Therapeutic targeting of Src-kinase Lyn in myeloid leukemic cell growth

Combination of midostaurin and ATRA exerts dose-dependent dual effects on acute myeloid leukemia cells with wild type FLT3

Background

Midostaurin combined with chemotherapy is currently used to treat newly diagnosed acute myeloid leukemia (AML) patients with FMS-like tyrosine kinase 3 (FLT3)-mutations. However, midostaurin acts as an antagonist to some chemotherapeutic agents in leukemia cell lines without FLT3 mutations. All-trans retinoic acid (ATRA) induces apoptosis when used in combination with midostaurin in FLT3-mutated AML cells. This combination has been shown to be safe in AML patients. However, the effect of this combination has not been investigated in AML without FLT3 mutations.

Methods

Cell proliferation was assessed by a cell counting assay. Cell death was evaluated by cell viability and Annexin-V assays. Cell differentiation was assessed by CD11b expression profiling and morphological analysis. To explore the underlying mechanisms, we studied the role of caspase3/7, Lyn, Fgr, Hck, RAF, MEK, ERK, AKT, PU.1, CCAAT/enhancer binding protein β (C/EBPβ) and C/EBPε by Western blot analysis and immunoprecipitation assays. Antitumor activity was also confirmed in mouse xenograft models established with AML cells.

Results

In this study, 0.1 − 0.25 μM midostaurin (mido(L)) combined with ATRA induced differentiation while 0.25 − 0.5 μM midostaurin (mido(H)) combined with ATRA triggered apoptosis in some AML cell lines without FLT3-mutations. Midostaurin combined with ATRA (mido-ATRA) also exhibited antitumor activity in mouse xenograft models established with AML cells. Mechanistically, mido(H)-ATRA-induced apoptosis was dependent on caspase-3/7. Mido(L)-ATRA inhibited Akt activation which was associated with decreased activity of Lyn/Fgr/Hck, resulted in dephosphorylation of RAF S259, activated RAF/MEK/ERK, along with upregulating the protein levels of C/EBPβ, C/EBPε and PU.1. A MEK specific inhibitor was observed to suppress mido(L)-ATRA-induced increases in the protein levels of C/EBPs and PU.1 and mido(L)-ATRA-induced differentiation. Furthermore, inhibition of Akt activity promoted mido(L)-ATRA-induced downregulation of RAF S259 phosphorylation and mido(L)-ATRA-induced differentiation. Therefore, Lyn/Fgr/Hck-associated Akt inhibition activated RAF/MEK/ERK and controlled mido(L)-ATRA-induced differentiation by upregulation of C/EBPs and PU.1. Mido(L)-ATRA also promoted assembly of the signalosome, which may facilitate RAF activation.

Conclusions

Midostaurin combined with ATRA exerts antitumor activity against AML with wild-type FLT3 mutations in vitro and in vivo. These findings may provide novel therapeutic strategies for some AML patients without FLT3 mutations and imply a new target of midostaurin.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09828-2.

HSP90 inhibitor NVP-BEP800 affects stability of SRC kinases and growth of T-cell and B-cell acute lymphoblastic leukemias

T-cell and B-cell acute lymphoblastic leukemias (T-ALL, B-ALL) are aggressive hematological malignancies characterized by an accumulation of immature T- or B-cells. Although patient outcomes have improved, novel targeted therapies are needed to reduce the intensity of chemotherapy and improve the prognosis of high-risk patients. Using cell lines, primary cells and patient-derived xenograft (PDX) models, we demonstrate that ALL cells viability is sensitive to NVP-BEP800, an ATP-competitive inhibitor of Heat shock protein 90 (HSP90). Furthermore, we reveal that lymphocyte-specific SRC family kinases (SFK) are important clients of the HSP90 chaperone in ALL. When PDX mice are treated with NVP-BEP800, we found that there is a decrease in ALL progression. Together, these results demonstrate that the chaperoning of SFK by HSP90 is involved in the growth of ALL. These novel findings provide an alternative approach to target SRC kinases and could be used for the development of new treatment strategies for ALL.

Preclinical efficacy for a novel tyrosine kinase inhibitor, ArQule 531 against acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) is the most common type of adult leukemia. Several studies have demonstrated that oncogenesis in AML is enhanced by kinase signaling pathways such as Src family kinases (SFK) including Src and Lyn, spleen tyrosine kinase (SYK), and bruton's tyrosine kinase (BTK). Recently, the multi-kinase inhibitor ArQule 531 (ARQ 531) has demonstrated potent inhibition of SFK and BTK that translated to improved pre-clinical in vivo activity as compared with the irreversible BTK inhibitor ibrutinib in chronic lymphocytic leukemia (CLL) models. Given the superior activity of ARQ 531 in CLL, and recognition that this molecule has a broad kinase inhibition profile, we pursued its application in pre-clinical models of AML.

METHODS

The potency of ARQ 531 was examined in vitro using FLT3 wild type and mutated (ITD) AML cell lines and primary samples. The modulation of pro-survival kinases following ARQ 531 treatment was determined using AML cell lines. The effect of SYK expression on ARQ 531 potency was evaluated using a SYK overexpressing cell line (Ba/F3 murine cells) constitutively expressing FLT3-ITD. Finally, the in vivo activity of ARQ 531 was evaluated using MOLM-13 disseminated xenograft model.

RESULTS

Our data demonstrate that ARQ 531 treatment has anti-proliferative activity in vitro and impairs colony formation in AML cell lines and primary AML cells independent of the presence of a FLT3 ITD mutation. We demonstrate decreased phosphorylation of oncogenic kinases targeted by ARQ 531, including SFK (Tyr416), BTK, and fms-related tyrosine kinase 3 (FLT3), ultimately leading to changes in down-stream targets including SYK, STAT5a, and ERK1/2. Based upon in vitro drug synergy data, we examined ARQ 531 in the MOLM-13 AML xenograft model alone and in combination with venetoclax. Despite ARQ 531 having a less favorable pharmacokinetics profile in rodents, we demonstrate modest single agent in vivo activity and synergy with venetoclax.

CONCLUSIONS

Our data support consideration of the application of ARQ 531 in combination trials for AML targeting higher drug concentrations in vivo.

Targeting Tyrosine Kinases in Acute Myeloid Leukemia: Why, Who and How?

Acute myeloid leukemia (AML) is a myeloid malignancy carrying a heterogeneous molecular panel of mutations participating in the blockade of differentiation and the increased proliferation of myeloid hematopoietic stem and progenitor cells. The historical "3 + 7" treatment (cytarabine and daunorubicin) is currently challenged by new therapeutic strategies, including drugs depending on the molecular landscape of AML. This panel of mutations makes it possible to combine some of these new treatments with conventional chemotherapy. For example, the FLT3 receptor is overexpressed or mutated in 80% or 30% of AML, respectively. Such anomalies have led to the development of targeted therapies using tyrosine kinase inhibitors (TKIs). In this review, we document the history of TKI targeting, FLT3 and several other tyrosine kinases involved in dysregulated signaling pathways.

The role of SRC family kinases in FLT3 signaling

The receptor tyrosine kinase FLT3 is expressed almost exclusively in the hematopoietic compartment. Binding of its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. This leads to autophosphorylation of FLT3 on several tyrosine residues which constitute high affinity binding sites for signal transduction molecules. Recruitment of these signal transduction molecules to FLT3 leads to the activation of several signal transduction pathways that regulate cell survival, cell proliferation and differentiation. Oncogenic, constitutively active mutants of FLT3 are known to be expressed in acute myeloid leukemia and to correlate with poor prognosis. Activation of the receptor mediates cell survival, cell proliferation and differentiation of cells. Several of the signal transduction pathways downstream of FLT3 have been shown to include various members of the SRC family of kinases (SFKs). They are involved in regulating the activity of RAS/ERK pathways through the scaffolding protein GAB2 and the adaptor protein SHC. They are also involved in negative regulation of signaling through phosphorylation of the ubiquitin E3 ligase CBL. Initially studied as the SFKs, as if they were a homogenous group of kinases, recent data suggest that each SFK has its own specific signaling capabilities where some are involved in positive signaling, while others are involved in negative signaling. This review discusses some recent insights into how SFKs are involved in FLT3 signaling.

