Src-Related Protein Tyrosine Kinases in Hematopoiesis

Molecular Cloning, Tissue Distribution and Antiviral Immune Response of Duck Src

As a founding member of the Src family of kinases, Src has been confirmed to participate in the regulation of immune responses, integrin signaling, and motility. Ducks are usually asymptomatic carriers of RNA viruses such as Newcastle disease virus and avian influenza virus, which can be deadly to chickens. The beneficial role of Src in modulating the immune response remains largely unknown in ducks. Here, we characterized the duck Src and found that it contains a 192-base-pair 5' untranslated region, a 1602-base-pair coding region, and a 2541-base-pair 3' untranslated region, encoding 533 amino acid residues. Additionally, duSrc transcripts were significantly activated in duck tissues infected by Newcastle disease virus compared to controls. The duSrc transcripts were notably widespread in all tissues examined, and the expression level was higher in liver, blood, lung, pancreas, and thymus. Moreover, we found the expression levels of IFN-β, NF-κB, IRF3, and Src were significantly increased in DEFs after infection with 5'ppp dsRNA, but there was no significant difference before and after treatment in DF1 cells. Furthermore, overexpression of duSrc followed by stimulation with 5'ppp dsRNA led to an elevation of IFN-β levels. The SH3 and PTKc domains of duSrc contributed to promoting the activity of IFN-β and NF-κB in DEFs stimulated by 5'ppp dsRNA.

Inhibition of HSP90 overcomes melphalan resistance through downregulation of Src in multiple myeloma cells

Multiple myeloma (MM) is the second most common hematologic malignancy. In spite of the development of new therapeutic agents, MM remains incurable due to multidrug resistance (MDR) and the 5-year survival rate is approximately 50%. Thus, further study is needed to investigate the mechanism of MDR and improve MM prognosis. Heat shock protein 90 (HSP90) is a molecular chaperone that is responsible for the stability of a number of client proteins, most of which are involved in tumor progression. Therefore, HSP90 inhibitors represent potential new therapeutic agents for cancer. Furthermore, inhibition of HSP90 leads to degradation of client proteins, overcoming acquired anti-cancer drug resistance. In this study, we assessed the role of HSP90 in MDR using established melphalan-resistant MM cells. We found that expression of HSP90 was higher in melphalan-resistant MM cells than in parent cells and that HSP90 inhibitors KW-2478 and NUV-AUY922 restored drug sensitivity to the level observed in parent cells. Activation of the unfolded protein response is a hallmark of MM, and expression of endoplasmic reticulum stress signaling molecules is reduced in melphalan-resistant cells; however, KW-2478 did not affect endoplasmic reticulum stress signaling. We demonstrated that treatment with KW-2478 decreased expression of Src, a client of HSP90, and suppressed the activity of ERK, Akt, and NF-κB. Our findings indicate that inhibition of HSP90 results in suppression of Src and its downstream effectors, including ERK, Akt, and NF-κB, and therefore that HSP90 inhibitors could be useful for treatment of MDR MM.

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.

Signalling through Src family kinase isoforms is not redundant in models of thrombo-inflammatory vascular disease

The Src family kinases (SFK) are a group of signalling molecules with important regulatory functions in inflammation and haemostasis. Leucocytes and platelets express multiple isoforms of the SFKs. Previous studies used broad‐spectrum pharmacological inhibitors, or murine models deficient in multiple SFK isoforms, to demonstrate the functional consequences of deficiencies in SFK signalling. Here, we hypothesized that individual SFK operate in a non‐redundant fashion in the thrombo‐inflammatory recruitment of monocyte during atherosclerosis. Using in vitro adhesion assays and single SFK knockout mice crossed with the ApoE^−/− model of atherosclerosis, we find that SFK signalling regulates platelet‐dependent recruitment of monocytes. However, loss of a single SFK, Fgr or Lyn, reduced platelet‐mediated monocyte recruitment in vitro. This translated into a significant reduction in the burden of atherosclerotic disease in Fgr^−/−/ApoE^−/− or Lyn^−/−/ApoE^−/− animals. SFK signalling is not redundant in thrombo‐inflammatory vascular disease and individual SFK may represent targets for therapeutic intervention.

Src activation decouples cell division orientation from cell geometry in mammalian cells

Orientation of cell division plane plays a crucial role in morphogenesis and regeneration. Misoriented cell division underlies many important diseases, such as cancer. Studies with Drosophila and C. elegance models show that Src, a proto-oncogene tyrosine-protein kinase, is a critical regulator of this aspect of mitosis. However, the role for Src in controlling cell division orientation in mammalian cells is not well understood. Using genetic and pharmacological approaches and two extracellular signals to orient cell division, we demonstrated a critical role for Src. Either knockout or pharmacological inhibition of Src would retain the fidelity of cell division orientation with the long-axis orientation of mother cells. Conversely, re-expression of Src would decouple cell division orientation from the pre-division orientation of the long axis of mother cells. Cell division orientation in human breast and gastric cancer tissues showed that the Src activation level correlated with the degree of mitotic spindle misorientation relative to the apical surface. Examination of proteins associated with cortical actin revealed that Src activation regulated the accumulation and local density of adhesion proteins on the sites of cell-matrix attachment. By analyzing division patterns in the cells with or without Src activation and through use of a mathematical model, we further support our findings and provide evidence for a previously unknown role for Src in regulating cell division orientation in relation to the pre-division geometry of mother cells, which may contribute to the misoriented cell division.

