Bcr-Abl acquires its transforming ability through its up-regulated Abl tyrosine kinase activity. Bcr is a phosphoprotein with a novel serine/threonine kinase activity encoded by its first exon. Over-expression of BCR in K562 cells produces a phosphoserine form of Bcr and interferes with the oncogenic effects of BCR-ABL in mice (Lin et al., Oncogene 2001). We have recently shown the inhibitory effects of Bcr on Bcr-Abl, in a nude mouse solid tumor model. Expression of BCR/GFP in TonB210 cells used for injection delayed tumor formation and tumors were 50% smaller compared to the TonB210/GFP control. In contrast two point mutants in the BCR kinase domain (Y360F and S354A), not only blocked Bcr’s inhibitory effects but enhanced the oncogenic effects of BCRABL (Perazzona et.al. Oncogene, 2008). Similar Bcr effects were observed in a mouse leukemia model. We are investigating the mechanism of the interaction between Bcr and Bcr-Abl proteins. Using TonB210 cells, in which BCR-ABL expression is controlled by a tetracycline-inducible promoter and Bcr is stably transduced by lentivirus infection, we observed that increasing levels of Bcr-Abl expression increased the levels of the Bcr protein. Treatment of TonB210 cells with imatinib mesylate decreased the levels of Bcr- Abl and surprisingly the Bcr protein as well, indicating that the tyrosine kinase function of Bcr-Abl is required to up-regulate Bcr protein expression. In addition, withdrawal of doxycycline also reduced Bcr-Abl and Bcr protein levels, confirming that Bcr-Abl is required for increased expression of the Bcr protein. In order to examine the levels of Bcr in cells lacking Bcr-Abl, we transduced BCR/GFP with lentivirus infection into BaF3 and 32D cells. Surprisingly, these cell lines expressed extremely low levels of Bcr, despite 90% expression of the GFP marker. Expression of Bcr was restored by overnight treatment with the proteasome inhibitor calpain inhibitor I. Forced expression of Bcr-Abl in BCR- transduced cells restored high expression of Bcr protein, confirming that Bcr- Abl is required for preventing degradation of the Bcr protein. Together these findings indicate that Bcr-Abl up-regulated Bcr expression by interfering with proteasomemediated degradation of the Bcr protein. Additional studies indicated that Bcr increases expression of the myeloid membrane surface marker Mac-1 in Bcr-Abl TonB210 cells, which originated from the mouse pro-B cell BaF3. We propose that Bcr may play a role in generating the myeloid phenotype caused by Bcr-Abl in CML patients and may be an important player in the chronic phase of CML by down-modulating Bcr-Abl.

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