Matrine inhibits BCR/ABL mediated ERK/MAPK pathway in human leukemia cells

The BCR/ABL fusion gene and its downstream signaling pathways such as Ras/Raf/MAPK, JAK/STAT3, and PI3K/AKT pathways play important roles in malignant transformation of leukemia, especially chronic myelogenous leukemia (CML). Our previous study showed that matrine, an alkaloid extracted from a Chinese herb radix sophorae, significantly inhibited the proliferation of human CML K562cells, induced cell cycle arrest in G0/G1, and promoted cell apoptosis. In the present study, we investigated the molecular mechanism of matrine in the growth inhibition of leukemia cells using K562 and HL-60 cell lines. RT-PCR and Western blot assay demonstrated that the expression of BCR/ABL in K562 and HL-60 cells was significantly inhibited by matrine treatment. Phosphorylation of MEK1, ERK1/2, and their upstream adaptor molecules Shc and SHP2 were significantly downregulated. The protein and mRNA expression of components of the ERK/MAPK signal pathway, and Bcl-xL, Cyclin D1, and c-Myc, were dramatically reduced. Conversely, the expression of p27, a negative regulator of cell cycle progression, increased after matrine treatment. These results indicated that the inhibition of ERK/MAPK and BCR/ABL signaling pathway was associated with matrine's suppressive effects on the growth of K562 and HL-60 cells. In in vivo study, matrine significantly decreased the mortality rate of tumor-baring mice and suggested that matrine could exert its anti-leukemia effect in vivo.

Proteomic analysis of imatinib-resistant CML-T1 cells reveals calcium homeostasis as a potential therapeutic target

Chronic myeloid leukemia (CML) therapy has markedly improved patient prognosis after introduction of imatinib mesylate for clinical use. However, a subset of patients develops resistance to imatinib and other tyrosine kinase inhibitors (TKIs), mainly due to point mutations in the region encoding the kinase domain of the fused BCR-ABL oncogene. To identify potential therapeutic targets in imatinib-resistant CML cells, we derived imatinib-resistant CML-T1 human cell line clone (CML-T1/IR) by prolonged exposure to imatinib in growth media. Mutational analysis revealed that the Y235H mutation in BCR-ABL is probably the main cause of CML-T1/IR resistance to imatinib. To identify alternative therapeutic targets for selective elimination of imatinib-resistant cells, we compared the proteome profiles of CML-T1 and CML-T1/IR cells using 2-DE-MS. We identified eight differentially expressed proteins, with strongly upregulated Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) in the resistant cells, suggesting that this protein may influence cytosolic pH, Ca²⁺ concentration or signaling pathways such as Wnt in CML-T1/IR cells. We tested several compounds including drugs in clinical use that interfere with the aforementioned processes and tested their relative toxicity to CML-T1 and CML-T1/IR cells. Calcium channel blockers, calcium signaling antagonists and modulators of calcium homeostasis, namely thapsigargin, ionomycin, verapamil, carboxyamidotriazole and immunosuppressive drugs cyclosporine A and tacrolimus (FK-506) were selectively toxic to CML-T1/IR cells. The putative cellular targets of these compounds in CML-T1/IR cells are postulated in this study. We propose that Ca²⁺ homeostasis can be a potential therapeutic target in CML cells resistant to TKIs. We demonstrate that a proteomic approach may be used to characterize a TKI-resistant population of CML cells enabling future individualized treatment options for patients.

Regulation of mitochondrial functions by protein phosphorylation and dephosphorylation

The mitochondria are double membrane-bound organelles found in most eukaryotic cells. They generate most of the cell’s energy supply of adenosine triphosphate (ATP). Protein phosphorylation and dephosphorylation are critical mechanisms in the regulation of cell signaling networks and are essential for almost all the cellular functions. For many decades, mitochondria were considered autonomous organelles merely functioning to generate energy for cells to survive and proliferate, and were thought to be independent of the cellular signaling networks. Consequently, phosphorylation and dephosphorylation processes of mitochondrial kinases and phosphatases were largely neglected. However, evidence accumulated in recent years on mitochondria-localized kinases/phosphatases has changed this longstanding view. Mitochondria are increasingly recognized as a hub for cell signaling, and many kinases and phosphatases have been reported to localize in mitochondria and play important functions. However, the strength of the evidence on mitochondrial localization and the activities of the reported kinases and phosphatases vary greatly, and the detailed mechanisms on how these kinases/phosphatases translocate to mitochondria, their subsequent function, and the physiological and pathological implications of their localization are still poorly understood. Here, we provide an updated perspective on the recent advancement in this area, with an emphasis on the implications of mitochondrial kinases/phosphatases in cancer and several other diseases.

Cytotoxicity of the Sesquiterpene Lactones Neoambrosin and Damsin from Ambrosia maritima Against Multidrug-Resistant Cancer Cells

Multidrug resistance is a prevailing phenomenon leading to chemotherapy treatment failure in cancer patients. In the current study two known cytotoxic pseudoguaianolide sesquiterpene lactones; neoambrosin (1) and damsin (2) that circumvent MDR were identified. The two cytotoxic compounds were isolated using column chromatography, characterized using 1D and 2D NMR, MS, and compared with literature values. The isolated compounds were investigated for their cytotoxic potential using resazurin assays and thereafter confirmed with immunoblotting and in silico studies. MDR cells overexpressing ABC transporters (P-glycoprotein, BCRP, ABCB5) did not confer cross-resistance toward (1) and (2), indicating that these compounds are not appropriate substrates for any of the three ABC transporters analyzed. Resistance mechanisms investigated also included; the loss of the functions of the TP53 and the mutated EGFR. The HCT116 p53^-/- cells were sensitive to 1 but resistant to 2. It was interesting to note that resistant cells transfected with oncogenic ΔEGFR exhibited hypersensitivity CS toward (1) and (2) (degrees of resistances were 0.18 and 0.15 for (1) and (2), respectively). Immunoblotting and in silico analyses revealed that 1 and 2 silenced c-Src kinase activity. It was hypothesized that inhibition of c-Src kinase activity may explain CS in EGFR-transfected cells. In conclusion, the significant cytotoxicity of 1 and 2 against different drug-resistant tumor cell lines indicate that they may be promising candidates to treat refractory tumors.

Src family kinases and their role in hematological malignancies

The Src family protein tyrosine kinases (SFKs) are non-receptor intracellular kinases that have important roles in both hematopoiesis and leukemogenesis. The derangement of their expression or activation has been demonstrated to contribute to hematological malignancies. This review first examines the mechanisms of SFK overexpression and hyperactivation, emphasizing the dysregulation of the upstream modulators. Subsequently, the role of SFK up-regulation in the initiation, progression and therapy resistance of many hematological malignancies is also analyzed. The presented evidence endeavors to highlight the influence of SFK up-regulation on an extensive number of hematological malignancies and the need to consider them as candidates in targeted anticancer therapy.