Caspase-8: regulating life and death

Roles for cell death in development, homeostasis, and the control of infections and cancer have long been recognized. Although excessive cell damage results in passive necrosis, cells can be triggered to engage molecular programs that result in cell death. Such triggers include cellular stress, oncogenic signals that engage tumor suppressor mechanisms, pathogen insults, and immune mechanisms. The best-known forms of programmed cell death are apoptosis and a recently recognized regulated necrosis termed necroptosis. Of the two best understood pathways of apoptosis, the extrinsic and intrinsic (mitochondrial) pathways, the former is induced by the ligation of death receptors, a subset of the TNF receptor (TNFR) superfamily. Ligation of these death receptors can also induce necroptosis. The extrinsic apoptosis and necroptosis pathways regulate each other and their balance determines whether cells live. Integral in the regulation and initiation of death receptor-mediated activation of programmed cell death is the aspartate-specific cysteine protease (caspase)-8. This review describes the role of caspase-8 in the initiation of extrinsic apoptosis execution and the mechanism by which caspase-8 inhibits necroptosis. The importance of caspase-8 in the development and homeostasis and the way that dysfunctional caspase-8 may contribute to the development of malignancies in mice and humans are also explored.

Apolipoprotein E mimetic peptide, CN-105, improves outcomes in ischemic stroke

Objective

At present, the absence of a pharmacological neuroprotectant represents an important unmet clinical need in the treatment of ischemic and traumatic brain injury. Recent evidence suggests that administration of apolipoprotein E mimetic therapies represent a viable therapeutic strategy in this setting. We investigate the neuroprotective and anti‐inflammatory properties of the apolipoprotein E mimetic pentapeptide, CN‐105, in a microglial cell line and murine model of ischemic stroke.

Methods

Ten to 13‐week‐old male C57/BL6 mice underwent transient middle cerebral artery occlusion and were randomly selected to receive CN‐105 (0.1 mg/kg) in 100 μL volume or vehicle via tail vein injection at various time points. Survival, motor‐sensory functional outcomes using rotarod test and 4‐limb wire hanging test, infarct volume assessment using 2,3,5‐Triphenyltetrazolium chloride staining method, and microglial activation in the contralateral hippocampus using F4/80 immunostaining were assessed at various time points. In vitro assessment of tumor necrosis factor‐alpha secretion in a microglial cell line was performed, and phosphoproteomic analysis conducted to explore early mechanistic pathways of CN‐105 in ischemic stroke.

Results

Mice receiving CN‐105 demonstrated improved survival, improved functional outcomes, reduced infarct volume, and reduced microglial activation. CN‐105 also suppressed inflammatory cytokines secretion in microglial cells in vitro. Phosphoproteomic signals suggest that CN‐105 reduces proinflammatory pathways and lower oxidative stress.

Interpretation

CN‐105 improves functional and histological outcomes in a murine model of ischemic stroke via modulation of neuroinflammatory pathways. Recent clinical trial of this compound has demonstrated favorable pharmacokinetic and safety profile, suggesting that CN‐105 represents an attractive candidate for clinical translation.

Shockingly: the loss of Lyn leads to leakiness

In this issue of Blood, Han et al demonstrate that endotoxin-induced mortality in a murine model of acute lung injury (ALI) was associated with increased vascular permeability attributable to loss of the Src family kinase (SFK) Lyn.

Bafetinib inhibits functional responses of human eosinophils in vitro

Eosinophils play a prominent role in the process of allergic inflammation. Non-receptor associated Lyn tyrosine kinases generate key initial signals in eosinophils. Bafetinib, a specific Abl/Lyn tyrosine kinase inhibitor has shown a potent antiproliferative activity in leukemic cells, but its effects on eosinophils have not been reported. Therefore, we studied the effects of bafetinib on functional and mechanistic responses of isolated human eosinophils. Bafetinib was more potent than non-specific tyrosin kinase comparators genistein and tyrphostin inhibiting superoxide anion triggered by N-formyl-Met-Leu-Phe (fMLF; 100 nM) (-log IC50=7.25 ± 0.04 M; 6.1 ± 0.04 M; and 6.55 ± 0.03 M, respectively). Bafetinib, genistein and tyrphostin did not modify the [Ca(2+)]i responses to fMLF. Bafetinib inhibited the release of EPO induced by fMLF with higher potency than genistein and tyrphostin (-log IC50=7.24 ± 0.09 M; 5.36 ± 0.28 M; and 5.37 ± 0.19 M, respectively), and nearly suppressed LTC4, ECP and chemotaxis. Bafetinib, genistein and tyrphostin did not change constitutive apoptosis. However bafetinib inhibited the ability of granulocyte-monocyte colony-stimulating factor to prevent apoptosis. The activation of Lyn tyrosine kinase, p-ERK1/2 and p-38 induced by fMLF was suppressed by bafetinib and attenuated by genistein and tyrphostin. In conclusion, bafetinib inhibits oxidative burst and generation of inflammatory mediators, and reverses the eosinophil survival. Therefore, future anti-allergic therapies based on bafetinib, could help to suppress excessive inflammatory response of eosinophils at inflammatory sites.

