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.

We and others have shown that the Bcr-Abl oncoprotein binds activators of the Ras pathway such as Grb2 and Shc. Grb2 binding is mediated through a phosphorylated tyrosine residue (Y177) located within a consensus Grb2 binding site encoded by the ®rst exon of the BCR gene. Our results indicate that P160 BCR is tyrosine phosphorylated at the same site by Bcr-Abl in kinase assays (Puil et al., 1994). We performed experiments to determine whether Bcr, which was tyrosine phosphorylated within cells by activated c-Abl, could also bind Grb2, and whether phosphotyrosine 177 was the major binding site. Complexes between Bcr and Abl were detected in a hemopoietic cell line lacking Bcr-Abl and in COS1 cells coexpressing both Bcr and Abl proteins. P160 BCR was tyrosine phosphorylated in COS1 cells coexpressing Abl and Bcr proteins. Similarly, various deletion mutants of Bcr including BCRN553, BCRN413 and BCRN221 were tyrosine phosphorylated by activated c-Abl whereas BCRN159 was not. Wild-type Bcr and Bcr Y177F were examined under these conditions for their ability to co-precipitate with Grb2. The results showed that while wild-type tyrosine phosphorylated Bcr e ciently bound Grb2, tyrosine phosphorylated Bcr Y177F had greatly reduced Grb2-binding ability. Studies with GST-SH2 (Grb2) revealed that tyrosine phosphorylated Bcr was able to bind to GST SH2 (Grb2) but tyrosine phosphorylated Bcr Y177F was de®cient in binding. These results indicate that the Bcr protein when phosphorylated at tyrosine 177 binds Grb2, thereby implicating Bcr as a potantial activator of the Ras pathway.

During the past decades, immunotherapy, especially the antibody-mediated immune checkpoint blockade (ICB) has shown durable tumor inhibition and changed the paradigm of cancer treatment. However, a growing body of evidence suggests that ICB treatment induces severe immune-related adverse events (irAEs), and the side effect even leads to the discontinuation of lifesaving treatment. Here, we found that ICB treatment induces colitis in melanoma patients and promotes the infiltration of CD8+ effector T cells into colitic lesions. Further transcriptomic dissection indicated the PI3K-AKT-mTOR pathway was highly activated in CD8+ effector T cells of colitic lesions. Moreover, we developed a mouse melanoma model to recapitulate the gastrointestinal toxicity of anti-PD-1 treatment in clinical settings. Anti-PD-1 treatment significantly contributed to the infiltration of CD8+ T cells, and correspondingly induced severe enteritis. Immunohistochemistry experiments showed that the PI3K-AKT-mTOR pathway of T cells was activated by anti-PD-1 treatment. Blockade of the pathway with mTOR inhibitor sirolimus not only inhibits tumor growth but also suppresses the T cell infiltration in colitic lesions. More importantly, combination with sirolimus and anti-PD-1 synergistically inhibits tumor growth via inducing the immunogenic cell death of tumor cells in vivo. In summary, our research demonstrated the principle of mTOR inhibitor and anti-PD-1 combinatorial therapeutic regimen, which provided a novel therapeutic strategy for irAEs in clinics.