Regulation of SRC kinases by microRNA-3607 located in a frequently deleted locus in prostate cancer

Genomic studies suggest that deletions at chromosome (chr) 5q region (particularly chr5q14-q23) are frequent in prostate cancer, implicating this region in prostate carcinogenesis. However, the genes within this region are largely unknown. Here, we report for the first time the widespread attenuation of miR-3607, an miRNA gene located at chr5q14 region, in prostate cancer. Expression analyses of miR-3607 in a clinical cohort of prostate cancer specimens showed that miR-3607 is significantly attenuated and low miR-3607 expression is correlated with tumor progression and poor survival outcome in prostate cancer. Our analyses suggest that miR-3607 expression may be a clinically significant parameter with an associated diagnostic potential. We examined the functional significance of miR-3607 in prostate cancer cell lines and found that miR-3607 overexpression led to significantly decreased proliferation, apoptosis induction, and decreased invasiveness. Furthermore, our results suggest that miR-3607 directly represses oncogenic SRC family kinases LYN and SRC in prostate cancer. In view of our results, we propose that miR-3607 plays a tumor-suppressive role in prostate cancer by regulating SRC kinases that in turn regulates prostate carcinogenesis. To our knowledge, this is the first report that: (i) identifies a novel role for miR-3607 located in a frequently deleted region of prostate cancer and (ii) defines novel miRNA-mediated regulation of SRC kinases in prostate cancer. Because SRC kinases play a central role in prostate cancer progression and metastasis and are attractive targets, this study has potential implications in the design of better therapeutic modalities for prostate cancer management.

Expression of LYN and PTEN genes in chronic myeloid leukemia and their importance in therapeutic strategy

Tyrosine kinase inhibitors (TKIs), imatinib, nilotinib and dasatinib, are the current treatment of chronic myeloid leukemia (CML). BCR-ABL1 point mutations are the principal cause of resistance to treatment; however other mechanisms could be involved in failure to TKI therapy. LYN is a src kinase protein that regulates survival and responsiveness of tumor cells by a BCR-ABL1 independent mechanism. PTEN tumor suppressor gene is downregulated by BCR-ABL1 in CML stem cells and its deletion is associated with acceleration of disease. In this study we evaluated the expression of LYN, PTEN and the ratio of both genes in 40 healthy donors (HD) and in 139 CML patients; 88 of them resistant to TKI in different phases of disease and 51 in chronic phase classified as optimal responders (OR) to TKI [40 treated with imatinib or nilotinib (OR-IN) and 11 treated with dasatinib (OR-D) therapy]. When we analyzed the gene expression values of LYN, an increase was observed only in advanced stages of the disease, however, when we analyzed the ratio between LYN and PTEN genes, the group of resistant patients in chronic phase in imatinib or nilotinib treatment (CP-IN) also showed a significant increase. Resistant patients treated with dasatinib, a src kinase inhibitor, presented a similar ratio to the observed in HD. In addition, the LYN/PTEN ratio and the LYN expression showed a direct significant correlation with BCR-ABL1 transcript levels in unmutated resistant patients treated with non-src kinase inhibitors. We were able to identify 8/35 (23%) of cases in CP-IN and 4/12 (33%) in accelerated phase and blast phase (AP/BC-IN), in which resistance could be associated with an increase in the ratio of the LYN/PTEN. Our data suggest that the LYN/PTEN expression ratio may be a sensitive monitor of disease progression in unmutated CML patients under imatinib or nilotinib treatment. This ratio could detect cases when resistance is related to altered LYN expression, suggesting that the treatment change to a src kinase inhibitor would be most suitable to overcome resistance.

The Src-family kinase inhibitor PP2 rescues inducible differentiation events in emergent retinoic acid-resistant myeloblastic leukemia cells

Retinoic acid is an embryonic morphogen and dietary factor that demonstrates chemotherapeutic efficacy in inducing maturation in leukemia cells. Using HL60 model human myeloid leukemia cells, where all-trans retinoic acid (RA) induces granulocytic differentiation, we developed two emergent RA-resistant HL60 cell lines which are characterized by loss of RA-inducible G1/G0 arrest, CD11b expression, inducible oxidative metabolism and p47^phox expression. However, RA-treated RA-resistant HL60 continue to exhibit sustained MEK/ERK activation, and one of the two sequentially emergent resistant lines retains RA-inducible CD38 expression. Other signaling events that define the wild-type (WT) response are compromised, including c-Raf phosphorylation and increased expression of c-Cbl, Vav1, and the Src-family kinases (SFKs) Lyn and Fgr. As shown previously in WT HL60 cells, we found that the SFK inhibitor PP2 significantly increases G1/G0 cell cycle arrest, CD38 and CD11b expression, c-Raf phosphorylation and expression of the aforementioned regulators in RA-resistant HL60. The resistant cells were potentially incapable of developing inducible oxidative metabolism. These results motivate the concept that RA resistance can occur in steps, wherein growth arrest and other differentiation events may be recovered in both emergent lines. Investigating the mechanistic anomalies in resistant cell lines is of therapeutic significance and helps to mechanistically understand the response to retinoic acid’s biological effects in WT HL60 cells.

Phase II pilot study of oral dasatinib in patients with higher-risk myelodysplastic syndrome (MDS) who failed conventional therapy

Given evidence for the role of Src family kinases, especially Lyn kinase, in myeloblast proliferation and the in vitro inhibitory activity of dasatinib on Src and Lyn, we conducted a phase II study to assess overall response to 100mg/day dasatinib in patients with higher-risk myelodysplastic syndrome (MDS), chronic myelomonocytic leukemia, or acute myeloid leukemia arising from MDS and who had failed prior treatment with azanucleoside analogs. Among 18 patients treated, 3 responded, 4 had stable disease, and 10 experienced disease progression. Toxicities were limited and consistent with previous reports. Dasatinib appears to be safe but with limited efficacy.

Expression and activity of Fyn mediate proliferation and blastic features of chronic myelogenous leukemia

The BCR-ABL1 oncogene is a tyrosine kinase that activates many signaling pathways, resulting in the induction of chronic myeloid leukemia (CML). Kinase inhibitors, such as imatinib, have been developed for the treatment of CML; however, the terminal, blast crisis phase of the disease remains a clinical challenge. Blast crisis CML is difficult to treat due to resistance to tyrosine kinase inhibitors, increased genomic instability and acquired secondary mutations. Our recent studies uncovered a role for Fyn in promoting BCR-ABL1 mediated cell growth and sensitivity to imatinib. Here we demonstrate that Fyn contributes to BCR-ABL1 induced genomic instability, a feature of blast crisis CML. Bone marrow cells and mouse embryonic fibroblasts derived from Fyn knockout mice transduced with BCR-ABL1 display slowed growth and clonogenic potential as compared to Fyn wild-type BCR-ABL1 expressing counterparts. K562 cells overexpressing constitutively active Fyn kinase were larger in size and displayed an accumulation of genomic abnormalities such as chromosomal aberrations and polyploidy. Importantly, loss of Fyn protected mouse embryonic fibroblast cells from increased number of chromosomal aberrations and fragments induced by BCR-ABL1. Together, these results reveal a novel role for Fyn in regulating events required for genomic maintenance and suggest that Fyn kinase activity plays a role in the progression of CML to blast crisis.