Conditional deletion of Jak2 reveals an essential role in hematopoiesis throughout mouse ontogeny: implications for Jak2 inhibition in humans

Germline deletion of Jak2 in mice results in embryonic lethality at E12.5 due to impaired hematopoiesis. However, the role that Jak2 might play in late gestation and postnatal life is unknown. To understand this, we utilized a conditional knockout approach that allowed for the deletion of Jak2 at various stages of prenatal and postnatal life. Specifically, Jak2 was deleted beginning at either mid/late gestation (E12.5), at postnatal day 4 (PN4), or at ∼2 months of age. Deletion of Jak2 beginning at E12.5 resulted in embryonic death characterized by a lack of hematopoiesis. Deletion beginning at PN4 was also lethal due to a lack of erythropoiesis. Deletion of Jak2 in young adults was characterized by blood cytopenias, abnormal erythrocyte morphology, decreased marrow hematopoietic potential, and splenic atrophy. However, death was observed in only 20% of the mutants. Further analysis of these mice suggested that the increased survivability was due to an incomplete deletion of Jak2 and subsequent re-population of Jak2 expressing cells, as conditional deletion in mice having one floxed Jak2 allele and one null allele resulted in a more severe phenotype and subsequent death of all animals. We found that the deletion of Jak2 in the young adults had a differential effect on hematopoietic lineages; specifically, conditional Jak2 deletion in young adults severely impaired erythropoiesis and thrombopoiesis, modestly affected granulopoiesis and monocytopoiesis, and had no effect on lymphopoiesis. Interestingly, while the hematopoietic organs of these mutant animals were severely affected by the deletion of Jak2, we found that the hearts, kidneys, lungs, and brains of these same mice were histologically normal. From this, we conclude that Jak2 plays an essential and non-redundant role in hematopoiesis during both prenatal and postnatal life and this has direct implications regarding the inhibition of Jak2 in humans.

Interleukin 2-inducible T cell kinase (ITK) facilitates efficient egress of HIV-1 by coordinating Gag distribution and actin organization

Interleukin 2-inducible T cell kinase (ITK) influences T cell signaling by coordinating actin polymerization and polarization as well as recruitment of kinases and adapter proteins. ITK regulates multiple steps of HIV-1 replication, including virion assembly and release. Fluorescent microscopy was used to examine the functional interactions between ITK and HIV-1 Gag during viral particle release. ITK and Gag colocalized at the plasma membrane and were concentrated at sites of F-actin accumulation and membrane lipid rafts in HIV-1 infected T cells. There was polarized staining of ITK, Gag, and actin towards sites of T cell conjugates. Small molecule inhibitors of ITK disrupted F-actin capping, perturbed Gag-ITK colocalization, inhibited virus like particle release, and reduced HIV replication in primary human CD4+ T cells. These data provide insight as to how ITK influences HIV-1 replication and suggest that targeting host factors that regulate HIV-1 egress provides an innovative strategy for controlling HIV infection.

The role of Src kinase in macrophage-mediated inflammatory responses

Src kinase (Src) is a tyrosine protein kinase that regulates cellular metabolism, survival, and proliferation. Many studies have shown that Src plays multiple roles in macrophage-mediated innate immunity, such as phagocytosis, the production of inflammatory cytokines/mediators, and the induction of cellular migration, which strongly implies that Src plays a pivotal role in the functional activation of macrophages. Macrophages are involved in a variety of immune responses and in inflammatory diseases including rheumatoid arthritis, atherosclerosis, diabetes, obesity, cancer, and osteoporosis. Previous studies have suggested roles for Src in macrophage-mediated inflammatory responses; however, recently, new functions for Src have been reported, implying that Src functions in macrophage-mediated inflammatory responses that have not been described. In this paper, we discuss recent studies regarding a number of these newly defined functions of Src in macrophage-mediated inflammatory responses. Moreover, we discuss the feasibility of Src as a target for the development of new pharmaceutical drugs to treat macrophage-mediated inflammatory diseases. We provide insights into recent reports regarding new functions for Src that are related to macrophage-related inflammatory responses and the development of novel Src inhibitors with strong immunosuppressive and anti-inflammatory properties, which could be applied to various macrophage-mediated inflammatory diseases.

Interactions among HCLS1, HAX1 and LEF-1 proteins are essential for G-CSF-triggered granulopoiesis

We found that hematopoietic cell-specific Lyn substrate 1 (HCLS1 or HS1) is highly expressed in human myeloid cells and that stimulation with granulocyte colony-stimulating factor (G-CSF) leads to HCLS1 phosphorylation. HCLS1 binds the transcription factor lymphoid-enhancer binding factor 1 (LEF-1), transporting LEF-1 into the nucleus upon G-CSF stimulation and inducing LEF-1 autoregulation. In patients with severe congenital neutropenia, inherited mutations in the gene encoding HCLS1-associated protein X-1 (HAX1) lead to profound defects in G-CSF-triggered phosphorylation of HCLS1 and subsequently to reduced autoregulation and expression of LEF-1. Consistent with these results, HCLS1-deficient mice are neutropenic. In bone marrow biopsies of the majority of tested patients with acute myeloid leukemia, HCLS1 protein expression is substantially elevated, associated with high levels of G-CSF synthesis and, in some individuals, a four-residue insertion in a proline-rich region of HCLS1 protein known to accelerate intracellular signaling. These data demonstrate the importance of HCLS1 in myelopoiesis in vitro and in vivo.

