Bcr: a negative regulator of the Bcr-Abl oncoprotein

Abstract

Chronic myelogenous leukemia is typically characterized by the presence of the Philadelphia chromosome (Ph) in which 5' portions of the BCR gene are fused to a large portion of the ABL gene. Our studies and those of others indicate that Bcr sequences within the Bcr-Abl oncoprotein are critically involved in activating the Abl tyrosine kinase and actively participate in the oncogenic response, which is generated by the Bcr-Abl oncoprotein. We investigated the role of the Bcr protein in the oncogenic eects of Bcr-Abl. Reduction of the level of the Bcr protein by incubating cells with a 3' BCR anti-sense oligodeoxynucleotide increased the growth rate and survival of hematopoietic cell lines expressing Bcr-Abl. Also, enforced expression of Bcr in Bcr-Abl cell lines strongly reduced transformation eciency. Induction of Bcr expression drastically reduced the phosphotyrosine content of Bcr-Abl in Rat-1 ®broblasts transformed by P185 BCR-ABL and in hematopoietic cells expressing P210 Bcr-Abl within days following induction of Bcr. Rat-1/P185 cells maintained for three weeks after Bcr induction had dramatically reduced amounts of phosphotyrosine proteins compared to cells in which Bcr expression was repressed by the addition of Tet. In contrast Bcr expression did not decrease the phosphotyrosine content of either v-Src or activated Neu tyrosine kinase. Importantly, the phosphotyrosine content of total P160 BCR (induced plus endogenous) was strongly reduced by inducing expression of Bcr, indicating that the induced Bcr protein was not a target of the tyrosine kinase activity of Bcr-Abl but instead functioned as an inhibitor of Bcr-Abl. These results show that the Bcr protein can function as a negative regulator of Bcr-Abl, but that the inhibitory eects of Bcr are dependent on achieving an elevated level of Bcr expression relative to Bcr-Abl.

Kinase domain mutants of Bcr enhance Bcr-Abl oncogenic effects

“…We have previously showed that overexpression of Bcr relative to Bcr-Abl is required for Bcr's inhibitory effects (Wu et al, 1999;Lin et al, 2001). In addition, it has been reported that BCR-ABL/BCR transcript ratios can be used as an important diagnostic parameter for aggressive behavior of CML patients, suggesting a possible role of normal Bcr in CML pathogenesis (Moravcova´et al, 2005).…”

“…The first exon also contains two serine-rich domains that bind specifically to the SH2 domain of Abl in a non-phosphotyrosine-dependent manner (Pendergast et al, 1991). These serine-rich sequences in the phosphoserine form are involved in downregulating Bcr-Abl oncogenic activity (Wu et al, 1999;Lin et al, 2001;Hawk et al, 2002).…”

“…In this tyrosine-phosphorylated form, Bcr serine/threonine kinase activity is reduced (Liu et al, 1996). However, overexpression of Bcr can interfere with Bcr-Abl oncogenic effects (Wu et al, 1999). In fact, Bcr becomes resistant to tyrosine phosphorylation in BcrAbl-positive cells when its expression is in molar excess.…”

“…In fact, Bcr becomes resistant to tyrosine phosphorylation in BcrAbl-positive cells when its expression is in molar excess. In this condition, the excess Bcr protein is mostly in the phosphoserine form and reduces the phosphotyrosine content of Bcr-Abl, strongly inhibiting its oncogenic activity (Wu et al, 1999;Lin et al, 2001). This phosphoserine form of Bcr is also predominant after overexpression of Bcr in soft agar clones of the CML K562 line containing an inducible BCR gene.…”

Bcr: a negative regulator of the Bcr-Abl oncoprotein in leukemia

“…Because of these findings, several induceable vector systems were tested to introduce a transcriptionally silent form of the BCR cDNA into Bcr-Abl cells, which would permit induction of BCR gene expression to determine the initial effects of Bcr on the Bcr-Abl oncogenic phenotype. The tetracycline (Tet) repression system was chosen (Wu et al, 1999). In order to maximize the induction of the Bcr protein and to insure a low level of synthesis of exogenous Bcr in the non-induced condition, various clones of transfected Bcr-Abl expressing cells were selected in soft agar (as a means to select oncogenic Rat-1 cell clones) under neo selection conditions.…”