Functions of the Lyn tyrosine kinase in health and disease

Src family kinases such as Lyn are important signaling intermediaries, relaying and modulating different inputs to regulate various outputs, such as proliferation, differentiation, apoptosis, migration and metabolism. Intriguingly, Lyn can mediate both positive and negative signaling processes within the same or different cellular contexts. This duality is exemplified by the B-cell defect in Lyn-/- mice in which Lyn is essential for negative regulation of the B-cell receptor; conversely, B-cells expressing a dominant active mutant of Lyn (Lynup/up) have elevated activities of positive regulators of the B-cell receptor due to this hyperactive kinase. Lyn has well-established functions in most haematopoietic cells, viz. progenitors via influencing c-kit signaling, through to mature cell receptor/integrin signaling, e.g. erythrocytes, platelets, mast cells and macrophages. Consequently, there is an important role for this kinase in regulating hematopoietic abnormalities. Lyn is an important regulator of autoimmune diseases such as asthma and psoriasis, due to its profound ability to influence immune cell signaling. Lyn has also been found to be important for maintaining the leukemic phenotype of many different liquid cancers including acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML) and B-cell lymphocytic leukaemia (BCLL). Lyn is also expressed in some solid tumors and here too it is establishing itself as a potential therapeutic target for prostate, glioblastoma, colon and more aggressive subtypes of breast cancer. LAY To relay information, a cell uses enzymes that put molecular markers on specific proteins so they interact with other proteins or move to specific parts of the cell to have particular functions. A protein called Lyn is one of these enzymes that regulate information transfer within cells to modulate cell growth, survival and movement. Depending on which type of cell and the source of the information input, Lyn can positively or negatively regulate the information output. This ability of Lyn to be able to both turn on and turn off the relay of information inside cells makes it difficult to fully understand its precise function in each specific circumstance. Lyn has important functions for cells involved in blood development, including different while blood cells as well as red blood cells, and in particular for the immune cells that produce antibodies (B-cells), as exemplified by the major B-cell abnormalities that mice with mutations in the Lyn gene display. Certain types of leukaemia and lymphoma appear to have too much Lyn activity that in part causes the characteristics of these diseases, suggesting it may be a good target to develop new anti-leukaemia drugs. Furthermore, some specific types, and even specific subtypes, of solid cancers, e.g. prostate, brain and breast cancer can also have abnormal regulation of Lyn. Consequently, targeting this protein in these cancers could also prove to be beneficial.

Targeting Lyn tyrosine kinase through protein fusions encompassing motifs of Cbp (Csk-binding protein) and the SOCS box of SOCS1

The tyrosine kinase Lyn is involved in oncogenic signalling in several leukaemias and solid tumours, and we have previously identified a pathway centred on Cbp [Csk (C-terminal Src kinase)-binding protein] that mediates both enzymatic inactivation, as well as proteasomal degradation of Lyn via phosphorylation-dependent recruitment of Csk (responsible for phosphorylating the inhibitory C-terminal tyrosine of Lyn) and SOCS1 (suppressor of cytokine signalling 1; an E3 ubiquitin ligase). In the present study we show that fusing specific functional motifs of Cbp and domains of SOCS1 together generates a novel molecule capable of directing the proteasomal degradation of Lyn. We have characterized the binding of pY (phospho-tyrosine) motifs of Cbp to SFK (Src-family kinase) SH2 (Src homology 2) domains, identifying those with high affinity and specificity for the SH2 domain of Lyn and that are preferred substrates of active Lyn. We then fused them to the SB (SOCS box) of SOCS1 to facilitate interaction with the ubiquitination-promoting elongin B/C complex. As an eGFP (enhanced green fluorescent protein) fusion, these proteins can direct the polyubiquitination and proteasomal degradation of active Lyn. Expressing this fusion protein in DU145 cancer cells (but not LNCaP or MCF-7 cells), that require Lyn signalling for survival, promotes loss of Lyn, loss of caspase 3, appearance of an apoptotic morphology and failure to survive/expand. These findings show how functional domains of Cbp and SOCS1 can be fused together to generate molecules capable of inhibiting the growth of cancer cells that express high levels of active Lyn.

Managing resistance in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disorder that affects 5000 new patients per year in the United States. Prior to 10 years ago, durable remission was rare and patients often underwent bone marrow transplantation with substantial morbidity and mortality. Fortunately, CML has been the epicenter of exciting advances in cancer therapy with the discovery of the Bcr-Abl gene fusion and the subsequent development of imatinib mesylate, a small molecule tyrosine kinase inhibitor, to target the kinase activity of the bcr-abl protein product. Despite unprecedented durability for complete hematologic, cytogenetic, and molecular responses seen with front-line imatinib therapy, many patients require alternative therapy because of drug intolerance, suboptimal response, primary resistance, secondary resistance, or progression to advanced phase disease. Further, up to 5% of patients present with advanced disease that does not sustain a durable response to tyrosine kinase inhibitors. Thus, up to one third of CML patients require alternate therapy. Chronic myeloid leukemia has become an exemplary model system for understanding molecular targeting and overcoming mechanisms of drug resistance. This review will discuss potential mechanisms of resistance and ongoing research into novel targets and agents for CML resistant to standard of care.

Dasatinib treatment for imatinib resistant or intolerant patients with chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) is a genetically associated malignancy of haematopoietic stem cells, characterized by a t(9;22) translocation that forms the Philadelphia chromosome and creates a novel fusion gene, BCR-ABL. Treatment with molecular-targeted therapy is usually initiated with imatinib, an inhibitor of BCR-ABL tyrosine kinase. Imatinib resistance is, however, observed in some CML patients, especially in those with advanced disease. Through computerized literature searches, a systematic analysis was conducted to examine the efficacy and benefits of dasatinib therapy for imatinib resistant or intolerant CML patients in the chronic phase (CP), accelerated phase (AP) and fatal blast crisis phase (BC). In terms of major haematological and cytogenetic responses, this meta-analysis showed no significant differences in dasatinib treatment between myeloid BC-CML and lymphoid BC-CML patients with imatinib resistance or intolerance. Dasatinib therapy was, however, significantly more effective in improving major haematological and cytogenetic responses for CP-CML patients than for AP-CML patients with imatinib resistance or intolerance.

Breast cancer patients' clinical outcome measures are associated with Src kinase family member expression

Background:

This study determined mRNA expression levels for Src kinase family (SFK) members in breast tissue specimens and assessed protein expression levels of prominent SFK members in invasive breast cancer to establish associations with clinical outcome. Ki67 was investigated to determine association between SFK members and proliferation.

Methods:

The mRNA expression levels were assessed for eight SFK members by quantitative real-time PCR. Immunohistochemistry was performed for c-Src, Lyn, Lck and Ki67.

Results:

mRNA expression was quantified in all tissue samples. SRC and LYN were the most highly expressed in malignant tissue. LCK was more highly expressed in oestrogen receptor (ER)-negative, compared with ER-positive tumours. High cytoplasmic Src kinase protein expression was significantly associated with decreased disease-specific survival. Lyn was not associated with survival at any cellular location. High membrane Lck expression was significantly associated with improved survival. Ki67 expression correlated with tumour grade and nuclear c-Src, but was not associated with survival.

Conclusions:

All eight SFK members were expressed in different breast tissues. Src kinase was highest expressed in breast cancer and had a negative impact on disease-specific survival. Membrane expression of Lck was associated with improved clinical outcome. High expression of Src kinase correlated with high proliferation.

Suppression of ADAM17-mediated Lyn/Akt pathways induces apoptosis of human leukemia U937 cells: Bungarus multicinctus protease inhibitor-like protein-1 uncovers the cytotoxic mechanism

Cell surface proteases have been demonstrated to play an important role in facilitating cell invasion into the extracellular matrix and may contribute significantly to extracellular matrix degradation by metastatic cancer cells. Abundant expression of these enzymes is associated with poor prognosis. Thus, protease inhibitors that repress cell surface proteases may be applicable to cancer therapy. Because soybean Kunitz-type trypsin inhibitor has been found to induce apoptotic death of human leukemia Jurkat cells, anti-leukemia activity of Bungarus multicinctus protease inhibitor-like protein-1 (PILP-1) is thus examined. PILP-1 induced apoptosis of human leukemia U937 cells, characteristic of loss of mitochondrial membrane potential, degradation of procaspase-8, and production of t-Bid. FADD down-regulation neither restored viability of PILP-1-treated cells nor attenuated production of active caspase-8 and t-Bid in PILP-1-treated cells, suggesting that the death receptor-mediated pathway was not involved in the cytotoxicity of PILP-1. It was found that PILP-1-evoked p38 MAPK activation and ERK inactivation led to PILP-1-induced cell death and down-regulation of ADAM17. Knockdown of ADAM17 by siRNA induced death of U937 cells and inactivation of Lyn and Akt. Immunoprecipitation suggested that ADAM17 and Lyn form complexes. Overexpression of ADAM17, LynY507F (gain of function), and constitutively active Akt suppressed the cytotoxic effects of PILP-1. PILP-1-elicited inactivation of Lyn and Akt was abrogated in cells with overexpressed ADAM17 or LynY507F. Taken together, our data indicate that ADAM17-mediated activation of Lyn/Akt maintains the viability of U937 cells and that suppression of the pathway is responsible for PILP-1-induced apoptosis.