Leukemia and benzene

Excessive exposure to benzene has been known for more than a century to damage the bone marrow resulting in decreases in the numbers of circulating blood cells, and ultimately, aplastic anemia. Of more recent vintage has been the appreciation that an alternative outcome of benzene exposure has been the development of one or more types of leukemia. While many investigators agree that the array of toxic metabolites, generated in the liver or in the bone marrow, can lead to traumatic bone marrow injury, the more subtle mechanisms leading to leukemia have yet to be critically dissected. This problem appears to have more general interest because of the recognition that so-called "second cancer" that results from prior treatment with alkylating agents to yield tumor remissions, often results in a type of leukemia reminiscent of benzene-induced leukemia. Furthermore, there is a growing literature attempting to characterize the fine structure of the marrow and the identification of so called "niches" that house a variety of stem cells and other types of cells. Some of these "niches" may harbor cells capable of initiating leukemias. The control of stem cell differentiation and proliferation via both inter- and intra-cellular signaling will ultimately determine the fate of these transformed stem cells. The ability of these cells to avoid checkpoints that would prevent them from contributing to the leukemogenic response is an additional area for study. Much of the study of benzene-induced bone marrow damage has concentrated on determining which of the benzene metabolites lead to leukemogenesis. The emphasis now should be directed to understanding how benzene metabolites alter bone marrow cell biology.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

AHI-1: a novel signaling protein and potential therapeutic target in human leukemia and brain disorders

Progress in the understanding of the molecular and cellular mechanisms of human cancer, including human leukemia and lymphomas, has been spurred by cloning of fusion genes created by chromosomal translocations or by retroviral insertional mutagenesis; a number of oncogenes and tumor suppressors involved in development of a number of malignancies have been identified in this manner. The BCR-ABL fusion gene, originating in a multipotent hematopoietic stem cell, is the molecular signature of chronic myeloid leukemia (CML). Discovery of this fusion gene has led to the development of one of the first successful targeted molecular therapies for cancer (Imatinib). It illustrates the advances that can result from an understanding of the molecular basis of disease. However, there still remain many as yet unidentified mutations that may influence the initiation or progression of human diseases. Thus, identification and characterization of the mechanism of action of genes that contribute to human diseases is an important and opportune area of current research. One promising candidate as a potential therapeutic target is Abelson helper integration site-1(Ahi-1/AHI-1) that was identified by retroviral insertional mutagenesis in murine models of leukemia/lymphomas and is highly elevated in certain human lymphoma and leukemia stem/progenitor cells. It encodes a unique protein with a SH3 domain, multiple SH3 binding sites and a WD40-repeat domain, suggesting that the normal protein has novel signaling activities. A new AHI-1-BCR-ABL-JAK2 interaction complex has recently been identified and this complex regulates transforming activities and drug resistance in CML stem/progenitor cells. Importantly, AHI-1 has recently been identified as a susceptibility gene involved in a number of brain disorders, including Joubert syndrome. Therefore, understanding molecular functions of the AHI-1 gene could lead to important and novel insights into disease processes involved in specific types of diseases. Ultimately, this knowledge will set the stage for translation into new and more effective diagnostic and treatment strategies.

HIV-1 Nef perturbs the function, structure, and signaling of the Golgi through the Src kinase Hck

The interaction between HIV-1 Nef and the Src kinase Hck in macrophages has been shown to accelerate the progression to AIDS. We previously showed that Nef disturbed the N-glycosylation/trafficking of Fms, a cytokine receptor essential for maintaining macrophages in an anti-inflammatory state, in an Hck-dependent manner. Here, we show the underlying molecular mechanism of this effect. Using various Hck isoforms and their mutants and Golgi-targeting Hck mutants, we confirmed that Hck activation at the Golgi causes the Nef-induced Fms N-glycosylation defect. Importantly, we found that both the co-expression of Nef and Hck and the expression of a Golgi-targeted active Hck mutant caused alterations in the distribution of GM130, a Golgi protein that was shown to be required for efficient protein glycosylation. Moreover, the activation of Hck at the Golgi caused strong serine phosphorylation of the GM130-interacting Golgi structural protein GRASP65, which is known to induce Golgi cisternal unstacking. Using pharmacological inhibitors, we also found that the activation of Hck at the Golgi followed by the activation of the MAP kinase ERK-GRASP65 cascade is involved in the Fms N-glycosylation defect. These results suggest that Nef perturbs the structure and signaling of the Golgi by activating Hck at the Golgi, and thereby, induces the N-glycosylation/trafficking defect of Fms, which is in line with the idea that Src family kinases are crucial Golgi regulators.

Integrating novel signaling pathways involved in erythropoiesis

Many extrinsic and intrinsic factors control the development of red blood cells from committed progenitors, with the Erythropoietin-receptor (Epo-R) signaling network being the primary controlling molecular hub. Although much is understood about erythroid signaling pathways, new and intriguing factors that influence different aspects of erythroid cell development are still being uncovered. New extrinsic effectors include hypoxia and polymeric IgA1 (pIgA1), and new Epo-R signaling pathway components include Lyn/Cbp and Lyn/Liar. Hypoxia directly activates committed erythroid progenitors to expand, whereas pIgA1 activates the Akt and MAP-Kinase (MAPK) pathways through transferrin receptors on more mature erythroid cells. The Lyn/Cbp pathway controls the activity and protein levels of Lyn through recruitment of Csk and SOCS1, as well as feeding into the control of other pathways mediated by recruitment of ras-GAP, PI3-kinase, PLCγ, Fes, and EBP50. Nuclear/cytoplasmic shuttling of Lyn and other signaling molecules is influenced by Liar and results in regulation of their intersecting signaling pathways. The challenge of future research is to flesh out the details of these new signaling regulators/networks and integrate their influences during the different stages of erythropoiesis.

