotein activates the Stat5 pathway independent of Jak2 (Ilaria and Van Etten, 1996;Klejman et al, 2002). In addition, Bcr-Abl activates Jak2 in a manner that does not lead to activation of Stat5 (Xie et al, 2001(Xie et al, , 2002, and this Bcr-Abl/Jak2 pathway leads to activation of c-Myc (Samanta et al, 2006). Therefore, we hypothesized that fibroblasts would become susceptible to oncogenic transformation by BCR-ABL following forced expression of cDNAs of the a and b chains of the IL-3 receptor.…”

STAP-2 interacts with and modulates BCR-ABL-mediated tumorigenesis

“…This BCR-ABLdependent phosphatidylinositol 3-kinase activation is in part mediated by GAB2 and/or c-CBL (Sattler et al, 1996;Chu et al, 2007). Furthermore, STAT5, which regulates CYCLIN D1 and BCL-xL, is activated by BCR-ABL independent of Jak family proteins (Xie et al, 2001). STAT5a/b-deficient fetal liver hematopoietic progenitors fail to generate leukemia in recipient mice after retroviral transduction with BCR-ABL (Hoelbl et al, 2006).…”