Src kinases as targets for B cell acute lymphoblastic leukaemia therapy

Inhibitors of SRC kinases impair antitumor activity of anti-CD20 monoclonal antibodies

Clinical trials with SRC family kinases (SFKs) inhibitors used alone or in a combination with anti-CD20 monoclonal antibodies (mAbs) are currently underway in the treatment of B-cell tumors. However, molecular interactions between these therapeutics have not been studied so far. A transcriptional profiling of tumor cells incubated with SFKs inhibitors revealed strong downregulation of MS4A1 gene encoding CD20 antigen. In a panel of primary and established B-cell tumors we observed that SFKs inhibitors strongly affect CD20 expression at the transcriptional level, leading to inhibition of anti-CD20 mAbs binding and increased resistance of tumor cells to complement-dependent cytotoxicity. Activation of the AKT signaling pathway significantly protected cells from dasatinib-triggered CD20 downregulation. Additionally, SFKs inhibitors suppressed antibody-dependent cell-mediated cytotoxicity by direct inhibition of natural killer cells. Abrogation of antitumor activity of rituximab was also observed in vivo in a mouse model. Noteworthy, the effects of SFKs inhibitors on NK cell function are largely reversible. The results of our studies indicate that development of optimal combinations of novel treatment modalities with anti-CD20 mAbs should be preceded by detailed preclinical evaluation of their effects on target cells.

Src family kinases mediate cytoplasmic retention of activated STAT5 in BCR-ABL-positive cells

Persistent activation of the Abl tyrosine kinase in the BCR-ABL fusion protein is the major cause of chronic myeloid leukemia (CML). Among many other substrates BCR-ABL phosphorylates STAT5 and Src family kinases (SFK). Activated pSTAT5 is essential for initial transformation and maintenance of the disease. Cytokine-induced phosphorylation on tyrosine 694 typically leads to nuclear accumulation of pSTAT5 and target gene expression. We verified that in BCR-ABL-positive progenitor cells from a CML patient and in K562 cells pSTAT5 is cytoplasmic. However, upon ectopic expression of BCR-ABL p210 in non-myeloid cells, co-transfected STAT5A is phosphorylated on Y694 and localized in the nucleus arguing for an additional factor mediating cytoplasmic retention in CML cells. Expression of the SFK v-Src, Hck or Lyn together with STAT5A results in phosphorylation on Y694 and cytoplasmic retention. Upon coexpression of BCR-ABL and individual SFK the cytoplasmic retention of activated STAT5A mediated by v-Src and Hck but not Lyn is dominant over nuclear translocation induced by BCR-ABL. Cytoplasmic retention depends on the kinase activity of SFK and is mediated through the interaction of the SH2 domain of STAT5A with the SFK. Interestingly, nuclear accumulation of STAT5A as a result of activation by FLT3-ITD, an oncogene found in acute myeloid leukemia, cannot be prevented by coexpression of SFK. Importantly, inhibition of SFK in K562 cells restored nuclear accumulation of pSTAT5A, enhanced STAT5 target gene expression and increased colony formation. Thus, SFK mediate cytoplasmic retention of pSTAT5A in BCR-ABL-positive cells. Cytoplasmic pSTAT5A in CML cells might balance the controversial functions of STAT5 in cellular senescence and differentiation versus G1/S progression and survival.

Targeted therapy of chronic myeloid leukemia

Inhibition of BCR-ABL with kinase inhibitors has become a well-accepted strategy for targeted therapy of Philadelphia-positive (Ph(+)) chronic myeloid leukemia (CML) and has been shown to be highly effective in controlling the disease. However, BCR-ABL kinase inhibitors do not efficiently kill leukemic stem cells (LSCs), indicating that this therapeutic strategy does not lead to a cure of CML. Development of curative therapies of CML require the identification of genes/pathways that play critical roles in survival and self-renewal of LSCs. Targeting of these key BCR-ABL downstream genes provides an opportunity to eradicate LSCs, as shown in our work that identifies the Alox5 gene as a key regulator of the function of CML LSCs. Immediate clinical trials are necessary to test the effectiveness of targeting a key BCR-ABL downstream gene in eradicating LSCs in CML patients. In this review, we will discuss current targeted therapies of CML using BCR-ABL kinase inhibitors, with a focus on the importance of developing a targeted therapy of CML through identification of target genes in CML LSCs.

Src-family kinases in the development and therapy of Philadelphia chromosome-positive chronic myeloid leukemia and acute lymphoblastic leukemia

The BCR-ABL kinase inhibitor imatinib has shown significant efficacy in chronic myeloid leukemia (CML) and is the standard front-line therapy for patients in chronic phase. However, a substantial number of patients are either primarily refractory or acquire resistance to imatinib. While a number of mechanisms are known to confer resistance to imatinib, increasing evidence has demonstrated a role for BCR-ABL–independent pathways. The Src-family kinases (SFKs) are one such pathway and have been implicated in imatinib resistance. Additionally, these kinases are key to the progression of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). The dual SFK/BCR-ABL inhibitor dasatinib is now clinically available and has markedly greater potency compared with imatinib against native BCR-ABL and the majority of imatinib resistant BCR-ABL mutants. Therefore, this agent, as well as other dual SFK/BCR-ABL inhibitors under development, could provide added therapeutic advantages by overcoming both BCR-ABL– dependent (i.e., BCR-ABL mutations) and – independent forms of imatinib resistance and delaying transition to advanced phase disease. In this review, we discuss the preclinical and clinical evidence demonstrating the involvement of SFKs in imatinib resistance and the progression of CML and Ph+ ALL, as well as the potential role of dual SFK/BCR-ABL inhibition in the management of these diseases.

Src kinase signaling in leukaemia

Role of Src kinases in acute lymphoblastic leukaemia has been recently demonstrated in leukaemia mouse model. Retained activation of Src kinases by the BCR-ABL oncoprotein in leukaemic cells following inhibition of BCR-ABL kinase activity by imatinib indicates that Src activation by BCR-ABL is independent of BCR-ABL kinase activity and provides an explanation for reduced effectiveness of the BCR-ABL kinase activity inhibitors in Philadelphia chromosome-positive acute lymphoblastic leukaemia. Simultaneous inhibition of kinase activity of both BCR-ABL and Src kinases results in long-term survival of mice with acute lymphoblastic leukaemia. Leukaemic stem cells exist in acute lymphoblastic leukaemia, and complete eradication of this group of cells would provide a curative therapy for this disease.

Recent advances in targeted therapy of human myelogenous leukaemia

Despite major advances in the diagnosis and treatment of myelogenous leukaemia during the past few decades, this group of diseases remains a serious medical concern with > 15,000 new cases each year and a mortality rate of approximately 10,000 in the US alone. Current available conventional therapies, including chemotherapy and bone marrow transplantation, often cause severe side effects owing mainly to the lack of specificity of the treatment. In the past years, significant progress has been made towards understanding the pathogenesis of myelogenous leukaemia from the molecular standpoint. To this end, a growing number of approaches are being exploited for the identification and validation of new therapeutic targets suitable for more potent and specific or 'targeted' intervention. In this review, the authors focus their discussion on the four most promising myelogenous leukaemia-associated molecular targets currently being pursued by major pharmaceutical and biotechnology companies, fms-like tyrosine kinase 3 (FLT3), CD33, farnesyl transferase and BCR-Abl, with emphasis on recent progress on the clinical development of therapeutic agents, including both kinase inhibitors and monoclonal antibodies, to these targets.