G-CSF receptor activation of the Src kinase Lyn is mediated by Gab2 recruitment of the Shp2 phosphatase

Src activation involves the coordinated regulation of positive and negative tyrosine phosphorylation sites. The mechanism whereby receptor tyrosine kinases, cytokine receptors, and integrins activate Src is not known. Here, we demonstrate that granulocyte colony-stimulating factor (G-CSF) activates Lyn, the predominant Src kinase in myeloid cells, through Gab2-mediated recruitment of Shp2. After G-CSF stimulation, Lyn dynamically associates with Gab2 in a spatiotemporal manner. The dephosphorylation of phospho-Lyn Tyr507 was abrogated in Shp2-deficient cells transfected with the G-CSF receptor but intact in cells expressing phosphatase-defective Shp2. Auto-phosphorylation of Lyn Tyr396 was impaired in cells treated with Gab2 siRNA. The constitutively activated Shp2E76A directed the dephosphorylation of phospho-Lyn Tyr507 in vitro. Tyr507 did not undergo dephosphorylation in G-CSF-stimulated cells expressing a mutant Gab2 unable to bind Shp2. We propose that Gab2 forms a complex with Lyn and after G-CSF stimulation, Gab2 recruits Shp2, which dephosphorylates phospho-Lyn Tyr507, leading to Lyn activation.

Dasatinib in solid tumors

IMPORTANCE OF THE FIELD

Dasatinib is an oral, potent adenosine triphosphate-competitive inhibitor of multiple tyrosine kinases including BCR-ABL, c-KIT, platelet-derived growth factor receptor, and Src family kinases (SFKs). It has gained much attention for its use in chronic myeloid leukemia and for the treatment of adults with Philadelphia chromosome-positive acute lymphoblastic leukemia. However, dasatinib is also being explored in solid tumors in ongoing Phase I and II clinical trials.

AREAS COVERED IN THIS REVIEW

The clinical efficacy of dasatinib in a wide variety of solid tumors and important Phase I/II studies utilizing dasatinib and the optimal dosage used in solid tumors. A literature search was conducted using PubMed/MEDLINE, www.clinicaltrials.gov , and the American Society of Clinical Oncology websites to find relevant Phase I/II clinical trials during 1987-2009.

WHAT THE READER WILL GAIN

The understanding that the biology and mechanism of Src activation in tumors are not well understood and finding the optimal use of SFK inhibitors in the clinical setting requires further investigation.

TAKE HOME MESSAGE

In reviewing the clinical safety data of dasatinib in its current use as a Src inhibitor in a wide variety of solid malignancies, dasatinib appears to be safe and is a promising agent for the treatment of metastatic solid tumors refractory to standard therapies.

Interleukin-6, CD45 and the Src-Kinases in Myeloma Cell Proliferation

Multiple myeloma (MM) is a proliferative disorder of monoclonal plasma cells which accumulate in human bone marrow, and myeloma cells proliferate in response to a cytokine, interleukin-6 (IL-6). We recently found that MPC-1- CD49e- immature myeloma cells expressing CD45 form a proliferating population in MM. IL-6 activates at least two intracellular pathways including signal transducer and activator of transcription 3 (STAT3) and extracellular signal-regulated kinase 1/2 (ERK1/2) following the activation of Janus kinases (JAKs) via its receptor complexes composed of the IL-6 receptor alpha chain and gp130. Although the roles of CD45 have been extensively studied for antigen receptors in B and T cells, its physiological consequences in other hematopoietic cells remain largely unknown. Myeloma cells expressing CD45 antigens which contain the activation of src family protein-tyrosine kinases (PTKs) independent of IL-6 stimulation proliferate in response to IL-6, whereas the proliferation of CD45- cells which lack a considerable activity of the src family PTKs is not promoted by IL-6. The STAT3 and ERK1/2 pathways are similarly activated by IL-6 in both cells either expressing or not expressing CD45. In this review, we argue a novel mechanism of proliferation of myeloma cells, in that the activation of both STAT3 and ERK1/2 is not sufficient for IL-6-induced proliferation which further requires IL-6-independent activation of the src family kinases associated with CD45 phosphatase. We propose that the cellular context, such as CD45 expression and src family kinase activation, is crucial for myeloma cells to proliferate in response to IL-6.

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.

A novel signaling pathway associated with Lyn, PI 3-kinase and Akt supports the proliferation of myeloma cells

Interleukin-6 (IL-6) is a growth factor for human myeloma cells. We have recently found that in myeloma cells the activation of both signal transducer and activator of transcription (STAT) 3 and extracellular signal-regulated kinase (ERK) 1/2 is not sufficient for the IL-6-induced proliferation, which further requires the activation of the src family kinases, such as Lyn. Here we showed that the Lyn-overexpressed myeloma cell lines had the higher proliferative rate with IL-6 and the enhanced activation of the phosphatidylinositol (PI) 3-kinase and Akt. The IL-6-induced phosphorylation of STAT3 and ERK1/2 was not up-regulated in the Lyn-overexpressed cells, indicating that the Lyn-PI 3-kinase-Akt pathway is independent of these pathways. The PI 3-kinase was co-precipitated with Lyn in the Lyn-overexpressed cells of which proliferation with IL-6 was abrogated by the specific inhibitors for PI 3-kinase or Akt, suggesting that the activation of the PI 3-kinase-Akt pathway associated with Lyn is indeed related to the concomitant augmentation of myeloma cell growth. Furthermore, the decreased expression of p53 and p21(Cip1) proteins was observed in the Lyn-overexpressed cells, implicating a possible downstream target of Akt. This study identifies a novel IL-6-mediated signaling pathway that certainly plays a role in the proliferation of myeloma cells and this novel mechanism of MM tumor cell growth associated with Lyn would eventually contribute to the development of MM treatment.