Longitudinal studies of SRC family kinases in imatinib- and dasatinib-resistant chronic myelogenous leukemia patients

This report aims to more accurately define the frequency of the involvement of SRC Family Kinases (SFKs) in imatinib- and dasatinib-resistant CML patients. Clinical samples were analysed during in vivo treatment. We confirmed the high frequency of SFKs involvement in Tyrosine kinase inhibitor-resistant CML (52% of the cases) and even further in progressive disease and blast crises (60% of the cases). The SFKs deregulation is also observed in patients harboring BCR-ABL mutations. In T315I and F317L mutated patients, CML-resistance appears to be promoted by SFKs kinase protein reactivation once the BCR-ABL mutated clone has decreased on Omacetaxine.

Gaining insights into the Bcr-Abl activity-independent mechanisms of resistance to imatinib mesylate in KCL22 cells: a comparative proteomic approach

Imatinib mesylate is a potent inhibitor of Bcr-Abl tyrosine kinase, an oncoprotein that plays a key role in the development of chronic myeloid leukemia. Consequently, imatinib is used as front-line therapy for this disease. A major concern in imatinib treatment is the emergence of resistance to the drug. Here we used the imatinib-resistant KCL22R and imatinib-sensitive KCL22S cells in which none of the known resistance mechanisms has been detected and hence novel Bcr-Abl activity-independent mechanisms could be envisaged. We characterized proteins that were differentially expressed between the KCL22R and KCL22S cells. Using two-dimensional differential gel electrophoresis coupled with mass spectrometry and Western blot analysis we identified 51 differentially expressed proteins: 27 were over-expressed and 24 were under-expressed in KCL22R versus KCL22S cells. Several of these proteins are likely to be involved in such survival mechanisms as modulation of redox balance and activation of anti-apoptotic pathways mediated by NF-kappaB and Ras-MAPK signaling. The data reported may be useful for further studies on mechanisms of imatinib resistance and for the screening of biomarkers to develop new combinatorial therapeutic approaches.

Dasatinib inhibits the growth of molecularly heterogeneous myeloid leukemias

PURPOSE

Dasatinib is a dual Src/Abl inhibitor recently approved for Bcr-Abl+ leukemias with resistance or intolerance to prior therapy. Because Src kinases contribute to multiple blood cell functions by triggering a variety of signaling pathways, we hypothesized that their molecular targeting might lead to growth inhibition in acute myeloid leukemia (AML).

EXPERIMENTAL DESIGN

We studied growth factor-dependent and growth factor-independent leukemic cell lines, including three cell lines expressing mutants of receptor tyrosine kinases (Flt3 or c-Kit) as well as primary AML blasts for responsiveness to dasatinib.

RESULTS

Dasatinib resulted in the inhibition of Src family kinases in all cell lines and blast cells at approximately 1 x 10(-9) mol/L. It also inhibited mutant Flt3 or Kit tyrosine phosphorylation at approximately 1 x 10(-6) mol/L. Mo7e cells expressing the activating mutation (codon 816) of c-Kit were most sensitive to growth inhibition with a GI(50) of 5 x 10(-9) mol/L. Primary AML blast cells exhibited a growth inhibition of <1 x 10(-6) mol/L. Cell lines that showed growth inhibition at approximately 1 x 10(-6) mol/L showed a G(1) cell cycle arrest and correlated with accumulation of p21 and p27 protein. The addition of rapamycin or cytotoxic agents enhanced growth inhibition. Dasatinib also caused the apoptosis of Mo7e cells expressing oncogenic Kit.

CONCLUSIONS

Although all of the precise targets for dasatinib are not known, this multikinase inhibitor causes either growth arrest or apoptosis in molecularly heterogeneous AML. The addition of cytotoxic or targeted agents can enhance its effects.

Dasatinib in the treatment of chronic myeloid leukemia in accelerated phase after imatinib failure: the START a trial

PURPOSE Patients with chronic myelogenous leukemia in accelerated phase (CML-AP) that is resistant or intolerant to imatinib have limited therapeutic options. Dasatinib, a potent inhibitor of BCR-ABL and SRC-family kinases, has efficacy in patients with CML-AP who have experienced treatment failure with imatinib. We now report follow-up data from the full patient cohort of 174 patients enrolled onto a phase II trial to provide a more complete assessment of the efficacy and safety of dasatinib in this population. PATIENTS AND METHODS Patients with imatinib-resistant (n = 161) or -intolerant (n = 13) CML-AP received dasatinib 70 mg orally twice daily. Results At a median follow-up of 14.1 months (treatment duration, 0.1 to 21.7 months), major and complete hematologic responses were attained by 64% and 45% of patients, respectively, and major and complete cytogenetic responses were achieved in 39% and 32% of patients, respectively. Responses were achieved irrespective of imatinib status (resistant or intolerant), prior stem-cell transplantation, or the presence of prior BCR-ABL mutation. The 12-month progression-free survival and overall survival rates were 66% and 82%, respectively. Dasatinib was generally well tolerated; the most frequent nonhematologic severe treatment-related adverse event was diarrhea (52%; grade 3 to 4, 8%). Cytopenias were common, including grade 3 to 4 neutropenia (76%) and thrombocytopenia (82%). Pleural effusion occurred in 27% of patients (grade 3 to 4, 5%). CONCLUSION Dasatinib is effective in patients with CML-AP after imatinib treatment failure.

Opposite expression pattern of Src kinase Lyn in acute and chronic haematological malignancies

Lck/yes-related novel (Lyn) tyrosine kinase overexpression has been suggested to be important for leukaemic cell growth making it an attractive target for therapy. By contrast, Lyn deficiency was shown to be responsible for a phenotype resembling myeloproliferative neoplasm (MPN) in mice. We aimed to shed more light on Lyn's role in haematological neoplasm and systematically investigated Lyn expression in MPN, acute and chronic leukaemia subtypes (n = 236). On top, B-cell chronic lymphocytic leukaemia (B-CLL) and chronic myeloid leukaemia significantly overexpressed Lyn when compared to de novo acute lymphoblastic leukaemia, de novo acute myeloid leukaemia (AML) and Philadelphia-chromosome-negative myeloproliferative neoplasms (p < 0.001). Most of acute leukaemia subtypes showed a notable down-regulation of Lyn mRNA but anyhow individual cases were labelled for the active form of Lyn protein. Intriguingly, secondary AML evolved in myelodysplastic syndromes revealed almost undetectable Lyn. Overexpression of Lyn in B-CLL was associated with a significant down-regulation of microRNA-337-5p suggesting that aberrant expression of this particular microRNA could be involved in post-transcriptional control of Lyn mRNA fate. We conclude that tyrosine kinase Lyn contributes to the malignant phenotype in certain leukaemia subtypes and therefore attracts targeted therapy.

