Jak2 is involved in c-Myc induction by Bcr-Abl

Abstract

We have previously shown that the Jak2 tyrosine kinase is activated in Bcr-Abl positive cell lines and blood cells from CML blast crisis patients by tyrosine phosphorylation. We are searching for downstream targets of Jak2 in Bcr-Abl positive cells. It is known that c-Myc expression is required for the oncogenic effects of BcrAbl, and that over-expression of c-Myc complements the transformation defect of the Bcr-Abl SH2 deletion mutant. Moreover, the Bcr-Abl SH2 deletion mutant and an Abl C-terminal deletion mutant are deficient in activating c-Myc expression. Since the Jak2 binds to the C-terminal domain of Bcr-Abl and optimal Jak2 activation requires the SH2 domain, we tested whether Jak2 was involved in c-Myc protein induction by BcrAbl. We treated the 32Dp210 Bcr-Abl cells with the Jak2 specific tyrosine kinase inhibitor, AG490, and found that this drug, like the Abl tyrosine kinase inhibitor STI-571, inhibited c-Myc protein induction by Bcr-Abl. Treatment of 32Dp210 Bcr-Abl cells with AG490 also inhibited c-MYC RNA expression. It is also known that c-Myc protein is a labile protein that is increased in amounts in response to various growth factors by a mechanism not involving new Myc protein formation. Treatment of 32Dp210 Bcr-Abl cells with both the proteasome inhibitor MG132 and AG490 blocked the reduction of the c-Myc protein observed by AG490 alone. An adaptor protein SH2-Bb is involved in the enhancement of the tyrosine kinase activity of Jak2 following ligand/receptor interaction. In this regard we showed that the Jak2/Bcr-Abl complex contains SH2-Bb. Expression of the SH2-Bb R555E mutant in 32Dp210 Bcr-Abl cells reduced c-Myc expression about 40% compared to a vector control. Interestingly, we found the reduction of the c-Myc protein in several clones of dominant-negative (DN) Jak2 expressing K562 cells correlated very well with the reduction of tumor growth of these cells in nude mice as compared to vector transfected K562 cells. Both STI-571 and AG490 also induced apoptosis in 32Dp210 cells. Of interest, IL-3 containing medium reversed the STI-571 induced apoptosis of 32Dp210 cells but did not reverse the induction of apoptosis by AG490, which strongly supports the specificity of the inhibitory effects of AG490 on the Jak2 tyrosine kinase. In summary, our findings indicate that Jak2 mediates the increase in cMyc expression that is induced by Bcr-Abl. Our results indicate that activated Jak2 not only mediates an increase of c-MYC RNA expression but also interferes with proteasome-dependent degradation of c-Myc protein.

Kinase domain mutants of Bcr enhance Bcr-Abl oncogenic effects

“…Although the effects of c-Myc reduction are not observed in cell culture, lower levels of c-Myc correlate with lower tumor induction in our mouse studies . The c-Myc protein is known to be required for the leukemic effects of Bcr-Abl (Sawyers et al, 1992;Xie et al, 2002). In this report, we were able to show that TonB210/BCR-ABL/BCR/GFP has decreased levels of c-Myc (Figure 5), and this decrease correlated with a decrease in tumor formation.…”

ReSETting PP2A tumour suppressor activity in blast crisis and imatinib-resistant chronic myelogenous leukaemia

“…Since ectopic SET expression antagonised the effects of exogenous PP2A, it is possible that, in CML-BC progenitors, SET-dependent suppression of PP2A activity represents one of the main mechanisms used by BCR/ABL to prevent inactivation of mitogenic and survival signals required for its leukaemogenic activity. The importance of loss of PP2A activity for CML disease progression is also supported by the ability of PP2A to induce Rb dephosphorylation (Avni et al, 2003) and suppress Jak2 kinase, which was found activated in CML-BC (Xie et al, 2002).…”

Sphingosine kinase-1 mediates BCR/ABL-induced upregulation of Mcl-1 in chronic myeloid leukemia cells

“…As shown in Figure 2, Wortmannin, PD98059 and AG490 could partly abolish the BCR/ABL-induced expression and cellular activity increase of SPK1, indicating that multiple signals are involving in BCR/ABL-induced SPK1 upregulation in CML cells. Among them, JAK2 signal has been proved to plays a critical role in the signal transduction of Bcr-Abl/Jak2/Gab2/PI3K/Akt/ GSK-3b network and mediates BCR/ABL-induced upregulation of c-Myc protein (Xie et al, 2002;Samanta et al, 2006). As AG490 abolishes the BCR/ABLinduced SPK1 expression mostly, JAK2 seems to be also the major signal in this process.…”