Src kinases as therapeutic targets for cancer

Src family kinases (SFKs) have a critical role in cell adhesion, invasion, proliferation, survival, and angiogenesis during tumor development. SFKs comprise nine family members that share similar structure and function. Overexpression or high activation of SFKs occurs frequently in tumor tissues and they are central mediators in multiple signaling pathways that are important in oncogenesis. SFKs can interact with tyrosine kinase receptors, such as EGFR and the VEGF receptor. SFKs can affect cell proliferation via the Ras/ERK/MAPK pathway and can regulate gene expression via transcription factors such as STAT molecules. SFKs can also affect cell adhesion and migration via interaction with integrins, actins, GTPase-activating proteins, scaffold proteins, such as p130(CAS) and paxillin, and kinases such as focal adhesion kinases. Furthermore, SFKs can regulate angiogenesis via gene expression of angiogenic growth factors, such as fibroblast growth factor, VEGF, and interleukin 8. On the basis of these important findings, small-molecule SFK inhibitors have been developed and are undergoing early phase clinical testing. In preclinical studies these agents can suppress tumor growth and metastases. The agents seem to be safe in humans and could add to the therapeutic arsenal against subsets of cancers.

Identification of tyrosine kinase, HCK, and tumor suppressor, BIN1, as potential mediators of AHI-1 oncogene in primary and transformed CTCL cells

AHI-1 is an oncogene often targeted by provirus insertional mutagenesis in murine leukemias and lymphomas. Aberrant expression of human AHI-1 occurs in cutaneous T-cell lymphoma (CTCL) cells and in CD4+CD7− Sezary cells from patients with Sezary syndrome. Stable knockdown of AHI-1 using retroviral-mediated RNA interference in CTCL cells inhibits their transforming activity in vitro and in vivo. To identify genes involved in AHI-1–mediated transformation, microarray analysis was performed to identify differentially expressed genes in AHI-1–suppressed CTCL cells. Fifteen up-regulated and 6 down-regulated genes were identified and confirmed by quantitative reverse transcription-polymerase chain reaction. Seven were further confirmed in a microarray analysis of CD4+CD7− Sezary cells from Sezary syndrome patients. HCK and BIN1 emerged as new candidate cooperative genes, with differential protein expression, which correlates with observed transcript changes. Interestingly, changes in HCK phosphorylation and biologic response to its inhibitor, dasatinib, were observed in AHI-1–suppressed or –overexpressed cells. The tumor suppressor BIN1 physically interacts with MYC in CTCL cells, which also exhibit differential MYC protein expression. In addition, aberrant expression of alternative splicing forms of BIN1 was observed in primary and transformed CTCL cells. These findings indicate that HCK and BIN1 may play critical roles in AHI-1–mediated leukemic transformation of human CTCL cells.

New targets and treatments in multiple myeloma: Src family kinases as central regulators of disease progression

Multiple myeloma is a malignant condition that most commonly occurs in the seventh decade of life. Recent improvements in treatment may result in a more favourable outlook for recently diagnosed patients. Multiple myeloma is an incurable clonal B-cell malignancy, which is initially responsive to conventional chemotherapy; one-third of the patients achieve complete remission but multidrug resistance eventually develops. Although autologous stem cell transplantation remains an important option, many older patients are less tolerant to the toxicity associated with conditioning treatment, as well as being intrinsically less likely to do well after transplantation. Most patients eventually relapse with or without transplantation, and salvage therapy is only moderately effective. Thalidomide and subsequently, lenalidomide and bortezomib, have demonstrated improved outcomes for these patients, as well as proving efficacious in front-line regimens. A deeper understanding of the molecular mechanisms underlying multiple myeloma has given rise to novel targeted approaches. This review will focus in particular on Src-dependent signalling pathways, reflecting the expanding realisation of the critical and ubiquitous role of Src family kinases (SFKs) in normal and abnormal hematopoiesis.

Csk-binding protein can regulate Lyn signals controlling cell morphology

The Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors, it interacts with COOH-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators COOH-terminal Src kinase (Csk) and suppressor of cytokine signaling-1 (SOCS1). Lyn phosphorylates Cbp on several tyrosine residues, including Tyr314, which recruits Csk/SOCS1, as well as Tyr381 and Tyr409 that bind Lyns own SH2 domain. We show that Cbp alters not only the ability of erythroid cells to differentiate but also their colony morphology. Consequently, we detailed the ability of Cbp to interact with and influence Lyns ability to initiate changes in cellular architecture, which affect cell-cell and cell-substratum interactions. Over-expression of active Lyn promotes filopodia formation while inactive Lyn promotes lamellipodia formation. Conversely, Cbp over-expression, which inhibits Lyn activity, promotes lamellipodia formation, while Cbp mutants preventing its interaction/signaling consequently allow Lyn to promote filopodia formation. Thus, the Lyn-Cbp pathway and subsequent regulation of Lyn signaling and cell morphology involves a dynamic and complex series of interactions.

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.