Crystal structures of the Lyn protein tyrosine kinase domain in its Apo- and inhibitor-bound state

The Src-family protein-tyrosine kinase (PTK) Lyn is the most important Src-family kinase in B cells, having both inhibitory and stimulatory activity that is dependent on the receptor, ligand, and developmental context of the B cell. An important role for Lyn has been reported in acute myeloid leukemia and chronic myeloid leukemia, as well as certain solid tumors. Although several Src-family inhibitors are available, the development of Lyn-specific inhibitors, or inhibitors with reduced off-target activity to Lyn, has been hampered by the lack of structural data on the Lyn kinase. Here we report the crystal structure of the non-liganded form of Lyn kinase domain, as well as in complex with three different inhibitors: the ATP analogue AMP-PNP; the pan Src kinase inhibitor PP2; and the BCR-Abl/Src-family inhibitor Dasatinib. The Lyn kinase domain was determined in its "active" conformation, but in the unphosphorylated state. All three inhibitors are bound at the ATP-binding site, with PP2 and Dasatinib extending into a hydrophobic pocket deep in the substrate cleft, thereby providing a basis for the Src-specific inhibition. Analysis of sequence and structural differences around the active site region of the Src-family PTKs were evident. Accordingly, our data provide valuable information for the further development of therapeutics targeting Lyn and the important Src-family of kinases.

Therapy options in imatinib failures

Chronic myelogenous leukemia (CML) is defined by the presence of the constitutively active tyrosine kinase breakpoint cluster region/Abelson (Bcr-Abl), which activates numerous signal transduction pathways leading to uncontrolled cell proliferation. The development of the Bcr-Abl-targeted imatinib represents a paradigm shift in the treatment of CML, because treatment with imatinib resulted in significantly better patient outcome, response rates, and overall survival compared with previous standards. Despite this advance, not all patients benefit from imatinib because of resistance and intolerance. Resistance to imatinib can develop from a number of mechanisms that can be defined as Bcr-Abl-dependent (e.g., most commonly resulting from point mutations in the Abl kinase domain) and Bcr-Abl-independent mechanisms (including the constitutive activation of downstream signaling molecules, e.g., Src family kinases), which could result in the activation of the pathway regardless of Bcr-Abl inhibition. Clearly, new treatment approaches are required for patients resistant to or intolerant of imatinib, which can be dose escalated in patients who demonstrate resistance. This does not result in long-term responses. Hematopoietic stem cell transplantation is limited by the availability of matched donors and the potential for morbidity. Dasatinib, a dual Bcr-Abl/Src kinase inhibitor, has shown efficacy against all imatinib-resistant Bcr-Abl mutations except for T315I. A large trial program showed that dasatinib is effective in patients previously exposed to imatinib and has a manageable safety profile in all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, resulting in its approval. Nilotinib, an analogue of imatinib, also has demonstrated activity in a similar patient population. These agents and less clinically advanced strategies are discussed in this review.

Targeting Lyn inhibits tumor growth and metastasis in Ewing's sarcoma

Src family tyrosine kinases (SFK) play an important role in growth and metastasis of many types of human malignancies. However, their significance in Ewing's sarcoma remains to be elucidated. The purpose of this study was to evaluate the role of Lyn, one member of the SFK, in Ewing's sarcoma growth and metastasis and to determine whether a SFK inhibitor can induce Ewing's tumor regression. Lyn was expressed and activated in TC71, A4573, and SK-ES human Ewing's sarcoma cells. Lyn expression was seen in 13 of 15 patient tumor samples, 6 of which showed Lyn activation. Specific inhibition of Lyn using small interfering RNA significantly decreased primary tumor growth and lytic activity, and also reduced lung metastases in vivo. Down-regulation of Lyn resulted in decreased invasive capacity of tumor cells in vitro. AP23994, a small-molecule SFK inhibitor, decreased Lyn kinase activity and suppressed TC71 cell growth in vitro in a dose-dependent manner. Furthermore, treatment of mice bearing s.c. TC71 tumors with AP23994 or with polyethylenimine/Lyn-small interfering RNA gene therapy resulted in reduced Lyn kinase activity and significant tumor growth suppression. EWS/FLI-1, which is translocation fusion protein associated with Ewing's sarcoma, regulated Lyn gene expression and kinase activity. These data suggest that targeting Lyn may be a new therapeutic approach in treatment of Ewing's sarcoma.

Association between imatinib-resistant BCR-ABL mutation-negative leukemia and persistent activation of LYN kinase

BACKGROUND

Imatinib is a tyrosine kinase inhibitor that is used to treat chronic myelogenous leukemia (CML). BCR-ABL mutations are associated with failure of imatinib treatment in many CML patients. LYN kinase regulates survival and responsiveness of CML cells to inhibition of BCR-ABL kinase, and differences in LYN regulation have been found between imatinib-sensitive and -resistant CML cell lines.

METHODS

We evaluated cells from 12 imatinib-resistant CML patients with mutation-negative BCR-ABL and from six imatinib-sensitive patients who discontinued therapy because of imatinib intolerance. Phosphorylation of BCR-ABL and LYN was assessed in patient cells and cell lines by immunoblotting with activation state-specific antibodies, co-immunoprecipitation studies, and mass spectroscopy analysis of phosphopeptides. Cell viability, caspase activation, and apoptosis were also measured. Mutations were analyzed by sequencing. The effect of silencing LYN with short interfering RNAs (siRNAs) or reducing activation by treatment with tyrosine kinase inhibitors was evaluated in cell lines and patient cells.

RESULTS

Imatinib treatment suppressed LYN phosphorylation in cells from imatinib-sensitive CML patients and imatinib-sensitive cell lines. Imatinib treatment blocked BCR-ABL signaling but did not suppress LYN phosphorylation in cells from imatinib-resistant patients, and persistent activation of LYN kinase was not associated with mutations in LYN kinase or its carboxyl-terminal regulatory domains. Unique LYN phosphorylation sites (tyrosine-193 and tyrosine-459) and associated proteins (c-Cbl and p80) were identified in cells from imatinib-resistant patients. Reducing LYN expression (siRNA) or activation (dasatinib) was associated with loss of cell survival and cytogenetic or complete hematologic responses in imatinib-resistant disease.

CONCLUSIONS

LYN activation was independent of BCR-ABL in cells from imatinib-resistant patients. Thus, LYN kinase may be involved in imatinib resistance in CML patients with mutation-negative BCR-ABL and its direct inhibition is consistent with clinical responses in these patients.

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.

Strategies for overcoming imatinib resistance in chronic myeloid leukemia

Imatinib was the first treatment for chronic myeloid leukemia (CML) that specifically targeted the causative BCR-ABL oncoprotein, and represented a major therapeutic advance in this disease; however, some patients develop resistance or intolerance. Resistance can be classified as BCR-ABL-dependent (e.g., mutation in the BCR-ABL gene) or BCR-ABL-independent (alternative pathways of disease progression, e.g., SRC-family tyrosine kinases). The investigation of therapeutic options post-imatinib failure resulted in the development and regulatory approval of dasatinib, a BCR-ABL and SRC-family kinase inhibitor. Dasatinib is active across all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, and demonstrates activity in almost all imatinib-resistant mutations. Other therapeutic options are also under investigation, with nilotinib being the most clinically advanced. Nilotinib is an analog of imatinib with similar multiple kinase targets, but without inhibition of SRC, and reduced in vitro activity against BCR-ABL P-loop mutations compared with dasatinib. Similar to dasatinib, nilotinib has no activity against T315I mutations. The availability of dasatinib and development of other tyrosine kinase inhibitors provide positive prospects for patients with imatinib-resistant or -intolerant CML. Here, we discuss several of these new strategies for treating patients after imatinib failure.