Nuclear localization of Lyn tyrosine kinase mediated by inhibition of its kinase activity

Src-family kinases, cytoplasmic enzymes that participate in various signaling events, are found at not only the plasma membrane but also subcellular compartments, such as the nucleus, the Golgi apparatus and late endosomes/lysosomes. Lyn, a member of the Src-family kinases, is known to play a role in DNA damage response and cell cycle control in the nucleus. However, it is still unclear how the localization of Lyn to the nucleus is regulated. Here, we investigated the mechanism of the distribution of Lyn between the cytoplasm and the nucleus in epitheloid HeLa cells and hematopoietic THP-1 cells. Lyn was definitely detected in purified nuclei by immunofluorescence and immunoblotting analyses. Nuclear accumulation of Lyn was enhanced upon treatment of cells with leptomycin B (LMB), an inhibitor of Crm1-mediated nuclear export. Moreover, Lyn mutants lacking the sites for lipid modification were highly accumulated in the nucleus upon LMB treatment. Intriguingly, inhibition of the kinase activity of Lyn by SU6656, Csk overexpression, or point mutation in the ATP-binding site induced an increase in nuclear Lyn levels. These results suggest that Lyn being imported into and rapidly exported from the nucleus preferentially accumulates in the nucleus by inhibition of the kinase activity and lipid modification.

Potential molecular mechanism for c-Src kinase-mediated regulation of intestinal cell migration

The ubiquitously expressed Src tyrosine kinases (c-Src, c-Yes, and c-Fyn) regulate intestinal cell growth and differentiation. Src activity is also elevated in the majority of malignant and premalignant tumors of the colon. The development of fibroblasts with the three ubiquitously expressed kinases deleted (SYF cells) has identified the role of Src proteins in the regulation of actin dynamics associated with increased cell migration and invasion. Despite this, unexpectedly nothing is known about the role of the individual Src kinases on intestinal cell cytoskeleton and/or cell migration. We have previously reported that villin, an epithelial cell-specific actin-modifying protein that regulates actin reorganization, cell morphology, cell migration, cell invasion, and apoptosis, is tyrosine-phosphorylated. In this report using the SYF cells reconstituted individually with c-Src, c-Yes, c-Fyn, and wild type or phosphorylation site mutants of villin, we demonstrate for the first time the absolute requirement for c-Src in villin-induced regulation of cell migration. The other major finding of our study is that contrary to previous reports, the nonreceptor tyrosine kinase, Jak3 (Janus kinase 3), does not regulate phosphorylation of villin or villin-induced cell migration and is, in fact, not expressed in intestinal epithelial cells. Further, we identify SHP-2 and PTP-PEST (protein-tyrosine phosphatase proline-, glutamate-, serine-, and threonine-rich sequence) as negative regulators of c-Src kinase and demonstrate a new function for these phosphatases in intestinal cell migration. Together, these data suggest that in colorectal carcinogenesis, elevation of c-Src or down-regulation of SHP-2 and/or PTP-PEST may promote cancer metastases and invasion by regulating villin-induced cell migration and cell invasion.

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

Csk-binding protein mediates sequential enzymatic down-regulation and degradation of Lyn in erythropoietin-stimulated cells

We have shown previously that the Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors. The importance of Lyn to red cell maturation has been highlighted by Lyn-/- mice developing anemia. Here we show that Lyn interacts with C-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators C-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk). Lyn phosphorylated Cbp on several tyrosine residues, including Tyr314, which recruited Csk/Ctk to suppress Lyn kinase activity. Intriguingly, phosphorylated Tyr314 also bound suppressor of cytokine signaling 1 (SOCS1), another well characterized negative regulator of cell signaling, resulting in elevated ubiquitination, and degradation of Lyn. In Epo-responsive primary cells and cell lines, Lyn rapidly phosphorylated Cbp, suppressing Lyn kinase activity via Csk/Ctk within minutes of Epo stimulation; hours later, SOCS1 bound to Cbp and was involved in the ubiquitination and turnover of Lyn protein. Thus, a single phosphotyrosine residue on Cbp coordinates a two-phase process involving distinct negative regulatory pathways to inactivate, then degrade, Lyn.

Lyn kinase promotes erythroblast expansion and late-stage development

Lyn kinase is known to modulate the formation and function of B cells, monocytes, and mast cells. However, Lyn-/- mice also develop erythrosplenomegaly, and cases for both negative and positive erythropoietic actions of Lyn recently have been outlined. In phenylhydrazine-treated Lyn-/- mice, extramedullary splenic erythropoiesis was hyperactivated, but this did not lead to accelerated recovery from anemia. Furthermore, ex vivo analyses of the development of bone marrow-derived Lyn-/- erythroblasts in unique primary culture systems indicated positive roles for Lyn at 2 stages. Late-stage Lyn-/- erythroblasts exhibited deficit Ter119(pos) cell formation, and this was paralleled by increased apoptosis (and decreased Bcl-xL expression). During early development, Lyn-/- erythroblasts accumulated at a Kit(pos)CD71(high) stage, possessed decreased proliferative capacity, and were attenuated in entering an apparent G1/S cell-cycle phase. In proposed compensatory responses, Lyn-/- erythroblasts expressed increased levels of activated Akt and p60-Src and decreased levels of death-associated protein kinase-2. Stat5 activation and Bcl-xL expression, in contrast, were significantly decreased in keeping with decreased survival and developmental potentials. Lyn, therefore, is proposed to function via erythroid cell-intrinsic mechanisms to promote progenitor cell expansion beyond a Kit(pos)CD71(high) stage and to support subsequent late-stage development.

Involvement of Golgi-associated Lyn tyrosine kinase in the translocation of annexin II to the endoplasmic reticulum under oxidative stress

Src-family tyrosine kinases, known to participate in signaling pathways of a variety of receptors at the plasma membrane, are found in cellular endomembranes such as the Golgi apparatus and endosomes. Recently, we showed that Lyn, a member of the Src kinases, accumulates on the Golgi apparatus and then traffics to the plasma membrane. We show here that a majority of endogenous Lyn but not c-Src is accumulated in Golgi-enriched heavy-membrane fractions on a sucrose-density gradient, whereas a small amount of endogenous Lyn is present in light-membrane fractions containing the plasma membrane. Inducible expression of kinase-active Lyn, which biosynthetically reaches the Golgi apparatus, triggers tyrosine phosphorylation of proteins including annexin II. Coimmunoprecipitation analyses reveal that Lyn physically associates with annexin II, and an in vitro kinase assay shows that Lyn phosphorylates annexin II directly. Furthermore, stimulation of cells with H2O2 induces tyrosine phosphorylation of annexin II on the Golgi apparatus in a manner that is dependent on the kinase activity of Src kinases, leading to the translocation of annexin II from the Golgi apparatus to the endoplasmic reticulum. Thus, these results suggest that endomembranes containing the Golgi apparatus where Lyn is anchored can serve as a signaling platform under oxidative stress.

Linking tumor cell cytotoxicity to mechanism of drug action: an integrated analysis of gene expression, small-molecule screening and structural databases

An integrated, bioinformatic analysis of three databases comprising tumor-cell-based small molecule screening data, gene expression measurements, and PDB (Protein Data Bank) ligand-target structures has been developed for probing mechanism of drug action (MOA). Clustering analysis of GI50 profiles for the NCI's database of compounds screened across a panel of tumor cells (NCI60) was used to select a subset of unique cytotoxic responses for about 4000 small molecules. Drug-gene-PDB relationships for this test set were examined by correlative analysis of cytotoxic response and differential gene expression profiles within the NCI60 and structural comparisons with known ligand-target crystallographic complexes. A survey of molecular features within these compounds finds thirteen conserved Compound Classes, each class exhibiting chemical features important for interactions with a variety of biological targets. Protein targets for an additional twelve Compound Classes could be directly assigned using drug-protein interactions observed in the crystallographic database. Results from the analysis of constitutive gene expressions established a clear connection between chemo-resistance and overexpression of gene families associated with the extracellular matrix, cytoskeletal organization, and xenobiotic metabolism. Conversely, chemo-sensitivity implicated overexpression of gene families involved in homeostatic functions of nucleic acid repair, aryl hydrocarbon metabolism, heat shock response, proteasome degradation and apoptosis. Correlations between chemo-responsiveness and differential gene expressions identified chemotypes with nonselective (i.e., many) molecular targets from those likely to have selective (i.e., few) molecular targets. Applications of data mining strategies that jointly utilize tumor cell screening, genomic, and structural data are presented for hypotheses generation and identifying novel anticancer candidates.

Increased susceptibility to apoptosis in CD45(+) myeloma cells accompanied by the increased expression of VDAC1

Expression of CD45 is quite variable in human myeloma cells and cell lines, such as U266, and CD45(+) U266 proliferates in response to a growth factor, interleukin-6. Here, we show that CD45(+) myeloma cell lines were more sensitive to various apoptotic stimuli, such as oxidative stress and endoplasmic reticulum (ER)-stress, than CD45(-) cells. Reactive oxygen species and calcium ion seemed to be involved in the susceptibility to apoptosis of CD45(+) U266. The activation of the src family kinases associated with CD45 phosphatase played an important role in the augmented apoptosis in CD45(+) U266 by oxidative stress. These results indicate that the CD45-expression renders myeloma cells competent for not only mitogenic but also apoptotic stimuli, resulting in either proliferation or apoptosis of CD45(+) myeloma cells dependently upon the circumstantial stimuli. Furthermore, voltage-dependent anion channel (VDAC) 1 was identified as a gene highly expressed in CD45(+) U266 by cDNA subtraction. The increased expression of VDAC1 seemed to augment the sensitivity to the ER-stress because the VDAC1-transfected U266 was more susceptible to the thapsigargin-induced apoptosis. Thus, CD45 expression accompanied by the increased VDAC1 expression sensitizes myeloma cells to the various extracellular stimuli that trigger apoptosis via the mitochondrial pathways.

The S2 accessory gene of equine infectious anemia virus is essential for expression of disease in ponies

Equine infectious anemia virus (EIAV) is a macrophage-tropic lentivirus that persistently infects horses and causes a disease that is characterized by periodic episodes of fever, thrombocytopenia, and viremia. EIAV encodes only four regulatory/accessory genes, (tat, rev, ttm, and S2) and is the least genetically complex of all known lentiviruses. We sought to determine the role of the EIAV S2 accessory gene of EIAV by introducing mutations that would prevent S2 expression on the p19/wenv17 infectious molecular clone. Virus derived from the p19/wenv17 molecular clone is highly virulent and routinely fatal when given in high doses (J. Virol. 72 (1998) 483). In contrast, an S2 deletion mutant on the p19/wenv17 background is unable to induce acute disease and plasma virus loads were reduced by 2.5 to 4.0 logs at 15 days post-infection. The S2 deleted virus failed to produce any detectable clinical signs during a 5-month observation period. These results demonstrate that S2 gene expression is essential for disease expression of EIAV.