A critical role for Lyn in acute myeloid leukemia

Receptor or nonreceptor tyrosine kinases (TKs) are known to play an important role in leukemogenesis. Here we studied the level of protein tyrosine phosphorylations in a series of fresh AML samples and evaluated the effect of TK inhibitors. Compared with normal hematopoietic progenitors, a high level of tyrosine phosphorylation was detected in most acute myeloid leukemia (AML) samples. The Src family kinases (SFKs) appeared constitutively activated in most cases, including in the CD34+CD38−CD123+ compartment as revealed by the level of phosphorylated tyrosine 416. Lyn was the major SFK family member expressed in an active form in AML cells where it was abnormally distributed throughout the plasma membrane and the cytosol as opposed to normal hematopoietic progenitors. The SFK inhibitor, PP2, strongly reduced the global level of tyrosine phosphorylations, inhibited cell proliferation, and induced apoptosis in patient samples without affecting normal granulomonocytic colony forming units. Moreover, silencing Lyn expression by small interfering RNA in primary AML cells strongly inhibited proliferation. Interestingly, a link between Lyn and the mTOR pathway was observed as PP2 and a Lyn knockdown both affected the phosphorylation of mTOR targets without inhibiting Akt phosphorylation. Lyn should be considered as a novel pharmacologic target for AML therapy.

Lyn regulates BCR-ABL and Gab2 tyrosine phosphorylation and c-Cbl protein stability in imatinib-resistant chronic myelogenous leukemia cells

Lyn kinase functions as a regulator of imatinib sensitivity in chronic myelogenous leukemia (CML) cells through an unknown mechanism. In patients who fail imatinib therapy but have no detectable BCR-ABL kinase mutation, we detected persistently activated Lyn kinase. In imatinib-resistant CML cells and patients, Lyn activation is BCR-ABL independent, it is complexed with the Gab2 and c-Cbl adapter/scaffold proteins, and it mediates persistent Gab2 and BCR-ABL tyrosine phosphorylation in the presence or absence of imatinib. Lyn silencing or inhibition is necessary to suppress Gab2 and BCR-ABL phosphorylation and to recover imatinib activity. Lyn also negatively regulates c-Cbl stability, whereas c-Cbl tyrosine phosphorylation is mediated by BCR-ABL. These results suggest that Lyn exists as a component of the BCR-ABL signaling complex and, in cells with high Lyn expression or activation, BCR-ABL kinase inhibition alone (imatinib) is not sufficient to fully disengage BCR-ABL-mediated signaling and suggests that BCR-ABL and Lyn kinase inhibition are needed to prevent or treat this form of imatinib resistance.

Src family tyrosine kinases phosphorylate Flt3 on juxtamembrane tyrosines and interfere with receptor maturation in a kinase-dependent manner

The receptor tyrosine kinase (RTK) Flt3 is expressed in early hematopoietic progenitor cells and stimulates their growth. Due to frequent mutations in the Flt3 gene in patients with acute myeloid leukemia (AML), Flt3 is regarded as a potential therapeutic target, but the underlying mechanisms are still poorly understood. Therefore, we investigated interactions of Flt3 and some Src family tyrosine kinases (SFKs), which are expressed predominantly or exclusively in hematopoietic cells and known to be involved in signal transduction by various RTKs. Employing sets of wt and mutant Flt3 and Hck, we analyzed protein binding as well as Flt3 phosphorylation and maturation in HEK-293 cells cotransfected with expression constructs encoding both binding partners. Kinase-inactive Hck-K269R was recruited to phosphotyrosine residues located in the juxtamembrane (JM) region of activated Flt3 via its SH2 domain. Several of the JM domain tyrosines were phophorylated by Hck and other SFKs. As apparent from the distribution of mature and hypoglycosylated Flt3, SFKs interfered with Flt3 maturation in a kinase-dependent manner. Together, these findings show a complex role of SFKs in Flt3 signaling and reveal a new function of SFKs in the maturation of RTKs.

Expression of sialidase Neu2 in leukemic K562 cells induces apoptosis by impairing Bcr-Abl/Src kinases signaling

Chronic myeloid leukemia is a hematopoietic stem cell cancer, originated by the perpetually "switched on" activity of the tyrosine kinase Bcr-Abl, leading to uncontrolled proliferation and insensitivity to apoptotic stimuli. The genetic phenotype of myeloid leukemic K562 cells includes the suppression of cytosolic sialidase Neu2. Neu2 transfection in K562 cells induced a marked decrease (-30% and -80%) of the mRNA of the anti-apoptotic factors Bcl-XL and Bcl-2, respectively, and an almost total disappearance of Bcl-2 protein. In addition, gene expression and activity of Bcr-Abl underwent a 35% diminution, together with a marked decrease of Bcr-Abl-dependent Src and Lyn kinase activity. Thus, the antiapoptotic axis Bcr-Abl, Src, and Lyn, which stimulates the formation of Bcl-XL and Bcl-2, was remarkably weakened. The ultimate consequences of these modifications were an increased susceptibility to apoptosis of K562 cells and a marked reduction of their proliferation rate. The molecular link between Neu2 activity and Bcr-Abl signaling pathway may rely on the desialylation of some cytosolic glycoproteins. In fact, three cytosolic glycoproteins, in the range 45-66 kDa, showed a 50-70% decrease of their sialic acid content upon Neu2 expression, supporting their possible role as modulators of the Bcr-Abl complex.

Aberrant DNA methylation of the Src kinase Hck, but not of Lyn, in Philadelphia chromosome negative acute lymphocytic leukemia

Hck and Lyn are required in Philadelphia chromosome (Ph) positive acute lymphocytic leukemia (ALL). Here, we present evidence that the promoter CpG island of Hck, but not of Lyn, is aberrantly methylated in leukemia. Hck promoter DNA methylation was detected in 13 out of 23 (56.5%) hematopoietic and eight out of 10 (80%) non-hematopoietic cell lines, but not in normal controls. Treatment with 5-aza-2'-deoxycytidine induced demethylation and restoration of Hck mRNA and protein expression. Hck methylation (> or =15%) was detected in nine out of 44 (20%) patients with Ph negative ALL, and in one out 16 (6%) patients with Ph positive ALL, but not in patients with AML or chronic myelogenous leukemia. In this subset of patients, low levels of Hck methylation (10-15%) were observed in 26-30% of patients. Lyn methylation was observed in three out of 28 (10.7%) cell lines, but only in one out of 71 (1.4%) patients. Patients with Ph negative ALL and Hck methylation had a poorer prognosis. These data indicate that Hck may have tumor suppressor properties in BCR-ABL negative leukemia.

Dasatinib induces significant hematologic and cytogenetic responses in patients with imatinib-resistant or -intolerant chronic myeloid leukemia in accelerated phase

Treatment options are limited for patients with imatinib-resistant or -intolerant accelerated phase chronic myeloid leukemia (CML-AP). Dasatinib is a novel, potent, oral, multitargeted kinase inhibitor of BCR-ABL and SRC-family kinases that showed marked efficacy in a phase 1 trial of patients with imatinib-resistant CML. Results are presented for 107 patients with CML-AP with imatinib-resistance or -intolerance from a phase 2, open-label study further evaluating dasatinib efficacy and safety. At 8 months' minimum follow-up, 81%, 64%, and 39% of patients achieved overall, major (MaHR), and complete hematologic responses, respectively, whereas 33% and 24% attained major and complete cytogenetic remission. Of 69 patients who achieved MaHR, 7 progressed. Seventy-six percent of patients are estimated to be alive and progression-free at 10 months. Response rates for the 60% of patients with baseline BCR-ABL mutations did not differ from the total population. Dasatinib was well tolerated: most nonhematologic adverse events (AEs) were mild to moderate; no imatinib-intolerant patients discontinued dasatinib because of AEs. Although common (76% of patients with severe neutropenia), cytopenias were manageable through dose modification. In summary, dasatinib induced significant hematologic and cytogenetic responses in patients with imatinib resistance or intolerance, was well tolerated, and may represent a potent new therapeutic option for CML-AP. Further follow-up is warranted. This trial was registered at www.clinicaltrials.gov as #CA180005.

Dasatinib induces complete hematologic and cytogenetic responses in patients with imatinib-resistant or -intolerant chronic myeloid leukemia in blast crisis

The prognosis for patients with chronic myeloid leukemia (CML) in myeloid blast crisis (MBC) or lymphoid blast crisis (LBC) remains poor. Although imatinib can induce responses in a subset of these patients, resistance to the drug develops rapidly. Dasatinib is a novel, oral, multitargeted kinase inhibitor of BCR-ABL and SRC family kinases. After promising phase 1 results, we report the results of phase 2 clinical trials of dasatinib in patients with imatinib-resistant or -intolerant blast crisis CML (MBC, n = 74; LBC, n = 42). At the 8-month follow-up, dasatinib induced major hematologic responses (MaHRs) in 34% and 31% of MBC- and LBC-CML patients and major cytogenetic responses (MCyRs) in 31% and 50% of these patients, respectively. Most (86%) of these MCyRs were complete cytogenetic responses (CCyRs). Responses were rapid and durable: 88% and 46%, respectively, of MBC- and LBC-CML patients achieving MaHR had not experienced disease progression at the 8-month follow-up. Response rates were similar in patients with and without BCR-ABL mutations known to confer resistance to imatinib. Dasatinib was well tolerated. Nonhematologic adverse events were mild to moderate. Cytopenias were common and could be managed by dose modification. Dasatinib is highly active and produces hematologic and cytogenetic responses in a significant number of patients with imatinib-resistant or -intolerant MBC- and LBC-CML. These trials were registered at www.clinicaltrials.gov as #CA180006 and #CA180015.

The duplicitous nature of the Lyn tyrosine kinase in growth factor signaling

The Lyn tyrosine kinase is a unique member of the Src family of non-receptor protein tyrosine kinases whose principal role is to regulate signals through inhibitory receptors thereby promoting signal attenuation. Lyn is renowned for its role in B cell antigen receptor and FcepsilonRI signaling; however, it is becoming increasingly apparent that Lyn also functions in signal transduction from growth factor receptors including the receptors for GM-CSF, IL-3, IL-5, SCF, erythropoietin, CSF-1, G-CSF, thrombopoietin and Flt3 ligand. Numerous studies have implicated Lyn in growth factor receptor signal amplification, while a number also suggest that Lyn participates in negative regulation of growth factor signaling. Indeed Lyn-deficient mice are hyper-responsive to myeloid growth factors and develop a myeloproliferative disorder that predisposes the mice to macrophage tumours, with loss of negative regulation through SHP-1 and SHIP-1 thought to be the major contributing factor to this phenotype. Developing a clear understanding of Lyn's role in establishing signaling thresholds in growth factor receptor signal amplification and signal inhibition may have important implications in the management of leukemias that may depend on Lyn activity.

The role of Src in solid and hematologic malignancies

c-Src was the first protooncogene described and was among the first molecules in which tyrosine kinase activity was documented. c-Src has been defined as a common modular structure that participates in much of the crosstalk between the cytoplasmic protein tyrosine kinases and tyrosine kinase receptors. Understanding the structure and function of this important class of protein kinases and elucidating the molecular signaling events mediated by c-Src are important not only for identifying the critical pathways but also for designing new strategies to block or inhibit the action of these kinases. Despite the large amount of information available on c-Src, its precise functions in cancer remain to be elucidated. Recently, there has been renewed interest in c-Src as a molecular target for cancer therapy, and multiple c-Src inhibitors are entering clinical trials. In this review, the authors describe the function and expression of c-Src in human malignancies and the novel c-Src inhibitors and their potential applications for cancer treatment.

Targeted therapy: for kids, too

Innovative hypothesis-driven clinical trials have achieved major successes over the past several decades in treating children and adolescents with cancer. DNA-damaging cytotoxic agents have cured children with cancer. While the mission is not yet accomplished, chemotherapy has been validated. None of these drugs were designed specifically for a pediatric disease. Continued progress will require new strategies. Now being tested for adult cancers, these strategies include gene therapy, immunotherapy, cancer prevention, and signal transduction inhibitor (STI) therapy. Of these, the most promising is STI therapy, also known as molecular therapeutics or targeted therapy. For this therapy to succeed, components of signal transduction (i.e., candidate drug targets) must be identified, the targets relevant to cancers, and the drugs available for trial. Because STI therapy is biologically driven and because therapy will be tailored depending on the molecular profile of a specific patient's tumor, clinical pediatric oncologists will need to acquire greater understanding of signaling pathways and their therapeutic relevance. With examples drawn from pediatric oncology, the critical steps in the pre-clinical development of targeted therapy are reviewed here.

Synthesis and Src Kinase Inhibitory Activity of 2-Phenyl- and 2-Thienyl-7-phenylaminothieno[3,2-b]pyridine-6-carbonitriles

2-phenyl-7-phenylaminothieno[3,2-b]pyridine-6-carbonitriles were recently reported to be inhibitors of Src kinase activity. In this study we present structure-activity relationships for additional thieno[3,2-b]pyridine-6-carbonitriles, modifying the substituents on the C-2 phenyl and C-7 phenylamino groups. Derivatives with various aminomethyl and aminoethyl substituents on the para position of the C-2 phenyl group retained the activity of the initial analogues. However, direct attachment of an amino group led to decreased activity. A 2,4-dichloro-5-methoxyphenylamino group at C-7 provided superior inhibition of Src enzymatic activity. Replacement of the C-2 phenyl group with a 3,5-substituted thiophene led to improved Src inhibitory activity compared to the parent compound, but other thiophene isomers were less active. One of the analogues reported here exhibited in vivo activity comparable to that of SKI-606, a related 3-quinolinecarbonitrile currently in clinical trials.

Src family tyrosine kinases are activated by Flt3 and are involved in the proliferative effects of leukemia-associated Flt3 mutations

OBJECTIVE

The hematopoietic growth factor receptor, Fms-like tyrosine kinase-3 (Flt3), modulates survival and proliferation of myeloid and B-cell precursors. Activating mutations of Flt3 are the most common molecular abnormalities in acute myeloid leukemia (AML) and have an apparent role in leukemogenesis. However, signaling pathways mediating Flt3 effects are incompletely understood. The role of Src kinases is unknown, although some, such as Lyn, have also been linked to leukemogenesis. This study examines the role of Src kinases in Flt3 signaling and the oncogenic effects of leukemia-associated Flt3 mutations.

MATERIALS AND METHODS

We examined the activation and functional roles of Src kinases in human leukemic myeloid cell lines expressing wild-type Flt3 or a constitutively active mutant, and in cells stably transduced with human wild-type or mutant Flt3.

RESULTS

Flt3 ligand stimulation of wild-type Flt3 increased phosphorylation of Src kinase Lyn. Constitutive Lyn phosphorylation and activation was found in cells expressing constitutively active Flt3 mutants. Src kinases are implicated in downregulation of closely related receptors, but Src inhibitors had no effect on ligand-stimulated Flt3 degradation, or on the rapid degradation of an Flt3 mutant. However, growth-factor-independent proliferation resulting from mutant Flt3 expression did depend on the activity of Src kinases.

CONCLUSION

Our studies reveal for the first time the involvement of Src kinases in Flt3 signaling, with activation of Lyn by constitutively active Flt3 mutants as well as ligand-stimulated wild-type receptor, and show that Src kinase inhibitors block proliferative effects of Flt3 mutants found in AML. Thus, Src kinases may represent targets for inhibitor therapy in Flt3-related AML.